Bruce W. D. Yardley

The petrologic case for a dry lower crust

Fluid pressure in the crust may be controlled by different mechanisms according to depth, temperature, and the mineralogy of the host rocks. Where rocks are fluid-saturated, fluid pressure may approach lithostatic or hydrostatic pressure depending on the ductility of the wall rocks and the connectivity of pores and fractures. However, if the host rocks contain minerals formed at temperatures higher than those currently prevailing, they will react with fluids to produce hydrated (or carbonated) retrograde minerals, and the fluid pressure will be limited by thermodynamic equilibrium between high-grade reactant minerals and retrograde products. The thermodynamically constrained parameter, water fugacity, may have a value of tens to hundreds of bars in the lower crust. In practice, this means that for typical igneous or high-grade metamorphic rocks now occurring in stable lower crust, notional fluid pressures are substantially (1 to 3 orders of magnitude) lower than lithostatic. No free, connected fluid phase can be present in deep stable crust, and alternative explanations must be sought for the relatively high electrical conductivity of such rocks. The proposal that high lower crustal conductivity is due to thin grain boundary films of graphite is also unlikely to be generally true because films of sufficient thickness would be readily visible on broken surfaces of hand specimens. An alternative explanation of the discrepancy between laboratory and field measurements of the conductivity of high-grade rocks is that laboratory measurements are not normally made under appropriate conditions of rock-buffered fluid pressure.

The chemistry of brines from an Alpine thrust system in the Central Pyrenees: An application of fluid inclusion analysis to the study of fluid behaviour in orogenesis

Abstract Quartz filled veins and fractures which formed late in the Alpine thrusting of the Central Pyrenees contain inclusions of hypersaline Na-Ca-Cl brines with total dissolved salts of up to 25 wt%. The total salinity is similar in all samples, irrespective of the vein or the wall rocks, but there are large variations (particularly in the Na Ca ratio) in the chemistry of the fluids between samples. With one exception, each sample contains only a single dominant fluid population. Crush-leach extraction and chemical analysis of the inclusion electrolytes for Na, K, Ca, Mg, Ba, B, Li, Sr, Rb, Fe, Mn, Zn, Pb, F, Cl, Br, and Sr and Pb isotopes reveals that the fluid chemistry is strongly influenced by the local rocks. Of the four different lithologies in the thrust stack sampled, the Triassic mudstones and Cretaceous limestones or Silurian phyllites acted as sources for the vein fluids during the late thrusting. The composition of the fluid in the veins was dependent on the proximity to these lithologies. For example, fluids from the Trias were dolomite saturated, whilst those close to limestone were calcite saturated. Strontium and lead isotopic analysis of inclusions and host rocks confirm that the more Narich fluids were in equilibrium with Triassic redbeds while Ca-rich fluids have been in isotopic equilibrium with either Cretaceous limestones or Silurian phyllites. Some samples have intermediate compositions due to mixing of the two endmember fluids prior to trapping as inclusions. The similarity of the Br Cl ratio (approximately twice seawater) and the consistent high salinity of all the inclusion fluids in the thrust stack indicate that they were all originally derived from a single source but progressively changed their cation and isotope chemistry through interaction with different host rocks. This ultimate source is likely to have been Triassic connate waters. We conclude that a local increase in permeability occurred when the veins formed and that fluid movement was over short distances. No evidence was found for a significant input of either surface or metamorphic fluids during thrusting.

Chlorine isotopes in fluid inclusions: determination of the origins of salinity in magmatic fluids

Abstract Thermal ionisation mass spectrometry (TIMS) of the Cs 2 Cl + ion has been used to determine the δ 37 Cl value of paleofluids trapped in fluid inclusions. Modifications to the existing published method have lowered the precision by a factor of 2–0.09‰ (1σ) thereby allowing real differences to be measured with greater certainty. The method has been applied to determine δ 37 Cl of two sets of magmatic fluids, trapped in fluid inclusions. Samples were studied from the Capitan pluton in New Mexico and the SW-England batholith, both of which have high temperature fluids whose δD and δ 18 O signatures are distinctly magmatic. Relative to standard mean ocean chloride (SMOC) the δ 37 Cl values of the Capitan samples cluster around 0‰, whereas those from SW-England cluster around +1.8‰. The Br/Cl ratios of the Capitan fluid inclusions are low, indicative of halite dissolution, and are comparable to local high salinity waters derived from leaching Permian evaporites. The Br/Cl ratios of the SW-England fluid inclusions are much higher and within the range of values normally considered to be “magmatic.” We conclude that the δ 37 Cl value of the Capitan magmatic fluids confirms the evidence from Br/Cl ratios that Cl was derived from an evaporite source, whereas the δ 37 Cl value of the SW-England fluids appears to be more representative of a deep magmatic source.

