Basalt was recovered at Site 1160 from two holes that penetrated ~22-Ma crust near the base of a large seamount within Zone A. This site is the farthest east of all Leg 187 sites and was selected to provide a baseline Pacific-type sample for this region at 22 Ma. One glass from Hole 1160A and two glasses and one whole-rock sample from Hole 1160B were analyzed for major and trace elements by ICP-AES. Nine whole-rock samples from Hole 1160B were analyzed for major and trace elements by XRF (Table T4). The highly altered nature of the whole-rock samples from Hole 1160A precluded XRF analysis. Hole 1160B whole-rock XRF powders were not analyzed by ICP-AES.
A single lithologic unit of slightly to moderately altered aphyric pillow basalt rubble was recovered from Hole 1160A (see "Hole 1160A" in "Igneous Petrology"). The single glass sample analyzed from this hole is identical in composition to younger Southeast Indian Ridge (SEIR) glasses with comparable MgO contents (7.7 wt%) from Segments A2 and A3 (Figs. F22, F23).
From Hole 1160B seven lithologic units of aphyric and moderately plagioclase ± olivine phyric basalt were defined in an alternating sequence of pillow basalt and massive flows. As a group, Hole 1160B lavas range from high-MgO (~9.0 wt%) primitive basalt glass and massive flow whole rocks to low-MgO (~5.5 wt%) pillow basalt whole rocks. Glasses from Units 1 and 5 have ~3.5 wt% higher MgO contents than their whole-rock counterparts. Whole rocks from Units 1, 3, 5, and 7 (i.e., the pillow lavas) have the lowest MgO contents. CaO, TiO2, Zr, Y, and Cr contents show little variation with respect to MgO content, and variations of Al2O3, Na2O, and CaO/Al2O3 are unsystematic. The evolved whole-rock compositions cannot be explained by simple magmatic processes, and we believe that they represent alteration effects. Massive flow whole rocks from Units 2, 4, and 6 are very fresh and overlap the Hole 1160B glass data. These fresh whole-rock and glass samples have high MgO contents that cluster at the least-fractionated end of the Zone A compositional range.
We see no compositional traits in Site 1160 basalts that would indicate significant temporal variability in ridge crest processes in this region. The similarities of Site 1160 glasses and unaltered massive whole rocks to the compositions and trends in Segments A2 and A3 are remarkable, given that these samples are from the flank of a large seamount. The high-MgO-content samples from Hole 1160B are as primitive as high-MgO Zone-A propagating rift tip lavas, but they do not have the enriched Na2O, Ba, and Sr contents or low CaO/Al2O3 of the latter. The lower Na2O, Ba, and Sr and high CaO/Al2O3 suggest a higher degree of melting, and the majority of Zone A-type lavas could have parental magmas with similar composition.
Basalts from Site 1160 are clearly derived from a Pacific-type mantle source. The glasses have Zr/Ba ratios >23 and low Ba contents, placing them well within the Pacific-type field defined by dredged Zone A glasses (Fig. F24A). The primitive Site 1160 lavas also overlap the high-MgO end of the Na2O/TiO2 vs. MgO trend shown by Segment A2 and A3 lavas, consistent with a Pacific-type mantle source (Fig. F24B).