Alejandro Rodríguez

Manfred J Van Bergen

H G M Eggenkamp

Experimental evaporation of hyperacid brines: Effects on chemical composition and chlorine isotope fractionation

Hyperacid brines from active volcanic lakes are some of the chemically most complex aqueous solutions on Earth. Their compositions provide valuable insights into processes of elemental transfer from a magma body to the surface and interactions with solid rocks and the atmosphere. This paper describes changes in chemical and δ 37 Cl signatures observed in a 1750 hour isothermal evaporation experiment on hyperacid (pH 0.1) sulphate-chloride brine water from the active lake of Kawah Ijen volcano (Indonesia). Although gypsum was the only evaporite mineral identified in the evolving brine, decreasing Si concentrations may ultimately result in amorphous silica precipitation. Geochemical simulations predict the additional formation of elemental sulphur at lower water activities ( ≤ 0.65) that were not reached in the experiment. Absence of other sulphates and halides despite the high load of dissolved elements (initial TDS ca. 100 g/kg) can be attributed to increased solubility of metals, promoted by extensive formation of complexes between the variety of cations and the major anions (HSO4 -, Cl - , F -) present. Chlorine deviations from a conservative behaviour point to losses of gaseous hydrogen chloride (HCl(g)) and consequently an increase in Br/Cl ratios.

Chlorine isotope fractionation that accompanied the escape of HCl(g) showed a marked change in sign and magnitude in the course of progressive evaporation of the brine. The calculated factor of fractionation between HCl(g) and dissolved Cl for the initial interval (before 500 hours) is positive ( ), indicating that, at first, the escaping HCl(g) was isotopically heavier than the dissolved Cl remaining in the brine.

Conversely, fractionation shifted to the opposite direction in the subsequent interval ( ), in agreement with values reported in literature. It is proposed that Cl isotopic fractionation in highly acidic brines is controlled by the distribution of dissolved chlorine species, which changes from Cl -to HCl o dominance with the progressive pH decline.

The Kawah Ijen lake acquired its extreme composition through influx of sulphur and halogenrich magmatic gas components and extensive rock dissolution. If hyperacid brines with comparable chemical composition existed on Mars, evaporation processes up to the extent reported here ( 0.85), were likely accompanied by losses of gaseous HCl. The resulting changes in Cl isotope compositions, Br/Cl, S/Cl and other ratios in the residual brine might be potentially recorded in assemblages of halogen-bearing secondary evaporation minerals. Also, volcanic-hydrothermal brines as these would extend the stability of liquid water on the Martian surface down to a temperature of -90°C.

INTRODUCTION

Many craters hosted by active volcanoes contain SO4-Cl hyperacid brines, which are complex aqueous solutions from a chemical point of view, being often marked not only by extreme acidity (pH<2) but also by a high load of dissolved components (TDS>2%) [START_REF] Rowe | Fluidvolcano interaction in an active stratovolcano: the crater lake system of Poás volcano, Costa Rica[END_REF][START_REF] Christenson | Evolution of a vent-hosted system beneath Ruapehu Crater Lake, New Zealand[END_REF][START_REF] Delmelle | Geochemistry, mineralogy and chemical modelig of the acid crater lake of Kawah-Ijen Volcano, Indonesia[END_REF][START_REF] Kempter | Leakage of active crater lake brine through the north flank at Rincón de la Vieja volcano, northwest Costa Rica, and implications for crater collapse[END_REF][START_REF] Varekamp | Volcanic lake systematics II. Chemical constraints[END_REF]2009;Rouwet el al., 2014). Evaporation is a major process, contributing to an increase in the concentration of total dissolved components and a decrease of volatile species. Under highly acidic conditions gaseous HCl will escape from the lake surface [START_REF] Rowe | Fluidvolcano interaction in an active stratovolcano: the crater lake system of Poás volcano, Costa Rica[END_REF][START_REF] Rouwet | Isotope fractionation and HCl partitioning during evaporative degassing from active lakes[END_REF], with volatilization being enhanced by increasing H2SO4 abundance (lowering pH), water temperature or Cl-content [START_REF] Truesdell | Origin and transport of chloride in superheated geothermal steam[END_REF][START_REF] Capaccioni | HCl degassing from extremely acidic crater lakes: preliminary results from experimental determinations and implications for geochemical monitoring[END_REF].

Limited chlorine isotope data available for acid crater lakes show a modest δ 37 Cl range between 0.0 and +1.2‰ [START_REF] Eggenkamp | The Geochemistry of chlorine isotopes[END_REF][START_REF] Sharp | An experimental determination of chlorine isotope fractionation in acid systems and applications to volcanic fumaroles[END_REF]Rodríguez et al., 2016a), in which any potential effects from fractionation induced by evaporative loss of gaseous HCl are difficult to deduce. So far, empirical insight into isotopic fractionation between aqueous and gaseous Cl compounds comes from experimental studies, which have been restricted to simple synthetic systems [START_REF] Sharp | An experimental determination of chlorine isotope fractionation in acid systems and applications to volcanic fumaroles[END_REF]. Equilibrium evaporation experiments on 1 M HCl performed by these authors resulted in a chlorine isotope fractionation factor between HCl gas and aqueous chloride ( of +1.4‰ to +1.8‰ (50-79°C), closely matching theoretical values reported by [START_REF] Schauble | Theoretical estimates of equilibrium chlorine-isotope fractionations[END_REF] (+1.7‰; 50-100°C) and [START_REF] Czarnacki | Isotope fractionation in aqua-gas systems: Cl2-HCl-Cl -, Br2-HBr-Brand H2S-S 2[END_REF] (+1.55‰ to +1.69‰; 60°C). However, [START_REF] Sharp | An experimental determination of chlorine isotope fractionation in acid systems and applications to volcanic fumaroles[END_REF] also conducted evaporation experiments on concentrated hydrochloric acid (11.6 M) in an open system at room temperature, which resulted in a negative value of -4.0‰. They proposed that, in this case, HCl(g) is relatively depleted in 37 Cl because the kinetic effect associated with the higher translational velocity of H 35 Cl predominates over the equilibrium fractionation between HCl(g) and Cl - (aq) in which 37 Cl preferentially partitions into the HCl(g).

This paper extends this experimental work to more complex natural aqueous solutions, and describes the effects of evaporation on the chemical and Cl isotopic composition of SO4-Cl hyperacid brine from an active volcanic crater lake. The results are potentially relevant for interpretations of the mineralogical marks that fluids have left on the surface of Mars. Firstly, the former presence of SO4-Cl acidic brines on Mars has been invoked to explain the high concentrations of sulphur and chlorine found by Viking, Pathfinder and MER (Mars Exploration Rovers) Opportunity and Spirit in soils and rock weathering rinds [START_REF] Brass | Stability of brines on Mars[END_REF][START_REF] Clark | The salts of Mars[END_REF][START_REF] Bell | Mineralogic and compositional properties of Martian soil and dust: Results from Mars Pathfinder[END_REF][START_REF] Clark | Chemistry and mineralogy of outcrops at Meridiani Planum[END_REF][START_REF] Tosca | Chemical divides and evaporite assemblages on Mars[END_REF][START_REF] Zolotov | Timing of acid weathering on Mars: A kineticthermodynamic assessment[END_REF][START_REF] Schmidt | Hydrothermal origin of halogens at Home Plate, Gusev Crater[END_REF].

These findings indicate that probably substantial volcanic degassing of volatile-rich magmas occurred on Mars throughout time [START_REF] Gellert | Chemistry of rocks and soils in Gusev crater from the Alpha Particle X-ray Spectrometer[END_REF][START_REF] Haskin | Water alteration of rocks and soils on Mars at the Spirit rover site in Gusev crater[END_REF][START_REF] Gellert | Alpha particle X-ray spectrometer (APXS): Results from Gusev crater and calibration report[END_REF]. The water-gas-rock interaction processes that once operated at or near the surface of Martian sulphate-rich terrains may have been similar to those of active terrestrial systems with hyperacid lakes, in view of similarities in alteration mineralogy and element distributions (Rodríguez andvan Bergen 2015, Rodríguez et al. 2016b;[START_REF] Rodríguez | Superficial alteration mineralogy in active volcanic systems: An example of Poás volcano, Costa Rica[END_REF]. The analogy is supported by experimental constraints on the evaporative evolution of acid-sulphate solutions equivalent to those derived from the chemical weathering of synthetic Martian basalt [START_REF] Tosca | Experimental constraints on the evaporation of partially oxidized acid-sulfate waters at the Martian surface[END_REF]. Furthermore, acidic brines are particularly relevant because large amounts of dissolved components could strongly increase the stability of liquid H2O on Mars by depressing the freezing point and extending its P-T field (Mölhmann and Thompsen, 2011;[START_REF] Elsenousy | Effect of evaporation and freezing on the salt paragenesis and habitability of brines at the Phoenix landing site[END_REF]. Finally, minerals formed as result of evaporation or freezing processes might be an important H2O-sink on Mars´ surface and could have played a major role in controlling acidity and salinity of Martian paleo-fluids and hence the habitability on this planet [START_REF] Knauth | Eutectic Brines on Mars: Origin and Possible Relation to Young Seepage Features[END_REF][START_REF] Toner | Modeling salt precipitation from brines on Mars: Evaporation versus freezing origin for soil salts[END_REF].