REE composition of an aqueous magmatic fluid: A fluid inclusion study from the Capitan Pluton, New Mexico, U.S.A.

Abstract The REE content of aqueous magmatic-derived fluids trapped in fluid inclusions, has been determined by ICP-MS after crush-leach extraction of the fluids in 4 samples. The total REE concentration varies between 200 and 1300 ppm and is dominated by the LREE, especially La, Ce and Nd. Fluids were released at different times from a melt, which changed composition as it underwent fractional crystallisation, and this is reflected in the concentration of REE in solution. Early formed quartz-fluorite veins, hosted by granophyre, contain the highest concentration of REE, and appear to be in equilibrium with aplite melt from which the fluid was inferred to have been derived since calculated fluid/melt distribution coefficients are in broad agreement with experimentally derived values. Variation in the REE content of the fluids is independent of salinity which remains constant at ∼ 80 wt% total salts. Later veins, hosted by aplite, contain fluid derived from a porphyritic melt and have lower REE concentrations, reflecting the greater incorporation of REE into mineral phases crystallising from the melt: titanite and allanite occur in these later veins. REE mineral/fluid distribution coefficients have been calculated for these minerals and show there is a strong preference for REE to partition into the minerals.

The role of water in the evolution of the continental crust

Abstract: Aqueous fluids have a profound influence on the evolution of the crust, both as agents of chemical mass transfer and mineral reactions, and through modifying its rheology. This paper is particularly concerned with the composition and role of fluids through the crustal cycle of burial, metamorphism and uplift. Information now comes from both conventional geological studies and from geophysics, which has documented both the presence of fluid in active areas of the crust, and also its migration in response to faulting. This contrasts with the overall slow rate of some fluid processes: for example, devolatilization reactions are endothermic and limited by the rate of heat supply. There is no clear distinction between diagenetic and metamorphic fluids, and extensive equilibration with host rocks means that few chemical or isotopic characteristics survive to provide tracers for deep fluid origins. After the metamorphic peak, remaining pore fluid is consumed in hydration reactions resulting in strong, dry rock. Subsequently, infiltration may reintroduce fluids but these normally survive only briefly until consumed by hydration reactions. Metamorphic rocks are strongly overpressured during prograde metamorphism and therefore have a low permeability. Considerable doubt is therefore cast on some claims of widespread and pervasive large fluid fluxes accompanying metamorphism.

Deep penetration of sedimentary fluids in basement rocks from southern Norway: Evidence from hydrocarbon and brine inclusions in quartz veins

Abstract This paper presents evidence for fluid flow and fluid-rock interaction at upper crustal levels within the crystalline basement of southern Norway. In the high-grade Modum Complex postmetamorphic veins of quartz occur in association with albitisation of metagabbros and metasediments. Pressure-temperature conditions for the formation of these veins are in the range 250–300°C and ca. 1–2 kbar. Primary and pseudosecondary fluid inclusions in the quartz veins show two fluids: (1) hydrocarbon ± C0 2 inclusions and (2) aqueous inclusions with variable salinities. Dark carbonaceous solid inclusions are also present. The hydrocarbon inclusions are methane dominated (ca. 80–100 mot%), and the presence of higher, complex hydrocarbons is demonstrated. The aqueous inclusions in the metagabbro-hosted veins show more saline compositions than the metasediment-hosted veins. A salinity range of ca. 23-0 wt% NaCl eq is found. Some of the aqueous inclusions may contain NaHCO 3 . Crush-leach analyses of inclusion fluids show Na-Ca-K-CI dominated compositions, with Na⪢Ca>K. Hydrocarbon-poor concentrates yield Br/Cl ratios close to seawater, while hydrocarbon-rich concentrates show higher values. The types and chemistry of the hydrocarbons in these veins indicate a biogenic origin of the hydrocarbon fluids. The inclusion fluids are interpreted as derived from an overlying sedimentary basin in late Precambrian or Permian times. The veining is interpreted as the deep expression of the percolation of basinal fluids into the metamorphic basement during crustal extension.

Rare Earth Element Speciation in Geothermal Fluids from Yellowstone National Park, Wyoming, USA

