GLORIA

GEOMAR Library Ocean Research Information Access

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
Filter
  • Articles  (4,451)
Document type
  • Articles  (4,451)
Source
Publisher
Years
Topic
  • 1
    Publication Date: 2018-07-05
    Description: Publication date: 15 September 2018 Source: Earth and Planetary Science Letters, Volume 498 Author(s): M. Heath, D. Phillips, E.L. Matchan Since its inception in the mid-1960s, the 40 Ar/ 39 Ar dating technique has been the premier method for determining the eruption ages of basaltic rocks, providing valuable insights into a plethora of terrestrial and planetary processes. Advances in multi-collector mass spectrometry and improved sample preparation procedures are enabling ever-improving analytical precision and clearer evaluation of the isotopic disturbances that affect many basaltic samples and cause discordant 40 Ar/ 39 Ar age spectra. Here, we present 40 Ar/ 39 Ar step-heating data for multiple samples from two Quaternary basalt flows (0.8038 ± 0.0017 and 2.309 ± 0.009 Ma) of the intraplate Newer Volcanic Province, southeast Australia. A small proportion of these samples give concordant 40 Ar/ 39 Ar results, but most are variably discordant. The factors controlling these disturbances and implications for accurate age determination are examined and modelled in both step-heating spectra and inverse isochron space. We demonstrate that the proportion of radiogenic 40 Ar ( 40 Ar ⁎ ) present in these samples strongly influences the nature of the discordance reflected in 40 Ar/ 39 Ar data. Mass-dependent fractionation appears to have a major influence on low- 40 Ar ⁎ samples, whereas 39 Ar recoil loss/redistribution effects are evident in samples with higher 40 Ar ⁎ proportions. The impact of mass fractionation is quantified via step-heating analyses of unirradiated basalt, whereby a ∼4% difference in 38 Ar/ 36 Ar ratios is observed between low- and high-temperature heating steps. On an inverse isochron plot ( 39 Ar/ 40 Ar vs 36 Ar/ 40 Ar), isotopic disturbance for groundmass samples primarily manifests as isochron rotation, leading to a negative correlation between initial 40 Ar/ 36 Ar ([ 40 Ar/ 36 Ar] i ) values and associated 40 Ar/ 39 Ar ages. We propose a new framework for the interpretation of 40 Ar/ 39 Ar step-heating data for basaltic samples, through judicious evaluation of inverse isochron data, ( 40 Ar/ 36 Ar) i ratios and inverse isochron ages. Results from this study suggest that only samples exhibiting both flat 40 Ar/ 39 Ar age spectra and atmospheric ( 40 Ar/ 36 Ar) i ratios yield accurate eruption ages; in the case of more discordant age spectra, intermediate temperature steps with atmospheric ( 40 Ar/ 36 Ar) i ratios may provide the closest approximation of the eruption age. Graphical abstract
    Print ISSN: 0012-821X
    Electronic ISSN: 1385-013X
    Topics: Geosciences , Physics
    Published by Elsevier
    Location Call Number Limitation Availability
    BibTip Others were also interested in ...
  • 2
    Publication Date: 2018-07-05
    Description: Publication date: 15 September 2018 Source: Earth and Planetary Science Letters, Volume 498 Author(s): Johannes Buchen, Hauke Marquardt, Sergio Speziale, Takaaki Kawazoe, Tiziana Boffa Ballaran, Alexander Kurnosov Earth's transition zone at depths between 410 km and 660 km plays a key role in Earth's deep water cycle since large amounts of hydrogen can be stored in the nominally anhydrous minerals wadsleyite and ringwoodite, ( Mg,Fe ) 2 SiO 4 . Previous mineral physics experiments on iron-free wadsleyite proposed low seismic velocities as an indicative feature for hydration in the transition zone. Here we report simultaneous sound wave velocity and density measurements on iron-bearing wadsleyite single crystals with 0.24 wt-% H 2 O . By comparison with earlier studies, we show that pressure suppresses the velocity reduction caused by higher degrees of hydration in iron-bearing wadsleyite, ultimately leading to a velocity cross-over for both P-waves and S-waves. Modeling based on our experimental results shows that wave speed variations within the transition zone as well as velocity jumps at the 410-km seismic discontinuity, both of which have been used in previous work to detect mantle hydration, are poor water sensors. Instead, the impedance contrast across the 410-km seismic discontinuity that is reduced in the presence of water can serve as a more robust indicator for hydrated parts of the transition zone.
