Clinopyroxene trace element chemistry as a proxy for magma compositional variations in the Izu Bonin rear arc over the last 15…

This study presents major and trace element chemistry of clinopyroxene (CPX) in 0-15 Ma core material recovered from Site U1437 during IODP Expedition 350. Because no fresh glass is present in the core samples older than 4.4 Ma, and hence there is no way to directly determine magma compositions, my study presents the development of a novel method for calculating liquid compositions from CPX grains in volcaniclastic sediments using distribution coefficients and trace elements measured in CPX. Geochemical data from CPX grains was acquired using a laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS). These data were used to calculate trace element ratios for intervals in each of the core's seven lithostratigraphic units. Calculated ratios from Unit I and II (< 4.4 Ma) were compared to measured ratios in coexisting glass to test the validity of the method and showed that the calculated melt chemistries can be used to track magma compositions through time. In turn, the calculated melt chemistries were used to evaluate competing hypotheses regarding the rear arc formation. The calculated melt chemistries for locally-sourced samples showed a change with increasing age from a rear arc seamount chain (RASC) signature (average La/Yb = 3.6 ±0.30 for Units II-V) (4-9 Ma) to a rift-related signature (average La/Yb = 2.0 ±0.16 for Units VI and VII) (9-15 Ma). These older active rift signatures agree with bulk rock chemistry of volcanic clasts recovered from Units VI and VII. These results show that there was a pronounced change in magma source characteristics at around 10 Ma. However, calculated melt chemistries of a small subset of Unit III and Unit V intervals were more enigmatic, reflecting a volcanic front signature with an overall average La/Yb of 0.52 ± 0.042. The progressive change in melt chemistries from a RASC signature to a rift-related signature with depth through the core as seen in the locally sourced intervals suggests a 'from the middle' hypothesis for the RASC formation. We interpret the change in signature was due to slab roll-back; the location of site U1437 resided within a rift-related environment during the Miocene, presumably just behind the volcanic front, then became the site of the modern RASC in the present, even further removed from the volcanic front. The few intervals with volcanic front signatures are hypothesized to indicate either 1) reworking of Oligocene volcanic front material that may underly the modern RASC at a depth greater than was sampled by the IODP rear arc core; or 2) continued sampling of volcanic front type magmas in this region even as the volcanic front migrated trench ward.