Low H2O/Ce ratios and δ18O values for continental basalts in eastern China: Geochemical evidence for involvement of the dehydrated crustal component in the mantle source

Abstract

Crustal components derived from subducted oceanic crust have been increasingly identified in the mantle sources of intraplate basalts. The geochemical processes in transferring the crustal signatures into the mantle sources of these basalts has been documented to be similar to those for the origin of island arc basalts. However, the dehydration melting of subducting oceanic slabs is assumed to occur at postarc depths, releasing less water than the dehydration that occurs at subarc depths. This study examines this assumption using a combination of mineral H2O/Ce and O isotope ratios for Cenozoic continental basalts in eastern China, an area where the western Pacific plate is subducting beneath its continental margin. Clinopyroxene phenocrysts from these continental basalts show H2O/Ce ratios of 108–204 for their magma and δ18O values of 3.9‰-5.0‰ for the clinopyroxene, both of which are lower than the values of the depleted MORB mantle. This suggests that the basalts were derived from a mantle source enriched by the dehydrated lower oceanic crust-derived component. Mineral H2O/Ce thermometry yields high temperatures of 1055 °C–1267 °C that represent the conditions of chemical metasomatism at the slab-mantle interface, significantly higher than those in generating the mantle sources of island arc basalts. On the other hand, lithochemical thermometry of the basalts yields much higher temperatures of 1456 °C–1522 °C that are indicative of the condition of mantle melting. While the substantial differences between these two temperatures indicates two-stage processes for origin of the continental basalts, the considerably higher metasomatic temperatures testify the dehydration melting of subducting oceanic slabs at postarc depths. These interpretations are verified by quantitative modeling of geochemical differentiation and mixing in an oceanic subduction zone. Therefore, the origin of intraplate basalts generally involves dehydration melting of subducting oceanic slabs at much greater depths than that of island arc basalts.