Lueder Mona, Tamblyn Renée, Rubatto Daniela, Hermann Jörg
Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, 3012 Bern, Switzerland.
Institute of Earth Sciences, University of Lausanne, Géopolis, Quartier Mouline, 1015 Lausanne, Switzerland.
Contrib Mineral Petrol. 2024;179(3):26. doi: 10.1007/s00410-024-02107-2. Epub 2024 Mar 10.
The trace-element and isotope geochemistry of rutile are robust tools to determine metamorphic temperatures, age, and host-/source lithologies. The use of rutile as single grain indicator for pressure, temperature, time and composition (P-T-t-X) of the host rock, which is vital in the use of detrital rutile to trace plate-tectonic regimes throughout Earth's history, requires the identification of a pressure dependent trace element in rutile. We investigate the pressure dependence of hydrogen in rutile using polarized in-situ Fourier Transform Infrared (FTIR) spectroscopy. HO contents in rutile vary between < 10-2500 μg/g HO with higher contents in samples with higher peak metamorphic pressures, making HO-in-rutile a viable pressure indicator. The highest HO contents at ~ 450-2000 μg/g are observed in mafic low temperature eclogite-facies rutile related to modern-style cold subduction conditions. Hydrogen zoning in FTIR maps indicates that H is retained at temperatures below 600-700 °C. Ratios of HO/Zr, using HO as pressure indicator and Zr as temperature proxy, are a proxy for thermal gradients of metamorphic rutile (i.e. P/T). Low temperature eclogite samples are also characterized by high Fe contents and therefore Fe/Zr-ratios might be used as a first order approximation for HO/Zr-ratios to identify mafic low temperature eclogite facies rutile. Based on common discrimination diagrams, Nb, W, and Sn can be used to distinguish different host/source rock lithologies of rutile. Combining both HO/Zr-ratios and Nb, W, and Sn contents can thus identify modern-style cold subduction signatures in rutile. The developed systematics can consequently be used to trace cold-subduction features in the (pre-Proterozoic) detrital record.
The online version contains supplementary material available at 10.1007/s00410-024-02107-2.
金红石的微量元素和同位素地球化学是确定变质温度、年龄以及主岩/源岩岩性的有力工具。将金红石用作指示主岩压力、温度、时间和成分（P-T-t-X）的单颗粒指标，这在利用碎屑金红石追踪地球历史上的板块构造演化过程中至关重要，而这需要识别金红石中一种与压力相关的微量元素。我们利用偏振原位傅里叶变换红外（FTIR）光谱研究了金红石中氢的压力依赖性。金红石中的HO含量在<10-2500μg/g HO之间变化，在峰值变质压力较高的样品中含量更高，这使得金红石中的HO成为一种可行的压力指标。在与现代冷俯冲条件相关的镁铁质低温榴辉岩相金红石中，观察到最高的HO含量约为450-2000μg/g。FTIR图谱中的氢分带表明，H在600-700°C以下的温度下得以保留。以HO作为压力指标、Zr作为温度代理，HO/Zr比值可作为变质金红石热梯度（即P/T）的代理指标。低温榴辉岩样品还具有高Fe含量的特征，因此Fe/Zr比值可作为HO/Zr比值的一阶近似值，用于识别镁铁质低温榴辉岩相金红石。基于常见的判别图，Nb、W和Sn可用于区分金红石的不同主岩/源岩岩性。因此，结合HO/Zr比值以及Nb、W和Sn含量，可以识别金红石中的现代冷俯冲特征。由此建立的系统方法可用于追踪（前寒武纪）碎屑记录中的冷俯冲特征。
在线版本包含可在10.1007/s00410-024-02107-2获取的补充材料。
