Age of zircons from chromites in the residual ophiolitic rocks as a reflection of upper mantle magmatic events     

G. N. Savelieva  P. V. Suslov  A. N. Larionov  N. G. Berezhnaya 《Doklady Earth Sciences》2006,411(9):1401-1406

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Focused ion beam technique and transmission electron microscope studies of microdiamonds from the Saxonian Erzgebirge, Germany     

Larissa F. Dobrzhinetskaya  Harry W. Green  Matthew Weschler  Mark Darus  Young-Chung Wang  Hans-Joachim Massonne  Bernhard St  ckhert 《Earth and Planetary Science Letters》2003,210(3-4):399-410

A focused ion beam of Ga ions is a relatively new technique that has been developed for microelectronic industries. Now researchers of the Earth sciences find it to be a promising tool for studying various geological materials. Using the FIB technique and an FEI Strata DB 235 dual beam system, we have successfully prepared several electron-transparent foils, which crossed μm-sized diamonds included in host minerals such as zircon and garnet from quartzofeldspathic rocks of the Saxonian Erzgebirge, Germany. Scanning and transmission electron microscopy applied to these foils revealed that the diamonds contain crystalline nanometric inclusions. These inclusions consist of minerals of known stoichiometries such as SiO₂ and Al₂SiO₅, whereas others are characterized by different combinations of Si, K, P, Ti, and Fe in the presence of oxygen (stoichiometries are not clear at this stage of research). One suite of inclusions is assumed to be represented by archerite, KH₂PO₄, which is known to be stable at pressures of 4–22 GPa, and one nanocrystal containing Pb, oxygen and carbon is interpreted to be Pb_xO_y or PbCO₃. Along with solid crystalline inclusions, the diamonds contain cavities filled by liquid/gas that escaped during sample preparation. These are associated with dislocations of diamond growth. Our data are consistent with the concept of diamond crystallization from a COH-rich multicomponent supercritical fluid and suggest that the composition of such a fluid is more consistent with a local crustal source rather than that of a mantle origin.  相似文献   

Vegetation and climate changes in northwestern Russia during the Lateglacial and Holocene inferred from the Lake Ladoga pollen record     

Larisa A. Savelieva  Andrei A. Andreev  Raphael Gromig  Dmitry A. Subetto  Grigory B. Fedorov  Volker Wennrich  Bernd Wagner  Martin Melles 《Boreas: An International Journal of Quaternary Research》2019,48(2):349-360

The new pollen record from the upper 12.75 m of a sediment core obtained in Lake Ladoga documents regional vegetation and climate changes in northwestern Russia over the last 13.9 cal. ka. The Lateglacial chronostratigraphy is based on varve chronology, while the Holocene stratigraphy is based on AMS ¹⁴C and OSL dates, supported by comparison with regional pollen records. During the Lateglacial (c. 13.9–11.2 cal. ka BP), the Lake Ladoga region experienced several climatic fluctuations as reflected in vegetation changes. Shrub and grass communities dominated between c. 13.9 and 13.2 cal. ka BP. The increase in Picea pollen at c. 13.2 cal. ka BP probably reflects the appearance of spruce in the southern Ladoga region at the beginning of the Allerød interstadial. After c. 12.6 cal. ka BP, the Younger Dryas cooling caused a significant decrease in spruce and increase in Artemisia with other herbs, indicative of tundra‐ and steppe‐like vegetation. A sharp transition from tundra‐steppe habitats to sparse birch forests characterizes the onset of Holocene warming c. 11.2 cal. ka BP. Pine forests dominated in the region from c. 9.0 to 8.1 cal. ka BP. The most favourable climatic conditions for deciduous broad‐leaved taxa existed between c. 8.1 and 5.5 cal. ka BP. Alder experiences an abrupt increase in the local vegetation c. 7.8 cal. ka BP. The decrease in tree pollen taxa (especially Picea) and the increase in herbs (mainly Poaceae) probably reflect human activity during the last 2.2 cal. ka. Pine forests have dominated the region since that time. Secale and other Cerealia pollen as well as ruderal herbs are permanently recorded since c. 0.8 cal. ka BP.  相似文献   

Vendian tectono-magmatic events in mantle ophiolitic complexes of the polar Urals: U-Pb dating of zircon from chromitite   总被引：1，自引：0，他引：1  

