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  • 1
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Detrital spinel is a widespread heavy mineral in sandstones from the Maastrichtian–Middle Eocene sedimentary basins in the SE Alps. Chemistry of detrital spinels from the Claut/Clauzetto and Julian Basins (N Italy and NW Slovenia) is used to constrain petrological and geochemical affinities and tectonic provenance of the source rocks. In addition, we have analysed melt inclusion compositions in the detrital volcanic spinels to better constrain the nature of their parental magmas. This is the first study of melt inclusions in detrital spinels. Two principal compositional groups of detrital spinels are recognised based on their TiO2 and Fe2+/Fe3+; one derived from peridotites, the other from basaltic volcanics. Peridotitic spinels are more abundant and have TiO2 〈 0.2 wt% and high Cr/Cr + Al (40–90), characteristic of suprasubduction zone harzburgites. Significant chemical variations among volcanic spinels (TiO2 up to 3 wt%, Al2O3 12–44 wt%) suggest multiple sources, with geochemically distinct characteristics, including MORB-type and backarc basin basalts, subduction-related magmas and tholeiites produced during early continental rifting. Compositions of homogenised melt inclusions in spinels with TiO2 〉 0.2 better distinguish the differences between the compositions of their host spinels and help to further clarify the geodynamic provenance of extrusive source rocks. Several compositional groups of melt inclusions have been recognised and represent diverse magmatism of marginal basins, including MORB- and subduction-related geochemical types, as well as magmas characteristic of early continental rifting. These results, combined with the data on regional ophiolitic complexes and tectonic reconstructions favour the Internal Dinarides of Yugoslavia as a possible source area for the SE Alps sediments.
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2013-06-01
    Description: Two synthetic series of spinels, MgCr2O4–Fe2+Cr2O4 and MgCr2O4–MgFe2 3+O4 have been studied by Raman spectroscopy to investigate the effects of Fe2+ and Fe3+ on their structure. In the first case, where Fe2+ substitutes Mg within the tetrahedral site, there is a continuous and monotonic shift of the Raman modes A1g and Eg toward lower wavenumbers with the increase of the chromite component into the spinel, while the F2g modes remain nearly in the same position. In the second series, for low Mg-ferrite content, Fe3+ substitutes for Cr in the octahedral site; when the Mg-ferrite content nears 40 %, a drastic change in the Raman spectra occurs as Fe3+ starts entering the tetrahedral site as well, consequently pushing Mg to occupy the octahedral one. The Raman spectral region between 620 and 700 cm−1 is associated to the octahedral site, where three peaks are present and it is possible to observe the Cr–Fe3+ substitution and the effects of order–disorder in the tetrahedral site. The spectral range at 500–620 cm−1 region shows that there is a shift of modes toward lower values with the increase of the Mg-ferrite content. The peaks in the region at 200–500 cm−1, when observed, show little or negligible Raman shift. ©2013 Springer-Verlag Berlin Heidelberg
    Print ISSN: 0342-1791
    Electronic ISSN: 1432-2021
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 3
    Publication Date: 2013-07-01
    Description: The influence of Al–Cr substitution on the spinel structure was studied in synthetic single crystals belonging to the FeCr2O4–FeAl2O4 series produced by flux growth at 1,000–1,300 °C in controlled atmosphere. Samples were characterized by single-crystal X-ray diffraction, electron microprobe analyses and Mössbauer spectroscopy. Crystals of sufficient size and quality for single-crystal X-ray diffraction were obtained in the ranges Chr0–0.45 and Chr70–100 but not for intermediate compositions, possibly due to a reduced stability in this range. The increase in chromite component leads to an increase in the cell edge from 8.1534 (6) to 8.3672 (1) Å and a decrease in the u parameter from 0.2645 (2) to 0.2628 (1). Chemical analyses show that Fe2+ is very close to 1 apfu (0.994–1.007), Al is in the range 0.0793–1.981 apfu, Cr between 0 and 1.925 apfu. In some cases, Fe3+ is present in amounts up to 0.031 apfu. Spinels with intermediate Cr content (Chr component between 40 and 60) are strongly zoned with Cr-rich cores and Cr-poor rims. Mössbauer analyses on powdered spinels of the runs from which single crystal has been used for X-ray structural data show values of Fe3+/Fetot consistently larger than that calculated by EMPA on single crystals, presumably due to chemical variation between single crystals from the same runs. The synthesis runs ended at a temperature of 1,000 °C, but it is possible that cation ordering continued in the Cr-poor samples towards lower temperatures, possibly down to 700 °C. ©2013 Springer-Verlag Berlin Heidelberg
    Print ISSN: 0342-1791
    Electronic ISSN: 1432-2021
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 4
    Publication Date: 2017-05-20
    Print ISSN: 0077-7757
    Topics: Geosciences
    Published by Schweizerbart
