Philip S. Neuhoff

Timing and structural relations of regional zeolite zones in basalts of the East Greenland continental margin

Zeolite isograds delineating regionally extensive metamorphic mineral zones occur throughout most of the early Tertiary flood basalts in central East Greenland. Mineralogic zones defined by isograds are essentially uniform in thickness throughout the province, parallel paleotopographic surfaces, and transgress lava stratigraphy. The distribution of index zeolite minerals is consistent with a thermal gradient of ∼40 ± 5 °C/km and heat flow of 1.7 ± 0.5 heat flow units. Zeolite facies metamorphism occurred rapidly during and just after the bulk of volcanism (57–55 Ma) and prior to hydrothermal alteration associated with emplacement of the ca. 55 Ma Skaergaard intrusion and mafic dikes of the coastal dike swarm. Consequently, zeolite isograds are temporal markers for distinguishing synvolcanic and postvolcanic deformation events. Discordance between dips of zeolite isograds and lava flows suggests a pre-metamorphic (and probably synvolcanic) component to tilting of the lava pile. Later deformation, likely associated with dikes and faults near the continental margin, tilts the zeolite isograds ∼ 2.5°SE.

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.

Enhanced resolution and quantitation from `ultrahigh' field NMR spectroscopy of glasses

Abstract The utilization of higher magnetic fields for nuclear magnetic resonance (NMR) (e.g., 14.1 and 18.8 T) can enhance both resolution and sensitivity in studies of disordered materials. When used in conjunction with fast magic angle spinning (MAS), high-field NMR can yield structural information not always available from NMR experiments of glasses at lower fields. Examples include site distinction of multiple boron groupings in 11 B MAS NMR spectra of borate glasses, the spectroscopic observation of low concentrations of five- and six-co-ordinate aluminum species in 27 Al MAS NMR spectra of aluminosilicate glasses, and new insight into magnesium co-ordination environments in silicate glasses using 25 Mg MAS NMR. Also presented are 17 O MAS NMR spectra of a zeolite in which high-field experiments permit resolution and quantification of bound molecular water. Comparisons of spectral data collected at 18.8 T with those at lower fields demonstrate these principles and indicate the potential of high-field MAS NMR applications to amorphous solids.

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.

Pyroclastic deposits within the East Greenland Tertiary flood basalts

Stratigraphic, geochemical and mineralogical characterization of pyroclastic deposits on the Gronau West Nunatak of East Greenland indicates that both alkaline and basaltic tephras occurred during the eruption of flood basalts associated with the opening of the North Atlantic ocean in the early Tertiary. Within the exposed section of Gronau West Nunatak, c. 1 km thick representing c. 1.6 Ma, 17 of the horizons are phreatomagmatic basaltic tephras. Near the top of the section, an alkaline tuff was dated at 53.8±0.3 Ma by 40Ar/39Ar method. This tuff contains a distinctive mineralogy (sanidine, Mg-katophorite, aegirine) and geochemistry (melt inclusion compositions, and trace and rare earth element abundances) that indicate it was erupted from the Gardiner melanephelinite-carbonatite volcanic complex located 175 km to the SE, which was active from c. 55 to 52 Ma. The alkaline tuff can be correlated to age-equivalent, compositionally similar alkaline pyroclastic horizons reported in North Atlantic sediment cores and in outcrops in Northern Europe, making it an important regional time-stratigraphic marker. This study indicates that North Atlantic explosive volcanic events were prevalent, regionally widespread, and originated in East Greenland during continental rifting, suggesting the need for reappraisal of the impact of North Atlantic volcanism on climate change during the early Tertiary.

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.

Si-Al disorder and solid solutions in analcime, chabazite, and wairakite

Abstract Quantitative determination of the abundance of Si(nAl) tetrahedral structural units (where n = 0, 1, 2, 3, or 4) through analysis of 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectra was used to assess the state of Si-Al disorder in the zeolites analcime [(NaAl)xSi48-xO96·16H2O], chabazite [(Ca0.5,Na,K)xAlxSi12-xO24·12H2O], and wairakite [CaAl2Si4O12·2H2O]. Short-range Si-Al ordering in chabazite and analcime is a regular function of Al mol fraction and is fully consistent with Al avoidance, as has generally been reported for zeolites not subjected to heat treatment. The results of this study and previously reported 29Si MAS NMR spectra suggest that natural analcime samples are more Si-Al ordered than either their synthetic counterparts or chabazite. Cluster variation method (CVM) calculations were used to calculate the configurational entropy (SCON) due to Si-Al disorder in chabazite and analcime. The calculations predict that long-range Si-Al ordering develops when Al occupies 5 out of 12 tetrahedral sites in chabazite and synthetic analcime and 17 out of every 48 tetrahedral sites in natural analcime. The difference between the calculated entropies and ideal entropies of mixing was used to derive activity-composition relationships for Si-Al substitution in these frameworks. Comparison between calculated values of SCON and the results of calorimetric and phase equilibrium studies on analcime indicate that the CVM accurately assesses SCON. The 29Si MAS NMR spectrum obtained for natural wairakite indicates that this mineral is largely Si- Al ordered, but comparison with a previously published spectrum indicates that natural and synthetic wairakites can exhibit significant variation in Si-Al disorder.

Comparison of FAM mixing to single-pulse mixing in 17O 3Q- and 5Q-MAS NMR of oxygen sites in zeolites

Fast amplitude modulation (FAM) shifted-echo pulse sequences for 3Q- and 5Q-MAS NMR experiments were compared for resolution of 17O on chemically and/or crystallographically distinct framework oxygen sites (i.e., Si–O–Si, Si–O–Al) in zeolites 13X, 4A, and a natural analcime (Na1.01Al1.01Si1.99O6·H2O). Significantly better sensitivity is afforded by fast amplitude modulation sequences. Resolution among different sites is also better in 5Q-MAS than in 3Q-MAS spectra, although sites with large quadrupolar coupling constants are more efficiently excited and reconverted in the latter experiments. No Al–O–Al linkages were detected in these zeolites.

Partial dehydration of laumontite: thermodynamic constraints and petrogenetic implications

Abstract Laumontite is a common zeolite mineral indicative of low-grade metamorphism of lavas and volcaniclastic sediments. Stoichiometric laumontite (CaAl2Si4O1· 4.5H2O) dehydrates in air at 298.15 K, 1 bar to leonhardite (CaAl2Si4O12 ·3.5H2O) via loss of water from the W1 crystallographic site. Consideration of reported X-ray diffraction and calorimetric data indicates that the standard molal volume and entropy for dehydration of laumontite to leonhardite + liquid water are ~13 cc/mol and ~8 cal/molK, respectively, at 298.15 K, 1 bar. Equilibrium between laumontite and leonhardite occurs at 70 to 80% relative humidity at 298.15 K, 1 bar, corresponding to a standard molal Gibbs energy and enthalpy of reaction of ~170±40 and ~2630±100 cal/mol, respectively. Calculated univariant equilibrium for this reaction is nearly linear from 46±3°C at 1 bar to 235±3°C at 5000 bar. Comparison of geological observations with these results suggests that laumontite forms as ‘leonhardite’ during metamorphism and diagenesis. Consideration of ‘leonhardite’, instead of laumontite, in low-grade metamorphic phase relations facilitates prediction of the relative stabilities of zeolites in natural and geologic systems, where calculated entropies of reaction incorrectly predict that assemblages bearing fully hydrated laumontite are stable at lower temperatures than the zeolites heulandite and stilbite.

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.