Anatexis at the roof of an oceanic magma chamber at IODP Site 1256 (equatorial Pacific): an experimental study
Anatexis at the roof of an oceanic magma chamber at IODP Site 1256 (equatorial Pacific): an experimental study
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DOI：
10.1007/s00410-015-1136-5
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发表时间：
2015-04
影响因子：
3.5
通讯作者：
M. Erdmann;L. Fischer;L. France;Chao Zhang;M. Godard;J. Koepke
中科院分区：
文献类型：
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作者：
M. Erdmann;L. Fischer;L. France;Chao Zhang;M. Godard;J. Koepke
在快速扩张的洋中脊，补充的轴向熔体透镜体可能向上移动并侵入上覆的热液蚀变席状岩脉，导致高级接触变质作用，并有可能引发顶板岩石的深熔作用。假设这个过程的产物是长英质成分的深熔熔体和花岗变晶、二辉石角岩，代表部分熔融后的残余物。综合大洋钻探计划第309、312和335号考察队在1256号站点（赤道东太平洋）取样了这样一个石化的海洋岩浆房。在本研究中，我们通过使用来自Site 1256席状岩脉杂岩的六种不同原岩进行部分熔融实验来模拟岩浆房屋顶岩石深熔过程，涵盖从弱蚀变到强蚀变玄武岩到部分或完全重结晶的花岗变角岩的岩性范围。结果表明，缺乏原生岩浆矿物的广泛蚀变的起始材料不能再现在海脊环境中回收的天然长英质岩石的化学性质，特别是对热液蚀变敏感的元素（例如，K、Cl）。天然地球化学趋势通过下部席状岩脉中中度蚀变玄武岩的部分熔融而再现。二辉石角岩，假定的残留物，仅在低熔融度（<20体积%）再现。实验中角闪石的整体缺失证实了角闪石在峰期变质作用中不产生的自然观察结果。比较实验产品与天然等价物表明，水活性（aH 2 O）显着降低在回热过程中，主要是基于较低的熔融氧化铝和较低的斜长石钙长石含量在loweraH 2 O。在预期温度（1000-1050 °C;两辉石角岩的峰值热叠印）下的高二氧化硅熔体只能在aH 2 O = 0.1下进行的实验系列中再现。
Replenished axial melt lenses at fast-spreading mid-oceanic ridges may move upward and intrude into the overlying hydrothermally altered sheeted dikes, resulting in high-grade contact metamorphism with the potential to trigger anatexis in the roof rocks. Assumed products of this process are anatectic melts of felsic composition and granoblastic, two-pyroxene hornfels, representing the residue after partial melting. Integrated Ocean Drilling Program Expeditions 309, 312, and 335 at Site 1256 (eastern equatorial Pacific) sampled such a fossilized oceanic magma chamber. In this study, we simulated magma chamber roof rock anatectic processes by performing partial melting experiments using six different protoliths from the Site 1256 sheeted dike complex, spanning a lithological range from poorly to strongly altered basalts to partially or fully recrystallized granoblastic hornfels. Results show that extensively altered starting material lacking primary magmatic minerals cannot reproduce the chemistry of natural felsic rocks recovered in ridge environments, especially elements sensitive to hydrothermal alteration (e.g., K, Cl). Natural geochemical trends are reproduced through partial melting of moderately altered basalts from the lower sheeted dikes. Two-pyroxene hornfels, the assumed residue, were reproduced only at low melting degrees (<20 vol%). The overall amphibole absence in the experiments confirms the natural observation that amphibole is not produced during peak metamorphism. Comparing experimental products with the natural equivalents reveals that water activity (aH2O) was significantly reduced during anatectic processes, mainly based on lower melt aluminum oxide and lower plagioclase anorthite content at loweraH2O. High silica melt at the expected temperature (1000–1050 °C; peak thermal overprint of two-pyroxene hornfels) could only be reproduced in the experimental series performed ataH2O = 0.1.