G. N. Savelieva  P. V. Suslov  A. N. Larionov 《Geotectonics》2007,41(2):105-113

Geological and isotopic evidence of Late Vendian magmatic events in restitic ultramafic mantle rocks of the Voikar-Syn’ya ophiolitic massif are considered and correlated with events at the eastern margin of the East European Platform. The geological and isotopic data show that the ophiolitic complexes of the Polar Urals were formed during several stages. The percolation of melts through peridotites was recorded in the newly formed mineral assemblages, for example, olivine + chromite ± zircon. Zircon crystallized from the residual fraction of the evolved basic melt that impregnated peridotite. The active interaction of hot restitic harzburgite with the migrating melt resumed repeatedly and could have led to the formation of several generations of chromite-bearing dunite. An important geological inference can be made from this suggestion: There is a high probability that isotopic markers of different age have been retained in restitic mantle complexes of ophiolites. The U-Pb dating of zircons with a SHRIMP-2 ion microprobe has shown that the isotopic age of seven grains is 585.3 ± 6 Ma (MSWD is 0.036 and the probability of concordance is 0.85). The obtained age of zircon from chromitite marks a Vendian tectonomagmatic event that occurred in the upper mantle of the transitional zone between the East European Plate and the oceanic basin. The island-arc complexes of the Polar Urals developed on the tectonically juxtaposed fragments of the Early Paleozoic and pre-Paleozoic oceanic crust. These crustal rocks were reworked during younger magmatic events related to the origin of the Middle Paleozoic island arcs. As a result, the rocks that formed in different geological epochs were locally retained in the restitic mantle complexes of a spatially indivisible ophiolitic association.  相似文献   

Refashioning place and new-build gentrification: The material and symbolic redevelopment of Three Kings,Auckland     

Larissa Naismith  Laurence Murphy 《New Zealand geographer》2023,79(1):3-14

Auckland's post-suburban landscape is associated with new mixed-use apartment developments. When located in former industrial sites, these emergent residential and retail spaces constitute new-build gentrification and carry the potential to rupture existing people/place dynamics. This article examines material and socio-cultural processes associated with the redevelopment of an industrial quarry in Three Kings, Auckland. It is argued that the redevelopment constitutes a physical refashioning of a former industrial space located within a traditional working-class community. The development involves a remaking of the material and socio-cultural fabric of the neighbourhood and aligns with new-build gentrification practices.  相似文献   

The Mineral Adsorption and The Application of a Metamorphic Felsic Rock as Aggregate in Asphalt Pavements     

Lago  Larissa Neves  Casagrande  Michéle Dal Toé  da Conceição Ribeiro  Roberto Carlos  de Albuquerque e Silva  Ben-Hur 《Geotechnical and Geological Engineering》2022,40(3):1585-1595

Geotechnical and Geological Engineering - The aggregates consist approximately 85% of the total volume in asphalts pavements, and their properties influence directly their behavior. Therefore, it...  相似文献   

Sediment transport on the Freiston Shore managed realignment site: An investigation using environmental magnetism     

Robin Rotman  Larissa Naylor  Rachael McDonnell  Conall MacNiocaill 《Geomorphology》2008,100(3-4):241-255

This paper presents the first attempt to use environmental magnetism to quantify sediment provenance on a managed realignment (MR) site. MR is one technique for creating/restoring intertidal habitats and improving the standard of coastal flood protection. Monitoring of MR sites is essential for determining how successful they are at achieving these aims, and normally includes measurement of sedimentation rates through the use of accretion plates and sediment erosion tables (SETs). One limitation of this technique is that the provenance of sediment accreting on realigned surfaces cannot readily be quantified. Our results demonstrate that magnetics-based fingerprinting can successfully apportion sediment source contributions to created/restored coastal saltmarshes with an efficiency similar to that of existing un-mixing models applied to other habitats (82.87%). Analysis of mineral magnetic properties (χ_FD, IRM_soft and SIRM) demonstrated that the majority of sediment accreting on the Freiston Shore MR Site (Lincolnshire, UK) is derived from established saltmarshes seaward of the breached embankment. This indicates that the MR site may be growing at the expense of neighbouring saltmarshes—thereby undermining the habitat-creation objectives used to justify MR. It is recommended that future MR monitoring programmes consider the provenance of post-breach deposits to help identify any adverse impacts that wetland creation/restoration may have on existing habitats. Further testing of environmental magnetism in coastal saltmarshes and MR sites is necessary to validate wider use of the technique.  相似文献   

Microdiamonds — Frontier of ultrahigh-pressure metamorphism: A review     

Larissa F. Dobrzhinetskaya^{Author Vitae} 《Gondwana Research》2012,21(1):207-223

This is a comprehensive review paper devoted to microdiamonds from ultrahigh-pressure metamorphic (UHPM) terranes incorporated in orogenic belts formed at convergent plate boundaries in Paleozoic-Mesozoic-Alpine time. When in 1980 the first small diamonds were discovered within “amphibolite-granulate facies” metamorphic rocks, it came as a great surprise that buoyant continental crust could be subducted to depths of hundreds of kilometers and then subsequently exhumed. Since then, much progress has been made in understanding the mechanism of these diamonds' formation, and the number of new diamond-bearing UHPM terranes was significantly increased, especially within European orogenes. Moreover, new variations in tectonic settings in which UHP rocks can be formed and exhumed came to the attention of geologists simply due to the finding of diamonds in places previously “forbidden” for their formation—e.g., oceanic islands, ophiolites, and forearc environments. Over the past decade, the rapidly moving technological advancement has made it possible to examine microdiamonds in detail and to learn that part of them has a polycrystalline nature; that they contain nanometric, multiphase inclusions of crystalline and fluid phases; and that they keep a “crustal” signature of carbon isotopes. Scanning and transmission electron microscopy, focused-ion-beam techniques, synchrotron infrared spectroscopy, micro X-ray diffraction, and nano-secondary ion mass spectrometry studies of these diamonds provide evidence that they keep traces of fluid originated from both crustal and mantle reservoirs, and that they probably interacted with deep mantle plumes. Hypotheses proposed for diamond formation in subduction zones founded on both analytical and experimental studies are discussed. The paper also emphasizes that the discovery of these microdiamonds (as well as coesite) triggered a major revision in the understanding of deep subduction processes, leading to a clear realization of how continental materials can be recycled into the Earth's mantle and geochemically rejuvenate it.  相似文献   

