New attempts to identify core-mantle interactions in plume-derived materials using ultra-high precision tungsten isotope measurements

Some mantle plume derived materials show coupled 187,186Os enrichments relative to upper-mantle materials that have been interpreted by some to reflect core-mantle interaction (Brandon et al., 1999, 2003, Puchtel et al., 2005). In addition to osmium, tungsten is another element whose isotopic composition can potentially be used to trace core-mantle interactions. Tungsten has one radiogenic isotope, 182W, a decay product of 182Hf, with a half-life of ~9 Myr. Like Os, W is siderophile, under reducing conditions, and, hence, is preferentially incorporated into Earth’s core, whereas Hf is lithophile and is retained in the mantle. Fractionation of Hf from W during core formation is predicted to have led to large differences in 182W/184W between the core and mantle. The use of W isotopes as tracers of core-mantle interaction has been hampered by limitations in the ability to measure W isotopic ratios at the level of ± 10 ppm or better. Within analytical uncertainty, MC-ICP-MS measurements of terrestrial rocks investigated so far show no resolvable 182W anomalies (Scherstén et al., 2004). Over the past year, we have developed a new ultra-high precision 182W/184W measurement protocol using a Triton thermal ionization mass spectrometer, allowing us to resolve 182W anomalies at a ± 6 ppm level (2σ, n=40). All W isotope measurements are performed in a negative ionization mode (WO3-) using a dynamic acquisition scheme. This precision improvement allows us to more rigorously interrogate the W isotopic compositions of materials with potentially deep mantle origins. A major problem in this application of W isotopes is the acquisition of mantle-derived materials that have not been contaminated with crustal W. Here we present W abundances, measured using isotope dilution, and corresponding ultra-high precision W isotopic composition measurements of Archean komatiites from the Kostomuksha greenstone belt (Baltic Shield), for which coupled 186Os-187Os enrichment has been previously reported (Puchtel et al., 2005). Except for two samples with significantly higher W abundances (120 and 190 ppb), Kostomuksha komatiites show a limited range in W contents, averaging 86 ± 15 ppb. Although W normally behaves as a strongly incompatible trace element in silicate systems, variations in W content are not correlated with variations of other incompatible element contents (e.g., REE) or proxies of differentiation (e.g., MgO) in the Kostomuksha komatiites. In addition, the W contents are much higher that predictions (~30 ppb) from partial melting models of a primitive mantle containing ~16 ppb W (Arevalo and McDonough, 2009). However, Nd, Pb isotopes and trace element data show no evidence for crustal contamination. Open-system W systematics cannot be excluded, but the high W content might also reflect an anomalously high W content in the mantle source of the Kostomuksha komatiites. High-precision tungsten isotope measurements are currently in progress.