Pre-eruptive water content and volatile degassing processes in the southern Okinawa Trough magma: Implications for subduction zone water recycling and magmatic contributions to hydrothermal systems
Pre-eruptive water content and volatile degassing processes in the southern Okinawa Trough magma: Implications for subduction zone water recycling and magmatic contributions to hydrothermal systems
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DOI：
10.1016/j.lithos.2023.107145
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发表时间：
2023-03
期刊：
影响因子：
3.5
通讯作者：
Yuxiang Zhang;G. Gaetani;B. Monteleone;Z. Zeng;Zuxing Chen;Xuebo Yin;Xiaoyuan Wang;Shuai Chen
中科院分区：
文献类型：
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作者：
Yuxiang Zhang;G. Gaetani;B. Monteleone;Z. Zeng;Zuxing Chen;Xuebo Yin;Xiaoyuan Wang;Shuai Chen
挥发分可以作为一个关键环节，连接岩浆和热液系统在俯冲带。在这里，我们提出了挥发性的第一个分析（例如，研究了冲绳海槽南部一座现代弧前火山（布穹丘）和两个弧后扩张中心（八重山地堑和与那国地堑）火山岩熔体包裹体和基质玻璃中的H2O、CO2和S）浓度。使用熔融包裹体分析和斜长石熔融测湿法相结合的岩浆喷发前的H2O浓度（2.2%至3.1%）进行了估计。从弧后到弧外，岩浆的H2O/Ce随δ11B值的增加而单调增加，δ11B值是俯冲蛇纹岩的良好地球化学示踪物。通过与其他俯冲带（马里亚纳弧、马里亚纳海槽、汤加弧、劳盆地）和MORB的数据相结合，我们建立了H2O/Ce和δ 11 B之间的强相关性，表明俯冲蛇纹岩可能对弧岩浆的水收支有重要贡献。我们的熔体包裹体中的低CO2浓度（45-225 μg/g，汽泡校正后）和基质玻璃（<1 μg/g，低于检测限），而原始弧岩浆中>3000 μg/g，表明在岩浆上升期间和/或岩浆库中发生了显著的CO2脱气，这一过程释放出的CO2足以支持冲绳海槽热液系统的高CO2通量。这也意味着在这个地区的热液CO2是不可能来自流体-岩石相互作用。此外，岩浆中S的浓度随着H2O的逐渐损失而降低，而F和Cl的浓度保持不变。因此，我们的研究结果表明，岩浆脱气是一个潜在的重要来源的CO2，S和H2O的热液系统在冲绳海槽。
Volatiles can act as a critical link that connects the magmatic and hydrothermal systems in subduction zones. Here, we present the first analyses of volatile (e.g., H2O, CO2, and S) concentrations in melt inclusions and matrix glasses of volcanic rocks from a modern arc front volcano (Irabu Knoll) and two back-arc spreading centers (Yaeyama Graben and Yonaguni Graben) in the southern Okinawa Trough, a nascent back-arc basin that is characterized by widespread volcanic and hydrothermal activity. The pre-eruptive H2O concentrations of magmas (∼2.2 to ∼3.1 wt%) were estimated using a combination of melt inclusion analyses and plagioclase-melt hygrometry. From the back-arc to the arc, the H2O/Ce of lavas increase monotonically with the increasing δ11B values, a good geochemical tracer for subducted serpentinites. By combining our data with those from other subduction zones (Mariana Arc, Mariana Trough, Tonga Arc, Lau Basin) and MORB, we established a robust correlation between H2O/Ce and δ11B, suggesting that subducted serpentinites might have a significant contribution to the water budget of arc magmas. The low CO2concentrations in our melt inclusions (∼45–225 μg/g, after vapor bubble correction) and matrix glasses (<1 μg/g, below detection limit) as compared to >3000 μg/g in primary arc magmas, suggests that significant CO2degassing has occurred during magma ascent and/or in the magma reservoir, which can release sufficient CO2to support the high CO2flux of the hydrothermal system in the Okinawa Trough. This also implies that hydrothermal CO2in this area is not likely derived from fluid-rock interactions. In addition, the concentration of S in the magma decreases with the progressive loss of H2O, whereas the concentrations of F and Cl remain constant. Our results thus suggest that magma degassing is a potentially important source of CO2, S, and H2O to the hydrothermal system in the Okinawa Trough.