Barium Partitioning Research Articles

Barium partitioning in calcite and aragonite as a function of growth rate

Calcite and aragonite growth rate experiments have been performed in the presence of aqueous Ba at 25 °C using the constant addition technique as a function of CaCO3 mineral growth rate (mol/m2/s). The partitioning of Ba in calcite at −8.4 ≤ Logrcalcite ≤ −7.3 exhibits a weak dependence on growth rate that can be expressed as:LogDBa,calcite=0.2477±0.0543×Logrcalcite-0.2949±0.4222;R2=0.81In the case of aragonite, Ba partitioning at −9.0 ≤ Lograragonite ≤ −7.8, exhibits a much stronger dependence on growth rate that can be described as:LogDBa,aragonite=0.4458±0.0563×Lograragonite+3.3407±0.4668;R2=0.84The determined DBa,aragonite values are systematically lower than unity and come in contrast to previous experimental works where Ba partitioning studied during aragonite nucleation. They are, however, in excellent agreement with the theoretical free energy correlation model by Wang and Xu (2001) that considers the effect of ionic radii size on DBa,aragonite. The defined growth rate dependence of Ba incorporation in aragonite provides new insights on the role of ionic radii size during the incorporation of trace elements and the formation of solid-solutions from aqueous solutions at low oversaturation degrees. The data presented here shed light on the process controlling the elevated DBa,aragonite values occurring at high saturation degrees of natural fluids with respect to aragonite that have been recorded in natural samples. Furthermore, the trend experimentally obtained herein has the potential to record variations in growth rate regimes in natural occurring aragonites and to provide information on the environmental conditions of natural waters in the past.

On the effect of aqueous barium on magnesite growth – A new route for the precipitation of the ordered anhydrous Mg-bearing double carbonate norsethite

The easiness of norsethite [BaMg(CO3)2] precipitation at ambient conditions is in vast contrast to the growth problems of other anhydrous Mg containing carbonate minerals such as magnesite [MgCO3] and dolomite [CaMg(CO3)2]. The striking structural similarity of dolomite and norsethite makes the different growth behavior even more noteworthy. In order to investigate this remarkable discrepancy, this study focuses on the fate and behavior of aqueous barium during growth of magnesite as magnesium endmember in the BaCO3-MgCO3 system. Knowledge of barium partitioning between carbonate minerals and solution, moreover, has important implications for the assessment of the mobility of toxic elements such as radium.Growth experiments have been conducted on magnesite seeds in hydrothermal mixed-flow reactors (T=100°C, pH~7.8, 0–100μM Ba2+, supersaturations Ω with respect to magnesite: ~100–200) and by hydrothermal atomic force microscopy (T=100°C, pH~8.2, 0–50μM Ba2+, Ω~60–90). The experiments showed that aqueous barium leaves magnesite growth rates unaffected but leads to norsethite precipitation. At the conditions of the experiments, norsethite growth rates were found to be controlled by the aqueous Ba2+ concentration. Given enough Ba2+, Mg2+ withdrawal from solution by norsethite clearly exceeded the withdrawal by magnesite growth. The logarithm of the solubility product of norsethite was found to be lower than −18.2 at 100°C. Microscopic investigations of the growth on the (104) surface of magnesite did not reveal any signs of Ba2+ incorporation yielding a partitioning coefficient of barium between magnesite and solution in the range of 10−2 or smaller.Preferential formation of the ordered phase norsethite over a solid solution is presumably facilitated by the large difference in Mg2+ and Ba2+ ionic radii. Due to the likely very high free energy of formation of the solid solution, ordering into distinct Ba- and Mg-layers is the only way to combine both cations within one phase. The lack of a solid solution, therefore, is one important prerequisite for the direct and unimpeded growth of an ordered double carbonate from additive free solution at ambient conditions. This behavior is in great contrast to the CaCO3-MgCO3 system where solid solution occurrence is common and growth of the ordered double carbonate (i.e. dolomite) is largely inhibited below 120°C.

