It is well documented that portions of continental crust can be returned to the surface after being  subducted to mantle depths..  However, evidence of metamorphism at the ultrahigh-pressure (UHP) conditions is commonly obscured by later processes. The primary objectives of this research were to determine what factors favor (or hinder) preservation of the UHP record and to develop new methods to identify UHP rocks that retain little evidence of the transformation. The Rhodope Metamorphic Complex (Greece) is ideally suited for this work because multiple localities across the study region contain diamond-bearing rocks, but the pressure, temperature, and deformation conditions recorded in each locality differ, thereby allowing the separate evaluation of many of the variables that affect the preservation of UHP metamorphism. The research was conducted by a team of PIs, graduate students, and undergraduates from Bowdoin College and the University of Massachusetts Amherst.  

   The team completed two field seasons during which they collected field data and rock samples that were then analyzed in the laboratory to decipher which processes overprinted and obscured the record of UHP metamorphism. A particular emphasis of the research involved characterizing the major and trace element evolution of accessory minerals from peak conditions through overprinting reactions. These minerals are particularly valuable because diffusion of key elements is slow and many can be dated to provide timing constraints on metamorphic and deformation events. These data were correlated with compositional maps of slow-diffusing trace elements in major minerals (e.g., P and Ti), and then tested with petrologic and geochemical modelling.

Some of the key findings include the following:

1) The development (including testing and refining) of a non-destructive analytical approach for trace element characterization of the mineral zircon. This represents a significant new approach for accessing petrological archives that were previously inaccessible due to the minimum size of most microbeam techniques, typically 10 – 25 micrometers. 

2) An anomalous, yet ubiquitous domain within zircon grains preserves evidence of metamorphism at UHP conditions. Trace element and isotopic characterization of this domain across the Rhodope Metamorphic Complex yields a robust and reproducible signal. Because zircon is hosted by different minerals that preserve components of the rock’s history, documentation of geochemistry and geochronology are required to extract these complex histories. These zircon domains may be a useful tool for identifying UHP metamorphism in other locations because they persist, even in rocks that are largely recrystallized. 

3) Micrometer-scale imaging and trace element mapping has revealed multiple domains in the mineral kyanite that can be correlated to multiple metamorphic events, spanning the earliest to latest stages. These findings indicate that kyanite is an underutilized mineral for investigating the record of metamorphism in high-pressure terranes. 

4) The ultrachron microprobe (UMass) was used to characterize compositions and dates from the mineral monazite.  Results from the Sidironero locality contain up to five compositional/age domains, ranging from ca. 170 to ca. 35 million years ago, each of which corresponds to a specific set of reactions and/or geologic conditions. In addition to demonstrating monazite behavior during UHP metamorphism, these data are an important demonstration of Bayesian age sequence modeling by which zircon, monazite, and xenotime data can be integrated into a single age model for a sample or region. The results of this modeling will likely impact how other researchers approach rocks with complex metamorphic and deformation histories. 

5) Because of the impacts of the covid-19 pandemic, the PIs experimented with different modalities of engaging students in the research – virtual vs. physical laboratory work. Student reflections and the PIs’ assessments of student learning demonstrated that students were most actively engaged when they began a project by physically collecting data; subsequent virtual laboratory work then built upon the questions and data provided by the students’ initial work. This sequenced approach bolsters engagement, and both elevates and contextualizes the subsequent virtual laboratory work. The most recent iterations of course modules offer a template for future projects that intend to engage undergraduates in active research. 

6) Undergraduate and graduate students were integral to both the field and laboratory work. Eight undergraduate research projects and work-study positions culminated in poster presentations at research symposia and co-authored presentations at national conferences; 75% of these students identify as members of historically marginalized groups. Dozens of undergraduate students completed course modules and/or course-based research projects that advanced the research and also trained the students in how to approach scientific problems, design and carry-out research, use scientific instruments, and communicate their findings—essential skills for the modern workforce. Graduate students at UMass gained valuable mentoring skills through running workshops, participating in field activities, and engaging in research group meetings. This collaborative research project also provided opportunities undergraduates to use instruments at a research institution and interact with graduate students outside of this project. 

Last Modified: 08/18/2023
Modified by: Michael L Williams