My Research Philosophy
The principal focus of my research is to evaluate the connections between mineralogy and crustal evolution. I capitalize on those connections with geochemical microanalysis to understand dynamic crustal and tectonic processes. My work thus far has been characterized by international, multidisciplinary collaborations working to solve complex geologic problems. The predominant theme of my research is to interrogate igneous rocks and their constituent minerals for their chemical, thermal, and structural histories. I study the effects of pressure, temperature, composition, and time (P-T-X-t) on mineral chemistry, preservation of primary magmatic signature, and the relationship between whole-rock chemistry and mineralogy. I believe strongly in the importance of improving diversity, equity, and inclusion in geoscience; my goal as a mentor is to recruit, support, and uplift undergraduate and graduate students from historically underrepresented backgrounds in geoscience.
My goal in research is to approach complex problems with the knowledge that, as humans, we tend toward bias and presumption in the interpretation of our data. I employ rigorous skepticism and thoughtful moderation as I attack scientific questions at the frontiers of geologic research. I believe taking a humble approach to exciting, complex problems not only serves to produce high-quality research in the moment, but also sets strong foundations as we enter a new era of analytical and computational capabilities in the geosciences.
Current Research
Reconstructing Crustal Hydration with Lower Crustal Xenoliths
Map of Colorado Plateau region after Selverstone et al. (1999; CO—Colorado, NM—New Mexico, UT—
Utah, AZ—Arizona). The Colorado Plateau is outlined with the bold dashed line. Locations of minette and serpentinized ultramafic breccia pipes, which contain crustal xenoliths, are noted with stars. Seismic lines and stations used to constrain crustal thickness, layers, and elastic properties are noted.
The Colorado Plateau is a high-elevation region in the Southwestern United States that was uplifted in the late-Cretaceous to mid-Tertiary. The average elevation of the plateau is nearly 2000m, and most of its elevation cannot be explained by tectonic shortening and thickening alone. It is hypothesized that a hydration event in the late Cretaceous or early Tertiary led to hydration of the lithospheric mantle and lower crust, decreasing overall density of the crustal column and leading to isostatic uplift of the plateau. The Navajo Volcanic Field in the four-corners region consists of multiple locations of minette and serpentinized ultramafic pipes that erupted ca. 28 million years ago. These volcanic pipes brought to the surface numerous mid- and lower-crustal xenoliths, including amphibolites, granulites, and altered igneous rocks ranging from mafic cumulates to felsic metagranitoids. My research, along with my supervisor Kevin Mahan and colleagues in Utah and France, seeks to reconstruct the timing and extent of lower crustal hydration using the petrology and geochemistry of these xenoliths. We are in the early stages of this research, and are currently surveying the mineralogy and alteration history off a suite of xenoliths from the NVF provided by Doug Smith from UT-Austin. perform high spatial resolution electron microprobe analyses of these xenoliths to examine the hydrated mineral assemblages in altered xenoliths, and reconstruct the extent of crustal hydration and the potential influence it had on the uplift of the plateau. In summer 2021, I mentored an undergraduate intern with the RESESS program, who worked with me to use compositional data from a subset of xenoliths to construct pre- and post-hydration density models of those lithologies. In addition to petrology and geochemistry, this project involves seismic velocity modeling and construction of model crustal columns pre- and post-hydration.
Geothermobarometry with Quartz and Zircon
Titanium (Ti) is incorporated into the quartz lattice as a function of temperature and pressure, and into zircon as a function of temperature. By measuring the Ti concentration of a quartz inclusion inside a zircon crystal, and the Ti concentration of the zircon itself, the temperature and pressure of crystallization can be constrained. This relationship has been experimentally calibrated and can be applied to a variety of systems to reconstruct the depth of crystallization of a magma, and by extension the minimum crustal thickness at the time of crystallization can be identified. In combination with U-Pb zircon ages, this method offers the opportunity to reconstruct the P-T-t paths of a suite of magmatic rocks, and potentially aid in reconstructing crustal thickness through time. I am currently working on applying this method to Lhasa terrane granitoids to reconstruct the pre- and syn-collisional crustal crustal thickness of Southern Tibet. I use oxygen isotopes and textural analyses to understand complicating factors that may modify the primary magmatic signature in these inclusion-host pairs.
