JGR paper: Magma-magma interaction in the mantle beneath eastern China

In addition to magma-rock and rock-rock reaction, magma-magma interaction at mantle depth has recently been proposed as an alternative mechanism to produce the compositional diversity of intraplate basalts. However, up to now no compelling geochemical evidence supports this novel hypothesis. Here, we present geochemistry for the Longhai basalts from Fujian Province, southeastern China, which demonstrates the interaction between two types of magma at mantle depth. At Longhai, the basalts form two groups, low-Ti basalts (TiO₂/MgO < 0.25) and high-Ti basalts (TiO₂/MgO > 0.25). Calculated primary compositions of the low-Ti basalts have compositions close to L+Opx+Cpx+Grt cotectic, and they also have low CaO contents (7.1–8.1 wt.%), suggesting a mainly pyroxenite source. Correlations of Ti/Gd and Zr/Hf with the Sm/Yb ratios, however, record binary mixing between the pyroxenite-derived melt and a second, subordinate source-derived melt. Melts from this second source component have low Ti/Gd, and high Zr/Hf and Ca/Al ratios, thus likely representing a carbonated component. The Sr, Nd, Hf and Pb isotopic compositions of the high-Ti basalts are close to the low-Ti basalts. The Sm/Yb ratios of the high-Ti basalts, however, is markedly elevated and characterized by crossing REE patterns at Ho, suggesting that they have source components comparable to the low-Ti basalts, but that they have experienced garnet and clinopyroxene fractionation. We posit that mingling of SiO₂-saturated tholeiitic magma with SiO₂-undersaturated alkaline magma might trigger such fractionation:

SiO₂-saturated tholeiitic magma + SiO₂-undersaturated alkaline magma → SiO₂-saturated alkaline magma + garnet + clinopyroxene

Therefore, the model of magma-magma interaction and associated deep evolution of magma in the mantle is proposed to explain the formation of Longhai basalts. It may moreover serve as a conceptual model for the formation of tholeiitic to alkaline intraplate basalts worldwide. The related paper titled “Magma-magma interaction in the mantle beneath eastern China” was recently published in Journal of Geophysical Research: Solid Earth (JGR). (122, doi:10.1002/2017JB014023)

Variation in Ti/Gd versus Sm/Yb for the Longhai basalts, their various source components, and invoked mixing relations

(Zeng, G., Chen, L.-H., Yu, X., Liu, J.-Q., Xu, X.-S., and Erdmann, S. (2017), Magma-magma interaction in the mantle beneath eastern China, J. Geophys. Res. Solid Earth, 122, doi:10.1002/ 2017JB014023.)