A recipe for the use of rutile in sedimentary provenance analysis

Abstract

Rutile has received considerable attention in the last decade as a valuable petrogenetic indicator mineral. Based on both new and previously published data, we carve out advantages and pitfalls regarding TiO2-minerals in sedimentary provenance analysis. This results in a recipe for the use of rutile in provenance studies. The main points are:Rutile geochemistry from different grain size fractions does not differ systematically, and hence rutiles should be extracted from the fraction containing the most rutile grains (usually 63–200μm). Similarly, different magnetic susceptibility of rutile does not systematically imply different trace element composition.Before interpretation of TiO2-mineral data, it is important to determine the polymorph type. Rutile, anatase and brookite appear to differ systematically in trace element composition. As an alternative to Raman spectroscopy, chemical classification according to Nb, Cr, Sn, Fe, V, and Zr concentrations can be applied.For rutile, a new host lithology discrimination scheme based on Cr–Nb systematics is introduced (x=5∗(Nb [ppm]−500)−Cr [ppm]), which leads to better classification results than previously published discrimination methods. According to this equation, metamafic rutiles have negative values of x, while metapelitic rutiles have positive values.Evaluation of the growth temperature calculations of metamorphic rutile after different authors shows that the equations given by Tomkins et al. (2007) should be applied to both metamafic and metapelitic rutiles. Although there is a pressure effect on the Zr incorporation in rutile, the pressure range for most rutiles of 5–15kbar introduces an uncertainty in calculated temperature of no more than ±35°C. The distribution of calculated temperatures from detrital rutiles is crucial; only well-defined temperature populations should be used for thermometry interpretation.