Petrology and shock metamorphism of the olivine-phyric shergottite Yamato 980459

Geochimica et Cosmochimica Acta, Vol. 68, No. 10, pp. 2359 –2377, 2004 Copyright © 2004 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/04 $30.00 ⫹ .00

Pergamon

doi:10.1016/j.gca.2003.11.022

Petrology and shock metamorphism of the olivine-phyric shergottite Yamato 980459: Evidence for a two-stage cooling and a single-stage ejection history ANSGAR GRESHAKE,* J¨ORG FRITZ, and DIETER STO¨ FFLER Museum fu¨r Naturkunde, Institut fu¨r Mineralogie, Humboldt-Universita¨t zu Berlin, Invalidenstrasse 43, 10115 Berlin, Germany (Received August 15, 2003; accepted in revised form November 21, 2003)

Abstract—The basaltic Martian meteorite Yamato 980459 consists of large olivine phenocrysts and often prismatic pyroxenes set into a fine-grained groundmass of smaller more Fe-rich olivine, chromite, and an interstitial residual material displaying quenching textures of dendritic olivine, chain-like augite and sulfide droplets in a glassy matrix. Yamato 980459 is, thus, the only Martian meteorite without plagioclase/ maskelynite. Olivine is compositionally zoned from a Mg-rich core to a Fe-rich rim with the outer few micrometers being especially rich in iron. With Fo84 the cores are the most magnesian olivines found in Martian meteorites so far. Pyroxenes are also mostly composite crystals of large orthopyroxene cores and thin Ca-rich overgrowths. Separate pigeonite and augites are rare. On basis of the mineral compositions, the cooling rates determined from crystal morphologies, and crystal grain size distributions it is deduced that the parent magma of Yamato 980459 initially cooled under near equilibrium conditions e.g., in a magma chamber allowing chromite and the Mg-rich silicates to form as cumulus phases. Fractional crystallization at higher cooling rates and a low degree of undercooling let to the formation of the Ca-, Al-, and Fe-rich overgrowths on olivine and orthopyroxene while the magma was ascending towards the Martian surface. Finally and before plagioclase and also phosphates could precipitate, the magma was very quickly erupted quenching the remaining melt to glass, dendritic silicates and sulfide droplets. The shape preferred orientation of olivine and pyroxene suggests a quick, thin outflow of lava. According to the shock effects found in the minerals of Yamato 980459, the meteorite experienced an equilibration shock pressure of about 20 –25 GPa. Its near surface position allowed the ejection from the planet’s surface already by a single impact event and at relatively low shock pressures. Copyright © 2004 Elsevier Ltd basaltic and olivine-phyric shergottites with abundant olivine megacrysts (Goodrich, 2002). Lherzolitic shergottites have more olivine and less plagioclase than the basalts and are cumulate rocks formed in a plutonic environment. The nakhlites (clinopyroxenites) show characteristic cumulate textures as do the dunite (Chassigny) and the orthopyroxenite (ALH 84001). With a crystallization age of ⬃4.5 Ga, ALH 84001 is the only representative of old Martian crust found so far. All Martian meteorites have been ejected from the planet’s surface by large-scale impacts. The ejection velocity must have exceeded the escape velocity of Mars, which is ⬃5 km/s (e.g., Nyquist et al., 2001). Extreme physical conditions experienced by the meteorites during ejection caused significant changes in the textures, mineralogy, and possibly even the isotopic compositions of constituent mineral phases. Intensive studies of shock effects, especially quantitative estimates of the peak shock pressure based on refractive index measurements of plagioclase and maskelynite proved that all Martian meteorites experienced shock pressures ranging between ⬃5 and 45 GPa (Fritz et al., 2003). Understanding the type and intensity of shock metamorphism of Martian meteorites is essential for the interpretation of the ejection and possible impact-induced relocation processes, which relate to some extent to the problem of the geological provenance, to the interpretation of analyzed isotope systems, and even to a possible transfer of early life from Mars to Earth (Nyquist et al., 2001; Horneck et al., 2001). A new Martian meteorite, Yamato 980459, of 82.46 g was found on the Yamato ice field, Antarctica by an official Japanese meteorite expedition on December 4th, 1998 and initially

1. INTRODUCTION

Due to the intensive search for meteorites, especially in the North African and Arabic deserts and in Antarctica, the number of Martian meteorites has increased to 30 unpaired samples (Meyer, 2003). These meteorites are basaltic and ultramafic igneous rocks that share mineralogical, geochemical, and isotopical characteristics indicating a common mantle source region of a large planetary body which is clearly distinct from the HED parent body (e.g., McSween, 1994; Folco et al., 2000). The generally young crystallization ages (⬍1.3 Ga), characteristic C, N, O, and noble gas isotopic compositions, distinct geochemical signatures, and most convincingly the fact that the meteorites contain shock-implanted noble gases closely matching the Martian atmosphere abundances determined by the Viking landers led to the now widely accepted conclusion that these rocks represent samples from Mars (e.g., Bogard and Johnson, 1983; Bogard et al., 1984; Becker and Pepin, 1984, 1986; Nyquist et al., 2001 and references therein). The Martian meteorites are divided into five groups: nakhlites, basaltic shergottites, lherzolitic shergottites, dunites, and orthopyroxenites—where the two latter groups are represented by only one meteorite each. Basaltic shergottites, the most abundant among the Martian meteorites, are pyroxene-rich basalts that probably formed in near-surface dykes, shallow intrusions or larger lava flows (e.g., Zipfel et al., 2000). Recently, basaltic shergottites were subdivided into olivine-poor

* Author to whom correspondence should be addressed (ansgar. [email protected]). 2359

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described as a porphyritic basalt with large olivine megacrysts set into a matrix of pyroxene and glassy material (Kojima and Imae, 2002). First petrological and mineralogical studies of this meteorite, all emphasizing the absence of plagioclase/maskelynite, indicate, that Yamato 980459 had an unusual magmatic history (Ikeda, 2003; Mikouchi et al., 2003; McKay and Mikouchi, 2003; Greshake et al., 2003). All studies note the prominent features of this meteorite, i.e., the strong chemical zoning of olivines and pyroxenes, the Mg-rich nature of olivine cores suggesting a phenocryst origin, the presence of olivine and pyroxene dendrites in the quenched residual material, and propose a crystallization sequence starting with early precipitation of Mg-rich olivine phenocrysts at 1450°C, followed by orthopyroxene and finally Ca-pyroxene. Calculations by McKay and Mikouchi (2003) show that the remaining melt was oversaturated with plagioclase which did not nucleate due to very quick cooling. Whereas the mineralogy of Yamato 980459 is different from the other known olivine-phyric shergottites and not typical for a basalt, its bulk chemistry is clearly basaltic and very similar to that of Sayh al Uhymir 005 (Dreibus et al., 2003; Misawa, 2003; Shirai and Ebihara, 2003). Isotopic studies reveal a Rb-Sr age of 304 ⫾ 82 Ma similar to QUE 94201 (Shih et al., 2003). They also indicate that Yamato 980459 may have crystallized from a magma type closely related to that of Dar al Gani 476, Sahy al Uhymir 005, and QUE 94201 (Shih et al., 2003). The cosmic-ray exposure age of Yamato 980459 is around 2.1–2.5 Ma (Nagao and Okazaki, 2003). Considering the terrestrial ages of Antarctic Martian meteorites (0.007– 0.29 Ma), the ejection age would be 2.1–2.8 Ma, which is comparable with QUE 94201 and NWA 480 (Nagao and Okazaki, 2003). Here we report a detailed study of petrology and shock history of this unusual Martian meteorite. 2. ANALYTICAL METHODS Three doubly polished thin sections prepared with acetone-soluble epoxy were studied by transmitted and reflected light microscopy. After optical inspection the sections were investigated by a JEOL JSM-6300 scanning electron microscope (SEM). Backscattered electron images and the image analysis software “Scion Image” (Scion Corporation) were used to determine the modal composition and the textural characteristics of the meteorite. Based on the digitized images, this software measures the pixel number of each phase to calculate the corresponding equivalent circle diameter (ECD). This diameter is used as grain size. The orientation of the longest dimension of each crystal was used to determine the degree of orientation of olivine and pyroxene. Quantitative mineral analyses were performed with a JEOL JXA-8800L electron microprobe operating at 15 kV and a current of 15 nA. Suitable mineral standards including anorthoclase, basaltic glass, chromite, chromium augite, diopside, ilmenite, microcline, and plagioclase, all certified by the United States National Museum as reference samples for electron microprobe analysis (Jarosewich et al., 1980) were applied to calculate the mineral compositions. X-ray elemental maps were acquired on the same electron microprobe at 15 kV accelerating voltage, 15 nA beam current, and a beam size of 1–2 ␮m. Following detailed petrographic studies, slotted Cu grids were glued on areas of interest of each thin section. The grids were then removed from the thin sections and thinned to perforation by argon ion-beam bombardment at 4.5 kV and an incidence angle of 12° using a GATAN 600 DIF duo ion mill. After the first holes appeared, the samples were thinned at a very shallow angle (3°) for several minutes to remove any amorphous films from the sample surface. Transmission electron microscope (TEM) studies were performed using a 200 kV PHILIPS CM 20 STEM equipped with a TRACOR Northern energy dispersive X-ray detector sensitive to elements with atomic numbers ⬎ 5.

