Sr and Nd isotopic systematics in ALHA 77005: Age of shock metamorphism in shergottites and magmatic differentiation on Mars

GQ16-7037/89/s3.00

Gecxhimica d Cosmochimxa Acla Vol. 53. pp. 2429-2441 Copylight 0 1989 Pergamon Press Printed m U.S.A.

+ .OO

plc.

Sr and Nd isotopic systematics in ALHA 77005: Age of shock metamorphism in shergottites and magmatic differentiation on Mars E. JAGOUTZ Max-Planck-Institut fur Chemie, Saarstrasse 23, D-6500 Mainz, F.R. Germany (Received August 30, 1988; accepted in revisedform June 30, 1989) Abstract-The meteorite ALHA 77005 belongs to the group of SNC meteorites which are thought to be fragments of the planet Mars. New Sr and Nd isotopic data are reported for ALHA 77005 whole rock and petrographically distinct mineral separates. Plagioclase crystallized from a plagioclase shock melt, fractionating the Rb/Sr ratio at the time of the shock event. A Rb/Sr age of 15 If: 15 Ma for the shock event is measured using this Rb/Sr fractionation. This shock age is, within error, identical to the exposure age of 2.5 Ma. Using augite and pigeonite a Rb/Sr crystallization age of 154 ? 6 Ma is measured, which is identical to the crystallization age of the Shergotty mesostasis-plagioclase. Petrographic evidence indicates that the cumulus olivine in ALHA 77005 is derived from a different source than the intercumulus liquid which crystallized the pyroxenes and the plagioclases. Petrographic comparison of all SNC meteorites suggests that most SNC meteorites are orthocumulates in which the cumulus phase comes from a different source than the intercumulus liquid. The interrelation of the different isotopic systems indicates that the SNC meteorites can be explained by mixing three isotopically distinct sources. These three sources were differentiated early in Martian history. In contrast to the Moon, where plagioclase fractionation is the dominant magmatic process, the major magmatic processes on Mars are mafic magmatism and mixing. Comparison of Martian and terrestrial isotopic systematics suggests that Mars accreted from a material chemically similar to Cl and ordinary chondrites, whereas the Earth has lower than chondritic Si/Mg and Rb/Pb ratios. A similar feature is observed when comparing C2, C3 and CV chondrites with Cl or ordinary chondrites: C2, C3, CV meteorites and the Earth have less Rb for their apparent Pb inventory.

morphism. SCHMITT and AHRENS(1983) demonstrated that the temperature distribution at the end of the shock is very heterogeneous, excluding predictions of Sr isotopic homogenization, by diffusion calculations. Rb-Sr isotopic data for shergottites form linear arrays but generally do not yield isochrons. As a result, complicated scenarios have been devised to explain apparent discrepancies between the exposure ages and shock metamorphic ages inferred for SNC meteorites (JONES, 1986). Igneous crystallization ages for SNC meteorites are also under debate. NYQUIST et al. ( 1979) reported a I65 + 12 Ma Rb-Sr shock age for Shergotty and SHIH ef al. (1982) suggested a crystallization age of 1.3 Ga for the shergottites. JAGOUTZ and W~;NKE (1986) suggested that Shergotty was an open system for the Sr and Nd isotopes and reported two age trends: 360 Ma from a pyroxene acid-leachates residue Nd isochron and 167 Ma from a plagioclase mesostasis Sr isochron. They preferred 360 Ma as the age of crystallization. CHEN and WASSERBURG( 1986) reported U-Th-Pb data on acid leachates and residues of whole rock fragments and minerals from Shergotty. The leachate residue age for minerals is about 400 Ma and for the whole rock around 200 Ma. This age is, within error, the same as the Nd ages. However, CHEN and WASSERBURG(1986) suggested that the 200 Ma age was caused by an impact into an ancient regolith on Mars, whereas the 400 Ma age might be an artifact due to laboratory contamination caused by mineral separation handling. PAPANASTASSIOUand WASSERBURG(1974) realized that the Rb-Sr isotopic systematics of Nakhla did not yield an

1. INTRODUCTION

NINE ACHONDRITEmeteorites with chemical, isotopic and petrographic characteristics suggesting an origin from a single parent body have apparent crystallization ages of 1.3 Ga or younger, which is some 3.0 Ga younger than most other meteorites (NYQUIST et al., 1979; BOGARDet al., 1984; SHIH et al., 1982; OTT, 1988). These achondrite meteorites are grouped together as the SNC meteorites (shergottites-nakhlites-chassignites) and are widely believed to be from Mars (see SHERGOTTYCONSORTIUM, 1986). Considerable debate has arisen concerning the conditions required to eject material from Mars (O’KEEFE and AHRENS, 1977; MELOSH, 1984; NYQUIST, 1983; REHFUSS, 1972; O’KEEFE and AHRENS, 1986). However, many of these concerns have been dispelled with the discovery of meteorites of lunar origin on Earth (LUNAR METEORITE CONSORTIUM, 1983). There has also been much debate about the ages of SNC meteorites, specifically with regard to the significance of their exposure, shock and crystallization ages. Exposure ages for the shergottites are given in Table 1. With the exception of EETA 7900 1, all shergottites have exposure ages of 2.5 Ma. Nakhla and Chassigny have exposure ages of 11 Ma. EETA 79001 has a significantly younger exposure age of 0.6 Ma. These exposure ages probably date the ejection of SNC meteorites from Mars. The shock metamorphic ages of SNC meteorites are more difficult to interpret, because there are little or no available data on the extent of resetting of Nd and Sr isotope systems during shock meta2429

2430

E. Jagoutz

isochron. However, GALE et al. (1975), using the Rb-Sr system, and NAKAMURA et al. (1982), using the Sm-Nd and UPb systems, proposed a 1.36 Ma crystallization age for Nakhla, despite the fact that several of their measurements did not plot on the mineral isochron. It is clear from these studies that additional work is needed to understand the significance of differences in exposure, shock and crystallization ages of SNC meteorites. Considerable progress in this discussion would result from the identification of the shock event age. This study was designed to help resolve the age of shock metamorphism in shergottites. A detailed isotopic and petrographic study of ALHA 77005 has accordingly been undertaken. Petrographic descriptions of ALHA 77005 are given elsewhere (MCSWEEN et al., 1979a,b; BERKELEY and KEIL, 1981; SMITH and STEEL, 1984). The most noteworthy feature in this sample is the presence of numerous olivine and plagioclase shock melts.

In fact, ALHA 77005 has suffered the most intense shock metamorphism of all shergottites (43 GPa; LAMBERT, 1985). This same shock event may also have been responsible for maskelynization in Shergotty and Zagami. In this study, I utilize variations in the “Rb/*%r ratios of plagioclase shock melts and quench crystals (plagioclase) contained in them to date the shock event in shergottites.

in a shock melt matrix. Grains of olivine shock melt (OM) were analyzed. A non-magnetic fraction of 22.4 mg consisted of a shock glass (produced by melting of plagioclase) and secondary plagioclase crystallized from this melt. This fraction was subjected to selective handpicking under the binocular microscope using normal reflection illumination. After rejecting all compound intergrowths, a sample of 17.74 mg was obtained. The sample was then examined under crossed nicols, in order to contrast birefringent crystalline plagioclase against isotropic glass. Most of the grains were completely glassy, but some of the grains were partly crystalline. The “best” feldspaxs were sep arated in polarized light and these were then split into two aliquots according to grain-shape. Tlie vesicular and curve-shaped grains are denoted as VB (the “very best” grains (0.25 mg)) and others are denoted SB (0.2 mg) (the “second best” grains). The rest of the nonmagnetic fraction (17.29 mg), consisting of shock melt glasses, was denoted as PM. Under routine inspection in cathodoluminescence, it was realized that a few grains of sample PM also contained whitIockite, so these were leached out with HCl (PM-L plagioclase Ieachate; PM-R plagioclase melt residue). All mineral separates were finally washed using weak HCL followed by several HZ0 rinses, in order to remove all surface contamination introduced by handling. Washing procedures were carried out within the dissolution teflon beaker in order to avoid additional handling. After drying, an isotopic tracer was added. Although the concentration data for Sr and Rb might suffer from weighing errors of these very small samples, the Rb/Sr ratio was not affected since a mixed spike was used. Both the mineral separates and the whole rock sample were analyzed for Sr and Nd isotope compositions. Chemical leaching procedures and analytical procedures have been documented elsewhere (JAGOUTZand WKNKE, 1986; JAGOUTZ,1987; ZINDLERand JACOUTZ, 1987). 3. PETROGRAPHY Petrographically, ALHA 77005 is a plagioclase lherzolite with remnant igneous textures. Important petrographic and mineral compositional features are described below. The major element compositions ofthe individual phases are given in Table 2; these results are similar to those in previous studies (MCSWEEN ez al., 1979b; BERKELEY and I&IL, 198 1; SMITH and STEEL, 1984).

Olivine The olivine in ALHA 77005 is subhedral

2. EXPERIMENTAL

METHOD

A petrographic study was canied out on a single thin section of ALHA 77005. Electron microprobe analyses were obtained at the Research School of Earth Sciences ANU, Canberra and scanning electron microscopy (SEM) was carried out at MPI, Mainz. The Na20 contents of shock melts proved difficult to measure because of volatilization under the electron beam. Even with a broad beam and a low specimen current (about 200 PA), there was still some loss of Na,O. Therefore, reported Na10 concentrations should he considered as a lower limit. The sample ALHA 77005.8 I, weighing 4.52 g, is currently under study by a consortium for trace element and geochronological studies. 614 mg of the whole rock sample was powdered. The remaining material (3.9 1 g) was carefully crushed in order to prepare mineral separates in the grain size fraction of 100-200 pm. The crushing resulted in 0.44 g fine powder, 0.803 g of a 50- IO0 pm fraction and 2.502 g of a 100-200 pm fraction. The sample loss from this prep aration was about 4%. All sample preparation procedures were carried out under clean-room conditions. The 100-200 pm fraction was processed on a modified Franz magnetic separator, using a stepwise-increasing magnetic field. Weight fractions obtained by this separation are shown in Fig. 1; the major peak for the olivine fraction is indicated (KM peak no. 8). Grain mounts revealed two distinct types of olivine: (1) large grains with a (Mg +lOO)/(Fe + Mg) ratio of 7 I .5, and (2) micro-crystalline olivine

colour.

Its chemical

composition

"" ALHA wi t

and brown in is uniform (Ca:Mg:Fe

B/KM

1

2 A

FIG. 1. A modified Franz magnetic separator was used for the mineral separation. The magnetic gradient was modified and adjusted to a closed rail. This diagram illustrates the selectivity of this separator. Weight fractions are plotted ver.yuSmagnetic steps (A = Amp).

2431

Origin and history of SNC meteorite ALHA 77005 'rable: 2

Chemical

camwsition

of major

minerals

in ALHA

77OE

II!-+

5’.Ei

0.2: I.18 0.51 0.91 12.45

‘4.86

5.29

ro.03 54.23 to.03 0.67 CO.01

38.52 0.75 CO.05 5.30 0.09

0.09

3.21 9.88 48.15 3.54

magmatic rocks. Augites range from lo- 15% hedenbergite, which is also low for magmatic augites. More than 10% of Fe in the augites is Fe3+, whereas the pigeonites have less than 10% Fe3+. The rims of the pyroxenes have more ferric iron than the cores. TiOz is higher in interstitial pyroxenes, but Cr203 and Alz03 are higher in poikilitic pyroxenes.

29.63 0.44 CO.05 4.22 0.06

= 0.3:72:27.7; FeO/MnO = 48 + 2; CaO content: rim = 0.14%, center = 0.24%) with slightly lower CaO contents near grain boundaries. According to OSTERTAG et al. (1984), 5% of the total iron in ALHA 77005 olivine is in the ferric state. Some of the grains are pseudorhombic due to a pronounced development of (110) (BERKLEYand KEIL, 198 1). One of the pseudorhombic olivine grains contains inclusions parallel to the grain boundaries (( 110) faces) suggesting that the inclusions are comagmatic with olivine (Fig. 2). These inclusions are comprised of chromite, glass and pyroxene and will be discussed later. Similar inclusions appear in many olivines (SHIH et al., 1982), but in most cases are randomly distributed. Some alteration of olivine can be observed (SMITH and STEEL, 1984). In some areas olivine is replaced by a fine intergrowth of silica and Fe-hydroxides which contain high concentrations of SO3 and P205. This alteration assemblage is occasionally found to be in direct contact with the plagioclase shock melt. However, diffusion or mixing of alteration products into the shock melt is absent. These observations suggest that the alteration is caused by terrestrial weathering in Antarctica, where the meteorite was found (YANAI et al., 1978).

Pyroxenes occur in two textural relationships: interstitial to plagioclase and olivine, or as oikocrysts surrounding olivine (MCSWEEN et al., 1979b; BERKLEYand KEIL, 1981; SMITH and STEEL, 1984). All pyroxenes display undulose extinction due to shock, and all (in the section investigated) are monoclinic. Clear augite is slightly green in colour, whereas the pigeonite is yellow-brown, microfractured and has a higher birefiingence (yellow) than the augite (grey). Augites commonly have an epitaxial overgrowth of pigeonite. Pyroxene compositional variations are shown in Fig. 3 and representative analyses are given in Table 2. Structural formulae indicate that the pyroxenes do not have Al in six-fold coordination, consistent with equilibration with plagioclase at low pressures. The pigeonites are occasionally compositionally zoned, with cores displaying higher CaO contents than rims, whereas their Fe0 contents are essentially constant. These pigeonites, containing 20% of the ferrosilite end member, have the lowest iron content of any pigeonite found in

Whitlockites Whitlockites have a spongy appearance in thin section. CaO and PzOs appear to be present in 3:2 stoichiometric proportions, but compared to whitlockites in the Shergotty meteorite, the ALHA 77005 whitlockites contain considerably less Na and Fe and more Mg (Table 3). The low Na concentration is restricted to the well preserved central parts of the whitlockite. Whitlockite-maskelynite boundaries record diffusion of P and lesser amounts of Ca from the whitlockite into the melt and trace amounts of Na from the melt into the whitlockite (Fig. 4) whereas on the maskelynite-pyroxene boundary there is no detectable phosphorus in the maskelynite.

‘70

*...............*: / *Q 010

FIG. 2. Pseudorhombic olivine from ALHA 77005. BERKLEY and that these grains are formed by pronounced de-

KEIL (198 1) stated

velopment of the (I 10) faces, due to fast growth. The inclusions in the center of the olivine are randomly distributed, whereas on the rims, an alignment of the inclusions along the grain boundaries is observed. This suggests that these inclusions are comagmatic.

2431

E. Jagoutz

Dir-

n A n

6

., n

2

-‘*

LA.GV

/ ,,’

n n

n ,Hd

-Al' ,,‘.

,‘&GV

1

Fo

ETA

-“r;;FTC

FIG. 3. Compilation of published data on pyroxene compositions from SNC meteorites. Shergotty (SH) and MT.4 79001 (Lit. A ETA; Lit. B ETB) shows a two-pyroxene compositional evolution, whereas Nakhla (NA) Goven:ador Valadares (GV) and Lafayette(LA) crystallized from a differentiatedmagma. The chemical composition of the pyroxenes in ALHA 77005 (AL) are close 10 the most primitive composition and show no chemical fractionation trend. AT * nrt the pyroxenes within the olivine inclusions in ALHA 77005 (see also Fig. 6). C‘hromite and ilmenite

The shock melts

Shergotty crystallized at an oxygen fugacity close to the FMQ-buffer, as estimated bq the ulviispinel-hematite solid solution in titanomagnetite and ilmenite (STOLPER and MCSWEEN, 1979; SMITH and HERVIG, 1979). Interstitial ilmenite in ALHA 77005 does not have a hematite component and therefore cannot be used as an oxygen fugacity indicator. However, O’NEIL and WALL ( 1987) have calibrated an oxygen barometer using the reaction olivine -t magnetite = orthopyroxene + oxygen, using the magnetite component in spine1 (chromite) as an oxygen fugacity indicator. In ALHA 77005 chromite occurs either as inclusions in olivine, or together with ilmenite in interstices (MCSWEEIV d al., 1979b, SMITH and STEEL, 1984). Chromite inclusions in olivine have about 62% of the chromite end member, whereas their ulvijspinel and magnetite components (generally around 10%) are slightly variable. The interstitial chromites are zoned, with rims similar in composition to chromitc inclusions in olivine and their cores contain 82% of the chromite end member and only around 2”L ulviispinel and magnetite end members. The magnetite component of the large interstitial chromites from ALHA 77005 varies from less than 2% in the center to 4% on the rims. Furthermore, the coexistence of Fe3+-olivine (as mentioned above) and Fe3+-free ilmenites indicate that ALHA 77005 is not equilibrated m its oxidation state. The relatively low magnetite content in the cores of interstitial chromites and the hematite-free ilmenite indicate crystallization of the ALHA 77005 mesostasis under low oxygen fugacity, possibly only slightly above the IW-buffer, whereas the olivine crystallized in an environment with considerably higher oxygen fugacity. The rimming of the interstitial chromites might be caused by an incomplete re-equilibration of the mesostasis with the olivincs.

The shock melts of ALHA 77005 can he ldenttfied it:, .; brown olivine melt (vitrophyre) and a colourtess plagiociasc melt (MCSWEEN et al., 19?9b; SMITH and STEEL, I984!, compositions are given in Table 4. These melts are sometimes in contact with each other, but aside from a hmited diffusion profile, there is no evidence of mixing between them. 1II SOera1 cases vesicles transect the contact of the two melts, if, dicating that the two liquids co-existed. Plagioclase melt All plagioclase has been completely melted by the sho& event, with many melt pockets having rhe forni of a plaglii

ccc\ p;0,

FIG. 4. The log P205 concentration 1s plotted W.W the dlstanLc from the whit&kite-maskelynite contact. Phosphorus (and possibi) other elements) diffuses into the plagioclase melt (PM). The REE budget of the maskelynite may have been largely contribuled by thr whit&kite. The leachate and residue of the maskelynite separate have identical Nd isotopic composition, whereas their Sm/Nd ratios are vastly different. This can be explained by Sm diffusing faster into the maskelynile than Nd.

2433

Origin and history of SNC meteorite ALHA 77005 *able

4: CoaDosition

of the different

shock

melts

within

ALLHA 77005

_ PM-melt %

OH I-xKl+

center

olivlne

melt+

clase crystal. Quench crystals of plagioclase commonly form on the outer rims of these pockets. Since the NazO content of these melts is uncertain due to volatilization in the electron beam, Ca was plotted against Al in order to estimate the original feldspar composition (Fig. 5). The plagioclase from which this melt formed was probably An 62, whereas secondary plagioclase, which crystallized on the outer rims of these melts is, on average, An 67. The K20 content in the melt core is five times higher in the secondary plagioclase than at the rim; this is compatible with the observed Rb-Sr fractionation in the plagioclase melt, which is used for dating the shock event, as discussed later. As shown in Fig. 5, the plagioclase melt and the quench plagioclase do not plot on the stoichiometric plagioclase line. This might indicate that the SNC plagioclases have non-stoichiometric compositions. Non-stoichiometric plagiociase has also been observed in lunar rocks (WEIL et al., 1970; BEATY and ALBEE, 1980). Maskelynite as well as shock glasses formed from lunar plagioclase accordingly show deviations from the stoichiometric plagioclase composition (VON ENGELHART, pers. commun.)

1.0

Olivine melt The ohvine melt is partially recrystallized, and contains spinifex and skeletal textured olivine crystallites that are compositionally zoned. Some also contain spine1 in a glassy matrix. The olivine melts appear to have been capable of digesting the other minerals in ALHA 77005. Thus pockets of olivine melt form irregular patches, partly invading the rock. These invading veinlets display comparatively higher TiOz and PzOs contents and lower CaO/A1203 ratios than the plagioclase melt.

pure Anorthife CaA12Slt0,, Lunar

FIG. 6. Internal structure of a large inclusion in an ALHA 77005 olivine. Similar inclusions are also shown in Fig. 2. Most of these inclusions contain vesicles. Feathery pyroxene (light grey) are in a glassy matrix (dark grey). The black inclusions within the glassy matrix are nearly 100% SiO*. (The refractive index of the black inclusions indicate that they are possibly coesite.)

plagioclase

Ca

Inclusions in olivine Inclusions in olivine (Fig. 2) range from several microns to more than 100 pm in diameter (Fig. 6) and comprise zoned,

0.2Albite Na Al Si308

- 2.0

1.8

1.6

1.k

1.2

1.0

Al [Atoms per 8 oxygensl FIG. 5. The Ca atoms versus the Al atoms are plotted for ALHA 77005 maskelynite and lunar plagioclase. (The structural formula for plagioclase was used for the analysis of maskelynite.) Most terrestrial and meteoritic plagioclase plot on the theoretical line. Nonstochiometric plagioclases have also been observed in lunar rocks.

