Origins and evolution of rhyolitic magmas in the central Snake River Plain: insights from coupled high-precision geochronology,oxygen isotope,and hafnium isotope analyses of zircon     

Dylan?P.?ColónEmail author  Ilya?N.?Bindeman  J?rn-Frederik?Wotzlaw  Eric?H.?Christiansen  Richard?A.?Stern 《Contributions to Mineralogy and Petrology》2018,173(2):11

We present new high-precision CA-ID-TIMS and in situ U–Pb ages together with Hf and O isotopic analyses (analyses performed all on the same grains) from four tuffs from the 15?10 Ma Bruneau–Jarbidge center of the Snake River Plain and from three rhyolitic units from the Kimberly borehole in the neighboring 10?6 Ma Twin Falls volcanic center. We find significant intrasample diversity in zircon ages (ranges of up to 3 Myr) and in δ¹⁸O (ranges of up to 6‰) and ε_Hf (ranges of up to 24 ε units) values. Zircon rims are also more homogeneous than the associated cores, and we show that zircon rim growth occurs faster than the resolution of in situ dating techniques. CA-ID-TIMS dating of a subset of zircon grains from the Twin Falls samples reveals complex crystallization histories spanning 10⁴–10⁶ years prior to some eruptions, suggesting that magma genesis was characterized by the cyclic remelting of buried volcanic rocks and intrusions associated with previous magmatic episodes. Age-dependent trends in zircon isotopic compositions show that rhyolite production in the Yellowstone hotspot track is driven by the mixing of mantle-derived melts (normal δ¹⁸O and ε_Hf) and a combination of Precambrian basement rock (normal δ¹⁸O and ε_Hf down to ??60) and shallow Mesozoic and Cenozoic age rocks, some of which are hydrothermally altered (to low δ¹⁸O values) by earlier stages of Snake River Plain magmatism. These crustal melts hybridize with juvenile basalts and rhyolites to produce the erupted rhyolites. We also observe that the Precambrian basement rock is only an important component in the erupted magmas in the first eruption at each caldera center, suggesting that the accumulation of new intrusions quickly builds an upper crustal intrusive body which is isolated from the Precambrian basement and evolves towards more isotopically juvenile and lower-δ¹⁸O compositions over time.  相似文献   

Portability of semantic and spatial–temporal machine learning methods to analyse social media for near-real-time disaster monitoring     

Havas  Clemens  Resch  Bernd 《Natural Hazards》2021,108(3):2939-2969

Natural Hazards - Up-to-date information about an emergency is crucial for effective disaster management. However, severe restrictions impede the creation of spatiotemporal information by current...  相似文献   

Reply to discussion of Gabrielsen et al. (2010) by Nielsen et al. (this volume): Latest Caledonian to present tectonomorphological development of southern Norway     

Roy H. Gabrielsen  Jan Inge Faleide  Christophe Pascal  Alvar Braathen  Johan Petter Nystuen  Bernd Etzelmuller  Sejal O'Donnell 《Marine and Petroleum Geology》2010

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Chronology of the last deglaciation and Holocene environmental changes in the Sisimiut area,SW Greenland based on lacustrine records     

Bernd Wagner  Ole Bennike 《Boreas: An International Journal of Quaternary Research》2012,41(3):481-493

The Sisimiut area was deglaciated in the early Holocene, c. 11 cal. ka BP. At that time the lowlands were inundated by the sea, but the isostatic rebound surpassed the global sea‐level rise, and the lowlands emerged from the sea. The pioneer vegetation in the area consisted of mosses and herbaceous plants. The oldest remains of woody plants (Empetrum nigrum) are dated to c. 10.3 cal. ka BP, and remains of Salix herbacea and Harrimanella hypnoides are found in slightly younger sediments. The maximum occurrence of statoblasts of the bryozoan Plumatella repens from c. 10 to 4.5 cal. ka BP probably reflects the Holocene thermal maximum, which is also indicated in geochemical proxies of the lake sediments. A maximum in organic matter accumulation in one of the three studied lakes c. 3 cal. ka BP can probably be ascribed to a late Holocene short‐duration temperature maximum or a period of increased aridity. Cenococcum geophilum sclerotia are common in the late Holocene, implying increased soil erosion during the Neoglaciation. A comparison with sediment and macrofossil records from inland shows similar Holocene trends and a similar immigration history. It also reveals that there has been a significant gradient throughout the Holocene, from an oceanic and stable climate at the outer coast to a more continental and unstable climate with warmer summers and drier conditions close to the margin of the Greenland ice sheet, where the buffer capacity of the sea is lower.  相似文献   

Biomarkers in sedimentary sulfides of precambrian age     

Bernd Mycke  Walter Michaelis  Egon T. Degens 《Organic Geochemistry》1988,13(4-6)

Samples of the Mt Isa formation (Australia, c, 1.5 Ga), and the Shungit formation (U.S.S.R., c. 2.3 Ga) were studied by organic geochemical means. All samples were freed of any low-molecular-weight solvent soluble organic material in order to insure the authenticity of the analysed material. The isolation of biochemical compounds entrapped in sulfides was the major goal of this work.Organic compounds that were entrapped during the early stages of sulfide formation may obviously survive extended periods of time and can be released by a mild hydrogenation of the sulfides. Preliminary investigations of the hydrocarbon fraction indicate n-alkanes, monomethylalkanes, cycloalkanes, and an unknown series of branched alkanes as major constituents. Their distribution patterns show great selectivity with respect to structures and chain lengths of individual compounds. Differences between the hydrogenation reaction product and the sample extract may arise from the release of a different kind of lipid material through dissolution of the sulfides.  相似文献   

Steroid and triterpenoid distributions in bransfield strait sediments: Hydrothermally-enhanced diagenetic transformations     

Martine Brault  Bernd R.T. Simoneit 《Organic Geochemistry》1988,13(4-6)

Recent sediments from Bransfield Strait, Antarctica have been analyzed for triterpenoid and steroid hydrocarbons, sterols and steroid ketones to consider the effects of hydrothermal fluids on the sedimentary organic matter. The steroid distributions in unaltered and altered sediments are controlled more by inputs from source organisms than by the effects of hydrothermal activity, which is suggested to be limited to low temperature alteration. Nevertheless, chemical reactions occurred in altered sediments and include dehydration of sterols to sterenes, isomerization of triterpenes and sterenes, rearrangement of sterenes to diasterenes and reductive processes leading to generation of phytane from phytol via phytenes.  相似文献   

Brine Discharge and Salinization, Concho River Watershed, West Texas     

Alan R. Dutton  Bernd C. Richter  Charles W. Kreitler 《Ground water》1989,27(3):375-383

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Settling-velocity distributions and sorting processes on a longitudinal dune: A case study     

Daniel Hartmann  Christian Christiansen 《地球表面变化过程与地形》1988,13(7):649-656

The processes acting on a longitudinal dune are inferred from the response in the area size-sorting characteristics of the sediment of the active layer. The sediment size distributions are determined by settling and are best described by a log-hyperbolic model which provides the most information on size-sorting processes. Size-sorting characteristics are markedly different across the low round-crested part of the dune from cross-sections of the high more sharp-crested part. This results in changes in textural parameters along the lee-side consistent with changes in the lee-side separation vortex. This is the first time that the effect of wind speed up on the lee-side is observed to be reflected in the sediment; it can only be detected with the very sensitive log-hyperbolic parameters.  相似文献   

A three dimensional perspective on the evolution of Archaean crust: LITHOPROBE seismic reflection images in the southwestern Superior province     

John Ludden  Claude Hubert  Arthur Barnes  Bernd Milkereit  Edward Sawyer 《Lithos》1993,30(3-4):357-372

15011993

Abstract

In 1990–1991 the LITHOPROBE project completed 450 km of seismic reflection profiles across the late Archaean crust of the southwestern Superior province. The results define a broad three-fold division of crust: upper crust in the Abitibi greenstone belt is non-reflective and is a 6–8 km veneer of volcanic and plutonic supracrustal rocks, whereas, in the sediment-gneiss dominated Pontiac subprovince, upper crust comprises shallow northwest-dipping turbidite sequences; mid-crust, in both the Abitibi and the Pontiac subprovinces, is interpreted as imbricate sequences of metasedimentary and metaplutonic rocks; lower crust in both subprovinces has a horizontal layer parallel strycture which may represent interleaved mafic-intermediate gneisses. The seismic signature of the northern Abitibi greenstone belt may be represented in an exposed 25 km crustal section in the Kapuskasing stuctural zone.

