My research is primarily rooted in field work. Much of this field work is located the exposed intrusive igneous roots of ancient volcanic arcs. I complement this field work with a range of analytical techniques and also multiple numerical modeling approaches to better understand the fundamental chemical and physical processes in volcanics arcs and to constrain their timescales. Through this work, we can better understand the fundamental processes active within modern subduction zone systems, and how these systems have contributed to evolution of continental crust over Earth’s history.

Arc processes exposed in the southernmost Sierra Nevada

The Sierra Nevada Batholith in California was formed more than 80 million years ago in a volcanic arc system much like the present day Andes. Today, the Sierra Nevada exposes the root of this volcanic system, primarily comprising granites and other shallowly emplaced felsic lithologies. The southernmost Sierra Nevada are exceptional in that they expose a 30 km thick arc crust section from these shallow granites to gabbros emplaced near the base of the crust. As a result, this section offers a rare opportunity to study how magmas are transported from the mantle through a volcanic arc, and how the magma cools, fractionates, and evolves along this path.

Differentiation and stratification

The crustal section in the Sierra Nevada reveals a distinct dichotomy between the lower crust and the middle and upper crust. In my work I have shown that the lower crust exposes melt poor, dense mafic cumulates, while the middle and upper crust consist of less dense felsic granitoids approaching melt compositions. Additionally, I have shown that these rocks preserve a different magmatic structures, with the lower crust primarily exposing horizontal magmatic foliations while steep to vertical foliations are exposed in the middle and upper crust.

The contrasting lithologies and structures in the upper and lower crust result in a density stratified crustal profile, and have allowed me to infer the dominant processes and controls on magma differentiation and emplacement.

Rapid construction of a crustal section

As part of my study of the Sierra Nevada crustal section, I have undertaken a CA-TIMS U/Pb zircon geochronology project to constrain the construction timescales for this section. In this work, I have dated samples from all crustal levels and the full range of exposed lithologies, and show that the crustal section was constructed over a remarkably short timescale of 1-1.5 million years. This result has exciting implications for the thermal history of the arc and for the flux rates of magma in arc settings.

Metasediment processes

Metasedimentary material are exposed throughout the Sierra Nevada crustal section, and provide a record of transport of material in the solid state within the arc system. By combining a detrital zircon study of these metasediments with detailed metamorphic petrology, I aim to constrain their crustal source regions and the processes by which they are delivered to the lower crust.

Evidence for arc differentiation processes revealed in global arc lava datasets

Motivated by my observations from the Southern Sierra Nevada, I have recently started to think more broadly about subduction zone magmatism, and to use what we have learned from studying arc crustal sections, combined with constraints from experimental studies, global datasets of arc lavas, and phase equilibria modeling to constrain processes at arcs globally. Using these methods, I have shown that hydrous fractional crystallization is significantly more efficient for the generation of felsic arc magmas than partial melting, and that many of the common arguments for magma differentiation via partial melting are equally permissive of fractional crystallization (Jagoutz and Klein, 2018; American Journal of Science). I am currently extending these same observations and constraints to interrogate a dataset of global arc magmas to characterize the role of (hydrous) fractional crystallization in the generation of evolved melts at active arcs, and to better understand how this process differs in continental and island arcs.

Changes to subduction zone processes over Earth’s history

I am particularly interested in understanding how subduction zone processes may have varied over Earth’s history. Too often, the argument over early Earth plate tectonics is distilled to either there were subduction zones identical to what we observe in modern systems, or subduction zones did not exist in the early Earth. This discourse leaves little room for the possibility that subduction zones were active on the early Earth, but that, due to a range of factors, this process and its products are not identical to what we observe today. By using both phase equilibria and geodynamic modeling, I have shown that subducted slabs in the early Earth would have rarely if ever stagnated in the mantle transition zone, a behavior that we observe in many slabs today (Klein et al., 2017; EPSL). 

Model development

As part of my modeling work, I have been involved in the development of SiStER, an easy to use, flexible, and efficient finite difference marker-in-cell geodynamic modeling code implemented in MATLAB (Olive et al., 2016; GJI). You can download SiStER here.