Unequilibrated Enstatite Chondrites Research Articles

Silicon and Oxygen Isotope Evolution of the Inner Solar System

Abstract Enstatite chondrites have been regarded as major building blocks of the Earth and other differentiated inner planetary bodies due to the similarity of Δ17O (deviation of the δ 17O value from the terrestrial silicate fractionation line) and nucleosynthetic isotope anomalies. However, this hypothesis has been rebutted by the fact that the Earth and enstatite chondrites show distinct Si isotopic compositions. It has been debated whether the origin of this Si isotope difference is the result of nebular or planetary processes. Here we show that the δ 30Si (deviation of 30Si/28Si relative to NBS 28 standard) and the Δ17O values of chondrules in unequilibrated enstatite chondrites are between −0.20‰ and −0.54‰ and −0.36‰ and +0.26‰, respectively. Furthermore, the chondrules with higher Δ17O values tend to have lower δ 30Si. The data exhibit values consistent with most of the noncarbonaceous group differentiated planetary bodies. This consistency suggests that the Si and O isotopic compositions of enstatite chondrules record those of the major precursors that formed the differentiated planetary bodies in the inner solar system. Model calculations based on the results reveal that the Si and O isotope variations of the enstatite chondrite chondrules were generated by an interaction between the evaporation-driven SiO-rich gas and partially or fully melted forsterite-rich precursor chondrules. The Mg/Si of the evaporated dust-gas mixtures increased with increasing silicate/metal ratio in the evaporated dust, which may have increased the bulk Mg/Si and δ 30Si value of the inner planetary bodies.

Metal‐rich nodules in anomalous EL3 chondrite Northwest Africa (NWA) 8785

AbstractNWA 8785 is a remarkable, recently identified, unequilibrated enstatite chondrite. It was classified as an EL3 but contains highly unusual characteristics not observed in any other EL3, including a high abundance of FeO‐rich matrix and metal‐rich nodules that are texturally and mineralogically different from those in other EL3s. We characterized the mineral assemblages and compositions of metal‐rich nodules in a thin section of NWA 8785 and compared them to nodules in other EL3s to evaluate models for formation of metal‐rich nodules in EL3s. Of a total of 40 metal‐rich nodules, 10 were selected for detailed study. These metal‐rich nodules vary in their physical structure, texture, and mineral assemblages. Some contain the rare Al‐poor, alkali‐rich silicate mineral roedderite, a first discovery in an EL3, as well as the Cl‐bearing sulfide djerfisherite. The diversity of metal‐rich nodules in NWA 8785 suggests each nodule formed independently and supports their origin by primary processes prior to accretion. The high abundance of FeO‐rich matrix and the unique qualities of its metal‐rich nodules call into question classification of NWA 8785 as an EL3, but the Si content in its kamacite and Cr and Ti content in its troilite, and the presence of alabandite, support its classification as an EL3; thus, it is an EL3‐anomalous. Although alternative hypotheses exist, the presence of roedderite, as well as a magnetite‐rich matrix and sodalite, may provide the first evidence of extensive metasomatic alteration on the EL3 parent body.

Micro-distribution of oxygen isotopes in unequilibrated enstatite chondrites

We report petrology and high precision, in situ oxygen isotope analyses of silicates in chondrules, fragments, metal-rich nodules and refractory inclusions from the ALH 81189 (EH3), ALH 85159 (paired with ALH 81189) and from the MAC 88136 (EL3) chondrite. This is the first report of oxygen isotope ratios for individual objects in an EL3 and for the silicates associated with the metal-rich nodules that are characteristic of unequilibrated enstatite (E3) chondrites. The oxygen isotopic data from the chondrules and other objects form a trend, on a 3-isotope plot, that coincides with the slope ∼1 primitive chondrule mineral (PCM) line (initially defined by chondrules from the Acfer 094 primitive carbonaceous chondrite), with most objects clustering at the intersection of the PCM line with the terrestrial fractionation (TF) line, near whole rock E3. The data from EH3 and EL3 overlap and show a similar distribution, suggesting they formed from a similar pool of precursors or in similar gaseous environments, but their mineral compositions suggest differences in their nebular environments and/or parent bodies. Silicates in the metal-rich nodules we analyzed (in both EH3 and EL3) have oxygen isotope ratios (as well as mineral compositions) similar to the silicate (metal-free) chondrules. This is consistent with formation of the metal-rich nodules prior to chondrite accretion, in an environment and from a process similar to that which formed the coexisting chondrules, but from more metal-rich mixtures of precursors. Olivine in an AOA from ALH 81189 is 16O-rich with δ18O = −46.5‰, δ17O = −48.0‰, similar to the AOAs and refractory inclusions previously reported in E3 and in all other chondrite groups. There is a clear distinction in oxygen isotopic compositions between the chondrules in the E3 chondrites and those in the LL and R as well as those in CV and CM chondrite groups. Chondrules from CR and E chondrites plot closer to the PCM line than all other chondrite groups with E3 chondrules having a different distribution toward more 16O-poor compositions. Chondrules in other chondrite groups form trends above and below the PCM. From the distribution of EC chondrules along the PCM line, we propose that similar pools of chondrule precursors were present in the different (carbonaceous, CR and Acfer 094 and non-carbonaceous, E) chondrule forming regions in the protoplanetary disk but with different amounts of 16O-rich refractory materials, prior to development of the postulated Jupiter divide that potentially separated inner (non-carbonaceous) from outer (carbonaceous chondrite) Solar System materials or Jupiter was inefficient in completely separating these materials.

