Magnesium isotope geochemistry in arc volcanism
Magnesium isotope geochemistry in arc volcanism
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DOI：
10.1073/pnas.1518456113
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发表时间：
2016-06
期刊：
影响因子：
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通讯作者：
F. Teng;Yan Hu;C. Chauvel
中科院分区：
文献类型：
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作者：
F. Teng;Yan Hu;C. Chauvel
马提尼克岛弧熔岩具有非地幔脊玄武岩镁同位素组成，这与俯冲镁的掺入是一致的。据我们所知，这是第一个报告的幔源熔岩与镁同位素组成重于大洋玄武岩，迄今为止，被证明是同位素均匀。更重要的是，我们的研究结果提供了深入了解马提尼克岛弧熔岩的强烈争议的起源，并建议镁的贡献，从流体提供的俯冲板可能会发挥显着的控制弧熔岩的镁同位素系统学。弧火山作用中俯冲板片的混入对弧熔岩的地球化学和同位素变化起着重要作用。然而，板状材料结合的机制和过程仍然不确定。在这里，我们报告，据我们所知，第一组镁同位素数据的一套从马提尼克岛在小安的列斯群岛弧，这显示了一个最极端的地球化学和同位素范围的弧熔岩样品，虽然这种变化的起源仍然是高度争论。我们发现马提尼克岛熔岩的δ 26 Mg值在−0.25到−0.10之间变化，而地幔的δ 26 Mg值范围很窄（−0.25 ± 0.04，2 SD）。这些高δ 26 Mg值表明俯冲板片中的重Mg同位素的掺入。橄榄岩、玄武岩和沉积物之间MgO含量的巨大差异使得沉积物和橄榄岩之间的直接混合或弧壳沉积物的同化不太可能是改变Mg同位素的主要机制。相反，马提尼克岛弧熔岩的重镁同位素特征要求地幔楔的整体组成受到缓冲，并通过在流体-地幔相互作用期间从蚀变俯冲板片释放的流体中优先加入重镁同位素而改变。这反过来又表明了大量的流体活动元素从俯冲板片转移到地幔楔，使镁同位素成为深部流体迁移的良好示踪剂。
Significance Arc lavas from Martinique have nonmidocean ridge basalt Mg isotopic composition, which is consistent with the incorporation of subducted Mg. This is, to our knowledge, the first report of mantle-derived lavas with Mg isotopic composition heavier than oceanic basalts, heretofore shown to be isotopically homogenous. More importantly, our results provide insight into the strongly debated origins of Martinique arc lavas and suggest that contributions of Mg from fluids supplied by the subducted slab may play a significant control in the Mg isotopic systematics of arc lavas. Incorporation of subducted slab in arc volcanism plays an important role in producing the geochemical and isotopic variations in arc lavas. The mechanism and process by which the slab materials are incorporated, however, are still uncertain. Here, we report, to our knowledge, the first set of Mg isotopic data for a suite of arc lava samples from Martinique Island in the Lesser Antilles arc, which displays one of the most extreme geochemical and isotopic ranges, although the origin of this variability is still highly debated. We find the δ26Mg of the Martinique Island lavas varies from −0.25 to −0.10, in contrast to the narrow range that characterizes the mantle (−0.25 ± 0.04, 2 SD). These high δ26Mg values suggest the incorporation of isotopically heavy Mg from the subducted slab. The large contrast in MgO content between peridotite, basalt, and sediment makes direct mixing between sediment and peridotite, or assimilation by arc crust sediment, unlikely to be the main mechanism to modify Mg isotopes. Instead, the heavy Mg isotopic signature of the Martinique arc lavas requires that the overall composition of the mantle wedge is buffered and modified by the preferential addition of heavy Mg isotopes from fluids released from the altered subducted slab during fluid−mantle interaction. This, in turn, suggests transfer of a large amount of fluid-mobile elements from the subducting slab to the mantle wedge and makes Mg isotopes an excellent tracer of deep fluid migration.