ISOTOPIC AND GEOCHEMICAL EVIDENCE OF DE-CARBONATION OF SUBDUCTING SLAB IN THE MANTLE WEDGE AS RECORDED IN ULTRAMAFIC XENOLITHS FROM THE KAMCHATKA ARC

The calc-alkaline lavas of the Kharchinsky and Sheveluch volcanoes in the central segment of the Kamchatka arc that formed as a result of northwestward subduction of the Mesozoic Pacific Plate contain peridotite xenoliths, representing mantle wedge samples beneath Kamchatka. The lavas are enriched in the incompatible LREE with characteristic negative Nb-Ta anomalies and ⁸⁷Sr/⁸⁶Sr ratios of 0.70339-0.70358. The ultramafic xenoliths, mostly dunites, hornblende-bearing wehrlites and harzburgites in the spinel facies, show strong evidence of plastic deformation of all constituent minerals, along with abundant fluid inclusions decorating the kink band boundaries and deformation lamellae in olivines, ortho- and clinopyroxenes.

The whole–rock xenoliths show conspicuous LREE depletion, Nb-Ta anomaly, large Ba enrichments relative to HFSE and a narrow range of ⁸⁷Sr/⁸⁶Sr ratios of 0.70342-0.70352. These trace element and Sr-isotopic signatures of the ultramafic xenoliths most probably represent a subducting slab-derived component, that is added to the depleted mantle, and which is dominated by a hydrous and CO₂-rich fluid. He contained in the CO₂ rich fluid inclusions of olivines and clinopyroxenes have ³He/⁴He ratios consistent with western Pacific volcanic arcs (R/R_a=6.8-8.0) with dominantly mantle source for the He. The CO₂/³He range from 2x10¹⁰- 6x10¹⁰, more than an order of magnitude greater than typical mantle values, and in the range of values usually observed in emanations from volcanic arcs. The enrichment of CO₂ relative to He in the xenoliths, representing the sub-arc mantle, suggests that the CO₂ evolved from the de-carbonation of the subducting oceanic crust. The CO₂ evolved from de-carbonation contain small amounts of ⁴He (CO₂/He ~10⁷), a result consistent with the arc fumaroles throughout the Pacific rim.