CONCLUSIONS

Minerals in MORB samples from the AAD along the SEIR were analyzed, and the following conclusions were obtained.

1. Olivine phenocrysts and even olivine grains in the groundmass are rich in Mg (Mg# > 80). Olivine phenocryst centers contain up to 0.3 wt% NiO, whereas olivine grains in the groundmass contain up to 0.2 wt% NiO.
2. Olivine fractional crystallization pathways were calculated for primitive MORB samples, five samples for Indian-type MORB and two samples for Pacific-type MORB. During this calculation, olivine compositions equilibrated with melt composition were calculated. Compositions of olivine phenocrysts in some Indian-type MORB are not equilibrated with those of the calculated olivine. Precision of whole-rock analysis, sampling bias in lava flow, or xenocrystic origin were considered to explain the disequilibrium. In these samples, large plagioclase phenocrysts show reversed compositional zoning. It is plausible that these phenocrysts are xenocrystic.
3. The number of calculation steps required to equilibrate olivine compositions to mantle olivine composition (i.e., degree of fractionation) suggests that Pacific-type MORB is more fractionated than Indian-type MORB.
4. Estimated compositions of primary magma determined by the olivine fractionation calculation were compared to previously reported results from experimental petrology, and the results of this comparison suggest that Indian-type MORB segregated from mantle material at 10 kbar (~30 km depth), whereas Pacific-type MORB segregated at 15 kbar (~45 km depth).
5. Olivine equilibrated with Pacific primary magma has Mg# = ~90, whereas olivine equilibrated with Indian primary magma has Mg# = 86.8-88.5. Composition of equilibrated olivine with calculated primary magma suggests that the mantle source for Indian-type MORB was mixture of mantle peridotite and material derived from melt equilibrated with low Mg#-low NiO olivine.