The brine used in the experiment comes from the hyperacid lake of Kawah Ijen, an active andesitic stratovolcano located at the rim of Ijen caldera, the easternmost volcanic complex on Java, Indonesia. This crater lake is one of the world's largest bodies of natural hyperacid brine, and is characterized by a high load of dissolved elements (often >100 g/l), very low pH (<1) and very high concentrations of chlorine (often >20000 mg/kg), sulphate (often >60000 mg/kg) and rock-forming elements Al, Fe, Ca, Mg, Na, K and Si [START_REF] Delmelle | Geochemistry, mineralogy and chemical modelig of the acid crater lake of Kawah-Ijen Volcano, Indonesia[END_REF][START_REF] Delmelle | Geochemistry of the magmatic-hydrothermal system of Kawah Ijen volcano, East Java, Indonesia[END_REF][START_REF] Van Hinsberg | Extreme alteration by hyperacidic brines at Kawah Ijen volcano, East Java, Indonesia I. Textural and mineralogical imprint[END_REF][START_REF] Caudron | Stratification at the Earth's largest hyperacidic lake and its consequences[END_REF]. The Kawah Ijen lake water type is typically found on active volcanic crater lakes. These waters normally present pH values between 2 and -1, and concentrations of Cl -+SO4 2-between 3000 and 300000 mg/kg [START_REF] Varekamp | Volcanic lake systematics II. Chemical constraints[END_REF].

METHODS

Evaporation experiment

The Kawah Ijen brine selected for the evaporation experiment was collected on October 30 th , 2002 and has been employed as in-house standard for hyperacid brine analysis at the Department of Earth Sciences at Utrecht University. The total amount of brine used for the experiment (1.4 l) was filtered through a 0.2 µm pore size cellulose acetate membrane to remove amorphous silica and gypsum that had precipitated upon storage for many years. A total of 26 plastic Greiner® tubes were filled with approximately 50 ml brine, and were collectively placed on a plastic rack that was partially submerged in a water bath at 60°C (Fig. 1). The initial weight of all samples was recorded before the evaporation experiment started.

The experiment lasted 1750 hours (73 days) during which duplicate samples were removed from the water bath at certain time intervals. Directly after removal, the final weight, pH and electrical conductivity, measured at room temperature (20±1°C), were determined for each sample. Next, six aliquots of approximately 1.6, 0.6 and 0.2 grams were diluted 10, 25 and 100 times with deionised water for IC (ion-chromatography) analysis, and 10, 25 and 100 times with 2% Suprapur® HNO3 solution for ICP-OES (inductively coupled plasma optical emission spectrometry) analysis. Towards the end of the evaporation experiment, some samples had to be further diluted up to 650 times for IC analysis. The pH and conductivity measurements were performed with a WTW® 3430 portable multimeter at room temperature (20°C). The pH electrode was calibrated with pH 1, 4 and 7 buffers. The conductivity electrode was calibrated with a 0.01 M KCl standard. Both electrodes were calibrated before every measurement. The acicular crystals formed during the experiment where separated from the solutions by filtration through a 0.2 µm pore size cellulose acetate membrane, washed with deionized water and finally dried overnight at 60°C.

Analytical methods

Major elements

The fluoride concentrations were determined by ion chromatography (IC), using a Dionex ® ICS-3000, equipped with a Dionex ® IonPac ® AS 19 column. A gradient elution of 10 -50 mM KOH was utilized. Total sulphur (ST), Cl, Al, B, Br, Ca, Fe, K, Mg, Na, Si, Sr and Ti were analysed by inductively coupled plasma optical emission spectrometry (ICP-OES) with a Spectro ® Ciros ® instrument. The average relative uncertainties on the analysis where the following: 1% (ST and Si); 2% (B, Br, Ca, Fe, K, Mg, Na and Sr); 3% (Al and Cl) and 4% (Ti and F). All the aforementioned analyses where carried out at the Department of Earth Sciences, Utrecht University.

Chlorine isotopes

Samples for δ 37 Cl analysis were prepared following a variation of the method described by [START_REF] Kaufmann | Chlorine in groundwater: stable isotope distribution[END_REF] and [START_REF] Eggenkamp | The Geochemistry of chlorine isotopes[END_REF]. Based on the Cl concentrations determined by ICP-OES, an aliquot of an undiluted sample from the evaporation experiment was treated with 4 ml of 1 M KNO3, 1 ml of H2O2 (30 wt.%) and 1 ml of concentrated HNO3 (65 wt.%). Then this mixture was heated to 80°C for 30 minutes before 1 ml of 0. [START_REF] Kaufmann | Chlorine in groundwater: stable isotope distribution[END_REF]. The analytical accuracy was ±0.16‰ (1σ) based on longterm analyses of this standard. Nevertheless, samples were analysed at least in duplicate and the averages were accepted if the difference between duplicates was ≤

(1)

Chemical composition of the precipitate

Raman spectra were obtained from the precipitate (acicular crystals) formed during the experiment using a 532 nm wavelength Nd-YAG laser. An optical microscope with a 50 x long working distance objective lens provided a laser spot size of ~2 µm at the sample surface. The scattered light was dispersed using a grating of 600 grooves/mm before being collected on a charge coupled device camera. Each spectrum was acquired 20 times with an integration time of 0.2 seconds. Detailed images were also obtained from carbon-coated crystals with a JEOL JCM-6000 Benchtop SEM. Spot analysis were performed on carbon-coated crystals with a JEOL JXA-8530F field-emission-electron-microprobe. The beam conditions were 15 kV, 10 nA and 20 µm diameter; scans were also done with a defocused 10 µm beam.

Geochemical modelling

The geochemical modelling software PHREEQC, version 3.1 [START_REF] Parkhurst | User's guide to PHREEQC (Version 2), A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations[END_REF], was used to calculate aqueous species distributions and mineral saturation states. The Lawrence Livermore National Laboratories thermodynamic database (llnl.dat) was used. Additionally, ion interaction parameters from many ion pairs were included [START_REF] Pitzer | Thermodynamics of electrolytes II. Activity and osmotic coefficients for strong electrolytes with one or both ions univalent[END_REF]. The pH values at 60°C were recalculated by charge balance with PHREEQC, as it is recommendable for extremely acid waters [START_REF] Nordstrom | Negative pH and extremely acidic mine waters from Iron Mountain, California[END_REF].

RESULTS

Evaporation rates

The total duration of the evaporation experiment was 1750 hours (~73 days). The weight loss of the brine virtually stopped after 1340 hours (samples 12 and 13) when a maximum of 58% was reached. (Fig. 2; Table 1). Evaporation rates were calculated taking the transversal area of the Greiner ® tubes (0.0601 dm 2 ) into account. The brine exhibited a decrease in evaporation rates throughout the experiment. In particular, around 400 hours, the brine showed a marked decrease (from 1.01 to 0.83 g/h/dm 2 ), after which there was a smooth decline until the end of the experiment.

Chemical composition, mineral saturation states and speciation in the brine

The hyperacid SO4-Cl composition of the Kawah Ijen brine is a result of the condensation of magmatic SO2, HCl, HF and HBr gases in water of largely meteoric origin. The acidity comes mainly from HSO4 -dissociation, together with contributions of HCl(aq), HF(aq) and HBr(aq) [START_REF] Giggenbach | Variations in the carbon, sulfur and chlorine contents of volcanic gas discharges from White Island, New Zealand[END_REF][START_REF] Giggenbach | Redox processes governing the chemistry of fumarolic gas discharges from White Island, New Zealand[END_REF]. Because of the strong acidity, the fluid contains high concentrations of rock-forming elements such as Ca, Al, Fe, Mg, Na, K and Si derived from the dissolution of primary minerals and glass. At the start of the experiments, the initial concentrations of total sulphur (ST), Cl, F, Br, Al, Fe, K, Na, Ca, and Mg in the brine were 21800,22100,1410,56,5730,1970,1230,1110,690 and 690 mg/kg, respectively.

Total dissolved solids (TDS) were 96 g/kg and pH was 0.10 (20±1°). At the end of the experiment, upon 58 wt. % of evaporation, the final concentrations were (in the same order): 52400,45900,3110,124,13800,4920,2920,2610,700 and 1630 mg/kg. The total dissolved solids increased up to 234 g/kg, and pH decreased to -0.41 (20±1°) (Table 2).

In order to explore elemental losses by mineral precipitation or evaporation of volatile phases, time series were plotted for ratios of element concentrations against Mg, adopted as a perfectly conservative solute (Fig. 3). Notable negative deviations from a horizontal trend in the ratio plots indicate removal of Cl, Ca, Sr and Si during some intervals in the evaporation process. The Cl/Mg ratio shows a steady decline virtually from the start (Fig. 3a), indicating an escape of gaseous HCl from the brine throughout the experiment, in absence of a Cl-bearing precipitating solid phase. There is a concomitant gradual increase of log values in the course of the experiment (Fig. 4a).

