Impact features of enstatite-rich meteorites

Enstatite-rich meteorites include EH and EL chondrites, rare ungrouped enstatite chondrites, aubrites, a few metal-rich meteorites (possibly derived from the mantle of the aubrite parent body), various impact-melt breccias and impact-melt rocks, and a few samples that may be partial-melt residues ultimately derived from enstatite chondrites. Members of these sets of rocks exhibit a wide range of impact features including mineral-lattice deformation, whole-rock brecciation, petrofabrics, opaque veins, rare high-pressure phases, silicate darkening, silicate-rich melt veins and melt pockets, shock-produced diamonds, euhedral enstatite grains, nucleation of enstatite on relict grains and chondrules, low MnO in enstatite, high Mn in troilite and oldhamite, grains of keilite, abundant silica, euhedral graphite, euhedral sinoite, F-rich amphibole and mica, and impact-melt globules and spherules. No single meteorite possesses all of these features, although many possess several. Impacts can also cause bulk REE fractionations due to melting and loss of oldhamite (CaS) – the main REE carrier in enstatite meteorites. The Shallowater aubrite can be modeled as an impact-melt rock derived from a large cratering event on a porous enstatite chondritic asteroid; it may have been shock melted at depth, slowly cooled and then excavated and quenched. Mount Egerton may share a broadly similar shock and thermal history; it could be from the same parent body as Shallowater. Many aubrites contain large pyroxene grains that exhibit weak mosaic extinction, consistent with shock-stage S4; in contrast, small olivine grains in some of these same aubrites have sharp or undulose extinction, consistent with shock stage S1 to S2. Because elemental diffusion is much faster in olivine than pyroxene, it seems likely that these aubrites experienced mild post-shock annealing, perhaps due to relatively shallow burial after an energetic impact event. There are correlations among EH and EL chondrites between petrologic type and the degree of shock, consistent with the hypothesis that collisional heating is mainly responsible for enstatite-chondrite thermal metamorphism. Nevertheless, the apparent shock stages of EL6 and EH6 chondrites tend to be lower than EL3-5 and EH3-5 chondrites, suggesting that the type-6 enstatite chondrites (many of which possess impact-produced features) were shocked and annealed. The relatively young Ar–Ar ages of enstatite chondrites record heating events that occurred long after any ²⁶Al that may have been present initially had decayed away. Impacts remain the only plausible heat source at these late dates. Some enstatite meteorites accreted to other celestial bodies: Hadley Rille (EH) was partly melted when it struck the Moon; Galim (b), also an EH chondrite, was shocked and partly oxidized when it accreted to the LL parent asteroid. EH, EL and aubrite-like clasts also occur in the polymict breccias Kaidun (a carbonaceous chondrite) and Almahata Sitta (an anomalous ureilite). The EH and EL clasts in Kaidun appear unshocked; some clasts in Almahata Sitta may have been extensively shocked on their parent bodies prior to being incorporated into the Almahata Sitta host.     

Diopside-bearing EL6 EET 90102: insights from rare earth element distributions

EET 90102 is the first known diopside-bearing EL6 chondrite. Diopside occurs in most aubrites and is occasionally found as rare small grains in unequilibrated enstatite chondrites, but is unknown from equilibrated enstatite chondrites. We have carried out a study of the rare earth element (REE) distributions in EET 90102, with a specific emphasis on diopside, in order to better understand its origin in this meteorite. We also present data for Ca-rich pyroxenes from two unequilibrated (EH3) enstatite chondrites for comparison.Our data show that diopside and other silicates in EET 90102 exhibit volatility-related anomalies indicative of formation under highly reducing conditions. Such anomalies have not previously been observed in EL6 chondrites, although they are common in unequilibrated enstatite chondrites. Diopside in EET 90102 probably formed by metamorphic equilibration of enstatite and oldhamite. The REE compositions of some grains, in particular the presence of positive Yb anomalies, indicate that they inherited their REE characteristics largely from CaS. Other grains have REE patterns that are more consistent with a derivation of diopside primarily from enstatite.In contrast to other EL6 chondrites, which experienced slow cooling, EET 90102 was quenched from high metamorphic temperatures. Thus, there may have been insufficient time to completely homogenize diopside REE compositions.The presence of diopside in EET 90102 simplifies one outstanding problem of aubrite formation. Melting of a diopside-bearing enstatite chondrite protolith provides a source for the abundant diopside in aubrites without requiring the oxidation of oldhamite, as suggested by previous research.     

