William M. Lamb

P-V-T properties of fluids in the system H2O ± CO2 ± NaCl: New graphical presentations and implications for fluid inclusion studies

Abstract Understanding the role of fluids in geologic processes requires a knowledge of the P-V- T properties of fluids over a wide range of conditions. Comparisons of several published equations of state with available experimental data for fluids composed of H2O and CO2 lead to the conclusion that the hard-sphere modified Redlich-Kwong equation of state of Kerrick and Jacobs (1981) most accurately predicts the P-V- T properties in this binary system. To model the volumetric properties in the H2OCO2NaCl system a formulation is presented involving a linear (ideal) interpolation between a pure-CO2 isochore predicted by the equation of state of Kerrick. and Jacobs (1981) and an H2O-NaCl isochore predicted by an empirical equation derived from the regression of available P-V-T data for the H2O-NaCl system. This formulation is applicable over a wide range of temperatures (>350°C) and pressures (2–10 kbars) and is especially suitable for high pressures and low-to-moderate temperatures (fluid densities ≥ 1.0 cm3). Determination of the appropriate isochore for an H2OCO2NaCl fluid inclusion requires (1) the relative salinity (NaCl/H2O + NaCl), (2) bulk density of the combined gas and liquid CO2 phases, and (3) volume percent estimate of the aqueous p the total homogenization temperature. The commonly encountered problem of estimating the volume percents of phases in inclusions may be avoided in some applications, and several new P-X(CO2) diagrams have been constructed and contoured with (a) the solvi in the mixed volatile system and (b) the measured density of the CO2 phase. The effects of H2OCO2 clathrates during microthermometric observations in the laboratory are evaluated and in most instances can be minimized or avoided. Application of these results to fluid inclusion studies have led to improved determinations of (1) pressures and temperatures of fluid entrapment in a variety of geologic settings and (2) pressures and temperatures of cooling and uplift following the peak of metamorphism. Proper interpretation of fluid inclusion data (from the literature) for two granulite fades terranes suggests that the uplift P-T-t path is more nearly initially isobaric than has sometimes been presented.

Deformation processes in a peridotite shear zone: reaction-softening by an H2O-deficient, continuous net transfer reaction

The Turon de Tecouere peridotite, in the North Pyrenean Zone, is composed of protomylonites grading to a 20–40 m wide zone of ultramylonites within a 0.6 km diameter exposure. The progressive mylonitization is marked by increasing volume fractions of very fine-grained matrix that comprise up to 90% of the ultramylonite. Deformation of the fine-grained matrix took place by grain size sensitive creep, as suggested by a very fine grain size (<10 μm), lack of dislocations in matrix grains, a weak crystallographic preferred orientation, and the alignment of grain boundaries parallel to the foliation. As the percentage of fine-grained matrix increased, weakening and localization resulted from a change in the dominant deformation mechanism from dislocation creep in the porphyroclasts to grain size sensitive creep in the fine-grained matrix. Production of the matrix grains took place by the nucleation of a number of different phases at the margins of porphyroclasts, indicating that the grain size reduction resulted primarily from reaction, and not from dynamic recrystallization. The nucleation of many phases along a single porphyroclast margin can be explained by a syntectonic continuous net transfer reaction associated with the spinel- to plagioclase-lherzolite transition. This continuous net transfer reaction produced new matrix grains with the same mineralogy as the original assemblage (olivine, orthopyroxene, clinopyroxene, spinel), with new compositions, plus plagioclase. Preliminary geothermobarometry indicates that the reaction took place over a range of temperatures and pressures (750–850°C, and possibly as high as 950°C and 0.5–1.1 GPa). The presence of only small amounts of amphibole, the lack of primary fluid inclusions, and no relation between the presence of amphibole and the intensity of mylonitic deformation led Vissers et al. [Tectonophysics 279 (1997) 303–325] to conclude that the deformation took place in an H2O-deficient environment. Reaction-enhanced softening may occur in the upper mantle wherever rocks move in pressure–temperature space and cross-reaction boundaries. Reaction boundaries are often modeled as univariant (lines in pressure–temperature space), yet mantle minerals are solid solutions so that reactions are continuous (multivariant) and take place over a broader region of pressure–temperature space than end-member reactions. It is therefore likely that shear zone deformation in polymineralic rocks will involve reaction-enhanced ductility over much of pressure–temperature space in the lithospheric mantle.

