Role of Partial Melting in the Asthenosphere
Abstract
The presence of partial melting in the asthenosphere has been supported by the occurrences of the seismic low velocity zone, the high electric conductive layer and the experimentally determined peridotite solidus in the presence of H2O and CO2 (e.g., Wyllie, 1988 JGR). More recently however, the significance of partial melting in the asthenosphere has been questioned based on laboratory measurements on Vs and Qs of mantle material as a function temperature and pressure (e.g. Faul and Jackson, 2005 EPSL). The recent discovery of young (ca.1Ma) alkali basalt magmatism on the 140 Ma subducting Pacific plate (Hirano et al., 2006 Science) revealed a strong evidence for the ubiquitous presence of partial melts under the oceanic lithosphere. Based on the petrologic study of the submarine alkali basalt described by Hirano et al. (2006), we estimated a primary magma composition for the young alkalic basalts near the Japan Trench (JPT-1CW; 44.3 wt% SiO2, 17.3 wt% MgO, 9.9 wt% FeO, 2.9% K2O, 1.6 wt% H2O and 1.9 wt% CO2). High-pressure melting experiments were carried out on JPT-1CW composition and we found that this magma can be in equilibrium with mantle peridotite at 3 GPa and 1400 degreeC (100 degreeC below the solidus of dry peridotite KLB-1, Takahashi, 1986 JGR). In order to study the role of partial melting under the oceanic lithosphere, experiments were carried out using a mixture of 97 wt% peridotite KLB-1 and 3 wt% JPT-1CW in the pressure range between 2.2 and 3.5 GPa and the temperature range between 1250 and 1500 degreeC. Typical run durations are 100 hrs for low- temperature and 20 hrs for high-temperature runs. Experiments were conducted using piston-cylinder apparatuses with talc-Pyrex-graphite assembly. Two types of sample containers (graphite-Pt and Re-Pt) were used and experimental results were duplicated for most P-T conditions. Three melting regimes were recognized in our experiments; 1) within the stability of phlogophite (lower than 1300 degC) neither signature of melting nor that for mobilization of elements were detected, 2) between 1300 and 1400 degC, almost all potassium has been lost from the peridotite matrix by migration of H2O-rich fluid, 3) at higher than 1400 degC, K-rich partial melts were formed and distributed throughout the peridotite matrix. Although the dry solidus for the peridotite KLB-1 varies from 1420 degC at 2.2 GPa to 1600 degC at 3.5 GPa (Takahashi, 1986 JGR), the three melting regimes for the wet peridotite are insensitive to temperature in the studied pressure range. Based on our experiments, we propose a model for the role of partial melting in the asthenosphere; 1) The upper bound of partially molten asthenosphere is most probably controlled by inflected peridotite solidus due to the stabilization of carbonate and breakdown of phlogophite at around 3GPa and 1300 degC. 2) In the asthenosphere, super-critical fluids promote segregation of alkali elements from deep mantle regime. 3) Oceanic plate would act as an impermeable lid for the asthenosphere and a thin melt-enriched layer would be produced beneath the oceanic lithosphere after the long term melt segregation. 4) This partial-melt layer may act as a slip-plane and would define the mechanical thickness of oceanic plate. 5) Ubiquitous occurrence of K-rich metasomatism at the keel of the lithosphere supports the existence of such partial-melt layer. Therefore, partial melting in the asthnosphere plays fundamentally important roles both in geochemical evolution and geodynamics of the Earth.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFMMR31D..08T
- Keywords:
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- 3619 Magma genesis and partial melting (1037);
- 3630 Experimental mineralogy and petrology;
- 7218 Lithosphere (1236);
- 8120 Dynamics of lithosphere and mantle: general (1213);
- 8124 Earth's interior: composition and state (1212;
- 7207;
- 7208;
- 8105)