From source to surface: An Os isotope study of the transfer of mantle source signatures in subduction-related melts

Osmium isotopic data for subduction zone magmas span a wide range from mantle-like to very radiogenic values. While passage through continental crust likely influences a magma's ¹⁸⁷Os/¹⁸⁸Os signature towards higher values, especially in evolved, low-Os magmas, what is unclear, in situations where slab-melting has been proposed, is the influence of the subducted slab on ¹⁸⁷Os/¹⁸⁸Os at the magma source. Because osmium is much more concentrated in the mantle than in most subduction zone magmas, and because of the compatible nature of osmium in mantle peridotites, it is possible that any slab contribution to ¹⁸⁷Os/¹⁸⁸Os will be completely obliterated during percolation of the magma through the mantle wedge, i.e., the magma reaching the crust is buffered to mantle wedge values. Examination of osmium isotopic systematics in peridotite-hosted pyroxenites in the Beni Bousera massif, Morocco suggests that this expectation may be unfounded. These pyroxenite layers have previously been shown to represent the crystallized products of magmas derived from remelted lithosphere in an ancient subduction zone (Pearson et al, 1993). While the ¹⁸⁷Os/¹⁸⁸Os ratios in the margins of the pyroxenite layers have been buffered by interaction with the host peridotite, the interior portions of pyroxenite layers are generally much too radiogenic to have been significantly altered by mantle buffering. This indicates that while initial melts flowing through veins in the mantle interact and are affected by the peridotite wall rock, subsequent melt flow is armoured from these effects by the reacted margins. The highly radiogenic ¹⁸⁷Os/¹⁸⁸Os of the pyroxenite interiors must reflect the composition of the source, rather than crustal contamination. These, together with previously published observations (Becker et al., 2004) show how highly radiogenic melt signatures in mantle melts may be transferred to the crust without losing their identity. Recent analyses of osmium isotopic compositions of adakites from Mindanao, Philippines reveal surprisingly low ¹⁸⁷Os/¹⁸⁸Os ratios - too low to have been derived from remelting 50-million-year old oceanic crust. The results from the Beni Bousera pyroxenites indicate the likelihood that the mantle-like ¹⁸⁷Os/¹⁸⁸Os signatures of the Mindanao adakites reflect their source composition. Becker et al., 2004. Chem. Geol. 208:141-156 Pearson et al., 1993 J. Petrol. 34:125-172