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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Parental basaltic melts and fluids in eastern Manus backarc Basin: implications for hydrothermal mineralisation
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Parental basaltic melts and fluids in eastern Manus backarc Basin: implications for hydrothermal mineralisation

机译：东部马努斯后弧盆地的亲本玄武质熔体和流体：对热液成矿的影响

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The eastern Manus Basin is an actively forming backarc extensional zone behind the New Britain Island arc, which hosts a number of submarine volcanic edifices and hydrothermal fields. Isotopic and trace element geochemical characteristics of the edifices are comparable with those of the adjacent subaerial New Britain arc, and differ significantly from those of MORB-like lavas on and near the Manus Spreading Ridge in the central part of the basin. Fractional crystallisation dominates magma evolution from primitive basalts to andesites, dacites and rhyodacites in the eastern Manus Basin, but several lineages with differing trace element enrichment have been delineated. Melt inclusions within olivine phenocrysts (Fo_(82-92)) of two representative east Manus basalts, respectively, with modest (0.2 wt%) and high (0.8 wt%) potassium contents, host ubiquitous CO_2-bearing vapour bubbles, denoting presence of an immiscible fluid phase at early stages of crystallisation. Bubbles often carry precipitate phases whose abundance is broadly proportional to the bubble size reaching a maximum in fluid bubbles with little or no melt. Among the precipitates, detected by laser Raman spectroscopy and EDS-scanning electron microscopy, carbonates are common and include magnesite, calcite, ankerite, rhodochrosite and nahcolite (NaHCO_3). Gypsum, anhydrite, barite, anglesite, pyrite, and chalcopyrite have also been found. Some amorphous precipitates recrystallise after bubbles are opened to Na-Ca carbonates, halite and Na-K-Ca alumino-silicates. Copper abundances decrease from basalt to dacite across the eastern Manus fractionation spectrum, whereas Pb behaves as an incompatible element, increasing to highest values in the dacites. Zinc abundance reaches maximum concentrations in andesite, and decreases during further fractionation. Loss of Cu especially form the fractionating magmas, in the absence of immiscible sulphide liquid, strongly implies metal partitioning into CO_2-H_2O fluid, which is degassed significantly during magma fractionation. Hydrothermal fluids in the PACMANUS system may carry a direct contribution of the magmatic metal-bearing fluid, exsolved from the crystallising arc-like magmas at this immature backarc basin, and are able to transport and concentrate major amounts of ore metals, particularly Cu.
机译：东部的马努斯盆地是新不列颠岛弧后的一个活跃形成的弧形伸展带，该弧带拥有许多海底火山大厦和热液田。建筑物的同位素和微量元素地球化学特征与相邻的新不列颠弧下的弧线相当，并且与盆地中部Manus Spread Ridge上及其附近的MORB状熔岩明显不同。在马努斯盆地东部，从原始玄武岩到安山岩，dacits和Rhydacites的部分岩浆演化占主导地位的岩浆演化，但是已经描绘出几种具有不同微量元素富集的谱系。两种代表性的东部Manus玄武岩的橄榄石表晶（Fo_（82-92））中的熔融夹杂物分别具有适度（0.2 wt％）和高（0.8 wt％）钾含量的宿主普遍存在的带有CO_2的蒸气泡，表明存在结晶初期的不相溶的液相气泡通常带有沉淀相，其丰度与气泡大小成正比，而在没有或几乎没有熔化的流体气泡中，气泡达到最大。通过激光拉曼光谱和EDS扫描电子显微镜检测到的沉淀物中，碳酸盐是常见的，包括菱镁矿，方解石，方铁矿，菱锰矿和Nahcolite（NaHCO_3）。还发现了石膏，硬石膏，重晶石，角铁，黄铁矿和黄铜矿。气泡打开后，一些无定形沉淀物重结晶为Na-Ca碳酸盐，盐岩和Na-K-Ca铝硅酸盐。整个东部Manus分馏光谱中，铜的丰度从玄武岩到菊红岩逐渐减少，而Pb表现为不相容元素，在镁矿中上升到最高值。锌的丰度在安山岩中达到最大浓度，并在进一步分馏过程中降低。在不存在不混溶的硫化物液体的情况下，特别是分馏岩浆中Cu的损失强烈暗示了金属会分配到CO_2-H_2O流体中，而在岩浆分馏过程中会大量脱气。 PACMANUS系统中的热液可以直接携带岩浆金属的流体，这些流体是从这个不成熟的后弧盆地的结晶弧状岩浆中溶解出来的，并且能够运输和浓缩大量的矿石金属，尤其是Cu。

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