Chemical and physical studies of type 3 chondrites—V: The enstatite chondrites

We report instrumental neutron activation analysis determinations of 19 major, minor and trace elements in three enstatite chondrites. Based on these, and literature data on the bulk and mineral composition of enstatite chondrites, we discuss the history of the type 3 or unequilibrated enstatite chondrites, and their relationship with the other enstatite chondrites. The type 3 enstatite chondrites have E chondrite lithophile element abundances and their siderophile element abundances place them with the EH chondrites, well resolved from the EL chondrites. Moderately volatile chalcophile elements are at the low end of the EH range and Cr appears to be intermediate between EH and EL. We suggest that the type 3 enstatite chondrites are EH chondrites which have suffered small depletions of certain chalcophile elements through the loss of shock-produced sulfurous liquids. The oxygen isotope differences between type 3 and other enstatite chondrites is consistent with equilibration with the nebula gas ~30° higher than the others, or with the loss of a plagioclase-rich liquid. The mineral chemistry of the type 3 chondrites is consistent with either low temperature equilibration, or, in some instances, with shock effects.     

Shock effects in “EH6” enstatite chondrites and implications for collisional heating of the EH and EL parent asteroids

Of the six chondrites that were listed as EH6 or EH6-an during the course of this study, we confirm the EH classification of Y-8404, Y-980211 and Y-980223 and the EH-an classification of Y-793225; two chondrites (A-882039 and Y-980524) are reclassified as EL (the former contains ferroan alabandite and both contain kamacite with ∼1 wt% Si). All of the meteorites contain euhedral enstatite grains surrounded by metal ± sulfide (although this texture is rare in Y-793225), consistent with enstatite crystallizing from a mixed melt. All contain enstatite with <0.04 wt% MnO; the three EH chondrites average 0.25 wt% Mn in troilite. (Literature data show that typical EH3-EH5 chondrites contain enstatite with 0.13-0.20 wt% MnO and troilite with 0.05-0.11 wt% Mn.) The three EH chondrites contain keilite [(Fe_>0.5,Mg_<0.5)S], which has been interpreted in the literature as a product of impact melting. Y-8404 and Y-980223 contain abundant silica (∼13 and ∼10 wt%, respectively), a rare phase in most enstatite chondrites. We suggest that all six meteorites have experienced impact melting; Mn was preferentially partitioned into sulfide during subsequent crystallization. The silica-rich samples may have become enriched in the aftermath of the impact by a redox reaction involving FeO and reduced Si. A-882039, Y-8404, Y-980211, Y-980223 and Y-980524 were incompletely melted; they contain rare relict chondrules and are classified as impact-melt breccias; Y-793225 is a chondrule-free impact-melt rock. If these EH and EH-an chondrites (which were previously listed as petrologic type 6) have, in fact, been impact melted, it seems plausible that collisional heating is generally responsible for EH-chondrite metamorphism. This is consistent with literature data showing that a large fraction (?0.7) of those chondrites classified EH5-7 and a significant fraction (?0.3) of those chondrites classified EH4 and EH4/5 possess textural and mineralogical properties suggestive of impact melting. In addition, ∼60% of classified EL6-7 chondrites (now including A-882039 and Y-980524) appear to have formed by impact melting. It thus seems likely that collisional heating is mainly responsible for EL- and EH-chondrite metamorphism.     

