Petrogenesis of shergottite meteorites inferred from minor and trace element microdistributions

Geochimica et Cosmochimica Acta,Vol. 58,No. 19,pp. 4213-4229,1994 Copyright0 1994ElsevierScience Ltd Printedin the USA. All rightsresewed

Pergamon

0016-7037/94 $6.00+ .OO

0016-7037(94)00202-9

Petrogenesis of shergottite meteorites inferred from minor and trace element microdistributions MEENAKSHI WADHWA,’

HARRY Y. MCSWEEN JR.,*

and GHISLAINE CROZAZ ’

‘McDonnell Center for the Space Sciences and Department

*Department

of Earth and Planetary Sciences, Washington University, St. Louis, MO 63 130, USA of Geological Sciences, University of Tennessee, Knoxville, TN 37996, USA

(Received December 22, 1993; accepted in revisedform April 13, 1994)

an extension of our previous work on the lherzolitic shergottites (ALHA and LEW88516), ion microprobe measurements of REEs and other selected trace and minor elements were made in individual minerals of the basaltic shergottites (Shergotty, Zagami, and EETA7900 1). Whole rock REE abundances in these achondrites are dominated by whitlockite, which is usually present only in trace amounts. One of the most significant features is the extensive zoning of trace and minor elements in pyroxenes. This zonation, which provides information on the crystallization histories of these meteorites, is of primary magmatic origin, and not the result of secondary processes such as diffusive reequilibration and/or metasomatic infiltration. Coherent trends in element-element plots for low-Ca pyroxenes indicate that each shergottite formed essentially by progressive (closed-system) fractional crystallization. The calculated REE patterns of the parent melts of all shergottites are LREE-depleted and parallel to their whole rock REE patterns (also indicative of closed-system fractional crystallization). On the basis of observations reported in this study, as well as previously determined petrologic and geochemical (including isotopic) characteristics, we present a model that suggests possible relationships among all five known shergottites.

Abstract-As

Trace elements, and in particular the rare earth elements (REEs), are among the most sensitive indicators of petrogenetic processes, and their distributions among the various mineral phases are very useful in geochemically modelling magma generation and evolution processes that resulted in the formation of various rock types, whether they are terrestrial or meteoritic. In previous work on shergottites, we have made detailed studies of the two lherzolites, ALHA and LEW88516 (LUNDBERG et al., 1990; HARVEY et al., 1993)) in which we were able to constrain the crystallization histories and the REE compositions of the parent melts for these meteorites. These studies also showed that pyroxene zoning in these rocks provides information about their petrogenesis. Here, we extend this work to the basaltic shergottites, Shergotty, Zagami, and EETA79001 (lithologies A and B, including the xenocrysts in lithology A). Although LUND BERGet al. ( 1988) characterized REEs in most of the minerals present in Shergotty, the emphasis of their study was on constraining the chronology of that meteorite; they made only three analyses of REEs in pyroxenes (of the core and rim of a pigeonite, and of the core of an augite). In the present work, the main emphasis is on characterizing the extent of zoning, not only of the REEs, but also of other trace and minor elements such as Y, Zr, SC, Cr, and Ti, in pyroxenes, in order to constrain the geochemical evolution of the magmas from which these rocks formed, and also to determine how these meteorites may be petrogenetically related. To this end, we have made a total of eighty-three pyroxene analyses in Shergotty, Zagami and EETA7900 1. In addition, we present new REE data for other minerals in all three basaltic shergottites, in order to characterize the REE microdistri-

INTRODUCTION SNC (shergottites, nakhlites, and chassignite) meteorites have been of great interest to meteoriticists, particularly for the last decade, as their identification as martian samples became widely accepted ( MCSWEEN, 1985; references therein). Of particular importance is the fact that their unique petrologic and geochemical characteristics provide valuable information about magmatic evolution on a large planetary body other than Earth. Of the ten presently known SNCs, five (Shergotty, Zagami, EETA79001, ALHA77005, and LEW88516) are classified as shergottites; ALH8400 1, an orthopyroxenite which was recently identified as a SNC meteorite by MITTLEFEHLDT ( 1994), will not be discussed here. Unlike the three nakhlites, the five shergottites are quite diverse in composition. The two “classic” shergottites, Shergotty and Zagami, are basaltic; EETA79001 is also basaltic, but it is unique, since it is the only known achondrite with two texturally distinct lithologies (one of which contains large xenocrysts of olivine and orthopyroxene) separated by an igneous, nonbrecciated contact; the last two shergottites, ALHA and LEW88516, are lherzolites. In addition, isotopic characteristics of each of these shergottites ( SHIH et al., 1982; WOODEN et al., 1982; NYQUIST et al., 1984) are distinct. Despite these differences, the five meteorites share some striking similarities, such as severe shock effects represented by maskelynitization of their plagioclase, relatively young crystallization ages, and LREE-depleted whole rock patterns. The last two characteristics, in particular, suggest that the petrogenesis of these meteorites may be similar. 4213

4214

M. Wadhwa, H. Y. McSween Jr., and G. Crozaz ( 1984), and JAGOUU ( 1989). while those for the most recently discovered shergottite, LEW885 16, were reported by DELANEY ( 1992) and HARVEY et al. ( 1993). Petrographic observations made by us, as well as data previously published by others, are briefly summarized below. Basaltic shergottites are comprised mostly of pyroxene (pigeonite and a&e) and maskelynite (i.e., glass of plagioclase composition). These three samples are distinctly different from the Iherzolitic shergottites, ALHA and LEW885 16, which are comprised of almost 50% modal olivine along with the clinopyroxene and maskelynite (see HARVEY et al., 1993, for a comparative petro~phic description of these two meteorites). The modai mineraiog~es of shergottites are summarized in Table 1. STOLPER and MCSWEEN ( 1979) described Shergotty and Zagami as being almost identical petrologically, except for their grain sizes (on average -0.46 mm for Shergotty, as opposed to 0.24 mm for Zagami). However, recently, MCCOY et al. ( 1992) examined thin sections from two large pieces (19.5 and 352 g, respectively) of Zagami and found grain sizes ranging from 0.19 mm to 0.36 mm. There is no phenocrystic olivine in Shergotty and Zagami, although there is usually some minor, late-stage fayalite, either associated with magnetite-ilmenite grains or in the mesostasis ( SMMITN and HERVIG, 19’79; STOLPER and ~CSWEEN, 1979). An exception is an unusual cm-sized mineral assemblage in Zagami that was recently identified by VKTISEN et al. ( 1992) and referred to as “Zagami DN.” it consists of -40 ~01% of a fayalite-rich intergrowth, along with - 14 ~01% clinopyroxene. IO-20 ~01% maskelynite, - 11 ~01% phosphates, -8 volY0 opaques, and -8 ~01% mesostasis and is believed to represent the solidified products of a late-stage melt pocket (MCCOY et al., 1993). Ion microprobe data have confirmed this interpretation ( WADHWAet al., 1993). EETA79001 is the only known achondrite with two texturally distinct lithologies (designated EETA79OOlA and EETA79~lB by SCORE et al., 198 I ) joined by a planar, nonbr~ciated contact, Both iitholo~~ are basaltic, consisting mostly of ctinopyroxene and maskelynite (much like Shergotty and Zagami); however, lithology A contains xenocrysts of olivine. orthopyroxene, and chromite, and its groundmass has an average grain-size ( -0.15 mm) that is finer-grained than that of lithology B (-0.28-0.37 mm) ( MCSWEEN and JAROSEWICH, 1983). Orthopyroxene xenocrysts are often

butions and to compare the abundances of REEs in particular phases in these meteorites. This information is also used to constrain the crystallization histories of these meteorites and the possible relationships between them. Finally, taking into consideration the data presented here as well as previously determined petrologic and geochemical (including isotopic) characteristics of the shergottites, we present a model for possible petrogenetic links between them. EXPERIMENTAL

METHODS

The following thin sections were used in this study: Shergotty (USNM321-2}, Zagami (USNM6471-I, UNM992), ~ETA7~lA (90,317), and EETA79~lB (,318). These C-coated samples were jnitiaily studied with a JSM-840A scanning electron microscope (SEM ). EnergydispersiveX-ray ( EDX ) spectra of variousphases of interest were obtained, mainly to confirm the preliminary mineral identification by optical microscopy. In addition, backscattered electron images at low magnification (30x) were obtained for each section. This was done to assess qualitatively the nature of major element pyroxene zoning in the shergottites, which allowed us to select spots (for ion microprobe analyses) that had a

range of Fe/Mg ratios. Concentrations of REEs and other selected trace and minor elements were then measured in situ in various minerals present in each

shergottite with the W~hin~on University modified CAMECA IMS3f ion microprobe. Ex~~men~l techniques have been described in detail by ZINNERand CROZAZ( 1986) and LUNDBERGet al. ( 1988). As reportedby HARVEYet al. ( 1993), the trace element me~urement program has recently been modified to record the count rate at each measured mass throughout the analysis. This allows us to detect contributions from adjacent phases or inclusions, and also to identify chemical variations within the mineral being analysed. Additional software, that allows easy manipulation of the data, has been developed to identify contaminated analyses (and, in some cases, eliminate cycles from an analysis that show evidence of contamination). MINERALOGY

AND PETROGRAPHY

The mineralogy of shergottites is well characterized and has been reviewed by MCSWEEN (1985). Major element compositions of minerals in the basaltic shergottites (i.e., Shergotty, Zagami, and EETA79~1) are given in the following references: SMITHand HERVIG ( 1979), STOLPERand MCSWEEN (1979),

STEELE and SMITH (1982),

and JAROSEWKH (1983),

SMITH et al, (1983),

MCSWEEN MCCOY et

al.

( 1992), and TREIMAN and SUTTON ( 1992); major element mineral compositions for ALHA can be found in ISHI et al. (1979), MCSWEEN et al. (1979), SMITH and STEELE

TABLE 1. Modal Miieraiogv of Shergottites (voI.%). Cfinopyroxene 1 Low-CaPyx Hi@Ka Pyx SHERGOTTY [ 65.5-751 33.52 36.32 ZAGAMI 74.3-8O.43 36.52 36S2 EETA79Wl (A) 54.5-62.S4 3.2-8.54 EETA79001 (B)

’

31.8-54.44

ALHA

Oiivine

-

Chtbopyroxene

b&i ffa) f

20-27.2l 10.3-21.?*,3

7.2-10.3*4

3.4-7.3*4

43.55

LEN’88516

IO-12.35.*

44.2-525~6 116 $0..598.9

358 12-139

IO-159

15.9-18.34 28.2-29.64

1 1.6-24S4

266

Maskelynite

-

8.1S*~y

Mesostasis

Phosphates

2.1-3.22.3

2.1-3.72,-1

IT-o.4(w)4

2.2-4.0s4

U-O.34

0.2-0.7 (w)~

3.4-3.84

OS-I.14

tr(W)’

1.7(w)Y

2.2-4.S1

2.4~5.2l

lr-2.4 (w)’ tr-0.1 (ap)l tr-1.3 (Wf2.3

16

Y7

0.7-2839

try

---___.---* Includes minerals in the xenocryst assemblage (i.e., olivine, orthopyroxene and chromite). (fa) = fayaiite; (w) = whitiockite; (ap) = apatite; (tr) = trace amounts. References (numbers given in superscripts): 1. St&iffieret al. (1986) and refereafes therein. 2. Stoiper and McSween(1979). 3. McCoy et at. (1992). 4. McSween and Jarosewich (1983). 5. Lundberg et uf. (1990). 6. Ma et al. (1981). 7. McSween et al. (1979). 8. Boynton et al. f1992). 9. DeIaney (1992).

