Research

I am no longer conducting academic research, but here you can read about some of the systems and projects I studied as a geologist!

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, but also sets strong foundations as we enter a new era of analytical and computational capabilities in the geosciences.

Projects

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, sought to reconstruct the timing and extent of lower crustal hydration using the petrology and geochemistry of these xenoliths. We performed high spatial resolution electron microprobe analyses 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 2020 and 2021 I mentored undergraduate interns 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

Schematic illustration of a transcrustal, vertically heterogeneous magma system where crystal growth can occur at multiple stages of magma fractionation, segregation, cooling, recharge, and emplacement or eruption. My research asks: where do quartz and zircon crystallize in the crustal column, and what does that tell us about crustal thickness?

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 applied this method to Lhasa terrane granitoids to reconstruct the pre- and syn-collisional crustal crustal thickness of Southern Tibet. I used oxygen isotopes and textural analyses to understand complicating factors that may modify the primary magmatic signature in these inclusion-host pairs.

Screen Shot 2018-08-09 at 4.23.06 PM — The importance of barometry (depth) in reconstructing the history of an orogen. We can only directly reconstruct crustal thickness if we can identify the pressure (depth) of magmatism.

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 combined 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.

I developed the metric “∆DM”, which is the difference between model Moho depths of two pseudobarometers (La/Yb and Sr/Y) for the same whole-rock analyses. These figures show the ∆DM of magmatic rocks from Southern Tibet, compared to two isotopic tracers that are typically used to track assimilation between crustal and mantle material in magmas. Figures A and C show ∆DM vs. ε_Nd , which is the part-per-10,000 deviation of the ¹⁴³Nd/¹⁴⁴Nd isotope ratio from chondrite. Modern depleted mantle has ε_Nd ≈ +8, while old continental crust is as low as -20. Figures B and D show ∆DM vs. ⁸⁷Sr/⁸⁶Sr_i, which is the age-corrected ratio of radiogenic ⁸⁷Sr to ⁸⁶Sr. The depleted mantle has ⁸⁷*Sr/*⁸⁶Sr_i ≈ 0.704, while crustal material, due to higher concentration of the radioactive parent isotope ⁸⁷Rb, has ⁸⁷*Sr/*⁸⁶Sr_i as high as 0.715.
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.

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).

epsHf in zircon RAFC v3 — Schematic representation of the effect of crustal thickness and magma recharge on the chemistry of zircon growth zones. A zircon grown in a magma body in a thin crust (left) will have a homogeneous, juvenile magmatic signal, as the country rock is too cold to assimilate appreciably with the magma. Periodic magma recharge will extend the time during which zircon can grow and enable more assimilation, leading to a heterogeneous and overall more crust-like chemical signature. Zircon grown in a deep magma body will have the opportunity for greater assimilation, leading to monotonic chemical zoning, from juvenile to crust-like from core to rim. Recharge further enables assimilation, with long residence times and frequent recharge leading to almost 100% isotopic assimilation of the country rock. Zircon Hf and O isotopic compositions may therefore function as an indirect proxy for crustal thickness.

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.

Tang et al., PNAS, 2019

Borlina et al., 2020, Science Advances