David Dolejš

Deciphering the petrogenesis of deeply buried granites: whole-rock geochemical constraints on the origin of largely undepleted felsic granulites from the Moldanubian Zone of the Bohemian Massif

The prominent felsic granulites in the southern part of the Bohemian Massif (Gfohl Unit, Moldanubian Zone), with the Variscan (∼340 Ma) high-pressure and high-temperature assemblage garnet+quartz+hypersolvus feldspar ± kyanite, correspond geochemically to slightly peraluminous, fractionated granitic rocks. Compared to the average upper crust and most granites, the U, Th and Cs concentrations are strongly depleted, probably because of the fluid and/or slight melt loss during the high-grade metamorphism (900–1050°C, 1·5–2·0 GPa). However, the rest of the trace-element contents and variation trends, such as decreasing Sr, Ba, Eu, LREE and Zr with increasing SiO 2 and Rb, can be explained by fractional crystallisation of a granitic magma. Low Zr and LREE contents yield ∼750°C zircon and monazite saturation temperatures and suggest relatively low-temperature crystallisation. The granulites contain radiogenic Sr ( 87 Sr/ 86 Sr 340 = 0·7106–0·7706) and unradiogenic Nd ( = − 4·2 to − 7·5), indicating derivation from an old crustal source. The whole-rock Rb–Sr isotopic system preserves the memory of an earlier, probably Ordovician, isotopic equilibrium. Contrary to previous studies, the bulk of felsic Moldanubian granulites do not appear to represent separated, syn-metamorphic Variscan HP–HT melts. Instead, they are interpreted as metamorphosed (partly anatectic) equivalents of older, probably high-level granites subducted to continental roots during the Variscan collision. Protolith formation may have occurred within an Early Palaeozoic rift setting, which is documented throughout the Variscan Zone in Europe.

Magnetic fabric and rheology of co-mingled magmas in the Nasavrky Plutonic Complex (E Bohemia): implications for intrusive strain regime and emplacement mechanism

Abstract The fabrics of mafic microgranular enclaves (MME) and of the host granodiorite of the old granodiorite intrusion of the Nasavrky Plutonic Complex (E Bohemia) were investigated by means of magnetic anisotropy. The magnetic fabric in MME is oriented coaxially with the magnetic fabric of the host granodiorite which is undoubtedly magmatic (intrusive) in origin. Consequently, the magnetic fabric in MME originated during the same process as the magnetic fabric in granodiorite, i.e. during the granodiorite emplacement. In addition, also the degree of anisotropy and the shapes of susceptibility ellipsoids are very similar in MME and in granodiorite, which probably indicates that the MME had probably a similar viscosity to that of granodiorite and behaved as passive markers whose shapes resembled those of the intrusive strain ellipsoids. Petrological and geochemical approaches coupled with rheological modelling allow the prediction of the physical states of magmas and of the development of their internal fabrics. Observed fabrics were compared to the results of numerical modelling of multiparticle systems slowly moving in viscous fluid. The steep magnetic foliations and almost vertical magnetic lineations suggest that todays erosion level represents a relatively deep intrusive level where magma flowed vertically and its intrusion was controlled by the shape of the feeder zone used for magma ascension.

Iron-carbon interactions at high temperatures and pressures

We have performed experiments in the Fe–C system at 2200–3400K and 25–70GPa using a multianvil press and laser-heated diamond anvil cell in order to constrain the stability of Fe3C. Iron carbide was observed experimentally as a stable phase using both experimental methods and independently confirmed by thermodynamic calculations. Our results imply that pure iron and carbon cannot coexist in a stable equilibrium at high pressure and high temperature. The high reactivity between metallic iron and the diamond requires a careful design of diamond anvil cell experiments in order to avoid carbon transport to the sample.

Garnet as a major carrier of the Y and REE in the granitic rocks: An example from the layered anorogenic granite in the Brno Batholith, Czech Republic

