Dr. Wendy A. Bohrson
Ph.D. 1993, University of California at Los Angeles
Current Interests and Projects
Wendy documents the physical and chemical changes magmas
experience as they ascend through and are stored in crustal
storage and transport zones. Specific interests include
(1) quantifying the roles that magma processes (e.g., crystal-liquid separation,
crustal assimilation, and magma mixing/recharge) play in magma evolution,
(2) documenting the timescales that influence melt and crystal aggregation,
(3) describing the physical disposition (size, shape location) of crustal magma bodies,
and (4) at active volcanoes, exploring the relationship between petrologic
records of magma processes and geophysical signals that presage eruption.
I explore these interests in two ways. The first is via
development and application of energy- and mass-constrained open-system
thermodynamic computational tools that track compositional, dynamical and phase
equilibria changes in evolving crustal magmas. The second is through integration
of textural and compositional data on whole-rocks and minerals from volcanic centers
(e.g., Etna, Steens Mountain, Aeolian Islands, volcanoes in the Cascades). These approaches
address larger themes in the earth sciences including mantle and crustal contributions to
crustal growth and evolution, time-transgressive thermal, mass and dynamical changes to
magmatic systems and their associated crustal columns, and precursor signals to volcanic eruption.
Development of energy-and mass-constrained open-system thermodynamic computational toolsTwo decades of collaboration with Dr. Frank Spera (UCSB) has led to the development and application of computational models that reflect (some of) the complexity that attends the formation and evolution of terrestrial magmas. The first phase of this work, which was funded by the National Science Foundation, led to development of the Energy-Constrained Recharge, Assimilation and Fractional Crystallization (EC-RAFC) model, which tracks trace element and isotopic evolution of a magmatic system undergoing fractional crystallization, recharge/mixing and crustal assimilation (RAFC). Compared to earlier models, EC represents significant advancement in modeling open-system magma processes because energy and mass are exchanged and conserved. This research effort, which resulted in 7 publications in Journal of Petrology and Geochemistry, Geophysics, Geosystems, has had significant impact on the igneous petrology community. More information about EC-RAFC can be found at the following link.
Because the EC approach is limited to trace elements and isotopes and does not accommodate phase equilibria responses to RAFC, Wendy, Frank Spera, GUy Brown, and Mark Ghiorso (OFM-West) undertook the task of developing a new and improved mass and energy-constrained model called the Magma Chamber Simulator (MCS). MCS is a thermodynamic model that quantifies the impact of RAFC through mass and enthalpy exchange in a composite system composed of magma body, recharge reservoir(s) and wallrock (i.e., crust) (Bohrson et al. 2014). What distinguishes this model from existing models is that open-system mass and energy constraints are applied in a rigorous thermodynamic framework. The formulation, which reflects a marriage of MELTS (see http://melts.ofm-research.org/ for more information) and EC, tracks major and trace element, isotope, thermal and mass constraints of melt and minerals in the magma, recharge magma(s) and wallrock. Simulations provide a quantitative description of processes that control compositional diversity that can be compared with plutonic or volcanic data. MCS has enormous potential because it can be applied to any plutonic or volcanic system that shows evidence of open-system processes, and forward modeling can link quantitative open-system scenarios with petrologic outcomes, producing a taxonomy/library of petrologic indicators of open-system processes. In addition to developing this taxonomy, plans for MCS include (1) applying the model in collaboration to a number of natural systems using a range of data (whole-rock, in situ mineral,, etc.), and (2) enhancing the code to provide increasingly sophisticated and realistic constraints on magma evolution.
