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

Chromite separates within the Peridotite Zone of the 2,700 Ma-old Stillwate r Complex are characterized by low Re-187/Os-188 (0.009 to 1.74) and relati vely high Os concentrations (8.74 to 78.2 ppb). Their calculated initial Os isotopic compositions likely reflect the compositions of the magmas from w hich they crystallized. The chromites show variable initial Os isotopic com positions (gamma (Os) of + 2.0 to + 16.4) over the vertical extent of the P eridotite Zone, implicating at least two sources of Os. Both the range of g amma (Os) and values of gamma (Os) decrease upsection. These variations in gamma (Os) were caused by mixing of variable proportions of two magmas havi ng 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 assimi lation of crust, perhaps of sedimentary rocks beneath the Stillwater Comple x. The gradual decrease in the initial gamma (Os) values of the chromite la yers with increasing stratigraphic height implies a decreasing relative con tribution from the contaminated magma throughout the growth of the Peridoti te Zone. Small variations in gamma (Os) between different chromite occurren ces within the H multicyclic unit reflects the petrologic requirement that chromite layers crystallize from slightly different proportions of the magm as, compared to chromite from olivine- and orthopyroxene-rich layers.