Site 1157 is located on ~22-Ma crust formed within Zone A ~230 km south of Site 1153 (~23 Ma). Two holes were drilled, and in each hole the rocks were assigned to a single lithologic unit. Two whole-rock samples from Hole 1157A and two from Hole 1157B were analyzed for major and trace elements by inductively coupled plasma-atomic emission spectrometry (ICP-AES) and X-ray fluorescence (XRF). One glass sample from each hole was analyzed by ICP-AES (Table T3). The ICP-AES and XRF results for whole rocks agree well, although Cr and Ni remain problematic.
Hole 1157A recovered mixed aphyric and moderately plagioclase-olivine phyric pillow basalt rubble as well as basalt-carbonate breccia. Samples analyzed from this hole include an aphyric whole rock, a plagioclase-olivine phyric whole rock, and a glass rind from a basalt clast recovered in the core catcher of the wash barrel. MgO contents range from ~7.0 to 8.5 wt%; the aphyric sample is the most evolved, and the glass is the least evolved (Fig. F38). The three samples generally define linear trends on most MgO variation diagrams, consistent with low-pressure crystal fractionation. However, Na2O varies orthogonally to a simple crystal fractionation trend which can only be explained by loss of MgO and/or mobility of Na2O during alteration of the whole-rock samples.
Hole 1157B recovered moderately plagioclase-olivine phyric pillow basalt. The samples from 1157B show a similar range of MgO contents (i.e., ~7 to 8 wt%), and the glass composition lies along the same low-pressure crystal fractionation trends as the samples from Site 1157A, but the whole-rock data have significantly lower TiO2 and CaO/Al2O3 values and higher Al2O3 and Cr at a given MgO content. As observed for previous sites, the glass is nearly 1.0 wt% higher in MgO content than are the associated whole rocks from this core. However, most other major and trace constituents are similar to the glass composition; only Ni is significantly different.
Together, the Site 1157 basalts are generally a coherent group, except that whole-rock Sample 187-1157A-2R-1, 86-90 cm, is low in Sr relative to the rest of the samples. Compared to 0- to 7-Ma Zone A basalts, most Site 1157 lavas have similar CaO, Zr, Y, and Cr contents, but lower Fe2O3, CaO/Al2O3, and TiO2 and higher Al2O3, Ni, Ba, and Sr (Figs. F38, F39). Low Fe2O3, CaO/Al2O3, and TiO2 values and high Al2O3, Ni, Ba, and Sr are characteristics typical of lavas dredged within the AAD and are traits uncommon in Zone A.
Comparison of Zone A axis and off-axis basalts with Site 1157 basalts reveals an interesting temporal variability in Ba, Sr, and CaO/Al2O3. The three groups define parallel trends with decreasing MgO, suggesting variations in source parameters for this part of the Southeast Indian Ridge (SEIR). For instance, if high CaO/Al2O3 content indicates a high degree of melting (Falloon and Green, 1987; Klein and Langmuir, 1987), then the Site 1157 lavas were by formed by lower degrees of melting than the axis and off-axis Zone A lavas, which formed by intermediate and high degrees of melting, respectively. Site 1157 basalts appear to have been produced during a particularly lean magmatic period. Variations in Ba and Sr are consistent with this melting scenario, but the low Na2O content for Site 1157 lavas is not. We speculate that such variations in melting are caused by the repeated arrival and demise of propagating rift-dying rift pairs along this section of the SEIR. Site 1157 lava could have erupted during a waning magmatic cycle associated with a dying rift.
In addition, major and trace element variations at Site 1157 are quite different from those at Site 1153 (see "Geochemistry," in the "Site 1153" chapter), the most interesting contrasts being that Fe2O3, TiO2, Zr, and Y contents at Site 1153 are relatively high. These compositional variations indicate a continual evolution of mantle conditions along the eastern border of the AAD.
The Zr/Ba systematics of Site 1157 basalts indicate that Indian- to Transitional-Pacific-type mantle was present east of the AAD at ~22 Ma (Fig. F40A). The Na2O/TiO2 values, relative to the overall variation for Leg 187 basalts, also indicate an Indian-type source (Fig. F40B). An interesting aspect of these diagrams is shown by whole-rock Sample 187-1157A 2R-1, 86-90 cm. This whole-rock sample appears (Fig. F40B) to be of Pacific type, and it consistently lies along the fractionation trend defined by Segment A1 lavas. All other Site 1157 basalts have Indian-type characteristics, like the associated glasses. The significance of this sample will not be known until we better understand the Ba systematics of the whole-rock samples and can corroborate these conclusions with isotopic data.