Petrogenesisof shergottite

surrounded by coronas of pigeonite having the same composition as that in the groundmass, and irregular embayments in the xenocrystic olivine cut across internal zoning patterns (STEELE and SMITH, 1982; MCSWEEN and JAROSEWICH, 1983). It is interesting to note that major element compositions of these xenocrysts are similar to the compositions of corresponding minerals in ALHA ( MCSWEEN and JAROSEWICH,1983; MCSWEEN, 1985). A common feature of all shergottites is the extreme shock (at least 30 GPa ) which they experienced, as is evidenced by the m~kelynit~tion of the plagioclase, fracturing, mosaicism, and undulatory extinction of olivine and pyroxene, polysynthetic twinning in pyroxene, Fe oxidation in olivine, and presence of melt pockets and veins (MCSWEEN and ST~F’FLER, 1980; STEELE and SMITH, 1982; MCSWEEN and JAROSEWICH, 1983; OSTERTAG et al., 1985; ST~FFLER et al., 1986; KELLER et al., 1992).

meteorites

4215

REE abundances in apatite of Zagami ( WADHWA et al., 1993) are similar (La - 27 X CI). Maskelynite Figure 2 compares REE abundances in maskelynite of shergottites. The REE patterns are characterized by a large positive Eu anomaly (Eu/Eu* is -40, 35 and 55 for maskelynite in Shergotty, Zagami and EETA7900 1, respectively; for comparison, Eu/Eu* is -50 for maskelynite in ALHA and LEW885 16). Chond~te-Nordic abundances of the light REEs ( LREEs) are higher than for the heavy REEs (HREEs). HREE concentrations in maskelynite are usually difficult to determine, not only because they are relatively low, but also because of molecular interferences from the LREE oxides in the HREE mass region. Olivine

TRACE AND MINOR ELEMENTS IN MINERALS OF SHERGOTI-ITES In the following sections, we report REE and some selected trace and minor element ~n~ntmtions measured in mineral phases of the basaltic shergottites. Minerals analysed include whitlockite, maskelynite, and pyroxene (low- and high-Ca). Orthopyroxene and olivine from the xenocrystic assemblage in lithology A of EETA79001 were also measured. Representative REE concentrations are given in Table 2 (number of analyses for each mineral are given in parentheses). Data for the two lherzolitic shergottites, ALHA and LEW885 16, were obtained by us for another study (HARVEY et al., 1993) and have been included in this section for the sake of comparison.

Phosphates The main phosphate phase in shergottites is whitlockite, although chlorapatite is also found in trace amounts in Shergotty, Zagami, and EETA79001. Of all the phases analysed, whitlockite is consistently the mineral with the highest REE concentrations. Figure 1 shows average chondrite-normalized REE abundances for this mineral in the shergottites. REE patterns in Shergotty and Zagami are almost identical, absolute REE concentrations, however, are higher in Shergotty (La-600XCI)thaninZagami(La-460XCI).Inboth lithologies of EETA7900 1, the whitlockite REE patterns are the same, although absolute con~nt~tions are slightly bigher in Iithology B (La - 120 X CI ) than in lithology A (La - 100 X CI). REE patterns in both lithologies of EETA79001 are different from those in Shergotty and Zagami, but similar to those in ALHA and LEW88516, in that they are relatively more LREE-depleted. The chondrite-normalized Gd/La ratio in Shergotty and Zagami whitlockites is - 1, whereas this ratio is -3.5-4.0 in EETA79001, ALHA77005, and LEW88516. All whitlockite REE patterns have small negative Eu anomalies, with Eu/Eu * (where Eu* is the interpolated value between the chondrite-normalized abundances of Sm and Gd) between 0.7-O-8. REE con~ntmtions in apatite of Shergotty were reported by LUNRBERGet al. ( 1988), who determined that this mineral accounted for less than 2% of the REEs in this meteorite.

Olivine in the xenocrystic assemblage in lithology A of EETA7900 I has, as expected, a pattern enriched in HREEs (Fig. 2). In fact, the LREEs arc usually present in such low con~ntmtions that they are below the detection limit of the ion microprobe. As shown in Fig. 2, HREE concentrations in xenocrystic olivine in EETA79OOlA are almost the same as those in poikilitically enclosed olivines in ALHA and LEW885 16. As mentioned earlier, there is no phenocrystic olivine in Shergotty and Zagami, although there is usually some minor late-stage fayalite. These fayalite grains are usually too finegrained to be analysed with the ion microprobe, the only exception to this being fayalite grains in Zagami DN (see Petrography section), with sizes up to tens of microns. Compared to the early-formed, phenocrystic olivine in the lherzolitic shergottites, this Fe-rich olivine is considerably enriched in minor and trace elements such as Ti, Zr, Y, and REEs ( WADHWA et al., 1993), confirming that it was a latestage mineral in the crystallization history of Zagami. Pyroxenes REE concentrations in low- and high-Ca pyroxenes in shergottites vary by more than an order of magnitude. Figure 3a,b shows representative REE abundances in pyroxenes in the three basaltic shergottites; for comparison, typical REE concentrations in pyroxenes of lherzolitic shergottites are shown in Fig. 3c,d. All pyroxene REE patterns in shergottit~ show a smooth increase from light to heavy REEs, and REE patterns for low-Ca pyroxenes are generally steeper than those for high-Ca pyroxenes. In addition, REE concentrations are typically higher in high-Ca than in low0 pyroxene. Pyroxene REE patterns in all three basaltic shergottites are characterized by small to moderate negative Eu anomalies (Eu/Eu * - 0.95-0.3). Figure 3 also illustrates the fact that a few pyroxenes in shergottites have Ce anomalies. These anomalies arc found only in Antarctic shergottites and never in Shergotty and Zagami. Previously, we reported Ce anomalies in pyroxenes of ALHA (LUNDBERG et al., 1990) and LEW88516 (HARVEY et al., 1993). In this study, we have identified Ce anomalies in both low- and high-Ca pyroxenes of

LEW88516

ALWA77OOS

EETA7QOOlA Xenocrysts EETA79001 B

EETA79001 A Groundmass

ZAGAMI

SHERGO-ITY

reflects

whitlo&te (6) Olivine (1)

LCa Px (9) HCa Px (3) Maskelynite (2) Whitlockite (4) LCa Px (17) HCa Px (8) M~~ly~te (2) Wbitlockite (4) LCa Px (20) HCaPx(l) Maskelynite (2) Whitlockite (3) Olthopyroxene (14) Olivine (8) LCa Px (20) HCa Px (5) Maskelynite (3) Whitlockite (4) LCa Px Poik (29) HCa Fx Poik (4) LCaPx Non-@oik (21: HCa Px Non-Poik (3) Maskelynite (3) Whitlockite (4) Olivine (1) Low-Ca Px Poik (23) High-Ca l?x Poik (3) LCaPx Non-Poik (151 HCa Px Non-Poik i2)’ Maskelynite (6) U2.5 19f-24 5-121 43168 70220 4124

4.9132 122/135 44ia3 185/403 140213 10625

9153 78197 30211 3024 lf4 301157 11150 37f 133 62+5 27~2

al&8

9.1162 22Of264 311133 3841610 13223

96/15S 2571360 3226 75214 8147 27li654 521252 439f559 68+ 14 54kl

441161 97f644 42~30 74211 4121 147i53 i 481154 22511769 130257 6522

113& 10 3.5113 24 332 1 24+6 -/8

Nd 7’0,296 2901696 81 k 10 23929 42/158 2201336 83511 177 + 12 29/l 48 162 ss+s 63~22 4042

Ce 53f221 1471410 17828 3562 13 361149 238/363 160236 280t 11 15160 76 64~3 64+17 3150

La 14151 33/97 8923 147+5 10150 671102 70235

7548

13t2 4526

231218 5141565 74f274 700/1112 33+4

4.4122 75185 8140 781124 468 j$SO

3&2

82f242 373fS43 2229 91+8 841422 721 331.3 72~23 22/436 5 f 0.9 2451406 5471970 2lf9 812 10 161122 46919 18 841612 816/1020 34Lt 1s 5521

9.5163 175l205 28il24 337i535 22+12

Gd 103/486 3 171837 22+3 120&9

19+2 13163 68 44S&lO 13k7 -I53 1 It:0.3 31i70 64182 3514.130 141-2 3i12 821278 19169 14SilS9 443 + 110 lO+t

Fill

1ai63 saia9 337 ;r 47 22&l 10139 38iS4 247 + 130

47/2&i 1621534 27 + 1 81&6 351106 112/171 23~11 6226 32i206 303 1822 42212 s/154 220.8 991167 1911384 18 + 10 45 2 6 If53 256fS44 571243 463f464 20+8 3122

Sm

13~2 1 + 0.5

26/l 14 72f2OS 4*1 2412 2Oi6 1 86/l 19 422 18+2 28f94 164 4+1 15&2 71109 1 f 0.3 68ilO6 1541212 4f2 18&2 4/‘30 104l24s 23152 2151219 4;tl 11&l 0.8 f 0.7 5.7157 122/121 2arl2 1661263 521

Tb

98*10 19~2

Dy 22911128 52611322 2123 15723 194i619 69 l/922 1922 128 + 18 23Si950 792 23&I 107 ;t 22 4Of768 la& 1 617i713 120211748 IS+5 1182 16 301273 73211826 23311214 156511683 21&t 75~2 1823 66157 1 812!842 2Otl68 1359l2157 26+ 14 20&2 5&l

a&2 161143 16oi2OS 46l216 3 111425 6+1

16~1

22~6 lli160 5kOo.6 1541173 2671365 31t2 24+3 9184 1871412 a91265 33ot394

32_t2 51/141 1431225 6+1 25&t 631231 270

6~1

521264 112,245

Ho

8+1 6/42 72/l 10 501127 12Oi154

62+4 44r296 555/l 100 32419 14 1031/1106

52k.7 27k3

Szlzl

421 9.2/71 69i72 24185 1461206

6~1

a+2 IO/67 5 + 0.6 6li99 136/141

6228 441624 28+2 452l622 925f 1046

41+ 28&4 591453 585f571 180/615 93211999 13+7

58+7 2531673 843

lo+1 29f80 91

37+4 S7&5

312 65 & 7 831538 681/632 172nOQ 87211488

4823

4624 561583 48&2 413f566 8741962

@.?+a 165/700 4401-760

Yb 236/1210 340/943 1121 24i86 69/l 15

Tm 281143 451141

Er 186BOl 38818SO 1413 8427 155is19 436J783 lO+l 70+6 179i661 784

REE concentrations in pyroxene, maskelynite, and olivine are in &, while those in whitlockite are in m. The number of the error from counting statistics. analyses of each mineral type is indicated in parentheses. For pyroxenes and olivines, the number of analyses usually exceeds the number of individual grains analysed but, for all other minerals, the numbers in parentheses correspond to the number of individual grains analysed. Data for ALHA and LEW88516 are from the recent comparative stu pof these two mete01rite:s (Harvey ef al., 1993). -rREE SHBRGO’lTITE MINERAL

