Leg 187 undertook to trace the boundary between Indian and Pacific, ocean-scale mantle provinces across 10- to 30-Ma seafloor of the Southeast Indian Ocean between Australia and Antarctica. The boundary is sharply defined on young seafloor within the Australian Antarctic Discordance (AAD), where it is 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 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 were able to assign each of the 23 holes drilled at 13 sites to an Indian, Pacific, or Transitional-Pacific upper mantle type. At three sites we encountered lavas from two of these three mantle types, indicating mixed or transitional mantle sources.
From these shipboard identifications of mantle domain, three fundamental observations can be made:
These observations lead to the following tentative conclusions which require careful testing as isotopic data become available. A discrete mantle boundary comparable to the present-day boundary in the AAD cannot be mapped through the entire 14- to 28-Ma time interval encompassed by Leg 187 sites, although comparable boundaries have existed for relatively short, discrete time intervals. We conclude that, in the long term, the eastern limit of the Indian mantle province corresponds closely to the eastern edge of the residual depth anomaly. Its locus must lie close to the -500-m residual depth contour that tracks south to connect with the known location of the Indian/Pacific boundary on younger seafloor of the AAD. West of this boundary, sporadic occurrences of Transitional-Pacific-type and even Pacific-type mantle are interspersed with the predominant Indian-type mantle. The western limit of Pacific or Transitional-Pacific mantle is not closely defined by our data, but it is most likely associated with the western boundary of the depth anomaly. The alternation of Indian-type sites with Pacific- and Transitional-Pacific-type sites in western Zone A on time scales of a few million years can be interpreted in terms of discrete incursions, either of Indian mantle beneath Zone A or, perhaps more likely, of Pacific mantle into the dominantly Indian region of the depth anomaly.
Samples from Leg 187 will undergo extensive elemental and isotopic analysis 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 involved with biodegradation of basaltic glass and their habitats.
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