Steven B. Simon

Mineralogy and Petrology of Comet 81P/Wild 2 Nucleus Samples

The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk.

Major element chemical and isotopic compositions of refractory inclusions in C3 chondrites: The separate roles of condensation and evaporation

Literature data for major element oxide compositions of most coarse-grained Types A and B inclusions in CV3 chondrites may be in error due to non-representative sampling of spinel relative to other phases because of small sample sizes. When reported compositions are corrected to the solar CaO/Al2O3 ratio by addition or subtraction of spinel, distinct trends result on oxide–oxide plots. These trends lie close to trajectories of bulk compositions of equilibrium condensates calculated for solar or dust-enriched gases under various conditions, except on a plot of MgO vs. SiO2 contents, where there is considerable scatter of the data points to the MgO-poor side of the condensation trajectory. The irreversible process of evaporative mass loss from a liquid droplet into an unsaturated H2 gas is modeled as a series of small equilibrium steps. This model is used to show that evolutionary paths of CMAS liquid compositions are identical for evaporation at all PH2 from 1 × 10−15 to 1 bar, with the ratio of the fraction of the SiO2 evaporated to that for MgO increasing both with increasing temperature from 1700 to 2000 K and with increasing SiO2 content of the starting composition. Such calculations show that compositions of most Type B inclusions can be explained by non-equilibrium evaporation of 10 to 30% of the MgO and 0 to 15% of the SiO2 into an H2 gas at 1700 K from liquid droplets whose compositions originated on any one of many possible equilibrium condensation trajectories. Some Type As may have suffered similar evaporative losses of MgO and SiO2 but at higher temperature. This degree of evaporation is consistent with the amount of Mg and Si isotopic mass fractionation observed in Types A and B inclusions. Evaporation probably happened after most Mg and Si were removed from the nebular gas into lower-temperature condensates.

Radar-Enabled Recovery of the Sutter’s Mill Meteorite, a Carbonaceous Chondrite Regolith Breccia

The Meteor That Fell to Earth In April 2012, a meteor was witnessed over the Sierra Nevada Mountains in California. Jenniskens et al. (p. 1583) used a combination of photographic and video images of the fireball coupled with Doppler weather radar images to facilitate the rapid recovery of meteorite fragments. A comprehensive analysis of some of these fragments shows that the Sutters Mill meteorite represents a new type of carbonaceous chondrite, a rare and primitive class of meteorites that contain clues to the origin and evolution of primitive materials in the solar system. The unexpected and complex nature of the fragments suggests that the surfaces of C-class asteroids, the presumed parent bodies of carbonaceous chondrites, are more complex than previously assumed. Analysis of this rare meteorite implies that the surfaces of C-class asteroids can be more complex than previously assumed. Doppler weather radar imaging enabled the rapid recovery of the Sutter’s Mill meteorite after a rare 4-kiloton of TNT–equivalent asteroid impact over the foothills of the Sierra Nevada in northern California. The recovered meteorites survived a record high-speed entry of 28.6 kilometers per second from an orbit close to that of Jupiter-family comets (Tisserand’s parameter = 2.8 ± 0.3). Sutter’s Mill is a regolith breccia composed of CM (Mighei)–type carbonaceous chondrite and highly reduced xenolithic materials. It exhibits considerable diversity of mineralogy, petrography, and isotope and organic chemistry, resulting from a complex formation history of the parent body surface. That diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to C-class asteroids are interpreted.

Fassaite composition trends during crystallization of Allende Type B refractory inclusion melts