3. RESULTS

3.1. Petrography and Mineralogy In thin sections Yamato 980459 shows a dominantly olivineporphyric texture with large (up to 1–2 mm-sized) euhedral to subhedral olivine crystals or clusters of crystals set into a more fine-grained groundmass of euhedral to anhedral prismatic pyroxenes and interstitial quenched residual melt (Fig. 1). Pyroxene and olivine often clump together to form glomerophyric textures (Fig. 2b, c). Olivine shows a bimodal grain size distribution with most grains ranging between 30 and 350 ␮m and fewer larger grains varying in grain-size from 600 to 1150 ␮m. Pyroxene grain size distribution is more gaussian with a mean diameter of 125 ␮m and a range of 44 –300 ␮m. Image analyses reveal that ⬃70% of the olivine megacrysts are glomerocrysts and ⬃30% occur as separate grains (Fig. 2b, c). One olivine megacryst was found showing corroded forms indicating partial resorption by the residual melt (Fig. 2a). Rarely pyroxenes were encountered as overgrowth on olivine. The large olivine crystals are magmatically zoned and show— where in contact with the quenched material—a characteristic Fe-rich rim. This overgrowth is discontinued at crystal faces where the olivine is in contact with chromite or pyroxene (Fig. 2a, b). The vast majority of matrix pyroxene in Yamato 980459 is present as composite crystals of either orthopyroxene-pigeonite-augite composition or—significantly less abundant—as smaller pigeonite-augite grains without orthopyroxene core (Fig. 2d); only two separate matrix augites were found in the sections studied. The chemical zoning of pyroxene is mostly irregular but continuous from a Mg-rich orthopyroxene core to an intermediate pigeonite layer and finally to an outer augite rim. Similar to olivine, the Ca-rich rim is discontinued where pyroxene is detached to a neighboring crystal (Fig. 2a, b). Some smaller olivines and pyroxenes show both perfectly developed and ragged crystal faces indicating that a rapid drop in temperature prevented crystallization to euhedral shapes. An impressive example for this is shown in Figure 3. Here a chemically zoned Fe-rich olivine (core Fa49.3; rim Fa61.7) is set into the quenched residual material containing tiny Fe-rich olivine nuclei. The grain displays both angular and ragged crystal faces with several tiny Fe-rich olivines nucleating on the longest angular face. At the largest ragged face, small olivines with grain sizes decreasing with increasing distance from the larger olivine seem to migrate to the large crystal. This feature may either be the result of resorbtion of the larger olivine in the residual material or document the ongoing crystallization of the Fe-rich rim regions of the olivine which was “frozen in” during rapid cooling. Since all olivines in the meteorite precipitating from the residual material are Fe-rich (up to Fa71.5), it seems rather unlikely that the melt dissolves the Fe-rich rim of the large olivine. We, thus, suggest that Figure 3 shows the crystallization of the outer regions of the large olivine which was stopped due to quenching. Image analysis also revealed a shape-preferred orientation of pyroxene and—less pronounced— of olivine which becomes apparent in a rose diagram (Fig. 1). Since Yamato 980459 contains textural evidence for a rapid eruption which would

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Fig. 1. Backscattered electron image of a Yamato 980459 thin section. Large olivine megacrysts and often prismatic pyroxenes are embedded into a fine-grained groundmass. Digital image analysis measuring the orientation of the longest dimension of 49 olivine and 741 pyroxene crystals shows a shape-preferred orientation of both phases as can be seen in the rose diagrams.

have been disruptive for a preferred orientation due to settling of cumulus crystals, the observed shape-preferred orientation is most probably the result of flow alignment. Olivine frequently contains chromite and melt inclusions as well as injections of residual melt into fractures (Fig. 2a-c). The melt inclusions are either composed of glassy material with sometimes tiny dendritic crystals or of chromite-pyroxeneglass assemblages and often display cracks radiating into the host olivine (Fig. 2a-c). Several melt inclusions show characteristic Fe-rich rims at the contact to their host crystal (Fig.

2a-c). Pyroxene also contains chromite and melt inclusions. Fe-rich rims at the contact inclusion-host are present but less pronounced compared to olivine. Minor phases in Yamato 980459 include matrix olivine, chromite, and sulfides. The small matrix olivines are commonly also chemically zoned and generally more Fe-rich than the megacrysts (Fig. 4d); only a few compositionally homogeneous olivines were found. Chromite is present as single, mostly euhedral matrix grains and as smaller inclusions in olivine and less frequently in pyroxene. Chromite grains range in size

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Fig. 2. Backscattered electron images of olivine and pyroxene morphologies in Yamato 980459. (a) Large, partially resorbed megacryst with melt inclusion and smaller more Fe-rich olivines. (b) Cluster of three olivine crystals with melt inclusions showing Fe-rich aureoles. Note that the Fe-rich rims of olivine and the Ca-rich rims of pyroxene are discontinued where both crystals are attached. (c) Incompletely compacted olivine glomerocryst. The crystals show a pronounced Fe-rich rim and contain numerous melt inclusions. (d) Pyroxene crystals displaying both euhedral and anhedral shapes. Note the irregular shaped orthopyroxene core of the grain on the right.

Fig. 3. Backscattered electron image of a zoned olivine which was quenched during growth. The crystallization process seems to be “frozen in.”

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Fig. 4. Backscattered electron images (a-e) and one brightfield TEM image (f) of the residual material in Yamato 980459. (a) Overview of an dendrite-rich area at enhanced contrast. (b) Close up view showing the tree-like texture of olivinedendrites and two sulfide droplets. (c) Image illustrating olivine-rich areas (upper right side) and pyroxene–rich areas (lower left side) in the residual material. (d) Larger Fe-rich olivine which was prevented from developing a totally euhedral shape and small baby swallowtail olivines. (e) Feathery olivine dendrites and chain-like pyroxenes in the residual material. (f) TEM image showing some larger and numerous tiny precipitates in the glass.

between 5 to 67 ␮m with a mean diameter of 15 ␮m. Sulfides dominantly occur in the quenched residual material forming small 5–30 ␮m-sized droplets, which crystallized due to liquid imiscibility of silicate- and sulfide-rich melts (Fig. 4b). Rarely tiny sulfide inclusions are present in olivine. Neither phosphates, ilmenite, or Ti-Mg-rich chromites, all commonly observed in basaltic shergottites, have been found in the sections studied. The modal composition of the rock was determined by image analysis (in vol.%): 15.7% olivine, 24.7% clinopyroxene, 27.9% orthopyroxene, 30.9% residual glassy material with dendrites, 0.5% chromite, 0.3% sulfide, and 0.1% melt inclusions in olivine and pyroxene (Table 1 and 2). On the basis of the mineral mode the meteorite is thus an ultramafic rock and must be classified as an olivine websterite.

