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Influence of temperature, pressure, and fluid salinity on the distribution of chlorine into serpentine minerals
Accepted Manuscript Influence of temperature, pressure, and fluid salinity on the distribution of chlorine into serpentine minerals Ruifang Huang, Weidong Sun, Wenhuan Zhan, Xing Ding, Jihao Zhu, Jiqiang Liu PII: DOI: Reference:
S1367-9120(17)30193-1 http://dx.doi.org/10.1016/j.jseaes.2017.04.022 JAES 3058
To appear in:
Journal of Asian Earth Sciences
Please cite this article as: Huang, R., Sun, W., Zhan, W., Ding, X., Zhu, J., Liu, J., Influence of temperature, pressure, and fluid salinity on the distribution of chlorine into serpentine minerals, Journal of Asian Earth Sciences (2017), doi: http://dx.doi.org/10.1016/j.jseaes.2017.04.022
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Influence of temperature, pressure, and fluid salinity on the distribution of chlorine into serpentine minerals Ruifang Huanga,b,*, Weidong Suna, Wenhuan Zhanb, Xing Dingc, Jihao Zhud, Jiqiang Liud
a
Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of
Geochemistry, Chinese Academy of Sciences, 510640 Guangzhou, PR China; b
Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology,
Chinese Academy of Sciences, 510301 Guangzhou, PR China; c
State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of
Geochemistry, Chinese Academy of Sciences, 510640 Guangzhou, PR China; d
The Second Institute of Oceanography, SOA, Hangzhou 310012, PR China
*Corresponding author: [email protected] (R. Huang)
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ABSTRACT Serpentinization produces serpentine minerals that have abundant water and fluid-mobile elements (e.g., Ba, Cs, and Cl). The dehydration of serpentine minerals produces chlorine-rich fluids that may be linked with the genesis of arc magmas. However, the factors that control the distribution of chlorine into serpentine minerals remain poorly constrained. We performed serpentinization experiments at 80-500 °C and pressures from vapor saturated pressures to 20 kbar on peridotite, orthopyroxene, and olivine with <5% pyroxene. The results show that the concentrations of chlorine in serpentine minerals were up to 1.2 wt% at 200 °C, whereas they decreased slightly at 311-400 °C and 3.0 kbar and became significantly lower at 485 °C and 3.0 kbar, ~0.1 wt%. Fluid salinity greatly decreased chlorine concentrations of olivine-derived serpentine produced at 400 °C and 3.0 kbar, which was associated with a decrease in silica mobility during serpentinization. By contrast, influence of fluid salinity at 311 °C and 3.0 kbar is minor. Moreover, chlorine distribution into serpentine can be influenced by primary minerals of serpentine. Serpentine formed in olivine-only experiments at 311 °C and 3.0 kbar had 0.08 ± 0.03 wt% Cl, which was significantly lower than chlorine concentrations of serpentine minerals (0.49 ± 0.36 wt%) produced in orthopyroxene-only experiments. By contrast, olivine-derived serpentine formed in experiments at 311 °C and 3.0 kbar on peridotite contained comparable chlorine with orthopyroxene-derived serpentine. In particular, olivine-derived serpentine had 0.16 ± 0.09 wt% Cl that was slightly higher than chlorine concentrations of serpentine formed in olivine-only experiments, whereas orthopyroxene-derived serpentine had 2
significantly lower chlorine concentrations than that formed in orthopyroxene-only experiments. The contrast may be associated with releases of aluminum and silica from pyroxene minerals, which possibly results in a decrease in chlorine concentrations of serpentine. The concentrations of chlorine in serpentine formed in experiments at 311 °C and 3.0 kbar were slightly lower than those in serpentine produced at 300 °C and 8.0 kbar, which may be associated with influence of pressure on the mobility of iron and silica. The experimental results of this study indicate that serpentine minerals are important carriers of chlorine in subduction zones. It also suggests that chlorine is significant for the redistribution of cations during serpentinization.
Key-words: serpentinization, chlorine, peridotite, aluminum, silica
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1. Introduction Serpentinization is the hydration of ultramafic rocks (typically peridotite and komatiite) at relatively low temperatures (≤500 °C), forming serpentine minerals, (±) brucite, (±) talc, and (±) magnetite. It occurs at various geological settings, including the ocean floor, mid-ocean ridges, and subduction zones (e.g., Charlou et al., 1996, 2000; Maekawa et al., 2001; Hyndman and Peacock, 2003; Mével, 2003; Evans et al., 2013). Serpentinization can greatly modify the chemical and physical properties of oceanic lithosphere (e.g., Escartín et al., 1997, 2001; Scambelluri et al., 1995; Mével, 2003; Guillot and Hattori, 2013). Serpentinites (with >90% serpentine) have significantly lower density than primary peridotite, and their magnetic susceptibility is up to two orders of magnitude higher (Mével, 2003; Evans et al., 2013). As suggested by deformation experiments, a very low degree of serpentinization can greatly decrease the strength of olivine (Escartín et al., 1997, 2001). Analyses of natural serpentinites show that serpentine minerals can incorporate not only water (up to 13.5 wt%) but also chlorine and fluid-mobile elements, such as Cs, Ba, and Sr (Scambelluri et al., 1995; Hattori and Guillot, 2003; Deschamps et al., 2012; Guillot and Hattori, 2013). Compared to fresh peridotite (with <30 ppm Cl), serpentinites have around one to two orders of magnitude higher chlorine concentrations (e.g., Bonifacie et al., 2008; Barnes and Straub, 2010). Thermodynamic and experimental studies suggest that serpentine minerals can be stable at depths of greater than 200 km (Ulmer and Trommsdorff, 1995; Schmidt and Poli, 1998). Therefore, serpentinization may play an important role for the transfer of H2O, chlorine, and fluid-mobile 4
elements to the mantle.
In spite of the importance, the mechanisms that control the incorporation of chlorine into serpentine minerals are poorly understood (Rucklidge, 1972; Rucklidge and Patterson, 1977; Anselmi et al., 2000; Scambelluri et al., 2004; Sharp and Barnes, 2004; Barnes and Sharp, 2006; Bonifacie et al., 2008). Chlorine can be hosted in a structurally-bound site, i.e., substitute for the hydroxyl group in serpentine (e.g., Anselmi et al., 2000) or in a water-soluble site (e.g., Rucklidge and Patterson, 1977; Sharp and Barnes, 2004; Barnes and Sharp, 2006). Hydrothermal experiments show that chlorine in the water-soluble site is readily leached by water (Sharp and Barnes, 2004; Barnes and Sharp, 2006), and it may increase greatly in a saline fluid (e.g., Rucklidge, 1972). Fluid inclusions of natural serpentinites have largely varied salinities (e.g., Philippot et al., 1998; Scambelluri et al., 1997, 2004), indicating that chlorine in serpentine can vary greatly under the same T-P conditions. Analyses of natural serpentinites show that oceanic serpentinites, possibly formed at ~250 °C, had abundant chlorine, up to 0.62 wt%, whereas antigorite serpentinites with the experience of eclogite-facies metamorphic grade contained significantly lower chlorine concentrations (Scambelluri et al., 2004). However, natural serpentinites commonly experience multistage metamorphic history, and consequently the obtained chlorine may not necessarily represent a single T-P condition. In particular, chlorine in natural serpentinites may be greatly influenced by lateral metamorphic fluids. However, there has been no hydrothermal experiment to investigate influence of 5
temperature, pressure, and salinity of starting fluids on chlorine incorporation into serpentine minerals.
In this study, we performed experiments at 80-500 °C and pressures ranging from vapor saturated pressures to 20 kbar with peridotite, orthopyroxene, and olivine with <5% pyroxene. The objectives are to (1) quantify the concentrations of chlorine in serpentine formed under different conditions, (2) investigate influence of temperature, pressure, fluid salinity, and water/rock ratios on chlorine distribution into serpentine minerals, and (3) assess the importance of serpentinization for the recycling of chlorine in subduction zones.
2. Materials and methods 2.1 Preparation of starting materials A non-altered peridotite, sampled at Panshishan (Jiangsu Province, China) where it occurs as xenoliths in alkaline basalts (Chen et al., 1994; Sun et al., 1998; Xu et al., 2008; Yang, 2008), was taken as the starting material. Peridotite is composed of ~65 vol% olivine, 20 vol% orthopyroxene, 15 vol% clinopyroxene and ~2 vol% spinel. The composition of the primary minerals in peridotite determined by electron microprobe has been described in a previous experimental study (Huang et al., 2015a). The peridotite is fresh, as evidenced by a very small loss on ignition (<0.3%) upon heating at 1200 °C (Yang, 2008). Olivine and orthopyroxene were picked from ground peridotite power (<60 mesh) under binocular microscope, and their fractions 6
include <5% of other mineral phases. Peridotite, olivine, and orthopyroxene were ground in an agate mortar, and then sieved into grain sizes of 100-177 µm. Saline solutions (0.5 and 3.3 mol/L NaCl) were prepared in order to investigate influence of fluid salinity on chlorine distribution into serpentine minerals.
2.2 Hydrothermal experiments All the experiments were conducted at high temperature and high pressure laboratory, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (Table 1). Experiments at 80 °C were performed in a water bath kettle, and the solid reactants and a saline starting solution were loaded into a 15 ml polypropylene centrifuge tube. Teflon-lined steel vessels were used in experiments at 200 °C (Psat ~16 bar). When finished, the tubes and steel vessels were quenched in ice water.
Experiments at 311-485 °C and 3.0 kbar were carried out in cold-seal hydrothermal vessels. Solid reactants and a saline starting solution were loaded into gold capsules (4.0 mm O.D., 3.6 mm I. D. and 30 mm in length), which were welded shut by a tungsten inert gas high-frequency pulse welder (PUK3). Leaks were always checked by putting capsules into a drying furnace at 100 °C for several hours, and those with mass differences from initial mass before heating less than 0.1% were used in experiments. The prepared capsules were placed into the end of hydrothermal vessels, followed by a filler rod (~6 cm long). Water was used as the pressure medium. Pressures were achieved by pumping water into the vessel, and they were measured 7
by a pressure gauge with a precision of ±200 bar. The temperature was monitored with an external K-type thermocouple that was inserted into a hole near the end of the vessel with the accuracy of ±2 °C. When finished, the vessels were immersed in ice water and temperature decreased to <100 °C within several seconds.
Experiments at 300-500 °C and 8-20 kbar were performed in a piston-cylinder apparatus (Quickpress, Depth of the Earth Co.). Solid reactants and a starting saline solution were loaded into gold capsules (around 10 mm in length, 3.0 mm outer diameter and 0.2 mm of wall thickness), and the capsules were welded with a tungsten inert gas high-frequency pulse welder (PUK3). Glass-salt assemblies were used. Temperature was monitored by an S-type thermocouple with an accuracy of ±10 °C. Pressure calibration has been done according to albite-breakdown reaction at 600 °C, and the results showed less than 8% of friction correction.
