Lithium isotopic systematics of submarine vent fluids from arc and back-arc hydrothermal systems in the western Pacific
Lithium isotopic systematics of submarine vent fluids from arc and back-arc hydrothermal systems in the western Pacific
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西太平洋弧和弧后热液系统海底喷口流体的锂同位素系统学
DOI：
10.1002/2016gc006355
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发表时间：
2016
期刊：
影响因子：
--
通讯作者：
H.
中科院分区：
文献类型：
--
作者：
Araoka;D.;Nishio;Y.;Gamo;T.;Yamaska;K.;Kawabata;H.
海底喷口流体中的锂浓度和同位素组成 (δ7Li) 对于海洋锂预算很重要，并且对于研究海底深处的热液系统可能有用，因为热液喷口流体相对于海水富含锂。尽管锂同位素地球化学已经在洋中脊（MOR）热液站点进行了研究，但在弧和弧后环境中，锂同位素组成尚未得到系统研究。在这里，我们确定了西太平洋 5 个岛弧和弧后热液系统的 11 种端元流体的 δ7Li 和 87Sr/86Sr 值，并研究了不同地质环境下高温水-岩石相互作用期间的 Li 行为。在缺乏沉积物的热液系统（马努斯盆地、伊豆-博宁岛弧、马里亚纳海槽和北斐济盆地）中，端元流体的Li浓度（0.23-1.30mmol/kg）和δ7Li值（+4.3‰至+7.2‰）主要通过稳态高温海水-岩石相互作用过程中的溶解-沉淀模型来解释。低锂浓度可归因于岩石中锂与温度相关的分配到流体相和相分离过程。 MOR站点之间Li的微小变化可能是由海底扩散热液流体的低温蚀变过程引起的。相比之下，冲绳海槽端元流体的最高锂浓度（3.40–5.98 mmol/kg）和最低δ7Li值（+1.6‰至+2.4‰）表明锂主要来自海洋沉积物。沉积物中锂的变化可以通过与这些热液位点上的海洋沉积物厚度相关的热液-沉积物相互作用程度的差异来解释。
The Li concentration and isotopic composition (δ7Li) in submarine vent fluids are important for oceanic Li budget and potentially useful for investigating hydrothermal systems deep under the seafloor because hydrothermal vent fluids are highly enriched in Li relative to seawater. Although Li isotopic geochemistry has been studied at mid‐ocean‐ridge (MOR) hydrothermal sites, in arc and back‐arc settings Li isotopic composition has not been systematically investigated. Here we determined the δ7Li and87Sr/86Sr values of 11 end‐member fluids from 5 arc and back‐arc hydrothermal systems in the western Pacific and examined Li behavior during high‐temperature water‐rock interactions in different geological settings. In sediment‐starved hydrothermal systems (Manus Basin, Izu‐Bonin Arc, Mariana Trough, and North Fiji Basin), the Li concentrations (0.23–1.30 mmol/kg) and δ7Li values (+4.3‰ to +7.2‰) of the end‐member fluids are explained mainly by dissolution‐precipitation model during high‐temperature seawater‐rock interactions at steady state. Low Li concentrations are attributable to temperature‐related apportioning of Li in rock into the fluid phase and phase separation process. Small variation in Li among MOR sites is probably caused by low‐temperature alteration process by diffusive hydrothermal fluids under the seafloor. In contrast, the highest Li concentrations (3.40–5.98 mmol/kg) and lowest δ7Li values (+1.6‰ to +2.4‰) of end‐member fluids from the Okinawa Trough demonstrate that the Li is predominantly derived from marine sediments. The variation of Li in sediment‐hosted sites can be explained by the differences in degree of hydrothermal fluid‐sediment interactions associated with the thickness of the marine sediment overlying these hydrothermal sites.