Geochimica d Cosmochimica &a Vol. 53, pp. 937-944 Copyright 0 1909 Rrgunon Rex pk. Printed in U.S.A.

0067037/89/$3.00 + .oO

LETTER

Extraterrestrial spherules in glacial sediment from the Transantarctic Mountains, Antarctica: Structure, mineralogy, and chemical composition

CHRISTIAN KOEBERL’~~ and ERIK H. HAGEN~

‘Institute of Geochemistry, University of Vienna, Dr.-Karl-Lueger-Ring 1, A- 10 10 Vienna, Austria *Lunar and Planetary Institute, 3303 NASA Road One, Houston, TX 77058, U.S.A.

3Department of Geology and Mineralogy and Byrd Polar Research Center, The Ohio State University, Columbus, OH 43210, U.S.A.

(Received January 3, 1989; accepted in revised form March 20, 1989)

Abstract-Silicate and glassy spherules of probably extraterrestrial origin have been discovered in glacial till from localities in the Beardmore glacier region, Tramantarctic Mountains, Antarctica. These spherules are similar in structure, appearance, and composition to spherules found in deepsea sediments and in Greenland. Most spherules have a shiny and smooth dark surface, while some show a characteristic brickwork pattern composed of intergrown olivine crystals and magnetite dendrites. Several spherules have been sectioned after trace element analysis by INAA. The interiors of the spherules contain olivine crystals of varying size (predominantly of forsteritic composition) set in a glassy Fe-rich and Mg-poor matrix. The matrix contains very fine-grained dendritic magnetite crystals and rare accessory wustite. SEM and microprobe studies show that most of the spherules have not experienced significant weathering. Trace element analyses for individual particles confirm the extraterrestrial nature of the spherules. They have undifferentiated chondritic rare earth element (REE) patterns and are enriched in Ir and other siderophile elements (with Ir contents between 10 and > 1000 ppb). Some spherules have experienced loss of volatile elements and some differentiation of siderophile elements (e.g., Ni). The Antarctic spherules-which might be micrometeorites that melted during passage through the atmosphere, or ablation spherules-are a significant addition to the collection of extraterrestrial microparticles. Of key importance is their high abundance in the sediment and the fact that they have been concentrated by glacial action.

INTRODUCTION

SUBMILLIMETER-SIZED SmERuLEs of presumed extraterres- trial origin have been reported during the past century from different terrestrial environments, most notably in deep-sea sediments. The first magnetic spherules in deepsea sediments were discovered about 100 years ago (MURRAY and RENARD, 189 1). With the development of microanalytical techniques, especially the electron microprobe, studies of the microstruc- ture and composition revealed the extraterrestrial origin of these particles (CASTAING and FREDRIKSSON, 1958; FRED- RIKSSON and MARTIN, 1963; SCHMIDT and KEIL, 1966). Similar cosmic particles have been found in beach sand (MARVIN and EINAUDI, 1967) and a single spherule on a mesa mountain deposit (FREDRIKSSON and GOWDY, 1963); some rare occurrences have been reported from Antarctic ice (THIEL and SCHMIDT, 196 1; SCHMIDT, 1964; KING and WAGSTAFF, 1980; WAGSTAFF and KING, 198 1; KUMAI et al.,

1983). More recently, MAURETTE et al. (1986) reported the dis-

covery of a large number of spherules from blue lakes on the Greenland ice sheet. The spherules in Greenland have been concentrated by a process that leads to the formation of sea- sonal melt water lakes and to the deposition of the extrater- restrial particles in sediments at the bottoms of the lakes. In addition to the spherules, which are similar in appearance and composition to spherules from deepsea sediments, ob- viously unmelted extraterrestrial particles have been found in the same deposits.

Three main groups of cosmic spherules hae been recog- nized (e.g., BLANCHARD et al., 1980; BROWNLEE, 1981): (1)

iron spherules, (2) silicate spherules, and (3) glassy spherules. Iron spherules consist mostly of iron (with other metals) in the form of metal and/or oxides. These spherules comprise up to half of the deepsea particles and sometimes contain Ni-rich metal cores. Some iron particles were found to contain “nuggets” of platinum group metals (BROWNLEE et af., 1984), which were taken as an indication of melting and differen- tiation and segregation within the spheres. The silicate spher- ules make up the largest fraction in the deep-sea collection and usually consist of intergrown olivine and magnetite of varying grain size.