.._. cao

TiOz

22.73 2.82

21.26 3.18

7.42

0.23

10.63

0.91

2434

Rk

WR

I’s!?.

WR WR

L T

a6.31

PM PM PM

R L T

I?.'?

241.6 27 4 269 3572 130 1,822

0.71'7 0.066 0.783 7 OGi G.0715 7.0726

Cm

la.bt

Pig Aug

19.1" 16.84

1528 48C

I?.." 0.1713

0.25 0.2

4500 J765

I.034 I.533

VB SE

3.4767

quench chnopyroxenes and occasionally chromites in a glassy matrix. However, the chromites discussed above also frequently appear as isolated inclusions in olivine. The bulk chemical composition of a silicate inclusion, together with analyses of its pyroxenes and glass matrix, are given in Table 5. The bulk analysis was obtained by rastering the electron beam over the entire inclusion using an energy dispersive system on the microprobe. The high Si02 contained in these inclusions is not expected to be in equilibrium with olivine, whereas the quench pyroxenes seem to represent the reaction products formed by incomplete re-equilibration. Experiments on the coexistence of olivine, pyroxene, and quartz are reported by NAFZINGER and MUAN (1967). Normally, only fayalitic ohvines can coexist with silica-rich liquids. SiOzrich phases are reported in all SNC meteorites. SiOZ coexisting with olivine has been reported in Govemador Valadares and Nakhla (BERKLEYet al., 1980). Those meteorites have a Farich olivine (Fa = 66-78) and the pyroxenes plot in the “forbidden zone” (LINDSLEY, 1983). The coexistence of olivine and SiOz may reflect the breakdown of ferrosilite to olivinc and quartz as observed in Shergotty (which does not contain primary olivine). However, ALHA 77005 and EETA 7900 I ~ which have Fo-rich olivines, can only metastably coexist with an SiOz-rich liquid. Rapidly crystallizing and overabundant olivine may coexist with SiO,-rich residual liquids, even in systems with low ferrous iron contents (RINGWOOD, 1966). Such a metastable coexistence has been frequently reported in meteorites as well as in terrestrial komatiites, On the other hand, the inclusion (Fig. 6) has a molar Fe/

TABLE:6b ____._.____~_ ALHA

77005

Sr

and

Wd_~lsotopl.c_~~~~~it~=on

i

./i

!. i

08L

14.042

/ .I526

2.341 16.389

0

5.0027 3827

Nd

99.'35 J.260 lO2.61

9668 I.119

,<:

. 526” i .-

-.i986 ; 2061 f'4046

2,_! :

(_ 6355

j,

r.0302 ‘.0254

.**a

./ 1,

.‘. ;-

Mg ratio of 1.3. which is close to that predtcted tot- a melt j:, equilibrium with the olivine ( I .47) and p?roxene i i .Zii t~~ ALHA 77005 (for these calculations a k’: of 0.3 for oiivrnc’ (ROEDDERand EMSLIE, 1970) and 0.2 for pyroxene (C~ovt. and BENCE. 1977) were assumed). The iron content IS U:G low to be a simple residual liquid remaining after olivine and pyroxene crystallization, which in turn is hard to expkuu Microprobe analyses of these glasses have conststently lov, totals (~95%) which suggests that they contam volatile specter such as HZ0 or COZ. This IS also indicated by rhe freqtien; occurrence of vesicles in these inclusrons. Small dark patches in the glassy matnx iF~g. hj are dom inantly SiOz, with up to 10% A&O, as analyzed. Howeve: some or all of this Alz03 content may be due to beam ovcr~aya onto neighbouring regions containing alummosilicate glas\ Under the optical microscope these SiO&ch patches arc \uil, weakly birefringent and have a higher refractive Index thaj the surrounding glass. The aluminosilicate matrix glass wouit: be expected to have a refractive index between 1.iZ and 1 i-l whereas anisotropic shocked quartz has a refractt\c in&~ between 1.46 and 1.48 (OSTER’TAG. 1983: 5mtmi_~R t’l ,,? 1986). The higher refractive mdex of the sihca-rich inciusmrj:I as compared to the matrix suggests that the. may bc coes;:c possibly crystallized during shock. 4. TRACE ELEMEN’I’ CEOCHEMIS’TR~ Trace element data for restdues and trachates horn UP: whole rock and minerals are given in Table 6a. Total whole: rock data, WR-T. were obtained by summmg data i‘or I& residue (WR Res) and leachate (WR I.;. Measured abundances of Sm, Nd. and Sr in ALHA 77005 arc 35-40$ hi&c! than those reported by SHIH et ui (19821 This is attributed to a higher modal abundance of olivine m their particul:~i sample. The 2 N HCl whole rock leachatc contains the bufi, of the REE, i.e., 86% Nd and 83% Sm. but only 14%.of rh total Sr. This probably reflects dissolution of a whitiochir4. phase, which is readily leached. In contrast, in Shergotty mo: :’ than 90% of the whole rock Nd and Sm appear to ha\,<: hce9? contained in whitlockite. The Rb/Sr and Sr/Nd ratios of ALHA 7?005 lcachates art comparable with those found in Shergotty (JAG~UTZ anr.i W;SNKE, 1986) but the Sm/Nd ratio in this leachate is 50’:; higher than that for Shergotty. Additionally, the ALHA 7?(K)?

2435

Origin and history of SNC meteorite ALHA 77005

residue has a 30% greater Sm/Nd ratio than the leachate, whereas in the Shergotty residue the Sm/Nd ratio is 64% greater than its leachate (Table 6). It seems Sm diffuses more readily than Nd from the whitlockite into the shock melt. This will be discussed later in more detail. Whole rock (WR-L) and plagioclase shock melt (PM-L) leachates have, within error, the same Nd and Sr isotopic composition, reflecting the whitlockite component. However, a difference of 27% in the Rb/Sr ratio and about 2% in the Sm/Nd ratio between these two leachates is observed. A Sr/ Nd ratio of 2.4 in the WR-L is similar to that in the whitlockite. The WR Sr/Nd ratio may reflect the plagioclase to whitlockite ratio, given that Nd is concentrated in wbitlockite and Sr is concentrated in plagioclase. The OM has a Sr/Nd ratio of 2 1, which is higher than the whole rock Sr/Nd ratio of 15. 5, AGE RELATIONS Sr and Nd isotopic data are shown in Figs. 7 and 8, respectively. The data do not form simple isochrons. Like the other SNC meteorites, the isotopic systematics of ALHA

