Halogen Abundances Research Articles

Heavy halogen compositions of peridotite massifs in the Ivrea-Verbano Zone and implications for strong modification of mantle rocks

The lithospheric mantle affected by metasomatism is an important reservoir for halogens and often preserves significant heterogeneity in halogen abundances. However, the halogen compositions of the non-metasomatized mantle rocks with depleted geochemical features have not been well studied, impeding a comprehensive understanding of the halogen distribution in the mantle. The Balmuccia (BM) and Baldissero (BD) peridotite massifs in the Ivrea-Verbano Zone, Italian Alps, represent fragments of the sub-continental lithospheric mantle (SCLM) that are fresh and devoid of subduction-related mantle metasomatism. Here we examine the heavy halogen contents of well-characterized spinel-facies mantle peridotites (n = 20) and pyroxenites (n = 16) from the BM and BD massifs. The results indicate significant variation in whole rock halogen contents of peridotites and pyroxenites, with chlorine (Cl) ranging from <30 to 244 μg/g, bromine (Br) from <0.10 to 0.94 μg/g, and iodine (I) from <0.015 to 0.12 μg/g. Some peridotites and pyroxenites show rather low halogen contents (below the detection limit), consistent with their depleted features. However, a large number of samples display halogen contents far higher than the depleted MORB mantle (i.e., Cl: 3–10 μg/g) and peridotite xenoliths from the strongly metasomatized mantle, although ∼30 % of the halogens were leached out by ultrapure water or dilute nitric acid. The data suggest variable halogen enrichment in many mantle rocks. The halogen contents of different types of mantle rocks do not vary with major elements and incompatible trace elements (e.g., Nb), indicating that halogen compositions are not controlled by magmatic processes such as melting and melt-peridotite reaction. Instead, secondary fluid inclusions are abundant and mainly distributed as trails within olivine and pyroxene in both peridotites and pyroxenites, which were likely trapped during the exhumation of these mantle rocks. Their elevated halogen/Nb ratios compared with MORB mantle and strong correlations with Ba/Nb support the effect of fluids. Micro-Raman analyses identify hydrous serpentines within fluid inclusions. Moreover, mass balance calculation indicates that halogen enrichment is mainly caused by secondary fluids (> 90 %). Together, our findings highlight that the fluid percolation in the mantle rocks during exhumation can intensely modify the halogens, to a similar degree as mantle metasomatism, suggesting the sensitivity of halogens to secondary modification.

Halogen Cycling in the Solid Earth

Each of the halogens constrains a different aspect of volatile cycling in the solid Earth. F is moderately incompatible in the mantle and has a low mobility at Earth's surface, meaning that it is preferentially retained in the mantle and continental crust. In contrast, Cl, Br, and I are strongly incompatible and highly soluble. Chloride is the dominant anion in seawater and many geofluids and a major component of evaporite minerals. Br and I are essential for life and significantly incorporated into organic matter that accumulates in marine sediments. Surficial fluids circulated into continental and oceanic crust incorporate surface-derived halogens into alteration minerals. As a result, subducting slabs and arc lavas are weakly enriched in F and strongly enriched in Cl, Br, and I. Subduction has maintained mantle Cl and Br concentrations at relatively constant levels since Earth's early differentiation, but mantle I/Cl has decreased over time. ▪Halogen abundances on the early Earth were affected by I partitioning into Earth's core and possible loss of hydrophilic Cl, Br, and I in an early formed ocean.▪Halogens are powerful tracers of subduction zone processes on the modern Earth, with Cl, Br, and I having a dominantly subducted origin in Earth's mantle.▪The deep subduction cycles of Cl, Br, and I are more similar to that of H2O than they are to F, but the geochemical cycle of each halogen differs in detail.▪Halogen abundance ratios and stable isotope ratios vary systematically in Earth's surface reservoirs, meaning that halogens are powerful tracers of geological fluids and melts.

