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Now showing items 1 - 16 of 38

  • CO2 flux emissions from the Earth’s most actively degassing volcanoes, 2005–2015

    Aiuppa, Alessandro   Fischer, Tobias P.   Plank, Terry   Bani, Philipson  

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  • Chemical composition of sediments subducting at the Izu-Bonin trench

    Plank, Terry   Kelley, Katherine A.   Murray, Richard W.   Stern, Lacie Quintin  

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  • Emerging geothermometers for estimating slab surface temperatures

    Plank, Terry   Cooper, Lauren B.   Manning, Craig E.  

    Soon after its creation, the upper part of the oceanic lithosphere becomes hydrated owing to prolonged interaction with sea water. As oceanic slabs sink into the mantle at subduction zones and experience increasing temperatures and pressures, they dehydrate and release fluids. Such slab fluids drive mantle melting and return ocean water to the Earth's surface through arc volcanism. The efficiency of this process, as well as the chemical make-up of slab fluids, depends on the pressure and temperature conditions experienced by the slabs as they subduct. A growing body of experimental data provides the basis for new geothermometers (for example the ratio of water to cerium) that, when combined with data from melt inclusions in volcanic crystals, predict that slab-fluid temperatures vary from 750 to 950 C for different subduction zones. Such high values indicate that fluids that exit the slab when it is below the arc are likely to be like melts or solute-rich fluids. Slab surface temperatures inferred from these geothermometers are at the upper end of those predicted by thermal models, implying that fluids could be released at relatively shallow depths and efficiently returned to the surface.
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  • Effects of the melting regime on the composition of the oceanic crust

    Plank, Terry   Langmuir, Charles H.  

    The physical form of the melting regime and the mechanisms of melt extraction influence the composition of magmas erupted at ocean ridges. The authors investigate aspects of this relationship, beginning with the assumption that melts can be extracted from the melting regime without significant reequilibration during their passage to the surface. The ocean crust thus represents a mixture of the individual melts. Many melting regimes lead to the same `residual mantle column' (RMC), defined as a vertical section through the mantle external to the melting regime. The RMC is the integrated result of melt extraction and is useful in evaluating the geochemical effects of many different types of melting regimes
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  • Tracing trace elements from sediment input to volcanic output at subduction zones

    Plank, Terry   Langmuir, Charles H.  

    Reports the sedimentary fluxes into eight trenches around the globe of trace elements that are enriched in arc volcanics (Ba, Sr, K, Rb, Cs, La, Th and U). The authors show that the volcanic outputs clearly reflect the sediment inputs, once the effects of melting are taken into account. Where the sediment flux into a trench is high for a particular element, the associated volcanics are enriched in this same element. Thus, some of the geochemical characteristics of arc volcanics can be traced back to the sediments at the trench. A mass balance of the inputs and outputs will ultimately provide estimates for how much sediment is recycled to the arc, and how much to the deeper mantle
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  • A view from the Sunda arc

    Plank, Terry   Langmuir, Charles H.  

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  • The meaning of “mean\r F\r ”: Clarifying the mean extent of melting at ocean ridges

    Plank, Terry   Spiegelman, Marc   Langmuir, Charles H.   Forsyth, Donald W.  

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  • Geodynamics: The ups and downs of sediments

    Plank, Terry   van Keken, Peter E.  

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  • Why do mafic arc magmas contain similar to 4 wt% water on average?

    Plank, Terry   Kelley, Katherine A.   Zimmer, Mindy M.   Hauri, Erik H.   Wallace, Paul J.  

