Across-arc geochemical trends in the Izu-Bonin arc: Constraints on source composition and mantle melting

Across-are geochemical trends can constrain subduction zone dynamics by pro viding clues to the source composition and melting systematics of subductio n-related magmas over a wide portion of the mantle wedge. The Izu-Bonin are contains an active volcanic bent, an active extensional zone, and a series of 3-9 Ma southwest trending across-arc seamount chains. The volcanic fron t (VF) contains one of the most depleted suites of any volcanic are, with b asalt containing 0.2-0.7 ppm Nb, 25-50 ppm Zr, Nb/Zr<0.015, and Zr/Y<2.5. R atios and concentrations of mantle-derived elements change significantly ac ross the Izu-Bonin are. The westernmost portions of the across-are seamount chains (WS) contain much higher incompatible element concentrations and as sociated ratios: 1-8 ppm Nb, 50-130 ppm Zr, Zr/Y=2-7, and Nb/Zr=0.02-0.1. T he extensional zone contains intermediate concentrations and ratios of thes e elements. Trace element modeling shows that VF and WS compositions cannot be produced by different degrees of melting of a homogeneous source. Inste ad, heterogeneous sources are required, implying that enriched source mater ial must exist in the back are regions of the Izu-Bonin are. Melt extractio n of fractional melts from the WS source may produce a residual, depleted s ource capable of generating VF magma. Age dating studies show that the VF a nd WS suites have retained similar compositions over the last 15 million ye ars, implying that steady state processes have continuously produced these diverse suites of magmas.