Cl Isotopic Compositions Research Articles

The bromine and chlorine isotope composition of primary halite deposits and their significance for the secular isotope composition of seawater

We determined the chlorine and bromine isotope compositions of 83 halite samples from nine different geological periods between the Orosirian and the present in order to study the secular Cl and Br isotope variations in the ocean during the last 2 billion years. Relatively large Cl (-0.24 to +0.51%e vs. SMOC) and Br ( —0.24 to +1.08%e vs. SMOB) isotope variations are found in these halite samples. Two different methods, one in which the isotope fractionation between the brine and the salt is used, and a second in which the relationship between the isotope compositions and the Br/Cl ratios in the halite samples is used were applied to establish the original Cl and Br isotope compositions of the ocean. Both approaches showed that the Cl and Br isotope compositions of the ocean have always been close to the modern value (which is by definition Ogee for both isotope systems) and that at most very small variations in Br and Cl isotope composition of seawater have occurred during the last 2 billion years. This indicates that, unlike in other isotope systems that often show significant isotope variations over geologie time, Cl and Br isotope compositions can be used directly to determine processes that occurred in the deposits of interest without need for correction for secular variations

The chlorine isotopic composition of the Moon: Insights from melt inclusions

The Moon exhibits a heavier chlorine (Cl) isotopic composition compared to the Earth. Several hypotheses have been put forward to explain this difference, based mostly on analyses of apatite in lunar samples complemented by bulk-rock data. The earliest hypothesis argued for Cl isotope fractionation during the degassing of anhydrous basaltic magmas on the Moon. Subsequently, other hypotheses emerged linking Cl isotope fractionation on the Moon with the degassing during the crystallization of the Lunar Magma Ocean (LMO). Currently, a variant of the LMO degassing model involving mixing between two end-member components, defined by early-formed cumulates, from which mare magmas were subsequently derived, and a KREEP component, which formed towards the end of the LMO crystallization, seems to reconcile some existing Cl isotope data on lunar samples. To further ascertain the history of Cl in the Moon and to investigate any evolution of Cl during magma crystallization and emplacement events, which could help resolve the chlorine isotopic variation between the Earth and the Moon, we analysed the Cl abundance and its isotopic composition in 36 olivine- and pyroxene-hosted melt inclusions (MI) in five Apollo basalts (10020, 12004, 12040, 14072 and 15016). Olivine-hosted MI have an average of 3.3±1.4ppm Cl. Higher Cl abundances (11.9 ppm on average) are measured for pyroxene-hosted MI, consistent with their formation at later stages in the crystallization of their parental melt compared to olivines. Chlorine isotopic composition (δ37Cl) of MI in the five Apollo basalts have weighted averages of +12.8±2.4‰ and +10.1±3.2‰ for olivine- and pyroxene-hosted MI, respectively, which are statistically indistinguishable. These isotopic compositions are also similar to those measured in apatite in these lunar basalts, with the exception of sample 14072, which is known to have a distinct petrogenetic history compared to other mare basalts. Based on our dataset, we conclude that, post-MI-entrapment, no significant Cl isotopic fractionation occurred during the crystallization and subsequent eruption of the parent magma and that Cl isotopic composition of MI and apatite primarily reflect the signature of the source region of these lunar basalts. Our findings are compatible with the hypothesis that in the majority of the cases the heavy Cl isotopic signature of the Moon was acquired during the earliest stages of LMO evolution. Interestingly, MI data from 14072 suggests that Apollo 14 lunar basalts might be an exception and may have experienced post-crystallization processes, possibly metasomatism, resulting in additional Cl isotopic fractionation recorded by apatite but not melt inclusions.

Investigating magmatic processes in the early Solar System using the Cl isotopic systematics of eucrites

Generally, terrestrial rocks, martian and chondritic meteorites exhibit a relatively narrow range in bulk and apatite Cl isotope compositions, with δ37Cl (per mil deviation from standard mean ocean chloride) values between −5.6 and +3.8‰. Lunar rocks, however, have more variable bulk and apatite δ37Cl values, ranging from ∼−4 to +40‰. As the Howardite-Eucrite-Diogenite (HED) meteorites represent the largest suite of crustal and sub-crustal rocks available from a differentiated basaltic asteroid (4 Vesta), studying them for their volatiles may provide insights into planetary differentiation processes during the earliest Solar System history.Here the abundance and isotopic composition of Cl in apatite were determined for seven eucrites representing a broad range of textural and petrological characteristics. Apatite Cl abundances range from ∼25 to 4900 ppm and the δ37Cl values range from −3.98 to +39.2‰. Samples with lower apatite H2O contents were typically also enriched in 37Cl but no systematic correlation between δ37Cl and δD values was observed across samples. Modelled Rayleigh fractionation and a strong positive correlation between bulk δ66Zn and apatite δ37Cl support the hypothesis that Cl degassed as metal chlorides from eucritic magmas, in a hydrogen-poor environment. In the case of lunar samples, it has been noted that δ37Cl values of apatite positively correlate with bulk La/Yb ratio. Interestingly, most eucrites show a negative correlation with bulk La/Yb ratio. Recently, isotopically light Cl values have been suggested to record the primary solar nebular signature. If this is the case then 4 Vesta, which accreted rapidly and early in Solar System history, could also record this primary nebular signature corresponding to the lightest Cl values measured here. The significant variation in Cl isotope composition observed within the eucrites are likely related to degassing of metal chlorides.

