|Title||Marine carbonates in the mantle source of oceanic basalts: Pb isotopic constraints|
|Publication Type||Journal Article|
|Year of Publication||2018|
|Authors||Castillo P.R, MacIsaac C., Perry S., Veizer J.|
|Type of Article||Article|
|Keywords||continental-crust; craton; evolution; geochemistry; lead; nd; Science & Technology - Other Topics; stromatolitic carbonates; supergroup; trace-element; uranium|
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 Pb-206/Pb-204 coming from one of the end-components with a long time-integrated high U-238/Pb-204 or mu (HIMU). Most OIB and many mid-ocean ridge basalts (MORB) also have high Pb-206/Pb-204, indicating a HIMU-like source. Moreover, measured Th-232/U-238 (kappa) for most MORB are lower than those deduced from their Pb-208/Pb-204 and Pb-206/Pb-204. Such high mu and low kappa 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 1st and 2nd 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.
|Short Title||Sci Rep|
In summary, our results indicate the existence of Archaean carbonates with the appropriate Pb and Sr isotopes to generate the HIMU source, per the marine carbonate recycling hypothesis. Through qualitative modeling, we illustrate via mixing between recycled upper mantle and partial melts from 1) select ancient marine carbonates the compositional signature of HIMU OIB, and 2) ancient carbonated crust and sediment the solutions to both 1st and 2nd Pb paradoxes. Notably, a recent study also concludes that the high U/Pb and U/Th content of ancient seawater as the source of the Pb isotopic signature of the HIMU end-component. Instead of Archaean carbonatitic melt, however, the study proposes metasomatism of the source with U-enriched, supercritical liquid derived from Proterozoic (1.9 Ga) seawater.