TABLE 2. REE in Minerals of Shergottites. Core and rim compositions are given for ‘LCa Px’ (low calcium pyroxene) and ‘HCa Px’ (high calcium pyroxene) as ‘core/rim’; ‘Poti and ‘Non-Poik’, in the case of pyroxenes in AL.HA77005 and LEW88516, denotes ‘poikilitic’ and ‘non-poikilitic’. REE concentrations in maskelynite, whitlockite and olivine are averages of all measurements on these grains; ‘+’ for maskelynite and whitlockite reflects the range of concen~ations present in these minerals, but for olivine,

4217

Petrogenesis of shergottite meteorites

1000

Shergottite Whitlockite

1 Sm Eu Gd Tb Dy Ho Er Tm Yb

La Ce Pr Nd

FIG. 1. REE abundances (normalized to CI chondrite values of PALMEet al., I98 1) in whitlockite of shergottites. Inset shows chondrite-normalized REE abundances in the whole rock of each shergottite. The same symbol was used for whitlockite and whole rock (see inset) of each shergottite. Data for whitlockite of ALHA and LEW885 16 from HARVEYet al. ( 1993). Error bars are smaller than data points.

EETA79001. In lithology A, two analyses of low-Ca pyroxene (out of twenty-one) show Ce anomalies, both positive, with Ce/Ce* (where Ce* is the interpolated Ce value between chondrite-normalized abundances of I_a and Pr) - 1.7 and -2.5. In lithology B, of twenty-five pyroxene analyses, there were ten (seven of low-Ca and three of high-Ca pyroxene) with Ce anomalies. Of the seven Ce anomalies in low-Ca pyroxene, three are negative (Ce/Ce* ranges from -0.4 to -0.6) and four are positive (with Ce/Ce* ranging from -4.5

EETA79001A Xenacrystic Olivine

to -29).

Of the three Ce anomalies in high-Ca pyroxene, two were negative (Ce/Ce* -0.4 and -0.6) and one was positive (Ce/Ce* - 2.4). In a previous study (HARVEY et al., 1993), we showed that in pyroxenes of lherzolitic shergottites there is a good positive correlation of Ti abundances with Fe#s (i.e., Fe/ [ Fe+Mg] ), and also that while Fe#s vary by less than a factor of -2, Ti concentrations vary by a factor of -6. This indicates that Ti abundance is a more sensitive indicator of progressive crystallization than Fe# in these pyroxenes. Therefore, in Fig. 4, we show the concentrations of trace and minor elements (Y, Zr, SC, and Cr) in low-Ca pyroxenes of basaltic shergottites ( Shergotty, Zagami, EETA7900 1A groundmass, and EETA7900 1B ) plotted vs. their Ti concentrations. For comparison, data for pyroxenes in the two lherzolitic shergottites, ALHA and LEW885 16 (HARVEY et al., 1993), are shown in Fig. 5. Also plotted in Fig. 5 are the compositions of orthopyroxene xenocrysts in EETA7900 1A (which are included here because this xenocrystic assemblage is mineralogically similar to the poikilitic domain in the lherzolitic shergottites). Concentrations of these elements are giveri in tabulated form in WADHWA (1994). DISCUSSION

I I

I

1

I

La Ce Pr Nd

I

I

I

I

I

1

1

I

I

REE

Budget

Sm Eu Cd Tb Dy Ho Er Tm Yb

FIG. 2. REE abundances (normalized to CI chondrite values of PALMEet al., I98 1) in maskelynite of shergottites. Symbols for mas-

kelynite of each shergottite are the same as in Fig. I. Also shown are chondrite-normalized HREE abundances in olivine of the megacrystic assemblage in lithology A of EETA7900 1, and poikilitically enclosed olivine in ALHA and LEW885 16. Data for maskelynite and olivines ofALHA77005 and LEW88516 from HARVEYet al. ( 1993). Error bars for maskelynite analyses are smaller than data points. Vertical dashed line indicates that HREE abundances (Ho to Yb) in maskelynite are not shown (see text).

In shergottites, as can be seen in Table 2, modally abundant phases, such as pyroxene, maskelynite, and olivine (when present), contain low REE abundances and whitlockite (which is usually a minor mineral) is the most enriched in REEs. Concentrations of REEs in the other calcium phosphate, apatite, found in trace amounts in Shergotty, Zagami, and EETA79001, are lower than in whitlockite by approximately an order of magnitude (LUNDBERG et al., 1988; WADHWA et al., 1993). Other accessory phases such as

M. Wadhwa, H. Y. McSween Jr., and G. Crozaz

4218

10

1

0.1

F

0.01

o Shergotty Low-C88Pyroxene I 0.01 t

A EETA79OOlA Low-Ca Pyroxene

A EETA79001A High-Ca Pyroxene

0 Shergotty High-Cn Pyroxene

Fz 3

l

0.001 10

I

,

1,

I

I

Zagami Law-Ca Pyroxene

l 2agami High-Ca Pyroxene

I

I,

I

I,

1

I

0.001 10

’

’

g

’

’

A BBTA790QlB Low-Ca Pyroxene A BBTA79001B High-Ca Pyroxene

’

’

’

’

’

’

’

’

’

5 s $

l

S 0.1

0.01

LEWg8516 Low-Ca Poiktiitic

0 AL&%77005 High-Ca Poikilitic Cl ALHA Low-Ca Non-Poikilit 0.001

q LEWs8516 High-Ca PoikBitic

0.001 La Ce Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb

La Ce Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb

FIG. 3. REE abundances (normalized to CI chondrite values of PALMEet al., 198I ) in low- and highCa pyroxenes in (a) Shergotty and Zagami, (b) lithologies A and B of EETA79001 (c) poikilitic and non~ikiiitic lithologies of AL~A77~5 (after HARVEYet al., 1993) and (d) poikifitic and nonpoikilitic li~hologiesof LEW885 16 (after HARVEY et al., 1993).

opaques and mesostasis are essentially devoid of REEs (LUNDBERG et al., 1988). Therefore, in all shergottites, bulk rock REE concentrations are controlled by REE abundances in whitlockite. As is evident in Fig. 1, the REE pattern of whit&kite is almost parallel to the whole rock pattern for each shergottite. Mass balance calculations indicate that whole rock REE concentrations in these meteorites can be accurately accounted for with oniy -0.5-1.5 wt% whit&kite (which is the approximate range of observed abundance of this heterogenously distributed mineral in these meteorites). Cerium Anomalies in EETA79001, Antarctic Weathering

the Result of

The presence of Ce anomalies in pyroxene of EETA7900 1, which is consistent with similar observations in pyroxene of the two other Antarctic shergottites (LUNDBERG et al., 1990; HARVEY et al., 1993), is yet another illustration of REE mobilization caused by the weathering of these meteorites in Antarctica. Such effects, which have not been detected in the two non-An~ctic shergottites, Shergotty and Zagami, are in fact not limited to just Antarctic shergottites. Ce anomalies

in Antarctic meteorites were Srst documented in whole rock samples of achondrites by SHIMIZU and MASUDA ( 1982 ). A subsequent and extensive comparison of Antarctic and nonAntarctic eucrites studied by INAA (MITT~E~EHLDT and LINDSTROM, I99 I) revealed that Ce anomalies are common in those that were found in Antarctica and absent in the others. Moreover, in a detailed ion microprobe study of an Antarctic eucrite, FLOSS and CROZAZ f 199 1) showed that pyroxene (which had an extensive network of shock-induced cracks) was the mineral that was most affected by REE mobilization, just as it is in the highly shocked Antarctic shergottites. The Larger fraction of pyroxene with Cc anomalies in Iithology B than in lithology A of EETA79001 is not readily explained, because lithology B is coarser-grained than lithology A. It may be a sampling effect from our ion microprobe measurements, or possibly our thin section of Iithology A was derived from a more interior, and therefore less altered, piece of the meteorite than the section of lithology B. At any rate, the observation of Ce anomalies, as previously noted by FLOSS and CROZAZ f i99 1f, indicates that caution must be exercised when using absolute REE concentrations in

4219

Petrogenesis of shergottite meteorites

lb1

Ti (ppm)

Ti @pm)

10s

80

A

I_

0

2000

4000

6000

Be00

1WOO 0

Ti (ppm)

2000

4000

6060

8000

10600

'Mppm)

FIG. 4. (a) Y vs. Ti, (b) Zr vs. Ti, (c) SC vs. Ti, and (d) Cr vs. Ti in low-Ca pyroxenes in Shergotty, Zagami, groundmass of EETA79OOlA, and EETA79001B. In each of these meteorites, the number of analyses exceeds the number of individual pyroxene grains analysed. whole rocks or minerals of Antarctic shergottites (or other achondrites) to constrain parent magma compositions or crystallization histories of these meteorites. The Origin of Pyroxene Zoning Previous studies of major and some minor element distributions in pyroxenes in shergottites have interpreted the zoning features to be of igneous origin (STOLPER and MCSWEEN, 1979; SMITH et al., 1983; MCSWEEN and JAROSEWICH, 1983; TREIMAN and SUTTON, 1992). From our work, it is evident that one of the most striking geochemical characteristics of the shergottites is that their pyroxenes exhibit extensive trace (including REEs) and minor element zonation. Before we can make interpretations regarding the petrogenesis of these meteorites based on these zonation features, it is important to establish that these are, indeed, of primary igneous origin. As can be seen in Fig. 4, concentrations of incompatible elements such as Y (and, by analogy, the REEs), Zr, and Ti in low-Ca pyroxenes in the basaltic shergottites covary. However, it appears that SC behaves incompatibly

(i.e., its concentrations are positively correlated with Ti abundances), while Cr behaves compatibly (i.e., its concentrations are anticorrelated with Ti abundances) in pigeonites in these meteorites. The important question here is whether these zonation features are of primary magmatic origin, or whether they have been affected to a significant degree by secondary alteration processes such as subsolidus reequilibration and/or metasomatic infiltration. If, indeed, the original trace and minor element zonation in these pigeonites has been preserved, it carries information regarding the crystallization histories of these meteorites; however, if secondary processes have altered the original primary zoning, it will no longer be a useful indicator of the magmatic processes that formed these rocks. Provided that the zonation in pyroxenes in basaltic shergottites is of primary, igneous origin, we expect that incompatible elements such as Y, Zr, and Ti will covary, which is indeed the case (Fig. 4a,b). Moreover, it is expected that compatible elements will be anticorrelated with incompatible elements (as is the case in Fig. 4d, i.e., Cr vs. Ti). However,

