Fe-Ti oxide-silicate (QUIlF-type) equilibria in feldspathoid-bearing systems
Fe-Ti oxide-silicate (QUIlF-type) equilibria in feldspathoid-bearing systems
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DOI：
10.2138/am.2011.3596
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发表时间：
2011
影响因子：
3.1
通讯作者：
J. Schilling;B. Ronald Frost;M. Marks;T. Wenzel;G. Markl
中科院分区：
文献类型：
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作者：
J. Schilling;B. Ronald Frost;M. Marks;T. Wenzel;G. Markl
摘要硅酸盐-氧化物平衡（简称QUIlF）已被证明是重建含磁铁矿和钛铁矿与橄榄石、石英或辉石的岩浆系统的温度和氧逸度演化的非常有力的工具。在本文中，我们扩展这些QUILF平衡，包括岩石中的硅活性控制的平衡之间的斜晶石和斜晶石。我们提供了钛磁铁矿+钛铁矿+长石+霞石+成分可变的橄榄石的正岩浆组合的数据，我们称之为AUNIlF：AUNIlF参考曲线（具有钠长石，霞石和铁橄榄石的单位活动）在QUIlF表面以下的氧逸度≥2个对数单位，温度约为700至800 °C时是稳定的，这是晚岩浆阶段的典型温度。在> ~800 °C的温度下，AUNIlF参考组合只有在低于FMQ（FMQ是铁橄榄石-磁铁矿-石英缓冲层）5个对数单位以上的不切实际的低fO值条件下才稳定，这解释了正岩浆AUNIlF组合的罕见或缺失。我们确定了来自加拿大魁北克的圣伊莱尔山的AUNIlF组合在~800 °C时的最大还原条件为ΔFMQ = -1.15（橄榄石为Fa 67，aSiO 2 = 0.41），并得出结论，包含纯铁橄榄石的AUNIlF组合不稳定地出现在陆地岩浆系统中。天然存在的AUNIIF组合的稳定性场是钠长石、霞石和橄榄石组合物的函数，并且由二氧化硅活性与铁橄榄石活性的比率（aSiO_2/ aFa）控制。当aSiO_2/aFa的值高于~0.77时，当二氧化硅活性被霞石-钠长石平衡缓冲时，橄榄石的Fa < ~70。在这些情况下，AUNIlF在氧逸度≥ -1.15（ΔFMQ）时是稳定的。在低于aSiO 2/aFa ~0.77的值下，AUNIlF平衡向较低的氧逸度移动，并且钛铁矿相对于ulvøspinel变得不稳定。类似于AUNIlF的构建和应用，提出了一种涉及白榴石和碱性长石的钾质系统（KULIlF）的QUIlF型反应曲线，并将其应用于天然存在的组合。钾质岩石总是在钛铁矿和磁铁矿的存在下使镁橄榄石结晶，反映出结晶过程中比钠质岩石具有更高的氧逸度。在碱性长石、磁铁矿、白榴石、钛铁矿和橄榄石组成的钾质体系组合中，由于橄榄石中铁橄榄石含量低，aSiO_2/aFa比值≥4。
Abstract Silicate-oxide equilibria (abbreviated as QUIlF) have proven to be very powerful tools for reconstructing the temperature and oxygen fugacity evolution of magmatic systems containing magnetite and ilmenite with olivine, quartz, or pyroxenes. In this paper, we extend these QUIlF equilibria to include rocks where silica activity is controlled by equilibria between feldspars and feldspathoids. We present data on the orthomagmatic assemblage of titanomagnetite + ilmenite + feldspar + nepheline + compositionally variable olivine, which we call AUNIlF: The AUNIlF reference curve (with unit activities for albite, nepheline, and fayalite) is stable at oxygen fugacities ≥2 log units below the QUIlF surface at temperatures of about 700 to 800 °C, temperatures typical of late-magmatic stages. At temperatures > ~800 °C, the AUNIlF reference assemblage would only be stable at unrealistically low fO₂ conditions more than 5 log units below FMQ (where FMQ is the fayalite-magnetite-quartz buffer), which explains the rarity or absence of orthomagmatic AUNIlF assemblages. We determine the most reduced conditions indicated by displaced AUNIlF assemblage from Mont Saint-Hilaire (Quebec, Canada) to be ΔFMQ = -1.15 at ~800 °C (olivine is Fa67 and aSiO₂ = 0.41) and conclude that AUNIlF assemblages involving pure fayalite do not stably occur in terrestrial magmatic systems. The stability field of naturally occurring AUNIlF assemblages is a function of albite, nepheline, and olivine compositions and is controlled by the ratio of silica activity to fayalite activity (aSiO₂/ aFa). At values higher than ~0.77 for aSiO₂/aFa, olivine is Fa < ~70 when silica activity is buffered by the nepheline-albite equilibrium. In these situations, AUNIlF is stable at oxygen fugacities ≥ -1.15 (ΔFMQ). At values below aSiO₂/aFa ~0.77, the AUNIlF equilibrium is shifted to lower oxygen fugacities and ilmenite becomes unstable relative to ulvøspinel. Analogous to the construction and application of AUNIlF, a QUIlF-type reaction curve for potassic systems (KULIlF) involving leucite and alkali feldspar is presented and applied to naturally occurring assemblages. Potassic rocks invariably crystallize forsteritic olivine in the presence of ilmenite and magnetite, reflecting higher oxygen fugacities during crystallization than their sodic counterparts. As a result of low fayalite component in olivine, the aSiO₂/aFa ratio becomes ≥4 in assemblages of potassic systems consisting of alkali feldspar, magnetite, leucite, ilmenite, and olivine.