    Print ISSN: 0012-821X
    Electronic ISSN: 1385-013X
    Topics: Geosciences , Physics
    Published by Elsevier
    Location Call Number Limitation Availability
    BibTip Others were also interested in ...
  • 3
    Publication Date: 2018-07-04
    Description: Publication date: 15 September 2018 Source: Earth and Planetary Science Letters, Volume 498 Author(s): Vincent Busigny, JiuBin Chen, Pascal Philippot, Stephan Borensztajn, Frederic Moynier The transfer of trace elements from external terrestrial reservoirs ( i.e. atmosphere, hydrosphere, crust) to the mantle is mainly controlled by their geochemical behaviors during hydrothermal alteration of the oceanic crust and subsequent subduction metamorphism. Deciphering fluid–rock partitioning and element pathways during hydrothermal and subduction metamorphism is complicated due to a variety of pressure-temperature conditions and variable compositions of fluids interacting with the lithosphere. Fluid–rock interactions can, however, be traced by coupling multiple isotopic systems in a single and uniform type of lithology. Here we present Cu and N isotope relationships in Alpine metagabbros, which were hydrothermally-altered on the seafloor and subsequently buried to different depths in a subduction zone. Our sample set covers non-subducted, as well as blueschist- and eclogite-facies low-strain metagabbros, and veins and mylonites equilibrated in eclogite facies conditions. The low-strain metagabbros have Cu contents ranging from 22.5 to 73.2 ppm, and δ 65 Cu values from −1.66 to − 0.02 ‰ . In contrast, eclogite-facies veins and mylonites have lower Cu contents, below 11.1 ppm (except one sample at 27.7 ppm), but a very large range of δ 65 Cu values from −3.12 to 1.76‰. Nitrogen contents and δ 15 N values on the same samples were reported in a previous study and vary between 2.6 and 55 ppm, and from 0.8 to 8.1‰, respectively. In Cu vs δ 65 Cu and N vs δ 15 N diagrams, the low-strain metagabbros show clear trends, which are not related to subduction-zone metamorphic conditions, and more likely reflect hydrothermal processes. We propose that N and Cu were released concomitantly from metagabbros to hydrothermal fluids. This produced a decrease of N and Cu concentrations in the residual rocks, together with an increase of δ 15 N and a decrease of δ 65 Cu . The Cu isotope data, coupled to mineralogical analyses of the metagabbros by SEM, are compatible with partial dissolution of chalcopyrite under oxidative conditions. Nitrogen is not detected in chalcopyrite or in any major or minor mineral phases, probably reflecting the presence of ammonium (NH 4 + ) in trace amounts substituting for Na + and/or Ca 2+ in major phases. The release of Cu and N during hydrothermal alteration is best modeled by Rayleigh distillation in an open system, although a batch equilibrium model cannot be completely discarded. During subduction of the oceanic crust, N was further released to fluids, while Cu was mostly retained in the rocks, a conclusion supported from isotopic and mineralogical analyses. This step resulted in a decrease of N concentration with moderate increase of δ 15 N ( 〈 0.5 ‰ ), and constant Cu contents and δ 65 Cu values. Accordingly, Cu isotope variability inherited from hydrothermal alteration was preserved in subducted metagabbros, although 90% of the fluids were lost during eclogitization. The present results imply that Cu depletion associated with light isotope enrichments in metagabbros can record the oxidative nature of hydrothermal alteration on the seafloor and be preserved despite late metamorphic overprint.
    Print ISSN: 0012-821X
    Electronic ISSN: 1385-013X
    Topics: Geosciences , Physics
    Published by Elsevier
    Location Call Number Limitation Availability
    BibTip Others were also interested in ...
