Spreading-rate dependence of melt extraction at mid-ocean ridges from mantle seismic refraction data.
A variety of observations indicate that mid-ocean ridges produce less crust at spreading rates below 20 mm yr(-1) (refs 1-3), reflecting changes in fundamental ridge processes with decreasing spreading rate. The nature of these changes, however, remains uncertain, with end-member explanations being decreasing shallow melting or incomplete melt extraction, each due to the influence of a thicker thermal lid. Here we present results of a seismic refraction experiment designed to study mid-ocean ridge processes by imaging residual mantle structure. Our results reveal an abrupt lateral change in bulk mantle seismic properties associated with a change from slow to ultraslow palaeo-spreading rate. Changes in mantle velocity gradient, basement topography and crustal thickness all correlate with this spreading-rate change. These observations can be explained by variations in melt extraction at the ridge, with a gabbroic phase preferentially retained in the mantle at slower spreading rates. The estimated volume of retained melt balances the approximately 1.5-km difference in crustal thickness, suggesting that changes in spreading rate affect melt-extraction processes rather than total melting.
Lizarralde
Daniel
D
School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, USA. danl@eas.gatech.edu
Gaherty
James B
JB
Collins
John A
JA
Hirth
Greg
G
Kim
Sangmyung D
SD