L. Grossman

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. Iron-nickel alloys have higher condensation temperatures than forsterite and enstatite at all total pressures greater than 7.1 × 10−5 atmospheres. These data lend support to the origin of the core and mantle of the earth by a heterogeneous accumulation process. The temperature difference between the condensation points of iron and magnesium silicates increases with pressure allowing the possibility of greater fractionation of metal from silicate towards the center of the solar nebula where the pressure and temperature were highest.

A Component of Primitive Nuclear Composition in Carbonaceous Meteorites

The oxygen of anhydrous, high-temperature minerals in carbonaceous meteorites is strongly depleted in the heavy stable isotopes 17O and 18O. The effect is the result of nuclear rather than chemical processes and probably results from the admixture of a component of almost pure 16O. This component may predate the solar system and may represent interstellar dust with a separate history of nucleosynthesis.

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.

DISTRIBUTION OF THE PRE-SOLAR COMPONENT IN ALLENDE AND OTHER CARBONACEOUS CHONDRITES

Excess 16 O, relative to terrestrial abundances, has been found in all samples of C2, C3 and C4 carbonaceous chondrites which have been analyzed, amounting to nine meteorites thus far. Whole-rocks and mineral separates from all the C3 and C4 meteorites fall on a single mixing line consistent with admixture of 1–5% of excess 16 O. The anhydrous silicates of C2 meteorites fall on the same line, but the hydrous silicate matrix of C2s define a mass-fractionation trend parallel to the terrestrial trend, but displaced towards higher 16 O. All meteorites analyzed are isotopically heterogeneous on a sub-millimeter scale. Detailed analyses of separated phases of several Allende Ca Al-rich inclusions reveal a consistent pattern of large 16 O enrichments in spinel, pyroxene and sometimes olivine, and small 16 O enrichments in melilite, feldspathoids and grossular. The heterogeneous distribution of the 16 O excesses, together with their enhancement in minerals believed to be early solar nebular condensates, implies the existence of pre-solar “carriers” of the isotopic anomaly, probably grains or molecules with oxygen which was nearly pure 16 O. These carriers have not been unequivocally indentified, but pre-solar grains of corundum or spinel, and pre-solar molecules of SiO are possibilities. The isotopic anomalies may also have been disturbed by diffusion-controlled processes of exchange between early condensates and their surroundings, either in the nebula, or later in the parent body. No direct correlation has yet been observed between the oxygen isotope anomalies and recently observed isotope anomalies in neon, magnesium and xenon.