On the basis of similarities to meteorite fusion crusts and laboratory ablation experiment products (e.g., BLANCHARD and CUNNINGHAM, 1974; BLANCHARD and DAVIS, 1978), the spherules have been interpreted as meteorite ablation spherules (e.g., FREDRIKSSON and GOWDY, 1963; FRED- RIKSSON and MARTIN, 1963; MILLARD and RNKELMAN, 1970). Recent studies of the cosmogenic isotopes %e and 26A1 (RAISBECK et al., 1983, 1986; YIOU et al., 1985) indicate that a significant fraction of the spherules originated from small objects (micrometeorites). Small particles settle in the atmosphere without melting, but any micrometeoroid that is larger than about 50 to 100 pm will melt during the passage through the atmosphere (BROWNLEE, 1981, and references therein).

SPHERULES FROM ANTARCI-ICA

The polar regions have long been suggested as clean and efficient collection sites for extraterrestrial particles (e.g.,

937

938 C. Koeberl and E. H. Hagen

THIEL and SCHMIDT, 196 1). The recent discovery of con- centrated deposits in Greenland by MAURETTE et al. ( 1986) has added a large number of particles to the collection of extraterrestrial spherules. In Antarctica, despite its larger size, only isolated extraterrestrial particles have been found so far (SCHMIDT, 1964). More recently attempts have been made to isolate cosmic particles from Antarctic ice cores. Since the particles are diluted by ice, these investigations require melting and filtering of large quantities of ice in order to detect a few cosmic particles (KING and WAGSTAFF, 1980; WAGSTAFF and KING, 198 1). YIOU and RAISBECK (1987) found five mi- crospherules of suspected cosmic origin (50- 160 micrometers diameter) after melting 131 kg of ice from two ice cores at Dome C. THIEL et al. (1987) reported several cosmic particles after the melting of four tons of shelf ice and speculated about a possible concentration mechanism of cosmic matter in the shelf ice.

Last year we reported the discovery of concentrated de- posits of cosmic spherules in Antarctica (KOEBERL et al., 1988; HAGEN et al., 1989). These spherules were discovered accidentally during a geochemical study of the heavy-mineral fractions of till from the Dominion Range and other localities in the Beardmore Glacier/Walcott Neve region in the Trans- antarctic Mountains (HAGEN, 1988). The locations of our samples are given in Fig. 1. Samples of several hundred grams of sediment were sieved to isolate the 125 to 500 pm fractions within which up to 4000 spherules per 100 gram of sediment were recovered by hand-picking (HAGEN, 1988; HAGEN et al., 1989).

The largest concentration of spherules occurs in sediment from the so-called “Meteorite Moraine” close to Lewis Cliff of Mt. Achemar, where numerous meteorites have been re- covered during recent ANSMET [Antarctic Search for Me- teorites] expeditions (CASSIDY et al., 1987). The cosmic origin of these spherules has been indicated by mineralogical and compositional evidence (KOEBERL et al., 1988; HAGEN et al., 1989).

170’E

FIG. 1. Locality map of part of the Beardmore Glacier area in the Tramantarctic Mountains. The spherules were recovered from glacial sediment collected at Plunket Point, Dominion Range; Walcott Neve/ Lewis CliR, and Mt. Sirius.

SAMPLES AND ANALYTICAL METHODS

Glacial sediment samples from Plunket Point (Dominion Range, Transantarctic Mountains) and from Meteorite Moraine (Walcott Neve, Eeardmore Glacier area) (see Fig. 1) were sieved, and the spherules were recovered by hand-picking from the 125 to 500 Nrn fractions. Most spherules have a shiny dark (black to brown) surface, are magnetic, and have specific gravities larger than 2.89. In addition, several grey to white spherules (which are rare compared to the black spherules) were recovered from the Meteorite Moraine sediment.

