Prograde and retrograde fluid flow during contact metamorphism of siliceous carbonate rocks from the Ballachulish aureole,Scotland

 Siliceous dolomites and limestones contain abundant retrograde minerals produced by hydration-carbonation reactions as the aureole cooled. Marbles that contained periclase at the peak of metamorphism bear secondary brucite, dolomite, and serpentine; forsterite-dolomite marbles have retrograde tremolite and serpentine; wollastonite limestones contain secondary calcite and quartz; and wollastonite-free limestones have retrograde tremolite. Secondary tremolite never appears in marbles where brucite has replaced periclase or in wollastonite-bearing limestones. A model for infiltration of siliceous carbonates by CO₂-H₂O fluid that assumes (a) vertical upwardly-directed flow, (b) fluid flux proportional to cooling rate, and (c) flow and reaction under conditions of local equilibrium between peak temperatures and ≈400 °C, reproduces the modes of altered carbonate rocks, observed reaction textures, and the incompatibility between tremolite and brucite and between tremolite and wollastonite. Except for samples from a dolomite xenolith, retrograde time-integrated flux recorded by reaction progress is on the order of 1000 mol fluid/cm² rock. Local focusing of flow near the contact is indicated by samples from the xenolith that record values an order of magnitude greater. Formation of periclase, forsterite, and wollastonite at the peak of metamorphism also required infiltration with prograde time-integrated flux approximately 100–1000 mol/cm². The comparatively small values of prograde and retrograde time-integrated flux are consistent with lack of stable isotope alteration of the carbonates and with the success of conductive thermal models in reproducing peak metamorphic temperatures recorded by mineral equilibria. Although isobaric univariant assemblages are ubiquitous in the carbonates, most formed during retrograde metamorphism. Isobaric univariant assemblages observed in metacarbonates from contact aureoles may not record physical conditions at the peak of metamorphism as is commonly assumed. Received: 19 September 1995 / Accepted: 14 March 1996     

Palæohydrology of the calcsilicate aureole of the Beinn an Dubhaich granite, Skye, Scotland: a stable isotopic study

The contact aureole developed in siliceous carbonates surrounding the Beinn an Dubhaich granite, Skye, shows textural and stable isotope evidence for infiltration of aqueous fluids during both prograde and retrograde metamorphism. Strongly depleted isotope compositions of reaction-product calcite correlate with high silica and fluorine contents, demonstrating a strong link between isotopic alteration and metasomatism by fluids with a significant magmatic component, even at the margins of the aureole. The oxygen and carbon isotope compositions of the carbonates form a linear cluster with a positive slope of about five, consistent with the depletion of isotope compositions by the infiltration of magmatic and/or meteoric fluids. Rayleigh fractionation during devolatilization played a minor role in determining the final isotope composition. Stable isotope compositions of coexisting calcite–dolomite pairs show varying amounts of isotopic disequilibrium, which correlate with the inferred fluid infiltration mechanism. Much of the calcite in dolostones is the product of infiltration-driven reactions along fractures, and is greatly depleted isotopically relative to the host dolomite, especially at talc grade. At higher grades the calcite–dolomite fractionation is smaller, probably due to both increased fluid–rock interaction and a greater tendency for fluid infiltration to be pervasive on the grain-scale. Limestones generally show near-equilibrium fractionation of oxygen and carbon owing to the overwhelming compositional influence of the host calcite. Veins formed during late-stage hydrothermal circulation have strongly ¹⁸O-depleted compositions relative to the host rock. No small-scale spatial patterns to the isotopic depletion were observed, but the extent of fluid infiltration was greatest in the west of the aureole. Fluid infiltration was clearly highly heterogeneous, with no evidence of a consistent flow direction. It is not possible to determine fluid fluxes or flow directions from one-dimensional flow models based on continuum flow in the Beinn an Dubhaich aureole.     