The chlorine isotope composition of iron meteorites: Evidence for the Cl isotope composition of the solar nebula and implications for extensive devolatilization during planet formation

AbstractThe bulk chlorine concentrations and isotopic compositions of a suite of non‐carbonaceous (NC) and carbonaceous (CC) iron meteorites were measured using gas source mass spectrometry. The δ37Cl values of magmatic irons range from −7.2 to 18.0‰ versus standard mean ocean chloride and are unrelated to their chlorine concentrations, which range from 0.3 to 161 ppm. Nonmagmatic IAB irons are comparatively Cl‐rich containing >161 ppm with δ37Cl values ranging from −6.1 to −3.2‰. The anomalously high and low δ37Cl values are inconsistent with a terrestrial source, and as Cl contents in magmatic irons are largely consistent with derivation from a chondrite‐like silicate complement, we suggest that Cl is indigenous to iron meteorites. Two NC irons, Cape York and Gibeon, have high cooling rates with anomalously high δ37Cl values of 13.4 and 18.0‰. We interpret these high isotopic compositions to result from Cl degassing during the disruption of their parent bodies, consistent with their low volatile contents (Ga, Ge, Ag). As no relevant mechanisms in iron meteorite parent bodies are expected to decrease δ37Cl values, whereas volatilization is known to increase δ37Cl values by the preferential loss of light isotopes, we interpret the low isotope values of <−5‰ and down to −7.2‰ to most closely represent the primordial isotopic composition of Cl in the solar nebula. Similar conclusions have been derived from low δ37Cl values down to −6, and −3.8‰ measured in Martian and Vestan meteorites, respectively. These low δ37Cl values are in contrast to those of chondrites which average around 0‰ previously explained by the incorporation of isotopically heavy HCl clathrate into chondrite parent bodies. The poor retention of low δ37Cl values in many differentiated planetary materials suggest that extensive devolatilization occurred during planet formation, which can explain Earth's high δ37Cl value by the loss of approximately 60% of the initial Cl content.

Apatite as a tracer of the source, chemistry and evolution of ore-forming fluids: The case of the Olserum-Djupedal REE-phosphate mineralisation, SE Sweden

Abstract This study explores the suitability of apatite as a tracer of the source(s), chemistry, and evolution of ore-forming hydrothermal fluids. This is tested by analysing the halogen (F, Cl, Br, and I), stable Cl isotopic, and trace element compositions of fluorapatite from the regional-scale Olserum-Djupedal rare earth element (REE) phosphate mineralisation in SE Sweden, which is dominated by monazite-(Ce), xenotime-(Y), and fluorapatite. The primary hydrothermal fluid flow system is recorded in a sequence from proximal granite-hosted to distal metasediment-hosted fluorapatite. Along this sequence, primary fluorapatite shows a gradual increase of Cl and Br concentrations and in (Gd/Yb)N, a decrease of F and I concentrations, a decrease in δ37Cl values, in (La/Sm)N, and partly in (La/Yb)N and (Y/Ho)N. Local compositional differences of halogen and trace element concentrations have developed along rims and in domains adjacent to fractures of fluorapatite due to late-stage partial reaction with fracture fluids. These differences are insignificant compared to the larger deposit-scale zoning. This suggests that apatite can retain the primary record of the original ore-forming fluid despite later overprinting fluid events. The agreement between Br/Cl and I/Cl ratios of apatite and those of co-existing fluid inclusions at lower temperatures indicates that only a minor fractionation of Br from I occurs during apatite precipitation. The halogen ratios of apatite can thus be used as a first-order estimate for the composition of the ore-forming fluid. Taking the small fractionation factors for Cl isotopes between apatite and co-existing fluid at high temperatures into account, we propose that the Cl isotopic composition of apatite and the halogen ratios derived from the apatite composition can be used jointly to trace the source(s) of ore-forming fluids. By contrast, most trace elements incorporated in apatite are affected by the host rock environment and by fluid-mineral partitioning due to growth competition between co-crystallising minerals. Collectively, apatite is sensitive to changing fluid compositions, yet it is also able to record the character of primary ore-forming fluids. Thus, apatite is suitable for tracing the origin, chemistry, and evolution of fluids in hydrothermal ore-forming settings.

H and Cl isotope characteristics of indigenous and late hydrothermal fluids on the differentiated asteroidal parent body of Grave Nunataks 06128

The paired achondrites Graves Nunataks (GRA) 06128 and 06129 are samples of an asteroid that underwent partial melting within a few million years after the start of Solar System formation. In order to better constrain the origin and processing of volatiles in the early Solar System, we have investigated the abundance of H, F and Cl and the isotopic composition of H and Cl in phosphates in GRA 06128 using secondary ion mass spectrometry. Indigenous H in GRA 06128, as recorded in magmatic merrillite, is characterised by an average δD of ca. −152 ± 330‰, which is broadly similar to estimates of the H isotope composition of indigenous H in other differentiated asteroidal and planetary bodies such as Mars, the Moon and the angrite and eucrite meteorite parent bodies. The merrillite data thus suggest that early accretion of locally-derived volatiles was widespread for the bodies currently populating the asteroid belt. Apatite formed at the expense of merrillite around 100 million years after the differentiation of the GRA 06128/9 parent body, during hydrothermal alteration, which was probably triggered by an impact event. Apatite in GRA 06128 contains 5.4–5.7 wt.% Cl, 0.6–0.8 wt.% F, and ∼20 to 60 ppm H2O, which is similar to the H2O abundance in merrillite from which apatite formed. The apatite δD values range between around +100‰ and +2000‰ and are inversely correlated with apatite H2O contents. The Cl isotope composition of apatite appears to be homogeneous across various grains, with an average δ37Cl value of 3.2 ± 0.7‰. A possible scenario to account for the apatite chemical and isotopic characteristics involves interaction of GRA 06128/9 with fumarole-like fluids derived from D- and HCl-rich ices delivered to the GRA 06128/9 parent-body by an ice-rich impactor.

Multiple reservoirs of volatiles in the Moon revealed by the isotopic composition of chlorine in lunar basalts

The isotopes of chlorine (37Cl and 35Cl) are highly fractionated in lunar samples compared to most other Solar System materials. Recently, the chlorine isotope signatures of lunar rocks have been attributed to large-scale degassing processes that occurred during the existence of a magma ocean. In this study we investigated how well a suite of lunar basalts, most of which have not previously been analyzed, conform to previous models. The Cl isotope compositions (δ37Cl (‰) = [(37Cl/35Clsample/37Cl/35ClSMOC) − 1] × 1000, where SMOC refers to standard mean ocean chloride) recorded range from ∼+7 to +14‰ (Apollo 15), +10 to +19‰ (Apollo 12), +9 to +15‰ (70017), +4 to +8‰ (MIL 05035), and +15 to +22‰ (Kalahari 009). The Cl isotopic data from the present study support the mixing trends previously reported by Boyce et al. (2015) and Barnes et al. (2016), as the Cl isotopic composition of apatites are positively correlated with bulk-rock incompatible trace element abundances in the low-Ti basalts, inclusive of low-Ti and KREEP basalts. This trend has been interpreted as evidence that incompatible trace elements, including Cl, were concentrated in the urKREEP residual liquid of the lunar magma ocean, rather than the mantle cumulates, and that urKREEP Cl had a highly fractionated isotopic composition. The source regions for the basalts were thus created by variable mixing between the mantle (Cl-poor and relatively unfractionated) and urKREEP. The high-Ti basalts show much more variability in measured Cl isotope ratios and scatter around the trend formed by the low-Ti basalts. Most of the data for lunar meteorites also fits the mixing of volatiles in their sources, but Kalahari 009, which is highly depleted in incompatible trace elements, contains apatites with heavily fractionated Cl isotopic compositions. Given that Kalahari 009 is one of the oldest lunar basalts and ought to have been derived from very early-formed mantle cumulates, a heavy Cl isotopic signature is likely not related to its mantle source, but more likely to magmatic or secondary alteration processes, perhaps via impact-driven vapor metasomatism of the lunar crust.

