The nature of metasomatism in the sub-arc mantle wedge: evidence from Re–Os isotopes in Kamchatka peridotite xenoliths

Abstract

We have performed Re–Os isotope measurements on a suite of 21 Kamchatka mantle xenoliths including 19 harzburgites and two lherzolites, from the northern arc front (Valovayam Volcano), the southern arc front (Avachinsky Volcano), and behind the arc front in the south (Bakening Volcano). Os and Re concentrations vary from 0.02 to 8.2 and 0.003 to 0.437 ppb, respectively, and 187Re/ 188Os varies from 0.004 to 3.811. 187Os/ 188Os ratios range from 0.1226 to 0.1566. Regional variations in Re–Os isotope signatures are apparent, with peridotites from Avachinsky exhibiting the least radiogenic Os isotope signatures and lowest Re/Os ratios, and those from Bakening the most radiogenic Os and highest Re/Os. Peridotites from Valovayam span the distinct compositional fields defined by the Avachinsky and Bakening peridotites. All of the Kamchatka peridotites are, however, characterized by radiogenic 187Os/ 188Os compared to non-arc continental peridotites with comparable Re abundances or Re/Os ratios. The relatively radiogenic Os isotope signatures in the Kamchatka peridotites cannot easily be explained by contamination of the xenoliths by their host lavas, as this process would result in Re/Os ratios higher than observed in the xenoliths. In situ radiogenic ingrowth of high Re/Os mantle followed by recent Re depletion also cannot explain the observed radiogenic Os signatures in the Kamchatka peridotites, as the time required for radiogenic ingrowth would be significantly greater than the age of the lithospheric terranes that make up the respective regions of Kamchatka. The radiogenic Os isotope signatures in the Kamchatka peridotites are instead attributed to metasomatism of the Kamchatka sub-arc mantle wedge by radiogenic slab-derived fluids and melts. The regional variations in Re–Os isotope signatures are consistent with previous petrographic and geochemical studies of the Kamchatka mantle xenoliths that reveal multistage metasomatic histories resulting from interaction of the mantle wedge with a variety of slab-derived fluids and melts, including silicic slab-melt metasomatism associated with subduction of relatively hot, young (∼15–25 Ma) oceanic crust in the northern arc front, hydrous slab-fluid metasomatism associated with subduction of colder, old (∼100 Ma) oceanic crust in the southern arc front, and carbonate-rich slab-melt metasomatism in the southern segment behind the arc front, where the slab is deeper. Positive correlations between 187Os/ 188Os, La/Sm, and Ru/Ir in Avachinsky harzburgites support a model in which high f O 2 , Cl-rich, hydrous slab fluids transport LREE, Ru, and radiogenic Os into the mantle wedge beneath the southern arc front. Re is either not transported, or is not retained in the mantle during fluid–mantle interaction. Relatively higher Re and more radiogenic Os (but low Os abundances) in the Valovayam and Bakening peridotites indicate that both scavenging of mantle Os as well as exchange with radiogenic slab-derived Os, and incorporation of Re, occurs during interaction of the mantle wedge with oxidized, adakitic, and carbonate-rich slab melts. Similar ranges of Re–Os isotope signatures in peridotites from Avachinsky, Japan and Lihir, and from Valovayam and the Cascades, respectively, suggest that the age (temperature) and depth of subducting oceanic crust influences the Re–Os composition of metasomatized sub-arc mantle.