John M. Ferry

Experimental calibration of the partitioning of Fe and Mg between biotite and garnet

The cation exchange reaction Fe3Al2Si3O12 +KMg3AlSi3O10(OH)2 = Mg3Al2Si3O12+KFe3-AlSi3 O10(OH)2 has been investigated by determining the partitioning of Fe and Mg between synthetic garnet, (Fe, Mg)3Al2Si3O12, and synthetic biotite, K(Fe, Mg)3AlSi3O10(OH)2. Experimental results at 2.07 kbar and 550 °–800 ° C are consistent with In [(Mg/Fe) garnet/(Mg/Fe) biotite] = -2109/T(°K) +0.782. The preferred estimates for Δ¯H and Δ¯S of the exchange reaction are 12,454 cal and 4.662 e.u., respectively. Mixtures of garnet and biotite in which the ratio garnet/biotite=49/1 were used in the cation exchange experiments. Consequently the composition of garnet-biotite pairs could approach equilibrium values in the experiments with minimal change in garnet composition (few tenths of a mole percent). Equilibrium was demonstrated at each temperature by reversal of the exchange reaction. Numerical analysis of the experimental data yields a geothermometer for rocks containing biotite and garnet that are close to binary Fe-Mg compounds.

Metasomatism and fluid flow in ductile fault zones

Observed major element metasomatism in 5 amphibolite facies ductile fault zones can be explained as the inevitable consequence of aqueous fluid flow along normal temperature gradients under conditions of local chemical equilibrium. The metasomatism does not require the infiltration of chemically exotic fluids. Calculations suggest that metasomatized ductile fault zones are typically infiltrated by ∼105 moles H2O/cm2, fluid flow is in the direction of decreasing temperature, and fluids contain about 1.0 molal total chloride. Where available, stable isotopic alteration data confirm both flow direction and fluid fluxes calculated from major element metasomatism. The fluid fluxes inferred from metasomatism do not require large-scale fluid recirculation or mantle sources if significant lateral fluid flow occurs in the deep crust. Time-integrated fluid fluxes are combined with estimates of flow duration to constrain average flow rates and average permeabilities. Rocks in ductile fault zones are probably much more permeable during metasomatism (average permeabilities of 10-17 to 10-15 m2) than rocks normally are during regional metamorphism (10-21 to 10-18 m2). Estimated average fluid flow rates (3.5×10-3 to 0.35 m/yr) are insufficient, however, to significantly elevate ambient temperatures within ductile faults. Fluid flow in the direction of decreasing temperature may increase the ductility of silicate rocks by adding K to the rocks and thereby driving mica-forming reactions.

Fluid flow, mineral reactions, and metasomatism

A general model that relates fluid flow along a temperature gradient to chemical reaction in rocks can be used to quantitatively interpret petrologic and geochemical data on metasomatism from ancient flow systems in terms of flow direction and time-integrated fluid flux. The model is applied to regional metamorphism, quartz veins, and a metasomatized ductile fault zone.

Crystal size distribution (CSD) in rocks and the kinetics and dynamics of crystallization

Crystal size distributions (CSDs) measured in metamorphic rocks yield quantitative information about crystal nucleation and growth rates, growth times, and the degree of overstepping (ΔT) of reactions during metamorphism. CSDs are described through use of a population density function n=dN/dL, where N is the cumulative number of crystals per unit volume and L is a linear crystal size. Plots of ln (n) vs. L for olivine+pyroxene and magnetite in high-temperature (1000° C) basalt hornfelses from the Isle of Skye define linear arrays, indicating continuous nucleation and growth of crystals during metamorphism. Using the slope and intercept of these linear plots in conjunction with growth rate estimates we infer minimum mineral growth times of less than 100 years at ΔT<10° C, and nucleation rates between 10−4 and 10−1/cm3/s. Garnet and magnetite in regionally metamorphosed pelitic schists from south-central Maine have CSDs which are bell-shaped. We interpret this form to be the result of two processes: 1) initial continuous nucleation and growth of crystals, and 2) later loss of small crystals due to annealing. The large crystals in regional metamorphic rocks retain the original size frequency distribution and may be used to obtain quantitative information on the original conditions of crystal nucleation and growth. The extent of annealing increases with increasing metamorphic grade and could be used to estimate the duration of annealing conditions if the value of a rate constant were known. Finally, the different forms of crystal size distributions directly reflect differences in the thermal histories of regional vs. contact metamorphosed rocks: because contact metamorphism involves high temperatures for short durations, resulting CSDs are linear and unaffected by annealing, similar to those produced by crystallization from a melt; because regional metamorphism involves prolonged cooling from high temperatures, primary linear CSDs are later modified by annealing to bell shapes.

