Petrology and origin of the shergottite meteorites

Petrology and origin of the shergottite meteorites EDWARD STOLPER Department of Geological Sciences, Harvard University, Cambridge, MA 02138, U.S.A. and HARRY Y. MCSWEEN, JR Department of Geological Sciences, University of Tennessee, Knoxville, TN 37916, U.S.A. (Rtceived 16 August 1978; accepted in

revised form

2 May

1979)

Abstract-Shergottites contain cumulus pigeon&e and augite, probably without cumulus plagioclase, and crystallized under relatively oxidizing conditions. Shergotty and Zagami may differ in the relative proportions of cumulus pyroxenes and crystallized intercumulus liquid, but the compositions of pyroxenes and liquid are similar in both meteorites. Absence of olivine in melting experiments suggests that the shergottites crystallized from fractionated derivatives of primary liquids. Low-Ca pyroxene and augite apparently began to crystallize from these primary liquids prior to ptagioclase. Shergottites can be readily distinguished from other achondrite groups by their mineralogies, ~ystaliization sequences, and inferred source region com~sitions. However, the source regions of the shergottites may be related to those of other achondrite types by addition or loss of volatile components. The bulk composition of the Earth’s upper mantle overlaps that of permissible shergottite source regions. Shergottites and terrestrial basal% display similarities in oxidation state and concentrations of trace and minor elements with a wide range of cosmochemical and geochemical affinities. Accretion of similar materials to produce the terrestrial upper mantle and the shergottite parent body or accretion of the Earth’s upper mantle from planetesimals similar to the shergottite parent body may account for many of their similarities. Models of the origin of the Earth’s upper mantle which attribute its oxidation state, its siderophile element abundances, and its volatile element abundances to uniquely terrestrial processes or conditions, or to factors unique to the origin and differentiation of large bodies, are unattractive in light of the similarities between shergottites and terrestrial basalts.

I. INTRODUCTfON THE SHERGOTT~TES are achondritic meteorites with textures, mineralogies and chemical compositions similar to terrestrial diabases. Initially dist~n~is~ed from the eucrites and other basaltic achondrites by their maskelynite (TSCHERMAK,1872), recent study has demonstrated that they are mineralogically and compositionally distinct from all other known meteorites (BINN$ 1967; DUKE, 1968; LA~JL et al., 1972; MCCARTNEYet al., 1974; MCSWEEN and STOLPER, 1978; SMITH and HERWG, 1979). In this paper we report the results of pe~ographic studies and 1 atmosphere melting experiments on the two known shergottites, Shergotty and Zagami, both of which are falls. The implications of these results for the origin and evolution of the shergottites and their parent body, and the relationship of the shergottite parent body to the parent bodies of the other achondrites and to the Earth are also examined. II. ANALYTICAL AND EXPERIMENTAL TECHNIQUES Modal cal point Relative (pigeonite

analyses of shergottites were performed by opticounts on four thin sections, listed in Table 1. proportions of coexisting clinopyroxanes and augite) in two sections were determined

from three-element (Ca, Mg, Fe} microprobe analysis surveys taken on a grid pattern. Complete mineral compositions were determined using a MAC Model 400-S automated electron microprobe operated at 15 kV and 0.4-0.5 pA. Data were reduced on-line using the empirical matrix corrections of BENCEand ALBEE(1968) and ALBEE and RAY (1970). Analyses of diaplectic glasses were performed with a broad electron beam (50 pm dia) to prevent volatilization of alkalis. Determinations of REE in whitlockite were obtained by microprobe using synthetic glass standards. Reoresentative mineral comoositions are shown in Table 2. _ Fragments of Shergotty (- 300 mg; USNM 817) and of Zagami (1.3 P; BMNH 1966, 541 were around under aceto& in an a&e mortar to a maximum &in size of 30 pm. Samples were suspended on Fe-doped Pt wire loops in a vertical, Pt-wound quench furnace in CO-CO2 gas mixtures. Wire (5 ml) for the loops was soaked in a sample of Scourie dike basalt at 1350°C at an oxygen fugacity near the QFM buffer for 5 days, and cleaned by soaking in HF. This produced a homogeneous Fe-Pt alloy with - 7% Fe by weight. Loops were reused up to 11 times; after the charge was broken off the loop, the loop was cleaned in cold or warm HF for ten minutes to an hour and then washed in distilled water before being reused. Approximately lO-20mg of sample were attached to a loop in a slurry with acetone or polyvinyl alcohol and then moved in a single movement from the top of the furnace tube to the hot spot. Samples were quenched in water. Other aspects of the experimental technique and apparatus are described in WALKER et al. (1973). CO-C@ gas mixtures were used to control oxygen fuga-

1475

1476

E.

STOLPER

and H. Y.

Table 1. Modal compositions of shergottites (Optical point counts, volume percent)

1 Pyroxene Maskelynite Mesostasis Intergrowth Titanomagnetite Ilmenite Pyrrhotite Wbitl~kite Fayalite (No. Points)

2

3

4.

70.5 23.9 2.8 2.0 0.5 0.3 tr

69.1 22.7 5.2 2.5 0.3 0.2 tr

76.3 69.7 18.X 24.7 I.7 2.6 2.2 2.1 0.6 0.6 0.5 0.7

(G4)

(&)

(l&)

(l&

1, 2--Sections cut from Shergotty USNM 321. 3.4-Sections cut from Zagami BMNH 1966. 54.

cities. Oxygen fugacities were calculated from the gas mixtures using the tables in DE~NES et al.(1974). The oxygen fugacities of the gas mixtures were calibrated at the quartzfayalite-magnetite buffer at the melting point of gold. Temperatures were measured with a Pt-PtlORh thermocouple located i cm above the sample and calibrated at the melting point of gold (lO64C). Most experiments were synthesis experiments. Ptagioclase entry, however, was reversed for the Zagami composition. Preliminary attempts to reverse the tiquidi of these compositions were not successful. The solidus was only approximately located by noting the first run in which the sample was not sintered and did not have a glassy sheen. Samples were examined as grain mounts in transmitted light and in reflected light in polished mounts prepared for microprobe analysis. Most polished samples were point counted in reflected light. Phases were analysed using microprobe procedures similar to those discussed above.

III. PETROGRAPHY

Both Shergotty and Zagami exhibit foliated textures produced by preferential orientation of pyroxene prisms and maskelynite grains (Fig. Ia and lb). Quantitative petrofabric data on these rocks are difficult to obtain, because (i) there are two coexisting CIi~opyroxenes with different optic o~entations that are distin~uishabIe onfy by microprobe analyses, and (ii) the amorphous nature of the Table 2. Representative mineral compositions I Na,O MgO

4% SiO, PZOS R*O CaO TiO, Cr@, MnO Fe0 Total

0.78 0.03 1.56 97.0 0.01 0.00 0.1 1 0.20 0.00 0.04 0.12 99.76

maskelynites precludes determination of their optic orientations. However, the degree of alignment of these minerals can he estimated by counting the number of grain contacts crossed during traverses at various angles to the apparent plane of foliation (DAROT,1973). Figure 2 illustrates grain contacts encountered during traverses drawn at 15” intervals on thin section photo-mosaics. Minima and maxima in the number of grain contacts encountered correspond to the apparent plane of foliation and the plane perpendicular to it. It is not possible to tell from these data whether or not these foliation planes also exhibit hneations. The textural appearance of shergottites does not resemble other foliated achondrites, e.g. the nakhlites (Fig. lc). Although the fabrics of both shergottites are similar, Shergotty is much coarser-grained ( - 2 x ) than Zagami (Fig. la and 1b). The average grain size for each meteorite, determined by dividing the traverse distance by the average number of gram contacts encountered on 12 traverses (from Fig. 2), is 0.46 mm for Shergotty and 0.24 mm for Zagami. The textural features of shergottites suggest that crystal accumulation occurred and that their compositions represent liquids plus accumulated crystals. Although we cannot eliminate flow segregation as the mechanism which produced the observed foliation, we regard accumulation due to gravity settling as the most plausible mechanism. Throughout this paper we shall refer to crystals accumulated or concentrated in a liquid as cumulus or ucru~u~~r~d crystals, regardless of the mechanism by which they accumuiated, of whether they are zoned or unzoned, of their proportions in the consolidated rock, or of whether they were concentrated at the temporary Roor, ceiling, or interior of a body of magma. This usage extends that of WAGER and BROWN (1967) in which cumulus crystals referred only to unzoned crystals (primocrysts) accumulated on the temporary floor of the magma chamber through the action of gravity. The liquid in which the cumulus crystals concentrated and which once surrounded these crystals. is referred to as infercumulus liquid and the phases crysrallized from this liquid are intercumulus crystals. (a) Pyro.xencrs. The shergottites are composed primarily of gray-green clinopyroxenes and colorless maskelynite. The two clinopyroxenes (pigeonite and augite) cannot readily be distinguished opticaIly. but grid microprobe surveys of 100 pyroxene points on one thin section of each meteorite indicate that the two pyroxenes are present in roughly equal volume proportions (grains of pigeonite:

(Microprobe analyses, weight percent--elements not determined)

2

3

4

2.39

0.10 2.21

0.32

30.3

MCSWEEN JR

without numbers were

5

6

7

x

9

IO

I .42 I.48 0.30 0.08 46.1 0.08 45.7

5.1 I 0.10 27.9 55.6

20.7 0.57 50.7

11.2 0.45 50.5

17.0

11.7

0.83 52.0

0.90 50.2

16.7 0.23 0.70 0.41 12.2 100.07

IS.5 0.58 0.37 0.75 19.9 99.90

0.29 10.7

5.63 6.71 21.8 50.4 0.18 0.58 0.80 0.33 0.08 0.85 0.40 0.30 0.10 0.75 0.80 66.0 74.3 48.7 4.33 0.46 20.6 29.7 99.54 99.61 99.74 1oo.s7* 100.16 99.46 100.02 * Includes Zr-0.57, Y--0.34, La-JJ.01, Nd--0.02, Ce--0.04.

--~

I-Silica (Shergotty); 2--FayaIite (Shergotty); 3-Titanomagnetite (Shergotty); GIlmenite (Zagami); S-Whitlockite (Shergotty); bhifaskelynite (Zagami); 7-Pigeonite Core (Shergotty); 8-Pigeonite Rim (Shergotty); 9-Augite Core (Zagami); l&Augite Rim (Zagami).

Fig. 1. Textural features in shergottites in plane polarized light, except where noted otherwise. (A) Foliation in Shergotty. (B) Foliation in Zagami. Note variation in grain size between Shergotty and Zagami. (C} Cumulus dinopyroxene crystals in Nakhla. (D) Twinned ,ciinop~oxene crystal in Shergotty fractured by shock, crossed polars. (E) Plagiociase lath in Shergotty converted to m~kelynite by shock. (F) Titanomagnetite grain in Shergotty containing geometrically arranged biades of ilmenite, reflected light.

