My long-term goals are to develop an integrated model for crustal magmatism, particularly in volcanic arcs, by coupling geochemical data with the physical dynamics of crystal-melt evolution and interaction. I focus on (1) the process of magma production in the upper mantle followed by transport and interaction with the overlying crust prior to eruption, (2) the physical process of crystal-melt separation and the geochemical response to this process through time, (3) transport of volatiles from source to surface, and (4) excess or cryptic degassing in volcanic systems induced by decompression.
ONGOING PROJECTS AND FUTURE DIRECTIONS
Evolution of the continental crust: generation and maintenance of large, silicic magma reservoirs
Examination of how the continental crust forms is essential because it bears on our understanding of how the earth differentiates. Since the average composition of the continental crust is granodioritic, investigating silicic magmas is fundamental to studying crustal evolution. Examining the processes that govern crustal evolution requires a comprehensive research program that considers the wealth of previous work done on this subject, while, at the same time, incorporates modern advancements in analytical techniques.
I focus on comparing the textural, petrological, and geochemical variations of intermediate plutonic rocks to crystal-rich rocks (up to 70 vol. % crystals) from: 1) large cognate, crystal-rich blocks included in explosive melt-dominant eruptions (see Deering, 2009; Deering, 2010), 2) ring faults and hypabyssal intrusions, 3) zoned ignimbrites (Deering et al., in review), and 4) monotonous intermediates. In a recent contribution, Olivier Bachmann and I developed a model that integrates the physical process of crystal fractionation with key trace elements to identify cumulate residue in silicic magmatic systems (Deering & Bachmann, 2010). Using this model as a foundation, we are developing a more robust geochemical characterization using EMP analysis of minerals and melt inclusions. More detailed analyses using LA-ICP-MS on individual minerals will further help to determine the behavior of trace elements in the evolving magma. My research team is currently writing a proposal to utilize X-ray tomography, which can be used to build a 3-D image of the rocks to determine more precisely the crystal size distribution, textures, and crystal orientation. This provides a quantitative framework for a more targeted analysis of the crystals and ultimately more meaningful interpretation of the magmatic evolution.
Excess or cryptic degassing in volcanic systems
Volatile elements (e.g., H2O, CO2, S, CL, F) dissolved in magmas play a fundamental role in controlling explosivity and style of volcanic eruptions. Equally important is the significant impact that these volatile species have on the climate following large volume eruptions (e.g. Tambora 1815, Pinatubo 1991). The mass of volatiles (e.g. SO2, CO2, HCl) released during large, caldera-forming eruptions can significantly exceed the initial amount dissolved in the erupted magma. The origin of this hidden mass of volatiles remains a controversial topic of debate (e.g. “the Sulfur Paradox”).
My current research has identified P-T-ƒO2-ƒH2O conditions and geochemical variations that are common among several post-caldera eruptions globally, which suggests these changes are governed by a profound perturbation in the silicic source zone within the mid- to upper-crust. In particular, a significant loss of volatile elements must have occurred. I am examining the physical response of the magmatic ‘rootzone’ to massive decompression events to better understand the impact on the heat and mass (volatiles) transport during and after these events and the effects on the recurrence interval between the large, caldera-forming eruptions. A portion of this research has been funded by a grant awarded to myself and Olivier Bachmann (see my CV).
My strategy is to develop a link between surface chemistry of geothermal reservoirs and the ancient to modern state of the magmatic system to determine the heat and mass balance. Multivariate statistical analysis (polytopic vector analysis) is used to determine the geochemical end-members of surface fluids that can then be used to differentiate between magmatic and atmospheric contributions, which will also provide information on the ‘state of the magmatic system’ and composition of the magma at depth.
Source to surface transport of magmatic volatiles in geothermal systems
Future growth of high-enthalpy geothermal power generation hinges upon the discovery of viable resource targets that lack obvious surface features. Exploration and development of these targets requires an integrated geothermal research programme.
In a recently funded, 5-6 year, multi-organization project funded by Mighty River Power, New Zealand, we aim to elucidate the mechanisms and indicators of heat and mass transfer from magmatic source areas to geothermal reservoirs. The primary objective of the project is to improve, refine and develop new field, experimental and theoretical techniques to ultimately establish an integrated approach to well targeting, and the discovery of blind geothermal systems in the Taupo Volcanic Zone (TVZ). To accomplish this objective, we have designed a multi-faceted research program involving a number of graduate students, two of whom I am co-supervising, from both the US and New Zealand. The international collaboration includes a scientific team that includes US organizations: University of Washington, United States Geological Survey, and New Zealand organizations: University of Canterbury, GNS Science, and Mighty River Power.
To identify new drilling targets associated with blind geothermal systems, we are using measurements of key chemical indicators (e.g. CO2, Cl, F, Ar, He, N) to assess the physical and chemical characteristics of existing successful geothermal areas, and those at unsuccessful areas, to distinguish between these systems. These findings will then be applied to potential new exploration. The specific outcomes of our project will include: 1) production of high resolution soil-gas and dissolved inorganic carbon (DIC) surveys; 2) identification of blind fault and fracture hosted fluid flow, 3) documentation of potential high up-flow zones, 4) integration of geochemical and geophysical datasets, and 5) description of fluid evolution from source to surface using thermodynamic and geochemical datasets.
My research team is currently writing an NSF proposal that will complement the New Zealand project by quantifying the heat and mass transfer in magmatic systems using well-exposed, shallow plutonic cross-sections. We will use the data from volatile species and hydrogen and oxygen isotopes to develop multiphase numerical models for the transfer of heat and mass throughout the evolution of the shallow magma reservoir.
Deering, C., Gravley, D., Vogel, T., Cole, J. & Leonard, G. (2010). Origins of cold-wet-oxidizing to hot-dry-reducing rhyolite magma cycles and distribution in the Taupo Volcanic Zone, New Zealand. Contributions to Mineralogy and Petrology.
Deering, C. D. (2009). Cannibalization of an amphibole-rich andesitic progenitor induced by caldera-collapse during the Matahina eruption: Evidence from amphibole compositions. American Mineralogist 94, 1162-1174.
Deering, C. D. & Bachmann, O. (2010). Trace element indicators of crystal accumulation in silicic igneous rocks. Earth and Planetary Science Letters 297, 324-331.
Deering, C. D., Bachmann, O. & Vogel, T. A. (in review). Co-eruption of extracted liquid and complementary cumulate mush following mafic intrusion: the case of the zoned Ammonia Tanks ignimbrite. Geology.
Deering, C. D., Cole, J. W. & Vogel, T. A. (2008). A Rhyolite Compositional Continuum Governed by Lower Crustal Source Conditions in the Taupo Volcanic Zone, New Zealand. Journal of Petrology 49, 2245-2276.