Ronald J. Spencer

Melting behavior of fluid inclusions in laboratory-grown halite crystals in the systems NaClH2O, NaClKClH2O, NaClMgCl2H2O, and NaClCaCl2H2O☆

Abstract The chemical composition of aqueous fluid inclusions in crystals of halite can be accurately determined from observed melting behaviors of ice, hydrohalite, and sylvite. Some fluid inclusion melting behaviors observed in laboratory-grown halite crystals (systems NaClH 2 O, NaClKClH 2 O, NaClMgCl 2  H 2 O, and NaClCaCl 2 H 2 O) differ from predicted stable equilibrium relations. In the NaClH 2 O and NaClKClH 2 O systems, observed first melt temperatures are up to 15°C below the equilibrium eutectic temperatures of −21.2° and −22.9°C, respectively. The final melting temperature of ice, in the presence of hydrohalite, and the final melting temperature of hydrohalite are reproducible and match predicted melting temperatures. The limit of detection of sylvite daughter crystals in the NaClKClH 2 O system is approximately 5 wt% (≈ 1 molal) KCl. Final melting temperatures of sylvite match published equilibrium data to within 0.3°C. In the NaClKClH 2 O system at halite saturation, m KCl can be determined from the final sylvite dissolution temperature or from the final melting temperature of hydrohalite. Fluid inclusions in the NaClMgCl 2 H 2 O and NaClCaCl 2 H 2 O systems that form stable salt hydrates (MgCl 2 · 12H 2 O and CaCl 2 · 6H 2 O) during freezing first melt within 3°C of predicted eutectic temperatures (−37° and −52°C). However, fluid inclusions with MgCl 2 or CaCl 2 may also start melting at temperatures as low as −80°C. Such low first melt temperatures indicate the presence of metastable salt hydrates (presumably MgCl 2 · 8H 2 O, MgCl 2 · 6H 2 O or CaCl 2 · 4H 2 O). The formation of metastable phases during freezing of fluid inclusions can lead to misinterpretation of the chemical composition of fluid inclusions in natural samples. This is especially true for fluid inclusions with first melt temperatures below −37°C which may be erroneously interpreted as being rich in CaCl 2 . The final melting of ice in the presence of hydrohalite may vary by more than 15°C in fluid inclusions of different size but identical bulk composition, and occurs at lower temperatures than predicted in fluid inclusions from the NaClMgCl 2 H 2 O and NaClCaCl 2 H 2 O systems. However, the final melting temperature of ice in inclusions which fail to nucleate hydrohalite, and the final melting temperature of hydrohalite are reproducible to within ±0.1 ° C and can be used to determine MgCl 2 and CaCl 2 molalities.

The prediction of mineral solubilities in natural waters: A chemical equilibrium model for the NaKCaMgClSO4H2O system at temperatures below 25°C☆

Abstract A low temperature thermochemical model for the system NaKCaMgClSO4H2O is presented. Aqueous species and standard chemical potentials of solid-solution reactions are modeled from published data for binary and ternary solutions. The temperature range below 25°C (to near −60°C) is emphasized, although the model parameters are fitted to merge smoothly with those of higher temperature models at temperatures between 25 and 100°C. Binary and ternary specific ion interaction terms vary independently with temperature and are modeled using freezing point depression and mineral solubility measurements. The standard chemical potential of the ice-water reaction is fitted independent of the model (from vapor pressure and free energy data). Remaining standard chemical potentials of solidsolution reactions are fitted along with the specific ion interaction terms. Model predictions are tested against published data for minerals formed and brine compositions obtained by chilling seawater to the eutectic (about −54°C). The model predicts the sequence of solid phases observed to precipitate from chilled seawater (mice-mirabilite-hydrohalite-sylvite-MgCl2 · 12H2Oantarcticite). For all but mirabilite model temperatures are within the uncertainty of the measured temperature. The compositions of brines predicted by the model also closely follow the observed compositions. The model allows accurate predictions of the freezing points of simple and complex solutions in the system. Low temperature phase equilibria and mineral solubilities may also be predicted. The model may be used to determine the composition of brines in fluid inclusions in the multicomponent system based on low temperature phase equilibria.

Seismic geomorphology and sedimentology of a tidally influenced river deposit, Lower Cretaceous Athabasca oil sands, Alberta, Canada