Conversely, the Br/Mg ratio remains largely constant although a decreasing tendency is suggested for the starting interval (Fig. 3b). The increase of log values was relatively small (Fig. 4a).

The Ca depletion in the brine after 594 hours (Fig. 3c) coincides with the appearance of acicular crystals from sample 10 (762 hours) onward until the end of the experiment.

According to Raman spectra and electron backscatter imaging (Figs. 5a and5b), together with semi-quantitative EPMA-WDS analysis this phase is gypsum (CaSO4•2H2O), containing appreciable amounts of Sr (average 700±200ppm). The decrease of the Sr/Mg follows the same timing as that of Ca/Mg (Fig. 3c). Hence, precipitation of gypsum explains the gradual depletion of both Ca and Sr, since the latter can substitute Ca [START_REF] Ichikuni | Partition of strontium between gypsum and solution[END_REF][START_REF] Kushnir | The coprecipitation of strontium, magnesium, sodium, potassium and chloride ions with gypsum. An experimental study[END_REF]. Celestine (SrSO4) was not detected and remained far from saturation (Fig. 4b).

Silicon concentrations exhibit a significant decrease towards the end of the experiment as is expressed in the Si/Mg decline after 930 hours (Fig. 3d). Because no silica-bearing solid phase was directly detected by SEM analyses, it is conceivable that escape of gaseous could be responsible for the Si loss. However, this option is difficult to substantiate with the F/Mg trend, from which no obvious loss of fluorine can be deduced (Fig. 3b). On the other hand, it must be realized that the fluorine required to match the loss of silicon as is too little, compared to the amount of dissolved F (>1410 mg/kg), to be noticeable within the analytical error. Alternatively, the apparent Si loss resulted from the precipitation of amorphous silica particles, which remained unobserved because of their small size and quantity (see Discussion). Other element/Mg ratios display horizontal trends (Figs. 3e and 3f), suggesting a conservative behaviour.

As Figure 4b illustrates, Ca-sulphates and amorphous silica were the only major-element hosts that approached saturation closest in the PHREEQC models. Calculated fugacities of HCl(g), HBr(g) and SiF4(g) show increases with time (Fig. 4a). The behaviour of Ti is governed by anatase (TiO2), which is commonly saturated in the Kawah Ijen lake [START_REF] Delmelle | Geochemistry, mineralogy and chemical modelig of the acid crater lake of Kawah-Ijen Volcano, Indonesia[END_REF][START_REF] Delmelle | Geochemistry of the magmatic-hydrothermal system of Kawah Ijen volcano, East Java, Indonesia[END_REF]. During the experiment, the concentrations of dissolved Ti were close to equilibrium (Figs. In summary, the only phases that formed upon brine evaporation were HCl(g), gypsum and a Si-bearing phase (probably amorphous SiO2 ). Anatase (TiO2) was in equilibrium and its presence could not be detected by the analysis.

Total dissolved chlorine (Cldiss.) is present in the form of undissociated hydrogen chloride (HCl 0 ) and Cl -, with the activity of the latter species being dominant (Fig. 6).

When evaporation proceeded, the activity of HCl 0 increased strongly, reflecting displacement of the equilibrium of reaction (2) to the left hand side due to the increase of H3O + . The same mechanism favoured the formation of H2SO4 0 and HBr 0 , but activities of these species were several orders of magnitude lower than that of HCl 0 (reactions 3 and 4) (Fig. 6).

(2)

(3) (4)

Aluminium has a strong tendency to form complexes with F -, so that most of this element is present as AlF2 + , AlF 2+ and AlF3 (Fig. and Cl -such as MgCl + , MgSO4 0 , CaCl + and CaSO4 0 , where the sulphur-bearing species always have higher activities than their chlorine-bearing counterparts (Figs. d and e; EA-2). Finally, potassium and sodium tend to remain largely as K + and Na + , with the most significant complexes being KHSO4 0 and NaCl 0 , respectively (Figs. f and g; EA-2). The dominant strontium species in the early part of the experiment was Sr 2+ , which decreased rapidly in favour of the SrCl + and SrSO4 0 complexes (Figure h, EA-2). The apparent strong reduction of Sr 2+ availability in the course of the experiment may explain our observation that the precipitated gypsum incorporated 3-4 times less strontium than expected from low-temperature gypsum-solution partitioning data [START_REF] Ichikuni | Partition of strontium between gypsum and solution[END_REF][START_REF] Kushnir | The coprecipitation of strontium, magnesium, sodium, potassium and chloride ions with gypsum. An experimental study[END_REF].

Chlorine isotope trends and Cl losses

From the initial δ 37 Cl composition of the hyperacid brine (+0.36±0.05‰) until sample 9 (500 hours), a slight decrease down to +0.22±0.04‰ was observed. This trend was followed by a significant increase up to +0.68±0.05‰ in the last sample (Table 3, Fig. 7).

The changes in isotope composition are accompanied by chlorine losses through escaping HCl(g), given the absence of any solid precipitating Cl-bearing phase during the experiment. The HCl(g) degassing of the brines probably started early (Fig. 3a), but the significant increase in log values after 500 hours (Fig. 4a) suggests an enhancement from that moment on.

Chlorine losses by degassing HCl(g) were calculated from the differences between measured and calculated Cl concentrations (Table 3). The concentration differences were transformed into mass losses (mg) for each sample, based on the remaining liquid mass (Table 1). Fractionation between HCl(g) and total dissolved chlorine (Cldiss.) during the evaporation experiment can be calculated using the Rayleigh fractionation model:

(5)

Where δi and δf are the δ 37 Cl composition for the initial Kawah Ijen brine (+0.36±0.05‰; sample t=0) and the subsequent δ 37 Cl compositions throughout the evaporation experiment, respectively. The fractionation factor is α, and F is the total chlorine remaining in the liquid, defined by: (6)

Expressing equation ( 5) in a linear form gives:

(7)

Where α can be obtained from the slope of the line. The experiments yielded two distinctive trends for different time intervals (Table 3). The corresponding values are 1.00300±0.000002 (t=0-500 hours) and 0.994479±0.000004 (t=500-1750 hours) (Fig 8a). The fractionation factors ( ) before and after 500 hours, are +2.963±0.014‰ and -5.536±0.004‰, respectively (Fig 9a). This implies that the HCl(g) produced during the first 500 hours of the experiment was enriched in 37 Cl compared to the Cldiss. in the hyperacid brine, whereas after 500 hours it was depleted in 37 Cl.

An alternative approach to calculate these fractionation factors is by using F values derived from the ratio between Cl and a conservative element such as boron. For each sample, the Cl/B ratio was determined and normalized to the ratio in the initial brine (sample t=0). In this case:

(8) and then,

The resulting F values and fractionation factors were again obtained from equations ( 6) and ( 7) by linear regression, assuming Rayleigh fractionation (EA-3) (Figs. 8b and9b).

In a similar way, F values and fractionation factors were also calculated for Fe and Mg as conservative elements (EA-3), (Figs. 8c, 8d, 9c and 9d) and essentially the same results

were obtained (Table 4).

DISCUSSION

According to experiments and models reported in literature, calcium sulphates are often the first phases to precipitate upon evaporation of saline brine waters, followed by a variety of K, Mg, Fe 2+ and Fe 3+ -sulphates [START_REF] Tosca | Chemical divides and evaporite assemblages on Mars[END_REF]2009;[START_REF] Gamazo | Modeling the interaction between evaporation and chemical composition in a natural saline system[END_REF][START_REF] Elsenousy | Effect of evaporation and freezing on the salt paragenesis and habitability of brines at the Phoenix landing site[END_REF][START_REF] Toner | Modeling salt precipitation from brines on Mars: Evaporation versus freezing origin for soil salts[END_REF]. In evaporation experiments on a Cl- an equilibrium pathway predicted by thermodynamic models but were influenced by kinetic controls on the precipitation of copiapite-group minerals. [START_REF] Gamazo | Modeling the interaction between evaporation and chemical composition in a natural saline system[END_REF] modelled the evaporation of Mg-Na-Cl-SO4 brine water from Quero lake (Spain), and observed the appearance of an anhydrite-gypsum paragenesis at = 0.78, followed by epsomite-hexahydrite and hexahydrite-kieserite paragenesis at values of 0.57 and 0.50, respectively. Similarly, based on the Na-K-Ca-Mg-Cl-SO4-ClO4 brines that could have produced the salt assemblages at the Phoenix landing site on Mars, the evaporation models of [START_REF] Toner | Modeling salt precipitation from brines on Mars: Evaporation versus freezing origin for soil salts[END_REF] predicted the appearance of anhydrite-gypsum, epsomitehexahydrite and hexahydrite-kieserite paragenesis at values of 0.68, 0.56 and 0.52, respectively.

In comparison to this general picture, two outcomes of our evaporation experiment regarding the formation of new phases during evolution of the brine are noticeable: the small number of precipitating minerals despite the high load and diversity of dissolved elements, and the apparent escape of several volatile components other than water.