The compositional classification of chondrites: II The enstatite chondrite groups

We present new data from a neutron activation analysis of four enstatite chondrites including the taxonomically important St. Sauveur, and discuss the classification of enstatite chondrites. The enstatite chondrites can be divided into two compositionally distinct sets; in one set abundances of nonrefractory siderophiles and moderately volatile chalcophiles and alkalis are 1.5–2.0× higher than in the other. A well-resolved compositional hiatus separates these two sets. The differences in composition are as great as those between the groups of ordinary chondrites, and therefore it appears best to treat these sets as separate groups. By analogy with the symbols used for ordinary chondrites we propose to designate the high-Fe, high siderophile group EH and the low-Fe, low-siderophile group EL. Known members of the EH group belong to petrologic types 4 and 5, whereas all EL members are petrologic type 6. Within the EH group no correlation is observed between petrologic type and abundance of nonrefractory siderophiles or moderately volatiles or alkalis.Two physical properties show only modest overlap between the EH and EL groups. Cosmic-ray ages for EH chondrites are 0.5–7 Ma, while those for EL chondrites are 4–18 Ma. Relative to Bjurböle, I-Xe formation intervals are $\text{?1.3 ± 0.6}$ Ma for EH chondrites and $\text{2.9 ± 0.5}$ Ma for EL chondrites. The weight of the chemical and physical evidence indicates that the EH and EL groups formed separate bodies at similar distances from the Sun.The available evidence for Shallowater and Happy Canyon, two strongly recrystallized silicate-rich meteorites containing > 40 mg/g Fe-Ni, indicates that the former is an enstatite-clan chondrite altered by loss of sulfide- and plagioclase-rich melts, whereas the latter is intermediate in composition between EL chondrites and the chondritic silicates in the Pine River IAB-anomalous meteorite.     

清镇陨石（EH3）硫镁矿微量元素化学特征

本文应用电子探针和中子活化分析方法详细研究了清镇陨石(EH 3)中硫镁矿的化学组成和微量元素分布、硫镁矿携带了部分HREE、高度富集钪等难熔亲石元素,论证了该矿物的高温成因,REE丰度可能与陨硫钙石互补。该矿物含有钠-硒组分,可能是顽火辉石陨石独有的组分。铬归一化的钠-钴(原子比)相关关系具有CI一致的趋势,表明其母体来自太阳组成的气体星云。     

A reappraisal of the metamorphic history of EH3 and EL3 enstatite chondrites

The thermal history of a series of EH3 and EL3 chondrites has been investigated by studying the degree of structural order of the organic matter (OM) located and characterized in matrix areas by Raman micro-spectroscopy. By comparison with unequilibrated ordinary chondrites (UOCs) and CO and CV carbonaceous chondrites, the following petrologic types have been assigned to various E chondrites: Sahara 97096 and Allan Hills 84206: 3.1-3.4; Allan Hills 85170 and Parsa: 3.5; Allan Hills 85119: 3.7; Qingzhen, MacAlpine Hills 88136 and MacAlpine Hills 88184: 3.6-3.7. The petrologic type of Qingzhen is consistent with the abundance of the P3 noble gas component, a sensitive tracer of the grade of thermal metamorphism. The petrologic types are qualitatively consistent with the abundance of fine-grained matrix for the whole series. No significant effects of shock processes on the structure of OM were observed. However such processes certainly compete with thermal metamorphism and the possibility of an effect cannot be fully discarded, in particular in the less metamorphosed objects. The OM precursors accreted by the EH3 and EL3 parent bodies appear to be fairly similar to those of UOCs and CO and CV carbonaceous chondrites. Raman data however show some slight structural differences that could be partly accounted for by shock processes. The metamorphic history of EH3 and EL3 chondrites has often been described as complex, in particular regarding the combined action of shock and thermal metamorphism. Because OM maturity is mostly controlled by the temperature of peak metamorphism, it is possible to distinguish between the contributions of long duration thermal processes and that of shock processes. Comparison of the petrologic types with the closure temperatures previously derived from opaque mineral assemblages has revealed that the thermal history of EH3 and EL3 chondrites is consistent with a simple asteroidal onion shell model. Thermal metamorphism in enstatite chondrites appears to be fairly similar to that which takes place in other chondrite classes. The complex features recorded by mineralogy and petrology and widely reported in the literature appear to be mostly controlled by shock processes.     