In sharp contrast to all other Martian meteorites found so far, Yamato 980459 does not contain any plagioclase or maskelynite, respectively. Instead, the interstitial regions between olivine and pyroxene are filled with a quenched residual material consisting of dendritic feathery olivine, chain-like augite, and small sulfide droplets embedded in a structureless glassy groundmass (Fig. 4). Backscattered electron images show that in some areas olivine is the dominating crystalline phase, whereas in others pyroxene is more abundant (Fig. 4c). Dendrites and chains are often found to nucleate on olivine or pyroxene crystal faces or even on sulfide droplets. Glassy areas without any precipitates are present but rare. Image analysis of the interstitial areas shows that chain-like pyroxene is more predominant than olivine dendrites. The average modal composition of

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A. Greshake, J. Fritz and D. Sto¨ ffler Table 1. Modal composition of Yamato 980459 compared with other Martian meteorites (vol. %). Meteorite

Lithology Pyroxene (total) Clinopyroxene Orthopyroxene Olivine Plagioclase/Maskelynite glass with dendrites Opaques Chromite Ilmenite Sulfides Phosphates SiO2-polymorphs Impact glass Primary inclusions Alteration phases

Y 980459 52.6 24.7 27.9 15.7 — 30.9 0.8 0.5 0.3 — — — 0.1 —

Y 980459

SaU 094†

CIPW corrected* 63.4 31.8 31.6 19.4 12.6 — 1.1 0.5 0.3 0.3 0.5 2.7 — 0.1 —

Gabbro 58.2 22.1 13.7 — 1.1 1 tr 0.1 tr — 4.8 0.1 tr

DaG 476 and 489‡

EETA 79001A§

Basalt 55.3-64.6 47-65.7-? 1.5-4 10.4-24 12-17 — 0.9-3.9 0-0.6

Basalt 69-73 58.2-71.5 3.4-7.2 7-10 16-18 — 2.2-4.0

0-0.6 tr-1.5 — 4-7.2

tr-0.3

LEW 88516# Lherzolite ⬎20-44.5 39.2-57 ⬍10-16 — ⬍5 0.8-3 ⬍1 0.9-1

1-3.1

* Contains also 0.1% K-feldspar. Gnos et al. (2002). ‡ Zipfel et al. (2000); Folco et al. (2000); Wadhwa et al. (2001); Mikouchi et al. (2001). § McSween and Jarosewich (1983). # Gleason et al. (1997); Treiman et al. (1994); Lodders (1998). †

tites Dar al Gani 476/489 (Zipfel et al., 2000; Folco et al., 2000; Wadhwa et al., 2001; Mikouchi et al., 2001) and Sayh al Uhaymir 094 (Gnos et al., 2002) but significantly higher portions of orthopyroxene which are comparable to the lherzolitic shergottite LEW 88516 (Meyer, 2003).

the residual material is (in vol.%): 12.1% olivine, 22.1% augite, 65% glassy material, and 0.9% sulfide (Table 2). Using the mean composition of the glassy material without dendrites and chain pyroxenes as determined by defocused beam electron microprobe analyses, the phases to crystallize from this melt under dry conditions were calculated applying CIPW norm (Table 2). The normative vol.% of minerals in Yamato 980459 after complete crystallization would be: 19.4% olivine, 31.8% clinopyroxene, 31.6% orthopyroxene, 12.6% plagioclase, 2.7% SiO2-phases, 0.5% apatite, 0.5% chromite, 0.3% sulfide, 0.3% ilmenite, 0.1% K-feldspar and 0.1% melt inclusions in olivine and pyroxene (Tables 1 and 2). This corrected modal composition now allows the classification of Yamato 980459 as an orthopyroxene-rich melanocratic olivine basalt (Streckeisen, 1976). Compared to other Martian meteorites, Yamato 980459 contains similar amounts of olivine as the olivine-phyric shergot-

3.2. Mineral Composition Results of representative mineral and glass analyses are given in Tables 3 and 4. Compositions of pyroxene, olivine and chromite are compared to those of other Martian meteorites in Figure 5. Olivine—In Yamato 980459, three chemically distinct main types of olivine can be distinguished: large euhedral often glomerophyric olivine megacrysts which are chemically zoned from a Mg-rich core (Famin16) to a Fe-rich rim (Famax69.1), smaller more Fe-rich matrix olivines which are mostly also

Table 2. Modal composition of the bulk rock, the residual material, the CIPW corrected glass and bulk of Yamato 980459 (vol%). Y 980459

Bulk

Pyroxene (total) Clinopyroxene Orthopyroxene Olivine Plagioclase/Maskelynite Glass with dendrites Glass without dendrites Opaques Chromite Ilmenite Sulfides Phosphates SiO2-polymorphs K-feldspar Primary inclusions

52.6 24.7 27.9 15.7 — 30.9 — 0.8 0.5 0.3 — — — 0.1

Glass with dendrites

CIPW of glass without dendrites

Bulk CIPW corrected

22.1 22.1 — 12.1 — — 65 0.9 — — 0.9 — — — —

19.4 1.3 18.1 — 62.4 — — 1.8 0.1 1.3 0.4 2.6 13.4 0.4

63.4 31.8 31.6 19.4 12.6 — — 1.1 0.5 0.3 0.3 0.5 2.7 0.1 0.1

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Table 3. Representative analyses of silicates and chromite from Yamato 980459.

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO NiO CaO Na2O K2O Total Fa Fs Wo

Olivine core

Olivine close to rim

Olivine rim

Matrix olivine core

Matrix olivine rim

Unzoned olivine

39.5 b.d. b.d. 0.42 15.2 0.37 45.0 0.14 0.18 b.d. b.d. 100.81 16.0

37.6 b.d. b.d. b.d. 26.2 0.55 35.8 b.d. 0.29 0.04 b.d. 100.48 29.1

32.3 0.04 0.06 b.d. 53.1 0.89 13.3 b.d. 0.39 b.d. b.d. 100.08 69.1

37.7 b.d. 0.06 0.14 24.2 0.55 37.0 b.d. 0.32 b.d. b.d. 99.97 26.8

32.1 0.04 0.06 0.04 52.0 0.92 13.0 b.d. 0.53 b.d. b.d. 98.69 69.2

34.4 0.04 b.d. 0.09 45.2 n.d. 0.82 19.5 0.45 b.d. b.d. 100.5 56.5

Orthopyroxene

Pigeonite

Augite

Chromite

55.5 b.d. 0.28 0.39 11.5 0.42 29.8 b.d. 1.16 b.d. b.d. 99.05

52.8 0.14 1.26 0.59 17.6 0.64 21.8 b.d. 4.9 0.03 b.d. 99.76

45.1 1.73 8.5 0.07 19.1 0.52 8.5 0.05 15.4 0.21 b.d. 99.18

0.13 0.36 6.4 61.9 22.8 0.82 7.1 0.07 0.04 0.07 b.d. 99.69

17.4 2.3

28.1 10.0

35.5 36.4

Data in wt%. b.d.: below detection limit.

chemically zoned (Fa26.8 – 69.2) and only rarely compositionally homogeneous (Fa56.5), and olivine dendrites in the quenched residual material which are with Fa71.5 the most Fe-rich olivines in the meteorite. Considering the Fe-rich rim around the olivine megacrysts, which measures only between 1 to 10 ␮m as a result of late stage quenching, the primary magmatic zoning of the large olivines is from Fa16 to Fa29.1 only. In the zoned megacrysts and matrix olivines MnO is correlated with FeO and is zoned from 0.4 in the core to 0.9 wt.% in the rim. In the sections studied, only one olivine grain was found showing clearly corroded forms (Fig. 2a). Chemically, this grain is zoned from Fa25.3 to Fa60.4 and thus showing a much more Fe-rich core composition than the euhedral megacrysts. A microprobe traverse starting in an euhedral olivine megacryst and crossing the Fe-rich rim into the quenched residual material shows the depletion of the glassy material in

iron and manganese close to the Fe-rich olivine rim (Fig. 6). The FeO- and MnO-concentrations in the glass increase with increasing distance from the olivine indicating that no equilibrium between olivine and residual melt was achieved. Due to the Mg-rich cores, which are the most magnesian found so far in Martian meteorites, and the small Fe-rich dendrites, olivine in Yamato 980459 compositionally covers a much wider range than olivine in other shergottites (Fig. 5). Pyroxenes—The dominant large and often prismatic matrix pyroxenes show a strong mostly irregular but continuous chemical zoning from a Mg-rich orthopyroxene core (Fsmin17.4) to a pigeonite layer (Fs22.4 –37.6Wo5–18.2) and to an augitic rim (Fs22.7–37Womax36.5). The Ca-pyroxene layers vary in width from few to several tens of ␮m. The few small pigeonite-augite composite grains present have similar compositions as the Ca-rich layers around orthopyroxenes and the very rare isolated

Table 4. Representative analyses of olivine dendrites, chain-like pyroxenes and glassy groundmass in the residual material from Yamato 980459.

