Convergent Boundary Magmatism
Convergent plate boundaries are the sites of active volcanism which, over geologic time, is thought to have generated the silica-rich crust of the Earth's continents. Melting at convergent boundaries results from the interaction of water rich fluid released from the downgoing plate and hot mantle circulated through the wedge-shaped region above the plate by its motion. A primary goal of our work is to better understand how physical variables, for example convergence rate, mantle temperature, amount of water released, control the spatial extent of this melting and the rate at which melt can be produced. Theoretical and numerical modeling studies have the objective of making testable predictions of how various observable physical variables affect melting at convergent boundaries.
The figure to the right shows the results of a numerical model of solid state flow, melting, and melt migration in the mantle wedge above a downgoing plate (shaded in bottom two panels, motion shown by white arrow) at a convergent plate boundary. Water rich fluid generated by dehydration of oceanic crust migrates upward into hotter overlying mantle by buoyant, intergranular flow. Melting, as indicated by the depletion of the solid, occurs within a thin region where water interacts with fertile mantle. Only a small fraction of mantle flowing through the mantle wedge undergoes melting, and this fraction is controlled by the solid flow through its influence on fluid migration paths, temperature distribution, and transport of fertile mantle into the melting region.
Top panel of image to the right: a) Mantle temperature is shown by colors, b) solid lines show the stability limits of two primary hydrous mineral phase (serpentine and chlorite), and c) thin dark lines, included in each panel, show paths of melt/fluid migration.
Middle panel (and enlarged inset) of image on the right: a) Melt/fluid volume fraction present at each point shown by colors in the middle panel indicate the melt/fluid fraction present at each point, b) solid state mantle flow shown by solid white streamlines, and c) melt/fluid flow lines as described above. Dashed contours show mantle depletion, see below.
Bottom panel of image on right: a) volume fraction of solid mantle removed by melting (depletion) shown by colors and b) flow lines as described above.
A.-M. Cagnioncle, E.M. Parmentier, and L.T. Elkins Tanton, The effect of solid flow above a subducting slab on the water distribution and melting at convergent plate boundaries J. Geophys. Res., 112, B09402, doi:10.1029/2007JB004934, 2007.
A.-M. Cagnioncle, P. B. Keleman, E.M. Parmentier, and A.E. Saal, The Aleutians: a case study for fluid migration and melt production models at convergent plate boundaries, submitted to Geology 2008.
J. Phipps Morgan, J. Hasenclever, M. Hort, L. Rupke, and E. M. Parmentier, On subducting slab entrainment of buoyant asthenosphere, Terra Nova, 19, 167173, doi: 10.1111/j.1365-3121.2007.00737.x, 2007.
P.B. Kelemen, J.L. Rilling, E.M. Parmentier, L. Mehl and B.R. Hacker, Thermal structure due to solid-state flow in the mantle wedge beneath arcs, in Inside the Subduction Factory (ed. J. Eiler), AGU Geophysical Monograph 138, 293-311 (2003).
C. E. Hall, K. M. Fischer, E. M. Parmentier, and D. Blackman. The influence of plate motions on three dimensional back-arc mantle flow and shear wave splitting, J. Geophys. Res. 105, 28009-28034 (2000).
Brown faculty collaborators:
Karen M. Fischer
Other project collaborators:
See above list of publications.
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