Bulk Meteorite Research Articles

The porosity and permeability of chondritic meteorites and interplanetary dust particles

Abstract— We have investigated the porosity of a large number of chondritic interplanetary dust particles (IDPs) and meteorites by three techniques: standard liquid/gas flow techniques, a new, noninvasive ultrasonic technique, and image processing of backscattered images. The latter technique is obviously best‐suited to sub‐kilogram sized samples. We have also measured the gas and liquid permeabilities of some chondrites by two techniques: standard liquid/gas flow techniques, and a new, nondestructive pressure release technique. We find that chondritic IDPs have a somewhat bimodal porosity distribution. Peaks are present at 0 and 4% porosity; a tail then extends to 53%. Type 1–3 chondrite matrix porosities range up to 30%, with a peak at 2%. The bulk porosities for type 1–3 chondrites have the same approximate range as exhibited by the matrix, which indicates that other components of the bulk meteorites (including chondrules and aggregates) have the same average porosity as the matrix. These results reveal that the porosities of primitive materials at scales ranging from nanogram to kilogram are similar, which implies that similar accretion dynamics operated through 12 orders of size magnitude. Permeabilities of the investigated chondrites vary by several orders of magnitude, and there appears to be no simple dependence of permeability with degree of aqueous alteration, chondrite type or porosity.

Germanium isotopic compositions in Canyon Diablo spheroids

By using ICP-MS, we have measured the concentrations and isotopic abundances of Ge in the following samples: (1) the iron meteorites Camp Verde, Toluca, Picacho, and Canyon Diablo; (2) Canyon Diablo spheroids; and (3) oxide rims and metallic cores obtained by grinding Canyon Diablo spheroids. Whole Canyon Diablo spheroids contain appreciably more Ge than does the bulk meteorite. Germanium is enriched in the metallic cores of the spheroids where concentrations may exceed 500 ppm. It is depleted in the oxide rims compared to the metallic cores and to the bulk meteorite. The major isotope ratios, 72 Ge 70 Ge , 73 Ge 70 Ge , and 74 Ge 70 Ge ea Ge, were determined with an uncertainty of ∼0.3% (1σ,). The isotopic abundances of Ge in the bulk iron meteorites and the metallic cores of spheroids match terrestrial values. However, the oxide rims of spheroids contain mass-fractionated Ge (4 ±‰/AMU), providing the first direct evidence for evaporative loss from these objects. Assuming Rayleigh distillation and GeO as the evaporating species, we estimate that about 50% of the Ge in the precursor of the oxide rims remains there.

Boron isotope ratios in meteorites and lunar rocks

The 11B/10B ratios of thirty-two meteorite falls and nine lunar rocks were measured as Cs2BO2+ using thermal ionization mass-spectrometry. The 11B/10B ratios of meteorites vary from 4.011 to 4.098, i.e., their δ11B values (relative to NIST SRM 951) range from −10.5 to +19.2%; however, excluding two outliers, Mokoia and Norton County, the range of most meteorites is smaller ( −10.5 to +7.5). The average of two C11 meteorites, Ivuna and Orgueil, is -3.3, in the middle of the range. The δ11B values of the lunar rocks vary less than those of meteorites, from −6.0 to −3.9.The average δ11B of CI1 chondrites is −3.3, similar to that of terrestrial fresh mid-ocean ridge basalts (−6.5 to −1.2) and to the estimated mantle value of +0.2 (Ishikawa and Nakamura, 1992), which is the best representative of the whole Earth.The similarity of δ11B values in meteorites, lunar rocks, and those parts of the Earth unaffected by water implies that the boron isotopic composition of the Solar System is rather homogeneous.Recently, Chaussidon and Robert (1995) reported larger variation of δ11B values in chondrules of three chondrites, from −50 to +40. This degree of heterogeneity is absent from bulk meteorites.

