Across-arc geochemical trends can constrain subduction zone dynamics by providing clues to the source composition and melting systematics of subduction-related magmas over a wide portion of the mantle wedge. The Izu-Bonin arc contains an active volcanic front, an active extensional zone, and a series of 3--9 Ma southwest trending across-arc seamount chains. The volcanic front (VF) contains one of the most depleted suites of any volcanic arc, with basalt containing 0.2--0.7 ppm Nb, 25--50 ppm Zr, Nb/Zr<0.015, and Zr/Y<2.5. Ratios and concentrations of mantle-derived elements change significantly across the Izu-Bonin arc. The westernmost portions of the across-arc seamount chains (WS) contain much higher incompatible element concentrations and associated ratios: 1--8 ppm Nb, 50--130 ppm Zr, Zr/Y=2--7, and Nb/Zr=0.02--0.1. The extensional zone contains intermediate concentrations and ratios of these elements. Trace element modeling shows that VF and WS compositions cannot be produced by different degrees of melting of a homogeneous source. Instead, heterogeneous sources are required, implying that enriched source material must exist in the back arc regions of the Izu-Bonin arc. Melt extraction of fractional melts from the WS source may produce a residual, depleted source capable of generating VF magma. Age dating studies show that the VF and WS suites have retained similar compositions over the last 15 million years, implying that steady state processes have continuously produced these diverse suites of magmas. ¿ 2000 American Geophysical Union