M. Wadhwa. H. Y. McSween Jr.. and G. Crozaz

4220

SC, which in a majority of terrestrial basaltic systems behaves compatibly in pigeonite, appears to behave incompatibly (i.e., SC concentrations are positively correlated with Ti abundances; Fig. 4~). We have previously explained the incompatibility of SC in low-Ca pyroxenes in ALHA and LEW885 I6 (Fig. 5c), as well as the unusual behaviour of Cr in these pyroxenes (i.e., Cr concentration increases with increasing Ti abundances up to Ti - 1700 ppm, but decreases for higher Ti concentrations; see Fig. 5d) in the context of a magmatic origin (HARVEY et al., 1993). Indeed, SC can behave incompatibly in pigeonites crystallizing from low-Al melts such as the parent melts of ALHA and LEW885 16, since the substitution of Sc into low-Ca pyroxene is charge-balanced by a coupled substitution of Al for Si in tetrahedral sites ( COBON et al., 1989); also, Cr concentrations in these pigeonites can be accounted for by co-crystallization of significant amounts of olivine (which would strongly exclude Cr) during the early part of the crystallization histories of ALHA and LEW885 16. These arguments are in good agreement with the predicted partition coefficients for SCand Cr in low-Ca pyroxenes and olivines, having the appropriate compositions, calculated using the program of COLSON et al. ( 1988). Using the same program, we determined that, indeed, SC is predicted to behave incompatibly in low-Ca pyroxenes in Shergotty, Zagami, and EETA79001, while Cr behaves compatibly in such pyroxenes (see Appendix A for details of the parameters used, as well as the results obtained from these calculations; results of similar calculations for ALHA and LEW885 16 are included for comparison). Therefore, the microdistributions of the trace and minor elements in shergottites are consistent with a magmatic origin, and no secondary processes such as reequilibration or metasomatism need to be invoked to explain them. Implications

for Shergottite

Crystallization

Histories

Earlier we showed that trace and minor element zonation in low-Ca pyroxenes in shergottites is of primary, magmatic origin, and therefore useful in constraining the petrogenesis of these meteorites. Certainly, the coherent trends for elemental compositions in pigeonites of each shergottite (Figs. 4, 5 ) strongly argue for progressive fractional crystallization. Also, from the large spread in trace and minor element abundances in low-Ca pyroxene, it is evident that this mineral was among the first to form and subsequently crystallized through most of the petrogenetic sequence in an essentially closed system. In each shergottite, trace element microdistributions indicate an identical sequence of crystallization for the REEbearing minerals. As illustrated in Fig. 3, pyroxenes with the lowest REE abundances have either no Eu anomalies or small negative Eu anomalies, and these anomalies become progressively more pronounced with higher REE abundances: thus, earlier-formed pyroxenes began crystallizing before plagioclase, while later-formed pyroxenes co-crystallized with this mineral. In all shergottites, whitlockite, the mineral with the highest REE concentrations, has small-to-moderate negative Eu anomalies, indicating that this mineral began crystallizing after plagioclase. The general crystallization sequence for shergottites described above is compatible with textural

interpretations and with the results of calculations using appropriate distribution coefficients (e.g., LUNDBERG et al., 1990; HARVEY et al., 1993) that demonstrate that earlyformed pyroxenes, plagioclase, and whitlockite, respectively, formed in equilibrium with melts of progressively increasing REE abundances. Another important point is that Y (and, by analogy, the REEs), Zr, and Ti compositions of pyroxenes in all five shergottites fall along similar trends (Figs. 4a,b, 5a,b) and, therefore, there is a strong possibility that these meteorites are petrogenetically related. This conclusion is not affected by the observation that the distributions of Sc and Cr are different for Shergotty, Zagami, and EETA7900 1B (Fig. 4c,d) on the one hand, and ALHA and LEW885 16 (Fig. 5c,d; note the sharp inflexions in the trends at Ti - 1700 ppm), on the other. As indicated in the previous section, these differences are probably due to crystallization of significant amounts of olivine during the early part of the crystallization history of ALHA and LEW885 16, an event which did not occur in basaltic shergottites (although the primary parent melts for the basaltic shergottites may well have been olivine saturated ) The similarity of trace and minor element microdistributions in low-Ca pyroxenes in the groundmass of EETA79001A (which is basaltic), and in nonpoikilitic pigeonites in lherzolitic shergottites is somewhat enigmatic, but nevertheless indicates that there may be a close relationship between the intercumulus material in the two lherzolites and the groundmass of EETA7900 IA (Figs. 4, 5; note slopes of trends defined by pigeonite compositions in EETA7900 I A groundmass are, in all cases, similar to those of non-poikilitic pigeonite compositions in ALHA and LEW885 16, but different from those of pigeonite compositions in the other basaltic shergottites). The possibility of a link between the lherzolitic shergottites and EETA7900 1A is also supported by the fact that (with the exception of Cr) the trace and minor element compositions of orthopyroxene xenocrysts in EETA79001A overlap those of poikilitic pigeonites in ALHA and LEW88516 (Fig. 5). Moreover, HREE abundances in xenocrystic olivine in EETA79001A are identical, within error, to HREE abundances in poikilitically enclosed olivines in the lherzolitic shergottites (Fig. 2). Therefort. the broad similarity of trace and minor element distributions in xenocrystic orthopyroxene and olivine in EETA7900 1A to corresponding minerals in ALHA and LEW885 16 indicates that these xenocrysts were derived from a rock similar to the lherzolitic shergottites. However, the differences in the Cr concentrations in the xenocrystic orthopyroxene in EETA7900 IA and the low-Ca poikilitic pyroxenes in the lherzolitic shergottites (Fig. 5d) suggests that the rock from which the xenocrysts were derived was not identical to these two lherzolites in its mineralogical and geochemical characteristics (as also indicated by isotopic data; SHIH et al., 1982; WODEN et al., 1982; JAGOUTZ, 1989; L. Nyquist, pers. commun.). REE Parent Magma Compositions

Having determined elemental concentrations in the earliest-formed pyroxenes. one can estimate the compositions

4221

Petrogenesis of shergottite meteorites

of the parent melts ofthe shergottites. LUNDBERGet al. ( 1988) derived the REE composition of the Shergotty parent melt, using core compositions of pigeonite and augite, assuming relative proportions of these minerals and intercumulus melt, and closed system conditions; their results indicated that the Shergotty parent melt had a REE pattern parallel to that of its whole rock. Here, we have used an alternate method to calculate REE abundances in the parent melts of the other two basaltic shergottites, Zagami and EETA7900 1 (lithologies A, B). This method assumes that the pigeonite cores were in equilibrium with the parent melt, and the melt composition is determined by dividing the measured REE concentrations in the earliest-formed pigeonite (i.e., those. with the lowest REE abundances) by the appropriate partition coefficients. Unfortunately, the choice of suitable partition coefficients is not straightforward, since it is well known that they vary considerably with factors such as temperature, oxygen fugacity, melt compositions, etc. (MCKAY, 1989). However, it is also recognized that although the absolute values of the REE partition coefficients for a particular mineral may vary significantly, their relative values (or the REE patterns of D values) usually remain the same. For example, REE partition coefficients for augite in Shergotty may be higher than those of augite in Nakhla by as much as a factor of -5, but the REE patterns of these D values are similar (MCKAY et al., 1992 ) . Therefore, due to uncertainties in the absolute values of the partition coefficients, we may not be able to accurately determine the absolute REE abundances in the parent melts of the shergottites, but the REE patterns of these melts can be well constrained. For our purpose, the most appropriate REE partition coefficients currently available in the literature are those determined experimentally by MCKAY et al. ( 1986) for pigeonite in Shergotty. It was recently determined that these synthetic pigeonites (with WO content < 25) are exsolved into fine, IO-50 nm augite/pigeonite lamellae ( MARINO et al., 1993), but it is believed that these lamellae are postcrystallization features and that the bulk pyroxene compositions still represents the composition that was in equilibrium with the melt (G. A. McKay, pers. commun.). Therefore, the REE partition coefficients determined by MCKAY et al. ( 1986) still appear to bc valid. However, errors in these experimentally determined D values for LREEs are as large as -50%. We have, therefore, chosen to use the pyroxene/melt partition coefficients determined by LUNDBERG et al. ( 1988) from ion microprobe measurements of Shergotty pigeonite (which are consistent, within error, with the aforementioned experimentally determined D values for REEs in Shergotty pigeonite). Using these D values, we find that REE patterns for melt compositions of Zagami and both lithologies of EETA7900 1 are LREE-depleted, and parallel to their respective whole rock REE patterns (Fig. 6). This is consistent with the LREEdepleted patterns determined for the parent melts of Shergotty (LUNDBERG et al., 1988), ALHA (LUNDBERG et al., 1990), and LEW88516 (HARVEY et al., 1993). This is an important result, since it supports closed-system fractional crystallization for each shergottite, and eliminates the need to invoke complex petrogenetic scenarios (as would be required if the shergottite parent melts were LREE-enriched,

as was calculated for the Shergotty parent melt by SMITH et al., 1984, using buik Shergotty analyses and partition coefficients as determined by NAKAMURA et al., 1982 ). It should be noted that the parallelism of the REE patterns of the shergottite parent melts and their respective whole rocks is not in conflict with the possibility of any of the shergottites being “cumulates,” because an enrichment of the “cumulus” phases (pyroxene, in the case of basaltic shergottites, or pyroxene and olivine, in the case of lherzolitic shergottites) would tend to lower the overall REE content in the cumulus pile (and therefore in the whole rock), although the relative REE pattern would remain almost the same as that of the parent melt. Therefore, provided that there was no infiltration of magma of a different composition than that of the intercumulus melt into the cumulus pile (i.e., if it did not behave as an open system), the system could subsequently evolve by progressive closed-system fractional crystallization and, upon solidification, the whole rock REE pattern would indeed be parallel to that of the parent melt. Petrogenetic Relationship A Model

Between the Shergottites:

There have been many petrological and geochemical studies of shergottites but, because of the geochemical and isotopic complexity of these meteorites, it has not been possible to relate them in any straightforward way through either their source region or their process of formation. As we have shown in an earlier section, REEs and other trace and minor element microdistributions in minerals (particularly in pyroxene) support a petrogenetic relationship between these meteorites. Therefore, we propose here a model that suggests some possible relationships between the shergottites. This model is consistent with what is currently known about the petrologic, geochemical, and isotopic characteristics of the shergottites, including the trace and minor element mineral abundances presented in this study, but is by no means uniquely required by the data. One extreme possibility, which is not incorporated in this model but cannot be ruled out, is that, although the shergottites may have originated on the same parent body and may even have formed by similar processes, they were formed completely independently of each other from distinct source regions and magmas. However, we consider this scenario unlikely because similar cosmic ray exposure ages ( B~GARD et al., 1984), combined with probability considerations (the occurrence of numerous, extremely high energy impacts over a relatively short period of time is unlikely), favor the possibility that these meteorites were derived from their parent body by a single impact, thus, implying geographic proximity of these rocks; this, in turn, suggests that they could have been part of the same “petrologic association.” Model constraints Our model specifically takes into account the following characteristics of the shergottites. Petrographic characteristics: 1) The shergottites are petrographically diverse: Shergotty and Zagami are basaltic; EETA79001 is also basaltic, but has two distinct lithologies in igneous contact (one of which con-

4222

M. Wadhwa, H. Y. McSween Jr., and G. Crozaz

loo

I4

Ti @pm)

Ti Wm)

a

1lee0

I 0

2000

4000

8000

6WO

ieeee

0

4890

2000

6000

8eoo

1OOeO

'Wvm)

Ti(ppm)

•I AL~~OO5

Poikilitic

0 ALMA77005

Non-Poikiiitic

x Orthopyroxene

’ LEW88516

Forte

m LEW88516

Non-Poik%tic

Xenocrysts

in EETA7MH)l

FIG. 5. (a) Y vs. Ti, (b) Zr vs. Ti, (c) SCvs. Ti, and (d) Cr vs. Ti in low-Ca poikilitic and nonpoikilitic pyroxenes in ALHA and LEW88516 (after HARVEYet al., 1993), and in orthopyroxene xenocrysts in EETA79001A ~com~sitiona1 ranges of xenncrysts are encircled). In each of these meteorites, the number of anaiyses exceeds the number of individual pyroxene grains analysed. tams xenocrysts); ALNA77005 and LEW885 16 are lherzolites. 2) EETA79001A contains large xenocrysts that petrographically resemble the ~ikilitic domains in ALHA77~5 ( MCSWEEN and JAROSEWICH,1983). 3 ) The mineralogies of both EETA7900 1 layers are alike (ignoring the megacrysts in A), except that lithology A has a higher proportion of pyroxene (65% vs. 59 ~01%in lithology B), and a higher pigeonite/augite ratio ( - 10 vs. -2 in lithology B); moreover, pigeonites in EETA7~OlA groundmass are, on average, more magnesian than those in EETA79OOlB (MCSWEEN and JAROSEWICH,1983). Geochemical characteristics: 1) All shergottites have LREE-depleted whole rock REE patterns; however, Shergotty and Zagami have shallower LREE depletions (i.e., higher LREE/HREE ratios), compared to the Antarctic shergottites ( BURGHELEet al., 1983; SHIH et al., 1982; SMITH et al., 1984; DREIBUSet al., 1992).

2) Major, minor, and trace element zoning has been preserved in minerals of shergottites (JONES, 1986, and references therein; TREIMAN and SUTTON, 1992; this study) in spite of the high shock pressures (230 GPa) they experienced. 3) Yttrium (and, by analogy, the REEs), zirconium, and titanium microdistributions in low-Ca pyroxenes in all shergottites define overlapping, coherent trends (this study). 4) Bulk major, minor, and siderophile trace elements in ALHA77005, EETA79OOlA, and EETA79001B show colinearity, with the composition of EETA7900 1A lying between those of ALHA and EETA79001B (MA et al., 1982; SMITH et al., 1984). 5 ) Mixing calculations based on major and minor element chemistry ( MCSWEEN and JAROSEWICH,1983) demonstrate that the composition of EETA79001A groundmass can be reproduced fairy we11by admixing -36% of an ALHA77~~like component to EETA7900 1B. 6) Major element compositions of the xenocrystic olivines

4223

Petrogenesis of shergottite meteorites and low-Ca pyroxenes in EETA79OOlA resemble compositions of similar minerals in the poikilitic domains of ALHA and LEW885 16 ( MCSWEEN and JAROSEWICH, 1983). Moreover, trace and minor element microdistributions (except Cr) in orthopyroxene xenocrysts in EETA79OOlA are similar to those in low-Ca poikilitic pyroxenes in ALHA and LEW885 16, and HREE abundances in xenocrystic olivine in EETA79OOlA are identical to those in poikilitically enclosed olivines in ALHA and LEW885 16 (this study). 7) SC and Cr microdistributions in low-Ca pyroxenes in the groundmass of EETA7900 1A are similar to those in nonpoikilitic pyroxenes in ALHA and LEW885 16, but different from low-Ca pyroxenes in the rest of the basaltic shergottites (this study). Isotopic characteristics: The isotopic evidence, although somewhat complex and controversial, provides the most stringent limitations on possible scenarios to explain the formation of shergottites and demonstrates that these objects are not simply related. Earlier work (SHIH et al., 1982; WOODEN et al., 1982) has shown that Sm-Nd data for whole rock samples of all the then-known shergottites lay on an “isochron” of - 1.3 b.y. that was interpreted as the igneous crystallization age. Rb-Sr data for mineral separates of each shergottite were found to plot on a line (“isochron”) corresponding to an age of 180 f 20 m.y. that was assumed to represent the time of the shock metamorphism which affected all shergottites. However, the RbSr (SHIH et al., 1982; WOODEN et al., 1982) and U-Th-Pb ( CHEN and WASSERBURG,1986) data for whole rocks failed to show any evidence for an event at - 1.3 b.y. Instead, they defined model ages of -4.3 * 0.2 b.y. taken to represent a time of major differentiation on the shergottite parent body. On the other hand, while evidence for the reality of the 180 m.y. event (above references and JAGOUTZ and WWNKE, 1986; JAGOUTZ, 1989) became overwhelming, its explanation was questioned. JONES ( 1986) suggested that the thermal effects of the shock event were insufficient for chemical and isotopic equilibration and that 180 m.y. was the crystallization age of these meteorites and not the time of shock metamorphism. His arguments were mainly based on the preservation of disequilibrium textures and on the presence of zoned minerals, two observations that are hard to reconcile with a complete resetting of the isotopic systems. JONES ( 1986) considered the possibility that the diffusitivities of major elements (which show distinct zoning, most clearly in pyroxene) and trace elements (such as those analysed isotopically, and for which data were not then available) might be decoupled. He concluded that this was unlikely and, certainly, our data, which abundantly illustrate the pervasive nature of trace element zoning in shergottites, strongly support his view. In addition, attempts to reset the Rb-Sr system, by exposing rocks to pressures characteristic of the shock level the shergottites experienced, were negative ( DEUTSCH and QUANDT, 1986; NYQUIST et al., 1987). Finally, the Rb-Sr dating of an ALHA plagioclase melt, most likely produced during the shock event, yielded a very young age of ( 15 -t 15) m.y. (JAGOUTZ, 1989), consistent with the cosmic ray exposure ages of shergottites (-2.5 m.y., with the exception of EETA7900 1, which was only exposed in space as a small

object for the last -0.6 m.y.). All this evidence leads us to believe that the much debated suggestion of JONES ( 1986) is on firm ground and that shergottites indeed crystallized approximately 180 m.y. ago. At that time, each shergottite had distinct strontium, neodymium, and lead isotopic ratios, which indicates that the various objects cannot by simply related. The large spread in initial isotopic ratios (including differences between the two lithologies in EETA79001; WOODENet al., 1982) suggests that either the shergottite parent magmas were derived from isotopically distinct source regions, or that they resulted from variable degrees of interaction of essentially isotopically uniform magmas with crustal materials en route to the surface (JONES, 1989). Deviations of the initial Nd isotopic ratios from the CHUR (i.e., Chondritic Uniform Reservoir) value at 180 m.y. (expressed as $,d = [ { ( ‘44Nd/‘43Nd)measu& ( ‘44Nd/ ‘43Nd)cHuR} - 1] X 104) are positive for the three Antarctic shergottites and negative for Shergotty and Zagami. Positive tNd values are the signatures of LREE-depleted sources (such as depleted mantle reservoirs) and are consistent not only with the observed whole rock REE patterns but also with the parent melt compositions that were derived in the present study. On the other hand, negative CNdvalues are

A”.[a1

l

Zagami Parent Melt Zagami Whole Rock (Smith et al., 1984)

EETA79OOlA Whole Rock (Burghele et al., 1983)

1

Parent Melt

La Ce Pr Nd

Sm Eu GdTbDy

Ho ErTmYb

FIG. 6. Calculated REE abundances in the parent melts of (a) Zagami, (b) EETA79001A groundmass, and (c) EETA79001B (normalized to CI chondrite values of PALME et al., 198I ). Solid lines show whole rock REE patterns for each of these shergottites.

4224

M. Wadhwa. H. Y. McSween Jr., and G. Crozaz

not compatible with the observed LREE depletions observed in Shergotty and Zagami, and inferred in their parent melts. This contradiction can be resolved if it is assumed that mantlederived Shergotty and Zagami magmas acquired a LREEenriched crustal component. This addition would have drastically lowered the ‘43Nd/‘44Nd ratios (to negative tNd values) and raised the LREE/HREE ratios. In this context, it is worth noting that, indeed, REE depletions in Shergotty and Zagami are much less pronounced than in the Antarctic shergottites (see inset in Fig. 1). Finally, regarding the isotopic characteristics of xenocrysts in EETA79OOlA, initial Rb-Sr isotopic measurements on an olivine separate from this meteorite indicated that the xenocrystic olivine did not lie on the 173 f 10 m.y. isochron defined by other mineral separates (WOODEN et al., 1982; NYQUIST et al., 1986) and was, therefore, not in isotopic equilibrium with the groundmass of EETA7900 1A. However, since then, the strontium blank for these measurements has been re-evaluated and, with this blank correction, the ohvine datum falls on the 173 m.y. isochron. This suggests that olivine xenocrysts may, in fact, be more closely related to the EETA7900 I A groundmass than was previously thought, although this has yet to be established with certainty (L. Nyquist, pers. commun.) The model According to our model (summarized in Fig. 7 ), shergottites formed - 180 m.y. ago (as indicated by their internal mineral isochrons) on a planetary-sized body (Mars); their whole rock “isochrons” do not have an age significance and are essentially mixing lines between two isotopically distinct reservoirs, such as the mantle and the crust (as previously suggested by JONES, 1989). Therefore, by the time the shergottites formed, the silicate-rich fraction of the shergottite parent body had effectively differentiated into a depleted mantle and an enriched crust. We suggest (as did JONES, 1989) that the variability found in initial radiogenic isotopic ratios of the shergottites is not a primary feature of the source regions from which their parent magmas were derived, but results mainly from varying degrees of contamination of shergottite parent magmas with an isotopically heterogeneous crust. The parent magmas of the lherzolitic shergottites (ALHA and LEW88516) were derived directly from a partly depleted mantle by partial melting (the degree of melting was sufficient to make the melts olivine-normative). The positive initial tNdvalue for ALHA (+ 14.9; JONES, 1989)) as well as the LREE-depletion in its whole rock REE abundances are consistent with this view. (The primary parent melts of the lherzolitic shergottites may or may not have been modified to some extent by a small amount of fractionation of olivine + chromite.) This episode of parent melt generation was followed by crystal accumulation and subsequent crystallization (lower right of Fig. 7). In the same temporal and spatial regime as these Iherzolites, there were other ALHA77005/LEW88516-like rocks being formed (lower left of Fig. 7), that differed to some extent in their mineral chemistry (and isotopic systematics) from

