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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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Africa
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North Africa
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Egypt (2)
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Antarctica
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Antarctic ice sheet
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East Antarctic ice sheet (1)
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Victoria Land (1)
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polar regions (1)
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commodities
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aggregate (2)
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elements, isotopes
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noble gases (1)
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geochronology methods
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terrestrial age (1)
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igneous rocks
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igneous rocks
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volcanic rocks
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glasses (1)
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meteorites
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meteorites
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iron meteorites
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octahedrite (1)
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micrometeorites (1)
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minerals
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alloys
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kamacite (1)
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plessite (1)
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silicates
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framework silicates
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silica minerals
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quartz (1)
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orthosilicates
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nesosilicates
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olivine group
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kirschsteinite (1)
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zircon group
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zircon (1)
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Primary terms
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Africa
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North Africa
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Egypt (2)
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Antarctica
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Antarctic ice sheet
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East Antarctic ice sheet (1)
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Victoria Land (1)
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asteroids (1)
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crystal chemistry (1)
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crystal growth (1)
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crystal structure (1)
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deformation (1)
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geochemistry (1)
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igneous rocks
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volcanic rocks
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glasses (1)
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metamorphism (2)
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meteorites
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iron meteorites
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octahedrite (1)
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micrometeorites (1)
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noble gases (1)
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Possible shock-induced crystallization of skeletal quartz from supercritical SiO 2 -H 2 O fluid: A case study of impact melt from Kamil impact crater, Egypt
ABSTRACT With an age of less than ~5000 yr and a diameter of 45 m, Kamil crater in Egypt is one of the youngest and smallest terrestrial impact craters known to date. Abundant evidence of shock-deformed sandstone has been reported from Kamil crater, including shatter cones, vesicular impact glass, high-pressure polymorphs of silica and carbon, planar deformation features (PDFs) and planar fractures (PFs) in quartz, dissociated zircon, melt veins, and intergranular melt, giving rise to a range of estimated shock pressures from ~20 to ~60 GPa. Here, we investigated shocked zircon from Kamil crater through characterization of microstructures in a centimeter-sized clast of shocked nonporous sandstone ejecta, previously described as containing quartz grains with PDFs and PFs, coesite, stishovite, diamond, and lechatelierite. Orientation analysis by electron backscatter diffraction (EBSD) showed that the quartz arenite consists of damaged detrital quartz grains surrounded by a matrix of either comminuted quartz or intergranular melt. Individual quartz grains are pervasively fractured (abundant PFs and PDFs); apparent isotropic crushing resulted in uniformly and highly dispersed orientation clusters on pole figures. Zircon grains are not abundant; however, four of 19 grains analyzed by EBSD contained {112} deformation twin lamellae, with individual lamellae ranging in length from 1 to 2 µm. Lengths of twin lamellae in Kamil zircon grains are anomalously short compared to those reported in shocked zircon from other impact structures, where individual lamellae are tens of micrometers long. Previous empirical studies have suggested that {112} twin lamellae in zircon form at ~20 GPa in non-porous target rocks, a finding supported by their coexistence, in some impactites, with high-pressure phases such as reidite. The only available experimental constraint, by diamond anvil cell, found {112} twins in zircon powder quenched at 20 GPa. The presence of coesite, stishovite, lechatelierite, and shocked quartz with PDFs in the studied sample is consistent with empirically derived pressure estimates of ~20 GPa for {112} twin formation in zircon in the ejecta sample from Kamil crater. Kamil thus represents the smallest and youngest impact structure where shock-twinned zircon has been reported. Given the apparent efficiency of {112} twin formation (21% of grains), shock-twinned zircon is here shown to provide a robust and readily identifiable record of shock deformation in a relatively common mineral at one of the smallest known terrestrial impact craters.
Cosmic Dust: Finding a Needle in a Haystack
Abstract The Earth accretes some 40,000 tonnes of extraterrestrial material each year. Most of this is interplanetary dust produced by collisions and evaporation of rocky and icy bodies in the Solar System. A fraction of this dust survives hypervelocity impact with the Earth’s upper atmosphere and is collected at the Earth’s surface in the form of microscopic particles (<2mm) called micrometeorites. Significant quantities of micrometeorites are recovered mainly from deep-sea sediments, and snow, ice and loose sediments in polar areas. Micrometeorites provide samples of a variety of dust-producing bodies in the Solar System for laboratory analysis, most notably primitive asteroids and comets which allow exploration of the first stages in the evolution of the protoplanetary disk. Furthermore, the systematic study of unbiased and time-constrained micrometeorite collections allows investigation of the cycles of extraterrestrial input to the global geochemical budget of planet Earth, including its bearing on the emergence of life. Lastly, knowledge of the physical and compositional properties of micrometeorites provides constraints for modelling the source regions and dynamic evolution of the cosmic-dust complex in the near-Earth space, as well as for assessing the potential hazard of dust in the vicinity of the Earth to space activities. This work provides basic information on micrometeoroid production in space and delivery to Earth, atmospheric entry and micrometeorite collections. It gives an overview of current knowledge of the diversity of micrometeorites in terms of their nature and origin, highlighting recent advances in the identification of new types and in the quantification of the flux of extraterrestrial matter to Earth.