1477

Petrology and origin of the shergottite meteorites

1479

o.;sj 0.2

0.3

0.4

0.5

0.6

0.7 0.6

Fe/(Fe+Mg) AI plane

4

of

Foliation e

Fig. 2. Number of grain contacts encountered during oriented traverses at 15” intervals from the apparent plane of foliation. The length of traverses in Shergotty was 12 mm, in Zagami was 6 mm. grains of augite = 52:48 Shergotty and 50:50 for Zagami). Interstitial mesostasis patches consist of vermicular intergrowths of glass and silica with magnetite, ilmenite, fayalite, pyrrhotite, and whitlockite (the last observed only in Shergotty). Modal analyses for both meteorites are very similar (Table 1). Euhedral to subhedral clinopyroxenes are commonly fractured or broken and may appear turbid as a result of shock (Fig. Id). In thin section the crystals are brown with yellowish-brown rims, reflecting Fe-enrichment at grain margins. Extinction is either normal, undulatory, or patchy. Twinning is common, especially on the (100) composition plane. Pigeonites and augites in both shergottites are generally compositionally zoned with homogeneous magnesian cores and Fe-rich rims (Fig. 3 and Table 2). The rims of pigeonites in Shergotty show a more pronounced enrichment of Fe than those in Zagami (Fig. 3), perhaps due to a larger proportion of intercumulus liquid relative to cumulus crystals in Shergotty (see below). The compositional variations for pyroxenes in Shergotty are as previously measured by BANNS (1967); however, Binns reported higher Ca contents in pigeonites and augites in Zagami than in the corresponding phases in Shergotty, a trend we have not been able to substantiate. We also have not observed pyroxenes of intermediate composition, as reported by DUKE (1968) from a microprobe traverse of

0.5

Cr

Fig. 4. Variation of non-quadrilateral components in pyroxenes. Symbols for Shergotty and Zagami pyroxenes as in Fig. 3. Other symbols are pyroxenes in: (m) Chassigny (FLORANet al., 1978X(Cl) Nakbla (BUNCHand REID, 1975), and (*) the Moama basaltic achondrite (LOVERING,1975). one zoned Shergotty pyroxene, but it is apparent that some pyroxenes are finely exsolved. The homogeneous, magnesian cores in pigeonites (En,,_,,Fs,,_,,Wo,,) and augites (En4,Fs10Wo,,) could be primocrysts and the Fe-rich rims, lafe-stage overgrowths or reaction products between primocrysts and intercumulus liquid. Some crystals show continuous zoning, and the cores of these grains are not as magnesian as are the homogeneous cores; these grains may not represent accumulated crystals. An alternative interpretation, preferred by SMITH and HERVIG(1979), is that the cumulus pyroxenes were already zoned at the time of accumulation. We have not been successful in making a modal estimate of the relative proportions of pyroxenes with homogeneous cores versus those that are continuously zoned. The variations of some minor components in pyroxenes are shown in Fig. 4. With increasing Fe/(Fe + Mg), TiO, increases and Cr,O, decreases. Although Al,O, contents are scattered, Ti/Al ratios increase progressively with Fe

Fig. 3. Microprobe analyses of pyroxenes in Shergotty and Zagami.

E.

1480

STOLPER

and H. Y. MCSWEENJR

wt %

Fe0

Fig. 5. Variation between MnO and Fe0 in shergottite pyroxenes. Diagonal lines represent pyroxene variations in lunar rocks and the Kapoeta howardite (DYMEKet al., 1976). Bulk analyses of Shergotty (MCCARTHYet al., 1974) and Zagami (EASTONand ELLIOTT,1977) are indicated by crosses and fall on the Kapoeta trend, as do many shergottite pyroxenes. However, the slope of this trend is different and extrapolates to 0.2MnO for zero FeO, as also found by SMITHand HIZRVIC; (1979) for Shergotty pyroxenes. enrichment. The rise and drop in Al relative to Cr and Ti and the drop in A120, contents of pyroxenes at Fe/(Fe c Mg) (molar) -0.55 (Fig. 4) may reflect the onset of plagioclase crystallization. Data for pyroxenes in nakhlites, chassignites, and basaltic achondrites fall approximately along these trends (Fig. 4). MnO and Fe0 also vary sympatheticafly in pyroxenes (Fig. 5). The observed MnO/FeO ratio is similar to the ratio for pyroxenes in the Kapoeta howardite, in nakhfites. and in chassignites, but not to lunar pyroxenes (DYMEKrt al.. 1976; STOLPER rt al., 1979). The silicate portions of H and L chondrites also have MnO/FeO ratios similar to shergottites (WANKE et cd., 1973). Some of the Fe in these pyroxenes may be trivalent; DUKE (1968) reported 0.58 wt% Fe,O, in a wetchemical analysis of a pyroxene fraction separated from Shergotty. (b) ~~s~~~~~~jr~.The meteorite Shergotty is the type specimen for maskelynite (T~H~~MAK, 1872). a plagioclase

glass formed by exposure to high shock pressures. Peak pressures experienced by shergottites are estimated at -3OOKbar (WTH and HEKVIG, 1979). MaskeIynite in shergottites lacks any crystal structure. as evidenced by isotropism and absence of diffraction lines in long-exposure X-ray powder photographs (BINNS,1967). The phase occurs as colorless anhedral grains or as fathlike thetomorphs after plagiocfase (Fig. le). DUKE (1968) noted that refractive indices and specific gravities of maskelynite grains are variable, and suggested that individual crystals were zoned. The composition of maskefynite could not be measured directly by microprobe point analysis because of rapid volatilization of Na; point analyses of maskelynites give non-stoichiometric compositions with low sums. JEANLOZ and AHRENS(1976) reported that Na X-ray intensity measurements are unstable for diaplectic glasses from shocked basalt (An,,_,,) and for synthetic plagioclase glasses with Anaso. VAN SCHMUS and

Ab

Fig. 6. Compositions of maskefynite and mesostasis intergrowths projected onto the plane orthoclasealbite-anorthite. Average wet-chemical analyses of maskelynite separates by BINNS(1967) and DUKE (1968) fall in the middle of the observed compositional range. The compositions of plagioclase and alkali feldspar in nakhlite intercumulus mesostasis (BUNCH and REID, 1975) fall at the ends of the shergottite mesostasis trend.

Petrology and origin of the shergottite meteorites

1481

Fig. 7. Oxygen fuga~ity-tem~rature conditions of ex~rimental runs and natural t~tanomagnetiteilmenite pairs (~lculated using the model of LINDSLEY and RUMBLE,1977). W~stjte-magnetite (WM), fayalite-magnetite-qu~tz (FMQ), and nickel-nickel oxide (NNO) buffer curves from HUEBNER (1971). The exact position of the FMQ buffer is in dispute (e.g. CHOU and WILLIAMS,1977).

R~BRE(1968) observed a similar effect when analyzing shocked plagioctase in chondritic meteorites. To minimize alkali loss, a defocussed beam (5Opm dia) was employed for maskelynite analyses, as advocated by JEANLOZand AHREN~(1976). Individual maskelynite grains in both meteorites are zoned from An~,Ab~~Or* cores to An4sAb,,0r, rims (Fig. 6) in many cases. [in a preliminary report of this data (MCSWEENand STOLPER,1978), we reported maskelynite as calcic as An,,, but this analysis was in error.] The average ~ompositioas of separates of maskelynite from Shergotty as determined by wet-chemical methods @INNS, 1967; DUKE, 1968) fall in the middle of this range (Fig. 6). Our maskelynite analyses also include OS-O.8 wt% Fe0 and 0.1-0.2 wt% MgO (Table 2). Molar Fe/(Fe + Mg) increases with albite content from 0.66-0.81 in the cores to 0.81-0.91 in the rims. Although many maskel~it~ grains are aligned parallel to the plane of foliation, this need not be due to accumulation of plagiociase. Most grains are highly irregular in shape and appear to fill interstices between pyroxene crystals, so the partial alignment of maskei~nit~ grains could result from interstitial crystallization of plagioclase in an already layered pyroxene-liquid mush. The observed drop in Al content of pyroxenes with increasing Fe/(Fe + Mg) is consistent with the suggestion that plagioclase crystallized later than the homogeneous cores of the pyroxene crystals and the inner portions of their zoned rims, and may be entirely crystallized from intercumulus liquids. The common association of maskelynite with late-stage mesostasis (discussed below) also suggests that much of the plagioclase crystallized in situ as an intercumulus phase. (c) Minor phases. Titanoma~netite (uiv~spinei) occurs as irregular or subhedral grains in both Shergotty and Zagami. Six analyses in each meteorite indicate consistent compositions of Pef~zFe&,Tid.&4 #ft37Usp63). llmenite occurs in subordinate i~mounts as anhedral grains intergrown with titanoma~etite or less commonly as thin lamellae geometrically arranged within magnetite grains (Fig. if). The ilmenite composition, as measured in 6 grains from each meteorite, is Ilrn~~Hrn~. nhis composition differs from our previously reported value (MCSWEENand STOLPER,1978).J These coexisting oxides define a temperature of 860°C and an f0, of 10m4 using the solution model of LINDSLEYand RUMBLE(1977). These values correspond approximately to the quartz-fayahte-magnetite buffer assemblage (Fig. 7). Fayahte occurs s~radicaliy as an interstitial phase. The composition measured for three grains (Table 2) is Fa,,_,,.

Rounded grains of pyrrhotite are intergrown with other interstitial phases, and analyses of 9 grains indicate a composition of PeO,,, S with Ni contents 20.3 wty;. DUKE (1968) and MCSWEEN and STOLPER (1978) previously reported that the sulfide present was troilite. an error corrected by SMITWand HERVIG(1979). The most prominent interstitial constituent is mesostesis formed of microscopic two-phase intergrowths of silica and feldspar glass. BINN~(1967) observed the material only in Shergotty, but it is present in Zagami as well, though in smaller amounts (Table 1). Binns proposed that this was a silica glass, and DUKE (1968) noted that extracted grains were either amorphous to X-rays or gave cristobahte powder lines. The patches vary in color from deep reddishbrown to light-tan and are often interleaved between pyroxene and maskelynite grains or extend into maskelynite (Fig. 8a). Less commonly mesostasis can be found between pyroxene grains (Fig. Sb). These patches are typically associated with opaque minerals, and oxides commonly have euhedral terminations extending into the intergrowth: this observation taken with the swirling texture which ‘wraps around’ the opaque crystals (Fig. 8~) suggests that the twophase intergrowth solidified after the oxide minerals. When viewed in reflected light under high magnification, the mesostasis patches are seen to consist of two phases intergrown in a vermicuIar network (Fig. 8d). The texture of this intergrowth is illustrated in detail in the SEM photomicrograph of Fig. Se. The light gray phase with high relief in silica, and the dark gray phase with lower relief is apparently glass of feldspar composition. An analysis of the silica polymorph is presented in Table 2. Direct analysis of the intergrown feldspar glass is hampered by alkali volatilization. However, the composition of the glass can be approximated by subtracting normative free silica from the bulk compositions of these two-phase areas as determined by microprobe defocussed beam analysis. Normative calculations of typical analyses of these inter~owths (Table 3) indicate that these areas consist almost entirely of silica and feldspar, although a small but persistent pyroxene (mostly ferrosilite) component is present. The calculated feldspar glass component does not have the same composition as the maskelynite grains in the meteorites, but is consistently richer in normative albite and orthociase (Fig. 6). Interstitial patches with deeper reddish-brown color are the more K-rich variety, and have lower silica contents than the patches with light-tan color (Table 3). These two-phase intergrowths may actually represent shocked mixtures of three groundma~s phases-,two coexisting alkali-rich feldspars (now diaplectic feldspar glass)

E.

1482 Table 3. Selected

STOLPER

and H. Y. MCSWEENJR

compositions of mesostasis analyses, weight percent)

Na,O MgO Al&), SiO, P*O5 K,O CaO TiOz Cr203 MnO Fe0 Total

2.62 0.00 12.5 16.7 0.03 5.79 1.41 0.39 0.00 0.02 0.66 100.12 *Includes Zr--O.l6, Y-0.19,

Normative minerals Or Ab An En Fs wo

Qz

Ilm Cor AP

3

2

1

3.92 0.03 12.4 78.7 0.11 0.31 3.23 0.61 0.01 0.00 0.89 100.21 La-0.00,

34.2 22.1 5.2 0.6 0.7 36.4 0.7 0. I

glasses (Microprobe 4

5

1.I2 3.32 1.78 0.13 0.02 0.02 11.8 10.7 13.0 76.3 81.2 13.4 0.08 0.04 0.09 6.19 0.28 6.02 0.95 2.82 2.25 0.53 0.2 1 0.54 0.00 0.00 0.00 0.05 0.03 0.00 2.87 0.51 1.00 100.62 99.13 98.52* Nd~~0.06, Ce----0.01.

1.9 33. I 15.2 0. I 0.6

36.4 14.4 4. I 0.3 4.5

1.7 28.3 13.6

36.7 15.6

1.9

1.o

0.7 0.1 55.1 0.4

47.6 1.1 0. I 0.3

38.4

33.2 1.1

0.7 0.2

0. I

0.2

11.3

l-Reddish-brown two-phase intergrowth (Zagami); 2----Tan twophase intergrowth (Zagami); 3-Reddish-brown two-phase intergrowth (Shergotty); G-Tan two-phase intergrowth (Shergotty); 5-Glass inclusion in whitlockite (Shergotty). and silica. The compositions of K-feldspar and sodic plagioclase occurring as intercumulus mesostasis in nakhlites (Bunch and Reid, 1975) plot at the ends of the linear trend of shergottite mesostasis glasses (Fig. 6), which may represent a mixing line between these two feldspars plus silica. These mesostasis compositions probably represent late-stage residual liquids in equilibrium or near-equilibrium with three phases: pyroxene, plagioclase, and silica. Eutectic points for these three phases in the systems diopside-leucite-silica (SCHAIRERand YODER. 1960) and diopside-nepheline-silica (BOWEN,1937) have very low pyroxene components and are similar in composition to the bulk intergrowths. Hexagonal crystals of whitlockite occur as an interstitial phase in Shergotty, but this mineral has not been observed in Zagami. Whit&kite has higher Na content (Table 2) than lunar and terrestrial whit&kites, and significantly lower rare earth contents than lunar analogs (FRONDEL, 1975, p. 156). Many whitlockite crystals contain elongated glass inclusions (Fig. 8f). These homogeneous glasses are identical in compositions to the bulk two-phase intergrowths except for slightly lower silica contents (Table 3). supporting the suggestion that the intergrowths may represent residual eutectic liquids. In several glass inclusions small euhedral titanomagnetite and pyrrhotite crystals appear to have settled to one end of the inclusion prior to solidification of the trapped melt (Fig. 8f). The direction of settling of trapped opaque minerals appears to be consistent from inclusion to inclusion and is normal to the apparent plane of foliation in the rock. This may serve as a ‘top and bottom’ criterion and provides evidence that the petrographic properties of Shergotty, and by extension Zagami, result from crystal settling due to gravity. IV. EXPERIMENTAL

RESULTS

The experimental conditions and results of each experiment are listed in Table 4 and summarized in Figs 7 and 9.

Analyses of selected experimental phases are presented in Tables 4 and 5. Natural and experimentally produced pyroxene compositions are compared in Fig. 10. Most experimental products were point counted to determine the proportions of each phase in each experiment. The modes of many experimental products were also determined by a least squares approach utilizing the bulk compositions of the starting materials and the microprobe analyses of the phases in the experimental products (WRIGHTand DOHERTY,1970). The modes of the experimental products determined in these two ways are summarized in Fig. 1I. No correction was made for the difference between weight percent modes obtained from the least

13001

1

1

I

.sj

1200T(Oc)

loo0

1

Zagclmi

Shergotty

]

Fig. 9. Summary of results of 1 atm quenching experiments on. shergottite meteorites. Oxygen fugacities near the quartz-fayalite-magnetite buffer. Minor spine1 was present . .. m an experiments.

Fig. 8. Photomicrographs of mesostasis in shergottites, in plane polarized light except where noted otherwise. (A) Patches of mesostasis invading maskelynite. (B) Mesostasis between pyroxene crystals. (C) Swirling texture of mesostasis wraps around associated opaque minerals, suggesting mesostassib crystallized after opaques. (D) Vermicular intergrowths of two phases, silica and feldspar glass, as seen in reflected light view of (C). (E) SEM enlargement of two-phase intergrowth texture. (F) Homogeneous glass inclusions in Shergotty whitlockite, by reflected light. Crystals of magnetite and pyrrhotite have settled to one end of the lower inclusion. In this photomicrograph the apparent plane of foliation in the rock is oriented approximately vertical.

1483

Petrology

and origin

of the shergottite

Table 4. Experimental Experiments Run No.

TV)

Sh-14 Sh-13 Sh-15 Sh-I 1 Sh-16 Sh-6

1228 1228 1230/1218 1218 1198 1176’

Sh-18

1158

- 8.97

21.9

Sh-7 Sh-10

1138 1108

-9.25 - 9.72

87.5 67.3

Sh-24 Sh-19 Sh-20 Sh-17 Sh-21 Sh-22 Sh-23

1093/l 108 1098 I109/1098 IO92 1068 1051 1033

Za-67 Za-68 Za-64 Za-69 Za-66 Za-7 I Za-65 Za-63 Za-75

1262 1262 1252 1261/1242 1240 126211232 1231 1212 1176’

- 7.43 - 7.43 - 7.55 -7.431-7.68 - 7.70 - 7.43/- 7.80 -7.81 -8.06 -8.7

2.1 16.0 5.1 5.013.4 -5’ 5.113.2 5.3 3.0 75.0

Za-5 I Za-76 Za-54 Za-60 Za-58

1170 1138 1132 1109 1082/l 105

- 8.63 - 9.25 -9.19 -9.54 -9.951-9.61

5.3 87.5 5.3 15.3 5.0163.8

Za-6 I Za-62 Za-59 Za-57 Za-55 Za-56

1103 1113/1102 1102 1082 1062 1062

-9.63 -9.471-9.65 -9.66 -9.97 - 10.29 - 10.29

5.0 6.7110.5 5.6 16.6 5.0 17.0

- 8.02 - 8.02 -7.98/-8.15 -8.14 -8.42 -8.70

2.0 19.0 2.Ol2.0 20.4 16.7 75.0

-9.941-9.72 -9.87 -9.701-9.87 -9.96 - 10.34 - 10.62 - 10.93

Experiments

results

completed on Shergotty Duration (hr)

fog,,fo,

(USNM817) Results gk oP gL oP gL oP g15, pig (Wo6En71, A.7, C.5, T.l), op g15, pig (Wo7En71, A.8, C.5, T.l), op gl’, pig (Wo9En68, A.5, C.2, T.l)*, aug (Wo35En53, A1.5, T.2)4, op gls, pig (Wol3En62, A.0, C.2, T.1)2, aug (Wo33En49, A1.9, C.5, T.2), op gl’, pig, aug (Wo34En47, Al.l, C.5, T.2)*, op g15aug (Wo33En41, A1.8, C.3, T.5), pig (WollEn54, A1.O, C.l, T.2) op gl, aug, pig, plag, op g15, aug, pig, plag, OP gl, aug, pig, ptag (An60), OP g15, aug, pig, plag, 0~ gl + xtals _ solidus subsolidus

8.4/16.0 65.0 8.4143.5 62.8 49.4 16.2 19.6

completed

on Zagami

(BMNH

I. Cumulus phases in the shergottites:

experimental

evidence

The compositions of natural and experimentally produced pyroxenes are compared in Fig. 10. The first pigeonites and augites to crystallize in experiments on the Shergotty and Zagami compositions are more magnesian than the most magnesian natural pigeonites and at&es.

1966, 54)

gt. oP gl, oP gl. oP gf. oP g15, pig (Wo7En71, A.5, C.3), op gl, cpx 0. OP g15, pig (Wo8En71, A.5, C.2). op gt5, pig, 0~ g15, pig (Wo9En68, A.5, C.2)2, aug (Wo37En51)‘, op gl, pig aug, 0~ g15. pig, aug (Wo32En46, A1.3, C.5, T.2)2, op gL pig, aug, OP g15, aug, pig, OP g15, aug (Wo30En43, Al.l, C.4, T.5)‘, pig, plag (An%), OP gl, aug, pig, plag, OP gl, aug, pig, plag (An60), OP gl, aug, pig, plag, OP gl, aug, pig, plag, op subsolidus gl (?)

Abbreviations: gl = glass, op = opaque, pig = pigeonite, aug = augite, AI,O,, Cr,Os, TiOr respectively in pyroxenes. ’ Duration only approximate. 2 Pyroxenes were heterogeneous. Representative composition given. 3 Temperature based on-control thermocouple. _ 4Augite only found in crystals rimmed with pigeonite. 5 Glass analyses given in Table 5. squares analyses and volume percent modes obtained from point counting. The ratio of pigeonite to augite in the experiments containing both pyroxenes was determined by least squares analysis only, as the two pyroxenes could not be distinguished by point counting. The least squares calculations of modes indicate that alkali loss during the experiments was 5 10% of the amount present and that iron gain or loss was 15% of the amount present.

1485

meteorites

plag = plagioclase.

