Bibliothek

Notizen: O, Sr and C isotopes from east-central Vermont are used to provide information on the timing and volume of metamorphic fluid flow. The results are then used to assess the evidence for redox transformations between C species. Oxygen profiles are homogenised on a metre scale; comparison with Sr isotopes suggest that O alteration may have occurred over a significantly larger timescale than that of Sr, possibly because O was modified during dewatering and diagenesis in addition to the high temperature alteration recorded by strontium. Sr isotope distributions are consistent with cross-layer fluid fluxes of 104−106 moles m−2; absolute values depend on the Sr fluid-rock distribution coefficient which is poorly known; however, reaction progress constraints suggest that fluxes were towards the lower end of this range. High δ13C values observed at lithological boundaries cannot be explained by volume loss or closed system processes and are taken to indicate reductive precipitation of graphite as a result of mixing between CO2 and CH4-bearing fluids. Mass balance calculations indicate that redox reactions occurring under metamorphic conditions convert a minimum of 10% of the CO2 released from limestones into graphite, thus providing a potentially important control on the average residence time of C within the crust with implications for C cycling models.

Notizen: A spatial association is observed between the size distribution of garnet porphyroblasts and the size distribution of quartz veins in greenschist facies metapelites from Troms, North Norway. The size distribution of quartz veins reflects the flow regime of metamorphic fluids. The hypothesis that the flow regime of metamorphic fluids is also responsible for the size distribution of garnet crystals was tested by ascribing empirical acceleration parameters to the nucleation and growth rates of garnet crystals.In regions where fluid flow was interpreted as pervasive’, acceleration parameters for nucleation were high, whereas in regions where fluid flow was interpreted as channelled’, acceleration parameters for growth were high. Accelerated crystal growth is further implied from the chemical zoning and crystal morphologies of garnets collected near discrete veins.This spatial association may imply that fluid flow can be instrumental in controlling garnet crystallization. Fluid flow could affect garnet crystallization kinetics by facilitating thermal advection and/or mass transfer. In the study area, rhodochrosite (MnCO3) veins provide evidence for mass transfer of Mn by fluid flow. An influx of Mn would expand the stability field of garnet to lower temperatures. The resulting thermal overstep could accelerate nucleation and/or growth of garnets.The corollary of this study is that size distributions and chemical zoning of garnets, or other porphyroblast phases, can be used to study metamorphic fluid flow.

Notizen: Vein-controlled retrograde infiltration of H2O-CO2 fluids into Dalradian epidote amphibolite facies rocks of the SW Scottish Highlands under greenschist facies conditions resulted in alteration of calcite-rich marble bands to dolomite and spatially associated 18O enrichment of about 10%. on a scale of metres. Fluid inclusion data indicate that the retrograde fluid was an H2O-salt mixture with a low CO2 content, and that the temperature of the fluid was about 400d̀ C. Detailed petrographic and textural (backscattered electron imaging) studies at one garnet-grade locality show that advection of fluid into marbles proceeded by a calcite-calcite grain edge flow mechanism, while alteration of non-carbonate wall-rock is associated with veinlets and microcracks.Stable isotopic analysis of carbonates from marble bands provides evidence for advection of isotopic fronts through carbonate wall-rocks perpendicular to dolomite veins, and fluid fluxes in the range 2.4–28.6 m3/m2 have been computed from measured advection distances. Coincidence of isotope and reaction fronts is considered to result from reaction-enhanced kinetics of isotope exchange at the reaction front. Front advection distances are related to the proportion of calcite to quartz in each marble band, with the largest advection distance occurring in nearly pure calcite matrix. This relationship indicates that fluid flow in carbonates is only possible along fluid-calcite-calcite grain edges. However, experimental constraints on dihedral angles in calcite-fluid systems require that pervasive infiltration occurred in response to calcite dissolution initiated at calcite-calcite grain junctions rather than to an open calcite pore geometry.The regional extent of the retrograde infiltration event has been documented from the high δ18O of dolomite-ankerite carbonates from veins and host-rocks over an area of least 50 × 50 km in the SW Scottish Highlands. Isotopically exotic 18O-rich retrograde fluids have moved rapidly upwards through the crust, inducing isotopic exchange and mineral reaction in wall-rocks only where lithology, pore geometry or mineral solubilities, pressure and temperature have been appropriate for pervasive infiltration to occur.

Notizen: Abstract  Infiltration of a metabasite sill from Islay, Scotland by an H2O-CO2 fluid caused (1) modification of δ18O and (2) carbonation at the sill margins. Maps of δ18O and reaction progress were constructed from a 20 × 47.7 metre sample grid across the sill. The grid consisted of 300 samples, spaced at m, dm and cm intervals, many of which were analysed for both δ18O and reaction progress. The δ18O was determined by laser fluorination of whole rock silicate powders and reaction progress was determined by rapid field-based measurement of % calcite (“fizz-o-meter”, Skelton et al. 1995). Reaction and isotope fronts outlined tube-like features that emanate from the sill margin and discrete nodes that, although detached from the sill margin in two dimensions, are thought to represent sections through similar tubes in three dimensions. We envisage that these protrusions are the fossil record of metamorphic “fluid pathways” whereby fluid permeated the sill. Isotope and reaction fronts are found to correlate spatially as predicted by a modified form of the chromatographic equation which describes this envisaged geometry, that is where isotopic and reactive transport in the fluid phase are facilitated by advection along specific fluid pathways and transverse diffusion in the surrounding rock. These fluid pathways consist of bundles of anastomosing grain boundary channels or micro-cracks, which are thought to propagate through transient cyclic infiltration, reaction, porosity enhancement and fracturing. This mechanism is self-perpetuating and accentuates random perturbations at the sill margin to form the observed tubes. We argue that this is the earliest stage of the infiltration process which has affected metabasites of the SW Scottish Highlands and that subsequent shear deformation of the reacted rims of these pathways, has caused their re-orientation and juxtaposition to form the reacted sill margins described by Skelton et al. (1995).