Geophysics: protons lead the charge.
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Greg Hirth
Publications and source records attributed to Greg Hirth.
Geophysical evidence from the MELT area for compositional controls on oceanic plates.
Magnetotelluric and seismic data, collected during the MELT experiment at the southern East Pacific Rise, constrain the distribution of melt beneath this mid-ocean-ridge spreading centre and also the evolution of the oceanic lithosphere during its early cooling history. Here we focus on structures imaged at distances approximately 100 to 350 km east of the ridge crest, corresponding to seafloor ages of approximately 1.3 to 4.5 million years (Myr), where the seismic and electrical conductivity structure is nearly constant and independent of age. Beginning at a depth of about 60 km, we image a large increase in electrical conductivity and a change from isotropic to transversely anisotropic electrical structure, with higher conductivity in the direction of fast propagation for seismic waves. Conductive cooling models predict structure that increases in depth with age, extending to about 30 km at 4.5 Myr ago. We infer, however, that the structure of young oceanic plates is instead controlled by a decrease in water content above a depth of 60 km induced by the melting process beneath the spreading centre.
Journal Article↗
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.
Journal Article↗