Lavas erupted at Indian Ocean spreading centers are isotopically distinct from those of the Pacific Ocean, reflecting a fundamental difference in the composition of the underlying upper mantle. Along the Southeast Indian Ridge (SEIR), the Indian Ocean and Pacific Ocean mantle isotopic provinces are separated by a uniquely sharp boundary (Klein et al., 1988). This boundary has been located to within 25 km along the spreading axis of the SEIR within the Australian Antarctic Discordance (AAD) (Pyle et al., 1992; Christie et al., 1998). Subsequent off-axis dredge sampling has shown that Pacific mantle has migrated rapidly westward during at least the last 4 m.y. The principal objective of Leg 187 was to delineate this boundary farther off axis, allowing us to infer its history over the last 30 m.y.
The AAD (Fig. F1) is a unique region, encompassing one of the deepest (4-5 km) regions of the global mid-oceanic spreading system. Its anomalous depth reflects the presence of unusually cold underlying mantle and, consequently, of thin crust (Marks et al., 1990; Forsyth et al., 1987; Sempéré et al., 1991; West et al., 1994). Despite a uniform, intermediate spreading rate, the SEIR undergoes an abrupt morphologic change across the eastern boundary of the AAD (Palmer et al., 1993). The region east of the AAD, known as Zone A, is characterized by an axial ridge with relatively smooth off-axis topography (characteristics usually associated with fast-spreading centers), whereas the AAD, also known as Zone B, is characterized by deep axial valleys with rough off-axis topography (characteristics usually associated with slow-spreading centers). Other anomalous characteristics of the AAD include a pattern of relatively short axial segments separated by long transforms with alternating offset directions, unusually thin oceanic crust, chaotic seafloor terrain dominated by listric extensional faulting rather than magmatism, high upper mantle seismic wave velocities, and an intermittent asymmetric spreading history (Weissel and Hayes, 1971, 1974; Forsyth et al., 1987; Marks et al., 1990; Sempéré et al., 1991; Palmer et al., 1993; West et al., 1994; West, 1997; Christie et al., 1998). The morphological and geophysical contrasts across the eastern boundary of the AAD are paralleled by distinct contrasts in the nature and variability of basaltic lava compositions (Pyle, 1994), reflecting fundamental differences in magma supply because of strong contrasts in the thermal regime of the spreading center.
The AAD appears to be the locus of converging asthenospheric mantle flows. This is suggested by multiple episodes of ridge propagation from both east and west toward the AAD (Vogt et al., 1984; Cochran et al., 1997; Sempéré et al., 1997; Sylvander, 1998; West et al., 1999) and by recent numerical model studies suggesting that significant convergent subaxial mantle flow is an inevitable consequence of gradients in axial depth and upper mantle temperature around the AAD (West and Christie, 1997; West et al., 1999).
Within Segment B5, the easternmost AAD segment, a distinct discontinuity in the Sr, Nd, and Pb isotopic signatures of axial lavas marks the boundary between Indian Ocean and Pacific Ocean mantle provinces (Klein et al., 1988; Pyle et al., 1990, 1992). The boundary is remarkably sharp, although lavas with transitional characteristics occur within 50-100 km of the boundary (Fig. F2). Along the axis of the SEIR, the boundary is located within 20-30 km of the ~126°E transform, the western boundary of Segment B5. The boundary has migrated westward across Segment B5 during the last 3-4 m.y. (Pyle et al., 1990, 1992; Christie et al., 1998).
