Geochemical Evidence for Subduction Fluxes, Mantle Melting and Fractional Crystallization Beneath the South Sandwich Island Arc
Geochemical Evidence for Subduction Fluxes, Mantle Melting and Fractional Crystallization Beneath the South Sandwich Island Arc
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DOI：
10.1093/petrology/36.4.1073
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发表时间：
1995-08
影响因子：
3.9
通讯作者：
J. Pearce;P. E. Baker;P. Harvey;I. W. Luff
中科院分区：
文献类型：
--
作者：
J. Pearce;P. E. Baker;P. Harvey;I. W. Luff
南桑威奇岛弧的火山有三个不同的系列：低钾拉斑玄武岩（其次是Zavodovski，Candlemas，Vindication，Montagu和布里斯托），拉斑玄武岩（其次是Visokoi，Sounders和Bellinghausen）和钙碱性（其次是Ltskov，Freezland和Cook和Thule的一部分）。通量计算表明，俯冲组分对所有三个系列地幔源区的贡献百分比从不可测（如Zr）到小（如Nd=20%）和中等（如La，Ce，Sr = 50-80%）到占主导地位（如Pb，K，Ba，Rb，Cs >90%），沿弧沿着变化很小。同位素系统学（Pb，Nd，Sr）表明，这俯冲组件获得了更大的贡献，从蚀变洋壳比远洋沉积。俯冲贡献较小的元素表明，在弧下熔融之前，地幔相对于N-MORB地幔已经亏损（相当于损失约2-5%的熔融部分）。在加入俯冲分量后，亏损地幔的动态熔融导致区分三个系列的K值变化。估计的部分熔融程度（~20%）略高于洋脊下的部分熔融程度，尽管地质测温表明原始岩浆温度（~1225°C）与原始MORB相似。大约一半的熔化可归因于挥发物的加入，一半归因于减压。可能需要涉及三维，两相流的动态熔化，以充分解释岛间的变化。
The volcanoes of the South Sandwich island arc follow three distinct series: low-K tholeiitic (followed by Zavodovski, Candlemas, Vindication, Montagu and Bristol), tholeiitic (followed by Visokoi, Sounders and Bellinghausen) and calcalkaline (followed by Ltskov, Freezland and part of Cook and Thule). Flux calculations indicate that the percentage contribution of the subduction component to the mantle source of all three series varies from undetectable (e.g. Zr) through small (e.g. Nd=20%) and moderate (e.g. La, Ce, Sr = 50-80%) to dominant (e.g. Pb, K, Ba, Rb, Cs >90%) with little change along the arc. Isotope systematics (Pb, Nd, Sr) show that this subduction component obtains a greater contribution from altered oceanic crust than from pelagic sediment. Elements for which the subduction contribution is small show that the mantle is already depleted relative to N-MORB mantle (equivalent to loss of an ~2-5% melt fraction) before melting beneath the arc. After addition of the subduction component, dynamic melting of this depleted mantle then causes the variationsin K that distinguish the three series. The estimated degree of partial melting (~20%) is slightly greater than that beneath ocean ridges, though geothermometry suggests that the primary magma temperature (~1225°C) is similar to that of primary MORB. About half of the melting may be attributed to volatile addition, and half to decompression. Dynamic melting involving three-dimensional, two-phase flow may be needed to explain fully the inter-island variations.