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Bradley R. Hacker - One of the best experts on this subject based on the ideXlab platform.
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Interpreting titanite U–Pb dates and Zr Thermobarometry in high-grade rocks: empirical constraints on elemental diffusivities of Pb, Al, Fe, Zr, Nb, and Ce
Contributions to Mineralogy and Petrology, 2019Co-Authors: Robert M. Holder, Bradley R. Hacker, Gareth Seward, Andrew R.c. Kylander-clarkAbstract:Length scales of compositional heterogeneity in titanite from 750 to 1000 °C metamorphic rocks from southern Madagascar were measured to provide empirical constraints on elemental diffusivities. The calculated Pb diffusivity is comparable to experimental estimates of Sr diffusivity; because of this, U–Pb dates from rocks that reached peak temperatures 900 °C; thus, Zr-in-titanite Thermobarometry should not be reset by diffusion in all but the smallest grains in the hottest rocks. Al and Nb diffuse at similar rates to Zr. Ce and Fe diffuse slower than Pb, but faster than Zr. Differences in empirical and experimental estimates of elemental diffusivities might be related to the complexity of most natural titanite solid solutions compared to the near-end-member titanite used in experiments.
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interpreting titanite u pb dates and zr Thermobarometry in high grade rocks empirical constraints on elemental diffusivities of pb al fe zr nb and ce
Contributions to Mineralogy and Petrology, 2019Co-Authors: Robert M. Holder, Bradley R. Hacker, Gareth Seward, Andrew R C KylanderclarkAbstract:Length scales of compositional heterogeneity in titanite from 750 to 1000 °C metamorphic rocks from southern Madagascar were measured to provide empirical constraints on elemental diffusivities. The calculated Pb diffusivity is comparable to experimental estimates of Sr diffusivity; because of this, U–Pb dates from rocks that reached peak temperatures 900 °C; thus, Zr-in-titanite Thermobarometry should not be reset by diffusion in all but the smallest grains in the hottest rocks. Al and Nb diffuse at similar rates to Zr. Ce and Fe diffuse slower than Pb, but faster than Zr. Differences in empirical and experimental estimates of elemental diffusivities might be related to the complexity of most natural titanite solid solutions compared to the near-end-member titanite used in experiments.
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Reconstruction of the Talkeetna intraoceanic arc of Alaska through Thermobarometry
Journal of Geophysical Research, 2008Co-Authors: Bradley R. Hacker, Luc Mehl, Peter B. Kelemen, Matthew Rioux, Mark D. Behn, Peter LuffiAbstract:[1] The Talkeetna arc is one of two intraoceanic arcs where much of the section from the upper mantle through the volcanic carapace is well exposed. We reconstruct the vertical section of the Talkeetna arc by determining the (re)crystallization pressures at various structural levels. The Thermobarometry shows that the tonalites and quartz diorites intruded at ∼5–9 km into a volcanic section estimated from stratigraphy to be 7 km thick. The shallowest, Tazlina and Barnette, gabbros crystallized at ∼17–24 km; the Klanelneechena Klippe crystallized at ∼24–26 km; and the base of the arc crystallized at ∼35 km depth. The arc had a volcanic:plutonic ratio of ∼1:3–1:4. However, many or most of the felsic plutonic rocks may represent crystallized liquids rather than cumulates so that the liquid:cumulate ratio might be 1:2 or larger. The current 5- to 7-km structural thickness of the plutonic section of the arc is ∼15–30% of the original 23- to 28-km thickness. The bulk composition of the original Talkeetna arc section was ∼51–58 wt % SiO2.
