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- Alasdair, Skelton, et al.
(author)
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Spatial coupling between spilitization and carbonation ofbasaltic sills in SW Scottish Highlands: evidence of amineralogical control of metamorphic fluid flow
- 2011
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In: Geofluids. - : Wiley. - 1468-8115 .- 1468-8123. ; 11:3, s. 245-259
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Journal article (peer-reviewed)abstract
- In a geochemical and petrological analysis of overprinting episodes of fluid–rock interaction in a well-studied metabasaltic sill in the SW Scottish Highlands, we show that syn-deformational access of metamorphic fluids and consequent fluid–rock interaction is at least in part controlled by preexisting mineralogical variations. Lithological and structural channelling of metamorphic fluids along the axis of the Ardrishaig Anticline, SW Scottish Highlands, caused carbonation of metabasaltic sills hosted by metasedimentary rocks of the Argyll Group in the Dalradian Supergroup. Analysis of chemical and mineralogical variability across a metabasaltic sill at Port Cill Maluaig shows that carbonation at greenschist to epidote–amphibolites facies conditions caused by infiltration of H2O-CO2 fluids was controlled by mineralogical variations, which were present before carbonation occurred. This variability probably reflects chemical and mineralogical changes imparted on the sill during premetamorphic spilitization. Calculation of precarbonation mineral modes reveals heterogeneous spatial distributions of epidote, amphibole, chlorite and epidote. This reflects both premetamorphic spilitization and prograde greenschist facies metamorphism prior to fluid flow. Spilitization caused albitization of primary plagioclase and spatially heterogeneous growth of epidote ± calcic amphibole ± chlorite ± quartz ± calcite. Greenschist facies metamorphism caused breakdown of primary pyroxene and continued, but spatially more homogeneous, growth of amphibole + chlorite ± quartz. These processes formed diffuse epidote-rich patches or semi-continuous layers. These might represent precursors of epidote segregations, which are better developed elsewhere in the SW Scottish Highlands. Chemical and field analyses of epidote reveal the evidence of local volume fluctuations associated with these concentrations of epidote. Transient permeability enhancement associated with these changes may have permitted higher fluid fluxes and therefore more extensive carbonation. This deflected metamorphic fluid such that its flow direction became more layer parallel, limiting propagation of the reaction front into the sill interior.
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| 2. |
- Skelton, Alasdair
(author)
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Flux rates for water and carbon during greenschist facies metamorphism
- 2011
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In: Geology. - 0091-7613 .- 1943-2682. ; 39:1, s. 43-46
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Journal article (peer-reviewed)abstract
- The time-averaged flux rate for a CO2-bearing hydrous fluid during greenschist facies regional metamorphism was estimated to be 10–10.2 ± 0.4 m3 m−2 s−1 by combining (1) Peclet numbers obtained by chromatographic analysis of the propagation of reaction fronts in 33 metamorphosed basaltic sills in the southwest Scottish Highlands (UK), (2) empirical diffusion rates for CO2 in water, and (3) calculated time-averaged metamorphic porosities. The latter were calculated using an expression obtained by combining estimated Peclet numbers with empirical porosity-permeability relationships and Darcy's law. This approach yielded a time-averaged metamorphic porosity of 10–2.6 ± 0.2 for greenschist facies conditions. The corresponding time scale for metamorphic fluid flow was 103.6 ± 0.1 yr. By using mineral assemblages to constrain fluid compositions, a time-averaged annual flux rate for carbon of 0.5–7 mol C m−2 yr−1 was calculated. This matches measured emission rates for metamorphic CO2 from orogenic hot springs and exceeds estimated rates of CO2 drawdown by orogenic silicate weathering, suggesting that orogenesis is a source rather than a sink of atmospheric CO2.
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