2.3 Analytical methods The surface morphology of solid products was characterized with a Zeiss Ultra 55 Field emission gun scanning electron microscope (SEM) at Second Institute of Oceanography, State Oceanic Administration of China. Samples were dispersed onto double-sided carbon tape and coated with a thin film of platinum for observation.
The mineralogy of solid products was identified with a Bruker Vector 33Fourier transform infrared spectrometer at the Analytical and Testing Center of South China 8
University of Technology. Infrared spectra were obtained at wavenumbers from 400 to 4000 cm−1 at a resolution of 4 cm−1, and 32 scans were accumulated for each spectrum. The KBr pellets were prepared by mixing around 1 mg of sample powder with 200 mg of KBr.
Some solid samples were mounted in epoxy resin and subsequently polished using Al2O3/SiC grit papers and diamond paste. To avoid the possibility that Cl-bearing phases were dissolved during sample preparation, we polished samples using ethanol instead of water. Comparisons of chlorine contents in samples polished using water and ethanol were also made. Compositions of solid products after experiments were determined by JEOL JXA 8100 electron microprobe with four wavelength-dispersive spectrometers at Second Institute of Oceanography, State Oceanic Administration. Operating conditions for serpentine analyses included 15 kV and 20 nA with a beam diameter of 15 µm. Calibration standards were jadeite (Si, Na), olivine (Mg), almandine garnet (Fe, Al), diopside (Ca), sanidine (K), chromium oxide (Cr), rutile (Ti), nickel silicide (Ni), cobalt metal (Co), rhodonite (Mn), and tugtupite (Cl). The counting time for Ni, Co, Cl and Mn was 30 s for peak and15 s for background, whereas other elements were analyzed with 10 s for peak and 5 s for background. The detection limit for chlorine is ~33 ppm.
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3. Results and discussions 3.1 Identification of secondary hydrous minerals As revealed by SEM imaging and Fourier transform infrared spectroscopy analyses (Fig. 1), the main secondary hydrous mineral produced at 311 °C and 3.0 kbar was fibrous chrysotile, and tabular shaped lizardite was formed in experiments at 80-200 °C and vapor saturated pressures, and at 400 °C and 3.0 kbar. Infrared spectra of solid products show typical infrared bands of serpentine at 954 cm−1, 1087 cm−1, and 3686 cm−1. The bands at 954 cm−1 and 1087 cm−1 were assigned to the Si-O group in serpentine, and the band at 3686 cm−1 was for the –OH group (e.g., Fuchs et al., 1998; Lafay et al., 2014). Composition of secondary hydrous minerals determined by electron microprobe also suggests that the major secondary hydrous mineral was serpentine (Table 2, Bonifacie et al., 2008). By contrast, talc and lizardite were produced in experiments at 485 °C and 3.0 kbar. The formation of talc was attested by infrared modes at 671 cm−1 and 3677 cm−1 (Liu et al., 2014). The band at 671 cm−1 represents the stretching mode of Si-O-Mg group in talc, and that at 3677 cm−1 represents a stretching vibration of the –OH group (Liu et al., 2014).
3.2 Influence of fluid salinity on chlorine incorporation into serpentine Experiments were conducted in low- and high-salinity starting fluids (0.5 and 3.3 mol/L NaCl) to study influence of fluid salinity on chlorine distribution into serpentine (Table 1). It shows that serpentine formed in experiments at 311 °C and 3.0 kbar with a low-salinity fluid had essentially the same quantities of chlorine as 10
serpentine produced in a high-salinity fluid. By contrast, olivine-derived serpentine produced in experiments at 400 °C and 3.0 kbar with a low-salinity fluid had significantly higher chlorine than that formed in a high-salinity fluid (Fig. 2). As shown in Fig. 3, an inverse correlation was found in olivine-derived serpentine between Cl and FeO contents of serpentine. Such negative correlation was associated with an inverse correlation between chlorine and SiO2 contents and a positive relation between FeO and SiO2 contents (Fig. 3). It suggests that chlorine is hosted in a structurally-bound site. On the other hand, the inverse correlation between Cl and FeO may be due to the formation of Fe2(OH)3Cl as proposed by Rucklidge and Patterson (1977), and the negative correlation between Cl and SiO2 may be caused by the inclusion of non-silicate minerals such as iowaite Mg6Fe2(OH)16Cl2. 4H2O. It was strongly argued by the following observations: First, SEM imaging and in situ analyses by electron microprobe show no evidence of the presence of these minerals. Moreover, compositions of serpentine minerals do not indicate a structural deficit in silica, which may exclude intergrowth with iowaite. It suggests that chlorine substitutes for –OH in serpentine minerals, which agrees with the Cl substitution for hydroxyl group as suggested previously for serpentine (Anselmi et al., 2000), mica and amphiboles (e.g., Volfinger et al., 1985). The decrease in chlorine concentrations of olivine-derived serpentine formed in experiments with a high-salinity fluid was associated with a great increase in Al2O3 content and a decrease in SiO2 content (Fig. 3). It indicates that fluid salinity greatly increased the mobility of aluminum but decreased the mobility of silica. Hydrothermal experiments show that the solubility of 11
SiO2 becomes significantly lower with higher salinity (Newton and Manning, 2000a, b). The increase in Al2O3 content and the decrease in the SiO2 content may distort the structure of olivine-derived serpentine, which possibly decreases chlorine concentrations of serpentine.
As reported by Rucklidge (1972), chlorine in serpentine minerals increased greatly when serpentine was equilibrated with a saline solution (5 wt% NaCl) at 462 °C and 2 kbar for 20 days, in contrast with experimental results of this study. The starting materials of their experiments were serpentine rather than primary olivine and peridotite, and therefore experimental results of Rucklidge (1972) reflect chlorine distribution into serpentine during post serpentinization. To illustrate post-serpentinization chlorine concentrations of serpentine minerals, we performed several experiments at ambient temperature and pressure by loading ~200 mg serpentinite powders (with 0.02 ± 0.01 wt% Cl) into a saline fluid (10 ml, 0.5 mol/L NaCl). Chlorine in serpentine increased to 0.08 ± 0.03 wt% after an experimental duration of 20 days, consistent with the observations of Rucklidge (1972). However, the incorporated chlorine was completely leached by water within 24 hours, indicating that chlorine hosted in a water-soluble site can be greatly influenced by fluid salinity. It also indicates that post-serpentinization chlorine concentrations are quite unrelated to those prevailing during serpentinization. Chlorine was mostly incorporated in a structurally-bound site in experiments of this study, which may explain the negligible influence of fluid salinity on the incorporation of chlorine into serpentine. 12
3.3 Chlorine in serpentine minerals versus reaction time Chlorine in serpentine minerals formed at 311 °C and 3.0 kbar decreased slightly with longer reaction time (Fig. 4). It suggests that chlorine is hosted in a structurally-bound site. Otherwise, chlorine in serpentine minerals should increase slightly with progressive serpentinization due to higher fluid salinity (e.g., Berndt et al., 1996). Chlorine in the structurally-bound site was also supported by an inverse correlation found in olivine-derived serpentine between chlorine and FeO contents and a negative correlation between chlorine and SiO2 contents in orthopyroxene-derived serpentine produced at 20 days (Fig. 5). Chlorine in orthopyroxene-derived serpentine formed after longer experimental duration was positively correlated with the Al2O3 content and was negatively correlated with the FeO content of serpentine. It suggests that chlorine is highly mobile during serpentinization, and that chlorine distribution into serpentine minerals greatly depends on the mobility of aluminum and iron. Although the concentrations of aluminum and iron in fluids derived from serpentinization are very low (e.g., Charlou et al., 2000; Okamoto et al., 2011), the mobility of aluminum and iron is supported by the following evidence: First, olivine had very low Al2O3 contents (0.01 wt%), whereas olivine-derived serpentine contained significantly larger quantities of Al2O3, up to 4.4 wt%. By contrast, orthopyroxene-derived serpentine had slightly lower Al2O3 than primary orthopyroxene. It indicates that orthopyroxene lost some of its aluminum to olivine during serpentinization, which agrees with previous studies on natural serpentinites (e.g., Dungan, 1979; Hébert et al., 1990). A decrease in the Al2O3 content of orthopyroxene-derived serpentine may lead to a decrease in 13
chlorine concentrations, due to a positive correlation between chlorine and Al2O3 contents (Fig. 5). Moreover, orthopyroxene-derived serpentine formed at 20 days had 6.28 ± 0.58 wt% FeO, comparable with the FeO contents of primary orthopyroxene (6.24 wt% FeO). By contrast, the FeO contents of orthopyroxene-derived serpentine increased slightly to 6.85 ± 0.63 wt% after longer experimental duration (Huang et al., 2017), indicating that orthopyroxene obtained some iron during serpentinization. An increase in the FeO content of orthopyroxene-derived serpentine may result in a decrease in the chlorine concentration, based on an inverse correlation between chlorine and FeO contents of serpentine (Fig. 5). However, the dependence of reaction time was not detected in experiments at 485 °C and 3.0 kbar, and chlorine in serpentine minerals was essentially unchangeable when experimental durations increased from 30 to 62 days (Table 1).
Analyses of natural serpentinites show that chlorine was abundant in serpentine minerals adjacent to the relict olivine, whereas it was relatively depleted in the centers of serpentine veins (e.g., Miura et al., 1981; Beard et al., 2009). It indicates that chlorine prefers to incorporate into serpentine formed at the reaction front, and it decreases with progressive serpentinization, consistent with experimental results of this study. By contrast, Stueber et al. (1968) reported that chlorine in some serpentinized peridotite decreased greatly with larger reaction progresses, whereas an inverse correlation between chlorine and reaction progresses was found for other samples, possibly reflecting different formation temperature of serpentinites. 14
3.4 Influence of starting materials on chlorine incorporation into serpentine To investigate influence of starting materials on chlorine distribution into serpentine minerals, we performed experiments at 311 °C and 3.0 kbar on olivine, orthopyroxene and peridotite. Serpentine minerals formed in olivine-only experiments had very low chlorine concentrations, 0.08 ± 0.03 wt% (Table 1). By contrast, serpentine minerals formed in orthopyroxene-only experiments contained significantly higher chlorine, 0.49 ± 0.36 wt% (Table 1, Fig. 6). It indicates that chlorine favors the structure of orthopyroxene-derived serpentine, which agrees with analyses of natural serpentinites (Bonifacie et al., 2008). Orthopyroxene-derived serpentine had 42.9 ± 0.7 wt% SiO2, which was much larger than the SiO2 content of olivine-derived serpentine (37.7 ± 1.7 wt%). Chlorine in orthopyroxene-derived serpentine was positively correlated with its SiO2 contents (Fig. 7). However, olivine-derived serpentine formed in experiments at 311 °C and 3.0 kbar on peridotite had comparable chlorine with orthopyroxene-derived serpentine. In particular, olivine-derived serpentine contained slightly higher chlorine concentrations than serpentine formed in olivine-only experiments (Fig. 6), whereas orthopyroxene-derived serpentine had much lower chlorine concentrations than serpentine formed in orthopyroxene-only experiments (Fig. 6). As indicated by Fig. 7, orthopyroxene-derived serpentine had a significantly smaller SiO2 content than serpentine formed in orthopyroxene-only experiments, resulting from releases of silica from orthopyroxene during serpentinization. The decrease in SiO2 contents of orthopyroxene-derived serpentine may lead to a 15
distortion of the serpentine structure, which possibly leads to a decrease in chlorine concentrations of serpentine minerals. It suggests that the process of olivine serpentinization differs greatly from the process of peridotite serpentinization, which is consistent with previous experimental studies (Huang et al., 2015b, 2017).
