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Fe3+ distribution between garnet and pyroxenes in metasomatised mantle wedge garnet peridotites
No full textOxygen-metal bonding in Ti-bearing compounds from O 1s spectra and ab initio full multiple-scattering calculations
No full textThe O K-edge spectra of a series of Ti-bearing compounds with Ti in different structural and chemical environments have been measured using electron energy-loss spectroscopy and analyzed using ab initio full multiple-scattering (MS) calculations. The near-edge structures arise mainly from covalency by direct and/or indirect interaction between O and metal atoms and between O and Si atoms. The coordination number of the cation and the site symmetry also influence the spectral shape and structures. Using different size clusters around the excited atom in the full MS simulation, it is possible to interpret and assign the features present in the spectra of each compound to its specific atomic arrangement and electronic structure.
Keywords: oxygen-metal bonding; Ti-bearing compounds; electron energy-loss spectroscopy; ab initio full multiple-scattering calculations; coordination numbers; site symmetry
The role of garnet and pyroxenes in redox processes in the mantle wedge
No full textRedox equilibria in the Earth’s mantle control many chemical and physical processes such as magma genesis, chemical differentiation and fluid-related metasomatism. The whole rock oxidation state is conventionally described in petrology by the oxygen fugacity (fO2). Many studies focussed on the redox state of low pressure assemblages in the mantle system, whereas the fO2 of supra-subduction mantle wedge is still poorly investigated. Orogenic garnet peridotites can be important witnesses of the processes occurring in the deep sub-arc mantle. However, the fO2 determination in most metasomatised garnet peridotites is a demanding task because of the large number of phases that may incorporate both ferric and ferrous iron, and that may show zonations. fO2 in high pressure peridotites is traditionally determined from the Fe2+-Fe3+ content of a garnet in equilibrium with olivine and orthopyroxene, but the Fe3+ partitioning among the peridotite mineral phases is often neglected. Works by [1-3] demonstrate that the increase of Fe3+ in garnet with increasing temperature and pressure is the consequence of the redistribution of Fe3+ from clinopyroxene into the garnet. This implies that the Fe3+ enrichment in garnet is not necessarily indicative of high whole-rock oxygen contents or of the interaction with more oxidised metasomatic agents.
We selected a sample of orogenic peridotite from the ultrahigh pressure Sulu belt (Eastern China) corresponding to a slice of metasomatised mantle wedge equilibrated at 5 Gpa and 900 °C. This shows phlogopite + magnesite in equilibrium with olivine, orthopyroxene, clinopyroxene and garnet. The Fe3+/ΣFe of garnet, clinopyroxene and phlogopite was measured by two combined techniques: the "Flank Method" on wavelength dispersive spectra, acquired on garnets with electron microprobe at the University of Milano, and electron energy loss spectroscopy, at the Bayerisches Geoinstitut, employed to analyse clinopyroxene and phlogopite, and to check possible zonations. The measurements indicate that the pyrope-rich garnets are zoned and contain Fe3+/ΣFe up to 0.12–0.14. These results are consistent with the relatively high oxygen fugacities (FMQ to FMQ+2) previously calculated for this sample [4]. Results on clinopyroxene and phlogopite indicate that only clinopyroxene contains some amounts of Fe3+ (Fe3+/ΣFe=0.48–0.51), while phlogopite is below the detection limits. Interestingly, also orthopyroxene contain some Fe3+/ΣFe, up to 0.10. Garnet/clinopyroxene and orthopyroxene/clinopyroxene qualitative partitioning apparently do not follow the same trend described by [1] for a suite of sub-cratonic garnet peridotite xenoliths. The determination of Fe2+/Fe3+ equilibria in mineral assemblages of metasomatised mantle-wedge peridotites therefore represents only the first step in unravelling the relationships between fO2 and phase assemblages in multi-component mantle systems.
References: [1] Canil D., O’Neill H.S.C. (1996): J Pet 37, 609–635; [2] Woodland A.B., Koch, M. (2003): Earth Planet Sci Lett 214, 295–310; [3] Woodland A.B. (2009): Lithos, in press; [4] Malaspina N., Poli S., Fumagalli P. (2009): J Petrol 50, 1533–1552
Fe3+ distribution between garnet and pyroxenes in mantle wedge carbonate-bearing garnet peridotites (Sulu, China) and implications for their oxidation state
No full textThis study presents new measurements of Fe3+ in garnet, olivine, clino- and orthopyroxene of a mantle-derived garnet peridotite from Donghai County, the southeastern end of the Sulu ultrahigh pressure terrane. These rocks correspond to a slice of supra-subduction lithospheric mantle wedge, tectonically emplaced into the crust. They record a multistage metasomatism by an alkali-rich silicate melt at high temperature, and a subsequent influx of a slab-derived incompatible element and silicate- rich fluid during the Triassic UHP metamorphism. We employed two "unconventional" techniques to measure the Fe3+/ΣFe content of mineral phases with high spatial resolution: (i) the Flank Method electron microprobe analyses for garnet, performing for the first time quantitative Fe2O3 map analyses on zoned garnets at the Dipartimento di Scienze della Terra, University of Milano, and (ii) the Electron Energy Loss Spectroscopy (EELS) for garnet, olivine and pyroxenes, at the Bayerisches Geoinstitut, University of Bayreuth.
