During and after drilling, the primary objective will be to locate the Indian/Pacific isotopic boundary and determine its configuration out to at least 30 Ma. From this information, we will infer the geometry and dynamics of these two mantle reservoirs and their boundary. In addition, there are number of subsidiary objectives. These can be divided into geochemical, geophysical, and microbiological categories for discussion, but we emphasize that these are strongly interrelated and that we will be seeking to thoroughly integrate all the results.
Geochemical analysis provides the principal tool for locating the isotopic boundary, even if the boundary proves to have a morpho-tectonic expression, as observed within the AAD (Christie et al., 1998). However, the geochemical objectives of Leg 187 extend well beyond this simple task to the problem of defining and understanding the nature and origin of the distinct Pacific and Indian geochemical signatures. Some specific questions that we will address are
What is the connection between the isotopic boundary and the known "Indian" samples from the DSDP sites near Tasmania and from the Lanyon et al. (1995) dredges northeast of the AAD? If a long-term migrating boundary is identified in Zone A, then these sites might be interpreted as representing Indian mantle that was present throughout the region before the influx of Pacific mantle began. If the boundary is shown not to have migrated across Zone A, then one might conclude that these sites are more influenced by their proximity to the Australian continent than to the Indian Ocean per se. The importance of these questions extends beyond the immediate region. They are relevant to our understanding of the origin of the isotopic signature of Indian Ocean mantle, and they will prove particularly important in considering the origin of recently identified "Indian" samples from western Pacific backarcs (Hergt and Hawkesworth, 1994) and from the Chile Rise in the eastern Pacific (Klein and Karsten, 1995; Karsten et al., 1996; Sherman et al., 1997).
The shape of the isotopic boundary can potentially contribute to our understanding of the origin of the AAD. Can it, for example, be traced back to some particular feature of the Australian and Antarctic continents, such as the eastern boundary of the Australian craton?
A secondary objective of the program will be to study the long-term petrologic history of the AAD. Have there been changes in depth and/or extent of melting through time? Can we infer temporal changes in mantle temperature beneath the AAD? Has the underlying cold mantle become warmer or colder through time? Have the petrological contrasts between Zone A and AAD lavas persisted through time?
Geophysical objectives will primarily focus on understanding the mantle dynamics of the region and their relation to the anomalous processes within the AAD. As part of the scientific effort associated with the 1996 cruise, West et al. (1997) and West and Christie (1997) have developed a suite of three-dimensional mantle-flow models specifically tailored to the tectonic history and segmentation characteristic of the eastern SEIR. In addition to integrating cooler-than-normal mantle temperatures beneath the AAD with along-axis asthenospheric flow toward the AAD, these models have a number of important features significant to this proposal:
Lateral mantle flow appears to be an inevitable consequence of the separation of the continents and mantle temperature gradients. During initial rifting of the continents, simple divergence is the sole force inducing flow, but as the continents separate, a mantle temperature gradient is required to maintain mantle flow consistent with known limits on the boundary configuration.
Along-axis asthenospheric flow is confined to a relatively narrow low-viscosity zone beneath the ridge axis (West et al., 1997), and the geometry of the overlying spreading system plays a significant role in channeling the along-axis flow where transforms are included in these models. Confining temperature-gradient-driven flow within the low-viscosity zone also results in a temperature inversion in the subaxial mantle that can significantly modify Na8.0 and Fe8.0 depth correlations.
Depth gradients in mantle viscosity inevitably lead to a mantle front that slopes in the direction of flow (West et al., 1997). This can lead to a decoupling of flow-related features that are controlled at different mantle depths. Thus, the isotopic boundary, as mapped at the seafloor, may differ in location and in geometry from a flow-driven propagating rift or from a chain of seamounts that form off-axis. Although no such chains are known east of the AAD, several occur to the west. And, although each of these surface features is a manifestation of mantle migration, none of them necessarily mimics in plan view the actual boundary between the two upper mantle provinces.
At the present state of development, modeling clearly demonstrates that hypothesized long-term mantle migration is consistent with, perhaps even favored by, our current understanding of the Pacific/Indian boundary (West et al., 1997, West and Christie, 1997). If the drilling proposed here allows us to identify the off-axis position of the isotope boundary, these models can be more precisely refined. Increasing refinement of the model will lead to stronger constraints on mantle dynamics of the region, including interactions among physical properties such as mantle temperature gradients, viscosity, flow velocities, and flow patterns. Also planned for continuing work are refinements in the resolution of some of the models. At present, the models are being developed to resolve local segment-scale details of flow, particularly the question of whether and why flow is stopped or impeded by major transform offsets as we have inferred from geochemical observations (West and Christie, 1997; Christie et al., 1998). Finally, perturbations in the temperature profile at depth can potentially influence the systematics of mantle melting, and the AAD flow models can be used to predict geochemical features, such as a departure of normalized sodium variations (Na8.0; Klein and Langmuir 1987) from predicted trends.
Subsurface Biosphere Objectives
The subsurface biosphere objectives are to isolate and study microbes present in the volcanic sequence of the upper oceanic crust and to study the diagenetic effects of this microbial activity. The specific microbiological objectives include:
Quantify microbes in the samples using fluorescent-labeled oligonucleotide probes.
Identify microbes responsible for biodegradation of basalt using molecular biological methods.
Isolate microbes participating in the biodegradation process.
The microbial diagenetic objectives will focus on how the microbiologic degradation of basalt progresses with time in the upper oceanic crust. Another objective of the cruise will be to further develop methods and procedures that will help to monitor and minimize microbiologic contamination of the core. Monitoring of drilling-induced contamination will be performed by adding 0.5-µ microspheres and per-fluro-methyl-cyclo hexane (PFT) to the drilling water when intervals to be sampled for microbiologic studies are cored.
To 187 Drilling Strategy
To 187 Table of Contents