ALHA and LEW885 16, and were similar to the xenocrystic assemblage in EETA7900 1A. Our preferred process for the formation of the parent magmas for Shergotty, Zagami, and EETA79OOlB is by fmctionation of olivine + chromite (and perhaps also pyroxene) from a melt similar to the parent magma of the lherzolitic shergottites (lower middle of Fig. 7 ). This process would result in the formation of more evolved (or fractionated) melts than the parent melts of the lhet-zolitic shergottites. Subsequently, the Shergotty and Zagami parent melts (with their considerably higher whole rock LREE / HREE ratios) could have incorporated a LREE-enriched (crustal ) component. As already mentioned, contamination by such a component may also provide a convenient explanation for the negative values of their initial tNd (-7.3 and -4.7, respectively; JONES, 1989). After crustal contamination, crystal accumulation may have occurred (or else, a process such as flow lineation on or near the surface may have produced the pyroxene orientation ), followed by crystallization (upper right of Fig. 7 ) Traditionally, Shergotty and Zagami have been considered cumulates based on experimental evidence and grain alignment ( STOLPERand MCSWEEN, 1979; MCCOY et al., 1992). The experimental evidence consists of the observation by STOL.PERand MGSWEEN ( 1979) that the first pyroxenes to crystallize in the experiments on Shergotty and Zagami compositions were more magnesian than the most magnesian natural pyroxenes. However, as noted by these authors, there could be alternate, and equally viable, interpretations for this discrepancy between experimental and natural pyroxenes, such as unreasonably high experimental fOz, re-equilibration of early-formed natural magnesian pyroxene cores, due to slow cooling, and/or disequilibrium growth, due to rapid crystallization of more Fe-rich pyroxenes under natural conditions. Moreover, the observed grain alignment or foliation in Shergotty and Zagami may have been produced by some process (flow segregation?) other than gravity settling. Therefore, it is not unequivocally known whether or not Shergotty and Zagami are cumulates. In any case, following crustal contamination, crystal accumulation may (or may not) have occurred and, subsequent crystallization resulted in the formation of Shergotty and Zagami. Unlike those of Shergotty and Zagami, the parent melt for EETA79001B did not undergo any significant amount of crustal contamination, since the value of the initial cNdfor bulk EETA79001 is positive (+18.3; JONES, 1989), and its whole rock has a low LREE/HREE ratio similar to that of the two lherzolitic shergottites. Like Shergotty and Zagami, it is not known with certainty whether EETA79001B contains excess (cumulus) pyroxene or not. Here, we will consider it to represent a magma composition based on the fact that there is no observed grain alignment, and it has the highest modal abundance of maskelynite of all the shergottites. EETA7900 1A could have formed by either of the following two processes (upper left of Fig. 7): 1) Assimilation of an ALHA /LEW885 16-like rock by a magma having the composition of EETA7900 I B. 2) Mixing of a phenocryst-bearing magma (olivine + chromite + orthopyroxene + intercumulus melt, having

Petrogenesis of shergottite meteorites

4225

IPARENTLELT~73~I

““‘TFOR

--__

‘%kiTALS

II

+ KM*

MULATZON

CRYSTAL ACCVMVZATZON PARENT MEL.T FOR ALH I LEW

t

I

PARTZAL MELTZNG

PARTLY DEPLETED MANTLE --I

FIG. 7. Schematic diagram of possible petrogenetic

relationships between the shergottites. ALH = ALHA77005; ratios than the

LEW = LEW88516; *ICM = Intercumulus melt; **melts now have relatively higher LREE/HREE original parent me&s; also have negative eNd values.

an overall composition similar to that of ALHA77005/ LEW88516) with a magma having the composition of EETA7900 1B. Available petrologic and geochemical data for EETA7900 1 and ALHA / LEW885 16 are consistent with either assimilation or magma mixing (see petrographic and geochemical constraints; strontium isotopic data are also compatible with this interpretation). The lack of clear evidence for either of these processes from neodymium isotopic compositions can easily be explained if the actual ALHA like component that was admixed with EETA7900lB was more like EETA7900 1B than ALHA in its neodymium isotopic composition or, alternatively, if it had lower REE abundances (by Z-50%) than ALHA77005. This is not inconsistent with our model, according to which EETA7900 1B was admixed with another component similar to ALHA77005, but not identical to it. This would also explain the subtle differences in the mineralogy and geochemistry of xenocrysts in EETA79OOIA and the poikilitic domain in ALHA /LEW885 16.

The higher proportion of pyroxene in EETA79OO 1A than in E~TA79~1 B (65 vs. 59 vol%), and the higher pigeoni~/ augite ratio (- 10 vs. -2) in EETA7900 1A ( MCSWEEN and JAROSEWICH, 1983) can be explained by assimilation of the magnesian olivine and orthopyroxene xenocrysts that would increase the Mg/( MgSFe) of the magma and promote crystallization of pigeonite at the expense of augite, but without significantly affecting other components such as the Ca/( Ca + Na) ratio; values of these molar ratios in both lithologies are: Mg/f Mg -t- Fe) -0.59 for A, 0.43 for B; Caf(Ca + Na) - 0.82 for A, 0.78 for B (data from MCSWEEN and JARO~EWICH, 1983; ratios for A were calculated after excluding xenocrysts). These petrographic and geochemical arguments for ~imilation could also be used to favor a magma mixing scenario. In this latter case, the xenocrysts in EETA79001A would represent phenocrysts present in one of the magmas prior to mixing. However, it should be noted that neither of the two scenarios mentioned above is perfect and each has its limitations. Of the two possibilities, assimilation (of as much as -3O-

M. Wadhwa, H. Y. McSween Jr., and G. CI’OZU

4226

of a refractory lherzolitic ~om~nent similar to ALHA77~5/LEW885 16 is fess likely because of thermal constraints (See Appendix B, part I). Assimilation of lherzolite by basaltic magma could proceed if the magma was superheated by hundreds of degrees, which does not seem geologically reasonable. One possible way out of this dilemma involves heating lherzolitic wall rocks to high temperatures (possibly 750-1000°C) prior to their incorporation as xenoliths in the magma. Prior heating might be accomplished by continuous passage of hot, ascending magma through a lherzolitic conduit near the surface. However, the megacrysts show no evidence of rec~s~lli~tion as typically observed for contact me~mo~hism. Shock heating of Iherzohte might also be invoked, but it seems a rather ad hoc scenario. The alternative, magma mixing (see Appendix B, part II, for results of a magma mixing calculation for EETA7900 1A, based on the petrologic mixing model of WRIGHT and DOHERTY, 1970)) avoids these thermal constraints; however, it too has its drawbacks. The most evident limitation of this scenario is that there does not appear to be any concrete textural evidence (such as resorption or quench features) for magma mixing in EETA79001A.

40 wt%)

CONCLUSIONS

In summary, the following conclusions can be drawn from the observations reported here. 1) REE and other trace and minor element microdistributions in minerals of shergottites suggest that these objects are petrogenetically related. 2) One of the most striking and informative features of shergottites is the extensive trace and minor element zonation in pyroxenes, which is of primary magmatic origin. 3) Trace and minor element trends in pyroxenes, as well as REE patterns of the calcuiated parent melts of the shergot&es, suggest that closed-system fractional c~stalIization played an important role in their petrogenesis. 4) Comparisons of trace and minor element microdistributions in minerals in EETA79OOlA with similar minerals in the two lherzolitic shergottites (ALHA and LEW885 16) indicate that EETA7900 IA may represent an admixture of an ALHA /LEW885 16-like component with a magma having the bulk composition of EETA7900 1B. We have presented a model for the possible relationships between shergottites that accounts for most of their known petrographic, geochemical, and isotopic characteristics. According to this model, their parent magmas were ultimately derived from partial melts of the partly depleted mantle of their parent planet, and acquired their distinct characteristics through processes such as crystal fractionation, crystal accumulation, magma mixing/assimilation, and crustal contamination. Acknowledgments-This work was supported by NASA grants NAG 9-55 to GC and NAGW 362 I to HYM. We would like to acknowledge the Smithsonian Institution, the University of New Mexico, and the Meteorite Working Group for the loan of the thin sections. We would also like to thank Russ Colson for the use of his programs. Constructive comments by J. Jones, A. Reid, and A. Treiman were greatly appreciated. ~d~t~~rialhandling: C. Koeberl

