RE-OS SYSTEMATICS IN FE-TI OXIDES FROM PROTEROZOIC ANORTHOSITE COMPLEXES

Initial ¹⁸⁷Os/¹⁸⁸Os ratios from oxide phases in Proterozoic Anorthosite Complexes (PACs) can elucidate magma sources for these enigmatic systems despite low (ppt range) concentrations of Re and Os in Fe-Ti oxides. Prior studies of Re-Os systematics in sulfides from PACs and associated mafic complexes reveal crustal ¹⁸⁷Os/¹⁸⁸Os, but also show that sulfides may reflect only local crustal contamination rather than the primary magmatic system. Oxides are found in all mafic complexes, either as large concentrated deposits or as accessory grains, and thus provide a more universal method of obtaining age and source information than more easily measured, but less reliable sulfides.

Magnetite samples from the Sybille Pit and Shanton Mines of the Laramie Anorthosite Complex (LAC) in Wyoming were analyzed for Re-Os data. LAC magnetites are coarse grained with microscopic exsolution of both ilmenite and in some cases ulvospinel. Re and Os concentrations in these samples have ranges of 50-240 ppt Re and 2-50 ppt Os. Magnetite and ilmenite samples from the Tellnes deposit in the Rogaland Anorthosite Complex in Norway were also analyzed and range from 50-300 ppt Re and 2-80 ppt Os. Pyrite intergrown with the analyzed magnetite yielded much higher concentrations (15 ppb Re and 2 ppb Os). The ilmenite sample is from a separate drill hole and has 140 ppt Re and 3 ppt Os. Ilmenite Re and Os concentrations are consistently lower than those in magnetite. Initial ¹⁸⁷Os/¹⁸⁸Os ratios are variable, possibly the result of incomplete homogenization of contaminated magmas. However, preliminary data from all samples, including sulfides, give elevated model initial ¹⁸⁷Os/¹⁸⁸Os ratios, reflecting a crustal source for the Os in both complexes.