Tungsten abundance and isotopic compositions of Ocean Island Basalts, an Oceanic Plateau, MORB samples: in search for a core-mantle interaction

Core-mantle interaction has been investigated using several geochemical tracers, which can put strong constraints on mantle convection of the Earth. To elucidate such interactions, siderophile elements that are highly concentrated in the metallic core rather than in the silicate mantle are the most favorable candidates to provide geochemical evidence. Platinum-group element (PGE) abundance and Os isotopes of oceanic basalts can work to deconvolve the contribution of core material. Hf-W system is another tracer to elucidate core-mantle interaction. Tungsten is a moderately siderophile, incompatible, and refractory element that partitions preferentially into the Earth's core during core formation. Collerson et al.(2002) reported a negative anomaly of the W isotope ratio in kimberlite of South Africa, and suggested the possibility of a core-mantle interaction. However, Scherstén et al.(2004) retested the samples of kimberlite of South Africa and Hawaiian picrites, observing no anomalies in W isotope ratio. Whether the inconsistent results were caused by different sample treatments or analytical difficulties in W purification remains unclear. In this study, we will report W abundances with isotopic compositions of OIBs, Ontong Java LIP and MORB. The W abundances in the OIBs analyzed in this study show a large variation: OIBs(South Polynesia Island, Kilauea, Loihi)=200-900ppb, OJPs=20-500ppb, MORB(Indian and EPR)=20-90ppb. Tungsten isotopic compositions were measured using multi-collector ICP mass-spectrometry. We examined the reproducibility of W isotopic analyses from 3-4 repeated measurements of four samples of OIBs and estimate the external reproducibility of isotope analyses as 0.1-0.27 epsilon unit. The analyzed OIBs have high tungsten abundance, but their tungsten isotopic composition shows no negative anomalies as indicates core-mantle interaction. The samples with high W abundance from OJPs do not have negative tungsten anomalies, either. These results suggest four possibilities about mantle convection and core-mantle interaction. 1) These sources of OIBs or LIPs analyzed, do not originate from CMB. 2) Negative W isotope ratio anomaly of the core has been later diluted with the entrainment of mantle material with normal W isotope composition. 3) W does not diffuse from outer core into mantle as has been pointed out by Humayun et al. (2004).