Isotopic composition of carbonates in the SNC meteorites Allan Hills 84001 and Nakhla

Abstract— We have measured the 13C/12C and 14C/12C ratios in CO2 released by acid etching of the carbonate‐bearing SNC meteorites Allan Hills 84001 and Nakhla. Most of the C released is strongly enriched in 13C. In 10 out of 12 samples, 15‰ <δ13C < 55‰. Terrestrial values of carbonateδ13C from weathering products are generally between −10 and +10‰. Two leachate samples especially rich in 13C, ALH 84001,27 and Nakhla 25, have elemental Si/Mg ratios much lower than those of the bulk meteorites and 14C activities that are much lower than the values expected for terrestrial carbonates. The former observation indicates that these leachates consist primarily of carbonates and, less likely, phosphates. The latter observation implies that heavy C was introduced not by terrestrial weathering but by extraterrestrial processes. For ALH 84001,121 (sample 27) and Nakhla (BM 1913,26) δ13C = +41‰ and +35‰, respectively. The measured 18O/16O ratios in the leaches are similar: δ18O ∼ 15 ± 5‰, contrasting with 4.2‰ in the bulk silicates. We infer that the C in the carbonates retains an extraterrestrial isotopic signature, but probably not O, due to its ease of isotopic exchange (Cole and Ohmoto, 1986).

Acfer 217‐A new member of the Rumuruti chondrite group (R)

Abstract— Previously, three meteorites from Australia and Antarctica were described as a new chondritic “grouplet” (Carlisle Lakes, Allan Hills (ALH) 85151, Yamato (Y) −75302; Rubin and Kallemeyn, 1989). This grouplet was classified as the “Carlisle Lakes‐type” chondrites (Weisberg et al., 1991). Recently, one Saharan sample and four more Antarctic meteorites were identified to belong to this group (Acfer 217, Y‐793575, Y‐82002, PCA91002, PCA91241). The latter two are probably paired. With the meteorite Rumuruti, the first fall of this type of chondrite is known (Schulze et al., 1994). We report here on the Saharan meteorite Acfer 217 which has chemical and mineralogical properties very similar to Rumuruti and Carlisle Lakes. All eight members of this group, Rumuruti, Carlisle Lakes, ALH85151, Y‐75302, Y‐793575, Y‐82002, Acfer 217, and the paired samples PCA91002 and PCA91241 justify the introduction of a new group of chondritic meteorites, the Rumuruti meteorites (R).Acfer 217 is a regolith breccia consisting of up to cm‐sized clasts (∼33 vol%) embedded in a fine‐grained, well‐lithified clastic matrix. The most abundant mineral is olivine (∼72 vol%), which has a high Fa‐content of 37–39 mol%. The major minerals (olivine, low‐Ca pyroxene, Ca‐pyroxene, and plagioclase) show some compositional variability indicating a slightly unequilibrated nature of the meteorite. Considering the mean olivine composition of Fa37.8 ± 5.7, a classification of Acfer 217 as a R3.8 chondrite would result; however, Acfer 217 is a regolith breccia consisting of clasts of various petrologic types. Therefore, we suggest to classify Acfer 217 as a R3–5 chondrite regolith breccia. The bulk meteorite is very weakly shocked (S2).The bulk composition of Acfer 217 and other R‐meteorites show that the R‐meteorites are basically chondritic in composition. The pattern of moderately volatile elements is unique in R chondrites; Na and Mn are essentially undepleted, similar to ordinary chondrites, while Zn and Se contents are similar to concentrations in CM chondrites. The oxygen isotopic composition in Acfer 217 is similar to that of Rumuruti, Carlisle Lakes, ALH 85151, and Y‐75302. In a δ17O vs. δ18O‐diagram, the R‐meteorites form a group well resolved from other chondrite groups.Acfer 217 was a meteoroid of common size with a radius between 15–65 cm and with a single stage exposure history. Based on 21Ne, an exposure age of about 35 Ma was calculated.

