Olivine-hosted melt inclusions as an archive of redox heterogeneity in magmatic systems
Olivine-hosted melt inclusions as an archive of redox heterogeneity in magmatic systems
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DOI：
10.1016/j.epsl.2017.09.029
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发表时间：
2014-12
影响因子：
5.3
通讯作者：
M. Hartley;O. Shorttle;J. Maclennan;Y. Moussallam;M. Edmonds
中科院分区：
文献类型：
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作者：
M. Hartley;O. Shorttle;J. Maclennan;Y. Moussallam;M. Edmonds
火山产物的氧化还原状态决定了它们对地球海洋和大气氧化的影响，为地球表面的氧气积累提供了长期的反馈。火山管道系统中的氧化还原条件档案，从岩浆的地幔来源，通过地壳储存，喷发，是在口袋里的熔体被困在晶体。虽然熔体包裹体长期以来一直被利用的能力，保留信息的岩浆的历史，他们的渗透性，快速扩散的元素，如氢，现在有据可查，他们保留的初始氧逸度（f O 2）可能是类似的扩散限制。为了验证这一点，我们已经测量了Fe 3+/Fe 2 O3-Fe显微XANES光谱在一套65橄榄石托管熔融包裹体和9基质玻璃AD 1783年冰岛拉基喷发。这次喷发经历了喷发前混合的化学成分不同的岩浆，同喷发脱气在通风口，和喷发后脱气过程中的熔岩流高达60公里以上的土地，提供了一个理想的测试是否在f O 2的岩浆的变化可能会通过其货物的晶体托管熔融包裹体。急冷火山灰熔体包裹体的Fe 3+/Fe 3 + Fe = 0.2 0 6 ± 0.0 0 8（ΔQFM =+0.7 ± 0.1），其fO 2与微量元素的富集或分异程度无关。这些包裹体保存了混合喷发前拉基岩浆的氧化还原条件。当将分级结晶校正为10重量%时，MgO，这些包裹体记录了母岩浆[Fe 3+/Fe 3 + Fe]（10）为0.18（ΔQFM为+ 0.4），比通常假定的冰岛玄武岩岩浆的Fe 3+/Fe 3 + Fe（0.10）氧化程度高得多。来自淬火熔岩边缘的熔融包裹体比火山灰的熔融包裹体还原程度更高，Fe 3+/Fe 3 + Fe介于0.133和0.177之间（ΔQFM介于− 0.4和+ 0.4之间）。这些包裹体已接近平衡与他们的载体熔岩，已减少硫脱气。夹杂物和载体熔体之间的逐步再平衡的f O 2发生在几个小时到几天的时间尺度上，导致硫化物饱和度（SCSS）的硫含量下降，并驱动夹杂物中不混溶的硫化物球的出溶。我们的数据表明，岩浆混合，渐进再平衡，和脱气在岩浆系统内的氧化还原演化的作用，以及在这些过程中的开放系统的熔体包裹体的性质F O 2。氧化还原的异质性，目前在时间的包裹体捕获可能会叠印的快速再平衡的熔融包裹体F O 2与外部环境，无论是在岩浆房和缓慢冷却的熔岩在表面。这可以解耦的F O 2，主要和微量元素化学的熔融包裹体档案，和掩模F O 2，岩浆分异和地幔源的异质性之间的关联，除非不同的岩浆的组装迅速其次是喷发。因此，我们的工具来了解岩浆的氧化还原条件是有限的，然而，仔细重建前和后喷发岩浆的历史，使我们能够确认相对氧化性质的洋中脊地幔相比，海洋岛屿型地幔。
The redox state of volcanic products determines their leverage on the oxidation of Earth's oceans and atmosphere, providing a long-term feedback on oxygen accumulation at the planet's surface. An archive of redox conditions in volcanic plumbing systems from a magma's mantle source, through crustal storage, to eruption, is carried in pockets of melt trapped within crystals. While melt inclusions have long been exploited for their capacity to retain information on a magma's history, their permeability to fast-diffusing elements such as hydrogen is now well documented and their retention of initial oxygen fugacities (f O 2) could be similarly diffusion-limited. To test this, we have measured Fe 3+/ΣFe by micro-XANES spectroscopy in a suite of 65 olivine-hosted melt inclusions and 9 matrix glasses from the AD 1783 Laki eruption, Iceland. This eruption experienced pre-eruptive mixing of chemically diverse magmas, syn-eruptive degassing at the vent, and post-eruptive degassing during lava flow up to 60 km over land, providing an ideal test of whether changes in the f O 2 of a magma may be communicated through to its cargo of crystal-hosted melt inclusions. Melt inclusions from rapidly quenched tephra samples have Fe 3+/ΣFe of 0.206±0.008 (ΔQFM of+ 0.7±0.1), with no correlation between their f O 2 and degree of trace element enrichment or differentiation. These inclusions preserve the redox conditions of the mixed pre-eruptive Laki magma. When corrected for fractional crystallisation to 10 wt.% MgO, these inclusions record a parental magma [Fe 3+/ΣFe](10) of 0.18 (ΔQFM of+ 0.4), significantly more oxidised than the Fe 3+/ΣFe of 0.10 that is often assumed for Icelandic basalt magmas. Melt inclusions from quenched lava selvages are more reduced than those from the tephra, having Fe 3+/ΣFe between 0.133 and 0.177 (ΔQFM from− 0.4 to+ 0.4). These inclusions have approached equilibrium with their carrier lava, which has been reduced by sulfur degassing. The progressive re-equilibration of f O 2 between inclusions and carrier melts occurs on timescales of hours to days, causing a drop in the sulfur content at sulfide saturation (SCSS) and driving the exsolution of immiscible sulfide globules in the inclusions. Our data demonstrate the roles of magma mixing, progressive re-equilibration, and degassing in redox evolution within magmatic systems, and the open-system nature of melt inclusions to f O 2 during these processes. Redox heterogeneity present at the time of inclusion trapping may be overprinted by rapid re-equilibration of melt inclusion f O 2 with the external environment, both in the magma chamber and during slow cooling in lava at the surface. This can decouple the melt inclusion archives of f O 2, major and trace element chemistry, and mask associations between f O 2, magmatic differentiation and mantle source heterogeneity unless the assembly of diverse magmas is rapidly followed by eruption. Our tools for understanding the redox conditions of magmas are thus limited; however, careful reconstruction of pre-and post-eruptive magmatic history has enabled us to confirm the relatively oxidised nature of ocean island-type mantle compared to that of mid-ocean ridge mantle.