Chlorine isotopic compositions of apatite in Apollo 14 rocks: Evidence for widespread vapor-phase metasomatism on the lunar nearside ∼4 billion years ago

Compared to most other planetary materials in the Solar System, some lunar rocks display high δ37Cl signatures. Loss of Cl in a H ≪ Cl environment has been invoked to explain the heavy signatures observed in lunar samples, either during volcanic eruptions onto the lunar surface or during large scale degassing of the lunar magma ocean. To explore the conditions under which Cl isotope fractionation occurred in lunar basaltic melts, five Apollo 14 crystalline samples were selected (14053,19, 14072,13, 14073,9, 14310,171 along with basaltic clast 14321,1482) for in situ analysis of Cl isotopes using secondary ion mass spectrometry. Cl isotopes were measured within the mineral apatite, with δ37Cl values ranging from +14.6 ± 1.6‰ to +40.0 ± 2.9‰. These values expand the range previously reported for apatite in lunar rocks, and include some of the heaviest Cl isotope compositions measured in lunar samples to date. The data here do not display a trend between increasing rare earth elements contents and δ37Cl values, reported in previous studies. Other processes that can explain the wide inter- and intra-sample variability of δ37Cl values are explored. Magmatic degassing is suggested to have potentially played a role in fractionating Cl isotope in these samples. Degassing alone, however, could not create the wide variability in isotopic signatures. Our favored hypothesis, to explain small scale heterogeneity, is late-stage interaction with a volatile-rich gas phase, originating from devolatilization of lunar surface regolith rocks ∼4 billion years ago. This period coincides with vapor-induced metasomastism recorded in other lunar samples collected at the Apollo 16 and 17 landing sites, pointing to the possibility of widespread volatile-induced metasomatism on the lunar nearside at that time, potentially attributed to the Imbrium formation event.

Using hydrochemical and stable isotopic (δ2H, δ18O, δ11B, and δ37Cl) data to understand groundwater evolution in an unconsolidated aquifer system in the southern coastal area of Laizhou Bay, China

This study illustrates the origin of Quaternary brines and groundwater salinization processes at Laizhou Bay using hydrochemical and δ2H, δ18O, δ11B and δ37Cl data. The characteristics of the major ionic components of these brines demonstrated that brines originated from evaporated seawater. The δ2H and δ18O values of brines showed that seawater underwent strong evaporation and that these brines underwent mixing with paleo-meteoric water or surface water. The fact that the Cl/Br mass ratios of the Laizhou Bay brines (180–291) are lower than that of halite dissolution brine (>1000) contradicts the hypothesis that these brines were derived from halite dissolution, and the fact that the δ37Cl values (0.14‰–0.21‰) of these brines are more enriched than those derived from seawater evaporation represents positive evidence that evaporated seawater in the lagoon infiltrated through clay layer, during which ion filtration resulted in the positive Cl isotopic compositions of these brines. The higher B isotopic compositions (49.88‰–64.42‰) and lower B/Cl molar ratios (8.5 × 10–5∼1.6 × 10–4) of these brines are consistent with the brine origin determined based on the analysis of Cl isotopes. As the evaporated water infiltrated the clay layer, the preferential adsorption of 10B onto clay dominated the B isotopic composition of the brine. Five different mixing processes were calculated using B contents and B isotopic compositions. These calculations illustrated that one shallow saline groundwater (No. 05) was derived from the mixing of fresh groundwater and deep brine, thus indicating that brine intrusion occurred in the northern area of Changyi, which was caused by the over-exploitation of fresh groundwater. While one shallow saline groundwater (No. 10) and one deep saline groundwater (No. 12) in the northern of Xiaying were derived from the mixing of brine and seawater. The δ11B values of the two shallow brackish groundwater samples were close to that produced by the desorption of B from marine clay, and their B/Cl molar ratios were relatively higher, which demonstrated that B-desorption dominated the characteristic B hydrochemical compositions of brackish groundwater.

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 δ37Cl signatures observed in a 1750 h 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 (aH2O ≤ 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 h) is positive (1000lnαHCl(g)-Cldiss.=+1.55±0.49‰to+3.37±1.11‰), 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 (1000lnαHCl(g)-Cldiss.=5.67±0.17‰to-5.64±0.08‰), 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 HClo dominance with the progressive pH decline.The Kawah Ijen lake acquired its extreme composition through influx of sulphur and halogen-rich 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 (aH2O=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.