A, C, T numbers

are wt%

This observation is consistent with the petrographically based inference that both of these meteorites contain cumulus pyroxene. The fact that this relationship exists both for pigeonites and au&es suggests that these meteorites contain both cumulus pigeonite and cumulus augite. This conclusion is supported by the observation that the most Mg-rich augites and the most Mg-rich pigeonites in these meteorites appear to have equilibrated with liquids + Mg) ratio (0.64-0.67 molar; with the same Fe*+/(Fe’+ based on Kr, values in Grove, 1978) and thus probably co-precipitated. Alternative interpretations of the discrepancy between the most magnesian natural and experimental pyroxenes can be envisioned. These could include: (i) experimental oxygen fugacities were unreasonably high; (ii) slow cooling

I486

E. STOLWR

EN

and H. Y. M~Swer:~i JH

FS

molar

Fig. 10. Compositions ofexperimentallyproduced pyroxenes (filled circles) and cores oCnatUrdl shergottite pyroxenes (stars). Solid curves show the trends of experimental pyroxene compositions. Numbers indicate the temperatures (“C) at which given pyroxene compositions equilibrated in the experiments. during the early portions of the crystallization history homogenized early natural magnesian cores; and (iii) rapid crystallization under natural conditions might produce more iron-rich pyroxenes than equilibrium experimental conditions (LOFGRENet u/.. 1974). However, we regard these alternatives as unlikely, especially in view of the petrographic observations cited above which strongly support a cumulus pyroxene component in the shergottites. It is difficult to establish with any certainty whether any of the plagioclase (now maskelyn~te) in the shergottites originated as a cumulus phase. The most calcic natural maskelynite is An,,. The first plagioclase to crystallize in the experiments varies between An,, and An,, in the experimental charges in which plagioclase has been analyzed (Table 4). This close correspondence between the experimental and natural plagioclase suggests that the amount of cumulus plagioclase, if there is any, must be minor. The rare earth patterns of Shergotty and Zagami (filled symbols

in Fig. 12) show no Eu anomaty and are also consistent with only minor cumulus piagioclase. The decrease in AlzO, contents of both pyroxenes at Fe/(Fe + Mg) molar -0.55 [this paper (Fig. 4) and SMITHand HEKVIG(1979)] may correspond to the onset of plagioclase crystallization. If so, this would indicate that plagioclase began to precipitate after au&e and pigeonite and would be consistent with all of the plagioclase being an intercumulus phase. However, if zoned pyroxene crystals accumulated to produce the shergottites, cumulus plagioclase could not be ruled out even if the A1,03 decrease were the result of the onset of plagioclase crystallization. The overall impression given by the textures of the shergottites is that pyroxenes are the only accumulated phases and that all of the plagiocIase crystallized from intercumulus liquid. This interpr~tatjon of shergottite petrography is consistent with pyroxene zoning trends, REE patterns, and the results of melting experiments. This is our

Table 5. Microprobe analyses of experimentally produced glasses’

Shergotty’ SiOz TiOz Al& Cr& FeO’ MnO MgG CaO Na,O KB Total

SiO TiO: Al,% Cr,% FeO* MnO MgG CaO Na,O R,G Total

50.4 &Xl 6.89 0.21 19.1

0.50 9.27 10.1 1.37 0.16 98.8

ZagamiJ ..5O.R 0.77 5.67 0.30

18.0 0.50 11.0 10.X 0.99 0.14 99.0

ChS

GIXSS

Sh-I I 50.2 0.78 6.98 0.10

Sh-16 49.7 0.9 I 7.29 0.1 I 19.3 0.49 7.87 IO.6 1.40 0.21 97.8

18.8 0.47 8.76 10.1 1.27 0.18 97.1

CibSS

Glass

Glass

Za-46

Za-65

Za-63

30.6 0.89 5.93 0.04 18.4 0.54 9.28

51.1 0.88 6.09 0.15

50.4

0.79 5.5x 0.10

17.9 0.66 IO.2 11.1

11.3

1.17 0.13 98.1

1.19 0.15 98.4

Glass Sh-6 SO.5 0.93 7.9 I 0.08 19.3 0.46 6.9 1

Glass Sh-18 49.9 0.93 8.49 0.08 19.5 0.45 5.95 10.5 I .73 0.22 97.8

10.8 1.55 0.21 98.7

’ Run numbers correspond to those in Table 4. * All Fe as FeO. 3 Bulk composition of sample based on probe analysis at 5 kb, 1350°C.

18.4 0.46 8.18 11.7 1.40 0.16 98.5 Glass Sh-7 50.1

I .0x 9.45 0.02 19.7 0.48 5.11

10.0 1.84 0.24 98.0

of

Glass Za-75

Glass Za-76

Glass Za-60

Glass Za-58

50.0 0.95

7.38 0.10 19.5 0.47 6.77 IO.7 1.s9 0.20 97.7

50.0 1.1s 9.53 0.08 19.2 (I.46 5.05 10.0 1.x9 0.25 97.7

49.7 1.30 10.8 0.03 18.1 0.61 4.26 9.7 1 2.34 0.32 97. I

50.7 I .3Y 10.4 0.00 18.3 0.40 3.X6 9.64 2.16 0.33 97. I

Glass Sh-10 49.9 1.21 IO.8 0.02 19.2 0.44 4.17 9.60 2.08 0.2x 97.7

Glass Sh-19 49.9 1.30 10.8 0.04 19.6 0.40 3.90 9.50 2.12 0.32 97.9

Glass Sh-17 50. I 1.50 10.5 0.07 20.0 0.41 3.56 9.37 2.25 0.37 98. I

glass from fusion of sample in graphite capsule

1487

Petrology and origin of the shergottite meteorites

wt,

%

Fig. Il. Summary diagram showing the proportions of phases present in the products of experiments on the Shergotty and Zagami meteorites. Modes of experiments were determined by point counting and least squares methods described in the text. Stars are percentages of glass; filled circles are percentages of augite; open circles are percentages of pigeonite; open squares are percentages of plagioclase. Percentages of spine1 were less than 1% in all experiments.

preferred interpretation and agrees with those of DUKE (1968) and SMITH and HERVIG(1979); however, we note that it is not possible to completely rule out minor cumulus plagioclase. 2. Modemsfor the for~tio~ of s~ergottite cumulates A wide range of petrologic models would be consistent with the ~trographical, geochemical, and experimental data available for the shergottites. While we have established with reasonable certainty that cumulus pigeonite and augite are present in these meteorites, we cannot rule out minor cumulus plagioclase, nor can we determine whether the zoning in the pyroxenes is a primary feature of the cumulus phases or whether this zoning is entirely due to crystallization of an intercumulus liquid. Nevertheless, specific petrologic models can be constructed which are consistent with available observations and which permit constraints to be placed on the conditions of formation of the shergottite cumulates, on the compositions of the intercumulus liquids, and on the proportions of cumulus phases and intercumulus liquid. Consider the following model: the shergottities were produced by the con~ntration of pigeonite and augite primocrysts (with compositions similar to the most magnesian pyroxenes in the shergottites) in intercumulus liquids with which they were in equilibrium. According to this model, the zoning observed in shergottite pyroxenes would be due to crystallization of these liquids. Petrographic support for this model comes from the observation that many pigeonites and augites in Shergotty and Zagami do indeed have homogeneous cores similar in composition to the most magnesian pyroxenes found in the meteorites. Within the framework of this model, the experimental data can be used to specify the temperatures of accumulation of the shergottites, the compositions of the intercumulus liquids. and the proportions of cumulus phases and intercumulus liquid. Since the cumulus pyroxenes are assumed to have been in equilibrium with the liquids in which they accumulated, the experiments in which the compositions of experimentally produced pyroxenes most nearly match the compositions of natural homogeneous pyroxene cores will have equilibrated at temperatures similar to the accumulation temperatures of the meteorites. Also, the compositions of the quenched experimental liquids and the proportions of phases in these experiments will be similar to the compositions of the intercumulus liquids and the proportions of cumulus and intercumulus phases in the meteorites. Reference to Fig. 10 shows that in experiments on the shergottites the Fe/Mg ratios of experimentally produced pyroxenes are similar to those of the natural magnesian pyroxene cores at _ 1140°C. The approximate proportions of cumulus and intercumulus

0 Shergotty u Zagami

12 lo

intercumulus intercumulus

Sm

Eu

liquid liquid

lb

(colt.)

fcalr. 1

Yb Lu SC

Fig. 12. REE and Sc abundances in shergottites measured by A. J. Irving using INAA on aliquots of the same powders used in our experiments, normalized to chondritic abundances (REE from FREYet al., 1968; Sc, Sppm, from SCHMITTet al., 1972). Absolute abundances of these and other elements measured by A. J. Irving are (in ppm, unless otherwise noted): Shergotty: La 2.18 f 0.06, Sm 1.36 + 0.03, Eu 0.53 f 0.01, Tb 0.36 + 0.03, Yb 1.59 + 0.06, Lu 0.262 + 0.008, Hf 2.0 f 0.1, Th 0.35 rt 0.05, Ta 0.27 + 0.04, SC 53 + 1, Co 35 f 0.7, Cr 1490 + 30, Ni 56 + 9, Fe0 18.3% f 0.3, NazO 1.30% + 0.02; Zagami: La 2.07 + 0.06, Sm 1.42 + 0.03, Eu 0.51 * 0.01, Tb 0.34 + 0.02, Yb 1.45 + 0.05, Lu 0.255 _t 0.008, Hf 1.9 f 0.1, Th 0.27 + 0.04, Ta 0.22 & 0.05, Sc 57 & 1, Co 37 t 0.8, Cr 2350 + 50, Ni 67 + 12, Fe0 l&00/, f 0.3. Na,O 1.147; + 0.02. The abundan~s calculated for shergottite intercumulus liquids are also shown. These were calculated using the proportions of cumulus pigeonite, cumulus augite, and intercumulus liquid given in the text and by assuming that the cumulus phases were unzoned and in equilibrium with the intercumulus liquid. Crystal-liquid partition coefficients used in the calculations are: Pigeonite: La 0.027, Sm 0.037, Eu 0.04, Tb 0.055, Yb 0.108, Lu 0.123, SC 1.4. Augite: La 0.084, Sm 0.193, Eu 0.22, Tb 0.27, Yb 0.32, Lu 0.30, SC 3.3. REE partition coefficients were taken from HASKIN and KOROTEV(1977). D(Eu) was interpolated between D(Sm) and D(Gd); a negligible Eu anomaly is expected for pyroxenes at the QFM buffer (GRUTZEKet al., 1974). D(Sc) for orthopyroxene from MCKAY and WEILL (1977) was used for pigeonite; D(Sc) for augite was taken from PASTERet al. (1974).

