Olivine chemistry of Cenozoic basalts in the South China Sea and the potential temperature of the mantle

The Grain size of mostly olivine phenocrysts in Cenozoic basalts dredged from the South China Sea are relatively small (0.2 similar to 0.6mm), the shape of that is idiomorphic or hypidomorphie with slightly resorption. There don't take on the peculiar ring-fracture and deformation structure which olivine xenocrysts have. In groundmass, the grain size of olivine microlites is just 0.01 similar to 0.05mm, and most of them were totally transformed into iddingsite. Earlier olivine phenocryst crystallizing from the magma are not precipitate and remain in deeper mantle, but entrained and transported towards the surface by host magma. During the ascent of magma, there may be have a restricted interaction between earlier olivine phenocryst and host magma, and a extremely small amount of olivine phenocryst still held the phenomena of weak zoning belt in composition. Therefore, olivine phenocrysts in the studied area are not xenocryst. Using Putirka(2005) method, combined with Fo value of core and mantle part of weak zoning olivine phenocryst and X-Fe value of host magma, we have estimated mantle potential temperature (T-p) below the central basin of the South China Sea. The results indicate that the mean value of T-p for the South China Sea is 1661 degrees C and hotter than that of OIs (ocean ridges) (1453 degrees C similar to 1475 degrees C), which lie in the middle part between Hawaii hotspot and Iceland hotspot. The contrast for T-p value between the South China Sea and normal OIs is 186 degrees C similar to 208 degrees C, which also lie in he middle part between T-p values of Hawaii hotspot and Iceland hotspot whose T-p values are 213 degrees C similar to 235 degrees C and 162 degrees C similar to 184 degrees C, respectively. The results show that the mantle below the South China Sea may be exist thermally anomalous, support the idea that there may be exist mantle plume around the South China Sea which is put forward in the light of those evidence from geophysical and/or petrologic data.