Thráinn Fridriksson

Zeolite Parageneses in the North Atlantic Igneous Province: Implications for Geotectonics and Groundwater Quality of Basaltic Crust

Zeolites are among the most common products of chemical interaction between groundwaters and the Earths crust during diagenesis and low-grade metamorphism. The unique crystal structures of zeolites result in large molar volumes, high cation-exchange capacities, and reversible dehydration. These properties influence both the stability and chemistry of zeolites in geologic systems, leading to complex parageneses and compositional relationships that provide sensitive indicators of physicochemical conditions in the crust. Observations of zeolite occurrence in Tertiary basaltic lavas in the North Atlantic region indicate that individual zeolite minerals are distributed in distinct, depth-controlled zones that parallel the paleosurface of the plateau basalts and transgress the lava stratigraphy. The zeolite zones are interpreted to have formed at the end of burial metamorphism of the lavas. Relative timing relations between various mineral parageneses and crustal-scale deformal features indicate that the minerals indicative of the zeolite zones formed within 1 million years after cessation of volcanism. Empirical correlation between the depth distribution of zeolite zones and the temperatures of formation of zeolites in geothermal systems provides estimates of regional thermal gradients and heat flow in flood-basalt provinces. Similarly, the orientations of zeolite zones can be used to distinguish synvolcanic and post-volcanic crustal deformation. Because zeolites that characterize the individual zones display different ion-exchange selectivities for various cations, reactions between groundwaters and zeolites in basaltic aquifers can result in depth-controlled zones where individual elements are concentrated in the crust. This is established for Sr, which is concentrated by at least an order of magnitude in heulandite, resulting in an overall Sr enrichment of lavas in the heulandite-stilbite zeolite zone.

Geological constraints on the thermodynamic properties of the stilbite—stellerite solid solution in low-grade metabasalts

Abstract Standard state thermodynamic data for stilbite (Ca2NaAl5Si13O36∗16H2O) and stellerite (Ca2Al4Si14O36∗14H2O), together with mixing properties of the stilbite -stellerite solid solution (stilbite SS) are derived through assessment of geological observations of stilbite SS compositions in metabasalts, experimental phase equilibria, and calorimetric observations. Measured compositions of stilbite SS in Tertiary metabasalts in Iceland and Icelandic geothermal systems change systematically towards the stellerite endmember with increasing stratigraphic depth and temperature. Standard molal volumes, heat capacities, and entropies for the endmembers of the solid solution are derived through critical review of published crystallographic and calorimetric data for natural stilbite SS. Standard molal Gibbs energies of formation at 298.15 K and 1 bar for stilbite (−4,946,475cal mol−1) and stellerite (−4,762,036 cal mol−1) and the mixing properties of the solid solution are retrieved from observed phase- and compositional-relations in metabasalts at Berufjordur, Iceland, measured temperatures of zeolite mineral distribution in active geothermal systems, and published observations of reversed phase equilibria. Mixing in stilbite SS can be described with an athermal solid solution model. Thermodynamic data resulting from our analysis provide close correlation between compositions of stilbite SS in Icelandic geothermal systems predicted from compositions of geothermal solutions and observed compositions of these minerals in low-grade metabasalts of Iceland, as well as the observed temperature of the stilbite SS to laumontite (leonhardite) transition in Icelandic geothermal systems. Stilbite SS composition in metabasalts is a sensitive function of temperature, fluid composition, coexisting minerals (especially silica polymorphs) and geothermal gradient.

Hydrogen-bonded water in laumontite I: X-ray powder diffraction study of water site occupancy and structural changes in laumontite during room-temperature isothermal hydration/dehydration

Abstract The response of the laumontite crystal structure to hydration/dehydration was evaluated using Rietveld refinements with XRD data collected under controlled PH₂O conditions at ~28.5 ℃. Refined water contents per unit cell (unit-cell formula: Ca4Al8Si16O48·nH2O) ranged between 12.5 H2O at 0.11 mbar PH₂O and 17.3 H2O at 37.6 mbar. The occupancy of the two awater sites hosting hydrogenbonded water molecules, W5 and W1, ranged from 13% to 100% and from 2% to 86%, respectively. During hydration of W5, between 0.11 and 5 mbar, the unit cell expanded continuously and reversibly from 1327 to 1348 Å3. The unit-cell volume remained nearly constant between 5 and 28 mbar. The hydration/dehydration of W1 exhibited hysteresis; hydration occurred at ~29 mbar and dehydration at ~24 mbar. During hydration of W1 at ~29 mbar the unit cell expanded from 1351 to 1384 Å3. Further hydration of W1 above 29 mbar resulted in gradual and reversible unit-cell expansion to 1386 Å3 at 37.6 mbar. Hydration/dehydration of W5 is a continuous reaction typical for zeolites. In contrast, the hydration/dehydration of W1 at room temperature is discontinuous, as manifested by the presence of two laumontite phases during hydration and dehydration. Unit-cell parameters of the two coexisting laumontite phases observed under these conditions are consistent with a vacant W1 site and ~80%-occupied W1 site, respectively. Gradual unit-cell expansion above 29 mbar due to increased PH₂O and increased occupancy of W1 indicate that hydration of the remaining 20% of the W1 site proceeds continuously.

Order/disorder in natrolite group zeolites: A 29Si and 27Al MAS NMR study

Abstract Disordering of Si and Al in natrolite, scolecite, mesolite, and gonnardite was investigated with 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The results indicate that with the exception of one sample of natrolite (from San Benito County, California), the natrolite, mesolite, and scolecite samples studied all exhibit small degrees (<10% Al occupancy of Si sites) of Si-Al disorder. The spectra for these samples are consistent with Al avoidance. Gonnardite is confirmed to have extensive Si-Al disorder, with only slight preferential Si occupation of the T1 site. Fits of 29Si MAS NMR spectra and mathematical relations based on Al avoidance were used to calculate Si and Al occupancies across the tetrahedral sites in these minerals. Configurational entropies arising from Si-Al disorder in natrolite, mesolite, and scolecite can add an addition 1-2% [up to 11 J/(mol·K)] to the total entropies of these phases at 298.15 K, whereas it may add as much as 7% to that of gonnardite [up to 27.7 J/(mol·K)]. These results also concur with previous observations of a gap in Si-Al disordering between orthorhombic and tetragonal natrolite samples and suggest that the state of disorder in natrolite is a function of temperature. The 29Si MAS NMR spectrum of gonnardite is consistent with a disordered natrolite framework structure, and not an intergrowth of thomsonite and natrolite structural domains.

Hydrothermal Minerals Record CO2 Partial Pressures in the Reykjanes Geothermal System, Iceland

