This summer I had the opportunity to experience experimental lab research first hand. Working with David Kohlstedt's Rock and Mineral Physics Lab at the University of Minnesota, allowed me to enter the world of experimental geology. Going to a small liberal school with only undergraduates there are not many opportunities to work in a research lab and to interact with graduate students. This summer was a great compliment to my undergraduate studies and a possible foreshadowing of the type of graduate work I would like to do. At Cornell I am getting great exposure to a concentrated, interdisciplinary major (double major in Chemistry and Geology). The summer PGGURP experience was an excellent addition that allowed me to work with grad students and introduced me to leading edge research. Working alongside graduate students on the cusp of geophysical knowledge opened my eyes to higher academic pathways.
Throughout the summer I prepared samples for torsion experiments in a Paterson gas-deformation apparatus. These mantle type rocks composed of olivine, chromite and mid-ocean ridge basalt (MORB) allow the examination of melt transport in the mantle specifically at mid-ocean ridges.
Melt transport is important in many geological processes such as the formation of oceanic crust at mid-ocean ridges and volcanoes overlying spreading centers, subduction zones and hot spots. These are areas which experience significant deformation. Melt has important implications in mantle flow since melt weakens the mantle and increases its permeability. Experiments indicate that an applied stress can lead to melt segregation. Highly permeable channels resulting from stress driven melt segregation may provide a mechanism for melt transport from within the mantle to the surface allowing melt to extrude from a planet's interior. At mid-ocean ridges, the basalt melt (MORB) surfaces in disequilibrium to the upper mantle peridotites through which it travels [5]. Chemical isolation of the melt during transport can occur in highly permeable channels. Field evidence for channels can be seen in ophiolites where dunites have been interpreted as dissolution channels in chemical equilibrium with MORB but not with surrounding mantle peridotites [1]. While stress leads to melt segregation in bands, surface tension redistributes melt homogeneously. The purpose of the experiments I worked on this summer was to investigate the stability of melt bands after removing the applied stress. Scaling experiments up to Earth scales will contribute to the understanding of the time scales necessary to rehomogenize features after melt segregation occurs.
This summer experience highlighted the need for improvement in the core-formation theory for rocky planets to lead us to a better understanding of how the Earth's core was formed and how the mantle interacts with the core. This understanding is essential, not only to understand our own planet Earth, but by extension to help develop the knowledge base for how other planets may be similar or different. At the center of the Earth exists an iron and nickel core. How did these metals concentrate to the center? One possible explanation is through density separation in a magma ocean. If the entire planet is molten, the heavier elements will sink to the bottom. However, this explanation leaves the question as to why iron and nickel both occur at the surface. Meteoric impacts can account for some, but is that process sufficient to account for all the actual deposits? Another possible and plausible explanation comes from the high permeable channels modeled in our experimental results. These channels could funnel the heavier elements to the center and when they subsequently become blocked they may leave some nickel and iron at the surface. A better understanding of melt transport can help examine planet formation.
Our sample developed this summer at the University of Minnesota, thanks to a PGGURPS grant, was developed to look into the role of surface tension in driving melt transport. The sample was deformed at the temperature of 1473 K and a confining pressure of 300 MPa. A constant twist rate of 0.18 mrad/sec was applied until a total shear strain of 1.693 was obtained. The sample was cut in half and polished using diamond lapping and colloidal silver polish. The sample displayed bands; therefore, half of the sample was prepared for static anneal for further investigation. The static anneal was performed at a temperature of 1473 K and a confining pressure of 200 MPa. Preliminary results obtained indicated that after a 4 hour static anneal band width doubled. Results indicate that surface tension can drive melt through mantle-type rocks -in the case of this experiment an olivine based matrix with chromite and MORB was used. Surface tension driven flow tends to homogenize melt distribution through time. Band dissipation occurs after a 4 hour anneal. In the annealed sample, the band width doubled and melt percentage increased by approximately 4%.