This work is is under revision for Geology.
Critical analysis of indirect geochemical proxies (“pseudobarometers”)
Recent geochemical studies of global geochemical databases have identified an empirical relationship between crustal thickness and a number of trace element ratios in magmatic rocks, including La/Yb, Sr/Y, and Gd/Yb. Pressure-dependent petrologic controls on trace element partitioning have been speculatively invoked to explain the association with crustal thickness; effects of assimilation, source contamination, and thermal history are largely disregarded. I am currently working to combine existing global trace element and isotope geochemical data with forward thermochemical models of magma recharge, crustal assimilation, and crystal fractionation to constrain the accuracy of these trace element “pseudobarometers”, and create a more nuanced interpretive framework for their future use in original research.
These figures show that magmas with more “mantle-like” isotopic signatures tend to have a more negative ∆DM (i.e., the calculated moho depth from La/Yb is much shallower than Sr/Y) than rocks that have experienced more crustal assimilation. These data suggest that assimilation is an important modifying factor to La/Yb and Sr/Y pseudobarometers, lending uncertainty to their application in many settings where crustal assimilation is a significant contributor to final magma composition.
PREVIOUS RESEARCH
Zircon Geochemistry and Geochronology
Zircon (ZrSiO4) acts as a time capsule, recording the unique geochemical, temperature, and pressure history of its host rock(s) as it grew. Its durability and resistance to chemical resetting makes it an ideal tool for a wide variety of applications in igneous and metamorphic petrology. Zircons contain micron-scale growth histories that can be interrogated using high spatial resolution in-situ methods such as secondary ion mass spectrometry (SIMS).
My PhD dissertation research involved reconstructing the crustal thickness and magmatic inflation history of southern Tibet before and during the India-Asia collision, using zircons from Gangdese Batholith granitoids. I used a combination of U-Pb ages, Ti-in-zircon thermometry, and Lu-Hf isotope geochemistry to reconstruct the time-temperature-composition history of magmatic rocks in the southern Lhasa terrane, which can be linked to the structural development of the Tibetan crust throughout collision.
Alexander et al., 2019, JGR-Solid Earth (accepted 17 October 2019).
As the world’s most durable material, zircons also serve as our only record for the conditions found on earliest Earth, in the Hadean eon (4.56-4.00 Ga). Because the oldest whole rocks found on Earth are no older than 4.03 Ga, detrital zircons are our only lens into the geologic history of the Earth prior to that time. Previous research by the Harrison group at UCLA has identified evidence of subduction, felsic crust, and a hydrosphere as early as 4.3-4.4 Ga (Hopkins et al., 2008; Harrison, 2009; Bell et al., 2011; etc.). Along with collaborators at UCLA, MIT, and the University of Cambridge, I am currently assisting with a project seeking to understand the formation of the Earth’s geodynamo (i.e., the onset of the Earth’s magnetic field). Analyses of carefully-selected Hadean zircons from Jack Hills, Australia suggests there is no evidence of a primary paleomagnetic signal in Hadean zircons.
Borlina et al., 2020, Science Advances
Future Research Interests
- The Andes as a modern analog to the precollisional Gangdese arc, including comparison of whole-rock and zircon geochemistry and geothermobarometry.
- Comparison of Ti temperature and pressure signatures in zircon-quartz inclusion-host pairs from volcanic, plutonic, and high-grade metamorphic terranes with well-constrained crustal thicknesses and/or P-T-X-t histories.
- Experimental and empirical calibration of Ti activity in rutile-undersaturated rocks (with applications to Ti-in-zircon thermometry, Ti-in-quartz thermobarometry, etc.)
Ellen W. Alexander 