Wet and Dry Basalt Magma Evolution at Torishima Volcano, Izu-Bonin Arc, Japan: the Possible Role of Phengite in the Downgoing Slab
Wet and Dry Basalt Magma Evolution at Torishima Volcano, Izu-Bonin Arc, Japan: the Possible Role of Phengite in the Downgoing Slab
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DOI：
10.1093/petrology/egm048
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发表时间：
2007-10
影响因子：
3.9
通讯作者：
Y. Tamura;K. Tani;Q. Chang;H. Shukuno;H. Kawabata;O. Ishizuka;R. Fiske
中科院分区：
文献类型：
--
作者：
Y. Tamura;K. Tani;Q. Chang;H. Shukuno;H. Kawabata;O. Ishizuka;R. Fiske
位于伊豆 - 小笠原弧中部的诹访之濑岛（北纬31°58′，东经140°8′）和鸟岛（北纬30°58′，东经140°38′）的弧前火山大小相似，作为相对孤立的山体从海底升起。它们共同提供了有关岩浆生成过程的有价值的沿弧信息。这些火山喷发了源于湿的和干的母源玄武岩浆（分别为低锆玄武岩和高锆玄武岩）的低钾玄武岩。基于涉及流体不迁移的不相容元素比值（镧/钐）的模型，母源玄武岩似乎是由同一源地幔不同程度的部分熔融形成的（湿玄武岩岩浆和干玄武岩岩浆分别为20%和10%）。假设湿玄武岩比干玄武岩含有更丰富的板片来源组分，对这两种玄武岩类型进行地球化学比较可以确定从俯冲板片流体传输所涉及的特定元素。利用来自鸟岛的大量新地球化学数据（在鸟岛处，俯冲板片顶部约100千米深），我们发现铯、铅和锶在低锆玄武岩中含量有变化地富集，这不能用分离结晶作用或地幔熔融程度的差异来解释。这些元素被解释为选择性地富集在板片来源的交代流体中。钾、高场强元素和稀土元素浓度的变化很容易用低锆玄武岩和高锆玄武岩之间熔融程度的变化来解释；这些元素不包含在板片来源的流体中。铷和钡在低锆玄武岩中表现出不同的行为，从像钾一样不迁移到在一些低锆玄武岩中轻度富集。我们认为富钾云母——多硅白云母在决定从俯冲板片释放的流体成分方面起着重要作用。在弧前环境中，板片深度为100千米，多硅白云母是稳定的，从板片释放的流体含钾很少。然而，在弧后环境中，板片深度为100 - 140千米，多硅白云母不稳定，释放出富钾流体。我们得出结论，弧玄武岩钾含量的跨弧变化可能与释放的流体或熔体的不同成分有关，而不是广泛认为的这种变化是由部分熔融程度控制的。
The arc-front volcanoes of Sumisu (31 58N, 1408E) andTorishima (30 58N, 140 38E) in the central Izu^Bonin arc are similar in size and rise as relatively isolated edifices from the seafloor.Together they provide valuable along-arc information about magma generation processes. The volcanoes have erupted low-K basalts originating from both wet and dry parental basaltic magmas (low-Zr basalts and high-Zr basalts, respectively). Based on models involving fluid-immobile incompatible element ratios (La/Sm), the parental basalts appear to result from different degrees of partial melting of the same source mantle ( 20% and 10% for wet and dry basalt magmas, respectively). Assuming that the wet basalts contain greater abundances of slab-derived components than their dry counterparts, geochemical comparison of these two basalt types permits the identification of the specific elements involved in fluid transport from the subducting slab. Using an extensive set of new geochemical data fromTorishima, where the top of the downgoing slab is about 100 km deep, we find that Cs, Pb, and Sr are variably enriched in the low-Zr basalts, which cannot be accounted for by fractional crystallization or by differences in the degree of mantle melting. These elements are interpreted to be selectively concentrated in slab-derived metasomatic fluids. Variations in K, high field strength element and rare earth element concentrations are readily explained by variations in the degree of melting between the lowand high-Zr basalts; these elements are not contained in the slab-derived fluids. Rb and Ba exhibit variable behaviour in the low-Zr basalts, ranging from immobile, similar to K, to mildly enriched in some low-Zr basalts.We suggest that the K-rich mica, phengite, plays an important role in determining the composition of fluids released from the downgoing slab. In arcfront settings, where slab depth is 100 km, phengite is stable, and the fluids released from the slab contain little K. In back-arc settings, however, where the slab is at 100^140 km depth, phengite is unstable, and K-rich fluids are released.We conclude that cross-arc variations in the K content of arc basalts are probably related to differing compositions of released fluids or melts rather than the widely held view that such variations are controlled by the degree of partial melting.