Nature of volatile depletion and genetic relationships in enstatite chondrites and aubrites inferred from Zn isotopes

Enstatite meteorites include the undifferentiated enstatite chondrites and the differentiated enstatite achondrites (aubrites). They are the most reduced group of all meteorites. The oxygen isotope compositions of both enstatite chondrites and aubrites plot along the terrestrial mass fractionation line, which suggests some genetic links between these meteorites and the Earth as well.For this study, we measured the Zn isotopic composition of 25 samples from the following groups: aubrites (main group and Shallowater), EL chondrites, EH chondrites and Happy Canyon (impact-melt breccia). We also analyzed the Zn isotopic composition and elemental abundance in separated phases (metal, silicates, and sulfides) of the EH4, EL3, and EL6 chondrites. The different groups of meteorites are isotopically distinct and give the following values (‰): aubrite main group (−7.08 < δ⁶⁶Zn < −0.37); EH3 chondrites (0.15 < δ⁶⁶Zn < 0.31); EH4 chondrites (0.15 < δ⁶⁶Zn < 0.27); EH5 chondrites (δ⁶⁶Zn = 0.27 ± 0.09; n = 1); EL3 chondrites (0.01 < δ⁶⁶Zn < 0.63); the Shallowater aubrite (1.48 < δ⁶⁶Zn < 2.36); EL6 chondrites (2.26 < δ⁶⁶Zn < 7.35); and the impact-melt enstatite chondrite Happy Canyon (δ⁶⁶Zn = 0.37).The aubrite Peña Blanca Spring (δ⁶⁶Zn = −7.04‰) and the EL6 North West Forrest (δ⁶⁶Zn = 7.35‰) are the isotopically lightest and heaviest samples, respectively, known so far in the Solar System. In comparison, the range of Zn isotopic composition of chondrites and terrestrial samples (−1.5 < δ⁶⁶Zn < 1‰) is much smaller ( [Luck et al., 2005] and [Herzog et al., 2009]).EH and EL3 chondrites have the same Zn isotopic composition as the Earth, which is another example of the isotopic similarity between Earth and enstatite chondrites. The Zn isotopic composition and abundance strongly support that the origin of the volatile element depletion between EL3 and EL6 chondrites is due to volatilization, probably during thermal metamorphism. Aubrites show strong elemental depletion in Zn compared to both EH and EL chondrites and they are enriched in light isotopes (δ⁶⁶Zn down to −7.04‰). This is the opposite of what would be expected if Zn elemental depletion was due to evaporation, assuming the aubrites started with an enstatite chondrite-like Zn isotopic composition. Evaporation is therefore not responsible for volatile loss from aubrites. On Earth, Zn isotopes fractionate very little during igneous processes, while differentiated meteorites show only minimal Zn isotopic variability. It is therefore very unlikely that igneous processes can account for the large isotopic fractionation of Zn in aubrites. Condensation of an isotopically light vapor best explains Zn depletion and isotopically light Zn in these puzzling rocks. Mass balance suggests that this isotopically light vapor carries Zn lost by the EL6 parent body during thermal metamorphism and that aubrites evolved from an EL6-like parent body. Finally, Zn isotopes suggest that Shallowater and aubrites originate from distinct parent bodies.     

Chemical composition and origin of the earth's primitive mantle

An attempt has been made to estimate the chemical composition of the earth's primitive mantle by a critical evaluation of data derived from ultramafic mantle samples and partial melting model calculations for mafic and ultramafic magmas of various ages.Compatible (Al, Ca, Si, Mg, Fe) and moderately incompatible (Ti, Zr, heavy and middle rare earth) elements in basaltic magma sources have not changed significantly since the early Archaean (~3.5 Byr). Estimated abundances for refractory lithophile elements (such as Al, Ca, Ti, Zr, Y, Se, REE etc.) in the primitive mantle are about 2.0 times ordinary chondrites (~ 1.1 times Cl chondrites relative to Mg). Highly incompatible volatile elements (K, Rb, Cs, Tl, Pb etc.) are depleted in the mantle throughout geological time. Abundances of Fe, Ni and Co are obtained on the basis of values for ultramafic nodules and model calculations using komatiites of various ages. The results show little (? 20%?) dispersion and there is no obvious secular variation since 3.5 Byr. Noble metals show similar effects. These data permit constraints to be placed on the timing of core formation.The estimated elemental abundances for the primitive mantle are normalized to Cl chondrites relative to Mg and plotted against the solar condensation temperature at 10^?4 atm. Above 700 K there are two parallel trends which are defined by lithophile elements (Al, Ca, REE, Ti, Mg, Si, Cr, Mn, Na, K, Rb, F, Zn etc.) and siderophile elements (W, Ni, Co, P, As, Ag, Sb and Ge) respectively. The depletion factor for the siderophile trend relative to the lithophile trend is about 0.085. Within each trend there is a continuous depletion towards lower temperature. A third trend is defined by noble metals (Ir, Os, Re, Pd, Pt and Au) with a depletion factor of about 0.003 relative to Cl chondrites. These trends are interpreted in terms of core-mantle differentiation and volatility-controlled processes operating before and during earth accretion.     

