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Geochemistry of intermediate olivine-phyric shergottite Northwest Africa 6234
No full textThe SNC meteorites are our only samples from Mars and are, therefore, the key source of information regarding its igneous history and geochemical evolution. Shergottites, which form the largest sub-group of the SNC meteorites, provide some of the most significant insights into the igneous history of Mars due in part to their petrological and geochemical diversity. Among petrological divisions of shergottites the olivine (ol) phyric shergottites are extremely useful in extending our knowledge of the interior of Mars, because they have many primitive characteristics. Here we present major- and trace-element geochemistry, Li isotope composition and abundance, and Re-Os isotope and highly-siderophile element abundance data for the ol-phyric shergottite NorthWest Africa 6234 (NWA 6234).
Meteorite NWA 6234 was found in 2009 at an undisclosed
location in Mali and purchased by an anonymous collector in February 2010. It was a 55.7 g partly fusion-crusted stone. It is thought to have experienced a moderate shock stage and limited weathering. For this study, we purchased a 3.3 gram slice of the meteorite from Marmet Meteorites, and confirmed that it matched the description of NWA 6234
Petrography, mineral chemistry, and crystallization history of olivine-phyric shergottite NWA 6234: a new melt composition
No full textKnowledge of Martian igneous and mantle compositions is crucial for understanding Mars’ mantle evolution, including early differentiation, mantle convection, and the chemical alteration at the surface. Primitive magmas provide the most direct information about their mantle source regions, but most Martian meteorites either contain cumulate olivine or crystallized from fractionated melts. The new Martian meteorite Northwest Africa (NWA) 6234 is an olivine-phyric shergottite. Its most magnesian olivine cores (Fo78) are in Mg-Fe equilibrium with a magma of the bulk rock composition, suggesting that it represents a melt composition. Thermochemical calculations show that NWA 6234 not only represents a melt composition but is a primitive melt derived from an approximately Fo80 mantle. Thus, NWA 6234 is similar to NWA 5789 and Y 980459 in the sense that all three are olivine-phyric shergottites and represent primitive magma compositions. However, NWA 6234 is of special significance because it represents the first olivine-phyric shergottite from a primitive ferroan magma. On the basis of Al/Ti ratio of pyroxenes in NWA 6234, the minor components in olivine and merrillite, and phosphorus zoning of olivine, we infer that the rock crystallized completely at pressures consistent with conditions in Mars’ upper crust. The textural intergrowths of the two phosphates (merrillite and apatite) indicate that at a very last stage of crystallization, merrillite reacted with an OH-Cl-F-rich melt to form apatite. As this meteorite crystallized completely at depth and never erupted, it is likely that its apatite compositions represent snapshots of the volatile ratios of the source region without being affected by degassing processes, which contain high OH-F content
Geochemistry of intermediate olivine-phyric shergottite Northwest Africa 6234, with similarities to basaltic shergottite Northwest Africa 480 and olivine-phyric shergottite Northwest Africa 2990
No full textThe newly found meteorite Northwest Africa 6234 (NWA 6234) is an olivine (ol)-phyric shergottite that is thought, based on texture and mineralogy, to be paired with Martian shergottite meteorites NWA 2990, 5960, and 6710. We report bulk-rock major- and trace-element abundances (including Li), abundances of highly siderophile elements, Re-Os isotope systematics, oxygen isotope ratios, and the lithium isotope ratio for NWA 6234. NWA 6234 is classified as a Martian shergottite, based on its oxygen isotope ratios, bulk composition, and bulk element abundance ratios, Fe ⁄Mn, Al ⁄ Ti, and Na⁄ Al. The Li concentration and δ 7Li value of NWA 6234 are similar to that of basaltic shergottites Zagami and Shergotty. The rare earth element (REE) pattern for NWA 6234 shows a depletion in the light REE (La-Nd) compared with the heavy REE (Sm-Lu), but not as extreme as the known ‘‘depleted’’ shergottites. Thus, NWA 6234 is suggested to belong to a new category of shergottite that is geochemically ‘‘intermediate’’ in incompatible elements. The only other basaltic or ol-phyric
shergottite with a similar ‘‘intermediate’’ character is the basaltic shergottite NWA 480. Rhenium-osmium isotope systematics are consistent with this intermediate character, assuming a crystallization age of 180 Ma. We conclude that NWA 6234 represents an intermediate compositional group between enriched and depleted shergottites and offers new insights into the nature of mantle differentiation and mixing among mantle reservoirs in Mars
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
The mainstream primary classroom as a language-learning environment for children with severe and persistent language impairment - implications of recent language intervention research
Get PDFMany UK children with severe and persistent language impairment (SLI) attend local mainstream schools. Although this should provide an excellent language-learning environment, opportunities may be limited by difficulties in sustaining time-consuming, child-specific learning activities; restricted co-professional working, and the complex classroom environment. Two language intervention studies in mainstream Scottish primary schools showed children with SLI receiving intervention from speech and language therapists (SLTs) or their assistants made more progress in expressive language than similar children receiving intervention from education staff. Potential reasons for this difference are sought in the amount of tailored language-learning activity undertaken; how actively school staff initiated contact with SLTs; and the language demands of the classroom. Tailored language learning appears to be a differentiating factor. A language support model, reflecting views of teachers and SLTs about encouraging language development for children with SLI within the ecology of the mainstream primary classroom, is also outlined
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