Carbonated sediment recycling and its contribution to lithospheric refertilization under the northern North China Craton

Abstract

Subduction of carbonated sediments is the principal mechanism for the replenishment of mantle carbon reservoirs, but the processes of transport of carbonated sediments into the mantle and their incorporation into mantle chemical and isotopic heterogeneities are poorly constrained. Here we use in-situ major and trace element and Sr isotopic compositions and mineral Sr-Nd isotopes of the peridotite xenoliths (lherzolites with minor harzburgites) in the Oligocene Fanshi basalts from the northern North China Craton (NCC) to characterize and decode two metasomatic events caused by subduction of carbonated sediments. Clinopyroxenes (Cpx) in the harzburgites are characterized by high (La/Yb)N (8.4–66), Ca/Al (4.7–6.4) and Zr/Hf (30–66) ratios but low Ti/Eu ratios (478–1268) coupled with strongly enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) but depleted high field-strength elements (HFSE), suggesting that they have been metasomatized by carbonatitic melt. Highly enriched Sr-Nd isotopic compositions of these Cpx (87Sr/86Sr=0.70640–0.70716, 143Nd/144Nd=0.512304) imply that metasomatic carbonatitic melt was derived from carbonated sediments. Compositional zonation of Cpx in a harzburgite shows decreasing 87Sr/86Sr ratio, increasing Sr content and decreasing Zr/Hf ratio from cores to the rims, documenting a later stage of metasomatism. In situ Sr isotopes in Cpx from both harzburgites and lherzolites converge in diagrams of 87Sr/86Sr vs Sr, Zr/Hf, Ti/Eu and (La/Yb)N, indicating that these peridotites could have been extensively affected by the second metasomatic event. Cpx in the lherzolites display lower (La/Yb)N (0.16–10.6) and Zr/Hf ratios (25–39), suggesting that the later metasomatic agent is probably a carbonate-rich silicate melt and not carbonatite. Sr and Nd isotopes of these Cpx (87Sr/86Sr=0.702075–0.706148, 143Nd/144Nd=0.512410–0.513286) exhibit negative correlation along a simple mixing line between depleted mantle and carbonated sediments. The low Ce/Pb ratio (<12) of the metasomatic agent suggests that the carbonate-rich silicate melt also originated from carbonated sediments. The timing of the two metasomatic events and the regional tectonic setting are consistent with derivation of the carbonated sediments from the Paleo-Asian Oceanic slab: low degree melting formed early carbonatite melts, which caused decreasing carbonate content of carbonated sediments, whereas subsequent higher degree melting of residual carbonated sediments produced the later carbonate-rich silicate melt. Both contributed significantly to lithospheric refertilization under the northern NCC.