The L3-6 chondritic regolith breccia Northwest Africa (NWA) 869: (I) Petrology, chemistry, oxygen isotopes, and Ar-Ar age determinations

The L3–6 chondritic regolith breccia Northwest Africa (NWA) 869: (I) Petrology,

chemistry, oxygen isotopes, and Ar-Ar age determinations

Knut METZLER1*, Addi BISCHOFF1, Richard C. GREENWOOD2, Herbert PALME3, MarkoGELLISSEN4, Jens HOPP5, Ian A. FRANCHI2, and Mario TRIELOFF5

1Westfalische Wilhelms-Universitat, Institut fur Planetologie, Wilhelm-Klemm-Strasse 10, 48149 Munster, Germany2Planetary and Space Sciences Research Institute, Open University, Walton Hall, Milton Keynes MK7 6AA, UK

3Sektion Meteoritenforschung, Forschungsinstitut und Naturmuseum Senckenberg, Senckenberganlage 25, 60325 Frankfurt,Germany

4Universitat Kiel, Institut fur Geowissenschaften, Ludewig-Meyn-Str. 10, 24118 Kiel, Germany5Ruprecht-Karls-Universitat, Institut fur Geowissenschaften, Im Neuenheimer Feld 234-236, 69120 Heidelberg, Germany

*Corresponding author. E-mail: [email protected]

(Received 01 May 2010; revision accepted 16 January 2011)

Abstract–Northwest Africa (NWA) 869 consists of thousands of individual stones with anestimated total weight of about 7 metric tons. It is an L3–6 chondrite and probablyrepresents the largest sample of the rare regolith breccias from the L–chondrite asteroid.It contains unequilibrated and equilibrated chondrite clasts, some of which display shock-darkening. Impact melt rocks (IMRs), both clast-free and clast-poor, are stronglydepleted in Fe,Ni metal, and sulfides. An unequilibrated microbreccia, two different lightinclusions and two different SiO2-bearing objects were found. Although the matrix of thisbreccia appears partly clastic, it is not a simple mixture of fine-grained debris formedfrom the above lithologies, but mainly represents an additional specific lithology of lowpetrologic type. We speculate that this material stems from a region of the parent bodythat was only weakly consolidated. One IMR clast and one SiO2-bearing object showD17O values similar to bulk NWA 869, suggesting that both are related to the host rock.In contrast, one light inclusion and one IMR clast appear to be unrelated to NWA 869,suggesting that the IMR clast is contaminated with impactor material. 40Ar-39Ar analysesof a type 4 chondrite clast yield a plateau age of 4402 ± 7 Ma, which is interpreted tobe the result of impact heating. Other impact events are recorded by an IMR clast at1790 ± 36 Ma and a shock-darkened clast at 2216 ± 40 Ma, demonstrating that NWA869 escaped major reset in the course of the event at approximately 470 Ma that affectedmany L–chondrites.

INTRODUCTION

Northwest Africa (NWA) 869 represents one ofthe largest meteorite finds from the Sahara. It consistsof thousands of individual stones from less than 1 gup to more than 20 kg. According to the meteoritedealer Dean Bessey (2007; personal communication),who handled most of the material, the total mass ofrecovered meteorites should be on the order of7 metric tons. Unfortunately, the strewn field isundocumented, and large amount of material has beendistributed without any information about its

provenance. For that reason, the NomenclatureCommittee of the Meteoritical Society demands:‘‘Scientists are advised to confirm the classification ofany specimen they obtain before publishing resultsunder this name’’ (Connolly et al. 2006). Thismeteorite has been classified as an L4–6 fragmentalbreccia (Connolly et al. 2006). Noble gas measurementslater revealed that it contains solar gases (Matsudaet al. 2006; Osawa and Nagao 2006) and hencerepresents a chondritic regolith breccia.

The first author of this paper has inspectedthousands of individual stones from the stocks of

� The Meteoritical Society, 2011. 652

Meteoritics & Planetary Science 46, Nr 5, 652–680 (2011)

doi: 10.1111/j.1945-5100.2011.01181.x

several Moroccan meteorite dealers during the past9 years and is convinced that most samples of NWA869 can be easily identified macroscopically. Typically,their fusion crusts have been removed by wind erosion,but some samples show fusion crust remnants. Fracturefaces, formed by ground collision, show a typical gray-green color and sometimes visible brecciations (lightand ⁄or dark clasts). In addition, the samples show verycharacteristic wind ablation features (Fig. 1a). Parts ofthe surfaces show patches of thin reddish iron oxide andadhering caliche, a typical yellowish fine-grainedsediment that sticks firmly. This is why ‘‘... dealers areconfident that most of the known masses are sufficientlydistinctive from other NWA meteorites ...’’ (Connollyet al. 2006).

There are some individual stones that aremacroscopically similar to NWA 869 but consist of onlyone single L-type lithology, e.g., of petrologic types 5 or6, shock-darkened clasts or impact melt rocks (IMRs).In these cases it is not possible to unambiguously assignthese stones to NWA 869 without doing extensivecosmogenic radionuclide measurements on each stone,which is extremely time-consuming. Although it seemsreasonable to assume that they represent large clasts,broken out of the breccia during passage of themeteoroid through the atmosphere, those samples havebeen excluded from this study.

AIMS OF THE CONSORTIAL STUDY

According to Bischoff and Schultz (2004) only 3%of L–chondrites are regolith breccias (12 of 405meteorites). Northwest Africa 869 possibly representsthe largest sample of these scarce rocks, since itcontains considerable amounts of solar noble gases(Matsuda et al. 2006; Osawa and Nagao 2006; Weltenet al. 2010). Furthermore, NWA 869 escaped thecatastrophic impact event about 470 Ma ago, whichdisturbed or reset the Ar-Ar ages of many L chondriticlithologies (see below). For these reasons a consortialstudy with participation of various research groups wasinitiated to decipher the formation history of NWA 869in detail. An overview of the sample allocation is givenin Table 1. The results are summarized in this paperand in a companion paper (Welten et al. 2011).Preliminary results have been published by Metzleret al. (2008) and Welten et al. (2010). The goals of thisstudy are the following:1. Although NWA 869 represents a large and well-

preserved lithified sample of the L–chondrite parentbody regolith, only minor research has been doneon it. Since further research is desirable to cover itsentire lithologic inventory, we try to definepetrographic and microchemical criteria for anunambiguous identification of NWA 869 samples.This should help to clarify pairings with otherNWA chondrites and to properly choose materialfor future research.

2. 40Ar-39Ar analyses were performed to date theevents that formed or modified some of theobserved lithologies.

3. Oxygen isotopes were measured on bulk samplesand several lithologies to confirm the L chondriticprovenance of the bulk meteorite and to identifyforeign clasts.

4. Measurements of noble gases and cosmogenicradionuclides on bulk samples and separatedlithologies were performed to obtain informationson the irradiation history of the meteoroid duringtransit (transit time, shielding depths) and of itsbreccia components on the parent asteroid prior tolithification. Measurements of 14C and 10Be wereperformed to estimate the terrestrial age of thismeteorite. The results of these investigations arepresented in Welten et al. (2011).

SAMPLES AND ANALYTICAL TECHNIQUES

Seven bulk samples of NWA 869 and 27 of itslithologies (see Table 1) have been studied in detail bythe following methods.

Fig. 1. Typical appearance of NWA 869. a) Individual stonewith remnants of fusion crust and wind ablation features(front). b) Cut surface showing the brecciation texture (light-dark structure) and some typical lithic clasts. Thermallymetamorphosed chondrite clasts of petrologic types 5 and 6are outlined. IMR, impact melt rock clast; type 3, chondriteclast of petrologic type 3; sd, shock-darkened chondrite clast.

The L3–6 chondritic regolith breccia NWA 869: (I) 653

Optical and Scanning Electron Microscopy Including

EDX Analysis

All samples were studied by optical and scanningelectron microscopy, using polished thin sections (seeTable 1). Mineral analyses were obtained using a JEOL840A scanning electron microscope, equipped with anenergy dispersive X-ray (EDX) analysis system (INCA,Oxford Instruments) and all data were normalized to100 wt%. Samples and appropriate mineral standardswere measured at an acceleration voltage of 20 KV andthe beam current constancy was always controlled by aFaraday cup. The standard ZAF correction procedureswere applied. Mineral standards were measured repeatedlyto ensure the reproducibility of the energy dispersive

system. EDX analyses of mineral phases were alwayschecked for stoichiometry. To estimate the relative errorsof the measurements we compared the measured data forthe mineral standard ‘‘Leedey olivine’’ obtained during 28measuring days and calculated the mean values andstandard deviations for MgO, SiO2, and FeO. We did thesame using the mineral standard ‘‘Leedey plagioclase’’ forAl2O3 and CaO. We found the following mean values andstandard deviations (wt%), compared to electron probemicroanalyses by Feldstein et al. (2001; data inparenthesis): MgO: 38.08 ± 0.37 (38.40); SiO2:38.40 ± 0.32 (38.10); FeO: 22.97 ± 0.39 (22.80); Al2O3:21.22 ± 0.36 (21.60); CaO: 2.10 ± 0.10 (2.22).

To obtain bulk elemental ratios for several lithicclasts, they were measured with 100-fold magnification

Table 1. List of investigated samples, analyzed parameters, and methods of investigation.

Lithology SamplePetrography(Pol. micr.)

Chemistry(SEM-EDX)

Chemistry(XRF)

Oxygenisotopes 40Ar-39Ar

Noblegasesa

Cosmogenicradionuclidesa

Bulk NWA 869 D XJ X

M-05-38-1 X XM-05-38-2 X XMS-04-1 XMB-13 X X X

SM-03-1 XClastic matrix D-a X X

M-05-38-2-e X X

MB-13-d X X XMB-13-l X XMB-13-N X X

Type less than 3.5chondrite clasts

M-06-44-1-a X XMB-13-N-11 X X

Type 4 chondrite clast M-05-38-2-b X X X X

Type 5 and 6chondrite clasts

M-05-38-2-d X X XMB-13-c X X X XMB-13-h X X

Shock-darkened

chondrite clasts

M-05-38-2-c X X X X

MB-13-a X X XMB-13-e X XMB-13-N-1 X X

MS-04-1-a X X XIMR clasts SM-03-1-1-a X X X X

M-05-38-1-a X X X X

SM-04-7-1-a X X XMB-13-f X XM-07-25-a X XMB-13-b X X

Light inclusions M-05-38-2-a X XM-07-23-a X X X

Unequilibrated

microbreccia

MB-13-O-3 X X

SiO2-bearingobjects

X-a X X XZ-08-2 X X

Pol. micr. = polarizing microscopy (transmitted and reflected light); SEM-EDX = scanning electron microscopy equipped with energy-

dispersive X-ray analysis; XRF = X-ray fluorescence analysis.aFor results see Welten et al. (2011).

654 K. Metzler et al.

https://www.researchgate.net/publication/227780065_Disequilibrium_partial_melting_on_the_Leedey_L6_chondrite_Textural_controls_on_the_melting_processes?el=1_x_8&enrichId=rgreq-3c6b563a-a86a-4ddf-a441-ad80189cae65&enrichSource=Y292ZXJQYWdlOzIyOTg3NzEyNTtBUzoxNTI4MjA5MDA3MDAxNjVAMTQxMzQ0Njc1MDM4Mg==

for 100 s, moving the sample stage under the electronbeam. Due to the coarse grain size of most lithologiessome shifts of element concentrations were anticipated.For that reason the values for SiO2, MgO, CaO, andAl2O3 for bulk NWA 869 were compared to thoseobtained by X-ray fluorescence (XRF) analysis. In thisway empirical correction factors (cf) were obtained,which turned out to be only marginal for SiO2 (cf0.959), MgO (cf 1.002) and CaO (cf 0.976). The valuefor Al2O3 shows a distinct deviation and needs a cf of0.715 to be corrected according to the XRF value. Dueto the various oxidation states of Fe in the samples thedata for this element were not used. Values for FeSwere calculated using the measured values for sulfur.

The modal composition of NWA 869 has beendetermined on large cut surfaces (591 cm2 in sum) bypoint counting techniques (Table 2).

X-ray Fluorescence Analysis

Major, minor, and some trace elements for bulkNWA 869 and three of its lithologies were determined byXRF analyses (see Table 1). Splits were cleaned in anultrasonic bath and homogenized by crushing in an agatemortar. About 120 mg of sample was fused with 3.6 g ofLi2B4O7. The fused beads were analyzed using a PhilipsPW 2400 XRF spectrometer at the Universitat zu Koln.The concentrations of the major elements Si, Mg, Al, andCa were determined with an accuracy of better than 3%,and the concentrations of the minor and trace elementsabundances (Ti, Cr, Mn, Fe, P, V, Ni, Zn) withaccuracies below 5%. These estimates are based on well-analyzed standard rocks and the Allende analyses ofJarosewich (1990). For details see Wolf and Palme (2001).Sodium, K, and S have not been determined because ofpossible volatilization during processing.

