超镁铁质岩石中所含的氧化物矿物是评估俯冲过程中蛇纹岩逐渐脱水释放的岩石和流体的氧化还原条件的有用工具。因为这些矿物质含有相关的氧化还原敏感元素,铁。先前的研究表明,磁铁矿在抗龟裂反应中占主导地位。然而,在较高的压力和温度条件下,磁铁矿和其他氧化物的命运仍未得到充分探索。我们提供了对超高岩中氧化物-硫化物-硅酸盐矿物组合的全面岩石学和地球化学研究,这些组合超出了反哥特和亚氯酸盐的反应(T=650-850°C和P=1-3GPa)。几个超镁铁质镜片,覆盖海洋蛇纹石化时不同的块状岩石成分和氧化程度,是从中央阿尔卑斯山调查的,瑞士。结果指向两种终端成员方案:(I)最常见的是,超橄榄岩具有橄榄石,Mg#为89-91(定义为摩尔Mg/(MgFetot)×100),并含有低氧化物模式(0.06-1.41vol。%),没有赤铁矿,氧化还原条件被邻苯二酚-橄榄石-磁铁矿弱氧化和缓冲。(ii)罕见事件,高橄榄石Mg#>94.5偏橄榄岩显示赤铁矿和磁铁矿共存,高氧化物模式(高达4卷。%),和氧化还原条件是赤铁矿-磁铁矿(HM)缓冲(Δlog10fO2,QFM为3至4)。尖晶石显示出从铬铁矿上的磁铁矿到Al-Cr尖晶石的成分演变,大致与温度升高有关。大多数由橄榄石-邻苯二酚-磁铁矿组合缓冲的样品都含有共存的五铁矿±磁黄铁矿,因此,对于这些弱氧化的样品(Δlog10fO2,QFM<2.5),可以鉴定出稳定的硫化物。在高度氧化的样品中没有发现硫化物。在700°C左右,磁铁矿向铬铁矿的转变伴随着fO2向较低值的转变。在弱氧化的偏沸石中普遍存在的氧气逸度下,共存流体中的硫始终以还原形式存在。然而,氧化的硫在高度氧化的蛇纹石释放的脱水液中可以是稳定的。
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Oxide minerals contained in ultramafic rocks are useful tools to assess the redox conditions of the rock and fluids liberated upon progressive serpentinite dehydration during subduction, as these minerals contain a relevant redox-sensitive element, iron. Previous studies have revealed that magnetite predominates across the antigorite-out reaction. However, the fate of magnetite and other oxides at higher pressure and temperature conditions has remained underexplored. We present a comprehensive petrological and geochemical study of
oxide-sulfide-silicate mineral assemblages in metaperidotites beyond antigorite- and chlorite-out reactions (T = 650-850 °C and P = 1-3 GPa). Several ultramafic lenses, covering different bulk rock compositions and extents of oxidation upon oceanic serpentinization, were investigated from the Central Alps, Switzerland. Results point to two endmember scenarios: (i) Most frequently, metaperidotites have olivine with a Mg# of 89-91 (defined as molar Mg/(Mg + Fetot) × 100) and contain low
oxide modes (0.06-1.41 vol.%), hematite is absent, and redox conditions are weakly oxidized and buffered by orthopyroxene-olivine-magnetite. (ii) Rare occurrence, high olivine Mg# > 94.5 metaperidotites display coexisting hematite and magnetite, high
oxide modes (up to 4 vol.%), and redox conditions are hematite-magnetite (HM) buffered (Δlog10fO2,QFM of + 3 to + 4). Spinel displays evolving compositions from magnetite over chromite to Al-Cr-spinel, roughly correlating with increasing temperature. Most of the samples buffered by the olivine-orthopyroxene-magnetite assemblage contain coexisting pentlandite ± pyrrhotite, thus identifying stable sulfides beyond antigorite dehydration for these weakly oxidized samples (Δlog10fO2,QFM < 2.5). No sulfides were recognized in the highly oxidized sample. The transition of magnetite to chromite at around 700 °C goes along with a shift in fO2 to lower values. At the prevailing oxygen fugacity in the weakly oxidized metaperidotites sulfur in a coexisting fluid is always present in its reduced form. However, oxidized sulfur can be stable in the dehydration fluids released from highly oxidized serpentinites.
UNASSIGNED: The online version contains supplementary material available at 10.1007/s00410-023-02032-w.