Collaborative Research: An Experimental and Analytical Investigation of the Parameters That Influence Measured CO2 in Plagioclase-Hosted Melt Inclusions in MORB

Oregon State University, Corvallis OR

Investigators

Abstract

Volcanoes are major Earth features and, when they erupt, can cause significant damage to life, infrastructure, and ecosystems. When they occur on the seafloor, they can also trigger tsunamis that have the potential to cause devastating damage to coastal communities. Thus, our understanding of magmatic processes is important for both understanding how the Earth works as well as for helping us understand the dangers magmatic events and volcanic eruptions can cause. This research focuses on the story about magmatic processes that occur deep in the Earth as told by tiny inclusions of melt trapped in crystals of plagioclase, one of the most ubiquitous minerals in the Earth's crust. Up until now, melt inclusions in plagioclase have not been considered reliable archives of primary information for volatile geochemical species like H2O, CO2, or S due to diffusion and post entrapment leakage. There have also been questions about the origin of melt inclusions and when and where they form in the history of the plagioclase in which they occur. This research combines innovative laboratory experiments and comprehensive controlled calibration studies to reconstruct the original volatile composition of melt inclusions in plagioclase from ultra-phyric lavas erupted on the seafloor in the Pacific Ocean on the Juan de Fuca Ridge, just west of the Washington and Canadian border. The experiments will be done on both plagioclase and coexisting olivines from the same suite of samples to compare against one another and assess the degree to which post entrapment crystallization and the process of reheating impact the measured volatile content of the inclusions. Time-series experiments will be conducted on a heating stage at various temperatures (from 1200 C to 1270 C) and heating rates. Analyses of the major element composition of the inclusions will be carried out via electron microprobe; trace element compositions will be determined using laser inductively coupled mass spectrometry; Fourier transform infrared spectroscopy will be used to verify CO2 and H2O compositions of large inclusions; and the ion microprobe and micro-Raman spectroscopy will be used to analyze CO2, H2O, S, and Mg. The proposed experiments that will be carried out will determine the mechanisms by which volatiles are lost from inclusions during transport in the crust or during experimental heating via process such as diffusion, formation of bubbles, and leakage through cracks in the crystal. Goals of the research are to produce better measurements of the depth of mineral crystallization from magmas in mid-ocean ridge environments for both olivine and plagioclase. Questions to be addressed include how post entrapment crystallization in the inclusion and the formation of bubbles affect the measured volatile contents of the melt inclusions; how different homogenization methodologies and quench rates influence measured volatile contents; what the measured values of different volatile species indicate about the depth of formation of the crystal cargo and the CO2 budget of the ocean crust; and how does the information from plagioclase melt inclusions compare to that coming from olivine melt inclusions. Broader impacts of the work include development of new experimental techniques that have potential implications to other fields, societal impacts in terms of better understanding volcanic processes and understanding magmatically generated ore deposits. It will also involve the training of two graduate students and two undergraduates.