Abstract Garnet and other rock-forming minerals from A-type granite dikes in the Pre-Variscan Brno Batholith were analyzed to determine relative contributions of individual minerals to whole-rock Y and REE budget and to assess incorporation mechanisms of these elements in garnet. Minor to accessory garnet (<2 vol%) is the essential reservoir for Y+REE in the Hlína granite accounting ∼84% Y and 61% REE of the total whole-rock budget. Zircon is another important carrier of REE with ∼13% Y and ∼11% REE. At least ∼21% REE and 1% Y were probably hosted by Th- and U-rich monazite that has been completely altered to a mixture of secondary REE-bearing phases. The contribution of major rockforming minerals (quartz and feldspars) is low (∼1% Y; 10% LREE; ∼1% HREE) excluding Eu, which is hosted predominantly by feldspars (∼90%). Minor to accessory muscovite and magnetite incorporate ∼1% Y and ∼2% REE of the whole-rock budget. Magmatic garnet Sps41-46Alm28-44And0-13 Grs6-12Prp0-1 is Y- and HREE-rich (up 1.54 wt% Y; up ∼1 wt% ΣREE), and the Y+REE enter the garnet structure via the menzerite-(Y) substitution. The Y and REE show complex zoning patterns and represent sensitive indicator of garnet evolution, in contrast to a homogeneous distribution of major divalent cations. General outward decrease of Y+REE is a common feature due to the strong partitioning of Y+HREE in the garnet relative to the other phases. REE underwent significant fractionation during growth of early garnet I; the YbN/NdN ratio generally decreases from the core to rim of garnet I. Higher Mn and Al, lower Ca, and Y+REE contents, as well as higher YbN/NdN ratio and more negative Eu anomaly in garnet II overgrowths indicate its crystallization from a more evolved melt. Application of zircon saturation geothermometry provides upper temperature limit of 734 ± 14 °C for the closed-system crystallization. Mineral equilibria reveal that crystallization started at QFM + 1.2, and preferential sequestration of Fe3+ into garnet and magnetite was responsible for progressively reducing conditions. Equilibrium between magnetite, garnet, quartz, and plagioclase, representing the final crystallization stage of the granitic magma, occurred at 658-663 °C and QFM 0 to + 0.8, hence at undercooling of ∼75 °C.

Phase transitions and volumetric properties of cryolite, Na3AlF6: Differential thermal analysis to 100 MPa

Abstract Cryolite, Na3AlF6, is the most abundant aluminofluoride mineral in highly evolved felsic suites and their pegmatites, but its phase transitions and thermodynamic properties at elevated pressures are unknown. We used a simple modification of the TZM pressure vessel to perform differential thermal analysis of cryolite at high pressures. Temperatures of the α-β transition are as follows: 559.30 ± 0.23 °C (1 atm), 562.10 ± 0.28 °C (47 MPa), and 567.33 ± 0.23 °C (101 MPa). Cryolite melting temperatures increase as follows: 1011.4 ± 0.2 °C (1 atm), 1019.2 ± 0.4 °C (50 MPa), and 1028.7 ± 0.4 °C (100 MPa). Both pressure-temperature relationships are linear: (dT/dp)α-β = 78.4 ± 8.4 °C/GPa and (dT/ dp)m = 174 ± 12 °C/GPa. Application of the Clapeyron relationship leads to the following volumetric changes: ΔVα−β = 0.089 ± 0.019 J/(mol·bar) and ΔVm = 1.49 ± 0.12 J/(mol·bar). Despite the significant self-dissociation in the cryolite liquid, melting sensu stricto (without dissociation) dominates the heat and volumetric changes during melting in comparable amounts: 83.3 ± 6.7 % ΔHm and 68 ± 15 % ΔVm and suggests that the degree of dissociation has no significant effect on the (dT/dp)m. Evaluation of previous and current volumetric data for cryolite polymorphs leads to Vβ,1284 = 8.49 ± 0.17 J/(mol·bar); coeficients for the volumetric thermal expansion in the form of the third-order polynomial equation are: V298 = 7.080 ± 0.012 J/(mol·bar), a1 = (1.39 ± 0.20)·10-4 K-1, a2 = (.2.15 ± 0.51)·10-7 K-2, and a3 = (2.68 ± 0.34)·10-10 K-3. The total (dT/dp)m of cryolite is very similar to that of villiaumite (NaF), whereas ΔVm/Vβ,1284 of cryolite is smaller than for other alkali halides (NaF, NaCl).