Mineralogy, chemistry, and noble gas contents of Adzhi‐Bogdo– an LL3–6 chondritic breccia with L‐chondritic and granitoidal clasts

Abstract— Adzhi‐Bogdo is an ordinary chondrite regolith breccia (LL3–6) which fell on 1949 October 30 in Gobi Altay, Mongolia. The rock consists of submm‐ to cm‐sized fragments embedded in a fine‐grained clastic matrix. The polymict breccia contains various types of clasts, some of which must be of foreign origin. Components of the breccia include chondrules, melt rock clasts (some of which are K‐rich), highly recrystallized rock fragments (“granulites”), breccia clasts, pyroxene‐rich fragments with achondritic textures, and alkali‐granitoids. The composition of olivine in most fragments is in the range of LL‐chondrites. However, olivine in some components has significantly lower fayalite contents, characteristic of L‐chondrites. The bulk meteorite is very weakly shocked (S2).Based on the bulk chemical composition, Adzhi‐Bogdo is an ordinary chondrite. The concentrations of Fe and Ni are somewhat intermediate between L‐ and LL‐chondrites.The contents of solar gases indicate that Adzhi‐Bogdo is a regolith breccia. Most of the solar He and probably a part of the solar Ne of Adzhi‐Bogdo has been lost. It is suggested that Adzhi‐Bogdo experienced an (impact‐induced) thermal event early in its history, because most of the radiogenic 40Ar is retained.

Nitrogen isotopic compositions of iron meteorites

Iron meteorites analyzed in this study have nitrogen concentrations ⪕-70 μg/g and δ 15 N from −90 to +150%.. Although the iron meteorites have a large range of δ 15 N, most have values more negative than −50%.. The nitrogen isotopic compositions were established by cosmochemical processes and were little modified by fractional crystallization or other chemical processes within the parent bodies. The data do not suggest the existence of a well-mixed solar nebular reservoir for nitrogen, as was already inferred from data from stony meteorites. The range of > 1100%. observed for δ 15 N in bulk meteorites is probably too large to be accounted for by physical and chemical mass-dependent fractionation processes in the solar nebula, and thus reflects nebular inhomogeneities of nucleosynthetic origin. Nitrogen isotopes may serve as fingerprints for different reservoirs, preserved in various parent bodies, in a manner similar to that observed for oxygen isotopes in stony meteorites, and oxygen-bearing phases of stony irons.

Variations of the isotopic composition of sulfur in enstatite and ordinary chondrites

High-precision sulfur isotopic analyses (δ 33S, δ 34S, and δ 36S) of bulk ordinary and enstatite chondrites demonstrate that systematic variations exist. The average δ 34S values are −0.26 ± 0.07, −0.02 ± 0.06, and 0.49 ± 0.16%. for enstatite and ordinary and carbonaceous chondrites, respectively. Isotopic variations of different sample specimens of primitive meteorites, e.g., Qingzhen and Abee, have been observed which may be attributed to heterogeneity in the early solar nebula. Sulfur isotopic fractionations in both bulk samples and mineral separates are mass-dependent, and no nuclear isotopic anomalies have been detected. The sulfur isotopic compositions of both mineral and density separates have been measured. The sulfur isotopic compositions of separated chondrules from Chainpur and Bjurbole are reported. Significant isotopic difference for the chondrules from the bulk meteorite are noted for both meteorites.