Methane-bearing aqueous fluid inclusions: Raman analysis, thermodynamic modelling and application to petroleum basins

Abstract Calibration for the determination of the CH4/H2O ratio using Raman spectroscopy is carried out using synthetic fluid inclusions with 0 m NaCl. Spectra of the symmetric stretching band of methane (ν1,CH4), and the bending (ν2,H2O) and stretching (νS,H2O) bands of water were obtained in the aqueous phase co-existing with the vapour phase at variable temperatures from 25° to a few degrees above the homogenisation temperature. A software program, based on the model of Duan et al. (1992a) [Duan, Z., Moller, N., Greenberg, J.H., Weare, J.H. 1992a. The prediction of methane solubility in natural waters to high ionic strength from 0°C to 250°C and from 0 to 1600 bar. Geochim. Cosmochim. Acta, 56, 1451–1460.] has been developed to calculate the composition of the aqueous liquid phase co-existing with the vapour phase in the synthetic fluid inclusions. Application to natural samples from the Alwyn South area (North Sea) shows that Raman analyses permit discrimination between inclusion populations that are not recognised using only microthermometric measurements. These results provide important constraints on the P–T conditions of inclusion formation and suggest that oil migration occurred at fluid pressures of approximately 240 bars.

Fluid inclusions in Adirondack granulites: Implications for the retrograde P-T path

Investigation of fluid inclusions in granitic and cale-silicate gneisses from the Adirondack Mountains, New York, has revealed the presence of various types, including: (1) CO2-rich, (2) mixed H2O−CO2±salt and (3) aqueous inclusions with no visible CO2. Many, if not all, of these inclusions were trapped or modified after the peak of granulite facies metamorphism, as shown by textural relations or by the lack of agreement between the composition of the fluids found in some inclusions and the composition of the peak-metamorphic fluid as estimated from mineral equilibria. Many fluid inclusions record conditions attained during retrograde cooling and uplift, with minimum pressures and temperatures of 2 to 3 kbar and 200 to 300°C. The temperatures and pressures derived from the investigation of these inclusions constrain the retrograde P-T path, and the results indicate that a period of cooling with little or no decompression.

The determination of phase relations in the CH4H2ONaCl system at 1 kbar, 400 to 600°C using synthetic fluid inclusions

Synthesis of fluid inclusions in the CH4H2ONaCl system was accomplished by subjecting fractured quartz, along with known quantities of CH4, H2O, and NaCl, to a pressure of 1 kbar and temperatures of 400, 500, or 600°C, in sealed Au capsules. Under the elevated P−T conditions some of the fractures healed, trapping fluids as inclusions. Microthermometric measurements conducted on the fluid inclusions show that at 1 kbar and 400 to 600°C, there are very broad regions of fluid unmixing in the CH4H2ONaCl system. For those bulk fluid compositions that lie in the two-phase (i.e., immiscible fluids) field, the high density phase is enriched in NaCl, whereas the low density phase is enriched in CH4. For any given bulk composition, the degree of NaCl enrichment in the high density phase increases, whereas the degree of CH4 enrichment in the low density phase decreases, as temperature increases from 4.00 to 600°C.

Oxy-amphibole equilibria in Ti-bearing calcic amphiboles: Experimental investigation and petrologic implications for mantle-derived amphiboles

