俯冲蛇纹岩中铁含量和氧逸度的实验约束

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Lisa Eberhard, Daniel J Frost, Catherine A McCammon, David Dolejš, James A D Connolly
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引用次数: 0

摘要

蛇纹岩在向俯冲带输送水方面起着重要作用。此外,蛇纹岩还含有铁,并能传输显著的氧化还原电位。为了量化两相中Fe3+/Fetot比值与氧逸度f(O2)之间的关系,我们对不同氧逸度平衡的天然蜥蜴石和反长岩样品进行了高压、高温实验和Mössbauer光谱测量。在反长岩中,Fe3+被分散到八面体位置,并被四面体Al平衡电荷。在蜥沙石中,Fe3+仅在低温和高f(O2)下观察到四面体,而在温度超过500℃和3 ~ 5 GPa时,Fe3+倾向于八面体位置。虽然蜥蜴石是亚稳态的,但在我们的实验中,蜥蜴石在300°C以上的条件下仍保持氧化还原平衡,并表现出与反长花岗岩相似的稳定性。发现蜥蜴石的Al浓度与温度有关,并且可以通过使用铁金属、石墨、石墨-方解石、Re-ReO2和Ru-RuO2等氧化还原缓冲液将蜥蜴石的Fe3+/Fetot比从0.1重新平衡到0.9。我们对反长花岗岩的实验表明,反长花岗岩在实验时间尺度上不调整其铝浓度。由于Fe3+被Al电荷平衡,因此也不可能操纵反长岩的Fe3+/Fetot比。然而,共存的相显示出与该反长花岗岩组成的化学平衡。反演了反长岩和蜥蜴岩的Fe3+-和al端元的标准吉布斯能,计算了俯冲蛇纹岩的变质演化过程。在f(O2)缓冲条件下,与在某些自然环境下的观察结果相比,蜥蜴石向反长铁矿的转变不会导致Fe3+/Fetot比的降低,但由于反长铁矿在平衡状态下的Fe3+/Fetot比低于蜥蜴石,因此确实导致了额外磁铁矿的形成。如果反长岩蛇纹岩的f(O2)在俯冲过程中受到缓冲,如石墨和碳酸盐的存在,则总体Fe3+/Fetot比值逐渐降低。另一方面,在封闭体系中,整体蛇纹岩Fe3+/Fetot比值保持不变,f(O2)在俯冲过程中增加。在这种情况下,典型Fe3+/Fetot比值为0.4的反长辉长蛇纹岩的f(O2)在脱水过程中从费长辉石-磁铁矿-石英缓冲剂增加到赤铁矿-磁铁矿f(O2)缓冲剂。这些f(O2)结果证实了先前的推断,即反长花岗岩脱水产生的流体可能不含有足够的氧化硫物质来氧化地幔楔。然而,如果封闭系统的行为在蜥蜴石到反长岩的相变过程中保持较高的整体氧化还原电位,则在反长岩脱水过程中可能达到足够高的f(O2)水平,以动员氧化硫物种。另外,地幔楔的氧化可能是由俯冲的海洋地壳和沉积物中的氧化剂来实现的。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Experimental constraints on the ferric Fe content and oxygen fugacity in subducted serpentinites
ABSTRACT Serpentinites play an important role in the delivery of water into subduction zones. In addition, serpentinites also contain ferric Fe and can transport significant redox potential. We present high-pressure and high-temperature experiments and Mössbauer spectroscopy measurements on natural lizardite and antigorite samples equilibrated at various oxygen fugacities in order to quantify the relationship between the oxygen fugacity f(O2) and the Fe3+/Fetot ratio in these two phases. In antigorite, Fe3+ partitions into the octahedral site and is charge balanced by tetrahedral Al. In lizardite, tetrahedral Fe3+ is observed only at low temperature as well as under high f(O2), whereas Fe3+ prefers the octahedral site at temperatures exceeding 500 °C and at 3 to 5 GPa. Although metastable, lizardite remains in redox equilibrium in our experiments at conditions above the lizardite to antigorite phase transformation at 300 °C and demonstrates a similar stability to antigorite. The Al concentration of lizardite is found to be temperature dependent, and it was possible to reequilibrate the Fe3+/Fetot ratio of lizardite from 0.1 to 0.9 by using redox buffers such as Fe metal, graphite, graphite–calcite, Re–ReO2 and Ru–RuO2. Our experiments on antigorite demonstrate that antigorite does not adjust its Al concentration on experimental time scales. Since Fe3+ is charge balanced by Al, it was also not possible to manipulate the Fe3+/Fetot ratio of antigorite. The coexisting phases, however, show chemical equilibration with this antigorite composition. We have retrieved the standard Gibbs energy for Fe3+- and Al-endmembers of antigorite and lizardite and calculated the metamorphic evolution of subducting serpentinites. The lizardite to antigorite transformation does not cause a decrease in the bulk Fe3+/Fetot ratio under f(O2) buffered conditions, in contrast to observations from some natural settings, but does result in the formation of additional magnetite due to antigorite having a lower Fe3+/Fetot ratio than lizardite at equilibrium. If the f(O2) of antigorite serpentinite is buffered during subduction, such as due to the presence of graphite and carbonate, the bulk Fe3+/Fetot ratio decreases progressively. On the other hand, in a closed system where the bulk serpentinite Fe3+/Fetot ratio remains constant, the f(O2) increases during subduction. In this scenario, the f(O2) of an antigorite serpentinite with a typical Fe3+/Fetot ratio of 0.4 increases from the fayalite–magnetite–quartz to the hematite–magnetite f(O2) buffer during dehydration. These f(O2) results confirm earlier inferences that fluids produced by antigorite dehydration may not contain sufficient oxidised sulphur species to oxidise the mantle wedge. Sufficiently high levels of f(O2) to mobilise oxidised sulphur species may be reached upon antigorite dehydration, however, if closed system behaviour maintains a high bulk redox potential across the lizardite to antigorite phase transformation. Alternatively, oxidation of the mantle wedge might be achieved by oxidising agents from sources in subducted oceanic crust and sediments.
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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