Relative to the evaporative enrichment factor of a perfectly conservative element such as Mg (2.37), the EF values of Ca (1.01), Si (1.72), Sr (1.97)

and Cl (2.08) are significantly lower, indicating partial removal of these elements into phases, inferred to be gaseous HCl (for Cl) and gypsum (for Ca and Sr) and possibly amorphous silica (for Si) as solids. Due to its elevated concentration in the brine, the total sulphur loss associated with gypsum formation was too little to be measurable outside the analytical error.

Hyperacid brine evolution and evaporite minerals

The observation that gypsum and possibly amorphous silica were the only precipitating phases in the course of the experiment is grossly consistent with thermodynamic equilibrium, since the models indicate that other phases stayed far from saturation (Fig. 4b). Gypsum was the precipitating calcium-sulphate although anhydrite tended to be closer to saturation. Reported temperatures of 42-60°C for the transition between gypsum and anhydrite [START_REF] Posnjak | The system, CaSO4-H2O[END_REF][START_REF] Posnjak | Deposition of calcium sulphate from seawater[END_REF][START_REF] Hardie | The gypsum-anhydrite equilibrium at one atmosphere[END_REF][START_REF] Innorta | The gypsum-anhydrite equilibrium by solubility measurements[END_REF] encompass our 60°C experimental temperature. Also, the transition temperature depends on overall solution chemistry and water activity [START_REF] Farrah | Gypsum-anhydrite transformation in hot acidic manganese sulphate solution. A comparative kinetic study employing several analytical methods[END_REF][START_REF] Li | Model-based construction of calcium sulphate phase transition diagrams in the HCl-CaCl2-H2O system between 0 and 100 °C[END_REF][START_REF] Azimi | Modelling of calcium sulphate solubility in concentrated multi-component sulphate solutions[END_REF]. A plausible explanation for the appearance of gypsum instead of anhydrite is a kinetic control. Since the nucleation rate of anhydrite is considerably slower, gypsum might remain as a metastable phase up to 120°C [START_REF] Ostroff | Conversion of gypsum to anhydrite in aqueous salt solutions[END_REF][START_REF] Freyer | Invited review, crystallization and phase stability of CaSO4 and CaSO4-based salts[END_REF][START_REF] Azimi | Mechanism and kinetics of gypsum-anhydrite transformations in aqueous electrolyte solutions[END_REF][START_REF] Ossorio | The gypsum-anhydrite paradox revised[END_REF].

In absence of unambiguous evidence for fluorine loss, which would support possible , we tentatively attribute the concentration decline of Si to formation of amorphous silica, a phase that is commonly saturated in the Kawah Ijen lake [START_REF] Delmelle | Geochemistry, mineralogy and chemical modelig of the acid crater lake of Kawah-Ijen Volcano, Indonesia[END_REF][START_REF] Delmelle | Geochemistry of the magmatic-hydrothermal system of Kawah Ijen volcano, East Java, Indonesia[END_REF]. However, because the brine sample had been filtered and the experimental temperature was some 20 o C higher than during its storage, the solution must have been undersaturated at least at the start of the experiment. Similar to gypsum, the PHREEQC models do not predict attainment of saturation for amorphous silica (Fig. 4b) but its SI value reached a maximum approximately at the time when the Si concentration decrease started (cf. Fig. 3d).

Calculations based on [START_REF] Gunnarsson | Amorphous silica solubility and the thermodynamic properties of H4SiO 0 4 in the range of 0° to 350°C at Psat[END_REF] suggest that solubility at 60°C would be almost three times the maximum concentration measured in the experiment, but the temperature equations of these authors describe the solubility in pure water, making its applicability to the hyperacid highly saline brine questionable.

We speculate that nanoparticles did form under the extreme acidic conditions and eventually reached a sufficiently large size to be filtered out, since increasing ionic strength, decreasing water activity and decreasing pH suppress amorphous silica solubility and promote polymerization and particle flocculation rates [START_REF] Marshall | Amorphous silica solubilities II. Effect of aqueous salt solutions at 25°C[END_REF][START_REF] Gorrepati | Silica precipitation in acid solutions: mechanism, pH effect and salt effect[END_REF].

In order to explore the possible stability of phases beyond the lowest water activity Considering these observations, the absence of precipitating halide and sulphate phases other than gypsum in the course of the experiment is largely in line with equilibrium modelling results. Apparently, the elevated concentrations reached by Cl and ST (total sulphur) promoted complexation with Mg and Fe, mainly as FeCl + , FeCl 2+ , FeSO4 0 , MgCl + and MgSO4 0 , which increased the solubilities of these cations and prevented them from precipitating as salts (Figs. 6, 11 and EA-2, EA-5). Similar arguments explain the absence of Al-, K-or Na-bearing phases. Our experimental and theoretical findings are also consistent with field evidence, since gypsum is usually the only major sulphate found in volcanic lake settings where hyperacid brine is subjected to simple evaporation alone (e.g., Rodríguez et al. 2016b;[START_REF] Rodríguez | Superficial alteration mineralogy in active volcanic systems: An example of Poás volcano, Costa Rica[END_REF]. Amorphous silica is also a common phase in these surface expressions of volcanic-hydrothermal systems, where its precipitation is largely governed by a drop of the brine temperature under ambient conditions.

Hyperacid brine evolution and volatile phases

The progressive water loss and concomitant pH decline not only caused an overall elemental concentration increase, but according to equation ( 2), it also promoted the formation of HCl(aq) at the expense of Cl -. The Br/Cl ratios increased slightly (Fig. 12) due to stronger volatilization of HCl(g) than HBr(g), since in aqueous solutions HBr(ac) is a stronger acid than HCl(ac) and thus tends to remain dissociated to a larger extent [START_REF] Marcus | Ionic dissociation of aqueous hydrobromic acid. Part 1.-Estimate from vapour pressure and activity coefficient data[END_REF]. The model predicts that Br/Cl ratios become even more pronounced at higher degrees of evaporation than covered by our experiment, with molar Br/Cl ratios in the remaining solution ultimately increasing up to more than 30 times the starting value. A simultaneous increase in the ST/Cl ratio further confirms the HCl(g) loss (Fig. 12). Both the experimental and modelling results thus provide evidence for unequal rates of halogen volatilization from highly acidic solutions as an efficient mechanism to fractionate Cl from Br, two elements that commonly show a similar hydrophilic behaviour in most terrestrial environments [START_REF] Aiuppa | Halogens in volcanic systems[END_REF].

Complexities in the partitioning behaviour of Cl and Br among co-existing brine and vapour have also been highlighted in studies of phase separation in sub seafloor hydrothermal systems. Results from field studies and autoclave experiments are not conclusive as to whether Br or Cl preferentially partitions into the low-salinity vapour phase fluids relative to the brines. Although most experiments and measurements on natural vent fluids tend to favour preferential partitioning of Br in the vapour phase (e.g., Von [START_REF] Damm | Extraordinary phase separation and segregation in vent fluids from the southern East Pacific Rise[END_REF]Foustoukos and Seyfried, 2007;[START_REF] Seo | Fractionation of Cl/Br during fluid phase separation in magmatic-hydrothermal fluids[END_REF], others reported an opposite trend (e.g., [START_REF] Liebscher | Br/Cl signature of hydrothermal fluids: liquid-vapor of bromine revised[END_REF]. Factors that may influence the partitioning behaviour include P-T conditions at which phase separation occurs, density contrast between vapour and brine, and alkali metals present in the solution [START_REF] Seo | Fractionation of Cl/Br during fluid phase separation in magmatic-hydrothermal fluids[END_REF].

Chlorine isotope fractionation -changing mechanism during evaporation

Theoretical equilibrium values at 60°C reported by [START_REF] Czarnacki | Isotope fractionation in aqua-gas systems: Cl2-HCl-Cl -, Br2-HBr-Brand H2S-S 2[END_REF] are +1.55‰ and +1.69‰, based on data from [START_REF] Urey | Isotopic exchange equilibria[END_REF] and [START_REF] Schauble | Theoretical estimates of equilibrium chlorine-isotope fractionations[END_REF], respectively. Experimental results of [START_REF] Sharp | An experimental determination of chlorine isotope fractionation in acid systems and applications to volcanic fumaroles[END_REF] for equilibrium Cl isotope fractionation between HCl(g) and hydrochloric acid are +1.4 to +1.8‰ between 50 and 79 o C. These authors attributed the preference of the heavy Cl isotope for the gas phase to the differences in bond types, because the bond between Cl and H in the HCl(g) molecule will be stronger than that of a solvated Cl -ion in an aqueous solution and therefore more capable to hold the heavier 37 Cl.

Our experimental results before 500 hours are consistent with the tendency that 37 Cl preferentially escapes in the evaporating HCl(g). However, the magnitude of fractionation is higher (average of four estimates is +2.5‰, Table 7), although for one approach (Cl mass losses based on B/Cl ratios) the result of +1.55±0.49‰ is closer to the theoretical values of [START_REF] Czarnacki | Isotope fractionation in aqua-gas systems: Cl2-HCl-Cl -, Br2-HBr-Brand H2S-S 2[END_REF]. This enhanced isotopic fractionation effect can be explained by the experimental conditions of open-system evaporation, instead of the closed-system equilibrium cases on which the previous theoretical and experimental results were based.