The Reykjanes Peninsula in southwest Iceland is the landward extension of the Mid-Atlantic Ridge spreading center. At present seawater penetrates the coastal Reykjanes geothermal system at depth, where the highest recorded temperature is ∼320°C. It mixes with magmatic volatiles and reacts with the basaltic host rock to form secondary hydrothermal minerals in progressively higher-grade mineral alteration zones with increasing depth. Within the epidote-chlorite and portions of the epidote-actinolite zones of alteration, epidote-prehnite-calcite-quartz-fluid constitutes a quadra-variant assemblage that, under conditions of specified temperature, pressure, and activity of H2O allows prediction of geothermal fluid PCO2 as a function of the composition of the solid solution minerals epidote or prehnite. This assemblage is typically found at temperatures >250°C and ≲310°C, and potentially provides a mineralogical recorder that constrains fluid CO2 concentrations based on compositional zoning in hydrothermal epidote. Analysis of epidote crystals separated from drillhole-cuttings from three geothermal wells (RN-9, RN-10, RN-17) display complex chemical zoning, generally with Fe(III)-rich cores and Al-rich rims. The Fe(III)-mol fraction of epidote at depths between 0.5 to 1 km ranges from 0.21 to 0.38, between 1 to 2 km depth the range is 0.17 to 0.48 and between 2 to 3 km it is 0.17 to 0.30. The Fe(III)-mol fraction of prehnite ranges from 0.11 to 0.59 in the upper portions of drillhole RN-17, where the highest Fe(III) content in epidote, 0.36, serves as the upper Fe(III) limit for epidotes coexisting with prehnite in this study. Because most observed prehnite crystals in the drillhole-cuttings are too small for electron microprobe analyses (<20μm), we employed a sigmoidal correlation of available compositional data from active geothermal systems to calculate the Fe(III)-Al composition of prehnite using measured compositions of epidote in the Reykjanes system. In drill cuttings that contain epidote, prehnite, quartz and calcite, using measured epidote compositions between the reference temperatures of 275°C and 310°C, calculated values of PCO2 for the geothermal fluids range from ∼0.6 to ∼6.2 bars. When only epidote, prehnite and quartz are observed in the drill cuttings, the calculated range of PCO2 is from ∼1.3 to ∼6.8 bars, which provides the maximum value of PCO2 at which calcite will not be present. The present day PCO2 values of geothermal fluids from the Reykjanes system were derived from analytical data on liquid and vapor samples collected at the surface from wet-steam well discharges using both the WATCH and SOLVEQ speciation programs. The geothermal fluids at reference temperature between 275°C and 310°C have PCO2 concentrations ranging from 1.3 bars to 4.0 bars. The calculated PCO2 values based on epidote compositions are in close agreement with present-day fluid PCO2 in the Reykjanes geothermal system. 72 percent of the calculated PCO2 values based on epidote compositions where the assemblage of epidote, prehnite, quartz and calcite are observed in drill cuttings are within the range of measured present-day fluids, while 58 percent of the calculated PCO2 values fall within the range when calcite is not present in the drill cuttings. Therefore, our method for calculating fluid PCO2 is proven quite reliable when all four index minerals are present. Additionally, if only epidote, prehnite and quartz are observed, our model calculations still serve as a moderately accurate predictive proxy for maximum fluid PCO2 composition in the Reykjanes geothermal system. Ultimately, these correlations between the measured and calculated PCO2 fluid compositions will in the future provide a method, based on compositional variation and paragenesis of hydrothermal minerals in mafic lithologies, by which to characterize spatial and temporal concentrations of CO2 in both active and fossil hydrothermal systems and in low-grade metamorphic environments. The generally observed zoning pattern in epidotes in the Reykjanes geothermal system from Fe(III)-rich cores to Al-rich rims suggests that if the zoning formed while equilibrium was maintained among the epidote-prehnite-calcite-quartz assemblage under near isothermal conditions, there must have been an increase in PCO2 with time. This geochemical signature may then be employed to make large scale inferences concerning the evolution of the Reykjanes geothermal system. Analysis of geothermal fluids collected at the wellhead suggest that the four index minerals that comprise the assemblage are in equilibrium with the fluids, thus enabling the application of the PCO2 predictive method discussed in this study to modern epidote samples. In addition to aiding in understanding the history of reactions that involved natural sequestration of CO2 derived from magmatic degassing, this study may also provide useful insights into reactions that could result from the injection of industrial CO2-rich fluids into hydrothermal environments in basaltic rocks.

Hydrogen-bonded water in laumontite II: Experimental determination of site-specific thermodynamic properties of hydration of the W1 and W5 sites

Abstract Isothermal vapor sorption experiments under controlled partial pressures of H2O (between 0.1 and 30 mbar, at 23.4, 30.1, 49.5, 64.5, and 79.3 °C) and liquid water immersion calorimetry experiments at 25.0 °C were conducted to determine standard molar thermodynamic properties of hydration of the W1 and W5 sites in laumontite that host hydrogen-bonded water. A Langmuir adsorption model was used in a thermodynamic analysis of the isothermal adsorption data for the W5 site together with a symmetrical regular solution model. Resulting values for the standard molar Gibbs energy and entropy of hydration of the W5 site relative to liquid water are -8430 ± 113 J/mol and -16.7 ± 2.1 J/(mol·K), respectively, and the Margules parameter, WG, is 1590 ± 63 J/mol. The standard enthalpy of hydration of the W1 site was determined by liquid-water immersion calorimetry experiments on laumontite containing vacant W1 and fully occupied W5, W2, and W8 sites. Discontinuous hydration and dehydration of W1 at 23.4 ± 0.7 °C and 24 ± 1 mbar PH₂O was used to constrain the molar Gibbs energy of hydration of this site. Resulting values for standard molar Gibbs energy of hydration and enthalpy of W1 relative to liquid water are -380 ± 170 and -8800 ± 1150 J/ mol, respectively. Isothermal adsorption at 23.4 °C and isobaric thermogravimetric experiments indicate that during dehydration of W1, only 0.83 moles of water are released from the crystal structure and 0.17 moles are relocated to a disordered site that has energetic properties similar to the W8 site. Calculations using the thermodynamic data determined in this study indicate that the water content of laumontite in equilibrium with liquid water ranges from ~4.5 H2O per 12 framework O atoms at room temperature and one bar pressure to ~3.5 H2O at 250 °C and at liquid-vapor saturation pressure for water.