Dehydration of melt inclusions in olivine and implications for the origin of silica-undersaturated island-arc melts
Dehydration of melt inclusions in olivine and implications for the origin of silica-undersaturated island-arc melts
复制标题
橄榄石中熔体包裹体的脱水及其对硅不饱和岛弧熔体起源的影响
DOI：
10.1016/j.epsl.2019.04.021
复制
发表时间：
2019
影响因子：
5.3
通讯作者：
F. Holtz
中科院分区：
文献类型：
--
作者：
M. Portnyagin;N. Mironov;R. Botcharnikov;A. Gurenko;R. Almeev;Cornelia Luft;F. Holtz
与俯冲有关的原生岩浆形成了现代大陆地壳，并抵消了在这种构造背景下发生的地壳物质向深部地幔的大规模再循环。富镁橄榄石中的熔体包裹体被认为是寻找与俯冲有关的原始熔体的最佳探针。然而，出乎意料的是，大多数这样的包裹体是SiO2-不饱和的，主要是SiO2-饱和岛弧岩。这些熔体的成因已被解释为熔融的角闪石的辉石在下地壳或上地幔。然而，目前的模型不能解释高丰度的SiO2-不饱和熔体以及它们的成分差异与寄主岩石的主要元素，但不是不相容的微量元素。在这里，我们报告的结果，从岩石和熔融包裹体的橄榄石从堪察加克柳切夫斯基火山的调查。我们发现，在300 MPa压力和1200 °C下，橄榄石Fo 85 − 90中SiO2不饱和熔体包裹体的实验再水化导致熔体中H2O和SiO2的伴随富集，使得再水化包裹体（4-5 wt% H2O）变得像原始克柳切夫斯基岩一样硅饱和。先前再水化夹杂物的实验脱水也导致在H2O和SiO2熔体的耦合耗尽。估计的化学计量的SiO2和H2O的增益/损失是一致的溶解/结晶的金属缺陷橄榄石包体壁。红外光谱分析表明，水的迁移机制是氢通过橄榄石结构在八面体金属（Mg，Fe）空位中扩散控制的。我们的结论是，以前报道的SiO2-不饱和组合物的许多熔体包裹体从紫苏辉石规范岛弧岩石可以解释的耦合损失高达几个重量百分比的H2O和SiO2从最初被困原始SiO2-饱和的含水熔体。因此，SiO2-不饱和熔体包裹体可能不是原始岛弧岩浆的代表。夹杂物中SiO2和H2O损失的发现，使我们能够提出一种方法来重建初始水含量，即使是完全脱水的夹杂物。本研究结果可能表明，大多数原始岛弧包裹体没有保留其初始H2O含量，并且原始弧熔体平均含有≥ 4wt%的H2O。较高的H2O浓度在主弧熔体意味着存在一个“地壳过滤器”控制的水含量，这可以保存在熔融包裹体，也较低的地幔熔融温度和更高的输出H2O通量在俯冲带比以前估计的基础上直接测定H2O在潜在的脱水熔融包裹体。
Primary subduction-related magmas build up modern continental crust and counterbalance massive recycling of crustal material into the deep mantle occurring at this tectonic setting. Melt inclusions in Mg-rich olivine are believed to be the best probes of primary subduction-related melts. However, unexpectedly, most of such inclusions are SiO2-undersaturated, in contrast to predominantly SiO2-saturated island-arc rocks. The origin of these melts has been explained by melting of amphibole-bearing pyroxenites in the lower crust or upper mantle. The current models fail, however, to explain the high abundance of SiO2-undersaturated melts as well as their compositional difference with host rocks for the major elements but not for incompatible trace elements. Here we report results from the investigation of rocks and melt inclusions in olivine from Klyuchevskoy volcano in Kamchatka. We show that experimental re-hydration of SiO2-undersaturated melt inclusions in olivine Fo85−90at 300 MPa pressure and 1200 °C causes a concomitant enrichment of melt in H2O and SiO2so that re-hydrated inclusions (4–5 wt% H2O) become as silica-saturated as primitive Klyuchevskoy rocks. An experimental dehydration of previously re-hydrated inclusions also resulted in coupled depletion of melt in H2O and SiO2. The estimated stoichiometry of SiO2and H2O gain/loss is consistent with dissolution/crystallization of metal-defect olivine on inclusion walls. The migration mechanism of water is controlled by hydrogen diffusion in the octahedral metal (Mg, Fe) vacancies through olivine structure as confirmed by FTIR spectroscopy. We conclude that the previously reported SiO2-undersaturated composition of many melt inclusions from hypersthene-normative island-arc rocks can be explained by the coupled loss of up to several weight percent of H2O and SiO2from the initially trapped primitive SiO2-saturated hydrous melts. Thus, SiO2-undersaturated melt inclusions may not be representative of primitive island-arc magmas. The discovery of the coupled SiO2and H2O loss from inclusions allowed us to propose a method for reconstruction of the initial water content even for completely dehydrated inclusions. The results of this study may indicate that the majority of primitive island-arc inclusions have not preserved their initial H2O content, and that primary arc melts contain on average ≥4 wt% H2O. The higher H2O concentration in primary arc melts implies the existence of a ‘crustal filter’ controlling the water content, which can be preserved in melt inclusions, and also the lower mantle melting temperatures and higher output H2O fluxes in subduction zones than previously estimated based on direct determination of H2O in potentially dehydrated melt inclusions.
堪察加半岛克柳切夫斯科伊火山原始含气泡熔体包裹体的挥发物含量：质量平衡与实验均质化测定的挥发物含量的比较
影响因子：
2.9
作者：
Moore, Lowell R.;Mironov, Nikita;Portnyagin, Maxim;Gazel, Esteban;Bodnar, Robert J.
通讯作者：
Bodnar, Robert J.
影响因子：
5
作者：
E. Ferriss;T. Plank;M. Newcombe;D. Walker;E. Hauri
通讯作者：
E. Ferriss;T. Plank;M. Newcombe;D. Walker;E. Hauri