In contrast, the experimental results after 500 hours point to a fractionation in opposite direction, i.e. with a strong preference of light 35 Cl for the gas phase (average of four estimates is -4.9‰, Table 4). [START_REF] Sharp | An experimental determination of chlorine isotope fractionation in acid systems and applications to volcanic fumaroles[END_REF] found a similar partitioning behaviour in a kinetic evaporation experiment with concentrated hydrochloric acid (11.6 M) at room temperature, which yielded a value of -4.0‰. They ascribed this to a kinetic effect associated with the higher translational velocity of H 35 Cl, which will thus escape more readily from the solution.

The change in overall fractionation behaviour in the course of the evaporation experiment can be explained by individual fractionation steps during the two successive reactions needed to form gaseous HCl(g) from the solution:

(10) (11) The formation of aqueous HCl 0 (reaction 10) is associated with a positive fractionation factor, and the evaporation step (reaction 11) with a negative one. Competition between these opposing effects thus determines the sign and magnitude of the overall isotopic fraction between gaseous HCl(g) and total dissolved Cl.

According to our speciation calculations, Cl -was the dominant Cl species in solution before 500 hours (80.5% of the total chlorine molality at t=0 hours, Fig. 13), which suggests that reaction (10) was the rate limiting step in the formation of HCl(g).

Apparently, the effect of the associated isotopic fractionation overwhelmed that of the subsequent evaporation step, so that the first escaping hydrogen chloride consisted mainly of H 37 Cl(g). After 500 hours, HCl 0 became the dominant species (60.5% at t=1750 hours, Fig. 12), which turned kinetic fractionation accompanying reaction (11) into the determining factor. Hence, as soon as much of the chlorine in the solution was converted into HCl 0 , the bonding effect became less important in fractionating the isotopes during HCl(g) evaporation in comparison to the kinetic effect associated with translational velocity differences between H 35 Cl 0 and H 37 Cl 0 . Consequently, from then on, hydrogen chloride preferentially escaped as H 35 Cl(g). According to PHREEQC calculations, at 25°C, HCl 0 accounts for 63.1% of the total chlorine concentration, and 36.9% for Cl -in a concentrated hydrochloric acid solution, which implies that the kinetic evaporation experiments of [START_REF] Sharp | An experimental determination of chlorine isotope fractionation in acid systems and applications to volcanic fumaroles[END_REF] mainly measured the fractionation between HCl 0 and HCl(g) as a product of reaction (11), similar to the conditions in our experiment after 500 hours. The correspondence in sign and magnitude of the isotopic fractionation between the two experiments supports this conjecture.

Following this reasoning, the apparent fractionation of chlorine isotopes between gaseous HCl and total Cl in an aqueous solution depends on the speciation of dissolved Cl, i.e. on the proportions of Cl -and HCl 0 . As the speciation calculations demonstrate, the experimental brine may also contain substantial amounts (up to 25%) of various complexed Cl species (Fig. 12). Because fractionation factors among all these dissolved species are unknown, a rigorous quantitative approach is not feasible, so that the inferred magnitudes for the initial and final parts of the experiment should be taken as approximations for the two different fractionation regimes. Further experimental and theoretical work is needed to substantiate these findings and to clarify the underlying controls.

Implications for Cl isotope signatures of volcanic crater lakes

The Cl isotope signature of hydrothermal fluids that feed an active volcanic lake is initially determined by the source(s) of chlorine, commonly a shallow magma body undergoing exsolution, and potentially also by fractionation during subsequent subsurface rock-gas-liquid interactions in the hydrothermal system or by a mixture of chlorine from shallow reservoirs such as seawater, groundwater or evaporites [START_REF] Li | Chlorine isotopes of thermal springs in arc volcanoes for tracing shallow magmatic activity[END_REF]Rodríguez et al., 2016a). Field observations revealed that gaseous HCl occasionally escapes from the surface of warm, highly acidic volcanic lakes [START_REF] Rowe | Fluidvolcano interaction in an active stratovolcano: the crater lake system of Poás volcano, Costa Rica[END_REF][START_REF] Rouwet | Isotope fractionation and HCl partitioning during evaporative degassing from active lakes[END_REF][START_REF] Rouwet | Dynamic fluid recycling at Laguna Caliente (Poás, Costa Rica) before and during the 2006-ongoing phreatic eruption cycle (2005-10)[END_REF]Rodríguez et al., 2016a), with volatilization being enhanced by increasing H2SO4 abundance (lowering pH), water temperature or Cl -content [START_REF] Truesdell | Origin and transport of chloride in superheated geothermal steam[END_REF][START_REF] Capaccioni | HCl degassing from extremely acidic crater lakes: preliminary results from experimental determinations and implications for geochemical monitoring[END_REF]. Hence, any fractionation associated with evaporative HCl(g) loss may have a superimposed effect on the isotope signature of the lake water body. According to our experimental results, the sign of fractionation depends on the proportions of dissolved Cl-species in the lake water, so in theory, this may become either enriched or depleted in 37 Cl.

However, in a steady state situation with a continuous supply of hydrothermal fluid needed to sustain the volume of a warm evaporating lake [START_REF] Pasternack | Volcanic lake systematics 1. Physical constraints[END_REF], the loss of HCl(g) will be small relative to the amount of dissolved chlorine in the lake reservoir, making a major isotopic shift unlikely. The restricted δ 37 Cl range between 0.0 and +1.2‰ found in acid crater lakes of volcanoes in subduction settings to date [START_REF] Eggenkamp | The Geochemistry of chlorine isotopes[END_REF][START_REF] Sharp | An experimental determination of chlorine isotope fractionation in acid systems and applications to volcanic fumaroles[END_REF]Rodríguez et al., 2016a) falls within the range between -3 and +3‰ reported for subduction-related volcanics [START_REF] Barnes | Chlorine isotope geochemistry[END_REF][START_REF] Liotta | Chlorine isotope composition of volcanic rocks and gases at Stromboli volcano (Aeolian Islands, Italy): Inferences on magmatic degassing prior to 2014 eruption[END_REF], suggesting that input signatures will generally be more important than evaporation effects in creating isotopic variations. Nevertheless, possible indications for an evaporation-induced Cl isotope shift come from the hyperacid crater lake of Poás volcano (Costa Rica) where long-term monitoring during almost three decades yielded δ 37 Cl between +0.02±0.06‰ and +1.15±0.09‰, with some of the highest values recorded during an active period when the lake volume was reduced, temperature high and pH<0 (Rodríguez et al., 2016a). According to the fractionation mechanism inferred from our experiments, this strong acidity favoured the prevalence of HCl 0 as dissolved chlorine species, which should have promoted the predominant escape of the lighter Cl isotope as gaseous HCl. The evaporative HCl(g) loss is also reflected in increased Br/Cl ratios in the lake brine during the same time interval (Rodríguez et al., 2016a), in agreement with the lower volatilization rate of HBr(g) under extremely acid conditions seen in our experiments.

