Oxygen isotope evidence for the formation of silicic Kermadec island arc and Havre-Lau backarc magmas by fractional crystallisation
Oxygen isotope evidence for the formation of silicic Kermadec island arc and Havre-Lau backarc magmas by fractional crystallisation
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DOI：
10.1016/j.epsl.2011.07.014
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发表时间：
2011-09
影响因子：
5.3
通讯作者：
K. Haase;S. Krumm;M. Regelous;M. Joachimski
中科院分区：
文献类型：
--
作者：
K. Haase;S. Krumm;M. Regelous;M. Joachimski
硅质岩浆在俯冲相关的设置被认为是通过极端的晶体分馏或部分熔融的地壳岩石，特别是热液变质的镁铁质岩石。氧同位素结合Cl浓度和比率在新鲜的火山玻璃提供了一种手段来评估和区分热液蚀变玄武岩和沉积物的相对贡献。来自两个Kermadec岛弧火山的安山质到流纹岩样品以及来自Lau和Havre弧后裂谷的玄武岩到英安岩样品的δ 18 O值在5.4和6.5‰ VSMOW之间，即非常接近幔源岩浆的组成。岛弧地壳或弧后地壳中的高温热液变质岩石的部分熔融可以排除，因为这些岩石通常亏损18 O和具有高Cl/Nb比值。海相沉积物的δ 18 O>10‰，因此沉积熔体对弧和弧后地区岩浆的形成不太可能有重要贡献。无论是岛弧英安岩和流纹岩，也不弧后安山岩和英安岩表明，在其岩石成因的地壳的显着参与。弧和弧后玄武岩熔体中δ 18 O的变化范围较窄，这是由于结晶分馏过程和少量沉积物的俯冲蚀变玄武岩/蛇纹岩的水化流体的贡献。
Silicic magmas in subduction-related settings are believed to form either by extreme crystal fractionation or by partial melting of crustal rocks, particularly hydrothermally metamorphosed mafic rocks. Oxygen isotopes in combination with Cl concentrations and ratios in fresh volcanic glasses provide a means to evaluate and discriminate the relative contributions from hydrothermally altered basalt and sedimentary material. Andesitic to rhyolitic samples from two Kermadec island arc volcanoes and basalts to dacites from the Lau and Havre backarc rifts yield δ18O between 5.4 and 6.5‰ VSMOW, i.e. very close to the composition of mantle-derived magmas. Partial melting of high-temperature hydrothermally metamorphosed rocks in the island arc crust or the backarc crust can be ruled out because such rocks typically are depleted in18O and have high Cl/Nb ratios. Marine sediments have δ18O >10‰ and sedimentary melts are therefore unlikely to contribute significantly to the formation of the silicic magmas in the arc and backarc region. Neither the island arc dacites and rhyolites nor the backarc andesites and dacites indicate a significant involvement of the crust in their petrogenesis. The narrow range of δ18O in the arc and backarc silicic melts is due to crystal fractionation processes and contributions of hydrous fluids from subducted altered basalt/serpentinite with little sediments.