Abstract An experimental study was carried out to investigate the equilibrium between Fe oxy-component and hydroxy-component in Ti-bearing calcic amphiboles, as described in the dehydrogenation/oxidation reaction Fe2+ + OH- = Fe3+ + O2- + 1/2 H2, for which the equilibrium constant (K) can be expressed as where □ = H-vacancy on the O3 anion position, Φ is the activity coeficient term, and Kx represents the thermodynamic mole fraction term (i.e., the K expressed as mole fractions rather than activities). The variation in Kx was quantified experimentally by annealing experiments on amphiboles of two different compositions: a mantle-derived kaersutite from Greenland, and a crustal pargasite from the Tschicoma Formation from the Jemez Mountains, New Mexico, volcanic complex. The conditions of the experiments ranged from 700.1000 °C, 1.10 kbar, and fH2 from that of the HM to GM solid buffer assemblages. The results, combined with similar data for a titanian pargasite from Vulcanʼs Throne, Arizona (Popp et al. 1995a), define the variation in log Kx as a function of T, P, and amphibole composition as given by the equation: If the T, P, and amphibole composition are known, values of log Kx calculated from the equation predict the equilibrium logfH2 of any experiment to within ~0.1 to 0.3 log units. It is assumed that a similar uncertainty in log fH2 would also to apply to the conditions of formation of natural amphiboles in the same composition range. If log fO₂ at the time of equilibration can be estimated independently for natural samples (e.g., mantle-derived amphiboles), the H2O activity also can be estimated. An alternate approach for estimating H2O activity from amphibole-bearing mantle rocks is to use a variety of H2O-buffering equilibria among end-member components in olivine, two-pyroxenes, amphibole, and other phases: e.g., 2 tr +2 fo = 5 en + 4 di + 2 H2O. A self-consistent thermodynamic database (THERMOCALC, Holland and Powell 1990) can be used to determine the aH₂O of such univariant H2O-buffering equilibria as a function of P and T. A mantle amphibole assemblage from Dish Hill (sample DH101-E, McGuire et al. 1991) was used to calculate aH₂O using the two different methods. The mean value of log aH₂O determined from seven different dehydration reactions is .1.70, with a 1σ range of ±0.50. That range of water activity is in good agreement with the value of log aH₂O = .1.90 ± 0.3 obtained using the dehydrogenation/oxidation equilibrium, along with an estimate of log fO₂. The use of xenolith amphiboles to infer values of aH₂O in the mantle requires that the H content of the amphibole does not change significantly during ascent or eruption. Changes in H content have significantly different effects on the dehydration and dehydrogenation equilibria, such that, comparison of the aH₂O estimates from the two different methods may permit quantification of H loss.

Comparison of calcite + dolomite thermometry and carbonate + silicate equilibria: Constraints on the conditions of metamorphism of the Llano uplift, central Texas, U.S.A.

Abstract Temperatures based on the composition of calcite coexisting with dolomite (calcite + dolomite thermometry) range from 475 to 600 °C for 63 marbles from the Llano uplift of central Texas. The highest temperatures, ∼600 °C, were obtained by carefully reintegrating calcite containing exsolved lamellae of dolomite. In some cases, these high temperatures were determined for marbles that contain an isobarically invariant assemblage consisting of calcite + dolomite + tremolite + diopside + forsterite. At a pressure of 3 kbar, these five minerals are stable at 630 °C and XCO2 = 0.62. In contrast, relatively low calcite + dolomite temperatures of 475-480 °C were obtained for marbles containing the assemblage calcite + dolomite + tremolite + talc. This talc-bearing assemblage is stable at ≤475 °C, depending on fluid composition, at a pressure of 3 kbar. Additional isobarically univariant equilibria are stable at intermediate temperatures (generally between 535 and 630 °C), and these are also generally consistent with results obtained from calcite + dolomite thermometry. Agreement between the calcite + dolomite temperatures and those inferred from silicate + carbonate equilibria in the marbles indicates that the temperatures generally reflect peak conditions of metamorphism, although some resetting has occurred. The marbles having the highest calcite-dolomite temperatures, as well as those containing the high- temperature isobarically invariant assemblage, are generally found close to post-tectonic pluton contacts, indicating that some of the amphibolite-facies assemblages are related to the emplacement of the granitic intrusions. Relatively low temperatures are recorded within approximately 2 km of pluton contacts, suggesting a possible thermal aureole. However, relatively high temperatures of 550-600 °C are recorded in marbles that are not spatially related to post-tectonic granites (>2 km from pluton contacts). These temperatures may be relicts of an earlier metamorphic event, although they could be related to granites that were emplaced above or below the current erosional level.

Amphibole equilibria in mantle rocks: Determining values of mantle aH2O and implications for mantle H2O contents

