Highly variable H2O/Ce ratios in the Hainan mantle plume

Abstract

Mantle plume-generated basalts have long been utilized to assess the large-scale heterogeneity of the deep mantle. Their chemical and isotopic enrichment has been thought to result from the incorporation of recycled sediments or oceanic crust in their sources. Variable extents of dehydration or rehydration of recycled materials during subduction can induce different H2O/Ce ratios in plume-related basalts. The Hainan plume has been regarded as the cause of the massive Cenozoic basaltic volcanism in southeastern Asia. In this study, we measured bulk-rock major and trace element compositions of basalts from a volcano at Heishanling Hill, northern Hainan Island, and made FTIR analyses of water in clinopyroxene phenocrysts to determine magmatic water contents. In addition, we determined Sr-Nd isotope compositions of basalts at Heishanling Hill and other three localities.Water contents of Cpx phenocrysts vary from 20 to 254 ppm, which were used to calculate the water contents in the corresponding equilibrated melts, yielding values from 0.22 to 2.42 wt% with an average of 1.07 ± 0.47 wt%. Considering the effect of magma differentiation on water contents, the primary melts of Heishanling basalts were estimated to have a range of 0.67–1.40 wt% for individual Heishanling basalts, with the average of 1.03 ± 0.33 wt%. The calculated water contents in the mantle source of the Heishanling basalts vary from 279 to 582 ppm, falling in the range of global OIBs.Combined with the results previously reported, H2O/Ce ratios of Hainan basalts were estimated to vary from 50 to 425, covering the reported range from EM-type OIBs to NMORBs. The enriched Sr-Nd isotopic compositions of the studied samples resemble those of other Hainan plume-related basalts, and their 143Nd/144Nd ratios are well correlated with incompatible trace element ratios (e.g., Th/La, Ba/Nb). Bulk-rock compositions are consistent with a contribution of pyroxenite in the mantle source of the Hainan basalts and mixing modeling suggests coupled contribution of recycled sediment and oceanic crust. H2O/Ce displays a negative correlation with 143Nd/144Nd and a positive correlation with (Rb/Nb)n and (Th/La)n, meaning that an increasing amount of recycled oceanic crust and sediment in the source increases its H2O/Ce. The recycled material incorporated in the Hainan plume source may have undergone variable degrees of dehydration or deep rehydration by fluids released from subcrustal hydrous minerals at grater depths during subduction.