Compositional differences in enstatite chondrites based on carbon and nitrogen stable isotope measurements

Carbon and nitrogen abundance and isotopic compositions, from four EH4, one EH5, five EL6 chondrites and one aubrite, were determined by using stepped pyrolysis (N only) and combustion (N and C) extractions in attempts to distinguish the components present. Carbon contents range from 0.15 to 0.70 wt%, with no systematic relationship between carbon content and meteorite group or petrologic type. Whole-rock δ¹³C values range from −28.5 to −4.1 %., Most C occurs as graphite and when temperature steps above 700°C are considered, there is a difference between EH4,5 (δ¹³C = −9.1 to -5.8%.) and EL6 chondrites (δ¹³C = −6.7 to +4.2%.). Carbon in Bustee aubrite is isotopically lighter (δ¹³C = −24%.) than in any enstatite chondrite.

Nitrogen occurs as osbornite, sinoite and in isostructural substitution for oxygen in silicate lattice sites. Nitrogen abundances and isotopic compositions are more variable than C, due to the heterogenous distribution of N-bearing minerals. Three EL6's containing osbornite have higher N concentrations than other type 6 enstatite chondrites. Sinoite, where present, is depleted in ¹⁵N relative to osbornite. Nitrogen in the Bustee aubrite has a similar abundance and δ¹⁵N value to those of EL6's, again dominated by the presence of osbornite.

In addition to the refractory C-and N-bearing minerals there is also organic material (largely terrestrial contamination) and evidence for at least two “exotic” components. The first is a host for Xe (HL) and is characterized by δ¹³C <-−47%. and δ¹⁵N ≤−73%., whereas the second is less well-defined, but is marked by δ¹⁵N = +269%.     

REE and Other Trace Element Chemistry of Oldhamite（CaS） in the Qingzhen Chondrite（EH3） and Their Genetic Implications

Instrumental neutron activation analysis(INAA) of 14 single oldhamite grains separated from the Qingzhen chondrite (EH3) for refractory(La,Ce,Sm ,Eu,Yb,Lu,Ca,Sc,Hf, and Th),volatile (Na,Cr,Zn,Se,Br,etc.)and siderophile elements (Fe,Ni,Co,Ir,Au ,and As) revealed that oldhamite is highly rich in refractory elements.The mineral serves as the principal carrier of REE and contains about 80% of the REEs in the Qingzhen enstatite chondrite .Furthermore, the large enrichment of LREE relative to HREE is noticed in oldhamite from the Qingzhen .In general, the oldhamite from metal-sulfide assemblages is richer in REE than that from the matrix,i.e.,the earlier the oldhamite grains condensed, the richer they are in REE. Meanwhile.oldhamite is also rich in vol-atile elements such as Se,Br, etc.In terms of the distribution of trace elements in oldhamitc from the Qingzhen ,the chondrite is srggested to have resulted from high-temperature condensation of solar nebula.     

Trace elements in the Allende meteorite—I. Coarse-grained,Ca-rich inclusions

INAA of ten coarse-grained, melilite-spinel-bearing inclusions in the Allende meteorite for Ca, Sc, Hf, Ta, W, Os, Ir, Ru, La, Ce, Sm, Eu, Tb, Dy, Yb, Fe, Co, Cr and Au reveals that all of the refractory elements are enriched by a mean factor of 18.6 relative to their concentrations in Cl chondrites, consistent with a high-temperature condensation origin for the inclusions. Os, Ir and Ru were probably incorporated by the inclusions as tiny nuggets of an alloy in which they were dissolved in cosmic proportion to one another. Sc and Hf entered the inclusions in a separate phase, also in cosmic proportion, accompanied by a fraction of the REE. Bulk REE abundances are independent of the major minerals in the inclusions; yet, data from mineral separates suggest that the REE were partitioned between coexisting melilite and pyroxene according to crystal structure controls. A two-stage model is proposed in which the REE first entered the inclusions as trace, refractory condensate phases and then re-distributed themselves between the crystallizing major phases after the inclusions were melted in the nebula.     