1488

E. STCWER and H. Y. MCSWEEN JR

phases in the shergottites inferred from the modes of experimental charges (Fig. 11) are: Zagami-55% intercumulus liquid, 23% cumulus augite. 227; cumulus pigeonite; Shergotty--72% intercumulus liquid, 1I”/;; cumulus augite. 17% cumulus pigeonite. With these proportions, the accumulated primocrysts would not have formed a closepacked network with intercumulus liquid in the interstices, since this would require approximately 65% cumulus crystals (WAGER and BROWN, 1967). According to this model, the compositions of the intercumulus liquids in Zagami and Shergotty would be approximated by the glass compositions of experiments Za-76 and Sh-7 listed in Table 5. These two glasses are virtually identical in composition, as are the homogeneous pyroxene cores in the two meteorites. This suggests that within the framework of this model of shergottite genesis, Shergotty and Zagami were produced by accumulation of similar pyroxenes in similar parent liquids: they differ only in their proportions of cumulus pigeonite, cumulus augite, and crystallized intercumulus liquid. According to this model, the shergottites contain no cumulus plagioclase, since plagioclase had not yet begun to precipitate in our experiments at the temperatures at which the compositions of experimental pyroxenes most closely match the compositions of natural pyroxene cores. Other models can be constructed which would be equally consistent with available data. For example, SMITH and HERVIG (1979) suggest that the shergottites formed by the accumulation of zoned pyroxcne crystals, perhaps accompanied by minor plagioclase a~cumulatioll. Because the amounts of zoned pyroxenes formed prior to accumulation and the amounts produced by crystallization of intercumulus liquids are unknown. neither the proportions of cumulus material in the shergottites, nor their accumulation temperatures, nor their intercumulus liquid compositions can be estimated for this model. However. our estimates of accumulation temperatures and proportions of intercumulus liquids given in the last paragraph. based on a homogeneous cumulus crystal model, give upper limits for the values consistent with a zoned cumulus crystal model. If the amounts of cumulus plagioclase in the shergottites are assumed to be nil or small. then the results of the experiments in which plagioclase first crystallized (e.g. Za-58, Sh-19) gjve approximate lower limits on the proportions of ~ter~umulus liquids (Zagami, 379;; Shergotty, 52%; see Fig. II) and accumu~tion temperatures ( _ 1lOYC) consistent with a zoned phenocryst model. The compositions of glasses in experiments Za-58 and Sh-19 provide approximate compositional limits for intercumulus liquids consistent with a zoned phenocryst mode. Temperatures of accumulation, intercumulus liquid compositions, and proportions of cumulus and intercumulus material for zoned phenocryst models would be between estimates based on a homogeneous cumulus crystal model given in the last paragraph and the limits given in this paragraph. The amounts of pre-accumulation zoning in the pyroxenes would have to be known before precise estimates could be made. At this time, it does not appear possible to choose between models involving accumulation of homogeneous pyroxenes and those involving accumulation of zoned pyroxenes. The REE patterns of the shergottttes and estimates of those of their intercumulus liquids are shown in Fig. 12. REE patterns of the intercumulus liquids were calculated by assuming that the cumulus crystals were unzoned and in equilibrium with these liquids. Proportions of intercumulus liquid and cumulus phases were taken from our estimates given above for the unzoned cumulus crystal model; crystal-liquid partition coefficients are given in the caption of Fig. 12. The calculated REE patterns are approximately flat relative to chondrites and have no Eu anomaly, but the chondrite normalized SC abundances are lower than the REE abundances. These REE-SC patterns

and levels of enrichment are similar to those in cucritic meteorites (CONSOLMAGNO and DRAKF. 1977; FUKL:OKA <*f at., 1977). REE abundances in intercumulus liquids for a zoned cumulus crystal model cannot be esti&tcd but would be light-REE enriched relative to and higher than the patternsshown in Fig. 12. SC abundances would hc lower than those shown in Fig. 12. The Ca/Al ratios of the intercumulus liquids assuming unzoned cumulus crystals would be approximately 1.4. They could be lower, approaching estimates of the cosmic ratio (1.10, AHRENS,1970; 1.26. CAMERON,1973) if models involving zoned cumulus crystals are assumed. The CrZOJ abundances of the intercumulus liquids wc‘rc probably less than O.l% based on the compositions of glasses in our experiments (this result is dependent on thu relevance of the oxygen fugacities of our experiments). These Cr abundances arc similar to those in terrestrial basalts which evolved at similar oxygen fugacities, hut much lower than those in lunar and eucritic liquids, which evolved at lower oxygen fugacities. The higher Cr abundances in lunar basalts and eucrites relative to shergottites and terrestrial basalts may largely reflect the fact that partition coefficients for Cr between crystals and liquid and the Cr contents of spine1 saturated liquids are functions of oxygen fugacity (e.g. SCHKIEIIER and HASKIN. 1976).

V. PARENTAL

LIQUIDS

SHER~O~~TES

FROM WERE

WHICH

THE

DERIVED

There can be little doubt that in the case of the shergottites, the sample base-two nearly identical accumulative rocks---is not sufficient to provide detailed or tightly constrained answers to petrogenetic questions. Nevertheless, a synthesis of the results of experimental studies and geochemical data, and analogy with basalt genesis on the Earth, the Moon. and the eucrite parent body. provide a basis for speculating about the genesis of basalts on the shergottite parent body. We will nssume that primary magmas on the shergottite parent body were generated by low pressure partial melting of plagioclasebearing peridotites; i.e. of source regions consisting dominantly of oiivine, with smaller amounts of pyroxene and plagioclase (STOLPER ef uf., 1979). Subsequent differentiation of these primary magmas will also bc assumed to have occurred at low pressures. ‘Pseudo-liquidus’ diagrams provide a framework for the interpretation of basalt petrogenesis. Figure 13 represents a basis for discussions of the origins of shergottite meteorites. The shergottites and our experimentally produced liquids lie almost exactly in this plane; that is, they have only minor normative silica or olivine. An olivine projection has been utilized to facilitate later comparisons with oliv~ne-~turated liquid compositions. The cotectic curves are based on the compositions of experimentally produced, muitiply saturated liquids (Table 5). From a petrogenetic standpoint, it is significant that olivine was not found in any of the experimental products. It is likely that olivine was a major phase in the source regions which on partial melting gave rise to primary magmas on the shergottite parent body. This is by analogy with terrestrial, lunar and

Petrology and origin of the shergottite meteorites

1489

PLAG

PLAGIOCLASE

Fig. 13, “Pseudo-liquidus” diagram for shergottite compositions. Molar units are used throughout. Compositions may be projected into this diagram by recalculating SiOr, Al&, FmO (Fe0 +

MnO + MgO), CaO, NazO, and I&O into the coordinates Fm$iO,

(FmO + AlzOo + 5(Na20 +

KaO)-Si02 + CaO), CaFmSizOd (CaO-Al,O, + Na20 + I&O), Fm,Si,O, (1/2[2 x Si02-FmO-3 x CaO-1 l(Nar0 + K20)-A1,03]),CaA12Si2O6(A120,-Na2O-K2O),NaAlSiJOs(2 x Na20),andKAlSi,08 (2 x KaO) and then recalculating the DI(CaFmSiz06), EN(FmzSizOs), and PLAG(CaAl,Si,Os + NaAlSi,Os + KAlSisO, components to 100%. The filled circles show the projected compositions of experimentally produced liquids (e.g. Table 5) saturated with pigeonite and augite. Phase boundaries are based on these compositions and analogy with other natural and synthetic systems. The ranges of augite and pigeonite solid solutions found in our experiments are also shown. Point A shows the projected composition of glasses in our experiments saturated with pigeonite, augite, and plagioclase. Point P is the projected composition of our estimate of the intercumulus liquids in the shergottites. Shergotty and Zagami bulk compositions are from DUKE (1968). EAWON and ELLIOT (1977) and our own analyses of h~rliquidus glasses (Table 5). All Fe has been calculated as Fe0 in these projections. The shaded region shows the projected compositions of liquids which on fractionation could yield the intercumulus liquids in the shergottites. The limits of this region are discussed in the text. The compositions of eucrite meteorites (MCCARTHYet of., 1973) and of primitive ocean ridge tholeiite (sample 527-l-1, LANGMUIRet nl., 1977) are shown for comparison.

basalt source regions, and based on cosmochemical considerations which suggest that most planetary interiors are dominantly olivine. It is therefore probable that primary magmas on the shergottite parent body were olivine-saturated. Thus lack of olivine-saturation of the intercumulus liquids in Shergotty and Zagami suggests that these intercumulus liquids were fractionated derivatives; that is, these intercumulus liquids were produced by ~ystallization differentiation of primary liquids and that at some point during this differentiation, residual liquids were no longer olivine-saturated because olivine went into reaction relation with the liquid. For the parent liquids of the shergottites, the relevant reaction relation was probably ohvine + liquid -+ low-calcium pyroxene. The pigeonite saturation of the intercumulus liquids in the shergottites is consistent with this suggestion. et&tic

2. Primary liquids on the shergottite parent body It is not possible to specify with any precision the

compositions intercumulus

of the primary liquids from which the liquids in the shergottites evolved. How-

ever, these primary liquid compositions probably project within the shaded region of Fig. 13. Only liquids with compositions projecting within this region would have had the crystallization sequence which we have inferred for the shergottites and their parent liquidspigeonite and augite before plagioclase-and could have produced residual liquids between points A and P by low pressure fractionation. P is the projection of the intercumulus liquid composition assuming unzoned cumulus crystals; A is the pigeonite + augite + plagioclase cotectic and is an approximate compositional limit to permissible intercumulus liquids. Intercumulus liquid compositions for models involving zoned cumulus crystals would project on the cotectic between P and A. The lower limit of the shaded region in Fig. 13 is based on the maximum amounts of pyroxene fractionation from primary liquids consistent with the REE-Sc abundances inferred for the intercumulus liquids (see below), and

