11:00 AM - 11:15 AM
[SIT06-08] High precision analysis of Os and W for early evolution of core-mantle
Keywords:core-mantle interaction, W isotope, Os isotope, early earth chemical evolution
In the early stage of applications of the 182Hf-182W isotope system has been used as a chronometer of the Earth’s core formation (Lee et al., 1995) to use that iron meteorites possess lower 182W/184W isotope ratios than those of chondrites and Earth’s rocks. Variation of this isotope ratio is presented as ε value using W isotope standard solution (NIST SRM-3163).
High precision isotope analysis of W (< 0.3 ε) was difficult until 2010 even if extremely sensitive and precise mass spectrometers of N-TIMS and MC-ICP-MS were used. Recently, the mass spectrometry in MC-ICP-MS or N-TIMS and chemical separation methods were significantly improved (Touboul et al., 2011), and high precision tungsten isotope analysis of ±5 μ (=0.05ε) is possible at present. These high precision analyses lead to findings of W isotope anomaly in the early Earth’s rocks such as komatiite (older than 2.8 billion years). Kostomuksha komatiite has positive anomaly (+20μ) (Touboul et al., 2012). Variation of these W isotope ratios was discussed together with PGE abundance and Os or Nd isotopes of mantle rocks. The PGE and Os isotopes could give constraints on the timing and processes of early mantle evolution such as Late Veneer, the core-mantle or mantle-silicate differentiation.
In our study, we are trying to develop the methodology of extremely high precision measurements of W, Nd and Os isotopes using N-TIMS and MC-ICP-MS and will try to reveal the core mantle coevolution on the early Earth.