Compositions of hydrothermal silicates and carbonates as indicators of physicochemical conditions in the Reykjanes geothermal system, Iceland

This paper presents results of a quantitative investigation of mineral chemistry in the Reykjanes geothermal system, Iceland, designed to assess the relative influence of various hydrothermal minerals on the observed bulk-rock mass changes of elements. Detailed petrography and electron microprobe analyses (n=∼1400) were performed on samples of rock cuttings from eleven wells that intersect different parts of the hydrothermal system. An important finding was the strong partitioning of barium into adularia, which becomes incorporated through “capture” substitution into the 12-fold coordination site, coupled with Al3+ substitution into the tetrahedral site. Adularia has average and maximum Ba contents of 0.8 and 3.5wt.%, respectively. In addition, there is no relationship between Ba concentration in adularia and crystal size, occurrence type (replacement versus vein/vug), or rock composition. There is also no correlation between the Ba content of replacement adularia with that of the igneous plagioclase being altered, demonstrating that Ba concentration in the altered rocks of the Reykjanes geothermal system is being controlled by large-scale hydrothermal redistribution processes and not primary chemostratigraphic variability. Calcite exerts a major control on strontium proportions in the bulk rock, and contains up to 1wt.% Sr. Concentrations of many minor and trace elements and some major elements in hydrothermal minerals at Reykjanes correlate positively with measured downhole temperature (to varying degrees), e.g.: Ba and Rb in adularia; Al(iv) in chlorite; Fe, Mg, Mn, and Sr in calcite; and the andradite component in garnet. The Fe/(Fe+Mg) of chlorite is unrelated to the precursor host rock ratio. Chlorite geothermometry, based on Al(iv) substitution and Fe/(Fe+Mg) content, predicts temperatures of 94–313°C (average=236°C). These temperatures are similar to or less than the present-day temperatures measured for corresponding sample depths in all wells (with the exception of a shallow interval from RN10). This may indicate a prolonged and ongoing heating trend in the deep liquid-phase region of the Reykjanes reservoir.

Crystallization of zoned (Ba, Pb)SO 4 single crystals from aqueous solutions in silica gel

Zoned single crystals of the BaSO 4–PbSO 4 solid solution have been grown from aqueous solutions in a diffusing reacting system at high supersaturation levels. The crystals have been characterized by scanning electron microscopy, EDAX and electron microprobe. Under the experimental conditions used, nucleation and crystal growth was influenced by multiple kinetic and thermodynamic factors. The low solubility of barite (BaSO 4) compared to anglesite (PbSO 4) implies a strong preferential partitioning of barium to the solid phase, but the distribution of the substituting ions between the solid and aqueous phases is also influenced by supersaturation that tends to soften this preferential partitioning. The chemical evolution of the system during crystal growth implies a morphological and compositional evolution of the solids that is recorded as “internal morphologies,” which have been observed in central sections of the crystals.

Contribution of cold seep barite to the barium geochemical budget of a marginal basin

We investigated barium partitioning in a marginal basin where large barite structures occur associated with cold seeps on the San Clemente fault, off the coast of southern California. The barium flux to San Clemente basin sediments is two to three times greater than fluxes measured in surrounding California Borderland basins where there is no evidence of seeps. Barium is accumulating in San Clemente sediments at an average rate of approximately 11 μmol m −2 day −1. Analyses of sediment trap material, water column samples, sediments and pore water indicate that expected barium-bearing mineral phases (e.g. detrital aluminosilicates, biogenic material and Fe/Mn oxyhydroxides) cannot account for all the barium accumulating in the San Clemente Basin. The excess barium, which represents transport of cold seep barite, is comparable to the combined flux of detrital and biogenic material to the basin floor. The magnitude of this flux and the occurrence of cold seeps throughout the oceans necessitate consideration of this phase in the development of barium geochemical budgets and the variety of paleoproxies involving barium in marine sediments.

