Date of Award
6-26-2026
Date Published
August 2026
Degree Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Earth & Environmental Sciences
Advisor(s)
Jay Thomas
Keywords
diffusion;garnet;geology;metamorphism;petrology;thermobarometry
Subject Categories
Earth Sciences | Geology | Physical Sciences and Mathematics
Abstract
Garnet is a relatively common, rock-forming mineral which crystallizes in a wide variety of geologic settings. For example, garnet is a major phase of eclogites, rocks that record a journey from more than 40 kilometers depth when exposed at the Earth’s surface. Additionally, garnet is commonly found in metasedimentary rocks deformed and buried to upper- and mid-crustal depths during major episodes of mountain-building and can even be a magmatic phase in granitic rocks, crystallizing from evolved, alumina-rich melts. Because garnet is physically robust and stable over a large range of pressure-temperature conditions, oftentimes it records a protracted history of metamorphic events within the deep Earth, through major element chemistry, entrapped mineral inclusions, and crystallization age, which can be radiometrically dated. Understanding and interpretation of pressure-temperature-time histories recorded by garnet requires assumptions of mechanical and geochemical equilibrium, which oftentimes are violated in geologic systems. As such, novel approaches testing paradigms regarding the timing and pressure-temperature conditions of garnet crystallization are of paramount interest to understanding deep lithospheric processes. This dissertation contains three chapters that apply and develop novel methods of thermobarometry, geochronology, and geochemistry to interpret the crystallization history of garnet in light of deep Earth processes. The first chapter is an experimental study which evaluates the geologic utility of zircon-in-garnet (ZiG) elastic thermobarometry, a relatively new method of determining the pressure-temperature conditions of garnet crystallization which had never been experimentally investigated at geologically relevant pressures. By growing garnet with zircon inclusions in piston-cylinder experiments at known pressure-temperature conditions, this experimental work demonstrated that ZiG host-inclusion pairs can faithfully record their conditions of entrapment in a simple configuration. Additionally, these results suggest that preservation of original crystallization conditions is path-dependent, and perhaps best applied to geological settings which crystallize garnet at near-peak temperatures. The second chapter of this dissertation studies paired suites of garnet-bearing metasedimentary and metaigneous rocks from the New England Appalachians, USA. This second study uses Sm-Nd geochronology to establish the crystallization history of two chemically distinct populations of garnet from the same outcrop or unit with different nucleation densities. Results of Sm-Nd geochronology from three pairs of samples show that manganese-rich high nucleation density garnet crystallized ~5–18 million years prior to lower nucleation density, iron-rich counterparts, in contrast to previous pressure-temperature estimations. Manganese tracer diffusion modelling in garnet demonstrates that episodes of heating experienced by small-radius, high nucleation density garnet must have been highly transient in nature, adding to a growing body of work which recognizes the prevalence of metamorphic events <10 million years in duration. The third chapter of this dissertation applies the novel techniques of quartz-in-garnet and zircon-in-garnet elastic thermobarometry to garnet-bearing granites to estimate their depth of emplacement in the lithosphere. This study focused on Cretaceous granites of central Idaho, USA which record the collision of an oceanic island arc, transpressional shearing along a plate boundary, and voluminous arc and crustal melt generation of the Idaho Batholith. New elastic thermobarometry data obtained in this study agree with previous estimations of the pressure-temperature conditions experienced by synchrononously deformed metamorphic rocks and estimate emplacement at ~18–27 kilometers depth, demonstrating that elastic thermobarometry is a viable method of constraining the emplacement pressure of granites when garnet is a late-crystallizing phase.
Access
Open Access
Recommended Citation
Koch, Megan Marie, "Novel approaches using garnet to understand metamorphic cycles and crustal architecture" (2026). Dissertations - ALL. 2359.
https://surface.syr.edu/etd/2359
