Chemically Reactive Fluid Flow During Regional Metamorphism

Johns Hopkins University, Baltimore MD

Investigators

Abstract

Ferry EAR-0229267 In this project, the PI and his graduate students will conduct field and laboratory studies on the origin and the mineralogical, isotopic, and chemical evolution of metamorphic rocks. Infiltration of rocks deep in the Earth's crust during episodes of mountain building by chemically reactive H2O-CO2 fluids is now recognized as an essential driving force of mineral reactions during metamorphism. Much progress has been made over the last decade in understanding the sources, amounts, geometry, and geological controls on reactive fluid transport in contact aureoles. Details of the physical and chemical behavior of reactive fluids during regional metamorphism, however, have remained more obscure and are the subject of research supported by this grant. A fundamental characteristic of regional metamorphic terrains is the common occurrence in siliceous carbonate rocks of mineral reactants and products in proportions that vary widely on scales ranging from several mm to several 100 m. The specific, pivotal question that will be addressed is whether these occurrences result from flow of variable amounts of reactive fluid within different, chemically isolated layers or whether they result from variations in initial amounts and compositions of minerals prior to reaction in different layers that maintain perfect or almost perfect chemical communication by diffusion. The answer to the question, in turn, will lead to an understanding of the mechanism (diffusion vs. flow) and geometry of reactive fluid transport during regional metamorphism, the source(s) of reactive fluid, the driving force for reaction, and the amount of fluid involved. In order to draw general conclusions, investigations will be made of carbonate rocks in medium-pressure terrains in eastern Vermont and the southern Swiss Alps and in a low-pressure terrain in south-central Maine. Research will involve mapping and sample collection in the field, chemical analysis of minerals with an electron microprobe, chemical analysis of whole-rock samples by X-ray fluorescence, O and C stable isotope analysis of carbonates and silicates by mass spectrometry, and calculations based on geochemical thermodynamics and transport theory. The project will support the training of three women Ph.D. students at Johns Hopkins University.