Chlorine isotope composition of volcanic rocks and gases at Stromboli volcano (Aeolian Islands, Italy): Inferences on magmatic degassing prior to 2014 eruption

Among the magmatic volatiles, chlorine (Cl) is degassed at shallow depths offering the opportunity to investigate the behavior of magmatic degassing close to the surface, and the possible occurrence of chemical and isotopic fractionation related to gas/melt partitioning. However, it is still unclear if the isotopic composition of Cl (δ37Cl) can be used as a proxy of magmatic degassing. In this work, we investigate the concentrations of chlorine and sulfur, and the Cl isotope composition of rocks and plume gases collected at Stromboli volcano, Aeolian Islands, Italy. This volcano was chosen because it is characterized by persistent eruptive activity (i.e., Strombolian explosions) and by the presence of magma at very shallow levels in the conduits. Rocks belonging to the different magmatic series erupted throughout the formation of the volcano have δ37Cl values ranging between −1.0 and +0.7‰. The isotopic composition seems independent of the Cl concentration of the rocks, but shows a negative correlation with SiO2 content. Plume gases have a greater isotopic compositional variability than the rocks (−2.2‰≤δ37Cl≤+1.5‰) and the composition seems related to the level of volcanic activity at Stromboli. Gases collected in 2011–2013 during days of ordinary eruptive activity are characterized by δ37Cl values ranging from +0.3 to +1.5‰ and S/Cl molar ratios between 1.4 and 2.2, similar to previous S/Cl measurements performed at Stromboli with other techniques. Plume gases collected in July 2014, in days of high-level eruptive activity preceding the onset of the 2014 effusive eruption, have negative δ37Cl values (−2.2‰≤δ37Cl≤−0.1‰) and S/Cl between 0.9 and 1.2, which are among the lowest S/Cl values measured at this volcano. The amplitude of the volcanic tremor and the variation in the inclination of very long period (VLP) seismic signal polarization clearly indicate that in July 2014 the intensity and frequency of Strombolian explosions, as well as the level of magma in the conduits, were higher than normal. This suggests that when magmatic degassing occurs at very shallow depths (magma residing at few tens of meters below the craters), isotopic fractionation between gaseous and dissolved chlorine (lnαgas-melt>0) can occur, resulting in a depletion of 37Cl in the melt. Finally, we argue that the range of δ37Cl values measured in rocks and gases at Stromboli overlaps most of the known mantle reservoirs (i.e., DMM, HIMU and EM-EM1), indicating that the transfer of chlorine from the mantle to the surface occurs without significant isotopic fractionation. This work opens new perspectives for better constraining the isotope signature of chlorine of different magmatic systems and mantle reservoirs on the Earth.

Evidence of multiple halogen sources in scapolites from iron oxide-copper-gold (IOCG) deposits and regional Na[sbnd]Cl metasomatic alteration, Norrbotten County, Sweden

Scapolites from barren regional NaCl metasomatic assemblages (RM), iron oxide-copper-gold deposits (IOCG), scapolite altered metabasic rocks (IOCG-M), and from IOCG-proximal alteration/Na-Skarns (IOCG-PS) from Norrbotten County in Northern Sweden have been analysed for halogen content and Cl stable isotope composition. The aim of the study was to constrain the source of halogens within alteration assemblages, and to investigate the possible fractionation of Cl isotopes between scapolite and the hydrothermal fluid. Scapolite separates were analysed for Cl, Br, and major oxide concentrations using electron probe micro-analysis (EPMA) and micro-X-ray fluorescence (XRFM) spectrometry. Chlorine was extracted from the scapolite separates via pyrohydrolysis and then analysed for their stable Cl isotope compositions by isotope ratio mass spectrometry (IRMS).All samples of scapolite investigated in this study are marialitic in composition. One of the scapolites from the Gruvberget deposit (IOCG-PS) had a Cl/Br molar ratio of 2363, which is the highest amongst all scapolites reported in the literature to date. Cl/Br molar ratios lower than seawater (650), were identified in two IOCG-PS scapolite samples (Cl/Br=554 and 271), as well as in two IOCG-M scapolites (Cl/Br=393 and 565). Three RM scapolites had Cl/Br molar ratios very close to, or slightly higher than, seawater values (639 to 770). Samples with Cl/Br molar ratios less than seawater are inferred to have halogens derived from evaporative residual brines; whereas samples with molar ratios higher than seawater may have halogens derived from fluids that have dissolved halite and/or are from magmatic systems. Considering the wide variation of the Cl/Br molar ratios in the IOCG-PS and IOCG-M scapolites compared to the restricted composition of the regional alteration (RM), it is proposed that the hydrothermal fluids interacted with several different protoliths to generate the IOCG alteration.RM alteration scapolites had δ37Cl values from −0.1‰ to +0.3‰, two IOCG-M scapolites had values of 0.2‰ and IOCG-PS scapolites had δ37Cl values from −0.1‰ to +1.0‰. Using a previously published δ37Cl value from fluid inclusion leachates (−1.7‰) from the IOCG-M mineralisation at Pahtohavare and the δ37Cl value of co-existing scapolite measured in this study (0.2‰), an empirical fluid-scapolite fractionation factor was calculated to be +1.9‰. This large fractionation factor is not supported by previous predictions for monovalent chlorides and, assuming equilibrium, indicates that 37Cl was preferentially accommodated in the A site of the scapolite structure. This indicates that either the stable Cl isotope partitioning between the CaCl2-rich brine and the scapolite may differ from currently available estimates for NaCl brines in equilibrium with silicate minerals, or that the scapolite and brine are not in isotopic equilibrium.Overall, the data in this study suggest that halogens in early scapolites were derived from residual brines and halite, during metamorphism of evaporites linked to the RM alteration. Later in the history of the Norrbotten district components of the RM alteration were recycled and mixed during magmatic and local metasomatic events to varying extents, resulting in the brines associated with IOCG alteration.