3.5 Influence of water/rock ratios, temperature and pressure on chlorine distribution into serpentine In order to study the effect of water/rock ratios on chlorine distribution into serpentine minerals, we performed experiments at 80 °C and vapor saturated pressures with water/rock ratios ranging from 0.29 to 19.5 and at 485 °C and 3.0 kbar with water/rock ratios varied from ~1.0 to 10.0. The concentrations of chlorine in serpentine minerals were essentially unchangeable (Table 1), suggesting that influence of water/rock ratios on chlorine distribution into serpentine minerals is minor. It also indicates that water/rock ratios had a negligible influence on the mobility of aluminum, iron and silica during serpentinization. Consistently, thermodynamic modelings show that the concentrations of iron, aluminum and silica in fluids derived from harzburgite serpentinization are quite comparable over a range of water/rock ratios (McCollom and Bach, 2009).
Influence of temperature on chlorine concentrations of serpentine minerals was illustrated in Fig. 8. Olivine-derived serpentine formed at 80 °C had very low chlorine concentrations, 0.06 ± 0.04 wt%, whereas olivine-derived serpentine produced at 16
200 °C contained around one order of magnitude higher chlorine concentrations. By contrast, chlorine in olivine-derived serpentine decreased slightly at 311-400 °C and became significantly lower at 485 °C (Fig. 8). The decrease in chlorine concentrations of serpentine minerals with increasing temperature was also observed in orthopyroxene-only experiments (Table 1). Serpentine minerals produced at 311 °C had 0.49 ± 0.36 wt% Cl, whereas serpentine minerals formed at 485 °C contained significantly lower chlorine, 0.09 ± 0.04 wt%.
The concentrations of chlorine in serpentine are illustrated in Fig. 9 as a function of pressure. Serpentine formed at 311 °C and 3.0 kbar had slightly lower chlorine concentrations than serpentine produced at 300 °C and 8.0 kbar (Fig. 9a). An inverse correlation in olivine-derived serpentine was found between chlorine and SiO2 contents (Fig. 9b), which was associated with a positive correlation between chlorine and FeO contents and a negative relation between FeO and SiO2 contents (Fig. 9c, d). It suggests that chlorine is hosted in a structurally-bound site. The decrease in chlorine with increasing pressures may be linked with an increase in SiO2 contents and a decrease in FeO contents. By contrast, influence of pressure on chlorine distribution into serpentine minerals in experiments at 400-500 °C is minor.
Few studies have examined the incorporation of chlorine into serpentine formed at different temperatures and pressures (Scambelluri et al., 2004; Bonifacie et al., 2008). These two studies have investigated high-pressure metaperidotites from Erro-Tobbio 17
Unit of the Voltri Massif, which experienced serpentinization in the oceanic environment, followed by subduction to HP-LT conditions of ~20 kbar and ~550 °C (Scambelluri et al., 2004; Bonifacie et al., 2008). The temperature of oceanic serpentinites was determined based on oxygen isotope fractionation between serpentine and magnetite, which were varied from 27 to ~300 °C (Bonifacie et al., 2008). The concentrations of chlorine in oceanic serpentinites are very high with large variations, whereas they decrease significantly at higher pressures and temperatures (Fig. 10, Scambelluri et al., 2004; Bonifacie et al., 2008). The experimental results of this study are consistent with those of Scambelluri et al. (2004) and Bonifacie et al. (2008), suggesting that our experimental results can be applied to natural systems.
3.6 Geological implications Experimental results of this study show that chlorine in serpentine minerals can be preserved at relatively high pressures and high temperatures, which suggests that serpentine minerals are important carriers of chlorine in subduction zones. The enrichment patterns of fluid-mobile elements in natural serpentinites are strikingly similar to those of arc magmas (Hattori and Guillot, 2003; Guillot and Hattori, 2013). It indicates that the dehydration of serpentinites is linked to the genesis of arc magmas. Experimental studies show that serpeninite dehydration at relatively higher temperatures (e.g., ≥600 °C) releases chlorine-bearing aqueous fluids that have abundant fluid-mobile elements (Tenthorey and Hermann, 2004). The fluids may play an important role for the metasomatism of peridotite in the mantle wedge, which is 18
supported by saline fluid inclusions derived from peridotite in the mantle wedge (Kawamoto et al., 2013). It possibly leads to partial melting of the peridotite and consequently the formation of arc magmas (Hattori and Guillot, 2003; Guillot and Hattori, 2013). Arc magmas have much larger quantities of chlorine than magmas from other tectonic settings (e.g., De Hoog et al., 2001). The correlation between Cl and FeO contents of serpentine suggests that chlorine is significant for the redistribution of elements. Indeed, chlorine is an important agent for mobilizing and transporting metals, such as Fe, Cu, and Au. Therefore, chlorine-bearing fluids released by serpentinites dehydration may be important for the formation of giant ore deposits at convergent margins (e.g., Sun et al., 2013, 2015).
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4 Conclusions Hydrothermal experiments were performed at 80-500 °C and pressures ranging from vapor saturated pressures to 20 kbar to study influence of temperature, fluid salinity, water/rock ratios, and pressure on chlorine distribution into serpentine minerals. The results show that chlorine was hosted in a structurally-bound site of serpentine, and it strongly depends on the mobility of iron, aluminum and silica during serpentinization. Chlorine concentrations of serpentine formed at 80 °C were very low, which increased by around one order of magnitude in experiments at 200 °C. By contrast, chlorine in serpentine minerals decreased slightly at 311-400 °C and it became significantly lower at 485 °C. Chlorine in olivine-derived serpentine formed at 400 °C and 3.0 kbar decreased greatly in a high-salinity fluid, which was associated with a decrease in SiO2 contents and an increase in Al2O3 contents of serpentine. By contrast, for experiments at 311°C and 3.0 kbar, influence of fluid salinity on chlorine incorporation into serpentine was negligible. Moreover, the presence of pyroxene minerals in peridotite greatly influenced the mobility of aluminum, iron and silica during serpentinization, and consequently chlorine distribution into serpentine minerals can be influenced. Orthopyroxene-derived serpentine formed in experiments at 311 °C and 3.0 kbar on peridotite had significantly lower chlorine concentrations than serpentine produced in orthopyroxene-only experiments, whereas olivine-derived serpentine contained much larger quantities of chlorine than serpentine formed in olivine-only experiments. Composition of serpentine minerals suggests that pyroxene minerals released some of its aluminum and silica to olivine during serpentinization. 20
As a consequence, orthopyroxene-derived serpentine formed in experiments on peridotite had smaller quantities of SiO2 than that produced in orthopyroxene-only experiments, which possibly leads to a decrease in chlorine concentrations of serpentine. Chlorine concentrations of serpentine formed in experiments at 311 °C and 3.0 kbar were slightly lower than those of serpentine produced at higher pressures, which may be associated with influence of pressure on the mobility of iron and silica. By contrast, pressure had a minor influence on chlorine concentrations of serpentine produced at 400-500 °C. Experimental results of this study suggest that chlorine incorporated into serpentine can be preserved at relatively high temperatures and pressures. Therefore, serpentinization is significant for the transfer of chlorine to the mantle.
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Acknowledgements: This research was financially supported by the Natural Science Foundation of China (91328204, 41603060), postdoctoral Science Foundation of China (2015M570735, 2016T90805), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB06030100), and the scientific research fund of the Second Institute of Oceanography, SOA (JG1405). We thank S. Jiang at South China University of Technology for the help during FTIR analyses.
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29
Table 1 Experimental conditions and chlorine concentrations of serpentine minerals Sample
T
P
Solid a
Starting
W/R
solutions
b
Time
Chlorine in
ratio
(day)
serpentine(wt%)
Chlorine in c
serpentine(wt%)d
No.
(°C)
(kbar)
reactant
HR25
311
3.0
Prt
0.5 mol/L
1.4
19
0.16(9)
0.22 (12)
HR37
311
3.5
Ol
0.5 mol/L
1.1
26
0.08(3)
—
HR42
311
3.0
Opx
0.5 mol/L
0.82
18
—
0.49(36)
HR61
311
3.0
Prt
0.5 mol/L
0.82
120
0.09(4)
0.14(5)
HR66
300
8.0
Prt
3.3 mol/L
0.50
22
0.42(18)
—
HR77
311
3.0
Prt
3.3 mol/L
1.2
28
0.25(8)
0.24(15)
HR36
400
1.0
Prt
0.5 mol/L
1.6
38
0.06(4)
0.19(16)
HR78
400
3.0
Prt
3.3 mol/L
1.0
29
0.13(13)
0.13(7)
HR79
400
3.0
Prt
0.5 mol/L
0.86
27
0.70(35)
0.12(4)
HR67
400
8.0
Prt
3.3 mol/L
0.50
22
0.08(2)
0.14(7)
HR21
485
3.0
Prt
0.5 mol/L
1.0
30
0.08(4)
—
HR27
485
3.0
Prt
0.5 mol/L
0.74
62
0.12(6)
—
HR45
485
2.0
Opx
0.5 mol/L
1.5
27
—
0.09(4)
HR23
500
8.0
Prt
0.5 mol/L
0.83
20
0.21(11)
0.16(10)
HR32
500
20
Prt
0.5 mol/L
0.20
23
0.08(2)
0.07(5)
HRL1
200
0.016
Prt
0.5 mol/L
7.1
19
1.21(13)
0.37(49)
HRL21e
80
—
Prt
0.5 mol/L
19.5
30
0.02(3)
—
HRL23
80
—
Prt
0.5 mol/L
2.4
30
0.02(2)
—
HRL26
80
—
Prt
0.5 mol/L
2.7
60
0.07(4)
0.04(5)
HRL34
80
—
Prt
0.5 mol/L
0.29
22
0.06(3)
—
a
Prt: peridotite, Ol: olivine, Opx: orthopyroxene.
b
Mass ratios between starting saline solutions and solid reactants
c
Olivine-derived serpentine. Chlorine concentrations were the average of ~6-30 analyses.
d
Orthopyroxene-derived serpentine. Numbers in parentheses are one standard deviation.
e
Vapor saturated pressure at 80 °C is 0.5 bar.
