Magnetite stability in ultramafic systems undergoing subduction plays a major role in controlling redox states of the fluids liberated upon dehydration reactions, as well as of residual rocks. Despite their relevance for the evaluation of the redox conditions, the systematics and geochemistry of oxide minerals have remained poorly constrained in subducted ultramafic rocks. Here, we present a detailed petrological and geochemical study of magnetite in hydrous ultramafic rocks from Cerro del Almirez (Spain). Our results indicate that prograde to peak magnetite, ilmenite–hematite solid solution minerals, and sulfides coexist in both antigorite-serpentinite and chlorite-harzburgite at c. 670 °C and 1·6 GPa, displaying successive crystallization stages, each characterized by specific mineral compositions. In antigorite-serpentinite, magnetite inherited from seafloor hydration and recrystallized during subduction has moderate Cr (Cr2O3 < 10 wt%) and low Al and V concentrations. In chlorite-harzburgite, polygonal magnetite is in textural equilibrium with olivine, orthopyroxene, chlorite, pentlandite, and ilmenite–hematite solid solution minerals. The Cr2O3 contents of this magnetite are up to 19 wt%, higher than any magnetite data obtained for antigorite-serpentinite, along with higher Al and V, derived from antigorite breakdown, and lower Mn concentrations. This polygonal magnetite displays conspicuous core to rim zoning as recognized on elemental maps. Cr–V–Al–Fe3+ mass-balance calculations, assuming conservative behavior of total Fe3+ and Al, were employed to model magnetite compositions and modes in the partially dehydrated product chlorite-harzburgite starting from antigorite-serpentinite, as well as in the serpentinite protolith starting from the chlorite-harzburgite. The model results disagree with measured Cr and V compositions in magnetite from antigorite-serpentinites and chlorite-harzburgites. This indicates that these two rock types had different initial bulk compositions and thus cannot be directly compared. Our mass-balance analysis also reveals that new magnetite formation is required across the antigorite-breakdown reaction to account for the mass conservation of fluid-immobile elements such as Cr–V–Al–Fe3+. Complete recrystallization and formation of new magnetite in equilibrium with peak olivine (Mg# 89–91), chlorite (Mg# ∼95), orthopyroxene (Mg# 90–91), and pentlandite buffer the released fluid to redox conditions of ∼1 log unit above the quartz–fayalite–magnetite buffer. This is consistent with the observation that the Fe–Ti solid solution minerals (hemo-ilmenite and ilmeno-hematite) crystallized as homogeneous phases and exsolved upon exhumation and cooling. We conclude that antigorite-dehydration reaction fluids carry only a moderate redox budget and therefore may not be the only reason why the magmas are comparatively oxidized.

中文翻译：

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俯冲过程中蛇纹石分解产生磁铁矿的结构和地球化学证据

俯冲超镁铁质系统中磁铁矿的稳定性在控制脱水反应释放的流体以及残余岩石的氧化还原状态方面发挥着重要作用。尽管它们与氧化还原条件的评估相关，但氧化物矿物的系统学和地球化学在俯冲超镁铁质岩中仍然受到很差的限制。在这里，我们对来自 Cerro del Almirez（西班牙）的含水超镁铁岩中的磁铁矿进行了详细的岩石学和地球化学研究。我们的研究结果表明，在 c. 时，在叶蛇纹石-蛇纹石和绿泥石-方镁石中，磁铁矿、钛铁矿-赤铁矿固溶体矿物和硫化物共存于峰值。670 °C 和 1·6 GPa，显示连续的结晶阶段，每个阶段都有特定的矿物成分。在蛇纹石-蛇纹石中，从海底水化继承并在俯冲过程中重结晶的磁铁矿具有中等的 Cr (Cr2O3 < 10 wt%) 和低的 Al 和 V 浓度。在绿泥石-方辉石中，多边形磁铁矿与橄榄石、斜方辉石、绿泥石、镍铁矿和钛铁矿-赤铁矿固溶体矿物处于结构平衡状态。这种磁铁矿的 Cr2O3 含量高达 19 wt%，高于从蛇纹石-蛇纹石中获得的任何磁铁矿数据，同时具有较高的 Al 和 V（源自叶蛇纹石分解）和较低的 Mn 浓度。这种多边形磁铁矿在元素图上显示出明显的核心到边缘分区。Cr–V–Al–Fe3+ 质量平衡计算，假设总 Fe3+ 和 Al 的保守行为，被用来模拟从叶蛇纹石-蛇纹石开始的部分脱水产物绿泥石-方镁石中的磁铁矿组成和模式，以及从绿泥石-方镁石开始的蛇纹石原岩中的磁铁矿组成和模式。模型结果与叶蛇纹石-蛇纹石和绿泥石-方镁石磁铁矿中测量的 Cr 和 V 成分不一致。这表明这两种岩石类型具有不同的初始体积组成，因此无法直接比较。我们的质量平衡分析还表明，在叶蛇纹石分解反应过程中需要新的磁铁矿形成，以解释流体固定元素（如 Cr-V-Al-Fe3+）的质量守恒。完全重结晶并形成与峰值橄榄石（Mg# 89-91）、绿泥石（Mg#∼95）、斜方辉石（Mg# 90-91）平衡的新磁铁矿，和 pentlandite 将释放的流体缓冲到石英 - 铁橄榄石 - 磁铁矿缓冲液上方约 1 log 单位的氧化还原条件。这与 Fe-Ti 固溶体矿物（赤铁钛矿和钛铁赤铁矿）以均质相结晶并在折返和冷却时溶解的观察结果一致。我们得出结论，叶蛇纹石脱水反应流体仅携带适度的氧化还原预算，因此可能不是岩浆相对氧化的唯一原因。