Halogen and Cl isotopic systematics in Martian phosphates: Implications for the Cl cycle and surface halogen reservoirs on Mars

The Cl isotopic compositions and halogen (Cl, F, Br, and I) abundances in phosphates from eight Martian meteorites, spanning most rock types and ages currently available, have been measured in situ by Secondary Ion Mass Spectrometry (SIMS). Likewise, the distribution of halogens has been documented by x-ray mapping. Halogen concentrations range over several orders of magnitude up to some of the largest concentrations yet measured in Martian samples or on the Martian surface, and the inter-element ratios are highly variable. Similarly, Cl isotope compositions exhibit a larger range than all pristine terrestrial igneous rocks. Phosphates in ancient (>4 Ga) meteorites (orthopyroxenite ALH 84001 and breccia NWA 7533) have positive δ37Cl anomalies (+1.1 to +2.5‰). These samples also exhibit explicit whole rock and grain scale evidence for hydrothermal or aqueous activity. In contrast, the phosphates in the younger basaltic Shergottite meteorites (<600 Ma) have negative δ37Cl anomalies (−0.2 to −5.6‰). Phosphates with the largest negative δ37Cl anomalies display zonation in which the rims of the grains are enriched in all halogens and have significantly more negative δ37Cl anomalies suggestive of interaction with the surface of Mars during the latest stages of basalt crystallization. The phosphates with no textural, major element, or halogen enrichment evidence for mixing with this surface reservoir have an average δ37Cl of −0.6‰, supporting a similar initial Cl isotope composition for Mars, the Earth, and the Moon. Oxidation and reduction of chlorine are the only processes known to strongly fractionate Cl isotopes, both positively and negatively, and perchlorate has been detected in weight percent concentrations on the Martian surface. The age range and obvious mixing history of the phosphates studied here suggest perchlorate formation and halogen cycling via brines, which have been documented on the Martian surface, has been active throughout Martian history.

Chlorine and hydrogen degassing in Vesta's magma ocean

The hydrophilic nature of halogens makes these elements ideal for probing potential hydrous geologic processes. Generally, in magmatic settings the stable isotopes of Cl may fractionate when H is in low concentrations and little fractionation occurs when the H concentration is high. We determined the Cl isotope composition and halogen content (F, Cl, Br, and I) of apatite and merrillite in seven basaltic eucrites, which are meteorites linked to the asteroid 4-Vesta, by using secondary ion mass spectrometry. We compare our halogen results with H isotope data, existing bulk rock concentrations, and petrologic models. The inferred Cl isotope composition of eucrites from this study, expressed in standard δ37Cl notation, which ranges from −3.8 to 7.7‰, correlates with the bulk major- and trace-element content, e.g., the Cl isotope composition positively correlates with Mg and Sc, while Cl isotope composition negatively correlates with K, V, and Cr. Here we suggest that eucrites preserve evidence of a degassing magma ocean as evidenced by the decreasing bulk rock K content with increasing δ37Cl. If the eucrite parent body, 4-Vesta, accreted with a negative δ37Cl of −3.8±1.1‰, at least some parts of the solar nebula would have been isotopically light compared to most estimates of the Earth, which on average is close to 0‰.

Early degassing of lunar urKREEP by crust-breaching impact(s)

Current models for the Moon's formation have yet to fully account for the thermal evolution of the Moon in the presence of H2O and other volatiles. Of particular importance is chlorine, since most lunar samples are characterised by unique heavy δ37Cl values, significantly deviating from those of other planetary materials, including Earth, for which δ37Cl values cluster around ∼0‰. In order to unravel the cause(s) of the Moon's unique chlorine isotope signature, we performed a comprehensive study of high-precision in situ Cl isotope measurements of apatite from a suite of Apollo samples with a range of geochemical characteristics and petrologic types. The Cl-isotopic compositions measured in lunar apatite in the studied samples display a wide range of δ37Cl values (reaching a maximum value of +36‰), which are positively correlated with the amount of potassium (K), Rare Earth Element (REE) and phosphorous (P) (KREEP) component in each sample. Using these new data, integrated with existing H-isotope data obtained for the same samples, we are able to place these findings in the context of the canonical lunar magma ocean (LMO) model. The results are consistent with the urKREEP reservoir being characterised by a δ37Cl ∼+30‰. Such a heavy Cl isotope signature requires metal-chloride degassing from a Cl-enriched urKREEP LMO residue, a process likely to have been triggered by at least one large crust-breaching impact event that facilitated the transport and exposure of urKREEP liquid to the lunar surface.