87Sr *% 0.7130

77005 have been affected by multiple processes. Therefore, it is necessary to prepare petrographically distinct mineral separates in order to understand the isotopic systematics (JAGOUTZ and WIINKE, 1986). Age of the shock event The microprobe data demonstrated that the olivine and the plagioclase shock melts experienced limited chemical exchange across their contacts. This suggests that then there is no isotopic resetting between these phases during the shock event, and thus these phases cannot yield age information concerning this event. However, given a homogeneous source for the Rb-Sr and/or Sm-Nd isotope systems prior to the shock event, chemical fractionation within these phases may yield an isochronous relationship. There is no fractionation of Rb-Sr or Sm-Nd in the olivine shock melt (because crystallization of olivine does not change these ratios) and therefore the age of the shock event is not recorded by it. In contrast, the plagioclase shock melt shows some fractionation of the Rb/Sr ratio caused by crystallization of secondary pla-

ALHA 77005 Isochron I,= 0.71042+2

151,?6my

shock age 0 5 30my

/

/

,

I

_

I

,I

0.7120 0.7108

0.7106

0.2 0

this Study 0 Shih et al.(1962)

1

I

2.0 87Rb *% FIG. 7. The Rb-Sr isochron diagram shows results for various mineral separates from ALHA 77005. A crystallization age of 154 + 6 Ma is given by the pyroxenes, which are nearly unaffected by the shock event. Additionally, these pyroxenes have textures and chemical compositions indicative of crystaUixation from a magma. The age of the shock event ( 15 + 15 Ma) is given by the Rb-Sr fractionation in the plagioclase melt. An explanation of the various mineral separates is given in the text.

E. Jagoutz

2436

ALHA 77005’

0.51335

0.51330 1

1.=51306?10

I

0.35

,

0.60

~

1

O.&S

Of~O~~~ in this study. The Sm-Nd results are con&tent with the mu&s obtained from the RbSr systematics. An explanation of the various mineral separates is given in ~.8.S~-~~~~~~~

separates from ALHA

77005 measured

the text.

gioclase (see also petrogmphic section and Experimental Method). Major element compositions of the plagioclase melts (Table 4) document a compositional gradient, especially in the I&O, from core to rim. This compositional gradient is also found in “Rb/%r in different fractions of the plagioclase melt (see Experimental Method). Figure 7 (insert) details the “Rb/*6sr versus g7Sr/84Srvariation in these PM fractions. A regression of these data yields an 15 + 15 Ma age for the plagioclase melt, indicating that Rb-Sr fractionation, hence the shock melting event, was recent. This age is also consistent with the exposure age (see Table 1) but is at odds with an earlier proposed shock age of 187 + 12 using the Rb-Sr isotope system (SHIH et al., 1982). There is aho a fractionation of the Sm/Nd ratio in the PM relative to the PM-leachate. This fractionation might be caused by diffision of REE from the whitlockite into the PM melt as already discussed in the petrographic section. A Sm-Nd age calculated from the plagio&se melt residue and leachate is t25 Ma (Fig. 6a), which is also consistent with the young age of the shock event. The concordance of exposure and shock ages in this SNC meteorite may indicate that other proposed shock ages require reevaluation. The c~st~iization

age of shergottites

For ALHA 77005 an internal Rb-Sr isochron age of 154 +, 6 Ma is given by two pyroxenq total whole rock and olivine melt (Fig 5b). This age agrees with the Rb-Sr age for maskelynite and mesostasis in Shergotty ( 167 Ma; JAGOUTZ and WXNKE, 1986). The plagioclase melt and its various fractions do not lie on this isochron, probbly due to fractionation of the Rb/Sr ratio by the shock melt, as discussed above. However, the isotopic composition of the plagioclase, prior to the shock, might be on the intersection of the shock isochron and the 154 Ma isochron. Actually, one of the “non-magnetic fractions” from SHIH et al. (1982) has the Rb/Sr ratio of this intersection (Fts 7). Furthermore, the leachates (PM-L, WRL) plot above the isochron. This illustrates the major

problem faced in isotopic dating of Shqotty or ALHA 77005. There are two phases with low Rb/Sr ratio, namely whitlockite and maskelynite; however, tktWOphWSbW cli@xmt Sr isotopic compositions. This cannot be interpmed as an “age” because there is not su5cient Rb in either phase to explain their isotopic difference. In the case of Shergotty the whole rock leachates may rep resent whitlocki~ and am not on the Sr isochron. Moreover, they are less radiogenie than the 4 initial value determined from the regmssion of the 167 Ma isochron. JAGOUTZ and W.&NICE (1986) suggested that plagioclase and, to a lesser extent, whitlockite exchanged their Sr with an external reservoir. A crystallization age of 167 Ma for Shergotty must be advocated assuming that the Sr in the maskelynite is not a&ted by an external exchange and the phosphateswere chemically an open system. in the case of ALHA 77005 the whitlockite (LUNDBERGet al., 1987) has possibly been affected by terrestrial weathering in the Antarctic, based on its spongy appearance in thin sefztions (see above). The disequilibrium of Sr isotopes between whitlockite and maskelynite must be kept in mind when looking at U-ThPb systematics. U and Th reside nearly entirely in whitlockite, whereas the host phase for Pb is maskelynite. Since whitlockite and maskelynite am not in equilibrium, as shown by their Sr isotopes, the “age” obtained by the U-Th-Pb systematics must be carefully analyzed. The divine inclusions and their relation to isotopes The olivine melt (ON) plots on the Rb-Sr isochron, but, as neither Rb nor Sr are contained in olivine, both elements must be derived from other sources. As d&ussed in the petrographic section, it is likely that pm-existing inclusions within olivine are dissolved in the olivine melt. However, the trace elements of the olivine melt might be dominated by other minerals dissolved in it, placing the olivine melt close to the whole rock on the isochron (Fig. 7). SHIH et al. ( 1982) reported RbSr measurements on olivine separates from ALHA 77005 which, according to their interpretation, are dominated by the Rb-Sr systematics of the olivine inclusions (since there is neither Sr nor Rb in clean olivine). These measurements fall off the pyroxene isochron obtained in this study (Fig 7). Since NYQUIST(1983) and SHIH ef al. ( 1982) have argued that the “ 180 Ma” age might represent the age of the shock event, the inchrsions in ALHA 77005 are thought to represent migratory shock melts which were injected during the shock event (KIEPP~R, 1974). They, therefore, implied that these melt inclusions are xenolithic to the olivine. However, several observations contradict this interpretation: 1) The inclusions are exclusively found in ohvine. ff they were produced by jetting, they should be randomly distributed in all minerals. 2) The inclusions are round and not associated with fractunes in the olivine. These fractures transect the inclusions and are not infilled by them, unlike the situation normally observed in olivine shock melts. 3) Alignment of inclusions along growth plains were found (Fig. 2).

