Fossil subduction zone origin for magmas in the Ferrar Large Igneous Province, Antarctica: Evidence from PGE and Os isotope systematics in the Basement Sill of the McMurdo Dry Valleys

Abstract Mantle plumes provide an attractive mechanism for generating short-duration, voluminous magmas in large igneous provinces (LIPs) while at the same time providing an explanation for the frequently associated break-up of supercontinents. This model has also been invoked for the Ferrar large igneous province (FLIP) in Antarctica, which zircon and baddeleyite U–Pb dating shows was emplaced over a short duration at 182.7 ± 0.5 Ma, contemporaneously with fragmentation of the supercontinent Gondwanaland. Here, we present platinum-group-element (PGE) and Os-isotopic data for the Basement Sill in the McMurdo Dry Valleys – a part of the FLIP – that challenge the plume interpretation. The Basement Sill samples studied are cumulate-textured gabbro to norite, and pyroxenite with minor ferro- or leuco-lithofacies with MgO ranging from 2 to 19 wt%. The 187 Os/ 188 Os values range from 0.1609 ± 0.003 (2 σ ) to 8.100 ± 1.600 (2 σ ); the minimum value overlaps with a previously published estimated initial 187 Os/ 188 Os ratio for Ferrar magmas of 0.145 ± 0.049 (2 σ ). The PGE abundance patterns for the Basement Sill define positive, convex-shaped slopes between the IPGE (Os, Ir and Ru) and PPGE (Pt, Pd and Rh). The most significant feature of the entire data set is the extreme sub-chondritic Os/Ir ratios ( 2 O is expelled from the source rocks and into the melt.