    The last 15 yr have seen an explosion of data on the volatile contents of magmas parental to arc volcanoes. This has occurred due to the intense study of melt inclusions trapped in volcanic phenocrysts, aliquots of magma that have presumably escaped degassing during eruption. The surprising first-order result is the narrow range in H2O concentrations in the least degassed melt inclusions from each volcano. Nearly all arc volcanoes are sourced with mafic magmas that contain 2-6 wt% H2O. The average for each arc varies even less, from 3.2 (for the Cascades) to 4.5 (for the Marianas), with a global average of 3.9 +/- 0.4 wt% H2O. Significant variations occur from volcano to volcano within each arc, but the means are indistinguishable within one s.d. The narrow range and common average value for H2O are in stark contrast to the concentrations of most other subduction tracers, such as Nb or Ba, which vary by orders of magnitude. A modulating process, either in the crust or mantle, is likely responsible for the restricted range in the H2O contents of arc melt inclusions. One possibility is that melt inclusion H2O values reflect vapor saturation at the last storage depth in the crust prior to eruption. In this scenario, magmas rise from the mantle with variable H2O contents (> 4 wt%), become vapor-saturated and start degassing, and continue to degas up until the depth at which they stall. If the stalling depths are similar to 6 km, which is common for storage depths beneath volcanoes, magmas would be saturated at similar to 4 wt% H2O, and melt inclusions, most of which become closed during further ascent, would thus record <= 4 wt% H2O. Another possibility is that the mantle melting process modulates water content in the melt such that magmas rise out of the mantle with similar to 4 wt% H2O. A strong relationship between the water content of the source, H2O(o) and the degree of melting (F) maintains nearly constant water contents in the melt for a restricted range in mantle temperature. Magmas with 3-4 wt% H2O can be generated at similar to 50 degrees below the dry solidus for a wide range in F and H2O(o). The narrow range in wedge temperatures may be another manifestation of a planet with average upper mantle of 1400 degrees C potential temperature. The characteristic mean and range of H2O contents of arc magmas has implications for both the volatile fuel for explosive eruptions and the mass balance of H2O recycled through subduction zones. (C) 2012 Elsevier B.V. All rights reserved.
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  • Magnesium isotopic composition of subducting marine sediments

    Hu, Yan   Teng, Fang-Zhen   Plank, Terry   Huang, Kang-Jun  

    Subducted sediments have been suggested to be a possible cause for local Mg isotopic variation in the overall homogeneous mantle based on anomalous Mg isotopic compositions detected in some mantle-derived rocks. Here we examine the Mg isotopic systematics of 77 bulk marine sediment samples from ten DSDP and ODP drill sites adjacent to the world's major subduction zones to assess their potential to cause mantle heterogeneity. In contrast to the limited mantle range (delta Mg-26 =3D -0.25 +/- 0.07 parts per thousand), delta Mg-26 of these sediments varies widely from -1.34 parts per thousand to + 0.46 parts per thousand, which is similar to values measured in continental sediments and appears to be linked to sediment lithology. Calcareous oozes are generally isotopically lighter than seawater; however, the presence of detrital impurities can shift their delta Mg-26 to higher values. In comparison to calcareous oozes, Mg-rich aluminosilicate sediments typically have higher delta Mg-26 values, although they may be either isotopically heavier or lighter than the mantle. Detrital sediments are the dominant sediment type and display large variation (-0.90 parts per thousand to + 0.18 parts per thousand) that reflects provenance heterogeneity and textural/mineralogical sorting. Sediments derived from juvenile terranes have delta Mg-26 similar to mantle values while those derived from mature, and thus more strongly weathered sources, often have higher delta Mg-26 values. On the other hand, turbidites may have lower delta Mg-26 values if containing isotopically light dolomite and/or garnet. In addition, clay-size fractions in turbidites commonly have higher delta Mg-26 than the associated sands. Hydrogenetic clays have a relatively high and uniform delta Mg-26 value of + 0.02 +/- 0.14 parts per thousand (2SD, n =3D 14), suggesting preferential uptake of heavy Mg isotopes during authigenic clay formation while hydrothermal clays have highly variable delta Mg-26 values (-0.57 parts per thousand to + 0.46 parts per thousand) that likely reflect variable exchange with seawater/hydrothermal fluids. Overall, considerable variations exist in both regional Mg mass flux and the weighted delta Mg-26 average of sedimentary sections subducting at different trenches. The flux-weighted delta Mg-26 of -0.336 parts per thousand is estimated for Global Subducting Sediments (GLOSS-II), which is isotopically lighter than the mantle average because of the presence of thick turbidite sequences with relatively low delta Mg-26 values and local carbonate enrichment in a few trenches near the equator. Therefore, it appears that subducting marine sediments are a source of heterogeneous delta Mg-26 values in the mantle, and they may be manifested locally in volcanic rocks that form in regions having a high flux of sediment melts to the mantle.
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  • A preliminary assessment of the symmetry of source composition and melting dynamics across the Azores plume