Fassaite is a major phase in Type B refractory inclusions and its wide compositional variability makes it an excellent recorder of crystallization-induced changes in liquid composition. Traverses across zoned crystals with electron and ion microprobes show that Ti, V, Sc, Zr and Hf decrease from core to rim, while Mg, Si, REEs, and other trace elements increase. Fassaite/liquid distribution coefficients derived from the compositions of early fassaite and calculated parent liquid compositions are higher than previously reported fassaite Ds, e.g., 0.2–0.8 vs. 0.08–0.5 for the trivalent REEs, possibly due to the high Ti and Al contents of the fassaite we analyzed. Log-log plots of concentrations of elements vs. those of Sc give linear relationships with slopes of (Dixl/L − 1)/(DScxl/L − 1), where Dixl/L- is the bulk crystal/liquid distribution coefficient for element i. From this relationship, we derive effective fassaite/liquid Ds of 2.8 for Sc, 1.5 for Hf, 1.1 for Zr, 0.52 for Y, 0.34 for Ta, 0.29 for Nb, and 0.31–0.48 for trivalent REEs. The effective Ds are applicable to fassaite crystallizing from melts having the compositions and thermal histories of Allende Type B refractory inclusions, and incorporate any equilibrium and kinetic effects that modify Ds during the course of crystallization. Observed fassaite REE contents can be reproduced by a fractional crystallization model in which the effective Ds are used. Ti3+ is more compatible in fassaite than Ti4+ and, without equilibration of the residual liquid with an external reducing gas, Ti3+Titot ratios (where Titot = Ti3+ + Ti4+ should decrease from core to rim in fassaite crystals. Comparison of profiles of Ti3+Titot ratios in several crystals with those calculated for fractional crystallization suggests that the trend in one crystal was controlled by fractional crystallization alone, but in two others the virtually constant Ti3+Titot ratios may imply that the liquids in these inclusions were able to maintain equilibrium with the solar nebular gas throughout crystallization. Attempts to determine the ƒo2 of fassaite crystallization on the basis of Ti3+Titot ratios must be based on fassaite grains which are not zoned with respect to Ti3+Titot. The range of observed REE abundances in subliquidus fassaite (7–800 × Cl for La and 30–2000 × Cl for Lu) now encompasses those in grains previously identified as relict, removing a major reason for their identification as such. Rounded fassaite grains with fairly sharp Mg-rich rims and Ti-rich cores, poikilitically enclosed in mantle melilite in TS34 are candidates for relict grains. The grains are so Scrich that they would require extreme prior crystallization of spinel and melilite, which is inconsistent with their very low REE contents (e.g., La at 3–7 × Cl). We suggest that the cores of these grains are relict and were trapped with liquid during formation of the mantle.

Chemical evolution of metal in refractory inclusions in CV3 chondrites

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to measure distributions of the siderophile elements V, Fe, Co, Ni, Mo, Ru, Rh, Pd, W, Re, Os, Ir, Pt, and Au in Fremdlinge with a spatial resolution of 15 to 25 m. A sulfide vein in a refractory inclusion in Allende (CV3-oxidized) is enriched in Rh, Ru, and Os with no detectable Pd, Re, Ir, or Pt, indicating that Rh, Ru, and Os were redistributed by sulfidation of the inclusion, causing fractionation of Re/Os and other siderophile element ratios in Allende CAIs. Fremdlinge in compact Type-A inclusions from Efremovka (CV3-reduced) exhibit subsolidus exsolution into kamacite and taenite and minimal secondary formation of V-magnetite and schreibersite. Siderophile element partitioning between taenite and kamacite is similar to that observed previously in iron meteorites, while preferential incorporation of the light PGEs (Ru, Rh, Pd) relative to Re, Os, Ir, and Pt by schreibersite was observed. Fremdling EM2 (CAI Ef2) has an outer rim of P-free metal that preserves the PGE signature of schreibersite, indicating that EM2 originally had a phosphide rim and lost P to the surrounding inclusion during secondary processing. Most Fremdlinge have chondrite-normalized refractory PGE patterns that are unfractionated, with PGE abundances derived from a small range of condensation temperatures, 1480 to 1468 K at Ptot 10 3 bar. Some Fremdlinge from the same CAI exhibit sloping PGE abundance patterns and Re/Os ratios up to 2 CI that likely represent mixing of grains that condensed at various temperatures. Copyright

