Primordial compositions of refractory inclusions

Bulk chemical and O-, Mg- and Si-isotopic compositions were measured for each of 17 Types A and B refractory inclusions from CV3 chondrites. After bulk chemical compositions were corrected for non-representative sampling in the laboratory, the Mg- and Si-isotopic compositions of each inclusion were used to calculate its original chemical composition assuming that the heavy-isotope enrichments of these elements are due to Rayleigh fractionation that accompanied their evaporation from CMAS liquids. The resulting pre-evaporation chemical compositions are consistent with those predicted by equilibrium thermodynamic calculations for high-temperature nebular condensates, but only if different inclusions condensed from nebular regions that ranged in total pressure from 10⁻⁶ to 10⁻¹ bar, regardless of whether they formed in a system of solar composition or in one enriched in dust of ordinary chondrite composition relative to gas by a factor of 10 compared to solar composition. This is similar to the range of total pressures predicted by dynamic models of the solar nebula for regions whose temperatures are in the range of silicate condensation temperatures. Alternatively, if departure from equilibrium condensation and/or non-representative sampling of condensates in the nebula occurred, the inferred range of total pressure could be smaller. Simple kinetic modeling of evaporation successfully reproduces observed chemical compositions of most inclusions from their inferred pre-evaporation compositions, suggesting that closed-system isotopic exchange processes did not have a significant effect on their isotopic compositions. Comparison of pre-evaporation compositions with observed ones indicates that 80% of the enrichment in refractory CaO + Al₂O₃ relative to more volatile MgO + SiO₂ is due to initial condensation and 20% due to subsequent evaporation for both Types A and B inclusions.     

Refractory trace elements in Ca-Al-rich inclusions in the Allende meteorite

The condensation temperatures are calculated for a number of refractory trace metals from a gas of solar composition at 10^?3 and 10^?4 atm. total pressure. Instrumental neutron activation analysis of Ca-Al-rich inclusions in the Allende carbonaceous chondrite reveals enrichments of 22.8 ± 2.2 in the concentrations of Ir, Sc and the rare earths relative to Cl chondrites. Such enrichments cannot be due to magmatic differentiation processes because of the marked differences in chemical behavior between Ir and Sc, exhibited by their distributions in terrestrial igneous rocks and meteorites. All of these elements should have condensed from a cooling gas of solar composition above or within the range of condensation temperatures of the major mineral phases of the inclusions, which suggests that these inclusions are high-temperature condensates from the primitive solar nebula. Gas-dust fractionation of these materials may have been responsible for the depletion of refractory elements in the ordinary and enstatite chondrites relative to the carbonaceous chondrites.     

Petrologic study of SJ101, a new forsterite-bearing CAI from the Allende CV3 chondrite

The forsterite-bearing Type B (FoB) CAI SJ101 consists of three major structural units: (1) light patches of sector-zoned, poikilitic Al-rich clinopyroxene (Cpx) with numerous inclusions of small spinel grains and aggregates and subordinate amounts of Mg-rich melilite (Mel) and anorthite (An) (Sp-Cpx lithology), (2) dark sinuous bands of Al-rich clinopyroxene with large (up to ∼300 × 60 μm) poikilitically enclosed euhedral forsterite (Fo) crystals (Fo-Cpx lithology), and (3) the external Cpx-Sp-An rim overlying the entire inclusion. The two major lithologies are always separated by a transition zone of clinopyroxene poikilitically enclosing both forsterite and spinel. The patches of the Sp-Cpx lithology exhibit significant textural and mineralogical variability that is size-dependent. Small patches typically consist of Cpx and spinel with minor remnants of melilite and/or its alteration products. Large patches contain Mel-An-rich cores with either equigranular-ophitic-subophitic or ‘lacy’ textures reminiscent of those in Types B or C CAIs, respectively. All silicates poikilitically enclose numerous spinel grains of identical habit. Both melilite and anorthite gradually disappear toward the boundary with the Fo-Cpx lithology. Neither the evaporation mantle of Al-rich melilite typical of other FoBs nor the Wark-Lovering rim is present. Secondary minerals include grossular, monticellite, magnetite, and a few grains of wollastonite, andradite, and nepheline.Being a rather typical FoB mineralogically and chemically, texturally SJ101 differs from other FoBs in displaying the nearly complete segregation of forsterite from spinel which occur only in the Fo-Cpx and Sp-Cpx lithologies, respectively. The complex, convoluted internal structure of SJ101 suggests that the coarse-grained Sp-An-Mel-Cpx cores and Fo-Cpx lithology represent the precursor materials of FoBs, proto-CAIs and Fo-rich accretionary rims. While the inferred chemistry and mineralogy of the Fo-rich rims are fairly typical, the high Åk content in SJ101 melilite (78.7-82.3 mol.%) implies that the SJ101 proto-CAIs represent a new type of CAIs that has not been sampled before. This type of CAIs might have formed by remelting of spinel-rich condensates.The Group II REE pattern, slightly negative δ²⁹Si and δ²⁵Mg values, and nearly solar ratios of the major elements in the bulk SJ101 suggest that its precursors, proto-CAIs and Fo-rich rims, could have formed by a non-equilibrium condensation in a closed system of solar composition somewhat depleted in a super-refractory evaporation residue. The proposed formation scenario of SJ101 invokes a non-steady cooling and condensation of the nebular gas interrupted by at least two distinct melting episodes required to account for the igneous textures of the Mel-An-Cpx-rich cores (proto-CAIs) and the Fo-Cpx lithology.     

