The Avacah peridotite xenolith suite from Kamachatka arc as a window to sub-volcanic front mantle processes
|Location||International Geological Congress,oslo 2008|
|Author||Arai, Shoji; Ishimaru, Satoko|
|Holding Date||29 September 2008|
Peridotite xenoliths obtained from recent volcanics of Avacha volcano, the Kamchatka arc, Russia, provide us with unrivaled information about upper mantle materials and processes beneath a volcanic front of an arc system. The volcanic front is the very locus where magma generation is most active in arcs. What are the mantle processes beneath such a special place of the arc system? We would like to answer this question based on our results (Arai et al., 2003; Ishimaru et al., 2007; Ishimaru & Arai, 2008a,b,c).
The Avacha xenolith suite is mostly composed of harzburgites with small amounts of pyroxenites, dunites and gabbros. The peridotites are characteristically metasomatized (addition of orthopyroxenes) and sheared to various degrees. Petrographically unmetasomatized harzburgites (mostly C-type = coarse-grained type) show low amounts of Cpx (< 2 vol. %) and relevant refractory mineral chemical characteristics. Olivines are magnesian (around Fo91), and chromian spinels are Cr-rich (Cr# = 0.5-0.7), indicating their high degrees of melting. The Fo content of olivine is rather constant over the range of spinel Cr#, suggesting the relevant melting was not simple melting but fluid-/melt-assisted melting.
The metasomatic addition of Opx at the expense of olivine is marked in some harzburgites. The metasomatic Opxs are poor in Ca, Al and Cr relative to primary ones. Metasomatism is especially prominent in F-type peridotites (sheared), producing Ca-amphiboles (tremolites to pargasites), Al spinels, Ni-Fe monosulfide solid solutions (MSS) in addition to Opxs. Metasomatic Ni enrichment of olivine (up to 5.3 wt% of NiO!) by some Ni-rich agent (possibly Ni-rich silicate melt?) was recognized in one harzburgite. Fe diarsenide was also found in addition of MSS. Almost all harzburgites examined show relatively high fO2. Δlog(fO2)FMQ (fO2 difference relative to the FMQ buffer in log unit) at 1.5 GPa varies from 0.49 (averaged) for apparently unmetasomatized C-type harzburgite via 0.68 for metasomatized C-type to 1.06 for F-type (strongly metasomatized) harzburgite. Note that the average value ranges from -1 to +1 for sub-arc peridotite xenoliths from Western Pacific (Arai et al., 2007), and from -2.5 to +0.5 for abyssal peridotites (Bryndzia & Wood, 1990).
As expected from the active volcanism on the surface, the degree of melting is high in the upper mantle beneath the volcanic front. Supply of fluids (H2O, S and As), which assist partial melting, is also quite active in the mantle there.