  • 4
    Publication Date: 2018-07-04
    Description: Publication date: 15 September 2018 Source: Earth and Planetary Science Letters, Volume 498 Author(s): Erin H. Phillips, Kenneth W.W. Sims, Janne Blichert-Toft, Richard C. Aster, Glenn A. Gaetani, Philip R. Kyle, Paul J. Wallace, Daniel J. Rasmussen The multiple, proximal, young and/or active volcanic centers of Ross Island, Antarctica, provide a unique opportunity to investigate both deep and shallow processes of alkaline magma genesis and the length scales of mantle heterogeneity. Ross Island, Antarctica is an assembly of four silica-undersaturated alkaline volcanic centers, including the active phonolitic Erebus volcano (1.2 to 0 Ma). Mt. Terror, Mt. Bird, and Hut Point Peninsula surround Erebus on the periphery of Ross Island and are mostly older (∼0.3 to 4 Ma) and mainly basanitic in composition. While the geochemical compositions and HIMU isotopic signature of Erebus lavas are well characterized, the geochemistry of the peripheral volcanic centers was, until this study, poorly known. To further investigate the nature of mantle sources and magmatic processes beneath Ross Island, we therefore measured major and trace element and Sr, Nd, Hf, and Pb isotope compositions on fifty-seven samples from the three volcanoes peripheral to Erebus volcano and compared the results to previous geochemical and isotopic studies of the latter. Mt. Terror, Mt. Bird, and Hut Point Peninsula have 87 Sr/ 86 Sr ranging from 0.702907 to 0.703147, ε Nd ranging from +4.3 to +6.3, ε Hf ranging from +5.6 to +8.6, and 206 Pb/ 204 Pb ranging from 19.282 to 20.241. The Sr, Nd, Hf, and Pb isotope compositions of all four volcanoes (Erebus, Terror, Bird, and Hut Point Peninsula) fall on a mixing line between the HIMU and DMM mantle end-members, with a minor contribution from an EM component. Small, but distinct differences in the isotopic compositions of the four volcanoes, most notably Mt. Bird, imply mantle source heterogeneity beneath Ross Island on a length scale comparable to or smaller than inter-volcano distances (i.e., less than 50 km). The major and trace element systematics and isotopic signatures of the Ross Island volcanic samples are distinct from those for samples from Zealandia and older Antarctic samples that typify magmas from the Cenozoic alkaline magmatic province. The chemical and isotopic compositions of the Ross Island lavas and tephras are best explained by small-degree melting of an ancient asthenospheric source containing residual garnet. Additionally, recent global tomographic modeling indicates low shear wave velocity anomalies with depths down to ∼1200 km beneath Ross Island. This suggests a deep-seated upwelling plume impinging on shallower metasomatized mantle which has been gradually eroded away and is no longer a potential source of Ross Island magmatism. Rather, the source of Ross Island lavas is old (Archean to early Proterozoic) material upwelling from the deep mantle as opposed to melting of subcontinental lithospheric mantle metasomatized by Paleozoic subduction. While our findings have local significance in terms of understanding Ross Island volcanism and providing a new perspective on the evolution of volcanism in the surrounding region during the Phanerozoic, they also have important global implications for gauging the nature, age, and evolution of the HIMU mantle source.
    Print ISSN: 0012-821X
    Electronic ISSN: 1385-013X
    Topics: Geosciences , Physics
    Published by Elsevier
    Location Call Number Limitation Availability
    BibTip Others were also interested in ...
  • 5
    Publication Date: 2018-07-04
    Description: Publication date: 1 September 2018 Source: Earth and Planetary Science Letters, Volume 497 Author(s): Zhaode Yuan, Jing Liu-Zeng, Wei Wang, Ray J. Weldon, Michael E. Oskin, Yanxiu Shao, Zhanfei Li, Zhigang Li, Peng Wang, Jinyu Zhang Long records of paleoearthquakes are essential for understanding earthquake recurrence behavior of active faults and for evaluating regional seismic hazard. However, paleoseismic data on the Altyn Tagh fault (ATF), one of the longest strike-slip fault in Asia, are scarce. We document a long paleoseismic record along the Xorkoli section of the central ATF. Eight and probably nine earthquakes are identified based on event evidence in the form of open fissures, folds, unconformities, and upward fault terminations, with modeled mean (95% confidence) ages of A.D. 1598 (1491–1741) yr (event A), A.D. 797 (676–926) yr (B), B.C. 668 (732–589) yr (C), B.C. 956 (1206–715) yr (D), B.C. 1301 (1369–1235) yr (E), B.C. 2105 (2233–1987) yr (F, probable), B.C. 2663 (2731–2601) yr (G), B.C. 2818 (2878–2742) yr (H), B.C. 3396 (3522–3205) yr (I). The mean recurrence interval is 620 ± 410 yr with a coefficient of variation (COV) of 0.67, indicating that earthquake recurrence is weakly periodic, with individual intervals ranging from as short as 150 yr to as long as 1460 yr. A global compilation of 35 strike-slip paleoseismic sites yields a similar average COV of 0.69. Synthesis of paleoseismic sites from the central ATF indicates that not all earthquakes ruptured to the eastern end of the Xorkoli section, within the Aksay restraining bend. Given that the 420-yr elapsed time since the most recent event, well within a COV of mean interval, a large surface-rupturing earthquake could occur at any time along the central ATF.
    Print ISSN: 0012-821X
    Electronic ISSN: 1385-013X
    Topics: Geosciences , Physics
    Published by Elsevier
    Location Call Number Limitation Availability
    BibTip Others were also interested in ...