REE and actinide microdistribution in Sahara 97072 and ALHA77295 EH3 chondrites: A combined cosmochemical and petrologic investigation

We report the results of rare earth elements (REEs) and U-Th inventory of individual minerals (oldhamite, enstatite and niningerite) in two of the most unequilibrated and primitive EH3 known so far, ALHA77295 and Sahara 97072. Under the highly reducing condition that prevailed during the formation of enstatite chondrites, REEs are mainly chalcophile and concentrated in oldhamite. The study is guided by detailed petrographic investigations of the individual minerals in chondrules, complex sulfide-metal clasts and enstatite-dominated matrices.We developed two textural parameters in order to resolve the evolution of oldhamite condensates and their residence in the solar gas prior to their accretion in the individual objects or in matrices and relate these textural features to the measured REE patterns of the individual oldhamite crystals. These textural parameters are the crystal habit of oldhamite grains (idiomorphic or anhedral) and their host assemblages. REE concentrations were measured by SIMS and LA-ICPMS.Oldhamite grains display REE enrichments (10-100 × CI). Four types of REE patterns are encountered in oldhamite in ALHA77295. In general the REE distributions cannot be assigned to a specific oldhamite-bearing assemblage. The most represented REE pattern is characterized by both slight to large positive Eu and Yb anomalies and is enriched in light REEs relative to heavy REEs. This pattern is present in 97% of oldhamite in Sahara 97072, suggesting a different source region in the reduced part of the nebula or different parental EH asteroids for the two EH3 chondrites. Different parental asteroids are also supported by MgS-FeS zoning profiles in niningerite grains adjacent to troilite revealing both normal and reverse zoning trends and different MnS contents. The observed homogeneity of REE distribution in oldhamite grains in Sahara 97072 is not related to the mild metamorphic event identified in this meteorite that caused breakdown of the major K- and Rb-bearing sulfide (djerfisherite).REE concentrations in enstatite range between 0.2 and 8 × CI. Hence, enstatite is an important REE host next to oldhamite. Most patterns are characterized by negative Eu and Yb anomalies. Niningerites are negligible contributors to bulk EH3 REE inventory. Average positive Eu and Yb anomalies observed in most oldhamite are complimentary to the negative ones in enstatite thus explaining the flat patterns of the bulk meteorites. The condensation calculations based on cosmic abundances predict that the first oldhamite condensates should have flat REE patterns with Eu and Yb depletions since Eu and Yb condense at lower temperature than other REE. However, this pattern is seen in enstatite. Our findings are at odds with the predicted negative Eu and Yb anomalies in oldhamite earliest condensates from a closed system in a reduced solar source. Our petrographic, mineral chemistry and REE abundances of oldhamite, enstatite and niningerite discards an origin of oldhamite by impact melting (Rubin et al., 2009).Our results do not support in first order the scenario of the incorporation of REE in the Earth’s core to explain ¹⁴²Nd excess in terrestrial samples relative to chondrites because oldhamite is the major REE carrier phase and has super-chondritic Sm/Nd ratios.     

Condensation in the primitive solar nebula

The distribution of the major elements between vapor and solid has been calculated for a cooling gas of cosmic composition. The assumption is made that high temperature condensates remain in equilibrium with the vapor, affecting the temperatures of appearance of successively less refractory phases. The model suggests that the major textural features and mineralogical composition of the Ca, Al-rich inclusions in the C3 chondrites were produced during condensation in the nebula characterized by slight departures from chemical equilibrium due to incomplete reaction of high temperature condensates. Fractionation of such a phase assemblage is sufficient to produce part of the lithophile element depletion of the ordinary chondrites relative to the cosmic abundances.     

