Mantle plume capture, anchoring, and outflow during Galápagos plume-ridge interaction

Compositions of basalts erupted between the main zone of Galápagos plume upwelling and adjacent Galápagos Spreading Center (GSC) provide important constraints on dynamic processes involved in transfer of deep-mantle-sourced material to mid-ocean ridges. We examine recent basalts from central and northeast Galápagos including some that have less radiogenic Sr, Nd, and Pb isotopic compositions than plume-influenced basalts (E-MORB) from the nearby ridge. We show that the location of E-MORB, greatest crustal thickness, and elevated topography on the GSC correlates with a confined zone of low-velocity, high-temperature mantle connecting the plume stem and ridge at depths of ∼100 km. At this site on the ridge, plume-driven upwelling involving deep melting of partially dehydrated, recycled ancient oceanic crust, plus plate-limited shallow melting of anhydrous peridotite, generate E-MORB and larger amounts of melt than elsewhere on the GSC. The first-order control on plume stem to ridge flow is rheological rather than gravitational, and strongly influenced by flow regimes initiated when the plume was on axis (>5 Ma). During subsequent northeast ridge migration material upwelling in the plume stem appears to have remained “anchored” to a contact point on the GSC. This deep, confined NE plume stem-to-ridge flow occurs via a network of melt channels, embedded within the normal spreading and advection of plume material beneath the Nazca plate, and coincides with locations of historic volcanism. Our observations require a more dynamically complex model than proposed by most studies, which rely on radial solid-state outflow of heterogeneous plume material to the ridge.We thank Galápagos National Park authorities and CDRS for permitting fieldwork in Galápagos. D. Villagomez and D. Toomey generously shared their extensive seismic data set for Galápagos, and D. McKenzie kindly provided help with temperature calculations. End-member compositions of Galápagos mantle reservoirs in Figure 4 were estimated from principal component analysis; data related to these calculations are available in the supporting information. We are grateful to Kaj Hoernle and two anonymous reviewers for their constructive comments on an earlier version of this manuscript. The research was funded by the University of Cambridge, Geological Society of London, NERC (RG57434), and NSF (EAR 0838461, EAR 0944229, and EAR-11452711).This is the final published version of the article. It first appeared at http://dx.doi.org/10.1002/2015GC00572

Source lithology of the Galápagos plume

We have measured the contents of Ni, Ca, and Mn in olivine phenocrysts from volcanoes in the Galápagos archipelago to infer the mantle source lithologies. Results show that peridotite is the dominant source lithology for Fernandina, Floreana, Genovesa, Marchena, Pinta, Wolf Island, and Darwin Island. These volcanoes largely characterize the PLUME, WD, FLO and DUM Nd, Sr, and Pb isotopic endmembers of Harpp and White (2001). Only a minor pyroxenite component contributes to Fernandina and Floreana. Peridotite is also the dominant source lithology for Volcan Wolf, Alcedo, and Cerro Azul, and that these have isotopic compositions that can be defined by mixing of the 4 endmembers. Peridotite is therefore the dominant source lithology of the Galápagos plume. However, pyroxenite melting is significant in two spatially separated domains which are also isotopically distinct: Roca Redonda, Volcan Ecuador, Sierra Negra in the enriched western part of the archipelago and Volcan Darwin, Santiago, Santa Cruz, and Santa Fe in the depleted east. An implication is that the western and eastern pyroxenite domains likely represent two separate bodies of recycled crust within the Galápagos mantle plume. Isotopically enriched and depleted domains of the archipelago melted from both peridotite and pyroxenite, and there is no relationship between source lithology and its isotopic characteristics. The identification of peridotite source melting in volcanoes with isotopic characteristics that have been attributed to recycled crust points to the importance of mixing in OIB genesis, in agreement with studies on the Canary Islands.M.S.Includes bibliographical referencesby Christopher Allen Vidit

A Calcium-in-Olivine Geohygrometer and its Application to Subduction Zone Magmatism