    Beier, Christoph   Turner, Simon   Plank, Terry   White, William  

    In order to undertake a preliminary assessment of the extent of symmetry in source composition and melting dynamics in the Azores plume we present new ICP-MS trace element data along with Sr, Nd, Pb, and U-Th-Ra isotope data for samples from the islands Flores and Corvo west of the Mid-Atlantic Ridge. We also present data from a picrite from Faial and new ICP-MS trace element data for 28 basaltic lavas from the eastern Azores Plateau to augment data published previously from these samples. Rare earth element data for primitive lavas (MgO >= 5% and Mg # >= 60) have La/YbN similar to 10 and variable Ce and/or Eu anomalies. Multi-incompatible trace element patterns normalized to primitive mantle are convex upward with small negative Th and K +/- Pb anomalies. While lavas to the east are characterized by low Nb/Zr and generally lower La/Yb ratios (with the notable exception of eastern Sao Miguel), lavas from the western islands have slightly higher Nb/Zr and La/Yb inferred to reflect smaller degrees of partial melting. The Sr-Nd-Pb isotope systematics imply that Corvo and Flores sample components which range from an isotopic source commonly found in the Azores (e. g., at the eastern island of Graciosa) to a more depleted, MORB-like mantle sampled at the Mid-Atlantic Ridge. However, in common with uncontaminated samples from Sao Miguel, the Corvo and Flores samples appear to have slightly lower 230 Th excesses and higher La/Yb, Tb/Yb than the other Azores islands or the Mid-Atlantic Ridge samples. The trace element and isotope data indicate a relatively symmetric pattern with distance across the MAR, while U-Th disequilibria, and thus inferred melting dynamics, appear less symmetric. Nevertheless, the data suggest that heterogeneities in source composition do not have a large effect on melting dynamics, at least within the Azores islands.
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  • Mantle melting as a function of water content beneath back-arc basins

    Kelley, Katherine A.   Plank, Terry   Grove, Timothy L.   Stolper, Edward M.   Newman, Sally   Hauri, Erik  

    Subduction zone magmas are characterized by high concentrations of H(2)O, presumably derived from the subducted plate and ultimately responsible for melting at this tectonic setting. Previous studies of the role of water during mantle melting beneath back-arc basins found positive correlations between the H(2)O concentration of the mantle (H(2)O(o)) and the extent of melting (F), in contrast to the negative correlations observed at mid-ocean ridges. Here we examine data compiled from six back-arc basins and three mid-ocean ridge regions. We use TiO(2) as a proxy for F, then use F to calculate H(2)O(o) from measured H(2)O concentrations of submarine basalts. Back-arc basins record up to 0.5 wt % H(2)O or more in their mantle sources and define positive, approximately linear correlations between H(2)O(o) and F that vary regionally in slope and intercept. Ridge-like mantle potential temperatures at back-arc basins, constrained from Na-Fe systematics (1350 degrees-1500 degrees C), correlate with variations in axial depth and wet melt productivity (similar to 30-80% F/wt % H(2)O(o)). Water concentrations in back-arc mantle sources increase toward the trench, and back-arc spreading segments with the highest mean H(2)O(o) are at anomalously shallow water depths, consistent with increases in crustal thickness and total melt production resulting from high H(2)O. These results contrast with those from ridges, which record low H(2)O(o) (<0.05 wt %) and broadly negative correlations between H(2)O(o) and F that result from purely passive melting and efficient melt focusing, where water and melt distribution are governed by the solid flow field. Back-arc basin spreading combines ridge-like adiabatic melting with nonadiabatic mantle melting paths that may be independent of the solid flow field and derive from the H(2)O supply from the subducting plate. These factors combine significant quantitative and qualitative differences in the integrated influence of water on melting phenomena in back-arc basin and mid-ocean ridge settings.
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  • Global variations in H2O/Ce: 2. Relationships to arc magma geochemistry and volatile fluxes

