Jacques Gros

Extinct superheavy element in the Allende meteorite

An effort has been made to identify the extinct superheavy element that was present in meteorites and decayed to 131-136Xe by spontaneous fission. To characterize its chemical properties, we have measured 26 trace elements in six mineral fractions from the Allende C3 chondrite that were enriched up to 180- fold in fission Xe. The superheavy element turned out to reside mainly in a rare mineral associated with chromite (probably a Fe, Ni, Cr, Al-sulfide), comprising only 0.04 percent of the meteorite. It is accompanied by volatile, sulfide- seeking elements such as Tl, Bi, Pb, Br, I, and the heavy noble gases Ar, Kr, and Xe, all of which apparently condensed with this mineral when it formed in the solar nebula at some temperature between 4000 and 5000K. Of the nine volatile superheavy elements 111 to 119, only 115, 114, and 113 are expected to condense as sulfides in that temperature interval. Finally, presumably at least one of these elements has an isotope with a half-life in the range 107 to 108 years: too short to survive to the present day, but long enough to leave detectable effects in meteorites.

Ries impact crater, southern Germany: search for meteoritic material

Abstract Twenty-three samples from the Ries crater, representing a wide range of shock metamorphism, were analyzed for seven siderophile elements (Au, Ge, Ir, Ni, Os, Pd, Re) and five volatile elements (Ag, Cd, Sb, Se, Zn). Taking Ir as an example, we found siderophile enrichments over the indigenous level of 0.015 ppb Ir occur in only eight samples. The excess is very modest; even the most enriched samples (a weakly shocked biotite gneiss and a metal-impregnated amphibolite) have Ir, Os corresponding to ~4 × 10 −4 C1 chondrite abundances. Of five fladle glasses analyzed only one shows excess Ir. Suevite matrix and vesicular glass have slight enrichment, but homogenous glass from the same rock does not. In fladle glasses, Ni and Se are strongly correlated and apparently reside in Ir, Os-poor Sulfides [pyrrhotite, chalcopyrite, pentlandite(?)]of terrestrial, probably sedimentary, origin. The Ir, Os and Ni enrichments of the metal-bearing amphibolite are compatible with chondritic ratios, but these are ill-defined because of uncertainty in Ni. In the other samples enriched in siderophiles Ir(Os), Ni and Se are mutually correlated; Ni Ir and Ni Os ~ 11 × C1 and are much higher than any chondritic ratios; Se Ni ~ 2 × C1 and suggests a sulfide phase, rather than metal may be the host of the correlated elements. Lacking a plausible local source, this material is apparently meteoritic in origin. The unusual elemental ratios, coupled with the very low enrichments, tend to exclude chondrites and most irons as likely projectile material. Of the achondrites, aubrites seem slightly preferable. Ratios of excess siderophiles in Ries materiel match tolerably those of an aubrite (possibly atypical) occurring as an inclusion in the Bencubbin meteorite, Australia. The Hungaria group of Mars-crossing asteroids may be a source of aubritic projectiles.

Volatile elements in chondrites - Metamorphism or nebular fractionation

Abstract Three of the most highly metamorphosed meteorites of their respective classes, Shaw (LL7), Karoonda (C5), and Coolidge (C4), were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Ni, Os, Pd, Rb, Re, Sb, Se, Te, Tl, U, and Zn. Comparison with data by Lipschutz and coworkers on artificially heated primitive meteorites shows that the natural metamorphism of meteorites cannot have taken place in a system open to volatiles. Shaw, metamorphosed at 1300°C for >10 6 yr, is less depleted in In, Bi, Ag, Te, Zn, and Tl than Krymka heated at 1000°C for 1 week. Karoonda, metamorphosed at 600°C for many millennia, is less depleted in Bi and Tl than Allende heated at 600°C for 1 week. Data on primordial noble gases also show that the volatile-element patterns of ordinary and carbonaceous chondrites were established by nebular condensation, and changed little if at all during metamorphism. For enstatite chondrites, the evidence is still incomplete, but seems to favor a nebular origin of the volatile pattern. The general constancy of Tl/Rb, Tl/Cs and Tl/U ratios in terrestrial and lunar rocks suggests that loss of volatile metals such as Tl is rare during normal magmatism or metamorphism. Only impact melts show such loss with any frequency.

Gas-rich minerals in the Allende meteorite - Attempted chemical characterization

A ten-step etching experiment with HNO 3 was performed on a chromite-carbon residue from Allende, in order to characterize the HNO 3 -soluble, minor phase “Q” that contains most of the primordial Ar, Kr, Xe. Each etch fraction was analyzed by neutron activation analysis for Cr, Fe, Co, Ir, and Au. The results suggest that Q consists of two minerals, each comprising about 5% of the residue. Phase Q1, of Fe/Cr ratio >20, in soluble in cold, dilute HNO 3 , and seems to contain most of the heavy noble gases. It may be an HCl-insoluble sulfide of nominal composition (Fe 84 Ni 12 Cr 4 )S x . Phase Q2, of Fe/Cr ratio ∼0.5 and somewhat enriched in Co, is slowly soluble in hot, conc. HNO 3 , and seems to be at least an order of magnitude poorer in heavy noble gases than Q1. It may be daubreelite, or an acid-soluble variety of chromite.