Barbara Ratschbacher
Research areas
Amphibole minerals as drivers and recorders of magmatic differentiation
Amphibole minerals are common igneous minerals in hydrous arc magmas of broad compositional range and are stable over large pressure and temperature conditions. Amphibole chemistry has been used extensively to understand a wide range of magmatic processes due to the mineral´s capability of recording crystallization temperatures and pressures, as well as melt compositions.
In particular, I am interested in whether Fe3+/FeT ratios (Fe3+/(Fe3+ + Fe2+) measured in amphibole reflect the Fe3+/FeT ratios of the melt from which they crystallized (i.e., amphibole functions as a proxy for magmatic oxygen fugacity). In order to determine Fe3+/FeT ratios in amphibole, we use single-crystal synchrotron Mössbauer spectroscopy (SMS). Experiments are conducted at beamline 3ID at Argonne National Laboratory, USA. SMS is an analytical technique that provides the spatial resolution to avoid averaging heterogeneous grains (e.g., containing iron-bearing inclusions) and detecting intra-grain Fe3+/FeT ratios variations. It uses the physics of nuclear forward scattering on single crystals and therefore it does not require reference spectra for data interpretation as required for other techniques determining Fe3+/FeT ratios (e.g., X-ray absorption near edge structure spectroscopy).
Ongoing projects focus on determining Fe3+/FeT ratios of volcanic amphiboles and evaluate whether these correlate with magmatic oxygen fugacity during crystallization. These volcanic amphiboles crystallized under a range of magmatic oxygen fugacity conditions, independently constrained using thermodynamic oxybarometry.
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Magma dynamics and temporal evolution of upper crustal magma reservoirs
Disentangling the processes that create the chemical and isotopic characteristics of upper crustal magma reservoirs (e.g., fractional crystallization, magma mixing, assimilation, sub-solidus alteration) and investigating the extent that these processes take place at the emplacement level is important for understanding the formation of economically important porphery-related deposit and the volcanic-plutonic connection.
Trace element compositional trends in zircons separated from single hand samples have been used to infer dynamic processes in magma reservoirs. A recently published study found that volcanic and plutonic hand samples span a wide range of variability (i.e., wide range of coefficients of variation), but there is no systematic difference in the average variability between plutonic and volcanic samples (i.e., no difference in the mean coefficient of variation). This indicates that dynamic processes related to eruption are not necessarily required as a fundamental process to create hand sample-scale compositional heterogeneity beyond what is present due to dynamic processes in the reservoir recorded in plutonic samples (Ratschbacher et al. 2024).
Past work has been focused on the Jurassic bimodal (gabbro and granite) and upper crustal Guadalupe igneous complex, Sierra Nevada, USA, where we combine whole rock and mineral major, trace element, and isotopic compositions with high-precision U-Pb ID-TIMS zircon geochronology to evaluate differentiation processes and their timescales (Ratschbacher et al., 2018, 2024)
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Spatial, structural, and chemical evolution of magmatic arcs over time
Magmatic activity in arcs involves the transfer of mass, heat, elemental, and volatile species from the mantle to the crust and ultimately to the atmosphere. Thus understanding how magmatic arcs evolve chemically, spatially, and temporally is important to understanding the budgets of global heat transfer, volatile cycling, ore formation, and the mass balance and isostatic behavior of the arc crustal column, which variably influences mountain building, erosion rates, and climate.
Research in this area focuses on better constraining magma addition rates to the crust using extinct and exposed arc crustal sections (Ratschbacher et al. 2019), how arc-related igneous rocks are deformed during and after peak magmatism (Lusk et al. 2021, Ratschbacher et al. 2021), and investigate processes in transcrustal magmatic systems (Delph et al. 2021).
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