30
Table 2 Representative composition of serpentine minerals determined by electron microprobe (in wt%)
a
SiO2
TiO2
Cr2O3
Al2O3
MgO
FeO
CaO
MnO
NiO
CoO
Na2O
K2O
Cl
Total
Notea
HRL23-1
38.91
-
0.01
6.20
30.36
8.52
0.24
0.10
0.23
0.07
0.13
0.17
0.02
84.94
ol
HRL23-2
42.26
-
0.06
3.39
32.09
7.99
0.09
0.09
0.31
0.02
0.15
0.42
0.01
86.86
ol
HRL23-3
43.30
0.02
0.05
0.15
31.80
7.75
0.13
0.04
0.39
0.01
0.04
0.06
0.07
83.81
ol
HRL23-4
42.43
-
0.04
1.09
34.07
7.45
0.09
0.10
0.16
-
0.05
0.08
0.02
85.57
ol
HRL23-5
42.16
0.07
0.05
1.88
35.73
5.71
0.06
0.09
0.22
-
0.06
0.09
0.02
86.13
ol
HRL1-1
41.54
0.02
-
1.53
30.51
8.72
0.04
0.08
0.28
0.09
0.23
0.02
1.13
83.93
ol
HRL1-2
42.57
-
0.00
1.25
29.21
9.74
0.04
0.07
0.39
0.02
0.36
0.03
1.36
84.74
ol
HRL1-3
42.15
-
-
1.09
30.78
9.64
0.05
0.13
0.32
0.04
0.23
0.02
1.14
85.34
ol
HRL1-4
38.49
0.13
0.15
9.22
32.87
3.46
0.38
0.13
0.04
-
0.30
0.10
0.19
85.42
opx
HRL1-5
39.00
0.11
0.11
8.45
31.81
3.89
0.12
0.13
0.10
0.05
0.26
0.10
0.15
84.24
opx
HR25-22
39.14
0.10
0.45
3.57
34.16
6.84
0.12
0.14
0.13
0.03
0.25
0.01
0.31
85.23
opx
HR25-23
38.81
0.06
0.46
4.08
32.47
6.88
0.11
0.10
0.13
0.02
0.34
0.01
0.22
83.68
opx
HR25-17
40.74
0.10
0.17
2.92
35.57
7.43
0.06
0.08
0.16
-
0.17
0.01
0.03
87.43
opx
HR25-70
39.77
0.06
0.53
2.97
35.46
6.93
0.15
0.14
0.13
0.02
0.28
-
0.25
86.69
opx
HR25-29
40.58
0.01
0.03
0.61
39.42
6.82
0.07
0.09
0.40
0.04
0.19
0.01
0.13
88.39
ol
HR25-43
39.35
0.10
0.04
2.42
34.95
8.81
0.10
0.10
0.24
0.03
0.19
0.01
0.10
86.42
ol
HR25-44
39.82
0.02
0.04
2.87
35.63
9.63
0.06
0.05
0.32
0.02
0.12
0.02
0.05
88.62
ol
HR25-45
39.67
-
0.01
3.02
35.75
8.94
0.08
0.10
0.31
-
0.14
0.01
0.11
88.14
ol
HR77-20
39.64
0.16
0.61
2.76
34.72
5.27
0.15
0.13
0.16
0.05
0.22
0.02
0.26
84.15
opx
HR77-21
41.14
0.09
0.45
3.27
34.98
5.42
0.14
0.14
0.12
0.01
0.21
0.03
0.26
86.26
opx
HR77-22
40.95
0.09
0.49
3.49
34.28
5.69
0.10
0.09
0.07
0.03
0.26
0.03
0.25
85.81
opx
HR77-25
40.47
0.01
0.07
0.49
34.68
7.36
0.04
0.14
0.36
0.02
0.04
0.01
0.12
83.80
ol
HR77-27
39.72
0.01
0.00
0.74
33.93
5.66
0.03
0.07
0.28
0.02
0.12
0.01
0.27
80.85
ol
HR77-28
39.58
0.01
0.03
0.71
33.10
6.74
0.04
0.12
0.34
0.03
0.15
0.01
0.32
81.19
ol
HR78-1
40.62
0.04
0.01
2.28
33.28
5.68
0.43
0.14
0.14
0.01
0.21
0.03
0.13
83.00
opx
HR78-2
40.24
0.10
0.08
3.29
32.50
5.99
0.25
0.10
0.07
0.02
0.23
0.04
0.11
83.01
opx
HR78-3
40.20
0.05
0.09
4.13
32.75
6.29
0.10
0.11
0.12
0.01
0.20
0.03
0.08
84.14
opx
HR78-21
41.20
0.05
0.03
1.90
32.30
7.80
0.06
0.17
0.18
0.01
0.27
0.04
0.06
84.08
ol
HR78-22
41.36
0.08
0.03
4.15
33.05
6.67
0.08
0.07
0.24
0.01
0.39
0.07
0.07
86.28
ol
HR78-23
40.62
0.07
0.04
4.30
32.99
8.14
0.04
0.12
0.35
0.04
0.29
0.06
0.07
87.11
ol
HR79-12
44.95
0.03
0.00
0.66
33.26
7.18
0.31
0.15
0.37
0.00
0.34
0.08
1.29
88.61
ol
HR79-13
43.65
0.03
0.00
0.62
31.57
7.09
0.24
0.16
0.55
0.00
0.22
0.05
0.35
84.52
ol
HR79-14
43.22
0.04
0.04
0.47
32.18
6.98
0.21
0.14
0.52
0.03
0.23
0.07
0.56
84.68
ol
HR79-7
39.78
0.07
0.21
3.72
34.75
5.99
0.08
0.07
0.13
0.02
0.19
0.06
0.13
85.19
opx
HR79-8
40.05
0.07
0.16
3.88
34.70
5.80
0.07
0.09
0.16
0.04
0.17
0.05
0.12
85.37
opx
HR79-9
40.38
0.06
0.54
3.39
34.03
5.95
0.10
0.11
0.15
0.04
0.17
0.06
0.17
85.17
opx
Primary minerals of serpentine. Ol: olivine, Opx: orthopyroxene
31
Figure captions: Figure 1 (a) Representative infrared spectra of experimental products. The main secondary hydrous mineral produced at 311 °C and 3.0 kbar was fibrous chrysotile. By contrast, tabular shaped lizardite was produced in experiments at 400 °C and 3.0 kbar, whereas lizardite and talc formed in experiments at 485 °C and 3.0 kbar. Serpentine is characterized by infrared modes at 954 cm-1, 1083 cm-1, and 3686 cm-1 (e.g., Fuchs et al., 1998; Lafay et al., 2014), and talc has typical infrared bands at 671 cm-1, and 3677 cm-1(Liu et al., 2014). (b), (c) and (d) Scanning electron microscope imaging of experimental products showing the formation of fibrous chrysotile (Ctl) at 311 °C and 3.0 kbar, lizardite (Lz) at 400 °C and 3.0 kbar, and talc (Tlc) at 485 °C and 3.0 kbar.
Figure 2 Influence of fluid salinity on chlorine distribution into serpentine minerals. Chlorine concentrations of serpentine formed in experiments at 311 °C and 3.0 kbar with a low-salinity fluid were essentially the same as chlorine concentrations of serpentine produced in a high-salinity fluid. By contrast, chlorine concentrations of olivine-derived serpentine formed in experiments at 400 °C and 3.0 kbar decreased greatly in a high-salinity fluid.
Figure 3 Chlorine in olivine-derived serpentine formed at 400 °C and 3.0 kbar versus (a) FeO contents, (b) SiO2 contents, and (c) Al2O3 contents. (d) The correlation between FeO and SiO2 contents of olivine-derived serpentine. 32
Figure 4 Influence of experimental duration (in day) on chlorine distribution into serpentine minerals formed in experiments at 311 °C and 3.0 kbar.
Figure 5 (a) Chlorine versus FeO contents of olivine-derived serpentine, and (b) chlorine versus SiO2 contents of orthopyroxene-derived serpentine formed in experiments at 311 °C and 3.0 kbar after 20 days. (c) Chlorine versus FeO contents and (d) chlorine versus Al2O3 contents of serpentine minerals produced in experiments at 311 °C and 3.0 kbar after 120 days.
Figure 6 Influence of primary minerals on chlorine concentrations of serpentine (in wt%) formed at 311 °C and 3.0 kbar. “Ol-only” represents serpentine formed in olivine-only experiments. “Opx-only” represents serpentine formed in orthopyroxene-only experiments. “Prt, ol-srp” stands for olivine-derived serpentine formed in experiments on peridotite. “Prt, opx-srp” stands for orthopyroxene-derived serpentine produced in experiments on peridotite.
Figure 7 Compositional differences between serpentine formed in orthopyroxene-only experiments and orthopyroxene-derived serpentine produced during peridotite serpentinization. (a) Chlorine versus SiO2 contents of serpentine and (b) chlorine versus Al2O3 contents of serpentine.
33
Figure 8 Influence of temperature on chlorine incorporation into serpentine minerals. Experiments at 311-485 °C were conducted at 3.0 kbar with a saline solution (0.5 mol/L) and comparable water/rock ratios. It shows that chlorine contents of serpentine were abundant at 300-400 °C, whereas they decreased greatly at 485 °C.
Figure 9 (a) Influence of pressure on chlorine contents of serpentine. (b) An inverse correlation between Cl and SiO2 contents of serpentine formed at 300 °C and 3-8 kbar. (c) The correlation between Cl and FeO contents, and (d) the correlation between FeO and SiO2 contents of serpentine.
Figure 10 Comparison between experimental results of this study and those based on natural serpentinites (Scambelluri et al., 2004; Bonifacie et al., 2008).
