Modal abundances

• ~70 vol% Fe,Ni-metal (Fig. 1)

• ~25 vol% silicates (chondrules & fragments)

• ~4 vol% troilite; <1 vol% merrillite, Cl-apatite & chromite

• No CAIs/AOAs or Al-rich chondrules have been found

• No fine-grained matrix

Textures and mineral assemblages

• Chondrules: mostly porphyritic (PO, POP, PP); rare non-porphyritic, mostly fragmented (BO, SO,

CC) (Figs. 2a,c, 3)

• apparent mean chondrule diameter: 370 μm, range: 60‒1200 μm (Fig. 4)

• Mesostasis contains abundant high-Ca pyroxene crystallites (Fig. 3)

• Troilite occurs as coarse grains, occ. associated with tetrataenite (Fig. 2d)

• Metal is mainly unzoned kamacite, some grains show chemical zoning with M-shaped Ni-profiles;

taenite and tetrataenite are rare (Figs. 2b, 5)

• Cl-apatite, merrillite and chromite occur in metal and peripheral parts of some chondrules, some

chondrules have P-rich rims (Figs. 2a,e,f, 6)

• One bleached CC chondrule was found (Fig. 2a)

• Low-Ca pyroxene surrounded by metal and peripheral parts of chondrules is replaced to various

degrees by ferroan olivine (Figs. 6a,b)

General Information on NWA 12379

• Found between 2017 Aug and 2018 Sep

• Total mass: 765 g

• Main mass with Luc Labenne

• Type specimen (20 g) at Natural History Museum of

Denmark in Copenhagen

Objectives

• Characterization & classification

• Genetic link to other chondrite groups

• Origin & formation history

Methods

• Optical microscopy of thick section (Fig. 1)

• Petrographic characterization by SEM imaging

• Mineral chemistry and X-ray mapping by EPMA

• O-isotope composition by IR-laser fluorination

• Preliminary Fe-isotope analysis of Fe,Ni-metal

PETROLOGY, MINERALOGY AND OXYGEN ISOTOPIC COMPOSITION OF NWA 12379 A NEW METAL-RICH CHONDRITE WITH AFFINITY TO ORDINARY CHONDRITES

C. A. Jansen1, F. E. Brenker1, A. N. Krot1,2,3, J. Zipfel4, A. Pack5, L. Labenne6, M. Bizzarro3, M. Schiller3

1Institute for Geosciences, Goethe-University, Frankfurt/M., Germany. 2Hawai‘i Institute of Geophysics & Planetology, University of Hawai‘i at Manoa , Honolulu, USA.3Centre for Star & Planet Formation, Copenhagen, Denmark. 4Senckenberg Forschungsinstitut & Naturmuseum, Frankfurt am Main, Germany. 5Georg-August-University, Gottingen, Germany. 6Labenne Meteorites, Paris, France. .

Figure 1. Reflected light photomicrograph of a thick section of NWA 12379 studiedshowing the locations of chondrules analyzed and areas of acquired X-ray maps.

Figure 6. (left) BSE images of silicates and phosphates in metal. (a, b) Low-Ca pyroxene (px) isreplaced to various degrees by ferroan olivine (ol); (c) chromite occurs at grain boundarybetween PO chondrule and metal; (d) merrillite occurs in both metal and chondruleperipheries as small, rounded inclusions.

Figure 3. Some porphyritic chondrules typical for NWA 12379. Porphyritic olivine (PO),olivine-pyroxene (POP), and pyroxene (PP) are the most common types. While nearly allolivine (ol) and low-Ca pyroxene (px) phenocrysts have uniform, equilibratedcompositions, some chemically-zoned and dusty olivine grains have been found.Mesostasis (mes) contains abundant high-Ca pyroxene (cpx) crystallites; secondary olivine(sec ol) replaces low-Ca pyroxene. Fine-grained matrix and matrix-like chondrule rims areabsent. What appears white, is Fe,Ni-metal, and some porosities can be seen (black).

Figure 9 (right) O-isotope compositionof a single chondrule from NWA 12379compared to OCs and two analysesfrom NWA 122379 suggested forpairing with NWA 12379. Values areconsistent with L or LL chondrites. Datafor OCs and NWA 12273 from [4] and[5], respectively.

Pyroxene Fs#

mean=14.7±3.7, n=19

mea

n

mea

n

Olivine Fa#

mean=25.3±3, n=36

a b

30

20

10

0

freq

uen

cy,

%

chondrule diameter, µm

400 800 12000

mean Chondrule diameter

mean=370±250, n=22

Figure 7. Histograms of (a) Fa content of olivine and (b) Fs contents of low-Capyroxene from porphyritic chondrules in NWA 12379 including mean values andstandard deviation (±1𝜎)

Figure 2. Combined RGB and single element Kα X-ray maps of “X-ray map 1”-section of NWA12379 (cf., Figure 1) in (a) Mg (red), Ca (green) and Al (blue) with indicated chondruletextures. Note that merrillite occurs as isolated grains as well as in chondrule rims; (b) Nimap. Most metal grains are unzoned kamacite (km), taenite (tn) and tetrataenite (ttn) arerarer. Some grains show chemical zoning with M-shaped Ni-profiles; (c) Si map showingsilicate abundance and chondrule textures; (d) S map showing the distribution of troilite; (e)P map showing the distribution of phosphates. Note that some chondrules are enriched in P,especially in their peripheral parts; (f) Cr map showing the distribution of chromite.