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  • 5
    Publication Date: 2014-01-03
    Description: We present the results of a comparative study in which we have measured Fe3+/ΣFe ratios in chromites from mantle chromitites in the Oman ophiolite using Mössbauer spectroscopy and single-crystal X-ray diffraction. We have compared these results with ratios calculated from mineral stoichiometry and find that mineral stoichiometry calculations do not accurately reflect the measured Fe3+/ΣFe ratios. We have identified three groups of samples. The majority preserve Fe3+/ΣFe ratios which are thought to be magmatic, whereas a few samples are highly oxidized and have high Fe3+/ΣFe ratios. There is also a group of partially oxidized samples. The oxidized chromites show anomalously low cell edge ( a _0) values and their oxygen positional parameters among the lowest ever found for chromites. Site occupancy calculations show that some chromites are non-stoichiometric and contain vacancies in their structure randomly distributed between both the T and M sites. The field relationships suggest that the oxidation of the magmatic chromitites took place in association with a ductile shear zone in mantle harzburgites. Primary magmatic Fe3+/ΣFe ratios measured for the Oman mantle chromitites are between 0.193–0.285 (X-ray data) and 0.164–0.270 (Mössbauer data) and preserve a range of Fe3+/ΣFe ratios which we propose is real and reflects differences in the composition of the magmas parental to the chromitites. The range of values extends from those MORB melts (0.16 ± 0.1) to those for arc basalts (0.22–0.28). ©2014 Springer-Verlag Berlin Heidelberg
    Print ISSN: 0010-7999
    Electronic ISSN: 1432-0967
    Topics: Geosciences
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  • 6
    Publication Date: 2011-12-01
    Description: Thirteen Cr-bearing spinels from major horizons of magma replenishment in the open-system Rum Layered Suite have been analysed by X-ray single crystal diffraction and electron microprobe analyses. On the basis of the structural parameters and the chemistry of these spinels the so-called Rum trend, in which Al-content increases at the expense of Cr and Fe3+, has been easily recognised. In addition, natural spinels with Fe3+ content similar to synthetic spinels on the MgCr_2O_4–MgFe_2O_4 join have been analysed for the first time. Layers of chromitite, anorthosite and peridotite situated within several cm of one another have yielded different intracrystalline exchange temperatures using an intercrystalline spinel-olivine thermometer. The Rum anorthosite Cr-spinels are interpreted as having crystallised within the cumulus pile following rejuvenation of the crystal mush. Their low Al-content is a function of simultaneous plagioclase crystallisation, reducing the amount of Al3+ present for the Cr-spinel. By contrast, Cr-spinels in well-known Archean anorthosites (e.g. Ujaragssuit nunât and Fiskenæsset, western Greenland) and Sittampundi (southern India) are very aluminous in composition, attributed to crystallisation of Cr-spinel from high-alumina basalts in lower crustal magma chambers and linked to the control exerted by plagioclase crystallisation on Al content of the melt, in the absence of clinopyroxene crystallisation. The compositional differences between the Rum anorthosite Cr-spinels and the Fiskenæsset and Sittampundi Cr-spinels suggest that postcumulus reaction of Cr-spinel and melt to low (800–900°C) temperatures, as invoked for the Rum crystals, may not have been as important a process in the Archean anorthosites. ©2011 Springer-Verlag
    Print ISSN: 0010-7999
    Electronic ISSN: 1432-0967
    Topics: Geosciences
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  • 7
    Publication Date: 2009-03-01
    Description: The Šambron–Kamenica Zone is situated on the northern margin of the Levočské vrchy mountains and Šarišskà vrchovina Highland, where the Central Carpathian Paleogene joins the Pieniny Klippen Belt. Sandstone outcrops in this area. From Cretaceous to Late Oligocene in age, these sediments suggest transport directions from S and SE. The heavy mineral assemblages of this sandstone include Cr-spinel grains, mainly displaying types II and III alpine-peridotite affinities, and are representative of Ocean Island Basalt volcanism. A sample from Upper Eocene sediments at Vit’az shows a clear change in Cr-spinel composition, which turns out to have types I and II peridotite affinities, and to derive from arc and Middle Ocean Ridge Basalt volcanism, with sediment transport directions from SW and WSW. These data indicate major variations in the Upper Eocene tectonic setting, giving constraints to paleogeographic reconstruction of the Slovak Central Carpathians. ©2007 Springer-Verlag
    Print ISSN: 1437-3254
    Electronic ISSN: 1437-3262
    Topics: Geosciences
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  • 8
    Publication Date: 2000-10-25
