Ureilite petrogenesis: A limited role for smelting during anatexis and catastrophic disruption

Abstract— A popular model for ureilites assumes that during anatexis in an asteroidal mantle, pressure‐buffered equilibrium smelting (partial reduction coincident with partial melting) engendered their conspicuous mafic‐silicate‐core mg diversity (75–96 mol%). Several mass‐balance problems arise from this hypothesis. Smelting inevitably consumes a large proportion of any plausible initial carbon while generating significant proportions of Fe metal and copious proportions of CO gas. The most serious problem concerns the yield of CO gas. If equilibrium smelting produced the ureilites’ entire 21 mol% range in olivine‐core mg, the proportion of gas within the asteroidal mantle (assuming plausibly low pressure <～80 bar) should have reached ≥85 vol%. Based on the remarkably stepwise cooling history inferred from ureilite texture and mineralogy, a runaway, CO‐leaky process that can loosely be termed smelting appears to have occurred, probably triggered by a major impact. The runaway scenario appears likely because, by Le Chǎtelier's principle, CO leakage would tend to accelerate the smelting process. Also, the copious volumes of gas produced by smelting would have led to explosive, mass‐leaky eruptions into the vacuum surrounding the asteroid. Loss of mass would mean diminution of interior pressure, which would induce further smelting, leading to further loss of mass (basalt), and so on. Such a disruptive runaway process may have engendered the ureilites’ distinctive reduced olivine rims. But the only smelting, according to this scenario, was a short‐lived disequilibrium process that reduced only the olivine rims, not the cores; and the ureilites were cooling, not melting, during the abortive “smelting” episode.     

Ureilite petrogenesis: A limited role for smelting during anatexis and catastrophic disruption

Abstract— A popular model for ureilites assumes that during anatexis in an asteroidal mantle, pressure‐buffered equilibrium smelting (partial reduction coincident with partial melting) engendered their conspicuous mafic‐silicate‐core mg diversity (75–96 mol%). Several mass‐balance problems arise from this hypothesis. Smelting inevitably consumes a large proportion of any plausible initial carbon while generating significant proportions of Fe metal and copious proportions of CO gas. The most serious problem concerns the yield of CO gas. If equilibrium smelting produced the ureilites' entire 21 mol% range in olivine‐core mg, the proportion of gas within the asteroidal mantle (assuming plausibly low pressure <˜80 bar) should have reached ≥85 vol%. Based on the remarkably stepwise cooling history inferred from ureilite texture and mineralogy, a runaway, CO‐leaky process that can loosely be termed smelting appears to have occurred, probably triggered by a major impact. The runaway scenario appears likely because, by Le Châtelier's principle, CO leakage would tend to accelerate the smelting process. Also, the copious volumes of gas produced by smelting would have led to explosive, mass‐leaky eruptions into the vacuum surrounding the asteroid. Loss of mass would mean diminution of interior pressure, which would induce further smelting, leading to further loss of mass (basalt), and so on. Such a disruptive runaway process may have engendered the ureilites' distinctive reduced olivine rims. But the only smelting, according to this scenario, was a short‐lived disequilibrium process that reduced only the olivine rims, not the cores; and the ureilites were cooling, not melting, during the abortive “smelting” episode.     

Ureilite Compaction

Abstract— Experimental partial melting of Allende in a modest thermal gradient produces compacted crystalline residues on the cold side and silicate liquid segregations on the hot side of the gradient. The mechanism leading to this segregation is a type of zone refining: thermal migration. Chemical diffusion produces relative migration of phases along a saturation gradient induced by a temperature gradient. The compacted crystals have many features reminiscent of ureilite meteorites: olivine major element compositions, surface-equilibrium-controlled adcumulus textures, ripened grain sizes, virtually complete silicate melt removal, and textural elongation. Sulfide melt fails to segregate from crystals by this mechanism. If thermal migration is responsible for the textural features of ureilite compaction, then often-assumed constraints about ambient gravity fields and cooling regimes do not apply to ureilite origins irrespective of whether the compaction is of a melting residue or a cumulus crystal pile. Certain ingrained expectations relating the geochemistry of complementary liquids to these crystals also will be unsatisfied. Because virtually any crystal consolidation regime operates through a thermal gradient, it is likely that thermal migration must play at least a small role in the process. It would be a particularly appropriate mechanism for consolidating ureilites within planetesimals undergoing vigorous primordial heating by radioactivity or electromagnetic induction—circumstances in which the efficacy of the process is not truncated by cooling. The possible existence of this mechanism does not invalidate less unusual suggestions for the origin of ureilites. However, some of the constraints on ureilite origin and complementary liquid geochemistry are now considerably relaxed.     

Carbon,Nitrogen, and Noble Gases in the Diamond Fractions of the Novo Urei Ureilite

We measured the contents and isotopic compositions for C, N, and noble gases in the diamond fractions separated in a heavy liquid ( = 2.9 g/cm³) from a sample enriched with diamond from the Novo Urei ureilite. The results show that the concentrations of nitrogen and noble gases in the diamond fraction isolated from the supernatant (the fraction is named DNU-1) are more than a factor of 1.5 higher than those in the diamond fraction from the residue (DNU-2). This difference is probably caused by smaller sizes of grains and (or) clusters of smaller grains as well as by larger defectiveness of the crystal lattice of the diamond in the DNU-1 fraction as compared to DNU-2. Both fractions are similar in the isotopic composition of C and N and in the ratios of trapped chemical elements. The results obtained and the published data concerning C, N, and noble gases in different fractions of other ureilites allow us to conclude the following. (1) The ureilite diamond was most likely formed from graphite and the fine-grained crystalline (or semiamorphous) carbonaceous phase as a result of shock transformation in the parent bodies. (2) The negative result in the search for the isotopically light component of nitrogen (¹⁵N is about –100) in the Antarctic unshocked ureilite ALH 78019 (Rai et al., 2002), which introduced serious difficulties for explaining the origin of the ureilite diamond in the parent bodies during the impact, is most likely caused by the absorption of atmospheric nitrogen by the carbonaceous material in the processes of terrestrial weathering. (3) The source of light nitrogen (¹⁵N –100) in the ureilite diamond was probably the presolar diamond in the initial carbonaceous material of the ureilite parent bodies, because the impurity elements, including nitrogen (¹⁵N < –350), in this diamond could be trapped in the magmatic processes by the carbonaceous material, which became a precursor of the ureilite diamond in the shock event.