Closed system behaviour of argon in osumilite records protracted high-T metamorphism within the Rogaland–Vest Agder Sector, Norway
© 2019 John Wiley & Sons Ltd Here, we present results of the first 40Ar/39Ar dating of osumilite, a high-T mineral that occurs in some volcanic and high-grade metamorphic rocks. The metamorphic osumilite studied here is from a metapelitic rock within the Rogaland–Vest Agder Sector, Norway,...
| Main Authors: | , , , , , , , , |
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| Format: | Journal Article |
| Language: | English |
| Published: |
WILEY
2019
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| Online Access: | http://purl.org/au-research/grants/arc/DE120103067 http://hdl.handle.net/20.500.11937/80263 |
| _version_ | 1848764192437305344 |
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| author | Blereau, Eleanor Clark, Chris Jourdan, Fred Johnson, Tim Taylor, Richard Kinny, Pete Danišík, Martin Hand, M. Eroglu, Ela |
| author_facet | Blereau, Eleanor Clark, Chris Jourdan, Fred Johnson, Tim Taylor, Richard Kinny, Pete Danišík, Martin Hand, M. Eroglu, Ela |
| author_sort | Blereau, Eleanor |
| building | Curtin Institutional Repository |
| collection | Online Access |
| description | © 2019 John Wiley & Sons Ltd
Here, we present results of the first 40Ar/39Ar dating of osumilite, a high-T mineral that occurs in some volcanic and high-grade metamorphic rocks. The metamorphic osumilite studied here is from a metapelitic rock within the Rogaland–Vest Agder Sector, Norway, an area that experienced regional granulite facies metamorphism and subsequent contact metamorphism between 1,100 Ma and 850 Ma. The large grain size (~1 cm) of osumilite in the studied rock, which preserves a nominally anhydrous assemblage, increases the potential for large portions of individual grains to have remained essentially unaffected by the effects of diffusive argon loss, potentially preserving prograde ages. Step-heating diffusion experiments yielded a maximum activation energy of ~461 kJ/mol and a pre-exponential factor of ~8.34 × 108 cm2/s for Ar diffusion in osumilite. These parameters correspond to a relatively high closure temperature of ~620°C for a cooling rate of 10°C/Ma in an osumilite crystal with a 175 µm radius. Fragments of osumilite separated from the sample preserve a range of ages between c. 1,070 and 860 Ma. The oldest ages are inferred to date the growth of coarse-grained osumilite during prograde granulite facies regional metamorphism, which pre-date contact metamorphism that has historically been ascribed to the growth of osumilite in the region. The majority of fragments record ages between c. 920 and 860 Ma, inferred to reflect the growth of osumilite and/or diffusive argon loss during contact metamorphism. The retention of old 40Ar/39Ar dates was facilitated by the low diffusivity of Ar in osumilite (i.e. a closed system), large grain sizes, and anhydrous metamorphic conditions. The ability to date osumilite with the 40Ar/39Ar method provides a valuable new thermochronometer that may constrain the timing and duration of high-T magmatic and metamorphic events. |
| first_indexed | 2025-11-14T11:15:27Z |
| format | Journal Article |
| id | curtin-20.500.11937-80263 |
| institution | Curtin University Malaysia |
| institution_category | Local University |
| language | English |
| last_indexed | 2025-11-14T11:15:27Z |
| publishDate | 2019 |
| publisher | WILEY |
| recordtype | eprints |
| repository_type | Digital Repository |