Formation of refractory inclusions by evaporation of condensate precursors

Berman’s (1983) activity-composition model for CaO-MgO-Al2O3-SiO2 liquids is used to calculate the change in bulk chemical and isotopic composition during simultaneous cooling, evaporation, and crystallization of droplets having the compositions of reasonable condensate precursors of Types A and B refractory inclusions in CV3 chondrites. The degree of evaporation of MgO and SiO2, calculated to be faithfully recorded in chemical and isotopic zoning of individual melilite crystals, is directly proportional to evaporation rate, which is a sensitive function of PH2, and inversely proportional to the droplet radius and cooling rate. When the precursors are partially melted in pure hydrogen at peak temperatures in the vicinity of the initial crystallization temperature of melilite, their bulk chemical compositions evolve into the composition fields of refractory inclusions, mass-fractionated isotopic compositions of Mg, Si, and O are produced that are in the range of the isotopic compositions of natural inclusions, and melilite zoning profiles result that are similar to those observed in real inclusions. For droplets of radius 0.25 cm evaporating at PH2 = 10−6 bar, precursors containing 8 to 13 wt.% MgO and 20 to 23% SiO2 evolve into objects similar to compact Type A inclusions at cooling rates of 2 to 12 K/h, depending on the precise starting composition. Precursors containing 13 to 14 wt.% MgO and 23 to 26% SiO2 evolve into objects with the characteristics of Type B1 inclusions at cooling rates of 1.5 to 3 K/h. The relatively SiO2-poor members of the Type B2 group can be produced from precursors containing 14 to 16 wt.% MgO and 27 to 33% SiO2 at cooling rates of 15% than are found on any condensation curve. The characteristics of fluffy Type A inclusions, including their reversely zoned melilite, can only be understood in the context of this model if they contain relict melilite.

Refractory inclusions from the Leoville, Efremovka, and Vigarano C3V chondrites - Major element differences between Types A and B, and extraordinary refractory siderophile element compositions

We have measured the bulk major and trace element compositions often CAIs from Leoville, Efremovka, and Vigarano, members of the reduced subgroup of C3V chondrites, by INAA. Like CAIs from the reduced subgroup studied previously and unlike CAIs from Allende, a member of the oxidized subgroup, nine of the ten CAIs studied here possess only small amounts of secondary phases and, hence, preserve their pre-alteration bulk major element compositions. Using those compositions and model pre-alteration compositions of Allende CAIs, we cannot distinguish Type B1s clearly from B2s, nor some CTAs from FTAs. All Type As and most Type Bs have superchondritic CaOAl2O3 ratios, suggesting that their precursors condensed from a solar gas after ~20% of the Al was removed by high-temperature, Al-rich phases. Type Bs possess higher concentrations of MgO and SiO2 than do Type As because the precursor phases of Type Bs reacted with the cooling nebular gas to form fassaite, while those of Type As did not. n nEight of the ten CAIs have unusual or unique refractory trace element characteristics compared to those seen in Allende inclusions. Vig1, L2, Ef1, and Ef2 have remarkably high concentrations of Re and Os (~57–145 × Cl) because they sampled 4–9 times more hcp condensate metal alloy than did most Allende inclusions. Vig1, L2, Ef1, Ef2, L1, and Vig2 have unusually large OsRu ratios (up to 6.9 × Cl) because the removal temperatures of their hcp alloy precursors were unusually high (up to ~ 1655 K at 10−3atm). L2 and L4 have low ZrHf ratios (~0.4 × Cl); L1 and Ef2, low V/Yb ratios (~0.25 × Cl); Ef2, high ThLa (~1.6 × Cl), and SrYb (~-2.4 × Cl) ratios; and ED, high HfLu (~2.6 × Cl), ScLu (~3.1 × Cl), and ZrLu (~2.8 × Cl) ratios. Most of these fractionations are explained by non-representative nebular sampling of one or more of hibonite, zirconium oxide, perovskite, and melilite, which may be condensates or evaporation residues. Of twenty-five inclusions from the reduced subgroup now studied in this laboratory, seventeen have at least one pair of refractory trace elements which are more fractionated relative to one another compared to Cl chondrites than are the same elements in Allende CAIs, indicating that the host phases of these elements were not as thoroughly mixed together in the nebular region where CAIs of the reduced subgroup accreted as where those in Allende did. Parts of the nebula sampled by C3V chondrites of the reduced subgroup were not well sampled by Allende, and vice versa. n nLike most coarse-grained inclusions from the reduced subgroup studied previously, most of those analyzed here have lower concentrations of Na, Au, Fe, Zn, and Mn than their Allende counterparts, consistent with the idea that they experienced secondary alteration at a higher temperature or for a shorter time than did Allende inclusions.

