submitted to Journal of Geophysical Research, December 1995
Buoyant Mantle Upwelling And Crustal Production At Oceanic Spreading Centers -- On-Axis Segmentation And Off-Axis Melting
Kopal Barnouin-Jha, E. M. Parmentier
(Department of Geological Sciences, Brown
University, Providence, Rhode Island)
David W. Sparks
(Lamont-Doherty Earth Observatory of Columbia
University, Palisades New York)
Numerical experiments are used to examine the effect of buoyant mantle flow on melt generation near spreading centers. Buoyancy results from the 1) depletion of residual mantle in Fe relative to Mg (mantle-depletion buoyancy), 2) the presence of low density melt (melt-retention buoyancy) and 3) thermal expansion (thermal buoyancy). Thermal buoyancy drives convection with an along-axis wavelength about twice the asthenosphere thickness, and which develops closer to the spreading axis at slower spreading rates. Melt-retention buoyancy enhances this segmentation, by focusing upwelling into isolated melting centers. With increasing spreading rate the amplitude of on-axis crustal segmentation decreases, the melting region being essentially two-dimensional at full spreading rates greater than 100 km/My. These numerical experiments predict centers of mantle upwelling that are more widely spaced than the actual wavelength of inferred crustal thickness variations. This discrepancy indicates that some process not included in our models is responsible for the short wavelength of segmentation of at slow spreading rates. At fast spreading rates, when the lithosphere is thinner, melt-retention buoyancy gives rise to isolated, focused centers of off-axis upwelling and melt production. Off-axis melting centers begin to form at about 300 km from the axis, with a characteristic along-axis spacing of 200 km. Successive off-axis melting centers are shifted along-axis, so that crustal thickness far from the axis is increased at a 100 km wavelength.