Origin and history of SNC meteorite ALHA 77005 It is accordingly proposed that these inclusions are not xenolithic material introduced during a shock metamorphic event, but that they were formed in situ. The olivines in ALHA 77005, however, are strongly affected by terrestrial alteration (DREIBUSand W~~NKE,1983; SMITH and STEELE, 1984; this work). It is difficult to obtain an olivine separate which is not affected by alteration and which contains sufficient inclusions to be analyzed. Nd isotopic systematics Nd isotopes measured in ALHA 77005 are shown in Fig. 8. ‘Excluding the residue of the plagioclase melt (for reasons discussed above) the measured isochron gives an “age’” of 135 -+ 40 Ma which coincides, within error, with the age derived from the Sr isotopes. The Nd isotopes are +22~ units more radiogenic in ALHA 77005 than in Shergotty. ALHA 77005 forms a “two-point isochron” with Shergotty and Zagami giving an age of 1.3 Ga, and there is a rumour (WOODEN et al., 1982) that EETA 79001 is also on this isochron. Since this age coincides with the internal age observed in Nakhla, SHIH et al. (1982) interpreted this as the age of crystallization, but I think this might be a coincidence. Furthermore, the whole-rock Sr and Pb isotopes contradict the assumption of a common source for the shergottites at 1.3 Ga. This will be discussed later.

6. MAGMATlSM

ON MARS

The moment of inertia of Mars indicates that the Martian mantle is of higher density than the terrestrial mantle. This requires a higher iron content in the Martian mantle. These two constraints combined with the observations on SNC meteorites makes it a challenge to model Martian magma evolution.

Mineralogy of the Martian mantle

A comparison of the compositional evolution of SNC pyroxenes is given in Fig. 3. Although these meteorites may come from different localities on Mars, there is a consistent mineralogical trend observed, suggesting crystallization through similar magmatic processes (MCSWEEN, 1985). Shergotty and EETA 79001 show a crystallization trend in pyroxene compositions, whereas ALHA 77005 and the nakhlites show no evolution in Fe content during crystallization. ALHA 77005 is the most primitive cumulate of all shergottites, whereas the nakhlites are cumulates from more evolved magmas. Generally, SNC meteorites show a two-pyroxene evolution. My impression is that after olivine and chromite crystallized, the next phase is augite (MCSWEEN, 1985). In Shergotty and ALHA 77005 pigeonite forms an epitaxial overgrowth on iron poor a&es. The first pigeonites to crystallize in SNC meteorites have 20% FeO, which is lower than low Fe pigeonite found in some terrestrial komatiite pigeonite cores. The most magnesian olivines with a Mg/Mg + Fe ratio of 0.75 are found in EETA 79001, which must be close to the Martian mantle Mg/Mg + Fe ratio. The model of Martian mantle composition based on element correlations in SNC meteorites also reveals a Martian mantle with a Mg/Mg

2437

+ Fe ratio of 0.76, which is in excellent agreement (DREIBUS et al., 1982). This ratio is consistent with the moment of inertia and the Martian mass (~GW~D and CLARK, 197 11, revealing an independent indication for the Martian origin of the SNC meteorites. In contrast to the Earth’s mantle, the Mg/Si ratio on Mars is nearly chondritic. This would indicate that the Martian mantle is close to enstatite normative. The density requirement, however, calls for so much iron in the Martian mantle that the olivine/pyroxene ratio in the Martian mantle might be. close to that of the terrestrial mantle. As mentioned by JAGOUTZ and WKNKE ( 1986) the higher abundance of volatiles (like K, Na, P and others) in the Martian mantle would possibly lead to stable mantle phases like sanidine and whitlock&e on Mars. Crystal accumulates

All SNC meteorites clearly show the accumulation of certain minerals. A cumulate rock consists of cumulus phases (for SNC meteorites olivine or pyroxene) and the intercumulus phase (or trapped liquid) which also might sofidify as olivine or pyroxene. The chemical composition of the cu-. muius and intercumulus phases may be recognized by whole rock inhomogeneities within the cumulate rock, In Shergotty or in ALHA 77005 (see trace element geochemistry section), for example, the incompatible elements as a group are different in different whole-rock samples, whereas the Sr/Nd ratio is, within error, the same. This can be understood if St and Nd are both in the intercumulus liquid and the cumulus phase is only pyroxene with no significant contribution to the inventory of Sr or Nd, respectively. The differences in the incompatible elements are caused by different abundances of the cumulus phase in the particular whole rock split. Plagioclase did not occur as a cumulus phase but crystallized from the intercumulus liquid. This is also evident from the texture. Furthermore there is no significant Eu anomaly observed in SNC whole-rock analyses. This also strongly argues against plagioclase being a cumulus phase in SNC meteorites. The lack of an Eu anomaly is not caused by a high oxygen fugacity; a strong positive Eu anomaly in the analysis of maskelynite gives an indication of Eu+’ existing on Mars. This contrasts with lunar samples, in which the Eu anomalies are a rule and plagioclase is the major mineral fractionating during lunar differentiation. In terrestrial samples, however, plagioclase fractionation occurs as an exception, mainly if a magma fractionates at crustal levels. The ultimate cause for crystal accumulation is a density contrast between magmatic liquid and crystal phases. PETERA (1987) reported experiments where olivine floats in melts of assumed Martian mantle compositions. They suggest that these melts, especially if they are generated below the spine1 stability field, would have a high density and probably not ascend to the surface. Olivine and pyroxene would goat and result in a mafic cap in the Martian mantle strata. As a consequence Ca and Al, the feldspar elements, would be concentrated in a lower section in the Martian mantle and therefore shallow-level plagioclase fractionation would not be a major magmatic process on Mars. However, these experiments might be a first step, rather than a final answer to this problem.