High-precision electron microprobe analyses were obtained on olivine grains from Klyuchevskoy, Shiveluch and Gorely volcanoes in the Kamchatka Arc; Irazu, Platanar and Barva volcanoes of the Central American Arc; and mid-ocean ridge basalt (MORB) from the Siqueiros Transform. Calcium contents of these subduction zone olivines are lower than those for olivines from modern MORB, Archean komatiite and Hawaii. A role for magmatic H2O is likely for subduction zone olivines, and we have explored the suggestion of earlier workers that it has affected the partitioning of CaO between olivine and silicate melt. We provide a provisional calibration of DCaO Ol/L as a function of magmatic MgO and H2O, based on nominally anhydrous experiments and minimally degassed H2O contents of olivine-hosted melt inclusions. Application of our geohygrometer typically yields 3–4 wt % magmatic H2O at the Kamchatka and Central American arcs for olivines having 1000 ppm Ca, which agrees with H2O maxima from melt inclusion studies; Cerro Negro and Shiveluch volcanoes are exceptions, with about 6% H2O. High-precision electron microprobe analyses with 10–20 lm spatial resolution on some olivine grains from Klyuchevskoy and Shiveluch show a decrease in Ca content from the core centers to the rim contacts, and a sharp increase in Ca in olivine rims. We suggest that the zoning of Ca in olivine from subduction zone lavas may provide the first petrological record of temporal changes that occur during hydration of the mantle wedge and dehydration during ascent, and we predict olivine H2O contents that can be tested by secondary ionization mass spectrometry analysis

Abrupt transition from fractional crystallization to magma mixing at Gorely volcano (Kamchatka) after caldera collapse

A series of large caldera-forming eruptions (361–38 ka) transformed Gorely volcano, southern Kamchatka Peninsula, from a shield-type system dominated by fractional crystallization processes to a composite volcanic center, exhibiting geochemical evidence of magma mixing. Old Gorely, an early shield volcano (700–361 ka), was followed by Young Gorely eruptions. Calc-alkaline high magnesium basalt to rhyolite lavas have been erupted from Gorely volcano since the Pleistocene. Fractional crystallization dominated evolution of the Old Gorely magmas, whereas magma mixing is more prominent in the Young Gorely eruptive products. The role of rechargeevacuation processes in Gorely magma evolution is negligible (a closed magmatic system); however, crustal rock assimilation plays a significant role for the evolved magmas. Most Gorely magmas differentiate in a shallow magmatic system at pressures up to 300 MPa, ∼3 wt% H2O, and oxygen fugacity of ∼QFM + 1.5 log units. Magma temperatures of 1123–1218 °C were measured using aluminum distribution between olivine and spinel in Old and Young Gorely basalts. The crystallization sequence of major minerals for Old Gorely was as follows: olivine and spinel (Ol + Sp) for mafic compositions (more than 5 wt% of MgO); clinopyroxene and plagioclase crystallized at ∼5 wt% of MgO (Ol +Cpx + Plag) and magnetite at ∼3.5 wt% of MgO (Ol + Cpx + Plag +Mt). We show that the shallow magma chamber evolution of Old Gorely occurs under conditions of decompression and degassing. We find that the caldera-forming eruption(s) modified the magma plumbing geometry. This led to a change in the dominant magma evolution process from fractional crystallization to magma mixing. We further suggest that disruption of the magma chamber and accompanying change in differentiation process have the potential to transform a shield volcanic system to that of composite cone on a global scale

Transforming Undergraduate Education in the Geosciences Using Remotely-Operated Electron Probe Microanalyzer and Scanning Electron Microscope: CUREs

Geoscience faculty at Florida International University, University of South Florida, Florida Gulf Coast University and Valencia College have been collaborating to expand undergraduate in-class use of a remotely operable electron probe microanalyzer and SEM housed at the Florida Center for Analytical Electron Microscopy (FCAEM; https://fcaem.fiu.edu). Use of remotely operated research instrumentation as a pedagogical tool in Geoscience CUREs is an effective strategy for promoting student achievement, fostering self-directed research, and encouraging the transition from passive learning to independent inquiry. In-class use of analytical instruments has commonly served small numbers of students because of costs, access limitations, and training. For faculty, changes to classroom practices are time consuming and carry a risk of failure, which can impede the spread of promising educational strategies. To overcome these barriers, our NSF/TUES funded project developed implementable CUREs, and specimen sets available for undergraduate Geoscience courses. A facilitator has been available to train and assist faculty in tandem with step-by-step written and video guidelines. Interactive exhibits demonstrating the remote capabilities of the instruments were presented at CUR, GSA, AGU, EER, MSA and NSF meetings. To date, the user base has expanded to eight different domestic and one international academic institution. Data gathered in our assessments indicate improved undergraduate understanding of key course concepts and better class performance including a deeper understanding of course subject matter. Most importantly, post-course benefits assessment suggests increased student enthusiasm towards science, greater willingness to undertake independent research and to enroll in additional STEM courses because of their CURE experiences. Anecdotal evidence suggests undergraduates are more inclined to apply to graduate school given their exposure to research practices. As the project moves forward, we are addressing new challenges in understanding what influences and promotes faculty motivation to adopt these interventions, and how best to support a growing and active instructional user base