    Description: Detrital spinel is a widespread heavy mineral in sandstones from the Maastrichtian–Middle Eocene sedimentary basins in the SE Alps. Chemistry of detrital spinels from the Claut/Clauzetto and Julian Basins (N Italy and NW Slovenia) is used to constrain petrological and geochemical affinities and tectonic provenance of the source rocks. In addition, we have analysed melt inclusion compositions in the detrital volcanic spinels to better constrain the nature of their parental magmas. This is the first study of melt inclusions in detrital spinels. Two principal compositional groups of detrital spinels are recognised based on their TiO_2 and Fe2+/Fe3+; one derived from peridotites, the other from basaltic volcanics. Peridotitic spinels are more abundant and have TiO_2 〈 0.2 wt% and high Cr/Cr + Al (40–90), characteristic of suprasubduction zone harzburgites. Significant chemical variations among volcanic spinels (TiO_2 up to 3 wt%, Al_2O_3 12–44 wt%) suggest multiple sources, with geochemically distinct characteristics, including MORB-type and backarc basin basalts, subduction-related magmas and tholeiites produced during early continental rifting. Compositions of homogenised melt inclusions in spinels with TiO_2 〉 0.2 better distinguish the differences between the compositions of their host spinels and help to further clarify the geodynamic provenance of extrusive source rocks. Several compositional groups of melt inclusions have been recognised and represent diverse magmatism of marginal basins, including MORB- and subduction-related geochemical types, as well as magmas characteristic of early continental rifting. These results, combined with the data on regional ophiolitic complexes and tectonic reconstructions favour the Internal Dinarides of Yugoslavia as a possible source area for the SE Alps sediments. ©2000 Springer-Verlag Berlin Heidelberg
    Print ISSN: 0010-7999
    Electronic ISSN: 1432-0967
    Topics: Geosciences
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  • 9
    Publication Date: 2013-10-01
    Description: Oxygen fugacity ( $$ f{{{ ext{O}}{ 2} }} $$ f O 2 ) is a fundamental but little known intensive variable in mantle processes. It influences the P/T position of a mantle solidus and the composition of mantle-derived melts and fluids and constrains mantle-core equilibria and a number of geophysical properties of the mantle. An important source of information on oxidation states is the ferric–ferrous iron ratio in mantle spinels. Since the magnetite component is low in mantle spinels, normal analytical errors translate into considerable $$ f{{{ ext{O}}{ 2} }} $$ f O 2 uncertainties. In this study, we analyzed the Fe3+–Fetot ratio of chromites present as inclusions in diamond and other mantle-related occurrences by point-source Mössbauer spectroscopy using single-crystal absorbers as well as conventional Mössbauer spectroscopy using powder absorbers. The studied spinels have been previously analyzed by single-crystal X-ray diffraction and electron microprobe. The ferric–ferrous ratios found are normally similar to the different techniques apart from some samples where a large number of grains have been used for the analyses (powder absorbers). The general agreement between the different techniques allows us to conclude that the studied chromites are stoichiometric. However, conventional Mössbauer spectroscopy on powder absorbers should be conducted with great care, since the method requires a relatively large amount of sample material. Spinel frequently occurs as small grains, and the large number of crystals required may possess different degrees of oxidation/alteration and, consequently, different ferric–ferrous ratio leading to possible errors in the interpretation of the results. ©2013 Springer-Verlag Berlin Heidelberg
    Print ISSN: 0342-1791
    Electronic ISSN: 1432-2021
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 10
    Publication Date: 2006-10-01
    Description: The influence on the spinel structure of Fe3+ → Cr substitution was studied in flux-grown synthetic single crystals of the magnesiochromite–magnesioferrite (MgCr2O4–MgFe2O4) solid solution series. Samples were analysed by single-crystal X-ray diffraction, electron microprobe analyses, optical absorption and Mössbauer spectroscopy. With the exception of iron-poor samples (3–12 mol-% MgFe2O4), optical absorption and Mössbauer spectra show that iron occurs almost exclusively as trivalent Fe in the present samples. A very intense and broad absorption band at ca 7,800 cm−1 dominates the optical absorption spectra of samples with higher Fe-contents. The appearance of this band is related to a distinct structural disorder of Fe3+ and a development of magnetic ordering as demonstrated by Mössbauer spectra. Profound composition-related changes are observed in the Mössbauer spectra, which are magnetically unsplit in the range 2–41 mol-% magnesioferrite, but become magnetically split in the range 59–100 mol-% magnesioferrite. Structural parameters a 0 and M–O increase with magnesioferrite content and inversion degree, while u and T–O decrease. Our study confirms the previously reported (Lavina et al. 2002 ) influence of Fe3+ at the M site on T–O bond lengths in the spinel structure. ©2006 Springer-Verlag
    Print ISSN: 0342-1791
    Electronic ISSN: 1432-2021
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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