Abstract H2O can affect the thermophysical properties of the mantle, and nominally anhydrous mantle minerals, such as olivine, pyroxenes, and garnet, may be an important reservoir of mantle H2O. However, the H2O content of nominally anhydrous mantle minerals now at the Earth’s surface may not always reflect mantle values. It is, therefore, desirable to develop different techniques to estimate mantle H2O contents, or values of the activity of H2O (ɑH₂O) at the conditions of equilibration in the mantle. To examine the potential of amphibole equilibria to determine values of mantle ɑH₂O, the chemical compositions of co-existing amphibole, olivine, two-pyroxenes, and spinel from a mantle xenolith, sample DH101E of McGuire et al. (1991), were used to estimate values of pressure (P), temperature (T), and ɑH₂O. A value of ɑH₂O was estimated from pargasite dehydration equilibria using chemical compositions of minerals as the basis for estimating activities of end-members in the natural phases (e.g., the activity of forsterite in olivine). These calculations were performed with the THERMOCALC software package and, at an estimated maximum T and P of 900 °C and 20 kbar, they yield an estimated value of ɑH₂O ≈ 0.02 for sample DH101E. The application of oxy-amphibole equilibrium, as described by Popp et al. (2006a, 2006b), using the composition of the amphibole in DH101E yields a value of the log of the hydrogen fugacity (fH₂) of -1.37. This value of fH₂ together with the estimated log fO₂ of -9.9 yields a value of ɑH₂O ≈ 0.0005 for sample DH101E. The lower estimated ɑH₂O compared to that estimated from dehydration equilibria may reflect a slight loss of H from amphibole in the post-formation environment, but both types of amphibole equilibria are consistent with a low value of ɑH₂O. Values of mantle ɑH₂O can be used to predict the H2O content of mantle olivines. At 900 °C and 20 kbar, the olivine in a sample that equilibrates at ɑH₂O <0.04, as estimated for sample DH101E, should contain <10 wt ppm H2O. This value is consistent with the lower end of the range of measured H2O contents of mantle olivines (≈4-400 wt ppm). Thus, estimates of values of ɑH₂O from amphibole equilibria can produce useful predictions of both the activity of H2O as well as the H2O content of nominally anhydrous mantle minerals.

Phase relations in the CH4-H2O-NaCl system at 2 kbar, 300 to 600°C as determined using synthetic fluid inclusions

Abstract Synthesis of fluid inclusions in the CH4-H2O-NaCl system was accomplished by subjecting fractured quartz or fluorite, along with known quantities of CH4, H2O, and NaCl, to a pressure of 2 kbar and temperatures of 300, 400, 500, or 600°C, in sealed Au capsules. Under the elevated P-T conditions, some of the fractures healed, trapping fluids as inclusions. Microthermometric measurements conducted on the fluid inclusions show that at 2 kbar and 400 to 600°C, there are very broad regions of fluid unmixing in the CH4-H2O-NaCl system. For those bulk fluid compositions that lie in the two-phase (i.e., immiscible fluids) field, the high-density phase is enriched in NaCl, whereas the low-density phase is enriched in CH4. For any given bulk composition, the degree of NaCl enrichment in the high-density phase increases, whereas the degree of CH4 enrichment in the low-density phase decreases, as temperature increases from 400 to 600°C. Our experimental constraints on the size of the two-phase field are generally consistent with results generated using the equation-of-state GEOFLUIDS (available at http://geotherm.ucsd.edu/geofluids/). However, when comparing the compositions of coexisting immiscible fluids, as determined experimentally vs. calculated using GEOFLUIDS, we find that some relatively small but probably significant differences exist between our experiments and this equation of state.

Sequential vein growth with fault displacement : An example from the Austin Chalk Formation, Texas

To determine the opening and precipitation history and characteristics of vein-forming fluids, analyses of oxygen and carbon isotopes and trace elements were carried out on multilayered crack-seal calcite veins in the Austin Chalk Formation near San Antonio, Texas. The veins developed within the normal fault zones possessing unique chemical and textural characteristics which indicate sequential vein development. They are composed of alternating millimeter- to submillimeter-thick calcite veinlets and host lithons, occasionally crosscut by coarse, equant-grained secondary calcite veins. Regular changes in δ18O (e.g., −2.6 to −5.6‰, Pee Dee belemnite (PDB)) of the calcite veinlets along the length of veins suggest that the individual calcite veinlets were sequentially developed. A systematic δ18O decrease in the vein opening direction primarily resulted from a continuous increase in temperature of the ascending fluids delivered to the Austin Chalk. Relatively constant δ13C (approximately +1.4±0.4‰, PDB) for the multilayered veins and most secondary veins indicates that the composition of fluids from which the calcite veins precipitated was initially buffered by the bulk chalk. There is no spatial variation in trace element composition of the calcite veinlets along the length of veins. Low Sr concentrations in both calcite veinlets and secondary veins relative to those of the host chalk reflect a low partition coefficient of Sr in calcite during vein formation. Normal faults in the Cretaceous Austin Chalk were conduits to upwardly mobile vein-forming fluids.