The unequilibrated enstatite chondrites

The enstatite chondrites formed under highly reducing (and/or sulfidizing) conditions as indicated by their mineral assemblages and compositions, which are sharply different from those of other chondrite groups. Enstatite is the major silicate mineral. Kamacite is Si-bearing and the enstatite chondrites contain a wide variety of monosulfide minerals that are not present in other chondrite groups. The unequilibrated enstatite chondrites are comprised of two groups (EH3 and EL3) and one anomalous member (LEW 87223), which can be distinguished by differences in their mineral assemblages and compositions. EH3 chondrites have >1.8 wt.% Si in their kamacite and contain the monosulfide niningerite (MgS), whereas EL3 chondrites have less than 1.4 wt.% Si in their kamacite and contain the monosulfide alabandite (MnS). The distinct mineralogies, compositions and textures of E3 chondrites make comparisons with ordinary chondrites (OCs) and carbonaceous chondrites (CCs) difficult, however, a range of recrystallization features in the E3s are observed, and some may be as primitive as type 3.1 OCs and CCs. Others, especially the EL3 chondrites, may have been considerably modified by impact processes and their primary textures disturbed. The chondrules in E3 chondrites, although texturally similar to type I pyroxene-rich chondrules, are sharply different from chondrules in other chondrite groups in containing Si-bearing metal, Ca- and Mg–Mn-rich sulfides and silica. This indicates formation in a reduced nebular environment separate from chondrules in other chondrites and possibly different precursor materials. Additionally the oxygen isotope compositions of E3 chondrules indicate formation from a unique oxygen reservoir. Although the abundance, size distribution, and secondary alteration minerals are not always identical, CAIs in E3 chondrites generally have textures, mineral assemblages and compositions similar to those in other groups. These observations indicates that CAIs in O, C and E chondrites all formed in the reservoir under similar conditions, and were redistributed to the different chondrite accretion zones, where the secondary alteration took place. Thus, chondrule formation was a local process for each particular chondrite group, but all CAIs may have formed in the similar nebular environment. Lack of evidence of water (hydrous minerals), and oxygen isotope compositions similar to Earth and Moon suggest formation of the E chondrites in the inner solar system and make them prime candidates as building blocks for the inner planets.     

Rhodium, gold and other highly siderophile element abundances in chondritic meteorites

The abundances of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, Rh, Pd and Au, and ¹⁸⁷Os/¹⁸⁸Os isotope ratios have been determined for a set of carbonaceous, ordinary, enstatite and Rumuruti chondrites, using an analytical technique that permits the precise and accurate measurement of all HSE from the same digestion aliquot. Concentrations of Re, Os, Ir, Ru, Pt and Pd were determined by isotope dilution ICP-MS and N-TIMS analysis. The monoisotopic elements Rh and Au were quantified relative to the abundance of Ir.Differences in HSE abundances and ratios such as Re/Os, ¹⁸⁷Os/¹⁸⁸Os, Pd/Ir and Au/Ir between different chondrite classes are further substantiated with new data, and additional Rh and Au data, including new data for CI chondrites. Systematically different relative abundances of Rh between different chondrite classes are reminiscent of the behaviour of Re. Carbonaceous chondrites are characterized by low average Rh/Ir of 0.27 ± 0.03 (1s) which is about 20% lower than the ratio for ordinary (0.34 ± 0.02) and enstatite chondrites (EH: 0.33 ± 0.01; EL: 0.32 ± 0.01). R chondrites show higher and somewhat variable Rh/Ir of 0.37 ± 0.07.Well-defined linear correlations of HSE, in particular for bulk samples of ordinary and EL chondrites, are explained by binary mixing and/or dilution by silicates. The HSE carriers responsible for these correlations have a uniform chemical composition, indicating efficient homogenization of local nebular heterogeneities during or prior to the formation of the host minerals in chondrite components. Excepting Rumuruti chondrites and Au in carbonaceous chondrites, these correlations also suggest that metamorphism, alteration and igneous processes had negligible influence on the HSE distribution on the bulk sample scale.Depletion patterns for Rh, Pd and Au in carbonaceous chondrites other than CI are smoothly related to condensation temperatures and therefore consistent with the general depletion of moderately volatile elements in carbonaceous chondrites. Fractionated HSE abundance patterns of ordinary, enstatite and Rumuruti chondrites, however, are more difficult to explain. Fractional condensation combined with the removal of metal phases at various times, and later mixing of early and late formed metal phases may provide a viable explanation. Planetary fractionation processes that may have affected precursor material of chondrite components cannot explain the HSE abundance patterns of chondrite groups. HSE abundances of some, but not all Rumuruti chondrites may be consistent with solid sulphide-liquid sulphide fractionation processes during impact induced melting.     