Silicate–SiO reaction in a protoplanetary disk recorded by oxygen isotopes in chondrules

The formation of planetesimals and planetary embryos during the earliest stages of the solar protoplanetary disk largely determined the composition and structure of the terrestrial planets. Within a few million years of the birth of the Solar System, chondrule formation and the accretion of the parent bodies of differentiated achondrites and the terrestrial planets took place in the inner protoplanetary disk 1,2 . Here we show that, for chondrules in unequilibrated enstatite chondrites, high-precision Δ17O values (where Δ17O is the deviation of the δ17O value from a terrestrial silicate fractionation line) vary significantly (ranging from −0.49 to +0.84‰) and fall on an array with a steep slope of 1.27 on a three-oxygen-isotope plot. This array can be explained by the reaction between an olivine-rich chondrule melt and an SiO-rich gas derived from vaporized dust and nebular gas. Our study suggests that a large proportion of the building blocks of planetary embryos formed by successive silicate–gas interaction processes: silicate–H2O followed by silicate–SiO interactions under more oxidized and reduced conditions, respectively, within a few million years of the formation of the Solar System.

Osmium isotopic homogeneity in the CK carbonaceous chondrites

Variable proportions of isotopically diverse presolar components are known to account for nucleosynthetic isotopic anomalies for a variety of elements (e.g., Ca, Ti, Cr, Ni, Sr, Zr, Mo, Ru, Pd, Ba, Nd, and Sm) in both bulk chondrites and achondrites. However, although large Os isotopic anomalies have been measured in acid leachates and residues of unequilibrated chondrites, bulk chondrites of various groups, iron meteorites, and pallasites exhibit Os isotopic compositions that are indistinguishable from terrestrial or bulk solar isotopic abundances. Since the magnitude of nucleosynthetic anomalies is typically largest in the carbonaceous chondrites, this study reports high-precision Os isotopic compositions and highly siderophile element (HSE) concentrations for ten CK chondrites. The isotope dilution concentration data for HSE and high-precision Os isotope ratios were determined on the same digestion aliquots, to precisely correct for radiogenic contributions to 186Os and 187Os. While acid leached bulk unequilibrated carbonaceous chondrites show deficits of s-process Os components to the same extent as revealed by unequilibrated enstatite, ordinary, and Rumuruti chondrites, equilibrated bulk CK chondrites exhibit no resolvable Os isotopic anomalies. These observations support the idea that acid-resistant, carbon-rich presolar grains, such as silicon carbide (SiC) or graphite, are major carriers for nucleosynthetic isotopic anomalies of Os. The destruction of these presolar grains, which are omnipresent in unequilibrated meteorites, must have occurred during aqueous alteration and thermal metamorphism, early in the CK chondrite parent body history. The dispersal of CK chondrites along the IIIAB iron meteorite isochron on a 187Os/188Os versus 187Re/188Os diagram, with Re/Os ratios from 0.032 to 0.083, in combination with the observed redistribution of other HSE (e.g., Pt, Pd), highlights the influence of parent body processes, overprinted by effects of recent terrestrial alteration. Under the oxidizing conditions prevalent on the CK parent body, evident from high abundances of magnetite and limited Fe-Ni metal in CK chondrites, these parent body processes made all isotopically anomalous Os, originally hosted in reduced presolar grains, accessible. The absence of Os isotopic anomalies in ordinary, enstatite, and now also carbonaceous chondrites, implies that the carriers of s- and r-process Os must have been effectively homogenized across the region of chondrite formation, and possibly even the entire solar protoplanetary nebula, as suggested by the Os isotopic compositions of iron meteorites and non-anomalous ureilites. Except for a limited number of ureilites, the relative proportions of presolar s- and r-process carriers of Os (and other elements such as W) in chondrites, and most other planetary bodies, must have remained constant during all subsequent nebular and planetary processes, which appears not to have been the case for other siderophile elements, including Mo, Ru, and Pd. The existence of Mo, Ru, Pd and other siderophile element isotopic anomalies thus appears to be in part controlled by the chemical properties of these elements (e.g., volatility), their host phase(s) (e.g., SiC, graphite, metal, sulfides), and the nature of the nebular or planetary processes experienced in the early solar system.