The results indicate that the pyrope-rich metasomatic garnets present a chemical zoning, with the complementary decrease in Al2O3 from ~23 to ~21 wt.%, relative to the increase of Fe2O3 from ~0.8 to ~2.5 wt.%. Such a trend is likely related to the Fe3+-Al substitution in the garnet octahedral site, which is sensitive to the garnet oxidation state. Clinopyroxenes are diopsidic in composition, whereas olivine and orthopyroxene have ~92 mol.% of forsterite and enstatite, respectively. The EELS measurements show that clinopyroxene contains relatively high Fe3+/ΣFe ratios and Na contents, ranging from 0.48 to 0.51 and from 0.13 to 0.17 a.p.f.u., respectively. Interestingly, also orthopyroxene may contain Fe3+/ΣFe up to 0.10 (±0.05), a percentage comparable to that of garnet, with important consequences in the study of redox processes in mantle rocks and in the application of many geothermometers. Garnet/clinopyroxene and orthopyroxene/clinopyroxene qualitative partitioning indicates a minimum redistribution of Fe3+ from clinopyroxene to garnet. The enrichment in Fe3+ of Ca-clinopyroxene requires the incorporation of a NaFe3+Si2O6 (aegerine) component, particularly in garnet peridotites where the Al content of clinopyroxene is buffered by its coexistence with garnet. The coupled Na-Fe3+ enrichment of our clinopyroxene likely suggests a corresponding enrichment in the whole rock. The Fe3+ substitution mechanism into clinopyroxene as aegerine component could be therefore favoured by the influx of Fe2O3- alkali-rich metasomatic fluid phases. This suggests that a possible net bulk oxidation and the redistribution of Fe3+ between garnet and pyroxenes could depend on additional variables besides temperature and pressure. Such mechanisms open new possibilities to unravel the redox processes occurring in the portion of mantle wedge interfacing the subducting slab, which is a key location where the mantle redox reactions likely occur
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
Slab-to-mantle “oxidation” transfer: insights from diamond-bearing majoritic garnets (Western Norway)
No full textThe majoritic garnet-bearing websterites from Bardane (Western Gneiss Region, Norway) are unique examples of metasomatised mantle wedge that interacted with C-saturated COH subduction fluid phases at 200 km depth. These peridotites represent slices of former Archean transition zone mantle that upwelled, melted and accreted to a thick cratonic lithosphere, where it cooled until the Middle Proterozoic (stages M1-M2). During the subsequent Caledonian to Scandian subduction cycle (stage M3) these depleted mantle rocks were dragged into deep portions of the supra-subduction mantle wedge, where the infiltration of crustal fluids initiated diamond and majoritic garnet crystallisation [1, 2]. The infiltration of subduction fluids in these rocks at increasing depth promoted crystallisation of microdiamonds in polyphase, fluid-related, inclusions and of majoritic garnet in veins. We studied the peak (stage M3) mineral assemblage garnet + orthopyroxene + olivine ± clinopyroxene ± phlogopite, where garnet turns to majoritic with pressure increase from 3 to 6.5 GPa and 800-1000 °C temperature. Majoritic garnet contains polyphase inclusions with daughter Cr-spinel + phlogopite/K-amphibole + dolomite/magnesite + graphite/diamond. They witness COH-fluid/mineral interaction closely related to the oxidation state of the rock and responsible for diamond formation. We determined the fO2 in the M3 assemblage starting from Fe3+ analyses in majoritic garnet which results progressively enriched in Fe3+/ΣFe up to 0.15. The fO2 values obtained for the Bardane mantle wedge garnet peridotites are up to -2 log units lower than the fayalite-quartz-magnetite (FMQ) buffer along a trend from arc lavas (FMQ+1.5 – FMQ+3) [3, 4] to mantle wedge garnet peridotites equilibrated at 4-5 GPa (FMQ – FMQ+2) [5]. Such a trend record the same systematic ΔFMQ decrease with depth as subcratonic xenoliths from South Africa [6], but shifted towards higher fugacities.