The bitumen of the Lower Cretaceous McMurray Formation in Alberta arguably represents one of the most important hydrocarbon accumulations in the world. In-situ development relies on heat transfer through the reservoir via horizontal steam injection wells placed 4 to 6 m (13–20 ft) above horizontal producers near the base of the sandstone reservoirs. Given this technology, understanding the distribution of the resource is paramount for a successful development program. Sedimentary facies provide a direct control on bitumen distribution and recovery. Most facies models developed to describe and predict sedimentary units of the McMurray Formation consider fluvial, estuarine, and/or deltaic depositional settings. In-situ development, however, requires a particularly high-resolution sedimentologic interpretation. High-quality three-dimensional seismic reflection data and extensive drill cores from acreage located approximately 50 km (31 mi) south of Fort McMurray provide important insights into the sedimentologic organization of reservoir and nonreservoir deposits in the upper one third (40 m [131 ft]) of the reservoir interval. Geomorphologic characteristics of the strata observed in seismic time slices reveal that a fluvial depositional setting was prevalent. Ichnologic and palynologic data, as well as sedimentary structures suggestive of tidal processes, indicate a marine influence in the upper reaches of a fluvial system characterized by channels that were 390 to 640 m (1280–2100 ft) wide and 28 to 36 m (92–118 ft) deep. The complex stratigraphic architecture consists of a mosaic of large-scale depositional elements, including abandoned channels or oxbow lake fills, point bars associated with lateral accretion, point bars associated with downstream accretion, counter point bars, and sandstone-filled channels. Reservoir deposits are primarily associated with point bars and sandstone-filled channels.

Change in the size of Walker Lake during the past 5000 years

Benson, L. V., Meyers, P. A. and Spencer, R. J., 1991. Change in the size of Walker Lake during the past 5000 years. Palaeogeogr., Palaeoclimatol., Palaeoecol., 81: 189-214. In 1984, a 12-m sediment core (WLC84-8) was taken from the deepest part of Walker Lake. Samples of the core were analysed for diatoms, pollen, carbonate mineralogy, magnesium content, 6180 and 613C values of the total inorganic fraction, 6180 and 613C values of Limnocythere ceriotuberosa, 613C values of the total organic fraction, grain size, and magnetic susceptibility. The data indicate that Walker Lake became shallow and probably desiccated between i> 5300-4800 and 27002100 yr B.P. Each of the organic and inorganic proxy indicators of lake size discussed in this paper was useful in determining the presence of the shallow-lake intervals. However, none of the indicators was useful in determining the cause of the shallowlake intervals. Instead, the types of fish living in Walker Lake prior to 1940 were used to demonstrate that shallow-lake intervals resulted from diversion of the Walker River and not from climatic aridity. Major changes in mineralogy and magnesium content of carbonates and major changes in diatom populations with time were found to be a function of the chemical evolution of Walker Lake combined with changing lake size. The stable isotopes of oxygen and carbon were found to be good indicators of lake volume changes. A lake-level record for Walker Lake constructed from stable-isotope data was found to be similar to a lake-level record constructed using tufa and tree-stump data. Both records indicate relatively high lake levels between 4800-2700 yr B.P., at 1250 yr B.P., and within the last 300 yr. Substantial declines in lake level occurred ~2000 and ~ 1000 yr B.P.

Origin of Ancient Potash Evaporites: Clues from the Modem Nonmarine Qaidam Basin of Western China

Modern potash salt deposits and associated brines of the Qaidam Basin, western China, demonstrate that some anomalous marine evaporites may have formed from nonmarine brines instead of seawater. Qaidam Basin brines are derived from meteoric river inflow mixed with small amounts of CaCl spring inflow similar in composition to many saline formation waters and hydrothermal brines. Evaporation of springenriched inflow yields a predicted mineral sequence including carnallite, bischofite, and tachyhydrite that is identical to several anomalous marine evaporites. Other mixtures of river and spring inflow produce the salt assemblage expected from evaporation of seawater.

Model of seawater composition for the Phanerozoic

We present an inverse model of Phanerozoic seawater com- position calibrated against updated paleoseawater compositions from fluid inclusions in marine halites. The model considers step- wise alteration of seawater composition via: (1) variable input of river water, (2) variable rates of alteration of seawater through reactions at mid-ocean ridges, and (3) variable rates of alteration of seawater through reactions on ridge flanks and across the ocean floor in general. The model achieves agreement with paleoseawater fluid inclusion data for Na 1 ,C a 2 1 ,S O 4 2 2 , and K 1 , particularly when variable runoff is considered. Variable rates of basalt- seawater interactions at both ridges and ridge flanks are required to understand the evolution of seawater, particularly the observed, near-constant concentration of K 1 through time.

Origin of CaCl brines in Devonian formations, western Canada sedimentary basin

Abstract The majority of waters in Devonian formations of the western Canada sedimentary basin are high salinity Ca Cl brines. This relatively rare type of brine is of economic interest because of its association with both oil and hydrothermal ore deposits. These brines are important sources of Ca 2+ , Mg 2+ and ore metals in sedimentary basins. It is possible to generate the Ca Cl brines by modification of residual evaporite brines. The evaporation of Devonian sea water produced large quantities of gypsum and halite such as the Prairie Formation. Residual brines were not retained in the evaporating system. Quantities of dense, halite-saturated brine, sufficient to account for the high salinity formation waters in the basin, were lost syndepositionally. The Cl − and Br − content of waters within Devonian formations in the basin can be explained by dilution of the residual evaporite brines. Brine composition was modified as a result of density driven reflux to depths of several kilometers. This placed concentrated brines in contact with Precambrian basement rocks at elevated temperatures (100–300°C). The most important reactions between the brine and basement rocks appear to involve albitization of feldspars. Further modification of the brines occured as cements formed in Devonian carbonates. Volumetrically, the most important of these is dolomite, in addition some Devonian carbonates were dolomitized. During ascent along faults Mg 2+ was lost from hot brines as dolomite formed.

