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- Alasdair, Skelton, et al.
(författare)
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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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Ingår i: Geofluids. - : Wiley. - 1468-8115 .- 1468-8123. ; 11:3, s. 245-259
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Tidskriftsartikel (refereegranskat)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. |
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| 3. |
- Pitcairn, Iain K., et al.
(författare)
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Structurally focused fluid flow during orogenesis : the Islay Anticline, SW Highlands, Scotland
- 2010
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 167:4, s. 659-674
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Tidskriftsartikel (refereegranskat)abstract
- Displacement of isotopic compositions at boundary layers across strata of contrasting composition is commonly used to investigate hydrothermal fluid flow during orogeny. This study investigates whether hydrothermal fluid flow was focused along the Islay Anticline, Islay, SW Highlands of Scotland, as shown in the axial zone of the neighbouring Ardrishaig Anticline. Four localities from the limb to the axial plane of the Islay Anticline were investigated for isotopic homogenization of metacarbonate units to silicate values. At Mull of Oa on the limb of the anticline, metacarbonate samples show limited isotopic resetting and the fluid flux is estimated to be <1 m(3) m(-2). Within the axial zone of the Islay Anticline, metacarbonate units from Port a' Chotain and Bagh an Da Dhoruis show complete isotopic homogenization to silicate values indicating higher fluid fluxes. Fluid flow was enhanced along localized parasitic folds such as at Port an t-Sruthain, where metacarbonates have been isotopically reset, and there are abundant quartz-carbonate veins that precipitated during D-1-D-2 deformation. Metamorphic fluid flow was higher in the axial zone of the Islay Anticline and in localized antiformal structures. Fluid fluxes are estimated to be considerably lower than at the neighbouring Ardrishaig Anticline.
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| 5. |
- Skelton, Alasdair, et al.
(författare)
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Fluid–rock interaction at a carbonatite-gneiss contact, Alnö, Sweden
- 2007
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 154, s. 75-90
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Tidskriftsartikel (refereegranskat)abstract
- We evaluate balanced metasomatic reactionsand model coupled reactive and isotopic transportat a carbonatite-gneiss contact at Alno, Sweden.We interpret structurally channelled fluid flow alongthe carbonatite-gneiss contact at ~640C. This caused(1) metasomatism of the gneiss, by the reaction:biotite + quartz + oligoclase + K2O + Na2O +/- CaO +/-MgO +/- FeO = albite + K-feldspar + arfvedsonite +aegirene-augite + H2O + SiO2, (2) metasomatism ofcarbonatite by the reaction: calcite + SiO2 = wollastonite+ CO2, and (3) isotopic homogenization of themetasomatised region. We suggest that reactiveweakening caused the metasomatised region to widenand that the metasomatic reactions are chemically(and possibly mechanically) coupled. Spatial separationof reaction and isotope fronts in the carbonatiteconforms to a chromatographic model which assumeslocal calcite–fluid equilibrium, yields a timescale of10^2–10^4 years for fluid–rock interaction and confirmsthat chemical transport towards the carbonatiteinterior was mainly by diffusion. We conclude thatmost silicate phases present in the studied carbonatitewere acquired by corrosion and assimilation of ijolite, as a reactive by-product of this process and bymetasomatism. The carbonatite was thus a relativelypure calcite–H2O–CO2–salt melt or fluid.Keywords Carbonatite, Fluid flow, Metasomatism, Alno
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| 6. |
- Skelton, Alasdair, et al.
