Re–Os isotopic constraints on magma mixing in the Peridotite Zone of the Stillwater Complex, Montana, USA

Abstract

Chromite separates within the Peridotite Zone of the 2,700 Ma-old Stillwater Complex are characterized by low 187Re/188Os (0.009 to 1.74) and relatively high Os concentrations (8.74 to 78.2 ppb). Their calculated initial Os isotopic compositions likely reflect the compositions of the magmas from which they crystallized. The chromites show variable initial Os isotopic compositions (γOs of +2.0 to +16.4) over the vertical extent of the Peridotite Zone, implicating at least two sources of Os. Both the range of γOs and values of γOs decrease upsection. These variations in γOs were caused by mixing of variable proportions of two magmas having different Os isotopic compositions. One of the magmatic components was a more primitive magma with a nearly chondritic Os isotopic composition. The other magma had a radiogenic Os isotopic composition as a result of assimilation of crust, perhaps of sedimentary rocks beneath the Stillwater Complex. The gradual decrease in the initial γOs values of the chromite layers with increasing stratigraphic height implies a decreasing relative contribution from the contaminated magma throughout the growth of the Peridotite Zone. Small variations in γOs between different chromite occurrences within the H multicyclic unit reflects the petrologic requirement that chromite layers crystallize from slightly different proportions of the magmas, compared to chromite from olivine- and orthopyroxene-rich layers.