Using a Remotely-Accessed Scanning Electron Microscope (SEM) to Evaluate Sands in a 2-Year College Earth Science Earth Class

In this project, earth science students at Valencia College in Orlando, FL evaluated the mineral composition and depositional history and environments a variety of sands, including campus and local sands, by remotely accessing a scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) housed at the Florida Center for Analytical Electron Microscopy (FCAEM; https://fcaem.fiu.edu/). The project was piloted in Spring, 2016 and subsequently revised and modified. In Fall of 2016, 46 students in 3 non-majors, non-lab earth science classes completed 4 sands activities. The 4 activities were presented in weeks 1, 6, 9, and 14 (SEM/EDS used in all but the first week) with progressively more complex tasks. On the first day of class, students collected campus sands and used reflective microscopes to make observations. For week 6, students rotated between using the SEM/EDS to determine elemental content and mineralogy of unknown sands grains and evaluating sands with reflective microscopes. For weeks 9 and 14 activities, students reviewed relevant literature on quartz sand textures and related environmental/climate interpretations. They completed jigsaw-type activities to calibrate observation criteria and to develop “local” expertise, then rotated between using the SEM/EDS and working on topic-related group activities. Students completed pre- and post-assessments looking at the project’s impact on student learning. The learning assessment included 6 questions developed by using student responses to open-ended questions from Spring, 2016 to create multiple choice questions used in Fall, 2016. Results for 2 of the 6 questions were statistically significant, both showing improvement. Students also completed attitude surveys at the end of the semester, with strongest positive responses towards working in groups and on how the activities and content were presented

MR imaging field strength: prospective evaluation of the diagnostic accuracy of MR for diagnosis of multiple sclerosis at 0.5 and 1.5 T.

PURPOSE: To compare the diagnostic efficacy of middle-field-strength and high-field-strength magnetic resonance (MR) imaging in diagnosis of multiple sclerosis. MATERIALS AND METHODS: One hundred thirty-two patients with suspected multiple sclerosis underwent MR imaging at 0.5 and 1.5 T. Imaging parameters were identical except for band width optimization at middle field strength. Images were interpreted by radiologists expert in MR imaging who were blinded to diagnosis and field strength. The diagnosis of multiple sclerosis was made by experienced neurologists, and indeterminate cases and patients without clinical evidence of multiple sclerosis were followed up for 6 months to 1 year. RESULTS: There was no difference in accuracy, sensitivity, or specificity between scanners in the diagnosis of multiple sclerosis or white matter disease. Equal numbers of lesions were detected at both field strengths in all parts of the brain. Image quality was always good or adequate at middle field strength. CONCLUSION: Higher field strength does not confer higher accuracy in the diagnosis of multiple sclerosis with current-generation MR imagers

MR imaging field strength: prospective evaluation of the diagnostic accuracy of MR for diagnosis of multiple sclerosis at 0.5 and 1.5 T.

PURPOSE: To compare the diagnostic efficacy of middle-field-strength and high-field-strength magnetic resonance (MR) imaging in diagnosis of multiple sclerosis. MATERIALS AND METHODS: One hundred thirty-two patients with suspected multiple sclerosis underwent MR imaging at 0.5 and 1.5 T. Imaging parameters were identical except for band width optimization at middle field strength. Images were interpreted by radiologists expert in MR imaging who were blinded to diagnosis and field strength. The diagnosis of multiple sclerosis was made by experienced neurologists, and indeterminate cases and patients without clinical evidence of multiple sclerosis were followed up for 6 months to 1 year. RESULTS: There was no difference in accuracy, sensitivity, or specificity between scanners in the diagnosis of multiple sclerosis or white matter disease. Equal numbers of lesions were detected at both field strengths in all parts of the brain. Image quality was always good or adequate at middle field strength. CONCLUSION: Higher field strength does not confer higher accuracy in the diagnosis of multiple sclerosis with current-generation MR imagers