Trace elements in anatectic products at the roof of mid-ocean ridge magma chambers: An experimental study
Trace elements in anatectic products at the roof of mid-ocean ridge magma chambers: An experimental study
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DOI：
10.1016/j.chemgeo.2017.03.004
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发表时间：
2017-05
期刊：
影响因子：
3.9
通讯作者：
M. Erdmann;L. Fischer;E. Deloule;J. Koepke
中科院分区：
文献类型：
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作者：
M. Erdmann;L. Fischer;E. Deloule;J. Koepke
在快速扩张的大洋中脊(MORS)，轴向熔融透镜(AML)和上覆席状岩脉之间的层位具有广泛的深熔作用。AML的热流与来自上方的热液共同作用导致了高级接触变质作用，这可能导致AML上方的顶板岩石深熔。这一过程的产物是富含二氧化硅的深熔熔体，有可能污染MOR玄武岩和残留的角岩。在这里，我们通过水合部分熔融实验模拟了AML顶板发生的复杂的火成岩和变质过程，并提供了相应的微量元素在熔体和残留物之间的分配系数，这对量化这些过程是有用的。我们给出了实验深熔长英质熔体和不同类型AML顶板岩石水合部分熔融产生的相关残留物的微量元素模式。使用的起始材料是由综合大洋钻探计划钻探的1256D孔的片状岩脉和角石。结果与同一地点直接相关的自然岩性(即长英质岩脉和花岗角砾岩)进行了比较。微量元素含量通常与天然样品重叠，在低水活度下产生的实验熔体(aH2O；lt；(0.5))尽管二氧化硅含量相对较低(58.9至65.7wt%)，但仍可高度富含微量元素。要再现在天然富硅岩石中观察到的低Al_2O_3含量，需要较低的aH2O。然而，低水表明，尽管在模拟深熔作用时，残留角闪石经常被用作解释MORS形成的相关长英质岩石中微量元素特征的重要相，但在深熔作用过程中，残留角闪石的存在并不是必要条件。由于角闪石中没有任何实验残留物，这与1256站点的岩脉/辉长岩过渡过程中的天然角闪石是一致的，因此我们假设AML顶板岩石内的部分熔融是在没有角闪石作为残余相参与的情况下进行的。我们给出了一套全面的微量元素组成，以及从我们的不同潜在原岩在大范围的温度和不同的H_2O_3下的无角闪石实验结果获得的体相和矿物/熔体的微量元素分配系数。
At fast-spreading mid-ocean ridges (MORs), the horizon between the axial melt lens (AML) and the overlying sheeted dikes is characterized by extensive anatectic processes. The heat flux of the AML in combination with hydrothermal fluids from above causes high-grade contact metamorphism, which may result in anatexis of the roof rocks above the AML. The products of this process are silica-rich anatectic melts that have the potential to contaminate MOR basalts and residual hornfels. Here, we simulate the complex igneous and metamorphic processes occurring at the AML roof by hydrous partial melting experiments and provide corresponding trace element partition coefficients between melt and residues, which are useful to quantify those processes. We present trace element patterns from experimental anatectic felsic melts and the related residue produced by hydrous partial melting of various types of AML roof rocks. The starting materials used are sheeted dikes and hornfelses from Hole 1256D drilled by the Integrated Ocean Drilling Program. Results are compared with directly-related natural lithologies (i.e., felsic veins and granoblastic hornfels) from the same site. The trace element contents generally overlap with natural examples and experimental melts produced at low water activity (aH2O < 0.5) can be highly enriched in trace elements despite relatively low SiO2contents (58.9 to 65.7 wt%). A lowaH2O is required to reproduce the low Al2O3contents observed in natural silica-rich rocks. However, lowaH2O implies that the presence of residual amphibole is not required for anatectic processes Even though residual amphibole is often used as an important phase for explaining trace element characteristics in relevant felsic rocks formed at MORs when modeling anatexis. Because amphibole is lacking in any experimental residue, which is in agreement with natural hornfelses from the dike/gabbro transition at Site 1256, we assume that partial melting within the AML roof rocks proceeds without the participation of amphibole as residual phase. We present a comprehensive set of trace element compositions as well as bulk and mineral/melt trace element partition coefficients obtained from our amphibole-free experimental results for different potential protoliths over a large range of temperature and at differentaH2Os.