Chlorine concentrations and stable isotope ratios in Izu Bonin tephra

d37Cl values were determined for Izu Bonin arc magmas erupted 0-44 Ma in order to better understand the time-dependent processing of volatiles in subduction zones. Pristine ash-sized particles (glass, pumice, scoria, and rock fragments) were handpicked from tephra drilled at ODP Site 782. d37Cl values for these particles span a large range from -2.1 to +1.7 per mil (error = ± 0.3 per mil) vs. SMOC (Standard Mean Ocean Chloride, defined as 0 per mil). The temporal data extend the previously reported range of d37Cl values of -2.6 to 0.4 per mil (bulk ash) and -5.4 to -0.1 per mil (volcanic gases) from the Quaternary Izu Bonin–Mariana volcanic front to more positive values. Overall, the temporal data indicate a time-progressive evolution, from isotopically negative Eocene and Oligocene magmas (-0.7 ± 1.1 per mil, n = 10) to Neogene magmas that have higher ?37Cl values on average (+0.3 ± 1.1 per mil; n = 13). The increase is due to the emergence of positive d37Cl values in the Neogene, while minimum d37Cl values are similar through time. The range in d37Cl values cannot be attributed to fractionation during melt formation and differentiation, and must reflect the diversity of Cl present in the arc magma sources. Cl clearly derives from the slab (> 96% Cl in arc magmas), but d37Cl values do not correlate with isotope tracers (e.g. 207Pb/204Pb and 87Sr/86Sr) that are indicative of the flux from subducting sedimentary and igneous crust. Given the steady, high Cl flux since at least 42 Ma, the temporal variability of d37Cl values is best explained by a flux from subducting isotopically positive and negative serpentinite formed in the ocean basins that mingles with and possibly overprints the isotopically negative flux from sediment and igneous crust at arc front depths. The change in the d37Cl values before and after backarc spreading may reflect either a tectonically induced change in the mechanism of serpentinite formation on the oceanic plate, or possibly the integration of isotopically positive wedge serpentinite as arc fluid source during the Neogene. Our study suggests that serpentinites are important fluid sources at arc front depth, and implies the return of isotopically positive and negative Cl from the Earth surface to the mantle.为深入理解俯冲带内挥发分的随时间演化地球化学过程，本研究针对0~44 Ma喷发的伊豆-小笠原弧（Izu Bonin arc）岩浆测定了d37Cl值。研究人员从大洋钻探计划（Ocean Drilling Program, ODP）782站位的钻探火山碎屑中手工拣选出原始的火山灰级颗粒（包括玻璃质、浮石、火山渣及岩屑）。相较于标准海洋氯离子平均值（Standard Mean Ocean Chloride, SMOC，定义为0‰），这些颗粒的d37Cl值分布范围跨度极大，为-2.1‰~+1.7‰（误差为±0.3‰）。本次研究的时序数据将此前报道的第四纪伊豆-小笠原-马里亚纳火山带的d37Cl值范围拓展至更偏正的数值区间：此前块状火山灰样品的d37Cl值范围为-2.6‰~0.4‰，火山气体样品则为-5.4‰~-0.1‰。整体而言，时序数据呈现出随时间推进的同位素演化趋势：始新世与渐新世岩浆的d37Cl值偏负（-0.7±1.1‰，n=10），而新近纪岩浆的平均d37Cl值更高（+0.3±1.1‰，n=13）。该数值升高主要源于新近纪出现了正的d37Cl值，而不同时期的d37Cl最小值基本保持一致。本次观测到的d37Cl值范围无法用熔体形成与分异过程中的同位素分馏解释，其本质反映了弧岩浆源区中氯同位素组成的多样性。弧岩浆中超过96%的氯均源自俯冲板片，但d37Cl值与指示俯冲沉积物和火成地壳物质通量的同位素示踪剂（如²⁰⁷Pb/²⁰⁴Pb、⁸⁷Sr/⁸⁶Sr）并无相关性。鉴于至少自42 Ma以来氯通量始终保持较高且稳定的水平，d37Cl值的时序变化可通过以下机制得到最佳解释：大洋盆地中形成的、兼具正负同位素组成的俯冲蛇纹岩所贡献的氯通量，在弧前深度与源自沉积物和火成地壳的负同位素氯通量发生混合，并可能对后者产生同位素叠加改造。弧后扩张前后的d37Cl值变化，既可能反映洋盆板块上蛇纹岩形成机制受构造作用驱动的改变，也可能与新近纪时期以具正同位素组成的楔状蛇纹岩作为弧流体源有关。本研究表明，蛇纹岩是弧前深度的重要流体源，同时也暗示了地球表面的正负同位素氯可循环进入地幔。