On the kinetics of textural equilibration in forsterite marbles

In the aureole of the Beinn an Dubhaich granite, Skye, the minimum observed median forsterite-calcite-calcite dihedral angle varies from 110° at the olivinein isograd to about 165° (the equilibrium value) at the granite-limestone contact. Laboratory experiments were performed to investigate the kinetics of this textural change. It was found that the rate of change of the forsterite-calcite-calcite dihedral angle followed approximately first-order kinetics with an activation energy of 48±4 kJ mol^-1 for fluid-present conditions, and 90 ±4 kJ mol^-1 for fluid-absent conditions. Scanning ion imaging demonstrated that, at least in the early stages of textural change, solution-reprecipitation of the calcite was the rate determining step in the fluid-present runs. The latter result and the value of the activation energy are both consistent with the activation energy found by previous authors for (albeit zeroth order) silicate-aqueous solution solution-reprecipitation reactions. The value of activation energy for the dry data does not correspond to those for either grain boundary or volume diffusion of oxygen or magnesium in forsterite. The mechanism for textural equilibration in the fluid-absent runs is uncertain. Application of the experimentally-derived rate equation to the Beinn an Dubhaich marbles gave activation energies higher than those obtained experimentally. It is concluded from consideration of grain growth effects that activation energies derived from the Beinn an Dubhaich marbles probably reflect textural equilibration under predominantly fluid-absent conditions.     

Fluid flow paths and mechanisms of fluid infiltration in carbonates during contact metamorphism: the Beinn an Dubhaich aureole, Skye

A textural study of marbles from the Beinn an Dubhaich granite contact aureole, Skye, has shown that mass transport by diffusion was probably negligible during the metamorphic event, and that the bulk of the carbonates reacted as a consequence of silica metasomatism, permitting the use of calcsilicates as a tracer for fluid infiltration pathways. Fracture-controlled infiltration was predominant in undeformed marbles, whereas pervasive infiltration occurred during synmetamorphic ductile deformation. Some calcite marbles contain disseminated unoriented calcsilicate minerals that are associated with neither fractures nor a ductile deformation fabric, consistent with an origin via infiltration of fluid along an interconnected grain-edge porosity. The inference of limited pervasive infiltration of undeformed carbonates is consistent with predictions based on experimentally determined fluid–solid dihedral angles.     

Mineralogical Evidence for Fluid-Rock Interaction Accompanying Prograde Contact Metamorphism of Siliceous Dolomites: Alta Stock Aureole, Utah, USA

Contact metamorphism of siliceous dolomite in the southern partof the metamorphic aureole of the Alta stock (Utah, USA) producedthe prograde isograd sequence: talc (Tc), tremolite (Tr), forsterite(Fo), and periclase (Per). Calcite (Cc)–dolomite (Do)geothermometry and phase equilibria define a general progradeT–X(CO₂) path of decreasing X(CO₂) with rising temperaturefor the dolomite. High-variance assemblages typify the aureole.Per + Cc and Fo + Cc + Do characterize the inner aureole (Perand Fo zones), and Tr + Do + Cc and Tc + Do + Cc are widespreadin the outer aureole (Tr and Tc zones). Low-variance assemblagesare rare and the thickness of reaction zones (coexisting reactantand product minerals) at the isogradic reaction fronts are narrow(tens of metres or less). The mineral assemblages, calculatedprogress of isograd reactions, and the prograde T–X(CO₂)path all indicate that massive dolomite was infiltrated by significantfluxes of water-rich fluids during prograde metamorphism, andthat the fluid flow was down-temperature and laterally awayfrom the igneous contact. Fluid infiltration continued throughat least the initial retrograde cooling of the periclase zone.Down-T fluid flow is also consistent with the results of Cc–Dogeothermometry and patterns of ¹⁸O depletion in this area. Theclose spatial association of reacted and unreacted chert nodulesin both the tremolite and talc zones plus the formation of tremoliteby two reactions indicate that the outer aureole varied in X(CO₂),and imply that fluid flow in the outer aureole was heterogeneous.The occurrence of dolomite-rich and periclase (brucite)-absent,high-     

Metamorphism and Fluid Infiltration of the Calc-silicate Aureole of the Beinn an Dubhaich Granite, Skye