Condensation of major elements during chondrule formation and its implication to the origin of chondrules

Two glassy refractory Al-rich chondrules in Semarkona (LL3.0), the most primitive unequilibrated ordinary chondrite, provide direct evidence for condensation of Si and Mg on melt droplets during cooling. The chondrules are completely rounded, rich in Ca and Al, and poor in Fe and alkalis. They have extraordinarily abundant glass (70-80 vol%) with a subordinate amount of forsterite as the only crystalline phase that occurs mostly rimming the chondrule edge. The groundmass glass is concentrically zoned in terms of Si with an outward increase, which is overlapped with local heterogeneity of Mg and Al induced by crystallization of forsterite. The outward increase of Si, mostly compensated by Al, cannot be formed solely by crystallization of forsterite from a homogeneous melt in a closed system. Combined with skeletal or dendritic morphology and sector zoning of forsterite, it is suggested that Si condensed onto totally molten droplets (“initial melts”) accompanied by nucleation and rapid growth of forsterite with lowering temperature. The “initial melts”, the compositions of which were estimated from the Ca contents of the first crystallized forsterite, are very similar to Type C CAI but are notably poorer in Mg and Si than the bulk chondrules, indicating condensation of Mg in addition to Si with an atomic ratio of Mg:Si ∼ 3:2. The condensation after the nucleation of forsterite took place below ∼1300 °C under cooling at ∼70 °C/h and amounted to 30 wt% of the current chondrule. This study suggests a model that a short-time and local shock heating event induced melting of Type C CAI and concomitant evaporation of dusts, ferromagnesian chondrules of earlier generation, and their fragments to generate Mg and Si-rich gas, which condensed onto the melt droplets upon cooling accompanying condensation of Type I chondrules.     

High-temperature condensates in chondrites and the environment in which they formed

Chemical compositions of melilitee and titaniferous pyroxenes in calcium- and aluminum-rich inclusions in carbonaceous chondrites are consistent with their origin as hightemperature condensates from a gas of solar composition. Thermodynamic calculations indicate that the highest temperature minerals equilibrated with the gas at temperatures in excess of 1400°K. The lack of evidence for direct condensation of gas to liquid enables us to set an upper limit to the pressure when the inclusions formed which may be as low as 2.2 × 10^?3atm. Glasses, which are commonly found in chondrules, are interpreted as quench products of liquids formed by secondary reheating of primary solid condensates. The high-temperature inclusions constitute evidence that accretion of grains to cm-sized objects occurred at a very early stage in the evolution of the solar nebula.     

Crystallization sequences of Ca-Al-rich inclusions from Allende: An experimental study

The equilibrium crystallization sequence at 1 atmosphere in air of a melt corresponding in composition to the average composition of Type B Ca-Al-rich inclusions from the Allende meteorite is: spinel (1550°C) → melilite (1400°C; Åk22) → anorthite (1260°C) → Ti-Al-rich clinopyroxene (1230°C; “Ti-fassaite”). The melilite becomes increasingly åkermanitic with decreasing temperature. The pyroxene is similar in composition to fassaites from Type B inclusions. Preliminary results suggest that the crystallization sequence is similar at oxygen fugacities near the iron-wüstite buffer.The results of these experiments have been integrated with available phase equilibrium data in the system CaO-MgO-Al₂O₃-SiO₂TiO₂ and a phase diagram for predicting the crystallization sequences of liquids with compositions of coarse-grained Ca-Al-rich inclusions has been developed.Available bulk compositions of coarse-grained inclusions form a well-defined trend in terms of major elements, extending from Type A and Bl inclusions near the spinel-melilite join to more pyroxene-rich Type B2 inclusions. The trend deviates from the expected sequence of solid condensates from a nebular gas at P = 10^?3 atm if pure diopside is assumed to be the clinopyroxene that condenses. The Type A-B1 end of the trend is similar in composition to calculated equilibrium condensates at 1202–1227°C and the trend as a whole parallels the sequence of condensates expected from diopside condensation at ~ 1170°C. The trend is consistent to first order with the condensation of solid Ti-rich fassaite in place of pure diopside at higher temperatures than those at which pure diopside is predicted to condense. Partially molten condensates may be likely in this case or if the nebular pressure is higher than 10^?3 atm.     

Timescales determining the degree of kinetic isotope fractionation by evaporation and condensation

A first order characteristic of the relative abundance of the elements in solar system materials ranging in size from inclusions in primitive meteorites to planetary sized objects such as the Earth and the Moon is that they are very much like that of the Sun for the more refractory elements but systematically depleted to varying degrees in the more volatile elements. This is taken as evidence that evaporation and and/or condensation were important processes in determining the distinctive chemical properties of solar system materials. In some instances there is also isotopic evidence suggesting evaporation in that certain materials are found enriched in the heavy isotopes of their more volatile elements. Here model calculations are used to explore how the relative rates of various key processes determine the relationship between elemental and isotopic fractionation during partial evaporation and partial condensation. The natural measure of time for the systems considered here is the evaporation or condensation timescale defined as the time it would take under the prevailing conditions for evaporation or condensation to completely transfer the element of interest between the two phases of the system. The other timescales considered involve the rate of change of temperature, the rate at which gas is removed from further interaction with the condensed phase, and the rates of diffusion in the condensed and gas phases. The results show that a key determinant of whether or not elemental fractionations have associated isotopic effects is the ratio of the partial pressure of a volatile element (P_i) to its saturation vapor pressure (P_i,sat) over the condensed phase. Systems in which the rate of temperature change or of gas removal are slow compared to the evaporation or condensation timescale will be in the limit P_i ∼ P_i,sat and thus will have little or no isotopic fractionation because at the high temperatures considered here there is negligible equilibrium fractionation of isotopes. If on the other hand the temperature changes are relatively fast, then P_i ≠ P_i,sat and there will be both elemental and isotopic fractionation during partial evaporation or partial condensation. Rapid removal of evolved gas results in P_i ? P_i,sat which will produce isotopically heavy evaporation residues. Diffusion-limited regimes, where transports within a phase are not sufficiently fast to maintain chemical and or isotopic homogeneity, will typically produce less isotopic fractionation than had the phases remained well mixed. The model results are used to suggest a likely explanation for the heavy silicon and magnesium isotopic composition of Type B CAIs (as due to rapid partial melting and subsequent cooling at rates of a few °C per hour), for the uniformity of the potassium isotopic composition of chondrules despite large differences in potassium depletions (as due to volatilization of potassium by reheating in regions of large but variable chondrules per unit volume), and that the remarkable uniformity of the potassium isotopic composition of solar system materials is not a measure of the relative importance of evaporation and condensation but rather due to the solar nebula having evolved sufficiently slowly that materials did not significantly depart from chemical equilibrium.     