Geochemistry of great Salt Lake, Utah II: Pleistocene-Holocene evolution

Sedimentologic and biostratigraphic evidence is used to develop a geochemical model for Great Salt Lake, Utah, extending back some 30,000 yrs. B.P. Hydrologie conditions as defined by the water budget equation are characterized by a lake initially at a low, saline stage, rising by about 17,000 yrs. B.P. to fresh water basin-full conditions (Bonneville level) and then, after about 15,000 yrs. B.P., dropping rapidly to a saline stage again, as exemplified by the present situation. Inflow composition has changed through time in response to the hydrologie history. During fresh-water periods high discharge inflow is dominated by calcium bicarbonate-type river waters; during saline stages, low discharge, NaCl-rich hydrothermal springs are significant solute sources. This evolution in lake composition to NaCl domination is illustrated by the massive mirabilite deposition, free of halite, following the rapid drawdown until about 8,000 years ago, while historic droughts have yielded principally halite. Hydrologic history can be combined with inferred inflow composition to derive concentration curves with time for each major solute in the lake. Calcium concentrations before the drawdown were controlled by calcite solubility, and afterwards by aragonite. Significant amounts of solutes are removed from the lake by diffusion into the sediments. Na+, Cl− and SO42− are also involved in salt precipitation. By including pore fluid data, a surprisingly good fit has been obtained between solute input over the time period considered and the amounts actually found in lake brines, pore fluids, salt beds and sediments. Excess amounts are present for calcium, carbonate and silica, indicating detrital input.

Paleotemperatures preserved in fluid inclusions in halite

A variety of paleoclimate proxy records allow determination of relative warming or cooling. However, if we are to understand climate change, quantification of past temperature fluctuations is essential. Our research indicates that fluid inclusions in halite can yield homogenization temperatures that record surface brine temperatures at the time of halite precipitation. To avoid problems with stretching, leaking, and initial trapping of air, samples with primary, single-phase (liquid) fluid inclusions are chilled in a freezer to nucleate vapor bubbles. We tested the reliability of this method of obtaining fluid-inclusion homogenization temperatures using modern salts precipitated at Badwater Basin, Death Valley, California. Homogenization temperatures correlate well with measured brine temperatures. The same method is applied to fluid inclusions in Pleistocene halite from a core taken at the same location in Death Valley. Results are at several scales, recording diurnal temperature variations, seasonal temperature fluctuations, and longer-term warming and cooling events that correlate with major changes in the sedimentary environment related to climate. This technique is uniquely instrumental for paleoclimate studies because it offers actual, not just proxy, paleotemperature data. 27 refs., 17 figs.

Stable isotopes of lake and fluid inclusion brines, Dabusun Lake, Qaidam Basin, western China: Hydrology and paleoclimatology in arid environments

The Qaidam Basin, underlain by salt, is the largest (120,000 km2) on the Qinghai-Tibet Plateau, western China. Numerous shallow to ephemeral saline lakes and dry saline pans are present on the Qarhan Salt Plain. Dabusun Lake, the largest (about 200 km2), contains high salinity NaMgCl brines. Whereas it precipitates halite, it is fringed by a potash salt flat. The dominant inflow to Dabusun Lake, the Golmud River, contains dilute Na+HCO−3-rich meteoric waters. Dabusun Lake brines fall on an evaporation trend given by δD(‰) = 3.3δ18O−43. Both δD and δ18O values increase with salinity which in turn varies considerably with flooding and evaporation. The isotope compositions of the fluid inclusion brines from modern halite formed along the lakes edge are intermediate to those of Dabusun Lake brines and those from the salt flat. Shallow sediments beneath Qarhan consist of interbedded salts and mud. A short core section (1.3–1.7 m depth) from the northern edge of Dabusun Lake, was found to contain three dissolution surfaces and three mud partings. The δ18O values for fluid inclusions in 22 primary halite samples from this section show a record of episodic flooding (lower δ18O values) followed by evaporation (gradual increase in δ18O values). Primary fluid inclusions in halite crystallized initially at the surface provide a geochemical record of surface brines. Their major element compositions varied through time. More concentrated fluids indicate more arid conditions in the basin, whereas wetter conditions prevailed during intervals of non-salt deposition when laminated muds accumulated. The isotope compositions together with the activity of H2O of fluid inclusions in primary halite were used to determine isotope variations in regional precipitation and hence paleoclimatic changes on the Qinghai-Tibet Plateau during the past 50,000 years.