Talking to my advisor and the PhD student I was working with, we have decided that we could continue work from this summer. The role of surface tension in driving melt transport remains a rich area for further study. The next step is to relate the empirical experimental results to a theoretical model of surface tension driven flow. In order to gain better understanding of band stability after stresses are removed. Further study is required on band dissipation. Additional data and study is required to determine how different annealing times affect results, whether band dissipation occurs at the same rate between samples of different strains and how static anneals completed on samples deformed to varying strains compare.
At the conclusion of the summer, I was privileged to present at two "poster" sessions. The campus wide session featured about 85 students presenting topics ranging from multicultural relationship to civil and mechanical engineering. I was the only geology student present. The session gave me the opportunity to explain geology concepts and areas of study to non-geologists. I realized the importance of good communication across disciplines and was very conscious of the specialized vocabulary that develops in each field and the need to explain complex concepts in common language with out relying on technical "jargon.". Later that afternoon the geology and geophysics interns gave a poster session in the geology department. At this session the atmosphere was quite a bit different. Everyone who attended had varying amounts of geology background so the concepts discussed and the explanations became much more in-depth. Discussions arose analyzing the validity of results and attempts to tie in themes between various projects from the summer. It was definitely quite a change from the previous session and demanded a more rigorous defense, and more detailed and exiting explanation of the intricacies of the experiment.
The summer experience was personally and academically rewarding for me. It expanded my knowledge of leading edge geophysics, gave me valuable lab experience, reinforced my conviction for the need for interdisciplinary study to understand complex processes and allowed me the pleasure to make professional/academic contacts and mentors. I would like to thank the National Aeronautical and Space Agency (NASA) for the PGGURPs grant and special thanks to Dr. Kohlstedt and Mark Zimmerman and everyone of the Rock and Mineral Physics Lab (especially PhD student Dan King but also Sylvie Demouchy and Lauren Larkin) for their patience, guidance and willingness to share their knowledge and time.
References
[1]Aharonov, E., Whitehead, J.A., Kelemen, P.B., Spiegelman, M., 1995. Channeling instability of upwelling melt in the mantle. Journal of Geophysical Research-Solid Earth.102, 14821-14833.
[2]Holtzman, B.K., Groebner, N.J., Zimmerman, M.E., Ginsberg, S.B., Kohlstedt, D.L., 2003a. Stress-driven segregation in partially molten rocks. Geochemistry Geophysics Geosystems. 4, 8607.
[3]Holtzman, B.K., Kohlstedt, D.L., Zimmerman, M.E., Heidelbach, F., Hiraga, T., Hustoft, J., 2003b. Melt segregation and strain partitioning: implications for seismic anisotropy and mantle flow. Science, 301, 1227-1230.
[4]Hustoft, J.W., and Kohlstedt, D.L., 1996. Metal-silicate segregation in deforming dunitic rocks. Geochemical Geophysics Geosystems. 7, Q02001.
[5]Kelemen, P.B., Shimizu, N., Salters, V.J.M., 1995. Extraction of mid-ocean-ridge basalt from the upwelling mantle by focused flow of melt in dunite channels. Nature. 375, 747-753.
[6]Kohlstedt, D.L. and Zimmerman, M.E., 1996. Rheology of partially molten mantle rocks. Annual Review of Earth and Planetary Sciences. 24, 41-62.
[7]Parsons, R.A., Nimmo, F., Hustoft, J.W., Holtzman, B.K., Kohlstedt, D.L., 2006. An experimental and numerical study of surface tension-driven melt flow. Preprint submitted to EPSL.
[8]Paterson, M.S., and Olgaard, D.L., 2000. Rock deformation tests to large shear strains in torsion. Journal of Structural Geology. 22, 1341-1358.
[9]Riley, G.N., Kohlstedt, D.L., 1991. Kinetics of melt migration in upper mantle-type rocks. Earth and Planetary Science Letters. 105, 500-521.