Ca,Al-rich inclusions, amoeboid olivine aggregates, and Al-rich chondrules from the unique carbonaceous chondrite Acfer 094: I. mineralogy and petrology

Based on their mineralogy and petrography, ∼200 refractory inclusions studied in the unique carbonaceous chondrite, Acfer 094, can be divided into corundum-rich (0.5%), hibonite-rich (1.1%), grossite-rich (8.5%), compact and fluffy Type A (spinel-melilite-rich, 50.3%), pyroxene-anorthite-rich (7.4%), and Type C (pyroxene-anorthite-rich with igneous textures, 1.6%) Ca,Al-rich inclusions (CAIs), pyroxene-hibonite spherules (0.5%), and amoeboid olivine aggregates (AOAs, 30.2%). Melilite in some CAIs is replaced by spinel and Al-diopside and/or by anorthite, whereas spinel-pyroxene assemblages in CAIs and AOAs appear to be replaced by anorthite. Forsterite grains in several AOAs are replaced by low-Ca pyroxene. None of the CAIs or AOAs show evidence for Fe-alkali metasomatic or aqueous alteration. The mineralogy, textures, and bulk chemistry of most Acfer 094 refractory inclusions are consistent with their origin by gas-solid condensation and may reflect continuous interaction with SiO and Mg of the cooling nebula gas. It appears that only a few CAIs experienced subsequent melting. The Al-rich chondrules (ARCs; >10 wt% bulk Al₂O₃) consist of forsteritic olivine and low-Ca pyroxene phenocrysts, pigeonite, augite, anorthitic plagioclase, ± spinel, FeNi-metal, and crystalline mesostasis composed of plagioclase, augite and a silica phase. Most ARCs are spherical and mineralogically uniform, but some are irregular in shape and heterogeneous in mineralogy, with distinct ferromagnesian and aluminous domains. The ferromagnesian domains tend to form chondrule mantles, and are dominated by low-Ca pyroxene and forsteritic olivine, anorthitic mesostasis, and Fe,Ni-metal nodules. The aluminous domains are dominated by anorthite, high-Ca pyroxene and spinel, occasionally with inclusions of perovskite; have no or little FeNi-metal; and tend to form cores of the heterogeneous chondrules. The cores are enriched in bulk Ca and Al, and apparently formed from melting of CAI-like precursor material that did not mix completely with adjacent ferromagnesian melt. The inferred presence of CAI-like material among precursors for Al-rich chondrules is in apparent conflict with lack of evidence for melting of CAIs that occur outside chondrules, suggesting that these CAIs were largely absent from chondrule-forming region(s) at the time of chondrule formation. This may imply that there are several populations of CAIs in Acfer 094 and that mixing of “normal” CAIs that occur outside chondrules and chondrules that accreted into the Acfer 094 parent asteroid took place after chondrule formation. Alternatively, there may have been an overlap in the CAI- and chondrule-forming regions, where the least refractory CAIs were mixed with Fe-Mg chondrule precursors. This hypothesis is difficult to reconcile with the lack of evidence of melting of AOAs which represent aggregates of the least refractory CAIs and forsterite grains.     

Ca–Al-rich inclusions in carbonaceous chondrites: the oldest solar system objects

This paper presents a review of recent available data on the first solid condensates of the Solar System, which include refractory CAIs (Ca–Al-rich Inclusions) mostly composed of Ca, Al, Mg, and Ti minerals. A theoretical condensation sequence calculated from thermodynamic data confirmed that CAIs formed as fine-grained aggregates in the protoplanetary disk from an ¹⁶О-rich gas of solar composition at temperatures >1300° K and pressures <10^–4 bar. Based on the diversity of CAI types, their mineralogical, bulk chemical, and isotopic compositions, it can be concluded that CAIs experienced melting and evaporation, possibly by shock waves, which may have occurred in the protoplanetary disk within a brief time interval. Some CAIs may have experienced multiple events such as melting, evaporation, and recycling back to the disk by means of a bipolar outflow. The CAIs having an absolute age of 4567.30 ± 0.16 Myr are the oldest objects in the Solar System. The study of CAIs revealed two distinct oxygen isotope reservoirs (¹⁶О-rich and ¹⁶О-poor) and established a chronology of the sequence of processes forming individual CAI components using Mg–Al, Cr–Mn and Pb–Pb isotopic systematics.     