    Ruscitto, Daniel M.   Wallace, Paul J.   Cooper, Lauren B.   Plank, Terry  

    We compiled a data set of 100 primitive arc magma compositions from melt inclusion and whole rock analyses to compare volatile contents, slab tracers, and calculated subduction component compositions between 18 subduction zone segments spanning the global range in slab thermal structure. The average primitive magma H2O content in our data set is 3.3 +/- 1.2 wt.% (1 s.d.) for melts erupted within 50 km of the volcanic front. While there is a wide range of volatile contents in magmas within individual arcs, the highest values occur in magmas erupted from vents along the volcanic front, where the subducting slab is located 104 +/- 29 km (avg +/- 1 s.d.) beneath the surface. This observation, coupled with positive correlations between H2O, Cl, S, and B contents and predictions from geodynamic models, provides strong evidence for the active supply of volatile-rich slab-derived components from the subducting oceanic plate beneath volcanic arcs. We also show that temperature-sensitive ratios (e. g., H2O/Ce) for both primitive arc magmas and calculated subduction components are similar and display monotonic behavior with slab thermal parameter. Furthermore, calculated subduction component compositions have higher trace element to H2O ratios in arcs with hotter slabs (lower thermal parameter), suggesting that hydrous melts of differing compositions are added beneath different arcs. Finally, we present new volatile outflux estimates for Central Cascades magmatism and then compare these to estimates for the Central American and Kamchatka-Kurile arcs to create a combined data set spanning a large range in slab thermal parameter.
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  • Sources of Fe to the equatorial Pacific Ocean from the Holocene to Miocene

    Ziegler, Christa L.   Murray, Richard W.   Plank, Terry   Hemming, Sidney R.  

    Biological productivity in the modern equatorial Pacific Ocean, a region with high nutrients and low chlorophyll, is currently limited by the micronutrient Fe. In order to test whether Fe was limiting in the past and to identify potential pathways of Fe delivery that could drive Fe fertilization (i.e., dust delivery from eolian inputs vs. Fe supplied by the Equatorial Undercurrent), we chemically isolated the terrigenous material from sediment along a cross-equatorial transect in the central equatorial Pacific at 140 degrees W and at Ocean Drilling Program Site 850 in the eastern equatorial Pacific. We quantified the contribution from each potential Fe-bearing terrigenous source using a suite of chemical- and isotopic discrimination strategies as well as multivariate statistical techniques. We find that the distribution of the terrigenous sources (i.e., Asian loess, South American ash Papua New Guinea and ocean island basalt) varies through time latitude and,,,, climate. Regardless of which method is used to determine accumulation rate, there also is no relationship between flux of any particular Fe source and climate. Moreover, there is no connection between a particular Fe source or pathway (eolian vs. Undercurrent) to total productivity during the Last Glacial Maximum, Pleistocene glacial episodes, and the Miocene "Biogenic Bloom". This would suggest an alternative process, such as an interoceanic reorganization of nutrient inventories, may be responsible for past changes in total export in the open ocean, rather than simply Fe supply from dust and/or Equatorial Undercurrent processes. Additionally, perhaps a change in Fe source or flux is related to a change in a particular component of the total productivity (e.g., the production of organic matter, calcium carbonate, or biogenic opal). (c) 2008 Elsevier B.V. All rights reserved.
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  • Mantle temperature variations beneath back-arc spreading centers inferred from seismology, petrology, and bathymetry

