An Experimental Study of Trace Element Fluxes from Subducted Oceanic Crust
An Experimental Study of Trace Element Fluxes from Subducted Oceanic Crust
复制标题
俯冲洋壳微量元素通量的实验研究
DOI：
10.1093/petrology/egv046
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发表时间：
2015-08-01
影响因子：
3.9
通讯作者：
Elliott, T.
中科院分区：
文献类型：
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作者：
Carter, L. B.;Skora, S.;Elliott, T.
我们通过实验确定了海底玄武岩原岩在与俯冲带熔融相关的压力和温度（3 GPa，750-1000摄氏度）下的含水相关系和微量元素分配行为。为了避免与起始材料的微量元素掺杂相关的潜在复杂性，我们使用了天然的原始洋中脊玄武岩（来自Kolbeinsey Ridge的MORB）和蚀变洋壳（来自深海钻探项目第46航段的AOC，类似于大西洋北纬20度）。将大约15wt%的水加入到起始材料中以模拟来自大洋地壳下面的脱水蛇纹岩的流体流动。蒸汽饱和固相线对玄武岩K2 O含量敏感，从MORB中的825 +/- 25 ℃（类似于0.04wt%K2O）降至AOC中的约750 ℃（类似于0.25wt%K2O）。纹理证据表明，近固相线流体是次临界的性质。MORB和AOC实验中的残余固体组合以石榴石和单斜辉石为主，副产物为蓝晶石、绿帘石、铁钛氧化物和金红石（固相线以下还有石英柯石英、多硅白云母和磷灰石）。淬火富硅熔体的微量元素分析显示出很强的温度依赖性的关键微量元素。与先前研究的微量元素掺杂的起始材料相比，我们没有观察到残留的褐帘石。相反，丰富的残余绿帘石为钍和轻稀土元素（LREE）提供了宿主，阻止了LREE的释放（Sigma LREE 1500和La/Sm-PUM（PUM表示原始上地幔）类似于1，最接近于全球弧熔岩的地球化学信号，产生于800-850 ℃的AOC。
We have determined experimentally the hydrous phase relations and trace element partitioning behaviour of ocean floor basalt protoliths at pressures and temperatures (3 GPa, 750-1000 degrees C) relevant to melting in subduction zones. To avoid potential complexities associated with trace element doping of starting materials we have used natural, pristine mid-ocean ridge basalt (MORB from Kolbeinsey Ridge) and altered oceanic crust (AOC from Deep Sea Drilling Project leg 46, similar to 20 degrees N Atlantic). Approximately 15 wt % water was added to starting materials to simulate fluid fluxing from dehydrating serpentinite underlying the oceanic crust. The vapour-saturated solidus is sensitive to basalt K2O content, decreasing from 825 +/- 25 degrees C in MORB (similar to 0.04 wt % K2O) to approximate to 750 degrees C in AOC (similar to 0.25 wt % K2O). Textural evidence indicates that near-solidus fluids are sub-critical in nature. The residual solid assemblage in both MORB and AOC experiments is dominated by garnet and clinopyroxene, with accessory kyanite, epidote, Fe-Ti oxide and rutile (plus quartz-coesite, phengite and apatite below the solidus). Trace element analyses of quenched silica-rich melts show a strong temperature dependence of key trace elements. In contrast to the trace element-doped starting materials of previous studies, we do not observe residual allanite. Instead, abundant residual epidote provides the host for thorium and light rare earth elements (LREE), preventing LREE from being released (Sigma LREE 1500 and La/Sm-PUM (where PUM indicates primitive upper mantle) similar to 1, most closely matching the geochemical signal of arc lavas worldwide, were generated from AOC at 800-850 degrees C.