| spelling | curtin-20.500.11937-802632021-01-19T02:12:52Z Closed system behaviour of argon in osumilite records protracted high-T metamorphism within the Rogaland–Vest Agder Sector, Norway Blereau, Eleanor Clark, Chris Jourdan, Fred Johnson, Tim Taylor, Richard Kinny, Pete Danišík, Martin Hand, M. Eroglu, Ela Science & Technology Physical Sciences Geology Ar-40 Ar-39 thermochronology closure temperature high-T metamorphism osumilite Rogaland ULTRAHIGH-TEMPERATURE METAMORPHISM MINERAL EQUILIBRIA CALCULATIONS CANYON SANIDINE STANDARD K-40 DECAY CONSTANTS AR-40/AR-39 GEOCHRONOLOGY JOINT DETERMINATION IMPROVED ACCURACY K-AR GRANULITES PHASES © 2019 John Wiley & Sons Ltd Here, we present results of the first 40Ar/39Ar dating of osumilite, a high-T mineral that occurs in some volcanic and high-grade metamorphic rocks. The metamorphic osumilite studied here is from a metapelitic rock within the Rogaland–Vest Agder Sector, Norway, an area that experienced regional granulite facies metamorphism and subsequent contact metamorphism between 1,100 Ma and 850 Ma. The large grain size (~1 cm) of osumilite in the studied rock, which preserves a nominally anhydrous assemblage, increases the potential for large portions of individual grains to have remained essentially unaffected by the effects of diffusive argon loss, potentially preserving prograde ages. Step-heating diffusion experiments yielded a maximum activation energy of ~461 kJ/mol and a pre-exponential factor of ~8.34 × 108 cm2/s for Ar diffusion in osumilite. These parameters correspond to a relatively high closure temperature of ~620°C for a cooling rate of 10°C/Ma in an osumilite crystal with a 175 µm radius. Fragments of osumilite separated from the sample preserve a range of ages between c. 1,070 and 860 Ma. The oldest ages are inferred to date the growth of coarse-grained osumilite during prograde granulite facies regional metamorphism, which pre-date contact metamorphism that has historically been ascribed to the growth of osumilite in the region. The majority of fragments record ages between c. 920 and 860 Ma, inferred to reflect the growth of osumilite and/or diffusive argon loss during contact metamorphism. The retention of old 40Ar/39Ar dates was facilitated by the low diffusivity of Ar in osumilite (i.e. a closed system), large grain sizes, and anhydrous metamorphic conditions. The ability to date osumilite with the 40Ar/39Ar method provides a valuable new thermochronometer that may constrain the timing and duration of high-T magmatic and metamorphic events. 2019 Journal Article http://hdl.handle.net/20.500.11937/80263 10.1111/jmg.12480 English http://purl.org/au-research/grants/arc/DE120103067 WILEY restricted |
| spellingShingle | Science & Technology Physical Sciences Geology Ar-40 Ar-39 thermochronology closure temperature high-T metamorphism osumilite Rogaland ULTRAHIGH-TEMPERATURE METAMORPHISM MINERAL EQUILIBRIA CALCULATIONS CANYON SANIDINE STANDARD K-40 DECAY CONSTANTS AR-40/AR-39 GEOCHRONOLOGY JOINT DETERMINATION IMPROVED ACCURACY K-AR GRANULITES PHASES Blereau, Eleanor Clark, Chris Jourdan, Fred Johnson, Tim Taylor, Richard Kinny, Pete Danišík, Martin Hand, M. Eroglu, Ela Closed system behaviour of argon in osumilite records protracted high-T metamorphism within the Rogaland–Vest Agder Sector, Norway |
| title | Closed system behaviour of argon in osumilite records protracted high-T metamorphism within the Rogaland–Vest Agder Sector, Norway |
| title_full | Closed system behaviour of argon in osumilite records protracted high-T metamorphism within the Rogaland–Vest Agder Sector, Norway |
| title_fullStr | Closed system behaviour of argon in osumilite records protracted high-T metamorphism within the Rogaland–Vest Agder Sector, Norway |
| title_full_unstemmed | Closed system behaviour of argon in osumilite records protracted high-T metamorphism within the Rogaland–Vest Agder Sector, Norway |
| title_short | Closed system behaviour of argon in osumilite records protracted high-T metamorphism within the Rogaland–Vest Agder Sector, Norway |
| title_sort | closed system behaviour of argon in osumilite records protracted high-t metamorphism within the rogaland–vest agder sector, norway |
| topic | Science & Technology Physical Sciences Geology Ar-40 Ar-39 thermochronology closure temperature high-T metamorphism osumilite Rogaland ULTRAHIGH-TEMPERATURE METAMORPHISM MINERAL EQUILIBRIA CALCULATIONS CANYON SANIDINE STANDARD K-40 DECAY CONSTANTS AR-40/AR-39 GEOCHRONOLOGY JOINT DETERMINATION IMPROVED ACCURACY K-AR GRANULITES PHASES |
| url | http://purl.org/au-research/grants/arc/DE120103067 http://hdl.handle.net/20.500.11937/80263 |