Magma ascent rate and initial water concentration inferred from diffusive water loss from olivine-hosted melt inclusions
Magma ascent rate and initial water concentration inferred from diffusive water loss from olivine-hosted melt inclusions
复制标题
根据橄榄石熔融包裹体的扩散失水推断的岩浆上升速率和初始水浓度
DOI：
10.1007/s00410-012-0821-x
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发表时间：
2013
影响因子：
3.5
通讯作者：
N. Cluzel
中科院分区：
文献类型：
--
作者：
Yang Chen;A. Provost;P. Schiano;N. Cluzel
在岩浆上升过程中，随着岩浆中水含量的降低，橄榄石中的熔融包裹体将通过氢在橄榄石中的扩散与岩浆重新平衡。以前的模型表明，对于一个单一的球形熔融夹杂物在一个球形橄榄石的中心，扩散再平衡的速率取决于分配系数和氢在橄榄石中的扩散系数，熔融夹杂物的半径，和半径的橄榄石。这个过程发生在几个小时内，必须考虑解释水的浓度时，橄榄石托管熔融包裹体。水的浓度和熔体包裹体半径之间的相关性是预期的，因为小的熔体包裹体比大的，当其他条件相同时，更迅速地再平衡。本研究探讨了天然样品中的扩散失水的影响，通过探索这样的水浓度和熔融包裹体半径之间的相关性，并表明相关性可以用来推断初始水浓度和岩浆上升速率。拉曼光谱和傅里叶变换红外光谱研究表明，伊奥利亚群岛Vulcano岛La Sommata地区6个橄榄石中的31个熔融包裹体（半径3.6 - 63.9 μ m）含水量为0.93 - 5.28 wt %，基质玻璃含水量为0.17 wt %。熔体包裹体中的水含量比以往研究中的数据（1.8 - 4.52wt%）显示出更大的变化。它与熔融包裹体半径呈正相关，但不相关的主要元素的浓度在熔融包裹体，这是一致的假设，水的浓度已扩散失水的影响。在一个简化的假设情况下，岩浆上升的初始水浓度和岩浆上升速率推断的扩散失水过程的数值模拟。每个橄榄石中的熔融包裹体被假定为具有相同的初始水浓度和岩浆上升速率。熔融包裹体被认为是在喷发后淬火的（即，不考虑喷发后的扩散性失水）。模型结果表明，熔融包裹体初始含水量为3.9-5.9wt%，喷发前上升速率为0.002-0.021MPa/s。上升速率的总体变化范围接近以往对玄武岩上升速率估计的下限。
As the water concentration in magma decreases during magma ascent, olivine-hosted melt inclusions will reequilibrate with the host magma through hydrogen diffusion in olivine. Previous models showed that for a single spherical melt inclusion in the center of a spherical olivine, the rate of diffusive reequilibration depends on the partition coefficient and diffusivity of hydrogen in olivine, the radius of the melt inclusion, and the radius of the olivine. This process occurs within a few hours and must be considered when interpreting water concentration in olivine-hosted melt inclusions. A correlation is expected between water concentration and melt inclusion radius, because small melt inclusions are more rapidly reequilibrated than large ones when the other conditions are the same. This study investigates the effect of diffusive water loss in natural samples by exploring such a correlation between water concentration and melt inclusion radius, and shows that the correlation can be used to infer the initial water concentration and magma ascent rate. Raman and Fourier transform infrared spectroscopy measurements show that 31 melt inclusions (3.6–63.9 μm in radius) in six olivines from la Sommata, Vulcano Island, Aeolian Islands, have 0.93–5.28 wt% water, and the host glass has 0.17 wt% water. The water concentration in the melt inclusions shows larger variation than the data in previous studies (1.8–4.52 wt%). It correlates positively with the melt inclusion radius, but does not correlate with the major element concentrations in the melt inclusions, which is consistent with the hypothesis that the water concentration has been affected by diffusive water loss. In a simplified hypothetical scenario of magma ascent, the initial water concentration and magma ascent rate are inferred by numerical modeling of the diffusive water loss process. The melt inclusions in each olivine are assumed to have the same initial water concentration and magma ascent rate. The melt inclusions are assumed to be quenched after eruption (i.e., the diffusive water loss after eruption is not considered). The model results show that the melt inclusions initially had 3.9–5.9 wt% water and ascended at 0.002–0.021 MPa/s before eruption. The overall range of ascent rate is close to the lower limit of previous estimates on the ascent rate of basalts.