Although the recent history of this uniquely sharp boundary between ocean basin-scale upper mantle isotopic domains has been reasonably well defined by mapping and conventional dredge sampling, its long-term relationship to the remarkable geophysical, morphological, and petrological features of the AAD had not been determined prior to Leg 187. The AAD is a long-lived major tectonic feature (Veevers, 1982). Its defining characteristic is its unusually deep bathymetry, which stretches across the ocean floor from the Australian to the Antarctic continental margins. The trend of this depth anomaly forms a shallow, west-pointing V-shape, cutting across the major fracture zones that currently define the eastern AAD segments (Figs. F1, F3). This V-shape implies that the depth anomaly has migrated westward at a long-term rate of ~15 mm/yr (Marks et al., 1991), which is much slower than either the recent migration rate of the isotopic boundary or the majority of the known propagating rifts along the SEIR (i.e., 25-30 mm/yr). Further, the relatively rapid northward absolute motion of the SEIR requires that the mantle "source" of the depth anomaly be linear and oriented approximately north-south. Recently, Gurnis et al. (1998) have suggested that the source of this cold linear anomaly lies in a band of subducted material that accumulated at the 660-km mantle discontinuity beneath a long-lived western Pacific subduction zone before ~100 Ma.
Prior to Leg 187, the locus and history of the isotopic boundary before ~4 Ma were almost completely unknown. Possible long-term relationships between the isotopic boundary and the morphologically defined AAD could be divided into two distinct classes (schematically illustrated in Fig. F3). Either the recent (0-4 Ma) isotopic boundary migration simply reflects a localized (~100 km) perturbation of a geochemical feature that has been associated with the eastern boundary of the AAD since the basin opened, or the migration is a long-lived phenomenon that only recently brought Pacific mantle beneath the AAD. In the first case, the boundary could be related either to the depth anomaly or to the eastern bounding transform but not, in the long term, to both. The second possibility, that the isotopic boundary only recently arrived beneath the AAD, was first proposed by Pyle et al. (1992), building on a suggestion by Alvarez (1982, 1990), that Pacific mantle began migrating westward when the South Tasman Rise first separated from Antarctica at 40-50 Ma. Limited geochemical support for this hypothesis came from the Indian and transitional isotopic signatures of altered ~38- and ~45-Ma basalts dredged to the north and east of the AAD by Lanyon et al. (1995) and from 60- to 69-Ma Deep Sea Drilling Project (DSDP) basalts that were drilled close to Tasmania (Pyle et al., 1995). Unfortunately, neither sample set is unequivocal. The dredged samples are from sites within the residual depth anomaly and therefore support two of the three possible configurations. The DSDP samples lie far to the east of the depth anomaly but very close to the continental margin. Their apparent Indian affinity is suspect because of the possibility that their mantle source has been contaminated by nearby subcontinental lithosphere. Finally, the oldest (~7 Ma) off-axis dredge sample from Zone A is of Pacific type (Christie et al., 1998), constraining possible loci of the Indian/Pacific boundary to intersect the eastern AAD transform north of approximately 47°45´S.
The principal objective of Leg 187 was to locate the Indian/Pacific mantle boundary through its expression in the geochemistry of mid-ocean-ridge basalt (MORB) lavas from 8- to 28-Ma seafloor to the north of the AAD. The clearest definition of this boundary can be seen in the Pb isotopic ratios, but it is clear in Sr and apparent in Nd isotopic ratios as well (Fig. F2). Although there are also clear overall differences in the major and trace element compositions between the lava populations of the two provinces, there are few elements that can reliably determine the mantle source affinity of individual lavas. Two elemental plots that can assign >90% of our current collection of young lavas are Zr/Ba vs. Ba and Na2O/TiO2 vs. MgO (see "Mantle Domain Recognition" in "Geochemistry"). These elements were measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) aboard the JOIDES Resolution throughout the leg. The Ba and Zr systematics appear to enable us to discriminate between basalts of Pacific affinity and their Indian and transitional counterparts, but Na2O/TiO2 proved to be less useful for this purpose because the boundary between Indian and Pacific types appears to be at lower Na2O/TiO2 values for the Leg 187 samples (see "Geochemistry" and "Summary" in the site chapters).