    Wiens, Douglas A.   Kelley, Katherine A.   Plank, Terry  

    Variations in seismological structure, major element composition, and axial depth between different back-arc spreading centers provide constraints on physical conditions associated with back-arc melt production. We invert vertical and transverse seismograms from several representative paths traversing the Mariana, Lau, North Fiji, and East Scotia back-arc basins. Seismic velocity varies substantially at depths of 40-100 km, with differences of up to 7% between the slowest (Lau) and the fastest (Mariana) structures. These mantle seismological structures correlate with major element systematics and the elevations of the ridge axes, consistent with differences in average upper mantle temperatures. In contrast to the temperature correlation, mantle seismic structure shows no apparent correlation with petrologically inferred water content. Petrological indicators suggest a similar to 100 degrees C range in mantle potential temperature, consistent with the seismic velocity variations, assuming experimentally determined temperature derivatives. The temperature variations, however, must extend throughout the upper similar to 200 km of the mantle wedge to produce the observed ridge elevation differences. Fast stab rollback and the influx of hot Samoan mantle may contribute to high temperatures in the Lau Basin. (c) 2006 Elsevier B.V. All rights reserved.
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  • High water contents in basaltic magmas from Irazu Volcano, Costa Rica

    Benjamin, Ezra R.   Plank, Terry   Wade, Jennifer A.   Kelley, Katherine A.   Haun, Erik H.   Alvarado, Guillermo E.  

    Irazu volcano, in Costa Rica, erupts magmas unusually enriched in incompatible trace elements (e.g., K, REE) relative to most other arc volcanoes worldwide. Previous studies place this enrichment in the mantle, with minimal inputs from the subducting slab. In order to test the subduction vs. mantle hypotheses, we present here the first published measurements of the pre-eruptive volatile content of lrazu magmas. Olivine-hosted melt inclusions from basaltic-andesite scoria from the 1723 eruption are volatile-rich, containing > 3 wt.% H2O, > 200 PPM CO2, > 2500 ppm S, > 2200 ppin Cl and > 1800 ppin F. The average composition of the 1723 melt inclusions is very similar to that of the host scoria (SiO2=54% SiO2), although inclusions include more mafic (48% SiO2) and felsic (57% SiO2) compositions. The 1723 melt inclusions have the same trace element characteristics (e.g., Ba/La) as the host scoria, ruling out exotic crustal or mantle sources. Together, the melt inclusions and their host olivines (Fo(87-79)) define a closed-system ascent path (150-20 MPa) of coupled degassing, crystallization, and cooling (1075-1045 degrees C). The maximum H2O measured in the melt inclusions and the shape of the degassing path together constrain the pre-eruptive H2O content to 3.2-3.5 wt. %, significantly higher than in ocean island basalts, but typical of arc magmas. The high H2O in Irazu melts, coupled with their high Cl/K2O, are inconsistent with enriched mantle with minimal slab fluid addition. We propose instead that subducting input is the dominant contributor to Irazu's geochemical compositions. Galapagos-derived seamounts and volcaniclastics are currently entering the trench near lrazu and provide to the Irazu source both volatiles (from seafloor hydration and chlorination) and ocean-island-type trace elements and isotopes. A few percent of subducted Galapagos volcanics added to MORB mantle can create Irazu compositions quantitatively, provided elements are further fractionated according to solute-rich liquid or melt-eclogite partition coefficients. Subduction of seamount chains may create high-K arc volcanism elsewhere, such as in the northern Marianas. (c) 2007 Elsevier B.V. All rights reserved.
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