| 1. |
- Bernardi, Francesco, et al.
(författare)
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OH-Defects in Detrital Quartz Grains from the Julian Basin (NE Italy and Slovenia): A Fourier Transform Infrared Study
- 2022
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Ingår i: Geosciences. - : MDPI AG. - 2076-3263. ; 12
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we analyzed up to 80 detrital quartz grains from four lithic greywackes along the stratigraphic column of the Julian Basin, a synorogenic basin in the southeastern Alps between Italy and Slovenia. Fourier transform infrared spectroscopy of detrital quartz was used to investigate the sample set with interest to its OH-defect speciation and content of each associated substitution. According to several recent studies, OH-defects in quartz are correlated to petrogenetic conditions of the source material and can be used as a provenance tool. The aim of this study is to compare results based on this method with previous studies that used other methods, to better constrain the palaeogeographical reconstruction of sedimentary fluxes. Detrital quartz within the samples of the basin shows different patterns of OH-defects and water content, indicating substantial petrogenetic differences between the sediment source rocks. For the oldest analyzed sample (ca. 66 Ma), the distribution of OH-defects suggests a mixed source between igneous and non-igneous rocks, with a predominance of metamorphic material supply. Another sample (56 Ma) reveals a great variability of OH-defects and water content, indicating that the magmatic component dominates over the metamorphic component. The distribution of OH-defects in the samples at the top of the sequence (52–53 Ma) suggests an almost solely metamorphic source. These results are in line with previous studies based on heavy minerals and geochemistry.
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| 2. |
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| 3. |
- Lenaz, Davide, et al.
(författare)
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Crystal structure refinement of chromites from two achondrites, their T-f(O2) conditions, and implications
- 2017
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Ingår i: Meteoritics and Planetary Science. - : Wiley. - 1086-9379 .- 1945-5100. ; 52:9, s. 1763-1775
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Tidskriftsartikel (refereegranskat)abstract
- Six Cr-spinel grains from NWA 6077 brachinite-like and NWA 725 winonaite achondrites have been studied by single-crystal X-ray diffraction and structural refinement. From a chemical point of view, spinels from NWA 6077 show Cr/(Cr + Al) (i.e., Cr#) and Mg/(Mg + Fe2+) (i.e., Mg#) values similar to other brachinites, while the Cr# of NWA 725 is lower than that of literature winonaites. Spinels from NWA 6077 and NWA 725 meteorites show similar cell edges, while the oxygen positional parameter is rather different being about 0.2629 for NWA 6077 and 0.2622 for NWA 725. Considering both parameters, NWA 725 shows structural features that are close to some terrestrial spinel occurrences as in komatiites, kimberlites, or included in diamonds; those from NWA 6077 show values that have no terrestrial analogs. Olivine-chromite closure temperature ranges from ~737 to ~765° C for NWA 725, being similar to that of literature winonaites and ~846 to ~884° C for NWA 6077. The logfO2 ranges from -19.8 to -20.5 and -17.0 to -17.9 for the two meteorites, respectively. The u values for terrestrial samples can give information about the cooling history of the samples. For the extraterrestrial samples, it seems that it can give information about the cooling only for spinels where it is lower than 0.2625. For higher values, it appears related only to the chemistry of the spinels.
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| 4. |
- Lenaz, Davide, et al.
(författare)
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First crystal-structure determination of chromites from an acapulcoite and ordinary chondrites
- 2015
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Ingår i: Mineralogical Magazine. - : Mineralogical Society. - 0026-461X .- 1471-8022. ; 79:3, s. 755-765
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Tidskriftsartikel (refereegranskat)abstract
- We report the first crystal-structure determinations of chromites from an acapulcoite and from ordinary chondrites. Cell edges range from 8.3212 (3) to 8.3501 (1) angstrom, while the oxygen positional parameters are in the range 0.2624 (3) to 0.26298 (9). Their compositions show they are very close to the chromite end-member FeCr2O4 with limited Al and Mg content. Titanium oxide content exceeds 1 wt.%, whereas the amount of Fe3+ is negligible. Extraterrestrial chromite is distinguished readily from terrestrial analogues on the basis of the cell edge and oxygen positional parameter. These distinctions will facilitate ongoing attempts to reconstruct the palaeoflux of meteorites to Earth from resistant extraterrestrial spinel grains recovered from ancient sediments.
