Revisiting the South China Sea MORBs: Mg isotope and whole-rock geochemical constraints

Mid-Ocean Ridge Basalts (MORBs) from the South China Sea (SCS) have attracted great attention from geologists in the past decade, but their petrogenesis still remains controversial. Here, we show that the Mg isotopes of MORBs from Integrated Ocean Drilling Program (IODP) Sites U1431, U1433 and U1434 are heterogeneous and present the mechanisms causing such heterogeneity. The MORBs from Sites U1431, U1433 and U1434 have delta Mg-26 values ranging from -0.35 parts per thousand to -0.21 parts per thousand, from -0.34 parts per thousand to -0.13 parts per thousand, and from -0.30 parts per thousand to -0.20 parts per thousand, respectively. Together with all the published data, we found the MORBs at Site U1431 with high MgO contents (> 9 wt%) usually have delta Mg-26 values that are generally lower than the normal mantle (-0.25 parts per thousand +/- 0.04 parts per thousand), whereas some of the Site U1433 MORBs are higher than the normal mantle. Shallow-level geological processes like seawater alteration have negligible effects on the Mg isotope fractionation of our samples. Elemental ratio indicative of the degree of partial melting (like Sm/Nd) from the same site also shows no relationship with the delta Mg-26 variations. Olivine is proposed to have light Mg isotopes. Considering the high MgO (> 9 wt%) contents of low-delta Mg-26 samples and the petrographic evidence, we propose that their slightly light Mg isotopic compositions of U1431 MORBs may result from the enrichment of olivine, rather than reflecting the mantle source. Moreover, low Ce/Pb ratios and most variable delta Mg-26 values of some U1433 MORBs suggest that lower continental crustal materials have contributed to their source. As all the previously published MORBs are evolved, we use the Revpet to estimate the mantle melting conditions of SCS MORBs, including the Site U1500 samples recovered from the northern margin of SCS. Compared with the global MORBs data, there is no thermal anomaly in SCS MORBs. Together with our new Mg isotope data, therefore, we propose that the role of the Hainan mantle plume in the opening and spreading of the SCS may be insignificant.

Automated summary

Mid-Ocean Ridge Basalts (MORBs) from the South China Sea (SCS) have attracted attention from geologists, but their petrogenesis is still disputed. Heterogeneous Mg isotopes in MORBs suggest olivine enrichment and contribution from lower continental crust. The study proposes that the role of the Hainan mantle plume in the opening and spreading of the SCS may be insignificant.