Preliminary tectonic models based on the seismic data are consistent with a plate-tectonic scenario involving oblique subduction and imbrication of sedimentary, plutonic and volcanic sequences. The northern Abitibi supracrustal sequences either represent an allochthon, or overlie an allochthonous underthrust metasedimentary and plutonic sequence which may be equivalent to a metasedimentary subprovince such as the Pontiac or Quetico.

Seismic velocities have yet to be defined. However, crustal thicknesses are relatively constant at 35–40 km. The thinnest crust is adjacent to the Grenville Front where Moho is very well defined.  相似文献   

Deformation fabric and microstructures in ophiolitic chromitites and host ultramafics,Sultanate of Oman     

F. G. Christiansen 《International Journal of Earth Sciences》1985,74(1):61-76

Harzburgite and dunite from the Oman ophiolite display a strong olivine fabric and microstructures typical for high temperature mantle deformation. The syntectonic, magmatically formed, chromitites occurring throughout the mantle sequence vary from almost undeformed (resembling chromitite from stratiform complexes) to highly deformed. Usually the interstitial silicates in the deformed ore possess a fabric similar to those of the host rocks, whereas a weak or poikilitic fabric is observed on relatively undeformed deposits. Chromite grains are elongated and flattened, defining strong foliations and lineations, in disseminated ore, and pull-apart-like structures perpendicular to the lineation form in the coarser massive ore. No preferred crystallographic orientation of chromite has been detected with the X-ray-texture goniometer, and it is proposed that chromite mainly deforms by passive rotation in the weaker silicate matrix, cataclasis and diffusion creep.

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Zusammenfassung Die Dunite und Harzburgite, die im Ophiolit des Oman auftreten, zeigen ein Olivingefüge und Mikrostrukturen, die typisch sind für Manteldeformationen bei hohen Temperaturen. Die syntektonischen, magmatisch gebildeten Chromitite sind über die gesamte Abfolge der Mantelgesteine zu finden. Das Ausmaß der Deformation der Chromitite reicht von gering bis stark. Fast undeformierte Chromitite gleichen denen stratiformer Komplexe. Im deformierten Erz zeigen die in Zwickeln auftretenden Silikatminerale im allgemeinen ein ähnliches Gefüge wie das Wirtsgestein; in relativ undeformierten Vorkommen zeigen sie ein schwaches oder poikilitisches Gefüge. Die Chromitkörner haben einen plattigen Habitus und bilden im Sprenkelerz ein deutliches Flächen- und Lineargefüge. Im grobkörnigen Derberz entstehen hingegen Dehnungsstrukturen (pull-apart textures) senkrecht zur Lineation. Röntgentexturgoniometrische Untersuchungen ergaben keine Regelung des Chromits, und es wird daher angenommen, daß der Chromit aufgrund passiver Rotation in der schwächeren Silikatmatrix deformiert wurde, weiterhin kommen auch Kataklase und Diffusionsmechanismen in Frage.

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Résumé Les dunites et harzburgites du complexe ophiolitique d'Oman présentent une forte fabrique de l'olivine et des microstructures typiques de déformations mantelliques de haute température. Les chromitites, d'origine magmatique syntectomque, qui se trouvent le long de la séquence mantellique varient entre des termes presque non déformés (ressemblant aux chromitites des complexes stratiformes) jusqu'à des termes très déformés. Les silicates interstitiels du minerai déformé présentent fréquemment une fabrique semblable à celle des minéraux de la roche hôte alors que l'on observe une fabrique peu développée ou bien une texture poecilitique dans les minerais des gisements relativement peu déformés. Les cristaux de chromite sont allongés et aplatis, définissant une foliation et une linéation très marquées dans les minerais disséminés; des structures d'étirement (pull-apart) perpendiculaires à la linéation apparaissent dans les minerais plus massifs. Aucune orientation cristallographique préferentielle n'a pu être mise en évidence au goniomètre de texture; on propose donc que la chromite se déforme principalement par rotation passive dans une matrice silicatée plus fragile, par cataclase et par fluage-diffusion.

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