Site 1154 basalts were recovered from ~28-Ma seafloor formed within Zone A of the Southeast Indian Ridge. Three whole-rock powders were analyzed by both X-ray fluorescence (XRF) and inductively coupled plasma-atomic emission spectrometry (ICP-AES), and two samples of fresh basalt glass chips were analyzed by ICP-AES only. The results are shown in Table T3. The ICP-AES analyses are of marginal quality because of dissolution problems and inexperience with the JY2000 instrument early in Leg 187. The major element data for the glass samples are compromised the most because SiO2 concentrations could only be estimated by difference from 100%. Despite the analytical difficulties, XRF and ICP-AES analyses agree well, with the exception of Ni and Cr, which are both lower in the ICP-AES analyses, and Ba, which is below the detection limit of the XRF analysis.
Samples from Hole 1154A are assigned to a single plagioclase-olivine phyric basalt unit based on macroscopic and microscopic examination (see "Igneous Petrology"). The whole-rock samples contain ~6 wt% MgO (Fig. F10). Like Sites 1152 and 1153, the glass samples from Hole 1154A have higher MgO contents (7.6-7.8 wt%) and lower K2O (0.11-0.14 wt%) than the whole-rock samples (Table T3). Except for MgO and K2O, all other element concentrations are similar for whole-rock and glass samples. The compositional differences between the whole-rock and the glass samples cannot result from simple low-pressure crystal fractionation, as the whole rock-glass tie lines are oblique to the fractionation trends for 0- to 7-Ma Zone A lavas (Figs. F10, F11). These observations are similar to those at Sites 1152 and 1153, again suggesting that alteration or a nonequilibrium magmatic process has affected the whole-rock compositions.
Site 1154 glasses are, for the most part, comparable to Zone A axial lavas. These glasses have slightly high Fe2O3 and Y concentrations, similar to Site 1153 glass, relative to Zone A axial samples. The most primitive Site 1155 glass has low CaO/Al2O3 values, suggesting it was erupted in an environment similar to the Zone A propagating rift tip (PRT) lavas. This is only weakly supported by slightly higher concentrations of moderately incompatible elements like Y and Zr but not by the low concentrations of highly incompatible elements like Ba and Sr. The similarity of Na2O contents between Site 1154 glasses and 0- to 7-Ma Zone A glasses indicates that the extent of mantle melting was the same to slightly higher. High Fe2O3 contents in Site 1154 glasses suggest fractionation from a parental magma derived at higher mean pressures of melting relative to the present Zone A lavas, although little else distinguishes them. Therefore, the magmatic environment appears to have been similar to that beneath the present Zone A spreading axis.
The Ba/Zr systematics of Site 1154 basalts (Fig. F12) suggest that Pacific-type mantle was present beneath Zone A at ~28 Ma. The Na2O/TiO2 vs. MgO diagram is also consistent with a Pacific-type mantle source.