2438

E Jagoutz

There is considerable debate on the compressibility ot mantle melts, and whether at increasing pressure melts become denser than olivine under terrestrial conditions. Experiments show that Fe-rich melts float Fe-poor olivine. The compressibility of a melt depends on the chemical composition. Primary melts under Martian mantle conditions are supposed to be considerably more Fe-rich than terrestrial analogues. However, since the presence of Fe is thought to reduce the compressibility of a melt, there is less probability that olivine will float in Martian primary mantle melts than in terrestrial mantle melts. D’nequilibrated mineral assemh1age.s SNCs are orthocumulates In orthocumulates the cumulus phases and the mtercumulus liquid come from different magmas. There is petrographic evidence for SNCs being orthocumulates. Mineral disequilibrium is observed in nakhlites and in EETA 7900 1 and ALHA 77005, whereas in EETA 79001 olivine and orthopyroxene xenocrysts are reported (MCSWEEN and JAROSEWICH, 1983). BERKLEY et al. (1980) and TREIMANN ( 1986) reported disequilibrium in nakhlites between olivine and pyroxene. In ALHA 77005 the oxidation states of cumulus and intercumulus phases are different (see Section 3) Along with this petrographic evidence, there is also isotopic evidence that SNC meteorites are mixtures of isotopically different components. From these observations it is evident that mixing must be the dominant process responsible for the chemistry and isotopic inventory of SNC meteorites. 4ge relation qf cumulus and intcrcumulu~ pha.~The isotopic signature of the intercumulus phase in shcr.. gottites reveals an age of 150- I60 Ma. The initial ratios for Sr and Pb are vastly different for the different shergottites. indicating that the intercumulus liquids are not coming from the same source. In order to produce a mixing line for each meteorite, yielding these similar ages, it would be necessary to call for a very specific mixing partner for each meteorite. which is very unlikely. Therefore. the 160 Ma age might represent the age of the intercumulus phase. Cumulus phases. which are in petrographic disequilibrium with the intercumums phase (see above), consist of olivine. chromite. augite. and pigeonite. These cumulus phases may he significantly older than the intercumulus liquid. Suggestion of an older age for the cumulus phases could be gained from the 40Ar-39Ar degassing pattern. The hightemperature fraction possibly indicates old components within the shergottites (SHIH er al.. 1983). Nakhlites do not have an old argon component (PODOSEK. 1973: BOGARD and HUSEIN, 1977). In Shergotty the intercumulus phase consists ot‘augite, pigeonite and possibly whitlockite with a crystallization age of 350 Ma (JAGOUTZ and WANKE. 1986). whereas mesostasis and maskelynite formed by the intercumulus phase give an age of 167 Ma. In the case of ALHA 77005, the intercumulus phase age ( 154 + 6 Ma) might coincide with the pyroxene isochron, since the intercumulus phase preferentially forms oikocrysts and interstitial phases. This is consistent with the observed

pyroxene texture. The olivme might represent the c~m~uiu~ phase. The compositional differences of the chromites mai reflect a higher oxygen fugacity for the cumulus phase tha;; for the intercumulus phase. whereas the rim composition <,;t the intercumulus chromite may reflect partial re-equilibratiori during the shock event. The 3YAr-404r release pattern c$r ALHA 77005 contains an appreciable amount of excess ‘““A.: (SHIH et al., 1982) suggesting an ancient component. l‘he~c. observations indicate that ALHA 77005 i< also an orthltcb mulate and that an age difference possibly exists betweeri cumulus and intercumulus phases. However. it is difficut: “I~. understand how these orthocumulate textures arc fonne~i petrographically. 7. MIXING MODEL Although 1 am well aware of the rather limited number t.:; SNC meteorites and their chemical complexity. I will attempr to evaluate a simple isotopic mixing relation to elucidate ?hts chemical evolution on Mars. This discussion will be restricted to the isotopic i.ornpri sition of the intercumulus phase. There 1s no cumulus plagioclase, since there are no whole rock Eu anomalies observer; in SNC meteorites caused by cumulus plagioclase (1.n~ I al.. 1986). The plagioclase. on the other hand. is the bus: phase for Sr and Pb and results in low Rb/Sr and LJ!Pb ratit)\ This is the reason that the age correction rbr the measureil Pb and Sr isotopes in plagioclase is small. compared to the isotopic variations of different SNC meteorites. in order :* evaluate mixing relations, the isotopic nntial ratios must bc used. because the initial ratio represents the isotopic ~unr. position at the time of emplacement. The age of the :RICI cumulus phase in shergottites might be 160 Ma, whereas thi intercumulus phase in Nakhla might be ~3 old as 1 1 Cisi Therefore, the initial ratio of Nakhla has 10 he age_corrertrLi in order to be compared with the shergottites .A single-stag< model-age evolution was applied to extrapolate the Nakhia source to match the shergottite ages. A plot of the best estimates for initial “i”Pb/:‘“Pb and ‘,?,I 86Sr ratios is shown in Fig. 9. Mixing line\ m this diagrarr, depend on the Sr/Pb ratio in the reservoirs. %mce plagioclasc may be present, and Sr as well as Pb IS mainly hosted r:, plagioclase, it is assumed that the Sr/Pb ratio IS similar IIJ ;lii reservoirs and the mixing lines can be assumed to he straigh! lines.

The range of isotope compositions observed III the mi SNC meteorites can be explained by the mtumg ofthree :ei ervoirs. The need for these three reservoirs can also be WC:‘: in a 20’Pb/204Pb versus 206Pbi2”Pb diagram. as well as 1i-1., Sr versus Nd isotope diagram. Multidimensional factor anai ysis indicates that the data form a two dimensional array (i I: a plane), similar to the case of terrestrial oceanic basalts (ZIY+ DLER et al., 1982), and demonstrate that the mixing of threereservoirs (A, B and C, in Figs. 9 and 10) can expiarn the total range of isotopic variation. The comparison of :h;. terrestrial and the Martian isotopic evolutrun is shown 5~’ Fig. IO. From the Pb isotope data, but also rndicated by the S{ isotope data, it is clear that these three reservoirs on Mar> were isolated for more than 4 Ga, possibly during an eari: differentiation (e.g., accretion, core-mantle differemiatuul

Origin and history of SNC meteorite ALHA 77005

Uokhlo

0.70,;

1 12

I

13

16

FIG. 9. The best estimates for initial =Pb/-Pb

I

15

and “Sr/%

16

ratios

are shown. The range of isotope compositions observed in the five SNC meteorites can be explained by the mixing of three reservoirs. (A, B and C). Mixing lines can be assumed to be straight in this diagram. Since plagioclase may be present, and Sr as well as Pb is mainly hosted in plagioclase, it is assumed that the Sr/Pb ratio is similar in the three reservoirs.

of Mars. On Earth, however, the three reservoirs could originate from a common reservoir at much later time, perhaps between 1 and 2 Ga. From Fig. 10 it is apparent that the isotopic variation on Mars is larger than in the terrestrial mantle.

2439

Other important differences between Martian and terrestrial isotopic systematics are noticeable when compared to the “meteoritic trend”. As shown in Fig. 10 the “meteoritic trend” is defined by Cl and C2-3, as well as a two-point average of H-chondrites. The two dashed lines indicate different cosmic fractionation trends, with both the C 1 and C23 trend linked at an achondritic point having a very radiogenic Pb and unradiogenic Sr isotopic composition. The ordinary chondrites generally scatter around the cosmic f~ctionation line starting from Cl chondrites, as does the bulk Mars composition, which is close to the reservoir C. The “bulk Earth” (BE), in contrast, plots close to the analogous evolutionary line which originates from the C2-3 chondrite end members. This observation suggests that the Earth accreted from material with a volatile element pattern similarto C2-3 chond&es, whereas Mars might have accreted from material with a volatile element abundance pattern similar to ordinary chondrites. Assuming that U/Sr ratio and Sm/Nd ratio in both planets are chondritic their pronounced isotopic difference must be caused by different Rb/Pb ratios in the accreting materials (JAGOUTZ, 1986). This difference is also apparent between Cl and C2-3 chondrites. It is noteworthy in this context that the Si/Mg ratio of the Earth (JAGOUTZ ef al., 1979) and in C2-3 chondrites are also lower than in Cl and ordinary chondrites. The isotopic systematics of Mars and Earth suggest that Mars accreted from ordinary chondrites, whereas the Earth may have accreted from material which