The Tertiary Beinn an Dubhaich granite intruded at 75OC and05 kb into siliceous dolostones and limestones of the UpperDurness Group in Strath, Skye, with the consequent developmentof talc, tremolite, diopside, olivine, and periclase in thebulk of the aureole, and abundant fluoro-borosilicate skarnsimmediately adjacent to the pluton. With increasing grade thelimestones develop the mineral sequence talc, tremolite, diopside,and olivine, whereas the dolostones develop the sequence talc,tremolite, olivine, and periclase. The abundant chert nodulesin the dolostones take either of the two reaction paths, dependenton their size. Those below 2–7 mm in dimension followthe dolostone reaction path, whereas larger nodules follow thelimestone reaction path. The presence of monticellite in thehighest-grade rocks points to the flushing of the contact byvolumes of water-rich fluid, derived presumably from the granite.Consideration of low-grade olivine-bearing veins and fracturesin the dolostones also points to the presence of extremely water-richfluid in the more distal parts of the aureole. Using simplebox models with constant porosity, it is shown that the observedreaction progress in oli vine-grade and talc-grade rocks canonly be accounted for if the rocks were infiltrated by substantialvolumes of water-bearing fluid. Minimum estimates for volumesof infiltrated fluid show that rocks nearest the pluton wereprobably infiltrated by greater amounts of fluid than thosefurther away. Low-grade rocks which suffered greatest amountsof infiltration are the brecciated dolostones nearest the Thrust.     

In situ X-ray observations of the decomposition of brucite and the graphite–diamond conversion in aqueous fluid at high pressure and temperature

 An experimental technique to make real-time observations at high pressure and temperature of the diamond-forming process in candidate material of mantle fluids as a catalyst has been established for the first time. In situ X-ray diffraction experiments using synchrotron radiation have been performed upon a mixture of brucite [Mg(OH)₂] and graphite as starting material. Brucite decomposes into periclase (MgO) and H₂O at 3.6 GPa and 1050 °C while no periclase is formed after the decomposition of brucite at 6.2 GPa and 1150 °C, indicating that the solubility of the MgO component in H₂O greatly increases with increasing pressure. The conversion of graphite to diamond in aqueous fluid has been observed at 7.7 GPa and 1835 °C. Time-dependent X-ray diffraction profiles for this transformation have been successfully obtained. Received: 17 July 2001 / Accepted: 18 February 2002     

The effect of thermal history on the development of mineral assemblages during infiltration-driven contact metamorphism

 A one-dimensional model for coupled heat flow, fluid flow, and mineral reaction predicts the identity and spatial and temporal distributions of mineral assemblages produced during contact metamorphism of siliceous dolomite. Results are compared to mineral distributions predicted by transport models for fluid flow along steady-state temperature profiles and to mineral distributions observed in contact aureoles to assess whether a detailed analysis of heat flow is required to extract meaningful information of fluid flow history from field data. Results identify several mineral assemblages whose interpretation in terms of fluid flow is dependent on an understanding of their thermal history. Certain key mineral assemblages, however, are sensitive records of the geometry and amount of fluid flow but are insensitive to thermal history. The presence or absence of these mineral assemblages constrains the history of fluid flow during contact metamorphism of siliceous dolomite regardless of the details of heat flow. Occurrences of the key assemblages record both magmatic fluid flow in the direction of decreasing temperature (Alta, Elkhorn aureoles) and metamorphic fluid flow in the direction of increasing temperature (Beinn an Dubhaich, Kasuga-mura aureoles) during contact metamorphism. Time-integrated input fluid flux averages on the order of 100 mol/cm² with a range of ±1 order of magnitude. Received: 13 October 1995 / Accepted: 20 March 1996     

The Rare Earth Elements and Uranium in Garnets from the Beinn an Dubhaich Aureole, Skye, Scotland, UK: Constraints on Processes in a Dynamic Hydrothermal System

Garnets from skarns in the Beinn an Dubhaich granite aureole,Isle of Skye, Scotland, have a large range of concentrationsof uranium (0·2–358 ppm) and the rare earth elements(REE) (23–4724 ppm). Variations in these concentrationscorrelate with major element zonation within the garnets, andwith changes in the shape of REE patterns. Typical patternsin most garnets display light REE (LREE) enrichment, flat heavyREE (HREE) distribution and a negative Eu anomaly. These patternsare interpreted to represent equilibrium trace element exchangebetween pre-existing pyroxene, hydrothermal fluid and calcicgarnets. Iron-rich zones are characterized by positive Eu anomaliesand an increase in the abundance of the LREE relative to theHREE. These patterns are interpreted as resulting from changesin REE speciation related to the introduction of externallybuffered fluid to the skarn system. Relatively Fe-poor zonesshow strongly HREE-enriched patterns with negative Eu anomaliesand in some instances depletions in Y relative to Ho and Dy,which are interpreted as resulting from surface sorption ofthe REE during rapid, disequilibrium garnet growth. Strong correlationsbetween U abundance and the REE patterns indicate that the sameprocesses have affected U distribution. Both types of patterncan be modified by the effects of closed-system crystallizationon REE abundance in the fluid, and changes in fluid major elementchemistry. KEY WORDS: fractionation; garnet; hydrothermal; rare earth elements; skarn     