Heavy halogen compositions of lamprophyres derived from metasomatized lithospheric mantle beneath eastern North China Craton

Abstract Halogens and other volatiles are widely recycled into the deep mantle by subduction and are key components to metasomatize the sub-continental lithospheric mantle (SCLM). Lamprophyres are well known to be rich in volatiles and are important for understanding the halogen characteristics of the meta-somatized SCLM and/or the mobilization of halogens during the ascent of such volatile-rich, low-degree partial melts. The North China Craton (NCC) hosts lamprophyre dikes coeval with extensive thinning of the eastern NCC in the Mesozoic and generated from lithosphere metasomatized by multiple-stage subduction components. Here we report bulk-rock heavy halogens (Cl, Br, and I) of 16 lamprophyres from the eastern NCC. The bulk-rock halogen concentrations are overall very low (Cl = 58–170 μg/g, Br = 285–559 ng/g, and I &amp;lt;5 ng/g), comparable with depleted Mid-Ocean ridge basalts (N-MORBs). Volatile-rich minerals (e.g., amphibole and biotite) are abundant (20–30 vol%) in these lamprophyres, however, electron probe microanalyses (EPMA) data indicate that amphiboles are mainly rich in OH and F but display very low Cl concentrations (0.01–0.04 wt%). The bulk rock and amphibole data consistently indicate low abundances of heavy halogens in the lamprophyres, which is difficult to reconcile with the remarkable enrichment of fluid-mobile large ion lithophile elements such as Ba, Rb, and K. Based on low Cl/Nb and Br/Nb but high Ba/Nb and K/Nb ratios, the low halogen concentrations likely resulted from extensive volatile loss (&amp;gt;90%) during melt ascent. The low Cl concentrations in early-stage amphiboles (Mg# 60–64) further indicate that such loss would have occurred before amphibole crystallization at a depth of ~15 km. We thus propose that crystallization of early olivines and pyroxenes and reaction with surrounding mantle rocks likely induced volatile saturation and exsolution, leading to strong partitioning of the halogens into the exsolved aqueous volatile phases and thus the extensive loss of halogens from the rising melt. These results reveal that significant volatile loss of halogens not only occurs during surficial low-pressure eruption but also at much deeper levels in the crust, as also identified for some kimberlites. Consequently, it would be difficult to constrain the primitive halogen components of the mantle sources via lamprophyres or similar magmas.

Noble gas evolution of the martian atmosphere in the last 4 Gyr recorded by regolith breccia NWA 8114

The martian meteorite Northwest Africa (NWA) 8114 is a regolith breccia grouped with the NWA 7034 (‘Black Beauty’) stone and others. The meteorite, with its complex rock and mineral load, records over 4.4 billion years of martian geological and atmospheric history. In this work we present new analyses of noble gases in NWA 8114, and consider the constraints they impose on the evolution of the martian atmosphere over the past 4 billion years. We also report a petrographic overview, halogen abundances, and an argon isotope age, which provide context for interpreting the noble gas data.The krypton and xenon elemental signature of NWA 8114 is elementally fractionated with respect to the present-day martian atmosphere as measured in shergottite glasses; there is no requirement for a contribution from the ancient martian atmosphere in our data. The xenon isotopic composition incorporates (i) a component enriched in 129Xe (maximum 129Xe/132Xe = 2.450 ± 0.045 compared with a solar ratio of ∼ 1), which is similar to the present day martian atmosphere, (ii) a cosmic-ray spallation component dominated by production from barium, and (iii) a fission component. We estimate a cosmic ray exposure (CRE) age of 5.7 ± 1.3 Myr from cosmogenic 21Ne and 38Ar.Understanding how the martian atmosphere has changed through the planet’s history is a key part of understanding the planet’s geological history and evolution. We develop a model for the evolution of the martian atmosphere constrained by the amount of spallation-derived xenon in the atmosphere today and the evolution of the 129Xe/132Xe ratio over time. A baseline model in which the early atmosphere collapsed 3.7 Gyr ago (and assuming no further loss) requires a constant degassing of the crustal budget of spallation xenon of 0.034 % Myr−1 to accumulate sufficient spallation-derived xenon in the atmosphere. Combining constraints imposed by the 129Xe/132Xe ratio with the spallation budget requires loss of xenon from the martian atmosphere over the last 3.7 Gyr, with the present day budget being as little as 20 % of that at the start of this period.

Global halogen flux of subducting oceanic crust

In order to constrain the geochemical cycling of halogens (F, Cl, Br, I) during subduction of altered ocean crust (AOC), this study compares bulk halogen concentrations from seven seafloor AOC drill cores (n=21) and exhumed eclogites (n=22) from three paleo-subduction settings (Raspas Complex, Ecuador; Zambezi Belt, Zambia; Cabo Ortegal, Spain). As ocean crust undergoes hydrothermal alteration, basalts and gabbros preferentially incorporate different halogens. Fluorine is predominantly hosted in basalts (averaging 155 μg/g); Cl and Br are enriched in gabbros (averaging 241 μg/g and 225 ng/g, respectively). During prograde metamorphism of AOC, F behaves compatibly and is decoupled from the heavy halogens (Cl, Br, I). Eclogite samples retain F in concentrations (30–160 μg/g) similar to seafloor values (20–190 μg/g). Chlorine and Br are strongly coupled and effectively mobilized during prograde metamorphism, with high-pressure (HP) samples containing between 5–15 μg/g Cl and 17–69 ng/g Br. Elevated F/Cl, Br/Cl, and I/Cl in eclogites relative to MORB and AOC ratios support the decoupling of halogens during metamorphism as Cl is most efficiently expelled from the slab; the mobility of halogens in subducting AOC is as follows: Cl≈Br>I≫F. In conjunction with published values of halogen abundances in seafloor serpentinites, marine sediments, and HP metaserpentinites and metasediments, the results of this study are used to estimate global halogen fluxes. Mass balance estimates indicate that up to 2% of the initial F, 50% of the initial Cl, 93% of the initial Br, and 97% of initial I entering subduction zones is lost between the trench and eclogite facies. Subducted AOC represents a substantial halogen reservoir to arc magma depths and may represent the most significant carrier of halogens to the deep mantle.

Metasomatized mantle lithosphere and altered ocean crust as a fluid source for orogenic gold deposits

Whether auriferous hydrothermal fluid is generated during crustal metamorphism or by subcrustal processes to form orogenic gold deposits is hotly debated. Noble gas isotope and halogen abundance ratios can provide important constraints to help resolve this controversy. The Danba gold deposit is a rare example of a Jurassic age hypozonal orogenic gold deposit on the western margin of the Yangtze Craton, China. It is characterized by thick barren quartz veins with fractures infilled by thin auriferous quartz veinlets. The syn-gold quartz veinlets are associated with plagioclase and pyrrhotite and are distinguished from the barren quartz vein hosts on the basis of cathodoluminescence, trace-element content and Raman spectroscopy and microthermometry of fluid inclusions. The dominant fluid inclusions in the gold-bearing quartz and plagioclase are H2O–CH4 with vapor fill of ∼ 15–45 vol% and high pressure-corrected trapping temperatures of ∼ 500 °C, low salinities (<4 wt% NaCl equiv.) and high Mg/Fe ratios. Crushing syn-gold quartz and plagioclase released fluid inclusions with 40Ar/36Ar ratios of between 490 and 710, molar Br/Cl ratios of (0.3–0.7) × 10−3 and molar I/Cl ratios of (18–60) × 10−6. Crushing syn-gold pyrrhotite released fluid inclusions with 3He/4He ratios of 0.21–0.38 Ra (Ra = air value, 1.39 × 10−6) that are higher than typical crustal values of < 0.1 Ra, and 40Ar/36Ar of 330–640 that are similar to the gold-related quartz-hosted inclusions. The combined noble gas and halogen data do not favor an auriferous fluid derived from K- and U-rich Proterozoic granitic basement, which is expected to have radiogenic 40Ar/36Ar and 3He/4He ratios. Instead, the data are more consistent with a fluid source from underlying mantle lithosphere containing a high proportion of altered ocean crust (AOC). The auriferous fluid that formed the Danba gold deposit is thus considered to have been derived from a source comprising both metasomatized mantle lithosphere that had been fertilized in a Neoproterozoic subduction event and a portion of relict AOC beneath the western margin of the Yangtze Craton. In contrast with syn-gold fluid inclusions, those in pre-gold barren quartz are H2O–CO2 dominated. Crushing pre-gold quartz released fluid inclusions with 40Ar/36Ar ratios of 2410–5450, Br/Cl ratios of (1.2–1.5) × 10−3 and I/Cl ratios of (60–80) × 10−6 that are distinctly different to the gold-ore stage. The higher 40Ar/36Ar ratios of the pre-gold fluids are consistent with a source for the barren host quartz veins from prograde greenschist- to amphibolite-facies metamorphism of the Proterozoic granitic basement.