Abstract Elevated concentrations (20–1133 nmol/kg) of rare earth elements (REE) are present in acid-sulphate and acid-sulphate-chloride hydrothermal waters of the Yellowstone National Park (YNP). We used recently estimated thermodynamic data ( Haas et al 1995 ) to speciate seventeen YNP hydrothermal fluids with the EQ3NR code. The fluids show a range in pH (2.0–4.0) and temperature (70°–93°C) and are of varied chemistry, with TDS = 155–2,075 ppm, sulphate = 100–10,325 μmol/kg, chloride = 190–24,580 μmol/kg, fluoride = 26–1,790 μmol/kg, and SO 4 /F = 0.8–323. Field temperature and pH measurements were used in the modelling and saturation with kaolinite and quartz was assumed, although quartz was actually supersaturated. Where possible, oxygen fugacity was calculated from the analytical sulphate/sulphide ratios, otherwise it was set above the hematite-magnetite buffer and pyrite saturation (although speciation calculations show that this is not critical). Carbonate and phosphate levels were set at the analytical detection limit, with the exception of 4 waters for which analytical data for phosphate existed. The waters show little fractionation of REE relative to their host rhyolitic volcanics; it appears that the REE abundances of hydrothermal fluids resulting from alteration of YNP rhyolites are unaffected by the presence of potential complexing species, i.e., that acid-alteration completely strips REE from the portion of the rocks that it affects without any fractionation across the REE series. The main control over REE speciation is the relative abundances of potential complexing agents; however, pH and absolute abundances are also important. In the most acidic waters (pH ∼ 2.0) the free ion is the major species when salinity and SO 4 /Cl are low (60–80% of each REE), and REE complexes with chloride can be significant (up to 5%). For higher SO 4 /Cl values, sulphate complexes dominate (80–90%). For less acid waters (pH 2.8–4.0) fluoride is the main complexing agent in low SO 4 /F fluids, but as the SO 4 /F ratio increases the sulphate species become dominant, especially for the light REE (LREE).

Validation of LA-ICP-MS fluid inclusion analysis with synthetic fluid inclusions

Abstract Laser ablation.inductively coupled plasma.mass spectrometry (LA-ICP-MS) has become recognized as a sensitive, efficient, and cost-effective approach to measuring the major-, minor-, and trace-solute compositions of individual fluid inclusions in minerals. As a prerequisite for the routine analysis of natural inclusions in our laboratory, the precision and accuracy of the technique was assessed using sets of multi-element synthetic fluid inclusions. Five multi-element standard solutions were prepared, and incorporated as fluid inclusions in quartz crystals at 750 °C and 7 kbar. Fluid inclusions were ablated with a 193 nm ArF excimer laser and analyzed with a quadrupole ICP-MS, equipped with an octopole reaction cell for the removal of Ar-based interferences. The internal standard used in all cases was Na. Analytical precision for K, Rb, and Cs is typically better than 15% RSD, whereas Li, Mg, Ca, Sr, Ba, Mn, Fe, Cu, Zn, and Cl analyses are typically reproducible within 30% RSD. Measured concentrations approximate a Gaussian distribution, suggesting that analytical errors are random. Analyses for most elements are accurate within 15%. Limits of detection vary widely according to inclusion volume, but are 1 to 100 µg/g for most elements. These figures of merit are in excellent agreement with previous studies. We also demonstrate that, over the range investigated, precision and accuracy are insensitive to inclusion size and depth. Finally, the combination of our LAICP- MS analyses with microthermometric data shows that charge-balancing to NaCl-H2O equivalent chloride molality is the most valid approach to LA-ICP-MS data reduction, where chloride-dominated fluid inclusions are concerned.

Crush-leach analysis of fluid inclusions in small natural and synthetic samples☆

Abstract The accuracy and precision of crush-leach analysis of fluid inclusions in quartz, performed using an acidified lanthanum chloride leach solution for analysis of polyvalent cations, has been evaluated. The reproducibility of analysis for thirteen major and trace elements was assessed by comparing results from two splits of each of two samples, separated prior to crushing and cleaning, and was found to be better than 5% in most cases. Crush-leach analysis has also been performed on small samples of ca. 70 mg quartz, and produced results for K, Mg, Fe, Mn, and Zn relative to Na that are indistinguishable from those obtained on portions of several grams of the same samples. This makes possible the selective analysis of inclusions from portions of microthermometry wafers. The miniaturised procedures have been applied to a segment of a synthetic fluid inclusion sample and gave K Na = 0.579–0.664 (true value 0.656) and Ca Na = 0.364–0.426 (true value 0.375). This provides proof that crush-leach analysis can accurately yield the fluid inclusion chemistry (provided samples contain one dominant generation of inclusions), although low yields gave bigger uncertainties than for analysis of natural samples.

The Evolution of Fluids Through the Metamorphic Cycle

The aim of this chapter is to present an overview of the behaviour of fluids during the metamorphic cycle, i.e. from sedimentation and diagenesis through metamorphism to post-metamorphic cooling, uplift and cratonization of an orogenic belt. In particular, the chapter will attempt to put metamorphic fluids, in the sense of those fluids actually generated in situ from metamorphic devolatilization reactions, in the context of other fluid inputs into the metamorphic pile before, during and after metamorphism. In the course of this review, there are three issues to which particular attention will be drawn: the role of original sedimentary fluids in dictating the character of subsequent metamorphic fluids; the rates at which the fluids may be released by reaction; and the extent to which fluids may be drawn down from high crustal levels into crystalline rocks during post-metamorphic uplift.