Why do Some Mantle Peridotites Contain Little Osmium and Iridium?

In peridotites that undergo moderate degrees of partial melting, Os and Ir behave as compatible elements (Morgan, 1986). Deviations from this rule have been noted recently for dunites and harzburgites that represent former high-permeability pathways of primitive melts in mantle-derived peridotites. Here we briefly discuss occurrences from three different tectonic settings and their implications. (1) Layered dunite-pyroxenite rocks occur in spinel-garnet peridotite massifs in lower Austria, remnants of mantle from a Paleozoic convergent plate margin. The dunite-pyroxenite rocks are characterized by suprachondritic γ Osi (7.2 to 13.9), indicating addition of radiogenic Os from percolating melts. Assuming the dunites originally had a composition similar to harzburgites at the same outcrop (γ Osi of -2.7 to -3.9), the change in γ Os requires melt/rock ratios between 10 and 400. Os and Cr are depleted in the dunites by at least 50-70 % compared to normal harzburgites at the same outcrop, and enriched in associated orthopyroxenites (up to 3.8 ppb Os, 5700 ppm Cr), indicating substantial transfer of peridotitic Os and Cr into melt. (2) Similar rocks occur in the Lanzo spinel-plagioclase peridotite massif (Italian Alps), a MORB-mantle fragment emplaced during the opening of the Thetys ocean 200 Ma ago. Dunite and harzburgite layers show anomalously low Ir abundances (0.36-1.8 ppb) compared to normal lherzolites (typically near 3-4 ppb Ir). Some Ir may have been deposited in websterite layers (0.8-4 ppb Ir). (3) Alkaline basalts from two off-ridge axis seamounts (<5 Ma) close to the Jan Mayen fracture zone (North Atlantic) contain a large proportion of mantle-derived harzburgitic xenocrysts and xenoliths. Separated xenocryst aggregates (predominantly olivine and orthopyroxene) contain little Os (6-150 ppt), have low Re/Os, and show γ Os between 0 and 106. The composition of these rocks is inconsistent with simple closed-system melting. Significant quantities of Mg-rich melt must have reacted with these peridotites and dissolved and removed a major proportion of the initial budget of Os, Ir, and to some extent Cr. Unknown fractions of these elements were re-deposited in Mg-Cr-rich pyroxenites. At low pressures, similar processes may play a role during the formation of tabular dunites and associated podiform chromitite deposits. The physical and chemical parameters that control these dissolution and precipitation processes are not well understood and require further study.