Buoyant Decompression Melting

Spreading center segmentation and interplate volcanism due to buoyant decompression melting: Divergent plate boundaries, where tectonic plates spread apart, are the site of mantle upwelling and melting which creates the basaltic crust of the Earth's ocean basins. Since the melting temperature of mantle rock increases with pressure, melting occurs if mantle upwells too rapidly to allow cooling by heat conduction. This is the case even at spreading rates as slow as ~ 10 km/Myr. For a plate boundary that is continuous along strike, uniform upwelling due to plate spreading should create melt, and therefore crust, uniformly along the spreading axis. However in global gravity images like those shown in Figure 1, seafloor generated at slow spreading rates show regularly spaced lineations that can be interpreted as crustal thickness variations resulting from along axis variations in upwelling and melting. Along-axis crustal thickness variations created at the spreading axis are carried off the axis resulting in seafloor lineated in the spreading direction. In constrast, fast spreading results in very uniform crustal thickness along long segments of spreading axis between large offset transform faults.

Numerical experiments show that buoyant upwelling columns develop beneath a spreading center at slow spreading rates. An example from numerical experiments of Choblet and Parmentier (2001) is shown in Figure 2. Here instability is caused by the nonlinear feedback between upwelling and melting. Buoyancy generated by melting causes upwelling to concentrate into columns beneath the spreading axis and enhanced upwelling in the buoyant columns increases buoyancy due to melting. Spontaneous buoyant decompression melting, illustrated by this example, has also been explored as a mechanism of intraplate volcanism (Raddick, et al., 2002).

Beneath spreading centers, in the presence of upwelling generated by plate motion, the buoyant decompression melting convective instability occurs only at sufficiently slow spreading rates. The spreading rate below which instability occurs depends on mantle viscosity as shown in Figure 3. This offers an explanation for the striking difference in the structure and morphology of seafloor generated at slow and fast spreading rates shown in Figure 1.

Publications cited:
E.M. Parmentier, The Dynamics and Convective Evolution of the Oceanic Upper Mantle, in Treatise on Geophysics, Chapter 7, (eds. G. Schubert and D. Bercovici), 305-323 (2007).

M. J. Raddick, E.M. Parmentier, and D. Scheirer, Buoyant decompression melting: a possible mechanism for intraplate volcanism, J. Geophys. Res. 107, 2228, doi:10.1029/2001jb000617 (2002).

G. Choblet and E.M. Parmentier, Mantle upwelling and melting beneath slow spreading centers: effects of variable rheology and melt productivity, Earth Planet. Sci. Lett. 184, 589-604 (2001).

M. Braun, G. Hirth, and E.M. Parmentier, Effects of deep damp melting on mantle flow and melt generation beneath mid-ocean ridges, Earth and Planet. Sci. Lett. 176, 339-356 (2000).

J. Phipps Morgan and E.M. Parmentier, Crenulated seafloor: evidence for spreading rate dependent structure of mantle upwelling and melting beneath a mid-oceanic spreading center, Earth Planet. Sci. Lett. 129, 73-84 (1995).

E.M. Parmentier and J. Phipps Morgan, The spreading rate dependence of three-dimensional oceanic spreading center structure, Nature 348, 325-328 (1990).

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