During Leg 187, 23 holes were drilled at 13 sites (Sites 1152-1164) to recover mid-ocean-ridge basalt (MORB) in the Southeast Indian Ocean between Australia and Antarctica (Fig. F1). The 13 drill sites are located on 14- to 28-Ma seafloor within the area of 126°-135°E and 41°-46°S, which is ~400 km north of the SEIR and on the eastern boundary of the Australian Antarctic Discordance (AAD). The AAD is a segment of the SEIR between 120° and 128°E that is characterized by anomalously deep bathymetry, low gravity, and rugged seafloor topography (Weissel and Hayes, 1971; Hayes and Conolly, 1972). By using contrasts in these physical and morphologic properties, Weissel and Hayes (1971) classified the SEIR into three longitudinal zones, A, B, and C, from east to west; Zone B incorporates the AAD. The eastern boundary of the AAD is an approximately north-south-tracing fracture zone that separates Zone A, with smooth off-axis topography to the east, from Zone B, with rough off-axis topography to the west. Geochemical analyses of dredge samples from the SEIR reveal that the eastern border of AAD is indeed a remarkable isotopic boundary between "Pacific-type" and "Indian-type" upper-mantle provinces (e.g., Klein et al., 1988; Pyle et al., 1992; Christie et al., 1998). Aeromagnetic data collected in this area show that the magnetic anomalies are of large amplitude east of the AAD (Zone A), low amplitude within the AAD (Zone B), and normal amplitude west of the AAD (Zone C) (e.g., Anderson et al., 1980; Vogt et al., 1984). It has been suggested that the high magnetic anomaly amplitudes found along the easternmost segment of the Southeast Indian Ridge (Zone A) are caused by the high NRMs of basalts, which are in turn caused by an increase of titanomagnetite concentration in the Zone A basalts (Anderson et al., 1980).
The purpose of Leg 187 was to locate the boundary between Indian and Pacific mantle provinces in the off-axis area. On the basis of the geochemical data obtained from the recovered MORB samples, each of the 23 holes was assigned to the Indian, Pacific, or Transitional-Pacific upper mantle type (Shipboard Scientific Party, 2001) (Fig. F1). Detailed descriptions including objectives, shipboard analyses, and achievements of Leg 187 can be found in the Leg 187 Initial Reports volume (Christie, Pedersen, Miller, et al., 2001).
A total of 94 slab samples, each measuring ~5 cm (length) x 4 cm (width) x 1 cm (height), were collected from the recovered cores for the present study. Most of the samples are pillow basalts, either aphyric or phyric, with sparsely to moderately distributed phenocrysts of plagioclase, clinopyroxene, or olivine. Plagioclase is the most abundant phenocryst in the phyric basalts. A few samples are basalt rubble or basaltic breccia. Three massive basalt samples from Hole 1160B (Samples 187-1160B-4R-1, 72-77 cm [#63], 7R-1, 59-62 cm [#65], and 9R-1, 111-116 cm [#67]) are intercalated with pillow basalt samples. A metadiabase sample from Hole 1162A (Sample 187-1162A-5R-1, 33-37 cm [#74]) shows greenschist facies metamorphism. Most samples have been subjected to alteration and are lighter bluish gray or brownish rather than dark gray or black, like fresh basalts. The pillow basalt samples show curved or convex outlines and generally have a light-colored core embraced by a darker outer zone (~0.5- to 1-cm-wide rim zone) on the cutting sections. For detailed studies, we further sampled the rim zone and core zone from many basalt specimens; a total of 114 specimens were obtained. Samples used for magnetic measurements were taken ~0.5 cm from pillow margins for the rim zone samples and ~1-3 cm from pillow margins for the core zone samples. The identification numbers and the attributed mantle provinces (Shipboard Scientific Party, 2001, table T2) of all the collected samples are shown in Table T1.