A thermodynamic investigation of barium and calcium sulfate stability in sediments at an oceanic ridge axis (Juan de Fuca, ODP legs 139 and 169)

doi:10.1016/S0016-7037(03)00177-7

Geochimica et Cosmochimica Acta

Volume 67, Issue 16, 15 August 2003, Pages 2965-2976

https://doi.org/10.1016/S0016-7037(03)00177-7 Get rights and content

Abstract

We have used a new thermodynamic model of barium and calcium sulfate solubilities in multicomponent electrolyte solutions (Monnin, 1999) to investigate the stabilities of barite and anhydrite in seawater or in marine sediment porewaters at high temperature and pressure. As a further test supplementing those previously carried out during model development, we have calculated the temperature at which standard seawater becomes saturated with respect to anhydrite. The model predicts that, upon heating at 500 bars, standard seawater becomes saturated with respect to anhydrite at 147 ± 5°C, which compares well with the literature value of 150°C (Bishoff and Seyfried, 1978). At 20 bars the calculated saturation temperature is 117 ± 3°C. This points to a non negligible pressure effect even at moderate pressures.

We have calculated the barite and anhydrite saturation indices for the in situ temperatures and pressures, from the composition of porewaters collected at ODP Sites 855, 856, 857, 858, 1035 and 1036 during ODP Legs 139 and 169 (Juan de Fuca and Gorda ridges, NE Pacific). Calculated saturation indices for porewater samples collected at depths corresponding to temperatures between 70° and 110–120°C at an in situ pressure of about 260 bars yield equilibrium values for anhydrite and barite. Saturation indices of samples collected at depths where the temperature exceeds 110–120°C, however, yield values indicating supersaturation with respect to anhydrite and undersaturation with respect to barite. This result is consistent with the redissolution of anhydrite during cooling, leading to the well documented sampling artifact affecting porewater compositions in high temperature marine sediments: anhydrite dissolution increases the porewater sulfate content, which in turn induces a loss of barium from solution through barite precipitation (the common ion effect). We postulate that this redissolution occurs in sediment samples for which the in situ temperature exceeds 110–120°C: below this limit anhydrite remains at equilibrium or does not have time to significantly dissolve before porewaters are sampled.

Introduction

Anhydrite is commonly found in high temperature oceanic environments linked to hydrothermal activity at spreading centers (Chang et al., 1996). Its occurrence is reported in altered sediments and mineral deposits forming at ridge axis, such as the Juan de Fuca ridge (NE Pacific), which has been the target of two drilling legs (139 and 169) of the Ocean Drilling Program Davis et al 1992, Fouquet et al 1998). In the laboratory, anhydrite solubility in aqueous solutions is well documented by numerous measurements at various temperatures and pressures which showed that its solubility decreases when temperature increases. This body of experimental data has been summarized and critically evaluated in the recent development of solubility models based on Pitzer’s ion interaction formalism Harvie et al 1984, Greenberg and Moller 1989, Monnin 1990. Such models which are for now limited to temperatures of about 200°C and to pressures up to 1 kbar, allow the calculation of mineral solubilities in a large range of concentration and composition, including seawater-type solutions. Due to its retrograde solubility, anhydrite precipitates when seawater is heated. Using her model of the Na-Ca-Cl-SO4-H₂O system as a thermodynamic model for standard seawater, Moller (1988) has calculated that standard seawater reaches equilibrium with anhydrite at 108°C at water vapor saturation pressure. On another hand, Bischoff and Seyfried (1978) experiments show that the seawater-anhydrite equilibrium temperature is 150°C at 500 bars.

When present in oceanic sediments, anhydrite is likely to dissolve when high temperature hydrothermal fluids cool down during mixing with low-temperature fluids linked to off-axis circulation (Sleep, 1991). Conversely, it may precipitate when seawater is heated in downwelling or recharge zones. Anhydrite precipitation may then clog the porosity and hence affect the flow geometry and the depth to which hydrothermal fluids may penetrate the crust, as extensively discussed by Sleep (1991). Anhydrite will also dissolve when sediment cores brought back to the surface cool down. This may then lead to high porewater calcium and sulfate concentrations that look anomalous when compared to those normally encountered in marine environments. This sampling artifact has been invoked to explain Ca and SO₄ concentrations of porewaters from sediment cores recovered at ODP Sites 856 and 1035 drilled during ODP Legs 139 and 169 at the Dead Dog and Bent Hill areas of the Juan de Fuca Ridge Davis et al 1992, Fouquet et al 1998. In addition, high porewater calcium and sulfate concentrations at ODP Site 856 have been attributed to an in-situ natural cause, i.e., present day in-situ dissolution of anhydrite deposited when sediment temperatures were higher due to sill intrusions (Davis et al., 1992). In another environment, the stability of anhydrite in contact with the Red Sea brines has been investigated by Monnin and Ramboz (1996) and by Anschutz et al. (2000).

Another common sulfate found in the marine environment is barite (barium sulfate). Its occurrence is widespread in suspended matter in the water column of the oceans where its formation is linked to biologic activity Dehairs et al 1980, Hanor 2000. It accumulates on the seafloor as micrometer-scale crystals during sedimentation. In oceanic hydrothermal environments, barite is an important constituent of hydrothermal chimneys and mounds Chang et al 1996, Hanor 2000. Barium sulfate solubility has also been extensively investigated by laboratory experiments. In aqueous solutions containing less than 0.25M NaCl, barite solubility increases with temperature up to 150°C, then it decreases above this temperature. This solubility maximum is not observed for more concentrated solutions (Blount, 1977). Barite solubilities in various aqueous solutions have recently been used to construct a solubility model as a function of temperature, pressure and solution composition to 200°C and to 1 kbar (Monnin, 1999).

For a given temperature and pressure, the stability of solid sulfates in the marine environment will depend on the concentrations of sulfate and of the constituent cation in the porewaters. The sulfate concentration can decrease, either by bacterial sulfate reduction at low temperatures (below about 110°C) or by thermal sulfate reduction (above about 125°C). The decrease of the porewater sulfate content may induce redissolution of solid sulfates. This has been well documented for barite, leading to so-called barium remobilization (e.g., McManus et al 1994, Torres et al 1996; Monnin et al., 2000).

Up to now the stability of sulfate minerals in the marine environment has been inferred either from petrographic observations, or from changes in porewater compositions. In this paper, we use a recent model of alkaline earth sulfate solubilities in complex electrolyte solutions as a function of temperature, pressure and solution composition (Monnin, 1999) to investigate the stability of Ca, Sr and Ba sulfates in sediments collected at a sedimented ridge axis (Juan de Fuca Middle Valley, ODP Leg 139 and Gorda Ridge Escabana Trough, ODP Leg 169).

Section snippets

In situ borehole temperatures at Middle Valley and Escanaba Trough

Middle Valley and Escanaba Trough are two sections of the Pacific ridge situated respectively at the northernmost section of the Juan de Fuca ridge and at the southern end of the Gorda ridge (Fig. 1). Because of the proximity of the North American continental margin, Middle Valley and Escabana Trough have been rapidly filled over geologic times by terrigenous sediments consisting of turbidite units of variable thickness, irregularly interbedded with biogenic pelagic sediments. Active discharge

Calculation of the Ba, Sr and Ca sulfate saturation indices from the porewater compositions

A mineral saturation index is defined here as the ratio of Q, the ionic product of the considered mineral, to its solubility product K_sp: $SI=Q/K_{sp}$

The ionic product Q of a MSO₄ mineral is defined as: $Q = m_{M}^{2+} (aq),T · m_{SO}_{4}^{2−}_{(aq),T} · γ_{MSO}_{4(aq),T}^{2}$ where m_M²⁺_(aq),T and mSO42−_(aq),T designate the total (measured) cation and sulfate molalities and γ_MSO₄_(aq),T the total (or stoichiometric) activity coefficient of the aqueous electrolyte.

Pitzer’s ion interaction formalism has allowed recent advances in

Application: barite and anhydrite stabilities in high temperature marine sediments

During ODP Leg 139 and 169, most porewaters have been retrieved by sediment squeezing following the usual ODP procedure. Porewater samples of indurated sediments obtained by the GRIND technique (Wheat et al., 1994) did not show any difference in composition with those obtained by squeezing. Major element concentrations are reported in the ODP Leg 139 (Davis et al., 1992) and Leg 169 Initial Report volumes (Fouquet et al., 1998), while Ba data for Leg 139 (Sites 855 to 858) are reported by Wheat

Discussion

One of the first questions to address in water–rock interaction modeling is to know whether calculated departures from equilibrium (indicated by saturation indices different from 1) are real phenomena associated to the dynamics of the system under study, or only model artifacts. This outlines the importance of model validation, a step in which data independent from those used in the model construction are used to test model predictions. Here we have supplemented this step by showing the good

Conclusions

1.
It is commonly accepted that standard seawater becomes saturated with respect to anhydrite at a temperature of 150°C, as shown experimentally by Bischoff and Seyfried (1978). Such an assumption overlooks the effect of pressure. The Bischoff and Seyfried (1978) experiments have been carried out for a pressure of 500 bars. We show that reducing the pressure from 500 bars to 20 bars lowers the equilibrium temperature of anhydrite in standard seawater from 147 to 117°C.
2.
We have calculated the barite

Acknowledgements

We acknowledge the support of S. Balleur by Schlumberger-Clamart. Many thanks are due to Geoff Wheat for his comments and for his interest at an early stage of this work, to Joris Gieskes and to the two anonymous reviewers for their suggestions leading to substantial improvements of the manuscript.

Associate editor: L. Walter

References (31)

P. Anschutz et al.
Geochemical dynamics of the Atlantis II Deep (Red Sea)II. Composition of metalliferous sediment pore waters
Geochim. Cosmochim. Acta
(2000)
P.A. Baker et al.
The origin of celestite in deep-sea carbonate sediments
Geochim. Cosmochim. Acta
(1988)
F. Dehairs et al.
Discrete suspended particles of barite and the barium cycle in the open ocean
Earth Planet. Sci. Lett.
(1980)
J.M. Gieskes et al.
Geochemistry of fluid phases and sedimentsRelevance to hydrothermal circulation in Middle Valley, ODP Leg 139 and 169
Appl. Geochem.
(2002)
J.P. Greenberg et al.
The prediction of mineral solubilities in natural watersA chemical equilibrium model for the system to high concentration from 0 to 250°C
Geochim. Cosmochim. Acta
(1989)
C. Harvie et al.
The prediction of mineral solubilities in natural watersThe system to high ionic strengths at 25°C
Geochim. Cosmochim. Acta.
(1984)
J. McManus et al.
Remobilization of barium in continental margin sediments
Geochim. Cosmochim. Acta
(1994)
N. Moller
The prediction of mineral solubilities in natural watersA chemical equilibrium model for the system to high temperature and concentration
Geochim. Cosmochim. Acta
(1988)
C. Monnin
An ion interaction model for the volumetric properties of natural watersDensity of the solution and partial molal volumes of electrolytes to high concentration at 25°C
Geochim. Cosmochim. Acta
(1989)
C. Monnin
The influence of pressure on the activity coefficients of the solutes and on the solubility of minerals in the system Na-Ca-Cl-SO₄-H₂O to 200°C and 1kbar, and to high NaCl concentration
Geochim. Cosmochim. Acta
(1990)

C. Monnin

A thermodynamic model for the solubility of barite and celestite in electrolyte solutions and seawater from 0 to 200°C and to 1 kbar

Chem. Geol.

(1999)

C. Monnin et al.

The anhydrite saturation index of the ponded brines and sediment pore waters of the Red Sea deeps

Chem. Geol.

(1996)

C. Monnin et al.

The barite saturation state of the world’s ocean

Mar. Chem.

(1999)

A. Paytan et al.

Benthic Ba fluxes in the central Equatorial PacificImplications for the oceanic Ba cycle

Earth Planet. Sci. Lett.

(1996)

M.E. Torres et al.

Barite fronts in continental margin sedimentsA new look at barium remobilisation in the zone of sulfate reduction and formation of heavy barites in diagenetic fronts

Chem. Geol.

(1996)

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ArticleA thermodynamic investigation of barium and calcium sulfate stability in sediments at an oceanic ridge axis (Juan de Fuca, ODP legs 139 and 169)

Abstract

Introduction

Section snippets

In situ borehole temperatures at Middle Valley and Escanaba Trough

Calculation of the Ba, Sr and Ca sulfate saturation indices from the porewater compositions

Application: barite and anhydrite stabilities in high temperature marine sediments

Discussion

Conclusions

Acknowledgements

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Article
A thermodynamic investigation of barium and calcium sulfate stability in sediments at an oceanic ridge axis (Juan de Fuca, ODP legs 139 and 169)