Constraining Mantle Heterogeneity beneath the South China Sea: A New Perspective on Magma Water Content

The nature of upper mantle is important to understand the evolution of the South China Sea (SCS); thus, we need better constrains on its mantle heterogeneity. Magma water concentration is a good indicator, but few data have been reported. However, the rarity of glass and melt inclusions and the special genesis for phenocrysts in SCS basalts present challenges to analyzing magmatic water content. Therefore, it is possible to estimate the water variations through the characteristics of partial melting and magma crystallization. We evaluated variations in Fe depletion, degree of melt fractions, and mantle source composition along the fossil spreading ridge (FSR) using SCS basalt data from published papers. We found that lava from the FSR 116.2 degrees E, FSR 117.7 degrees E, and non-FSR regions can be considered normal lava with normal water content; in contrast, lava from the FSR 117 degrees E-carbonatite and 114.9-115.0 degrees E basalts have higher water content and show evidence of strong Fe depletion during the fractional crystallization after elimination of the effects of plagioclase oversaturation. The enriched water in the 117 degrees E-carbonatite basalts is contained in carbonated silicate melts, and that in the 114.9-115.0 degrees E basalts results from mantle contamination with the lower continental crust. The lava from the 117 degrees E-normal basalt has much lower water content because of the lesser influence of the Hainan plume. Therefore, there must be a mantle source compositional transition area between the southwestern and eastern sub-basins of the SCS, which have different mantle evolution histories. The mantle in the west is more affected by contamination with continental materials, while that in the east is more affected by the Hainan mantle plume.