Oxygen Isotope Variations at the Margin of a CAI Records Circulation Within the Solar Nebula

Isotope measurements within an inclusion in a meteorite reveal a record of processes in the early solar system. Micrometer-scale analyses of a calcium-, aluminum-rich inclusion (CAI) and the characteristic mineral bands mantling the CAI reveal that the outer parts of this primitive object have a large range of oxygen isotope compositions. The variations are systematic; the relative abundance of 16O first decreases toward the CAI margin, approaching a planetary-like isotopic composition, then shifts to extremely 16O-rich compositions through the surrounding rim. The variability implies that CAIs probably formed from several oxygen reservoirs. The observations support early and short-lived fluctuations of the environment in which CAIs formed, either because of transport of the CAIs themselves to distinct regions of the solar nebula or because of varying gas composition near the proto-Sun.

Formation of spinel-,hibonite-rich inclusions found in CM2 carbonaceous chondrites

Abstract We report petrography, mineral chemistry, bulk chemistry, and bulk isotopic compositions of a suite of 40 spinel-rich inclusions from the Murchison (CM2) carbonaceous chondrite. Seven types of inclusions have been identified based on mineral assemblage: spinel-hibonite-perovskite; spinelperovskite- pyroxene; spinel-perovskite-melilite; spinel-hibonite-perovskite-melilite; spinel-hibonite; spinel-pyroxene; and spinel-melilite-anorthite. Hibonite-bearing inclusions have Ti-poor spinel compared to the hibonite-free ones, and spinel-hibonite-perovskite inclusions have the highest average bulk TiO2 contents (7.8 wt%). The bulk CaO/Al2O3 ratios of the inclusions range from 0.005 to 0.21, well below the solar value of 0.79. Hibonite-, spinel-rich inclusions consist of phases that are not predicted by condensation calculations to coexist; in the equilibrium sequence, hibonite is followed by melilite, which is followed by spinel. Therefore, hibonite-melilite or melilite-spinel inclusions should be dominant instead. One explanation for the .missing melilite. is that it condensed as expected, but was lost due to evaporation of Mg and Ca during heating and melting of spherule precursors. If this theory were correct, melilite-poor spherules would have isotopically heavy Mg and Ca, assuming Rayleigh fractionation accompanied evaporation. Except for one inclusion with FMg = 4.3 ± 2.6./amu and another with isotopically light Ca (FCa = -3.4 ± 2.0./amu), however, all the inclusions we analyzed have normal isotopic compositions within their 2σ uncertainties. Thus, we found no evidence for significant mass-dependent fractionation. Conditions necessary for non-Rayleigh evaporation are unlikely if not unrealistic, and our preferred explanation for the general lack of melilite among hibonite-, spinel-bearing inclusions is kinetic inhibition of melilite condensation relative to spinel. Because of similarities between the crystal structures of hibonite and spinel, it should be easier for spinel than for melilite to form from hibonite.

Calcium Tschermak's pyroxene, CaAlAlSiO6, from the Allende and Murray meteorites: EBSD and micro-Raman characterizations

Abstract Calcium Tschermak’s pyroxene (CaTs), CaAlAlSiO6, is well known as an important component in pyroxene. It is a member of the Ca clinopyroxene group in which Al dominates in the M1 site. Pyroxenes with more than 80 mol% CaTs were observed previously in Ca-,Al-rich refractory inclusions (CAI) from five carbonaceous chondrites. This study re-investigated the near end-member CaTs in the Allende and Murray chondrites. Electron backscatter diffraction (EBSD) is used to establish that its crystal structure is monoclinic, C2/c; a = 9.609 Å, b = 8.652 Å, c = 5.274 Å, β =106.06°, V = 421.35 Å3, and Z = 4. Its EBSD pattern is an excellent match to that of synthetic CaAlAlSiO6 with the C2/c structure. MicroRaman is also carried out to confirm the crystal structure. The Allende CaTs, with 46.00 wt% Al2O3 and 97 mol% Al in the M1 site, has the formula Ca1.02(Al0.97Fe0.01Mg0.01)Σ0.99(Si1.00Al1.00)Σ2.00O6. It occurs as micrometer-sized crystals along with melilite, hibonite, perovskite, spinel, corundum, Ti3+-rich pyroxene, and grossular in a fluffy Type A CAI. It is probably a secondary phase resulting from the alteration of gehlenitic melilite. The CaTs in Murray, with a formula Ca0.98(Al0.81Mg0.16Ti4+0.04)Σ1.01 (Si1.11Al0.89)Σ2.00O6, occurs with hibonite and Al-rich diopside in a glass-free refractory spherule. This sample formed by solidification of a once-molten droplet early in the history of the solar system.