Experimental Investigation Of The Deformation Of Mg₂GeO₄ At High Temperatures And High Pressures

Olivine is the most abundant mineral in the upper mantle and as its high pressure polycrystal ringwoodite in the mantle transition zone. It has long been reconginzied that the rheological properties of these minerals play important roles in governing the dynamics of Earth’s mantle as well as its thermal evolution. Thus, the rheological properties of the constituent mienrals for these regions are of great significience in modeling dynamics activities occurring within Eath’s interior. With the development of techonology on the study of rheological propertis under high pressure and high temperature, there are lots of researches focusing on the rheological properties of mantle minerals, but lack of studies on ringwoodite. The apparatus of rhelogical studies can now reach the P-T condition of the mantle transition zone (MTZ), but there still have large uncertainties in stress measurement. It is still not possible to conduct reliable quatitative deformation experiments on ringwoodite. As a conquence, use of analogues remains attractive to gain significant insight into the rheological properties of the deep mantle by extrapolating laboratory data to the condition appropriate in mantle transition zone. We select Mg2GeO4 as the analogues of Mg2SiO4 and use a 5GPa modified Griggs to study its rheological properties under high pressure and high temperature. The main subject of this dissertation are:1. the microstructure of phase transformation induced earthquake and their sliding mechanism; 2 the strength of Mg2GeO4 spinel and primary study of its fabric; 3. the effects of a layering structure on the seismic properties of a serpentinized peridotite.(1)Earthquakes are known to occur by two distinctly different mechanisms. In the low-temperature, low-pressure environment of the upper few tens of km of the lithosphere, earthquakes are envisioned to occur by brittle processes, (e.g., fracturing, comminution) that are activated along pre-existing fault zones. However, at greater depths in the crust and down to depths of 700 km in subducting lithosphere, the high temperature and high pressure prohibit brittle failure. Shear failure at high pressure requires a small amount of "fluid"-producing mineral reactions that enable both shear initiation and propagation. The "fluid" can be either a true fluid (eg. H2O, CO2, melt) or a nanocrystalline solid exhibiting an extremely low viscosity in the solid state and serves as a constantly renewing lubricant during fault propagation. We conducted transformation-induced faulting in Mg2GeO4 and successfully prepared speciments with well developed anticracks, nanofaults and through-going shear fractures (all of them filled with nanometric spinel). We compared these objects at nanometer resolution for comparison with high speed friction results, the results show that:1).both experiments involve mineral phase transformations; 2) both experiments lack amorphous material; 3) both experiments lack evidence of melting; 4) both experiments have equant, rounded, nanograins lacking preferred orientation in sliding zones; 5) both samples show no grain-boundary porosity in sliding zones; 6) both experiments display very low frictional resistance; 7) the dominant sliding mechanism for the nanometric is grain boundary sliding or superplastic.These results indicate that the fault-weakening mechanism that shallow crustal earthquake and deep subduction zone earthquake have in common.(2) Previous studies’ show that Mg2GeO4 spinel has a weaker strength than that of olivine due to the fine grain sizes, led to a dominant deformation mechanism of grain boundary sliding or superplastic. In this study we use oxide powders to synthesizes the spinel phase, and overcome the difficult that olivine transforms to spinel with fine grain sizes. The dominant grain size of spinel in our study is 20-50 μm, ensuring the dominant deformation mechansim is dislocation creep. The flow law we determined for the spinel phase is ε=10-477σ3.1exp(-205/RT).Compared with previous work on Mg2GeO4 olivine under similar conditions, the strength of the spinel phase is 2-3 times greater. This moderate increase in viscosity is consistent with recent MTZ modeling and geophysical inversion, both of which also show a larger increase in viscosity between the MTZ and the lower mantle. Recent MTZ modeling yields results suggesting that the upper mantle low-viscosity zone (LVZ) could be either within the MTZ or above it. Our results suggest that the LVZ should be above the MTZ unless the upper mantle phase transformations at 410 and/or 660km lead to narrow LVZs or the MTZ contains significant H2O.(3) We conducted shear experiment on Mg2GeO4 spinel under high pressure and high temperature, and measured the lattice preferred orientation (LPOs) of the experimental specimens. The results show that the Mg2GeO4 spinel has random fabric. This is consistant with previous simulation results. The single crystal of ringwoodite show nearly istropic properties. Combining with the LPOs results, we propose that the LPOs of ringwoodite are not the reason for the seismic anisotropy in the mantle transition zone.(4) Seismic anisotropy is an ubiquitous feature of most geological materials, the primary causes of seismic anisotropy are the shape preferred orientation (SPO) and the lattice-preferred orientation (LPO) of earth materials. Previous studies have been focused mostly on the effects of LPOs on seismic anisotropy. In this study, we investigated the effect of a layering structure on the seismic anisotropy of subduction zone and the LPO evolution of a serpentinized peridotite. Our results indicate that:1). The LPOs of antigorite can inherite from the olivine that was not deformed in the later deformation, which has also been reported by previous TEM observations; 2) Olivine embedded in the antigorite matrix shows a random fabric, suggesting that the rheological strength of antigorite is much lower than that of olivine; 3). When a layering structure is formed, the polarization direction of the fast seismic velocity changes from trench-perpendicular to trench parallel after the volume of antigorie reached about 20%. This can explain why near the trench the polarization of seismic anisotropy is parallel the trench but away from the trench, the polarization of seismic anisotropy is perpendicular the trench. A ～20% antigorite is also consistent with the seismic velocities of most subduction zone; 4).The LPOs are the dominant factor for the seismic properties of mantle wedge. The layering structure can enhance the seismic anisotropy.