Figure 4. Histogram of apparent non-fragmented chondrule sizes includingmean value and standard deviation (±1σ).

Figure 5. (a-d) BSE images of zoned Fe,Ni-metal grains.

References:[1[ Jones, R. H. (1998) LPS XXIX, Abstract#1397. [2] Krot, A. N. et al. (2014) In Meteoritesand Cosmochemical Processes (ed. A.M. Davis)Vol. 1, Treatise on Geochemistry (eds. H. D.Holland and K. K. Turekian), Elsevier, Oxford, pp.1–63. [3] Weisberg, M. K. et al. (2015)Geochimica et Cosmochimica Acta, 167:269–285. [4] Clayton, R. N. et al (1991) Geochimicaet Cosmochimica Acta, 55(8):2317–2337. [5]Agee, C. B. et al. (2019) LPS XLX, Abstract#1176. [6] Grossman, J. N. et al. (2000) Sci.,Meteoritics & Planet. Meteorites and the EarlySolar System II, 35, 467-486. [7] Lewis, J. A.and Jones, R. H. (2018) LPS XLIX, Abstract#1254. [8] Huss, G. R. et al. (2006) in pp. 567–586. [9] Jones, R. H. (1999) LPS XXIX, Abstract#1397. [10] Trinquier, A. et al. (2009) Science,324, 374–376. [11] Warren, P. H. (2011) EarthPlanet. Sci. Lett., 311, 93–100. [12] Budde, G.et al. (2016) Earth Planet. Sci. Lett., 454, 293–303. [13] Cook, D. L. and Schönbächler, M.(2017) Astronomical Journal, 154, 172. [14]Nanne, J. A. M. et al. (2019) Earth Planet. Sci.Lett., 511, 44–54. [15] van Kooten, E. M. et al.(2016) Proc. Nation. Acad. Sci., 113, 2011–2016.

[Abstract #2741]

Figure 8. (right) Fa vs. Fs ofcorresponding pairs of olivine and low-Ca pyroxene in porphyritic chondrulescompared to silicate compositions ofequilibrated (type 4-6) OCs, CB and Gchondrites (data from [1], [2] and [3],respectively). Silicates in NWA 12379are much more oxidized than in CB andG chondrites and not equilibrated.

Chondrules

Electron microprobe analyses show a large spread of

silicate compositions in porphyritic chondrules

• olivine Fa25.3±3; range Fa18.1–28.5, median Fa26.5, PMD=11.9, Cr2O3 = 0.03±0.02 wt%, FeO/MnO = 53.6±5.9 (wt. ratio), n=36 (Figs. 7a, 8)

• low-Ca pyroxene Fs14.7±3.7Wo1.4±1.3; range Fs3.2–18.7

Wo0.2–4.5, median Fs15.8Wo0.8, n=19 (Figs. 7b, 8)

Oxygen isotopes

• Bulk analysis of a single 1.98 mg chondrule revealed δ17O = 3.69‰, δ18O = 5.14‰, ∆17O = 0.96‰ (Fig. 9)

Similarities to metal-rich carbonaceous (CH and CB) and G chondrites include high Fe,Ni-metal abundance and lack of matrix [2,3]. However,high Fa and Fs contents, the dominance of FeO-rich porphyritic chondrules and whole-rock ∆17O ~1‰ clearly distinguish this meteorite fromthese groups (e.g., [2]). These features, along with the rarity of Al-rich chondrules and refractory inclusions [2], the presence of bleachedchondrules [6], and Cl-apatite, merrillite and chromite [7] in combination with the larger spread of silicate compositions (unequilibrated type 3range [1,8]) suggest an affinity of the silicate fraction of NWA 12379 to mildly metamorphosed L3 chondrites. Its high metal-abundance andlack of matrix, however, are inconsistent with OC classification. Thus, we classify NWA 12379 as “ungrouped metal-rich chondrite” with affinityto OCs. Based on petrography and mineral chemistry, it seems to be paired with NWA 12273 (cf., [5]).

At this point, it remains unclear whether the two major components of this meteorite, chondrules and metal, have a common origin (i.e.,non-carbonaceous solar system reservoir), or were mixed together by some process, e.g., collision of a metal-rich with an OC-like asteroids. Ourpreliminary Fe-isotope data suggest an outer solar system origin for the metal fraction of NWA 12379. To answer this question, whole-rock Cr, Ti,Fe, Ni, and Mo isotopic measurements of the silicate and metal fractions are needed [10–15].

The meteorite experienced oxidation and mild fluid-assisted thermal metamorphism on its final parent-body, that resulted in near-equilibration of chondrule silicates, and the formation of phosphates, chromite, tetrataenite and secondary ferroan olivine that replaces low-Capyroxene. Timing of the latter and the source of the fluids do also remain open issues at this point. Oxygen-isotope measurements of thesecondary ferroan olivine by SIMS are needed.

Fs

(mol%

)

0

5

10

15

20

25

0 5 10 15 20 25 30

Fa (mol%)

H

L

LL

NWA 12379

equilibrated

OCs

a

CBs

G

Sources:

OCs: Jones, 1998

CBs: Krot et al., 2014

G: Weisberg et al., 2015

δ18O, ‰

δ1

7O

, ‰

NWA 12379

NWA 12273

LL

L

H

23 3.5 4.5 5.5 6.54 5 6

2.5

3.5

4.5

3

4

5

TF

(this study)

(Agee et al., 2019)

(Clayton et al., 1991)