(författare)
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Quantification of the rate of methane production by serpentinization
- 2008
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Ingår i: International Geological Congress, 33. ; , s. CD-ROM
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We used seismic velocity as a proxy for serpentinization of the mantle, which occurred beneath thinned but laterally continuous continental crust during continental break up, prior to opening of the Atlantic Ocean. The serpentinized sub-continental mantle is now exhumed, beneath the Iberia Abyssal Plain and was accessed by scientific drilling on Ocean Drilling Program legs 149 and 173. Chromatographic modelling of the serpentinization front yields a front displacement (z) of 2197 ± 89 m. We ignored diffusive broadening of this front in the direction of fluid flow and used the shape of the front to constrain a Damköhler number (ND). This was scaled such that ND = t, where is a rate constant for serpentinization in s-1 and t is time in s. We thus obtained ND = 6.0 ± 0.2. We then estimated values of for (1) surface reaction as rate-limiting and (2) chemical transport as rate-limiting. Combining these values with our estimate of the Damköhler number yielded comparable timescales of 10,000 – 1,000,000 years for serpentinization. Combining this timescale with the front displacement yielded a time-averaged volumetric rate for serpentinization of 0.002-0.2 m3-serpentinite.m-2.yr-1.We then referred to the experimental results of Janecky and Seyfried (1986), which predicted that serpentinization by reaction with seawater probably occurs by a coupled set of reactions, with olivine, orthopyroxene and clinopyroxene dissolving independently and at different rates. Of this set of reactions, olivine dissolution is predicted to occur by the reaction:2(Mg,Fe)2SiO4 + 2H+ + H2O = (Mg,Fe)3Si2O5(OH)4 + (Mg,Fe)2+ (1)Because lizardite, which is the most abundant of the serpentine minerals typically contains no more than 5 wt. % FeO (Wicks & O’ Hanley 1988), excess Fe2+ will preferentially (with respect to Mg2+) enter the solution. Oxidation of Fe2+ by H2O and CO2 can then produce magnetite and CH4 by the reaction:12Fe2+ + 14H2O + CO2 = 4Fe3O4 + 24H+ + CH4 (2)Fe3O4 (magnetite) can be used as a proxy for CH4 produced by reactions (1) and (2) and exiting the mantle. The average mode of magnetite in 16 samples of >95% serpentinized peridotite recovered by scientific drilling within the region of exhumed mantle is 4.5 vol. %. This is equivalent to 1000 mol-Fe3O4.m-3. Combining this value with the time-averaged volumetric rate for serpentinization, we obtain a time-averaged annual production rate for magnetite of 2-200 mol.m-2.yr-1. Finally, based on the stoichiometry of reaction (2), we thus obtain an annual flux rate for CH4 production of 0.5-50 mol.m-2.yr-1.
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| 7. |
- Skelton, Alasdair, et al.
(författare)
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Regional mapping of pre-metamorphic spilitization and associated chemical mobility in greenschist-facies metabasalts of the SW Scottish Highlands
- 2010
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 167:5, s. 1049-1061
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Tidskriftsartikel (refereegranskat)abstract
- Both spilitic and non-spilitic metabasaltic sills are hosted by greenschist-facies metasediments in the SW Scottish Highlands. Spilitization is mainly characterized by enrichment in Na2O, elevated modal plagioclase and epidote segregations. Mapping of the spatial distribution of spilitic metabasalts reveals an ancient sub-sea-floor fluid cell centred on the extrusive Tayvallich Volcanics. Fluid circulation was most extensive at shallow levels where most sills were spilitized. We attribute this to pervasive flow of saline fluid, which was thermally driven by the cooling suite of lava flows and sills. Spilitization below this lithostratigraphic depth was restricted to only a few sills. Their spilitization is largely unrelated to specific properties of these sills (e. g. width, chemistry or host lithology). We conclude that fluid channelling was an intrinsic property of sub-sea-floor fluid flow either at deeper levels or earlier during fluid circulation. By profiling of the size distributions of relic phenocrysts in a partly spilitized sill, we conclude that spilitization proceeds with the symmetric propagation of a spilitization front from the sill margins towards the sill interior. Based on chemical profiling across the margin of an epidote segregation, we conclude that spilitization is associated with chemical transport on scales ranging from 0.1 to 10 m.
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