Trace elements in the Allende meteorite—IV. Amoeboid olivine aggregates

INAA data for Ca, Sc, Hf, La, Ce, Sm, Eu, Tb, Yb, Lu, Os, Ir, Ru, Na, Cl, Br, Fe, Mn, Cr, Co, Au, As, and Sb are presented for ten amoeboid aggregates from the Allende meteorite. Only one lacks olivine. Seven of the remainder, as a group, have cosmic proportions of refractory lithophile and siderophile elements and appear to have formed when coarse-grained Allende inclusion material underwent partial reaction with a low-temperature nebular gas and mixture with FeO-rich olivine. The other two have highly fractionated abundances of refractory elements relative to one another compared to Cl chondrites, including Group II REE patterns, and probably formed by the mixing of fine-grained Allende inclusion material with FeO-rich olivine. Non-refractory siderophile components are also different in composition in each type of amoeboid olivine aggregate.     

Enstatite chondrites: Trace element clues to their origin

We have analyzed by RNAA 3 EH and 3 EL chondrites for 20 trace elements. Interelement correlations were examined visually and by factor analysis, to assess the effects of nebular fractionation and metamorphism.Refractory siderophiles (Ir, Os, Re) correlate with “normal siderophiles” (Ni, Pd, Au, Sb, and Ge) in EL's but not EH's; presumably these two element groups originally condensed on separate phases (CAI and metal), but then concentrated in metal during metamorphism. Sb and Ge are more depleted than the other three elements of the “normal” group, presumably by volatilization during chondrule formation.Volatiles are consistently more depleted in EL's than EH's, by factors >10× for the more volatile elements. Some of the stronger correlations are found for In-Tl, Tl-Bi, and Zn-Cd-In. These correlations are about equally consistent with predicted condensation curves for the solar nebula (especially for host phases with negative heats of solution, or for P = 0.1?1 atm) and with volatilization curves for artificially heated Abee, as determined by M E. Lipschutz and coworkers at Purdue. No decisive test between these alternatives is available at present, but the close correlation of Zn, Cd, In may eventually provide a crucial test.Factor analysis shows that 3 factors account for 93% of the variance; they seem to reflect volatile (F1), siderophile (F2), and chalcophile (F3) behavior. The element groupings agree largely with those recognized visually; they are listed with the inferred host phases. F1 (minor sulfide, probably ZnS): Zn, Cd, In, Br; F2 (CAI, later metal): Ir, Os. Re; F1, F2 (metal): Ni, Pd, Au, Ge, Sb; F3, F1 (FeS): Se, Te, Bi, Tl. These correlations differ to some extent from those obtained by Shaw (1974) in an earlier factor analysis, presumably because the new data are more homogeneous and extensive, especially for siderophiles. The new correlations also show that the cosmochemical behavior of some volatiles in E-chondrites differs from that predicted for ordinary chondrites, so that condensation curves for the latter are not strictly applicable.     

