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What garnet, clinopyroxene, and diamond potential can tell us about the evolution of sub-cratonic mantle sections: a case study of the Zagadochnaya kimberlite (Yakutia)
Get PDFThis PhD provides major and trace element compositions for a new suite of Cr-rich garnet xenocrysts and associated minerals from the diamond-free Zagadochnaya kimberlite, Daldyn Field, Yakutia (Russia). Interpreting the nature and evolution of the underlying lithospheric mantle from these samples requires a good understanding of relationships between pressures and temperatures of formation, upper mantle phase relations and trace element signatures of petrochemical processes. I addressed these issues by integrating three main lines of research.
i) Evaluation of single-clinopyroxene geobarometry for garnet peridotites. Single-clinopyroxene thermobarometry represents the most reliable method among single-mineral thermobarometric techniques for mantle rocks, but the geobarometer tends to produce considerable scatter in P–T estimates when applied to clinopyroxenes with unfavourable compositions. Multiple electron microprobe analyses on compositionally diverse clinopyroxenes, using different analytical conditions, demonstrated that this scatter is mostly related to propagation of analytical errors on the calculated Cr-in-cpx pressure. The results of this analytical tests were used to calculate model analytical errors and propagated P uncertainties for a large set of published analyses of mantle-derived, xenolith-borne clinopyroxenes. I found that the parameter aCr/Cr# [where aCr = Cr–0.81•Na•Cr#, Cr#=Cr/(Cr+Al), atoms per 6-oxygen formula unit] can be used to discriminate clinopyroxenes for which analytical errors alone will propagate unacceptable P uncertainties (i.e., higher than ±0.25 GPa) for several combinations of analytical conditions. I therefore defined a new optimized analytical procedure for single-clinopyroxene geobarometry, which significantly decreases the pressure uncertainties and allows a better definition of clinopyroxene-based geotherms.
ii) Thermodynamic modeling of natural peridotitic systems. Calculations of phase equilibria in Cr-bearing peridotitic systems were used to predict the effects of P, T and bulk compositional variations on garnet–spinel relations in fertile and depleted mantle compositions. Calculations showed that in cratonic lithospheric sections the width of the garnet–spinel transition strongly depends on bulk composition: in a fertile mantle, spinel can coexist with garnet to about 120 km depth, while in an ultra-depleted harzburgitic mantle it can be stable to over 180 km depth. In the garnet+spinel stability field the calculated modes of spinel are very low (0.1–2.8 %), suggesting that spinel grains may be easily overlooked in mantle xenoliths. The model also suggests a significant potential role of P–T conditions on the distribution of garnets in the popular Ca–Cr discrimination diagram.
iii) Interpretation of the mantle record in the Zagadochnaya kimberlite. A suite of 28 garnet xenocrysts, often containing polymineralic inclusions, were selected from a heavy-mineral concentrate of the Zagadochnaya kimberlite for detailed electron microprobe analysis and laser-ablation inductively-coupled mass spectrometry. Robust P–T estimates for the garnets and associated clinopyroxenes and the Ca and Cr contents in the garnets indicate that the kimberlite sampled only a shallow mantle portion ( 130 km, the absence of diamond, the Mg-rich composition of the kimberlite, and the pervasive reactions that produced the secondary assemblages
Ladinian magmatism of the South Alpine: Geochemical and isotopic data and their implications for the geodynamic setting of Adria plate
No full textPartial melting of an enstatite chondrite at 1 GPa: Implications for early planetary differentiation
Get PDFWe present new time series partial melting experiments performed on a natural enstatite chondrite (EL6), aimed at investigating the textural and geochemical changes induced by silicate-metal equilibration during early planetary differentiation. The starting material of our experiments consisted of small fragments (ca. 50 mg) obtained from the interior of the enstatite chondrite MCY 14005 (MacKay Glacier, Antarctica), collected during the XXX° Italian Expedition in Antarctica (PNRA). Experiments were performed in graphite capsules at a pressure of 1 GPa, at temperature ranging from 1100 to 1300 °C, with run durations from 1 to 24 h. The initial phase assemblage of the enstatite chondrite, mostly composed by granular enstatite and Fe-Ni metal (up to 400 μm in size) with minor amounts of sulphides and plagioclase, undergoes significant changes with increasing temperature and run duration. At 1100 °C, no silicate melt is produced and subsolidus reactions occur at the contact between the metal and silicate phases. At 1200 °C, small amounts of silicate melt are produced at the grain boundaries and enstatite grains in contact with the melt grow Fe-enriched rims. The metal portions are characterized by two immiscible liquid phases that exhibit rounded shapes when in contact with the silicate melt, whereas smaller (micrometric) liquid metal spheres occur isolated within the silicate melt throughout the experimental charges. These features are already observed in the 1 h experiment but become increasingly evident with increasing run duration, and at higher temperatures. In the experiments performed at 1300 °C, the amount of silicate melt increases and new silicate minerals form (olivine and low-Ca-pyroxene).