The effects of solid solution and diffusion on infiltration-driven metamorphism with application to metaperidotite,Val d’Efra,Central Swiss Alps

The complication introduced by solid solutions in the analysis of infiltration-driven mineral reactions is that the mole fraction of tracer component i in fluid (X _i) changes with reaction progress (ξ). The effect was incorporated into transport models for coupled fluid flow and mineral reaction by parameterizing the relation between X _i and ξ. With specific reference to carbonation and hydration during regional metamorphism of the peridotite body in Val d’Efra, whose constituent minerals are all solid solutions, infiltration of a disequilibrium fluid produces a single sharp reaction front if rock is assumed uniform in composition. The reaction front separates completely unreacted rock downstream from rock upstream with ξ at a steady-state limit (ξ_ss ≤ ξ_max) that depends on input fluid composition (ξ_max is the maximum possible value). Novel phenomena develop, however, if the flow medium, like the metaperidotite body, is composed of many small domains that differ in initial mineral modes and compositions but with X _i homogenized at a spatial scale larger than the size of the domains (e.g., by diffusion). In this case, infiltration of a disequilibrium fluid produces up to as many different reaction fronts along the flow path as there are domains with 0 ≤ ξ < ξ_ss in all domains except upstream from the slowest moving front where ξ = ξ_ss in all domains. Measured values of ξ in the metaperidotite, (all 0 < ξ < ξ_max) are best reproduced by down-temperature infiltration of a disequilibrium fluid with X_\textCO2 =  0. 1 9 6 X_{{{\text{CO}}_{2} }} = \, 0. 1 9 6 into a multi-domain medium with uniform X_\textCO2 X_{{{\text{CO}}_{2} }} at each spatial point along the flow path (homogenized across the domains at the m-scale by diffusion), and time-integrated fluid flux ≥1,836 mol fluid/cm² rock. Results resolve the paradox of the widespread spatial distribution of reactants and products of infiltration-driven decarbonation/dehydration reactions in regional metamorphic terrains (which in the absence of solid solution and compositional domains indicate up-temperature flow) and the prediction of hydrodynamic models that regional metamorphic fluid flow normally is directed vertically upward and down temperature.     

Contact metamorphism in the Los Santos W skarn (NW Spain)

Summary The intrusion of the Lower Permian Los Santos-Valdelacasa granitoids in the Los Santos area caused contact metamorphism of Later Vendian-Lower Cambrian metasediments. High grade mineral assemblages are confined to a 7 km wide contact aureole. Contact metamorphism was accompanied by intense metasomatism and development of skarns, and it generated the following mineral assemblages: diopside, forsterite, phlogopite (±clintonite) and humites and spinel-bearing assemblages or diopside, grossular, vesuvianite ± wollastonite in the marbles, depending on the bulk rock composition. Cordierite, K-feldspar, andalusite and, locally, sillimanite appear in the metapelitic rocks. Mineral assemblages of marbles and hornfelses indicate pressure conditions ranging from 0.2 to 0.25 GPa and maximum temperatures between 630 and 640 °C. ¹³C and ¹⁸O depletions in calcite marbles are consistent with hydrothermal fluid–rock interaction during metamorphism. Calcites are depleted in both ¹⁸O (δ¹⁸O = 12.74‰) and ¹³C (δ¹³C = −5.47‰) relative to dolomite of unmetamorphosed dolostone (δ¹⁸O = 20.79‰ and δ¹³C = −1.52‰). The δ¹³C variation can be interpreted in terms of Rayleigh distillation during continuous CO₂ fluid removal from the carbonates. The δ¹⁸O values reflect hydrothermal exchange with an externally derived fluid. Microthermometric analyses of fluid inclusions from vesuvianite indicate that the fluid was water dominated with minor contents of CO₂ (±CH₄ ± N₂) suggesting a metamorphic origin. Fluorine-bearing minerals such as chondrodite, norbergite and F-rich phlogopite indicate that contact metamorphism was accompanied by fluorine metasomatism. Metasomatism was more intense in the inner-central portion of the contact aureole, where access to fluids was extensive. The irregular geometry of the contact with small aplitic intrusives between the metasediments and the Variscan granitoids probably served as pathways for fluid circulation.     

