Geochemical evidence for the fractionation of iridium group elements at the early stages of crystallization of the Dovyren magmas (northern Baikal area, Russia)

We have first generalized geochemical and mineralogical data indicating the important role of crystallization of Os–Ir–Ru phases and fractionation of refractory iridium subgroup of PGE (IPGE) at the early stages of the evolution of parental magmas and primitive cumulates from the Yoko–Dovyren layered massif (northern Baikal area, Russia). The object of study was two types of plagioclase peridotites from the lower part of the intrusion, differing in the porosity of primary olivine cumulates: less melanocratic (but more primitive) type I and more melanocratic type II. Inclusions of refractory IPGE (Os, Ir, and Ru) discovered during LA–ICP-MS studies of aluminochromite from type I rocks are the first evidence for the presence of Os–Ir–Ru phases. Subsequent electron microscopy examinations revealed more than 25 grains of laurite and Ir-containing osmium in aluminochromite from plagioperidotites of both types. Attention is focused on the importance of the Ru/Cr₂O₃ and Ir/Cr₂O₃ ratios in rocks for the separation of IPGE at early and late fractionation stages. The conclusion is drawn that the higher Ru/Cr₂O₃ and Ir/Cr₂O₃ ratios in type I plagioperidotites indicate higher enrichment of aluminochromite in inclusions of refractory IPGE minerals. This is consistent with the fact that these rocks are assigned to the most primitive high-temperature ultramafites genetically related to the parental magma, which was in equilibrium with olivine Fo₈₈ at ~1290 ºC. We have established that the parental Dovyren magma was already depleted in IPGE and rhodium before its entrance into a chamber. No signs of early sulfide–silicate immiscibility have been detected.