The mantle represents 70% of the Earth's volume, and therefore plays a key role in the evolution of the Earth. Since early 20th century, a seismic discontinuity at 660-km depth in the mantle has been believed to be an important filter for material exchange (including water and carbon-dioxide) between the upper and lower mantle. Laboratory measurements have shown that the phase transitions in the dominant mantle minerals explain this 660-km discontinuity. However, there exist multiple phase transitions near this depth and the interactions among these phase transitions are not well understood. Recent high-resolution seismology studies reveal complex structures near the discontinuity and strong lateral variations in the properties of the discontinuity, suggesting interaction between mineral phase transitions occur in the mantle and providing motivation for high-resolution laboratory measurements.
In order to measure the interactions among different phase transitions near the 660-km discontinuity, the investigators will take full advantage of recent developments in (1) the diamond-anvil cell (a high pressure device) technique, which allows fine control of pressure, (2) the laser levitation synthesis technique, which allows the investigators to engineer the compositions of starting materials similar to those expected for mantle rocks, and (3) high-resolution synchrotron X-ray diffraction techniques, which allow detection of the phase transitions unambiguously. The results from this study will help geophysicists to understand the role of the 660-km discontinuity in mantle convection and chemical differentiation. Ths proposed work is timely because huge seismic datasets available from large digital seismic networks, such as USARRAY, are beginning to provide high-resolution images to resolve fine-scale structures near the 660-km discontinuity.
The mantle is the thickest layer in Earth's interior (3000-km thick) and the convection in the layer drives the geological processes on the surface, such as volcanic activities, earthquakes, and crust formation. Since 1930’s, seismologists recognized a global structure located at 660-km depth from the surface in the mantle. The origin of the structure is important in understanding the scale of convection and thermal evolution of the mantle. However, the origin has been debated for many decades. Recent developments in seismology have revealed that the structure is much more complex than previously thought. In this NSF supported project, we investigate the origin of the complex structures using high-pressure laboratory experiments combined with synchrotron X-ray techniques and atomic-resolution electron microscope techniques. We found that atomic scale changes in mineral structures may occur with different patterns at different temperatures and in different compositions, suggesting that the complex structure found by seismologists are related to lateral changes in composition and temperature. Furthermore, by providing new data on how seismic structures would change in response to changes in composition and temperature, we provide new tools for seismologists to relate their observations to changes in these parameters. The new interpretation will help Earth scientists to understand the scale of mantle convection and its variations at different regions in the mantle, which will allow us to understand the origin of hot spot volcanisms, such as Hawaii and Yellowstone, and thermal evolution of our planet.