Hydration of periclase at 350 ∘C to 620 ∘C and 200 MPa: experimental calibration of reaction rate

The hydration of periclase to brucite was investigated experimentally. Single crystals of periclase machined to millimeter sized cubes with (100) surfaces were reacted with distilled water at temperatures of 350 to 620 ^°C and a pressure of 200 MPa for run durations of 5 to 40 minutes. Hydration produced a layer of brucite covering the surface of periclase. While the shrinking periclase largely retained its cube shape a surface roughness developed on the μm scale and eventually outward pointing spikes bounded by (111) faces emerged on the retreating faces of the periclase due to kinetic selection of less reactive (111) and (110) surfaces. The periclase to brucite conversion followed a linear rate law, where the reaction rate increased from 350 to 530 ^°C and then decreased towards higher temperature and finally vanished at about 630 ^°C, where periclase, brucite, and water are in equilibrium at 200 MPa. The overall kinetics of the hydration reaction is conveniently described in terms of a phenomenological interface mobility. Measuring the velocity of the hydration front relative to the lattice of the reactant periclase, the temperature dependence of its mobility is described by an Arrhenius relation with pre-exponential factor 1.7.10^?12 m ⁴/s.J and activation energy of E_A=55 kJ/mol.     

Structurally controlled fluid flow during contact metamorphism in the Ritter Range pendant, California, USA

The mineralogy and O-isotope geochemistry of siliceous limestones from the Ritter Range pendant constrain the geometry and amount of fluid flow during contact metamorphism associated with emplacement of a pluton of the Sierra Nevada Batholith. Wollastonite (Wo) replaces calcite (Cal) + quartz (Qtz) on a layer-by-layer basis in homoclinal beds that strike NW and dip almost vertically. At the peak of metamorphism (P≈ 1500 bars, T≈ 600 °C) fluid in equilibrium with Cal, Qtz, and Wo has composition XCO₂=0.28, requiring that the Wo-forming reaction was driven by infiltration of reactive H₂O-rich fluid. The spatial distribution of Wo and Cal + Qtz records that peak metamorphic fluid flow was layer-parallel, upward. Bounds on the prograde time-integrated fluid flux associated with formation of Wo are set by: (1) the overlap in O-isotope composition between Wo-bearing and Wo-free rocks (>245 mol fluid/cm² rock); (2) the amount of fluid that would drive the Wo-reaction front upward to the present level of exposure from a point at depth where Cal, Qtz, and Wo would be in equilibrium with pure CO₂ (<1615 mol/cm²). Back-reaction of Wo to Cal + Qtz records an additional time-integrated retrograde fluid flux of ≈ 200–1000 mol/cm². The direction and amount of flow inferred from mineralogical and isotopic data agree with the results of the hydrologic model for metamorphic fluid flow in the area of Hanson et al. (1993). Fingers of Wo-bearing rock that extend farthest from the fluid source along contacts between limestone and more siliceous rocks point to strong control of flow geometry at the 0.1–100 m scale exerted by premetamorphic structures. Studies that neglect structural control at this scale may fail to predict correctly fundamental aspects of contact metamorphic fluid flow. Received: 27 January 1997 / Accepted: 2 October 1997     

H2O activity in concentrated NaCl solutions at high pressures and temperatures measured by the brucite-periclase equilibrium