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 >1.8wt.% Si in their kamacite and contain the monosulfide niningerite (MgS), whereas EL3 chondrites have less than 1.4wt.% 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.

Structure, composition, and location of organic matter in the enstatite chondrite Sahara 97096 (EH3)

Abstract– The insoluble organic matter (IOM) of an unequilibrated enstatite chondrite Sahara (SAH) 97096 has been investigated using a battery of analytical techniques. As the enstatite chondrites are thought to have formed in a reduced environment at higher temperatures than carbonaceous chondrites, they constitute an interesting comparative material to test the heterogeneities of the IOM in the solar system and to constrain the processes that could affect IOM during solar system evolution. The SAH 97096 IOM is found in situ: as submicrometer grains in the network of fine‐grained matrix occurring mostly around chondrules and as inclusions in metallic nodules, where the carbonaceous matter appears to be more graphitized. IOM in these two settings has very similar δ15N and δ13C; this supports the idea that graphitized inclusions in metal could be formed by metal catalytic graphitization of matrix IOM. A detailed comparison between the IOM extracted from a fresh part and a terrestrially weathered part of SAH 97096 shows the similarity between both IOM samples in spite of the high degree of mineral alteration in the latter. The isolated IOM exhibits a heterogeneous polyaromatic macromolecular structure, sometimes highly graphitized, without any detectable free radicals and deuterium‐heterogeneity and having mean H‐ and N‐isotopic compositions in the range of values observed for carbonaceous chondrites. It contains some submicrometer‐sized areas highly enriched in 15N (δ15N up to 1600‰). These observations reinforce the idea that the IOM found in carbonaceous chondrites is a common component widespread in the solar system. Most of the features of SAH 97096 IOM could be explained by the thermal modification of this main component.

Origin of Halogens and Nitrogen in Enstatite Chondrites

The EH and EL enstatite chondrites are the most reduced chondrite groups, having formed in nebular regions where the gas may have had high C/O and/or pH2/pH2O ratios. Enstatite chondrites (particularly EH) have higher CI- and Mg-normalized abundances of halogens (especially F and Cl) and nitrogen than ordinary chondrites and most groups of carbonaceous chondrites. Even relative to CI chondrites, EH and EL chondrites are enriched in F. We have found that literature values for the halogen abundance ratios in EH and EL chondrites are strongly correlated with the electronegativities of the individual halogens. We suggest that the most reactive halogens were the most efficient at forming compounds (e.g., halides) that were incorporated into EH-chondrite precursor materials. It seems plausible that, under the more-oxidizing conditions pertaining to the other chondrite groups, a larger fraction of the halogens remained in the gas. Nitrogen may have been incorporated into the enstatite chondrites as simple nitrides that did not condense under the more-oxidizing conditions in the regions where other chondrite groups formed. Literature data show that unequilibrated enstatite chondrites have light bulk N (δ 15N ≈ −20‰) compared to most ordinary (−5 to +20‰) and carbonaceous (+20 to +190‰) chondrites; this may reflect the contribution in enstatite chondrites of nitride condensates with δ15 N values close to the proposed nebular mean (~−400‰). In contrast, N in carbonaceous chondrites is mainly contained within 15N-rich organic matter. The major carrier of N in ordinary chondrites is unknown.