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prograde amphibolite facies to ultrahigh pressure transition along nordfjord western norway implications for exhumation tectonics
Tectonics, 2007Co-Authors: David Young, Bradley R. Hacker, Torgeir B Andersen, Fernando CorfuAbstract:[1] The Nordfjord area of western Norway hosts one of the best known provinces of ultrahigh-pressure (UHP) rocks in the world, yet the nature of the transition from the UHP rocks into surrounding amphibolite-facies crust has remained unclear. New Thermobarometry of phengite- and kyanite-bearing eclogites shows that this transition is unbroken by major structures: the pressure and temperature gradient increases smoothly over a horizontal distance of ∼20 km, from high-pressure amphibolite facies (∼1.5 GPa/600–700°C) through quartz eclogite (∼2.4 GPa/600°C) into UHP eclogite (>2.7 GPa/700°C). These data support a model in which the Norwegian UHP province remained attached to lower pressure crust during exhumation. This requires that a large area of the Western Gneiss Region was metamorphosed and exhumed ∼60 km through the mantle, as a relatively coherent body.
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THE CONUNDRUM OF SAMAIL : EXPLAINING THE METAMORPHIC HISTORY
Tectonophysics, 1997Co-Authors: Bradley R. Hacker, Edwin GnosAbstract:Abstract Recent thermochronology and Thermobarometry place new constraints on the tectonic evolution of the Samail ophiolite. Zircon and radiolarian ages indicate formation of the Samail ophiolite crust at 97-94 Ma. Hornblende ages demonstrate that the metamorphic sole beneath the ophiolite formed and, only 1–4 m.y. later, cooled to 550°C. Thermobarometry dictates that the sole formed at peak temperatures of 775–875°C and unexpectedly high pressures of 1.1 GPa. These data imply extremely rapid (∼200 km/m.y.) subduction beneath a very young ophiolite for a few m.y. to depths of 30–40 km, followed by equally rapid subduction of cold lithosphere beneath the ophiolite. High-pressure rocks beneath the ophiolite show subduction of the Arabian continental margin to depths as great as 70–80 km, but the temporal and genetic relationships between the metamorphic sole and the high-pressure rocks are poorly known. Key questions remaining are the magnitude, style, and age of extension of the ophiolite, and the timing of high-pressure metamorphic event(s).
K A Ouderkirk - One of the best experts on this subject based on the ideXlab platform.
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contact metamorphic p t t paths from sm nd garnet ages phase equilibria modelling and Thermobarometry garnet ledge south eastern alaska usa
Journal of Metamorphic Geology, 2001Co-Authors: Harold H Stowell, D L Taylor, D L Tinkham, S A Goldberg, K A OuderkirkAbstract:Sm–Nd garnet-whole rock geochronology, phase equilibria, and Thermobarometry results from Garnet Ledge, south-eastern Alaska, provide the first precisely constrained P–T–t path for garnet zone contact metamorphism. Garnet cores from two crystals and associated whole rocks yield a four point isochron age for initial garnet growth of 89.9 ± 3.6 Ma. Garnet rims and matrix minerals from the same samples yield a five point isochron age for final garnet growth of 89 ± 1 Ma. Six size fractions of zircon from the adjacent pluton yield a concordant U–Pb age of 91.6 ± 0.5 Ma. The garnet core and rim, and zircon ages are compatible with single-stage garnet growth during and/or after pluton emplacement. All garnet core–whole rock and garnet rim-matrix data from the two samples constrain garnet growth duration to ≤5.5 my. A garnet mid-point and the associated matrix from one of the two garnet crystals yield an age of 90.0 ± 1.0 Ma. This mid-point result is logically younger than the 90.7 ± 5.6 Ma core–whole rock age and older than the 88.4 ± 2.5 Ma rim-matrix age for this sample. A MnNaCaKFMASH phase diagram (P–T pseudosection) and the garnet core composition are used to predict that cores of garnet crystals grew at 610 ± 20 °C and 5 ± 1 kbar. This exceeds the temperature of the garnet-in reaction by c. 50 °C and is compatible with overstepping of the garnet growth reaction during contact metamorphism. Intersection of three reactions involving garnet-biotite-sillimanite-plagioclase-quartz calculated by THERMOCALC in average P–T mode, and exchange Thermobarometry were used to estimate peak metamorphic conditions of 678 ± 58 °C at 6.1 ± 0.9 kbar and 685 ± 50 °C at 6.3 ± 1 kbar, respectively. Integration of pressure, temperature, and age estimates yields a pressure-temperature-time path compatible with near isobaric garnet growth over an interval of c. 70 °C and c. 2.3 my.
