Rare earth element (REE) and yttrium concentrations of coexisting monazite and xenotime were determined from a suite of seven metapelites from the Variscan fold belt in NE Bavaria, Germany. The metapelites include a continuous prograde, mainly low-P (3–5 kbar) metamorphic profile from greenschist (c. 400 °C) to lower granulite facies conditions (c. 700 °C). The LREE (La–Sm) are incorporated preferentially in monoclinic monazite (REO9 polyhedron), whereas the HREE plus Y are concentrated in tetragonal xenotime (REO8 polyhedron). The major element concentrations of both phases in all rocks are very similar and do not depend on metamorphic grade. Monazite consists mainly of La, Ce and Nd (La0.20–0.23, Ce0.41–0.45, Nd0.15–0.18)PO4, all other elements are below 6 mol%. Likewise, xenotime consists mainly of YPO4 with some Dy and Gd solid solutions (Y0.76–0.80, Dy0.05–0.07, Gd0.04–0.06). In contrast, the minor HREE concentrations in monazite increase strongly with increasing metamorphic grade: Y, Dy and Gd increase by a factor of 3–5 from greenschist to granulite facies rocks. Monazite crystals often show zonation with cores low in HREE and rims high in HREE that is interpreted as growth zonation attained during prograde metamorphism. Similarly, Sm and Nd in xenotimes increase by a factor of 3–4 with increasing metamorphic grade. Prograde zonation in single crystals of xenotime was not observed. The XHREE+Y in monazite and XLREE in xenotime of the seven rocks define two limbs along the strongly asymmetric miscibility gap from c. 400 °C to 700 °C. The empirical calibration of the monazite miscibility gap limb coexisting with xenotime is appropriate for geothermometry. Due to its contents of U and Th, monazite has often been used for U–Pb age determination. The combination of our empirical thermometer on prograde zoned monazite along with possible age determination of zoned single crystals may provide information about prograde branches of temperature–time paths.

Monazite–xenotime miscibility gap thermometry. I. An empirical calibration

Compositional maps of orthopyroxene and garnet of contrasting grain size and in contact with different minerals were made from two paragneiss granulites from the Minto terrane of northern Quebec. The compositional maps provide clear evidence of late exchange of Fe/(Fe + Mg) after Ca in garnet and Al in orthopyroxene had been quenched-in. The extent of late Fe-Mg exchange was controlled by neighbouring minerals, with negligible Fe-Mg gradients against plagioclase and quartz, and substantial gradients against exchangeable Fe-Mg minerals. Cores of grains in contact with exchangeable Fe-Mg neighbours are progressively more reset in Fe/(Fe + Mg) as grain size decreases, whereas cores of even small grains surrounded by only plagioclase and quartz are not significantly different in Fe/(Fe + Mg) than cores of the largest grains. Gradients of Ca in garnet and of Al in orthopyroxene in grains of uniform Fe/(Fe + Mg) preserve a high-temperature retrograde history during which intergranular exchange effected compositional uniformity of mineral rims and intragranular Fe-Mg diffusion in garnet and orthopyroxene was rapid enough to homogenize Fe/(Fe + Mg). The transition from efficient intergranular exchange at relatively high temperatures to local Fe-Mg exchange at lower temperatures may have been controlled by loss of an intergranular exchange medium in the rock, possibly an internally generated dehydration melt phase. Implications for geothermometry of granulites include the following (numerical values are particular to this study): (1) core compositions of garnet and orthopyroxene grains in contact with exchangeable neighbours may be reset in Fe/(Fe + Mg) relative to the most refractory compositions by an amount equivalent to 120d C; (2) Fe-Mg exchange thermometry using even the most refractory Fe/(Fe + Mg) compositions may not record peak granulite conditions, possibly recording instead the temperature at which an intergranular exchange medium was lost from the rock; and (3) temperature-sensitive net transfer equilibria involving Al solubility in orthopyroxene yield temperatures up to 150d C higher than maximum Fe-Mg exchange temperatures, even in grains with flat Fe/(Fe - Mg) compositional profiles, making them a better means of estimating peak granulite temperatures than Fe-Mg exchange thermometry.

Zoning patterns in orthopyroxene and garnet in granulites: implications for geothermometry

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.

Patterns of mineral occurrence in metamorphic rocks

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 Earth's crust and in the mechanisms and rates of deformation.

A historical review of metamorphic fluid flow

The Ballachulish Igneous Complex consists of an outer quartz diorite and an inner granite, emplaced at about 300 MPa, initially at 1000 to 1050 °C. The contact aureole (0.5–2 km wide) occurs in metapelites and metapsammites plus minor graphitic slates, carbonate rocks and metaquartzites. A textural examination of the arkosic Appin Quartzite, which was previously believed to have melted only within a few metres of the intrusion, demonstrates that partial melting occurred up to 500 m away from the vertical eastern contact. Coupling petrographic observations with Qtz-Ab-Or-H2O phase relations, we determined both the amounts of actual melt and the maximum possible amounts of melt in the samples. Melting efficiency was everywhere less than 100% and decreased with distance from the intrusion. Though perhaps not the only possible source of fluid throughout the aureole, simple models demonstrate that H2O evolution from the pluton would have been volumetrically sufficient and persisted long enough to account for the observed partial melting. A time-integrated fluid flux of 7000 kg/m2 from the pluton is necessary to account for the observed amounts of partial melt in the Appin Quartzite. From its inefficiency, we infer that infiltration of the Appin Quartzite cannot have occurred along interconnected grain-edge channels. Rather, it was controlled by hydraulic fracturing, with fracture density determining melting efficiency. Bulk-rock permeability is calculated to be 10−20 m2, an order of magnitude lower than that necessary to permit pervasive flow of all the fluid exsolving from the pluton. There is little difference between the calculated time-integrated fluid flux through the Appin Quartzite on the eastern flank and an estimate of the infiltrating flux through the pelitic Leven Schist on the western flank in the time interval during which both rock types were above their solidus temperature, despite differences in their equilibrium quartz-H2O dihedral angles at temperatures immediately below the solidus, and differences in the attitude of the contact. The rates of H2O expulsion from the cooling pluton are consistent with highly efficient fracture-dominated flow, allowing insufficient time for textural equilibration.

Partial melting of the Appin Quartzite driven by fracture-controlled H[sub]2O infiltration in the aureole of the Ballachulish Igneous Complex, Scottish Highlands

In this chapter the petrography and mineral chemistry of carbonates of the Ballachulish aureole are presented. Carbonates are not widespread in the aureole, so the record of progressive metamorphism is incomplete. The outermost thermometamorphic reaction observed in the aureole leads to the appearance of talc + calcite in siliceous dolomites and impure limestones. In the inner aureole from the point where both forsterite and diopside are formed, the outcrop is continuous up to the formation of periclase.

L. Masch

Papers

Decarbonation Reactions in Siliceous Dolomites and Impure Limestones