34
Figure 1
35
Figure 2
36
Figure 3
37
Figure 4
38
Figure 5
39
Figure 6
40
Figure 7
41
Figure 8
42
Figure 9
43
Figure 10
Highlights: ●The incorporation of chlorine into serpentine was experimentally studied ●Temperature, pressure and fluid salinity greatly influence chlorine behavior ●Chlorine in serpentine reached maximum at 200 °C, which decreased at 300-500 °C ●Chlorine behavior is closely associated with the mobility of aluminum and silica ●Serpentine is an important carrier of chlorine in subduction zones
44
S1367-9120(17)30193-1 http://dx.doi.org/10.1016/j.jseaes.2017.04.022 JAES 3058
To appear in:
Journal of Asian Earth Sciences
Please cite this article as: Huang, R., Sun, W., Zhan, W., Ding, X., Zhu, J., Liu, J., Influence of temperature, pressure, and fluid salinity on the distribution of chlorine into serpentine minerals, Journal of Asian Earth Sciences (2017), doi: http://dx.doi.org/10.1016/j.jseaes.2017.04.022
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Influence of temperature, pressure, and fluid salinity on the distribution of chlorine into serpentine minerals Ruifang Huanga,b,*, Weidong Suna, Wenhuan Zhanb, Xing Dingc, Jihao Zhud, Jiqiang Liud
a
Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of
Geochemistry, Chinese Academy of Sciences, 510640 Guangzhou, PR China; b
Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology,
Chinese Academy of Sciences, 510301 Guangzhou, PR China; c
State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of
Geochemistry, Chinese Academy of Sciences, 510640 Guangzhou, PR China; d
The Second Institute of Oceanography, SOA, Hangzhou 310012, PR China
*Corresponding author: [email protected] (R. Huang)
1
ABSTRACT Serpentinization produces serpentine minerals that have abundant water and fluid-mobile elements (e.g., Ba, Cs, and Cl). The dehydration of serpentine minerals produces chlorine-rich fluids that may be linked with the genesis of arc magmas. However, the factors that control the distribution of chlorine into serpentine minerals remain poorly constrained. We performed serpentinization experiments at 80-500 °C and pressures from vapor saturated pressures to 20 kbar on peridotite, orthopyroxene, and olivine with <5% pyroxene. The results show that the concentrations of chlorine in serpentine minerals were up to 1.2 wt% at 200 °C, whereas they decreased slightly at 311-400 °C and 3.0 kbar and became significantly lower at 485 °C and 3.0 kbar, ~0.1 wt%. Fluid salinity greatly decreased chlorine concentrations of olivine-derived serpentine produced at 400 °C and 3.0 kbar, which was associated with a decrease in silica mobility during serpentinization. By contrast, influence of fluid salinity at 311 °C and 3.0 kbar is minor. Moreover, chlorine distribution into serpentine can be influenced by primary minerals of serpentine. Serpentine formed in olivine-only experiments at 311 °C and 3.0 kbar had 0.08 ± 0.03 wt% Cl, which was significantly lower than chlorine concentrations of serpentine minerals (0.49 ± 0.36 wt%) produced in orthopyroxene-only experiments. By contrast, olivine-derived serpentine formed in experiments at 311 °C and 3.0 kbar on peridotite contained comparable chlorine with orthopyroxene-derived serpentine. In particular, olivine-derived serpentine had 0.16 ± 0.09 wt% Cl that was slightly higher than chlorine concentrations of serpentine formed in olivine-only experiments, whereas orthopyroxene-derived serpentine had 2
significantly lower chlorine concentrations than that formed in orthopyroxene-only experiments. The contrast may be associated with releases of aluminum and silica from pyroxene minerals, which possibly results in a decrease in chlorine concentrations of serpentine. The concentrations of chlorine in serpentine formed in experiments at 311 °C and 3.0 kbar were slightly lower than those in serpentine produced at 300 °C and 8.0 kbar, which may be associated with influence of pressure on the mobility of iron and silica. The experimental results of this study indicate that serpentine minerals are important carriers of chlorine in subduction zones. It also suggests that chlorine is significant for the redistribution of cations during serpentinization.
Key-words: serpentinization, chlorine, peridotite, aluminum, silica
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1. Introduction Serpentinization is the hydration of ultramafic rocks (typically peridotite and komatiite) at relatively low temperatures (≤500 °C), forming serpentine minerals, (±) brucite, (±) talc, and (±) magnetite. It occurs at various geological settings, including the ocean floor, mid-ocean ridges, and subduction zones (e.g., Charlou et al., 1996, 2000; Maekawa et al., 2001; Hyndman and Peacock, 2003; Mével, 2003; Evans et al., 2013). Serpentinization can greatly modify the chemical and physical properties of oceanic lithosphere (e.g., Escartín et al., 1997, 2001; Scambelluri et al., 1995; Mével, 2003; Guillot and Hattori, 2013). Serpentinites (with >90% serpentine) have significantly lower density than primary peridotite, and their magnetic susceptibility is up to two orders of magnitude higher (Mével, 2003; Evans et al., 2013). As suggested by deformation experiments, a very low degree of serpentinization can greatly decrease the strength of olivine (Escartín et al., 1997, 2001). Analyses of natural serpentinites show that serpentine minerals can incorporate not only water (up to 13.5 wt%) but also chlorine and fluid-mobile elements, such as Cs, Ba, and Sr (Scambelluri et al., 1995; Hattori and Guillot, 2003; Deschamps et al., 2012; Guillot and Hattori, 2013). Compared to fresh peridotite (with <30 ppm Cl), serpentinites have around one to two orders of magnitude higher chlorine concentrations (e.g., Bonifacie et al., 2008; Barnes and Straub, 2010). Thermodynamic and experimental studies suggest that serpentine minerals can be stable at depths of greater than 200 km (Ulmer and Trommsdorff, 1995; Schmidt and Poli, 1998). Therefore, serpentinization may play an important role for the transfer of H2O, chlorine, and fluid-mobile 4
elements to the mantle.
In spite of the importance, the mechanisms that control the incorporation of chlorine into serpentine minerals are poorly understood (Rucklidge, 1972; Rucklidge and Patterson, 1977; Anselmi et al., 2000; Scambelluri et al., 2004; Sharp and Barnes, 2004; Barnes and Sharp, 2006; Bonifacie et al., 2008). Chlorine can be hosted in a structurally-bound site, i.e., substitute for the hydroxyl group in serpentine (e.g., Anselmi et al., 2000) or in a water-soluble site (e.g., Rucklidge and Patterson, 1977; Sharp and Barnes, 2004; Barnes and Sharp, 2006). Hydrothermal experiments show that chlorine in the water-soluble site is readily leached by water (Sharp and Barnes, 2004; Barnes and Sharp, 2006), and it may increase greatly in a saline fluid (e.g., Rucklidge, 1972). Fluid inclusions of natural serpentinites have largely varied salinities (e.g., Philippot et al., 1998; Scambelluri et al., 1997, 2004), indicating that chlorine in serpentine can vary greatly under the same T-P conditions. Analyses of natural serpentinites show that oceanic serpentinites, possibly formed at ~250 °C, had abundant chlorine, up to 0.62 wt%, whereas antigorite serpentinites with the experience of eclogite-facies metamorphic grade contained significantly lower chlorine concentrations (Scambelluri et al., 2004). However, natural serpentinites commonly experience multistage metamorphic history, and consequently the obtained chlorine may not necessarily represent a single T-P condition. In particular, chlorine in natural serpentinites may be greatly influenced by lateral metamorphic fluids. However, there has been no hydrothermal experiment to investigate influence of 5
temperature, pressure, and salinity of starting fluids on chlorine incorporation into serpentine minerals.
In this study, we performed experiments at 80-500 °C and pressures ranging from vapor saturated pressures to 20 kbar with peridotite, orthopyroxene, and olivine with <5% pyroxene. The objectives are to (1) quantify the concentrations of chlorine in serpentine formed under different conditions, (2) investigate influence of temperature, pressure, fluid salinity, and water/rock ratios on chlorine distribution into serpentine minerals, and (3) assess the importance of serpentinization for the recycling of chlorine in subduction zones.
2. Materials and methods 2.1 Preparation of starting materials A non-altered peridotite, sampled at Panshishan (Jiangsu Province, China) where it occurs as xenoliths in alkaline basalts (Chen et al., 1994; Sun et al., 1998; Xu et al., 2008; Yang, 2008), was taken as the starting material. Peridotite is composed of ~65 vol% olivine, 20 vol% orthopyroxene, 15 vol% clinopyroxene and ~2 vol% spinel. The composition of the primary minerals in peridotite determined by electron microprobe has been described in a previous experimental study (Huang et al., 2015a). The peridotite is fresh, as evidenced by a very small loss on ignition (<0.3%) upon heating at 1200 °C (Yang, 2008). Olivine and orthopyroxene were picked from ground peridotite power (<60 mesh) under binocular microscope, and their fractions 6
include <5% of other mineral phases. Peridotite, olivine, and orthopyroxene were ground in an agate mortar, and then sieved into grain sizes of 100-177 µm. Saline solutions (0.5 and 3.3 mol/L NaCl) were prepared in order to investigate influence of fluid salinity on chlorine distribution into serpentine minerals.
2.2 Hydrothermal experiments All the experiments were conducted at high temperature and high pressure laboratory, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (Table 1). Experiments at 80 °C were performed in a water bath kettle, and the solid reactants and a saline starting solution were loaded into a 15 ml polypropylene centrifuge tube. Teflon-lined steel vessels were used in experiments at 200 °C (Psat ~16 bar). When finished, the tubes and steel vessels were quenched in ice water.
Experiments at 311-485 °C and 3.0 kbar were carried out in cold-seal hydrothermal vessels. Solid reactants and a saline starting solution were loaded into gold capsules (4.0 mm O.D., 3.6 mm I. D. and 30 mm in length), which were welded shut by a tungsten inert gas high-frequency pulse welder (PUK3). Leaks were always checked by putting capsules into a drying furnace at 100 °C for several hours, and those with mass differences from initial mass before heating less than 0.1% were used in experiments. The prepared capsules were placed into the end of hydrothermal vessels, followed by a filler rod (~6 cm long). Water was used as the pressure medium. Pressures were achieved by pumping water into the vessel, and they were measured 7
by a pressure gauge with a precision of ±200 bar. The temperature was monitored with an external K-type thermocouple that was inserted into a hole near the end of the vessel with the accuracy of ±2 °C. When finished, the vessels were immersed in ice water and temperature decreased to <100 °C within several seconds.
Experiments at 300-500 °C and 8-20 kbar were performed in a piston-cylinder apparatus (Quickpress, Depth of the Earth Co.). Solid reactants and a starting saline solution were loaded into gold capsules (around 10 mm in length, 3.0 mm outer diameter and 0.2 mm of wall thickness), and the capsules were welded with a tungsten inert gas high-frequency pulse welder (PUK3). Glass-salt assemblies were used. Temperature was monitored by an S-type thermocouple with an accuracy of ±10 °C. Pressure calibration has been done according to albite-breakdown reaction at 600 °C, and the results showed less than 8% of friction correction.
