Previous geochemical studies reveal that two isotopically distinct mantle sources underlie the SEIR between Australia and Antarctica (e.g., Pyle at al., 1992; Christie et al., 1998). During ODP Leg 187 older oceanic crusts off axis of the AAD were drilled, and chemical analysis of sampled material shows that an isotopic boundary has existed there since at least ~30 Ma, with small-scale perturbations in timescale and in space (Shipboard Scientific Party, 2001). It is plausible that the Indian/Pacific isotopic boundary has slowly migrated westward, as opposed to the rapid-migration hypothesis suggested by Christie et al. (1998) based on geochemical results from ODP Leg 187 (see other chapters in this volume). Therefore, petrogenetic conditions inferred from petrological data should also reflect the essential differences between Indian and Pacific MORB sources.
Compositions of estimated primary magma indicate the depth of melt segregation from the surrounding mantle material. The compositions calculated in this study indicate that the depth for Indian-type MORB is shallower than that for Pacific-type MORB if compositions of source mantle do not differ significantly. Potassium components in mantle peridotites will lower the solidus temperature (Takahashi and Kushiro, 1983), and increased potassium and sodium components will shift the isobaric liquid compositional trend toward the silica apex on a CIPW norm projection (Sweeney et al., 1991). However, contents of potassium and sodium (or NaK# by Sweeney et al., 1991) are not significantly different between the two types of MORB analyzed here. Therefore, the calculated difference in the depth of melt segregation of Pacific-type and Indian-type MORB is realistic, although the absolute values of these depths may indicate a shallower limit of the depth.
Previous petrological studies reported that the AAD lavas have higher MgO contents and higher Na8.0, lower Fe8.0, and higher Si8.0 values than Zone A lavas (i.e., Pacific-type MORB), indicating lower degrees of partial melting and lower mean pressures of melting beneath the AAD (Klein et al., 1991). Compositions of glass samples from Leg 187 confirm this relationship (C. Russo, unpubl. data) (Fig. F4). Figure F7 illustrates a schematic model for petrogenetic conditions beneath the AAD, and details of the model including the previous results are described below. Beneath Zone A, the onset of partial melting starts and segregation of melt from mantle material occur at deeper level than beneath the AAD. Because the degree of partial melting for Indian-type MORB is lower than Pacific-type MORB, the hypothetical melting column beneath the AAD (Indian-type mantle) is shorter than that beneath Zone A (Pacific-type mantle).
Calculated olivine composition equilibrated with primary magma for the Indian-type MORB has a slightly Fe-richer composition (Mg# = 86.8-88.5) than that for Pacific-type MORB (88.9-90.2), indicating the heterogeneity of the MORB source. According to Ozawa (1997), the "mantle olivine array" includes two distinct origins of olivine in mantle. One is due to partial melting of original mantle peridotite. In this case, residual olivine occupies a high-Mg# portion (Mg# > 90) of the mantle olivine array. Another origin of the mantle olivine array is related to interaction between high Mg#-high NiO olivine in mantle peridotite and melt that is equilibrated with low Mg#-low NiO olivine. This interaction produces olivine that has a composition at a low-Mg# portion (Mg# < 90) of the mantle olivine array. Because primary magma for Indian-type MORB appears to be equilibrated with low-Mg# olivine, it is plausible that the Indian-type MORB source consists of mixture of mantle peridotite and material derived from melt that is equilibrated with low Mg#-NiO olivine. On the other hand, primary magma for Pacific-type MORB was equilibrated with residual mantle olivine by partial melting. Therefore, in terms of petrogenetic conditions inferred by mineral compositions and whole-rock major element composition, it is suggested that differences between the Indian- and Pacific-type MORB from around the AAD result from mantle source compositions.