Oxygen Isotope Analysis

Oxygen isotope analysis of bulk material and fourdifferent lithologies from NWA 869 (see Table 1) was

undertaken using an infrared laser fluorination system(Miller et al. 1999). Fresh interior chips weighingapproximately 100 mg were sampled from each of theanalyzed lithologies. These were then powdered andhomogenized and analyses were undertaken onapproximately 2 mg aliquots taken from this powder.O2 was liberated by heating these powders using aninfrared CO2 laser (10.6 lm) in the presence of 210 torrof BrF5. After fluorination, the O2 released was purifiedby passing it through two cryogenic nitrogen traps andover a bed of heated KBr. The O2 was analyzed using aMicromass Prism III dual inlet mass spectrometer.Overall system precision (1r), based on replicateanalyses of international (NBS-28 quartz, UWG-2garnet) and internal standards, is approximately±0.04& for d17O; ±0.08& for d18O; ±0.024& forD17O (Miller et al. 1999). The quoted precision (1r) forthe various lithologies from NWA 869 is based onreplicate analyses. Oxygen isotope analyses are reportedin standard d notation where d18O has been calculatedas: d18O = ([18O ⁄ 16Osample ⁄ 18O ⁄ 16Oref])1) · 1000 andsimilarly for d17O using 17O ⁄ 16O ratio. D17O has beencalculated as: D17O = d17O)0.52 d18O.

40Ar-39Ar Dating

40Ar-39Ar analysis followed standard proceduresgiven by Jessberger et al. (1980) and Trieloff et al.(1994, 1998, 2003, 2005). Samples were wrapped inhigh-purity (99.999%) Al-foil and irradiated for 20 daysin evacuated quartz ampoules, applying 1 mm cadmiumshielding to suppress the 37Cl(n,cb))38Ar reaction, at theGKSS-reactor in Geesthacht, Germany.

The J-value was 1.08 · 10)2 as determined by2657 ± 4 Ma old NL25 hornblende flux monitors(Schaeffer and Schaeffer 1977; Schwarz and Trieloff2007a). Correction factors for interfering isotopesdetermined by CaF2 monitors were (36Ar ⁄ 37Ar)Ca =(4.3 ± 0.2) x 10)4, (38Ar ⁄ 37Ar)Ca = (8.8 ± 0.2) · 10)5,and (39Ar ⁄ 37Ar)Ca = (9.8 ± 0.1)· 10)4. (38Ar ⁄ 39Ar)K =(5.8 ± 0.9) · 10)3 was determined via the NL25monitors, after subtraction of Cl-induced 38Ar.(40Ar ⁄ 37Ar)Ca = (3 ± 3) · 10)3 was taken from Turner(1971) and (40Ar ⁄ 39Ar)K = (1.23 ± 0.24) · 10)2 fromBrereton (1970). The samples were stepwise heated totemperatures from 500 �C to 1400 �C (up to 26temperature steps) using a resistance-heated furnacewith 40Ar blank values of (6–12) · 10)10 cm3 STP at850 �C and (16–30) · 10)10 cm3 STP at 1400 �C (10 minheating duration). Extraction temperatures werecontrolled based on heating power calibrated by athermocouple. Apparent ages were calculated using theSteiger and Jager (1977) conventions. Givenuncertainties of apparent ages are 1r. An anticipated

Table 2. Modal composition of NWA 869 (pointcounting, 1140 points, 591 cm2) and maximum clast sizeof lithologies.

LithologyAbundance(vol%)

Maximumclast size (mm)

Clastic matrix 73.6Type 5 and 6 chondrite clasts 20.3 55Shock-darkened chondrite clasts 3.9 33

IMR clasts 0.9 45Type 3 chondrite clasts 0.8 15Fe,Ni metal more than 2 mm 0.3 6

Sulfide more than 2 mm 0.2 5


https://www.researchgate.net/publication/256158482_Steiger_R_H_Jaeger_E_Subcommission_on_geochronology_convention_of_the_use_of_decay_constants_in_geo-_and_cosmochronology_Earth_Planet_Sci_Lett_36_359-362?el=1_x_8&enrichId=rgreq-3c6b563a-a86a-4ddf-a441-ad80189cae65&enrichSource=Y292ZXJQYWdlOzIyOTg3NzEyNTtBUzoxNTI4MjA5MDA3MDAxNjVAMTQxMzQ0Njc1MDM4Mg==

recalibration of the K decay constants (Begemann et al.2001; Mundil et al. 2006; Schwarz and Trieloff 2007b) isnot taken into account here, as such a recalibrationwould increase apparent ages by about 0.7–1.0%, i.e.,not significant within the uncertainties of our results.

The concentration of 36Artrapped in most sampleswas calculated by component deconvolution using the36Ar ⁄ 38Ar ratios that vary between a trappedcomponent of 5.35 and a cosmogenic component with36Ar ⁄ 38Ar = 0.65. Values lower than 0.65 indicatecontributions of 38Ar derived from neutron reactions onCl during reactor irradiation.

RESULTS

Weathering

Northwest Africa 869 has been attributed toweathering class W1 (Connolly et al. 2006), i.e.,weathering of typical samples is minimal. Very minorcalcite veining has been observed in the outer portionsof some individual stones. Typically, some oxidationoccurs in type 5 and 6 chondrite clasts, leading to theirspeckled brown-white appearance (e.g., Fig. 1b, lowerleft clast). Metals in shock-darkened chondrite claststend to rust more efficiently than other metals and theseclasts are frequently surrounded by thin brownishoxidation haloes. There are only a few samples of NWA869 which are nearly free of oxidation.

On the other hand large fragments (mostly morethan 1 kg) with remnants of fusion crust exist that aremore or less covered by caliche. These fragmentsprobably stem from large individuals that wereshattered during impact and buried in the soil, showinga more intense weathering.

Shock Effects

Shock ClassificationThe shock classification of this meteorite is S3

(Connolly et al. 2006), which means weakly shockedwith a corresponding shock pressure of 15–20 GPa(Stoffler et al. 1991). We did not observe components oflower shock stage and hence this meteorite seems to beshocked to this degree as an entity. Lithic clastsshocked to a higher degree than S3 prior to theiradmixture to the host breccia can be frequently foundand are described below (shock-darkened chondriteclasts and IMRs).

Offset Planes and Shock VeinsOffset planes were occasionally observed, indicating

some relative motions of rock components during shockexposure. Shock veins with thicknesses up to about

100 lm rarely occur, but large cut surfaces (e.g.,Fig. 1b) are free of these features. One lithic clasthaving a shock vein of an earlier generation with athickness of about 1 mm has been found.

Overall Texture, Modal Composition, and Lithologic

Inventory of NWA 869

Northwest Africa 869 represents a coarse-grainedchondritic breccia, consisting of various types of lithicclasts with different degrees of thermal and shockmetamorphism. It displays a light-dark structure madeof light-colored, metamorphosed chondrite clasts ofpetrologic type 5 and 6 set into a darker, partly clasticmatrix (Fig. 1b). This appearance is typical ofchondritic regolith breccias and similar to type Abreccias (Bischoff et al. 1982, 1983). The textures ofsome clastic matrix portions are shown in Fig. 2a.

We found chondrite clasts of many petrologic types(less than 3.5–6; Fig. 3), some of which are shock-darkened (Fig. 4). IMRs, both clast-free and clast-poor,occur in a wide variety (Fig. 5). In addition, oneunequilibrated microbreccia (Fig. 6), two lightinclusions (Figs. 7a and 7b), and two SiO2-bearingobjects (Figs. 7c and 7d) were found and investigated indetail.

Northwest Africa 869 consists of about 74 vol%clastic matrix, in which about 20 vol% chondrite clastsof petrologic types 5 and 6 and about 4 vol% shock-darkened chondrite clasts are embedded. Clasts ofIMRs (0.9 vol%), chondrite clasts of petrologic type 3(0.8 vol%), large (more than 2 mm) Fe,Ni metal(0.3 vol%) and sulfide grains (0.2 vol%) are minorconstituents (see Table 2).

Petrology and Mineral Chemistry of NWA 869 and Its

Lithologic Components

In the following paragraphs a short petrologicdescription is given for each lithology. The fayalite (Fa)and ferrosilite (Fs) values for olivines and low-Capyroxenes, respectively, are summarized in Table 3 andshown in Figs. 8–11. The petrologic types of chondriteclasts were determined using table 1 from Huss et al.(2006), table 3 from Weisberg et al. (2006), andtable 2.6 from Hutchison (2004).

Bulk NWA 869Northwest Africa 869 is currently classified as a

fragmental breccia of type L4–6 (Connolly et al. 2006).To document its unequilibrated status and to underscorethe proposed reclassification as an L chondritic regolithbreccia of type 3–6 all measured olivine and low-Capyroxene compositions are summarized in Fig. 8. The


https://www.researchgate.net/publication/4690657_Ca-Al-rich_chondrules_and_inclusions_in_ordinary_chondrites?el=1_x_8&enrichId=rgreq-3c6b563a-a86a-4ddf-a441-ad80189cae65&enrichSource=Y292ZXJQYWdlOzIyOTg3NzEyNTtBUzoxNTI4MjA5MDA3MDAxNjVAMTQxMzQ0Njc1MDM4Mg==

Fig. 2. Textures of the partly clastic matrix and embedded isolated chondrules of undisturbed appearance. a) Clastic portionwith strong variations in olivine chemistry (SEM-BSE image; Fa, fayalite content of olivine). b) Barred olivine chondrule (center)with dark glassy mesostasis and radial pyroxene chondrule (upper right center; transmitted light). c) Cryptocrystalline pyroxenechondrule (left) and large radial pyroxene chondrule (right; transmitted light). d) Porphyritic olivine chondrule with glassymesostasis (transmitted light).

Fig. 3. Textures of various types of chondrite clasts in NWA 869. a, b) Clasts of petrologic type less than 3.5. a) MB-13-N-11;b) M-06-44-1-a. c) Clast of petrologic type 4 (M-05-38-2-b). d) Highly equilibrated clast of petrologic type 5 or 6 (M-05-38-2-d).White: Fe,Ni metal and sulfide (SEM-BSE images).


variability of olivine compositions ranges from Fa0 toFa50, with a peak around Fa25, that of low-Ca pyroxenesfrom Fs0 to Fs35 with a peak around Fs21. Both peak

values are typical for metamorphosed L–chondrites (e.g.,Brearley and Jones 1998), here representing thelithologies of petrologic type 5 and 6 found in NWA 869.

Fig. 4. Textures of shock-darkened chondrite clasts. a) Clast of petrologic type less than 3.5 (MS-04-1-a). b) Clast of petrologictype 3.9 (MB-13-a). c) Clast of petrologic type 5 or 6 with networks of sulfide and Fe,Ni metal (M-05-38-2-c). d) Clast ofpetrologic type 5 or 6 with indications of postshock annealing (MB-13-e). White: Fe,Ni metal and sulfide (SEM-BSE images).

Fig. 5. Textures of IMRs. a) IMR clast with slightly zoned olivine phenocrysts (bottom) and skeletal Fe-rich olivine in a glassymesostasis (SM-03-1-1-a). b) IMR clast with laths of zoned olivine in a glassy mesostasis (M-05-38-1-a). c) IMR clast witheuhedral grains of zoned olivine in a Fe-poor, partly devitrified mesostasis (SM-04-7-1-a). d) IMR clast, consisting of euhedralzoned olivine grains set in a partly devitrified glassy mesostasis. Relictic FeO-rich olivine fragments are admixed (arrows) whichare overgrown by Mg-rich olivine (MB-13-b). White: Fe,Ni metal and sulfide (SEM-BSE images).


Fig. 6. Clast of an unequilibrated microbreccia (MB-13-O-3). a) Overview, showing alignment and layering of containing clasts.Black grain in the upper right center is nearly pure forsterite. b) Shocked clast within the microbreccia with sulfideimpregnations (upper half) and an extremely Fe-rich fine-grained matrix (upper right corner); the lower half of the image showsthe high diversity of olivine and pyroxene compositions in the microbreccia. c) Fayalite (Fa) and ferrosilite (Fs) contents ofvarious olivine and pyroxene grains. d) Texture of interstitials between olivine and pyroxene grains. These are either voids orfilled with small amounts of cementing material. White: Fe,Ni metal and sulfide (SEM-BSE images).

Fig. 7. Textures of some rare lithologies in NWA 869. a, b) Light inclusions. c, d) SiO2-bearing objects. a) Intergrowth ofslightly zoned olivine (ol), low-Ca pyroxene (px), and plagioclase (plag) in the light inclusion M-05-38-2-a. b) Crystals of mm-sized, slighly zoned olivine (ol) and Ca-poor pyroxene (px) set in a quenched mesostasis of Ca-rich pyroxene and Ca-richplagioclase in the light inclusion M-07-23-a. c) SiO2 ‘‘dendrites’’ (SiO2) and Ca-poor pyroxene (px) in the SiO2-bearing objectX-a. d) Former chondrule (right), partly replaced by SiO2 (SiO2) in the SiO2-bearing object Z-08-2. On the left the relict of abarred olivine chondrule (BOC) is outlined. The black horizontal feature is a crack (SEM-BSE images).


Table

3.Mineralchem

istryofolivineandlow-C

apyroxenein

theinvestigatedlithologies,

andinferred

petrologic

types.