Eight spherules from the Meteorite Moraine locality have been investigated by SEM-EDX (see Fig. 2), using the JEOL SEM at the NASA Johnson Space Center in Houston (20 kV acceleration voltage). In addition, samples from the Dominion Range have been analyzed for trace element compositions using instrumental neutron activation analysis (INAA). Subsequently, they were mounted and sectioned for electron microprobe studies using the Cameca Camebax electron microprobe at the NASA Johnson Space Center in Houston ( 15 kV acceleration voltage) with online ZAF correction procedures. The standardization was performed with well-characterized standard minerals available at the JSC Cameca microprobe. The electron beam has been focused to about 1 lrn diameter. Replicate analyses showed that the precision was generally better than 5 rel%. Results of the microprobe analyses are given in Tables 1, 2, and 3. Back-scatter electron images (see Fig. 3) were also obtained using a Robinson detector on the SEM at the NASA Johnson Space Center, Houston.

The INAA work on 15 spherules was reported earlier (KOEBERL et al., 1988; HAGEN et al., 1989). Following that initial characteriza- tion, we analyzed individual spherules and report here the lirst results from that study. The weight of the individual spherules was deter- mined with an ultra-micro balance. Before sealing the spherules in- dividually into quartz vials for irradiation, they were cleaned with water and alcohol (suprapure). The samples (together with natural and synthetic standards) were then irradiated for 110 hours at the 8 MW ASTRA reactor of the Austrian Research Center (Seibersdorf near Vienna) at a neutron flux of about 6. 10” n cm-’ s-‘. After cooling for four days, the spherules were transferred into suitable polyethylene vials for gamma spectrometry at the Institute of Geo- chemistry, University of Vienna. The samples were measured in four counting cycles, starting four days after the irradiation, and ending about eight weeks later. Measuring times ranged from about 50000 to 300000 seconds for each spherule.

This procedure allowed the measurement of 33 major and trace elements. For six of these elements (Br, Rb, Sr, Zr, Ba, and Ta) only upper limits could be determined. The precision of the analyses varies between the individual spherules due to different samples weights (and abundances), but for most elements it is better than +20% (for Na, SC, Cr, Mn, Co, Sm, Eu, Yb, Ir, and Au, better than 10%). The results of the INAA work are given in Table 4. After finishing the INAA measurements, the spherules were included in the electron microprobe analyses.

MORPHOLOGICAL AND MINERALOGICAL STUDIES

The external structure and appearance of the Meteorite Moraine spherules was studied by SEM (for the Dominion Range spherules, see HAGEN, 1988; HAGEN et al., 1989). Most spherules have a smooth surface even when using high magnifications, which indicates that these spherules have ex- perienced little weathering. A close examination of the sur- faces shows less etching compared to deep-sea spherules (which are often heavily corroded) or Greenland spherules as analyzed by CALLOT et al. (1987).

Some spherules display a characteristic brickwork pattern, similar to some deep-sea spherules (BLANCHARD et al., 1980) and Tunguska spherules (ZBIK, 1983). The brickwork pattern results from aligned olivine and magnetite crystals on the surface. Figure 2a gives an overall view of one of the large spherules with brickwork surface structure, while Fig. 2b shows a close-up of the same spherule. Large euhedml olivine

939 Extraterrestrial spherules in glacial sediment in Antarctica

FIG. 2. (a) SEM photograph of a spherule from Walcott Neve/ Lcwis Cliff The spherule consists of olivine and magnetite crystals and shows the typical brickwork pattern that is also observed in deep­sea or Tunguska spherules. (b) Close-up of the surface structure of the spherule in Fig. 2a. The large crystals that dominate the brickwork structure are euhedral olivine crystals offorsteritic composition. The space between the olivine crystals shows very fine-grained dendrites of magnetite. (c) Surface ofa greyish white spherule from the Walcott Neve/Lewis eli If. This is a new variety of cosmic spherules consisti ng of pure forsterite that have not been observed at any other terrestrial location. The very narrow brickwork pattern has no magnetite com­ponent.

crystals are intergrown with fine-grained dendritic magnetite. The individual magnetite crystals that make up the dendritic structure are of submicrometer size. The grain sizes of the olivine crystals are widely variable between and within in­dividual spherules and range from about I and 60 microm­eters. In addition to the olivine and magnetite crystals, a glassy Fe-rich matrix is present, but it is difficult to see from the exterior.