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO NiO CaO Na2O K2O P2O5 S Total Fa Fs Wo

Melt & olivine dendrites

Olivine dendrites

Melt at olivine dendrites

Melt & chain-like pyroxenes

Chain-like pyroxenes

Melt at chain-like pyroxenes

50.3 1.2 16.7 b.d. 17.5 0.39 1.14 b.d. 9.1 2.42 0.06 1.01 0.25 100.07

31.9 0.11 0.09 b.d. 54.7 1.02 12.2 b.d. 0.57 0.04 b.d. n.d. n.d. 100.63 71.5

53.9 1.3 17.1 0.16 11.0 0.33 0.74 b.d. 11.5 1.89 0.06 1.1 0.13 99.21

50.3 1.17 16.4 b.d. 17.9 0.38 1.05 b.d. 9.3 2.3 0.06 1.02 0.23 100.11

45.0 1.32 8.6 b.d. 23.2 0.72 8.4 0.03 11.8 0.18 b.d. n.d. n.d. 99.25

51.6 0.93 18.9 b.d. 16.1 0.32 0.43 0.06 8.4 2.11 0.06 1.33 0.16 100.4

Data in wt%. b.d.: below detection limit.

43.7 28.4

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Fig. 5. Composition of pyroxene, olivine, and chromite in Yamato 980459 compared to other Martian meteorites. All data in mol.%; see Table 1 for references.

matrix augites show chemical zoning with increasing Fe- and Al-contents towards their rim. X-ray elemental mapping and microprobe traverses across pyroxene crystals and extending into the neighboring glassy material illustrate the irregular zoning of the grains with respect to Fe, Mg, Ca, and Al (Figs. 7 and 8). It also is apparent that the orthopyroxene core regions have mostly irregular outlines and are characterized by increasing Fe-concentrations towards the augite-pigeonite contact. Some cores also show a symmetric zoning in calcium and aluminum (Fig. 7). Pigeonite and augite layers are also strongly zoned with increasing Fe-, Ca-, and Al- concentrations from the innermost part to the outer rim (Figs. 7 and 8). These rims are discontinued where the pyroxenes are in contact with olivine, indicating that the crystal clusters formed before augite and pigeonite precipitation. According to the molar Fe/(Fe ⫹ Mg) ratios of 0.26 for olivine and 0.25 for pyroxene at the aggregate interface, both silicates were almost in equilibrium when they clumped together. The increase in aluminum content in the Ca-rich pyroxene rims of Yamato 980459 to up to 8.5 wt.% Al2O3 is contrary to

all other Martian meteorites where the Al-concentration in pyroxene rims typically decreases. Since this decrease defines the entry of plagioclase, the increase in case of Yamato 980459 documents cooling of the melt too quickly to allow plagioclase to form. Suppression of plagioclase crystallization also caused increasing Ca-concentrations in the melt, leading to the formation of Ca-rich pyroxene overgrowth, few smaller pigeoniteaugite grains, and very rare separate augites (e.g., McSween et al., 1996). In the glassy material the Ca- and Al-contents decrease with increasing distance from the pyroxene, whereas Fe- and Mgconcentrations are variable without showing a clear compositional trend (Fig. 8). In terms of the molar Fe/(Fe⫹Mg)-ratios (fe-numbers), the onset of pigeonite at 0.24 (range 0.24 – 0.45) overlaps only marginally with the most Fe-rich orthopyroxene compositions at 0.25, while augite starting at 0.32 (range 0.32– 0.56) has significantly higher fe-numbers (Fig. 9). Pigeonite and augite compositionally overlap at fe ⫽ 0.32– 0.46, indicating that these two phases temporarily co-crystallized from the magma. Orthopyroxene starts at fe-number 0.18 (range 0.18 – 0.25)

Petrology and shock metamorphism of the olivine-phyric shergottite Yamato 980459

Fig. 6. Electron microprobe traverse across an olivine-glass interface. Note the almost identical behavior of Fe and Mn. See text for more details.

which is close to lowest value of 0.16 measured in a Mg-rich olivine core. This suggests that the most magnesian olivine is near but not exactly in equilibrium with the most magnesian orthopyroxene. Detailed SEM and TEM investigations of several pyroxene grains did not reveal any exsolution pattern or evidence for spinodal decomposition in pigeonite and augite. TEM investigations confirm that the Mg-rich pyroxene cores are predominantly of orthorhombic symmetry and very rarely contain regions of ortho-/clinoenstatite intergrowths. In contrast, the Carich pyroxene rims often consist of intimately intergrown augite and pigeonite (Fig. 13c). These intergrowths also cause a streaking in the SAED pattern (Fig. 13c inset). Pyroxene present as chain-like aggregates in the residual material is exclusively augite and with Fs⬃44Wo⬃28 (highest fe-number of 0.61) generally more Fe-rich but less Ca-rich than the augitic rims of matrix pyroxene; the high Al-content (ⱕ8.6 wt.% Al2O3) of the chains is similar to the augitic rims of matrix pyroxenes.

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Opaques—Chromite with TiO2-concentrations ⬍0.4 wt.% is the only spinel present in Yamato 980459 (Figs. 1, 2, and 5). The grains are generally homogeneous in composition and only one larger grain was found showing Al-enrichments towards the rim. Ti-Mg-rich members of the chromite-ulvo¨ spinel solid solution, which are typically found in other basaltic and also lherzolitic shergottites, are absent (Fig. 5). Sulfides are pyrrhotite with up to 2 wt.% Ni. Residual material—Defocussed beam microprobe analyses of the residual material covering glass, olivine dendrites and chain-like pyroxenes show almost identical compositions for areas with more olivine dendrites and for regions with more abundant pyroxenes indicating a primarily homogeneous bulk composition of the melt (Table 4). While this bulk composition is high in alkali elements and plots in the field of basaltic rocks, the glassy material alone is more silica-rich falling into the field of basaltic andesites (Fig. 10). TEM studies show that what appears to be entirely glassy at SEM scale often contains tiny 50 –200 nm sized rounded precipitates of olivine, pyroxene, and sulfide (Fig. 4f). The groundmass, however, is in fact glassy, as confirmed by the diffuse selected area diffraction pattern (SAED). Melt inclusions—Trapped melt inclusions frequently found in olivine and pyroxene can be mineralogically and compositionally divided into (1) orthopyroxene ⫹ glass inclusions, (2) vitrophyric clino-⫹ orthopyroxene ⫾ chromite ⫾ glass inclusions, (3) chromite ⫹ glass, and (4) glass ⫾ olivine dendrites ⫾ chain-like augites inclusions (Fig. 2a-c). Orthopyroxene in melt inclusions is characterized by low fe-numbers of about 0.19 which are almost identical with the most Mg-rich matrix orthopyroxenes (Table 5). Clinopyroxene is pigeonite, and chromite again has low Ti-concentrations. Glassy material present in trapped melt inclusions is highly variable in composition. While some glasses are compositionally similar to the glass present in the residual material, others are of more silicarich andesitic composition (Table 5). The latter and the vitrophyric inclusions are similar to magmatic inclusions found in other basaltic shergottites (e.g., Folco et al., 2000). Several melt inclusions show a Fe-rich aureole at the contact to the host crystal (Fig. 2b, c). 3.3. Bulk Chemistry The bulk chemistry of the rock was calculated by using the mineral mode combined with electron microprobe analyses of the individual phases. Considering the heterogeneity of the rock, the results of these calculations are in good agreement with the data obtained by wet chemical analysis (Misawa, 2003). Most importantly, both methods yield exactly the same molar Mg/(Fe ⫹ Mg) ratio for Yamato 980459. The data compared to whole-rock compositions of other olivine-phyric, basaltic, and lherzolitic shergottites, are given in Table 6 and illustrated in Figure 10. Diagnostic element ratios of Martian meteorites are, among others, Fe/Mn⫽40 and Na/Al ⫽ 0.22 (e.g., Wa¨ nke and Dreibus, 1988; Dreibus et al., 2000). The calculated element ratios for Yamato 980459 (Fe/Mn ⫽ 37 and Na/Al ⫽ 0.19) are well in the range of these characteristic values and comparable to other basaltic shergottites (Table 6). Also, the Ni/Mg ratio follows the trend defined by shergottites which is clearly dis-

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A. Greshake, J. Fritz and D. Sto¨ ffler

Fig. 7. X-ray elemental mapping of a pyroxene-rich area in Yamato 980459. (a) Back scattered electron image of the area. (b) Mg-map showing the pronounced and continuous zoning from an orthopyroxene core to an augite rim. (c) Al-map showing the symmetric zoning of the core regions of some pyroxenes and the strong Al-enrichment in the rims. (d) Ca-map illustrating the thin augite and pigeonite rims and also the internal zoning of some cores.

tinct from terrestrial and lunar rocks and from basaltic achondrites (Wa¨ nke and Dreibus, 1988). The whole-rock composition of Yamato 980459 shows strong similarities with the olivine-phyric shergottites Dar al Gani 476 and Sayh al Uhaymir 005 (Table 6). Although classified as a basaltic rock according to the Na2O⫹K2O/SiO2 ratio, its high molar Mg/(Mg⫹Fe) ratio of 0.67 indicates a strong affinity to lherzolitic shergottites. As Yamato 980459 shares this high mg-number with other olivine-phyric shergottites, this feature may be considered as a more general characteristic of this group of Martian meteorites. 3.4. Shock Metamorphism Yamato 980458 is an unbrecciated rock displaying solidstate shock effects in the silicate phases. In the thin sections studied here, no signs of shock related melting, i.e., melt veins or melt pockets, or of recrystallization were found. However, Mikouchi et al. (2003) report the occurrence of few heterogeneously distributed melt pockets in their sample. Olivine—At the scale of the optical microscope, olivine

megacrysts and smaller matrix olivines show multiple sets of crossing planar and irregular fractures as well as very strong undulatory extinction, as expressed by the maximum variation of the extinction angle of 14° corresponding to a pressure of ⬃25 GPa (Figs. 11a, 12a; Fritz et al., 2002, 2003). Several crystals display weak mosaicism (Fig. 12a). Some injections of residual melt in fractures are displaced, indicating that shearing played a role during shock metamorphism and clearly postdates the injection of the material (Fig. 12b). While olivine in the more strongly shock metamorphosed shergottite Dar al Gani 476 is fractured into many small blocks causing strong mosaicism (Greshake and Sto¨ ffler, 1999), our TEM investigations show that olivine in Yamato 980459 is characterized by a much lower fracture density (Fig. 12). The most abundant planar fracture orientations are (100), (010), and {130}, orientations which are diagnostic of shock damage (Mu¨ ller and Hornemann, 1969). At TEM scale these fractures generally appear as up to several ␮m-wide open cracks; only very few planar fractures with spacings ⱕ0.5 nm were found (Fig. 12d). Since the wide fractures are dominantly continuous,

Petrology and shock metamorphism of the olivine-phyric shergottite Yamato 980459

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Fig. 9. Weight % CaO versus the molar Fe/(Fe⫹Mg)-ratios for pyroxene in Yamato 980459. The onset of pigeonite crystallization overlaps only marginally with the formation of orthopyroxene while pigeonite and augite co-crystallized for longer time.

augite rims contain mostly straight twin lamella which are recognizable in high-resolution images and SAED pattern (Fig. 13d). Additionally, pyroxenes frequently display a low density of up to 1 ␮m long dislocations (Fig. 13e). Opaques—Chromite, which exhibits brittle behavior under shock metamorphism, shows few irregular fractures. Sulfides, which preferentially respond by partial melting, seem to be unaffected by shock. 4. DISCUSSION

4.1. Olivine: Phenocrysts or Xenocrysts? Fig. 8. Electron microprobe traverse across an olivine-glass interface. Note the strong Al-enrichments towards the rim. See text for more details.

clearly separating the lattice on both sides of the fractures, the small fractures tend to be discontinuous with variable lattice displacements (Fig. 12d). In olivine, shock metamorphism also induced numerous dislocations (Fig. 12c) whose density is, however, lower than in olivine from Dar al Gani 476 (Greshake and Sto¨ ffler, 1999) and of meteoritic olivine experimentally shocked to 35 GPa (Langenhorst et al., 1994; Schmitt, 2000). Pyroxenes—Pyroxenes are characterized by a high density of irregular fractures with commonly one set of fractures perpendicular to the prism (Fig. 13a). Pigeonite and augite show intense polysynthetic mechanical twinning subparallel (001). Several of the twin lamellae are slightly curved while most exhibit straight boundaries. Similar to olivine, the pyroxene crystals display strong undulatory extinction (mean ⫽ 13.3° corresponding to 24 GPa, Figs. 11b; Fritz et al., 2002, 2003) and weak mosaicism. Most pyroxenes in Yamato 980459 are intensively fractured into small blocks leading to strong asterism in the SAED pattern of those regions and probably causing the mosaicism observed in the optical microscope (Fig. 13b). Pigeonite and

There is considerable debate about the origin of large olivines in olivine-phyric shergottites, i.e., do they represent phenocryts or xenocrysts? While Zipfel et al. (2000) interpret olivine in Dar al Gani 476 as phenocrysts, Mikouchi et al. (2001) underline the apparent disequilibrium between olivine and melt in this meteorite and favor a xenocryst origin. Also

Fig. 10. Nomenclature diagram for several Martian meteorites. The residual glass found in Yamato 980459 is enriched in sodium, potassium, and silica compared to the bulk meteorite; 1this work, 2Misawa (2003).

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Table 5. Representative analyses of silicates and glass in melt inclusions of Yamato 980459.

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO NiO CaO Na2O K2O P2O5 S Total Fs Wo

Orthopxyroxene

Pigeonite

Glass

Andesitic glass

55.9 0.07 0.39 0.56 12.5 0.4 29.6 0.05 1.44 b.d. b.d. b.d. b.d. 100.91 18.9 2.8

53.9 0.18 2.0 0.78 14.5 0.52 22.7 b.d. 6.5 b.d. b.d. b.d. b.d. 101.08 23.0 13.2

55.7 1.17 15.1 0.04 10.0 0.28 1.67 b.d. 12.4 1.6 0.06 0.94 0.09 99.05

61.0 0.77 13.1 0.09 6.8 0.27 2.2 b.d. 14.5 1.08 0.04 0.68 0.07 100.60

Data in wt%. b.d.: below detection limit.

olivines in lithology A of EETA 79001 are interpreted as xenocrysts (e.g., McSween and Jarosewich, 1983). In Yamato 980459, euhedral olivine megacrysts have the most magnesian core compositions of all Martian olivines known so far (Meyer, 2003). Following calculations of Mikouchi et al. (2001) for Dar al Gani 476, a melt of about the bulk composition of Yamato 980459 would be in equilibrium with olivine of Fo⬃86. This is only slightly more magnesian than the observed olivine core composition of Fa84. It, thus, seems likely that olivine cores in Yamato 980459 are phenocrysts representing the first silicate precipitating from the melt. As described in the previous sections, one olivine megacryst was found displaying corroded forms and a Fe-rich core com-

Fig. 11. The degree of extinction in (a) olivine and (b) pyroxene in several Martian meteorites versus the shock pressure determined by refractive index measurements of maskelynite. For Yamato 980459 where no maskelynite is present a shock pressure of 20 –25 GPa is obtained from the degree of extinction.

