The lead isotope composition of the Earth´s mantle from double-spike analysis of abyssal peridotites
Final Report Abstract
This project set out to determine the lead isotope composition of the Earth’s upper mantle, from analysis of peridotites from oceanic lithosphere. These peridotites represent the residues of the melting process beneath mid-ocean spreading centres, and therefore provide important insights into mantle composition, which complement those from studies of melts derived from the mantle (mid-ocean ridge basalts). The project was inspired because several recent publications had suggested that the Pb isotope composition of abyssal peridotites differs (is less radiogenic) than that of midocean ridge basalts, and that analysis of basalts alone cannot be used to infer the composition of the mantle. If correct, this would mean that our current estimate of the Pb isotope composition of the Earth’s mantle is incomplete, which could explain several puzzling aspects of mantle geochemistry, including the ‚Pb paradox’. Direct geochemical studies of abyssal peridotites are rare, because such peridotites are often extremely altered, and because most contain extremely low concentrations of incompatible trace elements, including Pb. The focus of the project changed somewhat, as we encountered analytical difficulties, and found new, interesting aspects of peridotite chemistry to investigate. Nevertheless, the project was a success, leading to new insights into the melting processes beneath spreading ridges. Our finding that abyssal peridotite and mid-ocean ridge basalt compositions are correlated on a global scale, and that spreading rate influences the degree of mantle melting at spreading ridges are especially important results. Highlights of the results from this project: (a) publishing a new global database of abyssal peridotite compositions, to complement published datasets for mid-ocean ridge basalts, and allowing new insights into melting processes beneath spreading ridges, (b) demonstrating that on a global scale, variations in the degree of melting inferred from mid-ocean ridge basalt (MORB) compositions are correlated with the variations in the degree of melting as calculated from abyssal peridotite compositions, (c) showing that ‚hotspots’ are the dominant cause of mantle temperature variations in the ocean basins. Away from hotspots, variations in mantle temperature are much smaller than previously thought, (d) first demonstrating that variations in spreading rate have an important control on the degree of mantle melting at spreading ridges, as reflected in the compositions of both peridotites and basalts, (e) developing a new tectonic model for the origin of supra-subduction zone ophiolites as formed at RTT-RTF triple junctions, (f) showing that inorganic carbonates in ophiolites preserve a record of past seawater compositions, potentially allowing the record of seawater Mg/Ca and Sr/Ca to be extended to times before the oldest preserved in-situ oceanic crust.
Publications
- The lead isotope composition of abyssal peridotites. European Mineralogical Conference, Frankfurt, 2012
Weinzierl C., Regelous, M., Haase, K., Ehrlich, J., Galer, S., Abouchami, W.
- Low-temperature hydrothermal fluid evolution. Goldschmidt Geochemistry Conference, Florence, 2013
Weinzierl, C., Bach, W., Böhm, F., Regelous, M., Haase, K.
- Formation of the Troodos Ophiolite at a triple junction: evidence from trace elements in volcanic glass. Chemical Geology 386 (2014) 66-79
Regelous, M., Haase, K., Freund, S., Keith, M., Weinzierl, C., Beier, C., Brandl, P.A., Endres, T., Schmidt, H.
(See online at https://doi.org/10.1016/j.chemgeo.2014.08.006) - Tectonic controls on MORB melting from correlated peridotite-basalt compositions. Goldschmidt Geochemistry Conference, Prague, 2015
Weinzierl, C., Regelous, M., Haase, K.
- The Troodos Ophiolite was probably formed at a RTT/RTF triple junction. Goldschmidt Geochemistry Conference, Prague, 2015
Regelous, M., Haase, K., Beier, C., Brandl, P., Keith, M., Weinzierl, C.