1490

E. STOLPERand H. Y. MCSWEEN JR

is only approximate. The upper bounds of the shaded region in Fig. 13are also only approximate. The locations of the curves which separate liquid compositions crystallizing pigeonite and augite ‘before plagioclase (permissible primary liquid compositions) from those liquid compositions from which plagioclase is the second phase to crystallize (not permissible primary compositions) are unknown; we have approximated them by lines which probably overestimate the range of permissible primary liquid compositions. Orthopyroxene rather than pigeon&e could have been the low-calcium pyroxene fractionating early in the crystallization sequences of permissible primary liquids due to the lower Fe/(Fe + Mg) ratios of these liquids. We cannot determine the absolute amounts of fractionation by which the shergottite intercumulus liquids evolved from olivine-saturated primary liquids, nor can we determine the relative proportions of olivine. low-Ca pyroxene, and high-Ca pyroxene involved in this fractionation. However, several lines of evidence suggest to us that these intercumulus liquids could have evolved by moderate degrees of pyroxene fractionation from primary liquids and that large amounts of high-Ca pyroxene were not invohed in this fractionation or left in the source region residues: (i) Some low-Ca pyroxene fractionation is needed to account for the lack of olivine-saturation of the intercumulus liquids. The fact that addition of 5046 or more cumuius pyroxene to these liquid compositions does not produce olivine-saturated compositions suggests that the pyroxene fractionation involved in the evolution of these intercumulus liquids was not negligible. (ii) The Ca/Al ratios of the inferred intercumulus iiyuids are equal to or slightly higher than the cosmic ratio of 1.1%1.26, depending on whether a zoned or unzoned cumulus crystal model is assumed. Assuming that the source regions of the primary magmas had roughly cosmic Ca/Al ratios, only minor high-Ca pyroxene could have been involved in the fractionation of shergottite primary liquids or remained in the residues in the shergottite source regions. Otherwise, unreasonably high Ca/Al ratios would be implied for shergottite primary liquids and their source regions. Low-Cd pyroxene fractionation has only a minor effect on the Ca/Al ratios of residual liquids. (iii) The SC depletion of the intercumulus liquids (Fig. 12) is consistent with pyroxene fractionation from primary liquids, but may also indicate that residual pyroxene was left in their source regions. Using the pigeonite-liquid partition coefficients given in the caption of Fig. 12 and a Rayleigh fractionation law, we obtain that the REE-SC patterns of the intercumulus liquids shown in Fig. 12 could be produced by about 50°; fractionation of pigeonite from a parental liquid with a flat REE-SC pattern at about &7x chondrites. This liquid could in turn have been produced by lo- 15”; partial melting of an olivine-rich peridotite with chondritic absolute initial REE-Sc

abundances (COWXMAGNO and DRAKE, 1977). The La/Lu ratio of such a liquid produced by SOo/, pigeonite fractionation woutd be about 1.07: that is, approximately flat like the intercumulus liquids shown in Fig. 12. Substantially less low-Ca pyroxene fractionation would be consistent with the same REE-SC pattern if residual pyroxene were left in the source regions of the shergottite primary liquids, Significantly larger amounts of pyroxene fractionation would not. however. be consistent with the calculated intercumuIus liquid pattern, since larger amounts of fractionation would result in larger Sc depletion relative to the REE and would eventually produce detectable light REE enrichment. These arguments are not conclusive, but suggest to us that compositions of the primary liquids from which the shergottites evolved would project in the part of the shaded region of Fig. 13 which lies in the low-Ca pyroxene liquidus field. In other words, these primary liquids would have had the crystallization sequence [olivine] -+ low-Ca pyroxene -+ lowCa pyroxene + augite+ low-Ca pyroxene + augite + plagioclase. The cumulates formed during the early differentiation of these primary liquids would record this crystallization sequence. Their olivines and pyroxenes would have lower Fe/(Fe + Mg) ratios than the pyroxenes in the shergottites, but their intercumulus plagioclases would have approximately the same composition as those in shergottites, An,,. As noted above, the more magnesian low-Ca pyroxenes in these cumulates could be orthopyroxenes rather than pigeonites. The recently discovered Antarctic achondrite ALHA 77005 appears to contain all of the features expected in such cumulates, and may be related to the shergottites in this way (MCSWEENet ul., 1979). 3. Petrologp of source regions on the shergottite purent hod)

Our conclusions con~rning the crystallization sequences and evolution of primary magmas on the shergottite parent body permit several inferences about the mineralogy and compositions of the source regions on this body, assuming that these primary magmas were generated on asteroidal parent bodies by low pressure partial melting of plagioclase peridotites. Since we have inferred that the primary liquids crystallized low-calcium pyroxene and augite before plagioclase, plagioclase would not have been left in these source regions as a residual phase at the degrees of partial melting which produced the primary liquids. The composition of the plagioclase in the primitive source peridotite can thus be approximated by the composition of the normative plagioclase in the primary liquid, although if fractional fusion (f%ESNELI..1969) were involved in the evolution of the shergottite primary liquids, the plagioclase in the primitive source peridotite could be somewhat more sodic than the normative plagioclase in the primary liquid. The composition of the normative plagioclase in

1491

Petroiogy and origin of the shergottite meteorites

E!

EN

Fig. 14. Phase diagram iilustrating the differences between basalt source regions on the shergottite parent body, on the basaltic achondrite parent body, on the nakhlite and chassignite parent bodies, and on the Earth. Phase boundaries are for ohvine-saturated equilibria at low pressures; phase boundaries for peridotites with normative plagioclase of An35, An50, and An100 are shown. The An50 and An100 boundaries are taken from STOLPER et al.(1979).The An35 boundaries are interpolated between the An0 and An50 boundaries of STOLPER et al.(1979). The plagioclase compositions given in the legend are those of normative plagioclases in these source regions. The estimates of the basaltic achondrite source region compositions are referenced in STOLPERet al. (1979). References for the upper mantle composition field are given in the text.

Shergotty and Zagami, An,,, must be similar to that of the primary liquid since little or no addition or subtraction of plagioclase was involved in the evolution of these cumulates from primary liquids. The low pressure melting of peridotites with An,, plagioclase can be modelled with the phase boundaries labelled ‘An50 in Fig. 14. These phase boundaries are for olivine-saturated equilibria and thus differ from those in Fig. 13. The smaller extent of the plagioclase field in Fig. 13 can be accounted for by the difference between olivine-saturated equilibria and equilibria which are not olivine-saturated (O'HARA, 1968). The olivine-saturated augite + lowcalcium pyroxene boundary may be nearer to the augite corner than the two pyroxene boundary which is not saturated with olivine (O'HARA, 1968; IRVINE, 1970), but the effect does not appear to be great. However, the Fe/(Fe + Mg) ratio is a major factor in determining whether pigeonite or orthopyroxene is the low-calcium pyroxene. Only source regions with compositions projecting in the shaded region of Fig. 14 can produce low pressure partial melts with the inferred crystallization sequence of the shergottite primary liquids, i.e. augite and low-calcium pyroxene before plagioclase. Point X has been taken as the apex of the shaded region, rather than point A of Fig. 13, because fractional fusion may have occurred in these source regions. The

same uncertainties in the upper bounds of the shaded region of Figure 13 discussed above apply to Fig. 14. These source regions contain olivine, low~alcium pyroxene, augite and An,, plagioclase. The first liquid produced on melting of such peridotites plots at point X (Fig. 14). Plagioclase is the first phase exhausted from these source regions on melting; after plagioclase exhaustion, the liquid composition moves down the olivine + augite + low-calcium pyroxene reaction curve until the next phase, either augite or low-calcium pyroxene, is exhausted from the source region. On further melting, the liquid composition moves into the olivine + low-calcium pyroxene field or the olivine + augite field on a curved path towards the projected bulk composition of the source. The lowcalcium pyroxene in the source may have been pigeonite, orthopyroxene, or both; if it was initially pigeonite, it may have inverted to orthopyroxene with increased melting and on greater degrees of partial melting, orthopyroxene may have been the only lowcalcium pyroxene left in the residue in the source region. If our suggestion that major augite fractionation was not involved in producing shergottite parent liquids from primary magmas and that augite was probably not a major phase in the source region residues of shergottite primary magmas is valid, then augite would probably have been the second phase

I497

E. STOWERand H. Y. MCSWEENJR

exhausted from the source region on melting and the source region bulk composition would project in the olivine + low-calcium pyroxene field in Fig. 14. In this case, only ohvine i low-calcium pyroxene would have been left in the residues at high degrees of partial melting. In the terminology of SRXPER et al. (1979) source regions of the shergottites would have been &-type sources.

VI. COMPARISON

WITH OTHER ACHONDRITES

Four groups of meteorites produced by ‘basaltic’ igneous activity are known: the basaltic ~chon~i~~ the shergottites, the nakhlites, and the chassignites. STOLPERer al. (1979) assumed that these four meteorite groups developed from liquids produced on asteroidal parent bodies by low pressure partial melting of plagioclase-bearing peridotites and attempted to understand the petrological and chemical difference between them in terms of the compositional differences between their source peridotites. The source regions of eucritic magmas were modelled as alkalipoor, metal-bearing peridoties with Ana, plagioclase, in which low-Ca pyroxenes were the only pyroxenes (STOLPER,1975: 1977). Liquids produced by melting of these source regions had the following crystallization sequence: low-calcium pyroxene + plagioclase-+ low-calcium pyroxene + plagioclase + augite. Estimates of the composition of these source regions are shown in Fig. 14. We have suggested that the source regions of the shergottites contained plagioclase ( -. An,,) and both augite and low-calcium pyroxene, and that pigeonite -I- augite probably crystallized from shergottite primary liquids before plagioclase. The range of permissible shergottite source region compositions is shown in Fig. 14, STOLPER et al. (1979) have suggested that the source regions of the parent mamas of the nak~l~t~ and chassignites contained sodic plagioclase (An,,_,,) and that augite was the dominant pyroxene. The crystallization sequences of liquids produced by melting of these source regions were olivine + augite~oIivine + augite + plagioclase. STOLPERet al. (1979) concluded that the compositions of these source regions project in the indicated area in Fig. 14. Figure 14 shows that the achondrite groups can be readily distinguished in terms of their source region compositions, mineralogies, and melting sequences. It is evident from Figs 13 and 14 that the shergottites and their source regions are truly distinct from the eucrites and the other basaltic achondrites and their source regions. The shergottite source regions were more oxidized, .riclier in alkalies and volatiles, and had higher a~ite/low~alcium pyroxene ratios than the source regions of the basaltic achondrite magmas. It. is also clear from Fig. 14 that the source regions of the shergottites were probably distinct from the source regions of the nakhlites and chagsignites, which were richer in alkalies and had higher augite/low-~l~iurn pyroxene ratios.

