Marine Carbonates in the Mantle Source of Oceanic Basalts: Pb Isotopic Constraints

摘要

For almost fifty years, geochemists have been interpreting the clues from Pb isotopic ratios concerning mantle composition and evolution separately. The Pb isotopes of ocean island basalts (OIB) indicate that their mantle source is heterogeneous, most likely due to the presence of end-components derived from recycled crust and sediment. Some OIB have unusually high ²⁰⁶Pb/²⁰⁴Pb coming from one of the end-components with a long time-integrated high ²³⁸U/²⁰⁴Pb or μ (HIMU). Most OIB and many mid-ocean ridge basalts (MORB) also have high ²⁰⁶Pb/²⁰⁴Pb, indicating a HIMU-like source. Moreover, measured ²³²Th/²³⁸U (κ) for most MORB are lower than those deduced from their ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb. Such high μ and low κ features of oceanic basalts are inconsistent with the known geochemical behavior of U, Pb and Th and temporal evolution of the mantle; these have been respectively termed the 1^st and 2^nd Pb paradox. Here we show that subducted marine carbonates can be a source for HIMU and a solution to the Pb paradoxes. The results are consistent with the predictions of the marine carbonate recycling hypothesis that posits the Pb isotopes of oceanic basalts indicate a common origin and/or magma generation process.