Chemical and isotopic constrains on the origin of brine and saline groundwater in Hetao plain, Inner Mongolia.

The origin and evolution of brine and saline groundwater have always been a challenged work for geochemists and hydrogeologists. Chemical and isotopic data of brine and saline waters were used to trace the sources of salinity and therefore to understand the transport mechanisms of groundwater in Xishanzui, Inner Mongolia. Both Cl/Br (molar) versus Na/Br (molar) and Cl (meq/L) versus Na (meq/L) indicated that salinity was from halite dissolution or at least a significant impact by halite dissolution. The logarithmic plot of the concentration trends of Cl (mg/L) versus Br (mg/L) for the evaporation of seawater and the Qinghai Salt Lake showed that the terrestrial halite dissolution was the dominated contribution for the salinity of this brine. The stable isotope ratios of hydrogen and oxygen suggested that the origin of brine was from paleorecharge water which experienced mixing of modern water in shallow aquifer. δ(37)Cl values ranged from -0.02 to 3.43 ‰ (SMOC), and reflecting mixing of different sources. The Cl isotopic compositions suggest that the dissolution of halite by paleometeoric water had a great contribution to the salinity of brine, and the contributions of the residual seawater and the dissolution of halite by the Yellow River water could be excluded.

Cl/Br ratios and chlorine isotope evidences for groundwater salinization and its impact on groundwater arsenic, fluoride and iodine enrichment in the Datong basin, China

In order to identify the salinization processes and its impact on arsenic, fluoride and iodine enrichment in groundwater, hydrogeochemical and environmental isotope studies have been conducted on groundwater from the Datong basin, China. The total dissolved solid (TDS) concentrations in groundwater ranged from 451 to 8250mg/L, and 41% of all samples were identified as moderately saline groundwater with TDS of 3000–10,000mg/L. The results of groundwater Cl concentrations, Cl/Br molar ratio and Cl isotope composition suggest that three processes including water-rock interaction, surface saline soil flushing, and evapotranspiration result in the groundwater salinization in the study area. The relatively higher Cl/Br molar ratio in groundwater from multiple screening wells indicates the contribution of halite dissolution from saline soil flushed by vertical infiltration to the groundwater salinization. However, the results of groundwater Cl/Br molar ratio model indicate that the effect of saline soil flushing practice is limited to account for the observed salinity variation in groundwater. The plots of groundwater Cl vs. Cl/Br molar ratio, and Cl vs δ37Cl perform the dominant effects of evapotranspiration on groundwater salinization. Inverse geochemical modeling results show that evapotranspiration may cause approximately 66% loss of shallow groundwater to account for the observed hydrochemical pattern. Due to the redox condition fluctuation induced by irrigation activities and evapotranspiration, groundwater salinization processes have negative effects on groundwater arsenic enrichment. For groundwater iodine and fluoride enrichment, evapotranspiration partly accounts for their elevation in slightly saline water. However, too strong evapotranspiration would restrict groundwater fluoride concentration due to the limitation of fluorite solubility.

Tracing chlorine sources of thermal and mineral springs along and across the Cascade Range using halogen concentrations and chlorine isotope compositions

