HIGH-PRECISION PB ISOTOPE DATA FROM CRUSTAL XENOLITHS TO EXAMINE MAGMA SOURCE AND CRUSTAL INTERACTION, BEZYMIANNY VOLCANO, KAMCHATKA

Volcanic arcs built upon relatively thin, young, mafic crust may preserve only subtle geochemical signatures of upper crustal involvement because of limited chemical contrast between magma and crust. This is apparently the case for the active Kamchatka arc. We have obtained high-precision Pb isotope and mineral composition data for a rare suite of crustal xenoliths and their host rocks to better constrain magmatic processes of the Central Kamchatka Depression (CKD), Russia. Previous work¹ demonstrates that two proximal active volcanoes, Bezymianny and Klyuchevskoy, have different Pb isotope signatures and, therefore, either differ in mantle source or record different extents of magma-crust interaction. To discriminate between these two hypotheses, one must know the chemical and isotopic composition of the underlying crust; however, these data are not available for the CKD. We collected a suite of plag-opx ± cpx ± olv xenoliths and host juvenile eruptive material at Bezymianny. The xenoliths are either cogmagmatic material or direct crustal samples. Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb= 18.158-18.268, ²⁰⁷Pb/²⁰⁴Pb= 15.468-15.481, ²⁰⁸Pb/²⁰⁴Pb= 37.793-37.897) generally overlap Bezymianny magma compositions, with the exception of a few xenoliths that have a less radiogenic Pb signature than their host eruptive material. If the xenoliths represent the CKD crust, then the assimilation of crust may account for the less radiogenic composition of Bezymianny compared to Klyuchevskoy. Although the two volcanoes are adjacent and may originate from the same volcanic system, it is also possible they tap two different source regions that differ by incorporation of subducted sediment into the mantle source; however, Kamchatka is a unique setting in which a subducted sediment component can be effectively ruled out. On an arc scale, Pb isotope and beryllium-10 data² preclude the incorporation of sediment and/or corresponding fluids in the magmas of the CKD. More specific to Bezymianny, our new data further verify that there is little or no sediment incorporation in its mantle source.

1. Kayzar, T. K. et al. AGU Fall. Meet. Suppl. Abstract.43A, 2146 (2008).

2. Kersting, A. & Arculus, R. Earth & Planet. Sci. Lett. 136, 133 (1996).