Information about access to the
MCS v1.0 is available at http://melts.ofm-research.org/magma-chamber.html
Characterization of magmatic systems through integration of textural and compositional data on whole-rocks and minerals using instrumentation and field workThe overarching idea of the work my students and I have done, mostly on young volcanoes, is that as magmas chemically and physically evolve through crystal-melt separation, magma recharge/mixing, and crustal assimilation, the relative timing and mass contributions from these processes are recorded in the chronology of eruptions (i.e., samples) and in individual crystals as they grow from core to rim. To deconvolve the series of "events," we acquire well constrained sample collections (through our own field work or collaborations. Whole rocks and minerals are characterized using a range of analysis techniques (e.g., XRF, electron microprobe, laser ablation ICPMS, thermal ionization mass spectrometer) to develop this chronological record. Additional characterization is done to develop an image of the subvolcanic magma storage and transport system (e.g., geobarometry, crystal size distribution analysis). The ultimate product is a quantitative (i.e., mass-constrained) image of how the crustal magma system evolves thermally, compositionally, and dynamically (e.g., geometry, location, size). Current Projects include Mt. Etna, the Aeolian Islands, the Steens Basalt and volcanoes in the Cascades. Below, 2 current projects are briefly outlined.
Crustal and Mantle Contributions to Historical Etna MagmasVolcanological data (effusion rate, eruption volume, etc.) for basaltic eruptions at Etna since 1329 AD are well documented, thereby affording an opportunity to examine how the associated magma body/bodies change through time. Etna provides an even more tantalizing target because in 1971, lava composition began to show marked enrichment (compared to older lavas) in some geochemical parameters including K, Cs, Rb, Sr and Pb isotopes that has continued. Since this “alkali enrichment” was first observed, Etna researchers have been debating its source. The two end-member ideas include changes in the mantle source(s) or crustal assimilation (aqueous fluids or melts). Associated with these chemical changes are increased frequency of eruption, explosivity, and magma volatile content. Thus, understanding the source of this change has hazard implications. My students and I, with a group of collaborators, have documented the source and timing of this alkali enrichment through whole-rock and in situ mineral analysis. The idea is to isolate the relative timing and location (in the crust) where the alkali enrichment manifests in a range of minerals (core to rim) that form in the crustal magmatic column. We have completed detailed in situ textural, major and trace element and Sr isotopic investigations of plagioclase and clinopyroxene. What has emerged is evidence that the mantle source may be evolving via increased contribution from subduction slab fluids (as proposed by several researchers) coupled with shallow assimilation of carbonate. We have also developed, through phase equilibria studies, an image of the magma storage and transport system. The next stage of study is to model the chemical evolution of the Etna lavas using the Magma Chamber Simulator.
Crustal Modulation of Flood Basalts-The Steens BasaltAt Steens Mountain (SE OR) as many as 200 lava flows, spanning no more than a few 100,000 years, are spectacularly exposed in a nearly km-thick section that records the early petrologic evolution of a continental flood basalt event. Published data suggest that early, more mafic magmas gave way to later-erupted, more evolved ones. In collaboration with Anita Grunder and PhD student Nicole Moore (OSU), my students and I are documenting in detail the textural and chemical changes that occur in this stratigraphic section. Data collection includes whole-rock major and trace element and Sr, Nd, Pb, Os, and Hf isotopes, and textural and major and trace element mineral data. The goal is to document how mantle vs. crustal contributions to the magmas change with time. The hypothesis is that the early stages are dominated by mantle input via high rates of magma recharge/mixing; as the crust heats up, younger eruptions show increased contribution from crustal assimilation. Quantitative modeling using the Magma Chamber Simulator is an integral part of this project, which has been funded by the National Science Foundation. The end product will be quantification of how the roles of mantle and crust changed with time, and how the thermal and chemical state of the crust evolved during this large volume intrusion event. The results have implications for other flood basalt terranes and for models that document how crust grows and evolves.
Current Research StudentsKaitlyn Nelson, Sylvana Bendana and Megan Graubard are currently doing MS research under Wendy's supervision. Kaitlyn is exploring the timing and source of selected element and isotopic enrichments in historical and recent lavas from Mt. Etna, and Sylvana and Megan are working on the Steens Basalt with Nicole Moore (PhD student) and Professor Anita Grunder (Oregon State University). Jenna Adams is doing an undergraduate honors thesis on Arenal using the Magma Chamber Simulator.
Bohrson received her Ph.D. from UCLA in 1993. From 1994-1998, she held a University of California Office of the President Postdoctoral Fellowship and a National Science Foundation Postdoctoral Fellowship, and was employed in the Department of Geological Sciences at the University of California, Santa Barbara as a researcher and lecturer.