STOLPERet uf. (1979) suggested that the basaltic achondrite, shergottite, and nakhlite-chassignite source peridotites may have been related. Addition of a voiatile-rich, oxidized component to peridotites similar to the source regions of eucritic liquids could produce peridotites similar to those inferred for the shergottite source regions. The peridotites of the nakhlite and chassignite source regions could have been related to basaltic achondrite-ty~ source peridotites in a similar way, requiring greater addition of a volatile-rich component than the shergottite source peridot&es. Alternatively, the nakhlite-chassignite, shergottite, and basaltic achondrite source peridotites could represent a sequence produced by increasing volatile-loss from volatile-rich peridotites. Details of the petrological and geochemical evidence for these proposed relationships among the igneous achondrites may be found in STOLPERet al. (1979). Although these proposed relationships between the source regions of the achondrites are probably oversimplified, they and our conclusions concerning the petrologic differences between these source regions, summarized in Fig. 14, provide a framework for understanding the evolutions of the parent bodies of these achondrites. The crystallization ages of the eucrites ( - 4.57 AE, BWCK and ALL&GRE,1978) are consistent with their evolution on small bodies differentiated by heat sources such as decay of 26A1.However, the nakhlites and chassignites have ages of about 1.2.--1.4 AE (F~OCARDand HUSAIN, 1976; LANCET and LANCET, 1971) and shergottites have ages of -0.6 AE (NYQUKXer al., 1979). The shergottite age assumes that Sm/Nd systematics were not reset by a late collision; if this model is incorrect, then the igneous age could be older. If these meteorites are derived from asteroidal parent bodies, as we have assumed, these young ages present a problem because radioactively generated igneous activity appears difficult so late in the history of the solar system on small bodies (Hsut and TOKXX, 1977). Models requiring derivation of these meteorites from large bodies encounter the dynamic difficulties associated with ejecting material from the few large bodies which are available, i.e. Venus, Mars, Mercury, Earth (WETHERILL,1974). The absence of meteorites from the Moon supports the notion that meteorites are not derived from other Iarge bodies. Heat sources which may be potentially effective on small bodies at these later times are large impacts, collisions between parent bodies, or tidal heating. VII. CO~PAR~~N

WITH THE EARTH

A number of features of the shergottites suggest that a comparison with the Earth may be as meaningful as a comparison with other achondritic meteorites: The oxidation state of the shergottites is similar to that of terrestrial basafts. STOLPERet al. (1979) showed that there is a progression of increasing K/U

Petrology and origin of the shergottite meteorites ratios from the eucrites to the shergottites to the chassignites and nakhlites, and that the shergottite K/U ratio is similar to the K/U ratio of terrestrial basalts. Although shergottites and terrestrial basalts are clearly distinguished from each other by differences between their oxygen isotope ratios (CLAYTON,1977), and between their characteristic FeO-MnO concentrations (STOLPERet al., 1979), the similarities between shergottites and terrestrial basalts suggest that further comparisons might yield insights into the origins and evolutions of the Earth and the shergottite parent body. 1. The Earth’s upper made and shergottite source regions: a petrologic comparison Terrestrial basalts and their associates have a sufficiently broad range of compositions that examples plotting in each of the primary liquidus fields and on every multiple saturation boundary of Figs 13 and 14 are known. We have plotted the composition of a typical oceanic tholeiite in Fig. 13. This, like most terrestrial basalts, crystallizes high-calcium pyroxene and plagioclase before low~lcium pyroxene, in contrast to the parent liquids of the shergottites which we believe crystallized high-calcium pyroxene and low-calcium pyroxene before plagioclase. Terrestrial basalts with crystallization sequences similar to those inferred for shergottites are known, but they are not common (O’HARA, 1970; IRVINE, 1970). The differences in composition between common terrestrial basalts and shergottite parent liquids do not necessarily indicate major compositional differences between their source regions. These differences can probably be largely attributed to the influence of high pressures on terrestrial basalt petrogenesis as opposed to the entirely low pressure petrogenesis we have assumed for the shergottite parent liquids (e.g. O’HARA, 1968). Ideally, we would like to directly compare the composition of the Earth’s mantle-the source region of terrestrial basalts-with the range of permissible source regions of shergottite primary liquids. Unfortunately the composition of the Earth’s mantle is controversial; estimates plotting in almost every petrologically distinct region of Fig. 14 can be found in the literature. In part, this range of estimates probably reflects real heterogeneity in the Earth’s upper mantle. It is impossible to say whether any of this heterogeneity is primary and dates from the Earth’s own origin, or has all subsequently developed by igneous and metamorphic processes occurring in the upper mantle throughout geologic time. Is there now, or was there ever, a discrete composition of the upper mantle or the mantle as a whole which can be meaningfully compared with the range of permissible shergottite source region compositions? Indeed, the same question can be asked of the source regions on the shergottite parent body, which could have undergone several episodes of differentiation prior to the melting event which produced the shergottites.

1493

In Fig. 14 we show the area occupied by a number of estimates of the composition of the Earth’s upper mantle. This area includes pyrolite (RINGWOOD, 1977a), ‘undepleted’ mantle compositions based on analysis of ultramafic inclusions in basalt (HARRISet al., 1972; HUTCHI~ONet al., 1975), ‘sheared’ garnet lherzolite nodules from South African kimberlites (BOYD and NIXON, 1973), other ‘fertile’-looking garnet lherzolite compositions from South Africa (&EN, 1971; O'HARA et al., 1975), and an average garnet peridotite from CARS~L (1968) taken by SMITH (1977) as his preferred upper mantle composition estimate. The composition of the normative plagioclase of the low pressure equivalents of these mantle compositions varies from An,, to An,g; pyrolite, the sheared lherzolite nodules, and Carswell’s average peridotite have a narrower range, An,,_, 1. The range of plagioclase compositions overlaps the plagioclase composition in the inferred shergottite source regions. In addition, the area of Fig. 14 occupied by these estimates of the composition of the Earth’s upper mantle overlaps the area occupied by permissible shergottite source regions. In other words, low pressure partial melting of the Earth’s upper mantie could produce liquids which, in terms of their alkali content and phase relations, would be similar to inferred primary magmas on the shergottite parent body. We conclude that petrologically there appears to be overlap between permissible shergottite source regions and the Earth’s upper mantle (Fig. 14), both in terms of their low pressure melting sequences and their normative plagioclaase compositions. We emphasize, however, the uncertainties and controversies surrounding estimates of the composition of the Earth’s upper mantle and their relevance to a ‘primary’ mantle com~sition. 2. Trace and minor elements in shergottifes and terrestrial basalts

STOLPER(1979) showed that the average concentrations of a number of trace and minor elements in shergottites are similar to their average concentrations in terrestrial basalts. In Fig. 15, we compare the average concentrations of some trace and minor elements in shergottites with their average concentrations in terrestrial, eucritic, and lunar basalts as a function of equilibrium condensation temperature from a gas of solar ~om~sition. In all cases, the sh~gottite concentrations differ by less than a factor of 10 from average terrestriaf basalt concentrations (Fig. 15a). This close correspondence is striking, especially when one considers that the similarity extends across elements with widely differing cosmochemical and geochemical behaviors; from refractory to volatile elements, to typically siderophile, chalcophile, and lithophile elements, and from elements which are compatible to those which are incompatible during peridotite melting. The correspondence is even more remarkable in view of the cumulus phases in the shergottites, the elects of fractionation suffered by terrestrial basalts

1494

E. STOLPERand H. Y. M&WFI:N Jn

and shergottites, and the differences in the pressures of melting and the degrees of partial melting which produced the shergottites and terrestrial basalts, all of which might have some effect on the concentrations of the elements plotted in Fig. 15.

spondence, however, is not exact. The plagioclase in the shergottite source regions and in the low pressure equivalent of the Earths upper mantle is probably labradorite in both cases, but according to most estimates the plagioclase in the low pressure mantle norm would be somewhat more calcic than in sher3. linplications fir plunetury evolution gottite source regions. There are small, but apparently We have shown that the shergottites display a real differences in their oxygen isotopic characteristics number of similarities to terrestrial basalts: oxidation (CLAYTON,1977). SMITH and HERVIC (1979) note that state, concentrations of minor and trace elements with terrestrial basalts tend to be slightly hydrous while a wide range of geochemical affinities, and, possibly, shergottites have no detectable water. The Mn conlow pressure norms of their source regions. The correcentrations in shergottites and other igneous meteorites consistently differ from those of typical terrestrial , basalts by a factor of 2 ~3x On Fig. 15, this difference appears minor, but it probably reflects a real difference between &he compositions of basalt source regions on the Earth and the shergottite parent body. Though the concentrations of elements compared in Fig. 15 are similar in shergottites and terrestrial basalts, the use of an cwruge terrestrial basalt masks a number of complexities. For example, although K abundances in shergottites are similar to the average terrestrial basalt, K in terrestrial basalts shows a range of several orders of magnitude. This reflects, in part, the complexity of the Earth’s mantle-.-its continuous differentiation, initial heterogeneities, volatile transfer and metamorphism, and so on. Thus, aIthough the K abundances in shergottites are similar to the terrestrial average (especially when compared with abundances in eucritic meteorites and lunar basalts), we should not overlook the complexity of basalt chemistry and genesis on the Earth and the difficulties associated with choosing an ‘average’ terrestrial basalt. Despite these cautionary notes, we feel that the similarities between the shergottites and terrestrial CONDENSATION TEMP. t”K basaits and those inferred for their source regions are Fig. $5. Abundances of minor and trace elements in (a) striking, especially when contrasted with the equally shergottites. (b) eucrites, and (c) lunar basal& relative to striking differences between the Earth and the two average abundances in terrestrial basal@ as functions of other well cha~~teri~ed planetary bodies--the Moon equilibrium condensation temperatures from a gas of solar and the eucrite parent body (Fig. 15). We feel that composition (P = lo-’ atmospheres). Condensation temperatures from compilation of RINGWOODand KES~~N these similarities have important implications for the (t977), except Ir (1555°K; GROSSMAN,1973), and Cs and origin of the Earth’s upper mantle and the shergottite Rb (1000°K) which were arbitrarily assigned the same conparent body. densation temperature as Na and K. Average abundances Models of the origin of the Earth’s upper mantle in terrestrial basalts from sources cited in STOLPER(1979). which attribute its oxidation state, its siderophile eleAverage abundances in shergottites as given in ST~LPER (t979), except for Mn (3990 ppm; average of 8 values), Na ment abundances and its volatile element abundances (10110 ppm; average of 9 values), and Cr (1620 ppm; averto uniquely terrestrial processes and cond~tjons or to age of 6 values). Sources of shergottite data are: DUKE factors or processes unique to the origin and differen( 1968), LAULet nl. ( 1972), CHOUet al. (1976), JEROME(1970), SCXMITTez al. (1972), PHILPOTTSand SCHNETZLER (19701, tiation of large bodies (e.g. RINGW~D, 1975; RINGWOOD and KESSON,1977) are unattractive in light of MCCARTHYet ni. (i974), EA~TON and ELLIOTT(1977), NYQUISTer (II. (1978a), and IRVING (see caption of Fig. the similarities between shergottites and terrestrial 12, this work). Eucrite values are for Juvinas: abundances basatts. The shergottites clearly demonstrate that from Table 1 of MORGANet al. (1978). except for afkalis (STO~_PER, 1977). P (average of values in DUKE and SILVER, similar characteristics can exist on a body other than 1967, and MCCARTHYet al., 1973), Mn (DUKE and SILVER, the Earth, probably not derived from the Earth, and 1967) Cu (LAUL et al.. 1972) and Ga (average of values probably of smaller size. I

I

I

I

from LAUL er a/., 1972. and CH~U et nl., 1976). Average lunar basalt is from RINGWOOD and KEZB~N(1977), except Tl, Cs, and Rb which are based on abundances in lunar basalt 12002 (data from ANDERSPI rrl., 1971; WILLIS et izf., 1971; TAYLOR er ui.,1971).