  • 6
    Publication Date: 2018-07-01
    Description: Publication date: 15 September 2018 Source: Earth and Planetary Science Letters, Volume 498 Author(s): Haiying Hu, Lidong Dai, Heping Li, Wenqing Sun, Baosheng Li Magnetotelluric measurements reveal the presence of high conductivity anomalies (up to ∼1 S/m) in both the forearc and backarc regions of subduction zones as well as the continental middle–lower crust. Such anomalies are commonly interpreted as a consequence of aqueous fluid released from the dehydration of hydrous minerals. Amphibole is an important constituent of the continental mid-crust and a major hydrous phase in subduction zones, such that its dehydration at high temperature has been suggested to provide a significant source of aqueous fluid. We performed electrical conductivity measurements of a natural Fe-bearing amphibole at 623–1173 K and 0.5–2.0 GPa using a multi-anvil apparatus and an impedance spectroscopy. Our results show that pressure has a very weak effect on conductivity compared with temperature. An abrupt variation of the impedance semicircular arc followed by a remarkable increase of electrical conductivity is observed at temperature of 843 ± 20  K . However, the enhancement in conductivity is not attributed to conductive aqueous fluid but rather to amphibole oxidation–dehydrogenation, as confirmed by infrared spectroscopy and optical microscopy observations. A slight decrease in activation enthalpy from ∼0.80 eV to ∼0.70 eV suggests that the conduction mechanism does not change before and after dehydrogenation, and small polaron conduction (electron holes hopping between Fe 2+ and Fe 3+ ) is considered to dominate the conductivity of amphibole over the entire temperature range. Although amphibole dehydrogenation at high temperature cannot serve as a principal source of aqueous fluid, the enhanced electrical conductivity of amphibole after dehydrogenation is sufficient to account for the high conductivity anomalies observed in slab–mantle wedge interfaces and the continental lowermost mid-crust, particularly in local regions with high heat flow. Graphical abstract
    Print ISSN: 0012-821X
    Electronic ISSN: 1385-013X
    Topics: Geosciences , Physics
    Published by Elsevier
    Location Call Number Limitation Availability
    BibTip Others were also interested in ...
  • 7
    Publication Date: 2018-06-29
    Description: Publication date: 15 September 2018 Source: Earth and Planetary Science Letters, Volume 498 Author(s): Colin R. Meyer, Brent M. Minchew The flux of grounded ice from the Antarctic Ice Sheet (AIS) primarily occurs through narrow, fast-flowing outlet glaciers and ice streams. Shearing generates heat in the lateral glacier margins, where there is a transition from fast-flow to near-stagnant ice or rock. This heat warms the ice and can form zones of water-saturated porous ice at the melting temperature, known as temperate ice. Here we derive a one-dimensional thermomechanical model to infer which AIS shear margins may contain temperate ice. Our model provides expressions for the critical shear strain rate at which a zone of temperate ice initiates and the thickness of the temperate zone. Both of these quantities are described by two nondimensional numbers—the Brinkman and Péclet numbers—that are functions of observable parameters such as shear strain rate, surface temperature, surface accumulation rate, and ice thickness. The development of temperate zones is incited by shear strain rates (Brinkman number) and suppressed by the advection of cold ice from the surface (Péclet number), which scales with surface accumulation rate. We also include a parametrization for lateral advection, which can suppress the formation of temperate ice. Applying recent observations and outputs from a regional climate model, we show that many active glacier shear margins could contain temperate ice. The spatial distribution and thickness of temperate zones are controlled largely by shear strain rates and surface accumulation rates.
    Print ISSN: 0012-821X
    Electronic ISSN: 1385-013X
    Topics: Geosciences , Physics
    Published by Elsevier
    Location Call Number Limitation Availability
    BibTip Others were also interested in ...
