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INTRODUCTION (continued)

Locating the Isotopic Boundary
The principal objective of Leg 187 was to locate the Indian-Pacific mantle isotopic 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 Nd and Sr isotopic ratios as well (Fig. 2). 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 be relied on to accurately determine the affinity of most individual lavas. Two elemental plots that can reliably assign >90% of our current collection of young lavas are Ba vs. Zr/Ba and MgO vs. Na2O/TiO2. These elements were reliably measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) aboard the JOIDES Resolution throughout the leg. Ba and Zr appear to have enabled us to discriminate between basalts of Pacific affinity and their Indian and transitional counterparts, but Na2O/TiO2 did not prove useful for this purpose.

Beyond the Isotopic Boundary
Defining the off-axis configuration and migration history of the Indian/Pacific mantle isotopic boundary is not simply an end in itself. In addition to its interest as a mantle dynamics phenomenon, an improved understanding of this 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 other tectonic variables are relatively 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.

Subsurface Biosphere
Recent findings have extended the 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 glass that specifically were sampled 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.

Sterile rock and sediment samples collected during Leg 187 for microbial culturing, DNA analysis, and electron microscopic study range in age from 14 to 30 Ma, providing an opportunity to study temporal changes in microbial alteration.

Drilling Strategy
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 and talus or other rubble; 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 imaging during the Melville's site survey cruises Boomerang 5 (BMRG05) and Sojourn 5 (SJRN05). Three additional sites were surveyed during the transit from Site 1158 to 1159, and two 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 (PDR) and, in all but a few cases, the single-channel seismic system to confirm the location and suitability of the proposed site. When 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.

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 analyses of the recovered basalts.

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