Influences of the Tonga Subduction Zone on seafloor massive sulfide deposits along the Eastern Lau Spreading Center and Valu Fa Ridge
Influences of the Tonga Subduction Zone on seafloor massive sulfide deposits along the Eastern Lau Spreading Center and Valu Fa Ridge
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DOI：
10.1016/j.gca.2017.08.010
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发表时间：
2017-10
影响因子：
5
通讯作者：
G. Evans;M. Tivey;J. Seewald;C. Wheat
中科院分区：
文献类型：
--
作者：
G. Evans;M. Tivey;J. Seewald;C. Wheat
本文研究了东劳扩张中心（ELSC）和Valu Fa Ridge （VFR）六个弧后热液喷口场的海底块状硫化物（SMS）矿床的形态、矿物学和地球化学特征，并对其进行了研究。为了补充矿床地球化学，完成了Cu、Zn、Ba、Pb和H2 （aq）的喷口流体分析，以补充现有数据，并实现在特定条件下矿物饱和状态的热力学计算。结果表明，热液流体中地幔不相容元素（Ba和Pb）丰度和SMS矿床中地幔不相容元素（Ba、Pb、As和Sb）丰度向南增加，这也表现为SMS矿床中重晶石（BaSO4）和方铅矿（PbS）的丰度。这些增加对应于ELSC/VFR与汤加俯冲带之间的距离减小，这与地壳岩性从北部的弧后盆地玄武岩向南部的安山岩、流纹岩和英安岩混合转变有关。重晶石影响矿床形态，有助于形成水平凸缘和深蹲阶地。结果也与区域尺度上热液反应带温度从北向南降低（除了最南端的Mariner喷口场）相一致，这导致相对于扩张速率相似的洋中脊，温度更低，ph值更高（Mottl et al., 2011）。这些流体是贫铜和贫锌的，由这些流体形成的矿床贫铜富锌。相比之下，在Mariner喷口区，假设较高温度和较低pH值的喷口流体是由较高的反应区温度和酸性岩浆挥发物的局部添加造成的（Mottl et al., 2011）。Mariner流体为富铜、富锌流体，产自富铜、贫锌、含铅适中的SMS矿床。热力学计算表明，喷口流体和SMS矿床中金属含量的差异可以用喷口流体的ph值来解释。在高ph值、Zn-、Cu-和pb较差的ELSC/VFR喷口流体中，武锌矿/闪锌矿（(Zn, Fe)S）和方铅矿（PbS）在较高温度下处于饱和状态，但在Mariner喷口场的低ph值、Zn-、Cu-和pb较富的喷口流体中，它们在相似温度下处于不饱和状态。ELSC和VFR SMS矿床的pH指标包括：在高pH流体形成的矿床中，沿管道衬里存在共沉淀纤锌矿和黄铜矿；在总体地球化学分析中，锌和银的浓度存在不同的相关性。当热液最小pH值（25℃）< 3.3时，热泉田中矿床的整体地球化学Zn:Ag呈显著正相关，而当热泉田中最小pH值（25℃）小于3.6时，Zn:Ag呈弱相关或负相关。数据表明，直接从热液中析出的明管烟囱矿物衬里的组成（矿物存在，（Zn,Fe）S中FeS的mol%）密切反映了样品热液的温度和硫逸度。因此，这些矿物衬里组成可以作为热液温度和组成（pH、金属含量、硫逸度）的指标。
This study investigates the morphology, mineralogy, and geochemistry of seafloor massive sulfide (SMS) deposits from six back-arc hydrothermal vent fields along the Eastern Lau Spreading Center (ELSC) and Valu Fa Ridge (VFR) in the context of endmember vent fluid chemistry and proximity to the Tonga Subduction Zone. To complement deposit geochemistry, vent fluid analyses of Cu, Zn, Ba, Pb and H2,(aq)were completed to supplement existing data and enable thermodynamic calculations of mineral saturation states atin situconditions. Results document southward increases in the abundance of mantle-incompatible elements in hydrothermal fluids (Ba and Pb) and SMS deposits (Ba, Pb, As, and Sb), which is also expressed in the abundance of barite (BaSO4) and galena (PbS) in SMS deposits. These increases correspond to a decrease in distance between the ELSC/VFR and the Tonga Subduction Zone that correlates with a change in crustal lithology from back-arc basin basalt in the north to mixed andesite, rhyolite, and dacite in the south. Barite influences deposit morphology, contributing to the formation of horizontal flanges and squat terraces. Results are also consistent with a regional-scale lowering of hydrothermal reaction zone temperatures from north to south (except at the southernmost Mariner vent field) that leads to lower-temperature, higher-pH vent fluids relative to mid-ocean ridges of similar spreading rates (Mottl et al., 2011). These fluids are Cu- and Zn-poor and the deposits formed from these fluids are Cu-poor but Zn-rich. In contrast, at the Mariner vent field, higher-temperature and lower pH vent fluids are hypothesized to result from higher reaction zone temperatures and the localized addition of acidic magmatic volatiles (Mottl et al., 2011). The Mariner fluids are Cu- and Zn-rich and vent from SMS deposits that are rich in Cu but poor in Zn with moderate amounts of Pb. Thermodynamic calculations indicate that the contrasting metal contents of vent fluids and SMS deposits can be accounted for by vent fluid pH. Wurtzite/sphalerite ((Zn, Fe)S) and galena (PbS) are saturated at higher temperatures in higher-pH, Zn-, Cu-, and Pb-poor ELSC/VFR vent fluids, but are undersaturated at similar temperatures in low-pH, Zn-, Cu-, and Pb-rich vent fluids from the Mariner vent field.Indicators of pH in the ELSC and VFR SMS deposits include the presence of co-precipitated wurtzite and chalcopyrite along conduit linings in deposits formed from higher pH fluids, and different correlations between concentrations of Zn and Ag in bulk geochemical analyses. Significant positive bulk geochemical Zn:Ag correlations occur for deposits at vent fields where hydrothermal fluids have a minimum pH (at 25 °C) < 3.3, while correlations of Zn:Ag are weak or negative for deposits at vent fields where the minimum vent fluid pH (at 25 °C) > 3.6. Data show that the compositions of the mineral linings of open conduit chimneys (minerals present, mol% FeS in (Zn,Fe)S) that precipitate directly from hydrothermal fluids closely reflect the temperature and sulfur fugacity of sampled hydrothermal fluids. These mineral lining compositions thus can be used as indicators of hydrothermal fluid temperature and composition (pH, metal content, sulfur fugacity).