Submarine venting of magmatic volatiles in the Eastern Manus Basin, Papua New Guinea
Submarine venting of magmatic volatiles in the Eastern Manus Basin, Papua New Guinea
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DOI：
10.1016/j.gca.2015.04.023
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发表时间：
2015-08
影响因子：
5
通讯作者：
J. Seewald;E. Reeves;W. Bach;P. Saccocia;P. Craddock;W. Shanks;S. Sylva;T. Pichler;M. Rosner;Emily A. Walsh
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文献类型：
--
作者：
J. Seewald;E. Reeves;W. Bach;P. Saccocia;P. Craddock;W. Shanks;S. Sylva;T. Pichler;M. Rosner;Emily A. Walsh
SuSu Knolls和DESMOS热液田位于马努斯盆地东部弧后伸展转换带。 2006 年，在两个地点收集了高酸性且富含 ΣSO4 的喷口流体，并分析了主要和微量物质的化学和同位素组成。测量到从海底排出的液体温度为 48 至 215 °C，由于元素 S0 的沉淀，其外观呈乳白色。喷口流体中 Na、K 和 Mg 的浓度相对于海水减少了 30%，但相对丰度相同。相比之下，相对于海水，流体中溶解的ΣCO2、Cl、SiO2(aq)、Fe 和Al 高度丰富。测得的 pH (25 °C) 范围为 0.95 至 1.87，ΣSO4 水溶液范围为 35 至 135 mmol/kg。化学和同位素组成表明，海水与不含钠、钾、镁和钙的、富含挥发物的岩浆液在地下混合而形成，该岩浆液是在类似于地下喷气孔排放的过程中从地下岩浆体中溶出的。对岩浆端元组成的估计表明流体相为 H2O > SO2 > CO2 ≈ Cl > F。喷口流体中 H2O 的氢和氧同位素组成以及 ΣCO2 的碳同位素组成强烈表明来自板片的 H2O 和 CO2 对东马努斯火山带下方地幔中产生的熔体的贡献。丰富的岩浆衍生的 SO2 在上升流区域冷却过程中发生歧化，并为喷口流体提供丰富的酸度、SO42− 和 S0。这些高酸性流体与上流区高度改变的矿物组合的相互作用导致了 SiO2(aq)、Fe 和 Al 的广泛水流动。 1995 年至 2006 年间，DESMOS 喷口场中水性 S 物种的形态和丰度随时间变化，表明与地下海水混合的岩浆端元中 SO2 的相对丰度有所增加。这项研究的结果限制了在岩浆活跃的弧后环境中形成温泉流体的过程以及由此产生的岩石圈和水柱之间的化学交换。
The SuSu Knolls and DESMOS hydrothermal fields are located in the back-arc extensional transform zone of the Eastern Manus Basin. In 2006, highly acidic and ΣSO4-rich vent fluids were collected at both sites and analyzed for the chemical and isotopic composition of major and trace species. Fluids exiting the seafloor have measured temperatures from 48 to 215 °C and are milky white in appearance due to precipitation of elemental S0. Vent fluid concentrations of Na, K, and Mg are depleted by as much as 30% relative to seawater, but have the same relative abundance. In contrast, the fluids are highly enriched in dissolved ΣCO2, Cl, SiO2(aq), Fe, and Al relative to seawater. Measured pH (25 °C) ranged from 0.95 to 1.87 and aqueous ΣSO4ranged from 35 to 135 mmol/kg. The chemical and isotopic composition points to formation via subsurface mixing of seawater with a Na-, K-, Mg-, and Ca-free, volatile-rich magmatic fluid exsolved from subsurface magma bodies during a process analogous to subaerial fumarole discharge. Estimates of the magmatic end-member composition indicate a fluid phase where H2O > SO2> CO2≈ Cl > F. The hydrogen and oxygen isotopic composition of H2O and carbon isotopic composition of ΣCO2in the vent fluids strongly suggest a contribution of slab-derived H2O and CO2to melts generated in the mantle beneath the Eastern Manus volcanic zone. Abundant magmatically-derived SO2undergoes disproportionation during cooling in upflow zones and contributes abundant acidity, SO42−, and S0to the venting fluids. Interaction of these highly acidic fluids with highly altered mineral assemblages in the upflow zone are responsible for extensive aqueous mobilization of SiO2(aq),Fe, and Al. Temporal variability in the speciation and abundance of aqueous S species between 1995 and 2006 at the DESMOS vent field suggests an increase in the relative abundance of SO2in the magmatic end-member that has mixed with seawater in the subsurface. Results of this study constrain processes responsible for the formation of hot-spring fluids in magmatically active back-arc environments and the resulting chemical exchange between the lithosphere and water column.