FIG. 10. This is a comparison of the terrestrial and the Martian isotopic evolution in a thr~-dimen~onal diagram. It is apparent that the isotopic variation in the Martian mantle is larger than in the terrestrial mantle. The two dashed lines indicate different cosmic fractionation trends, with both the Cl and C2-3 trend linked at an achondritic point having a very radiogenic Pb and unradiogenic Sr isotopic composition. The bulk Mars composition, which is close to the reservoir C, might be similar to the chondritic trend. The “bulk Earth” (BE), in contrast, plots close to the analogous evolutionary line which originates from the C2-3 chondrite end members. Since the U/Sr and Sm/Nd ratios in both planets are chondritic, their pronounced isotopic difference must be caused by different Rb/Pb ratios in the accreting materials (JAGOUTZ, 1986). The isotopic systematics of Mars and Barth suggest that Mars accreted from ordinary chondrites, whereas the Barth may have accreted from material which falls along a cosmochemical fractionation trend from Cl to C2, C3, CV chondrites to the Earth (JAGOUTZet al., 1979).

2440

E. Jagoutz

falls along a cosmochemical

fractionation trend from Cl to C2. C3, CV chondrites to the Earth (JAGOUTZ et al.. 1979) 8. CONCLUSION The new data reported in this study demonstrate that the shock age of ALHA 77005 is less than 20 Ma. The precision of such a young age in the Sm-Nd or Rb-Sr system is relatively poor and therefore it is plausible that the shock age coincides with the exposure age. The coincidence of these ages removes the necessity for the scenario advocated by BOGARD et al. ( 1984) and others, in which the shergottites were ejected from Mars 180 Ma ago as a single fragment with a diameter of >6 m which broke? up in space at 2.5 Ma. On the other hand. it renews the question of the nature of the 160 Ma isotopic signature observed in shergottites. It is proposed that the 160 Ma is the crystallization age of the intercumulus phase, whereas the cumulus phases (or parts of the cumulus phases) might be older. The early differentiation of Mars was possibly dominated by fractionation of m&c minerals like olivine and pyroxene. In contrast, the lunar differentiation, which might coincide with the magma ocean, involved dominantly plagiociase fractionation forming the lunar crust. Moreover, DELANO (1980) and RINGWOOD er al. (1987) have recognized ultramafic magmatism on the Moon as well, caused by a high degree of partial melting of the lunar interior. This style of differentiation might be similar to terrestrial komatiites and the proposed differentiation of Mars. On Mars, fractionation of olivine, augite, and pigeonite was the dominant magmatic process. A regolith (CHEN and WASSERBURG, 1986) of these mafic minerals was produced either by impact or by gas explosions. Late, possibly more acidic magmatism incorporated these regolith minerals in extrusive flows which were cooled rapidly at the surface. The mantle sources of these late magmas were affected by the primary differentiation. The existence of this old mantle reservoir on Mars calls for a special tectonic environment to prevent the remixing by convection. ~4cknowledgements-The thinking for this paper was done during my stay at ANU in Canberra. Without the help of W. F. McDonougb, S. Kesson and A. E. Ringwood this publication would not exist. The constructive and critical atmosphere in the Research School of Earth Science is an experience I gratefully appreciate. Editorial handling: H. Y. McSween. Jr

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and origin of Govemador Valadares and other nakhlites. fioc. Lunar Planet. Sci. Conf 11th. 1089-I 102. B~GARDD. D. and HUSEINL. ( 1977) A new 1.03aeon young achond&e. Geophys. Res. L&t. 4, 69-7 1. BOGARDD. D., NYQUISTL. E. and JOHNSONP. (1984) Noble gas contents of shergottites and implications for the Martian origin of SNC meteorites. Geochim. Cosmochim. Acra 48, 1723- 1739. CHEN J. H. and WASSERBURGG. J. f 1986) Formation ages and eve

lution of Shergotty and its parent planet from L:-Th-Pb systemam:. Geochim. Cosmochim. Acta 50,955-968. DELANOJ. (1980) Chemistry and liquidus phase relations of Apotlo 15 red glass: Implications for the deep lunar Interior. Lunar 5i Cant XI 1,25 l-288. DREIBUSG., PALMEH., RAMMENSEE W., SPETTELB..WECKWER1’11 G. and WXNKEH. (1982) Composition of Shergotty parent body Further evidence of two component model of planet formation (abstr.). Lunar Sci. Conf XIII, 186-187. DREIBUSG. and WKNKEH. (1983) Halogenes rn Antarcnc meteontei Meteoritics 18, 29 1. GALEN. H., ARDENJ. W. and HUTCHWN R. ( I Y75)The chronolog> of Nakhla achondritic meteorite. Earth Planet Tci Let! 26, ! 9% 206. GROVET. L. and BENCEA. E. ( 1977) Experimental study of pyroxenr liquid interaction in quartz-normative basalt i 5597. Proc. &no: Sci. ConJ 8th. 1549- 1579. JAGOUTZE. (1986) Sm-Nd and Rb-Sr isotopic systemancs ot tire SNC meteorite ALHA 77005. (abstr.). Lunar Sci ConJ XVII. 3X4385. JAGOIJTZE. (1987) Nd and Sr isotopic systemattlcs m an eciogm xenolith from Tanzania: Evidence for frozen mineral equilibria in the continental lithosphere. Geochim. Cosmochim. Acfa 52. 126 1293. JAGOUTZE. and WKNKE H. ( 1986) Sr and Nd tsotopic systemaucs of Shergotty meteorite. Geochim. Cosmochim. Acta 50,939~953 JAGOUTZE., PALMEH., BADDENHAUSEN H., BLUMH.. CENDALL~ M., DREIBUSG., SPE~I-ELB., LORENZV. and WAENKEH. (1979’ The abundance of major, minor and trace elements in the Earth‘.5 mantle as derived from primitive ultramafic nodules. Priic. Lunu:Planet. Sci. Co@. IO, 203 l-2050. JONESJ. H. (1986) A d&u&on of isotopic systematlcs and mmera! zoning in the shergottites: Evidence to 180 m.y. igneous crystai lization age. Geochim. Cosmochim. Acta 50.969-977 KIEFFERS. W. (1974) From regolith to rock bv shock. T!w Mow 13,301-320.

LAMBERTP. (1985) Metamorphic record m shergotites. .I1~lc’orif.: 24690-69 I. LINDSLEYD. H. (1983) Pyroxene thetmometq ?rnc~r .kfmrrui 6X. 477-493. LUNARMETEORITECONSORTIUMf1983) Geophr:: /
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