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Straub S.M.,Lamont Doherty Earth Observatory | Straub S.M.,Academia Sinica, Taiwan | Gomez-Tuena A.,National Autonomous University of Mexico | Stuart F.M.,Scottish Universities Research and Reactor Center | And 5 more authors.
Earth and Planetary Science Letters | Year: 2011

Andesite magmatism at convergent margins is essential for the differentiation of silicate Earth, but no consensus exists as to andesite petrogenesis. Models proposing origin of primary andesite melts from mantle and/or slab materials remain in deadlock with the seemingly irrefutable petrographic and chemical evidence for andesite formation through mixing of basaltic mantle melts with silicic components from the overlying crust. Here we use 3He/4He ratios of high-Ni olivines to demonstrate the mantle origin of basaltic to andesitic arc magmas in the central Mexican Volcanic Belt (MVB) that is constructed on ~50km thick continental crust. We propose that the central MVB arc magmas are hybrids of high-Mg#>70 basaltic and dacitic initial mantle melts which were produced by melting of a peridotite subarc mantle interspersed with silica-deficient and silica-excess pyroxenite veins. These veins formed by infiltration of reactive silicic components from the subducting slab. Partial melts from pyroxenites, and minor component melts from peridotite, mix in variable proportions to produce high-Mg# basaltic, andesitic and dacitic magmas. Moderate fractional crystallization and recharge melt mixing in the overlying crust produces then the lower-Mg# magmas erupted. Our model accounts for the contrast between the arc-typical SiO2 variability at a given Mg# and the strong correlation between major element oxides SiO2, MgO and FeO which is not reproduced by mantle-crust mixing models. Our data further indicate that viscous high-silica mantle magmas may preferentially be emplaced as intrusive silicic plutonic rocks in the crust rather than erupt. Ultimately, our results imply a stronger turnover of slab and mantle materials in subduction zones with a negligible, or lesser dilution, by materials from the overlying crust. © 2011 Elsevier B.V.

Straub S.M.,Lamont Doherty Earth Observatory | Gomez-tuena A.,National Autonomous University of Mexico | Zellmer G.F.,Academia Sinica, Taiwan | Stuart F.M.,Scottish Universities Research and Reactor Center | And 5 more authors.
Journal of Petrology | Year: 2013

Andesite petrogenesis is inextricably linked to plate processing at convergent margins. The details of andesite formation, however, remain poorly understood because the signatures of the initial arc mantle melts are often modified in the overlying crust. To distinguish initial mantle from crustal signatures in arc magmas, we studied two compositionally zoned Holocene monogenetic volcanoes, Texcal Flow and Volcan Chichinautzin, in the central Mexican Volcanic Belt (MVB). Texcal Flow and V. Chichinautzin erupt ocean island basalt (OIB)-type, high-Nb (17-36 ppm), olivine-phyric basalts to basaltic andesites (49·4-57·3 wt SiO2; Mg# 68-50) that show an arc affinity in their major element oxides. At both volcanoes melt SiO2. increases with time. However, systematic changes of melt SiO2 with 87Sr/86Sr and 143Nd/144Nd, the overall low 87Sr/86Sr 0·70305-0·70453 and high 143Nd/144Nd 0·51273-0·51299 relative to continental crust, and the high 3He/4He 7-8 Ra of olivine phenocrysts preclude melt silica enrichment by crustal assimilation and fractional crystallization. Instead, the data require the existence of silicic initial mantle melts. The high Ni abundances of olivines suggest that the silicic melts originate from segregations of reaction pyroxenites that formed in the peridotite mantle wedge following multiple infiltrations of silicic slab components. Sequential melting of zoned silica-deficient to silica-excess pyroxenites can reproduce the time-progressive evolution of melt silica content at Texcal Flow and V. Chichinautzin. As initial melts always have high Mg# > 70 regardless of their SiO2. content, the low-Mg# values of the magmas erupted must reflect loss of moderate amounts (<15) of olivine and possibly pyroxenes at crustal levels. Fractional crystallization and recharge mixing nearly erase all mantle signatures in the most silicic V. Chichinautzin magmas, so that their origin can only be inferred from their association with the more mafic precursory melts. The pyroxenite model implies that ∼15-18 wt of the erupted melt mass, and possibly more, is slab-derived. We infer that the elements Fe, Mg, Ca and Ti are principally mantle-derived, whereas significant amounts of the elements Si, K, Na, P and possibly Al may be contributed from slab. As blends of mantle and slab materials, the OIB-type Texcal Flow and V. Chichinautzin magmas provide limited indication of the composition of the sub-arc mantle prior to subduction modification, which is inferred to be similar to primitive mantle, but less enriched than the sources of the intraplate magmas behind the MVB volcanic front. © The Author 2012. Published by Oxford University Press. All rights reserved.

