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Thermal metamorphism of mantle chromites and the stability of noble-metal nanoparticles

机译：亚铬铁矿的热变质作用和贵金属纳米粒子的稳定性

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The Loma Baya complex in south-western Mexico is a volume of chromitite-bearing oceanic mantle that records a complex metamorphic history, defined by a first stage of hydrous metamorphism overprinted by a short-lived thermal event associated with an Eocene granite intrusion. During the hydrous metamorphism, the primary magmatic chromite-olivine assemblage was replaced by a secondary, porous intergrowth of Fe2+ -rich chromite and chlorite. The heat supplied by an Eocene-age granite intrusion reversed the hydration reaction, producing chromite rims with perfectly developed crystal faces. This third-generation chromite is in equilibrium with highly magnesian (neoformed) olivine and defines a chemical trend analogous to the original magmatic one. The preservation of both reactions in the Loma Baya chromitite provides compelling evidence that the hydration of chromite can be reversed by either prograde metamorphism or any heating event, confirming previous thermodynamic predictions. Understanding these complex features is of particular interest due to the fact that changes in temperature and variable degrees of fluid/rock interaction during metamorphism and intrusion have also significantly affected the chromite-hosted IPGE carrier phases. Here, we propose that the metamorphic fluids involved in the hydrous metamorphism have caused the desulphurization of laurite RuS2, releasing minute particles of Ru-Os-Ir alloys <50 nm in diameter. The following short-lived thermal event that promoted dehydration in the chromitite had the opposite effect on nanoparticle stability, producing a significant coarsening of metal nanoparticles to dimensions larger than a micron. Based on such observations, we argue that IPGE nanoparticles can be exsolved and grown (or coarsen) from sulphide matrices during prograde metamorphism or heating and not exclusively upon cooling under magmatic conditions as it has been previously suggested. These results provide new insights on the relevant role of temperature and nanoparticle-host interaction phenomena in natural systems, shedding new light on the kinetic controls of nano-to micron-scale IPGE particle distributions during metamorphism.
机译：墨西哥西南部的洛马巴亚（Loma Baya）复合体是一卷含铬铁矿的海洋地幔，记录了一个复杂的变质历史，定义为含水期变质的第一阶段，该阶段由与始新世花岗岩侵入相关的短暂热事件覆盖。在含水变质过程中，主要的岩浆铬铁矿-橄榄石组合被富Fe2 +的铬铁矿和绿泥石的次生多孔共生所替代。始新世时代的花岗岩侵入体提供的热量逆转了水化反应，生成了具有完美结晶面的铬铁矿轮辋。该第三代亚铬铁矿与高镁质（新成岩）橄榄石处于平衡状态，并定义了与原始岩浆岩相似的化学趋势。 Loma Baya铬铁矿中两种反应的保存都提供了令人信服的证据，证明铬铁矿的水合可以通过变质变质或任何加热事件来逆转，从而证实了先前的热力学预测。由于在变质和侵入过程中温度的变化以及流体/岩石相互作用的可变程度也显着影响了以铬铁矿为载体的IPGE载体相，因此了解这些复杂的特征特别有意义。在这里，我们提出，参与含水变质作用的变质流体已经引起了月桂石RuS2的脱硫，释放出直径小于50 nm的Ru-Os-Ir合金的细小颗粒。下列短暂的热事件促进了铬铁矿中的脱水，对纳米颗粒的稳定性产生了相反的影响，从而使金属纳米颗粒显着变粗至尺寸大于微米。基于这样的观察，我们认为IPGE纳米粒子可以在硫化变质或加热过程中从硫化物基质中溶解并生长（或粗化），而并非如先前所建议的那样仅在岩浆条件下冷却。这些结果为温度和纳米粒子-宿主相互作用现象在自然系统中的相关作用提供了新的见解，为变质过程中纳米至微米级IPGE粒子分布的动力学控制提供了新的思路。

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