ABSTRACTLeg 187 undertook to trace the boundary between Indian and Pacific, ocean-scale mantle provinces across 10- to 30-m.y.-old seafloor of the southeast Indian Ocean between Australia and Antarctica. The boundary has been located on young seafloor of the Australian Antarctic Discordance (AAD), where it is sharply defined and migrating to the west at ~40 mm/yr.
The leg was built around a responsive drilling strategy in which real time shipboard geochemical analyses from one site were frequently used to guide the selection of subsequent sites from a slate of preapproved targets. This strategy proved highly effective, allowing us to maximize our time on site and to focus on sites that could potentially yield the best definition of the boundary configuration. Using Ba and Zr contents of basalt glasses referenced to our database of younger (0-7 Ma) lavas from the AAD and Zone A (east of the AAD), we assigned each of the 23 holes drilled at 13 sites to an Indian, Pacific, or Transitional-Pacific (TP) mantle domain. Three sites encountered lavas from two of the three domains.
From these shipboard identifications of mantle domain, three
fundamental observations can be made:
Samples from Leg 187 will undergo extensive geochemical and isotopic analyses to refine the definition of the isotopic boundary and to improve our understanding of the nature and origin of the AAD, the mantle boundary, and the distinctive Indian Ocean mantle province. In addition, a battery of samples collected as quickly as possible under conditions that were as sterile as possible were placed in a variety of media in order to characterize the microbial population of the deep seafloor. Complementary electron microscope studies will seek to characterize fossil and living microbes within samples and mechanisms involved with biodegradation of basaltic glass.
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