Dynamics associated with partial melting in Earth's uppermost and lowermost mantle and the structure and phase relationships in Earth's D" layer
Abstract
The development of buoyant decompression melting instabilities beneath diffusely extending lithosphere is studied in order to assess the consequences of this kind of phenomenon upon the timing, rate, and distribution of mantle melt production in this important geological setting. A combination of numerical modeling and linear analysis that include the effects of thermal, melt, and solid depletion buoyancy show that rapid extension suppresses this instability, and a "Rayleigh number" for this process is defined that quantitatively captures this behavior. Increased degrees of solid depletion buoyancy are found to enhance the instability of the partially molten layer. If conditions in Earth's asthenosphere are such that this Rayleigh number is less than a critical value, then instabilities will be suppressed during extension and develop only after extension slows or stops, producing two pulses of magma generation due to extension followed by the subsequent instability. The possibility that seismically anomalous thin patches of Earth's lowermost mantle, termed "ultralow-velocity zones," are partial melts derived from the mantle is investigated using regional scale numerical models of mantle convection. The results show that partial melting introduces a ubiquitous partially molten layer, with the thickest portions occurring at the base of upwelling plumes and a very thin layer occurring elsewhere. It appears that only a dense partially molten mixture produces partial melt distributions that are compatible with seismic observations, however, any non-zero degree of dense melt percolation leads to excessive degrees of melt accumulation at the core-mantle boundary. A recently discovered solid-solid phase transition at pressures of the D" layer is ideally situated to reveal the thermal structure of the lowermost mantle, where no phase transitions were previously known to exist. Here we show that a pair of seismic discontinuities observed in some regions of D" can be explained by the same phase transition as the result of a double-crossing of the phase boundary by the geotherm at two different depths. This simple model can also explain why a seismic discontinuity is not observed in some other regions, and provides new constraints for the magnitude of temperature variations within D".
- Publication:
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Ph.D. Thesis
- Pub Date:
- 2006
- Bibcode:
- 2006PhDT.......104H