60Fe– 60Ni and 53Mn– 53Cr isotopic systems in sulfides from unequilibrated enstatite chondrites

In situ measurements of 60Fe– 60Ni and 53Mn– 53Cr isotopic systems with an ion microprobe have been carried out for sulfide assemblages from unequilibrated enstatite chondrites (UECs). Evidence for the initial presence of 60Fe has been observed in nine sulfide inclusions from three UECs: ALHA77295, MAC88136, and Qingzhen. The inferred initial ( 60Fe/ 56Fe) [( 60Fe/ 56Fe) 0] ratios show a large variation range, from ∼2 × 10 −7 to ∼2 × 10 −6. The sulfide inclusions with high Fe/Ni ratios yield ( 60Fe/ 56Fe) 0 ratios of ∼(2–7) × 10 −7, similar to most of the ( 60Fe/ 56Fe) 0 values of troilite and pyroxene observed in unequilibrated ordinary chondrites (UOCs). Inclusions with high inferred ( 60Fe/ 56Fe) 0 ratios (∼1–2 × 10 −6) have low Fe/Ni ratios and the magnitude of the 60Ni excesses is similar in two MAC88136 assemblages in spite of a difference of a factor of two in their Fe/Ni ratios. The inferred high ( 60Fe/ 56Fe) 0 ratios were probably the result of Fe–Ni re-distribution in the sulfides during later alteration processes. The 53Mn– 53Cr system was measured in five of the sulfide assemblages that were examined for their 60Fe– 60Ni systematics. The 53Mn– 53Cr isochrons yielded variable initial ( 53Mn/ 55Mn) [( 53Mn/ 55Mn) 0] ratios from ∼(2–7) × 10 −7. There is no obvious correlation between the ( 60Fe/ 56Fe) 0 and ( 53Mn/ 55Mn) 0 ratios. The variable 53Mn– 53Cr isochrons probably also indicate later disturbance to the isotopic systems in these sulfides. Even though no chronological information can be extracted from the 60Fe– 60Ni and 53Mn– 53Cr systems in these UEC sulfides, our results indicate that 60Fe was present in the enstatite chondrite formation region of the early Solar System.

Oxygen isotope and 26Al‐26Mg systematics of aluminum‐rich chondrules from unequilibrated enstatite chondrites

Abstract— Correlated in situ analyses of the oxygen and magnesium isotopic compositions of aluminum‐rich chondrules from unequilibrated enstatite chondrites were obtained using an ion microprobe. Among eleven aluminum‐rich chondrules and two plagioclase fragments measured for 26Al‐26Mg systematics, only one aluminum‐rich chondrule contains excess 26Mg from the in situ decay of 26Al; the inferred initial ratio (26Al/27Al)o = (6.8 ± 2.4) × 10−6 is consistent with ratios observed in chondrules from carbonaceous chondrites and unequilibrated ordinary chondrites.The oxygen isotopic compositions of five aluminum‐rich chondrules and one plagioclase fragment define a line of slope ˜0.6 ± 0.1 on a three‐oxygen‐isotope diagram, overlapping the field defined by ferromagnesian chondrules in enstatite chondrites but extending to more 16O‐rich compositions with a range in δ18O of about ˜12‰. Based on their oxygen isotopic compositions, aluminum‐rich chondrules in unequilibrated enstatite chondrites are probably genetically related to ferromagnesian chondrules and are not simple mixtures of materials from ferromagnesian chondrules and calcium‐aluminum‐rich inclusions (CAIs).Relative to their counterparts from unequilibrated ordinary chondrites, aluminum‐rich chondrules from unequilibrated enstatite chondrites show a narrower oxygen isotopic range and much less resolvable excess 26Mg from the in situ decay of 26Al, probably resulting from higher degrees of equilibration and isotopic exchange during post‐crystallization metamorphism. However, the presence of 26Al‐bearing chondrules within the primitive ordinary, carbonaceous, and now enstatite chondrites suggests that 26Al was at least approximately homogeneously distributed across the chondrite‐forming region.