Paired Renazzo-type (CR) carbonaceous chondrites from the Sahara

Ten chondrites with chemical and mineralogical similarities to the carbonaceous chondrite Renazzo were recovered at two locations of the Sahara: Acfer 059, 087, 097, 114, 139, 186, 187, 209, 270 and El Djouf 001. Although the El Djouf location is more than 500 km away from the Acfer location, all samples appear to result from a single fall based on chemical and petrographic similarities and supported by light element stable isotope geochemistry, noble gas record, and similar 26Al contents. The Acfer-El Djouf meteorite is classified as a CR (Renazzo-type) carbonaceous chondrite. This group presently comprises three non-Antarctic members (Al Rais, Renazzo, Acfer-El Djouf) and five Antarctic meteorites. The major lithological components of the Acfer-El Djouf meteorite are large chondrules (up to 1 cm in size; mean diameter: 1.0 ± 0.6 mm), chondrule and mineral fragments, Ca,Al-rich inclusions, FeNi-metal (about 8–10 vol%) and dark inclusions embedded in a fine-grained fragment-bearing groundmass. Mineral compositions of the ten Acfer-El Djouf samples are similar to those of other CR chondrites. Most of the Ca,Al-rich inclusions are below 300 μm in size and rich in melilite and spinel. In some CAIs the rare phase CaAl 4O 7 is dominant. Fo-rich, Cr-bearing olivine (Fa 0–4) and enstatite (Fs 0–4) are the major phases of the chondrite. The meteorite is mildly shocked with a shock stage of S2 indicating a peak shock pressure of 5–10 GPa for the bulk meteorite. The oxygen isotopic compositions and carbon and nitrogen stable isotope geochemistry of the Acfer-El Djouf samples are very similar to those of the other CR-type chondrites. The major element composition of the Acfer-El Djouf meteorite is indistinguishable from CR chondrites. When compared to Renazzo the Acfer-El Djouf samples, however, have systematically lower contents of the moderately volatile elements Zn, Ga, As, Au, Sb, and Se and the highly volatile elements Br, C, and N. This is thought to reflect primary differences between Renazzo and the Acfer-El Djouf meteorite.

Thep-nuclei: abundances and origins

The review discusses the solar system (meteoritic) abundances and the possible modes of nucleosynthesis of the 30-oddp-nuclei from74Se to196Hg. In addition to a discussion of the abundances for bulk meteorites, isotopic anomalies related to thep-nuclei are discussed; e.g., the Xe-HL associated with the ‘interstellar’ diamonds and the extinct radionuclides146Sm and92Nb. Various proposed schemes of synthesizingp-nuclei are reviewed. It is noted that the 7-process (i.e., photoerosion) operating in SN Ia (exploding C-O white dwarfs) appears capable of accounting for the relative and absolute abundances of all but one or two of the rarest ofp-nuclei. Synthesis of these latter nuclei is also discussed.

A ferroan region of the lunar highlands as recorded in meteorites MAC88104 and MAC88105

MacAlpine Hills 88104 and 88105 (MAC88104/5) are paired meteorites of noritic anorthosite composition from the lunar highlands. MAC88105 is a breccia composed mainly of melt-breccia clasts in a fine-grained, fragmental, and partly glassy matrix. The most abundant melt lithologies are feldspathic and are similar in composition to the bulk meteorite. Other melt lithologies include feldspathic melt rocks, mafic melt breccias, and a rare melt breccia relatively enriched in incompatible trace elements. Subordinate lithic clasts are granulitic breccias and ferroan (relatively low Mg [Mg + Fe] ) igneous lithologies, including troctolitic anorthosite, anorthositic norite, gabbronorite, and anorthosite. Igneous clasts having mafic mineral compositions more magnesian than Fo 55 and En 60 were not observed. Rare fragments of glass spheres and shards as well as glass clasts indicate that the meteorite was derived from an immature regolith. The bulk composition of MAC88105 is characterized by a molar Mg (Mg + Fe) ratio of 0.62, at the extreme low end of the range for meteorites from the lunar highlands. Its low concentrations of incompatible trace elements and feldspathic bulk composition (29% Al 2O 3), suggest that it, like the other lunar meteorites, formed at a site far removed from the areas sampled by the Apollo missions. Similarities in mineral compositions among the different lithologies of the breccia (melt-breccia clasts, granulitic breccias, igneous fragments) and the distribution of mineral fragments suggest that most components of the meteorite were derived from a crustal section dominated by material with a noritic anorthosite composition and an affinity to the ferroan suite of plutonic rocks. The mafic melt breccias have roughly peritectic melt compositions, similar to many “low-K Fra Mauro” melt breccias in the Apollo collection but much more ferroan, and represent a source material distinct from the noritic anorthosite. Moderate Ce anomalies, both positive and negative, occur in some separated clasts and to a lesser extent in some small chips dominated by matrix, but not in powder prepared from 20 g of bulk meteorite. These anomalies are almost certainly terrestrial weathering effects. One granulitic breccia clast is rich in siderophile elements as a result of a large amount of Fe-Ni metal. The Au Ir ratio of the clast (0.64, CI-normalized) is distinctly different from that of metal-rich impact melts from Apollo 16 (~3, CI-normalized). The compositional range observed among the nonmare lunar meteorites ALHA81005, Y791197, Y82192/3, Y86032, and MAC88104/5 is equivalent to that observed among representative samples of < 1 mm fines from Apollo 16. Thus, compositional data provide little constraint on the number of impacts required to eject these meteorites from the Moon.

