Platinum group element fractionation in a komatiitic basalt lava lake

Precise PGE (Os, Ir, Ru, Pt, Pd) abundance data for bulk rock samples, and mineral separates, from a deep differentiated komatiitic basalt lava lake i n the Vetreny Belt (Baltic Shield) were obtained using an ICP-MS ID techniq ue. The composition of the erupted liquid (MgO = 15%) is characterized by a highly fractionated PGE pattern, (Pd/Os)(N) = 48, with subchondritic (Os/I r)(N) = 0.43. Analysis of separated olivine and chromite revealed that Ru, Os, and Ir were slightly compatible to moderately incompatible in olivine ( D = 1.7-0.8), and were compatible with chromite (D = 100-150). Platinum and Pd were highly incompatible with olivine (D = 0.08-0.03), and moderately c ompatible with chromite (D = 1.6-3.3). Bulk solid-liquid partition coeffici ents for PGEs, Ni, Cr, and Cu, estimated from the whole rock regressions, w ere: Os 7.9, Ir 6.6, Ru 4.5, Pt 0.53, Pd 0.09, Ni 6.2, Cr 4.4, and Cu 0.01. The highly incompatible behavior of Cu and Pd indicates that there was no separation of FeNiCu immiscible sulfide liquid during the differentiation o f the lava lake. Comparison of the bulk partition coefficients with those c alculated from the olivine and chromite compositions implies that the fract ionating mineral assemblage included similar to 10(-5 +/-1) mass fraction o f pyrrhotite. The data obtained provide new constraints on the effects of f ractional crystallization on the PGE compositions of mafic-ultramafic magma s. Copyright (C) 2001 Elsevier Science Ltd.