1,721,005 research outputs found
A common volatilization trend in Transantarctic Mountain and Australasian microtektites: Implications for their formation model and parent crater location
No full textWe studied the variations of the volatile major elements Na and K in Transantarctic Mountain microtektites and Australasian microtektites with distance from the putative source crater location in Indochina. The dataset includes 169 normal-type Australasian microtektites (101 from this study and 68 from the literature) from 24 deep-sea sediment cores up to 8000km from Indochina, and 54 Transantarctic Mountain microtektites from northern Victoria Land, 11000km due southeast of Indochina. Normal-type (MgO8000km are 4.27±0.67wt.% (n=84), 3.20±1.21wt.% (n=50), 2.10±0.25wt.% (n=35) and 1.25±0.25wt.% (n=54), respectively. The trend highlights a relationship between increasing loss of volatiles in microtektites with longer trajectories and higher temperature-time regimes which should be taken into account in microtektite formation modeling. The trend is consistent with a previous hypothesis that Transantarctic Mountain microtektites belong to the Australasian strewn field and that Indochina is the target region for the parent catastrophic impact
Coesite in a Muong Nong-type tektite from Muong Phin, Laos: Description, formation, and survival
No full textWe examined 16 white opaque inclusions exposed on two polished slices of a Muong Nong-type Australasian tektite from Muong Phin, Laos. The inclusions usually consist of a core, surrounded by a froth layer, and a quartz neoblast layer. The cores are composed primarily of a mixture of silica glass, coesite, and quartz in varying proportions. A thin (up to ~4 μm) layer of SiO2-poor glass enriched in FeO, MgO, CaO, Al2O3, and TiO2 is observed as a bright halo in backscattered electron images around the quartz neoblasts and in places contains μm-sized crystals, which may be Fe,Mg-rich spinel. The distribution and textural relationships between the coesite-bearing inclusions and the tektite matrix point to an in situ formation of the coesite due to an impact, rather than to infall, from a nearby impact, into tektite melt produced by the aerial burst of a bolide. The quartz neoblasts probably formed by crystallization of silica melt squeezed out of the inclusion core during the development of the froth layer. The bright halo may be the result of silica diffusing from the adjacent tektite melt into the growing quartz neoblasts. We propose that the survival of coesite was possible due to the froth layer that acted as a heat sink during bubble expansion and then as a thermal insulator
Evidence of subsolidus quartz-coesite transformation in impact ejecta from the Australasian tektite strewn field
No full textCoesite, a high-pressure silica polymorph, is a diagnostic indicator of impact cratering in quartz-bearing target rocks. The formation mechanism of coesite during hypervelocity impacts has been debated since its discovery in impact rocks in the 1960s. Electron diffraction analysis coupled with scanning electron microscopy and Raman spectroscopy of shocked silica grains from the Australasian tektite/microtektite strewn field reveals fine-grained intergrowths of coesite plus quartz bearing planar deformation features (PDFs). Quartz and euhedral microcrystalline coesite are in direct contact, showing a recurrent pseudo iso-orientation, with the [11 ̄1]* vector of quartz near parallel to the [0 1 0]* vector of coesite. Moreover, discontinuous planar features in coesite domains are in textural continuity with PDFs in adjacent quartz relicts. These observations indicate that quartz transforms to coesite after PDF formation and through a solid-state martensitic-like process involving a relative structural shift of {1 ̄011} quartz planes, which would eventually turn into coesite (0 1 0) planes. This process further explains the structural relation observed between the characteristic (0 1 0) twinning and disorder of impact-formed coesite, and the 101 ̄1 PDF family in quartz. If this mechanism is the main way in which coesite forms in impacts, a re-evaluation of peak shock pressure estimates in quartz-bearing target rocks is required because coesite has been previously considered to form by rapid crystallization from silica melt or diaplectic glass during shock unloading at 30–60 GPa. © 2019 The Author(s
Evidence for direct solid-state quartz-to-coesite transformation in shocked ejecta from the Australasian tektite/microtektite strewn field
No full textCoesite, a high-pressure silica polymorph, is a diagnostic indicator of impact cratering in quartz-bearing
target rocks. The formation mechanism of coesite during hypervelocity impacts has been debated since its
discovery in the 1960s. In impactites, coesite is preserved as a metastable phase in crystalline rocks that
experienced peak shock pressures above ~30-40 GPa (Stöffler & Langenhorst, 1994), and in porous sedimentary
rocks shocked at pressures as low as ~10 GPa (Kowitz et al., 2016). There is a general consensus that coesite
within impactites originates by crystallization from a dense amorphous phase during shock unloading, when
the pressure release path passes through the coesite stability field. Here, we present a combination of TEMbased electron diffraction analyses in order to obtain the necessary high-resolution images and crystallographic
data to unravel the spatial and temporal relations of quartz and coesite. Such nanoscale approach revealed
evidence for direct solid-state quartz-to-coesite transformation in shocked coesite-bearing quartz ejecta from
the Australasian tektite/microtektite strewn field, which is the largest and youngest (~0.8 Myr old) on Earth.
