Читать книгу Isotopic Constraints on Earth System Processes - Группа авторов - Страница 77

Studied Central American arc magmas have no calcium isotope evidence for carbonate recycling. This is observed even for magmas with strong trace element and radiogenic isotope signatures indicating sediment subduction. The decoupled signatures are important because they likely reflect different sources and processes, and demonstrate that sediment subduction is not a bulk mixing process. Fresh Oldoinyo Lengai carbonatites also have δ⁴⁴Ca values that are similar to unmelted peridotites, komatiites, and most basalts, which again implies a primitive mantle‐derived calcium source. The possible exception comes from the intrusive Laacher See carbonatite. Its calcium isotopic signature could be related to carbonate sediment recycling in a long deep convective cycle. Alternatively, the relatively light calcium isotopic composition (δ⁴⁴Ca = –0.39) measured in the Laacher See carbonatite, and recently found in other carbonatites, may be associated with a process in which an isotopically heavy Ca‐bearing fluid was extracted during dehydration of its lithospheric mantle source, mass‐fractionating its original BSE calcium isotopic composition.