The (optically) grey to white spherules show an interesting difference in structure and composition. Figure 2c is a close­up of the surface of a 280 llm greyish spherule. A brickwork pattern that is slightly different from the black spherules is visible. Here the brickwork pattern is composed only of par­allel bars of euhedral olivine crystals without any magnetite. The olivine crystals are much smaller, and the spacing be­tween the bars is narrower than for the black spherules. EDX analyses of the spherule in Fig. 2c gave a purely forsteritic olivine composition, without any indication of magnetite or an Fe-rich glass phase. To our knowledge no spherules of such composition have been reported from other occurrences. A more detailed description of these spherules with analytical data is in preparation.

In order to investigate the interior structure of the spherules, several have been sectioned. Although the outside surfaces of most spherules look alike, their interiors show widely varying textures. Figure 3a-e shows examples of different looking spherules, all of which are larger than 100 llm. In general, the microprobe analyses indicate that most spherules have the same structural components: olivine crystals (offor­steritic composition), a glassy Fe-rich and Mg-poor matrix, and magnetite. About 50% ofthe spherules have a fine-grained interior (similar to Fig. 3b, but sometimes without any visible structure), while the others display a pronounced crystal structure. Olivine crystals are usually of euhedral shape and often show quench textures (Fig. 3a-c). Some fine-grained spherules show structures like barred chondrules, which have also been observed in deep-sea spherules (BLANCHARD et aI., 1980). Several spherules contain voids (e.g., Fig. 3a) which probably originated from escaping volatile elements or com­pounds.

Numerous microprobe analyses of the interior components of about 20 spherules are summarized in Table I. The spher­ules CS-l, CS-2, CS-3, and CS-O have previously been ana­lyzed by INAA for trace elements. CS-2 is shown in Fig. 3b; CS-I and CS-3 have a similar appearance but with much smaller grain size, while CS-O is very similar to the spherule shown in Fig. 3e. It is obvious that the chemical compositions of the olivine crystals vary both between the individual spherules and within spherules (e.g., CS-ll; Fig. 3a). The glass (matrix) composition is also variable, depending on variable amounts of very fine-grained (submicrometer-sized) magnetite. The bright spots in CS-2 (Fig. 3b) are in fact mag­netite dendrites of about 2 llm length that are composed of submicron-sized magnetite crystals. All analyses are consistent with chondritic abundances and abundances known from deep-sea and Greenland spherules.

Most olivine crystals are zoned, being Fe-rich at the rim and Fe-poor in the center. Table 2 gives the results of a mi­croprobe profile across one of the large olivine crystals of spherule CS-12 (Fig. 3c), starting and ending in the glass/

940 C. Koeberl and E. H. Hagen

SiO2

n02

A’203

Fe0

Ml0

COO

MS0

NW

K20

0203

Ni

S

41.46 302 39.21 4a30 17.39 3a54 34.52 37.14 10.24 4x07 5l.W 37.52 1x51

0.W 0.009 0.16 I236 a07 0.3W 0.115 a0 a0 a0 ano am a21

&76 a391 4.76 7.55 207 7.11 4.37 aJ3J a063 a359 29w 266 4.36

20.68 323.5 3462 3337 1x69 4M4 4947 no2 IL12 649 2&w 19.39 71.29

0.459 a312 a29 0.57 0.41 a52 0.512 a292 a215 a534 a922 a17 aI5

3.30 a466 1.31 4.01 1.67 &al 2842 a235 a124 a25 235 227 a61

3208 2864 19.32 4.39 31.83 2.771 ha5 3483 47.52 4624 1x95 Lo2 7.36

0.0 a0 0.01 a01 a0 aoo5 0.11 a0 aof3 aOW 1.393 a16 aw

a0 a017 a0 0.0 0.0 a0 0.0 a0 a0 0.003 0.069 au 0.0 aw6 a084 a20 a73 a36 aBas 0.737 0.487 a327 0.210 al65 0.03 a90 a179 al85 au 0.02 a36 0.062 0.036 a 165 0.06 am7 a021 a0 0.22 0.0 a009 a0 0.19 0.01 0.644 0.341 0.0 a0 a0 aw3 a0 0.03

magnetite matrix (points 1 and 8). The points near the rim (2,6, 7) are clearly more Fe-rich and Mg-poor than the core. The zoning is not symmetrical, since the forsteritic core is not in the exact center of the crystal. In some spherules the grain size is so small that at the spot size of the electron beam the sample is almost homogeneous. The chemical zoning of the olivine crystals is variable in extent. Some crystals (es- pecially small ones) show only little variations from rim to core. This seems to be a function of the quench process that leads to the formation of crystals of different sizes. The cooling rates have been different for individual spherules, as indicated by different crystal sizes, zoning of olivines, and widths of magnetite dendrites (observations from chondrules seem to show that in some cases the cooling rate can be inferred from grain sizes and quench textures; HEWINS, 1988).

No FeNi spherules have been found so far in the new Ant- arctic collection, indicating that they are rare, which is con- sistent with new observations from Greenland (MAURETTE

et al., 1986). Among the about 40 spherules that have been sectioned, none contained any FeNi core (unlike cores in glassy or silicate deep-sea or Greenland spherules; FRED-

RKZKIN and G~WDY, 1963; BLANCHARD et al., 1980; ROBIN

et al., 1988). However, in a few spherules, small metal inclu-

Xl #Z x3 x4 w #6 w XI

902 5&% 5a57 u.W 4374 19.93 50.78 51.26 5a58

=02 a17 a05 a01 a0 a10 all 0.14 aIs

A’203 102 216 a79 0.54 205 221 247 275

Fe0 23.51 13.48 IO.13 9.59 1676 19.91 m.51 n.38

hf”0 1.11 0.75 0.62 aw 1.03 a99 a99 1.07

60 119 a95 0.40 0.29 1.70 1.91 1.77 281

MS0 15.17 30.56 4272 4472 MW 21.76 20.46 I136

N@ 1.48 0.44 a2o aw au 0.76 a73 1.05

K20 a07 0.66 0.02 a02 aor aos a04 a07

0203 0.19 0.16 0.22 0.20 0.16 0.14 0.m al5

Ni 0.02 0.03 0.05 0.65 0.01 aw 0.0 0.0

s a03 a0 a02 0.6 0.0 a0 0.w 0.0

sions have been found. Table 3 gives the composition of two small (3 pm diameter) metal droplets in spherule C?S- 15. They seem to consist (at least partly) of Ni-rich metallic Fe. The silicate component is probably caused by excitation of the surrounding areas by the electron beam. The two droplets, although only 15 pm apart, have distinctly different com- positions (one being very Cr-rich).

CHEMICAL COMPOSITION

The abundances of the major elements in a number of different spherules have already been discussed in the previous section. All abundances are in agreement with chondritic values. Figure 4 gives the relationship between Si, Ca, and Al which are diagnostic of chondritic abundances. The Ant- arctic spherules plot near the line defined by different chon- drites, together with deep-sea spherules (BLANCHARD et al.,

1980). The trace element composition (measured by INAA) of four individual spherules from Plunket Point/Dominion Range is given in Table 4. It is obvious that there are signif- icant differences between the individual spherules, although their composition (or the abundance ratios of selected trace elements) are close to chondritic abundances (or abundance ratios). The rare earth elements (REE) in the four spherules in Fig. 5 all have a flat pattern, which is characteristic of

F.?

Fe0

Ml0

%G3 Ni

SiO2

n02

A1203

CUO

0.309

a025

181

21.81

a69

4.13

2u

1831

59.58 W.30 2204

am5 a32 a31

1238 5684 54.84

1.43 a03 a05

483 a18 a37

a355 125 119

7.34 5.99 860

a667 0.01 a66

lo.34 54.4 7.91

941 Extraterrestrial spherules in glacial sediment in Antarctica

undifferentiated chondritic matter. Volatile elements show

the largest variations. The differences between the chondritic

abundances and the actual abundances in the spherules is

probably due to processes occurring during passage through

the atmosphere. Particles that are larger than about 50 to

100 !Lm melt when they pass through the atmosphere.

FIG. 3. (a) SEM-BSE photo of a sectioned spherule from Plunkel Point (spherule CS-II). The dark crystals are euhedral olivine. which is set in a glassy matrix that contains very fine-grained magnetite and accessory wustite (lower left). The voids were probably caused by the escape of volatiles. The olivine crystals are zoned with an Fe-poor core. (b) SEM-BSE photo of a sectioned spherule from Plunket Point (spherule CS-2; see Tables I and 4). The interior consists of small olivine crystals and micrometer-sized magnetite dendrites (bright spots). A crystallization texture is visible on the left side. (c) SEM­BSE photo of spherule CS-12 from Plunket Point. The spherule shows large zoned olivine crystals set in a glassy matrix with small magnetite dendrites (white). The zoning of the crystals (see Table 2) and their size indicates a cooling history that is different from that of spherule CS-2 (Fig. 3b). (d) SEM-BSE photo of spherule CS-16 from Plunket Point. The dark areas are Fe-poor forstcrite (FeO = 0.8-1.5 wt%). the lighter areas are glassy matrix. and the white spots are magnetite with chromite (12 wt'Ji Cr203)' (e) SEM-BSE photo of spherule CS­17 from Plunket Point. consisting of 5 to 10,um sized olivine crystals and long spear-shaped magnetite dendrites.

