A rutile and titanite record of subduction fluids: Integrated oxygen isotope and trace element analyses in Franciscan high-pressure rocks

IF 3.5 2区 地球科学 Q1 GEOLOGY Journal of Metamorphic Geology Pub Date : 2023-02-27 DOI:10.1111/jmg.12717
F. Zeb Page, Craig D. Storey, EIMF
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Abstract

In situ oxygen analysis of garnet in eclogite and related rocks is increasingly being used to probe the composition of subduction fluids. However, in many cases, these samples contain textural signs of both fluid flow and retrograde metamorphism, some of which may take place outside the garnet stability field. In order to test the connection between polymetamorphism and fluid infiltration, rutile rimmed by titanite from high-grade tectonic blocks of the Franciscan Formation (California, USA) was analysed for oxygen isotope ratios and trace element concentrations. Zirconium concentrations in rutile yield temperatures of ~600°C for eclogite and hornblende eclogite from three well-studied localities (Junction School, Tiburon and Ward Creek). Rutile trace element concentrations are generally low and consistent with a mafic protolith. Titanite surrounding rutile has inherited much of its trace element content from rutile, and Zr-in-titanite temperatures are spuriously high. Titanite in rutile-free samples (blueschist and eclogite from Jenner beach) have similar compositions suggesting that they were formed at the expense of rutile as well. Oxygen isotope ratios from rutile and titanite in the same sample are fortuitously similar, indicating disequilibrium between these minerals, which formed at different times and temperatures but in equilibrium with the same oxygen reservoir. Rutile in blocks with garnets zoned in oxygen isotopes are generally in equilibrium with the rims rather than the cores. Slow oxygen diffusion in rutile and the low temperatures of formation require that rutile recrystallized after fluid interaction and before blueschist facies metamorphism. External fluid interaction of Franciscan eclogites took place near the peak of metamorphism.

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俯冲流体的金红石和钛矿记录:方济各高压岩石的综合氧同位素和微量元素分析
榴辉岩及相关岩石中石榴石的原位氧分析越来越多地用于探测俯冲流体的成分。然而,在许多情况下,这些样品含有流体流动和逆变质作用的结构迹象,其中一些可能发生在石榴石稳定区之外。为了测试多聚变质作用与流体渗透之间的联系,分析了方济各会组(美国加利福尼亚州)高级构造块体中由钛矿镶边的金红石的氧同位素比率和微量元素浓度。来自三个研究充分的地区(Junction School、Tiburon和Ward Creek)的榴辉岩和角闪榴辉岩的金红石中的锆浓度产生温度为~600°C。金红石微量元素浓度通常较低,与镁铁质原岩一致。金红石周围的钛矿继承了金红石的大部分微量元素含量,并且Zr-in-钛矿的温度高得离谱。无金红石样品(詹纳海滩的蓝片岩和榴辉岩)中的钛矿成分相似,表明它们也是以牺牲金红石为代价形成的。同一样品中金红石和钛矿的氧同位素比例偶然相似,表明这些矿物之间存在不平衡,它们在不同的时间和温度下形成,但与同一氧气库保持平衡。含氧同位素石榴石块体中的金红石通常与边缘而非核心处于平衡状态。氧气在金红石中的缓慢扩散和形成的低温要求金红石在流体相互作用后和蓝片岩相变质作用前再结晶。方济各榴辉岩的外部流体相互作用发生在变质作用高峰期附近。
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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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