2.3 Analytical methods The surface morphology of solid products was characterized with a Zeiss Ultra 55 Field emission gun scanning electron microscope (SEM) at Second Institute of Oceanography, State Oceanic Administration of China. Samples were dispersed onto double-sided carbon tape and coated with a thin film of platinum for observation.
The mineralogy of solid products was identified with a Bruker Vector 33Fourier transform infrared spectrometer at the Analytical and Testing Center of South China 8
University of Technology. Infrared spectra were obtained at wavenumbers from 400 to 4000 cm−1 at a resolution of 4 cm−1, and 32 scans were accumulated for each spectrum. The KBr pellets were prepared by mixing around 1 mg of sample powder with 200 mg of KBr.
Some solid samples were mounted in epoxy resin and subsequently polished using Al2O3/SiC grit papers and diamond paste. To avoid the possibility that Cl-bearing phases were dissolved during sample preparation, we polished samples using ethanol instead of water. Comparisons of chlorine contents in samples polished using water and ethanol were also made. Compositions of solid products after experiments were determined by JEOL JXA 8100 electron microprobe with four wavelength-dispersive spectrometers at Second Institute of Oceanography, State Oceanic Administration. Operating conditions for serpentine analyses included 15 kV and 20 nA with a beam diameter of 15 µm. Calibration standards were jadeite (Si, Na), olivine (Mg), almandine garnet (Fe, Al), diopside (Ca), sanidine (K), chromium oxide (Cr), rutile (Ti), nickel silicide (Ni), cobalt metal (Co), rhodonite (Mn), and tugtupite (Cl). The counting time for Ni, Co, Cl and Mn was 30 s for peak and15 s for background, whereas other elements were analyzed with 10 s for peak and 5 s for background. The detection limit for chlorine is ~33 ppm.
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3. Results and discussions 3.1 Identification of secondary hydrous minerals As revealed by SEM imaging and Fourier transform infrared spectroscopy analyses (Fig. 1), the main secondary hydrous mineral produced at 311 °C and 3.0 kbar was fibrous chrysotile, and tabular shaped lizardite was formed in experiments at 80-200 °C and vapor saturated pressures, and at 400 °C and 3.0 kbar. Infrared spectra of solid products show typical infrared bands of serpentine at 954 cm−1, 1087 cm−1, and 3686 cm−1. The bands at 954 cm−1 and 1087 cm−1 were assigned to the Si-O group in serpentine, and the band at 3686 cm−1 was for the –OH group (e.g., Fuchs et al., 1998; Lafay et al., 2014). Composition of secondary hydrous minerals determined by electron microprobe also suggests that the major secondary hydrous mineral was serpentine (Table 2, Bonifacie et al., 2008). By contrast, talc and lizardite were produced in experiments at 485 °C and 3.0 kbar. The formation of talc was attested by infrared modes at 671 cm−1 and 3677 cm−1 (Liu et al., 2014). The band at 671 cm−1 represents the stretching mode of Si-O-Mg group in talc, and that at 3677 cm−1 represents a stretching vibration of the –OH group (Liu et al., 2014).
3.2 Influence of fluid salinity on chlorine incorporation into serpentine Experiments were conducted in low- and high-salinity starting fluids (0.5 and 3.3 mol/L NaCl) to study influence of fluid salinity on chlorine distribution into serpentine (Table 1). It shows that serpentine formed in experiments at 311 °C and 3.0 kbar with a low-salinity fluid had essentially the same quantities of chlorine as 10
serpentine produced in a high-salinity fluid. By contrast, olivine-derived serpentine produced in experiments at 400 °C and 3.0 kbar with a low-salinity fluid had significantly higher chlorine than that formed in a high-salinity fluid (Fig. 2). As shown in Fig. 3, an inverse correlation was found in olivine-derived serpentine between Cl and FeO contents of serpentine. Such negative correlation was associated with an inverse correlation between chlorine and SiO2 contents and a positive relation between FeO and SiO2 contents (Fig. 3). It suggests that chlorine is hosted in a structurally-bound site. On the other hand, the inverse correlation between Cl and FeO may be due to the formation of Fe2(OH)3Cl as proposed by Rucklidge and Patterson (1977), and the negative correlation between Cl and SiO2 may be caused by the inclusion of non-silicate minerals such as iowaite Mg6Fe2(OH)16Cl2. 4H2O. It was strongly argued by the following observations: First, SEM imaging and in situ analyses by electron microprobe show no evidence of the presence of these minerals. Moreover, compositions of serpentine minerals do not indicate a structural deficit in silica, which may exclude intergrowth with iowaite. It suggests that chlorine substitutes for –OH in serpentine minerals, which agrees with the Cl substitution for hydroxyl group as suggested previously for serpentine (Anselmi et al., 2000), mica and amphiboles (e.g., Volfinger et al., 1985). The decrease in chlorine concentrations of olivine-derived serpentine formed in experiments with a high-salinity fluid was associated with a great increase in Al2O3 content and a decrease in SiO2 content (Fig. 3). It indicates that fluid salinity greatly increased the mobility of aluminum but decreased the mobility of silica. Hydrothermal experiments show that the solubility of 11
SiO2 becomes significantly lower with higher salinity (Newton and Manning, 2000a, b). The increase in Al2O3 content and the decrease in the SiO2 content may distort the structure of olivine-derived serpentine, which possibly decreases chlorine concentrations of serpentine.
As reported by Rucklidge (1972), chlorine in serpentine minerals increased greatly when serpentine was equilibrated with a saline solution (5 wt% NaCl) at 462 °C and 2 kbar for 20 days, in contrast with experimental results of this study. The starting materials of their experiments were serpentine rather than primary olivine and peridotite, and therefore experimental results of Rucklidge (1972) reflect chlorine distribution into serpentine during post serpentinization. To illustrate post-serpentinization chlorine concentrations of serpentine minerals, we performed several experiments at ambient temperature and pressure by loading ~200 mg serpentinite powders (with 0.02 ± 0.01 wt% Cl) into a saline fluid (10 ml, 0.5 mol/L NaCl). Chlorine in serpentine increased to 0.08 ± 0.03 wt% after an experimental duration of 20 days, consistent with the observations of Rucklidge (1972). However, the incorporated chlorine was completely leached by water within 24 hours, indicating that chlorine hosted in a water-soluble site can be greatly influenced by fluid salinity. It also indicates that post-serpentinization chlorine concentrations are quite unrelated to those prevailing during serpentinization. Chlorine was mostly incorporated in a structurally-bound site in experiments of this study, which may explain the negligible influence of fluid salinity on the incorporation of chlorine into serpentine. 12
3.3 Chlorine in serpentine minerals versus reaction time Chlorine in serpentine minerals formed at 311 °C and 3.0 kbar decreased slightly with longer reaction time (Fig. 4). It suggests that chlorine is hosted in a structurally-bound site. Otherwise, chlorine in serpentine minerals should increase slightly with progressive serpentinization due to higher fluid salinity (e.g., Berndt et al., 1996). Chlorine in the structurally-bound site was also supported by an inverse correlation found in olivine-derived serpentine between chlorine and FeO contents and a negative correlation between chlorine and SiO2 contents in orthopyroxene-derived serpentine produced at 20 days (Fig. 5). Chlorine in orthopyroxene-derived serpentine formed after longer experimental duration was positively correlated with the Al2O3 content and was negatively correlated with the FeO content of serpentine. It suggests that chlorine is highly mobile during serpentinization, and that chlorine distribution into serpentine minerals greatly depends on the mobility of aluminum and iron. Although the concentrations of aluminum and iron in fluids derived from serpentinization are very low (e.g., Charlou et al., 2000; Okamoto et al., 2011), the mobility of aluminum and iron is supported by the following evidence: First, olivine had very low Al2O3 contents (0.01 wt%), whereas olivine-derived serpentine contained significantly larger quantities of Al2O3, up to 4.4 wt%. By contrast, orthopyroxene-derived serpentine had slightly lower Al2O3 than primary orthopyroxene. It indicates that orthopyroxene lost some of its aluminum to olivine during serpentinization, which agrees with previous studies on natural serpentinites (e.g., Dungan, 1979; Hébert et al., 1990). A decrease in the Al2O3 content of orthopyroxene-derived serpentine may lead to a decrease in 13
chlorine concentrations, due to a positive correlation between chlorine and Al2O3 contents (Fig. 5). Moreover, orthopyroxene-derived serpentine formed at 20 days had 6.28 ± 0.58 wt% FeO, comparable with the FeO contents of primary orthopyroxene (6.24 wt% FeO). By contrast, the FeO contents of orthopyroxene-derived serpentine increased slightly to 6.85 ± 0.63 wt% after longer experimental duration (Huang et al., 2017), indicating that orthopyroxene obtained some iron during serpentinization. An increase in the FeO content of orthopyroxene-derived serpentine may result in a decrease in the chlorine concentration, based on an inverse correlation between chlorine and FeO contents of serpentine (Fig. 5). However, the dependence of reaction time was not detected in experiments at 485 °C and 3.0 kbar, and chlorine in serpentine minerals was essentially unchangeable when experimental durations increased from 30 to 62 days (Table 1).
Analyses of natural serpentinites show that chlorine was abundant in serpentine minerals adjacent to the relict olivine, whereas it was relatively depleted in the centers of serpentine veins (e.g., Miura et al., 1981; Beard et al., 2009). It indicates that chlorine prefers to incorporate into serpentine formed at the reaction front, and it decreases with progressive serpentinization, consistent with experimental results of this study. By contrast, Stueber et al. (1968) reported that chlorine in some serpentinized peridotite decreased greatly with larger reaction progresses, whereas an inverse correlation between chlorine and reaction progresses was found for other samples, possibly reflecting different formation temperature of serpentinites. 14
3.4 Influence of starting materials on chlorine incorporation into serpentine To investigate influence of starting materials on chlorine distribution into serpentine minerals, we performed experiments at 311 °C and 3.0 kbar on olivine, orthopyroxene and peridotite. Serpentine minerals formed in olivine-only experiments had very low chlorine concentrations, 0.08 ± 0.03 wt% (Table 1). By contrast, serpentine minerals formed in orthopyroxene-only experiments contained significantly higher chlorine, 0.49 ± 0.36 wt% (Table 1, Fig. 6). It indicates that chlorine favors the structure of orthopyroxene-derived serpentine, which agrees with analyses of natural serpentinites (Bonifacie et al., 2008). Orthopyroxene-derived serpentine had 42.9 ± 0.7 wt% SiO2, which was much larger than the SiO2 content of olivine-derived serpentine (37.7 ± 1.7 wt%). Chlorine in orthopyroxene-derived serpentine was positively correlated with its SiO2 contents (Fig. 7). However, olivine-derived serpentine formed in experiments at 311 °C and 3.0 kbar on peridotite had comparable chlorine with orthopyroxene-derived serpentine. In particular, olivine-derived serpentine contained slightly higher chlorine concentrations than serpentine formed in olivine-only experiments (Fig. 6), whereas orthopyroxene-derived serpentine had much lower chlorine concentrations than serpentine formed in orthopyroxene-only experiments (Fig. 6). As indicated by Fig. 7, orthopyroxene-derived serpentine had a significantly smaller SiO2 content than serpentine formed in orthopyroxene-only experiments, resulting from releases of silica from orthopyroxene during serpentinization. The decrease in SiO2 contents of orthopyroxene-derived serpentine may lead to a 15
distortion of the serpentine structure, which possibly leads to a decrease in chlorine concentrations of serpentine minerals. It suggests that the process of olivine serpentinization differs greatly from the process of peridotite serpentinization, which is consistent with previous experimental studies (Huang et al., 2015b, 2017).
