Proterozoic lithosphere in Marie Byrd Land, West Antarctica: Re-Os systematics of spinel peridotite xenoliths

Marie Byrd Land, which forms part of the composite West Antarctica microplate, experienced a long history of subduction during Paleozoic and Mesozoic times, and has been argued to have been affected by at least one mantle plume. The age of the Marie Byrd Land lithosphere is not known, as the only lower crustal samples (granulite and pyroxenite xenoliths) have proven resistant to conventional radiogenic dating methods. The oldest exposed magmatic rocks are Cambrian, and any Precambrian history for Marie Byrd Land, while speculated upon, is unknown. Here we present the first geochemical data for a peridotite xenolith suite from Marie Byrd Land, including Re-Os isotope measurements for 17 samples. The 17 spinel peridotites are from three volcanic centers: Mt Hampton and Mt Cumming, in the Executive Committee Range, and Mt Aldaz, in the USAS Escarpment. The xenoliths are residual lherzolites and harzburgites, ranging from fertile to depleted (e.g.0.39-3.38 wt.% CaO) compositions. Re-Os isotopic systematics indicate a complex evolution for the Marie Byrd Land lithosphere. One sample records Re + Cu ± Os enrichment and two samples document localized Os enrichment, which is most likely related to supra-subduction zone processing, although enrichment due to plume interaction cannot be ruled out. Proterozoic lithosphere stabilization model ages of ca.1.1 and >1.3 Ga are recorded in xenoliths from the Executive Committee Range. The Proterozoic ages are significantly older than outcropping mid-crustal rocks, marking Marie Byrd Land as a third circum-Pacific Phanerozoic mobile belt with preserved Proterozoic lithospheric mantle. The Mesoproterozoic Os model ages are consistent with Proterozoic Nd model ages in Marie Byrd Land granites and orthogneisses, and with a significant detrital zircon age component (ca. 1.0-1.2 Ga) in the thick Swanson Formation sediments that are exposed along the coast. The simplest interpretation of the available mantle and crustal age data is that the lower crust beneath Marie Byrd Land is also Proterozoic in age. Alternatively, if the mantle and lower crust in Marie Byrd Land are not temporally coupled, possible origins for the Proterozoic mantle include older lithosphere impinging from the adjacent East Antarctic craton, or lithospheric mantle that was incorporated into younger oceanic lithosphere during "messy" continental breakup (e.g. within an oceanic plateau) and subsequently accreted to the Antarctic margin. © 2002 Elsevier Science B.V. All rights reserved.