In addition to its interest as a mantle dynamics phenomenon, an improved understanding of the Indian/Pacific mantle boundary is important for a broader general understanding of the oceanic mantle. In investigating the nature and origins of the AAD, the isotopic boundary and the mantle provinces that it separates, we are also investigating the importance of variations in geochemistry, isotopic composition, temperature, and other physical characteristics of the oceanic upper mantle in a setting where spreading rate is constant. Improved knowledge of the distribution of these chemical and physical characteristics in space and time will lead to a better understanding of the dynamics of the oceanic mantle and of its interaction with the magmatic processes of the mid-ocean-ridge system.
Recent findings have extended the known biosphere to include microbial life in deep subsurface volcanic regions of the ocean floor, and much attention has been focused on the nature of microbes that live on, and contribute to the alteration of, basaltic glass in oceanic lavas (Thorseth et al., 1995; Furnes et al., 1996; Fisk et al., 1998; Torsvik et al., 1998). The first evidence for this phenomenon was from textures in basaltic glass from Iceland (Thorseth et al., 1992). Similar textures were later found in basaltic glass from Ocean Drilling Program (ODP) Hole 896A at the Costa Rica Rift, and the microbial contribution to the alteration history was supported by the presence of DNA along the assumed biogenic alteration fronts (Thorseth et al., 1995; Furnes et al., 1996; Giovannoni et al., 1996). Microbes have recently been documented to inhabit internal fracture surfaces of basaltic glasses that were sampled specifically for microbiological studies during MIR submersible dives to the Knipovich Ridge (Thorseth et al., 1999). Dissolution textures directly beneath and manganese and iron precipitates adjacent to many individual microbes suggest that microbial activity plays an active role in the low-temperature alteration of ocean-floor basalts.
Rock and sediment samples collected during Leg 187 for microbial culturing, DNA analysis, and electron microscopic study range in age from 14 to 28 Ma, providing an opportunity to study temporal changes in microbial alteration.
In order to fulfill the primary objective of the leg (the location and characterization of the Indian/Pacific mantle isotopic boundary), our drilling strategy was focused on maximizing the number of sites rather than recovery or penetration at any one site. Although our goal for each site was ~50 m penetration into basaltic basement, this was achieved only at five sites. At most sites, drilling conditions were poor as we penetrated broken pillow flows, talus, or other rubble, and many holes were abandoned when they became unstable.
Much of the region is devoid of measurable sediment cover. Most sites were located on localized sediment pockets detected by single-channel seismic (SCS) imaging during the R/V Melville site survey cruises Boomerang 5 and Sojourner 5. Three additional sites were surveyed during the transit from Site 1158 to 1159, and two of these were subsequently drilled as Sites 1161 and 1162. Based on the seismic records, all sites were ranked on a scale of 1 to 3, depending on the clarity with which they were imaged and the width and depth of sediment cover. At highly ranked sites, sediment thickness predictions from site survey data proved to be reasonably accurate, so, whenever possible, we chose higher ranked sites. At Site 1152, only a few meters of soft sediment were encountered, and spud-in conditions were little better than those for bare rock. Two other low-ranked sites, AAD-2b and -3a (Sites 1159 and 1163), proved to have more than adequate sediment cover and were drilled successfully.
As the JOIDES Resolution approached each site, we ran a short survey using the 3.5-kHz precision depth recorder and, in all but a few cases, the SCS system to confirm the location and suitability of the proposed site. Whenever possible, these surveys were run obliquely to the original north-south survey lines, but in some cases weather conditions dictated that we run close to the original course. For several smaller sites we chose to run north-south lines to minimize out-of-plane reflections from the dominantly east-west trending topography.
Because sediments across the region were expected to be reworked and possibly winnowed and because basement penetration at as many sites as possible was the primary objective of this leg, we chose in most cases to wash through the sediment section. Wash cores containing significant sediment intervals were recovered at 10 sites. Site locations and data are summarized in Table T1.
During Leg 187 we used a responsive drilling strategy. At key points during the leg, subsequent sites were chosen from among the 19 preapproved sites according to the results of onboard geochemical analysis of the recovered basalts. Details of the analytical program and its use to distinguish Indian from Pacific mantle signatures can be found in "Geochemistry" and in the individual site summaries.