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| 5. |
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| 6. |
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| 7. |
- Lenaz, Davide, et al.
(författare)
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Structural parameters of Cr-bearing spinels and pleonaste from the Cuillin Igneous Complex (Isle of Skye, Scotland): Implications for metamorphic and cooling history
- 2016
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Ingår i: Mineralogical magazine. - : Mineralogical Society. - 0026-461X .- 1471-8022. ; 80:5, s. 749-764
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Tidskriftsartikel (refereegranskat)abstract
- The Outer Layered Suite of the Cuillin Igneous Complex (Isle of Skye, NW Scotland) comprises a Peridotite Series and a younger Allivalite Series (the latter comprising troctolites, eucrites and gabbros). Close to the junction between the Peridotite and the Allivalite Series (but wholly contained within the latter), an ultramafic breccia unit containing abundant peridotite xenoliths crops out. In the Peridotite Series, reddish-brown Cr-bearing spinels are present as disseminated crystals in the peridotite and also as chromitite seams, while in the peridotite xenoliths of the breccia unit, green pleonaste occurs in both of these modes of textural occurrence. Optical absorption spectroscopy reveals that the colour difference between the two spinel phases is related mainly to variable Al, Cr and Fe contents, while crystal structural analysis shows that the cooling rate calculated utilizing the oxygen positional parameter is comparable for all samples. The intracrystalline closure temperature for the Cr-spinel in the Peridotite Series is different for the disseminated and seam textural occurrences of the spinels, while the temperatures yielded by pleonaste in the peridotite xenoliths are the same for both textural occurrences. Our dataset suggests that the pleonaste in the peridotite xenoliths has been heated and equilibrated under subsolidus conditions, probably during breccia formation. During this heating, homogenization of the closure temperatures of pleonaste spinels occurred.
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| 8. |
- Lenaz, Davide, et al.
(författare)
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Terrestrial Cr-spinels in the Maiolica limestone : Where are they from?
- 2019
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Ingår i: 250 Million Years of Earth History in Central Italy: Celebrating 25 Years of the Geological Observatory of Coldigioco. - : Geological Society of America. - 0072-1077. ; 542, s. 121-131
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Bokkapitel (refereegranskat)abstract
- In total, 33 and 65 chrome-spinel (Cr-spinel) grains in the >63 and 32–63 µm size fractions, respectively, were recovered from 12 beds in two stratigraphically separated groups along the 240-m-thick Monte Acuto section of the Maiolica limestone in central Italy, spanning from the Berriasian to the early Hauterivian. The chemistry of these detrital spinels suggests they may represent windblown ophiolitic detritus, showing the evolution of an ophiolite evolving from a mid-ocean-ridge basalt setting (Bosso section) to a suprasubduction-zone setting, including a backarc and an arc setting (Monte Acuto section). The source of the Maiolica detrital minerals may have been the obducting ophiolites of the Albanides and/or the Hellenides, which show a similar evolution. In this case, it is particularly important to note how the Cr-spinel detritus in the Maiolica limestone records this evolution over a relatively short period of time, lasting ~10 m.y.
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| 9. |
- Lenaz, Davide, et al.
(författare)
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The Effects of High-Grade Metamorphism on Cr-Spinel from the Archean Sittampundi Complex, South India
- 2021
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Ingår i: Minerals. - : MDPI AG. - 2075-163X. ; 11:12, s. 1-13
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the crystal and structural behavior of Cr-bearing spinels from the Archean chromitites of Sittampundi (India), which had been subjected to very high-grade metamorphism. The structural data show that their oxygen positional parameters are among the highest ever recorded for Cr-bearing spinels with similar Cr# and Mg# and very similar to those found for other Archean occurrences. The general agreement between electron microprobe and Mössbauer data indicates that the analyzed spinels are stoichiometric. It is therefore most likely that the PH2O and Ptotal values as well as both the oxygen fugacity and the temperature reached during high-grade metamorphism inhibited the possibility of the non-stoichiometry of chromites, contrary to what can happen in ophiolites, where non-stoichiometry has recently been documented.
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| 10. |
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