Thermal effects of variable material properties and metamorphic reactions in a three‐component subducting slab

We explore the effects of variable material properties, phase transformations, and metamorphic devolatilization reactions on the thermal structure of a subducting slab using thermodynamic phase equilibrium calculations combined with a thermal evolution model. The subducting slab is divided into three layers consisting of oceanic sediments, altered oceanic crust, and partially serpentinized or anhydrous harzburgite. Solid-fluid equilibria and material properties are computed for each layer individually to illustrate distinct thermal consequences when chemical and mechanical homogenization within the slab is limited. Two extreme scenarios are considered for a newly forming fluid phase: complete retention in the rock pore space or instantaneous fluid escape due to porosity collapse. Internal heat generation or consumption due to variable heat capacity, compressional work, and energetics of progressive metamorphic and devolatilization reactions contribute to the thermal evolution of the slab in addition to the dominating heat flux from the surrounding mantle. They can be considered as a perturbation on the temperature profile obtained in dynamic or kinematic subduction models. Our calculations indicate that subducting sediments and oceanic crust warm by 40 and 70°C, respectively, before the effect of wedge convection and heating is encountered at 1.7 GPa. Retention of fluid in the slab pore space plays a negligible role in oceanic crust and serpentinized peridotites. By contrast, the large volatile budget of oceanic sediments causes early fluid saturation and fluid-retaining sediments cool by up to 150°C compared to their fluid-free counterparts.

Halogens in Silicic Magmas and Their Hydrothermal Systems

Halogens, mainly F and Cl, play key roles in the evolution and rheology of silicic magmas, magmatic-hydrothermal transition, partitioning of metals into aqueous fluids, and formation of ore deposits. Similarity of ionic radii of O, hydroxyl, and F, and a much greater size of Cl are responsible for (i) higher solubility, hence compatibility of F in silicate melts, (ii) greater lattice energies of fluorides, therefore their more refractory character and lower solubilities in fluids, and (iii) higher hardness of F as ligand for complexing, leading to a distinct spectrum of metal-fluoride versus metal-chloride complexes. In the F-rich systems, the interaction of F with rock-forming aluminosilicates corresponds to progressive fluorination by the thermodynamic component F2O−1. Formation of F-bearing minerals first occurs in peralkaline and silica-undersaturated systems that buffer F concentrations at very low levels (villiaumite, fluorite). The highest concentrations of F are reached in peraluminous silica-saturated systems, where fluorite or topaz are stable. Coordination differences and short-range order effects between [NaAl]–F, Na–F versus Si–O lead to the fluoride-silicate liquid immiscibility, which extends from the silica–cryolite binary to the peralkaline albite–silica–cryolite ternary and to peraluminous topaz-bearing systems, where it may propagate to solidus temperatures in the presence of other components such as Li. Differentiation paths of silicic magmas diverge, depending on the Ca-F proportions. In the Ca-rich systems, the F enrichment is severely limited by fluorite crystallization, whereas the Ca-poor magmas evolve to the high F concentrations and saturate with topaz, cryolite, or immiscible multicomponent fluoride melts (brines). These liquids preferentially partition and decouple high-field strength elements and rare-earth elements (REE), and are responsible for the appearance of non-chondritic element ratios and/or lanthanide tetrad effects. Continuous transition from volatile-rich silicate melts to hydrothermal fluids is unlikely, although two fluids—hydrous halide melts and solute-poor aqueous fluids—may often exsolve simultaneously. The fluoride ligand is responsible for the effective sequestration of hard cations, mainly REE, Th, U, and Zr, into the hydrothermal fluids. In the Cl-dominated systems, the maximum concentrations in silicate melts are significantly lower than those of F due to the absence of bonding between Cl and network-forming cations in the melt structure. The typical Cl-rich phase in felsic magmas is an aqueous ± carbonic fluid phase; the saturation of which limits the attainable concentration of Cl in the silicate melt. The more depolymerized the structure of the silicate melt is, the more easily metal-chloride species are accommodated. Therefore, metaluminous rhyolites are characterized by the highest fluid/melt partition coefficients for Cl as well as the lowest maximum dissolved Cl concentration. Chlorine is dominantly present as NaCl, KCl, CaCl2, FeCl2, and HCl species in aqueous magmatic fluids; their relative proportions are strongly influenced by silicate melt composition, pressure and total dissolved chloride concentration. The activity coefficients of metal-chloride species in the aqueous fluid are strongly dependent on pressure and total chloride concentration, and so is the volatile/melt partition coefficient of Cl. The increase of pressure strongly promotes Cl partitioning into the fluid phase, whereas increased chloride concentrations in the fluid work against it, especially if vapor-brine immiscibility occurs anchoring the activity of major chloride species in the system. Chloride ions are dominant, or at least take the form of significant complex forming ligands for a broad range of economically important elements found in magmatic-hydrothermal ore deposits such as Cu, Au, Mo, Pb, Zn, Sn, and W. Therefore, Cl has significant effect on the volatile/melt and vapor/brine partition coefficients of these elements, and at least partially controls the likelihood of the formation of economic ore mineralization.