Research Training Program

Erin Nicole DiMaggio
University of Michigan
Ann Arbor, Michigan

Sorena S. Sorensen, Ph. D.
Supervising Scientist
Department of Mineral Sciences

"The RTP program has given me a chance to perform meaningful scientific research, but more importantly it has taught me ways to overcome the everyday frustrations of investigation."

Evidence for Feldspar Replacement in Altered Metamorphic Tuffs in the Duck Lake Volcanic Suite

During metasomatism, fluids change the compositions, minerals and textures of rocks. Rocks in the Duck Lake Volcanic Suite (Sierra Nevada, east-central California) show evidence of metasomatism. The original tuffs, which are eruptive volcanic rocks made of ash, pumice and rock and crystal fragments, were deposited ~164 million years ago during the Jurassic Period, when the region was undergoing subduction-related volcanism. About ~50 million years after deposition, the rocks were heated and contact-metamorphosed by intrusions of granitic magma. Cathodoluminescence (CL) imaging reveals both textural and chemical patterns in the rocks. When bombarded by an electron beam, minerals in thin sections emit visible light (CL), which were captured with a highly sensitive digital camera. Different compositions of minerals luminesce different colors, so CL was used to differentiate stages of mineral growth and alteration. CL textures and mineral analyses with the electron microprobe show that two generations of metasomatism affected the rocks at Duck Lake. Early metasomatism was a low-temperature event during which fluids replaced the primary igneous feldspar minerals, sanidine and plagioclase, with nearly pure potassium and sodium feldspars, respectively. The second metasomatic event occurred during the waning stages of the emplacement of a pluton (magma cooled at depth). The crystallizing magma created a system of moving, hot fluids (a hydrothermal system) in the surrounding rocks, and hot fluids entered the tuffs. These fluids added calcium and partially replaced potassium and sodium feldspars by calcium-rich plagioclase feldspar. Patterns observed in thin sections show that this exchange probably happened while the pluton was cooling, making the metasomatism a retrograde, or late-stage feature. These results help understand how fluids move through rocks during metamorphism, and may eventually increase understanding of ore deposit formation, tectonic history and the geochemical evolution of continental crust during magmatism.

This research was supported by a grant from the National Science Foundation Research Experiences for Undergraduates program, Award Number DBI-9820303.

Letter of Gratitude