Two holes at Site 1162 penetrated ~18-Ma crust, 37 km south of the dying rift/transfer zone sampled at Site 1161 in Zone A. In both Holes 1162A and 1162B, we encountered dolomite-cemented breccia containing highly altered basalt clasts along with several other lithologies. Alteration was so pervasive that only one glass from Hole 1162B was available for major and trace element analysis by ICP-AES (Table T3). Two samples of the carbonate cement were also analyzed by ICP-AES to establish whether this authigenic material is high-Mg calcite or dolomite.
The single analyzed glass sample from Site 1162 contains ~10% spherulites. Its composition is typical for Zone A, Pacific-type basalt. It contains ~7.7 wt% MgO; Y and Ba are slightly high and Na2O and CaO/Al2O3 are relatively low, but, without additional data, this sample is not considered to be distinct from normal Zone A compositions.
Our limited Site 1162 data show no convincing evidence for temporal change beneath the Zone A spreading axis. Rather, the Site 1162 basalt glass documents a prior period of seafloor accretion similar to that occurring today (Figs. F26, F27).
The Zr/Ba vs. Ba (Fig. F28A) and Na2O/TiO2 vs. MgO (Fig. F28B) systematics of Hole 1162B glass indicate a Pacific-type mantle source. The glass composition lies slightly outside the tip of the Pacific-type mid-ocean-ridge basalt field with lower Zr/Ba than 0- to 7-Ma Pacific-type lavas and Ba at the high end of the 0- to 7-Ma range. Several samples from Leg 187 sites have this ambiguous trait, plotting outside both Pacific- and Indian-type fields on this diagram in a field that we have previously termed Transitional Pacific (see "Barium and Zirconium" in "Mantle Domain Recognition" in "Geochemistry" in the "Leg Summary" chapter). Nonetheless, we believe this sample to be unequivocally Pacific type because it is conceivably on a mixing trend toward the propagating rift lavas and/or has formed by slightly lower degrees of melting. The high Ba content could also result from the necessary inclusion of spherulitic material within the analyzed sample. Slightly lower Ba (i.e., glass without spherulites) would move this data point into the Pacific-type field. The Na2O/TiO2 diagram clearly indicates a Pacific-type source, well within the range of Segments A2 and A3 glasses.