Insights into the earliest slab inputs from the lithium systematics of Izu-Bonin forearc volcanic rocks recovered during IODP Expedition 352

IODP Expedition 352 to the Izu-Bonin forearc recovered sequences of basalts and boninites that provide a near-complete record of the igneous activity during subduction initiation. Forearc basalts (FAB) are decompression melts related to mantle upwelling during early slab foundering, while boninites record the fluid-fluxed melting of harzburgitic mantle depleted by FAB production. To try and document the onset of slab inputs during subduction initiation, we are examining lithium abundance and isotopic systematics of Expedition 352 recovered FABs and boninites. While lithium is enriched in arc lavas, its mobility during prograde metasomatic processes appears to be limited to higher temperature conditions. As well, the fact that Li is moderately incompatible in magmatic systems means significant Li is sequestered in phenocrysts, potentially permitting successful analysis in older and/or moderately seawater-altered samples, after a targeted leaching procedure.

Our samples analyzed include in-situ measured glasses, and "POOL sample" whole rocks which were leached in cold 6M HCl to remove seawater alteration effects. FAB Li concentrations vary from 5 to 13 ppm. Li/Yb ratios in "axial" FABs are MORB-like at 1.6 to 1.85. However, in the upper FAB sequences Li/Yb increases to up to 2.5, suggesting additions of a component that is richer in Li than in heavy REE. The low-silica boninites range in Li from 2.7 to 19.8 ppm, on average higher than FABs, with Li/Yb as high as 36.9 and averaging 8.9, far higher than any known arc volcanic rocks. On a plot of Li/Yb vs. Dy/Yb the boninites display a vertical array, consistent with additions of slab Li and/or with residual amphibole, which retains Yb relative to Li. 352 boninites form a broad negative array on a plot of K/Li versus La/Yb. Such trends indicate amphibole crystallization in volcanic arc lavas. However, as amphibole is not encountered in Exp. 352 boninites, and is not a stable crystallizing phase in boninite systems, these arrays most likely reflect the impact of inputs from an amphibolitized early down-going slab, consistent with inferences from radiogenic isotopes.