Paired lunar meteorites MAC88104 and MAC88105: A new “fan” of lunar petrology

New lunar meteorite MAC88104/5 represents an exciting new opportunity to study a potentially unsampled region of the Moon. We have analyzed six thin sections by electron microprobe and three bulk samples by Instrumental Neutron Activation (INA) in order to determine the chemical characteristics of this new lunar sample. Lunar meteorite MACS 8104/5 is dominated by lithologies of the ferroan anorthosite (FAN) suite and contains abundant granulitized highland clasts, devitrified glass beads of impact origin, and two small clasts which appear to be of basaltic origin. One of these “asaltic” clasts (clast E in MAC88105,84) is probably mesostasis material, whereas the second larger clast (clast G) may be similar to the Very Low-Ti (VLT) or low-Ti/high-alumina mare basalts. Impact melt clasts MAC88105,69 and 72 have major and trace element compositions similar to the bulk meteorite. There is little evidence of any LKFM (Low-K Fra Mauro or low-K KREEP) contribution to this meteorite, as MAC88104/5 and other brecciated lunar meteorites are Fe-rich and poor in the incompatible elements relative to Apollo 16 regolith and feldspathic breccias. While the exact site of origin for the lunar meteorites cannot be pinpointed, it is evident that they were derived from a relatively KREEP-free ferroan anorthosite terrain.

Aqueous alteration of the Nakhla meteorite

Abstract— Interior samples of three different Nakhla specimens contain an iron‐rich silicate “rust” (which includes a tentatively identified smectite), Ca‐carbonate (probably calcite), Ca‐sulfate (possibly gypsum or bassanite), Mg‐sulfate (possibly epsomite or kieserite), and NaCl (halite); the total abundance of these phases is estimated as &lt;0.01 weight percent of the bulk meteorite. Rust veins are truncated and decrepitated by fusion crust and are preserved as faulted segments in partially healed olivine crystals, indicating that the rust is pre‐terrestrial in origin. Because Ca‐carbonate and Ca‐sulfate are intergrown with the rust, they are also indicated to be of pre‐terrestrial origin. Similar textural evidence regarding origins of the NaCl and Mg‐sulfate is lacking. Impure and poorly crystallized sulfates and halides on the fusion crust of the meteorite suggest leaching of interior (pre‐terrestrial) salts from the interior after Nakhla arrived on Earth but coincidental addition of these same salts by terrestrial contamination cannot be excluded. At least the clay‐like silicate “rust,” Ca‐carbonate, and Ca‐sulfate were formed by precipitation from water‐based solutions on the Nakhla parent planet although temperature and pressure conditions of aqueous precipitation are unconstrained by currently available data. It is possible that aqueous alteration on the parent body was responsible for the previously observed disturbance of the Rb‐Sr geochronometer in Nakhla at or near 1.3 Ga.

Cooling rates and parent bodies of iron meteorites from group IIICD, IAB, and IVB

New metallographic cooling rates have been calculated for the iron meteorites Carlton (IIICD), Toluca (IAB), and Odessa (IAB) based on the central Ni concentration versus taenite width method. New kamacite bandwidth cooling rates have been calculated for the members of group IVB, the ungrouped iron meteorite Chinga, and the high-Ni members of IIICD and IAB. The cooling rates determined are up to several orders of magnitude faster than those previously reported.A difference of about a factor of 60 is found between the kamacite bandwidth cooling rate and the central Ni versus taenite width cooling rate of the same meteorite, Carlton. This discrepancy is interpreted as resulting from extensive regolith accretion on the parent body at the time of Widmanstätten pattern formation.No systematic trend is found in the cooling rates as a function of bulk meteorite Ni of the magmatic group IVB. Combined with the strong inter-element correlations found in group IVB [15], the constant cooling rate suggests a core origin of this group.No systematic cooling rate variation is found in neither of the non-magmatic groups IIICD or IAB. Combined with the weak or missing inter-element correlations in these groups, the results of the present work support the megaregolith melt pool scenario suggested by Wasson et al. [1] for the origin of the non-magmatic groups IIICD and IAB.

