Lithium, boron and lead isotope and trace element systematics of Quaternary basaltic volcanic rocks in northeastern Japan: mineralogical controls on slab-derived fluid composition

摘要

Across-arc Li-B-Pb isotope systematics, together with trace element geochemistry and phase relationships in subducting slabs are used to understand the petrogenesis of lavas from the northeastern Japan arc. Pb/Nb ratios and Pb isotope compositions of the arc lavas decrease with depth to slab whereas Li/Y, U/Nb, Th/Nb and La/Sm show an upward profile across the arc, with peaks at 180 km depth to the Wadati-Benioff zone (WBZ). The peaks in these profiles may reflect the influence of lawsonite breakdown in the slab at 180 km depth. B/Nb ratios of the arc lavas do not show clear across-arc variation and the delta(11)B values show a concave upward across-arc profile with minimum values at around 180 kin in depth of the WBZ. These results possibly reflect the decomposition of tourmaline at similar to180 km. Our results suggest that accessory minerals and minor phases in the subducting slab play important roles in determining the chemical compositions of fluid released by dehydration, and the resulting trace element and isotope composition of arc lavas. Li isotope compositions of the lavas are MORB-like and do not show systematic across-arc variation, contrasting with Izu arc lavas, which show maximum delta(7)Li values at the volcanic front and a systematic decrease towards the back arc. The strikingly different results for the northeastern Japan arc and the Izu arc are likely to be caused by differing extents of Li isotope fractionation due to differing subduction zone physical characteristics, such as thermal structure of the subducting slab and subduction angle. Thus, not only chemical characteristics of the subducting slab but also the physical, as well as chemical characteristics of the subduction zone could affect the Li isotope compositions of subducting slabs, producing heterogeneity in the Li isotope composition of the mantle. (C) 2004 Elsevier B.V. All rights reserved.