Chapter 4 The Generation and Migration of Partial Melt beneath Oceanic Spreading Centers
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Publisher Summary This chapter discusses the generation and migration of partial melt beneath oceanic spreading centers. The narrowness of the zone of crustal emplacement at oceanic spreading centers indicates that melt formed in the mantle beneath a spreading center must be focused toward the spreading axis; it can be accomplished by lateral flow in sloping decompacting boundary layers at the top of the melting region. These layers form by the decompaction or dilation of the solid matrix in response to an excess melt pressure that develops beneath an impermeable boundary. The extraction efficiency of decompacting layers is determined by the temperature and upwelling distribution in the mantle. Mantle flow and melting beneath an offset spreading center are investigated in the chapter using three-dimensional numerical experiments. The flow consists of the superposition of passive flow driven by plate spreading and buoyant flow driven by thermal expansion and compositional density gradients due to melt extraction. Buoyant flow affects the distribution of melt production by increasing the along-axis variation in upwelling beneath the spreading center and generating melting in the upwelling limbs of off-axis thermally driven convective rolls. Approximate paths for the extraction of melt along the decompacting layers at the top of the melting region are calculated in the chapter. These paths predict the division of the melting region into separate melt extraction regions for different spreading segments. Predicted along-axis distributions of crustal thickness, topography, and gravity have greater variation at slow spreading rates in agreement with observations.