The partitioning of barium and lead between silicate melts and aqueous fluids at high pressures and temperatures

The origin of subduction-related magmas is still a matter of debate in the Earth Sciences. These magmas are characterised by their distinctive trace element compositions compared to magmas from other tectonic settings, e.g. mid-ocean ridges or rifts. The distinct trace element composition of these magmas is generally attributed to alteration of the source region by a contaminating agent: either a silicate melt or a hydrous fluid, possibly chlorine-enriched. In this study, we have used μPIXE (proton induced X-ray emission) to analyse synthetic samples obtained from a micro-experimental petrology study that aims to determine the partitioning behaviour of two key elements, Ba and Pb, between silicate melt and both pure water and saline fluids. Our experiments were performed at high-pressure (>0.34–1.53 GPa) and high-temperature (697–1082 °C) in a hydrothermal diamond anvil cell, that was used as a transparent rapid quench autoclave. We observed that at high pressure and temperature, in the presence of pure water, Ba and Pb are not strongly fractionated into one phase or the other. The partition coefficient of Pb is ranging from 0.46 to 1.28. Results from one experiment performed at 0.83 GPa and 847 °C, in the presence of a saline fluid indicate that the presence of Cl induces strong fractionation of Pb and moderate fractionation of Ba both into the silicate melt. In addition, our data indicate that Cl is strongly partitioned into the fluid phase.

Trace-element partitioning at conditions far from equilibrium: Ba and Cs distributions between alkali feldspar and undercooled hydrous granitic liquid at 200 MPa

This study examines the effects of increasing supersaturation, attained by single-step liquidus undercooling (ΔT), on the partitioning of barium and cesium between potassic alkali feldspar (Afs) and hydrous granitic liquid at 200 MPa. The investigation is motivated by trace-element distribution patterns in granitic pegmatites which cannot be simulated by fractionation models using "equilibrium" partition coefficients, and thus its purpose is to assess if, how, and why partition coefficients for compatible and incompatible trace elements may vary when crystal growth commences far from the crystal–melt equilibrium boundary. Barium expands the liquidus stability field of potassic feldspar to higher temperatures, such that liquidi for the Ba-rich (~0.5 wt% BaO) compositions used are ~100 °C higher than for Ba-absent analogues. At low degrees of undercooling (ΔT~50 °C), values of DBaAfs/m. (~10–20) fall within the range of previous investigations, as do values of DCsAfs/m. (≤0.10) from experiments at all temperatures. Progressively greater undercooling is manifested in the run products by increasingly skeletal to cuneiform crystal morphologies, increased compositional zonation of Afs, and the development of compositional boundary layers in glass. Whereas the partitioning behavior of Cs (incompatible) is not measurably affected, strong undercooling apparently causes the partitioning of Ba (highly compatible) to deviate from equilibrium behavior. Feldspars produced by strong undercooling (ΔT≥100 °C) are heterogeneous, such that DBaAfs/m. versus K/K+Na varies linearly between the average value at 850 °C and the equilibrium value appropriate to the temperature of growth. Hence, high supersaturation accompanying undercooling produces feldspar compositions by isothermal growth which record a vestige of the liquid line of descent (i.e., an ontogeny within zoned crystals which approximately tracks the feldspar liquidus from high temperature to the final low temperature of actual crystal growth). Such zoning patterns may mimic normal patterns produced by fractionation with decreasing temperature under near-equilibrium (near-liquidus) conditions. Increasing fluorine contents tend to exacerbate the effects of undercooling by inhibiting feldspar nucleation, causing both K/K+Na ratios and compatible trace-element partitioning behavior in feldspar to deviate from equilibrium values. This effect continues until nucleation lag is overcome, whereupon a period of rapid growth from supersaturated liquid follows which is similar to that in F-absent compositions. Electronic Supplementary Material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1008/s00410-002-425-y.

Petrogenetic implications of Ba-sanidine in the Lionato Tuff (Colli Albani Volcanic District, Central Italy)

AbstractA vitric fiamme containing BaO-bearing sanidine (BaO = 11 wt.%) was sampled (via di Fioranello, Rome) in the pyroclastic products of the Lower Flow Unit (LFU) belonging to the Villa Senni Eruption Unit in the Colli Albani Volcanic District (Roman Comagmatic Province, Central Italy). The fiamme of the LFU is essentially made up of glass, in which scarce phenocrysts of BaO-bearing sanidine, leucite and Al2O3-bearing Ca-clinopyroxene occur. In this paper the various parameters controlling barium partitioning between alkali feldspar and LFU liquid, are discussed.

