Probing the metamorphic record using detrital rutile

Much of what is currently known about early Earth has been recovered from detrital minerals, most notably through detrital zircon studies. But while zircon mostly records high-temperature magma formation, crustal growth and recycling, other minerals must be investigated to study the metamorphic record. Rutile, one of a range of different heavy minerals, can be used as a fingerprint for deep-crustal metamorphic processes, becoming progressively more stable under (ultra)high-pressure (UHP) and HT metamorphic conditions. Since rutile can be dated using the U-Pb method and Zr is incorporated as a function of temperature, rutile has been extensively used to trace metamorphic temperatures during metamorphism. However, extracting barometric or grade information from rutile remains one of the main challenges. Also, a more detailed assessment of rutile stability and chemistry at LT-HP conditions was lacking. Titanite, as a Ti-bearing phase, is often present at blueschist facies while rutile can be absent. To investigate this, rutile and titanite from metamafic rocks formed under HT-LP and LT-HP metamorphic conditions were studied. Rutile is found stable at HP and also at <2 kbar, lower than the constraints suggested by experiments for rutile stability, because of the release of excess Ti from Ti-amphiboles. Using Nb/V as a pressure proxy distinguishes rutile grains formed at LP from those formed at eclogitic pressures. Together with recent advances in using H₂O concentrations in rutile (Lueder et al., 2024), rutile chemistry can unlock the potential of using detrital rutile to investigate the tectonometamorphic evolution of the crust.