Departmental Research in Earth, Environmental and Planetary Sciences Seminar
Graduate Interdisciplinary Earth Science Symposia (GIESS)
Speakers: James Eguchi and Sriparna Saha
Friday, March 2, KWGL Room 100, 12:00 Noon
James Eguchi: “C, O and tectonics”
CO2 and O2 are among the most important gases on the planet as they both play important roles in regulating the habitability of Earth. Importance to habitability is not the only link CO2 and O2 share; the cycling of CO2 and O2 are intimately linked through the process of photosynthesis. Understanding the relationship between the CO2 cycle and the O2 cycle allows us to make important interpretations about the evolution of these two gases on Earth. Two of the most dramatic events in the CO2 and O2 cycles are the Lomagundi event and the Great oxidation event which occurred about 2.5-2.0 billion years ago. There have been many proposals on what may have caused these events, however the debate is still open and there has been less efforts made on proposing a single mechanism that would result in both the Lomagundi event as well as the Great Oxidation event. Here we propose that a change in the Earth’s tectonic mode could cause both an increase in atmospheric oxygen as well and a positive C isotope excursion as seen in the geologic record.
Sriparna Saha: “Constraints on the Chemistry and Abundance of hydrous phases in Sub Continental Lithospheric Mantle: Implications for Mid-Lithospheric Discontinuities”
Origins of a 2-10 % reduction in seismic shear wave velocity (V S ) at depths of 60-160 km in sub continental lithospheric mantle (SCLM) regions, identified as the Mid LithosphericDiscontinuity (MLD) are highly debated. One of the proposed explanations is the presence of hydrous minerals such as amphibole and phlogopite at these depths. Although stability and compositions of these phases in peridotite + H 2 O ± CO 2 have been widely explored, variation in their composition and abundance as a function of permissible SCLM chemistry remain poorly understood. We have compiled peridotite phase equilibria experiments conducted over a range of pressure (0.5-8 GPa) and temperature (680-1300°C) for bulk H 2 O and CO 2 contents of 0.05-13.79 wt. % and 0.25-5.3 wt. %, respectively to understand how compositional parameters such as CaO and alkali/H 2 O affect the chemistry and abundance of amphibole and phlogopite. This study shows that the abundance of amphibole increases with CaO content and decreasing alkali/H 2 O. The abundance of phlogopite varies directly with K 2 O content. Unlike phlogopite compositions that remain consistent, amphibole compositions show variability (pargastitic to K-richterite) depending on bulk CaO and Na 2 O. Mineral modes (obtained by mass balance on a melt/fluid free basis were used to calculate, aggregate shear wave velocity, V S for the respective assemblages and compared with absolute values observed at MLD depths. Vs shows a strong inverse correlation with phlogopite and amphibole modes (particularly where phlogopite is absent). Assuming the Mg# of cratonic xenoliths, 5-10% phlogopite at MLD depths can match the observed Vs values, while CaO contents in cratonic xenoliths limit the amphibole abundance to 10%, which is lower than previous estimates based on heat flow calculations. In conjunction with the modes of hydrous phases and corresponding V s values, modes of orthopyroxene could be used to identify the process of volatile introduction and thus be linked to models of craton formation.