Trace element abundances in rutiles from eclogites and associated garnet mica schists

Abstract

We present electron microprobe and laser ablation microprobe (LAM) data for a range of high field strength (Zr, Nb, Mo, Sn, Sb, Hf, Ta, W) and other trace elements (Al, Si, Ca, V, Cr, Mn, Fe, Pb, Th, U) in rutile from eclogites and garnet mica schists from Trescolmen, Central Alps. Most analysed rutiles are homogeneous (at least for Nb, Cr, W, Zr, V and Fe), both on a single grain scale and between grains from a single thin section. Concentrations of V, Zr, Nb, Sb and W determined by both electron and laser ablation microprobe techniques yield similar results and confirm the reliability of the analytical methods within estimated precision. Measurements of trace element contents of coexisting phases in eclogites and their modal abundances show that rutile is the dominant carrier (>90% of whole rock content) for Ti, Nb, Sb, Ta and W as well as an important carrier (5–45% of the whole rock content) for V, Cr, Mo and Sn. The crystallographic implications are that, for relatively rigid crystal sites such as in rutile, trace elements with a similar ionic radius are preferred over trace elements with the same charge but deviating size. Our results demonstrate the utility of rutile chemistry in the following applications: (1) By using a combination of the measured TiO 2 content of the whole rock and the trace element concentration of rutile, precise whole rock data on elements that are either difficult to analyze by conventional techniques such as XRF or solution ICP-MS (Nb, Sb, Ta, W) or may be susceptible to late stage alteration (Sb) can be estimated. (2) Trace element contents of detrital rutile grains are a potentially powerful tool for sedimentary provenance studies since they reflect key element ratios (e.g., Nb/TiO 2 and Cr/TiO 2) of their source rocks. In addition, measurements of trace elements in detrital rutiles might help distinguish possible source rocks, e.g., high-grade metamorphic rocks such as eclogites and high-pressure granulites from hydrothermal ore deposits and kimberlites. In view of the dominance of rutile in the Sb budget of subducting oceanic crust, and the enrichment of Sb in the slab component of subduction zones, additional experimental studies on Sb-partitioning between rutile and fluid are needed in order to understand the behaviour of Sb in subduction zones.