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Contact metamorphic P–T–t paths from Sm–Nd garnet ages, phase equilibria modelling and Thermobarometry: Garnet Ledge, south-eastern Alaska, USA
Journal of Metamorphic Geology, 2001Co-Authors: Harold H Stowell, D L Taylor, D L Tinkham, S A Goldberg, K A OuderkirkAbstract:Sm–Nd garnet-whole rock geochronology, phase equilibria, and Thermobarometry results from Garnet Ledge, south-eastern Alaska, provide the first precisely constrained P–T–t path for garnet zone contact metamorphism. Garnet cores from two crystals and associated whole rocks yield a four point isochron age for initial garnet growth of 89.9 ± 3.6 Ma. Garnet rims and matrix minerals from the same samples yield a five point isochron age for final garnet growth of 89 ± 1 Ma. Six size fractions of zircon from the adjacent pluton yield a concordant U–Pb age of 91.6 ± 0.5 Ma. The garnet core and rim, and zircon ages are compatible with single-stage garnet growth during and/or after pluton emplacement. All garnet core–whole rock and garnet rim-matrix data from the two samples constrain garnet growth duration to ≤5.5 my. A garnet mid-point and the associated matrix from one of the two garnet crystals yield an age of 90.0 ± 1.0 Ma. This mid-point result is logically younger than the 90.7 ± 5.6 Ma core–whole rock age and older than the 88.4 ± 2.5 Ma rim-matrix age for this sample. A MnNaCaKFMASH phase diagram (P–T pseudosection) and the garnet core composition are used to predict that cores of garnet crystals grew at 610 ± 20 °C and 5 ± 1 kbar. This exceeds the temperature of the garnet-in reaction by c. 50 °C and is compatible with overstepping of the garnet growth reaction during contact metamorphism. Intersection of three reactions involving garnet-biotite-sillimanite-plagioclase-quartz calculated by THERMOCALC in average P–T mode, and exchange Thermobarometry were used to estimate peak metamorphic conditions of 678 ± 58 °C at 6.1 ± 0.9 kbar and 685 ± 50 °C at 6.3 ± 1 kbar, respectively. Integration of pressure, temperature, and age estimates yields a pressure-temperature-time path compatible with near isobaric garnet growth over an interval of c. 70 °C and c. 2.3 my.
Frank S. Spear - One of the best experts on this subject based on the ideXlab platform.
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Regional Quartz Inclusion Barometry and Comparison with Conventional Thermobarometry and Intersecting Isopleths from the Connecticut Valley Trough, Vermont and Massachusetts, USA
Journal of Petrology, 2020Co-Authors: Oliver M. Wolfe, Frank S. SpearAbstract:Abstract A comparative analysis of Raman shifts of quartz inclusions in garnet was made along two traverses across the Connecticut Valley Trough (CVT) in western New England, USA, to examine the regional trends of quartz inclusion in garnet (QuiG) Raman barometry pressure results and to compare this method with conventional Thermobarometry and the method of intersecting garnet core isopleths. Overall, Raman shifts of quartz inclusions ranged from 1·2 to 3·5 cm–1 over all field areas and displayed a south to north decrease, matching the overall decrease in mapped metamorphic grade. Raman shifts of quartz inclusions typically did not show systematic variation with respect to their radial position within a garnet crystal, and indicate that garnet probably grew at nearly isothermal and isobaric pressure–temperature (P–T) conditions. The P–T conditions inferred from conventional Thermobarometry were in the range of ∼500–575 °C and ∼7·4–10·3 kbar over the sample suite and are in good agreement with previous published Thermobarometry throughout the CVT. These P–T results are broadly consistent with QuiG barometry and also suggest that garnet grew isothermally and isobarically at near peak P–T conditions. However, P–T conditions and P–T paths inferred using either garnet core Thermobarometry or garnet core intersecting isopleths yield results that are internally inconsistent and generally disagree with the pressure results from QuiG barometry. Garnet core isopleth intersections consistently