CONCLUSIONBy any measure, Leg 187 was highly successful. We recovered samples from 23 holes at 13 sites with an average water depth of almost 5 km. In the process, a record 140 km of drill string was passed through the drill floor. In part, this success relates to the unusually mild weather that we encountered. In larger part, it relates to the extraordinary dedication of the JOIDES Resolution and ODP crews. Our reactive drilling strategy worked well. After very limited teething troubles with sample preparation and running of the new ICP-AES instrument, we were able to obtain the geochemical data that we needed within 12 hr of "core on deck" for critical samples.
Our principal objective was to determine the configuration of the Indian-Pacific mantle isotopic boundary beneath 10- to 30-Ma seafloor north of the AAD. Using the shipboard Ba and Zr data, we have tentatively outlined the boundary, but definitive answers (and the fulfillment of our objective) will require onshore isotopic analyses. The discrete isotopic boundary that has migrated across Segment B5 of the AAD over the last 3-4 m.y. is probably not a permanent feature, but we believe that we have identified two similar migration events in western Zone A at about 19 and 22 Ma. Over the long term, the Indian-Pacific boundary is probably represented by a transitional region that coincides with the regional depth anomaly. This region appears to be dominated by Indian-type mantle with common, discrete occurrences of TP- and even Pacific-type mantle. In western Zone A, along the eastern boundary of the depth anomaly, Pacific- and Indian-type mantle domains appear to alternate on a time and spatial scale that is comparable to the present-day migration in Segment B5. It is unclear whether this alternation represents short-lived incursions of (1) Indian mantle beneath Zone A or (2) Pacific mantle into the depth anomaly. By analogy with the present day and because Pacific mantle is most commonly associated with robust magmatism, the latter seems more likely.
Samples from Leg 187 will undergo extensive geochemical and isotopic analysis onshore. Initially, these studies will serve to confirm the mantle provenance of the lavas that we sampled and refine or understanding of the location and nature of the mantle isotopic boundary. Further study will allow us to better understand changes in mantle composition, temperature, dynamics, melting conditions, and magma evolution. Comparative studies with other regions worldwide will enable us to improve our understanding of the nature of both mantle domains and of the origin and evolution of the depth anomaly and the AAD.
Although results will not be available for some time, the microbiology objectives of the leg have been well served. Between two and six samples were taken at each site. The samples include a range of lithologies, including pillow rims, pillow interiors, massive flows, and various breccias. The samples also vary in the extent and type of alteration. Samples will be cultured in a variety of media, at low and high pressures. Electron microscope studies will attempt to characterize living and fossil microbes within samples.
References | Table of Contents