滇东地区碳酸盐岩上覆风化层的元素地球化学特征及其成因讨论

碳酸盐岩风化形成的红土保存着喀斯特发展演化历史证据,同时也是喀斯特地区土壤研究的重要对象。文章选取云 南石林地区的两处典型碳酸盐岩剖面为研究对象,对主量元素,微量元素及稀土元素在风化层的迁移特征及分布规律进行 研究,为探究风化层的成因提供依据。结果显示：（1） 以Ti为参比元素的剖面迁移特征表明,两剖面的主量元素在成土过 程中有相似的迁移规律,多数表现为淋失;微量元素略有差异,富集淋失程度不一。（2） UCC 标准化蜘蛛图显示,相对于 基岩,风化层中的Ca和Sr均出现亏损;与UCC相比,Fe、Ti等元素轻微富集,Mg、Ca、Na、K、P等元素显示了强烈的亏 损特征。（3） 基岩与风化层的REE分布模式相似,但风化层的稀土相对富集,轻稀土元素间的分异较大而重稀土元素间的 分异较小,且SJC剖面的轻、重稀土元素比值大于QST剖面;稀土元素球粒陨石标准化后,SJC剖面的Eu为负异常,剖面 上部和下部出现Ce负异常;QST剖面Ce负异常,Eu明显负异常。（4） 元素含量变化和元素对Al-Ti、Al-Fe及Zr-Hf相关性 说明剖面上覆红土是下伏基岩风化的结果。研究结果显示,两个剖面的元素地球化学特征与基岩存在很好的继承性,风化 层是基岩原位风化的产物。     

The Varre-Sai chondrite,a Brazilian fall: petrology and geochemistry

The Varre-Sai meteorite fell along the border of the states of Espirito Santo and Rio de Janeiro, Brazil; on 19 June 2010 at 5:40 pm. Petrography and X-ray powder diffraction (XRD) indicate that the rock is an L5 S4 chondrite, with blastoporphyritic texture that has not been previously described. Geochemical data based on major and rare-earth elements (REEs) show that Varre-Sai is highly similar to the other L chondrites. In Harker diagrams, Varre-Sai, L, and LL chondrites form a single group, suggesting no significant chemical differences between them and contributing to the long-standing debate of whether LL chondrites form a distinct group or whether they are a subset of the L group. Harker diagrams also define a trend from E to H and L/LL chondrites, similar to the cosmochemical trends suggested by other authors. The behaviour of Fe₂O_3t and NiO indicates a relationship with Fe-Ni alloys, and their trend in the diagram suggests some chemical differentiation in the ordinary chondrite parental bodies. The REE content in Varre-Sai, normalized to C chondrites, falls in the field of L chondrites and others, but with slight REE enrichment. The chemical differences in chondrites, mainly in REEs, Fe₂O_3t and NiO could be alternatively interpreted as variations in the inherited agglutinated materials as chondrules, Ca–Al-rich inclusions and Fe–Ni nodules.     

Physical properties of chondrules in different chondrite groups: Implications for multiple melting events in dusty environments

Chondrite groups (CV, CK, CR) with large average chondrule sizes have low proportions of RP plus C chondrules, high proportions of enveloping compound chondrules, high proportions of chondrules with (thick) igneous rims, and relatively low proportions of type-I chondrules containing sulfide. In contrast, chondrite groups (CM, CO, OC, R, EH, EL) with smaller average chondrule sizes have the opposite properties. Equilibrated CK chondrites have plagioclase with relatively low Na; equilibrated OC, R, EH and EL chondrites have more sodic plagioclase. Enveloping compound chondrules and chondrules with igneous rims formed during a remelting event after the primary chondrule was incorporated into a dustball. Repeated episodes of remelting after chondrules were surrounded by dust would tend to produce large chondrules. RP and C chondrules formed by complete melting of their precursor assemblages; remelting of RP and C chondrules surrounded by dust would tend to produce porphyritic chondrules as small dust particles mixed with the melt, providing nuclei for crystallizing phenocrysts. This process would tend to diminish the numbers of RP and C chondrules. Correlations among these chondrule physical properties suggest that chondrite groups with large chondrules were typically surrounded by thick dust-rich mantles that formed in locally dusty nebular environments. Chondrules that were surrounded by thick dust mantles tended to cool more slowly because heat could not quickly radiate away. Slow cooling led to enhanced migration of sulfide to chondrule surfaces and more extensive sulfide evaporation. These chondrules also lost Na; the plagioclase that formed from equilibrated CK chondrites was thus depleted in Na.