 H₂O activities in concentrated NaCl solutions were measured in the ranges 600°–900° C and 2–15 kbar and at NaCl concentrations up to halite saturation by depression of the brucite (Mg(OH)₂) – periclase (MgO) dehydration equilibrium. Experiments were made in internally heated Ar pressure apparatus at 2 and 4.2 kbar and in 1.91-cm-diameter piston-cylinder apparatus with NaCl pressure medium at 4.2, 7, 10 and 15 kbar. Fluid compositions in equilibrium with brucite and periclase were reversed to closures of less than 2 mol% by measuring weight changes after drying of punctured Pt capsules. Brucite-periclase equilibrium in the binary system was redetermined using coarsely crystalline synthetic brucite and periclase to inhibit back-reaction in quenching. These data lead to a linear expression for the standard Gibbs free energy of the brucite dehydration reaction in the experimental temperature range: ΔG° (±120J)=73418–134.95T(K). Using this function as a baseline, the experimental dehydration points in the system MgO−H₂O−NaCl lead to a simple systematic relationship of high-temperature H₂O activity in NaCl solution. At low pressure and low fluid densities near 2 kbar the H₂O activity is closely approximated by its mole fraction. At pressures of 10 kbar and greater, with fluid densities approaching those of condensed H₂O, the H₂O activity becomes nearly equal to the square of its mole fraction. Isobaric halite saturation points terminating the univariant brucite-periclase curves were determined at each experimental pressure. The five temperature-composition points in the system NaCl−H₂O are in close agreement with the halite saturation curves (liquidus curves) given by existing data from differential thermal analysis to 6 kbar. Solubility of MgO in the vapor phase near halite saturation is much less than one mole percent and could not have influenced our determinations. Activity concentration relations in the experimental P-T range may be retrieved for the binary system H₂O-NaCl from our brucite-periclase data and from halite liquidus data with minor extrapolation. At two kbar, solutions closely approach an ideal gas mixture, whereas at 10 kbar and above the solutions closely approximate an ideal fused salt mixture, where the activities of H₂O and NaCl correspond to an ideal activity formulation. This profound pressure-induced change of state may be characterized by the activity (a) – concentration (X) expression: a _{H 2}O=X _{H 2}O/(1+αX _NaCl), and a _NaCl=(1+α)^(1+α)[X _NaCl/(1+αX _NaCl)]^(1+α). The parameter α is determined by regression of the brucite-periclase H₂O activity data: α=exp[A–B/ϱ_{H 2}O ]-CP/T, where A=4.226, B=2.9605, C=164.984, and P is in kbar, T is in Kelvins, and ϱ_{H 2}O is the density of H₂O at given P and T in g/cm³. These formulas reproduce both the H₂O activity data and the NaCl activity data with a standard deviation of ±0.010. The thermodynamic behavior of concentrated NaCl solutions at high temperature and pressure is thus much simpler than portrayed by extended Debye-Hückel theory. The low H₂O activity at high pressures in concentrated supercritical NaCl solutions (or hydrosaline melts) indicates that such solutions should be feasible as chemically active fluids capable of coexisting with solid rocks and silicate liquids (and a CO₂-rich vapor) in many processes of deep crustal and upper mantle metamorphism and metasomatism. Received: 1 September 1995 / Accepted: 24 March 1996     

Geochemical self-organization of olivine-grade contact metamorphosed chert nodules in dolomite marble, Kilchrist, Skye

Chert nodules in dolostones in the aureole of the Tertiary Beinn an Dubhaich granite, Skye, have developed  mm-scale concentric monomineralic bands of alternating calcite and olivine during breakdown of a diopside core by the reaction 3 Dol+Di=4 Cal+2Fo+2 CO₂. A simple textural progression from thin (<1 cm) homogeneous olivine-calcite rims close to the olivine-in isograd, to total replacement of the diopside core by a ≤10 cm thick well-banded rim close to the carbonate-granite contact demonstrates the developmental stages of the patterning. Correlation of olivine grain size with band spacing, and a greater olivine grain size in banded compared with homogeneous rims support pattern development by post-nucleation geochemical self-organization.     

The direction of fluid flow during contact metamorphism of siliceous carbonate rocks: new data for the Monzoni and Predazzo aureoles,northern Italy,and a global review

Periclase formed in siliceous dolomitic marbles during contact metamorphism in the Monzoni and Predazzo aureoles, the Dolomites, northern Italy, by infiltration of the carbonate rocks by chemically reactive, H₂O-rich fluids at 500 bar and 565-710 °C. The spatial distribution of periclase and oxygen isotope compositions is consistent with reactive fluid flow that was primarily vertical and upward in both aureoles with time-integrated flux ~5,000 and ~300 mol fluid/cm² rock in the Monzoni and Predazzo aureoles, respectively. The new results for Monzoni and Predazzo are considered along with published studies of 13 other aureoles to draw general conclusions about the direction, amount, and controls on the geometry of reactive fluid flow during contact metamorphism of siliceous carbonate rocks. Flow in 12 aureoles was primarily vertically upward with and without a horizontal component directed away from the pluton. Fluid flow in two of the other three was primarily horizontal, directed from the pluton into the aureole. The direction of flow in the remaining aureole is uncertain. Earlier suggestions that fluid flow is often horizontal, directed toward the pluton, are likely explained by an erroneous assumption that widespread coexisting mineral reactants and products represent arrested prograde decarbonation reactions. With the exception of three samples from one aureole, time-integrated fluid flux was in the range 10²-10⁴ mol/cm². Both the amount and direction of fluid flow are consistent with hydrodynamic models of contact metamorphism. The orientation of bedding and lithologic contacts appears to be the principal control over whether fluid flow was either primarily vertical or horizontal. Other pre-metamorphic structures, including dikes, faults, fold hinges, and fracture zones, served to channel fluid flow as well.     

