Date of Award

8-22-2025

Date Published

September 2025

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth & Environmental Sciences

Advisor(s)

Jay Thomas

Second Advisor

Weiwei Zheng

Keywords

crystallization;fluid inclusions;geochemistry;granite;magma;thermobarometry

Subject Categories

Earth Sciences | Geology | Physical Sciences and Mathematics

Abstract

Granite is ubiquitously recognized by many people due to its common use as a building material and its abundance in the Earth's continental crust. From countertops to library steps, and as a source of economically important elements, granitic rocks are deeply intertwined with human development. Defining the pressure and temperature (P– T) conditions under which granitic composition magmas crystallize is critical for understanding their petrogenesis. This dissertation explores granite petrogenesis across three chapters, utilizing detailed analysis of rock-forming minerals, associated fluids, and geochemical variations to understand how compositional variability influences the lowest possible crystallization conditions. Chapter 1 investigates the physical and geochemical evolution of fluids and minerals in the shallow-level Rito del Medio pluton (RDMP), a high-silica hydrous granite located near Questa, New Mexico. The RDMP is a shallow-level medium- to coarse-grained equigranular granitic pluton with abundant miarolitic cavities, and has a “normal” composition that is not enriched in fluxing elements such as Li, B, P, or F. The RDMP has two distinct paragenetic contexts. The groundmass minerals crystallized from water-saturated melts and the miarolitic cavity minerals crystallized from hydrothermal fluids exsolved from the RDMP magma. The rock has a consistent mineralogy through both paragenetic contexts permitting a complete analysis of its magmatic-to-hydrothermal crystallization history. Chapter 2 tracks the changes in P–T throughout the crystallization history of the RDMP using a suite of thermobarometric methods including two-feldspar thermometry, Ti-in-quartz solubility, fluid inclusion microthermometry, Ti-saturation biotite thermometry, and quartz-in-garnet elastic thermobarometry on minerals and inclusions in both paragenetic contexts that accompanied the magmatic-to-hydrothermal transition. Unlike many modern thermobarometric analyses of natural samples that provide a simplistic snapshot of one rock forming mineral’s crystallization, this research uses a multimethodological approach to provide a complete history of crystallization, while limiting the ambiguity and bias inherent in the utilization of a single method. Many thermobarometric methods, including those used in this research, applied to granitic composition rocks yield temperature estimates ~75 to 100 °C lower than the widely used haplogranite water-saturated solidus (hereinafter referred to as “haplogranite solidus”) currently considered as defining the minimum P–T conditions at which melt can exist. Chapter 3 addresses these discrepancies by evaluating the lowest P–T conditions at which granitic rocks can contain melt. Granite crystallization experiments were conducted on 12 granite compositions spanning the range of pressures from 2 to 15 kbar relevant to continental crust formation. The experimental results are consistent with numerous modern thermobarometric results on natural samples and extend magmatic conditions to significantly lower temperatures than the haplogranite solidus that defined the lower limit of magmatic crystallization for >67 years. This phase boundary is a foundational reference curve for a myriad of solid-earth disciplines involving continental crust including studies in igneous and metamorphic petrology, crust formation, geochemical reservoir development, planetary structure, crustal rheology, tectonics, and ore deposits, therefore it is paramount that this phase boundary is accurately recalibrated using modern experimental methodologies.

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