Experimental study of barium partitioning between phlogopite and silicate liquid at upper-mantle pressure and temperature

Barium partition coefficients for phlogopite crystallized from a natural lamproite at 10–30 kbar and 900–1150°C show a three-fold decrease in partition coefficient ( D) with increasing pressure from 10 to 30 kbar (e.g., from 0.75 to 0.23), and 35–50% increase in D with increasing temperature from 1000° to 1100°C at 10 and 15 kbar (e.g., from 0.51 to 0.75). The variation with temperature is contrary to most recorded variations of D, and is attributed to a compositional control in the phlogopite. D correlates with substitution of titanium in the phlogopite. Thus as the titanium content increases with increasing temperature, the barium content, and hence D, increases. Application of partitioning data for barium in models for lamproite genesis suggests that phlogopite of metasomatic origin, and possibly some barium-rich accessory phases, are present in the source rocks for lamproite, and phlogopite plays a major role in the generation of lamproitic magma.

Barium partitioning between alkali feldspar and silicate liquid at high temperature and pressure

Barium partitioning between alkali feldspar and a natural trachyte liquid, enriched with barium, has been determined as a function of pressure and temperature from 10 to 25 kb and 900°–1100° C. Both long duration experiments and a “re-equilibration” experiment suggest close approach to equilibrium. Partition coefficients (D Ba) decrease as both temperature and pressure increase (e.g., D Ba changes from 8.71 at 10 kb, 900° C to 1.48 at 25 kb, 1100° C). Water activity also controls the barium partitioning with a marked decrease in D Ba af/liq for addition of less than 0.8 wt% H2O, but with no apparent additional effect for higher water contents in the bulk composition (e.g., from 0.8–4.2 wt% H2O). The composition of alkali feldspar also has a significant effect on D Ba af/liq , but the data obtained do not allow derivation of a complete D-Or relationship. These new data suggest that Henry's Law is obeyed for most of the barium concentrations examined, and the limit of Henry's Law behaviour for barium in alkali feldspar is as high as 6 wt% BaO in alkali feldspar and 1.2 wt% BaO in the melt, similar to the results of Long (1978). The experimental results broadly overlap with natural data for D Ba, determined from coexisting alkali feldspar phenocrysts and glass (or groundmass).

Barium partitioning during the transformation of corals from aragonite to calcite

Examination of fresh (aragonite) and altered (calcite) corals from the Pleistocene reef terraces exposed on Barbados, West Indies, reveals the natural partitioning of Ba2+ during the aragonite—calcite transformation. The Ba2+ decreased from typical values of 8–15 ppm in the aragonites to 1–3 ppm in the calcites, a finding consistent with the experimentally determined partition coefficient of Ba2+ into calcite (0.06 ± 0.02 at 25°C). Quantitatively, the degree of partitioning of Ba2+ in these samples was compatible with the simultaneous partitioning of Sr2+.

Partitioning of barium between coexisting K-feldspars and plagioclase in the Bushveld granites from the Zaaiplaats area, South Africa

Partitioning of barium between coexisting K-feldspars and plagioclase in the Bushveld granites from the Zaaiplaats area, South Africa

Barium Partitioning in Carbonates: Theory and Applications: ABSTRACT

The partition coefficient for Ba2+ into calcite has been established as 0.06 ± 0.01 at 25°C (77°F). The partition coefficient proved independent of rate of precipitation in a series of 19 runs of varying duration. This value is substantially lower than that reported by Kitano in 1971, reflecting our seeding technique, which focused the experiment on crystal growth rather than on spontaneous nucleation and growth. The 0.06 value is compatible with the values reported for Sr2+ (0.05 to 0.14), a cation of identical charge, but closer to Ca2+ in ionic radius. The natural partitioning of Ba2+ was examined in fresh (aragonite) and altered (calcite) corals from the Pleistocene reef terraces exposed on Barbados, West Indies. The Ba2+ concentrations ranged from 8 to 15 ppm in the aragonites and decreased to typical values of 1 to 3 ppm in the calcites. The partitioning of Ba2+ in these samples was quantitatively similar to the attendant partitioning of Sr2+. The partitioning of Ba2+ during the carbonate cycle offers new areas of investigation. The low concentrations of Ba2+ in sedimentary biograins (3 orders of magnitude less than Sr2+) make Ba2+ a sensitive monitor of externally derived, barium-bearing solutions involved in diagenetic reactions. Likewise, t e low concentrations ensure ideal partitioning behavior in diagenetic solutions of low ionic strength. End_of_Article - Last_Page 516------------