Amoeboid olivine aggregates and related objects in carbonaceous chondrites: records of nebular and asteroid processes

Amoeboid olivine aggregates (AOAs) are the most common type of refractory inclusions in CM, CR, CH, CV, CO, and ungrouped carbonaceous chondrites Acfer 094 and Adelaide; only one AOA was found in the CB_b chondrite Hammadah al Hamra 237 and none were observed in the CB_a chondrites Bencubbin, Gujba, and Weatherford. In primitive (unaltered and unmetamorphosed) carbonaceous chondrites, AOAs consist of forsterite (Fa_<2), Fe, Ni-metal (5-12 wt% Ni), and Ca, Al-rich inclusions (CAIs) composed of Al-diopside, spinel, anorthite, and very rare melilite. Melilite is typically replaced by a fine-grained mixture of spinel, Al-diopside, and ±anorthite; spinel is replaced by anorthite. About 10% of AOAs contain low-Ca pyroxene replacing forsterite. Forsterite and spinel are always ¹⁶O-rich (δ^17,18O∼−40‰ to −50‰), whereas melilite, anorthite, and diopside could be either similarly ¹⁶O-rich or ¹⁶O-depleted to varying degrees; the latter is common in AOAs from altered and metamorphosed carbonaceous chondrites such as some CVs and COs. Low-Ca pyroxene is either ¹⁶O-rich (δ^17,18O∼−40‰) or ¹⁶O-poor (δ^17,18O∼0‰). Most AOAs in CV chondrites have unfractionated (∼2-10×CI) rare-earth element patterns. AOAs have similar textures, mineralogy and oxygen isotopic compositions to those of forsterite-rich accretionary rims surrounding different types of CAIs (compact and fluffy Type A, Type B, and fine-grained, spinel-rich) in CV and CR chondrites. AOAs in primitive carbonaceous chondrites show no evidence for alteration and thermal metamorphism. Secondary minerals in AOAs from CR, CM, and CO, and CV chondrites are similar to those in chondrules, CAIs, and matrices of their host meteorites and include phyllosilicates, magnetite, carbonates, nepheline, sodalite, grossular, wollastonite, hedenbergite, andradite, and ferrous olivine.Our observations and a thermodynamic analysis suggest that AOAs and forsterite-rich accretionary rims formed in ¹⁶O-rich gaseous reservoirs, probably in the CAI-forming region(s), as aggregates of solar nebular condensates originally composed of forsterite, Fe, Ni-metal, and CAIs. Some of the CAIs were melted prior to aggregation into AOAs and experienced formation of Wark-Lovering rims. Before and possibly after the aggregation, melilite and spinel in CAIs reacted with SiO and Mg of the solar nebula gas enriched in ¹⁶O to form Al-diopside and anorthite. Forsterite in some AOAs reacted with ¹⁶O-enriched SiO gas to form low-Ca pyroxene. Some other AOAs were either reheated in ¹⁶O-poor gaseous reservoirs or coated by ¹⁶O-depleted pyroxene-rich dust and melted to varying degrees, possibly during chondrule formation. The most extensively melted AOAs experienced oxygen isotope exchange with ¹⁶O-poor nebular gas and may have been transformed into magnesian (Type I) chondrules. Secondary mineralization and at least some of the oxygen isotope exchange in AOAs from altered and metamorphosed chondrites must have resulted from alteration in the presence of aqueous solutions after aggregation and lithification of the chondrite parent asteroids.     

Oxygen isotopic compositions of chondrules from the metal-rich chondrites Isheyevo (CH/CBb), MAC 02675 (CBb) and QUE 94627 (CBb)

It has been recently suggested that (1) CH chondrites and the CB_b/CH-like chondrite Isheyevo contain two populations of chondrules formed by different processes: (i) magnesian non-porphyritic (cryptocrystalline and barred) chondrules, which are similar to those in the CB chondrites and formed in an impact-generated plume of melt and gas resulted from large-scale asteroidal collision, and (ii) porphyritic chondrules formed by melting of solid precursors in the solar nebula. (2) Porphyritic chondrules in Isheyevo and CH chondrites are different from porphyritic chondrules in other carbonaceous chondrites ( [Krot et al., 2005], [Krot et al., 2008a] and [Krot et al., 2008b]). In order to test these hypotheses, we measured in situ oxygen isotopic compositions of porphyritic (magnesian, Type I and ferroan, Type II) and non-porphyritic (magnesian and ferroan cryptocrystalline) chondrules from Isheyevo and CB_b chondrites MAC 02675 and QUE 94627, paired with QUE 94611, using a Cameca ims-1280 ion microprobe.On a three-isotope oxygen diagram (δ¹⁷O vs. δ¹⁸O), compositions of chondrules measured follow approximately slope-1 line. Data for 19 magnesian cryptocrystalline chondrules from Isheyevo, 24 magnesian cryptocrystalline chondrules and 6 magnesian cryptocrystalline silicate inclusions inside chemically-zoned Fe,Ni-metal condensates from CB_b chondrites have nearly identical compositions: Δ¹⁷O = −2.2 ± 0.9‰, −2.3 ± 0.6‰ and −2.2 ± 1.0‰ (2σ), respectively. These observations and isotopically light magnesium compositions of cryptocrystalline magnesian chondrules in CB_b chondrites (Gounelle et al., 2007) are consistent with their single-stage origin, possibly as gas-melt condensates in an impact-generated plume. In contrast, Δ¹⁷O values for 11 Type I and 9 Type II chondrules from Isheyevo range from −5‰ to +4‰ and from −17‰ to +3‰, respectively. In contrast to typical chondrules from carbonaceous chondrites, seven out of 11 Type I chondrules from Isheyevo plot above the terrestrial fractionation line. We conclude that (i) porphyritic chondrules in Isheyevo belong to a unique population of objects, suggesting formation either in a different nebular region or at a different time than chondrules from other carbonaceous chondrites; (ii) Isheyevo, CB and CH chondrites are genetically related meteorites: they contain non-porphyritic chondrules produced during the same highly-energetic event, probably large-scale asteroidal collision; (iii) the differences in mineralogy, petrography, chemical and whole-rock oxygen isotopic compositions between CH and CB chondrites are due to various proportions of the nebular and the impact-produced materials.     

Modal mineralogy of CM chondrites by X-ray diffraction (PSD-XRD): Part 2. Degree, nature and settings of aqueous alteration

Within 5 million years after formation of calcium aluminium rich inclusions (CAI), high temperature anhydrous phases were transformed to hydrous phyllosilicates, mostly serpentines, which dominate the matrices of the most primitive carbonaceous chondrites. CMs are the largest group of meteorites to provide samples of this material. To understand the nature of the availability, and role of H₂O in the early solar system - as well as the settings of aqueous alteration - defining CM petrogenesis is critical. By Position Sensitive Detector X-ray Diffraction (PSD-XRD), we determine the modal abundance of crystalline phases present in volumes >1% for a suite of CMs - extending Part 1 of this work that dealt only with CM2 falls (Howard et al., 2009) to now include CM2 and CM1 finds. CM2 samples contain 13-31% Fe,Mg silicates (olivine + pyroxene) and from 67% to 82% total phyllosilicate (mean 75% ± 1.3 2σ). CM1 samples contain 6-10% olivine + pyroxene and 86-88% total phyllosilicate. Magnetite (0.6-5.2%), sulphide (0.6-3.9%), calcite (0-1.9%) and gypsum (0-0.8%) are minor phases across all samples. Since phyllosilicate forms from hydration of anhydrous Fe,Mg silicates (olivine + pyroxene), the ratio of total phyllosilicate to total anhydrous Fe,Mg silicate defines the degree of hydration and the following sequence results (in order of increasing hydration): QUE 97990 < Y 791198 < Murchison < Murray < Mighei < ALHA 81002 < Nogoya ? Cold Bokkeveld ? Essebi < QUE 93005 < ALH 83100 < MET 01070 < SCO 06043. High activities of Al (mostly from reactive mesostasis) and Si help to explain the composition and structure of CM serpentines that are distinct from terrestrial standards. Our data allows inference as to CM mineralogy at the point of accretion and challenges the conceptual validity of progressive alteration sequences. Modal mineralogy also provides new insights into CM petrogenesis and hints at a component of aqueous alteration occurring in the nebula, in addition to on the CM parent body(ies).     

Major, trace element and oxygen isotope study of glass cosmic spherules of chondritic composition: The record of their source material and atmospheric entry heating