SIMS analyses of Mg, Cr, and Ni isotopes in primitive meteorites and short-lived radionuclides in the early solar system

SIMS analyses of 26 Al – 26 Mg , 60 Fe – 60 Ni , and 53 Mn – 53 Cr systems in unequilibrated enstatite chondrites provide evidence for the former existence of 26 Al , 60 Fe , and 53 Mn in this highly reduced meteorite group, suggesting the widespread of these short-lived radionuclides in the early solar system. Calcium–aluminum-rich inclusions and Al-rich chondrules in enstatite chondrites show 26 Al – 26 Mg characteristics similar to their counterparts in other types of chondrites. Assuming its homogenous distribution, fine-scale 26 Al relative chronology can be obtained for different components in meteorites. Well-defined isochrons yield initial 60 Fe / 56 Fe ratios of (0.3−1)×10 −6 in sulfides from enstatite chondrites. The former presence of 60 Fe suggests a stellar source and its possible high abundance points to a supernova origin. 26 Al and 60 Fe could be important heat sources for the early planetary processes. The large variations of initial 60 Fe / 56 Fe and 53 Mn / 55 Mn ratios and the discordance between 60 Fe – 60 Ni and 53 Mn – 53 Cr systems observed in sulfides indicate later disturbance of the two isotopic systems, and therefore, bear no strict chronological significance. SIMS isotopic analysis can be utilized to attain important information about the early evolution of the solar system.

Metal size distributions in EH and EL chondrites

EL chondrites exhibit larger average metal grain sizes than EH chondrites, a difference attributed by Easton [1983] to metamorphic coarsening, as most EL chondrites are equilibrated, and most EH chondrites are unequilibrated. In this paper, we present metal grain size data for three unequilbrated EL3 chondrites (PCA 91020; ALH 85119; MAC 88180) , and three EH3 chondrites (ALH 84170; PCA 91085; PCA 91238). We find that EL3 chondrites have larger average metal grain sizes than EH3 chondrites, and that grain sizes of the unequilibrated enstatite chondrites are similar to those of equilibrated chondrites of the same class. We thus interpret the metal size distributions of enstatite chondrites as primarily reflecting their pre‐metamorphic distributions. Shock processing appears to have had only minor influence on metal grain size distributions in these meteorites.

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.

Mineral chemistry and the origin of enstatite in unequilibrated enstatite chondrites

A comprehensive study of the petrography, cathodoluminescence (CL), rare earth element (REE) abundances, and magnesium and silicon isotopic compositions of enstatite, the major mineral in unequilibrated enstatite chondrites (UECs), is reported. For comparison, observations were also made in equilibrated enstatite chondrites (EECs) and aubrites. In UECs, there are no REE patterns or abundances that uniquely characterize the CL color and intensity, grain size, or occurrence of enstatite. Red enstatite is dominant in UECs and blue enstatite in EECs. Three REE patterns represent the diversity observed in UEC enstatite; many grains are depleted in one or more of the three most volatile REEs under reducing conditions (i.e., Yb, Eu, and Sm). All REE patterns are compatible with the fractional crystallization (during chondrule formation) of melts whose precursors either had flat REE patterns or were depleted in the most volatile REEs. No evidence was found to suggest that enstatite grains require nebular condensation or multiple reservoirs, as suggested by others. Magnesium and silicon isotopic ratios are normal, therefore excluding the role of evaporation in separating the REEs. This fractionation was most likely accomplished during condensation of the chondrule precursors. Our data, combined with synthetic experiments, support the idea that coexisting red and blue CL enstatites formed by crystallization from a single melt. Most of the red enstatite was converted to blue by diffusion and recrystallization to form the more equilibrated E chondrites. It is not clear why enstatite is depleted in the most volatile REEs while oldhamite, a highly refractory mineral which is the major carrier of REE in E chondrites, commonly has enrichments of these elements.

Geochemical and petrographic studies of oldhamite, diopside, and roedderite in enstatite meteorites

Abstract— In Qingzhen (EH3), oldhamite contains numerous types of inclusions and intergrows with other phases; but in equilibrated enstatite chondrites and aubrites, it usually occurs as individual grains. I suggest that oldhamite in unequilibrated enstatite chondrites (UECs) crystallized from a melt, probably during chondrule formation. Subsequent thermal metamorphism on the parent bodies further modified the oldhamite occurrences in enstatite chondrites. This suggestion is consistent with the results of melting experiments on UECs and aubrites and with the volatile element enrichments in this mineral.I analyzed minor and trace element abundances in diopside from two aubrites. These data and petrographic observations suggest that diopside formed by igneous crystallization.I report the first known occurrence of roedderite in an aubrite and its major, minor, and trace element concentrations. This mineral is rich in alkalis but is depleted in siderophile and refractory lithophile elements.A negative Sm anomaly was noted in albite from equilibrated enstatite chondrites and aubrites.

Experimental rare‐earth‐element partitioning in oldhamite: Implications for the igneous origin of aubritic oldhamite

Abstract— Aubritic oldhamite (CaS) has been the subject of intense study recently because it is the major rare‐earth‐element (REE) carrier in aubrites, has a variety of REE patterns comparable to those in unequilibrated enstatite chondrites and has an extraordinarily high melting point as a pure substance (2525 °C). These latter two facts have caused some authors to assert that much of the aubritic oldhamite is an unmelted nebular relict, rather than of igneous origin. We have conducted REE partitioning experiments between oldhamite and silicate melt using an aubritic bulk composition at 1200 °C and 1300 °C and subsolidus annealing experiments. All experiments produced crystalline oldhamite, with a range of compositions, glass and Fe metal, as well as enstatite, SiO2, diopside and troilite in some charges. Rare‐earth‐element partitioning is strongly dependent on oldhamite composition and temperature. Subsolidus annealing results in larger partition coefficients for some oldhamite grains, particularly those in contact with troilite. All experimental oldhamite/silicate melt partition coefficients are <20 and the vast majority are <5, which is similar to those reported in the literature and is two orders of magnitude less than those inferred for natural aubritic oldhamite. These partition coefficients preclude a simple igneous model, since REE abundances in aubritic oldhamite are greater than would be predicted on the basis of the experimental partition coefficients. Our experimental partition coefficients are consistent with a relict nebular origin for aubritic oldhamite, although experimental evidence that suggests melting of oldhamite at temperatures lower than that reached on the aubrite parent body are clearly inconsistent with the nebular model. Our experiments are consistent also with a complex igneous history. Oldhamite REE patterns may reflect a complex process of partial melting, melt removal, fractional crystallization and subsolidus annealing and exsolution. These mechanisms (primarily fractional crystallization and subsolidus annealing) can produce a wide range of REE patterns in aubritic oldhamite, as well as elevated (100–1000 × CI) REE abundances observed in aubritic oldhamite.

Manganese‐chromium systematics in sulfides of unequilibrated enstatite chondrites

Abstract— We measured with a secondary ion mass spectrometer Mn/Cr ratios and Cr isotopes in individual grains of Mn‐bearing sulfides (i.e., sphalerites, ZnS; alabandites, MnS; and niningerites, MgS) in nine unequilibrated enstatite chondrites (UECs). The goals were to determine whether live 53Mn (half‐life ∼3.7 Ma) was incorporated in these objects at the time of their isotopic closure and to establish whether Mn‐Cr systematics in sulfides in UECs can be used as a high‐resolution chronometer to constrain formation time differences between these meteorites. Sulfide grains analysed in four of these UECs, MAC 88136 (EL3), MAC 88184 (EL3), MAC 88180 (EL3), and Indarch (EH4), have clear 53Cr excesses. These 53Cr excesses can be very large (δ53Cr/52Cr ranges up to ∼18,400%, the largest 53Cr excess measured so far) and, in some grains, are well correlated with the Mn/Cr ratios. Thus, they were most likely produced by the in situ decay of 53Mn in the meteorite samples. In the remaining five meteorites, no detectable excesses of 53Cr were found, and only upper limits on the initial 53Mn/55Mn ratios could be established. The four meteorites with 53Cr excesses show variations in the inferred 53Mn/55Mn ratios in various sulfide grains of the same meteorite. The Mn‐Cr systematics in these sulfides were disturbed (during and/or after the decay of 53Mn) by varying degrees of reequilibration. Provided 53Mn was homogeneously distributed in the region of the early solar system where these objects formed, the data suggest that the time of the last isotopic equilibration of sulfides in EL chondrites occurred at least 3 Ma after a similar episode in EH chondrites.

The origin of oldhamite in unequilibrated enstatite chondrites

We report the results of an ion probe study of rare earth elements (REEs) in six unequilibrated enstatite chondrites. Oldhamite is confirmed as the major but not the only REE carrier in these meteorites. Data are also provided for other minerals (enstatite, niningerite, and alabandite) that carry detectable amounts of REEs. Although the variety of REE patterns in oldhamite is not as extreme as in hibonite, there are seven distinct REE patterns (four of which form a continuum) that characterize most oldhamite grains from E chondrites. Two of these patterns (F and G) were each found in only one weathered grain. There is no correlation between REE pattern and oldhamite petrologic setting. Our trace element data are used to evaluate earlier work and a number of elements (Zr, Y, Sc, REEs, Sr, Se, and probably Sb and Cs), including both refractory and volatile species under oxidizing conditions, are found enriched in oldhamite. A comparison of REE patterns with the results of equilibrium condensation calculations indicates that nebular processes were important in determining the composition of the oldhamite precursors, whereas metamorphic processes also affected oldhamite compositions. Enrichments of chalcophiles are not necessarily due to interaction of oldhamite with nebular gas at low temperatures, as previously suggested. Oldhamite, like hibonite, was formed from melts whose trace element compositions were determined by high temperature transient events in the solar nebula.

