H 2 O-rich rutile as an indicator for modern-style cold subduction.

UNLABELLED: 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. H2 O contents in rutile vary between < 10-2500 μg/g H2 O with higher contents in samples with higher peak metamorphic pressures, making H2 O-in-rutile a viable pressure indicator. The highest H2 O 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 H2 O/Zr, using H2 O 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 H2 O/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 H2 O/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.

SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00410-024-02107-2.