Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.430s&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 430
Computer Science
Chondrites, Eh Chondrites, Enstatite Chondrites, Neutron Activation Analysis, Nodules
Scientific paper
The contents of Na, Sc, Cr, Mn, Fe, Co, Ni, Zn, Ga, As, Se, Br, Sb, W, Ir, and Au in metallic nodules, magnetic and nonmagnetic fractions, and chondrules of the Qingzhen EH3 chondrite have been determined by instrumental neutron activation analysis (INAA). Five of the largest separated chondrules (0.07- 5.77 mg) were selected for INAA. After extraction of the chondrules, the residual sample was gently ground to reduce the grain size and sieved into the following fractions: >500 micrometers, 200-500 micrometers, 100-200 micrometers, 50-100 micrometers, 15-50 micrometers, and <15 micrometers. All fractions were separated by a hand magnet into a nonmagnetic fraction, consisting mainly of silicates and sulfides (mainly troilite), and a magnetic fraction consisting of metal, sulfide, and minor silicates. The separated magnetic nodules from the >500-micrometer fraction weighed between 0.49 mg and 3.99 mg. From all the powders, aliquots of 10 mg were irradiated at the German Cancer Research Institute at Heidelberg (TRIGA-HD II) and counted by using a large-volume, high-resolution Ge(Li) detector. In every irradiation step two samples of the Allende chondrite acted as primary standards. Results: Element concentrations vary with the grain size of the metal due to kamacite in the coarse and schreibersite +- perryite in the intermediate and fine fractions. The element contents (normalized to Fe) of Na, Sc, Cr, Mn, Ni, Se, W, Ir, and Au increase in the metal with decreasing grain size. Cobalt and As display a trend opposite to that of Ni and Au, decreasing with decreasing grain size of the metal in Qingzhen. Whereas the abundance ratios (relative to CI chondrites) of As, Au, and Co are very similar, the refractory siderophile elements Ir and W are depleted in the metal. Under high reducing conditions Ir and W belong to the most refractory siderophiles. The depletion of the refractory elements in Qingzhen with respect to carbonaceous chondrites has been attributed to a partial loss of an early condensate and of a minor metallic phase rich in Ir and W. The W/Ir ratio of the host materials was greater than the ratio in carbonaceous chondrites. This difference in element/Ni ratios observed in Qingzhen metal can be attributed to a loss of refractory-rich material at high temperatures, before Ni and Au had completely condensed. The constancy of the ratios of Ni/Au and As/Co suggests that these elements were not fractionated during the metal-silicate fractionation process and their ratios were already established prior to the loss of metal. This implies that the above elements were already condensed when the fractionation process took place. Because of its lithophile character at low temperature, W was already partly oxidized at the time of the separation of metal from silicates and fractionated. A fractionation of the elements took place before the formation of Qingzhen metal and W was separated as an oxide in an oxidizing environment. The chemical composition of Qingzhen shows that the chondritic material was fractionated in two processes before accretion of the parent body: (1) fractionation of refractory elements under relatively high oxidation conditions and (2) a metal-silicate fractionation. The first process was earlier and has been attributed to the depletion of the high-temperature condensates and a metallic component enriched in the refractory siderophile elements Ir and W. The second fractionation process took place below the condensation temperature of Fe-Ni metal.
Pernicka Ernst
Schmidt Georg
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