Lithology

Sample

Rem

arks

Olivines;fayalite

(Fa)contents

Low-C

apyroxenes;ferrosilite

(Fs)

contents

Petrologic

type

Famean

(mole%

)PMD

Famin–max

(mole%

)n

Fsmean

(mole%

)PMD

Fsmin–max

(mole%

)n

Clastic

matrix

MB-13-d

25.5

423.2–27.4

15

21.1

10

13.4–22.6

15

MB-13-N

Area#1

23.3

14

14.1–29.3

15

19.5

10

14.2–22.0

12

Area#2

20.7

32

10.3–26.3

16

17.4

29

7.0–21.2

14

Area#3

25.2

622.9–27.9

13

21.9

718.5–23.9

9Type3

chondrite

clasts

M-06-44-1-a

Chondrules

20.7

61

0.2–35.0

17

8.4

65

2.9–20.0

18

Lessthan3.5

Matrix

area#1

33.5

232.4–35.0

13

n.f.

Matrix

area#2

36.9

335.2–39.0

5n.f.

MB-13-N

-11

Chondrules

18.3

58

0.6–31.3

19

6.7

52

2.1–13.3

19

Lessthan3.5

Matrix

30.8

328.8–32.2

13

27.6

526.8–29.3

2

Type4

chondrite

clast

M-05-38-2-b

24.7

223.8–25.4

918.3

22

9.8–21.1

11

4

Type5and6

chondrite

clasts

M-05-38-2-d

26.3

225.5–26.8

10

21.8

320.9–23.0

10

5or6

MB-13-c

25.4

323.9–25.9

15

21.6

220.6–22.4

15

5or6

MB-13-h

25.2

124.8–25.5

13

21.0

120.6–21.5

10

5or6

Shock-darkened

chondrite

clasts

M-05-38-2-c

25.5

423.1–27.2

10

20.7

618.1–23.1

10

5or6

MB13-a

25.7

622.5–27.0

10

21.9

28

12.3–35.8

93.9

MB-13-e

26.1

225.4–26.9

25

22.0

221.3–25.7

25

5or6

MB-13-N

-123.9

323.0–25.2

12

21.3

917.9–24.6

94

MS-04-1-a

19.1

53

1.2–30.6

18

13.0

59

2.3–26.8

10

Lessthan3.5

IMR

clasts

SM-03-1-1-a

24.7

43

10.2–41.2

14

n.f.

Clast

within

IMR

24.6

223.9–25.2

519.8

13

15.3–21.5

54

Clast

within

IMR

25.2

224.6–25.5

521.3

220.9–22.1

55or6

M-05-38-1-a

19.0

34

10.3–32.5

40

n.f.

SM-04-7-1-a

17.1

15

12.4–21.3

14

n.f.

MB-13-f

13.8

23

10.5–21.0

14

n.f.

Clast

within

IMR

25.6

324.5–26.0

521.3

220.8–22.1

55or6

M-07-25-a

21.5

56

8.6–35.7

6n.f.

MB-13-b

11.8

22

9.8–17.9

14

n.f.

Clastswithin

IMR

24.2

322.6–24.8

720.5

020.4–20.6

75or6

Lightinclusions

M-05-38-2-a

23.6

12

19.3–28.3

15

21.7

121.5–22.3

10

M-07-23-a

23.2

419.1–24.5

30

19.2

417.8–20.6

22

Unequilibr.

microbreccia

MB-13-O

-324.9

54

1.2–49.2

71

15.1

49

1.7–35.9

26

SiO

2-bearing

objects

X-a

22.4

022.3–22.4

219.3

119.0–19.7

15

Z-08-2

15.7

22

5.5–17.8

19

11.0

41

3.7–18.8

21

3.7

PMD

=standard

deviation

expressed

asapercentageofthemean;this

term

isused

accordingto

table

2.6

ofHutchison

(2004),

itdoes

notcorrespond

exactly

totheoriginal

definitionofPMD

byDoddet

al.(1967);min–max=

minim

um

andmaxim

um

values

measured;n=

number

ofanalyses;n.f.=

notfound.


Clastic MatrixThe clastic matrix of NWA 869 makes up about

74 vol% of this meteorite (Table 2). Although termed‘‘clastic matrix’’ this groundmass in which the lithic clastsare embedded appears only partly clastic (Fig. 2a). Itdoes not seem to be a simple mixture of fine-graineddebris formed from the coherent lithologies mentionedabove, but seems to represent an additional specificlithology of low petrologic type. Isolated unequilibratedchondrules with perfect outer shapes, sometimes withglassy mesostasis (Figs. 2b and 2d), are admixed. Several

matrix areas were chosen to obtain olivine and pyroxenestatistics separately (Table 3). The chemical compositionsshow large variations with Fa10–28 for olivines, Fs7–24 forlow-Ca pyroxenes, and En47–48Fs8–10Wo43–46 for Ca-richpyroxenes. It turned out that matrix areas differ in theircorresponding statistics, even if they are located less than1 cm from each other. The mean compositions of olivineand pyroxene from four different portions of clasticmatrix are shown in Fig. 9.

Isolated Grains of Fe,Ni Metal, and Sulfide with SizesMore Than 2 mm

As is typical for ordinary chondrites, most metalsand sulfides reside outside of chondrules as isolatedgrains. Some of these grains reach sizes of more than2 mm and either occur monomineralic (kamacite, taenite,or sulfide) or as intergrowth. Isolated grains of Fe,Nimetal (mostly kamacite-taenite-intergrowth) and sulfidegrains more than 2 mm occur with abundances of 0.3 and0.2 vol%, respectively, and reach grain sizes of 6 mm inthe case of Fe,Ni metal and 5 mm in the case of sulfide(Table 2). In addition, several large sulfide grains werefound that were surrounded by an Fe,Ni metal rim.

Type 3 Chondrite ClastsWe found unequilibrated chondrite clasts of

petrologic type 3 with sizes up to 1.5 cm (Table 2),which mainly consist of olivine-pyroxene-richchondrules with only small amounts of interstitial fine-grained matrix (Figs. 3a and 3b). These clasts appeardark on cut surfaces and in thin sections. Chondrules ofmany types have been observed, some of which containpink and brownish chondrule glass. Fe,Ni metal mainlyoccurs as large single masses outside of chondrules.Accessory minerals in chondrules and matrices areplagioclase, sulfide, chromite, and seldom magnetite.

Fig. 8. Fayalite (Fa) contents of olivines (left) and ferrosilite (Fs) contents (right) of low-Ca pyroxenes in NWA 869. The datafrom all lithologies are summarized to demonstrate the unequilibrated state of this meteorite.

Fig. 9. Mean fayalite (Fa) and ferrosilite (Fs) values forolivine and low-Ca pyroxene, respectively, from various clastsand clastic matrix portions of NWA 869. For unequilibratedchondrite clasts the corresponding variation of Fa and Fs isindicated by solid lines with arrows (dashed lines meanextensions beyond the scale). The field for equilibratedL–chondrites (Brearley and Jones 1998) is shown forcomparison.


Two clasts of petrologic type less than 3.5 werechosen for further investigation (see Table 1). Olivine andlow-Ca pyroxene in chondrules show large chemicalvariations in composition (Fa0–35 and Fs2–20, respectively;see Table 3). In contrast to this the fine-grainedinterchondrule matrices show recrystallization texturesand their olivines and pyroxenes are nearly equilibrated(percentage mean deviation [PMD] of Fa and Fs: 2–5%;Table 3). The mean Fa-contents of olivines from theequilibrated interchondrule matrices vary from place toplace (e.g., Fa31, Fa34, Fa37; see Table 3). Even main Favalues for two matrix areas in a given clast (M-06-44-1-a;Table 3) differ significantly from each other. Either eachmatrix area has been equilibrated locally with itsimmediate surroundings or it is possible that eachchondrule originally carried its own matrix of distinctcomposition in the shape of chondrule rims, as is the case

for CM chondrites (e.g., Metzler et al. 1992; Metzler andBischoff 1996). In Figs. 9 and 10 the variability ofolivines and low-Ca pyroxenes is shown which documentsthe unequilibrated character of these lithologies.

Type 4 Chondrite ClastsOne clast has been separated due to its unusual

porous appearance. It turned out that its olivines arenearly equilibrated with a mean of about Fa25 and aPMD of Fa contents of 2, whereas pyroxenes are veryheterogeneous with a mean of about Fs18 and a PMDof Fs contents of 22 (Table 3), which is typical forpetrologic type 4 lithologies. This clast plots at the edgeof the field of equilibrated L–chondrites (Brearley andJones 1998) in Fig. 9. Its texture is shown in Fig. 3c. Inaddition, one shock-darkened chondrite clast of thispetrologic type has been found (see below).

Fig. 10. Fayalite (Fa) contents of olivines (left; 67 grains) and ferrosilite (Fs) contents of low-Ca pyroxenes (right; 39 grains) intwo chondrite clasts of petrologic type less than 3.5 from NWA 869. These data, obtained from chondrules and interchondrulematrices, demonstrate the unequilibrated state of these clasts.

Fig. 11. Fayalite (Fa) contents of olivines (left; 71 grains) and ferrosilite (Fs) contents of low-Ca pyroxenes (right; 26 grains) inthe unequilibrated microbreccia MB-13-O-3. The data show the unequilibrated state of this lithology.


Type 5 and 6 Chondrite ClastsHighly metamorphosed clasts of petrologic types 5

and 6 make up the majority of lithic clasts with a modalabundance of about 20 vol% (Table 2). The maximumclast size is 5.5 cm (see Fig. 1b, lower left). In this worktype 5 and type 6 chondrite clasts are not distinguishedand summed up in the following due to small petrologicdifferences and often small grain sizes. Their chondrulesand other primary components are blurred andextensive recrystallization textures can be observed(Fig. 3d). The measured mean composition of olivine isabout Fa26 and that of low-Ca pyroxene is about Fs22with very low PMD of Fa and Fs contents of 1-3(Table 3), indicative of strong equilibration. The meancompositions of olivine and pyroxene from various type5 and 6 clasts plot within the field of equilibratedL–chondrites (Brearley and Jones 1998) (Fig. 9).

Shock-Darkened Chondrite ClastsShock-darkened chondrite clasts occur with sizes up

to 3.3 cm and make up about 4 vol% of theinvestigated samples (Table 2). These clasts oftendisplay strong internal brecciations and are crosscut byfinely dispersed sulfide and Fe,Ni metal veins. Theseopaque minerals penetrate from the outside intochondrules and mineral clasts, and occupy grainboundaries and cracks (Fig. 4), leading to the darkappearance. This texture has been interpreted as theresult of mobilization of sulfides and metal due to theextensive increase of temperature and pressure causedby shock waves (e.g., Dodd 1981; Stoffler et al. 1991;Rubin 1992). Similar textures were also producedexperimentally by high strain-rate deformationexperiments (Van der Bogert et al. 2003).

Shock-darkening is generally restricted to isolatedclasts, i.e., shock-darkening always predated clastformation. We found that clasts of many petrologictypes are affected by this process. Five clasts have beeninvestigated in detail (Table 3), two of which areclassified as petrologic type 3 (less than 3.5, 3.9; Figs. 4aand 4b), one as type 4 (e.g., Fig. 4c), and two as type 5or 6 (Fig. 4d). The mean compositions of olivine andpyroxene from various shock-darkened chondrite clastsare shown in Fig. 9.

There are distinct differences in the opacitybetween various shock-darkened chondrite clasts in thinsections. It turned out that the darkest clasts showextremely branched networks of sulfides and someFe,Ni metal veins throughout the entire texture(Figs. 4a, 4b, and 4c). In contrast to this a moretranslucent variety (MB-13-e) shows a coarsening ofsulfides and the networks seem to have beentransformed to chains of sulfide beads with typicaldiameters of 10–50 lm (Fig. 4d).

IMRsThe abundance of IMRs in NWA 869 is about

1 vol% with clast sizes up to 4.5 cm (Table 2). Theselithologies represent a subsidiary but characteristic anddistinct component of NWA 869. The appliednomenclature follows the suggestions by Stoffler andGrieve (2007). Six IMR clasts (see Table 1) have beenanalyzed and are described separately in the following.Three of these clasts are clast-free and three of themare clast-poor. The latter are characterized by variousamounts of admixed mineral and lithic clasts,embedded within the crystallized melts. The meancompositions of crystallized olivines vary between Fa12(MB-13-b) and Fa25 (SM-03-1-1-a) (Table 3, Fig. 9).Pyroxenes were only observed as admixed relict grains,but not as grains crystallized from the melts. Sulfideand Fe,Ni metal are strongly depleted in all investigatedIMRs (see below) and show very characteristic texturesin the shape of fine-grained intergrowth, where Fe,Nimetal occurs as droplets within the sulfide. Thesetextures have been described as ‘‘fizzed troilite,’’indicating impact melting followed by rapidcrystallization (Scott 1982).

SM-03-1-1-a: is a clast-poor IMR with a size of1.2 cm consisting of skeletal olivine needles up to1.7 mm set into a glassy mesostasis. These olivinecrystals are strongly zoned with Mg-rich cores (Fa10–17)and Fe-rich rims (Fa20–41) (Table 3). The mesostasis isnearly opaque in transmitted light, containing a secondgeneration of fine-grained Fe-rich (Fa36–41) skeletalolivines with lengths up to 200 lm (Fig. 5a). Sulfide andFe,Ni metal droplets with sizes up to 30 lm arehomogeneously distributed throughout the mesostasis.This IMR contains admixed chondrite clasts ofpetrologic types 4–6.