At Central Washington University, Bohrson has been the Director of the Office of Undergraduate Research (2000-2003), and the Interim Director of Unviersity Research (2002-2003) and she was Chair of Geological Sciences (2007-2011). She is currently the Director of the CWU Science Talent Expansion Program.
•Bohrson WA, Spera FJ, Ghiorso, MS, Brown, GA, Creamer, J, and Mayfield, A, 2014, Thermodynamic Model for Energy-Constrained Open System Evolution of Crustal Magma Bodies Undergoing Simultaneous Assimilation, Recharge, and Crystallization: The Magma Chamber Simulator, Journal of Petrology 55: 1685-1717, doi:10.1093/petrology/egu036.
•Doherty, AL, Bodnar RJ, De Vivo B, Bohrson WA, Belkin HE, Messina A, Tracy RJ, 2012, Bulk rock composition and geochemistry of olivine-hosted melt inclusions in the Grey Porri Tuff and selected lavas of the Monte dei Porri volcano, Salina, Aeolian Islands, southern Italy, Central European Central European Journal of Geosciences, DOI: 10.2478/s13533-011-0066-7.
•Sbarbori E, Tauxe L, Gogichaishvili A, Urrutia-Fucugauchi J, Bohrson WA, 2009, Paleomagnetic behavior of volcanic rocks from Isla Socorro, Mexico, Earth Planets Space 61: 191-204.
•Salisbury MJ, Bohrson WA, Clynne M, Ramos FC, Hoskin P, 2008, Origin of the 1915 Lassen Peak eruption by magma mixing: Evidence for formation of
chemically distinct plagioclase populations from crystal size distribution and in situ chemical data, Journal of Petrology, doi: 10.1093/petrology/egn045.
• Bohrson, W. A., and F. J. Spera, 2007, Energy-Constrained Recharge, Assimilation, and Fractional Crystallization (EC-RAXFC):
A Visual Basic computer code for calculating trace element
and isotope variations of open-system magmatic systems, Geochem. Geophys. Geosyst., 8, Q11003, doi:10.1029/2007GC001781.
•Spera, FJ, Bohrson WA, Till CB, Ghiorso MS, 2007, Partitioning of Trace Elements among
Coexisting Crystals, Melt and Supercritical Fluid during Isobaric Fractional Crystallization
and Fractional Melting, American Mineralogist 92: 1881-1898.
•Bohrson WB, 2007,
Insight into subvolcanic magma plumbing systems, Geology 35: 767-768.
•Fowler, SJ, Spera FJ, Bohrson WA, Belkin HE, De Vivo B, 2007,
Phase Equilibria Constraints on the Chemical and Physical Evolution
of the Campanian Ignimbrite, Journal of Petrology 48: 459-493.
•Bohrson WA, Spera FJ, Fowler SJ, Belkin HE, DeVivo B, Rolandi G, 2006, Petrogenesis of the Campanian Ignimbrite: Implications for
Crystal-Melt Separation and Open-System Processes from Major and Trace Elements and Th Isotope Data,
in Volcanism in the Campania Plain, Vesuvius, Campi Flegrei and Ignimbrites, Developments in Volcanology 9, B. De Vivo (ed.), Elsevier, New York, pgs. 249-288.
•Spera FJ and Bohrson WA, 2004, Open-System
Magma Chamber Evolution: an Energy-Constrained Geochemical Model
Incorporating the Effects of Concurrent Eruption, Recharge,
Variable Assimilation and Fractional Crystallization (EC-E
RAÇFC), Journal of Petrology 45:2459-2480.
Teaching Interests and Schedule
Wendy teaches undergraduate major courses in Rocks and Minerals, as well as upper-level undergraduate/graduate level courses in Igneous Petrology, Volcanology, and Isotope Geochemistry. She also teaches graduate classes in her discipline and contributes to the core graduate classes, including research methods and field geology of the Pacfic Northwest. She has also taught in the Science Talent Expansion Program