CHEMICAL COMPOSITION OF THE BASALTS ON EAST PACIFIC RISE (1.5°N~1.5°S) AND SOUTH MID-ATLANTIC RIDGE (13.2°S)

Six samples of fresh basalts collected from the East Pacific Rise (EPRⅠarea and Ⅱarea) and the South Mid-Atlantic Ridge (SMAR) have been studied in petrology and geochemistry. The EPR basalts can be grouped into the categories of pyroxene basalt, vesicular basalt and tholeiitic basalt, and the SMAR are dominated by pyroxene basalts. The mineral composition of the EPR is similar to that of SMAR by the existance of quartz (Q) and hypersthene (Hy).They all belong to the typical tholeiite. Both of the basalt of EPRⅠand SMAR, however,are enriched in LREE possibly due to the constraints of the mantle source (E-MORB). These basalts may be formed by the enriched mantle before partial melting or only having weak partial melting in the early stage of evolution. The basalts from the EPRⅡarea are normal mid-ocean ridge basalts from the classical depleted mantle (N-MORB) partially melted in early stage and the upper mantle magma was differentiated to certain extent. The difference in sources of the basalts at EPRⅠand Ⅱillustrates that the mantle is heterogeneous on the East Pacific Rise.