Mantle Melting Conditions beneath the Basin and Range (Invited)
Abstract
It is yet debated what drives mantle melting and widespread mafic volcanism across the Basin and Range of the western US over the past 10 Ma. Possibilities include lithospheric extension, mantle hydration, and local convective upwellings. Also relevant is the temperature and compositional structure of the melting region, and the relationship to the continental lithosphere. Such debates have proliferated largely without knowledge as to the basic conditions of melting -- the temperature, pressure and water content in the melting region -- which are recorded in the chemical composition of magmas. Such quantitative information has been lacking, but is now becoming available due to improved melt-mantle thermobarometers [1,2], and new data on the water content and oxidation state (fO2) of magmas in the Western US. Our own work has focused on melt inclusions trapped in olivine from young cinders, which record undegassed H2O and sulfur contents, and the vanadium content of olivines, which, with sulfur [3], record magmatic fO2[4]. Such constraints are vital to obtaining accurate P-T estimates of melting, because 3 wt% H2O leads to ~100 °C drop in calculated temperatures, and 15% Fe3+ (reflected in fO2) leads to a ~75°C drop in temperature and a ~0.7 GPa drop in pressure (~ 20 km). Preliminary work on the Big Pine (CA), Crater Flat (NV) and Snow Canyon (UT) volcanic fields reveal large regional variations in H2O (1 - 4 wt% H2O) and estimated fO2 (FMQ+0.3 to +1.5, and ~ 18 - 28% Fe3+) in melt inclusions. When combined with thermobarometers based on the MgO and SiO2 content of primary liquids, these magmas require large variations in melting conditions across the region, from cool (< 1300°C) and shallow (40-50 km) melting beneath Big Pine in the west, to higher temperatures (> 1400°C) and deeper melting (65-70km) beneath Snow Canyon in the east. Such melting depths coincide with the top of the low velocity region in recent surface wave models using EarthScope data [5]. Thus, mantle melt generation beneath the Basin and Range appears to require 1) lateral temperature variations on the order of 150 degrees and 2) temperatures and seismic velocities in the melting regions that are more typical of asthenosphere than lithosphere. [1] Lee, C-T, et al., EPSL, 2009. [2] Putirka, Reviews in Mineralogy, 2008. [3] Jugo et al., 2009. [4] Canil, EPSL, 2002. [5] Yang et al., JGR, 2008.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V53A..06P
- Keywords:
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- 1033 GEOCHEMISTRY / Intra-plate processes;
- 3619 MINERALOGY AND PETROLOGY / Magma genesis and partial melting;
- 8120 TECTONOPHYSICS / Dynamics of lithosphere and mantle: general;
- 8415 VOLCANOLOGY / Intra-plate processes