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FIRST DISCOVERY OF ASIMOWITE IN AN ANTARCTIC METEORITE (GROSVENOR MOUNTAINS 17151, L6, ORDINARY CHONDRITE).
No full textOrdinary chondrites (OCs) preserve a record of impact events due to collision(s) among their parent asteroids that helps constrain the shock conditions and parameters (in particular, size and relative encounter velocity) of impactors and targets. In turn, the co-evolution of planetesimal sizes and their orbital excitation can distinguish among scenarios for the early evolution of the solar system. Shock parameters can be inferred from high-pressure (HP) minerals that are often found in melt veins (MV). Herein, we report the first discoveries of nine HP minerals in the recently recovered L6 chondrite, GRO 17151 (Grosvenor Mountains, season 2017-2018). Furthermore, we report the first discovery of the Fe-rich analogue of wadsleyite, asimowite, from an Antarctic meteorite (3rd known occurrence altogether).
One large main MV is visible in each of two studied sections. The vein contains HP polymorphs of olivine (ringwoodite + ahrensite + wadsleyite + asimowite), orthopyroxene (majorite), Na-pyroxene (albitic jadeite), phosphate (tuite), silica (stishovite) and oxide (xieite). Using these observations, we are able to constrain the impact record of this meteorite.
The GRO 17151 was recovered by the Antarctic Search for Meteorites (ANSMET) expedition in the 2017-2018 season and classified as type L6 [1]. Two single-polished thin sections (GRO 17151-8, GRO 17151-9) were examined for shock indicators, with a focus on the MVs and their HP phases. We used optical microscopy, a JEOL JSM-IT300LV scanning electron microscope (SEM), a dispersive confocal Renishaw inVia Reflex Raman microscope (514 nm laser), a Zeiss 1550VP field-emission SEM with energy-dispersive X-ray spectrometry (EDS) and electron backscatter diffraction (EBSD) capability, and a JEOL JXA 8530F electron probe micro-analyzer. Around 20 areas of the MV were analyzed for texture, mineral chemistry, and Raman spectroscopy (RS), and a few of them with EBSD.
Petrography, mineral chemistry, and structure: In the groundmass of GRO 17151, olivine grains show strong mosaicism and planar deformation features. The single thick MV is presumed to be the result of a strong shock event; it is in contact with olivine but occasionally also with pyroxene and metal grains. The width of the thickest MV is nearly constant (~500 to 600 μm). The MV consists of glass, silicate clasts (olivine, pyroxene, and plagioclase), sulfides, chromite, and Fe-Ni metal.
HP minerals. We have observed 9 HP phases including ringwoodite (rw), ahrensite (ahr), wadsleyite (wd), asimowite (asi), majorite (maj), albitic jadeite (jd), stishovite (stv), tuite (tu), and xieite (xie). The MV matrix is mostly a crystallized assemblage of maj+rw+wd+magnesiowüstite (Fig. 1).
The large green crystals, up to about 250 μm, have olivine stoichiometry and variable composition from Fa15 to Fa52. Co-located RS have strong wad peaks and minor rw peaks. Turning to EBSD analysis coupled with high-spatial resolution co-located EPMA, the small BSE-bright crystals in Figs 1B and 1D are Fe-dominant (Fa>50%), yield structure solutions most consistent with wadsleyite structure (though ahr structure is difficult to distinguish by EBSD), and strong wd Raman peaks. Hence these are asimowite (Fig. 2).
Crystals of albitic jadeite [2,3] are equigranular to irregularly shaped, up to ~20 μm, and by EPMA yield the formula (Na0.43-61Ca0.07-0.08K0.02-0.085□0.27-0.45)(Al0.83-0.86Si0.16-0.19Fe0.01-0.04)Si2O6, with Ca# [100×Ca/(Ca+ Na)] in the range of 10.4-15.1. The RS of GRO 17151 albitic jadeite has a distinct major peak at 701 cm–1 and minor peaks at 204, 377, 436, 525, 574, 989, and 1038 cm–1 (Fig. 3A). The RS of near-endmember jd has major peaks at 700, 991, and 1040 cm–1 and minor peaks at 204, 375, 385, 433, 525, 575 cm–1 (RRUFF R050220.2), which is a good match even though our EPMA analysis shows that the GRO 17151 material is not near-endmember jd. As expected but not often found, the jd coexists at fine scale with stishovite, based on mixed RS (spectrum A31 in figure 3A) showing the major stv peak at ~751 cm1 and one minor peak at 231 cm1. Further, the composition of phosphate has a merrillite-like chemistry with Na2O in the range 2.58-2.74, MgO 3.38-3.51 and FeO 0.72-1.35 wt.%.
P-T-t constraints. The RS of GRO 17151 phosphate shows a mix of merrillite and tu due to partial preservation of tu from high pressure; tu suggests P ≤ 22 GPa. The P for growth of albitic jadeite and asimowite [5], which lack any stability fields, cannot be assessed. The occurrence of wd, however, suggests P > 13 GPa to at most 22 GPa (depending on T and Fe content), coexisting wd and ahr bounds the P along the upper limit of the wd field. The existence of maj suggests P of 17–20 GPa and 1800–2100 °C [4].
Pending further work, it is clear that GRO 17151 is heavily shocked, hosting at least nine HP phases indicating peak shock conditions > 17 GPa and 1800 °C. It hosts a unique record of the passage of material through space-and-time-variable high-P,T conditions. This occurrence of asimowite is the first in an Antarctic meteorite and indicates a very different paragenesis from previous reports [5]. Moreover, all the fine-grained wadsleyite()-structured material is more Fe-rich than any experimentally synthesized stable wd, even though it is not all asimowite by the 50% rule. We suggest that Fe-rich (~Fo50) olivine suffered a shock strong enough to form ringwoodite(), which thereafter, during decompression avoided back-transformation to olivine() at the equilibrium - phase loop, reaching instead the metastable branch of β-γ loop and forming a series from ~Fa40 wd to ~Fa55 asi, with traces of even more Fe-rich ahr still remaining on quench
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Recensione a: Lexicon Monacense Anonymum (München, Bayerische Staatsbibliothek, lat. 17151, 17153, 17194), a cura di V. Lunardini, Firenze 2009 (in «Bollettino di Studi Latini» 40,2 [2010], pp. 893-898)
No full textRecensione dell'ediz., a cura di V. Lunardini, del «Lexicon Monacense Anonymum», testimoniato in tre codd. monacensi (München, Bayerische Staatsbibliothek, lat. 17151, 17153, 17194)
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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