Implications for Mars

The widespread existence of evaporate material across the Martian surface indicates that fluid evaporation played an important role in the geologic past of the planet [START_REF] Altheide | Experimental investigation of the stability and evaporation of sulfate and chloride brines on Mars[END_REF]. Hence, the results of our experiments are potentially significant for interpretations of δ 37 Cl signatures of secondary Cl-bearing minerals considered to have formed through evaporative processes. The hypothesis of two Cl reservoirs on this planet is represented by a 37 Cl-depleted mantle from olivine-phyric shergottites (δ 37 Cl = -3.8 to -2.0‰) and a 37 Cl-enriched crust from chassignites and nakhlites (δ 37 Cl up to +8.6‰) [START_REF] Sharp | The chlorine isotope composition of Martian meteorites 2. Implications for the early solar system and the formation of Mars[END_REF]Williams et al., 2016). The most negative values, found in olivine-phyric shergottites, have been taken to represent the primordial bulk composition of Mars, which the planet may have acquired during accretion, whereas the positive values, measured in other Martian meteorites, are suggestive of a 37 Cl enriched crust, possibly due to preferential loss of 35 Cl into space from the surface [START_REF] Sharp | The chlorine isotope composition of Martian meteorites 2. Implications for the early solar system and the formation of Mars[END_REF]Williams et al., 2016). However, [START_REF] Bellucci | Halogen and Cl isotopic systematics in Martian phosphates: Implications for the Cl cycle and surface halogen reservoirs on Mars[END_REF] reported an average δ 37 Cl content of -0.6‰ for apatites in shergottites with no textural, major element or halogen evidence for mixing with a surface reservoir. The authors concluded that the bulk Cl isotope compositions of Mars and Earth are not significantly different from each other and that the mixture of surface material with a crystallizing mafic or ultramafic magma is responsible for both negative (-5.6 to -0.2‰) and positive (+1.1 to +2.5‰) shifts in δ 37 Cl. [START_REF] Tosca | Chemical divides and evaporite assemblages on Mars[END_REF] have shown that, when starting from a solution that derived its cations from rock weathering, evaporation may develop a large spectrum of different brine compositions depending on the abundance and relative proportions of HCO3 -, SO4 -2 , and Cl -as principal anions. The authors proposed that saline mineral formation through evaporation on Mars, either proceeded in acid-sulphate dominated surficial environments or under conditions where acidity was buffered by basaltic weathering reactions and carbonates were produced. The evaporation path of the Kawah Ijen brine is more in line with the former option but is distinct in detail since the starting fluid is a hyperacid sulphate-chloride solution with an initial high load of cations due to rock dissolution. If halogen-rich brines with a similar hydrothermal origin existed on Mars (e.g., [START_REF] Schmidt | Hydrothermal origin of halogens at Home Plate, Gusev Crater[END_REF][START_REF] Yen | Hydrothermal processes at Gusev Crater: An evaluation of Paso Robles class soils[END_REF], our findings indicate that evaporation-induced degassing of HCl will have either increased or decreased δ 37 Cl in the residual solutions, since sign and magnitude of fractionation seem controlled by pHdependent distribution of the Cl-species. Cases of 37 Cl enrichment may signal a very strong acidity since a preferred escape of the lighter isotope presumably requires a prevalence of aqueous HCl 0 over Cl -. In view of this mechanism, HCl degassing alone is inadequate as explanation for the elevated δ 37 Cl signatures of nahklites, since the hydrothermal fluid inferred to have infiltrated the parent rock was CO2 rich and had a circum-neutral pH [START_REF] Bridges | The nakhlite hydrothermal brine on Mars[END_REF][START_REF] Treiman | High-temperature chlorine-rich fluid in the Martian crust: A precursor to habitability[END_REF]. More likely, fractionation accompanying precipitation and removal of a Cl-bearing phase may have driven δ 37 Cl to the positive values. Also, if the extremely light and highly variable δ 37 Cl values (from -51±5‰ to -1±7‰) measured in sedimentary rocks and sand in Gale

Crater by the SAM instrument on the Curiosity Rover [START_REF] Farley | Light and variable 37 Cl/ 35 Cl ratios in rocks from Gale Crater, Mars: Possible signature of perchlorate[END_REF] are correct, the Cl-hosting mineral(s) could not have been precipitated from an acidic brine that had undergone simple HCl evaporation. Although, according to our experiments, it is conceivable that HCl loss could drive the residual solution to lower δ 37 Cl, the fractionation factor would not be sufficient to reach the extreme negative values starting from a Martian mantle value, if Rayleigh-type fractionation is assumed. More likely, unusual low 37 Cl/ 35 Cl ratios are attributable to involvement of perchlorates, either as component in measured material or as its precursor [START_REF] Farley | Light and variable 37 Cl/ 35 Cl ratios in rocks from Gale Crater, Mars: Possible signature of perchlorate[END_REF][START_REF] Bellucci | Halogen and Cl isotopic systematics in Martian phosphates: Implications for the Cl cycle and surface halogen reservoirs on Mars[END_REF]. Our experiments also highlight the possibility that aqueous bromine and chlorine can become fractionated as a result of unequal evaporation rates of HCl(g) and HBr(g).

This mechanism, together with precipitation of halogen-bearing minerals, could explain the variability of Br/Cl ratios and frequent Br-enrichments in soils of Gusev Crater and Meridiani Planum (e.g., [START_REF] Gellert | Alpha particle X-ray spectrometer (APXS): Results from Gusev crater and calibration report[END_REF]Marion et al., 2009).

The modelled progressive reduction of water activity in our experimental liquid can be used to estimate the theoretical freezing temperature of a residual Martian brine that experienced extensive evaporation [START_REF] Chevrier | Low temperature aqueous ferric sulfate solutions on the surface of Mars[END_REF][START_REF] Elsenousy | Effect of evaporation and freezing on the salt paragenesis and habitability of brines at the Phoenix landing site[END_REF]:

(12) (13)
Where is the freezing temperature, is the ideal gas constant, is the activity of water, = 273.15 K; and is the enthalpy of fusion.

If similar hyperacid brines as that of Kawah Ijen would have been created on Mars (e.g., as a result of interaction of magmatic volatiles and ice, combined with intense waterrock interaction), the lowest value of 0.85 reached in our simulation predicts freezing temperatures down to 183 K (-90.15°C). This is close to the freezing point for the acids of Cl -and SO4 2- [START_REF] Clark | The salts of Mars[END_REF], which are among the strongest freezing point depressors. Taking the yearly average temperature of 240 K (-33.15°C) on

Mars´ surface into account [START_REF] Mellon | The presence and stability of ground ice in the southern hemisphere of Mars[END_REF], the stability of liquid water on the planet would thus be significantly enhanced by the existence of compositionally equivalent hyperacid brines. A final corollary is that the extreme chemistry of such evaporating brines, notably the combination of very low pH, low water activity and high salinity, has a negative effect on Martian habitability [START_REF] Fox-Powell | Ionic strength is a barrier to the habitability of Mars[END_REF].

SUMMARY AND CONCLUSIONS

A 73-days long experiment on crater lake water from Kawah Ijen volcano (Indonesia)

was carried out to study the chemical evolution of highly saline, hyperacid sulphatechloride brine during extended evaporation, with special attention to chlorine isotope fractionation associated with the escape of gaseous HCl. Up to 58% evaporative water loss from the brine (starting pH ca. 0.1, TDS ca. 100 mg/kg) was accompanied by increasing contents of dissolved rock-forming elements, sulphur and halogens. Despite the high load and diversity of solutes, newly formed solid phases were restricted to Srbearing gypsum and possibly amorphous silica. The limited capability of producing evaporite minerals relative to that of seawater and other brine types apparently signals strongly enhanced metal solubilities due to complexation with abundantly available HSO4 -, Cl -and F -.

Continuous loss of HCl(g) throughout the experiment permitted deriving quantitative insight into Cl-isotope fractionation between aqueous and gaseous chlorine. Monitoring the residual solution revealed a marked change in sign and magnitude of fractionation when evaporation proceeded. In the first three weeks of the experiment, the evaporating HCl(g) was isotopically heavier than the chlorine remaining in solution. Assuming Rayleigh-type behaviour, the calculated fractionation factor ( ) was from , depending on the method for determining the Cl losses from the brine. In contrast, the evaporating HCl(g) became isotopically lighter in the subsequent interval, with a corresponding fractionation factor between -5.67±0.17‰ and -3.97±0.08‰, more in line with existing experimental data [START_REF] Sharp | An experimental determination of chlorine isotope fractionation in acid systems and applications to volcanic fumaroles[END_REF]. The change in behaviour coincided with a transition from Cl -to HCl 0 as the dominant chlorine species in the solution when pH had dropped below ca. -0.2. It is inferred that the distribution of dissolved Cl species exerts a strong control on sign and magnitude of Cl isotope fractionation during HCl(g) evaporation from an acid brine.

Because evidence for HBr(g) loss is tenuous and the escape of HCl(g) is clearly more efficient, selective evaporation can also be an effective mechanism to fractionate Cl from Br and other solutes in acidic sulphate-chloride brines.

In most cases, surface evaporation of HCl(g) will have little effect on the Cl isotope compositions or Cl/Br ratios of sizable active volcanic lakes due to the large mass of dissolved chlorine compared to the evaporated fraction and the continuous supply of magmatic volatiles that usually mark these reservoirs. Detectible fractionation effects in hyperacid lakes may be limited to periods of enhanced evaporation and volume reduction.

The BRISOACTIONS). This is IPGP contribution 3884.

FIGURE CAPTIONS

Figure 1. Experimental setup. Twenty-six plastic Greiner® tubes, each containing approximately 50 ml of brine, were partially submerged in a water bath at 60°C. The solutions were allowed to evaporate in air. Sampling was performed after a pre-set time interval when two tubes were removed and processed separately for analysis. Table 3. Chlorine isotope evolution during experimental evaporation of the hyperacid Kawah Ijen brine, inferred from mass loss of the solution, δ 37 Cl of dissolved Cl and inferred HCl(g) loss.

Calculations of F values (fractions of Cl remaining in solution) based on differences between measured and calculated Cl concentrations, combined with water-vapour losses (see Table 1). values obtained from calculated fractionation factors, assuming Rayleigh fractionation (see Figs. 8 and9). The reported values assume a stepwise change in fractionation behaviour and are valid for the 0-500 hours and 500-1750 hours interval, respectively.

SUPPLEMENTARY ELECTRONIC ANNEX CAPTIONS

EA-1. Saturation indices of (a) halides, (b) Fe 2+ -sulphates and (c) Mg-sulphates during the evaporation experiment.

