At Site 1161, basaltic samples were recovered from two holes, 1161A and 1161B. The site is located in Zone A, 43 km south of Site 1158, on ~19-Ma crust that we interpret to be within the transferred lithosphere domain of an extinct westward-propagating rift. This site was selected to test whether Indian- or Pacific-type mantle was present during the waning stages of magmatism that resulted from abandonment by rift propagation.
In Hole 1161A we encountered a single lithologic unit of basaltic rubble with intervals of basaltic breccia. From Hole 1161B we recovered similar material that was likewise assigned to a single lithologic unit. One glass sample was removed from a breccia clast in Hole 1161A and analyzed for major and trace elements by ICP-AES, and one aphyric basalt whole-rock sample from Hole 1161A was analyzed for major and trace elements by XRF (Table T4). No samples from Hole 1161B were analyzed.
The Site 1161A glass has a relatively unfractionated composition (i.e., ~8.0 wt% MgO) with characteristics typical of Zone A lavas (Figs. F29, F30). The Hole 1161A whole rock contains less MgO than the glass and exhibits the same characteristics that suggest loss of Mg by alteration in whole-rock samples from every Leg 187 site. This conclusion is supported by thin-section examination of whole-rock Sample 187-1161A-4R-1, 105-109 cm (Fig. F7), which shows high degrees of alteration; not unexpectedly, a relatively high loss on ignition (1.44 wt%) was measured on this sample.
The glass is compositionally similar to unfractionated lavas dredged from Southeast Indian Ridge (SEIR) Segments A2 and A3. The high Ba and Sr and low CaO/Al2O3 ratios suggest a low degree of melting. The Hole 1161A glass composition plots near lavas recovered from the rift tip region of segments undergoing propagation (e.g., Ba, Y, and CaO/Al2O3), suggesting, as inferred from the seafloor terrain and magnetic anomaly offsets (Vogt et al., 1984), an association with the propagating rift tip tectonic environment.
The Zr/Ba systematics of Hole 1161A basaltic glass indicate an Indian-type mantle source, as the sample plots within the Indian-type mid-ocean-ridge basalt field in Figure F31. Even though the glass sample plots on the Pacific side of the Na2O/TiO2 vs. MgO diagram, the data are nonetheless consistent with an Indian mantle provenance since the glass overlaps the compositions of transitional Segment B5 axial lavas. Furthermore, Leg 187 Na2O/TiO2 vs. MgO relationships are consistently offset to lower Na2O/TiO2 values relative to zero-age SEIR basalt glass (see "Sodium and Titanium" in the "Leg Summary" chapter). Hole 1161A glass has high Na2O/TiO2 relative to other Leg 187 compositions and is therefore Indian type in composition. From these data, Site 1161 lavas appear to document another incursion of Indian-type mantle east of the AAD/Zone A transform boundary.