Jeffrey S. Hanor

Origin of saline fluids in sedimentary basins

Abstract Subsurface saline waters in sedimentary basins can be divided into three groups based on their anionic composition and salinity: (1) Waters with anions other than Cl dominant. These include Na-HCO3 and Na-acetate waters. Most such waters have salinities of less than 10 000 mg 1−1; (2) Cl-dominated, halite-undersaturated waters having salinities between 10 000 and 250 000–300 000 mg l−1. These include Na-Cl waters and, at higher salinities, Na-Ca-Cl waters; (3) Cl-dominated, halite-saturated waters with salinities typically in excess of 300 000 mg 1−1. Ca and K become increasingly dominant and Na decreases with increasing salinity. Subaerial evaporation of marine and continental waters and the subsurface dissolution of evaporites both have the potential for producing the range of salinities and dissolved chloride concentrations observed for most subsurface brines, but not their major cation compositions. The broad systematic increase in dissolved Na, K, Mg, Ca, and Sr and decrease in pH and alkalinity with increasing salinity support the hypothesis that the approach toward thermodynamic buffering by silicate-carbonate ± (halide) mineral assemblages is a first-order control on subsurface fluid compositions, even at temperatures well below 100°C. The chemical potential of chloride or, alternatively, the aqueous concentration of anionic charge, is a master variable which ranks in importance with such other variables as pressure and temperature in driving fluid-rock exchange and controlling bulk fluid compositions. This variable is in turn controlled largely by physical processes of fluid advection and dispersion. Dissolved organic acid anions are associated primarily with low salinity waters, but dissolved metals, such as Cu, Pb, and Zn are preferentially found in brines having salinities in excess of 200 g 1−1. The high chloride concentration and low pH of these saline waters may enhance solubilization of metals through chloride complexing.

Non-conservative behavior of barium during mixing of Mississippi River and Gulf of Mexico waters

The partitioning of trace elements between the aqueous and suspended fractions of estuarine waters can play a critical role in local problems of water quality and is a necessary parameter in establishing global mass balances for individual elements. A study has been made of the behavior of Ba during mixing of fresh and marine waters in a classic salt-wedge estuary — the lower Mississippi River. Samples were collected during low river stage, when much of the mixing was confined within the river channel. The behavior of dissolved Ba is strongly non-conservative, with excess Ba introduced into solution during mixing. The concentration of dissolved Ba in river water was 60 μg/l. During initial mixing (Cl= 0−5g/l), dissolved Ba values increased to 80 μg/l. With further mixing (Cl= 5−19g/l), dissolved Ba decreased to normal Gulf of Mexico surface values of 11 μg/l. The excess Ba appears to have been stripped off of suspended clays by exchange with major cations in seawater, and the behavior of Ba can be accurately described by a simple ion-exchange model. Average river waters at the time of sampling contained from 40 to 80 μg/l adsorbed, exchangeable Ba, depending on the concentration of suspended clays. Approximately half of the flux of dissolved Ba into the Gulf of Mexico was thus derived by desorption occurring near the mouth of the river. Clearly, river concentrations in the absence of exchange data are inadequate to characterize the continental input of dissolved Ba into the oceans.

Precipitation of beachrock cements; mixing of marine and meteoric waters vs. CO 2 -degassing

ABSTRACT The origin of calcite, Mg-calcite, and aragonite cements formed near the water table and in the intertidal zone of tropical and subtropical beach sediments has been the subject of extended debate. Following Field (1919), it is proposed here that much of this cement is precipitated as a consequence of loss of CO2 from carbonate-saturated beach groundwaters. Mass-transport calculations support the further proposal that vertical fluid dispersion in the phreatic zone resulting from tidal oscillation of the water table is sufficient to induce degassing of CO2 from a seaward-flowing groundwater. Loss of CO2 is further enhanced by tidal pumping of the gas phase in the vadose zone across the sediment-atmosphere interface. As sediment porosity is lowered by pre ipitation of cement, the ability of the groundwater system to degas and form new cements is reduced. As long as the system remains thermodynamically open with respect to CO2 and close to saturation with respect to calcite or aragonite, however, it remains an unlikely site for the precipitation of dolomite. The hypothesis that degassing alone is sufficient to cause supersaturation and cementation is supported by an experimental study of degassing of mixed beach and marine waters from St. Croix, U.S. Virgin Islands, in which 30 µm-thick, low-Mg calcite crusts were formed which closely resemble natural water-table cements. The waters were spiked with Hg2Cl2 to retard biological involvement. The maximum observed rate of precipitation of calcite in solutions in direct contact with the atmosphere was 10-9 moles cm-2 sec-1, which is a rate sufficient to indurate a beach sediment within a period of 12 hours. Thermodynamic calculations indicate that precipitation could not have been induced by mixing of marine and meteoric waters, as h s been proposed by others, but that loss of CO2, a process independent of mixing, was necessary to cause supersaturation.