The evolution of enstatite and chondrules in unequilibrated enstatite chondrites: Evidence from iron‐rich pyroxene

Abstract— FeO‐rich (Fs6–34) pyroxene lacking cathodoluminescence (CL), hereafter black pyroxene, is a major constituent of some of the chondrules and fragments in unequilibrated (type 3) enstatite chondrites (UECs). It contains structurally oriented zones of Cr‐, Mn‐, V‐rich, FeO‐poor enstatite with red CL, associated with mm‐sized blebs of low‐Ni, Fe‐metal and, in some cases, silica. These occurrences represent clear evidence of pyroxene reduction. The black pyroxene is nearly always rimmed by minor element (Cr, Mn, V)‐poor enstatite having a blue CL. More commonly, red and blue enstatites, unassociated with black pyroxene, occur as larger grains in chondrules and fragments, and these constitute the major silicate phases in UECs. The REE abundance patterns of the black pyroxene are LREE‐depleted. The blue enstatite rims, however, have a near‐flat to LREE‐enriched pattern, ∼0.5–4x chondritic. The petrologic and trace element data indicate that the black pyroxene is from an earlier generation of chondrules that formed in a nebular region that was more oxidizing than that of the enstatite chondrites. Following solidification, these chondrules experienced a more reducing nebular environment and underwent reduction. Some, perhaps most, of the red enstatite that is common throughout the UECs may be the product of solid‐state reduction of black pyroxene. The blue enstatite rims grew onto the surfaces of the black pyroxene and red enstatite as a result of condensation from a nebular gas.The evolutionary history of some of the enstatite and chondrules in enstatite chondrites can be expressed in a four‐stage model that includes: Stage 1. Formation of chondrules in an oxidizing nebular environment Stage 2. Solid‐state reduction of the more oxidized chondrules and fragments to red enstatite in a more reducing nebular environment Stage 3. Formation of blue enstatite rims on the black pyroxene as well as on the red enstatite. Stage 4. Reprocessing, by various degrees of melting, of many of the earlier‐formed materials.

Evidence for a late thermal event of unequilibrated enstatite chondrites: A Rb‐Sr study of Qingzhen and Yamato 6901 (EH3) and Khairpur (EL6)

Abstract— The Rb‐Sr whole rock and internal systematics of two EH3 chondrites, Qingzhen and Yamato 6901, and of one EL6 chondrite, Khairpur, were determined. Sulfides were separated using a stepwise dissolution technique. The mineral species in each fraction were estimated based on the chemical analyses of 12 major elements. The internal Rb‐Sr systematics of the EH3 chondrites are highly disturbed. Fractions corresponding to sulfide phases show excess 87Sr, while other fractions corresponding to silicate phases produce a linear trend on a Rb‐Sr evolution diagram. If these linear relations are interpreted as isochrons, the ages of the silicate phases are 2.12 ± 0.23 Ga and 2.05 ± 0.33 Ga with the initial Sr isotopic ratios of 0.7112 ± 0.0018 and 0.7089 ± 0.0032, for Qingzhen and Yamato 6901, respectively. The process of the isotopic disturbance probably involved the breakdown of the major K‐bearing sulfide (djerfisherite), and a lack of isotopic exchange between sulfide and silicate phases indicates moderate temperatures of reheating. Although a complete Sr isotopic re‐homogenization among silicate phases was not attained, we interpret the Rb‐Sr results as indicative of a late thermal event about 2 Ga ago on the parent bodies of these EH3 chondrites. These ages agree well with previously published K‐Ar ages. An older isochron age of 4.481 ± 0.036 Ga with a low initial Sr isotopic ratio of 0.69866 ± 0.00038 was obtained for the data from silicate fractions of Khairpur, indicating early petrological equilibration on the parent body of EL6 chondrites.