Mid-oceanic Ridge Basalts(MORBS) Chemistry And Characteristics Of Plagioclase-hosted Melt Inclusions In The South Atlantic Ridge 19°S And Implications For Magmatic Processes

To understand the characteristics of the melting and magmatic processes in slow spreading ridge further, detailed study of mineralogy, whole-rock major and trace elements, Sr and Nd-series isotopic compositions have been carried out in the south Mid-Atlantic Ridge 19°S(SMAR 19°) MORB. Meanwhile, for making clear the control of change dynamics on composition of plagioclase-hosted melt inclusions from MORB, we analyzed the major elements, homogeneous temperature and formation process of the plagioclase-hosted melt inclusions(MIs) in MORB from SMAR 19°. Innovative results and conclusions in this study are as follows:The mineralogical characters, major and trace element compositions and Sr and Ndseries isotopic compositions show that the MORBs in SMAR 19°, which derived from higher depleted mantle composed by Spinel lherzolite phase, is belong to low-K tholeiite series, MORB magmas have experienced extensive low-pressure crystallization of olivine plus plagioclase, whereas clinopyroxene has not taken part in crystallization. The phenosrysts are mainly plagioclase and less olivine, and pyroxene is basically absent. The effects of post-magmatic alteration on the SMAR19° basalts are not obvious.The characters that the higher Sm/Nd, 143Nd/144 Nd, εNd values in SMAR19° basalts, and the lower Rb/Sr, Th/Pb, La/Sm N values in Saint Helena basalts, and the results of 87 Sr/86Sr- 143Nd/144 Nd and εNd-εSr diagrams collectively indicate that the interaction between ridge and plumes is not obvious. There should be a asthenosphere area, which is far away from the effects of ridge suction and plumes between the SMAR19° ridge fragment and Saint Helena plumes.The research of MIs compositions in Petrological Database(Pet DB) shows that(1) the effect of post-entrapment crystallization process could lead to negative correlation between Fe O, Mg O and Al2O3 in plagioclase-hosted MIs compositions, and make plagioclase-hosted MIs less convincible for reflecting the original compositions than the olivine-hosted MIs in the same basaltic samples.(2) Ca/Al ratio can be regarded as a new indicator for the effect of post-entrapped processes on magmatic compositions: when the Ca/Al ratio of plagioclase-hosted MIs is less than 0.78, the melt inclusions components corrected for fractionation effect to Mg#=0.72 can represent the primary magma derived from partially melted lherzolite or evolved magma from anhydrous Mid-ocean ridge basalts(MORB); when 1.0≥Ca/A≥0.78, the melt inclusions may represent the original components after eliminating the PEC effects; when Ca/Al>1.0, the compositions of MIs components can’t reflect the compositions of parent basaltic magma.Ca/Al ratios, Ca O, Na2 O, Fe O, and Mg O contents indicate that the compositions of plagioclase-hosted MIs in SMAR 19° MORB, which have been effected by complex change processes, can’t represent the melts compositions at the moment of being entrapped. The results of homogeneous temperature and re-crystallization experiments indicate there might have a micro heterogeneous magma system under the SMAR 19° area.