Melting Phase Relations and Element Partitioning in MORB to Lowermost Mantle Conditions
Melting Phase Relations and Element Partitioning in MORB to Lowermost Mantle Conditions
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MORB 至最低地幔条件下的熔相关系和元素分配
DOI：
10.1029/2018jb015790
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发表时间：
2018
期刊：
影响因子：
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通讯作者：
Ohishi Yasuo
中科院分区：
文献类型：
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作者：
Tateno Shigehiko;Hirose Kei;Sakata Shuhei;Yonemitsu Kyoko;Ozawa Haruka;Hirata Takafumi;Hirao Naohisa;Ohishi Yasuo
利用激光加热金刚石砧细胞技术研究了135 GPa的洋中脊玄武岩体组成中的熔融相关系和晶体-熔体元素分配。利用场发射型电子探针（FE‐EPMA）、透射电子显微镜（TEM）和激光烧蚀电感耦合等离子体质谱仪（LA‐ICP‐MS），我们对共存的熔体和固相中的主要元素和微量元素进行了全面的分析。CaSiO3‐钙钛矿（Ca‐pv）在整个下地幔压力范围内为液相相。当压力超过100 GPa时，二氧化硅和镁钙钛矿是第二和第三个结晶相，在135 GPa时，钙钛矿紧随其后，钙钛矿紧随其后。微量元素在Ca‐pv和熔体之间的分配表现出强烈的压力效应，这可能是由于Ca‐pv中阳离子的高压缩性和熔体压缩效应的结合。Na和K的Ca - pv/melt分配系数（DNaandDK）随着压力的增加而增加，在最低地幔压力下，Na接近于1，dk大于1。此外，地壳地壳比地幔深处的地壳地壳更大（或在不确定范围内相同）。135 GPa洋中脊玄武岩部分熔融51%形成的部分熔体显示出明显的铁富集，因此在地幔底部应该具有负浮力。残余固体的密度与PREM密度几乎相同，因此很可能参与地幔对流并再循环到地表。
Melting phase relations and crystal‐melt element partitioning in a mid‐oceanic ridge basalt bulk composition were studied to 135 GPa using laser‐heated diamond‐anvil cell techniques. Using field‐emission‐type electron microprobe (FE‐EPMA), transmission electron microscope (TEM), and laser ablation‐inductively‐coupled plasma mass spectrometer (LA‐ICP‐MS), we obtained comprehensive analyses of major and trace elements in coexisting melt and solid phases. CaSiO3‐perovskite (Ca‐pv) was found to be the liquidus phase throughout the lower mantle pressure range. Whereas silica, followed by Mg‐perovskite, are the second and third crystallizing phases to pressures exceeding 100 GPa, postperovskite, closely followed by seifertite, succeed Ca‐pv at 135 GPa. The partitioning of trace elements between Ca‐pv and melts exhibited a strong pressure effect, possibly due to a combination of high compressibility of cations compared to the lattice site in Ca‐pv and melt compressional effects. The Ca‐pv/melt partition coefficients for Na and K (DNaandDK) increase with increasing pressure, withDNaclose to unity andDKgreater than unity at lowermost mantle pressures. Also,DNdbecomes larger (or identical within uncertainty) thanDSmin the deep lower mantle. Partial melt formed by 51% partial melting of mid‐oceanic ridge basalt at 135 GPa showed marked iron‐enrichment and should thus have negative buoyancy at the base of the mantle. The density of residual solid is almost identical to the PREM density, and therefore, it is likely to be involved in mantle convection and recycled to the surface.