  • 8
    Publication Date: 2018-06-29
    Description: Publication date: 1 September 2018 Source: Earth and Planetary Science Letters, Volume 497 Author(s): Andrea Rielli, Andrew G. Tomkins, Oliver Nebel, Massimo Raveggi, Heejin Jeon, Laure Martin, Janaína N. Ávila At convergent margins fluids liberated from subducting slabs metasomatise the overlying mantle wedge, enriching it in volatiles, incompatible elements and possibly ore-forming metals. Despite the genetic link between this process, the genesis of arc magmas, and formation of porphyry Cu–Au deposits, there is currently little understanding of the behaviour of chalcophile and siderophile elements during subduction-related mantle metasomatism. In this study, we report sulfur isotopic compositions and PGE concentrations of sulfides in a suite of garnet peridotites from the Western Gneiss Region of Norway, sampling mantle wedge from ∼100 to ∼250 km depth. Sulfides hosted in metasomatised samples have δ 34 S deviated from typical mantle values, ranging between −10.0 and +5.4‰, indicating derivation of sulfur from subducted crust. Sulfides in pervasively metasomatised samples have atypical PGE signatures, with strong enrichment in Os and Ru relative to Ir, whereas channelised fluid flow produced sulfides extremely enriched in Pd, up to 700 times the concentration found in non-metasomatised samples. These signatures are reconcilable with a high oxidation state of the metasomatising agents and demonstrate that subduction can recycle chalcophile and siderophile elements into and within the mantle, along with sulfur. We further show that because the solubility of Os and Ru in fluids is redox sensitive, and Pd is more soluble than the I-PGE, ratios such as Os/Ir, Ru/Ir plotted against Pd/Ir can be used to trace the metasomatic oxidation of mantle samples, mantle-derived magmas and porphyry Cu±Au deposits. This geochemical insight is used to show that Au-rich porphyry Cu deposits are derived from more oxidised mantle wedge than Au-poor porphyry deposits.
    Print ISSN: 0012-821X
    Electronic ISSN: 1385-013X
    Topics: Geosciences , Physics
    Published by Elsevier
    Location Call Number Limitation Availability
    BibTip Others were also interested in ...
  • 9
    Publication Date: 2018-06-29
    Description: Publication date: 15 September 2018 Source: Earth and Planetary Science Letters, Volume 498 Author(s): Kirsten Schulze, Hauke Marquardt, Takaaki Kawazoe, Tiziana Boffa Ballaran, Catherine McCammon, Monika Koch-Müller, Alexander Kurnosov, Katharina Marquardt Ringwoodite, the dominant mineral at depths between 520 km and 660 km, can store up to 2–3 wt.% of water in its crystal structure, making the Earth's transition zone a plausible water reservoir that plays a central role in Earth's deep water cycle. Experiments show that hydration of ringwoodite significantly reduces elastic wave velocities at room pressure, but the effect of pressure remains poorly constrained. Here, a novel experimental setup enables a direct quantification of the effect of hydration on ringwoodite single-crystal elasticity and density at pressures of the Earth's transition zone and high temperatures. Our data show that the hydration-induced reduction of seismic velocities almost vanishes at conditions of the transition zone. Seismic data thus agree with a wide range of water contents in the transition zone.
    Print ISSN: 0012-821X
    Electronic ISSN: 1385-013X
    Topics: Geosciences , Physics
    Published by Elsevier
    Location Call Number Limitation Availability
    BibTip Others were also interested in ...
  • 10
    Publication Date: 2018-06-28
    Description: Publication date: 15 September 2018 Source: Earth and Planetary Science Letters, Volume 498 Author(s): Mark G. Fox-Powell, Alan Channing, Daniel Applin, Ed Cloutis, Louisa J. Preston, Claire R. Cousins Silica-rich hydrothermal fluids that experience freezing temperatures precipitate cryogenic opal-A (COA) within ice-bound brine channels. We investigated cryogenic silicification as a novel preservation pathway for chemo- and photo-lithotrophic Bacteria and Archaea. We find that the co-partitioning of microbial cells and silica into brine channels causes microorganisms to become fossilised in COA. Rod- and coccoidal-form Bacteria and Archaea produce numerous cell casts on COA particle surfaces, while Chloroflexus filaments are preserved inside particle interiors. COA particles precipitated from natural Icelandic hot spring fluids possess similar biomorphic casts, including those containing intact microbial cells. Biomolecules and inorganic metabolic products are also captured by COA precipitation, and are detectable with a combination of visible – shortwave infrared reflectance, FTIR, and Raman spectroscopy. We identify cryogenic silicification as a newly described mechanism by which microbial biosignatures can be preserved within silica-rich hydrothermal environments. This work has implications for the interpretation of biosignatures in relic hydrothermal settings, and for life-detection on Mars and Enceladus, where opaline silica indicative of hydrothermal activity has been detected, and freezing surface conditions predominate. Graphical abstract
    Print ISSN: 0012-821X
    Electronic ISSN: 1385-013X
    Topics: Geosciences , Physics
    Published by Elsevier
    Location Call Number Limitation Availability
    BibTip Others were also interested in ...
Close ⊗
This website uses cookies and the analysis tool Matomo. More information can be found here...