The detectability of melt channels beneath slow- and fast-spreading mid-ocean ridges
Abstract
At mid-ocean ridges, the oceanic crust is created by the intrusion and extrusion of melt, drawn upwards from below the ridge axis in high-porosity channels which form by reactive flow. These channels have drastically different seismic properties to the surrounding mantle material, which has the potential to explain widespread observations of seismic anisotropy in surface and body waves travelling through mid-ocean ridges and the oceanic lithosphere. In order to investigate these processes, we use a 2D numerical model of coupled magma/mantle dynamics. The model is based on conservation statements for mass, momentum, energy and composition in a two-phase system with two components in local thermodynamic equilibrium. One component is more fusible than the other, and a network of magma-rich channels are nucleated when heterogeneities rich in fusible material are introduced into the system. Whilst most channels focus melt towards the axis, pockets of high porosity (analogous to melt) are also rifted away from the ridge axis. We model ridges with full spreading rates between 40 and 160 mm/a, reflecting most ridge systems currently active. Shear wave splitting measurements at mid-ocean ridges reveal a pattern of increasing splitting with distance from the ridge axis, and greater splitting beneath fast-spreading ridges. Surface wave studies show that horizontally-polarised waves (SH) travel faster than those polarised vertically (SV) in the region of 50-100 km beneath the ocean floor. Neither of these observations can be wholly explained by the alignment of mineral phases due to mantle flow beneath ridges. We convert the thermodynamic ridge model to elasticity using effective medium theories, taking porosity as a proxy for melt, and using melt flow lines to predict the orientation of the elasticity. Seismic forward modelling then predicts the amount and orientation of shear wave splitting and surface wave anisotropy, which can be compared to observations. Early results indicate that, at slow spreading rates, low porosities lead to a concentration of melt-induced anisotropy close to the ridge axis (< 50 km), whilst faster ridges show significant splitting to some distance (>250 km). Channels dip away from the axis more steeply at slow spreading centres (> 45°) than fast ones (∼30°). Melt-rich 'lenses' with long axes parallel to the horizontal are carried away from the ridge axis and potentially may explain the SH-faster-than-SV signature seen in surface waves. Melt-induced anisotropy alone is unlikely to explain the overall splitting measured at ridges however: we find a better fit to observations when previous models of mineral alignment are included in our calculations in addition to the new models we present here.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T12D..04W
- Keywords:
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- 1032 GEOCHEMISTRY / Mid-oceanic ridge processes;
- 3035 MARINE GEOLOGY AND GEOPHYSICS / Midocean ridge processes;
- 7208 SEISMOLOGY / Mantle;
- 7245 SEISMOLOGY / Mid-ocean ridges