Molybdenum and boron isotope evidence for fluid-fluxed melting of intraplate upper mantle beneath the eastern North China Craton

Abstract

Fluid-fluxed melting of the mantle is the principal mechanism for the generation of arc magmas above subduction zones, but it has rarely been documented as important in intraplate settings. Here, we present new molybdenum (Mo) isotopic data from a suite of well-characterized Cenozoic basalts from the eastern North China Craton (NCC) to constrain the mechanisms of mantle melting in the region. These basalts represent mixtures of three components, namely, nephelinites and basanites that variably mixed with alkali basalts. The alkali basalts have relatively low δ98/95Mo ranging from −0.56‰ to −0.22‰ relative to the NIST3134 standard, suggesting the presence of dehydrated pelagic sediments in their mantle source rocks. Interestingly, the basanites range to higher δ98/95Mo (−0.04 ± 0.02‰) compared with the nephelinites (−0.31‰ to −0.22‰) with no associated changes in their radiogenic isotopes, and they show trends of decreasing Dy/Yb, Ce/Mo and δ11B with increasing Ba/Nb and B/Nb ratios. These correlations indicate that Mo behaves as a fluid-mobile trace element in the intraplate mantle beneath the NCC, and that the mantle sources of the nephelinites and basanites were infiltrated by this fluid component. Existing data show that this fluid component has lower H2O/Ce ratios (110–130) than that of mid-ocean ridge basalts (H2O/Ce ≈ 200). This result, together with the low-SiO2 contents, relative depletion of Zr–Hf–Ti, and high δ66Zn characteristics of these strongly alkaline rocks, suggests derivation from a carbonated mantle source. Mo and B isotope systematics thus reveal carbonated fluid-fluxed mantle melting occurred beneath the eastern NCC. Increasing fluid inputs led to increases in the degree of melting of the mantle source that generated melts ranging in composition from nephelinite through basanite. The origin of such a fluid flux is enigmatic in an intraplate setting, but could be related to the decarbonization of subducted slabs stalled in the deep mantle.