http://www.sciencedirect.com/science/article/pii/S1674987113000315

The Panzhihua gabbroic intrusion,part of the plumbing system of the Emeishan large igneous province, intruded late-Proterozoic dolomites and marls about～263 Ma ago.The dolomites in the contact aureole were converted to brucite marbles and a diverse suite of forsterite,diopside and garnet skarns.The variation in mineralogy is explained in part by differences in the composition of the protolith,particularly the proportion of silica minerals and clay,and in part by transfer of elements from intruding magmas.The trace element compositions of most marbles and skarns are very similar to those of unmetamorphosed dolomites and marls,but some contain high Si,Ti,and Fe contents that are interpreted to have come from a magmatic source.Three brucite marbles sampled～10 m from the contact of the intrusion and named "enriched brucite marble" have trace element compositions very different from their dolomitic protolith:their rare earth elements are strongly enriched whereas levels of Nb-Ta,Zr-Hf and Ti are very low.These characteristics resemble those of carbonate liquid in equilibrium with silicate liquid or more probably with silicate minerals in the case of Panzhihua,a similarity we take to indicate that the sample underwent partial melting.Samples taken up to 300 m from the contact contain brucite indicating that high temperatures persisted well into the country rocks.However,other samples collected only tens of metres from the contact are only slightly recrystallized indicating that conditions in the aureole were highly variable.We suggest that temperatures within the aureole were controlled by conduction of heat from the main intrusion and by supply of additional heat from abundant small dykes within the aureole.Circulation of fluids derived from deeper levels in the aureole flushed the carbon dioxide from the dolomite,lowering temperature needed to partially melt carbonate to the temperatures attained near the intrusion.Irregular but extensive heating destabilized the carbonates of the aureole and decarbonation reactions associated with carbonate breakdown and melting emitted a large volume of CO₂,with potential impact on global climate.     

Transport of Pb and Sr in leaky aquifers of the Bufa del Diente contact metamorphic aureole, north-east Mexico

The Sr and Pb isotopes from the 31.6 ± 0.3 Ma (2σ) old Diente del Bufa alkali syenite, northeastern Mexico, and marbles of its contact aureole were used to trace the sources and the mobility of these metals during hydrothermal activity. Chert layers form aquifers within the marbles. The marbles represent aquitards. During fluid-wallrock reaction, the chert layers developed wollastonite rims. Early wollastonite rims have Sr and Pb isotopic compositions similar to those of their immediate host marbles, which indicates that the isotopic composition of Sr and Pb is initially buffered by the marble. Later wollastonite and other replacement minerals rimming the aquifer have Sr and Pb isotopic compositions that carry with time increasingly larger contributions from the high-salinity magmatic brine. The Sr and Pb contributions from the alkali syenite can be traced isotopically for more than 90 m away from the contact of the intrusion. In contrast, Sr and Pb originating from the alkali syenite are traceable within the marbles only for 3 to 5 cm from the aquifer-marble boundary. This distance is comparable to the spatial distribution of isotopic alterations of C and O implying that Sr and Pb were transported into the marbles through a fluid phase. The isotopic variation of Sr, Pb, C, and O across the aquifer-marble profiles reflects infiltration as a transport mechanism rather than diffusion. Because Sr and Pb are minor components in both the infiltrating fluid and the rock and because their concentrations are strongly affected by the distribution coefficients among the solid phases present, there is little correlation between the isotopic compositions of the trace elements Sr and Pb and those of C and O, which are major components in fluid and rock. Very thin meta-argillite rinds at the outer margin of the aquifer represent residual material after the dissolution of calcite. They are distinctly enriched in Rb, Sr, and U. The Rb and Sr are to some extent residual from the original limestone mineralogy, whereas U is dominantly derived from the magmatic fluid and leaked from the aquifer with the escaping immiscible CO₂-rich H₂O-CO₂ fluid that was produced by decarbonation. The ²³⁸U/²⁰⁴Pb values ranging from 100 to 250 and distinctly lowered Th/U in the meta-argillite rims (1) demonstrate that U was transported with the magmatic fluid along the aquifer and (2) imply that during unmixing of the highly saline magmatic fluid U fractionated into the CO₂-rich H₂O-CO₂ fluid from which it precipitated selectively in the meta-argillite band across the aquifer. Radioautographs demonstrate that the upper meta-argillite rim has 20 to 40 times more U than the lower rim, which implies that 20 to 40 times more CO₂-rich H₂O-CO₂ fluid has left through the upper aquifer contact. Received: 30 September 1997 / Accepted: 15 December 1997     

