Thermochemical behaviour of Pt, Ir, Rh and Ru vs fO2 and fS2 in a basaltic melt.: Implications for the differentiation and precipitation of these elements

This paper describes an experiment conducted at 1430 degrees C to study the behaviour of Pt, Ir, Rh and Ru in a silicate melt of basaltic composition in relation to fO(2) and fS(2). Previous results showing the existence of a passivation layer on the metals are confirmed and demonstrated by an electrochemical technique. The presence in the silicate melt of platinum and iridium at zero valency is used to explain the observed behaviour for these elements. On the other hand, the presence of Rh3+ and Ru3+ ions likely to enter the spinel lattice helps to explain the mineralogical observations made on the samples of a Tropoja chromite (Albania), formed in a sulphur-depleted system. While the variation in oxygen fugacity can explain the premature precipitation of IPGEs (Ir, Ru, Os), it is the variation in sulphur fugacity which explains the similar precipitation of PPGEs (Rh, Pt, Pd). Complex Pt (or Rh) compounds in the silicate melt are highlighted for a sulphur fugacity greater than 1.5 x 10(-2) atm. The PPGEs precipitate only when the base metal sulphides (BMS) precipitate. This occurs after precipitation of a large amount of iron and calcium, i.e., after a significant reduction in the sulphur capacity of the magmatic liquid and a concomitant increase in fS(2). The temperature and oxygen fugacity of a dunitic assemblage are determined from intrinsic oxygen fugacity measurements performed by means of a solid electrolyte cell with chromites and olivines from the Krasta (Albania) differentiated series. This enabled the validity of the thermochemical results to be checked. Finally, a PGE differentiation and precipitation scheme is proposed as a function of oxygen and sulphur fugacities and in the presence of a fluid phase.