plotted between the nominal garnet-in curve on mineral assemblage diagrams and the P–T conditions constrained by QuiG isomekes for the majority of the sample suite. Additionally, most samples’ P–T results from QuiG barometry and rim Thermobarometry show marked disagreement from those derived from garnet core Thermobarometry, compared with the minority that showed agreement within uncertainty. Pressures calculated from QuiG barometry ranged from 8·5 to 9·5 kbar along the traverses in western Massachusetts (MA) and central Vermont (VT) and from 6·5 to 7·5 kbar in northern VT indicating an increase in peak burial of 3–6 km from north to south. Along the western end of the central VT traverse, there are differences in measured Raman shifts and inferred peak pressures of up to 1 kbar across the Richardson Memorial Contact (RMC), indicating a possible fault contact with minor post-peak metamorphic shortening of up to ∼3 km. In contrast, along an east–west traverse in the vicinity of the Goshen Dome, MA, there was little observed variation in Raman shifts across the contact. By contrast, QuiG barometry clearly indicates significant discontinuities in peak pressure east of the Strafford Dome in central VT. This supports the interpretation that post-peak metamorphic shortening was necessary to juxtapose upper staurolite–kyanite zone rocks next to lower garnet zone pelites. Overall, it is concluded that garnet core Thermobarometry and garnet core isopleths may provide unreliable results for the P–T conditions of garnet nucleation and inferred P–T paths during garnet growth unless independently verified. The consistency of QuiG results with rim Thermobarometry indicates that peak metamorphic conditions previously reported for the CVT using garnet rim Thermobarometry are robust and that variation in QuiG barometry results is a valuable tool to analyze structural features within a metamorphic terrane.
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Experimental study of quartz inclusions in garnet at pressures up to 3.0 GPa: evaluating validity of the quartz-in-garnet inclusion elastic thermobarometer
Contributions to Mineralogy and Petrology, 2018Co-Authors: Jay B. Thomas, Frank S. SpearAbstract:Garnet crystals with quartz inclusions were hydrothermally crystallized from oxide starting materials in piston–cylinder apparatuses at pressures from 0.5 to 3 GPa and temperatures ranging from 700 to 800 °C to study how entrapment conditions affect remnant pressures of quartz inclusions used for quartz-in-garnet (QuiG) elastic Thermobarometry. Systematic changes of the 128, 206 and 464 cm^−1 Raman band frequencies of quartz were used to determine pressures of quartz inclusions in garnet using Raman spectroscopy calibrations that describe the P–T dependencies of Raman band shifts for quartz under hydrostatic pressure. Within analytical uncertainties, inclusion pressures calculated for each of the three Raman band frequencies are equivalent, which suggests that non-hydrostatic stress effects caused by elastic anisotropy in quartz are smaller than measurement errors. The experimental quartz inclusions have pressures ranging from − 0.351 to 1.247 GPa that span the range of values observed for quartz inclusions in garnets from natural rocks. Quartz inclusion pressures were used to model P–T conditions at which the inclusions could have been trapped. The accuracy of QuiG Thermobarometry was evaluated by considering the differences between pressures measured during experiments and pressures calculated using published equation of state parameters for quartz and garnet. Our experimental results demonstrate that Raman measurements performed at room temperature can be used without corrections to estimate garnet crystallization pressures. Calculated entrapment pressures for quartz inclusions in garnet are less than ~ 10% different from pressures measured during the experiments. Because the method is simple to apply with reasonable accuracy, we expect widespread usage of QuiG Thermobarometry to estimate crystallization conditions for garnet-bearing silicic rocks.