EA-2. Time series of calculated activities (PHREEQC) of the main aqueous species of (a) Al, (b) Fe(II), (d) Fe(III), (e) Mg, (f) Ca, (g) K, (h) Na and (i) Sr. Note that the Y-axis scales are variable. 3 but with F values calculated from Cl/B ratios assuming that B is perfectly conservative. The reported values assume a stepwise change in fractionation behaviour and are valid for the 0-500 hours and 500-1750 hours interval, respectively. Additionally, F values are calculated from Cl/Fe and Cl/Mg ratios assuming that both Fe and Mg are conservative.

EA-3. Same as Table

EA

  3f and 4b), taking analytical uncertainty into account. Fluorite (CaF2), cryolite (AlF3) and halite (NaCl) remained always well undersaturated, showing an initial increase in saturation indices (SI), followed by a levelling off (Fig. a, Supplementary Electronic Annex EA-1). Iron and magnesium sulphates remained undersaturated throughout the experiment. The saturation indices of szolmolnokite (FeSO4•H2O), rozenite (FeSO4•4H2O), siderotile (FeSO4•5H2O), ferrohexahydrite (FeSO4•6H2O) and melanterite (FeSO4•7H2O) show an initial increase, then a decline and finally an increase again towards the end of the experiment (Fig. b, EA-1). The SI values of kieserite (MgSO4•H2O), starkeyite (MgSO4•4H2O), pentahydrite (MgSO4•5H2O), hexahydrite (MgSO4•6H2O) and epsomite (MgSO4•7H2O) increased steadily with time (Fig. c, EA-1).

  bearing acid-sulphate fluid with a composition derived from chemical weathering of a synthetic olivine-bearing Martian basalt,[START_REF] Tosca | Experimental constraints on the evaporation of partially oxidized acid-sulfate waters at the Martian surface[END_REF] found gypsum, Mg-rich voltaite, an unidentified (Mg,Fe)-sulphate-heptahydrate and rhomboclase as precipitating minerals. The concomitant changes in solution chemistry were first a loss of Ca and a further depletion of K, Al, Fe, Mg and SO4 towards the end of the run when most of H2O was evaporated. No detectable fractionation of Cl occurred. The authors inferred that sulphate mineralogy and chemical evolution of the solution did not follow

  reached in the experiments ( = 0.85), an evaporation process was simulated in PHREEQC, whereby one litre of Kawah Ijen brine with the same composition as the starting experimental sample was evaporated by removing water in 1000 steps at 60°C. The run did not converge after the 990 th step, when 99% (985 g) of the initial water content was removed and the ionic strength of the brine reached 163 m. The values dropped from 0.95 to 0.32. The only phases formed were hydrogen chloride ( ≤ 0.86), gypsum ( ≤ 0.83) and elemental sulphur ( ≤ 0.65) (Figs. 10a and 10b). At the end of the simulation szolmolnokite (FeSO4•H2O) and rozenite (FeSO4•4H2O) together with kieserite (MgSO4•H2O) and starkeyite (MgSO4•4H2O) were among the iron and magnesium sulphates that approached the highest saturation levels, but none of these salts were predicted to precipitate, even at the lowest values of the simulation. The same applied for some halides as well (Figs. a, b and c; EA-4).

Figure 2 .

 2 Figure 2. Time series of (a) evaporation loss (wt. %), (b) evaporation rate (g/h/dm 2 ) and (c) pH recorded during the experiment on the Kawah Ijen hyperacid brine. The error bars are smaller than the size of the symbols.

Figure 3 .

 3 Figure 3. Element/Mg molar ratios recorded in the course of progressive evaporation for: (a) Cl, ST; (b) F, 10xBr, B; (c) Ca, Sr; (d) Al, Si; (e) Na, Fe; (f) K, Ti. Horizontal trends point to conservative behaviour of most elements, whereas declining ratios indicate loss of Cl, Ca, Sr and Si through separation of gaseous (HCl) and solid (gypsum and possibly amorphous silica) phases. Note that the Y-axis scales are variable.

Figure 4 .

 4 Figure 4. Time series trends for (a) fugacities of selected gaseous compounds and (b) saturation indices of selected minerals, calculated for the evaporating brine. Similar graphs for halides and sulphates with lower SI values are given in the Supplementary Electronic Annex EA-1.

Figure 5 .

 5 Figure 5. (a) Raman spectra of gypsum precipitated in the course of the experiment. Reference spectra for anhydrite (R040012) and gypsum (R040029) are from the RRUFF database (www.rruff.info). (b) SEM image of gypsum crystals.

Figure 6 .

 6 Figure 6. Time series of calculated activities (PHREEQC) for the main aqueous species of S(VI), Cl and Br in the residual evaporating brine. Similar graphs for metal-bearing species are given in the Supplementary Electronic Annex EA-2.

Figure 7 .

 7 Figure 7. Evolution of δ 37 Cl in the residual hyperacid brine in the course of progressive evaporation at T=60°C.

Figure 8 .

 8 Figure 8. Factors for chlorine isotope fractionation between HCl(g) and Cldiss. ( ), assuming Rayleigh distillation. Mass-losses of Cl (F values) based on: (a) Cl concentrations and water losses, (b) Cl/B ratios, (c) Cl/Fe ratios, and (d) Cl/Mg ratios. Note the significant change in α value after approximately 500 hours of evaporation.

Figure 9 .

 9 Figure 9. Measured δ 37 Cl for dissolved Cl (Cldiss,) and calculated δ 37 Cl for gaseous Cl (HCl(g)), based on (a) Cl mass loss, (b) Cl/B ratios, (c) Cl/Fe ratios, and (d) Cl/Mg ratios. Note the significant change in δ 37 Cl after approximately 500 hours of evaporation.

Figure 10 .

 10 Figure 10. Predicted amounts of secondary phases formed during progressive evaporation, based on PHREEQC simulation (T=60°C), as a function of (a) H2O loss (wt. %) and (b) water activity ().

Figure 11 .

 11 Figure11. Activities of the main species of S(VI), Cl and Br according to simulated evaporation (PHREEQC) of the hyperacid brine at T=60°C. Note that curves for the activities of HCl o and Cl - cross at high degrees of water loss. Similar graphs for activities of the main metal species are given in Supplementary Electronic Annex EA-5.

Figure 12 .

 12 Figure 12. Br/Cl and ST/Cl molar ratios recorded throughout the evaporation experiment. Increasing trends are consistent with loss of gaseous HCl.

Figure 13 .

 13 Figure13. Calculated distributions of dissolved Cl species in the course of the evaporation experiment on the hyperacid brine, expressed as percentage of the total Cl molality. Note that after 500 hours HCl 0 becomes more abundant than Cl -.

- 4 .

 4 Calculated changes in the saturation indices (PHREEQC) of: (a) hydrous iron (II) sulphates, (b) hydrous magnesium sulphates and (c) halides during progressive evaporation of the hyperacid brine at T=60°C. EA-5. Activities of the main species of (a) Al, (b) Fe(II), (b) Fe(III), (d) Mg, (d) Ca, (f) K, (g) Na and (h) Sr according to simulated evaporation (PHREEQC) of the hyperacid brine at T=60°C. Note that the Y-axis scales are variable.

  a, EA-2). On the other hand, Fe 2+ and Fe 3+ are predominantly complexed with Cl -, forming FeCl + and FeCl 2+ , respectively (Figs. b and c; EA-2). It should be mentioned that, due to the extreme acidity of the Kawah Ijen brine, iron is predominantly in its lower oxidation state (Fe 2+ ). Magnesium and calcium are mainly present as Mg 2+ and Ca 2+ , respectively, but also form complexes with SO4 2-

  results of this work may provide new clues on the origin of Cl isotope signatures and Cl/Br variability in Martian soils and minerals. If evaporation of highly acidic fluids played a role, preferential loss of HCl(g) would yield a relative enrichment of Br and a shift in Cl-isotope composition either in positive or in negative direction, depending on the speciation of aqueous chlorine and thus on pH. It is conceivable that such effects are preserved in halogen-bearing salts and other solid residues of evaporation. Finally, the (former) presence of hyperacid sulphate-chloride brines as studied here would extend the stability of liquid water on the planet's surface to extremely low temperatures. manuscript benefited from the valuable comments of the reviewers. Funding for this research was provided by NWO (Netherlands Organization for Scientific Research), project ALW-GO-PL/10-03. H.G.M. Eggenkamp has received funding from the European Union's Horizon 2020 research and innovation programme as Individual Fellow under

	the Marie	Skłodowska-Curie Actions (grant agreement N°702001,

TABLE CAPTIONS Table 1 .

 CAPTIONS1 Recorded data for mass loss, pH, conductivity and evaporation rate during experimental evaporation of the hyperacid Kawah Ijen brine.

Table 2 .

 2 Compositional changes in the hyperacid Kawah Ijen brine during experimental evaporation.

Table 4 .

 4 Summary of chlorine isotope fractionation factors obtained from the evaporation experiments at T=60°C, based on Rayleigh fractionation and different approaches for calculating F factors (see Figs.8 and 9, Table3and EA-3).

