Melts at the Lithosphere-Asthenosphere Boundary beneath the Basin and Range, US (Invited)
Abstract
The Transportable Array component of EarthScope is providing an unparalleled view of the seismic structure of the mantle beneath the North American continent. In volcanically active regions such as the Basin and Range province of the western US, petrological data can also be used to constrain the temperature, water content, and depth of melting within the mantle, all of which may contribute to seismic velocity anomalies. Of particular interest to dynamic models is the location and evolution of the lithosphere-asthenosphere boundary (LAB), for which petrological and seismological data yield complementary constraints. The LAB is a rheological boundary that may strongly relate to the locus and mode of melting, whether by upwelling, hydration or extension. Here we present a preliminary integration of mantle melting depths, derived from the chemical composition of basaltic scoria from recent cinder cones across the Basin and Range, with shear velocity structure derived from inversion of Rayleigh waves. Primitive basaltic magmas record in their major element composition the pressures and temperatures of last equilibration in the mantle. Specifically, the Fe content of primary melts scales with melting temperature (through olivine-melt equilibrium) and the Si content scales inversely with pressure (through olivine-orthopyroxene melt equilibrium). Independent of these relationships, the water content of magmas affects estimated temperatures (roughly 100 C per 3 wt percent H2O), and the ferric Fe component affects estimated pressures or depths (15-20 km per 15 percent Fe3+). Our efforts have thus gone into measuring the pre-eruptive H2O content of Basin and Range magmas, using undegassed melt inclusions trapped in olivine, and their oxidation state, based on sulfur and vanadium speciation. Our results thus far for volcanic fields in the Western Grand Canyon (AZ), St. George (UT), and Crater Flat (NV) regions, indicate melt equilibration depths around 55-70 km. These depths generally coincide with the top of the low velocity zone (LVZ), and not the velocity minimum, in these regions. The gradient in velocity at the top of the LVZ, transitioning to a high velocity lid, is consistent with the location of the LAB. Melts may equilibrate here because their upward progress is impeded by the rheological contrast, stalling below the stronger lithosphere. Melts formed at greater depths are likely responsible for velocities in the minimum, typically less than 4.1 km/s, too low to be caused by temperature alone without unreasonably high attenuation. These velocity constraints, along with temperatures of equilibration that are 1300-1400 C, support a melting region in upwelling asthenosphere, below the LAB. We do not find evidence for major melting within a cold, hydrous lithosphere, nor exclusively at the base of a thermally eroded or extended lithosphere.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.T42C..02P
- Keywords:
-
- 7218 SEISMOLOGY / Lithosphere;
- 8109 TECTONOPHYSICS / Continental tectonics: extensional;
- 8439 VOLCANOLOGY / Physics and chemistry of magma bodies