Table 6. Bulk chemistry of Yamato 980459 and of some olivine-phyric, basaltic and lherzolitic shergottites. References

Y 9804591

Y 9804592

SaU 0053

DaG 4764

Zagami5

LEW 885166

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO NiO CaO Na2O K2O P2O5 S Total Mg/(Fe⫹Mg) molar Fe/Mn Na/Al

49.4 0.48 6.0 0.71 15.8 0.43 18.1 0.03 7.2 0.8 0.02 0.31 0.07 99.35 0.67 37 0.19

48.7 0.54 5.27 0.71 17.32 0.52 19.64 0.03 6.37 0.48 ⬍0.02 0.29 — 99.89 0.67 33.4 0.13

47.2 0.42 4.53 0.79 18.07 0.46 20.49 0.04 5.46 0.6 0.02 0.31 0.16 98.55 0.67 40 0.19

48.91 0.42 4.67 0.83 17.17 0.48 20.75 0.04 5.84 0.55 0.04 0.34 — 100.04 0.68 35.9 0.17

50.5 0.79 6.05 0.33 18.1 0.5 11.3 0.01 10.5 1.23 0.14 0.5 0.19 100.14 0.53 36.3 0.28

46.0 0.39 3.31 0.86 19.0 0.49 25.0 0.04 4.2 0.56 0.03 0.39 0.1 100.28 0.7 38.9 0.24

Data in wt%. This work; data calculated using mineral mode and electron microprobe analyses. 2 Misawa (2003). 3 Dreibus et al. (2000); mean was used when INAA and XRF data were given. 4 Zipfel et al. (2000); CaCO3 and S-free renormalized data. 5 Lodders (1998). 1

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Fig. 12. Shock effects in olivine from Yamato 980459. (a) Optical microscope image showing different sets of irregular and planar fractures. (b) Backscattered electron image showing residual melt injected into a fracture in olivine and displaced by shearing during the impact event. (c) Darkfield TEM image showing numerous dislocations in olivine. (d) Highresolution TEM image of a discontinuous fracture. The displacement is up to 0.5 nm corresponding to about half a lattice plane.

position (Fa25.3), which was clearly not in equilibrium with a melt of Yamato’s bulk composition. It may be suggested that this grain represents a xenocryst which crystallized from a more Fe-rich magma and was mixed into Yamato’s parent magma. Since also this crystal shows the typical Fe-rich rim formed during ascent, it is likely that partial resorption took place in the magma chamber. Similar calculations for pyroxene at the same melt bulk composition predict an equilibrated atomic Mg/(Mg⫹Fe) ratio of ⬃0.88, a value which differs significantly from the most magnesian pyroxene composition of 0.82 observed in Yamato 980459. One possible explanation for this observation is that during initial olivine crystallization the melt became more and more Fe-rich leading to lower Mg/(Mg⫹Fe) ratios when orthopyroxene started to crystallize. Consequently, orthopyroxene co-precipitated with a more Fe-rich olivine and not with the Mg-rich cores. This view is also supported by the fact that olivine and pyroxene forming crystal clusters have very similar Mg/(Mg⫹Fe) ratios at the contact, indicating that both were almost in equilibrium when the clusters formed. 4.2. Cooling History from Olivine Morphologies Detailed investigations of olivine morphologies in crystallization experiments of various basaltic rock compositions allow to constrain the cooling history of Yamato 980459. According to the terminology used in these studies (Donaldson, 1976; Faure et al., 2003), olivine megacrysts and matrix olivines in

Yamato 980459 are either polyhedral with well-developed faces (euhedral) or more rarely anhedral (granular olivine) in shape. Dynamic crystallization experiments performed with starting materials of highly variable compositions and different oxygen fugacities at dry (1 atm) as well as at water-saturated and waterundersaturated conditions showed that, as a function of both cooling rate and degree of undercooling (⌬T), there is a consistent pattern of olivine shape variation (Donaldson, 1976). These morphologies are essentially independent from the chemical composition and the oxygen fugacity but may change when the starting material is water-saturated. In general, the presence of water disfavors the growths of angular grains resulting in more granular and rounded crystals (Donaldson, 1976). The dominance of angular, polyhedral olivines in Yamato 980459 may thus also indicate a low water content of the magma. According to the experiments, the growths of polyhedral olivine crystals is diagnostic for cooling rates of ⬃0.5°C/h (Donaldson, 1976). Crystallization experiments with Fe-free starting material suggest an upper limit for the formation of solely polyhedral olivine crystals at 2°C/h and ⫺⌬T ⫽ 116°C (Faure et al., 2003). The second type of olivine found in the meteorite is present in the residual glassy material and shows dendritic morphologies, i.e., baby swallowtails and feathery crystals which are indicative for rapid growth (Fig. 4; Faure et al., 2003). Using an Apollo 12 basalt as starting material, dendritic feathery olivine formed at cooling rates of 1450°C/h (Donaldson, 1976). At a cooling rate of 1890°C/h, the evolution of forsterite dendrite morphologies is strongly influenced by the degree of under-

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A. Greshake, J. Fritz and D. Sto¨ ffler

Fig. 13. Microstructural characteristics of pyroxene from Yamato 980459. (a) Backscattered electron image of a prismatic pyroxene with irregular and planar fractures dominantly perpendicular to the prism. (b) Brightfield TEM image of a fractured area with the corresponding selected area diffraction pattern (SAED; inset) showing strong asterism. (c) High-resolution TEM image of pigeonite/augite-intergrowths. Spacings are: d100pigeonite: ⬃0.92 nm and d200augite: ⬃0.46 nm. (d) High-resolution TEM image of a thin twin lamellae in clinopyroxene. (e) Darkfield TEM image showing dislocations in pyroxene.

cooling: baby swallowtails develop at ⫺⌬T ⫽ 68°C, swallowtails at ⫺⌬T ⫽ 122°C, and dendritic fibres at ⫺⌬T ⫽ 258°C (Faure et al., 2003). However, it should be pointed out, that at such high cooling rates the degree of undercooling is strongly affected by the water content and the values given above may only be valid for water-free systems (Donaldson, 1976). From the olivine morphologies found in Yamato 980459, it is thus likely that the polyhedral olivines grew at low waterpressure under a steady-state regime, e.g., in a magma chamber, at cooling rates of 0.5 to maximal 2°C/h. During continuing crystallization, slightly increasing cooling rates prevented chemical equilibration between melt and crystals and a low degree of undercooling made crystallization onto the forsterite cores favorable. Due to the low nucleation rates separate Fe-rich olivines are formed less often.

Frequently found small olivines and pyroxenes with both well-developed and ragged crystal faces document a further increase of cooling rates, inhibiting the growth of large crystals and preventing the formation of euhedral shapes. While rapidly ascending towards the Martian surface, the residual melt was finally quenched at least at 1450°C/h allowing the crystallization of feathery olivine dendrites. Assuming a water-free magma, the presence of baby swallowtail olivine may point to even higher cooling rates of 1890°C at a degree of undercooling of at least ⫺⌬T ⫽ 68°C. 4.3. Crystal Size Distribution of Pyroxenes and Olivine The theory of crystal size distributions (CSD) has been adapted for geoscience applications to study the kinetics and dynamics of crystallization of rocks by Marsh (1988, 1998).

Petrology and shock metamorphism of the olivine-phyric shergottite Yamato 980459

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Fig. 14. Pyroxene and olivine in Yamato 980459. (a, c, e) Histograms of number of grains versus width. Superimposed are the corresponding cumulative histograms. The scale given for the y-axis is valid for the bars only. (b, d, f) Crystal size distribution (CSD) plots based on data in (a, c, e), of ln(n) versus width, where n is dNV*/dL and NV* is the cumulative number of crystals per volume (NV ⫽ [NA]1.5).