Physical and chemical controls on the composition of waters in sedimentary basins

Abstract The composition of subsurface waters is determined not only by diagenetic reactions but also by physical processes of fluid convection and hydrodynamic dispersion. The south-west Louisiana Gulf Coast provides an instructive field example of the net effects of diagenetic reaction and solute transport on pore water compositions in a regional siliciclastic sequence. Most formation waters here have compositions totally unlike the compositions of the connate meteoric and marine fluids that were buried with their host sediments at the time of deposition. Dissolved chloride has been generated by the subsurface dissolution of salt domes and has been pervasively transported by fluid flow throughout most of the upper 3 km of the sedimentary section. The simultaneous systematic variation in dissolved Na, K, Mg, Ca and alkalinity with chloride in these waters supports the hypothesis that metastable thermodynamic buffering by silicate-carbonate mineral assemblages is a first-order control on fluid compositions. The chemical potential of chloride, or alternatively, total anionic charge, appears to be a master variable which ranks in importance with such other variables as pressure and temperature in driving diagenesis in this region. This variable is controlled largely by physical processes of advection and dispersion in the upper 3 km of the section and by dehydration reactions in deeper, mudstone-dominated sediments. Where the composition of the fluid is largely rock-buffered, the ultimate origin of the fluid and its pathway of chemical evolution may be obscured, at least in terms of major solute composition. Non-buffered components, such as Cl and Br, or isotopic compositions are more likely to retain information on original end-member fluid compositions and reaction pathways.

Determination of source ages and migration patterns of brines from the U.S. Gulf Coast basin using 129I

Abstract The long-lived cosmogenic and fissiogenic isotope 129I has been applied to the study of source ages and migration patterns of oil field brines from the U.S. Gulf Coast basin. 129 I I ratios were measured in sixty samples of saline formation water from depths of 660–4503 m in several oil fields in southeast Texas, south-central Louisiana, and offshore Louisiana. Comparisons between measured ratios and the decay curve for surface hydrospheric 129I result in minimum source ages much older than present host formation ages, indicating vertical migration of brine from older, deeper sources. Conservative corrections for a fissiogenic component due to spontaneous fission of 238U in the subsurface result in the following median source ages: Iberia field: 55 Ma, Port Barre field: 53 Ma, and Eugene Island Blocks 316 and 330: 55 Ma, pointing to Wilcox Group shales as source formations. Mesozoic sources cannot be ruled out, due to the uncertainties in estimating the fissiogenic component. Some of the ratios measured in Texas Gulf Coast brines show evidence of a greater fissiogenic component which indicates that the brines have resided in formations with locally high U concentrations.

Origin and timing of the metasomatic silicification of an early Archean komatiite sequence, Barberton Mountain Land, South Africa

Abstract Stratiform units of pervasively silicified early Archean ultramafic rocks occur near the boundary between the Onverwacht and Fig Tree groups in the Barberton Mountain Land, Republic of South Africa. The origin of these units has been variously ascribed to early Archean subaerial weathering or submarine alteration, to cataclastic metamorphism, and to the alteration of silicic tuffs. An exceptionally well-exposed example of such an alteration sequence along the Skokohla River valley in the central Barberton Mountain Land was studied to determine the nature of the precursor rocks and the timing and nature of the alteration processes. Well-preserved ghosts of spinifex- and culmulate-textured olivines and pyroxenes establish the komatiitic ancestry of the Skokohla rocks. Rock units of several distinctive lithofacies can be correlated over the entire 1.5 km strike of the sequence and were derived from the metasomatic conversion of a series of ultramafic flow units to rocks dominated by quartz, dioctohedral mica, and chlorite. The original ultramafics have been strongly enriched in Si, K, Rb, and Ba, and depleted in Fe, Mg, Ca, Na, Mn, Sr, Zn, and Ni. The elements Al, Ti, P, Cr, Zr, Y, and Nb were immobile. Alteration apparently occurred soon after extrusive activity and before any significant tectonic deformation. It is most probable that alteration resulted from the dynamic, hydrothermal upwelling of fluids through original fracture porosity in the ultramafic flows.

Isovolumetric Silicification of Early Archean Komatiites: Geochemical Mass Balances and Constraints on Origin

Stratiform quartz-phyllosilicate-carbonate alteration zones are a common feature of Archean greenstone terranes, and determining their origin is an important aspect of understanding the nature of shallow crustal alteration during the early history of the earth. Komatiites and komatiitic basalts of the uppermost Onverwacht Group (ca. 3.4-3.5 Ga), Barberton greenstone belt, South Africa, were regionally metasomatized prior to major tectonic deformation to the assemblage: quartz + dioctahedral mica + chlorite

Reactive transport involving rock-buffered fluids of varying salinity

\pm