Condensation in dust-enriched systems

Abstract Full equilibrium calculations of the sequence of condensation of the elements from cosmic gases made by total vaporization of dust-enriched systems were performed in order to investigate the oxidation state of the resulting condensates. The computations included 23 elements and 374 gas species, and were done over a range of Ptot from 10−3 to 10−6 bar and for enrichments up to 1000× in dust of Cl composition relative to a system of solar composition. Because liquids are stable condensates in dust-enriched systems, the MELTS nonideal solution model for silicate liquids (Ghiorso and Sack, 1995) was incorporated into the computer code. Condensation at 10−3 bar and dust enrichments of 100×, 500×, and 1000× occur at oxygen fugacities of IW-3.1, IW-1.7, and IW-1.2, respectively, and, at the temperature of cessation of direct condensation of olivine from the vapor, yields XFa of 0.019, 0.088, and 0.164, respectively. Silicate liquid is a stable condensate at dust enrichments >∼12.5× at 10−3 bar and >∼425× at 10−6 bar. At 500×, the liquid field is >1000 K wide and accounts for a maximum of 48% of the silicon at 10−3 bar, and is 240 K wide and accounts for 25% of the silicon at 10−6 bar. At the temperature of disappearance of liquid, XFa of coexisting olivine is 0.025, 0.14, and 0.31 at 100×, 500×, and 1000×, respectively, almost independent of Ptot. At 1000×, the Na2O and K2O contents of the last liquid reach 10.1 and 1.3 wt.%, respectively, at 10−3 bar but are both negligible at 10−6 bar. At 10−3 bar, iron sulfide liquids are stable condensates at dust enrichments at least as low as 500× and coexist with silicate liquid at 1000×. No sulfide liquid is found at 10−6 bar. At 10−3 bar, the predicted distribution of Fe between metal, silicate and sulfide at 1310 K and a dust enrichment of 560× matches that found in H-group chondrites, and at 1330 K and 675× matches that of L-group chondrites prior to metal loss. Only at combinations of high Ptot and high dust enrichment do the bulk chemical composition trends of condensates reach the FeO contents typical of type IIA chondrules at temperatures where dust and gas could be expected to equilibrate, ≥1200 K. Even under these conditions, however, the composition trajectories of predicted condensates pass through compositions with much more CaO + Al2O3 relative to MgO + SiO2 than those of most type IA chondrules. Furthermore, on a plot of wt.% Na2O vs. wt.% FeO, most chondrule compositions are too Na2O-rich to lie along trends predicted for the bulk chemical compositions of the condensates at Ptot ≤ 10−3 bar and dust enrichments ≤1000×. Together, these chemical differences indicate that individual chondrules formed neither by quenching samples of the liquid + solid condensates that existed at various temperatures nor by quenching secondary liquids that formed from such samples. With the exception of very FeO-poor, Na2O-rich glasses in type I chondrules and glasses with very high FeO and Na2O in type II chondrules, however, many chondrule glass compositions fall along bulk composition trajectories for liquids in equilibrium with cosmic gases at 10−3 bar and dust enrichments between 600× and 1000×. If these chondrules formed by secondary melting of mixtures of condensates that formed at different temperatures, nebular regions with characteristics such as these would have been necessary to prevent loss of Na2O by evaporation and FeO by reduction from the liquid precursors of their glasses, assuming that the liquids were hot for a long enough time to have equilibrated with the gas.

Petrography and mineral chemistry of Ca-rich inclusions in the Allende meteorite

There are two types of white, coarse-grained, Ca-Al-rich inclusions in Allende. Type A inclusions contain 80–85 per cent melilite, 15–20 per cent spinel, 1–2 per cent perovskite and rare plagioclase, hibonite, wollastonite and grossularite. Clinopyroxene, if present, is restricted to thin rims around inclusions or cavities in their interiors. Type B inclusions contain 35–60 per cent pyroxene, 15–30 per cent spinel, 5–25 per cent plagioclase and 5–20 per cent melilite. The coarse pyroxene crystals in Type Bs contain >15 per cent Al2O3 and >1.8 per cent Ti, some of which is trivalent. Type A pyroxenes contain <9 per cent Al2O3 and <0.7 per cent Ti. Electron microprobe analyses of 600 melilite, 39 pyroxene, 35 plagioelase, 33 spinel and 20 perovskite grains were performed in 16 Type A, 1 intermediate and 9 Type B inclusions in Allende and 1 Type A in Grosnaja. Melilite composition histograms from individual Type A inclusions are usually peaked between Ak10 and Ak30 and are 15–20 mole % wide while those from Type B inclusions are broader, unpeaked and displaced to higher akermanite contents. Most pyroxenes contain < 1 per cent FeO. All plagioclase is An 98 to An 100. Spinel is almost pure MgAl2O4. Perovskite contains small (< 1 per cent) but significant amounts of Mg, Al, Fe, Y, Zr and Nb. Inferred bulk chemical compositions of Type A inclusions are rather close to those expected for high-temperature condensates. Those of Type B inclusions suggest slightly lower temperatures but their Ca/Al ratio seems less than the Type As, indicating that the Type Bs may not be their direct descendants. Some textural features suggest that the inclusions are primordial solid condensetes while others indicate that they may have been melted after condensation. Fragmentation and metamorphism may have also occurred after condensation.

