Variety and origin of magmas on Shatsky Rise, northwest Pacific Ocean
Variety and origin of magmas on Shatsky Rise, northwest Pacific Ocean
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DOI：
10.1029/2012gc004235
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发表时间：
2012-08
期刊：
影响因子：
3.7
通讯作者：
T. Sano;K. Shimizu;A. Ishikawa;R. Senda;Q. Chang;J. Kimura;M. Widdowson;W. Sager
中科院分区：
文献类型：
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作者：
T. Sano;K. Shimizu;A. Ishikawa;R. Senda;Q. Chang;J. Kimura;M. Widdowson;W. Sager
沙茨基海隆由厚的（最大30公里）玄武质地壳组成，具有各种地球化学成分。地球化学数据表明，高原上存在四种岩浆类型，即正常型、低Ti型、高Nb型和U1349型。正常类型的数量最多，出现在高原的所有三个大型建筑物上：Tamu，Ori和Shirshov建筑物。正常型玄武岩的成分与正常洋中脊玄武岩（N-MORB）的成分相似，但不相容元素的相对富集程度较低。低钛型与正常型玄武岩的区别在于在给定的MgO下Ti含量略低。高铌型的组成特征是不相容微量元素含量明显高。U1349型玄武岩的成分较其他类型的玄武岩更为原始和亏损。正常类型玄武岩占最古老的塔穆地块熔岩单位的94%，非正常类型（即，其他三种类型）玄武岩在较年轻的Ori地块上占1.57%，这意味着地球化学成分可能随着时间的推移而变得不均匀。岩石学研究表明，正常、低Ti和高Nb型玄武岩的成分是通过浅岩浆房（<200 MPa）中橄榄石、斜长石和普通辉石的分离结晶演化而来的。不动微量元素的模型计算表明，正常型玄武岩可能是亏损地幔源区在残余石榴石存在下熔融约15%而形成的。这种熔融程度与N-MORB相似，但在岩石形成过程中残留石榴石的较大影响意味着更大的熔融深度。
Shatsky Rise consists of thick (∼30 km maximum) basaltic crust with various geochemical compositions. Geochemistry data indicate that four magma types exist on the plateau; namely normal, low‐Ti, high‐Nb, and U1349 types. The normal type is the most abundant in volume and appears on all three large edifices of the plateau: Tamu, Ori, and Shirshov massifs. Composition of the normal type is similar to normal mid‐ocean ridge basalt (N‐MORB) composition, but with slight relative enrichment of the more incompatible elements. The low‐Ti type is distinguished from the normal type basalt by slightly lower Ti content at a given MgO. Composition of the high‐Nb type is characterized by distinctively high contents of incompatible trace elements. U1349 type basalts are composed of more primitive and depleted compositions compared with the others. The normal type basalts constitute ∼94% of the lava units of the oldest Tamu Massif and non‐normal types (i.e., the other three types) basalts comprise ∼57% on the younger Ori Massif, implying that geochemical compositions may have become heterogeneous with time. Petrological examination demonstrates that compositions of the normal‐, low‐Ti‐, and high‐Nb‐type basalts evolved through fractional crystallization of olivine, plagioclase, and augite in shallow magma chambers (<200 MPa). Model calculations of immobile trace elements estimate that the normal type basalt can be formed by ∼15% melting of a depleted mantle source in the presence of residual garnet. This degree of melting is similar to N‐MORB, but the larger effect of residual garnet during petrogenesis implies that a greater depth of melting.