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exposure of the SNC achondrites and constraints on their derivation from Mars. Lunar Planet. Sci. XV, 68-69 (abstr.). BOWEN N. L. ( 1928) The Evolution of Igneous Rocks. Dover Publications, Inc. BOYNTON W. V., HILL D. H., and KRING D. A. (1992) The trace element composition of LEW885 16 and its relationship to SNC meteorites. Lunar Planet Sci. XXIII, 147- 148 (abstr.) . BURGHELEA. et al. ( 1983) Chemistry ofshergottitesand the Shergotty parent body (SPB): Further evidence for the two component model of planet formation. Lunar Planet. Sci. XIV, 80-8 i (abstr.). CHEN J. H. and WASSERBURGG. J. (1986) Formation ages and evolution of Shergotty and its parent planet from U-Pb-Th systematics. Geochim. Cos~oc~i~. Acfa 50,9X-968. COLSON R. O., MCKAY G. A., and TAYLOR L. A. ( 1988) Temperature and composition dependencies of trace element partitioning: Olivine/melt and low-Ca pyroxene/melt. Geochim Cosmochim. Acta 52, 539-553. COLSON R. O., MCKAY G. A., and TAYLOR L. A. ( 1989) Charge balancing of trivalent trace elements in olivine and low-Ca pyroxene: A test using experimental partitioning data. Geochim. Cosmochim. Acta 53,643-648. DELANEYJ. S. (1992) Petrological comparison of LEW88516 and A LHA 77005 shergottites. Meteorifics 27,2 I 3-2 14 (abstr.) , DEUTSCHA. and QUANM B. ( 1986) The response of the St isotopic system in geological samples to artificial shock pressure. Mefeorirics 21, 354-355 (abstr.). DREIBUSG., JOCHUMK. H., PALMEH., SPE~EL B., WLOTZKAF.. and WANRE H. ( 1992) LEW885 16: A meteorite com~sitionally close to the ‘*Martian mantle.” Mereoritics 27, 2 16-2 17 (abstr.). FLOSSC, and CROZAZG. ( 1991) Ce anomalies in LEW85300 eucrite: Evidence for REE mobilization during Antarctic weathering. Earth Pianet. Sci. Lett. 107, 13-24. HARVEY R. P., WA~BWA M., MCSWEEN H. Y., JR., and CROZAZ G. ( 1993) Petrography, mineral chemistry, and petrogenesis of Antarctic shergottite LEW885 16. Geochim. Cosmochim. Acta 57, 4769-4783. ISHII T., TAKEDA H.. and YANAI K. f 1979) Pyroxene geothermometry applied to a three-pyroxene achondrite from Allan Hills, Antarctica and ordinary chondrites. Mineral 1, 9.460-48 I. JAGOUTZE. ( 1989) Sr and Nd isotopic systematics in ALHA77005: Age of shock metamo~hism and magmatic differentiation on Mars. Ge~~him. ~osrnoc~~irn.Acta 53, 2429-244 1. JAGOUTZE. and WANKE H. ( 1986) Sr and Nd isotopic systematics of Shergotty meteorite. Geochim. (hsmochim. Acta SO, 939-953. JONESJ. H. ( 1986) A discussion of isotopic systematics and mineral zoning in the shergottites: Evidence for a 180 m.y. igneous crystallization age. Geochim. Cosmochim. Acta SO, 969-977. JONES J. H. ( 1989) Isotopic relationships among the shergottites, nakhlites, and Chassigny. Proc. 19th Lunar Planet. Sci. Can&, 465474. KELLERL. P., TREIMANA. H., and WENTWORTHS. J. ( 1992) Shock effects in the shegottite LEW88516: Optical and electron microscope observations. Meteoritics 27, 242 (abstr.). LONGHI J. and PAN V. ( 1989) The parent magmas ofthe SNC meteorites. Proc. 19th Lunar Planet. Sci. &or& 45 l-464. LIJNDBERGL. L., CROZAZ G., MCKAY G., and ZINNER E. ( 1988) Rare earth element carriers in the Shergotty meteorite and implications for its chronology. Geochim. ~osrn~h~rn. Acta 52,21472163. LUNI>BERGL. L., CRO’ZAZG., and MCSWEEN H. Y., JR. ( 1990) Rare earth elements in minerals of the ALHA shergottite and implications for its parent magma and crystallization history. Geochim. Cosmochim. Acta S4,2535-2547. MA M.-S.. LAUL J. C.. and SCHMITTR. A. ( 198 1) Complementary rare earth patterns in unique achondrites, such as ALHA and shergottites, and in the earth. Proc. 12th Lunar Planet. Sci. C’on~. 1349-l 358. MA M.-S., LA~JLJ. C., SMITH M. R., and SCHMITT R. A. (1982) Chemistry of the shergottites Elephant Moraine A79001 and Zagami. Lunar Pkznet Sci. XIII, 451-452 (abstr.).

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Petrogenesis of shergottite meteorites MARINO ‘I. A., MCKAY G. A., and TRE~MANA. H. ( 1993) Submicroscopic lamelar structures in REEdoped dinopyroxene. GSA Abstr. Prog. 25, A2 16-A2 17 (al&r.). MCCOY T. J., TAYLORG. J., and KEIL K. ( 1992) Zagami: Product of a two-stage magmatic history. Geochim. Cosmochim. Acta 56,

3571-3582. MCCOY T. J., KEIL K., and TAYLORG. J. ( 1993) The dregs ofcrystallization in Zagami. Lunar Planet. Sci. XXIV, 947-948 (abstr.). MCKAY G. A. (1989) Partitioning of rare earth elements between major silicate minerals and basaltic melts. In Geochemistry and ~ineraiogy ofRare Earth EIeme~ts (ed. B. R. LIPIN and G. A. MCKAY), pp. 45-75. Mineral. Sot. Amer. MCKAY G. A., WAG~TAFFJ., and YANG S.-R. ( 1986) Chnopyroxene REE distribution coefficients for shergottites: The REE content of the Shergotty melt. Geochim. Cosmochim. Acta 50,927-937. MCKAY G. A., LE L., and WAGSTAFFJ. ( 1992) REE partition coefficients for the Nakhla parent melt. Lunar Planet. Sci. XXIII, 889890 (abstr.) . MCSWEEN H. Y., JR. ( 1985) SNC meteorites: Clues to martian petrologic evolution? Rev. Geophys. 23, 39 1-4 16. MCSWEEN H. Y., JR. and JAROSEWICHE. (1983) Petrogenesis of the Elephant Moraine A79001 meteorite: Multiple magma pulses on the shergottite parent body. Geochim. Cosmochim. Acta 47,

1501-1513. MCSWEENH. Y., JR. and ST~FFLERD. ( 1980) Shock metamorphic features in Allan Hifls 77005 meteorite. Lunar Planet. Sci. XI, 717-719 (abstr.). MCSWEEN H. Y., JR., TAYLOR L. A., and STOLPERE. M. f 1979) Allan Hills 7700.5: A new meteorite type found in Antarctica. Science204, 1201-1203. MCSWEEN H. Y., JR., LUNDBERGL. L., and CROZAZ Cl. ( 1988) Crystallization of the ALHA shergottite: How closed is a closed system? Lunar Planet. Sci. X1X, 766-167 (abstr.). MI~LEFEHLDT D. W. (1994) ALH84001, a cumulate orthopyroxenite member of the martian meteorite clan. Meteoritics 29,214-

221. MITTLEFEWLDTD. W. and LINDSTROMM. M. ( 1991) Generation of abnormal trace element abundances in Antarctic eucrites by weathering processes. Geochim. Cosmochim. Acta 55,77-87. NAKAMURAN., UNRUH D. M., TATSUMOTOM., and HUT~HI~N R. ( 1982) Origin and evolution of the Nakhla meteorite inferred from the Sm-Nd and U-W svstematics and REE. Ba. Sr. Rb. and K abundances. Geochim. Cosmochim. Acta 46. 15% l573. NYQUISTL. E., WOODENJ., BANSALB., WIESM~NNH., and SHIH C.-Y, I 1984 ) Sr and Nd isotonic svstematics of EETA7900 I. Meteoritics 19,284 (abstr.). _ _ NYQUISTL., WIESMANNH., SHIH C.-Y., and BANSALB. ( 1986) Sr isotopic systematics of EETA79001 glass. Lunar Planet. Sci. XVII,

624-625. NYQUISTL., Hoftz F., WIESMANNH., SHIH C.-Y., and BANSALB.

( 1987) isotopic studies ofshergottite chronology: I. Effect ofshock metamorphism on the Rb-Sr system. Lunar PIanet Sci. XVIII, 732-733 (abstr.). OSTERTAGR., STO%LER D., JAMMES C., and PFANNSCHMIDTG. ( 1985) Shock effects in the Shergotty meteo~te-E~den~ for only one shock event. Lunar Planet. Sri. XVI, 19-20 (abstr.). PALMEH., SUESSH., and ZEH H. D. ( 198 1) Abundances of elements in the solar system. In Lando~t-Bornstein;Astronomy and Asrrophysics, Vol 2a, pp. 257-272. Springer-Verlag. ROBIE R. A., HEMINGWAYB. S., and FISHER J. R. (1978) Thermodynamic properties of minerals and related substances at 298.15 K and I bar ( IO5 Pascals) pressure and at higher temperatures. U.S. GS Bull. 1452, pp. I-456. SCORE R., SCHWARZC. M., KING T. V. V., MASON B., BOGARD D. D., and GABELE. M. ( 198 I ) In Antarctic Meteorite Descriptions; Johnson Space Center Publ. 54. pp. I33- 134. SHIH C-Y. et al. ( 1982) Chronology and petrology of young achond&es, Shergotty, Zagami and ALHA77005: Late magmatism on a geologically active planet. Geochim. Cosmochim. Acta 46,2323-

2344. SHIMIZUH. and MASUDAA. f 1982) REE characteristics of Antarctic eucrites. Mem. Natl. Inst. Polar Res. (Special Issue) 25, t45- 152.

SMITH J. V. and HERVIG R. L. ( 1979) Shergotty meteorite: Miner&g& petrography, and minor elements. hfefeorifics 14, 12 l- 142. SMITH J. V. and STEELEI. M. ( 1984) Achondrite ALHA77005: Alteration of chromite and olivine. Meteoritics 19, 12 l-l 33. SMITH J. V., STEELE1. M., and LEITCHC. A. ( 1983) Mineral chemistry of the shergottites, nakhlites, Chassigny, Brachina, pallasites and ureilites. J. Geophys. Res. 88, B229-B236. SMITH M. R. et al. ( 1984) Petroaenesis of the SNC (Sheraottites. Nakhlites, Chassiguites)‘meteohtes: Implications for the& origin from a large dynamic planet, possibly Mars. Proc. 14th Lunar

Planet. Sci. Co&; J. Geophys. Rex 89, B612-B630. STEELEI. M. and SMITH J. V. ( 1982) Petrography and mineralogy of two basalts and olivine-spine1 fragments in achondrite EETA79001. Proc. 13th Lunar Planet. Sci. Conf. _ Part I: J. Geophys. Res. 87, (suppl.), A375-A384. ST~FFLER D. et al. ( 1986) Shock metamorphism and petrography of the Shetgotty achondrite. Geochim. Cosmochim. Acta 50,889-

903. STOLPER E. M. and MCSWEEN H. Y., JR. (1979) Petrology and origin of the shergottite meteorites. Geochim. Cosmochim. Acta

43, 1475-1498. TREIMANA. H. and SUTTONS. R. ( 1992) Petragenesis of the Zagami meteorite: inferences from synchrotron X-ray (SXRF) microprobe and electron microprobe analyses of pyroxenes. Geochim. Cos-

mochim. Acta 56,4059-4074. VISTJSENL., PETERSENLt., and MADSENM. B. ( 1992) Mossbauer spectroscopy showing large-s&e inhom~eneity in the presumed Martian meteorite Zagami. Physica Scripta 46,94-96. WADHWAM. ( 1994) Geochemical studies of two unusual groups of meteorites: Trace element microdistributions in SNC meteorites and Mn-Cr isotopic systematics in unequilibrated enstatite chondrites. Ph.D. dissertation, Washington Univ. WADHWAM., MCCOY T. J., KEIL K., and CROZAZG. ( 1993) The chemical and physical evolution of late-stage melt in Zagami. Meteoritics 28, 453 (abstr.). WOODEN J. L. et al. ( 1982) Rb-Sr and Sm-Nd constraints on the origin of EETA79001: A second Antarctic shergottite. Lunar Planet. Sci. XIII, 879-880 (abstr.). WRIGHT T. L. and DOHERTYP. C. (1970) A linear programming and least squares computer method for solving petrologic mixing problems. G. S. A. BuIl. 81, pp. 1995-2008. ZINNER E. and CROZAZ G. ( 1986) A method for the quanti~tive measurement of rare earth elements in the ion microprobe. IntI.