A model for coupled fluid-flow and mixed-volatile mineral reactions with applications to regional metamorphism

The effect of fluid flow on mixed-volatile reactions in metamorphic rocks is described by an expression derived from the standard equation for coupled chemical-reaction and fluid-flow in porous media. If local mineral-fluid equilibrium is assumed, the expression quantitatively relates the time-integrated flux at any point in a flow-system to the progress of devolatilization reactions and the temperature- and pressure-gradients along the direction of flow. Model calculations indicate that rocks are generally devolatilized by fluids flowing uptemperature and/or down-pressure. Flow down-temperature typically results in hydration and carbonation of rocks. Time-integrated fluid fluxes implied by visible amounts of mineral products of devolatilization reactions are on the order of 5·102–5·104 mol/cm2. The model was applied to regionally metamorphosed impure carbonate rocks from south-central Maine, USA, to obtain estimates of fluid flux, flow-direction, and in-situ metamorphic-rock permeability from petrologic data. Calculated time-integrated fluxes are 104–106 cm3/cm2 at 400°–450° C, 3,500 bars. Fluid flowed from regions of low temperature to regions of high temperature at the peak of the metamorphic event. Using Darcys Law and estimates for the duration of metamorphism and hydrologic head, calculated fluxes are 0.1–20·10-4 m/year and minimum permeabilities are 10-10–10-6 Darcy. The range of inferred permeability is in good agreement with published laboratory measurements of the permeability of metamorphic rocks.

Patterns of mineral occurrence in metamorphic rocks

Abstract Patterns in the occurrence of minerals in metamorphic rocks suggest additional opportunities for investigating chemical and physical processes during metamorphism. Three such patterns are reviewed. First, trace minerals in metamorphic rocks commonly occur with regular distributions indicating their participation in prograde reactions that can be mapped as isograds. Examples include the distribution of allanite and monazite in pelitic rocks and of zircon and baddeleyite in siliceous dolomites. Recognition of these isograds points to the potential for developing a chronology of specific chemical reactions during metamorphism and for defining the P-T conditions of those reactions. Second, the mineralogical products of retrograde metamorphism in many cases occur in distinctive associations that are consistent with partial mineral-fluid equilibrium. Examples include the distribution of retrograde calcite, quartz, and tremolite in siliceous limestones and of retrograde tremolite, dolomite, brucite, and serpentine in siliceous dolomites from contact aureoles. Among other things, application of partial equilibrium to retrograde metamorphic rocks leads to constraints on the amount and direction of fluid flow in contact aureoles as they cool. Third, pseudomorphs are typically absent from prograde metamorphic rocks but are common in retrograde metamorphic rocks. The distribution may be explained by the effect of “force of crystallization.” The pattern of occurrence of pseudomorphs thus suggests novel phenomena during metamorphism that develop from an interplay between chemical and mechanical processes

Fluid flow and stable isotopic alteration in rocks at elevated temperatures with applications to metamorphism

Abstract A simple model quantitatively predicts the shifts in stable isotope composition that result from fluid flow through rocks along a temperature gradient in flow systems where local fluid-rock isotope exchange equilibrium is attained. Equilibrium fluid flow along a temperature gradient is a potent mechanism for stable isotopic alteration in rocks: for example, a time-integrated fluid flux of 1 · 105 mol H2O/ cm2 flowing through a carbonate or quartzo-feldspathic rock at 600° C along a temperature gradient of +25°C/km will causeδ 18O to decrease by ≈5‰. Using the model, measured isotopic shifts in rocks can be quantitatively interpreted as records of time-integrated fluid flux and of flow direction relative to fossil temperature gradients in metamorphic terranes and other high-temperature, deep-crustal fluid flow systems. Inferred whole-rock 18O-depletions of 5–8‰ in contact and regional metamorphic terranes can be explained by flow of 2–50 · 104 mol/cm2 aqueous fluids through rocks in the direction of increasing temperature if local mineral-fluid equilibrium is maintained. Stable isotope alteration in metamorphic rocks does not require infiltration of chemically exotic, non-equilibrium fluids, and therefore does not necessarily provide information about the source of infiltrating fluids.