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Thermobarometry in granulites: pitfalls and new approaches
Precambrian Research, 1992Co-Authors: Frank S. Spear, Frank P. FlorenceAbstract:Abstract Computer models of cooling granulite-facies rocks with the assemblage garnet+biotite+sillimanite+K-feldspar±plagioclase+quartz are presented that incorporate simultaneous reaction (net transfer and exchange) and multicomponent diffusion in garnet. In models where cooling is accompanied by exchange reactions plus diffusion only, the calculated garnet core+matrix biotite temperature is always lower than the metamorphic peak by an amount that depends on the cooling rate, the size of the garnet and the volumetric ratio of garnet to biotite. Maximum calculated core temperatures of approximately 700–750°C are to be expected from typical granulite-facies garnet-biotite assemblages and calculated core temperatures can easily be as low as 600°C. In models where both net transfer and exchange reactions are possible, calculated garnet core+matrix biotite temperatures are typically higher than the metamorphic peak temperatures (by more than 150°C) for large garnets (R > 2 mm) but may be lower than the metamorphic peak for small garnets (R The composition of matrix plagioclase in equilibrium with garnet is very sensitive to the amount of modal plagioclase that reacts and hence, calculated P-T trajectories are very sensitive to the petrologic interpretation of this recrystallization history. For models in which all the modal plagioclase reacts, the core and rim pressures may be faithfully recovered, provided that the temperature estimate for the core is reliable. For models in which only a small percentage of the modal plagioclase reacts, very small modal amounts of plagioclase may be in equilibrium with garnet rim, which, if incorrectly interpreted, may lead to grossly erroneous P-T path interpretations. A method to test an inferred P-T path model is presented that involves attempting to match the observed garnet and plagioclase zoning information with a forward model that involves simultaneous reaction and diffusion. An example from western New Hampshire, USA, is presented that closely matches the measured profile.
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Thermobarometry and P-T paths from granulite facies rocks: an introduction
Precambrian Research, 1992Co-Authors: Frank S. SpearAbstract:Abstract P-T paths from granulite terranes provide unique information about the thermal and tectonic evolution of the lower crust. Although methods for P-T path determination are well established, integration of the methods of Thermobarometry, reaction path analysis, mineral zoning studies and diffusion studies with geochronologic analysis of temperature-time histories is necessary to further interpret observed assemblages and textures. Granulite P-T paths are characterized by either a phase of nearly isothermal decompression, which is likely related to a period of tectonic denudation, or nearly isobaric cooling, which may be related to the thermal relaxation of the crust following either the addition of heat by magmas or rapid denudation. Prograde portions of the P-T path are required to distinguish further between these options.
Robert M. Holder - One of the best experts on this subject based on the ideXlab platform.
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Interpreting titanite U–Pb dates and Zr Thermobarometry in high-grade rocks: empirical constraints on elemental diffusivities of Pb, Al, Fe, Zr, Nb, and Ce
Contributions to Mineralogy and Petrology, 2019Co-Authors: Robert M. Holder, Bradley R. Hacker, Gareth Seward, Andrew R.c. Kylander-clarkAbstract:Length scales of compositional heterogeneity in titanite from 750 to 1000 °C metamorphic rocks from southern Madagascar were measured to provide empirical constraints on elemental diffusivities. The calculated Pb diffusivity is comparable to experimental estimates of Sr diffusivity; because of this, U–Pb dates from rocks that reached peak temperatures 900 °C; thus, Zr-in-titanite Thermobarometry should not be reset by diffusion in all but the smallest grains in the hottest rocks. Al and Nb diffuse at similar rates to Zr. Ce and Fe diffuse slower than Pb, but faster than Zr. Differences in empirical and experimental estimates of elemental diffusivities might be related to the complexity of most natural titanite solid solutions compared to the near-end-member titanite used in experiments.
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interpreting titanite u pb dates and zr Thermobarometry in high grade rocks empirical constraints on elemental diffusivities of pb al fe zr nb and ce
Contributions to Mineralogy and Petrology, 2019Co-Authors: Robert M. Holder, Bradley R. Hacker, Gareth Seward, Andrew R C KylanderclarkAbstract:Length scales of compositional heterogeneity in titanite from 750 to 1000 °C metamorphic rocks from southern Madagascar were measured to provide empirical constraints on elemental diffusivities. The calculated Pb diffusivity is comparable to experimental estimates of Sr diffusivity; because of this, U–Pb dates from rocks that reached peak temperatures 900 °C; thus, Zr-in-titanite Thermobarometry should not be reset by diffusion in all but the smallest grains in the hottest rocks. Al and Nb diffuse at similar rates to Zr. Ce and Fe diffuse slower than Pb, but faster than Zr. Differences in empirical and experimental estimates of elemental diffusivities might be related to the complexity of most natural titanite solid solutions compared to the near-end-member titanite used in experiments.
Valentin R. Troll - One of the best experts on this subject based on the ideXlab platform.
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Multi-level magma plumbing at Agung and Batur volcanoes increases risk of hazardous eruptions.
Scientific reports, 2018Co-Authors: Harri Geiger, Valentin R. Troll, Ester M. Jolis, Frances M. Deegan, Chris Harris, David R. Hilton, Carmela FredaAbstract:The island of Bali in Indonesia is home to two active stratovolcanoes, Agung and Batur, but relatively little is known of their underlying magma plumbing systems. Here we define magma storage depths and isotopic evolution of the 1963 and 1974 eruptions using mineral-melt equilibrium Thermobarometry and oxygen and helium isotopes in mineral separates. Olivine crystallised from a primitive magma and has average δ18O values of 4.8‰. Clinopyroxene records magma storage at the crust-mantle boundary, and displays mantle-like isotope values for Helium (8.62 RA) and δ18O (5.0–5.8‰). Plagioclase reveals crystallisation in upper crustal storage reservoirs and shows δ18O values of 5.5–6.4‰. Our new Thermobarometry and isotope data thus corroborate earlier seismic and InSAR studies that inferred upper crustal magma storage in the region. This type of multi-level plumbing architecture could drive replenishing magma to rapid volatile saturation, thus increasing the likelihood of explosive eruptions and the consequent hazard potential for the population of Bali.
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The magma plumbing system for the 1971 Teneguía eruption on La Palma, Canary Islands
Contributions to Mineralogy and Petrology, 2015Co-Authors: Abigail K. Barker, Valentin R. Troll, Juan Carlos Carracedo, Peter A. NichollsAbstract:The 1971 Teneguia eruption is the most recent volcanic event of the Cumbre Vieja rift zone on La Palma. The eruption produced basanite lavas that host xenoliths, which we investigate to provide insight into the processes of differentiation, assimilation and magma storage beneath La Palma. We compare our results to the older volcano magmatic systems of the island with the aim to reconstruct the temporal development of the magma plumbing system beneath La Palma. The 1971 lavas are clinopyroxene-olivine-phyric basanites that contain augite, sodic-augite and aluminium augite. Kaersutite cumulate xenoliths host olivine, clinopyroxene including sodic-diopside, and calcic-amphibole, whereas an analysed leucogabbro xenolith hosts plagioclase, sodic-augite-diopside, calcic-amphibole and hauyne. Mineral Thermobarometry and mineral-melt Thermobarometry indicate that clinopyroxene and plagioclase in the 1971 Teneguia lavas crystallised at 20–45 km depth, coinciding with clinopyroxene and calcic-amphibole crystallisation in the kaersutite cumulate xenoliths at 25–45 km and clinopyroxene, calcic-amphibole and plagioclase crystallisation in the leucogabbro xenolith at 30–50 km. Combined mineral chemistry and Thermobarometry suggest that the magmas had already crystallised, differentiated and formed multiple crystal populations in the oceanic lithospheric mantle. Notably, the magmas that supplied the 1949 and 1971 events appear to have crystallised deeper than the earlier Cumbre Vieja magmas, which suggests progressive underplating beneath the Cumbre Vieja rift zone. In addition, the lavas and xenoliths of the 1971 event crystallised at a common depth, indicating a reused plumbing system and progressive recycling of Ocean Island plutonic complexes during subsequent magmatic activity.
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Magma storage and plumbing of adakite-type post-ophiolite intrusions in the Sabzevar ophiolitic zone, northeast Iran
Solid Earth, 2015Co-Authors: K. Jamshidi, Valentin R. Troll, Habibollah Ghasemi, Mahmoud Sadeghian, Borje DahrenAbstract:Abstract. Subduction-related adakite-type intrusive rocks emplaced into the late Cretaceous–Paleocene Sabzevar ophiolite zone, northeast Iran, range from Mg-andesite to rhyodacite in composition. Here we investigate the magma supply system to these subvolcanic intrusive rocks by applying thermobarometric mineral and mineral–melt equilibrium models, including amphibole Thermobarometry, plagioclase–melt Thermobarometry and clinopyroxene–melt barometry. Based on the results of these thermobarometric models, plagioclase crystallized dominantly at pressures of ~350 (130 to 468) MPa, while amphiboles record both low pressures (~300 MPa) and very high pressures (>700 MPa) of crystallization. The latter is supported by the calculated pressures for clinopyroxene crystallization (550 to 730 MPa). The association of amphibole with clinopyroxene and no plagioclase in the most primitive samples (Mg-andesites) is consistent with amphibole fractionation from very hydrous magmas at deep crustal levels of the plumbing system, which may have been a key process in intensifying adakite-type affinities in this rock suite. Barometry, combined with frequent disequilibrium features such as oscillatory-zoned and sieve-textured plagioclase crystals with An-rich overgrowths in more evolved samples, implies that final magma differentiation occurred in an open upper crustal magma system that developed progressively stronger compositional modifications during high-level magma storage.
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magma plumbing beneath anak krakatau volcano indonesia evidence for multiple magma storage regions
Contributions to Mineralogy and Petrology, 2012Co-Authors: Borje Dahren, Valentin R. Troll, Ulf B Andersson, Jane P Chadwick, Mairi F Gardner, K Jaxybulatov, Ivan KoulakovAbstract:Understanding magma plumbing is essential for predicting the behaviour of explosive volcanoes. We investigate magma plumbing at the highly active Anak Krakatau volcano (Indonesia), situated on the rim of the 1883 Krakatau caldera by employing a suite of thermobarometric models. These include clinopyroxene-melt Thermobarometry, plagioclase-melt Thermobarometry, clinopyroxene composition barometry and olivine-melt thermometry. Petrological studies have previously identified shallow magma storage in the region of 2–8 km beneath Krakatau, while existing seismic evidence points towards mid- to deep-crustal storage zone(s), at 9 and 22 km, respectively. Our results show that clinopyroxene in Anak Krakatau lavas crystallized at a depth of 7–12 km, while plagioclase records both shallow crustal (3–7 km) and sub-Moho (23–28 km) levels of crystallization. These magma storage regions coincide with well-constrained major lithological boundaries in the crust, implying that magma ascent and storage at Anak Krakatau is strongly controlled by crustal properties. A tandem seismic tomography survey independently identified a separate upper crustal ( 7 km). Both petrological and seismic methods are sensitive in detecting magma bodies in the crust, but suffer from various limitations. Combined geophysical and petrological surveys, in turn, offer increased potential for a comprehensive characterization of magma plumbing at active volcanic complexes.
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The Brava seamount, Cape Verde: Beyond the spatial extent of EM1 and petrogenesis of highly evolved alkaline lavas.
2010Co-Authors: Abigail K. Barker, Valentin R. Troll, Axel Andersson, Thor H. Hansteen, Rob M. EllamAbstract:The Brava seamount, located southwest of the island of Brava, Cape Verde was sampled during research cruise 8/85 of the R.R.S. Charles Darwin in 1985. Two groups of highly evolved alkaline volcanics are distinguished from the Brava seamount: 1) pyroxene-phonolites containing clinopyroxene, amphibole, nepheline, ±biotite, and minor sanidine and 2) feldspathoid-phonolites containing nepheline, nausean, minor biotite and leucite. All of the samples have MgO between 0.8 and 2 wt%, comparable to the most evolved volcanics sampled in the Cape Verde archipelago. The feldspathoid-phonolites have NaO2 of 12-13 wt%. Alkaline lavas from the Brava seamount have higher Sr/Sr (0.70337 to 0.70347) at eNd of +6 to +7 than previously sampled in Cape Verde. Sr isotopes will be integrated with oxygen isotopes to establish magma and crust interactions in the magmatic plumbing system beneath the Brava seamount. Clinopyroxene-melt Thermobarometry will be presented to constrain the depths of equilibrium crystallisation. Sr-O isotopes and Thermobarometry will be combined to build a picture of the levels of magma stalling and interaction between magmas and the crust beneath the Brava seamount.