BRETT (1977) suggested that the concentrations of siderophiles in the terrestrial upper mantle and in the

lunar mantle were controlled by equilibration with sulfide-rich melts during core formation. If this is cor-

Petrology and origin of the shergottite meteorites rect, then the shergottites or their source regions also probably equilibrated with sulfide melts, perhaps during segregation of a core on the shergottite parent body or if residual sulfide remained in the source regions of shergottite magmas during magma genesis. Otherwise, it would be difficult to explain why their concentrations of elements which are chalcophile in the absence of metal (e.g. Ni, Co, Cu and possibly other elements shown in Fig. 15) are similar to those of terrestrial basalts. However, although the absolute abundance levels and the details of the interelement fractionations among many elements in terrestrial basalts and shergottites may have been strongly influenced by equilibration of silicates with molten sulfide, this probably could not have resulted in the similarities between shergottites and terrestrial basalts shown in Fig. 15a unless the abundances of siderophile and chalcophile elements in their source regions were already quite similar prior to sulfide segregation. Thus, despite the possible effects of sulfide segregation on the abundances of siderophiles and chalcophiles, we believe that the overall similarities between terrestrial basalts and shergottites shown in Fig. 15a reflect corresponding similarities in the primordial compositions of their source regions. The similarities between the shergottite source regions and the Earth’s upper mantle can be explained by models which suggest that they were largely built out of the same materials. That is, the similarities in the characteristics of the terrestrial upper mantle and the shergottite parent body which we have been comparing may have been inherited from the materials from which they accreted. Proponents of models of Earth formation of the heterogeneous accretion type (e.g. CLARK et at., 1972; KIMURA et al., 1974) have suggesged that the siderophile and volatile element characteristics of the outer regions of the Earth and its high oxidation state may be largely due to accretion of material with these characteristics, possibly as a veneer due to late bombardment of the Earth by volatile-rich material. Perhaps the shergottite parent body was largely or entirely composed of the same type of oxidized and volatile- and siderophile-rich material which accreted late on the Earth. The abundance patterns of siderophiles in the source regions of terrestrial basalts and shergottites could either directly reflect that of the volatile-rich material (CLARK et al., 1972; K&~URAet af., 1974), or, as mentioned above, they could have been modified by equilibration with molten sulfide. In the latter case, the abundances of siderophiles in these source regions would only indirectly reflect the primordial abundances of the volatile-rich material, though these would have been similar on the Earth and the shergottite parent body. One of the possibilities discussed above is that the source regions of the shergottites were produced by addition of an oxidized, low-temperature component to a volatile-poor, reduced peridotite such as that envisioned for the source regions of eucritic liquids

1495

1979). The terrestrial upper mantle could have formed from a similar mixture (RINGWOOD, 1977a). These two components could have been mixed prior to or during accretion, possibly by late bombardment of the Earth by volatile-rich material (KIMURAet al., 1974). The similarities between the planets which we discussed could reflect the fact that the low-temperature materials mixed with the volatilepoor, reduced material were similar in both cases. Another possible explanation of the similarities between the shergottite source regions and the Earth’s upper mantle, which need not be independent of those mentioned in the last two paragraphs, is that the shergottites are derived from a body left over from the swarm of planetesimals which accreted to form the outer portions, or perhaps the entire mantle, of the Earth. That is, perhaps the Earth, or at least its outer regions, was largely composed of a group of planetesimals similar to the shergottite parent body. Despite small differences, the overall similarities between the oxygen isotopic characteristics of the shergottites and the Earth (CLAYMN, 1977) are consistent with this sugg~tion. Indeed, WE~R~L (1977) suggested that the differentiated meteorites may be derived from a group of asteroids on the inner margin of the asteroid belt, left over from the swarm of planetesimah from which the Earth and Venus formed. Though the asteroids referred to by WETHERILL(1977) are largely S-type asteroids, which may contain substantial proportions of metal, it is likely that the Earth accreted from a range of planetesimal types, some of which were metal-bearing like the eucrite parent body and others which were metal-free like the shergottite parent body. Alternatively, the shergottite parent body could have been vertically or laterally heterogeneous, containing both metal-free and metal-rich areas. One interesting prediction of the sorts of explanations of the similarities between the shergottite source regions and the terrestrial mantle which we have proposed is that we might expect to encounter similar characteristics on other bodies. For example, if the shergottites are indeed derived from bodies left over frrom the Earth-Venus planetesimal swarm, we might expect venusian basalts to be similar in several respects to shergottites and the terrestrial basalts. In this case, however, it may be difficult to understand why the moon is not more similar to the Earth and the parent body of the shergottites. Alternatively, if the characteristics which we have focused on are related to the late accretion of low-temperature materials on planets, we should expect similar characteristics on other bodies in addition to Venus, such as Mars, which could have attracted late-accreting material.

(STOLPER et al.,

VIII. CONCLUSIONS The shergottite meteorites appear to represent a link between the other differentiated meteorites and the Earth. Their high Fe0 contents high FeO/MnO ratios, and their meteoritic origin tie them closely to

1496

E. STOLPER and H. Y. MCSWEENJR

other igneous meteorites: the basaltic achondrites, the nakhlites, and the chassignites (STOLPER et al., 1979). In addition, these four groups of meteorites appear to define a petrological and geochemical sequence which may suggest a relationship between their parent bodies. Modelling of such a relationshiD reinforces the importance, emphasized by many others, of a volatile-rich, oxidized component in models of the bulk compositions of planetary bodies. The parent bodies of these meteorite groups could be related by direct addition of variable amounts of such a cornponent to a volatile-poor, reduced component or by variable degrees of volatile loss from a single type of volatile-rich, oxidized component. On the other hand, the shergottites display certain remarkable similarities to terrestrial basalts. They evolved at similar oxidation states, their K/U ratios are similar, and they have similar abundances of a number of minor and trace elements with a wide range of cosmochemical and geochemical behaviors. In addition, the low pressure norm of the Earth’s upper mantle may be similar in important respects to permissible basalt source regions on the shergottite parent body. Hence, the shergottite parent body, generally presumed to be asteroidal, was in many important respects similar to the Earth, a major terrestrial planet. There are many implications of the link between the Earth and the differentiated meteorites provided by the shergottites. We believe, however, that the most important implication is that viable models of planetary origin must be able to account for the connection between a major planet, the Earth, and the presumably asteroidal parent bodv of the shergottites. In this light, models which attribute the oxidation state, volatile and siderophile element abundances, and aspects of major element chemistry of the Earth’s upper mantle to factors unique to the origin and diffeientiation of the Earth 0; large bodies in general (for example, to differentiation and core formation at high pressures) are unattractive. On the other hand, models which attribute the similarities between shergottites and terrestrial basalts to homogeneous accretion of the mantles of their parent bodies from similar materials, to addition possibly late in the accretionary histories of both parent bodies of a volatile-rich, oxidized component, or to predominance of (differentiated?) planetesimals similar to and related to the shergottite parent body in the accretion of the Earth’s mantle, all appear to be acceptable. I

_

REFERENCES AHRENSL. H. (1970) The composition of stony meteorites -1X. Abundance trends of the refractory elements in chondrites. basaltic achondrites and Apollo 1I fines. Earth Planer. Sri. Left. 10, l-6. ALBEEA. and RAY L. (1970) Correction factors of electron probe micro-analysis of silicates. oxides, carbonates. phosphates and sulfates. Analyt. Gem. 42, 1408-1414. ANDERSE., GANAPATXYR., KEAYSR. R., LAUL J. C. and MORGANJ. W. (1971) Volatile and siderophile elements in lunar rocks: comoarison with terrestrial and meteoritic basalts. Proc. Second Lunnar Sci. Conf. Geochim. Cosmochim. Acta Suppl. 2, 1021-1036. BENCEA. E. and ABEEA. (1968) Empirical correction factors of the electron microanalysis of silicates and oxides. J. Geol. 76, 382--403. BINNSR. W. (1967) Stony meteorites bearing maskelynite. Nuture 214, 1I1 l-1 112. BIRCK J. L. and ALL!&RE C. J. (1978) Chronology and chemical history of the parrent body of basaltic achondrites studied by the 87Rb-87Sr method. Earth Planet. Sci. Letf. 39. 37.-51. B~GARD D. d. and HL~SAINL. (1977) A new 1.3 Aeonyoung achondrite. Geophys. Res. Lett. 4, 69-71. BOWENN. L. (1937) Recent high-temperature research on silicates and its significance in igneous geology. Am. J. sci. 33, l-21. BRETTR. (1977) Equilibration of the upper mantle with sulfide-rich liquid during core formation, and its application to the moon. EOS 58, 430. BUNCHT. E. and REID A. M. (1975) The nakhlites part I: petrography and mineral chemistry. Meteoritic.7 10, 303-3 15.

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CHOG’I. M. and WILLIAMSR. J. (1977) Hydrogen fugacity sensor measurements on the quartz-fayaliie-ma&et& and hematite-magnetite buffer reactions. EOS 58. 520. CLARKS. P.. TUREK~AN K. and GROSSMAN L. (1972) Model for the early history of the earth. In Nuture of the Solid Earrh (ed. E. C. Robertson), pp. 3-18. McGraw-Hill. CLAYTON R. N. and MAYEDAT. K. (1975) Genetic relations between the moon and meteorites. Pror. Lunar Sci. Co@ hth, Geochim. Cosmochim. Acta Suppl. 6, 1761-1769. CI.AYTONR. N. (1977) Genetic relations among meteorites and planets. In Comets Asteroids Meteorites (ed. A. H. Delsemme), pp. 545-5.50, U. of Toledo. CONSOLMAC;NO G. and DRAKE M. J. (1977) Composition and evolution of the eucrite parent body: evidence from rare earth elements. Geochim. Cosmochim. Acra 41, 1271-1282. DAROTM. (1973) Methodes d’analyse structurale et cinemarique. Application a’letude du massif ultrabasique de lo

Acknowledgements-SamDIes were urovided for this studv by R. S. CLARKE,JR (USGM) and i. HUTCHI~ON (BMNHj. We are grateful to J. V. SMIIX and A. J. IRVING,both of whom generously shared their data on shergottites with us prior to publicaiion. Reviews by J. DELANO,M. DRAKE, and M. DUKE and discussions with R. BRETT.R. N. CLAYTON,and G. WE-ILL were also helpful. This work was partially supported by NASA grants NSG 7413 to H. Y. MCSWEEN,JR and NGL 22-007-247 to J. F. HAYS.

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