However, without IOBe_26Al measurements (which are

planned) we are unable to decide if the Antarctic spherules are individual micrometeorites or ablation spherules. Mea­surements of 53Mn in some deep-sea spherules by NISHIIZUMI

and ARNOLD (1982) have been interpreted as being in agree­ment with their interpretation as ablation spherules. However,

942 C. Koeherl and E. H. Hagen

NCr

SC

a Mn

Fe (w%)

co Ni (~96)

z/l

As

se Br

Rb

sr

ZI

llu

cd

Sb

BE

h

cc

Nd

Sm Eu lt m Lu

Hf ra OS

Ir

AlI

778

cl

-

330

156

904

1120

19.1

202

1.02

6S

1.2

LO5

C8

CEO

C&3

Go

0.82

1.66

0.073

-SO

a29

0.80

a61

am

aa91

0.052

0.235

0.033

Cl.7

<0.3

I.0

I.023

a149

@%

<a2

10300

9a8

919s

22m

262

247

0.58

117

<5

272

CW

CM0

<MO

C8O

a2

234

a28

C2OO

40

Il.0

as

10

1.2

a65

246

a40 4

<a9

<a2

a&s1

a019

1.6

J.S

240

291

I1

Is60

2s. 7

27.1

a06

IS2

<5

1.69

cw

C2%

CM0

<70

0.07

I.37

1.0

c2lm

1.25

10

20

a63

0.23

aI6

a673

am

<2

so.9

<a2

a010

a056

a7

<a8

5.0

7so

2370

19.7

Iso

0.83

105

<3

2.4

<IO

<I%

<I%

MO

a85

1.2

a25

<I%

0.80

22

O.SO

0.19

al0

aSo

a08

<I

<as

a92 a957

a19

co.2

<as

79%

7.8

3soo

2706

27.2

763

bSI

455

24

29

4

3.45

11.4

17

1.16

O.%

a21

3.6

0.367

a957

ani

0.231

ao87

a056

a246

a0381

a17

a02

0.945

a975

0.255

a051 QOl4

III, I 0.01 0.1

Ca/Si (wt%)

FIG. 4. Correlation diagram between Al, Ca, and Si in extrater- restrial spherules as compared to chondrites (after BLANCHARD et al., 1980). The Antarctic spherules (microprobe measurements ofthe sectioned spherules from Plunket Point) plot close to the correlation line defined by the three chondrite groups and are similar to the deep sea sphemles.

FIG. 5. Chrondrite-normalized REE plot for four individual spher- ules from Plunket Point..Spherule CS-2 is shown in Fig. 3b. All spherules have flat and undifferentiated chrondritic patterns (the uni- form enrichment in some spherules is probably due to the loss of volatiles).

RAISBECK e# al. (1983, 1986) have been able to show that a large fraction of the cosmic spherules in Greenland and in the deep-sea collection might have been individual micro- meteorites. Therefore, it is possible that the Antarctic spher- ules may have originated as micrometeorites too. The uniform enrichment of the REEs in some of the spherules is most probably due to the loss of volatile components (e.g., Na, K, Zn, As, Se).

The siderophile elements, such as Ir or OS, are enriched in most spherules and thus provide further proof for an ex- traterrestrial origin. The absolute abundances of Ir and OS are near the chondritic abundances in two of the spherules, while the other two still show significant Ir enrichments com- pared to terrestrial materials. The elemental contents are comparable to the abundances observed in deep-sea (GAN- APATHY et al., 1978) and Greenland spherules (CHEVALLIER et al., 1987; ROBIN et al., 1988). Similar large variations in the absolute abundances have been observed in these spher- ules.