These ejecta consist of a mixture of coesite and quartz in variable proportions, the latter showing planar
deformation features (PDFs) with typical {10-11} and {10-12} orientations. Coesite crystals range in size from
~500 nm to few nanometres, with rounded or elongated habit. They show evident twinning and planar disorder
along (010) planes. Where quartz and coesite are in contact, no appreciable amorphous or ‘glassy’ volume
was detected. Instead, quartz boundaries are always lobate or sawtooth-like, with euhedral coesite crystals
penetrating through the quartz boundaries. Moreover, PDFs in quartz clearly extend in the coesite domains,
suggesting that the latter forms directly at the expense of shocked quartz crystals.
Our observations indicate that quartz transforms directly to coesite after PDF formation and through a solidstate process without entering the silica liquid stability field. The recurrent pseudo iso-orientation between
the (1-11) vector in quartz and the (010) vector of neighbouring coesite crystals point to a martensitic-like
transformation as possible transition mechanism. Arguably, solid-state martensitic-like process could represent
the dominant mechanism of coesite formation in a wide range of cratering events, at least for those with porous
target rocks like at the Barringer (Kieffer et al., 1976) and Kamil craters (Folco et al., 2018). This implies lower
peak impact pressure and temperature conditions for the formation of impact coesite than previously thought
Impact coesite: Formation and survival
No full textThis is an overview of the results of our ongoing research (in part published, [1], in part under review, [2] and [3]) aiming at better understanding of the formation and survival of impact coesite - a debated issue in impact cratering and shock metamorphism studies. Impact coesite occurs in the form of nanometer-sized grains with polysynthetic twinning on
(010) grains, typically embedded in silica glass. Its presence in rocks that experienced shock conditions beyond the stability field is an intriguing and controversial issue. Models, widely accepted since its discovery in 1960 [4], predict that coesite forms during crystallization from highly densified silica melts [5], [6], [7] or from diaplectic glass [8] during shock unloading, when the decompression path intersects the coesite stability field (pressure 3–10 GPa, temperature <3000 K). In contrast to these mechanisms, we show mineralogical and petrographic evidence of subsolidus direct quartz-to-coesite transformation in quartzose impactites from different geological contexts, including a plausible mechanism for this polymorphic transformation. These results have implications on the reconstruction of the P-T-t paths experienced by target rocks and on the definition of impact scenario
(S)TEM analysis of quartz-coesite relations in impact ejecta from the Australasian tektite strew field
No full textShocked quartz and other mineral inclusions in Australasian microtektites
No full textWe report the occurrence of microscopic inclusions of shocked quartz plus a Zr phase and trace of Fe oxide crystallites in Australasian microtektites recovered from deep-sea sediment cores within 2000 km of Indochina. The shocked quartz and the Zr phase are interpreted as relicts of the target rock. Furthermore, the internal homogeneity of Australasian microtektites in terms of abundance of relict mineral inclusions, vesicles, and schlieren increases with distance from Indochina. This finding strengthens the current hypothesis that the source crater of the largest and youngest tektite strewn field on Earth is located in the Indochina region, as internal heterogeneity characterizes normal impact glass found in or near the source crater. This finding also indicates that the Australasian microtektites with the longest trajectories underwent the highest temperatures or were heated longer. The definition of microtektites should include the possible occurrence of microscopic relict inclusions as an indication of proximity to the source crater
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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