HR: 14:10h
AN: T43D-03    [Abstracts]
TI: 3-D Melt Migration Driven by Dynamic Pressure Gradients in a Passive, Mantle Flow Field Can Explain (Almost) Everything About Melt Delivery In a Mid-Ocean Ridge System Offset by Transform Faults
AU: * Forsyth, D W
EM: Donald_Forsyth@brown.edu
AF: Department of Geological Sciences, Brown University, 324 Brook Street, Providence, RI 02912 United States
AU: Saal, A E
EM: asaal@brown.edu
AF: Department of Geological Sciences, Brown University, 324 Brook Street, Providence, RI 02912 United States
AB: Dynamic pressure gradients developed in a deforming solid medium were one of the first mechanisms suggested for driving melt from a broad region of melt production to a narrow zone of crustal accretion at the ridge axis. This idea was never fully embraced because it requires high viscosities, greater than 10*21 Pa s; otherwise melt buoyancy drives the magma vertically. Perhaps dehydration of the mantle matrix accomplished by removal of the first small melt fraction can increase the viscosity to the level required to drive porous melt flow to the axis. Assuming that the dynamic pressure gradient in a deforming mantle is the dominant driving force for melt migration, new 3-D calculations show that this mechanism quantitatively explains a remarkable number of the fundamental observations of variations in crustal structure and mid-ocean ridge basalt (MORB) composition in a ridge-transform system. More melt is predicted to be delivered to the middle of ridge segments, accounting for the thickened crust near the center, the thin crust near fracture zones, and the resultant mantle Bouguer anomaly bulls-eyes on slow spreading ridges. Deeper, low-degree melts are also preferentially delivered to the segment center. Near the end of a segment, greater cooling caused by slower upwelling rates of the mantle matrix cause melting to terminate at greater depth. As suggested by Reynolds and Langmuir (1997), these phenomena combine to produce magmas near the transform that have much lower levels of highly incompatible elements but higher levels of moderately incompatible elements than segment-center magmas. If the mantle contains "plum pudding" style heterogeneities that melt at greater depth, this model can explain the isotopic anomalies at the centers of some of the longer spreading segments on the Mid-Atlantic Ridge (Michael et al., 1994). The 230Th deficiency and highly depleted character of MORBs from East Pacific Rise intra-transform spreading centers are also predicted. The predicted transform effect in these new calculations is more pronounced than in the Phipps Morgan and Forsyth (1988) 3-D passive flow models because that study employed a different model for melt migration.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting