Chemical exchange during hydrothermal alteration of basalt by seawater—I. Experimental results for major and minor components of seawater

Fresh mid-ocean ridge basalt of varying crystallinity has been powdered and reacted with seawater and an artificial Na-K-Ca-Cl solution at 200–500°C and 500–1000 bar in sealed gold capsules. Water/rock mass ratios of 1–3 were used and durations ranged from 2 to 20 months.These time periods were sufficient for most elements to approach a steady-state concentration in solution which was determined by equilibrium with alteration minerals (Mg, SiO₂, SO₄), by rate of formation of these minerals (Na, Ca), or by depletion from the rock (K, B, Ba). The resulting solutions closely resemble the brines from the basalt-seawater geothermal system at Reykjanes, Iceland. Mg was almost completely removed from seawater into the alteration products smectite, tremolite-actinolite, or talc. Sulfate also was removed to low concentrations, both by precipitation of anyhydrite and by reduction to sulfide. Net transfer of Na from seawater into solids occurred in most experiments by formation of sodic feldspar and possibly analcime. Sr was removed from seawater in some experiments but showed no change or a small gain in others. SiO₂, Ca, K, Ba, B and CO₂ were leached from basalt and enriched in solution. SiO₂ concentrations were controlled by saturation with quartz at 300°C and above. The principal Ca-bearing phases which formed were anhydrite, the hydrated Ca-silicate truscottite, tremolite-actinolite, and possibly wairakite. No K-rich phases formed. For some minerals the crystallinity of the starting basalt affected the amount which formed.Removal of Mg from seawater into solid alteration products occurred rapidly and was balanced largely by leaching of Ca from basalt. Net transfer of Na from seawater into solids occurred more slowly and was balanced mainly by leaching of additional Ca from basalt. Thus, reaction between seawater and basalt at low water/rock ratios can be considered to consist of two exchanges: Mg for Ca, and Na for Ca.     

Formation and chemical evolution of magnesium chloride brines by evaporite dissolution processes—Implications for evaporite geochemistry

The evolution of magnesium chloride brines with high bromide contents via a multistage reaction and dissolution process has been studied in brine seeps of a German potash mine. The observed chemical trends and phase equilibria can be modeled and interpreted in terms of a NaCl solution (cap rock brine) infiltrating into a potash zone characterized by the metamorphic mineral assemblage kieserite + sylvite + halite + anhydrite. Establishment of a persistent, stable equilibrium assemblage and constant fluid composition in the invariant point IP1 of the six component (Na-K-Mg-Ca-Cl-SO₄-H₂O) system of oceanic salts is prevented by the perpetually renewed input of NaCl-brine and by the intermittent exposure of incompatible kieserite. Instead, the solutions develop towards the metastable invariant point IP1(gy), with the mineral assemblage carnallite + polyhalite + sylvite + halite + gypsum, where gypsum takes the place of anhydrite (stage I). The temporary exposure of kieserite and the ensuing formation of polyhalite effectively buffer the solutions along the metastable polyhalite phase boundary during stages II and III. Eventually, in stage IV, polyhalite becomes depleted and admixture of more NaCl brine leads to low sulfate solution compositions, which are now only constrained by carnallite + sylvite + halite, and the once hexary system degenerates to a quaternary one (Na-K-Mg-Cl-H₂O) in point E. Bromide in brines shows equilibrium partitioning with respect to the wall rock minerals. The pattern of evolving brine compositions may serve as a model for similar brine occurrences, which in some cases may have been misinterpreted as remains of fossil, highly concentrated and chemically modified seawater. Similar magnesium chloride brines of salt lakes (e.g., Dead Sea, Dabusun Lake) show subtle differences and are constrained by fewer mineral equilibria (more degrees of freedom), and their low sulfate contents are due to gypsum precipitation, driven by calcium chloride input from dolomitization reactions. Finally, the observed reaction sequence is generalized, and a model for the formation of magnesium sulfate depleted, chloride-type potash salts and bischofite deposits by leaching of sulfate-type evaporites is proposed.     

An evaporated seawater origin for the ore-forming brines in unconformity-related uranium deposits (Athabasca Basin, Canada): Cl/Br and δCl analysis of fluid inclusions

Analyses of halogen concentration and stable chlorine isotope composition of fluid inclusions from hydrothermal quartz and carbonate veins spatially and temporally associated with giant unconformity-related uranium deposits from the Paleoproterozoic Athabasca Basin (Canada) were performed in order to determine the origin of chloride in the ore-forming brines. Microthermometric analyses show that samples contain variable amounts of a NaCl-rich brine (Cl concentration between 120,000 and 180,000 ppm) and a CaCl₂-rich brine (Cl concentration between 160,000 and 220,000 ppm). Molar Cl/Br ratios of fluid inclusion leachates range from ∼100 to ∼900, with most values between 150 and 350. Cl/Br ratios below 650 (seawater value) indicate that the high salinities were acquired by evaporation of seawater. Most δ³⁷Cl values are between −0.6‰ and 0‰ (seawater value) which is also compatible with a common evaporated seawater origin for both NaCl- and CaCl₂-rich brines.Slight discrepancies between the Cl concentration, Cl/Br, δ³⁷Cl data and seawater evaporation trends, indicate that the evaporated seawater underwent secondary minor modification of its composition by: (i) mixing with a minor amount of halite-dissolution brine or re-equilibration with halite during burial; (ii) dilution in a maximum of 30% of connate and/or formation waters during its migration towards the base of the Athabasca sandstones; (iii) leaching of chloride from biotites within basement rocks and (iv) water loss by hydration reactions in alteration haloes linked to uranium deposition.The chloride in uranium ore-forming brines of the Athabasca Basin has an unambiguous dominantly marine origin and has required large-scale seawater evaporation and evaporite deposition. Although the direct evidence for evaporative environments in the Athabasca Basin are lacking due to the erosion of ∼80% of the sedimentary pile, Cl/Br ratios and δ³⁷Cl values of brines have behaved conservatively at the basin scale and throughout basin history.     

Hydrothermal processes along mid-ocean ridges: An experimental investigation

An experimental investigation of high-temperature seawater/basalt interactions has been conducted in order to better evaluate the geochemical and economic implications of hydrothermal circulation of seawater in the oceanic crust along active mid-ocean ridges. The results indicate that, as seawater reacts with basalt between 200 ° C and 500 ° C at 500–800 bars, the fluid tends to change from an oxygenated, slightly alkaline, Na⁺, Mg⁺⁺, SO₄ ⁼, Cl^? solution to a reducing, acidic, Na⁺, Ca⁺⁺, Cl^?, solution with Fe, Mn and Cu concentrations up to 1500, 190 and 0.3 ppm respectively. Silica concentrations in the fluid reach concentrations of 200–600 ppm; however, Al abundances remain very low (～0.5 ppm). Gray and green smectites, anhydrite, albite, tremolite-actinolite, chalcopyrite, pyrrhotite and hematite were the dominant alteration products formed. These data imply that large-scale circulation of seawater in the oceanic crust could account for the Al-deficient metalliferous sediments associated with mid-ocean ridges and could be important in the genesis of certain Fe-Cu sulfide ore deposists. The process could also affect the geochemical budgets of certain elements and exert substantial control of the steady-state composition of seawater by removing excess Na and Mg and adding Ca, Si, and H to the oceans.     

Geochemistry and isotope chemistry of CaNaCl brines in Silurian strata,Michigan Basin,U.S.A.

Formation waters from Silurian-aged reefs in the northern and southern trends of lower Michigan were collected and analyzed for major, minor and isotope compositions. The results were combined with an analysis of an exceptionally concentrated (TDS 640 g/l) Silurian brine reported by Case in 1945 to determine the origin and possible evolutionary pathways for the chemical and isotope components of the brines. The waters are extremely concentrated(TDS> 450g/l) CaNaCl brines. Bromide values support that they originated from seawater concentrated into the MgSO₄ and possibly the KCl salt facies. The brines have, however, evolved considerably from an expected seawater composition and now contain a dominant CaCl composition. Dolomitization appears to have been very important in the brine evolution, but this process cannot explain all the Ca present in these brines. Four scenarios may explain the enrichment in Ca: (1) halite dissolution accompanied by the exchange of Na for Ca; (2) reactions involving aluminosilicate minerals, carbonates and halite; (3) an input of CaCl₂ solutions derived from altered MgCl₂ fluids released during the metamorphism of carnallite into sylvite; and (4) a pre-existing enrichment of CaCl in the Early Paleozoic seawater that filled the basin. All four are possible, but the favored explanation involves the diagenesis of the Salina A-1 potash salts. The isotope composition of the waters is consistent with evaporated seawater, perhaps enriched by exchange with carbonates or by the input of hydration water from evaporite minerals. The isotopic evolution, however, is equivocal but the brine composition does not indicate they have been diluted with meteoric water. This implies the waters have remained isolated from surface-controlled hydrological systems.     

Origin and geochemistry of Miocene marine evaporites associated with red beds: Great Kavir Basin,Central Iran

During the Cenozoic numerous shallow epicontinental evaporite basins formed due to tectonic movements in the Northern Province of the Central Iran Tectonic Zone (the Great Kavir Basin). During the Miocene, due to sea‐level fluctuations, thick sequences of evaporites and carbonates accumulated in these basins that subsequently were overlain by continental red beds. Development of halite evaporites with substantial thickness in this area implies inflow of seawater along the narrow continental rift axis. The early ocean basin development was initiated in Early Eocene time and continued up to the Middle Miocene in the isolated failed rift arms. Competition between marine and non‐marine environments, at the edge of the encroaching sea, produced several sequences of both abrupt and gradual transition from continental wadi sediments to marginal marine evaporites in the studied area. These evaporites show well‐preserved textures indicative of relatively shallow‐brine pools. The high Br content of these evaporites indicates marine‐derived parent brines that were under the sporadic influence of freshening by meteoric water or replenishing seawater. However, the association of hopper and cornet textures denotes stratified brine that filled a relatively large pool and prevented rapid variations in the Br profile. Unstable basin conditions that triggered modification of parent brine chemistry prevailed in this basin and caused variable distribution patterns for different elements in the chloride units. The presence of sylvite and the absence of Mg‐sulphate/chlorides in the paragenetic sequence indicate SO₄−depleted parent brine in the studied sequence. Petrographic examinations along with geochemical analyses on these potash‐bearing halites reveal parental brines which were a mixture of seawater and CaCl₂‐rich brines. The source of CaCl₂‐rich brines is ascribed to the presence of local rift systems in the Great Kavir Basin up to the end of the Early Miocene. Copyright © 2007 John Wiley & Sons, Ltd.     

Major,minor element chemistry and oxygen and hydrogen isotopic compositions of Marun oil‐field brines,SW Iran: Source history and economic potential

Cation and anion concentrations and oxygen and hydrogen isotopic ratios of brines in the Asmari Formation (Oligocene–early Miocene) from the Marun oil field of southwest Iran were measured to identify the origin of these brines (e.g. salt dissolution vs. seawater evaporation) as well as the involvement of water–rock reaction processes in their evolution. Marun brines are characterized by having higher concentrations of calcium (11 000–20 000 mg/L), chlorine (120 000–160 000 mg/L) and bromide (600–1000 mg/L) compared to modern seawater. Samples are also enriched in ¹⁸O relative to seawater, fall to the right of the Global Meteoric Water Line and local rain water, and plot close to the halite brine trajectory on the δD versus δ¹⁸O diagram. Geochemical characteristics of Marun brines are inconsistent with a meteoric origin, but instead correspond to residual evaporated seawater modified by water–rock interaction, most significantly dolomitization. In addition, anhydrite precipitation or sulphate reduction appears to be important in chemical modification of the Marun brines, as indicated by lower sulphate contents relative to evaporated seawater. Extensive dolomitization, the presence of anhydrite nodules and high salinity fluid inclusions in the upper parts of the Asmari Formation fit a model whereby the Marun brines likely originated from the seepage reflux of concentrated seawater during the deposition of the overlying Gachsaran Formation evaporites in the Miocene. Copyright © 2010 John Wiley & Sons, Ltd.     

Origin and geochemical reaction paths of sabkha brines: Sabkha Jayb Uwayyid, eastern Saudi Arabia

Sabkhas are ubiquitous geomorphic features in eastern Saudi Arabia. Seven brine samples were taken from Sabkha Jayb Uwayyid in eastern Saudi Arabia. Brine chemistry, saturation state with respect to carbonate and evaporate minerals, and evaporation-driven geochemical reaction paths were investigated to delineate the origin of brines and the evolution of both brine chemistry and sabkha mineralogy. The average total dissolved solids in the sabkha brines is 243 g/l. The order of cation dominance is Na⁺   >>  Mg²⁺ >>  Ca²⁺>K⁺, while anion dominance is Cl⁻ >> SO₄ ²⁻ >> HCO₃ ⁻. Based on the chemical divide principle and observed ion ratios, it was concluded that sabkha brines have evolved from deep groundwater rather than from direct rainfall, runoff from the surroundings, or inflow of shallow groundwater. Aqueous speciation simulations show that: (1) all seven brines are supersaturated with respect to calcite, dolomite, and magnesite and undersaturated with respect to halite; (2) three brines are undersaturated with respect to both gypsum and anhydrite, while three brines are supersaturated with respect to both minerals; (3) anhydrite is a more stable solid phase than gypsum in four brines. Evaporation factors required to bring the brines to the halite phase boundary ranged from 1.016 to 4.53. All reaction paths to the halite phase boundary follow the neutral path as CO₂ is degassed and dolomite precipitates from the brines. On average, a sabkha brine containing 1 kg of H₂O precipitates 7.6 g of minerals along the reaction path to the halite phase boundary, of which 52% is anhydrite, 35.3% is gypsum, and 12.7% is dolomite. Bicarbonate is the limiting factor of dolomite precipitation, and sulfate is the limiting factor of gypsum and anhydrite precipitation from sabkha brines.     

Strontium distribution and celestite occurrence in Zechstein (Upper Permian) anhydrites of West Poland

The previous study showed that the Zechstein (Upper Permian) anhydrites have about 0.2% strontium with a remarkably small sample scatter. Our study of three lower Zechstein anhydrite units (Lower Anhydrite, Upper Anhydrite and Basal Anhydrite) from West Poland indicate that although often the Sr content is 0.1–0.2%, there are common deviations. In particular, a considerable part (28%) of the studied samples is characterized by lower values (<0.1%), and on the other hand ca. 15% of samples are Sr-enriched, and in those samples celestite was recorded. Particular anhydrite levels differ especially in the frequency of samples showing great Sr content. The greatest variation was found in the Lower Anhydrite. This agrees well with the conclusion derived from the sedimentological studies indicating that there was the greatest differentiation of depositional environments during the deposition of the Lower Anhydrite. The Sr content is a good indicator of brine concentration during the gypsum precipitation and it seems that the subsequent gypsum–anhydrite transformation itself does not affect the strontium distribution. The histograms of Sr content in the Basal Anhydrite indicate a slightly higher brine concentration than it was during the Lower Anhydrite deposition, and the latter in turn was higher than brine concentration during the Upper Anhydrite sedimentation. Celestite veins are clearly diagenetic in origin. The form of celestite occurrence and the increased strontium content (1% or more) indicate an additional source of ions that occurred outside the anhydrite series. In the case of the Lower Anhydrite, the supposed additional source of Sr was related to aragonite-to-calcite transition and squeezing of CaCl₂ brines from reefs into anhydrite series due to increased pressure. For the Basal Anhydrite this source could be related to brines derived from the Older Halite deposits.     

Mineral equilibria in a six-component seawater system,Na-K-Mg-Ca-SO4-Cl-H2O,at 25°C

Phase relations in the 6-component system Na-K-Mg-Ca-SO₄-Cl-H₂O have been calculated for halite saturation, 25°C and 1 atm pressure. Using a Jänecke projection with the apices Ca-Mg-K₂-SO₄, 27 stable invariant points have been located which are connected by 69 univariant curves. Polyhalite is the only quaternary solid, but anhydrite occupies the bulk of the interior tetrahedral space. Consequently, 24 of the invariant points lie very close to the Ca-free base, Mg-K₂-SO₄. The remaining three points involve tachyhydrite and/or antarcticite. All points but two (20,27) represent peritectic conditions. Metastable equilibria have been calculated for the Ca-free system and yield relations corresponding to the solar diagram.Seawater lies in the subspace anhydrite-halite-carnallite-kieserite-bischofite (point 20) and its evaporation has been discussed for conditions of equilibrium and fractional crystallization. After gypsum is converted to anhydrite, halite precipitates. The next phase, under equilibrium conditions, is glauberite, crystallizing at the expense of anhydrite. Continued evaporation leads to glauberite resorption and eventual replacement by polyhalite. Then follow the magnesium sulfates epsomite, hexahydrite and kieserite, which are joined by carnallite. Polyhalite is replaced by anhydrite and bischoflte is added at the final invariant condition. Kainite does not appear as a primary phase under equilibrium conditions, but it is an important phase during fractional crystallization, where Ca-phases are not allowed to back-react with the brine.Up to the appearance of glauberite, thickness ratios of halite: anhydrite couplets (equilibrium or fractionation) can vary from 0 to 7, the relative amount of halite increasing with more intense evaporation. During evaporation, the activity of H₂O decreases from 0.98 (seawater) to 0.34 (final invariant brine). The data provided can be used to evaluate the effects of mineral precipitation, evaporation and brine mixing for a wide variety of natural brines.     

Low temperature basalt alteration by sea water: an experimental study at 70°C and 150°C

Basaltic glass and diabase were reacted with seawater at 70°C at 1 bar and 150°C at 500 bars to determine fluid composition and alteration mineralogy. All experiments were performed at a water/ rock mass ratio of 10.The changes in seawater chemistry depended on temperature and crystallinity of the basalt. The experiment at 70°C produced a slight but continuous loss of Mg, Na and K and enrichment of Ca and SiO₂ in the seawater while pH decreased slowly. At 150°C, in contrast, Mg and SO₄ were quickly and quantitatively removed while Ca, SiO₂, Na, K, Fe, Mn and Ba were added to the seawater. pH rose to values between 5.5 and 6.5 after an initial drop to lower values. Basalt glass reacted more extensively at 150°C than diabase.Smectite was the major alteration product (iron-rich saponite) at 150°C for both the glass and diabase experiments. Smectite from the diabase experiment was well crystallized while that from the glass experiment was poorly crystallized. The smectites are similar to smectites found in altered oceanic ophiolitic basalts.     

A strontium, oxygen and hydrogen isotopic composition of brines, Michigan and appalachian basins, Ontario and Michigan

⁸⁷Sr/⁸⁶Sr ratios of brine from samples from the Michigan and Appalachian Basins, in Ontario and Michigan, covering the stratigraphic interval from the Cambrian to Mississippian, vary from 0.708 to 0.711. With the exception of the salt units of the Salina Formation (Silurian), most values are greater than seawater for the time in question, indicating water-rock interaction. The sources of the radiogenic Sr has not been identified. All samples plot below the GMWL in δ¹⁸O−δ²H space, with the Cambrian and Ordovician samples closest to the line. Mixing of brines meteoric and glacial (Pleistocene) water is indicated in some cases. The more concentrated brines from each stratigraphic unit show a very narrow spread in values. All the Ordovician brines show a narrow range over a 200 km area for Sr, O and H isotopes, indicating extensive lateral migration of the fluids.Strontium in the brine has not equilibrated isotopically with its host rock. In some cases the late-stage minerals saddle dolomite, calcite and anhydrite have the same ⁸⁷Sr/⁸⁶Sr ratios as the brine, indicating that they precipitated from the brine in isotopic equilibrium.     

Insights to magmatic-hydrothermal processes in the Manus back-arc basin as recorded by anhydrite

Microchemical analyses of rare earth element (REE) concentrations and Sr and S isotope ratios of anhydrite are used to identify sub-seafloor processes governing the formation of hydrothermal fluids in the convergent margin Manus Basin, Papua New Guinea. Samples comprise drill-core vein anhydrite and seafloor massive anhydrite from the PACMANUS (Roman Ruins, Snowcap and Fenway) and SuSu Knolls (North Su) active hydrothermal fields. Chondrite-normalized REE patterns in anhydrite show remarkable heterogeneity on the scale of individual grains, different from the near uniform REE_N patterns measured in anhydrite from mid-ocean ridge deposits. The REE_N patterns in anhydrite are correlated with REE distributions measured in hydrothermal fluids venting at the seafloor at these vent fields and are interpreted to record episodes of hydrothermal fluid formation affected by magmatic volatile degassing. ⁸⁷Sr/⁸⁶Sr ratios vary dramatically within individual grains between that of contemporary seawater and that of endmember hydrothermal fluid. Anhydrite was precipitated from a highly variable mixture of the two. The intra-grain heterogeneity implies that anhydrite preserves periods of contrasting hydrothermal versus seawater dominant near-seafloor fluid circulation. Most sulfate δ³⁴S values of anhydrite cluster around that of contemporary seawater, consistent with anhydrite precipitating from hydrothermal fluid mixed with locally entrained seawater. Sulfate δ³⁴S isotope ratios in some anhydrites are, however, lighter than that of seawater, which are interpreted as recording a source of sulfate derived from magmatic SO₂ degassed from underlying felsic magmas in the Manus Basin. The range of elemental and isotopic signatures observed in anhydrite records a range of sub-seafloor processes including high-temperature hydrothermal fluid circulation, varying extents of magmatic volatile degassing, seawater entrainment and fluid mixing. The chemical and isotopic heterogeneity recorded in anhydrite at the inter- and intra-grain scale captures the dynamics of hydrothermal fluid formation and sub-seafloor circulation that is highly variable both spatially and temporally on timescales over which hydrothermal deposits are formed. Microchemical analysis of hydrothermal minerals can provide information about the temporal history of submarine hydrothermal systems that are variable over time and cannot necessarily be inferred only from the study of vent fluids.     

Carbonatization of oceanic crust by the seafloor hydrothermal activity and its significance as a CO2 sink in the Early Archean

Early Archean (3.46 Ga) hydrothermally altered basaltic rocks exposed near Marble Bar, eastern Pilbara Craton, have been studied in order to reveal geological and geochemical natures of seafloor hydrothermal carbonatization and to estimate the CO₂ flux sunk into the altered oceanic crust by the carbonatization. The basaltic rocks are divided into basalt and dolerite, and the basalt is further subdivided into type I, having original igneous rock textures, and type II, lacking these textures due to strong hydrothermal alteration. Primary clinopyroxene phenocrysts are preserved in some part of the dolerite samples, and the alteration mineral assemblage of dolerite (chlorite + epidote + albite + quartz ± actinolite) indicates that the alteration condition was typical greenschist facies. In other samples, all primary minerals were completely replaced by secondary minerals, and the alteration mineral assemblage of the type I and type II basalts (chlorite + K-mica + quartz + carbonate minerals ± albite) is characterized by the presence of K-mica and carbonate minerals and the absence of Ca-Al silicate minerals such as epidote and actinolite, suggesting the alteration condition of high CO₂ fugacity. The difference of the alteration mineral assemblages between basalt and dolerite is probably attributed to the difference of water/rock ratio that, in turn, depends on their porosity.Carbonate minerals in the carbonatized basalt include calcite, ankerite, and siderite, but calcite is quite dominant. The δ¹³C values of the carbonate minerals are −0.3 ± 1.2‰ and mostly within the range of marine carbonate, indicating that the carbonate minerals were formed by seafloor hydrothermal alteration and that carbonate carbon in the altered basalt was derived from seawater. Whole-rock chemical composition of the basaltic rocks is essentially similar to that of modern mid-ocean ridge basalt (MORB) except for highly mobile elements such as K₂O, Rb, Sr, and Ba. Compared to the least altered dolerite, all altered basalt samples are enriched in K₂O, Rb, and Ba, and are depleted in Na₂O, reflecting the presence of K-mica replacing primary plagioclase. In addition, noticeable CO₂ enrichment is recognized in the basalt due to the ubiquitous presence of carbonate minerals, but there was essentially neither gain nor loss of CaO. This suggests that the CO₂ in the hydrothermal fluid (seawater) was trapped by using Ca originally contained in the basalt. The CaO/CO₂ ratios of the basalt are generally the same as that of pure calcite, indicating that Ca in the basalt was almost completely converted to calcite during the carbonatization, although Mg and Fe were mainly redistributed into noncarbonate minerals such as chlorite.The carbon flux into the Early Archean oceanic crust by the seafloor hydrothermal carbonatization is estimated to be 3.8 × 10¹³ mol/yr, based on the average carbon content of altered oceanic crust of 1.4 × 10^-3 mol/g, the alteration depth of 500 m, and the spreading rate of 1.8 × 10¹¹ cm²/yr. This flux is equivalent to or greater than the present-day total carbon flux. It is most likely that the seafloor hydrothermal carbonatization played an important role as a sink of atmospheric and oceanic CO₂ in the Early Archean.     

The geochemistry of Ca,Sr, Ba and Ra sulfates in some deep brines from the Palo Duro Basin,Texas

The geochemistry of Ca, Sr, Ba and Ra sulfates in some deep brines from the Palo Duro Basin of north Texas, was studied to define geochemical controls on radionuclides such as ⁹⁰Sr and ²²⁶Ra. Published solubility data for gypsum, anhydrite, celestite, barite and RaSO₄ were first reevaluated, in most cases using the ion interaction approach of Pitzer, to determine solubility products of the sulfates as a function of temperature and pressure. Ionic strengths of the brines were from 2.9 to 4.8 m, their temperatures and pressures up to 40°C and 130 bars. Saturation indices of the sulfates were computed with the ion-interaction approach in one brine from the arkosic granite wash fades and four from the carbonate Wolfcamp Formation. All five brines are saturated with respect to gypsum, anhydrite and celestite, and three of the five with respect to barite. All are undersaturated by from 5 to 6 orders of magnitude with respect to pure RaSO₄. ²²⁶Ra concentrations in the brines, which ranged from 10^?11.3 to 10^?12.7 m, are not controlled by RaSO₄ solubility or adsorption, but possibly by the solubility of trace Ra solid solutions in sulfates including celestite and barite.     

Hydrothermal alteration of basalt by seawater under seawater-dominated conditions

Fresh mid-ocean ridge basalt glass and diabase have been reacted with seawater at 150–300°C, 500 bar, and water/rock mass ratios of 50, 62, and 125, using experimental apparatus which allowed on-line sampling of solution to monitor reaction progress. These experiments characterize reaction under what we have called “seawater-dominated” conditions of hydrothermal alteration.In an experiment at 300°C, basalt glass undergoing alteration removed nearly all Mg²⁺ from an amount of seawater 50 times its own mass. In the process, the glass was converted entirely to mixed-layer smectite-chlorite, anhydrite, and minor hematite. Removal of Mg from seawater occurred as a Mg(OH)₂ component incorporated into the secondary clay. This produced a precipitous drop in solution pH early in the experiment, accompanied by a dramatic increase in the concentrations of Fe, Mn, and Zn in solution. As Mg removal neared completion and the glass was hydrolyzed, pH rose again and heavy metal concentrations dropped.At water/rock ratios of 62 and 125 and 150–300°C, the mineral assemblage produced was similar to that at a water/rock ratio of 50. Solution chemistry, however, contrasted with the earlier experiment in that Mg concentrations in solution were greater and pH lower. This caused significant leaching of heavy metals. At 300°C nearly all of the Na, Ca, Cu, Zn, and CO₂ and most of the K, Ba, Sr, and Mn were leached from the silicates. H₂S, Al, Si, and possibly Co were also significantly mobilized, whereas V, Cr, and Ni were not. Little or no seawater sulfate was reduced.Although submarine hot spring solutions sampled to date along mid-ocean ridges clearly come from rock-dominated hydrothermal systems, evidence from ocean floor metabasalts and from heat flow studies indicates that seawater-dominated conditions of alteration prevail at least locally both in axial hightemperature systems and in ridge flank systems at lower temperatures.     

Calcium isotope (δCa) fractionation along hydrothermal pathways, Logatchev field (Mid-Atlantic Ridge, 14°45′N)

We investigate the Logatchev Hydrothermal Field at the Mid-Atlantic Ridge, 14°45′N to constrain the calcium isotope hydrothermal flux into the ocean. During the transformation of seawater to a hydrothermal solution, the Ca concentration of pristine seawater ([Ca]_SW) increases from about 10 mM to about 32 mM in the hydrothermal fluid endmember ([Ca]_HydEnd) and thereby adopts a δ^44/40Ca_HydEnd of −0.95 ± 0.07‰ relative to seawater (SW) and a ⁸⁷Sr/⁸⁶Sr isotope ratio of 0.7034(4). We demonstrate that δ^44/40Ca_HydEnd is higher than that of the bedrock at the Logatchev field. From mass balance calculations, we deduce a δ^44/40Ca of −1.17 ± 0.04‰ (SW) for the host-rocks in the reaction zone and −1.45 ± 0.05‰ (SW) for the isotopic composition of the entire hydrothermal cell of the Logatchev field. The values are isotopically lighter than the currently assumed δ^44/40Ca for Bulk Earth of −0.92 ± 0.18‰ (SW) [Skulan J., DePaolo D. J. and Owens T. L. (1997) Biological control of calcium isotopic abundances in the global calcium cycle. Geochim. Cosmochim. Acta61,(12) 2505-2510] and challenge previous assumptions of no Ca isotope fractionation between hydrothermal fluid and the oceanic crust [Zhu P. and Macdougall J. D. (1998) Calcium isotopes in the marine environment and the oceanic calcium cycle. Geochim. Cosmochim. Acta62,(10) 1691-1698; Schmitt A. -D., Chabeaux F. and Stille P. (2003) The calcium riverine and hydrothermal isotopic fluxes and the oceanic calcium mass balance. Earth Planet. Sci. Lett. 6731, 1-16]. Here we propose that Ca isotope fractionation along the fluid flow pathway of the Logatchev field occurs during the precipitation of anhydrite. Two anhydrite samples from the Logatchev Hydrothermal Field show an average fractionation of about Δ^44/40Ca = −0.5‰ relative to their assumed parental solutions. Ca isotope ratios in aragonites from carbonate veins from ODP drill cores indicate aragonite precipitation directly from seawater at low temperatures with an average δ^44/40Ca of −1.54 ± 0.08‰ (SW). The relatively large fractionation between the aragonite precipitates and seawater in combination with their frequent abundance in weathered mafic and ultramafic rocks suggest a reconsideration of the marine Ca isotope budget, in particular with regard to ocean crust alteration.     

Canadian Shield brine from the Con Mine, Yellowknife, NT, Canada: Noble gas evidence for an evaporated Palaeozoic seawater origin mixed with glacial meltwater and Holocene recharge

Dissolved noble gas concentrations were measured in high salinity (270 g/L) Ca(Na)-Cl groundwaters from the Con Mine, Yellowknife, Canada in an effort to discriminate between two possible origins, as either a brine generated by evaporative enrichment in a Paleozoic inland sea, or marine water concentrated by freezing during glacial times. Major ion and isotope geochemistry indicate that brines from the deepest level remain relatively undisturbed by mixing with modern water introduced by mining. Mixing calculations are used to quantify fractions of brine, glacial meltwater and modern water. From this, noble gas concentrations were corrected for excess air with Ne and normalized to 100% brine solution. Over-pressuring of helium and argon in the brine provide age constraints based on the accumulation of geogenic ⁴He and ⁴⁰Ar. Radiogenic age calculations together with the local geological history suggest brine emplacement during early Palaeozoic time, likely during the Devonian when evaporitic inland seas existed in this region. The concentrations of the atmospherically derived noble gases in the brine fraction (Kr = 1.4E-8, Xe = 8.5E-10 cc_STP/cc_H2O) are close to atmospheric equilibrium for brine at 25 °C (Kr = 7.3E-9, Xe = 8.0E-10 cc_STP/cc_H2O), but are far lower than would be expected for closed-system concentration of seawater by freezing (Kr = 2.8E-6, Xe = 4.2E-7 cc_STP/cc_H2O). Thus, despite the complicated mixing history of the brine, the atmospheric and geogenic noble gases provide strong evidence for an origin as air-equilibrated brine from evaporated Paleozoic seawater, which infiltrated via density displacement through existing fractures and faults into the Canadian Shield.     

Hydrothermal alteration of oceanic basalts by seawater

Hydrothermally altered pillow basalts dredged from the Mid-Atlantic Ridge, and belonging to the greenschist facies, have been studied in order to determine the mineralogical and corresponding chemical changes, that result from basalt-seawater interaction at elevated temperatures.The mineralogical transformations are predominantly to albite-actinolite-chlorite-epidote assemblages. Quartz and pyrite are common accessory minerals. On the basis of their mineralogy, the samples may be divided into chlorite-rich and epidote-rich assemblages. The chlorite-rich assemblages, which are the predominant variety, show the greatest chemical changes, while the epidote-rich samples show very little change in composition compared with their basaltic precursors.Mass balances across individual pillows in which the central portions are relatively unaltered allow the directions and ranges of elemental fluxes to be calculated. In general, SiO₂ and CaO are leached from the basalt, while MgO and H₂O are taken up. No consistent trends are observed for Na₂O and K₂O although they do show some variations in the core-and-rim analyses.Consideration of the elemental fluxes in terms of steady-state geochemical mass balances for oceanic inputs and outputs indicates that hydrothermal alteration provides a sink for Mg, which may be extremely important in solving the problem of apparent excess Mg input to the oceans. The amount of Ca that is leached from the rock may be of significance in the geochemical budget of that element. The amount of SiO₂ in the circulating fluid is controlled by the solubility of quartz or amorphous silica, depending on temperature, and considerable redistribution of silica takes place within the basaltic pile. The changes in redox conditions during hydrothermal alteration do not affect the present day oxidation state of the atmosphere and hydrosphere.