Re-Os Systematics in Proterozoic Anorthosite Complexes

Colorado State University, Fort Collins CO

Investigators

Abstract

Hannah EAR-0106874 The magma source for Proterozoic massif-type anorthosite complexes and the origin of massive sulfide bodies found in some of those complexes remain unresolved problems. We propose that rhenium-osmium (Re-Os) concentrations and Os isotope ratios for oxides, sulfides, and silicates from two well-studied anorthosite suites will shed light on these issues. Most models based on geochemical data suggest that anorthosite complexes are derived from mantle melts that pond and fractionate at the base of the crust, and are then variably contaminated by crustal materials during ascent. Considerations from phase equilibria, however, suggest that parental magmas are derived by partial melting of a gabbroic source, presumably in the lower crust. Os isotope ratios, which are particularly sensitive indicators of crustal melting or assimilation, should help discriminate between competing models. High the initial 187Os/188Os ratios in sulfides from Proterozoic anorthosite complexes suggest dominance of crustal Os. It is unclear, however, whether the crustal Os was acquired by selective volatilization and incorporation of crustal sulfide, wholesale assimilation of bulk crust, or derivation of parental melts by partial melting of a crustal gabbroic source. We will compare Os isotopic data from two systems - the apparently sulfide-free Laramie Anorthosite Complex in Wyoming, and the sulfide-bearing Hakefjorden Complex in SW Sweden - to estimate the most likely source for the sulfides and their contained metals. The results could offer an exploration tool for distinguishing sulfide-poor and sulfide-rich systems.