My principal research interest is to understand the chemical and physical processes and their tempos leading to continental crust formation in subduction zones. In particular, I am focusing on understanding the vertical chemical and structural architecture of magma plumbing systems developed during high magma flux periods in continental arcs and the interaction of magmatism and tectonism in Cordilleran orogens. I am working on different scales from mineral to arc crustal-wide studies and aim to integrate structural with geochemical processes to better understand feedbacks between magmatism and tectonism. I use a multidisciplinary approach including field observations, microstructural studies, whole rock and mineral chemistry, geochronology, experimental petrology, geochemical and thermal modeling and isostatic mass balance considerations.

Vertical variations in the oxidation state of the arc crustal column

Arc volcanic rocks are generally more oxidized than their counterparts erupting at mid-ocean ridges, however the process responsible for their oxidized nature is debated. The transfer of volatiles and/or oxidized species in hydrous fluids from the subducting slab to the mantle wedge is commonly inferred to cause the oxidized nature of arc magmas. However, an alternative hypothesis states that mantle-derived melts can change their oxidation state during ascent through the arc crustal column through differentiation mechanisms (assimilation, fractionation, degassing) before eruption. This can possibly create an associated vertical crustal stratification in fO2 throughout the crustal column. I am investigating a potential effect of crustal fractionation on magmatic fO2 by studying rocks from the exposed Talkeetna arc crustal section in Alaska.  Rocks from the Talkeetna arc span different crustal levels (lower crust to volcanics) encompassing a wide compositional spectrum, which allows to investigate the influence of differentiation on changes in magma oxygen fugacity. This work is in collaboration with Claire Bucholz at Caltech.

Histogram showing oxygen fugacity calculated from Fe-Ti oxide pairs relative to Ni-NiO buffer (ΔNNO) from lavas erupted at magmatic arc settings and mid-ocean ridge basalts.

Fe3+/FeT ratios in amphiboles – a tool for understanding the redox state of arc magmas

Vertical Divider

Emma Sosa and myself at the APS beamline 3 at the Advanced Photon Source at the Argonne National Laboratory analyzing amphibole grains with Mössbauer spectroscopy.

Thin section picture of brown-green amphiboles from the 1991 eruption of Mt. Pinatubo.

Plutonic rocks comprise the majority of magmas in subduction zones (volcanic to plutonic ratios of 1:20-1:30) but their magmatic fO2 is understudied. This partially arises due to the fact that plutonic rocks generally lack mineralogy amenable to quantitative estimates of fO2 using traditional oxybarometers (e.g. FeTi oxides), highlighting the importance of other approaches to constrain magmatic fO2. Amphibole is a common mafic phase in arc plutonic rocks, occurring in ultramafic cumulates and gabbros characteristic of the lower crust as well as in more evolved, high-silica intrusive rocks found in the middle to upper crust. Therefore, the Fe3+/FeT ratio in amphibole has the potential to provide quantitative constraints on variations in fO2 with depth in the arc crust, as well as the effects of differentiation on magmatic fO2. I am measuring Fe3+/FeT ratios using high-resolution synchrotron-based Mössbauer spectroscopy in natural and synthetic amphiboles. The latter of which have been synthesized from high-pressure and temperature piston-cylinder experiments to develop a calibration for amphibole Fe3+/FeT ratios as a function of fO2 and understand the accommodation mechanism of ferric iron in the amphibole structure. This work is in collboration with Claire Bucholz, Jennifer Jackson, Natalia Solomatova and Paul Asimow at Caltech. 

Magma addition rates to arc crust and its application for element and volatile cycling

Magma transfer from the mantle to the crust in arcs is an important step in the global cycling of elements and volatiles from earth’s interior to the atmosphere and influences a large variety of processes and crustal features: heat budget, elemental cycling, crustal deformation, igneous differentiation, volcanic-plutonic connections, isostatic responds and mass balance of the crust. Compared to extrusive magmatic fluxes, intrusive magmatic fluxes are less well constrained but likely play a significant role given their vast volumetric dominance. Exposed arc sections provide a means to estimate magma addition rates (MARs) to different crustal levels and highlight their spatial variability (e.g., fore-arc to rear-arc) by using a combination of field and geochronologic data. I investigated three such sections (Cascades Core, Washington State; Sierra Nevada, California; Famatinian arc, Argentina) and observed an increase in volume addition with depth. I also found significant spatial variability in magma addition, with less added in the fore-arc (~ 15 %) and rear-arc (~ 10 to 30 %) settings compared to the main arc. More results can be found in Ratschbacher et al., 2019.

Plot showing calculated CO2 degassing fluxes (Mt C/year) since 750 Ma, a ratio of flare‐up to lulls of 50:50 and an arc width of 97 km. Shown is also the relative importance of arc magmatism throughout the Phanerozoic (red curve) from zircon data and global greenhouse and icehouse climates after McKenzie et al. (2016). Ranges of global continental arc CO2 fluxes determined by other methods are shown in green.

Vertical temporal, chemical and physical characteristics of flare-up magmatism in thick continental arcs

Geologic map of the Cascades Crystalline Core composite are section, Washington State, USA (modified after Haugerud and Tabor, 2009 and Miller et al. 2009).