The composition of kamacite in iron meteorites investigated by accelerator mass spectroscopy, neutron activation analysis and analytical electron microscopy

Accelerator mass spectroscopy (AMS) at Oxford has been used to investigate the noble metal elemental composition of kamacite from twelve iron meteorites. W, Re, Os, Ir, Pt and Au abundances of kamacite samples isolated from iron meteorites has been determined by AMS. We have also determined Co, Ir and Au by instrumental neutron activation analysis, and Ni by energy dispersive X-ray analysis. AMS is found to be an accurate method of a threshold potentially much lower than neutron activation analysis. Due to large scatter in the bulk meteorite abundances, we are not able to estimate partition of elements between kamacite and taenite. We find that, within the uncertainties, our kamacite abundance determinations reflects the bulk meteorite abundances.

An interstellar dust component rich in 12C

Chondritic meteorites contain grains which condensed in circumstellar environments and carry unusual isotope signatures1 providing clues to the location in which grain formation took place. Isotope measurements on grains that are composed entirely of carbon or carbon compounds have, up to now, given compositions either close to typical Solar System values or enriched in 13C (refs 2–5). Despite the existence of astrophysical environments where 12C production predominates, highly 12C-enriched carbon com-ponents in meteorites are uncommon. Here we report the results of an analysis of an acid-resistant residue from the Allende (CV3) meteorite1,6, extensively combusted in oxygen at low temperatures (<500 °C) to remove isotopically normal carbon, which revealed the presence of a small amount (<0.1 p.p.m.) of carbon with a 12C/13C ratio close to 120, well above the value of 89 ± 3 found in terrestrial samples or bulk meteorites. This component is denoted Cλ. Stellar evolutionary processes favour lowering of the 12C/13C ratio; hence Cλ, which has a combustion temperature consistent with graphite, might be a primitive type of interstellar material.

Isotopic anomalies of Ne, Xe, and C in meteorites. I. Separation of carriers by density and chemical resistance

In an attempt to characterize the carriers of presolar noble gases, 19—mainly non-colloidal—separates from the Murray and Murchison C2 chondrites were isotopically analyzed for Ne, Xe, and in several cases for C and N. Although most samples were strongly depleted in Xe-HL and enriched in Ne-E(H), Ne-E(L), and Xe-S, the separation methods failed to isolate the noble-gas carriers in pure form. Nonetheless, several new properties of the carriers were established. The carriers of Ne-E(H) and Xe-S are resistant to HCl, HF, boiling HClO 4, and CrO 3-H 2SO 4, and thus must be either diamond or some resistant carbide or oxide (SiC, hibonite, etc.), but not spinel, graphite, or amorphous carbon. These two noble-gas components seem to be associated with two heavy carbon components ( δC 13 ⋍ +1000% . and +1400%.), combusting at ~ 900 and ~ 1200°C, but cannot yet be assigned to either. The carrier of Ne-E(L), Cα, is slowly oxidized by Cr 2O − 7 and HNO 3, and rapidly by boiling HC1O 4. It may be some form of amorphous carbon, of δC 13 ⋍+340% . A new carbon component, Cθ, was found as 0.2–2 μm inclusions in Murchison spinel. It is amorphous, has δC 13 = −50%, and contains little or no noble gas, though the surrounding spinel contains a component of Ne 20 Ne 22 ⋍ 10.7 ± 0.2, possibly Ne-C from solar flares. The Cθ grains require a gas phase of C/O ≥ 1 for their formation, well above the solar ratio of 0.6, and presumably the occluding spinel, too, formed from such a gas. Both Cθ and spinel are enriched by ~4% in C 12 and O 16, respectively. This is of interest in connection with the suggestion by Schramm and Olive (1982) that much of the C 12, O 16, and Ne 20 in the solar system was contributed by a massive, nearby supernova. A new heavy nitrogen component ( δN 15 ≥ +252%.) has been found. It has an abundance of ~ 1 ppm in the bulk meteorite, combusts at 450–500°C, and may be associated with isotopically normal carbon or with Cα.

