Intra-grain variability of hydrogen and trace elements in rutile

Abstract

Rutile is a commonly used petrogenetic indicator mineral to determine metamorphic temperatures, ages, host−/source lithologies, geochemical reservoirs, and subduction conditions. However, intra grain variabilities of trace elements in rutile are rarely considered. We performed trace element and hydrogen mapping of rutile to assess zoning and diffusion in natural rutile from various lithologies. Trace element and hydrogen show distinct zoning patterns, with mostly regular zoning in rutile from pegmatites and low-T hydrothermal clefts, and typically irregular zoning in rutile from high-P veins and metamorphic rocks. Whereas no clear patterns of trace element correlations can be identified, hydrogen, tri-, tetra- and pentavalent cations can show the same zoning patterns within single rutile grains, despite different substitution mechanisms. This indicates that hydrogen and trace element incorporation is externally controlled by availability and diffusivity of hydrogen and trace elements within the rock matrix, as well as rutile growth rates. H₂O mapping reveals that hydrogen is retained in rutile at temperatures of up to ∼650 °C. Coupled substitution of hydrogen with divalent and trivalent cations requires coupled diffusion processes for charge balance if hydrogen is diffusively re-equilibrated. Slow diffusion rates and thus relatively high temperatures for diffusive closure in rutile lead to retention of primary hydrogen and trace element zoning. At high-T conditions of >650 °C, diffusive re-equilibration of all trace elements can be observed. Complex zoning patterns of Zr in rutile show that Zr incorporation into rutile is not purely temperature dependent. In this study, Zr-undersaturation can lead to inaccurate Zr-in-rutile temperatures of up to ∼35 °C difference from peak formation temperatures within single rutile grains and might be a useful tool to evaluate rutile growth conditions. Niobium and Ta are highly zoned in rutile, leading to extremely variable Nb/Ta ratios within single rutile grains that cannot be reconstructed from single spot analyses. Overall, mapping offers a novel and promising tool to understanding trace element behavior in rutile.