Although Fe, Cr and Mn are close to chondritic abun- dances, other siderophile elements are not. Figure 6 shows the chondrite-normalized siderophile element patterns for the four spherules, indicating that they have experienced some fractionation. This may be due to compositional variations in the original micrometeorites or to fractionation processes during melting and quenching in the atmosphere. The spher- ules are also devoid of sulfur, which is normally present in chondritic material. This may indicate that some siderophile elements fractionated into sulfides, which were subsequently lost as volatile compounds. The exact process of siderophile element fractionation is, however, not known (which does not apply only for the Antarctic spherules but also for the Greenland and deep-sea spherules).

CONCLUSIONS

We have described silicate and glassy spherules from new localities in the Transantarctic Mountains. These spherules are very similar to spherules found in deep-sea sediments and in blue ice lakes in Greenland, and are most probably of extraterrestrial origin. SEM studies have shown that the

Extraterrestrial spherules in glacial sediment in Antarctica 943

I NI CO OS lr Au

FIG. 6. Chrondrite normalized patterns of some siderophile ele- ments in four cosmic spherules from Plunket Point,(Table 4). The pattern indicates that some siderophile elements underwent frac- tionation, perhaps into sulfide phases that were lost during passage through the atmosphere.

Antarctic spherules usually have smooth surfaces or show a

characteristic brickwork structure that is composed of inter- grown olivine (forsterite) and fine-grained magnetite crystals. Individual spherules have been sectioned and show a wide variety of internal structures, usually olivine crystals set in glassy matrix with dendritic magnetite crystals (of micrometer to submicrometer size). The grain sizes of the crystals vary between individual spherules as a function of their different cooling histories.

This is also evident from the chemical compositions of the

crystals obtained by microprobe analyses. Larger olivine crystals are zoned, with an Fe-rich rim and an Fe-poor core.

Most spherules show less evidence of weathering than the Greenland or deep-sea spherules. This is due to the environ-

mental conditions in Antarctica, where the spherules are only very rarely exposed to meltwater and biological weathering is absent. This indicates that weathering of the spherules is slower in Antarctica than in other terrestrial environments,

The major and trace element composition ofthe spherules provides further evidence for an extraterrestrial origin. Si,

Mg, Al, and Fe are very close to the chondritic abundance ratios. The REEs in individual spherules show flat chondrite normalized patterns, which are characteristic of undifferen- tiated chondritic material. The uniform enrichment of the REEs in some spherules is most probably due to the loss of volatile compounds. Siderophile elements, such as Ir or OS, are enriched in all four spherules (compared to terrestrial materials) and have chondritic abundances in two spherules. The siderophile element patterns indicate that some frac- tionation has taken place, probably after partitioning into sulfide phases and subsequent loss as volatile sulfide com- pounds.

An important factor is the high abundance of the Antarctic spherules in the glacial sediment. If only spherules larger than about 125 pm are considered, their abundance ranges up to several thousand spherules per 100 g of bulk sediment (HAGEN et al., 1989). This indicates that a very efficient mechanism has been active in concentrating these spherules from glacial ice into glacial sediment. Glacial action and wind

erosion-deposition (due to the strong katabatic winds) may contribute to the concentration of the spherules in the glacial sediment.

We conclude that the new Antarctic spherules are of ex- traterrestrial origin. They may represent individual micro- meteorites that melted during passage through the atmo- sphere. Alternatively, they may be ablation spherules, al- though we prefer the former explanation. Measurements of “Be and 26Al will be helpful to distinguish between the two possibilities. They represent a valuable addition to the col- lection of cosmic microparticles in terrestrial environments.

Acknowledgements-We are grateful to Gunter Faure (Ohio State University) for very helpful discussions during the work. C.K. is grateful to M. Zolensky (NASA-Johnson Space Center, Houston) for help with the SEM photography, to R. Bernhard and V. Yang (NASA-JSC) for help with microprobe and SEM-BSE work, and to Donna Jalufka-Chady (Lunar and Planetary Institute) for artwork. Reviews by K. Fredriksson and two anonymous reviewers have been helpful for improving the paper. This research has been supported by NSF grant DPP-87 14324. The Lunar and Planetary Institute is operated by the Universities in Space Research Association under contract NASW-4066 with the National Aeronautics and Space Ad- ministration. This is Lunar and Planetary Institute Contribution No. 702.

Editorial handling: G. Faure

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