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.

为了更深入地理解俯冲带内挥发分的时间演化过程，我们对0~44 Ma喷发的伊豆-小笠原弧岩浆测定了d37Cl值。研究人员从大洋钻探计划(Ocean Drilling Program, ODP)782站位的火山碎屑沉积物中手工挑选出原始的火山灰级颗粒（包括玻璃、浮石、火山渣和岩屑）。这些颗粒的d37Cl值相对于标准大洋氯(Standard Mean Ocean Chloride, SMOC，定义为0‰)的跨度极大，介于-2.1‰至+1.7‰之间（误差为±0.3‰）。本次的时间序列数据将此前报道的第四纪伊豆-小笠原-马里亚纳火山前锋带的d37Cl值范围——块状火山灰为-2.6‰~0.4‰，火山气体为-5.4‰~-0.1‰——拓展至更偏正的数值区间。总体而言，时间序列数据显示出随时间推进的同位素演化特征：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）。这种数值升高主要源于新近纪出现了正d37Cl值，而不同时期的最小d37Cl值基本保持一致。d37Cl值的分布范围无法用岩浆形成与分异过程中的同位素分馏来解释，其必然反映了弧岩浆源区中氯的多样性。弧岩浆中的氯几乎全部来自俯冲板片（占比超过96%），但d37Cl值与指示俯冲沉积岩和火成地壳物质通量的同位素示踪剂（如207Pb/204Pb和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值的时间变异性最好用洋盆中形成的、兼具正负同位素特征的俯冲蛇纹岩的通量来解释：这些蛇纹岩在弧前深度与来自沉积岩和火成地壳的负同位素通量混合，并可能对其产生叠加改造。弧后扩张前后d37Cl值的变化，可能反映了洋壳板片上蛇纹岩形成机制的构造诱导变化，也可能反映了新近纪时期弧流体源区纳入了具有正同位素特征的楔状蛇纹岩。本研究表明，蛇纹岩是弧前深度的重要流体源，同时也暗示了氯同位素从地球表面往返于地幔的循环过程。