Diffusive over-hydration of olivine-hosted melt inclusions
Diffusive over-hydration of olivine-hosted melt inclusions
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DOI：
10.1016/j.epsl.2015.06.008
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发表时间：
2015-06
影响因子：
5.3
通讯作者：
M. Hartley;D. Neave;J. Maclennan;M. Edmonds;T. Thordarson
中科院分区：
文献类型：
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作者：
M. Hartley;D. Neave;J. Maclennan;M. Edmonds;T. Thordarson
岩浆喷发前的含水量通常是用晶体熔融包裹体来估算的。然而，橄榄石托管的熔融包裹体是易于后捕获修改H+扩散，因为它们重新平衡与外部环境。这种效果是公认的H+损失的情况下，从橄榄石托管包体，在脱气岩浆中缓慢冷却。在这里，我们提出了相反的效果的证据：除了H+到包裹体中举行的熔体中富含H2 O相对于被困熔体。冰岛东部火山带拉基和Skuggafjöll喷发的211个橄榄石包裹体的成分变化表明，扩散H+增益控制不相容的微量元素贫化包裹体的H2 O含量。个别喷发单位包含橄榄石托管的包裹体，具有变化很大的不相容元素浓度，但接近恒定的H2O。此外，超过40%的包裹体的H2O/Ce> 380，明显高于原始冰岛熔体或洋中脊玄武岩的H2O/Ce（150-280）。事实上，最高的H2 O/Ce比率被发现在最不相容的元素贫化夹杂物表明，水化是一个并发的混合和结晶的组成不同的原生熔体的后果。水化发生时，橄榄石含有贫化包裹体与低H2O含量并列对更多的含水熔体在混合过程中。熔融包裹体从一个单一的喷发可能会保留扩散H+损失和H+增益的证据。因此，微量元素数据对于确定包裹体捕获时熔融包裹体的H2O含量以及最终确定地幔源区的H2O含量至关重要。
The pre-eruptive water content of magma is often estimated using crystal-hosted melt inclusions. However, olivine-hosted melt inclusions are prone to post-entrapment modification by H+ diffusion as they re-equilibrate with their external environment. This effect is well established for the case of H+ loss from olivine-hosted inclusions that have cooled slowly in degassed magma. Here we present evidence for the opposite effect: the addition of H+ into inclusions that are held in melts that are enriched in H 2 O with respect to the trapped melts. The compositional variability in a suite of 211 olivine-hosted inclusions from the Laki and Skuggafjöll eruptions in Iceland's Eastern Volcanic Zone indicates that diffusive H+ gain governs the H 2 O content of incompatible trace element depleted inclusions. Individual eruptive units contain olivine-hosted inclusions with widely varying incompatible element concentrations but near-constant H 2 O. Furthermore, over 40% of the inclusions have H 2 O/Ce> 380, significantly higher than the H 2 O/Ce expected in primary Icelandic melts or mid-ocean ridge basalts (150–280). The fact that the highest H 2 O/Ce ratios are found in the most incompatible element depleted inclusions indicates that hydration is a consequence of the concurrent mixing and crystallisation of compositionally diverse primary melts. Hydration occurs when olivines containing depleted inclusions with low H 2 O contents are juxtaposed against more hydrous melts during mixing. Melt inclusions from a single eruption may preserve evidence of both diffusive H+ loss and H+ gain. Trace element data are therefore vital for determining H 2 O contents of melt inclusions at the time of inclusion trapping and, ultimately, the H 2 O content of the mantle source regions.