A historical review of metamorphic fluid flow

Recognition of the importance of fluid flow in the process of metamorphism was an outgrowth of efforts by petrologists over the last 50–60 years to understand the mineralogical evolution of metamorphic rocks. Mineralogical, chemical, and isotopic data are now routinely used to identify where fluid has flowed in metamorphic terranes, to measure the amount of fluid, to constrain the direction of flow relative to temperature and pressure gradients and lithologic contacts, and to determine the age of flow. Fluid may flow through a static, interconnected network of microscopic tubes at grain corners only under special combinations of mineralogy, fluid composition, pressure, and temperature. Fluid flow typically is restricted to hydraulic fractures whose formation and maintenance require dynamic processes such as increasing temperature, active de volatilization reactions, and/or deformation. Hydraulic fracture flow is heterogeneous in both space and time. Metamorphic fluid transport may be driven by density differences between rock and fluid, by density variations in fluid generated from temperature gradients, by deformation, and by surface tension. Metamorphic fluid flow plays a significant role in heat and mass transfer in Earths crust and in the mechanisms and rates of deformation.

Reaction Progress: A Monitor of Fluid—Rock Interaction during Metamorphic and Hydrothermal Events

When fluid infiltrates a rock and they are not in chemical equilibrium, chemical reactions proceed between fluid and minerals in the rock. Once the stoichiometry of the mineral—fluid reaction and the composition of the fluid is taken into account, the progress of the reaction serves as a quantitative measure of how much fluid the rock chemically interacts with. Reaction progress therefore serves as a natural fossil flux meter for fluid—rock interactions during, for example, metamorphism and hydrothermal events. Numerous applications can be made. On an outcrop scale, reaction progress can determine whether fluid flow was pervasive or was channelized along bedding, fractures, or foliation. On a regional scale, reaction progress can identify metamorphic and hydrothermal infiltration fronts and the relationship between fluid—rock interaction and the degree of metamorphism or alteration. On the scale of an entire metamorphic belt, reaction progress may reveal whether fluid released during metamorphism flows to the surface in a single pass or is recirculated in crustal-scale metamorphic hydrothermal cells.

Hydrothermal alteration of Tertiary igneous rocks from the Isle of Skye, northwest Scotland

Hydrothermal alteration of Tertiary gabbros from Skye involved the reaction of igneous olivine, augite, hypersthene, plagioclase, magnetite, and ilmenite with aqueous fluid primarily to combinations of talc, chlorite, montmorillonite, calcic amphibole, biotite, and secondary magnetite. Lesser amounts of calcite, epidote, quartz, sphene, prehnite, and garnet also developed. During mineralogical alteration of gabbro there was a net addition to rock of K, Na, Sr, and H2O and a net loss of Mg. Gabbro was oxidized early in the hydrothermal event and later reduced. Iron and silicon were probably initially lost and later added. There is no evidence for significant change in the Al or Ca content of the gabbros. Hydrothermal alteration of Skye gabbro involved not only large-scale migration of 18O, 16O, D and H but also of K, Na, Sr, Mg, and probably Fe and Si.Mineral thermometry indicates that pyroxenes in the gabbros crystallized at 1000° C–1150° C and were very resistent chemically as well as isotopically to later hydrothermal alteration. Hypothetical equilibrium between primary and secondary mafic silicates suggests that mineralogical alteration of gabbro occurred at ∼450°–550° C. The lack of correlation between mineralogical and isotopic alteration of gabbro requires that much isotopic alteration occurred at temepratures above those at which the secondary minerals developed, 550°–1000° C. The chemical alteration of gabbro is correlated with its mineralogical alteration and therefore occurred at 450°–550° C.Measured progress of the mineral-fluid reactions was used to estimate the amount of H2O fluid that infiltrated the gabbro as primary olivine was converted to talc+magnetite at 525°–550° C. Calculated fluid-rock ratios are in the range 0.2–6 (volume basis) and are smaller than values estimated from isotopic data (fluid/rock ∼1–10, volume basis). Both isotopic and petrologic data point to pervasive flow of fluid through crystalline rock at elevated temperatures of 500°–1000° C. Isotopic fluid-rock ratios are larger than petrologic fluid-rock ratios because isotopic alteration of cooling gabbro began earlier and at higher temperatures than did the mineralogical alteration.