Straub S.M.,Lamont Doherty Earth Observatory | Straub S.M.,Academia Sinica, Taiwan | Gomez-Tuena A.,National Autonomous University of Mexico | Bindeman I.N.,University of Oregon | And 6 more authors.
Geochimica et Cosmochimica Acta | Year: 2015

Recycling of upper plate crust in subduction zones, or 'subduction erosion', is a major mechanism of crustal destruction at convergent margins. However, assessing the impact of eroded crust on arc magmas is difficult owing to the compositional similarity between the eroded crust, trench sediment and arc crustal basement that may all contribute to arc magma formation. Here we compare Sr-Nd-Pb-Hf and trace element data of crustal input material to Sr-Nd-Pb-Hf-He-O isotope chemistry of a well-characterized series of olivine-phyric, high-Mg# basalts to dacites in the central Mexican Volcanic Belt (MVB). Basaltic to andesitic magmas crystallize high-Ni olivines that have high mantle-like 3He/4He=7-8Ra and high crustal δ18Omelt=+6.3-8.5‰ implying their host magmas to be near-primary melts from a mantle infiltrated by slab-derived crustal components. Remarkably, their Hf-Nd isotope and Nd/Hf trace element systematics rule out the trench sediment as the recycled crust end member, and imply that the coastal and offshore granodiorites are the dominant recycled crust component. Sr-Nd-Pb-Hf isotope modeling shows that the granodiorites control the highly to moderately incompatible elements in the calc-alkaline arc magmas, together with lesser additions of Pb- and Sr-rich fluids from subducted mid-oceanic ridge basalt (MORB)-type altered oceanic crust (AOC). Nd-Hf mass balance suggests that the granodiorite exceeds the flux of the trench sediment by at least 9-10times, corresponding to a flux of ≥79-88km3/km/Myr into the subduction zone. At an estimated thickness of 1500-1700m, the granodiorite may buoyantly rise as bulk 'slab diapirs' into the mantle melt region and impose its trace element signature (e.g., Th/La, Nb/Ta) on the prevalent calc-alkaline arc magmas. Deep slab melting and local recycling of other slab components such as oceanic seamounts further diversify the MVB magmas by producing rare, strongly fractionated high-La magmas and a minor population of high-Nb magmas, respectively. Overall, the central MVB magmas inherit their striking geochemical diversity principally from the slab, thus emphasizing the importance of continental crust recycling in modern solid Earth relative to its new formation in modern subduction zones. © 2015 Elsevier Ltd.

Tarling G.A.,Natural Environment Research Council | Stowasser G.,Natural Environment Research Council | Ward P.,Natural Environment Research Council | Poulton A.J.,UK National Oceanography Center | And 4 more authors.
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2012