New geochemical data on cosmic spherules (187 major element, 76 trace element, and 10 oxygen isotope compositions) and 273 analyses from the literature were used to assess the chemical diversity observed among glass cosmic spherules with chondritic composition. Three chemical groups of glass spherules are identified: normal chondritic spherules, CAT-like spherules (where CAT refers to Ca-Al-Ti-rich spherules), and high Ca-Al spherules. The transition from normal to high Ca-Al spherules occurs through a progressive enrichment in refractory major elements (on average from 2.3 wt.% to 7.0 wt.% for CaO, 2.8 wt.% to 7.2 wt.% for Al₂O₃, and 0.14 wt.% to 0.31 wt.% for TiO₂) and refractory trace elements (from 6.2 μg/g to 19.3 μg/g for Zr and 1.6CI-4.3CI for Rare Earth Elements-REEs) relative to moderately refractory elements (Mg, Si) and volatile elements (Rb, Na, Zn, Pb). Based on a comparison with experimental works from the literature, these chemical groups are thought to record progressive heating and evaporation during atmospheric entry. The evaporative mass losses evaluated for the high Ca-Al group (80-90%) supersede those of the CAT spherules which up to now have been considered as the most heated class of stony cosmic spherules. However, glass cosmic spherules still retain isotopic and elemental evidence of their source and precursor mineralogy. Four out of the 10 normal and high Ca-Al spherules analysed for oxygen isotopes are related to ordinary chondrites (δ¹⁸O = 13.2-17.3‰ and δ¹⁷O = 7.6-9.2‰). They are systematically enriched in Ni and Co (Ni = 24-500 μg/g) with respect to spherules related to carbonaceous chondrites (Ni < 1.2 μg/g, δ¹⁸O = 13.1-28.0‰ and δ¹⁷O = 5.1-14.0‰). REE abundances in cosmic spherules, which are not fractionated according to parent body or atmospheric entry heating, can then be used to unravel the precursor mineralogy. Spherules with flat REE pattern close to unity when normalized to CI are the most abundant in our dataset (54%) and likely derive from homogeneous, fine-grained chondritic precursors. Other REE patterns fall into no more than five categories, a surprising reproducibility in view of the mineralogical heterogeneity of chondritic lithologies at the micrometeorite scale.     

Conditions of condensate rim formation on the surface of lunar regolith particles

Condensate objects observed in the lunar regolith are distinctly separated on the basis of morpho-logical and chemical characteristics into droplets condensed during the expansion of an impact-generated vapor cloud and films condensed on the relatively cold surface of mineral particles. Using the analyses of both condensate forms and experimental data on the evaporation of melt corresponding to a typical lunar highland rock of the gabbro-anorthosite composition from Apollo 16 sample 68415.40, the temperature conditions of vapor condensation during lunar impact events were estimated. The comparison of condensate compositions with the analyses of vapors from the evaporation experiment showed that, compared with the compositions of droplet-type condensates, the condensate rims were formed from a vapor with high contents of refractory CaO and Al₂O₃ and at very different condensation temperatures. The enrichment of vapor in CaO and Al₂O₃ could be attained only at high temperatures of melt evaporation (higher than ∼ 1850°C according to experimental data). The estimated condensation temperatures of droplets are significantly lower, ∼1750–1500°C. Rim-type condensates were produced by vapor quenching on the relatively cold surface of a solid mineral particle, which resulted in almost complete precipitation of all major components of the silicate vapor without fractionation in accordance with their individual volatilities.     

Chemical and isotopic fractionation of wet andesite in a temperature gradient: Experiments and models suggesting a new mechanism of magma differentiation

Piston-cylinder experiments were conducted to investigate the behavior of partially molten wet andesite held within an imposed temperature gradient at 0.5 GPa. In one experiment, homogenous andesite powder (USGS rock standard AGV-1) with 4 wt.% H₂O was sealed in a double capsule assembly for 66 days. The temperature at one end of this charge was held at 950 °C, and the temperature at the other end was kept at 350 °C. During the experiment, thermal migration (i.e., diffusion in a thermal gradient) took place, and the andesite underwent compositional and mineralogical differentiation. The run product can be broadly divided into three portions: (1) the top third, at the hot end, contained 100% melt; (2) the middle-third contained crystalline phases plus progressively less melt; and (3) the bottom third, at the cold end, consisted of a fine-grained, almost entirely crystalline solid of granitic composition. Bulk major- and trace-element compositions change down temperature gradient, reflecting the systematic change in modal mineralogy. These changes mimic differentiation trends produced by fractional crystallization. The change in composition throughout the run product indicates that a fully connected hydrous silicate melt existed throughout the charge, even in the crystalline, cold bottom region. Electron Backscatter Diffraction analysis of the run product indicates that no preferred crystallographic orientation of minerals developed in the run product. However, a significant anisotropy of magnetic susceptibility was observed, suggesting that new crystals of magnetite were elongated in the direction of the thermal gradient. Further, petrographic observation reveals alignment of hornblende parallel to the thermal gradient. Finally, the upper half of the run product shows large systematic variations in Fe-Mg isotopic composition reflecting thermal diffusion, with the hot end systematically enriched in light isotopes. The overall δ⁵⁶Fe_IRMM-14 and δ²⁶Mg_DSM-3 offsets are 2.8‰ and 9.9‰, respectively, much greater than the range of Fe-Mg isotope variation in high-temperature terrestrial samples.In contrast, no obvious chemical differentiation was observed in a similar experiment (of 33 days duration) where the temperature ranged from 550 to 350 °C, indicating the critical role of the melt in causing the differentiation observed in the 950-350 °C experiment. If temperature gradients can be sustained for the multi-million-year time scales implied by geochronology in some plutonic systems, thermal migration could play a heretofore unrecognized role in the development of differentiated plutons. Elemental distributions, dominated by phase equilibria, cannot be used to discriminate thermal migration from conventional magma differentiation processes such as fractional crystallization. However, the observation of Fe-Mg isotopic variations in partially molten portions of the experiment indicates that these isotopic systems could provide a unique fingerprint to this process. This result could also provide a possible explanation for the Fe-Mg isotope variations observed in high-temperature silicate rocks and minerals.     

