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  • 1
    Publication Date: 2019-01-11
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 2
    Publication Date: 2019-02-01
    Type: Article , PeerReviewed
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  • 3
    Publication Date: 2017-12-19
    Description: Thin oceanic crust is formed by decompression melting of the upper mantle at mid-ocean ridges, but the origin of the thick and buoyant continental crust is enigmatic. Juvenile continental crust may form from magmas erupted above intraoceanic subduction zones, where oceanic lithosphere subducts beneath other oceanic lithosphere. However, it is unclear why the subduction of dominantly basaltic oceanic crust would result in the formation of andesitic continental crust at the surface. Here we use geochemical and geophysical data to reconstruct the evolution of the Central American land bridge, which formed above an intra-oceanic subduction system over the past 70Myr. We find that the geochemical signature of erupted lavas evolved from basaltic to andesitic about 10Myr ago - coincident with the onset of subduction of more oceanic crust that originally formed above the Galápagos mantle plume. We also find that seismic P-waves travel through the crust at velocities intermediate between those typically observed for oceanic and continental crust. We develop a continentality index to quantitatively correlate geochemical composition with the average P-wave velocity of arc crust globally. We conclude that although the formation and evolution of continents may involve many processes, melting enriched oceanic crust within a subduction zone - a process probably more common in the Archaean - can produce juvenile continental crust.
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  • 4
    Publication Date: 2016-10-24
    Description: The origin and initiation of the Aleutian Subduction Zone, forming the boundary between the Pacific Ocean and Bering Sea, remains elusive. A continuous belt of arc volcanism extended from mainland Alaska to the Siberian margin with the youngest volcanism along the Beringian margin ranging from 54-50Ma (Davies et al., 1989, CanJEarthSci26). Magnetic anomalies indicate that plate motion changed from a northerly to a westerly direction at ~56-53Ma (chron 25-24). Thereafter volcanism shifted to the Aleutian Arc, presumably trapping a fragment of the Kula plate that formed the basement of the Bering Sea. Interestingly the oldest rocks from the forearcs of the Izu-Bonin-Mariana (IBM) and the Tonga subduction systems yield ages of 50-52Ma (e.g. Reagan et al. 2013, EPSL380; Meffre et al., 2013, G3_13), suggesting subduction initiation as a consequence of a Pacific-wide tectonic reorganization. Did the Aleutian Arc also form during this plate-wide event? The oldest Aleutian ages come from the submarine Murray Canyon in the central Aleutian Arc (46Ma; Jicha et al., 2006, Geology34) and from Medny, the smaller Komandorsky Island, in the westernmost Aleutians (47Ma). The earliest volcanism on Medny (47-21Ma) is tholeiitic with clear subduction-related incompatible element signatures, e.g. relative enrichments in mobile elements such as U, Sr and Pb and depletions in Nb and Ta. Isotopic compositions for Sr-Nd-Pb indicate contributions from subducted sediment and ocean crust, as well as Komandorsky mantle wedge. In the IBM and Tonga Arc systems, boninites have been found among the earliest lavas. In contrast, no boninites have been found in the Aleutians thus far, which could suggest that the oldest lavas have not been found and that subduction may also have initiated earlier, possibly also between 52-50Ma. In order to further constrain the age of arc initiation and the compositions of lavas formed during arc initiation, we have just completed detailed sampling of the lowermost portions of the Adak, Amchitka, Murray and Attu Canyons in the Aleutian forearc and at Kresta Ridge in the rear-arc on R/V SONNE Cruise 249/1. On SO249/2, we will sample forearc canyons in the westernmost Aleutians southeast of Medney Island. Results from the Komandorsky Islands and preliminary results from the cruises will be presented.
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  • 5
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    American Geophysical Union
    In:  In: Inside the Subduction Factory. , ed. by Eiler, J. Geophysical Monograph Series, 138 . American Geophysical Union, Washington, D.C., pp. 223-276. ISBN 0-87590-997-3
    Publication Date: 2018-10-08
    Type: Book chapter , NonPeerReviewed
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  • 6
    Publication Date: 2018-02-01
    Description: © The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geochimica et Cosmochimica Acta 181 (2016): 217-237, doi:10.1016/j.gca.2016.03.010.
    Description: Sediment transport from the subducted slab to the mantle wedge is an important process in understanding the chemical and physical conditions of arc magma generation. The Aleutian arc offers an excellent opportunity to study sediment transport processes because the subducted sediment flux varies systematically along strike (Kelemen et al., 2003) and many lavas exhibit unambiguous signatures of sediment addition to the sub-arc mantle (Morris et al., 1990). However, the exact sediment contribution to Aleutian lavas and how these sediments are transported from the slab to the surface are still debated. Thallium (Tl) isotope ratios have great potential to distinguish sediment fluxes in subduction zones because pelagic sediments and low temperature altered oceanic crust are highly enriched in Tl and display heavy and light Tl isotope compositions, respectively, compared with the upper mantle and continental crust. Here, we investigate the Tl isotope composition of lavas covering almost the entire Aleutian arc a well as sediments outboard of both the eastern (DSDP Sites 178 and 183) and central (ODP Hole 886C) portions of the arc. Sediment Tl isotope compositions change systematically from lighter in the Eastern to heavier in the Central Aleutians reflecting a larger proportion of pelagic sediments when distal from the North American continent. Lavas in the Eastern and Central Aleutians mirror this systematic change to heavier Tl isotope compositions to the west, which shows that the subducted sediment composition is directly translated to the arc east of Kanaga Island. Moreover, quantitative mixing models of Tl and Pb, Sr and Nd isotopes reveal that bulk sediment transfer of ~0.6-1.0% by weight in the Eastern Aleutians and ~0.2-0.6% by weight in the Central Aleutians can account for all four isotope systems. Bulk mixing models, however, require that fractionation of trace element ratios like Ce/Pb, Cs/Tl, and Sr/Nd in the Central and Eastern Aleutians occurs after the sediment component was mixed with the mantle wedge. Models of Sr and Nd isotopes that involve sediment melting require either high degrees of sediment melting (〉50%), in which case trace element ratios like Ce/Pb, Cs/Tl, and Sr/Nd of Aleutian lavas need to be produced after mixing with the mantle, or significant fluid additions from the underlying oceanic crust with Sr and Nd isotope compositions indistinguishable from the mantle wedge as well as high Sr/Nd ratios similar to that of low (〈20%) degree sediment melts. Thallium isotope data from Western Aleutian lavas exhibit compositions slightly lighter than the upper mantle, which implies a negligible sediment flux at this location and probably involvement of low-temperature altered oceanic crust in the generation of these lavas. In general, the lightest Tl isotope compositions are observed for the highest Sr/Y ratios and most unradiogenic Sr and Pb isotope compositions, which is broadly consistent with derivation of these lavas via melting of eclogitized altered oceanic crust.
    Description: This study was funded by NSF grants EAR-1119373 and -1427310 to SGN and EAR-1456814 to TP
    Description: 2017-03-07
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 7
    Publication Date: 2019-10-22
    Description: Seafloor lavas of the western Aleutian arc define a unique dataset for island arcs globally in 238U-230Th isotope space, with 230Th excesses up to 79% and 230Th/232Th activity ratios [(230Th/232Th)] up to ∼2.27. Basic modeling to produce the observed 230Th excesses is consistent with low Yb and fractionated Dy/Yb in end-member high-Mg# dacites reflecting a role for residual garnet. The models require use of partitioning data with DU ∼0.2 and DTh ∼0.1 in garnet as measured under experimental conditions appropriate for equilibration of hydrous and high-silica melts with eclogite at 〈1050° C and 2-4 GPa (Skora & Blundy, 2010 J Petrol; Kessel et al., 2005 Nature). Mixing relationships indicate that (230Th/232Th) is more sensitive to the presence of AOC (altered oceanic crust) in the source than is 87Sr/86Sr, so the coupling of unradiogenic Sr (87Sr/86Sr 〈0.7029) with (230Th/232Th) 〉2.0 points to a source with a modest but measureable contribution of U-enriched AOC. Most melting of the subducting plate is in Pacific MORB that has not been strongly affected by seawater alteration (Yogodzinski et al., 2017 EPSL). Key mixing relationships can only be observed in arc volcanic rocks lacking subducted sediment in their source which adds significant quantities of radiogenic Sr. This criterion is met in the western Aleutians but not by common arc rocks globally. An important caveat for the dataset is that the ages of most western Aleutian samples are unknown, so measured (230Th/232Th) and 230Th-excess are considered minimums. We cannot rule out a role for 230Th ingrowth to explain high (230Th/232Th), but strong correlations of (230Th/232Th) with long-lived isotopes suggest that elevated (230Th) in island-arc volcanic rocks globally (compilation of Huang et al., 2016 Chem Geol) is primarily an indicator of source composition, and not a product of ingrowth during melt transport. In the global dataset, we find that mixing of (230Th/232Th) and (238U/230Th) with 87Sr/86Sr, 208Pb/204Pb, 143Nd/144Nd, and with trace element ratios such as Th/U and Lu/Hf are all consistent with a role for an eclogite-melt source component (similar to that observed western Aleutian seafloor lavas) in island-arc volcanic rocks throughout the Aleutian arc and in arc volcanic rocks globally.
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  • 8
    Publication Date: 2020-02-06
    Description: Highlights • An eclogite-melt component (slab melt) is present in volcanic rocks throughout the Aleutian arc. • Fluids that drive slab melting are produced by dehydration of serpentinite in the subducting plate. • Slab melting encompasses a large section of mafic oceanic crust unaffected by seawater alteration. • The subducting plate beneath the Aleutian arc is hotter than indicated by most thermal models. Abstract High Mg# andesites and dacites (Mg# = molar Mg/Mg + Fe) from western Aleutian seafloor volcanoes carry high concentrations of Sr (〉1000 ppm) that is unradiogenic (87Sr/86Sr 〈 0.7029) compared to lavas from emergent volcanoes throughout the arc (200–800 ppm Sr, 87Sr/86Sr 〉0.7030). Data patterns in plots of 87Sr/86Sr vs Y/Sr and Nd/Sr imply the existence of an eclogite-melt source component – formed by partial melting of MORB eclogite in the subducting Pacific Plate – which is most clearly expressed in the compositions of western Aleutian andesites and dacites (Nd/Sr and Y/Sr 〈 0.02) and which dominates the source budget for Sr in volcanic rocks throughout the arc. When viewed in combination with inversely correlated εNdεNd and 87Sr/86Sr, these patterns rule out aqueous fluids as an important source of Sr because mixtures of fluids from altered oceanic crust with depleted mantle and sediment produce compositions with 87Sr/86Sr higher than in common Aleutian rocks. The unradiogenic nature of Sr in the western Aleutian andesite–dacite end-member may be understood if H2O required to drive melting of the subducting oceanic crust is transported in fluids containing little Sr. Mass balance demonstrates that such fluids may be produced by dewatering of serpentinite in the mantle section of the subducting plate. If the eclogite-melt source component is present throughout the Aleutian arc, melting of the subducting plate must extend into minimally altered parts of the sheeted dike section or upper gabbros, at depths 〉2 km below the paleo-seafloor. Oxygen isotopes in western Aleutian seafloor lavas, which fall within a narrow range of MORB-like values (δ18O=5.1–5.7δ18O=5.1–5.7), are also consistent with this model. These results indicate that the subducting Pacific lithosphere beneath the Aleutian arc is significantly hotter than indicated my most thermal models.
    Type: Article , PeerReviewed
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  • 9
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    In:  [Talk] In: KALMAR - Second Bilateral Workshop on Russian-German Cooperation on Kurile-Kamchatka and Aleutean Marginal Sea-Island Arc Systems, 16.05.-20.05.2011, Trier . KALMAR - Kurile-Kamchatka-Aleutean Marginal Sea - Island Systems : Program and Abstracts ; Workshop in Russian-German Cooperation, May 16 - 20, 2011 Trier, Germany ; pp. 23-24 .
    Publication Date: 2020-11-03
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  • 10
    Publication Date: 2020-11-05
    Description: Constraining the behaviour of Re and Os during eclogite melting is required to understand the Re and Os budget and 187Os/188Os of recycled slabs produced at warm subduction zones. It is particularly relevant to early Earth history, a period during which slab melting could have prevailed over dehydration due to higher mantle temperatures. There are however currently few constraints on Re and Os mobility during slab melting. Accordingly, we measured Os, Re and 187Os/188Os in primitive submarine lavas (Mg# ˃ 0.6) from the western Aleutian Arc. These include strongly adakitic rocks shown to be derived from eclogite melting (high-Mg# andesite, dacites and rhyodacites), as well as non-adakitic rocks (high-Mg# andesites, basaltic andesites and basalts) with variable sediment and fluid-derived slab contributions for comparison. The 187Os/188Os of the adakitic and non-adakitic volcanic rocks vary significantly but largely overlap. In both groups, the most radiogenic values occur in samples with the lowest Os concentrations, thus implicating crustal assimilation as the main cause of Os isotope variations. Adakitic and non-adakitic rocks least affected by crustal assimilation have overlapping 187Os/188Os of 0.141–0.149. We show that the source of the adakites is very unlikely to comprise significant eclogite-derived Os, which suggests no or minimal mobilization of Os during eclogite melting. Eclogitic Os is inferred to be retained in sulphides or replacement phases formed upon sulphide breakdown for which Os has high affinity, such as a platinum-group minerals (PGMs). The small Os budget of the adakites is most likely derived from limited reaction with the mantle wedge during ascent. Degassing has reduced Re contents in most samples, but not for end-member adakites (SiO2 〉 67% and Sr/Y 〉 200; n = 4) that were erupted at seafloor depths 〉 2500 m. These undegassed samples have elevated Re concentrations (0.8–1.5 ppb) that are positively correlated with Sr/Y and so are interpreted to be primary magmatic concentrations resulting from the mobilization of Re from the slab. Re could either be derived from the eclogites or from the serpentinite-derived fluids fluxing eclogites during melting. The former scenario would produce recycled residual crusts with lower Re/Os than in unmelted eclogites while the latter would result in Re/Os ranging from similar to higher than prior to melting. In both cases, the Re/Os and therefore the time-integrated 187Os/188Os of residual crust produced at warm subduction zones involving slab melting are likely to be different from that processed at cooler typical modern subduction zones. Therefore, if slab melting was an important process during the early Earth, the use of Re and Os partitioning in modern subduction zones to model the source of magmas comprising old recycled oceanic crust, such as the HIMU (high μ = 238U/204Pb) ocean island basalts (OIBs), might lead to erroneous interpretations.
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