Tollan Peter, Hermann Jörg
Institut für Geologie, University of Bern, Switzerland.
Nat Geosci. 2019 Jun 27;12(8):667-671. doi: 10.1038/s41561-019-0411-x. Epub 2019 Jul 29.
Elevated HO concentrations and oxygen fugacities are two fundamental properties that distinguish magmas formed in subduction zones from new crust generated at mid ocean ridges. However, the mechanism of magma oxidation, and how it relates to the increase in HO remains unclear. In this study, we use infrared spectroscopy of mantle wedge orthopyroxene to trace the temporal and spatial evolution of oxygen fugacity during transport of hydrous arc melts towards the crust. A positive correlation between equilibrium oxygen fugacity and orthopyroxene HO concentrations for the peridotite samples studied allowed the assignment of specific, commonly-observed absorption bands to redox-sensitive crystallographic defects. HO content associated with these redox-sensitive defects increases in concentration across individual crystals, uniquely preserving the time-dependent transition from reduced to oxidised conditions during the migration of hydrous melts through the mantle wedge. A separate, but related process of reaction with H occurring primarily during the earliest stages of melt-mantle reaction may be fundamental in generating the oxidised nature of hydrous melts. Our study proposes that the oxidised nature of arc magmas may not be a primary feature, but is instead acquired progressively as hydrous primary melts react with the surrounding mantle.
较高的水含量（HO）浓度和氧逸度是区分俯冲带形成的岩浆与大洋中脊新生成地壳的两个基本特征。然而，岩浆氧化的机制以及它与水含量（HO）增加的关系仍不清楚。在这项研究中，我们利用地幔楔斜方辉石的红外光谱来追踪含水弧岩浆向地壳运移过程中氧逸度的时空演化。对于所研究的橄榄岩样品，平衡氧逸度与斜方辉石水含量（HO）浓度之间的正相关关系使得可以将特定的、常见的吸收带归因于对氧化还原敏感的晶体缺陷。与这些对氧化还原敏感的缺陷相关的水含量（HO）在单个晶体中的浓度增加，独特地保留了含水熔体通过地幔楔迁移过程中从还原条件到氧化条件的时间依赖性转变。一个单独但相关的过程，即主要在熔体 - 地幔反应的最早阶段发生的与氢（H）的反应，可能是产生含水熔体氧化性质的根本原因。我们的研究提出，弧岩浆的氧化性质可能不是一个主要特征，而是在含水原生熔体与周围地幔反应时逐渐获得的。
