Multi-element isotope study of natrocarbonatites (1993 lava flows) from Oldoinyo Lengai volcano, Tanzania: Implications for core-mantle interactions: Implications for core-mantle interactions

We contribute to the debate of core-mantle interactions by providing tungsten (W) isotope (ε¹⁸²W) data from natrocarbonatite samples collected from the 1993 lava flows at Oldoinyo Lengai (OL) volcano, Tanzania. The ¹⁸²Hf-¹⁸²W system (half-life = 8.9 Ma) is well suited to constraining the timing of core formation and monitoring incursions of the core into the mantle. Most mantle-derived rocks are difficult to analyze because of their ultra-low W contents. We consider natrocarbonatites as suitable proxy for quantifying the core-mantle interactions given their unusually high W concentrations (published values range from ~40 to 115 ppm). The ε¹⁸²W values of 7 natrocarbonatites measured by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) represent the first dataset obtained for the OL lavas (OLLs). The ε¹⁸²W values calculated from W isotope ratios range from −0.01 to +0.16 and are indistinguishable within limits of uncertainty, from those of the ambient modern mantle sources (i.e., EM1, HIMU, and HIMU-like). The relatively uniform W isotope compositions associated with the lack of any ε¹⁸²W anomaly from the natrocarbonatites imply that there may have been no contribution from the core (i.e., ε¹⁸²W < 0) into the magma source of the OLLs. Alternately, the core signature could have been obscured either by the addition of mantle components that survived since the Earth's early accretion, or by an unknown region formed by an inefficient percolation of metal during core-mantle segregation that left parts of the mantle enriched in W. New C–O isotope data from the same samples (δ¹³C = −5.9 to −6.9‰; δ¹⁸O = 9.5–10.9‰) rule out any involvement of the continental crust into the OL magma and are consistent with other studies that show a mantle signature for most natrocarbonatites. The heavier δ¹⁸O values can be attributed to sub-solidus processes. The potential oxygen isotopic shift would not, however, affect the W isotopic compositions of the OLLs.