Lunar rock-rain: Diverse silicate impact-vapor condensates in an Apollo-14 regolith breccia

Apollo 14 regolith breccia 14076, long known to be uniquely endowed with high-alumina, silica-poor (HASP) material of evaporation-residue origin, has been found to contain a diverse suite of complementary condensates, dubbed GASP (gas-associated spheroidal precipitates). GASP occurs in two forms: as glassy or extremely fine grained quenched-melt spheroids, mostly less than 5 μm across; and as quenched textured clasts up to 200 μm across. In two of the clasts, origin by aggregation of spheroidal GASP is confirmed by the presence of relict spheroids. GASP is distinctively depleted in the same refractory major oxides that are characteristically enriched in HASP: Al₂O₃ and CaO. Among the larger GASP spheroids, Al₂O₃ is seldom >1 wt%; among the clasts, excluding two instances of apparent contamination by Na- and K-rich substrate-derived melt, bulk Al₂O₃ averages 0.3 wt%. Depletion of Al₂O₃ and CaO is also manifested by pyroxene compositions in some clasts; e.g., in the largest clast, En₈₂Wo_0.45 with 0.07 wt% Al₂O₃. Although GASP bulk compositions are nearly pure SiO₂ + MgO + FeO, they are nonetheless highly diverse. Spheroid compositions range in mg from 7 to 84 mol%, and in FeO/SiO₂ (weight ratio) from 0.002 to 0.67. Bulk compositions and textures of many GASP spheroids suggest that liquid immiscibility occurred prior to quenching; implying that these materials were, some time after condensation, at temperatures of ∼1680 °C. Textural evidence for immiscibility includes lobate boundaries between silicic and mafic domains, and a general tendency for quenched mafic silicates to be concentrated into a few limited patches rather than evenly dispersed. The parent melt of the largest clast’s pyroxene is inferred to have formed as a partial melt within the parent aggregation of GASP matter, compositionally near the pyroxene + cristobalite + melt eutectic and thus at ∼1500 °C. A few GASP spheroids show possible signs of in-flight collision-coalescence, but aggregation of the much larger clasts probably took place in mushy puddles on the lunar surface. Little mixing took place between these GASP puddles and the related HASP, probably because GASP condensation did not commence until after an intermediate stage during which, while neither net evaporation nor net condensation took place, expansion of the vapor cloud carried the eventual GASP matter well apart from the HASP. Considering the characteristic length-scale of lunar regolith mixing, the concentration of both GASP and HASP into this single unique regolith sample (14076) is most consistent with a parent crater size (diameter) of 10-100 km. I speculate that the 14076 regolith may have been unusually situated, almost directly uprange from an unusually oblique large impact. Mercurian analogs of the 14076 impact condensates may have significant implications for remote sensing.     

Estimation of temperature conditions for the formation of HASP and GASP glasses from the lunar regolith

Impact cratering on the Moon’s surface was accompanied by the high-temperature melting of rocks, melt evaporation, and silicate vapor condensation. Evidence for the extensive evaporative fractionation of melts was found in HASP (High-Alumina Silica-Poor) glasses from the lunar regolith. Numerous objects of condensation origin were found in the Apollo 14 regolith breccia. They are referred to as GASP (Gas-Associated Spheroidal Precipitates). With respect to chemical characteristics, namely FeO and SiO₂ contents, GASP were subdivided into Fe-rich (FeGASP) and Si-rich (SiGASP) condensates. Based on experimental data on the evaporation of aluminous basalt sample 68415.40 from the Apollo 16 collection and the calculated compositions of residual melts and complementary vapors at various temperatures, we compared the obtained compositions with the chemical analyses of the HASP glasses and GASP condensates. The comparison was aimed at estimating the temperature conditions of HASP and GASP formation. The comparison showed that the compositions of the HASP glasses and GASP condensates are consistent with the compositions obtained in the equilibrium experiment. In accordance with the experiment, the temperature range of the evaporation of HASP glasses was estimated as ∼1750–1870°C. The temperature interval of condensation, with allowance for the effect of vapor supercooling, is ∼1700–1500°C for FeGASP and no higher than 1700–1750°C for SiGASP. This paper discusses the problems of establishing interphase thermodynamic equilibrium during the dispersion of a vapor-melt cloud, vapor supercooling during its condensation, and the influence of the curvature of melt and condensate particles on the character of evaporation and condensation.