The chemical compositions of olivine, plagioclase, pyroxene, and spinel in lavas collected during Ocean Drilling Program Leg 187 in the Australian Antarctic Discordance, Southeast Indian Ridge (41°-46°S, 126°-135°E) were analyzed, and modeling of the theoretical equilibrium petrogenetic conditions between olivine and melt was conducted. The cores of larger olivine phenocrysts, particularly in the isotopic Indian-type mid-ocean-ridge basalt (MORB), are not equilibrated with melt compositions and are considered to be xenocrystic. Larger plagioclase phenocrysts with compositionally reversed zonation are also xenocrystic. The compositions of primary magma were calculated using a "maximum olivine fractionation" model for primitive MORB that should fractionate only olivine. Olivine compositions equilibrated with calculated primary magma and compositions of calculated primary magma suggest that (1) isotopic Pacific-type MORB is more fractionated than Indian-type MORB, (2) Pacific-type MORB was produced by higher degrees of partial melting than Indian-type MORB, and (3) primary magma for Indian-type MORB was segregated from mantle at 10 kbar (~30 km depth), whereas that for Pacific-type MORB was segregated at 15 kbar (~45 km depth).
1Sato, H., 2004. Mineral compositions of MORB from the Australian Antarctic Discordance (AAD): implications for mantle source characteristics. In Pedersen, R.B., Christie, D.M., and Miller, D.J. (Eds.), Proc. ODP, Sci. Results, 187 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/187_SR/202/202.htm>. [Cited YYYY-MM-DD]
2School of Business Administration/ Institute of Natural Science, Shenshu University, 2-1-1 Higashimata, Tama, Kawasaki, Kanagawa 214-8580, Japan. satohiro@isc.senshu-u.ac.jp
Initial receipt: 17 May 2002
Acceptance: 17 July 2003
Web publication:
17 February 2004
Ms 187SR-202