The vertical architecture, extent of magmatic processes and their tempos throughout arc crustal columns are difficult to constrain due to the lack of exposure. Exposed arc crustal sections allow studying temporal and chemical characteristics of the magma plumbing system developed during arc magmatism. The mid-Cretaceous flare-up period in the Cascades arc of Washington State is exposed as multiple intrusive complexes emplaced from ~ 5 to 35 km depth reflecting chemical and temporal changes throughout ~ 10 Ma of flare-up magmatism at different crustal levels. I use a combination of field observations, published high precision U-Pb ID-TIMS zircon ages, whole rock geochemistry and mineral chemistry to monitor magma addition rates at different crustal levels and throughout the flare-up period and understand where in the crustal column differentiation has taken place and how flare-up magmatism changes with time. Furthermore, syn-magmatic shortening during the flare-up period allows to investigate the deformational responds of magma addition at different crustal levels and to compare the tempos of magma addition and deformation. This work is in collaboration with Scott Paterson at USC and Lawford Anderson at BU. 

Timescale of magma reservoir construction and in-situ fractionation in the shallow crust

The size, shape, construction timescales, hypersolidus lifespan and extent of in-situ fractionation in shallow crustal magmatic reservoirs and their connection to volcanic lithologies erupted at the surface is highly debated. Central to this debate is the interpretation of zircon crystallization ages in the context of the emplacement of magma batches (i.e. construction timescale of magma reservoirs) and melt residence timescales at the emplacement level. We established a new workflow to better determine emplacement ages of compositionally different magma batches from high-precision U-Pb ID-TIMS-TEA zircon ages using MELTS modeling and Bayesian statistics. Results are combined with thermal modeling, whole rock geochemistry and zircon trace element compositions to determine the size, longevity, melt fraction and extent of in-situ fractionation in a magma reservoir. The Jurassic tilted, bimodal (gabbroic and granitic) Guadalupe igneous complex (GIC) in the Sierra Nevada arc presents an exceptional opportunity to test this workflow and determine the construction duration of a subvolcanic (~ 1 to 10 km) magma reservoir and estimate the timescale of emplacement and extent of in-situ differentiation to create compositional bimodality. This work is in collaboration with Brenhin Keller at the Berkeley Geochronology Center, Blair Schoene at Princeton University and Scott Paterson and David Okaya at USC. More information in Ratschbacher et al. 2018

Rank-order plot of U-Pb ID-TIMS zircon ages from units in the GIC, increasing from right to left according their stratigraphic position in the complex. Shown are also SiO2 whole rock compositions from dated samples and the saturation and solidus age of zircons for each sample determined by a Bayesian approach. Each bar represents one single grain analyses.

Results of 2D finite difference thermal modeling. Results are shown in 50 ka time steps and color-coded according to T. From panel 3 onward, the figures zoom into the pluton (box in panel 2).

Depth-dependent changes in deformation and magmatism in Cordilleran orogens

Results of isostatic mass balance modeling of the Famatinian orogen using field constrains and published data as input parameters

My work in the Famatinian arc in NW Argentina focuses on the consequences of magma addition to the crust of Cordilleran-type orogens and the effects thereof to crustal deformation, crustal-wide mass balance and the isostatic behavior of the orogen. The Ordovician Famatinian arc comprises a composite arc crustal section exposing depths from volcanics to approximately 30 km depths. Thus represents a great place to investigate depth-dependent changes in magma addition and deformation of igneous and host rocks at different crustal levels. These constrains can then be used as input parameters for a crustal-wide isostatic mass balance model to better understand changes in crustal thickness, elevation, exhumation and erosion during orogeny. Our work shows that magmatic thickening dominates the total crustal thickness and drives mountain building during arc magmatism, while after arc magmatism erosion rates dominate over tectonic thickening driving exhumation of the arc. Furthermore, field studies imply a change in deformation intensity with depth in both indigenous rocks and host rocks during active emplacement of magma at all crustal levels. This work is in collboration with Pablo Alasino and Mariano Laroverre at CRILAR and La Rioja University, Argentina and Scott Paterson at USC. 

The role of arc magmatism in strain (de)-localization in shear zones

The Sierras Pampeanas in northwest Argentina expose an extensive network of dominantly east-dipping ductile shear zones comprising reverse motion. These mylonite to ultramyonlite zones deform Early to Middle Ordovician arc intrusive and extrusive rocks during the late stages of the Famatinian orogeny emplaced along the Gondwana margin. They are interpreted to record ongoing shortening after magmatism terminated following the Ordovician magmatic flare-up event. The timing of mylonite formation relative to arc magmatism in intrusive rocks is controversial. We document a newly discovered ~ 10 km wide mylonite zone in the Cuesta de Randolfo area, southern Puna of Argentina. This area allows to investigate the transition from magmatic to solid-state deformation and the role of arc magmatism in localization of ductile deformation in the center of magma-rich Cordilleran  orogens such as the Famatinian system. This work is in collaboration with Tarryn Cawood at USC and a group of PIs from USC and Argentina.

Geologic map of the Cuesta de Randolfo mylonite zone with age data and solid-state foliation orientations.

Differentiation of peralkaline rocks and associated mineral deposits 

Compositional variation of eudialyte-group minerals plotted against their stratigraphic height in the complex (Ratschbacher et al. 2015).

The agpaitic rocks in the alkaline to peralkaline, 1.16 Ga Ilimaussaq igneous complex in West Greenland exhibit spectacular layering and are exceptionally rich in REE. The last intrusive phase of the Ilimaussaq igneous comprise the agpaitic, so called lujavrites, which were interpreted to form in a closest system by in-situ fractionation leading to an enrichment of REE into wt. % amounts. Eudialyte is an early crystallizing phase capable of monitoring the evolution of the melt during differentiation. We developed fractionation indicators based on eudialyte geochemistry to investigate the differentiation of the melt and detect recharge events in the lujavrite sequence. Based on field mapping and microstructures, we have also been able to present an emplacement model for the last intrusive phase of the Ilimaussaq igneous complex – more information in Ratschbacher et al. 2015.