Jeremy S. Delaney

Mass-dependent fractionation of Mg, Si, and Fe isotopes in five stony cosmic spherules

We have measured with an electron microprobe the Mg, Al, Si, Ca, Ti, Mn, and Fe contents of five strongly heated stony cosmic spherules (sCS) from the South Pole water well. We have also measured the isotopic compositions of Si, and when possible of Mg and of Fe in these objects by ion microprobe. Except for iron, the measured elemental compositions are chondritic within a factor of 2. In four samples, the ratio of 57Fe/56Fe exceeds the terrestrial value by 3.5‰ to 48‰. Mass-dependent fractionation of the isotopes of Si ranges from ∼2 to ∼8 ‰/AMU in three samples. Mg is clearly fractionated in only one sample, for which δ25Mg = ∼8 ‰. The extent of mass-dependent fractionation of the isotopes and, by implication, of evaporative loss generally follows a trend Mg < Si < Fe. The trend is similar to that found in laboratory heating experiments of charges with solar composition. Although the observed isotopic inhomogeneities within some samples call into question the strict validity of the Rayleigh equation for the sCS, its approximate application to our new and to previously published results for Mg suggests that evaporative losses of greater than 40 wt.% occur rarely from sCS, and that the precursor grains of the sCS had a CM-carbonaceous-chondrite-like complement of Mg, Si, Ca, and Al. Low Fe contents relative to CM abundances could reflect an unusual precursor composition, or, more probably, losses by processes that did not fractionate isotopes, i.e., ejection of immiscible FeS and FeNi beads from the melt or rapid, complete separation and decomposition of FeS at the surface.

Metamorphic reactions in mesosiderites: origin of abundant phosphate and silica

Abstract The high modal abundances of merrillite [Ca 3 (PO 4 ) 2 ] and tridymite in most mesosiderites are not the result of igneous fractionation but are attributed to redox reactions between silicates and P-bearing Fe-Ni metal within a limited T-fO 2 range at low pressure. The Emery mesosiderite is the most tridymite- and merrillite-rich mesosiderite so it is used as the model for this study. Examination of reactions in the system CaO-SiO 2 -MgSiO 3 -Fe-P-O indicate that essentially all of the present phosphorus in Emery should have been dissolved in the metallic portion (calculated to have contained 0.65 wt% P originally), and that it largely reacted to form phosphate. The thermodynamic calculations predict that the reactions would have occurred between 970°C, log fO 2 = −16.5 and 1030°C, log fO 2 = −15.0 for the range of phase compositions in Emery. A narrower range of conditions is expected for other mesosiderites. Phosphide (schreibersite) formed only later at temperatures

Cumberland Falls chondritic inclusions: III. Consortium study of relationship to inclusions in Allan Hills 78113 aubrite

Abstract We describe the mineralogy and report contents of Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U and Zn determined by RNAA in three primitive chondritic inclusions from the ALH A78113 aubrite. Comparison of these data with those for large, petrologic type 3 chondritic clasts from the Cumberland Falls aubrite and the discovery of small clasts in it like those in ALH A78113 indicate that all constitute a single chondritic suite. We also report thermoluminescence data for Cumberland Falls chondritic inclusions and achondritic host. These results, together with mineralogie, major, minor and trace element information, demonstrate that aubrite inclusions represent a different sort of type 3 chondrite, not an LL3 chondrite altered during equilibration with aubrite host. Instead, the aubrite inclusions represent a distinct chondrite class. These inclusions reflect nebular condensation/accretion over a broad redox range and at temperatures relatively high compared with those at which other type 3 chondrites formed. Limited metamorphism and reduction occurred during condensation/accretion, prior to incorporation into aubrite host. During the impact of the chondritic parent body with the aubrite parent body, chondrite fragments were strongly shocked and cooled rapidly. They then mixed with aubrite host, possibly in a regolith, so that these aubrites now represent impact breccias.

Copper and nickel partitioning in iron meteorites

Trace element analyses using proton induced x-ray emission (PIXE) and synchrotron x-ray fluorescence (SXRF) have been made on metal and troilite from 9 iron meteorites representing 5 geochemical groups. Nickel and copper distribution coefficients D (troilitemetal) vary by factors of 600 and 20, respectively, correlate positively with kamacite band width and correlate negatively with bulk nickel content. Meteorites with bulk Ni 1) while those with Ni > 10% have Cu-depleted troilite (DCu < 1). Since magmatic evolution and partial melting will produce Cu-enriched troilite only, the observation of Cu-depleted troilite in Ni-rich meteorites is evidence for subsolidus re-equilibration, a process which reduces DCu by transfer of Cu to exsolving metal. Elemental redistribution may have played an important role in establishing the present chemical trends in iron meteorites.