Ultramafic clasts from the South Chamorro serpentine mud volcano reveal a polyphase serpentinization history of the Mariana forearc mantle
Ultramafic clasts from the South Chamorro serpentine mud volcano reveal a polyphase serpentinization history of the Mariana forearc mantle
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DOI：
10.1016/j.lithos.2015.03.015
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发表时间：
2015-06
期刊：
影响因子：
3.5
通讯作者：
Wolf-Achim Kahl;N. Jöns;W. Bach;F. Klein;J. Alt
中科院分区：
文献类型：
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作者：
Wolf-Achim Kahl;N. Jöns;W. Bach;F. Klein;J. Alt
位于普遍断裂的马里亚纳前弧的外半部的蛇纹海山提供了一个很好的窗口到前弧脱挥发分过程，这可以强烈影响俯冲带的挥发分和微量元素的循环。从马里亚纳前弧的活泥火山中回收的蛇纹石化超镁铁质碎屑揭示了指示复杂的多相蚀变历史的微观结构、矿物组合和成分。岩石相关系和氧同位素表明，超镁铁碎屑在低于200 °C的温度下被蛇纹化。可识别出几个连续的蛇纹岩化事件，它们以不同的脉代为代表，具有不同的微量元素含量。原位测量的Rb/Cs比是相当均匀的范围在1和10之间，这是一致的Cs动员沉积物在较低的温度下，并提供进一步的可信度的低温条件下提出的模型中的热结构在弧前设置。晚期脉显示出较低的流体移动的元素（FME）浓度比早期脉，这表明流体排放的俯冲板片上的蛇纹流体的组合物的影响减少。在超镁铁质碎屑中观察到的连续微组构和矿物化学演化可能对蛇纹流体的起源和性质产生影响。我们推测，蛋白石和蒙脱石脱水产生石英饱和流体具有高FME含量和Rb/Cs之间的1和4，导致早期普遍的蛇纹石化。部分蛇纹化的物质随后可能从俯冲上地幔楔的基面被侵蚀。蛇纹石化继续进行，但相互作用的流体没有携带明显的沉积签名，无论是因为FMEs不再从板释放，或由于途中损失的FMEs。晚温石棉脉，文件增加了访问的流体在现在的流体为主的制度的特点是微量元素含量减少，略有增加Rb/Cs比接近10。这种缺乏沉积物为主的地球化学特征一致显示在所有晚期蛇纹石化阶段可能表明，沉积物衍生的流体已被完全重置（即FME过剩被删除）的俯冲通道内持续的水-岩反应。漂浮上升的基质和碎屑在导管的最后阶段的特点是水镁石为主的碎屑从碎屑边缘向内（独立的内部碎屑织物关系），这对应于重新平衡与碱性，低硅活性流体在上升的泥浆中的蚀变。
Serpentine seamounts located on the outer half of the pervasively fractured Mariana forearc provide an excellent window into the forearc devolatilization processes, which can strongly influence the cycling of volatiles and trace elements in subduction zones. Serpentinized ultramafic clasts recovered from an active mud volcano in the Mariana forearc reveal microstructures, mineral assemblages and compositions that are indicative of a complex polyphase alteration history. Petrologic phase relations and oxygen isotopes suggest that ultramafic clasts were serpentinized at temperatures below 200 °C. Several successive serpentinization events represented by different vein generations with distinct trace element contents can be recognized. Measured in situ Rb/Cs ratios are fairly uniform ranging between 1 and 10, which is consistent with Cs mobilization from sediments at lower temperatures and lends further credence to the low-temperature conditions proposed in models of the thermal structure in forearc settings. Late veins show lower fluid mobile element (FME) concentrations than early veins, suggesting a decreasing influence of fluid discharge from the subducting slab on the composition of the serpentinizing fluids. The continuous microfabric and mineral chemical evolution observed in the ultramafic clasts may have implications as to the origin and nature of the serpentinizing fluids. We hypothesize that opal and smectite dehydration produce quartz-saturated fluids with high FME contents and Rb/Cs between 1 and 4 that cause the early pervasive serpentinization. The partially serpentinized material may then be eroded from the basal plane of the suprasubduction mantle wedge. Serpentinization continued but the interacting fluids did not carry a pronounced sedimentary signature, either because FMEs were no longer released from the slab, or due to an en route loss of FMEs. Late chrysotile veins that document the increased access of fluids in a now fluid-dominated regime are characterized by reduced trace element contents with a slightly increased Rb/Cs ratio near 10. This lack of sediment-dominated geochemical signatures consistently displayed in all late serpentinization stages may indicate that the sediment-derived fluids have been completely reset (i.e. the FME excesses were removed) by continued water–rock reaction within the subduction channel. The final stage of buoyant rise of matrix and clasts in the conduits is characterized by brucite-dominated alteration of the clasts from the clast rim inward (independent of the intra-clast fabric relations), which corresponds to re-equilibration with alkaline, low-silica activity fluids in the rising mud.