Partial Melting and Assimilation of Dolomitic Xenoliths by Mafic Magma: the Ioko-Dovyren Intrusion (North Baikal Region, Russia)

A petrological study was carried out on Mg-skarn-bearing dunitecumulates that are part of the Neo-Proterozoic Ioko-Dovyrenintrusion (North Baikal region, Russia). Skarn xenoliths containbrucite pseudomorphs after periclase, forsterite and Cr-poorspinel. Fine-grained forsterite–spinel skarns occur withthe brucite skarns or as isolated schlieren. Field relationshipsreveal that the Mg-skarns formed from silica-poor dolomiticxenoliths by interaction with the mafic magma of the Ioko-Dovyrenintrusion. Rapid heating of dolomitic xenoliths by the maficmagma caused the decomposition of dolomite into calcite + periclase,releasing much CO₂. Further heating quantitatively melted thecalcite. A periclase-rich restite was left behind after extractionof the low-density, low-viscosity calcite melt. The extractedcalcite melt mixed with the surrounding mafic melt. This resultedin crystallization of olivine with CaO contents up to 1·67wt %. A local decrease in the silica concentration stabilizedCaAl₂SiO₆-rich clinopyroxene. Brucite/periclase-free forsterite–spinelskarns probably originated by crystallization from the maficmelt close to the xenoliths at elevated fO₂. The high fO₂ wascaused by CO₂-rich fluids released during the decompositionof the xenoliths. The above case study provides the first evidencefor partial melting of dolomite xenoliths during incorporationby a mafic magma. KEY WORDS: dunite; dolomite assimilation; partial melting     

Patterns of fluid flow in the contact aureole of the Late Miocene Monte Capanne pluton (Elba Island,Italy): the role of structures and rheology

Fluid–rock interaction was investigated in the inner aureole of the Late Miocene Monte Capanne pluton on Elba Island (Tuscany, central Italy) by integrating structural, petrological, fluid inclusion, and stable isotope analyses. In the north-western sector of the aureole (Procchio–Spartaia area), calc–silicates alternate with nearly pure carbonate layers at the metre scale. Close to the pluton, the prograde metamorphic sequence includes calc–silicates that transition within a few metres to overlying nearly pure calcite marbles. The calc–silicates are extensively metasomatised to form massive wollastonite-grossular-bearing exoskarn. The mineralogical assemblage found in the marbles and the unshifted carbon and oxygen isotopic ratios in calcite attest that the fluid phase was internally buffered. On the other hand, the calc–silicates constituted channels for infiltration of disequilibrium fluids of magmatic origin. Fluid infiltration was enhanced by hydrofracturing and structurally-controlled by existing planar anisotropies in calc–silicates (layering and lithological boundaries). At the metamorphic peak (∼600°C and 1.5–2 kbar), the marble–calc–silicate interface acted as a barrier to fluids exsolved from the crystallising intrusions, separating two different flow patterns in the inner aureole: a high fluid–flux region on its higher grade side (Wol-zone) and a low fluid–flux region on the lower-grade side (Cpx zone). Results of this study: (1) documented that fluid pathways in the aureole rocks at the top of the pluton were largely horizontal, controlled by the lithological layering and the pluton–host rock contact; and (2) elucidated the primary control exerted by the structural and rheological properties of the host rocks on the geometry of fluid flow during pluton emplacement.