HR: 16:15h
AN: V42H-02    [PDF]
TI: Field Studies of Mantle Melting at Ultra-slow Spreading Ridges
AU: * Dick, H J
EM: hdick@whoi.edu
AF: Dept. of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Standish, J
EM: jstandish@whoi.edu
AF: Dept. of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Michael, P J
EM: pjm@utulsa.edu
AF: University of Tulsa, 600 College Avenue, Tulsa, OK 74104 United States
AU: Lin, J
EM: jlin@whoi.edu
AF: Dept. of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Schouten, H
EM: hschouten@whoi.edu
AF: Dept. of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AB: Field studies of ultraslow spreading ridges put new constraints on the generation of abyssal basalts and the nature of the mantle. At the transition from slow to ultra-slow spreading there is a sharp discontinuity with an abrupt transition from continuous to discontinuous volcanism along the ridge. On the ultra-slow side of the discontinuities, the ridges consist of linked amagmatic and magmatic accretionary segments, and transform faults disappear despite often extreme oblique spreading. The amagmatic accretionary segments expose abundant mantle peridotites and exhibit sparse volcanism. There is no correlation between the volume of magmatism and further decreases in spreading rate across the discontinuities, with magmatism often locally increasing again with decreasing spreading rate. Magmas dredged across these discontinuities share similar major element characteristics, with an increase in Na$_{8}$ and decrease in Fe$_{8}$. Trace element compositions across these discontinuities, however, show different patterns of variation, though generally the basalts are trace element and isotopically enriched compared to typical MORB. One interpretation of this pattern is that there is bimodal melting behavior: suggesting a multi-component mantle source with an early melting vein or vein-like components. This is consistent with the hypothesis that the composition of a heterogeneous mantle many thousands of kilometers apart can be significantly different. While all the discontinuities lie at an effective spreading rate corrected for ridge geometry near 12 mm/yr full-rate, at the Gakkel Ridge the discontinuity matches a prominent bathymetric discontinuity associated with early rifting and the Morrice Jessup Rise and Jermak Plateau. The discontinuity at $16\deg$E on the SWIR, on the other hand, is migrating rapidly eastward along the ridge - suggesting that it is a plate driven discontinuity. In the third case, the discontinuity appears at the Melville F.Z. These contrasting observations indicate that while the discontinuities in mantle melting behavior may be a primary function of spreading rate, mantle composition and major physiographic discontinuities also play a major role in fixing their position.
DE: 3035 Midocean ridge processes
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting