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    Sep 2011 - Jun 2017
    Graduate Student
    University of Wisconsin–Madison · Department of Geoscience
    May 2003 - May 2005
    Undergraduate research assistant
    Wesleyan University · Department of Earth and Environmental Sciences
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    Hanna Bartram
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    Camila Pineda
    Brad Singer
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    Thiruchelvi R Reddy
    Hasti Mirkia
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    Evgenya S Shelobolina
    Liz Percak-Dennett
    Reinhard Kozdon
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    Tim Foltz
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    Meagan E Ankney
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    Research Items
    The Laguna del Maule (LdM) volcanic field in Chile is an exceptional example of postglacial rhyolitic volcanism in the Southern Volcanic Zone of the Andes. By interferometric analysis of synthetic aperture radar (SAR) images acquired between 2007 and 2012, we measure exceptionally rapid deformation. The maximum vertical velocity exceeds 280 mm yr-1. Although the rate of deformation was negligible from 2003 January to 2004 February, it accelerated some time before 2007 January. Statistical testing rejects, with 95 per cent confidence, four hypotheses of artefacts caused by tropospheric gradients, ionospheric effects, orbital errors or topographic relief, respectively. The high rate of deformation is confirmed by daily estimates of position during several months in 2012, as measured by analysis of signals transmitted by the Global Positioning System (GPS) and received on the ground at three stations around the reservoir forming the LdM. The fastest-moving GPS station (MAU2) has a velocity vector of [-180 ± 4, 46 ± 2, 280 ± 4] mm yr-1 for the northward, eastward and upward components, respectively, with respect to the stable interior of the South America Plate. The observed deformation cannot be explained by changes in the gravitational load caused by variations in the water level in the reservoir. For the most recent observation time interval, spanning 44 d in early 2012, the model that best fits the InSAR observations involves an inflating sill at a depth of 5.2 ± 0.3 km, with length 9.0 ± 0.3 km, width 5.3 ± 0.4 km, dip 20 ± 3° from horizontal and strike 14 ± 5° clockwise from north, assuming a rectangular dislocation in a half-space with uniform elastic properties. During this time interval, the estimated rate of tensile opening is 1.1 ± 0.04 m yr-1, such that the rate of volume increase in the modelled sill is 51 ± 5 million m3 yr-1 or 1.6 ± 0.2 m3 s-1. From 2004 January to 2012 April the total increase in volume was at least 0.15 km3 over the 5.2-yr interval observed by InSAR. The inflating region includes most of the 16-km-by-14-km ring of rhyolitic domes and coulees. The similarity of high-silica rhyolite compositions on opposite sides of the ring and the concentration of rhyolitic eruptions since ˜20 ka suggest that processes within a large silicic magma chamber are responsible for the current deformation.
    rhyolitic volcanism in the SouthernVolcanic Zone of theAndes.By interferometric analysis of synthetic aperture radar (SAR) images acquired between 2007 and 2012, we measure excep- tionally rapid deformation. The maximum vertical velocity exceeds 280 mm yr–1. Although the rate of deformation was negligible from 2003 January to 2004 February, it accelerated some time before 2007 January. Statistical testing rejects, with 95 per cent confidence, four hypotheses of artefacts caused by tropospheric gradients, ionospheric effects, orbital errors or topographic relief, respectively. The high rate of deformation is confirmed by daily estimates of position during several months in 2012, as measured by analysis of signals transmitted by the Global Positioning System (GPS) and received on the ground at three stations around the reservoir forming the LdM. The fastest-moving GPS station (MAU2) has a velocity vector of [–180 ± 4, 46 ± 2, 280 ± 4] mm yr–1 for the northward, eastward and upward components, respectively, with respect to the stable interior of the South America Plate. The observed deformation cannot be explained by changes in the gravitational load caused by variations in the water level in the reservoir. For the most recent observation time interval, spanning 44 d in early 2012, the model that best fits the InSAR observations involves an inflating sill at a depth of 5.2 ± 0.3 km, with length 9.0 ± 0.3 km, width 5.3 ± 0.4 km, dip 20 ± 3◦ from horizontal and strike 14 ± 5◦ clockwise from north, assuming a rectangular dislocation in a half-space with uniform elastic properties. During this time interval, the estimated rate of tensile open- ing is 1.1 ± 0.04 m yr–1, such that the rate of volume increase in the modelled sill is 51 ± 5 million m3 yr–1 or 1.6 ± 0.2 m3 s–1. From 2004 January to 2012 April the total increase in volume wasatleast0.15 km3 over the 5.2-yr interval observed by InSAR. The inflating region includes most of the 16-km-by-14-km ring of rhyolitic domes and coulees. The similarity of high-silica rhyolite compositions on opposite sides of the ring and the concentration of rhyolitic eruptions since ∼20 ka suggest that processes within a large silicic magma chamber are responsible for the current deformation.
    Accurate and precise ages of large silicic eruptions are critical to calibrating the geologic timescale and gauging the tempo of changes in climate, biologic evolution, and magmatic processes throughout Earth history. The conventional approach to dating these eruptive products using the ⁴⁰Ar/³⁹Ar method is to fuse dozens of individual feldspar crystals. However, dispersion of fusion dates is common and interpretation is complicated by increasingly precise data obtained via multicollector mass spectrometry. Incremental heating of 49 individual Bishop Tuff (BT) sanidine crystals produces ⁴⁰Ar/³⁹Ar dates with reduced dispersion, yet we find a 16-ky range of plateau dates that is not attributable to excess Ar. We interpret this dispersion to reflect cooling of the magma reservoir margins below ∼475 °C, accumulation of radiogenic Ar, and rapid preeruption remobilization. Accordingly, these data elucidate the recycling of subsolidus material into voluminous rhyolite magma reservoirs and the effect of preeruptive magmatic processes on the ⁴⁰Ar/³⁹Ar system. The youngest sanidine dates, likely the most representative of the BT eruption age, yield a weighted mean of 764.8 ± 0.3/0.6 ka (2σ analytical/full uncertainty) indicating eruption only ∼7 ky following the Matuyama−Brunhes magnetic polarity reversal. Single-crystal incremental heating provides leverage with which to interpret complex populations of ⁴⁰Ar/³⁹Ar sanidine and U-Pb zircon dates and a substantially improved capability to resolve the timing and causal relationship of events in the geologic record.
    Explosive eruptions of large-volume rhyolitic magma systems are common in the geologic record and pose a major potential threat to society. Unlike other natural hazards, such as earth-quakes and tsunamis, a large rhyolitic volcano may provide warning signs long before a caldera-forming eruption occurs. Yet, these signs—and what they imply about magma-crust dynamics—are not well known. This is because we have learned how these systems form, grow, and erupt mainly from the study of ash flow tuffs deposited tens to hundreds of thousands of years ago or more, or from the geophysical imaging of the unerupted portions of the reservoirs beneath the associated calderas. The Laguna del Maule Volcanic Field, Chile, includes an unusually large and recent concentration of silicic eruptions. Since 2007, the crust there has been inflating at an astonishing rate of at least 25 cm/yr. This unique opportunity to investigate the dynamics of a large rhyolitic system while magma migration, reservoir growth, and crustal deformation are actively under way is stimulating a new international collaboration. Findings thus far lead to the hypothesis that the silicic vents have tapped an extensive layer of crystal-poor, rhyolitic melt that began to form atop a magmatic mush zone that was established by ca. 20 ka with a renewed phase of rhyolite eruptions during the Holocene. Modeling of surface deformation, magnetotelluric data, and gravity changes suggest that magma is currently intruding at a depth of ~5 km. The next phase of this investigation seeks to enlarge the sets of geophysical and geochemical data and to use these observations in numerical models of system dynamics.
    The rear-arc Laguna del Maule volcanic field (LdM) in the Andean Southern Volcanic Zone, 36 S, is among the most active latest Pleistocene–Holocene rhyolitic centers globally and has been inflating at a rate of > 20 cm a–1 since 2007. At least 50 eruptions during the last 26 kyr allow for a thorough interrogation of changes in the physical and chemical state of this large, 20 km diameter, silicic system. Trace element concentrations and Sr, Pb and Th isotope ratios indicate that the mafic pre- cursors to the LdM rhyolites result from mixing between partial melts of garnet-bearing mantle and crust in Th-excess and partial melts of garnet-free crust in U-excess. The 238U/230Th ratios of the LdM lavas are decoupled from the slab fluid signature, similar to several recently studied fron- tal arc volcanic centers in the Southern Volcanic Zone. A narrow range of radiogenic isotope com- positions and increasing isotopic homogeneity with differentiation indicate that silicic magma is generated by magma hybridization and crystallization in the upper crust with limited involvement of older, radiogenic material. New 40Ar/39Ar and 36Cl ages reveal a wide footprint of silicic volcan- ism during the early post-glacial (25–19 ka) and Holocene (c. 8–2 ka) periods, but focused within a single eruptive center during the interim period. Subtle temporal variations in trace element com- positions and two-oxide temperatures indicate that these eruptions, issued from vents distributed within a similar area, tapped at least two physically discrete rhyolite reservoirs. This compositional distinction favors punctuated extraction and ephemeral storage of the erupted magma batches. Frequent mafic recharge incubates this long-lived, growing shallow silicic magma reservoir above the granite eutectic, which favors magma interactions over rejuvenation of near- to sub-solidus silicic cumulates. A long-term rate of mass addition—extrapolated from surface deformation accu- mulated over the past decade—is comparable with those that have produced moderate- to large- volume caldera-forming eruptions elsewhere.
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