M-05-38-1-a: is a clast-free IMR (1.0 cm indiameter) with chemically zoned olivines in the shape ofskeletal needles, set into a partly devitrified glassymesostasis (Fig. 5b). It shows a striking grain sizegradient with olivine needles of about 1 mm in thecenter and only 20 lm at its edge. This probably reflectsthe temperature gradient during cooling of this smallIMR unit. The olivine compositions vary between Fa10and Fa33 (Table 3).

SM-04-7-1-a: comes from an individual of NWA869 with a weight of about 40 g which consists almostentirely of this clast-free IMR (max. 4.9 cm indiameter). It is composed of small (less than 40 lm)euhedral zoned olivine crystals (Fa12–21) (Table 3), setinto a partly devitrified glassy mesostasis (Fig. 5c). Tinybeads of sulfide with sizes up to 4 lm are evenlydispersed in the mesostasis. This rock is furthercharacterized by the occurrence of large metal-sulfide-intergrowths with grain sizes up to 3 mm.


MB-13-f: is a clast-poor IMR with a size of 1.2 cmwhich consists of chemically zoned skeletal olivines, setinto a partly devitrified glassy mesostasis. The olivinecompositions vary between Fa11 and Fa21 (Table 3).Fe,Ni metal and sulfides occur with grain sizes up to50 lm. Admixed are pyroxene fragments with sizes upto 0.9 mm and equilibrated olivine fragments,overgrown by Mg-rich olivine (Fig. 5d). Oneequilibrated, chromite-bearing chondrite clast has beenfound within this IMR (see Table 3).

M-07-25-a: is a clast-free IMR with a diameter of2.8 cm and an ameboid outer shape. Euhedral zonedolivines (Fa9–36; Table 3) with sizes up to 100 lm are setinto an inhomogeneous glassy mesostasis with sulfide andFe,Ni metal droplets (less than 50 lm). The mesostasis ispartly devitrified, showing tiny chromite dendrites.

MB-13-b: represents a small (0.6 cm) clast-poor IMRwith porphyritic texture, consisting of chemically zonedolivine phenocrysts (less than 70 lm). The interstitialmesostasis is interspersed with Fe-rich skeletal olivinecrystals with sizes up to 20 lm. The olivine compositionsvary between Fa10 and Fa18 (Table 3). Fe,Ni metal andsulfides occur with grain sizes up to 200 lm. Equilibratedchondrite clasts (Table 3) with sizes up to 300 lm areadmixed, as well as isolated feldspar, chromite, andequilibrated olivine fragments, where the latter areovergrown by Mg-rich olivine (Fig. 5d).

Unequilibrated Microbreccia (MB-13-O-3)One angular clast with the longest dimension of

about 600 lm has been found (Fig. 6a), which is nearlyopaque in transmitted light due to the fine grain size(less than 5 lm) of metal and sulfide grains (Figs. 6b,6c, and 6d). It shows an internal layering, indicated bythe inhomogeneous distribution of opaque phases(Fig. 6a). This lithology consists mainly of tiny (lessthan 1–10 lm) olivine and pyroxene grains of verydifferent compositions (Table 3, Figs. 6c, 9, and 11),and Fe,Ni metal and sulfide grains. Interstitial areasbetween grains are either cavities or contain a Si-Al-Ca-Na-rich filling (Fig. 6d). Many olivines are zoned andolivine compositions vary between Fa1 and Fa49(Table 3), where Mg-rich olivines show thin rims of Fe-rich olivine (Fig. 6d). Low-Ca pyroxenes vary betweenFs2 and Fs36 (Table 3). A few grains of Ca-rich feldspar(An65–66) have been observed. One chondrite clast withthe longest dimension of 300 lm was found showingshock-impregnated sulfide veins (Fig. 6b). It consists ofMg-rich pyroxene grains, embedded into a fine-grainedmatrix of extremely Fe-rich (Fa45–49) olivines.

Light InclusionsTwo subrounded clasts were found which are

characterized by their light appearance on cut surfaces

and their small amounts of sulfide (Table 5) and Fe,Nimetal. Both inclusions differ from IMRs in severalaspects (see below) and hence are described separately.The mean compositions of their olivine and pyroxeneare shown in Fig. 9. Although these clasts are groupedtogether, they do not seem to be genetically linked.

M-05-38-2-a: This clast has the longest dimensionof 1.1 cm and shows a granular texture (Fig. 7a). Itconsists of zoned olivine (Fa19–28) and low-Ca-pyroxene(Fs22; Table 3) with typical grain sizes of about 200 lm.Zonation of olivine proves that this lithology hasescaped considerable thermal equilibration. Theinterstitial mesostasis is interspersed with sulfide andFe,Ni metal beads with sizes between less than 5 and100 lm.

M-07-23-a: This coarse-grained clast has the longestdimension of 1.7 cm, showing an ophitic to subophitictexture (Fig. 7b). It has a complex mineralogicalcomposition and consists of euhedral, mm-sized zonedolivine (Fa19–25) and low-Ca-pyroxene (Fs18–21; Table 3)crystals, Na-rich feldspar (An7–24), and K-rich feldspar(Ab74An3Or23) with interstitial Ca-rich mesostasis. Thelatter is characterized by a quench texture, i.e., cotecticcrystallization of Ca-rich plagioclase (An61–86) and Ca-rich pyroxene (En48–67Fs6–16Wo17–46). Overgrowth oflow-Ca pyroxene by Ca-rich pyroxene has beenfrequently observed. Accessories are Fe,Ni metal,chromite, merrillite, ilmenite, and small amounts ofsulfide.

SiO2-Bearing ObjectsTwo objects with small amounts of Fe,Ni metal and

sulfide (Table 5) have been found that contain freeSiO2, a rare component in chondritic constituents. Theirmean olivine and pyroxene compositions are shown inFig. 9.

X-a: This chondrule-like object with a diameter of6 mm appears nearly white on the cut surface andshows patchy domains in transmitted light. It consistsof a groundmass of fine-grained (less than 5–50 lm)and homogeneous low-Ca pyroxenes (Fs19) and a muchlower amount of olivine (Fa22), both of which are wellequilibrated. Embedded are ameboid ‘‘droplets’’ of SiO2

(10–300 lm in size) with dendritic shapes (Fig. 7c). Thepyroxenes contain tiny vugs (less than 5 lm) and theirmargins have a corroded appearance. Accessories arerelictic Ca-rich pyroxenes (En47–62Fs7–12Wo26–44),chromite, sodic feldspars (Ab85–90An1–4Or7–14),chlorapatite, and Fe,Ni metal. Sulfides seem to beabsent. A Na-Al-Si-rich component occurs between thegroundmass pyroxenes and as droplets (less than 5 lm)within the latter.

Z-08-2: This angular clast with a diameter of about3 cm has a medium gray appearance and shows some


layering. It represents an unusual, unequilibratedchondrite clast of type 3.7 with embedded chondrulerelicts (Fig. 7d). The texture is dominated bycontraction cracks. Most SiO2 grains are eitherisometric or outline relict chondrules (Fig. 7d). Someameboid SiO2 grains, similar to those in inclusion X-a(see above) were found. Olivines are unusually low inFe with a mean of Fa16, which is the lowest mean Fa-value of olivine among all investigated chondrite clasts.The compositions of olivine vary between Fa6 and Fa18,those of low-Ca pyroxene between Fs4 and Fs19. Someenstatite grains rimmed by Ca-rich pyroxene werefound. Accessories are Ca-rich feldspar (Ab35An65Or0),chromite, and fine-grained sulfide. Small amounts ofvery Ni-rich (56 wt% Ni) metal occur.

Al-Rich ObjectsThe investigated samples were optically searched for

Ca,Al-rich inclusions (CAIs), but none have been found.On the other hand four Al-rich chondrules were detectedwith Al2O3 values between 11 and 20 wt%, CaO valuesbetween 3 and 10 wt%, and Na2O values between 2 and6 wt%. These are similar to those described from otherordinary and enstatite chondrites (Bischoff and Keil,1983, 1984; Bischoff et al., 1984, 1985).

One Al-rich chondrule (MB-13-N-11-b) with theoverall shape of a porphyritic olivine chondrule showsdistinct alteration features. It has a diameter of about1 mm and is confined by a rim of formerly euhedralolivine crystals. These crystals (Fa24) show embayedcrystal margins which could be interpreted as corrosionfeatures. The olivine partly shows a pseudomorphicreplacement by feldspar with a composition of aboutAb81An15Or4. The inner part of the chondrule consistsof an Al-Na-Ca-rich mesostasis with tiny sulfide andchromite grains. Embedded are isolated, formerly

euhedral olivine crystals with similar composition andsimilar corrosion features. Some olivine crystals arenearly entirely replaced by feldspar-like material. Thealteration seems to predate the incorporation of thechondrule into the present breccia.

Bulk Chemistry of NWA 869 and Its Lithologic

Components

Since NWA 869 is only weakly weathered (W1),severe terrestrial alteration effects as observed in heavilyaltered samples from The Sahara (Stelzner et al. 1999)can be ruled out. The bulk chemical compositions ofNWA 869 and some of its lithologies, obtained by XRFanalyses, are shown in Table 4. The low sums are mainlydue to missing data for sodium and sulfur, which havenot been determined. From these XRF data andmicrochemical data obtained by SEM-EDX someimportant bulk elemental ratios for various lithologieshave been calculated. These element ratios (Mg ⁄Si,Ca ⁄Si, and Al ⁄Si) are summarized together with thecorresponding SiO2 and FeS values in Table 5 and arevisualized in Fig. 12. For data calculation and correctionsee above. Although it is beyond the scope of this paperto discuss chondrule chemistry, bulk data for 16 isolatedand perfectly preserved chondrules from the partly clasticmatrix have been measured. These data are from fourAl-rich chondrules, four cryptocrystalline chondrules,six olivine-pyroxene-rich chondrules, and two olivine-pyroxene-rich macrochondrules (for definitions seeBridges and Hutchison 1997) and have been included forcomparison with the analyzed lithologies.

Bulk NWA 869The bulk chemical data for NWA 869 obtained by

XRF analysis (Table 4) are well within the range for

Table 4. Bulk chemical compositions of NWA 869 and some clast types. Data were obtained by XRF analysis(mean of 2 measurements each).

MB-13 MB-13-c MS-04-1-a

M-05-38-1-aNWA869 bulk

Type 5 or 6chondrite clast

Shock-darkenedchondrite clast IMR clast

SiO2 wt% 39.03 39.15 39.08 45.06TiO2 wt% 0.12 0.11 0.11 0.13

Al2O3 wt% 2.06 2.07 2.02 2.45FeO wt% 28.43 26.51 26.94 17.4MgO wt% 24.11 24.30 23.38 27.89

MnO wt% 0.34 0.33 0.34 0.39CaO wt% 2.01 2.09 1.71 2.16P2O5 wt% 0.225 0.206 0.212 0.081

V ppm 84 55 84 90Cr ppm 4211 3849 3865 4179Ni ppm 13660 10417 12600 740Zn ppm 55 58 63 47

Sum wt% 99.09 97.20 96.43 97.00


L–chondrites (Jarosewich 1990). Nevertheless, comparedto the mean value of L–chondrites (Jarosewich 1990; 87meteorites) NWA 869 is characterized by a slightlyhigher value for CaO (2.01 versus 1.82 wt%) and aslightly lower value for Al2O3 (2.06 versus 2.26 wt%).This leads to a significantly higher atomic Ca ⁄Al-ratiocompared to mean L–chondrite chemistry (0.89 versus

0.73), which nevertheless lies within the variability ofL–chondrites. The values for TiO2, FeO, MgO, MnO,P2O5, V, Ni, and Zn appear typical for L–chondrites(MATESS 1988; Jarosewich 1990; Norman andMittlefehldt 2002). The value for Cr (4211 ppm) isslightly higher than the highest value from allL–chondrites listed in Jarosewich (1990).

Table 5. Bulk chemical compositions and elemental ratios of NWA 869 and various lithologies; for datacalculation and correction see text.

