P12-Chen Lihui_abstract+photo

Recycling of ancient ‘carbonate’ in Pitcairn lavas

Xiao-Jun Wanga, Li-Hui Chena, *, Albrecht W. Hofmannb, *, Takeshi Hanyuc, Hiroshi Kawabatad

a School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China

b Max-Planck-Institut für Chemie, Abteilung Klimageochemie, D-55128 Mainz, Germany

c Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan

d Faculty of Science and Technology, Kochi University, Kochi 780-8520, Japan

* Correspondence: chenlh@nju.edu.cn or albrecht.hofmann@mpic.de

Earth’s lower mantle delivers isotopically coded messages about its composition and evolution via oceanic island basalts. The extreme Sr, Nd, Hf, and Pb isotopic compositions found in Pitcairn Island basalts have been labeled EM1 (Enriched Mantle-1), characterizing it as one of the mantle endmembers. The EM1 origin has been vigorously debated over 25 years, with main interpretations ranging from delaminated subcontinental lithosphere, recycled lower continental crust, to recycled oceanic crust carrying ancient pelagic sediments, all of which may potentially generate the requisite radiogenic isotopic composition. Here we find that δ26Mg ratios in Pitcairn EM1 basalts are significantly lower than in normal mantle, and are among the lowest values so far recorded in oceanic basalts. Since marine carbonate is the most common and typical reservoir totally characterized by extremely low δ26Mg values, we therefore infer that the subnormal δ26Mg of Pitcairn EM1 component originates from subducted marine carbonates. This, combined with previous evidence yielding exceptionally unradiogenic Pb as well as S isotopes subjected to mass-independent fractionation, suggest that the Pitcairn EM1 component is most likely derived from late Archean subducted carbonate-bearing sediments.