3.5 Influence of water/rock ratios, temperature and pressure on chlorine distribution into serpentine In order to study the effect of water/rock ratios on chlorine distribution into serpentine minerals, we performed experiments at 80 °C and vapor saturated pressures with water/rock ratios ranging from 0.29 to 19.5 and at 485 °C and 3.0 kbar with water/rock ratios varied from ~1.0 to 10.0. The concentrations of chlorine in serpentine minerals were essentially unchangeable (Table 1), suggesting that influence of water/rock ratios on chlorine distribution into serpentine minerals is minor. It also indicates that water/rock ratios had a negligible influence on the mobility of aluminum, iron and silica during serpentinization. Consistently, thermodynamic modelings show that the concentrations of iron, aluminum and silica in fluids derived from harzburgite serpentinization are quite comparable over a range of water/rock ratios (McCollom and Bach, 2009).
Influence of temperature on chlorine concentrations of serpentine minerals was illustrated in Fig. 8. Olivine-derived serpentine formed at 80 °C had very low chlorine concentrations, 0.06 ± 0.04 wt%, whereas olivine-derived serpentine produced at 16
200 °C contained around one order of magnitude higher chlorine concentrations. By contrast, chlorine in olivine-derived serpentine decreased slightly at 311-400 °C and became significantly lower at 485 °C (Fig. 8). The decrease in chlorine concentrations of serpentine minerals with increasing temperature was also observed in orthopyroxene-only experiments (Table 1). Serpentine minerals produced at 311 °C had 0.49 ± 0.36 wt% Cl, whereas serpentine minerals formed at 485 °C contained significantly lower chlorine, 0.09 ± 0.04 wt%.
The concentrations of chlorine in serpentine are illustrated in Fig. 9 as a function of pressure. Serpentine formed at 311 °C and 3.0 kbar had slightly lower chlorine concentrations than serpentine produced at 300 °C and 8.0 kbar (Fig. 9a). An inverse correlation in olivine-derived serpentine was found between chlorine and SiO2 contents (Fig. 9b), which was associated with a positive correlation between chlorine and FeO contents and a negative relation between FeO and SiO2 contents (Fig. 9c, d). It suggests that chlorine is hosted in a structurally-bound site. The decrease in chlorine with increasing pressures may be linked with an increase in SiO2 contents and a decrease in FeO contents. By contrast, influence of pressure on chlorine distribution into serpentine minerals in experiments at 400-500 °C is minor.
Few studies have examined the incorporation of chlorine into serpentine formed at different temperatures and pressures (Scambelluri et al., 2004; Bonifacie et al., 2008). These two studies have investigated high-pressure metaperidotites from Erro-Tobbio 17
Unit of the Voltri Massif, which experienced serpentinization in the oceanic environment, followed by subduction to HP-LT conditions of ~20 kbar and ~550 °C (Scambelluri et al., 2004; Bonifacie et al., 2008). The temperature of oceanic serpentinites was determined based on oxygen isotope fractionation between serpentine and magnetite, which were varied from 27 to ~300 °C (Bonifacie et al., 2008). The concentrations of chlorine in oceanic serpentinites are very high with large variations, whereas they decrease significantly at higher pressures and temperatures (Fig. 10, Scambelluri et al., 2004; Bonifacie et al., 2008). The experimental results of this study are consistent with those of Scambelluri et al. (2004) and Bonifacie et al. (2008), suggesting that our experimental results can be applied to natural systems.
3.6 Geological implications Experimental results of this study show that chlorine in serpentine minerals can be preserved at relatively high pressures and high temperatures, which suggests that serpentine minerals are important carriers of chlorine in subduction zones. The enrichment patterns of fluid-mobile elements in natural serpentinites are strikingly similar to those of arc magmas (Hattori and Guillot, 2003; Guillot and Hattori, 2013). It indicates that the dehydration of serpentinites is linked to the genesis of arc magmas. Experimental studies show that serpeninite dehydration at relatively higher temperatures (e.g., ≥600 °C) releases chlorine-bearing aqueous fluids that have abundant fluid-mobile elements (Tenthorey and Hermann, 2004). The fluids may play an important role for the metasomatism of peridotite in the mantle wedge, which is 18
supported by saline fluid inclusions derived from peridotite in the mantle wedge (Kawamoto et al., 2013). It possibly leads to partial melting of the peridotite and consequently the formation of arc magmas (Hattori and Guillot, 2003; Guillot and Hattori, 2013). Arc magmas have much larger quantities of chlorine than magmas from other tectonic settings (e.g., De Hoog et al., 2001). The correlation between Cl and FeO contents of serpentine suggests that chlorine is significant for the redistribution of elements. Indeed, chlorine is an important agent for mobilizing and transporting metals, such as Fe, Cu, and Au. Therefore, chlorine-bearing fluids released by serpentinites dehydration may be important for the formation of giant ore deposits at convergent margins (e.g., Sun et al., 2013, 2015).
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4 Conclusions Hydrothermal experiments were performed at 80-500 °C and pressures ranging from vapor saturated pressures to 20 kbar to study influence of temperature, fluid salinity, water/rock ratios, and pressure on chlorine distribution into serpentine minerals. The results show that chlorine was hosted in a structurally-bound site of serpentine, and it strongly depends on the mobility of iron, aluminum and silica during serpentinization. Chlorine concentrations of serpentine formed at 80 °C were very low, which increased by around one order of magnitude in experiments at 200 °C. By contrast, chlorine in serpentine minerals decreased slightly at 311-400 °C and it became significantly lower at 485 °C. Chlorine in olivine-derived serpentine formed at 400 °C and 3.0 kbar decreased greatly in a high-salinity fluid, which was associated with a decrease in SiO2 contents and an increase in Al2O3 contents of serpentine. By contrast, for experiments at 311°C and 3.0 kbar, influence of fluid salinity on chlorine incorporation into serpentine was negligible. Moreover, the presence of pyroxene minerals in peridotite greatly influenced the mobility of aluminum, iron and silica during serpentinization, and consequently chlorine distribution into serpentine minerals can be influenced. Orthopyroxene-derived serpentine formed in experiments at 311 °C and 3.0 kbar on peridotite had significantly lower chlorine concentrations than serpentine produced in orthopyroxene-only experiments, whereas olivine-derived serpentine contained much larger quantities of chlorine than serpentine formed in olivine-only experiments. Composition of serpentine minerals suggests that pyroxene minerals released some of its aluminum and silica to olivine during serpentinization. 20
As a consequence, orthopyroxene-derived serpentine formed in experiments on peridotite had smaller quantities of SiO2 than that produced in orthopyroxene-only experiments, which possibly leads to a decrease in chlorine concentrations of serpentine. Chlorine concentrations of serpentine formed in experiments at 311 °C and 3.0 kbar were slightly lower than those of serpentine produced at higher pressures, which may be associated with influence of pressure on the mobility of iron and silica. By contrast, pressure had a minor influence on chlorine concentrations of serpentine produced at 400-500 °C. Experimental results of this study suggest that chlorine incorporated into serpentine can be preserved at relatively high temperatures and pressures. Therefore, serpentinization is significant for the transfer of chlorine to the mantle.
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Acknowledgements: This research was financially supported by the Natural Science Foundation of China (91328204, 41603060), postdoctoral Science Foundation of China (2015M570735, 2016T90805), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB06030100), and the scientific research fund of the Second Institute of Oceanography, SOA (JG1405). We thank S. Jiang at South China University of Technology for the help during FTIR analyses.
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29
Table 1 Experimental conditions and chlorine concentrations of serpentine minerals Sample
T
P
Solid a
Starting
W/R
solutions
b
Time
Chlorine in
ratio
(day)
serpentine(wt%)
Chlorine in c
serpentine(wt%)d
No.
(°C)
(kbar)
reactant
HR25
311
3.0
Prt
0.5 mol/L
1.4
19
0.16(9)
0.22 (12)
HR37
311
3.5
Ol
0.5 mol/L
1.1
26
0.08(3)
—
HR42
311
3.0
Opx
0.5 mol/L
0.82
18
—
0.49(36)
HR61
311
3.0
Prt
0.5 mol/L
0.82
120
0.09(4)
0.14(5)
HR66
300
8.0
Prt
3.3 mol/L
0.50
22
0.42(18)
—
HR77
311
3.0
Prt
3.3 mol/L
1.2
28
0.25(8)
0.24(15)
HR36
400
1.0
Prt
0.5 mol/L
1.6
38
0.06(4)
0.19(16)
HR78
400
3.0
Prt
3.3 mol/L
1.0
29
0.13(13)
0.13(7)
HR79
400
3.0
Prt
0.5 mol/L
0.86
27
0.70(35)
0.12(4)
HR67
400
8.0
Prt
3.3 mol/L
0.50
22
0.08(2)
0.14(7)
HR21
485
3.0
Prt
0.5 mol/L
1.0
30
0.08(4)
—
HR27
485
3.0
Prt
0.5 mol/L
0.74
62
0.12(6)
—
HR45
485
2.0
Opx
0.5 mol/L
1.5
27
—
0.09(4)
HR23
500
8.0
Prt
0.5 mol/L
0.83
20
0.21(11)
0.16(10)
HR32
500
20
Prt
0.5 mol/L
0.20
23
0.08(2)
0.07(5)
HRL1
200
0.016
Prt
0.5 mol/L
7.1
19
1.21(13)
0.37(49)
HRL21e
80
—
Prt
0.5 mol/L
19.5
30
0.02(3)
—
HRL23
80
—
Prt
0.5 mol/L
2.4
30
0.02(2)
—
HRL26
80
—
Prt
0.5 mol/L
2.7
60
0.07(4)
0.04(5)
HRL34
80
—
Prt
0.5 mol/L
0.29
22
0.06(3)
—
a
Prt: peridotite, Ol: olivine, Opx: orthopyroxene.
b
Mass ratios between starting saline solutions and solid reactants
c
Olivine-derived serpentine. Chlorine concentrations were the average of ~6-30 analyses.
d
Orthopyroxene-derived serpentine. Numbers in parentheses are one standard deviation.
e
Vapor saturated pressure at 80 °C is 0.5 bar.
