HR: 11:50h
AN: T32A-07    [Abstracts]
TI: Sensitivity of MORB Chemistry and Emplacement to Accretionary Tectonics at Ultraslow-Spreading Rates on the Southwest Indian Ridge
AU: * Standish, J J
EM: jstandish@whoi.edu
AF: Woods Hole Oceanographic Institution, 266 Woods Hole Rd., Woods Hole, MA 02540 United States
AU: Dick, H J
EM: hdick@whoi.edu
AF: Woods Hole Oceanographic Institution, 266 Woods Hole Rd., Woods Hole, MA 02540 United States
AU: Sims, K W
EM: ksims@whoi.edu
AF: Woods Hole Oceanographic Institution, 266 Woods Hole Rd., Woods Hole, MA 02540 United States
AB: The 600 km orthogonal supersegment between 16°-25° E on the Southwest Indian Ridge displays typical slow-spreading ridge morphology and segmentation, and dominantly erupts tholeiitic N-MORB. To the west, between 9° -16° E, the 400 km oblique supersegment has strikingly different morphology and segmentation. Highly enriched E-MORBs erupt at robust, orthogonally spreading magmatic segments, compared to both depleted N-MORB and moderately enriched E-MORB at directly adjacent obliquely spreading amagmatic segments. Major element, trace element, and isotopic compositions are strongly correlated with observed and estimated ridge characteristics such as, ridge segmentation, inferred upwelling rate (spreading geometry), crustal thickness, and lithologic distribution. These correlations suggest that MORB chemistry is not solely a function of "source", but is highly sensitive to "process", especially at ultraslow-spreading rates (< 20 mm/yr). Along-axis variations in inferred lithospheric thickness reflect segment-scale differences in ultraslow-spreading accretionary tectonics, and likely result in melt focusing and subsequently disproportionate volumes of enriched melts. Pressures representing initiation of 3-phase partial crystallization are significantly shallower beneath the orthogonal magmatic segments, suggesting less conductive cooling of the lithosphere from above. Numerical modeling presented at this meeting [Barry et al., 2005] supports this position, illustrating significant temperature differences at depth for an orthogonal spreading versus highly oblique spreading ridge segment. In addition to the variability along-axis, measurements of U-Th-Ra disequilibrium in 12 basalt glasses provide direct "age-dating" evidence of anomalously young lavas erupting throughout the rift valley and on the rift valley walls. This distribution of young lavas throughout the study area clearly indicates that volcanism and magmatic accretion occurring at significant distances from the axis of rifting (>10 km) play a crucial role in the formation of new ocean crust at ultraslow-spreading environments. In addition, a lava dredged from the rift valley wall 1000 meters above the rift axis contains excess ²²⁶Ra, making it younger than 8 Kyr. That this lava was emplaced well into the rift-mountains indicates that the most likely conduit for eruption is along faults. The significant temporal and spatial variability observed in this complex tectonic environment indicates, 1) that little is known about the controls and mechanisms involved in off-axis eruptions at slow spreading ridges, and 2) that the interplay between tectonic and magmatic accretion at ultraslow-spreading ridges is much different than at faster spreading ridges.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3035 Midocean ridge processes
DE: 3600 MINERALOGY AND PETROLOGY
DE: 8000 STRUCTURAL GEOLOGY
DE: 8400 VOLCANOLOGY
SC: Tectonophysics [T]
MN: Fall Meeting 2005