Lithology Remarks SampleSiO2

(wt%)FeS(wt%)

Mg ⁄Si(wt% ⁄wt%)

Ca ⁄Si(wt% ⁄wt%)

Al ⁄Si(wt% ⁄wt%)

Bulk NWA 869 MB-13a 39.0 5.3 0.796 0.079 0.060Clastic matrix MB-13-d 38.8 4.9 0.817 0.090 0.057

MB-13-l 38.0 6.1 0.828 0.075 0.057

Type 3 chondriteclasts

Type less than 3.5 M-06-44-1-a 39.7 5.1 0.807 0.093 0.063Type less than 3.5 MB-13-N-11 38.1 4.7 0.828 0.075 0.058

Type 4 chondrite

clast

M-05-38-2-b 39.7 4.3 0.796 0.076 0.063

Type 5 or 6chondrite clasts

M-05-38-2-d 38.1 6.8 0.744 0.076 0.084MB-13-ca 39.2 4.6 0.801 0.082 0.069

MB-13-h 38.7 6.3 0.796 0.081 0.062Shock-darkenedchondrite clasts

Type 5 or 6 M-05-38-2-c 37.1 5.9 0.849 0.077 0.075Type 3.9 MB13-a 37.4 6.8 0.838 0.069 0.054Type 5 or 6; post-shock

annealed

MB-13-e 38.7 5.7 0.807 0.076 0.054

Type 4 MB-13-N-1 37.0 9.8 0.765 0.072 0.061Type less than 3.5 MS-04-1-aa 39.1 5.0 0.772 0.067 0.059

IMR clasts Clast-poor SM-03-1-1-a 44.9 1.1 0.828 0.072 0.061Clast-free M-05-38-1-aa 45.1 1.7 0.799 0.073 0.062Clast-free SM-04-7-1-a 44.9 1.3 0.849 0.096 0.068

Clast-poor MB-13-f 43.4 2.1 0.817 0.084 0.051Clast-free M-07-25-a 45.3 1.3 0.765 0.084 0.058Clast-poor MB-13-b 44.9 1.9 0.765 0.085 0.056

Light inclusions M-05-38-2-a 44.3 1.2 0.786 0.112 0.069M-07-23-a 44.2 0.5 0.754 0.188 0.112

Unequilibratedmicrobreccia

MB-13-O-3 38.5 3.7 0.932 0.049 0.043

SiO2-bearingobjects

X-a 55.9 0.3 0.545 0.076 0.028Z-08-2 43.1 1.5 0.974 0.100 0.066

Chondrules Al-rich M-07-23-b 45.0 0.3 0.073 0.246 0.455

Al-rich; altered MB-13-N-11b 50.5 1.0 0.335 0.098 0.231Al-rich MB-13-N-26 42.9 1.2 0.482 0.181 0.285Al-rich MB-13-N-w 52.3 0.5 0.154 0.293 0.263

Cryptocrystalline MB-13-i 41.3 0.2 0.974 0.024 0.070Cryptocrystalline MB-13-N-7 48.7 0.5 0.712 0.041 0.025Cryptocrystalline M-04-38-4-c 47.8 0.4 0.733 0.150 0.037Cryptocrystalline M-04-38-4-d 46.5 0.4 0.775 0.120 0.050

Ol-px-rich MB-13-N-9 43.6 1.4 0.817 0.081 0.060Ol-px-rich MB-13-N-9a 49.6 1.8 0.796 0.080 0.065Ol-px-rich MB-13-N-19 50.6 0.3 0.639 0.077 0.040

Ol-px-rich MB-13-N-24 52.4 0.6 0.691 0.043 0.020Ol-px-rich MB-13-N-27 54.4 0.0 0.450 0.176 0.078Ol-px-rich MB-13-N-28 53.5 0.6 0.660 0.055 0.028

Ol-px-rich macrochondrule M-05-38-4-a 41.8 0.2 1.069 0.115 0.113Ol-px-rich macrochondrule M-05-38-4-b 47.4 0.2 0.828 0.088 0.077

aData obtained by XRF analysis, except for FeS (see Table 4).


Chondrite Clasts of Types 3–6 and Clastic MatrixSix chondrite clasts (petrologic type 3–6) and two

portions of clastic matrix have been analyzed (Table 5). Itturned out that the Mg ⁄Si, Ca ⁄Si, Al ⁄Si, and Ca ⁄Alelemental ratios of the clastic matrix and most clasts areclose to the values for the bulk meteorite (Fig. 12). Oneclast of petrologic type 5 or 6 (M-05-38-2-d) is somewhatexceptional since it seems to be slightly enriched infeldspar, leading to a lowered Ca ⁄Al (Fig. 12b) and anenhanced Al ⁄Si (Figs. 12c and 12d) value. The XRF datafor the type 5 or 6 clast MB-13-c are listed in Table 4.They are similar to bulk NWA 869 values with slighlylower contents of FeO, V, Cr, and Ni.

Shock-Darkened Chondrite ClastsThe data for shock-darkened chondrite clasts

scatter around the values for bulk NWA 869 andunshocked chondrite clasts (Fig. 12) with a slighttendency of SiO2 depletion and FeS enrichment(Figs. 12a and 12b). The FeS contents show a wider

variation compared to unshocked chondrite clasts witha maximum of 9.8 wt% for the shock-darkened clastMB-13-N-1 (Table 5). The XRF data for the clast MS-04-1-a are shown in Table 4. Like those of the type 5 or6 clast they are similar to bulk NWA 869 values withslighly lower contents of FeO, Cr, and Ni.

IMRsThe most striking chemical features of IMRs are their

low concentrations of FeS, which vary between 1.1 and2.1 wt%, in contrast to values between 4.3 and 6.8 wt%for chondrite clasts of petrologic type 3–6 (Table 5).Concurrently high SiO2 values can be observed, whichrange between 43.4 and 45.3 wt% for the six investigatedIMRs, compared to values between 38.1 and 39.7 wt%SiO2 for unshocked chondrite clasts (Table 5). Because ofthese differences IMRs can be easily distinguished frombulk NWA 869 and chondrite clasts of any petrologictype (Figs. 12a and 12b). On the other hand, Si-normalized elemental concentrations of Al, Mg, and Ca

Fig. 12. Chemical variation diagrams for the bulk compositions of NWA 869, various clasts, clastic matrix portions, and somechondrules. Data for bulk NWA 869 and three clasts are obtained by XRF analysis (see Table 4). a, b) FeS versus SiO2 andCa ⁄Al versus SiO2, respectively; due to their low FeS and high SiO2 contents IMRs, SiO2-bearing objects, and light inclusionsplot separately from chondrite clasts and clastic matrix portions. c, d) Al ⁄Si versus Mg ⁄Si and Al ⁄Si versus Ca ⁄Si; clastic matrixportions and most chondrite clasts plot near the bulk value for NWA 869. Values for Al-rich chondrules are not shown sincethey plot outside the diagram.


are roughly within the range of bulk NWA 869 and themeasured chondrite clasts (Figs. 12c and 12d).

The P2O5 and Zn concentrations in the IMR clastM-05-38-1-a are significantly lower than those in bulkNWA 869 and chondrite clasts (Table 4). This could bethe result of volatile depletion by evaporation duringIMR formation.

Unequilibrated MicrobrecciaThe SiO2 value and the Ca ⁄Al ratio of this lithology

lie in the range for the measured chondrite clasts, but itsFeS content and Ca ⁄Si and Al ⁄Si ratios are distinctlylower. The Mg ⁄Si ratio is higher than of any otherchondrite clast analyzed in this study (Table 5; Fig. 12).

Light InclusionsThese lithologies show some chemical affinities to

IMRs, especially in their contents of SiO2 and FeS(Fig. 12a). The Ca ⁄Al ratios are nearly identical in bothclasts and similar to those of IMRs (Fig. 12b). On theother hand their Al ⁄Si and Ca ⁄Si values differ frommost of the investigated IMRs (Figs. 12c and 12d).

SiO2-Bearing ObjectsBoth SiO2-bearing objects contain high, but

distinctly different SiO2 concentrations (43.1 and55.9 wt%; Table 5, Figs. 12a and 12b). Also the Ca ⁄Al,Al ⁄Si, Mg ⁄Si, and Ca ⁄Si ratios are quite different,proving that both objects have no common origin, as itis also indicated by their very different petrologicfeatures (see above). Clast X-a is the most SiO2-richlithology (55.9 wt% SiO2) found in this study.

ChondrulesThe chemical data for 16 chondrules are

summarized in Table 5 and shown in Fig. 12. Allchondrules are richer in SiO2 and poorer in FeS thanchondrite clasts of all petrologic types and most IMRs,but some are within the range for IMRs. The depletionof siderophile and chalcophile elements in chondrules isa well-known characteristic (e.g., Grossman et al. 1988).The Ca ⁄Al, Al ⁄Si, Mg ⁄Si, and Ca ⁄Si ratios scatterwidely (Figs. 12b, 12c, and 12d). In Figs. 12c and 12dthe data for the four analyzed Al-rich chondrules arenot shown, since their values plot outside the diagrams.

Oxygen Isotope Signatures

Oxygen isotope data for NWA 869 are given inTable 6 and plotted in Fig. 13. Also shown in Fig. 13are the fields for H, L, and LL group ordinarychondrites as defined by the analyses of Clayton et al.(1991). The bulk fraction for NWA 869 plots wellwithin the field of L group ordinary chondrites. TheIMR M-05-38-1-a also plots in the L group field veryclose to the bulk fraction. The SiO2-bearing object X-ahas a similar D17O value to the bulk rock fraction andthe IMR M-05-38-1 but is displaced to higher d18Ovalues. The data suggest that all three components maybe genetically linked. The IMR SM-04-7-1 and the lightinclusion M-07-23-a have distinct oxygen isotopecompositions compared to each other and to the othercomponents. This suggests that these two lithologieshave distinct origins and cannot be related to the otherthree lithologies (bulk, IMR M-05-38-1-a, SiO2-bearinginclusion X-a) by any mass fractionation process.

40Ar-39Ar Age Determinations

Detailed isotope data of 40Ar-39Ar analyses are givenin the Appendix tables. Apparent ages and age spectraare corrected for a small––nominal––routine correctionfor primordial argon with (40Ar ⁄ 36Ar)trapped = 1 ± 1. Ifisochrons yield distinct compositions of trapped excess40Ar, respective corrections were applied, which areexplicitly stated in the text, figure captions, and ⁄orlegends.

Type 4 Chondrite Clast (M-05-38-2-b)The age spectrum of the type 4 clast (Fig. 14)

displays apparent ages rising from about 3.4–4.4 Ga. Tenextractions (670–775 �C, release from chondritic feldspar,e.g., Trieloff et al. 2003b; Korochantseva et al. 2005;Pellas et al. 1997) define a small age plateau of4402 ± 7 Ma. High temperature extractions of thissample decrease in apparent ages (Fig. 14), accompaniedby a concomitant decrease of the K ⁄Ca ratio. Thisindicates disturbance by 39Ar recoil redistribution(Turner and Cadogan 1974; Huneke and Smith 1976),and is typical for fine-grained petrologic type 4lithologies (e.g., Trieloff et al. 2003b). As Ar-Ar dating

Table 6. Oxygen isotope data for bulk NWA 869, two IMR clasts, one light inclusion, and one SiO2-bearingobject. Data for IMRs and the SiO2-bearing object are mean values of two analyses each.

Lithology Sample d17O& 1r d18O& 1r D17O& 1r

Bulk NWA 869 M-05-38-2 3.52 – 4.67 – 1.09 –

IMR SM-04-7-1-a 2.85 0.09 4.64 0.19 0.44 0.01IMR M-05-38-1-a 3.47 0.02 4.50 0.03 1.13 0.00Light inclusion M-07-23-a 3.64 0.09 5.77 0.09 0.64 0.05

SiO2-bearing object X-a 4.67 – 6.57 – 1.20 –


requires conversion of part of the 39K into 39Ar duringirradiation in the research reactor, part of 39Ar may belost from fine-grained K feldspar or mesostasis into grainboundaries of more retentive minerals such as olivine orpyroxene. This 39Ar is then released in excess to 40Ar athigh temperatures, causing anomalously low apparentages without chronological significance.

An isochrone analysis of the 10 extractions from670 �C to 775 �C (not shown here) demonstrates thattrapped argon has a nonradiogenic composition of(40Ar ⁄ 36Ar)tr = 28 ± 295, i.e., zero within uncertainty.

This indicates that this sample did not incorporatesignificant amounts of terrestrial atmospheric argon(with 40Ar ⁄ 36Ar = 296) or excess argon componentssuch as identified in other L–chondrites (Korochantsevaet al. 2007).

IMR Clast (SM-03-1-1-a)The age spectrum of this IMR (Fig. 15) displays

apparent ages rising from about 3.9–4.6 Ga. Sevenextractions (580–640 �C) have an average age of4631 ± 88 Ma, followed by another segment of sevenextractions (660–720 �C) averaging 4668 ± 55 Ma,nominally older than the solar system age. Hightemperature extractions decrease substantially in age. Asfor the type 4 clast, these features can be explained byserious disturbance by 39Ar recoil redistribution or,alternatively or additionally, the presence of trapped orexcess 40Ar (i.e., 40Ar not from in situ decay of 40K)between 20 and 50% of the fractional 39Ar release.However, an isochrone analysis of the two segmentsdemonstrates that trapped argon has a nonradiogeniccomposition of (40Ar ⁄ 36Ar)tr = 18 ± 5238 and(40Ar ⁄ 36Ar)tr = 370 ± 3463, respectively, i.e., againzero within uncertainty. This would mean that––ifexcess ages are indeed related to excess 40Ar––thisexcess 40Ar is nearly purely radiogenic in composition,i.e., unsupported by trapped 36Ar.

On the other hand, the analysis of extractions athigh temperatures (960–1110 �C), yields a partial

Fig. 14. Age spectrum and K ⁄Ca spectrum of type 4chondrite clast M-05-38-2-b. Apparent ages increase withincreasing extraction temperature to a small plateau segmentcomprised of 10 extractions yielding an age of 4402 ± 7 Ma.Subsequent extractions with decreasing apparent ages arelikely disturbed by 39Ar recoil redistribution.

Fig. 13. Oxygen isotope results for NWA 869. Also shown arethe fields for H, L, and LL group ordinary chondrites asdefined by the analyses of Clayton et al. (1991).