In order to provide constraints on the sources of chlorine in spring waters associated with arc volcanism, the major/minor element concentrations and stable isotope compositions of chlorine, oxygen, and hydrogen were measured in 28 thermal and mineral springs along the Cascade Range in northwestern USA. Chloride concentrations in the springs range from 64 to 19,000 mg/L and δCl37 values range from +0.2‰ to +1.9‰ (average=+1.0±0.4‰), with no systematic variation along or across the arc, nor correlations with their presumed underlying basement lithologies. Additionally, nine geochemically well-characterized lavas from across the Mt. St. Helens/Mt. Adams region of the Cascade Range (Leeman et al., 2004, 2005) were analyzed for their halogen concentrations and Cl isotope compositions. In the arc lavas, Cl and Br concentrations from the volcanic front are higher than in lavas from the forearc and backarc. F and I concentrations progressively decrease from forearc to backarc, similar to the trend documented for B in most arcs. δCl37 values of the lavas range from −0.1 to +0.8‰ (average = +0.4±0.3‰). Our results suggest that the predominantly positive δCl37 values observed in the springs are consistent with water interaction with underlying 37Cl-enriched basalt and/or altered oceanic crust, thereby making thermal spring waters a reasonable proxy for the Cl isotope compositions of associated volcanic rocks in the Cascades. However, waters with δCl37 values >+1.0‰ also suggest additional contributions of chlorine degassed from cooling magmas due to subsurface vapor–liquid HCl fractionation in which Cl is lost to the aqueous fluid phase and 37Cl is concentrated in the ascending magmatic HCl vapor. Future work is necessary to better constrain Cl isotope behavior during volcanic degassing and fluid–rock interaction in order to improve volatile flux estimates through subduction zones.

Simultaneous determinations of fluorine, chlorine, and sulfur in rock samples by ion chromatography combined with pyrohydrolysis

We present a simplified method for simultaneous determinations of fluorine (F), chlorine (Cl), and sulfur (S) contents and isotopic abundances in rock samples. After using pyrohydrolysis to extract F, Cl, and S from a sample and transfer it into a sodium hydroxide (NaOH) solution, concentrations of F, Cl, and S as sulfate are measured by ion chromatography. The method is applicable to a variety of rock materials, including peridotite, basalt, gabbro, andesite, rhyolite, sulfide minerals, and possibly to sea salt, with F, Cl, and S contents of several μg g-1 to several tens of weight percent in 40 to 250 mg of rock powder. F, Cl, and S contents obtained by 5-7 repeated analyses of each of nine geological reference materials were in good agreement with previously reported values. In certain reference materials, sulfite peaks were observed, indicating incomplete oxidation to sulfate during S extraction by pyrohydrolysis. However, total S contents in those samples analysed by ICP-AES were also in good agreement with previous reported values. The RSDs of F, Cl, and S contents in reference materials were <6%, except for samples with F content <10 μg g-1 (∼20%) or S content <∼20 μg g-1 (<20%). In addition, Cl isotope compositions of seawater (IAPSO Standard Seawater and Pacific Ocean water) from sample solutions in this study were identical within error to reported values, indicating that Cl isotopes do not fractionate during pyrohydrolysis. After small isotopic fractionation corrections, multiple isotope compositions of S (32S, 33S, 34S and 36S) in solutions of sulfide (IAEA-S-1, IAEA-S-2, and IAEA-S-3), IAPSO Standard Seawater, and gabbro (JGb-1), also agree well with their respective reported values. Thus, this method is applicable to obtain accurate Cl and S contents as well as their isotopic compositions from a single aliquot of sample.

H and Cl isotope systematics of apatite in brecciated lunar meteorites Northwest Africa 4472, Northwest Africa 773, Sayh al Uhaymir 169, and Kalahari 009

AbstractWe have investigated the H and Cl systematics in apatite from four brecciated lunar meteorites. In Northwest Africa (NWA) 4472, most of the apatites contain ∼2000–6000 ppm H2O with δD between −200 and 0‰, except for one grain isolated in the matrix, which contains ∼6000 ppm H2O with δD of ∼500–900‰. This low‐δD apatite contains ∼2500–7500 ppm Cl associated with δ37Cl of ∼15–20‰, while the high‐δD grain contains ∼2500 ppm Cl with δ37Cl of ∼7–15‰. In NWA 773, apatites in a first group contain ∼700–2500 ppm H2O with δD values averaging around ∼0 ± 100‰, while apatites in a second group contain ∼5500–16500 ppm H2O with δD ∼250 ± 50‰. In Sayh al Uhaymir (SaU) 169 and Kalahari (Kal) 009, apatites are similar in terms of their H2O contents (∼600–3000 ppm) and δD values (−100 to 200‰). In SaU 169, apatites contain ∼6000–10,000 ppm Cl, characterized by δ37Cl of ∼5–12‰. Overall, most of the analyzed apatite grains have δD within the range reported for carbonaceous chondrites, similar to apatite analyzed in ancient (&gt;3.9 Ga) lunar magmatic. One grain in NWA 4472 has H and Cl isotope compositions similar to apatite from mare basalts. With an age of 4.35 Ga, this grain could be a representative of the oldest known lunar volcanic activity. Finally, since numerous evolved clasts in NWA 773 formed through silicate liquid immiscibility, the apatite grains with extremely high H2O contents, reaching pure hydroxylapatite composition, could provide insights into the effects of such process on the evolution of volatiles in lunar magmas.