Carbonates and sulfates in CI chondrites: formation by aqueous activity on the parent body.

Compositions and morphologies of dolomites, breunnerites, Ca-carbonates, Ca-sulfates and Mg, Ni, Na-sulfates, and their petrologic interrelations, in four CI chondrites are consistent with their having been formed by aqueous activity on the CI parent body. Radiochronometric data indicate that this activity took place very early in Solar-System history. No evidence for original ("primitive") condensates seems to be present. However, alteration apparently took place without change in bulk meteorite composition.

Isotopic composition of silicon in meteorites

Abstract Samples of bulk meteorites show only mass-dependent fractionation of silicon isotopes. No isotopic anomalies were found. The variation of the ratios 29 Si/ 28 Si and 30 Si/ 28 Si over the meteorite classes is small; 1%. per mass unit difference. The average Si isotopic composition for each class of meteorites is identical, within analytical uncertainties. This is quite unlike O, whose anomalous isotopic abundances in bulk samples differentiate among the classes of meteorites. The overlapping abundance ranges of Si isotopes among many classes of meteorites suggest closed-system behavior for this element prior to meteorite accretion and allow calculation of an average solar system Si isotope composition.

Chemical systematics of the shergotty meteorite and the composition of its parent body (Mars)

We report chemical data for 60 elements by INAA and RNAA in two bulk samples, for 30 elements in various mineral separates of Shergotty, and results of leaching experiments with 1M HCl on powdered aliquots of Shergotty and BETA 79001, lithologies A and B. Shergotty is homogeneous in major element composition but heterogeneous with respect to LIL trace elements (~20%). The heterogeneity is even greater for volatile and siderophile trace elements. The mineral data, including three clinopyroxene fractions with variable FeO contents, maskelynite and minor phases (Ti-magnetite, ilmenite, quartz, K-rich phase), show that major minerals do not account for the rare earth elements (REE) in the bulk meteorite. Instead, the REE are to a large extent concentrated in accessory whitlockite and apatite (shown by leaching with 1M HCl): together with the majority of REE (La, 96%, Yb 70%), Cl and Br are quantitatively dissolved by leaching. The REE patterns of the leachate of Shergotty and EETA 79001 are different. The Shergotty leachate may consist of two components. Component l is similar to that of EETA 79001 leachate (whitlockite), component 2 is enriched in light REE and may be responsible for the higher LREE contents of Shergotty in comparison to the other shergottites. There is some evidence that Shergotty was an open system and component 2 was introduced after crystallization.The REE patterns of the residues of Shergotty and EETA 79001 are identical indicating that the parent magmas of both meteorites are compositionally similar. Based on cpx separates with the lowest REE content, the REE pattern in the Shergotty parent magma was calculated. It is enriched in LREE and has a subchondritic NdSm ratio. The negative Eu anomaly in the phosphates indicates that at least some plagioclase crystallized before phosphate.Based on several element correlations in SNC meteorites, it was suggested (Dreibus and Wänke, 1984) that both the Shergotty parent body (SPB, very probably Mars), and the Earth accreted from the same two chemically different components: component A, highly reduced and devoid of volatile elements and an oxidized component B containing also volatile elements. The SPB (Mars) mantle is 2–4 times richer in volatile and moderately siderophile elements than the Earth, indicating a higher portion of component B in the SPB. The concentrations of chalcophile elements in the SPB mantle are low, reflecting equilibration with a sulfide phase and subsequent segregation of sulfide into the core. Unlike the Earth (Wänke, 1981), the SPB (Mars) may therefore have accreted almost homogeneously.