Based on the idea that continuous nucleation and growth of crystals in a steady-state system produce a gaussian grain size distribution, a cumulative histogram of the number of grains (N) vs. size (L) is produced. By calculating the change of the slope dN/dL (size), the so-called number density n (number of crystals in a given size class per unit volume; Cashman and Marsh, 1988) is determined. Plotting ln(n) again vs. the size, a linear relationship is found for all continuous nucleation and growth scenarios (Marsh, 1988, 1998; Lentz and McSween, 2000). While frequently used to study the crystallization conditions in terrestrial lava flows, crystal size distribution (CSD) analysis has recently been applied for the first time also to Martian meteorites (Lentz and McSween, 2000). Based on pyroxene grain size distributions the results allowed them to establish a single-stage crystallization history for the groundmass of the

olivine-phyric shergottites Dar al Gani 476 and EETA 79001 lithology A and for the basaltic shergottites QUE 94201 and EETA 79001 lithology B. In contrast, a pronounced kinking in the CSD plots of Zagami and Shergotty suggests a two-stage crystallization history for pyroxenes in these two meteorites (Lentz and McSween, 2000). The histograms, cumulative histograms, and corresponding crystal size distribution plots for pyroxene, pyroxene cores, and olivine in Yamato 980469 are shown in Figure 14. More detailed information is given in Table 7. Similar to Lentz and McSween (2000), we defined the width of the grains as grain size to avoid a geometric bias. Since the size distributions were measured in number per unit areas, a conversion had to be made to number per unit volume by raising the number measured per unit area to the 3/2 power (Cashman and Marsh, 1988).

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A. Greshake, J. Fritz and D. Sto¨ ffler Table 7. Crystal size distribution information.

Mineral

Avg width (mm)

St dev (mm)

Pyroxene Pyroxene cores Olivine

0.074 0.07 0.11

0.043 0.036 0.128

Dom width (mm) 0.034 0.028 0.039§ 0.334†

Number of grains

Area mm2

741 536 88 77 11

15.44 15.44 24.27

Slope*

Intcpt*

R2*

⫺29.78 ⫺35.31

8.91 8.97

0.954 0.977

⫺25.72 ⫺2.99

5.57 0.11

0.996 0.868

Bin interval (mm) 0.03-0.33 0.03-0.27 0-0.99 0-0.18 0.18-0.99

* Based on weighted least-squares regression for designated bin interval only. Using the upper three data points. Using the lower four data points.

§ †

The shape of the CSD plot of the Mg-rich pyroxene cores in Yamato 980459 is generally linear overall except the smallest grain sizes, implying that orthopyroxene dominantly crystallized under steady-state conditions of continuous nucleation and growths (Fig. 14c, d). The pronounced turnover at the smallest grain sizes suggests a period of crystallization where nucleation is discontinued but the growth of previously formed pyroxene crystals continued (Marsh, 1988; Lentz and McSween, 2000). The CSD plot obtained for the pyroxenes (cores and Ca-rich rims) shows almost the same shape as the plot for the cores just shifted to larger grain sizes (Fig. 14a, b). The plot indicates very similar crystallization conditions for both cores and rims characterized by continuous nucleation and growths and a period of discontinued nucleation. However, it must be pointed out that pigeonite and augite form— except few separate grains— only very thin layers around the Mg-rich cores neither changing the width nor the shape of the crystals significantly. Consequently, the shape of the pyroxene CSD plot is clearly dominated by the orthopyroxene cores. In neither of the two pyroxene CSD plots a significant kinking indicative for a two-stage cooling history is recognizable. The following scenario seems plausible for crystallization of pyroxenes in Yamato 980459: (1) formation of Mg-rich pyroxene cores at depth under steady-state conditions as indicated by the linear part of the core’s CSD plot, (2) during ascent nucleation of orthopyroxene almost stopped and the previously formed pyroxene cores were overgrown by thin layers of pigeonite and augite as reflected by the turnover at small grain sizes in the CSD plots; only few separate pigeonite-augite grains formed during this stage. It should be noted that due to quenching in the very late stage of crystallization, chain like augites were the last type of pyroxenes that formed in this rock. The CSD plot of olivine shows two regimes: first, a linear form with a negative slope of the three smallest grain sizes (encompassing 92% of all crystals) indicating crystallization under steady-state conditions of continuous nucleation and growths and second, a turnoff followed by a more horizontal part at larger grains sizes indicating an overabundance of large grains that may result from entrainment of cumulate crystals previously grown from the same magma (e.g., Marsh, 1988; Goodrich, 2003). The chemical composition of the olivines makes incorporation of major amounts of phenocryst unlikely (see previous sections). Compared to CSD plots of pyroxene from other olivine-

phyric shergottites, Yamato 980459 pyroxene is different by showing a pronounced turnover at the smallest grain sizes indicating a distinct crystallization history (Lentz and McSween, 2000). Olivine in Yamato 980459 shows slight similarities with olivine in SaU 005 where also continuous growth and an overabundance of larger crystals is observed (Goodrich, 2003). 4.4. Pyroxene Zoning Olivine-phyric shergottites generally contain separate grains of low- and high-Ca pyroxenes. Both are often zoned from magnesian cores to more calcic-ferroan rims (e.g., Goodrich, 2002; Meyer, 2003). In contrast, pyroxenes in Yamato 980459 are dominantly present as composite grains irregularly but continuously zoned from orthopyroxene to augite or much less frequently from pigeonite to augite. Additionally, TEM investigations revealed that pyroxenes in Yamato do not show any exsolved. augite and/or pigeonite lamellae as observed in other shergottites (e.g., Brearley, 1991; Mu¨ ller, 1993; Mikouchi et al., 1999). In the samples studied by TEM, not even evidence for the very initial stage of spinodal decomposition was found. From these observations the following crystallization history is deduced for the pyroxenes in Yamato 980459: As the first type of pyroxene, Mg-rich orthopyroxene crystallized from the melt under nearly equilibrium steady-state conditions. Based on its high modal abundance, orthopyroxene continued to grow undisturbed for a relatively long period of time and its crystallization only marginally overlapped the onset of pigeonite. The irregular shape and symmetric Ca-Alzoning pattern of several orthopyroxene cores, however, indicate that conditions changed to fractional crystallization and disequilibrium conditions at this early stage. During precipitation of Mg-rich silicates the melt evolved to a more Fe-, Ca-, and Al-rich composition, first causing increasing Fe-concentrations in orthopyroxene and finally the crystallization of pigeonite. While increasing cooling rates did not allow chemical equilibration of crystals and melt, often leading to irregular compositional zoning patterns, a low degree of undercooling hindered nucleation of separate pigeonite grains. Instead, the Ca-rich pyroxenes grew preferentially onto the preexisting orthopyroxene cores and only very few isolated pigeonite grains could form. As indicated by the relatively small amount of pigeonite, its stability field was quickly transgressed and augite immediately started to crystallize when the melt reached the augitepigeonite cotectic phase boundary. Again, the cooling rates