The biomass size structure of pelagic communities provides a system level perspective that can be instructive when considering trophic interactions. Such perspectives can become even more powerful when combined with taxonomic information and stable isotope analysis. Here we apply these approaches to the pelagic community of the Scotia Sea (Southern Ocean) and consider the structure and development of trophic interactions over different years and seasons. Samples were collected from three open-ocean cruises during the austral spring 2006, summer 2008 and autumn 2009. Three main sampling techniques were employed: sampling bottles for microplankton (0-50m), vertically hauled fine meshed nets for mesozooplankton (0-400m) and coarse-meshed trawls for macrozooplankton and nekton (0-1000m). All samples were identified to the lowest practicable taxonomic level and their abundance, individual body weight and biomass (in terms of carbon) estimated. Slopes of normalised biomass spectrum versus size showed a significant but not substantial difference between cruises and were between -1.09 and -1.06. These slopes were shallower than expected for a community at equilibrium and indicated that there was an accumulation of biomass in the larger size classes (10 1-10 5mgCind -1). A secondary structure of biomass domes was also apparent, with the domes being 2.5-3log 10 intervals apart in spring and summer and 2log 10 intervals apart in autumn. The recruitment of copepod-consuming macrozooplankton, Euphausia triacantha and Themisto gaudichaudii into an additional biomass dome was responsible for the decrease in the inter-dome interval in autumn. Predator to prey mass ratios estimated from stable isotope analysis reached a minimum in autumn while the estimated trophic level of myctophid fish was highest in that season. This reflected greater amounts of internal recycling and increased numbers of trophic levels in autumn compared to earlier times of the year. The accumulation of biomass in larger size classes throughout the year in the Scotia Sea may reflect the prevalence of species that store energy and have multiyear life-cycles. © 2011 Elsevier Ltd.

Straub S.M.,Lamont Doherty Earth Observatory | Zellmer G.F.,Academia Sinica, Taiwan | Zellmer G.F.,Massey University | Gomez-Tuena A.,National Autonomous University of Mexico | And 3 more authors.
Geological Society Special Publication | Year: 2014

A fundamental question in the formation of orogenic andesites is whether their high melt SiO2 reflects the recycling of silicic melts from the subducted slab or the processing of basaltic mantle melts in the overlying crust. The latter model is widely favoured, because most arc magmas lack the 'garnet' signature of partial slab melts. Here we present new trace element data from Holocene high-Mg# >64-72 calc-alkaline basalts to andesites (50-62 wt% SiO2) from the central Mexican Volcanic Belt that crystallize high-Ni olivines with the high 3He/4He = 7-8 of the upper mantle. These magmas have been proposed to be partial melts from 'reaction pyroxenites', which formed by hybridization of mantle peridotite (c. 82-85%) and heavy rare earth elementdepleted silicic slab melt (>15-18%). Forward and inverse models suggest that the absence of a garnet signature in these melts reflects the efficient buffering of the heavy rare earth elements (Ho to Lu) in the subarc mantle. In contrast, all elements more incompatible than H-excepting TiO2-are more or less strongly controlled by the silicic slab flux that also directly contributes to the silicic arc magma formation. Our study emphasizes the strong link between slab recycling and the genesis of orogenic andesites. © The Geological Society of London 2014.

PubMed | Scottish Universities Research and Reactor Center
Type: Journal Article | Journal: Environmental geochemistry and health | Year: 2013

Low level liquid radioactive waste discharges from the Sellafield nuclear fuel reprocessing plant in north west England had generated environmental inventories of about 3 10(16) Bq of(137)Cs, 6.8 10(14) Bq of(239,240)Pu and 8.9 10(14) Bq of(241)Am by 1990. Most of the(239,240)Pu and(241)Am and about 10% of the(137)Cs has been retained in a deposit of fine marine sediment close to the discharge point. The quantities of radionuclides discharged annually from Sellafield decreased by two orders of magnitude from the mid-1970s to 1990 but estimated critical group internal and external exposure decreased by less than one order of magnitude over this period. This indicates that during the period of reduced discharges, radionuclides already in the environment from previous releases continued to contribute to the critical group exposure and highlights the need to understand processes controlling the environmental distribution of the radionuclides.Redistribution of the contaminated marine sediment is potentially of major significance in this context, in particular if it results in transport of radionuclides to intertidal areas, where contact with the human population is relatively likely.A review is presented of published work relating to Sellafield waste radionuclides in Irish Sea sediments. Data on temporal and spatial trends in radionuclide concentrations and activity ratios are collated from a number of sources to show that the dominant mechanism of radionuclide supply to intertidal areas is by redistribution of the contaminated marine sediment. The implications of this mechanism of supply for trends in critical group radiation exposure are considered.

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