Probing terrestrial mantle evolution using Ru isotopes

A diversity of materials was likely added to Earth during the late stages of its accretion; however, the specific elemental and isotopic compositions of these contributors are poorly constrained. The efficiency with which these late accreted materials were mixed into the mantle also remains an open question. The highly siderophile element ruthenium (Ru) provides a potentially useful isotopic genetic tracer for late accretionary additions to Earth. Well resolved deficiencies in ¹⁰⁰Ru have been reported on the scale of whole-rock samples of meteorites, and are interpreted to stem from the heterogeneous distribution of s-process carrier phases in the solar nebula [1,2]. Isotopically diverse materials are, therefore, likely contributors of late accreted materials to Earth's mantle. Recent isotopic studies have found that major accretionary events (e.g., the Moon-forming giant impact) did not completely homogenize the mantle; thus, long-term preservation of isotopically distinct reservoirs in the mantle might be expected [3]. Identification of isotopically heterogeneous domains in the mantle using Ru isotopic analyses can potentially reveal the nature of different impactors. To investigate the genetics of late accretionary additions and to evaluate the veracity of late accretionary models, we have developed a refined analytical technique for the high-precision measurement of Ru isotopic composition using negative thermal ionization mass spectrometry (N-TIMS). Replicate analyses of an Alfa Aesar Ru standard (n = 56) over the period of several months indicate a current external precision of ×8 ppm (2σ SD) for ¹⁰⁰Ru/¹⁰¹Ru. Data are corrected for instrumental mass fractionation using the exponential law and ⁹⁹Ru/¹⁰¹Ru as the normalizing ratio. Data are also corrected for oxide interferences assuming a natural oxygen isotopic composition. No second order oxygen correction is required [3]. Prior to this study, the highest external precision achieved using standard N-TIMS or MC-ICP-MS techniques has typically been between ×25 and 30 ppm [1,2]. Thus, the external precision achieved in our study so far is a factor of ~4 higher than in any previous measurements. In the beginning stages of this study, a suite of chromitites from the 492 Ma old Shetland ophiolite complex has been analyzed to test the extent of mantle heterogeneity preserved in relatively young terrestrial materials. Repeat analyses of Ru fractions from these chromitites show no statistically significant deviations of ¹⁰⁰Ru/¹⁰¹Ru from the terrestrial standard, thus, indicating a homogeneous mantle composition during this late stage of mantle evolution. Isotopic heterogeneity, however, may be preserved in older terrestrial materials and we have begun the search for these heterogeneities. [1] Chen et al. (2010) Geochim. Cosmochim. Acta 74, 3851-3862. [2] Fischer-Gödde et al., (2013) 44th Lunar Planet. Sci. Conf. #2456 (abstr.) [3] Touboul et al. (2012) Science 335, 1065-1069.