Unique Behaviors of Highly Siderophile elements

Platinum group elements, Re and Au are called highly siderophile elements (HSE) because they have very high metal/silicate partition coefficients. Determination of their metal/silicate partition coefficients, however, is not easy. Low concentrations of HSE in silicates are not the only hamper. The suboptical size nuggets formed by HSE in silicate melts under reducing conditions are another factor affecting true determination of HSE in the silicates (1). The mechanism of formation of HSE nuggets is still under discussion. In this study a fire assay technique was used to extract metal elements from geological reference material BCR-1. Similar amounts of highly pure FeNi metal and BCR-1 were mixed and melted in a furnace at 1550° C and at a CO/CO₂ controlled oxygen fugacity. After fusion a big metal ball formed. Some tiny metal grains ( ~100 mm) were also found in the silicate glass. The big metal and several tiny metals were collected for neutron activation analysis. While the major compositions of the big metal and tiny metals are similar, concentrations of trace elements in the two sizes of metal are very different: trace elements are highly enriched in the tiny metal fraction. Cobalt and Cr come mainly from the BCR-1 reference material and their fractionation between big and tiny metals is reduced along with increasing melting time. Platinum, Au and Ir come from Pt wire used to hold corundum containers. The tiny/big metal concentration ratios of Pt, Au and Ir, however, increased with increasing melting time. A likely explanation is Pt, Au and Ir dissolved in the silicate melts via vapor forms and the dissolved Pt, Au and Ir are disseminated in the silicate and easily caught by tiny metal grains. If so the appearance of HSE nuggets is an experimental design problem. Avoiding formation and condensation of HSE vapors may eliminate the occurrence of HSE nuggets. Reference: (1) Borisov A and Palme H. GCA, 4349-4357, 1997.