Enstatite chondrites are characterized by an oxygen isotope composition similar to that of the bulk Earth and Moon, and are considered to have initially formed in the terrestrial planetary zone of the solar nebula. For this reason, they represent a suitable material to investigate the early planetary differentiation processes that occurred in the proto-Earth system. Preliminary results from our experiments indicate that, at the investigated oxygen fugacity (1-2 log units below the IW buffer), the Fe-Si exchange between the metal and silicate phases allows the formation of silicate melt and silicate phases such as olivine and low-Ca-pyroxene. At the same time, the change in shape of the metal grains (increasingly circular/spherical with increasing temperature) and the overall reduction of their number density with increasing experimental time point to rapid aggregation of the metal phase and, possibly, to fast silicate-metal differentiation in small planetesimals
Project DIVE (Drilling the Ivrea-Verbano zonE): A joint petrological, geochemical, and geophysical exploration of the lower continental crust
No full textDespite the structural complexity of the Alps at numerous scales, geological and geophysical investigations have respectively mapped and imaged tremendous amounts of information near the surface and at depth. However, there is an inherent gap between the two sets of approaches, leaving the middle and lower crust poorly constrained. This has been one of the main motivations to initiate the ICDP project DIVE (Drilling the Ivrea-Verbano zonE), in which three geological sites of the Ivrea-Verbano Zone will be explored through scientific drilling. In this zone, near-complete sections of the continental crust are exposed at the surface, and with careful geological preparation and geophysical site surveys we have targeted three areas with a great potential of further discoveries during DIVE. Almost all physical and chemical properties will be characterized on the recovered rock core samples, in borehole logging investigations, and additional surveys around each site. Taken together, these should cover a large range of spatial scales covering at least 6 orders of magnitude (mm to km), investigate structures and their variations in bulk properties within the lower crust, and the transition to mantle rocks in an unprecedented way. The interdisciplinary approach not only allows to correlate numerous geophysical and petrological properties, but with modelling it will also allow to investigate the causative relationships.
The detailed aims, preparatory steps, as well as the current status of project DIVE, will be presented at the conference. By that time, drilling of the first hole is expected to start near Ornavasso, followed by a second hole in Megolo (both in Val d’Ossola). For the third site near Balmuccia, which is planned for later, site survey results will be presented
Preliminary results from the ICDP - DIVE project: Hole DT-1b (Ornavasso, Italy)
No full textWe report preliminary results from drill cores and logging from the ICDP Drilling the Ivrea-Verbano zonE (DIVE) project, Hole DT-1b in Ornavasso (Val d’Ossola, northern Italy). Characterized by pronounced geophysical anomalies, the exposed Ivrea-Verbano Zone in the Italian Alps represents an archetypal lower continental crust section. The first phase of DIVE is dedicated to the drilling and the petrological and geophysical characterization of the lowermost continental crust. Specifically, Hole DT-1b was set in the in the hinge zone of the Massone Antiform to explore the pre-Permian lithologies of the lower continental crust. Hole DT-1b was drilled using the diamond double tube continuous wireline coring method, from October 6 to December 10, 2022, and the retrieved rock cores were inspected and classified by the DIVE drilling project science team. Core recovery was effectively 100% throughout the entire drill hole. In total, 578m of the upper part of the lower continental crust in the Ivrea-Verbano Zone were drilled and cored.
Here we summarize on site visual core descriptions and preliminary geophysical logging and microbiological investigations. The cores mostly consist of amphibolites and garnet-bearing metapelites with variable presence of migmatitic structures, and with local high and low temperature shear zones, pegmatitic dikes, and open fractures.
Continuous monitoring of borehole fluids and gases (OLGA and miniRuedi devices, see EGU abstract by Dutoit and coworkers), and a suite of borehole logging measurements (see abstract by Li, Greenwood, Caspari and coworkers) were performed. They match very well the core logs performed on site (magnetic susceptibility and natural gamma radiation). The most prominent, directly observable deformation feature was a high-temperature foliation, with a dip angle between 30 and 60° in the upper part of the hole, becoming increasingly steeper in the deeper part.
Along the entire drill hole fractures and open cracks were observed, some of them filled with precipitates of quartz, carbonates, sulphides and oxides. These fractures are potentially promising hosts for microbial communities and are currently under investigation. Additional samples for microbiological studies were taken every 20m from the drill cores and are currently cultivated for further investigations.
Hole DT-1b provides detailed insights into the compositional, structural and geophysical variation of metasedimentary continental lower crust, including the distribution of sulphides. Relamination of metamorphic pelitic and mafic rocks may produce an important reservoir of sulphur bearing minerals in the lower continental crust. Further results emerging between abstract submission and the conference will be presented
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
First crystal-structure determination of an olivine inclusion still trapped in a diamond: composition and implications for diamond source pressure
No full textAppropriate 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
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