30
Table 2 Representative composition of serpentine minerals determined by electron microprobe (in wt%)
a
SiO2
TiO2
Cr2O3
Al2O3
MgO
FeO
CaO
MnO
NiO
CoO
Na2O
K2O
Cl
Total
Notea
HRL23-1
38.91
-
0.01
6.20
30.36
8.52
0.24
0.10
0.23
0.07
0.13
0.17
0.02
84.94
ol
HRL23-2
42.26
-
0.06
3.39
32.09
7.99
0.09
0.09
0.31
0.02
0.15
0.42
0.01
86.86
ol
HRL23-3
43.30
0.02
0.05
0.15
31.80
7.75
0.13
0.04
0.39
0.01
0.04
0.06
0.07
83.81
ol
HRL23-4
42.43
-
0.04
1.09
34.07
7.45
0.09
0.10
0.16
-
0.05
0.08
0.02
85.57
ol
HRL23-5
42.16
0.07
0.05
1.88
35.73
5.71
0.06
0.09
0.22
-
0.06
0.09
0.02
86.13
ol
HRL1-1
41.54
0.02
-
1.53
30.51
8.72
0.04
0.08
0.28
0.09
0.23
0.02
1.13
83.93
ol
HRL1-2
42.57
-
0.00
1.25
29.21
9.74
0.04
0.07
0.39
0.02
0.36
0.03
1.36
84.74
ol
HRL1-3
42.15
-
-
1.09
30.78
9.64
0.05
0.13
0.32
0.04
0.23
0.02
1.14
85.34
ol
HRL1-4
38.49
0.13
0.15
9.22
32.87
3.46
0.38
0.13
0.04
-
0.30
0.10
0.19
85.42
opx
HRL1-5
39.00
0.11
0.11
8.45
31.81
3.89
0.12
0.13
0.10
0.05
0.26
0.10
0.15
84.24
opx
HR25-22
39.14
0.10
0.45
3.57
34.16
6.84
0.12
0.14
0.13
0.03
0.25
0.01
0.31
85.23
opx
HR25-23
38.81
0.06
0.46
4.08
32.47
6.88
0.11
0.10
0.13
0.02
0.34
0.01
0.22
83.68
opx
HR25-17
40.74
0.10
0.17
2.92
35.57
7.43
0.06
0.08
0.16
-
0.17
0.01
0.03
87.43
opx
HR25-70
39.77
0.06
0.53
2.97
35.46
6.93
0.15
0.14
0.13
0.02
0.28
-
0.25
86.69
opx
HR25-29
40.58
0.01
0.03
0.61
39.42
6.82
0.07
0.09
0.40
0.04
0.19
0.01
0.13
88.39
ol
HR25-43
39.35
0.10
0.04
2.42
34.95
8.81
0.10
0.10
0.24
0.03
0.19
0.01
0.10
86.42
ol
HR25-44
39.82
0.02
0.04
2.87
35.63
9.63
0.06
0.05
0.32
0.02
0.12
0.02
0.05
88.62
ol
HR25-45
39.67
-
0.01
3.02
35.75
8.94
0.08
0.10
0.31
-
0.14
0.01
0.11
88.14
ol
HR77-20
39.64
0.16
0.61
2.76
34.72
5.27
0.15
0.13
0.16
0.05
0.22
0.02
0.26
84.15
opx
HR77-21
41.14
0.09
0.45
3.27
34.98
5.42
0.14
0.14
0.12
0.01
0.21
0.03
0.26
86.26
opx
HR77-22
40.95
0.09
0.49
3.49
34.28
5.69
0.10
0.09
0.07
0.03
0.26
0.03
0.25
85.81
opx
HR77-25
40.47
0.01
0.07
0.49
34.68
7.36
0.04
0.14
0.36
0.02
0.04
0.01
0.12
83.80
ol
HR77-27
39.72
0.01
0.00
0.74
33.93
5.66
0.03
0.07
0.28
0.02
0.12
0.01
0.27
80.85
ol
HR77-28
39.58
0.01
0.03
0.71
33.10
6.74
0.04
0.12
0.34
0.03
0.15
0.01
0.32
81.19
ol
HR78-1
40.62
0.04
0.01
2.28
33.28
5.68
0.43
0.14
0.14
0.01
0.21
0.03
0.13
83.00
opx
HR78-2
40.24
0.10
0.08
3.29
32.50
5.99
0.25
0.10
0.07
0.02
0.23
0.04
0.11
83.01
opx
HR78-3
40.20
0.05
0.09
4.13
32.75
6.29
0.10
0.11
0.12
0.01
0.20
0.03
0.08
84.14
opx
HR78-21
41.20
0.05
0.03
1.90
32.30
7.80
0.06
0.17
0.18
0.01
0.27
0.04
0.06
84.08
ol
HR78-22
41.36
0.08
0.03
4.15
33.05
6.67
0.08
0.07
0.24
0.01
0.39
0.07
0.07
86.28
ol
HR78-23
40.62
0.07
0.04
4.30
32.99
8.14
0.04
0.12
0.35
0.04
0.29
0.06
0.07
87.11
ol
HR79-12
44.95
0.03
0.00
0.66
33.26
7.18
0.31
0.15
0.37
0.00
0.34
0.08
1.29
88.61
ol
HR79-13
43.65
0.03
0.00
0.62
31.57
7.09
0.24
0.16
0.55
0.00
0.22
0.05
0.35
84.52
ol
HR79-14
43.22
0.04
0.04
0.47
32.18
6.98
0.21
0.14
0.52
0.03
0.23
0.07
0.56
84.68
ol
HR79-7
39.78
0.07
0.21
3.72
34.75
5.99
0.08
0.07
0.13
0.02
0.19
0.06
0.13
85.19
opx
HR79-8
40.05
0.07
0.16
3.88
34.70
5.80
0.07
0.09
0.16
0.04
0.17
0.05
0.12
85.37
opx
HR79-9
40.38
0.06
0.54
3.39
34.03
5.95
0.10
0.11
0.15
0.04
0.17
0.06
0.17
85.17
opx
Primary minerals of serpentine. Ol: olivine, Opx: orthopyroxene
31
Figure captions: Figure 1 (a) Representative infrared spectra of experimental products. The main secondary hydrous mineral produced at 311 °C and 3.0 kbar was fibrous chrysotile. By contrast, tabular shaped lizardite was produced in experiments at 400 °C and 3.0 kbar, whereas lizardite and talc formed in experiments at 485 °C and 3.0 kbar. Serpentine is characterized by infrared modes at 954 cm-1, 1083 cm-1, and 3686 cm-1 (e.g., Fuchs et al., 1998; Lafay et al., 2014), and talc has typical infrared bands at 671 cm-1, and 3677 cm-1(Liu et al., 2014). (b), (c) and (d) Scanning electron microscope imaging of experimental products showing the formation of fibrous chrysotile (Ctl) at 311 °C and 3.0 kbar, lizardite (Lz) at 400 °C and 3.0 kbar, and talc (Tlc) at 485 °C and 3.0 kbar.
Figure 2 Influence of fluid salinity on chlorine distribution into serpentine minerals. Chlorine concentrations of serpentine formed in experiments at 311 °C and 3.0 kbar with a low-salinity fluid were essentially the same as chlorine concentrations of serpentine produced in a high-salinity fluid. By contrast, chlorine concentrations of olivine-derived serpentine formed in experiments at 400 °C and 3.0 kbar decreased greatly in a high-salinity fluid.
Figure 3 Chlorine in olivine-derived serpentine formed at 400 °C and 3.0 kbar versus (a) FeO contents, (b) SiO2 contents, and (c) Al2O3 contents. (d) The correlation between FeO and SiO2 contents of olivine-derived serpentine. 32
Figure 4 Influence of experimental duration (in day) on chlorine distribution into serpentine minerals formed in experiments at 311 °C and 3.0 kbar.
Figure 5 (a) Chlorine versus FeO contents of olivine-derived serpentine, and (b) chlorine versus SiO2 contents of orthopyroxene-derived serpentine formed in experiments at 311 °C and 3.0 kbar after 20 days. (c) Chlorine versus FeO contents and (d) chlorine versus Al2O3 contents of serpentine minerals produced in experiments at 311 °C and 3.0 kbar after 120 days.
Figure 6 Influence of primary minerals on chlorine concentrations of serpentine (in wt%) formed at 311 °C and 3.0 kbar. “Ol-only” represents serpentine formed in olivine-only experiments. “Opx-only” represents serpentine formed in orthopyroxene-only experiments. “Prt, ol-srp” stands for olivine-derived serpentine formed in experiments on peridotite. “Prt, opx-srp” stands for orthopyroxene-derived serpentine produced in experiments on peridotite.
Figure 7 Compositional differences between serpentine formed in orthopyroxene-only experiments and orthopyroxene-derived serpentine produced during peridotite serpentinization. (a) Chlorine versus SiO2 contents of serpentine and (b) chlorine versus Al2O3 contents of serpentine.
33
Figure 8 Influence of temperature on chlorine incorporation into serpentine minerals. Experiments at 311-485 °C were conducted at 3.0 kbar with a saline solution (0.5 mol/L) and comparable water/rock ratios. It shows that chlorine contents of serpentine were abundant at 300-400 °C, whereas they decreased greatly at 485 °C.
Figure 9 (a) Influence of pressure on chlorine contents of serpentine. (b) An inverse correlation between Cl and SiO2 contents of serpentine formed at 300 °C and 3-8 kbar. (c) The correlation between Cl and FeO contents, and (d) the correlation between FeO and SiO2 contents of serpentine.
Figure 10 Comparison between experimental results of this study and those based on natural serpentinites (Scambelluri et al., 2004; Bonifacie et al., 2008).
34
Figure 1
35
Figure 2
36
Figure 3
37
Figure 4
38
Figure 5
39
Figure 6
40
Figure 7
41
Figure 8
42
Figure 9
43
Figure 10
Highlights: ●The incorporation of chlorine into serpentine was experimentally studied ●Temperature, pressure and fluid salinity greatly influence chlorine behavior ●Chlorine in serpentine reached maximum at 200 °C, which decreased at 300-500 °C ●Chlorine behavior is closely associated with the mobility of aluminum and silica ●Serpentine is an important carrier of chlorine in subduction zones
44