Fluid Flow and Diffusion in the Waterville Limestone, South--Central Maine: Constraints from Strontium, Oxygen and Carbon Isotope Profiles
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Abstract:
Oxygen, carbon and strontium isotopic profiles across the margin of the Waterville limestone member are used to investigate advective and diffusive transport during metamorphism of the Waterville Formation in south–central Maine, USA. Rb–Sr isotopic systematics were homogenized on the ~10 cm hand-specimen scale at ages that are within error of the 366±6 Ma Rb–Sr whole-rock age of the syn-metamorphic Hallowell pluton. This is consistent with a plutonic heat source for this low-pressure andalusite- and sillimanite-grade Acadian metamorphic terrane. Advective displacements of all three isotope profiles at the garnet-grade Blue Rock Quarry indicate fluid flow to the east into the limestone, and the oxygen-isotope profile implies a time-integrated fluid flux of 3.2±1.5 m 3 /m 2 (2σ error). This cross layer flux is insufficient to cause the observed reaction progress of the muscovite+ankerite–Quartz to biotite+anorthite (in plagioclase)+calcite reaction in the ~100 m thick Waterville limestone member and much of the fluid flow responsible may have been layer parallel. The isotope profiles indicate advective–diffusive homogenization over distances of 1.5 m (δ 13 C) to 6 m (δ 18 O) and such homogenization distances are difficult to reconcile with observations of order of magnitude variations in reaction progression the centimetre scale or less. It is possible that infiltration occurred during events short lived compared with diffusion, that the reactions started at different temperatures dependent on bulk composition or that diffusion of water from layers with less reactants to layers with more reactants was important in driving the biotite-producing reaction. However, variations of fluid composition inferred from the mineral assemblages are apparently inconsistent with diffusion driving reactions progress, and models of precursor assemblages do not indicate significant compositional control of the temperature of the first appearance of biotite in the rocks. Irrespective of the details of flow and diffusive exchange on the centimetre scale, the average reaction progress in the Waterville limestone member requires significant layer-parallel fluid fluxes.Keywords:
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Stable strontium isotope,one of the non-traditional stable isotopes,has been focused on in isotope geochemistry since last ten years and has become one of the exciting frontier sciences in geo-scientific studies.Strontium has four stable isotopes:84Sr,86Sr,87Sr and 88Sr.TIMS or MC-ICP-MS can be employed to measure stable strontium isotopes,and the ratios are expressed as δ88/86Sr.Variations in δ88/86Sr in nature range from-1.73‰ to 0.66‰.The factors affecting strontium fractionation are indistinct,and more work needs to be done on the fractionation mechanism for further understanding.At present the applications of stable strontium isotope geochemistry are presented as follows:(1) Paleoceanographic temperaure reconstruction based on δ88/86Sr of cold-water corals;(2) combining radiogenic strontium and stable strontium fractionation to study geochemical cycling in the ocean;(3) paleodietary research with stable strontium in bones.
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