Cleavage of pyrimidine dimers in specific DNA sequences by a pyrimidine dimer DNA-glycosylase of M. luteus

Pyrimidine dimer formation in response to UV radiation is governed by the thymine content of the potential dimer and the two flanking nucleotides. An enzymatic activity can be purified from Micrococcus luteus that cleaves the N-glycosyl bond between the 5′ pyrimidine of a dimer and the corresponding sugar without rupture of a phosphodiester bond. We propose that strand scission at a dimer site by the M. luteus enzyme requires two activities, a pyrimidine dimer DNA-glycosylase and an apyrimidinic/apurinic endonuclease.

Condensation of CaOMgOAl2O3SiO2 liquids from cosmic gases

Abstract The Berman (1983) activity-composition model for non-ideal liquid solutions in the CaO MgO Al2O3 SiO2 (CMAS) system is incorporated into equilibrium condensation calculations which are used to explore the vapor-solid-liquid stability relations as a function of temperature and total pressure (Ptot) in a gas of solar composition, and as a function of temperature and dust/gas ratio at Ptot = 1 × 10−3 atm in gases produced by total vaporization of systems enriched in interstellar dust relative to the gas compared to solar abundances. Condensate liquids are very non-ideal, suggesting that results of previous attempts to model their formation using ideal solution models are highly inaccurate. As is the case for the Berman (1983) liquid model itself, results of the present calculations are in very good agreement with experimentally determined liquid-crystal phase relations except where intermediate members of solid solution series, such as melilite and fassaite, are predicted to be in equilibrium with liquid, in which cases liquid-crystal equilibration temperatures are overestimated by 50 to 100 K. CMAS liquids are stable in a solar gas at a Ptot at least as low as 5 × 10−2 atm and perhaps as low as 1 × 10−2 atnt, much lower than previous estimates for liquids of pure forsterite composition, due to the colligative effects of CaO and Al203. CMAS liquids are stable at Ptot = 1 × 10−3 atm in systems with dust/gas enrichment factors at least as low as 16 and perhaps as low as 5 relative to solar abundances. Results of these calculations suggest that, upon cooling, a solid melilite + spinel condensate assemblage, comparable to a Type A refractory inclusion, would react with the vapor to produce a liquid much richer in MgO and SiO2 than the starting material, at either elevated Ptot or enhanced dust/gas ratio. If this partial melt were isolated from further reaction with the nebular gas, it would solidify into a spinel + melilite + fassaite + anorthite assemblage, similar in chemical and mineralogical composition to a Type B refractory inclusion. Forsterite coexists stably with CMAS condensate liquids over wide ranges of Ptot and dust/gas ratio, extending to the lowest Ptot and dust/gas ratio at which liquids are stable. If the compositions of glass inclusions inside isolated forsterite crystals in the Murchison CM2 chondrite have been modified by precipitation of 25 wt% forsterite as a daughter mineral from the liquid precursors of those glasses, the inclusions could represent condensate liquids that were in equilibrium with forsterite at Ptot = 0.3 atm or at dust/gas enrichment factors of ∼70 at Ptot = 1 × 10−3 atm.

Origin of the high-temperature fraction of C2 chondrites

The coarse-grained fraction of C2 chondrites is composed mostly of single crystals and aggregates of crystals of Mg-rich olivine and pyroxene. They do not possess compelling textural evidence of being the solidification products of rapidly-quenched molten droplets. Metal inclusions in the silicates contain 3·82–8·88 mole% Ni, 0·16–0·70 per cent Co, 0·17–1·07 per cent Cr and up to 5·70 per cent P. Thermodynamic calculations show that alloys of these compositions may be condensates from the solar nebula. The implication is that the high-temperature fraction of C2 chondrites consists mostly of high-temperature condensates. Chemical data show that the high-temperature fraction has an Fe/Mg atomic ratio of ⩽ 0·31 compared to 1·3 in the matrix, indicating that much of the iron has been lost from the high-temperature fraction and converted to the troilito and oxidized iron of the low-temperature fraction. The presence of low-Ni metal grains in the aggregates and high Ni/Fe and Co/Fe ratios in the matrix of some C2s indicates preferential loss of early NiCo-rich metal from the high-temperature fraction during condensation.

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.