J. Mass Spectrom. Ion Proc. 69, 17-38. APPENDIX

A

The program Of COLSONet al. ( 1988) predicts partition coefficients for various trace and minor elements in Iow-Ca pyroxene and olivine crystallizing from basaltic melts under QFM conditions, based on a model in which temperature and composition dependencies ofequilibrium partitioning were constrained by evaluating the relationship [-In(K) = AH/RT - AS/R] for each element. Weight percent partition coefficients are calculated for input temperature, melt composition, and crystal com~sition. Using this program, we have calculated the low-Ca pyroxene/melt and olivine/melt ~uilib~um partition coefficients of Sc3+, Cr3+, and Cr2+ for the shergottite parent melt compositions listed in Table Al, for temperatures between I 100 and 1250°C. Composition of low-Ca pyroxene cores in the basaltic shergottites are assumed to be -En, (STOLPERand MCSWEEN, 1979; MCSWEENand JAROSEWICH, 1983; TREIMAN and SUTTON, 1992) while those in ALHA and LEW88516 are assumed to be -Ens0 and --EnTJ, respectively (LUNDBERG et al., 1990; HARVEY et al., 1993). Compositions of olivine cores (assumed to have grown in equilibrium with low-Ca pyroxene cores) in ALHA and LEW88516 are taken to be -For9 and -Fo,* respectively (LUNDBERGet al., 1990; HARVEYet al., 1993). The calculated partition coefficients are shown in Table A2. Scandium is predicted to be moderately incompatible in low-Ca pyroxene in all shergottites, except in Shergotty, in which it appears to be moderately compatible (it should be noted, however, that the parent melt for Shergotty estimated by STOLPER and M~SWEEN, 1979. has a

M. Wadhwa, II. Y. McSween Jr., and G. Crozaz

4228

Table Al. Parent meh compositions of shergottit~. SH (Sher~otty) from Stolper and McSween ( 1979); ZAG (Zapami) from Treiman and Sutton (1992) (composition derived assuming 8.5% cumulus pigeon&e and 8.5% cumulus augite); EETg (EETA79OOlA groundmass) from Loaghi and Pan (1989); ALHA (ALHA77005) from McSween et al. (1988) LEWa &EW88516) fmm Harvey er al. (1993)(composition a).

K;O0.54

Mg##

(I) Thermal Constraints

0.08 0.71

-0.07__

0.58

_

value will be assumed for harzburgite. From the caiculations of MCSWEEN and JAROSEWICH( 1983). to produce IO0 g of hybrid magma, 36 g of harzburgite must be assimilated, and the thermal input required to raise the temperature of this amount of harzburgitic xenolith by 900°C is -36 kj. Heats of fusion of olivine (Foes) and orthopyroxene are respectively 400 j/g and 6 16 j/g ( ROBIE et al., 1978). Assuming that the assimilated material consists of 26 g of orthopyroxene and IO g of olivine, the heat required for melting the harzburgitic xenolith is -20 kj. Heats of mixing are so small that they can be neglected. Thus, the total heat required to assimilate relatively cold harzburgite is -56 kj/ 100 g of hybrid magma. A potential problem with this calculation is that it is not clear whether harzburgite wotdd indeed melt at 1100°C.ISHIIet al. ( 1979) determined an equilibmtion temperature of -1160°C for ALHA77~5 using pyroxene g~thermome~. However, this meteorite contains both cumulus and postcumulus pyroxenes and, from their description, it appears that this equilibmtion temperature refers to crystallization of the fractionated intercumulus liquid rather than the harzburgite cumulate portion. The melting temperature of an olivine-orthopyroxene xenohth is probably at least several hundred degrees higher, but could be lowered if additional phases were present. It seems likely that assimilation of harzburgite by basaltic magma would have to proceed via incongruent reactions with the melt. Because we cannot specify what reactions may be involved, we cannot calculate precisely the heat required. However, because the necessary heat for incongruent reactions involving olivine and orthopyroxene is aImost certainly lower than fusion, we will adopt a conservative value of half that required for fusion at I iOO”C, - 10 kj. The estimated total heat for assimilation is thus -46 kj/ 100 g of hybrid magma produced; it probably is a lower limit. Because few magmas are superheated and crystallization is exothermic, the heat necessary to accomplish assimilation is likely to be supplied by precipitation of a thermally equivalent amount of crystals from the magma (BOWEN, 1928). 64 g of EETA79OOlB (corresponding to the amount of magma that must assimilate 36 g of harzburgite) consists of -5 1 g of clinopyroxenes (pigeonite and au&e) and - 13 g of plagioclase (calculated from modal analyses; MCSWEEN and JAROSEWICH,1983). The latent heat of crystallization for both clinopyroxenes is taken as -430 j/g, and for intermediate plagioclase -322 j/g (ROBIE et al., 1978 ). Total crystallization of

considerably lower Mg number and higher A&O3 compared to the parent melt compositions estimated for the other four shergottites (Table Al ); calculations using the program of CoLsoN et al. ( 1988, 1989) showed that if the Shergotty parent melt did have a higher Mg number and lower A1203comparable to the parent melts of the other shergottites, SCwould indeed be incompatible in the low-Ca pyroxene crystallizing from such a melt). Also, SC is strongly incompatible in olivine in the lherzolitic shergottites. As can be seen in Table A2, Cr3+ and Cr*’ are predicted to be compatible in low-Ca pyroxene in all shergottites. In olivine in the lherzolitic shergottites, Cr3+ IS . estimated to be incompatible, while Cr*+ ranges from incompatible to weakly compatible. However, since Cr3+ is expected to be the predominant Cr species in a basaltic melt at QFM, between 1100and 125O”C, the total olivine/melt partition coethcient of Cr (weighted average of pa~ition coefficients of Cr3+ and Cr2+) will be < 1. APPENDIX

--.~

3

on Assimilation

The petrologic mixing model of WRIGHT and DOHERTY( 1970) was used by MCSWEENand JAROSEWICH( 1983) to determine that the major and minor element composition of the groundmass of EETA79OOlA could be reproduced (with small residuals) by the addition of 10 wt.% olivine, 26 wt.% orthopyroxene, and 0.5 wt.% chromite (with compositions similar to minerals in the poikilitic areas in ALHA77005) to the EETA79001B bulk composition. Therefore, the assimilation model requires that at least -36% harzburgite be fully incorporated by EETA79~lB to produce the groundmass of EETA7900 I A. However, the refractory nature and high proportion of this harzburgitic material impose rather stringent thermal constraints on the assimilation process. An estimation of the heat required for assimilation is outlined below. First, the temperature of harzburgite xenoliths must be raised from some low value (say 200°C) to above the magma B solidus temperature. We will assume a temperature increase of 9OO”C, as the solidus temperatures of other shergottites are near 1100°C (STOLPER and MCSWEEN, 1979). Heat capacities of both olivine and orthopyroxene are -1.1 j/g.deg (ROBE et al., 1978), and this

Table A2. Estimated low-Ca p~oxen~mel~ and o~v~~melt partitioncoefficients for Sex+, Cr3+and Cr2+in shergottite parent melts (using the program of Co&on$ul., 1988). T (DC) &ment SR ZAG EEfg ALHA LEW _ _-_._ Pyx Pyx 01 Y Y 1100 sc3+ 1.78 0.078 -*_A0.114 ,97x, :5x8 - 0.37 1150

1200

1250

Cr3+ Cr2+ sc3+ Cr3+ Cr2+

1.87

W+

4.19 5.13 1.53 4.40 5.11 1.32

Cr3+ Cr2+

4.49 5.07

0.58 2.02 2.98

Sd+

1.15

0.50

3.04 0.67 1.95 3.01

1.35 2.23 0.49 1.41 2.20

0.95 1.07 0.32 0.99 1.07

0.157 0.751 0.087 0.184 0.83 1

1.39 1.97 0.47 I .45 1.Qh

0.193 1.12

0.42 1.46 2.18 0.36

0.27 1.03 1.07 0.24

0.095 0.214 0.913 0 104

0.41 1.50

0.13R 0.265 1.43 0.150

1.96 0.35

0.125 0.228 I .306

Petrogenesis of shergottite meteorites

4229

Table Bl. Petrologic mixing calculations for EETA79001 and ALHA77005 using the method of Wright and Doherty (1970). MagmaMixing Comparison Input data Model data Combined Bulk Bulk Bulk Residuals3 EETA(A)l Proportions3 EETA(B)l ALHA Si@ 49.0 43.4 48.5 1.90 Tie, 1 23 0.45 0.70 hon$e -0.17 BBTA(A)I -o.96 A2Q3 9.93 i:6i 5.68 0.07 Cr2% 0.14 0.97 0.58 n.d. n.d. 0.22 Fe203 ---L

Iz MS0 CaO Nay0

E&F@) 55.8% 0.06 O.-l6 0.47 16.3 0.52 18.9 ALL-0.04 14.0 16.6 -0.48 8.0 7.1 44.2% -0.28 1.2 0.84 -0.01 0.04 0.05 n.d. 0.65 -0.20 EETA(A) = EETA79OOlA:EETAIB) = EETA79OOlB:ALHA =

0.47 17.4 7.32 11.0 1.68

1 - Compositions taken from McSween and Jarosewich (1983). 2 - Composition from Smith et al. (1984). 3 - Combined proportions in weight 96; residuals are the mismatch between analysed bulk composition of EETA79OOlA and that calculated from the model (i.e., calculated wt.% observed wt.%).

64 g of EETA7000 1B magma would therefore liberate -26 kg (which is only -57% of the heat required for assimilation!). It should be noted that this is an upper limit of the heat liberated, because assimilation is no longer operable once a magma is -75% crystalline. (II) Magma Mixing Model Calculations

Magma mixing is preferred over assimilation as the process that gave rise. to EETA79001A, since it avoids the thermal constraints discussed in the above section. Therefore, we present results of calculations for a magma mixing scenario (based on the petrologic mixing model Of WRIGHTand DOHERTY, 1970), assuming that a magma with the composition of EETA7900 1B mixed with another magma that contained phenocrysts of olivine and orthopyroxene and had

an overall composition close to that of bulk ALHA77005. The combined proportions of EETA79OOlB and ALHA in the model (see Table Bl) are similar to those determined by NYQUIST et al. ( 1984) from stroutium isotopic data. If the residuals (i.e., observed wt% subtracted from calculated wt%) from the magma mixing model calculations are compared with those for assimilation (Table 4 in MCSWEEN and JAROSEWICH,1983), it can be seen that the match is not nearly as good for magma mixing as for assimilation. The largest errors are in SiO,, A1203, CaO, Na20, and P205, the components enriched in the groundmass relative to the megacrysts in EETA79OOlA. This mismatch is not serious, and may even be expected, since the bulk composition of ALHA probably does not reflect the exact composition of the one of the mixed magmas (which probably contained a greater proportion of intercumulus melt than ALHA77005).