Serpentinite dehydration at low pressures
Abstract Petrographic observations combined with mineral compositional analyses constrain the phase relations of prograde metamorphosed serpentinites in the Bergell contact aureole (Italy). In a 1500 m profile perpendicular to the north-eastern edge of the Bergell intrusion, seven dehydration reacti...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
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SpringerOpen
2022-04-01
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| Series: | Swiss Journal of Geosciences |
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| Online Access: | https://doi.org/10.1186/s00015-022-00415-y |
| _version_ | 1818274569921757184 |
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| author | Elias D. Kempf Jörg Hermann James A. D. Connolly |
| author_facet | Elias D. Kempf Jörg Hermann James A. D. Connolly |
| author_sort | Elias D. Kempf |
| collection | DOAJ |
| description | Abstract Petrographic observations combined with mineral compositional analyses constrain the phase relations of prograde metamorphosed serpentinites in the Bergell contact aureole (Italy). In a 1500 m profile perpendicular to the north-eastern edge of the Bergell intrusion, seven dehydration reactions ran to completion. Three previously undocumented reactions have been identified within 70 m of the intrusive contact: olivine + anthophyllite = orthopyroxene + H2O, tremolite + Cr–Al-spinel = olivine + Mg-hornblende + H2O and chlorite = olivine + orthopyroxene + Cr-Al-spinel + H2O. Petrological analysis indicates that these reactions occur over a narrow range of pressure and temperature, 300 ± 30 MPa and 720 ± 10 °C respectively. Computed phase diagram sections reproduce the observed mineral parageneses with one notable exception. Due to the underestimation of aluminium and sodium contents in Ca-amphibole models, plagioclase is predicted above 700 °C instead of Mg-hornblende. In comparison with natural grains, the aluminium content of computed chlorite compositions is overestimated for low grade parageneses while it is underestimated near the upper thermal stability limit of chlorite. In the computed sections, Fe partitioning relative to Mg between olivine and other silicates, suggests a clear preference for Fe in olivine, that therefore shows lower Mg#s. In contrast, microprobe analyses of natural mineral pairs indicate that orthopyroxene, Mg-hornblende and anthophyllite have lower Mg#s than equilibrium olivine. The inferred thermal profile of the metamorphic aureole is not consistent with simple heat conduction models and indicates a contact temperature of ~ 800 °C, which is 120–230 °C higher than previously estimated. Petrography also reveals extensive retrograde overprint of the prograde parageneses within 200 m of the contact. Retrogression is related to metamorphic fluids that were released by dehydration reactions during contact metamorphism and magmatic fluids expelled from the tonalite intrusion. The thermal gradient between the intrusion and the country rocks induced hydrothermal circulation of these fluids throughout the contact aureole, which beyond peak metamorphic conditions caused retrograde overprint of the prograde parageneses. The proposed phase relations for low and high pressures, and in particular, the transition from tremolite to Mg-hornblende, provides a complete representation of hydration and dehydration processes in serpentinites in subduction zones, along deep oceanic transform faults, and at passive continental margins. The latter has new implications, specifically for subduction initiation. |
| first_indexed | 2024-12-12T22:15:57Z |
| format | Article |
| id | doaj.art-9ac833a804004e1d89925ef4d7179ceb |
| institution | Directory Open Access Journal |
| issn | 1661-8726 1661-8734 |
| language | English |
| last_indexed | 2024-12-12T22:15:57Z |
| publishDate | 2022-04-01 |
| publisher | SpringerOpen |
| record_format | Article |
| series | Swiss Journal of Geosciences |
| spelling | doaj.art-9ac833a804004e1d89925ef4d7179ceb2022-12-22T00:10:06ZengSpringerOpenSwiss Journal of Geosciences1661-87261661-87342022-04-01115113010.1186/s00015-022-00415-ySerpentinite dehydration at low pressuresElias D. Kempf0Jörg Hermann1James A. D. Connolly2Institute of Geological Sciences, University of BernInstitute of Geological Sciences, University of BernInstitute of Mineralogy and Petrology, ETH ZurichAbstract Petrographic observations combined with mineral compositional analyses constrain the phase relations of prograde metamorphosed serpentinites in the Bergell contact aureole (Italy). In a 1500 m profile perpendicular to the north-eastern edge of the Bergell intrusion, seven dehydration reactions ran to completion. Three previously undocumented reactions have been identified within 70 m of the intrusive contact: olivine + anthophyllite = orthopyroxene + H2O, tremolite + Cr–Al-spinel = olivine + Mg-hornblende + H2O and chlorite = olivine + orthopyroxene + Cr-Al-spinel + H2O. Petrological analysis indicates that these reactions occur over a narrow range of pressure and temperature, 300 ± 30 MPa and 720 ± 10 °C respectively. Computed phase diagram sections reproduce the observed mineral parageneses with one notable exception. Due to the underestimation of aluminium and sodium contents in Ca-amphibole models, plagioclase is predicted above 700 °C instead of Mg-hornblende. In comparison with natural grains, the aluminium content of computed chlorite compositions is overestimated for low grade parageneses while it is underestimated near the upper thermal stability limit of chlorite. In the computed sections, Fe partitioning relative to Mg between olivine and other silicates, suggests a clear preference for Fe in olivine, that therefore shows lower Mg#s. In contrast, microprobe analyses of natural mineral pairs indicate that orthopyroxene, Mg-hornblende and anthophyllite have lower Mg#s than equilibrium olivine. The inferred thermal profile of the metamorphic aureole is not consistent with simple heat conduction models and indicates a contact temperature of ~ 800 °C, which is 120–230 °C higher than previously estimated. Petrography also reveals extensive retrograde overprint of the prograde parageneses within 200 m of the contact. Retrogression is related to metamorphic fluids that were released by dehydration reactions during contact metamorphism and magmatic fluids expelled from the tonalite intrusion. The thermal gradient between the intrusion and the country rocks induced hydrothermal circulation of these fluids throughout the contact aureole, which beyond peak metamorphic conditions caused retrograde overprint of the prograde parageneses. The proposed phase relations for low and high pressures, and in particular, the transition from tremolite to Mg-hornblende, provides a complete representation of hydration and dehydration processes in serpentinites in subduction zones, along deep oceanic transform faults, and at passive continental margins. The latter has new implications, specifically for subduction initiation.https://doi.org/10.1186/s00015-022-00415-yContact metamorphismSerpentinitesFe–Mg partitioningMg-hornblendeChloriteSpinel |
| spellingShingle | Elias D. Kempf Jörg Hermann James A. D. Connolly Serpentinite dehydration at low pressures Swiss Journal of Geosciences Contact metamorphism Serpentinites Fe–Mg partitioning Mg-hornblende Chlorite Spinel |
| title | Serpentinite dehydration at low pressures |
| title_full | Serpentinite dehydration at low pressures |
| title_fullStr | Serpentinite dehydration at low pressures |
| title_full_unstemmed | Serpentinite dehydration at low pressures |
| title_short | Serpentinite dehydration at low pressures |
| title_sort | serpentinite dehydration at low pressures |
| topic | Contact metamorphism Serpentinites Fe–Mg partitioning Mg-hornblende Chlorite Spinel |
| url | https://doi.org/10.1186/s00015-022-00415-y |
| work_keys_str_mv | AT eliasdkempf serpentinitedehydrationatlowpressures AT jorghermann serpentinitedehydrationatlowpressures AT jamesadconnolly serpentinitedehydrationatlowpressures |