Investigation of blueschist and serpentinized harzburgite from the Mariana forearc: Insights into the mechanisms of element mobilization in subductionzones and storage of fluid-mobile elements in the mantle wedge

Numerous serpentinite seamounts on the forearc of the Izu-Bonin-Mariana (IBM) subduction zone present the only known locations worldwide where mantle wedge serpentinites and blueschist-facies metamafic fragments can be directly sampled. These fragments have been transported diapirically in a low temperature fluid-mud matrix from within this active subduction zone from a depth of >20 km below seafloor, i.e., directly from the slab-mantle-interface. At South Chamorro Seamount (ODP Leg 195), ~85 km distal from the trench axis, the slab surface is at ~27 km depth, where estimated temperatures are chlorite + amphibole. Estimated concentrations of light elements in bulk rocks are in the same range as in altered oceanic crust and subducting sediments, demonstrating that the major amount remains in the subducting slab and is not released with fluids. However, moderate B loss is suggested by the light d11B values of phengite, chlorite and amphibole (–6 ±4 ‰). As B fractionation is most effective at low temperatures, this light B isotope signature can be explained by low fluid losses from the shallow slab, which originally had a slightly positive average d11B value. Due to B isotope fractionation, the released (Li- and B-enriched) slab-fluids that correlate with a slabresidue with –6 ±4 ‰, are positive reaching a d11B value of up to ~ +20 ‰. In variably serpentinized peridotites, the serpentine polymorphs lizardite, chrysotile and polygonal serpentine together with minor brucite are preferentially distributed between textures or serpentine generations. Li, Be and B abundances are variable and serpentine minerals show an enrichment for Li and B but also a depletion for Li compared to depleted mantle values. Be contents in serpentine are low and similar to primary minerals olivine, orthopyroxene and clinopyroxene. These results demonstrate that serpentinization contributes to a general light element enrichment in the mantle wedge. Early serpentine textures have high Li abundances (up to 30 µg/g) compared to the later serpentine, B abundances are variable. The dominant polymorph is lizardite, which indicates relatively low fluid-rock ratios. Late (youngest) serpentine generations are dominated by chrysotile, which indicates high fluid-rock ratios during serpentinization. This serpentine has lower Li abundances and is relatively enriched in B. The varying fluid-rock ratios between serpentine generations are suggested to be the key to explaining the large range in d11B values of –14 ‰ to +24 ‰ found in serpentine. During early peridotite hydration, all fluid is completely consumed in small-scale serpentine areas and the averaging (~25 mm spot size) SIMS analyses reveal the positive B isotope signature of the slab-derived fluids. The negative d11B values in serpentine can be well explained by strong B fractionation between fluid and serpentine at the low temperature and neutral to basic pH.