The determination of oxygen fugacity of the hydrate–carbonate-bearing M3 mineral association enabled to estimate the speciation of slab-derived metasomatic COH fluids responsible for polyphase inclusions precipitation. Such fluids are H2O-CO2 mixtures, whereby the H2O/CO2 ratio increases with increasing pressure. The peculiar composition of majorite-hosted diamond-bearing polyphase inclusions from Bardane and the speciation of its COH component point to an “oxidised” silicate-rich aqueous fluid contaminant from the subducted slab to the mantle wedge which could be regarded as carrier of dissolved oxidised components from the subducted slab to the mantle wedge [7, 5]. Such a mechanism opens new possibilities to unravel the redox processes occurring in arc mantle sources and their role in precipitating diamond.
References. [1] Spengler, D., Van Roermund, H.L.M., Drury, M.R., Ottolini, L., Mason, P.D., Davies, G.R. (2006): Nature, 440, 913–917; [2] Scambelluri, M., Pettke, T., van Roermund, H.L.M. (2008): Geology, 36, 59–62; [3] Carmichael, I.S.E., Frey, H.M., Lange, R.A., Hall, C.M. (2006): Bull. Volcanol., 68, 407–419; [4] Rohrbach, A., Schuth, S., Ballhaus, C., Münker, C., Matveev, S., Qopoto, C. (2005): Contrib. Mineral. Petrol., 149, 685–698; [5] Malaspina, N., Poli, S., Fumagalli, P. (2009): J. Petrol., 50, 1533–1552; [6] Woodland, A.B. & Koch, M. (2003): Earth Planet. Sci. Lett., 214, 295–310; [7] Hirschmann, M.M., (2009): Science, 325, 545–546
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
Silicate-rich aqueous fluids sources for diamond crystallisation in deep mantle wedge
No full textThe majoritic garnet-bearing websterites from Bardane (Western Gneiss Region, Norway) are unique examples of
metasomatised mantle wedge slices that interacted with C-saturated COH subduction fluid phases at 200 km depth.
These peridotites represent slices of former Archean transition zone mantle that upwelled, melted and accreted to
a thick cratonic lithosphere, where it cooled until the Middle Proterozoic (stages M1-M2). During the subsequent
Caledonian to Scandian subduction cycle (stage M3) these depleted mantle rocks were dragged into deep portions
of the supra-subduction mantle wedge, where the infiltration of crustal fluids initiated diamond crystallisation.
During this stage majoritic garnet crystallised at grain boundaries and in veins. Evidence of COH fluid influx is
provided by the precipitation of dolomite/magnesite, pyroxene, phlogopite, spinel and diamond/graphite in solid
multiphase inclusions within orthopyroxene and garnet, as weel as by centimetric veins of majoritic garnet.
We studied the peak M3 mineral assemblage garnet + orthopyroxene + olivine clinopyroxene phlogopite,
where garnet turns to majoritic with pressure increase from 3 to 6.5 GPa and 800-1000 °C temperature. Majoritic
garnet contains polyphase inclusions with daughter Cr-spinel + phlogopite/K-amphibole + dolomite/magnesite
+ graphite/diamond, witnessing fluid/mineral interaction closely related to the oxidation state of the rock and
responsible for diamond formation. We determined the fO2 in the M3 assemblage starting from Fe3+ analyses in
majoritic garnet and we estimated the speciation of the graphite/diamond-COH slab fluids. The Fe3+/Fe of M3-
majoritic garnet was measured with the "Flank Method" on wavelength dispersive spectra, acquired on garnets
with electron microprobe at the University of Milano. Natural and synthetic standards were used for calibration,
together with an additional pyrope-rich standard, measured with electron energy loss spectroscopy. The results
indicate a progressive enrichment in Fe3+/Fe up to 0.15 in M3-majoritic garnet. Majoritic garnet shows a core-torim
zonation with decreasing Fe3+/Fe from 0.15-0.10 in the core to 0.05-0.00 in the rim. The fO2 calculations
have been performed with an improved thermodynamic solution model for the Fe3+-bearing garnet skiagite. The
resulting fO2 is variably lower than the fayalite-quartz-magnetite (FMQ) buffer ( 1 log unit). However, these
values are higher than fO2 of garnet peridotite xenoliths from sub-cratonic mantle equilibrated at similar pressure
conditions ( FMQ-3), but lower than mantle wedge garnet peridotites from the ultrahigh pressure Sulu belt
(FMQ FMQ+2) equilibrated at 4-5 GPa.
The determination of oxygen fugacity of these hydrate–carbonate-bearing garnet peridotites enables us to estimate
the speciation of COH metasomatic fluids derived from the subducting slab, responsible for polyphase inclusions
precipitation. Our results indicate that the metasomatic COH fluid was a H2O+CO2 mixture, with increasing
H2O/CO2 with pressure. The peculiar composition of majorite-hosted diamond-bearing polyphase inclusions from
Bardane and the speciation of its COH component point to an “oxidised” silicate-rich aqueous fluid contaminant,
carrier of Fe3+ from the subducted slab to the mantle wedge
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
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