Petrology and shock metamorphism of the olivine-phyric shergottite Yamato 980459

were high enough to maintain chemical zoning but the degree of undercooling was too low to support formation of separate augite crystals. As a consequence of these low nucleation and high growth rates, augite dominantly developed thin rims around the Ca-poor pyroxenes. Overlapping compositions confirm temporary co-precipitation of augite and pigeonite, moving the melt composition along the cotectic phase boundary towards the augite-pigeonite-plagioclase eutectic. Before the melt reached the eutectic point, the rock was rapidly quenched so that plagioclase crystallization was suppressed. The high Caand Al-concentrations present in the augitic rims and in the chain-like augites of the quenched residual material are the result of this suppression. The extremely high cooling rates also prevented any unmixing process in pigeonite and augite as indicated by the lack of exsolution features. 4.5. Residual Material The interstitial material in Yamato 980459, consisting of dendritic olivine, chain-like augites, and sulfide droplets embedded into a glassy groundmass, is considered to represent the residual melt at the time of rapid cooling. Evidence for this conclusion is deduced from the mineralogy and chemical composition of the material: During crystallization of the large olivines and pyroxenes, the composition of the primary magma of Yamato 980458 became continuously enriched in Fe, Ca, and Al. Microprobe transverses across olivine- and pyroxene-melt interfaces indicate that quick cooling caused strong disequilibrium between glass and the outer regions of these crystalline phases. This compositional evolution of the melt is well documented in the very Fe-rich olivine dendrites and the Al-rich chain-like augites which are the last silicates precipitating from the residual magma during quenching. Due to immiscibility of sulfide- and silicate-rich melts, the entire sulfur content of the magma was deposited into small droplets of pyrrhotite immersed in the silicate glass. As several olivine dendrites and especially pyroxene chains nucleated on the droplets’ surfaces, their formation must predate silicate crystallization. The remaining glassy groundmass is high in silica, alkali elements, and phosphorus, and plott in the field of basaltic andesites. As indicated by the high phosphorus concentration, rapid cooling also suppressed the formation of phosphates which are typical late-stage crystallizates in other Martian meteorites (McSween et al., 1996). Alternatively, the P content may not have been high enough to cause phosphate saturation. 4.6. Shock Metamorphism By calibrating the degree of extinction of olivine and pyroxene with respect to the shock pressure determined by refractive index measurements of maskelynite in differently shocked Martian meteorites, an equilibration peak shock pressure of 20 –25 GPa is obtained for the maximum variation of the extinction angles in Yamato 980459. Due to the lack of plagioclase/maskelynite, more precise data cannot be obtained. Compared to the other Martian meteorites, Yamato 980459 is the least shock metamorphosed shergottite and is most similar to the olivine-phyric shergottite Dhofar 019 (30 ⫾ 7.9 GPa) and the dunite Chassigny (26.6 ⫾ 0.5 GPa). Considering the

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different ejection ages of Yamato 980459 (2.1–2.8 Ma, Nagao and Okazaki, 2003), Dhofar 019 (19.8 ⫾ 2.3 Ma; Nyquist et al., 2001), and Chassigny (11.3 ⫾ 0.6 Ma; Nyquist et al., 2001), it seems likely that Yamato was removed from the Martian surface by a distinct ejection event. The degree of shock metamorphism in Yamato 980459 is well within the range of shock pressures experienced by Martian meteorites which spans from ⬃5 GPa in two nakhlites (Lafayette and Yamato 000593) to 45 GPa in some basaltic and lherzolitic shergottites (Los Angeles, LEW 88516, and ALH 77005). This range is in good agreement with the boundary conditions (9 – 45 GPa) obtained from three-dimensional numerical simulations of oblique impacts on Mars, indicating that it seems impossible to transfer “unshocked” material from Mars to Earth (Artemieva and Ivanov, 2002). Crater statistics on Mars also suggest that source craters for near surface Martian rocks should be smaller than 3 km (Nyquist et al., 2001; Artemieva and Ivanov, 2002). 5. SUMMARY AND CONCLUSIONS

The results presented above allow us to constrain the crystallization and ejection history of Yamato 980459. According to the cooling rates of the large olivine phenocrysts, the primary geological setting is a relatively Mg-rich basaltic magma located in a shallow intrusion below the Martian surface: 1. Upon the onset of cooling, chromite, forsteritic olivine and only shortly later Mg-rich orthopyroxene were the first cumulus phases to crystallize. The high modal abundance of enstatite, the crystal size distribution of the pyroxene cores, and the presence of olivine and pyroxene crystal clusters suggest that crystallization initially took place at steadystate conditions of continuous nucleation and growth. Symmetric Ca-Al-zoning pattern of several orthopyroxene core regions indicate that fractional crystallization quickly became dominant over equilibrium crystallization. 2. While continuously evolving to a more iron-, calcium-, and aluminum-rich composition the magma migrated upwards to the planets’ surface. The cooling rates increased at a relatively low degree of undercooling and subsequent crystallization took place under strongly disequilibrium conditions. In Yamato 980459, the chemical evolution of the melt is recorded in the Fe-rich olivine rims and the Fe-, Ca-, and Al-rich pyroxene overgrowths. The high cooling rates did not allow equilibration between crystals and melt so that the compositional zoning was maintained. The low degree of undercooling inhibited the formation of abundant separate pigeonite and augite grains and led to Ca-rich overgrowths onto preexisting orthopyroxene cores. 3. In the final stage of magmatic evolution, Yamato 980459 was rapidly quenched and solidified as the only Martian meteorite without plagioclase/maskelynite known so far. A possible mechanism for such a rapid drop of temperature consistent with the observed flow alignment could be the quick eruption of a thin lava flow with contact to cold neighboring rocks. Petrologically, the quenching suppressed formation of plagioclase and phosphates, leaving an interstitial residual melt of dendritic olivines, chain-like pyroxenes, and sulfide droplets in

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a glassy groundmass. The high abundance of tiny crystallites reflects a high degree of undercooling at very high cooling rates favoring nucleation instead of crystal growth. Chemically, the silicates are again Fe, Ca-, and Al-rich rich and the glassy material has high concentrations of silica and phosphorus representing the residual of the Yamato 980459 parent magma. 4. Abrupt cooling in the last stage of its magmatic history strongly suggests that Yamato 980459 was finally located on the Martian surface. According to crater statistics the source craters for such near surface Martian rocks should be smaller than 3 km (Artemieva and Ivanov, 2002). The comparable low peak shock pressure of 20 –25 GPa experienced by Yamato is in excellent agreement with this scenario. In contrast to more deeply seated rocks, i.e., lherzolites and dunites, Yamato’s surface position allowed ejection in a single impact event at relatively low shock pressures. In summary, Yamato 980459 represents a Martian meteorite derived from a very Mg-rich primitive magma and showing clear evidence for a two-stage cooling history. Slow cooling followed by rapid is the only plausible sequence to explain the observed textural characteristics and most likely occurred when crystallization began in a magma chamber, followed by the opening of a conduit and migration of the magma to the surface. The quenching textures, the lack of vesicles and the flow alignment of olivine and pyroxene are convincing evidence that the magma finally erupted as a thin flow and was transported directly onto the Martian regolith from where the rock was ejected in a single impact event. Acknowledgments—We are grateful to H.-R. Kno¨ fler for sample preparation and H. Nier for photographic work. We thank the National Institute for Polar Research, Tokyo, for generously supplying the sample of Yamato 980459. Constructive reviews by Hap McSween, Nabil Boctor and the associate editor Christian Koeberl significantly improved the quality of the paper and are very much appreciated. This research was partly supported by the Deutsche Forschungsgemeinschaft (GR 1658/4-3). Associate editor: C. Koeberl REFERENCES Artemieva N. A. and Ivanov B. A. (2002) Ejection of Martian meteorites— can they fly? Lunar Planet. Sci. XXXIII, Lunar Planet. Inst., Houston. #1113 (abstr.). Becker R. H. and Pepin R. O. (1984) The case for the Martian origin of the shergottites: Nitrogen and noble gases in EETA 79001. Earth Planet. Sci. Lett. 69, 225–242. Becker R. H. and Pepin R. O. (1986) Nitrogen and light noble gases in Shergotty. Geochim. Cosmochim. Acta 50, 993–1000. Bogard D. D. and Johnson P. (1983) Martian gases in an Antarctic meteorite. Science 221, 651– 654. Bogard D. D., Nyquist L. E., and Johnson P. (1984) Noble gas contents of shergottites and implications for the Martian origin of SNC meteorites. Geochim. Cosmochim. Acta 48, 1723–1739. Brearley A. J. (1991) Subsolidus microstructure and cooling history of pyroxenes in the Zagami shergottite. Lunar Planet. Sci. XXII, Lunar Planet. Inst. Houston. 135–135(abstr.). Cashman K. V. and Marsh B. D. (1988) Crystal size distribution (CSD) in rocks and the kinetics and dynamics of crystallization. II: Makaopuhi lava lake. Contrib. Mineral. Petrol. 99, 292–305. Donaldson C. H. (1976) An experimental investigation of olivine morphology. Contrib. Mineral. Petrol. 57, 187–213.

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