Trace element concentrations in iron type cosmic spherules determined by the SR-XRF method

Abstract

The X-ray fluorescence method using synchrotron radiation (SR-XRF) was applied to determine trace element abundances in iron type (I-type) cosmic spherules collected at deep-sea sediments. Cr, Co, and Ni were detected from almost all spherules and average concentrations are 1336,2991ppm, and 4.0%, respectively. Ga, Ge, and Mn were detected from 20,30,and 20% of spherules and average concentrations are 4,4 and 486ppm, respectively. Spherules containing metallic cores were enriched in Ni and Co and depleted in volatile elements such as Ga and Ge relative to those without cores. The Ni and Co enrichments resulted from high concentrations of the elements in the metallic cores, while the Ga and Ge depletion might indicate that core-bearing spherules have experienced more severe heating during atmospheric entry than the core-lacking ones. On the basis of Mn concentration we divided spherules into two groups : high and low Mn groups. Trace element abundances of the low Mn group are systematically fractionated relative to those of iron meteorites : volatile elements are depleted and refractory ones are enriched in the spherules. This appears to be consistent with the hypothesis that I-type spherules are ablation products of iron meteorites that have lost volatile elements during melting. Production of the low Mn spherules by ablation of chondritic meteorites is also possible, but chondrite melting should have occurred under a limited range of oxygen fugacity in order to enrich Cr and remove Mn in the spherules. Spherules belonging to high Mn group show a complex trace element pattern when normalized to chondrites. A large Mn excess relative to Cr in this type of spherules can not be explained by any formation process, thus it is difficult to infer the precursor material of the Mn-rich spherules