Magmatism, Hydrothermal Cooling and Asymmetric Accretion at Slow-spreading Ridges

Asymmetric spreading is common at slow-spreading mid-ocean ridges when an active detachment fault accommodates a portion of the total plate separation. Basalts erupted along asymmetric segments have lower Ca, higher Fe, Na, K than the ones collected from symmetric segments, indicating higher pressures of fractionation and lower extents of partial melting of the mantle [Langmuir et al., AGU, 2013]. Seismic evidence also shows a thicker and colder axial lithosphere at asymmetric sections of the ridge [Escartín et al., 2008]. This phenomenon is most obvious when the asymmetric spreading centers are also oblique to its opening direction. The reduced melt supply beneath asymmetric spreading segments may be attributed to distorted mantle upwelling, enhanced hydrothermal cooling, and enriched compositional heterogeneities in the upper mantle.

We construct two-dimensional thermo-mechanical models of symmetric and asymmetric spreading centers, and test the effects of asymmetric accretion and hydrothermal circulation on mantle melting. A temperature-dependent mantle viscosity is used. The hydrothermal circulation is implemented as an enhanced thermal conductivity limited by cutoff depth and temperature. The effect of oblique spreading is incorporated in the model as reduced effective spreading rate. Mantle flow and thermal structure are solved in the commercial finite element software COMSOL Multiphysics®. Melt production and flux are estimated in Matlab® using a nonlinear melting function [Katz et al., 2003]. We show that the asymmetric accretion alone does not affect the extent of melting or reduce the melt flux significantly. Hydrothermal cooling can plays an important role in deepening the melting depth and lowering the melt extent. Therefore, the difference in the extent of melting between asymmetric and symmetric spreading models can be explained by an enhanced hydrothermal circulation at asymmetric segments. This correlation is supported by the observation made at Mid-Atlantic Ridge that hydrothermal activities are closely associated with asymmetric segments. In addition, Lavier and Buck [2002] showed that efficient cooling of the lithosphere favors the development of large offset detachments and therefore asymmetric spreading.