Fig. 15. Age spectrum and K ⁄Ca spectrum of IMR clast SM-03-1-1-a. The K content is relatively high, while apparent agesin intermediate temperature extractions exceed the age of thesolar system, which is suggestive of concomitant enrichment ofrelict 40Ar and K in impact melt phases. High temperatureextractions contain a distinct trapped argon component andobviously recorded an impact event about 1.8 Ga ago.


isochrone with a remarkable composition of trappedargon with (40Ar ⁄ 36Ar)tr = 25.7 ± 2.1, which issignificantly different from zero. Subtracting of thistrapped 40Ar from the respective extractions in the agespectrum yields a partial plateau segment of1790 ± 36 Ma (Fig. 15).

Shock-Darkened Clast (M-05-38-2-c)The age spectrum of this clast is similar to that of

the impact melt clast (see above), again with apparentages rising from about 3.8–4.6 Ga. Five extractions(650–700 �C) peak at an average age of 4829 ± 96 Ma(Fig. 16a). High temperature extractions decreasesubstantially in age, again indicating serious disturbanceby 39Ar recoil redistribution. The isochrone analysis inthis release regime does not yield a compositiondifferent from zero ([40Ar ⁄ 36Ar]tr = 133 ± 198)––unlikethe impact melt clast––but again a partial plateausegment yields an age of 2216 ± 40 Ma. This plateau iseven better visible when apparent ages are plottedagainst the 37Ar release, a plot which emphasizes theage of the Ca bearing phases, as 37Ar derived duringneutron irradiation from 40Ca is a tracer for Ca-bearingminerals (Fig. 16b).

DISCUSSION

Reclassification of NWA 869

Based on our oxygen isotope data NWA 869 clearlyderives from the L–chondritic asteroid. We foundchondrite clasts of petrologic type less than 3.5, 3.9, and4–6, some of which suffered shock-darkening byimpacts on the parent body. IMRs, both clast-poor andclast-free, occur in a wide variety. In addition, two lightinclusions, one unequilibrated microbreccia, and twoSiO2-bearing objects were investigated. Based on ourresults we propose to reclassify NWA 869 as an L3–6chondritic regolith breccia.

Possible Pairings of NWA 869 and Comparison with the

L–Chondrite MAC 87302

There are several other classified L–chondrites fromNorthwest Africa with textures very similar to NWA869. It is almost certain that material of NWA 869received many other NWA numbers (see Connolly et al.2006), but it is beyond the scope of this paper tospeculate on this. Retracing the pairings of NWA 869remains a wide field of future investigation.

On the other hand, there is an unpaired meteoritesample which closely resembles NWA 869. Welzenbachet al. (2005) investigated the Antarctic L–chondriteMacAlpine Hills (MAC) 87302 and found that it

consists of a very similar assortment of lithologies likeNWA 869 and contains solar-wind implanted noblegases, as well. They found that the matrix of MAC87302 also consists of unequilibrated material(petrologic type 3–4) containing distinctly outlinedchondrules. Fayalite and ferrosilite values of mostgrains are typical for metamorphosed L–chondrites

Fig. 16. Age spectrum and K ⁄Ca spectrum of shock-darkenedclast M-05-38-2-c. a) Apparent ages are plotted versus thefractional 39Ar release. Similar to the IMR clast (Fig. 15), thesample displays high apparent ages at intermediatetemperature extractions and a small plateau segment in thelast part of the spectrum at about 2.2 Ga. b) Here theapparent ages are plotted versus the fractional 37Ar release.This emphasizes the age of Ca-rich phases, and the plateaufrom Fig. 16a is better recognizable.


(Fa23–24; Fs20–21) but unequilibrated grains up to Fa5and Fs1 were also found. Light-colored metamorphosedclasts of petrologic type 5 with sizes of more than 1 cmare present with a modal abundance of 10–20%,compared to about 20% in NWA 869. Furthermore,cm-sized shock-darkened clasts and IMR clasts wereobserved, similar to the findings for NWA 869. Bothmeteorites may represent the lithified regolith from thesame or a similar region of the L–chondrite parentbody.

Lithified Regolith of the L–Chondrite Asteroid

Evidence for repeated impacts on meteorite parentbodies has been derived from the study of meteoriticbreccias from most meteorite groups (e.g., Bischoffet al. 2006). Many of these breccias are mechanicalmixtures of rocks, which had been metamorphosed tovarious degrees by thermal annealing and shock wavesprior to their admixture to the host breccia. The samecan be observed in NWA 869, which contains highlyrecrystallized type 5 and 6 metamorphosed lithologiesfrom greater depth and unequilibrated fragments fromthe upper surface of the parent body. Some of theselithologies had been shock-darkened in their specificenvironment prior to fragmentation and mixing. Inadddition, a large variety of IMRs can be found.Interactions with the solar wind on the asteroidalsurface are responsible for the high concentrations ofsolar type noble gases within this breccia (see Weltenet al. 2010). Northwest Africa 869 obviously formed byimpact-induced consolidation of loose regolith, aprocess which can be retraced for many other chondriticregolith breccias, as well (e.g., Kieffer 1975; Bischoffet al. 1983). According to the 40Ar-39Ar age of theyoungest lithology (lMR clast SM-03-1-1-a), the finalassembly and shock lithification of NWA 869 occurredless than 1.79 Ga ago.

Origin of Clastic Matrix and Its Chemical Heterogeneity

The partly clastic matrix of this meteorite ischemically very similar to bulk NWA 869. It is not asimple mixture of fine-grained debris formed from theabove lithologies, but seems to be a specific chondriticlithology with widely varying chemical compositions ofolivines and pyroxenes (see Table 3). It was found thatcertain areas of clastic matrix show chemical differences,even when these areas are less than a centimeter apart(see above). This could point to the existence ofpetrologically similar matrix units with small chemicaldifferences, where the boundaries between these unitsare optically difficult to detect. The matrix isinterspersed with perfectly preserved chondrules, which

were probably never part of consolidated rock that wascomminuted by impacts. One explanation is that thematrix material stems from a region or layer of theparent body that was unconsolidated or only weaklyconsolidated. In this scenario impact activity did notresult in the formation of lithic fragments but mainlyled to stirring up of loosely bonded material which wastransported in the shape of isolated components andadmixed to the NWA 869 breccia. This general type oftexture (clasts of coherent chondritic rock embeddedinto a clastic material and undisturbed chondrules) isnot only found in NWA 869 but also in a number ofother chondritic regolith breccias, e.g., Murchison,Murray, Adzhi-Bogdo, Study Butte, Acfer 094, Adrar003 (Metzler et al. 1992; Bischoff et al. 1993, 1996;Sokol and Bischoff 2006; Sokol et al. 2007). Althoughthe chemical compositions of most measured chondrulesfrom the partly clastic matrix clearly deviate from thebulk chemistry of NWA 869 (see Fig. 12), it cannot beruled out that some of them are impact melt droplets.

Genesis of a Type 4 Chondrite Clast

Ar-Ar measurements of M-05-38-2-b (type 4chondrite clast) reveal a small partial age plateau of4402 ± 7 Ma age (Fig. 14) which is significantly higherthan those of the majority of L–chondrites. Many ofthe latter have gas retention ages of about 470 Ma, e.g.,Ghubara, Mbale, McKinney, Bluff, Paranaiba, Taiban,Peace River, Chico (Heymann 1967; McConville et al.1988; Bogard et al. 1995; Kunz et al. 1997;Korochantseva et al. 2007), which was ascribed to amajor parent body collision leading to rapid transferand deposition of fossil L–chondrites on Earth (Schmitzet al. 1997, 2001; Heck et al. 2004; Korochantseva et al.2007). Others have Ar-Ar ages between 0.5 and 4 Ga,e.g., Walters, Ness County, Alfianello, Orvinio,Arapahoe, Lubbock (Bogard and Hirsch 1980; Kunzet al. 1997), indicating repeated episodes of impactmetamorphism. However, some L–chondrites likeBjurbole record Ar-Ar ages as high as 4.4 Ga (e.g.,Herrwerth 1982), similar to the age of the type 4 clastreported herein. Further evidence for early impactsbefore 4 Ga on the L–chondritic parent body comesfrom the formation ages of IMRs like Shaw, PatuxentRange 91501, and Miller Range 05029 (e.g., Bogard andHirsch 1980; Benedix et al. 2008; Wittmann et al. 2009).Our preferred interpretation of the 4402 Ma age iscooling after such an early impact rather than coolingafter parent body thermal metamorphism, ascomparable ages of H chondrites indicate that Ar-Arages of type 4 petrologic types due to cooling in anonion shell layered parent body are in the order of4.51–4.53 Ga (Trieloff et al. 2003b).


Formation of Shock-Darkened Chondrite Clasts

In NWA 869 shock-darkening is generallyrestricted to isolated clasts, i.e., shock-darkening alwayspredates clast formation. The FeS contents in theseclasts show a wider variation than in unshockedchondrite clasts. These variations can be explained bythe redistribution of sulfides due to impactmetamorphism, where sulfides may have beenmobilized and depleted or enriched in certain parts ofthe corresponding rock.

The darkest clasts show extremely branchednetworks of sulfides and some Fe,Ni metal veinsthroughout the entire objects. In contrast to this, amore translucent variety (MB-13-e) shows a coarseningof sulfides and the networks have been transformed tochains of sulfide beads with typical diameters of 10–50 lm (Fig. 4d). We explain this texture to be the resultof an annealing process after darkening.

Our interpretation of the Ar-Ar-data for the shock-darkened clast M-05-38-2-c (Fig. 16) is that the 40Arreleased at intermediate temperatures is related tophases, which have been enriched in K and 40Ar byshock effects. They seemingly remained a closed systemand did not lose their 40Ar effectively. The hightemperature extractions better represent an impact resetage of 2216 ± 40 Ma.

Formation of IMRs

IMRs are whole-rock melts containing variableamounts of admixed clastic debris. The formation ofthese rocks needs shock pressures in the order of 80–100 GPa, leading to postshock temperatures of at least1500 �C. The corresponding impact velocity ofimpacting bodies is more than 8 km s)1 (Stoffler et al.1988, 1991; Horz et al. 2005). IMRs are known fromseveral ordinary chondrites (e.g., Bischoff and Stoffler1992; Bischoff et al. 1993, 2006 and references therein).The ratios for Mg ⁄Si, Ca ⁄Si, Al ⁄Si, and Ca ⁄Al in thesix investigated IMRs are within the range for bulkNWA 869 (Table 5, Fig. 12), indicating that these rocksprincipally represent crystallized whole rock melts of Lchondritic composition. In addition, mineral and lithicclasts enclosed in the clast-poor IMRs belong to typicallithologies found in NWA 869, i.e., chondritic rocks oftype 4–6 (see Table 3). Furthermore, oxygen isotopedata for the IMR clast M-05-38-1-a are very similar tothat for bulk NWA 869 (Table 6; Fig. 13). The IMRclast SM-04-7-1-a seems to have incorporated a certainfraction of impactor material, since its O-isotope valuescannot be related to L chondritic lithologies by anymass fractionation process.

Depletion of Fe,Ni Metal and Sulfide by GravitationalSeparation

IMRs are distinctly depleted in Fe,Ni metal andFeS while SiO2 values are enhanced relative to bulkvalues of NWA 869 (Fig. 12a). This change is probablydue to a shift in the relative proportions betweensilicates and the metal-sulfide portion. This assumptionis confirmed by Figs. 12c and 12d, which show that theelemental ratios of Mg ⁄Si, Ca ⁄Si, and Al ⁄Si in IMRsare similar to bulk NWA 869 and to the majority ofchondrite clasts. The mean depletion of FeS in IMRclasts relative to the bulk meteorite is about 70% (seeTable 5). The depletion for Fe,Ni metal in the IMRclast M-05-38-1-a relative to bulk NWA 869 is about95%, calculated from its Ni concentration (Table 4) andassuming a constant Fe ⁄Ni ratio in the metals. Weassume that the depletion of Fe,Ni metal and sulfide isthe result of gravitational separation of heavy Fe,Ni-sulfide melts from silicate melts within superheatedimpact melt sheets with their very low viscosities.

Formation of Some IMRs as Melt DropletsSome IMRs in NWA 869 are not true clasts but

represent units with subrounded or even ameboid outershapes. In the case of M-05-38-1-a the striking gradientin grain size proves that this small melt unit cooledrapidly as an entity. This rules out a simple proveniencefor some IMRs as fragments from crystallized impactmelt sheets which had gravitationally lost most of theirFe,Ni metal and sulfide. We speculate that some IMRsare excavated droplets formed either by impacts intopre-existing crystallized impact melt units or by impactsinto still molten melt sheets. Should they have formedsimply by impacts into regular L chondritic rocks itwould be difficult to explain their strong depletion inFe,Ni metal and sulfide. On the other hand, somesiderophile depletion by vaporization cannot be ruledout in the case of small impact melt droplets.

Formation Age of IMR Clast SM-03-1-1-aThe Ar-Ar-method has been applied (Fig. 15) to

date the impact event that formed this IMR. Since itrepresents a rapidly cooled melt, we suggest thatpreviously accumulated 40Ar remained dissolved in themelt preventing effective loss of 40Ar. This 40Ar nowappears as excess 40Ar enriched in the Fe-richmesostasis. Ar released at high temperatures could beargon from recrystallized olivine or respective K-richinclusions in olivine that lost 40Ar due to theincompatible behavior of 40Ar in these phases. In thiscase, the 1790 Ma age would date the impact event,while the 4631–4668 Ma ages just give a rough idea ofthe time of previously accumulated 40Ar.


While Ca concentrations of the three samples usedfor Ar-Ar-measurements are similar (shock-darkenedclast: 1.21 ± 0.03%; type 4 clast: 1.43 ± 0.04%; IMRclast: 1.37 ± 0.03%), K concentrations are very different(shock-darkened clast: 2291 ± 43 ppm; type 4 clast:799 ± 15 ppm; IMR clast: 4482 ± 142 ppm). Only thetype 4 clast has a normal ordinary chondritic K value.The IMR clast shows the highest K concentration and asa tentative scenario we may envisage that the impactevent enriches both K and even more radiogenic 40Ar inthe impact melt, without major degassing of previouslyaccumulated radiogenic 40Ar.

Genesis of the Unequilibrated Microbreccia

This small clast (Fig. 6) represents an assortment ofthe chemically most diverse olivine and pyroxene grainsfound within a single lithic clast in this study (Table 3,Figs. 9 and 11). Mg-rich olivines show thin rims ofFe-rich olivine, which could be formed by mild thermalmetamorphism after formation (Fig. 6d). This lithologyshows an internal layering and contains chondrite clastswith shock-impregnated sulfides. The interstitialsbetween grains frequently contain a feldspathic material(Si-Al-Ca-Na-rich fillings), which is typical for lithifiedbreccias and occurs as cement in lithified ordinarychondrite regolith breccias (Bischoff et al. 1983). Thisclast may have been part of an impact breccia layerformed from unequilibrated chondritic components andcan be described as a breccia in the NWA 869 breccia.Similar clasts have also been observed in the LL3–6breccia Adzhi-Bogdo (Bischoff et al. 1993) and withinthe L3.2 ordinary chondrite Bishunpur (Weisberg andPrinz 1996). The latter authors termed these fine-grainedand dark to near opaque objects ‘‘agglomeraticchondrules’’ and interpreted them as the result ofagglomeration of grains in the solar nebula, sintered byshort-term heating events.

Interpretation of Light Inclusions

In principle, both light inclusions can be interpretedas either fragments of macrochondrules or IMRs. Theycontain zoned olivine crystals, proving that bothlithologies escaped thermal equilibration on their parentbody. Their SiO2- and FeS-contents are in the range ofthe measured IMRs (Table 5, Fig. 12a), as well as theirbulk Ca ⁄Al-ratio (Fig. 12b). On the other hand, theirCa ⁄Si- and Al ⁄Si-ratios differ from those of the IMRs(Fig. 12d). For this, we prefer a macrochondrule originfor the light inclusion M-05-38-2-a. The other lightinclusion (M-07-23-a) shows the highest Ca ⁄Si andAl ⁄Si values measured in all lithic clasts. It couldrepresent an IMR with an admixed foreign component

(see oxygen isotope data; Table 6 and Fig. 13), but dueto its strong chemical deviation from all other IMRs weinterpret this inclusion as a fragment of amacrochondrule or an admixed foreign igneous clast.

Interpretation of SiO2-Bearing Objects

SiO2-bearing objects have been described fromseveral carbonaceous and ordinary chondrites (e.g.,Fujimaki et al. 1981; Olsen 1983; Planner 1983;Brigham et al. 1986; Hezel et al. 2003, 2006). Theirouter shapes often resemble chondrules and they usuallyconsist of very pure silica polymorphs and a silicate ofpyroxene normative composition. Bridges et al. (1995)describe SiO2-bearing clasts from two ordinarychondrites and conclude that these objects are the resultof fractional crystallization on a chondritic parent body.

On the other hand, Hezel et al. (2003, 2006) arguethat, based on the low concentrations of Al, Ca, andREE in the pyroxenes, the flat CI-normalizedREE-patterns and the feldspar-free paragenesis of manySiO2-bearing objects, a planetary igneous origin appearsunlikely. According to these authors the bulk SiO2

contents of these objects vary between more than 50and 100 wt%, which cannot be explained byequilibrium condensation, but seems to be the result offractional crystallization in the solar nebula. Theyconclude that SiO2-rich objects can be interpreted asvery silica-rich chondrules, formed by reheating offormerly condensed SiO2-rich precursors during thechondrule-forming process, followed by rapid cooling.

The SiO2-bearing object X-a (Fig. 7c) obviouslybelongs to the same type of objects described by Hezelet al. (2003, 2006). It resembles a large chondrule, isnearly free of metal and sulfide and its bulk SiO2 contentof about 56 wt% (Table 5, Fig. 12a) is the highest valuefor all lithologies found in this study. Although its d18Ovalue differs from that of bulk NWA 869, their similarD17O values (Table 6; Fig. 13) suggest that both may begenetically linked. SiO2-rich chondrules with similarlyhigh d18O values have been found by Bridges et al. (1998)in the LL chondrites Parnallee and Chainpur. Hence, weinterpret this inclusion as a SiO2-rich chondrule.

The SiO2-bearing object Z-08-2 (Fig. 7d) representsa large (3 cm) angular chondrite clast of indigenous orforeign origin, which seems to have been altered byunknown processes. This chondrule-bearing lithology ofpetrologic type 3.7 shows some textural layering and itsolivine has the lowest mean Fa value found among allinvestigated chondrite clasts. The overall texture pointsto a volume reduction (contraction cracks) late in itshistory and the high Ni content (56 wt%) in the fewexisting metal grains indicate an oxidizing environmentduring alteration.


SUMMARY AND CONCLUSIONS

NWA 869 possibly represents the largest sample(about 7 metric tons) of the rare regolith breccias fromthe L–chondrite parent body. It consists of lithic clasts,embedded in a partly clastic matrix. Measurements ofits modal composition reveal 74 vol% clastic matrix,20 vol% chondrite clasts of petrologic types 4–6,4 vol% shock-darkened chondrite clasts, 0.9 vol% IMRclasts, and 0.8 vol% chondrite clasts of petrologic type3. The bulk chemical data obtained by XRF analysisand the O-isotope data are well within the range forL–chondrites. Most chondrite clasts of type less than3.5 to type 6 (unshocked and shock-darkened) and theclastic matrix are similar to bulk NWA 869 concerningtheir Mg ⁄Si, Ca ⁄Si, Al ⁄Si, and Ca ⁄Al ratios. Based onour results we propose to reclassify NWA 869 as anL3–6 chondritic regolith breccia.

The matrix of this meteorite is not a simple mixtureof fine-grained debris formed from typical chondritelithologies of petrologic types 3–6. It mainly representsan additional specific and unequilibrated lithology,containing isolated unequilibrated chondrules withperfect outer shapes. Our explanation is that the matrixmaterial stems from a region or layer of the parentbody that was unconsolidated or only weaklyconsolidated prior to its excavation and admixture tothe host breccia.

We found unequilibrated L–chondrite clasts ofpetrologic types less than 3.5 and 3.9, and equilibratedfragments of petrologic types 4–6. Shock-darkening isrestricted to isolated clasts of petrologic type 3 to 6, i.e.,shock darkening always predated clast formation. Localin situ shock darkening has not been observed. Thiscould be an important criterion to distinguish thismeteorite from other Saharan L–chondrites.

IMRs, both clast-free and clast-poor, occur in awide variety and are strongly depleted in Fe,Ni metaland sulfides with a mean FeS depletion on the order of70%. This could be the result of separation ofimmiscible Fe,Ni-sulfide melt and silicate melt in asuperheated impact melt volume due to asteroidalgravitation. Oxygen isotope measurements indicate thatone of the measured IMRs has values comparable tobulk NWA 869, whereas another IMR seems torepresent a mixture with impactor material.

Two light inclusions with coarse textures areprobably fragments of macrochondrules. Alternatively,one of them possibly represents an admixed foreignigneous clast. Two different types of SiO2-bearingobjects were found and are interpreted as a chondruleand an altered chondrite clast of petrologic type 3.7,respectively. A small clast of an extremelyunequilibrated microbreccia has been found, which

possibly originated from an impact breccia layer whichconsisted of unequilibrated chondritic components.

A type 4 chondrite clast yields an 40Ar-39Ar plateauage of 4402 ± 7 Ma which is interpreted to be the resultof heating by an early impact event. Furthermore, impactactivity around 2 Ga ago was detected for a shock-darkened clast (2216 ± 40 Ma) and an impact melt clast(1790 ± 36 Ma). Until now, little impact activity wasrecorded for the L–chondrite parent body during thisperiod (Bogard et al. 1995). We found that Ar release ofimpact lithologies at intermediate temperatureextractions displays apparent ages nominally higher, butvery similar to the solar system age of 4.56 Ga. As Kcontents vary significantly, it can be concluded that bothK and 40Ar may have been either lost or added in a verysimilar manner, i.e., may occasionally behavegeochemically coherent during impact events. Accordingto the 40Ar-39Ar age of the youngest lithology (lMR clastSM-03-1-1-a), the final assembly and shock lithificationof NWA 869 occurred less than 1.79 Ga ago. None of themeasured components indicate involvement in thecataclysmic event approximately 470 Ma ago, whichaffected many other L–chondrites and likely disruptedthe L–chondrite parent body.

The results of the corresponding measurements ofnoble gases and cosmogenic radionuclides on bulksamples and separated lithologies from NWA 869 (seeTable 1) are presented in a companion paper (Weltenet al. 2011).

All in all, NWA 869 represents an important andextraordinarily diverse sample of the lithified regolith ofthe L–chondrite parent body. Fortunately, this materialis available for study in large quantities and furthersystematic search will probably reveal an even largervariety of rock types of either indigenous or foreignorigin. This will help to broaden our knowledge of thelithic inventory of the L–chondrite parent body and toretrace impact and mixing processes in the asteroid belt.

Acknowledgments—We are grateful to Michael Hofmann,Moembris, Germany and to late Walter Zeitschel, Hanau,Germany for providing samples. We appreciate technicaland analytical assistance by Thomas Jording, WinfriedSchwarz, Sandra Eckardt, Lennart Rohrer, and FrankBartschat. We thank the Forschungszentrum Geesthacht(GKSS) for access to neutron irradiation facilities.We acknowledge support by the DeutscheForschungsgemeinschaft and the Klaus Tschira StiftunggGmbH. We appreciate the thorough and veryconstructive reviews by Catherine M. Corrigan and AxelWittmann. Comments by the associate editor EdwardScott were also very helpful in improving the manuscript.

Editorial Handling—Dr. Edward Scott


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APPENDIX

Appendix tables A1–A3 display measured argon isotopes corrected for mass discrimination, sensitivity, systemblanks, decay and relative neutron doses. All isotopes are also corrected for interfering isotopes produced on K andCa during irradiation. Remaining argon isotopes are given in cm3 STP g)1 and have the following composition:

36Ar = 36Aratm + 36Artrap + 36Arcos atm: terrestrial atmospheric argon

trap: trapped extraterrestrial argoncos: cosmogenic argon

37Ar = 37ArCa Ca: argon derived from Ca38Ar = 38Aratm + 36Artrap + 36Arcos +

38ArCl Cl: argon derived from Cl39Ar = 39ArK K: argon derived from K40Ar = 40Arrad + 40Aratm + 40Artrap rad: in situ radiogenic argon

Apparent ages in tables were calculated by subtracting an almost negligible amount of primordial trapped argon from 40Ar, assuming40Ar ⁄ 36Ar = 1 in each temperature extraction.


Table A1. NWA 869 type 4 chondrite clast M-05-38-2-b.

Temp (�C) 36Ar · 10)11 37Ar · 10)11 38Ar · 10)12 39Ar · 10)12 40Ar · 10)9 Age (Ma)

500 434 ± 7 429 ± 8 1035 ± 12 2009 ± 15 1657 ± 10 4131 ± 9

570 160 ± 6 883 ± 12 462 ± 12 5722 ± 38 2931 ± 16 3379 ± 5

600 37 ± 3 711 ± 12 183 ± 8 4274 ± 29 2772 ± 15 3745 ± 6

610 26 ± 2 669 ± 11 151 ± 8 4025 ± 28 3012 ± 16 3973 ± 7

620 19 ± 2 488 ± 8 97 ± 6 2953 ± 21 2416 ± 13 4117 ± 7

630 19 ± 2 485 ± 9 106 ± 6 2921 ± 21 2543 ± 14 4218 ± 7

640 11 ± 2 363 ± 8 76 ± 6 2145 ± 33 1918 ± 28 4262 ± 6

650 16 ± 2 464 ± 12 103 ± 7 2682 ± 41 2510 ± 37 4336 ± 5

660 26 ± 2 444 ± 10 114 ± 7 2527 ± 39 2411 ± 36 4368 ± 6

670 38 ± 2 403 ± 8 127 ± 6 2302 ± 36 2271 ± 34 4423 ± 8

680 30 ± 2 339 ± 8 107 ± 6 1873 ± 29 1843 ± 27 4419 ± 6

690 21 ± 2 347 ± 8 92 ± 6 981 ± 30 909 ± 28 4384 ± 6

700 17 ± 2 302 ± 7 74 ± 5 1669 ± 25 1630 ± 24 4406 ± 5

710 19 ± 2 250 ± 7 78 ± 5 1503 ± 23 1450 ± 22 4386 ± 6

720 19 ± 2 207 ± 7 78 ± 5 1226 ± 8 1189 ± 4 4395 ± 8

730 20 ± 2 171 ± 5 66 ± 5 1024 ± 5 986 ± 3 4382 ± 5

745 32 ± 2 286 ± 6 118 ± 6 1736 ± 8 1686 ± 6 4396 ± 5

760 26 ± 2 160 ± 5 86 ± 5 871 ± 5 845 ± 3 4395 ± 7

775 35 ± 2 182 ± 4 107 ± 5 858 ± 5 844 ± 3 4419 ± 6

795 32 ± 2 214 ± 5 102 ± 5 819 ± 5 776 ± 3 4358 ± 7

820 44 ± 2 317 ± 8 144 ± 5 868 ± 13 807 ± 12 4325 ± 7

850 56 ± 2 404 ± 9 179 ± 6 933 ± 14 836 ± 12 4264 ± 7

890 101 ± 2 783 ± 16 303 ± 7 1281 ± 19 1114 ± 16 4216 ± 6

930 99 ± 2 891 ± 15 300 ± 6 1055 ± 16 857 ± 12 4105 ± 8

980 78 ± 2 1017 ± 18 272 ± 6 987 ± 15 764 ± 11 4028 ± 8

1050 63 ± 2 1431 ± 25 282 ± 7 1335 ± 20 1072 ± 16 4086 ± 6

1130 89 ± 2 4081 ± 49 519 ± 9 2478 ± 27 2201 ± 23 4251 ± 5

1170 59 ± 2 3046 ± 38 377 ± 6 667 ± 8 599 ± 6 4270 ± 10

1250 390 ± 6 8400 ± 110 1178 ± 18 1219 ± 20 1026 ± 11 4162 ± 22

1350 323 ± 5 20,620 ± 260 1254 ± 24 3385 ± 38 3021 ± 31 4258 ± 6

1390 259 ± 6 4782 ± 73 709 ± 20 724 ± 13 634 ± 7 4226 ± 23

1395 96 ± 4 1317 ± 26 234 ± 10 236 ± 7 261 ± 4 4611 ± 42

Total 2695 ± 17 54890 ± 300 9112 ± 52 60290 ± 130 50790 ± 110 4165 ± 1.6

Table A2. NWA 869 IMR clast SM-03-1-1-a.


400 1364 ± 14 20 ± 1 2627 ± 26 153 ± 2 464 ± 4 6345 ± 15

450 389 ± 4 31 ± 1 810 ± 8 307 ± 1 238 ± 1 4024 ± 6

490 258 ± 3 69 ± 2 589 ± 9 632 ± 2 468 ± 1 3954 ± 5

510 131 ± 3 90 ± 1 387 ± 11 788 ± 3 675 ± 1 4192 ± 4

520 78 ± 3 84 ± 1 263 ± 10 730 ± 2 694 ± 1 4362 ± 4

530 50 ± 2 84 ± 2 192 ± 9 706 ± 2 716 ± 1 4469 ± 5

540 36 ± 2 84 ± 2 172 ± 8 670 ± 2 705 ± 1 4530 ± 5

550 30 ± 2 87 ± 2 146 ± 9 667 ± 2 724 ± 1 4581 ± 5

560 26 ± 2 80 ± 1 141 ± 7 625 ± 2 690 ± 1 4608 ± 5

570 16 ± 2 75 ± 1 113 ± 8 578 ± 4 641 ± 4 4619 ± 3

580 17 ± 2 91 ± 2 128 ± 8 696 ± 4 777 ± 5 4628 ± 4

590 40 ± 3 97 ± 1 171 ± 9 734 ± 5 822 ± 5 4633 ± 3

600 26 ± 1 84 ± 1 139 ± 9 651 ± 4 728 ± 4 4632 ± 4

610 36 ± 2 96 ± 2 175 ± 8 768 ± 5 860 ± 5 4633 ± 5

620 28 ± 2 102 ± 2 164 ± 8 784 ± 5 879 ± 5 4633 ± 5

630 18 ± 1 98 ± 2 135 ± 8 723 ± 3 803 ± 2 4620 ± 4

640 15 ± 1 99 ± 2 139 ± 8 749 ± 3 843 ± 2 4641 ± 4

650 25 ± 1 110 ± 2 166 ± 10 861 ± 3 985 ± 3 4669 ± 4

660 43 ± 2 101 ± 1 184 ± 11 769 ± 3 893 ± 2 4694 ± 5

670 31 ± 2 96 ± 2 153 ± 9 744 ± 3 859 ± 2 4685 ± 4

680 26 ± 2 93 ± 2 145 ± 9 748 ± 3 866 ± 2 4689 ± 5

690 19 ± 1 107 ± 1 157 ± 10 854 ± 3 983 ± 1 4680 ± 4


Table A2. Continued. NWA 869 IMR clast SM-03-1-1-a.


700 23 ± 2 120 ± 2 169 ± 10 937 ± 3 1079 ± 1 4679 ± 4

710 28 ± 2 117 ± 2 172 ± 10 913 ± 3 1048 ± 1 4674 ± 4

720 29 ± 2 130 ± 2 181 ± 12 981 ± 3 1122 ± 2 4668 ± 5

730 29 ± 1 114 ± 2 180 ± 9 896 ± 3 1018 ± 1 4657 ± 4

740 26 ± 1 108 ± 3 170 ± 10 812 ± 19 921 ± 22 4653 ± 4

750 29 ± 2 108 ± 3 186 ± 10 860 ± 20 967 ± 23 4639 ± 4

760 33 ± 2 100 ± 3 172 ± 9 769 ± 18 850 ± 20 4612 ± 3

770 43 ± 2 93 ± 3 189 ± 9 714 ± 17 780 ± 18 4593 ± 3

780 37 ± 1 74 ± 2 152 ± 7 583 ± 14 624 ± 15 4558 ± 3

790 29 ± 1 66 ± 2 124 ± 7 526 ± 12 542 ± 13 4496 ± 4

800 30 ± 1 63 ± 1 130 ± 7 474 ± 2 465 ± 1 4415 ± 5

810 35 ± 1 61 ± 1 141 ± 6 475 ± 2 444 ± 1 4334 ± 5

820 36 ± 1 57 ± 2 136 ± 5 404 ± 1 350 ± 1 4210 ± 4

830 38 ± 1 51 ± 1 136 ± 6 389 ± 2 308 ± 1 4067 ± 5

850 60 ± 1 69 ± 2 191 ± 6 474 ± 2 281 ± 1 3605 ± 4

880 116 ± 1 138 ± 2 371 ± 9 849 ± 3 300 ± 1 2827 ± 2

920 198 ± 2 239 ± 2 618 ± 14 1373 ± 5 272 ± 1 2061 ± 2

960 266 ± 2 308 ± 2 791 ± 17 1667 ± 7 248 ± 1 1726 ± 2

990 219 ± 2 233 ± 2 654 ± 12 1209 ± 5 194 ± 1 1806 ± 2

1020 189 ± 1 175 ± 2 549 ± 10 904 ± 4 155 ± 1 1888 ± 3

1050 203 ± 3 144 ± 3 541 ± 12 670 ± 10 123 ± 2 1965 ± 3

1080 226 ± 4 104 ± 2 545 ± 10 428 ± 6 77 ± 1 1935 ± 5

1110 309 ± 5 83 ± 2 689 ± 11 287 ± 4 52 ± 1 1962 ± 7

1150 214 ± 4 63 ± 2 497 ± 10 123 ± 2 28 ± 1 2208 ± 11

1200 136 ± 4 48 ± 2 356 ± 11 36 ± 1 12 ± 1 2769 ± 28

1270 626 ± 12 144 ± 3 1327 ± 24 113 ± 2 29 ± 1 2374 ± 31

1350 527 ± 4 183 ± 2 1132 ± 10 467 ± 2 144 ± 1 2634 ± 6

1390 183 ± 3 111 ± 2 458 ± 8 376 ± 2 118 ± 1 2662 ± 7

1395 85 ± 2 58 ± 2 218 ± 6 184 ± 1 55 ± 1 2603 ± 5

Total 6706 ± 24 5238 ± 14 18471 ± 75 33829 ± 48 28920 ± 48 4188 ± 1.2

Table A3. NWA 869 shock-darkened clast M-05-38-2-c.


500 808 ± 18 776 ± 19 1907 ± 46 1340 ± 25 911 ± 17 3820 ± 7

520 141 ± 7 488 ± 16 500 ± 18 1041 ± 19 712 ± 13 3831 ± 5

530 51 ± 3 330 ± 7 255 ± 9 727 ± 3 560 ± 1 4019 ± 5

540 45 ± 3 335 ± 11 234 ± 10 719 ± 3 615 ± 1 4188 ± 5

550 34 ± 3 312 ± 8 204 ± 9 642 ± 3 594 ± 1 4317 ± 7

560 32 ± 3 374 ± 9 218 ± 9 741 ± 3 733 ± 1 4427 ± 6

570 27 ± 3 306 ± 8 176 ± 8 592 ± 2 616 ± 1 4510 ± 5

580 19 ± 2 294 ± 10 159 ± 7 532 ± 2 575 ± 1 4572 ± 6

590 19 ± 3 320 ± 10 156 ± 7 519 ± 2 580 ± 1 4630 ± 5

600 9 ± 1 279 ± 12 121 ± 6 427 ± 2 490 ± 2 4676 ± 5

610 15 ± 2 324 ± 12 140 ± 6 451 ± 2 528 ± 2 4706 ± 5

620 19 ± 2 292 ± 8 144 ± 7 410 ± 2 492 ± 1 4749 ± 4

630 20 ± 2 293 ± 7 137 ± 6 370 ± 1 450 ± 1 4771 ± 4

640 20 ± 1 279 ± 9 136 ± 6 331 ± 1 412 ± 1 4809 ± 4

650 21 ± 2 257 ± 8 126 ± 5 299 ± 1 377 ± 1 4832 ± 5

660 37 ± 2 242 ± 8 152 ± 5 262 ± 1 339 ± 1 4878 ± 5

670 36 ± 2 214 ± 10 142 ± 5 227 ± 1 296 ± 1 4888 ± 5

680 20 ± 1 206 ± 11 110 ± 6 196 ± 3 256 ± 4 4889 ± 6

690 22 ± 1 219 ± 9 122 ± 6 207 ± 4 267 ± 5 4870 ± 9

700 19 ± 1 182 ± 8 96 ± 5 159 ± 3 202 ± 3 4847 ± 6

710 25 ± 1 224 ± 8 121 ± 5 174 ± 3 217 ± 4 4818 ± 6

720 19 ± 1 131 ± 8 90 ± 4 112 ± 2 140 ± 2 4817 ± 8

740 28 ± 2 204 ± 9 21 ± 6 139 ± 2 166 ± 3 4741 ± 6

780 51 ± 2 376 ± 9 219 ± 6 189 ± 3 198 ± 3 4523 ± 5

820 79 ± 2 461 ± 10 300 ± 7 199 ± 3 174 ± 2 4224 ± 4

860 89 ± 2 628 ± 14 345 ± 7 195 ± 3 119 ± 2 3655 ± 5

910 99 ± 2 825 ± 14 400 ± 8 176 ± 2 84 ± 1 3277 ± 4


Table A3. Continued. NWA 869 shock-darkened clast M-05-38-2-c.


980 57 ± 2 711 ± 17 279 ± 7 164 ± 2 53 ± 1 2690 ± 5

1050 47 ± 3 1054 ± 21 292 ± 8 301 ± 4 81 ± 1 2457 ± 6

1100 46 ± 3 1345 ± 25 278 ± 9 325 ± 5 78 ± 1 2302 ± 3

1130 44 ± 2 1336 ± 20 261 ± 7 286 ± 3 65 ± 1 2232 ± 4

1160 84 ± 3 4289 ± 50 726 ± 14 921 ± 10 216 ± 2 2268 ± 2

1180 71 ± 3 4571 ± 52 628 ± 13 791 ± 8 187 ± 2 2281 ± 3

1190 38 ± 3 2481 ± 34 323 ± 9 397 ± 4 88 ± 1 2203 ± 3

1205 57 ± 3 3555 ± 43 375 ± 11 594 ± 6 138 ± 1 2262 ± 2

1220 18 ± 3 1640 ± 27 127 ± 9 177 ± 2 39 ± 1 2188 ± 4

1250 55 ± 4 3788 ± 48 331 ± 11 511 ± 6 117 ± 1 2245 ± 3

1280 64 ± 4 3545 ± 47 345 ± 12 462 ± 5 108 ± 1 2271 ± 3

1310 66 ± 5 3113 ± 45 328 ± 13 371 ± 5 90 ± 1 2310 ± 4

1350 68 ± 5 3013 ± 46 316 ± 15 320 ± 4 84 ± 1 2413 ± 4

1385 61 ± 6 1912 ± 31 242 ± 16 198 ± 2 63 ± 1 2680 ± 5

1386 78 ± 6 898 ± 28 175 ± 16 97 ± 1 35 ± 1 2883 ± 10

Total 2659 ± 27 46420 ± 160 11856 ± 75 17292 ± 39 12547 ± 24 3924 ± 1.8


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The L3-6 chondritic regolith breccia Northwest Africa (NWA) 869: (I) Petrology, chemistry, oxygen isotopes, and Ar-Ar age determinations