HR: 10:50h
AN: V32A-03 INVITED    [Abstracts]
TI: On foundering lithosphere and volatile migration: Upside-down melting
AU: * Elkins-Tanton, L
EM: ltelkins@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Ave. 54-824, Cambridge, MA 02139, United States
AB: On Earth magmatism occurs on continents in the absence of subduction, often producing volatile-rich magmas such as those in the Leucite Hills, the Sierra Nevada, and Peru's Altiplano. The primary hypothesis to explain this volcanism is foundering of the lower lithosphere into the mantle. Here loss of the lower lithosphere is hypothesized to occur in a ductile manner in response to a density contrast such as would be caused by intruding mantle melts that freeze as eclogites. This mechanism requires no specific structural weakness beyond a dense region in the lithosphere that is gravitationally unstable with respect to the underlying mantle and that possesses a rheology conducive to flow. Density contrasts of as little as 1% are fully sufficient to drive gravitational instabilities. A gravitational instability forms when a perturbation in a boundary grows through lateral flow, causing the perturbation to grow. The growing instability begins to sink into the underlying mantle material as a drip, exactly analogous to but reversed in the sense of growth from an ascending plume head. The unstable material will sink more rapidly than lateral flow in the lower lithosphere can continue to add material to it, resulting in an annulus of thinned lithosphere centered on the instability. Thus the lithosphere is thinned slightly in the region around the drip, but no dome forms in the lower lithosphere during ductile delamination. Traditionally magmatism associated with instabilities has been attributed to return flow of the asthenosphere into such a dome, but maintaining a dome in the lithosphere requires unusual rheological conditions not expected in such a setting. Any volatile content in the sinking material may act in petrologically significant ways. The sinking lower lithosphere may contain 0.1 to 0.2 mass% of water if only nominally anhydrous minerals are present, and up to several weight percent of water if phlogopite or amphibole are present. The sinking lithospheric material heats conductively in the asthenosphere. Depending upon its rate of descent and volatile content, the sinking material may (1) devolatilize (as a descending slab in a subduction zone does), (2) carry volatiles to depth, sinking in some cases faster than slabs and thus carrying volatiles to depth more efficiently, or (3) heat sufficiently quickly to cross its solidus and itself produce magma. Because melting in instabilities would occur as they sink, we have termed this novel melting mechanism "upside-down melting." Upside-down melting has the potential to create primitive hydrous basaltic magmas with high alkali contents and lithospheric trace element signatures, typical of small-volume continental magmas worldwide. These magmas are compositionally distinct from the relatively dry adiabatic melts that result at mid-ocean ridges, and which the dry adiabatic melting created by the movement of the sinking instability would more closely resemble; the models successfully predict their range of mantle source conditions.
DE: 3621 Mantle processes (1038)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8148 Planetary volcanism (5480, 8450)
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8425 Effusive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting