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  • 1
    Publication Date: 2015-09-23
    Description: Olivine major and trace element compositions from 12 basalts from the southern Payenia volcanic province in Argentina have been analyzed by electron microprobe and laser ablation inductively coupled plasma mass spectrometry. The olivines have high Fe/Mn and low Ca/Fe and many fall at the end of the global olivine array, indicating that they were formed from a pyroxene-rich source distinct from typical mantle peridotite. The olivines with the highest Fe/Mn have higher Zn/Fe, Zn and Co and lower Co/Fe than the olivines with lower Fe/Mn, also suggesting contributions from a pyroxene-rich source. Together with whole-rock radiogenic isotopes and elemental concentrations, the samples indicate mixing between two mantle sources: (1) a pyroxene-rich source with EM-1 ocean island basalt type trace element and isotope characteristics; (2) a peridotitic source with more radiogenic Pb that was metasomatized by subduction-zone fluids and/or melts. The increasing contributions from the pyroxene-rich source in the southern Payenia basalts are correlated with an increasing Fe-enrichment, which caused the olivines to have lower forsterite contents at a given Ni content. Al-in-olivine crystallization temperatures measured on olivine–spinel pairs are between 1155 and 1243°C and indicate that the magmas formed at normal upper mantle (asthenospheric) temperatures of ~1350°C. The pyroxene-rich material is interpreted to have been brought up from the deeper parts of the upper mantle by vigorous asthenospheric upwelling caused by break-off of the Nazca slab south of Payenia during the Pliocene and roll-back of the subducting slab beneath Payenia. The pyroxene-rich mantle mixed with peridotitic metasomatized South Atlantic mantle in the mantle wedge beneath Payenia.
    Print ISSN: 0022-3530
    Electronic ISSN: 1460-2415
    Topics: Geosciences
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  • 2
    Publication Date: 2013-02-20
    Description: Asymmetrically zoned hotspot tracks in the Pacific Ocean are interpreted to have formed from zoned plumes originating from the large-scale, lower-mantle, low-seismic-velocity anomaly (superplume?) beneath the southern Pacific, providing direct information about lower-mantle compositional heterogeneity. New trace-element and Sr-Nd-Hf-Pb isotope data from the classic Tristan-Gough hotspot track in the South Atlantic also display a bilateral, asymmetric zonation with two distinct mantle source components, making it the first zoned plume to be recognized overlying the African superplume. The plume zonation can be traced for 70 m.y., four times longer than recognized for Pacific zoned hotspot tracks. These findings confirm that the proposed zonation of Pacific hotspots is not simply a geochemical oddity, but could be a major feature of plumes derived from lower-mantle superplumes. We propose that the enriched southern Gough subtrack source with elevated 207 Pb/ 204 Pb and 208 Pb/ 204 Pb at a given 206 Pb/ 204 Pb, but low 143 Nd/ 144 Nd and 176 Hf/ 177 Hf (DUPAL-like composition), may reflect the African superplume composition, whereas the more depleted northern Tristan subtrack source could represent a mixture of the superplume with the surrounding depleted mantle. Our results strengthen arguments that the enriched signature (DUPAL anomaly) in the South Atlantic could be derived from the lower mantle.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 3
    Publication Date: 2013-02-09
    Description: Corals are important marine archives for high-resolution reconstructions of low-latitude climate variability in preinstrumental and historical periods. Herein, we present monthly-resolved records of 18 O and Sr/Ca for the 20th century (1917–2007) from a Porites lutea colony from the Maldives (northwestern Indian Ocean). Previous studies of annual mean extension rates of this coral revealed a distinct dependency of coral growth on variations of sea surface temperature (SST) and hydraulic energy, driven by El Niño-Southern Oscillation (ENSO) and southwestern Indian monsoon forcing. This enables the investigation of coral 18 O and Sr/Ca ratios from the Maldives as archives of historical ENSO and Indian monsoon variability. Unlike other locations in the Indian Ocean, correlation of 18 O and SST is weak (r = –0.42; p 〈 0.001), suggesting interferences of SST and seawater 18 O at the location. These interferences probably caused significantly weaker interannual ENSO signatures in 18 O as evident in the extension rates of our coral and in geochemical proxies measured at other sites in the northwestern Indian Ocean. Sr/Ca ratios show bias by nontemperature effects, and were not used to estimate seawater 18 O and salinity. Strong decadal variability (10–14 years) in 18 O, indicative of a Pacific ENSO signal, is not found in instrumental SST, and could be explained by variations in salinity. Interannual and decadal monsoon variability (6–7 yrs and 18–19 yrs), found in the extension rates, was not found in 18 O, suggesting, in agreement with other studies, that geochemical coral proxy records from the northwestern Indian Ocean do not capture temporal variations of the Indian monsoon strength.
    Print ISSN: 0883-1351
    Electronic ISSN: 0883-1351
    Topics: Geosciences
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  • 4
    Publication Date: 2011-12-23
    Description: Potassium-enriched mafic lavas, and in particular basalts of the high-K series, are important end-members of subduction zone volcanism. Two petrogenetic models can explain the generation of mafic K-rich lavas, involving either melting of ancient, enriched lithospheric mantle sources (single-stage model) or melting triggered by recent refertilization by subduction-related components derived from subducted sediments or oceanic crust (multi-stage model). These two models are tested for post-collisional high-K rocks of Eocene–Oligocene age from the Rhodopes, SE Bulgaria, based on new major element, trace element and Sr–Nd–Hf–Pb isotope data. The single-stage model is evaluated by Sr–Nd isotope modelling assuming the presence of ancient enriched lithospheric domains whereas the multi-stage model is assessed by comparing the compositions of the Bulgarian lavas with those of lavas from Santorini. Santorini lavas are considered to sample the current trace element and isotope inventory of the long-lived Aegean subduction zone system. This northward facing system has been active most probably since late Jurassic or early Cretaceous times and was potentially involved in refertilizing the mantle sources of the Bulgarian lavas. In addition to the data for Bulgarian lavas, we present new major element, trace element and Sr–Nd–Hf–Pb isotope data for Santorini. These are evaluated together with previously published data to infer the mode of source enrichment in the Aegean realm. The Bulgarian lavas exhibit a broad range of compositions from medium-K to high-K and shoshonitic with radiogenic 87 Sr/ 86 Sr (0·706–0·709), and unradiogenic Nd (–5·7 to –1·9) and Hf (–3 to +3) isotope signatures. The trace element budget was apparently well buffered against shallow-level crustal assimilation as documented by the trace element and Sr–Nd isotope systematics. Modelling of the Sr–Nd isotope compositions of the Bulgarian lavas argues for recent (Mesozoic to Cenozoic) source enrichment. Therefore single-stage models involving melting of ancient (〉1 Ga) lithospheric mantle can be confidently ruled out, in agreement with tectonic models for the region. The enriched isotope signatures, together with a pronounced enrichment of incompatible elements, rather indicate mantle refertilization by a subduction component similar to continent-derived sediments subducted at the Hellenic Trench at present. The Bulgarian lavas record a predominant influx of subduction fluid components in comparison with the Santorini lavas. Collectively, the data presented for the Bulgarian lavas are clearly in favour of a multi-stage model.
    Print ISSN: 0022-3530
    Electronic ISSN: 1460-2415
    Topics: Geosciences
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  • 5
    Publication Date: 2012-08-01
    Description: Bowers Ridge is an ~700 km long arcuate ridge behind the Central Aleutian Arc in the Bering Sea. The lack of age and geochemical data for the ridge has hampered the development of geodynamic models for the evolution of the North Pacific and the Aleutian–Bering Sea region. Here we present the first geochemical and 40 Ar/ 39 Ar age data for the volcanic basement of Bowers Ridge and a seamount from the western end of the ridge sampled during R/V Sonne cruise SO201-1b. The northern Bowers Ridge basement (26–32 Ma) consists of mafic to intermediate calc-alkaline rocks with adakite-like (Sr/Y = 33–53, La N /Yb N = 3.3–7.8), high field strength element (HFSE)–depleted (e.g., Nb N /La N = 0.07–0.31) trace element patterns and Sr-Nd-Pb isotope compositions within the Western Aleutian Arc array, implying magma generation above an obliquely subducting slab. The seamount samples (22–24 Ma) are HFSE-rich alkaline olivine basalts (La N /Yb N = 3.3–3.9, Nb N /La N = 1.0–1.4) with minor arc-type trace element signatures (Pb N /Ce N = 1.4–1.6, K N /Nb N = 1.7–1.9) but with Pacific mid-oceanic-ridge basalt (MORB)–like isotopic compositions, pointing to an origin by small-degree decompression melting from slightly subduction-modified mantle. The geochemistry of the recovered rocks can be explained by highly oblique subduction along the northern part of Bowers Ridge in its present-day configuration, consistent with an in-situ origin of Bowers Ridge as a Cenozoic island arc.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 6
    Publication Date: 2013-05-30
    Description: Metamorphosed, vesuvianite-bearing dykes occur in serpentinised peridotites of the Raspas Complex (Ecuador), which represents a piece of oceanic lithosphere that has experienced high-pressure, subduction-related metamorphism. The serpentinite mantle protoliths are geochemically indistinguishable from modern oceanic lithosphere entering subduction zones. Positive Eu anomalies (Eu/Eu* = 1.3–7.2) and relative LREE enrichments (La N /Sm N = 1.2–5.5) point to hydrothermal alteration of the peridotite precursor rocks at or near the seafloor. Major mineral phases in the metamorphosed dykes include chlorite, diopside, amphibole and vesuvianite. In each dyke, only two of these phases – either amphibole + vesuvianite, diopside + chlorite, or amphibole + chlorite dominate the modal mineralogy with 〉~90 vol.%, suggesting metasomatic replacement at elevated P-T conditions during subduction, controlled by an external fluid. This fluid caused the decrease in coexisting mineral phases and overprinting of initial Sr isotope ratios (0.7025–0.7031). Preserved geochemical signatures from the dyke protoliths, including positive Eu anomalies (Eu/Eu* = 1.2–2.0) and Na enrichment due to spilitisation, reveal that the dykes originated as oceanic olivine gabbros and troctolites. Vesuvianite in the Raspas Complex formed by hydration and silica removal from gabbroic mineral assemblages during subduction. It has a wide stability in P-T space for hydrated and silica deficient bulk compositions so that it potentially represents a significant repository for the cycling of elements during subduction. In addition to Ca, Mg and Al, incorporation of significant amounts of Ti, Fe and Na (up to 2.4, 1.7 and 1.6 atoms per formula unit, respectively) in vesuvianite bears evidence for the potential of vesuvianite as petrogenetic indicator, although lack of relevant thermodynamic and experimental data precludes the extraction of quantitative information. For cold subduction zones in particular, vesuvianite appears to be able to carry significant amounts of water to mantle depths. Preferential incorporation of HREE (up to 2.2 ppm Yb), Sr (up to ~ 300 ppm) and Pb (up to 4.5 ppm) in vesuvianite underlines its potentially important role for the storage, transport and release of these key elements in radiogenic isotope geochemistry during subduction zone cycling.
    Print ISSN: 0935-1221
    Electronic ISSN: 1617-4011
    Topics: Geosciences
    Published by Schweizerbart
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  • 7
    Publication Date: 2013-01-30
    Description: At the present, the geochemical influence of the Galápagos hotspot (offshore South America) can be seen only along the Galápagos spreading center, north of the hotspot. It is possible, however, that Galápagos plume material also reached the East Pacific Rise in the past. Detecting such influence would be of particular importance for the interpretation of geochemical data from oceanic crust at Ocean Drilling Program (ODP) Site 1256, which formed ~15 m.y. ago at the East Pacific Rise during a Miocene period of superfast spreading, and is considered to be a reference site for oceanic crust produced at fast-spreading ridges. Here we present geochemical data from Miocene basaltic crust (23–7 Ma) drilled at several Deep Sea Drilling Project (DSDP), ODP, and Integrated Ocean Drilling Program (IODP) sites that formed along the East Pacific Rise between 3°S and 7°N. Lavas formed between ca. 22.5 and ca. 11 Ma show enriched, Galápagos plume–like Pb and Nd isotope ratios (with a peak in enrichment between ≥18 and 12 Ma) compared to lavas created shortly before or after this time interval. Despite their enriched isotope composition, these samples generally show depletion in more-incompatible, relative to less-incompatible, trace elements. Derivation from an enriched Galápagos plume source that had experienced recent melt extraction before it melted further beneath the East Pacific Rise can explain the combined incompatible-trace-element depletion and isotopic enrichment of the 22.5–11 Ma lavas. The influence of plume material correlates with the interval of superfast spreading along the equatorial East Pacific Rise, suggesting a causal relationship. Enhanced ridge-plume interaction ("ridge suction") due to superfast spreading could have facilitated the flow of Galápagos plume material to the ridge. On the other hand, the arrival of Galápagos-type signatures took place immediately after formation of the Galápagos spreading center, which could have provided a pathway for hot plume material to spread into the main ridge network.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 8
    Publication Date: 2013-08-20
    Description: We have investigated experimentally the partitioning of Au between solid and liquid sulfide phases and basaltic melts at 200 MPa, at redox conditions close to the sulfide-sulfate transition, over temperatures between 1050 and 1200 °C, which span the monosulfide solid solution (MSS) - sulfide liquid (SuL) solidus. The measured MSS/basalt partition coefficient of Au ( D Au MSS-sil ) is about 100–200, whereas the partition coefficient of sulfide liquid/basalt ( D Au SuL-sil ) is approximately 10 times larger at 2200. Although we find that temperature, pressure, and oxygen fugacity ( f O 2 ) exert relatively weak controls on Au partitioning, they exert major indirect influences on Au behavior by controlling the identity of the condensed sulfide phase and by affecting S solubility. These observations have important implications for the behavior of Au in the processes of partial melting in the mantle and magma crystallization in the crust. The occurrence of natural magmas with elevated concentrations of Au and presumably other highly siderophile and chalcophile elements requires predominance of MSS over SuL in the source or/and oxidizing conditions close to or above the sulfide-sulfate transition in the magma.
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
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  • 9
    Publication Date: 2000-03-01
    Print ISSN: 0361-0128
    Electronic ISSN: 1554-0774
    Topics: Geosciences
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  • 10
    Publication Date: 2013-02-08
    Print ISSN: 0883-1351
    Electronic ISSN: 0883-1351
    Topics: Geosciences
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