Site 1159 basalts were recovered from a single hole that yielded samples of ~14-Ma crust formed within Zone A of the Southeast Indian Ridge. Two whole-rock powders were analyzed for major and trace elements by XRF and ICP-AES, and two samples of fresh basalt glass chips were analyzed by ICP-AES (Table T3). Relative to the XRF analyses, Al2O3, Fe2O3, MgO, and Na2O contents are higher and Ni and Cr are lower for the ICP-AES analyses. All other elemental analyses are in good agreement between both the XRF and ICP-AES analyses.
From Hole 1159A we recovered samples from a single lithologic unit of aphyric pillow basalt (see "Igneous Petrology"). The glass samples from this site contain ~1 wt% more SiO2,~0.5 wt% more MgO, ~0.2 wt% less K2O, and 3 to ~4 ppm less Ba than the whole-rock samples (Table T3). All other elements are similar in whole-rock and glass samples. The slight compositional differences in CaO and Al2O3 between the whole-rock and the glass samples could be explained by fractional crystallization of plagioclase or by crystal sorting, as they parallel low-pressure crystal fractionation trends of axial Zone A lavas (Fig. F18). The whole-rock-glass variations on TiO2 and Fe2O3 vs. MgO plots, however, are oblique to these fractionation trends and are therefore inconsistent with low-pressure crystal fractionation. These observations are similar to previous sites, suggesting that alteration is the dominant cause of the low-MgO whole-rock compositions.
In Figures F18 and F19, Site 1159 glasses are compared to 0- to 7-Ma Zone A lavas from Segments A2 and A3. Site 1159 glasses have similar compositions to 0- to 7-Ma Zone A lavas, although Y contents are slightly higher and Sr slightly lower at the same MgO content. We note that Fe2O3 is significantly higher (~2 wt%) and Na2O is lower than the 0- to 7-Ma Zone A lavas, which could suggest slightly deeper melting conditions. However, higher degrees of melting are not supported by CaO/Al2O3. Therefore, the general similarity between Site 1159 and Segments A2 and A3 lavas suggests a comparable mantle source composition and comparable mantle melting conditions. The low Sr, Ba, and Na2O also indicate that Site 1159 lavas erupted along a section of the spreading axis away from propagating rift tips.
Site 1159 samples plot clearly in the Pacific field on the Zr/Ba vs. Ba diagram (Fig. F20A), suggesting a Pacific-type mantle source existed in this portion of Zone A at ~14 Ma. The Na2O/TiO2 vs. MgO characteristics for Site 1159 basalts are also consistent with a Pacific-type mantle source (Fig. F20B).