Temperature-dependent Rutile Solubility in Garnet and Clinopyroxene from Mantle Eclogite: Implications for Continental Crust Formation and V-based Oxybarometry
Temperature-dependent Rutile Solubility in Garnet and Clinopyroxene from Mantle Eclogite: Implications for Continental Crust Formation and V-based Oxybarometry
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DOI：
10.1093/petrology/egaa065
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发表时间：
2020-06-01
影响因子：
3.9
通讯作者：
Aulbach, Sonja
中科院分区：
文献类型：
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作者：
Aulbach, Sonja
尽管金红石在变质玄武岩中处于辅助矿物的地位，但它控制着全岩的Ti、Nb和Ta收支。这些都是用来追踪流体和熔体介导的质量转移跨越地幔-地壳边界的关键要素。金红石还含有大量的氧化还原敏感元素V，它越来越多地用于估计氧逸度。金伯利岩地幔榴辉岩捕虏体，这是经常金红石轴承，已被解释为残留物提取的部分熔体类似的平均大陆地壳的组成。已发表的榴辉岩捕虏体的矿物组成与地质温压计算相结合的结果表明，随着岩石圈地幔最后停留温度的升高，石榴子石和单斜辉石中的TiO 2含量增加，而表观单斜辉石-石榴子石分配系数减小。这意味着（1）榴辉岩石榴石或单斜辉石中TiO 2含量的增加并不是交代作用随深度增加的标志;（2）不含金红石的全岩榴辉岩随着温度的降低会越来越低估TiO 2、Nb和Ta的含量;（3）低温榴辉岩更可能含有游离金红石。只有大约三分之一的类似的250个样品在这里考虑将有全岩二氧化钛含量（重建计算金红石模式）所需的金红石饱和俯冲和部分熔融。如果有一个俯冲洋壳现在采样地幔榴辉岩的作用，在大陆地壳中的特征Ti-Nb-Ta耗尽可能需要流体为主的过程，这些元素没有有效地动员。在石榴石中，钛摄取的八面体网站上容纳主要通过耦合取代与钠和次要的二价金属阳离子，并没有证据取代的四面体网站。对于与传导性地热平衡的样品，除了Na富集之外，Ti可以指示金刚石稳定性场中的平衡。单斜辉石中的硬玉组分作为温度的函数是各种样品平衡的地热的良好指示剂，并且可以用于揭示每个套件中受等压加热影响的样品。钒在榴辉岩石榴石、单斜辉石和金红石中的分布受体相组成、温度和氧逸度的影响。在经过仔细检查的TiO 2含量> 0.8wt%的低温样品中，基于V的氧压法可以监测大洋地壳变质作用期间的氧化还原条件，或者在较低的TiO 2下，在（长期）冷却相关的金红石从石榴石或单斜辉石中出溶的过程中，而在较高温度的含钛铁矿的样品中，交代条件可能被记录下来。
Despite its accessory mineral status in metabasaltic rocks, rutile controls the whole-rock Ti, Nb and Ta budget. These are key elements used to trace fluid- and melt-mediated mass transfer across the mantle-crust boundary. Rutile also contains significant amounts of the redox-sensitive element V, which is increasingly used to estimate oxygen fugacity. Kimberlite-borne mantle eclogite xenoliths, which are frequently rutile-bearing, have been interpreted as residues from the extraction of silicic partial melt similar in composition to the average continental crust. Published mineral compositions for eclogite xenoliths from various cratons combined with geothermobarometrical calculations show that TiO2 contents in garnet and clinopyroxene increase with increasing temperature of last residence in the lithospheric mantle, whereas apparent clinopyroxene-garnet distribution coefficients decrease. This implies that (1) increasing TiO2 contents in eclogitic garnet or clinopyroxene are not a signature of increasing metasomatism with depth, (2) whole-rock eclogites reconstructed without rutile will increasingly underestimate TiO2, Nb and Ta contents with decreasing temperature, and (3) low-temperature eclogites are more likely to contain free rutile. Only about a third of the similar to 250 samples considered here would have whole-rock TiO2 contents (reconstructed with calculated rutile modes) required for rutile saturation during subduction and partial melting. If there is a role for subducting oceanic crust now sampled as mantle eclogite, the characteristic Ti-Nb-Ta depletion in continental crust may require fluid-dominated processes, where these elements are not efficiently mobilised. In garnet, Ti uptake on the octahedral site is accommodated primarily by coupled substitution with Na and subordinately with a divalent metal cation, and there is no evidence for substitution on the tetrahedral site. For samples equilibrated to the conductive geotherm, Ti in addition to Na enrichment may be indicative of equilibration in the diamond stability field. The jadeite component in clinopyroxene as a function of temperature is a good indicator of the geotherm to which the various samples equilibrated, and can be used to reveal samples within each suite that have been affected by isobaric heating. The distribution of V in eclogitic garnet, clinopyroxene and rutile is affected by bulk composition, temperature and oxygen fugacity. In carefully vetted, low-temperature samples with TiO2 contents >0.8 wt%, V-based oxybarometry may monitor redox conditions prevailing during metamorphism of oceanic crust or, at lower TiO2, during (secular) cooling-related exsolution of rutile from garnet or clinopyroxene, whereas in higher-temperature ilmenite-bearing samples metasomatic conditions may be recorded.