Polychronous formation of mantle complexes in ophiolites     

G. N. Savelieva  V. G. Batanova  N. A. Berezhnaya  S. L. Presnyakov  A. V. Sobolev  S. G. Skublov  I. A. Belousov 《Geotectonics》2013,47(3):167-179

The paper presents new determinations of the U-Pb zircon age of high-Al chromitite from dunite of the mantle section of the Voikar-Synya massif at the Kershor site in the boundary zone with rocks of the dunite-wehrlite-clinopyroxenite complex. The high-Cr chromitite from dunite in the central part of the same massif contains zircon dated at ca. 0.6 Ga [10]. It is suggested that Paleoproterozoic (2.0?1.9 Ga) zircons from chromitites of the mantle section near the petrological Moho boundary were formed in the course of partial melting of peridotites and/or their interaction with migrating MORB-type melts. The occurrence of Vendian and Paleoproterozoic zircons in chromitites from different parts of the mantle section, as well as previously published petrological, geochemical, and geological data [2, 11, 22] allow us to suggest a complex multistage evolution of the mantle section in ophiolites. The arguments stated below show that chromitites and host dunites could have been formed at different times and were probably related to different processes. Thus, not only various complexes of the pre-Paleozoic oceanic crust reworked in the suprasubduction setting differ in age, but also the mantle rock of similar petrography, vary in the time of their formation.  相似文献   

The Oligocene Lund Tuff, Great Basin, USA: a very large volume monotonous intermediate     

Larissa L. Maughan  Eric H. Christiansen  Myron G. Best  C. Sherman Gromm  Alan L. Deino  David G. Tingey 《Journal of Volcanology and Geothermal Research》2002,113(1-2)

Unusual monotonous intermediate ignimbrites consist of phenocryst-rich dacite that occurs as very large volume (>1000 km³) deposits that lack systematic compositional zonation, comagmatic rhyolite precursors, and underlying plinian beds. They are distinct from countless, usually smaller volume, zoned rhyolite–dacite–andesite deposits that are conventionally believed to have erupted from magma chambers in which thermal and compositional gradients were established because of sidewall crystallization and associated convective fractionation. Despite their great volume, or because of it, monotonous intermediates have received little attention. Documentation of the stratigraphy, composition, and geologic setting of the Lund Tuff – one of four monotonous intermediate tuffs in the middle-Tertiary Great Basin ignimbrite province – provides insight into its unusual origin and, by implication, the origin of other similar monotonous intermediates.The Lund Tuff is a single cooling unit with normal magnetic polarity whose volume likely exceeded 3000 km³. It was emplaced 29.02±0.04 Ma in and around the coeval White Rock caldera which has an unextended north–south diameter of about 50 km. The tuff is monotonous in that its phenocryst assemblage is virtually uniform throughout the deposit: plagioclase>quartz≈hornblende>biotite>Fe–Ti oxides≈sanidine>titanite, zircon, and apatite. However, ratios of phenocrysts vary by as much as an order of magnitude in a manner consistent with progressive crystallization in the pre-eruption chamber. A significant range in whole-rock chemical composition (e.g., 63–71 wt% SiO₂) is poorly correlated with phenocryst abundance.These compositional attributes cannot have been caused wholly by winnowing of glass from phenocrysts during eruption, as has been suggested for the monotonous intermediate Fish Canyon Tuff. Pumice fragments are also crystal-rich, and chemically and mineralogically indistinguishable from bulk tuff. We postulate that convective mixing in a sill-like magma chamber precluded development of a zoned chamber with a rhyolitic top or of a zoned pyroclastic deposit. Chemical variations in the Lund Tuff are consistent with equilibrium crystallization of a parental dacitic magma followed by eruptive mixing of compositionally diverse crystals and high-silica rhyolite vitroclasts during evacuation and emplacement. This model contrasts with the more systematic withdrawal from a bottle-shaped chamber in which sidewall crystallization creates a marked vertical compositional gradient and a substantial volume of capping-evolved rhyolite magma. Eruption at exceptionally high discharge rates precluded development of an underlying plinian deposit.The generation of the monotonous intermediate Lund magma and others like it in the middle Tertiary of the western USA reflects an unusually high flux of mantle-derived mafic magma into unusually thick and warm crust above a subducting slab of oceanic lithosphere.  相似文献