Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway)
Graphite formation in the deep crust during granulite facies metamorphism is documented in the Proterozoic gneisses of the Lofoten–Vesterålen Complex, northern Norway. Graphite schist is hosted in banded gneisses dominated by orthopyroxene-bearing quartzofeldspathic gneiss, including marble, calcsil...
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MDPI AG
2023-09-01
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| author | Ane K. Engvik Håvard Gautneb Pål Tore Mørkved Janja Knežević Solberg Muriel Erambert |
| author_facet | Ane K. Engvik Håvard Gautneb Pål Tore Mørkved Janja Knežević Solberg Muriel Erambert |
| author_sort | Ane K. Engvik |
| collection | DOAJ |
| description | Graphite formation in the deep crust during granulite facies metamorphism is documented in the Proterozoic gneisses of the Lofoten–Vesterålen Complex, northern Norway. Graphite schist is hosted in banded gneisses dominated by orthopyroxene-bearing quartzofeldspathic gneiss, including marble, calcsilicate rocks and amphibolite. The schist has major graphite (<modality 39%), quartz, plagioclase, pyroxenes, biotite (Mg# = 0.67–0.91; Ti < 0.66 a.p.f.u.) and K-feldspar/perthite. Pyroxene is orthopyroxene (En<sub>69–74</sub>) and/or clinopyroxene (En<sub>33–53</sub>Fs<sub>1–14</sub>Wo<sub>44–53</sub>); graphite occurs in assemblage with metamorphic orthopyroxene. Phase diagram modelling (plagioclase + orthopyroxene (Mg#-ratio = 0.74) + biotite + quartz + rutile + ilmenite + graphite-assemblage) constrains pressure-temperature conditions of 810–835 °C and 0.73–0.77 GPa; Zr-in-rutile thermometry 726–854 °C. COH fluids stabilise graphite and orthopyroxene; the high Mg#-ratio of biotite and pyroxenes, and apatite Cl < 2 a.p.f.u., indicate the importance of fluids during metamorphism. Stable isotopic <i>δ</i><sup>13</sup>C<sub>graphite</sub> in the graphite schist is −38 to −17‰; <i>δ</i><sup>13</sup>C<sub>calcite</sub> of marbles +3‰ to +10‰. Samples with both graphite and calcite present give lighter values for <i>δ</i><sup>13</sup>C<sub>calcite</sub> = −8.7‰ to −9.5‰ and heavier values for <i>δ</i><sup>13</sup>C<sub>graphite</sub> = −11.5‰ to −8.9‰. <i>δ</i><sup>18</sup>O<sub>calcite</sub> for marble shows lighter values, ranging from −15.4‰ to −7.5‰. We interpret the graphite origin as organic carbon accumulated in sediments, while isotopic exchange between graphite and calcite reflects metamorphic and hydrothermal re-equilibration. |
| first_indexed | 2024-03-10T21:02:03Z |
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| last_indexed | 2024-03-10T21:02:03Z |
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| spelling | doaj.art-de460379b168461b8abb4e17b6ccdef52023-11-19T17:30:08ZengMDPI AGMinerals2075-163X2023-09-011310127910.3390/min13101279Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway)Ane K. Engvik0Håvard Gautneb1Pål Tore Mørkved2Janja Knežević Solberg3Muriel Erambert4Geological Survey of Norway, P.O. Box 6315 Torgarden, N-7491 Trondheim, NorwayGeological Survey of Norway, P.O. Box 6315 Torgarden, N-7491 Trondheim, NorwayDepartment of Earth Science, University of Bergen, P.O. Box 7803, N-5020 Bergen, NorwayGeological Survey of Norway, P.O. Box 6315 Torgarden, N-7491 Trondheim, NorwayDepartment of Geosciences, University of Oslo, P.O. Box 1047, N-0316 Oslo, NorwayGraphite formation in the deep crust during granulite facies metamorphism is documented in the Proterozoic gneisses of the Lofoten–Vesterålen Complex, northern Norway. Graphite schist is hosted in banded gneisses dominated by orthopyroxene-bearing quartzofeldspathic gneiss, including marble, calcsilicate rocks and amphibolite. The schist has major graphite (<modality 39%), quartz, plagioclase, pyroxenes, biotite (Mg# = 0.67–0.91; Ti < 0.66 a.p.f.u.) and K-feldspar/perthite. Pyroxene is orthopyroxene (En<sub>69–74</sub>) and/or clinopyroxene (En<sub>33–53</sub>Fs<sub>1–14</sub>Wo<sub>44–53</sub>); graphite occurs in assemblage with metamorphic orthopyroxene. Phase diagram modelling (plagioclase + orthopyroxene (Mg#-ratio = 0.74) + biotite + quartz + rutile + ilmenite + graphite-assemblage) constrains pressure-temperature conditions of 810–835 °C and 0.73–0.77 GPa; Zr-in-rutile thermometry 726–854 °C. COH fluids stabilise graphite and orthopyroxene; the high Mg#-ratio of biotite and pyroxenes, and apatite Cl < 2 a.p.f.u., indicate the importance of fluids during metamorphism. Stable isotopic <i>δ</i><sup>13</sup>C<sub>graphite</sub> in the graphite schist is −38 to −17‰; <i>δ</i><sup>13</sup>C<sub>calcite</sub> of marbles +3‰ to +10‰. Samples with both graphite and calcite present give lighter values for <i>δ</i><sup>13</sup>C<sub>calcite</sub> = −8.7‰ to −9.5‰ and heavier values for <i>δ</i><sup>13</sup>C<sub>graphite</sub> = −11.5‰ to −8.9‰. <i>δ</i><sup>18</sup>O<sub>calcite</sub> for marble shows lighter values, ranging from −15.4‰ to −7.5‰. We interpret the graphite origin as organic carbon accumulated in sediments, while isotopic exchange between graphite and calcite reflects metamorphic and hydrothermal re-equilibration.https://www.mdpi.com/2075-163X/13/10/1279C-isotopefluidgranulitegraphiteVesterålen |
| spellingShingle | Ane K. Engvik Håvard Gautneb Pål Tore Mørkved Janja Knežević Solberg Muriel Erambert Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway) Minerals C-isotope fluid granulite graphite Vesterålen |
| title | Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway) |
| title_full | Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway) |
| title_fullStr | Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway) |
| title_full_unstemmed | Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway) |
| title_short | Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway) |
| title_sort | proterozoic deep carbon characterisation origin and the role of fluids during high grade metamorphism of graphite lofoten vesteralen complex norway |
| topic | C-isotope fluid granulite graphite Vesterålen |
| url | https://www.mdpi.com/2075-163X/13/10/1279 |
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