ACKNOWLEDGMENTS

All laboratory analyses were performed with the instrumental facilities of the Department of Earth Sciences, Utrecht University. We are grateful to Arnold van Dijk for carrying out the mass spectrometer measurements, and to Helen de Waard, Ton Zalm and Dineke van de Meent for help with experiments, ICP-OES and IC analyses, respectively. Our appreciation also goes to Helen King for expert support with Raman spectroscopy and SEM techniques, and to Sergei Matveev for the skilful EPMA work. The


Véronique Conédéra
email: conedera@laas.fr

Fabien Mesnilgrente
email: fabien.mesnilgrente@laas.fr

Paul Fadel
email: pfadel@laas.fr

Norbert Fabre
email: nfabre@laas.fr

New developments in inkjet of deposits carried out under localized extraction or helium

Keywords: ink-jet printing, helium flow, localized extraction, low temperature deposition

INTRODUCTION

The ink-jet deposition technology has been undergoing development for almost twenty years [START_REF] Clymer | A new approach for understanding dominant design: the case of the inkjet printer[END_REF], showing the difficulty of this technique which is now extensively used in graphics industries. Indeed more than 3,000 patents have been filed between 1989 and 2000. This technique which seems highly appealing when considered from the point of view of its principle nevertheless poses new problems in micro and nanotechnology applications, such as the safety issue of inks with nanoparticles, the clogging of ejection nozzles when low-boiling point solvents are employed and resolution problems associated with the print head fabrication technology and with deposition parameters. Line width can be predicted based on the well-known relationship reported in the literature [START_REF] Smith | Direct ink-jet printing and low temperature conversion of conductive silver patterns[END_REF]; and it is explained by the following equation: ω² = where ω is the as-printed track width, d is the drop diameter, θ is the contact angle, p is the spacing of adjacent droplets. This function takes account of the contact angle, the droplet diameter and spacing between two droplets. As a first approximation, to increase resolution, one has to increase the contact angle or spacing between two droplets. However, this entails inherent limitations as it cannot be repeated indefinitely. Beyond a certain point the printed line becomes unstable [START_REF] Schiaffino | Formation and stability of liquid and molten beads on a solid surface[END_REF][START_REF] Stringer | Limits to feature size and resolution in ink jet printing[END_REF]. Another solution consists in diminishing the droplet diameter; the latter depending on the nozzle diameter; in this case one is limited by the nozzle fabrication technology. However a number of methods have been studied to produce smaller droplets than the ejection nozzle hole diameter [START_REF] Rensch | Creation of small microdrops" Micro Fab Technologies[END_REF], but the implementation is sometimes difficult to address.

In this paper, three types of systems for which patents have been filed are presented. The first system based on helium flow injection allows the substrate temperature to be decreased during printing and to improve line resolution. A second system features a helium flow injected through use of a venturi-like design. It exhibits the same performance as the first system with the additional advantage of increasing droplet velocity without increasing the droplet diameter. The third system with localized extraction absorbs the gas emitted during solvent evaporation as well as the solid particles likely to be generated during evaporation. This system is equally used to enhance resolution.

EXPERIMENTAL RESULTS

To validate our equipment, we used an ethylene glycolbased ink with ZnO nanoparticles (10% in weight). Ethylene glycol was selected because of its high boiling point (198°C) which allows synthesis of inks which will not easily dry at the end of the ejection nozzle and therefore will not cause clogging during stop phases. But the disadvantage of working with a solvent at high boiling point involves a low writing velocity in order to allow the evaporation of solvent; in our case, the writing speed was fixed for all the operations at 0,4. 10 -3 m/sec. To achieve suspension of ZnO nanoparticles in ethylene glycol, there is no need to add a surfactant. Thus a deposition containing mainly pure ZnO is obtained following drying, supporting the fabrication of devices like gas sensors [START_REF] Conedera | Manufacturability of gas sensor with ZnO nanoparticles suspension deposited by ink jet printing SPIE Photonics West[END_REF][START_REF] Mesnilgrente | Geometric control of inkjet printing K L M 278µm 244µm 168µm features using ethylene glycol and diethylene glycol in ZnO nanoparticules ink[END_REF].An Altatech equipment fitted out with a single nozzle from MicroFab is used. The helium flow is injected at a rate of 300ml to 1,100ml per minute. Below 280ml per minute, the benefit of helium disappears. Between 300ml and 1,100ml per minute no improvement in terms of resolution, drying temperature or printing instability can be noticed. Figure 1 shows the resolution obtained with ZnO deposited on a substrate heated at 65°C: (A) without helium injection; (B) with helium injection (370ml/min); (C) with injection of nitrogen (370ml/min). Note that the line printed in (C) is thinner than in (A), but wider than in (B). In (B) the diffusion rate of the ethylene glycol vapour under helium atmosphere occurs faster than in air or nitrogen, leading to a higher evaporation rate.

Use of a system without droplet acceleration

The phenomenon associated with evaporation has been investigated and reported in the literature [START_REF] Dunn | A mathematical model for the evaporation of a thin sessile liquid droplet: comparison between experiment and theory[END_REF]. Starting from lines (D) to (I), the helium flow has been suppressed while the substrate temperature was gradually increased; (D:70°C); (E:85°C); (F:90°C); (G :95°C); (H :100°C). Up to line F, it can be noticed with the observation camera fitted onto our machine that ethylene glycol evaporates within a few seconds following deposition. In H, nothing occurs; in I (105°C), the annular structure of the deposition is typical of a rapid evaporation featuring deposition inhomogeneity. It can be considered that lines G and H best correspond to the aspect of line B, although the particle distribution seems less good. In this example, the temperature gain obtained with the use of helium is between 30°C and 35°C. A venturi-type system has been added at the outlet of the ejection nozzle. The results in terms of temperature or resolution are identical to those given by the previous system. On the other hand, we can increase the droplet velocity by increasing the helium output as shown in Figure 2. Thus this system enables us to increase droplet velocity without increasing the diameter as in the case of a voltage increase in the piezoelectric actuator. This is well accounted for in the literature [START_REF] Smith | Direct ink-jet printing and low temperature conversion of conductive silver patterns[END_REF].

Use of a venturi-type system with droplet acceleration

The benefit of a higher velocity lies in having more spacing between the ejection nozzle and the substrate with no loss of resolution, and being able to print on high-aspect ratio substrates.

In our example of Fig. 3 for a 20 Volt excitation voltage of the piezoelectric actuator the droplet diameter is 51µm with a velocity of 1.5m/s. To obtain a velocity of 4.5m/s, a 40 Volt excitation is needed. In this case a droplet with a 60µm diameter will be obtained. With the use of the venturi system, one can keep an excitation voltage of 25 Volts and get a velocity of 4.5 m/s with a helium output of 1,100ml/min.

System with gas extraction

Extraction occurs at the outlet of the nozzle or in an annular fashion. Output may range from 500ml/min to 1,000ml/min. The advantage of this system lies in the extraction of gas or solid particles at the time of evaporation preventing the latter particles from settling in the machine environment. Given the high scan on the substrate, evaporation occurs more quickly than without suction; the major output can create a vacuum locally which promotes evaporation. Also a significant gain is achieved in terms of resolution as can be seen in Fig 4 where the three printing methods have been compared. (K) without injection of helium and without vacuum; (L) with suction; (M) with injection of helium. In this experimental case, the line width decreases from 278µm for a deposition without any system to 244µm for a system equipped with localized extraction (D) to 168µm for a system fitted out with helium injection.

Helium injection remains more efficient in terms of resolution.

CONCLUSIONS

These new systems, particularly those with helium injection, offer advantages relative to the other systems devoid of such improvements. The print resolution is greatly improved (up to 40% in certain cases). Printing can be performed at a lower temperature and high boiling-point solvents can be employed. Their use can be made compatible with biological molecules or substrates that cannot withstand high temperatures. With the venturi system, one can print with more spacing between the source and the substrate.

Additionally, these improvements greatly simplify the design of these machines. Although the system equipped with an extraction facility does not offer the same resolution performance as systems with a helium injection, it offers the advantage of efficiency protecting operators.

Figure 1 :

 1 Figure 1: Optical picture of ZnO nanoparticles deposited on gold layer at different temperatures. Contact angle 40°; diameter of the nozzle 50µm; distance between two droplets 50µm.

Figure 2 :

 2 Figure 2: Droplet velocity vs helium flow with the venturi system.

Figure 3 :

 3 Figure 3: Drop velocity and diameter as a function of piezoelectric excitation voltage for an ink-jet 50µm dia. nozzle.

Figure 4 :

 4 Figure 4: Comparison between the 3 methods. ZnO nanoparticles deposited on Silicon substrate at 65°C; nozzle diameter 50µm; more spacing between two droplets: 50µm; contact angle 17°.(K) without injection of helium and without vacuum; (L) with suction; (M) with injection of helium. In this experimental case, the line width decreases from 278µm for a deposition without any system to 244µm for a system equipped with localized extraction (D) to 168µm for a system fitted out with helium injection.

Acknowledgements

This work has been supported by the French National Agency (ANR) in the frame of its technological Research NANO-INNOV/TR program (NANOCOMM, project N° ANR-09-NIRT-004) and the authors would like to thank Robert Plana, Daniela Dragomirescu and Sebastien Pacchini