Developments in Halogen Abundance and Isotope Measurements

The four stable halogens (F, Cl, Br, and I) are low-abundance elements that are widely distributed in nature. Two of the halogens, Cl and Br, each have two stable isotopes showing a range in natural isotope variation of up to a few parts per thousand. A variety of analytical techniques have been developed to determine the abundance and isotopic ratios of the halogens: these include in situ techniques for high spatial resolution studies and bulk determinations, and they have been applied to a range of materials, including whole rocks, minerals, glasses, and fluid inclusions. Here, we summarise some of the established methods for determining halogen abundances and isotopes and highlight key advances.

A halogen budget of the bulk silicate Earth points to a history of early halogen degassing followed by net regassing

Halogens are important tracers of various planetary formation and evolution processes, and an accurate understanding of their abundances in the Earth's silicate reservoirs can help us reconstruct the history of interactions among mantle, atmosphere, and oceans. The previous studies of halogen abundances in the bulk silicate Earth (BSE) are based on the assumption of constant ratios of element abundances, which is shown to result in a gross underestimation of the BSE halogen budget. Here we present a more robust approach using a log-log linear model. Using this method, we provide an internally consistent estimate of halogen abundances in the depleted mid-ocean ridge basalts (MORB)-source mantle, the enriched ocean island basalts (OIB)-source mantle, the depleted mantle, and BSE. Unlike previous studies, our results suggest that halogens in BSE are not more depleted compared to elements with similar volatility, thereby indicating sufficient halogen retention during planetary accretion. According to halogen abundances in the depleted mantle and BSE, we estimate that ∼87% of all stable halogens reside in the present-day mantle. Given our understanding of the history of mantle degassing and the evolution of crustal recycling, the revised halogen budget suggests that deep halogen cycle is characterized by efficient degassing in the early Earth and subsequent net regassing in the rest of Earth history. Such an evolution of deep halogen cycle presents a major step toward a more comprehensive understanding of ancient ocean alkalinity, which affects carbon partitioning within the hydrosphere, the stability of crustal and authigenic minerals, and the development of early life.

High-pressure, halogen-bearing melt preserved in ultrahigh-temperature felsic granulites of the Central Maine Terrane, Connecticut (U.S.A.)

Abstract Inclusions of relic high-pressure melts provide crucial information on the fate of crustal rocks in the deep roots of orogens during collision and crustal thickening, including at extreme temperature conditions exceeding 1000 °C. However, discoveries of high-pressure melt inclusions are still a relative rarity among case studies of inclusions in metamorphic minerals. Here we present the results of experimental and microchemical investigations of nanogranitoids in garnets from the felsic granulites of the Central Maine Terrane (Connecticut, U.S.A.). Their successful experimental re-homogenization at ~2 GPa confirms that they originally were trapped portions of deep melts and makes them the first direct evidence of high pressure during peak metamorphism and melting for these felsic granulites. The trapped melt has a hydrous, granitic, and peraluminous character typical of crustal melts from metapelites. This melt is higher in mafic components (FeO and MgO) than most of the nanogranitoids investigated previously, likely the result of the extreme melting temperatures—well above 1000 °C. This is the first natural evidence of the positive correlation between temperature and mafic character of the melt; a trend previously supported only by experimental evidence. Moreover, it poses a severe caveat against the common assumption that partial melts from metasediments at depth are always leucogranitic in composition. NanoSIMS measurement on re-homogenized inclusions show significant amounts of CO2, Cl, and F. Halogen abundance in the melt is considered to be a proxy for the presence of brines (strongly saline fluids) at depth. Brines are known to shift the melting temperatures of the system toward higher values and may have been responsible for delaying melt production via biotite dehydration melting until these rocks reached extreme temperatures of more than 1000 °C, rather than 800–850 °C as commonly observed for these reactions.

Magmatic volatiles and platinum-group element mineralization in the Stillwater layered intrusion, U.S.A.

Abstract The activity of volatile-rich fluids may be important in the evolution of basaltic magmatic systems and associated precious metal ore formation. There is evidence for Cl-rich fluids within the Stillwater Complex (Montana, U.S.A.), which have been linked to platinum and palladium mineralization in the economically important Johns-Manville (J-M) Reef ore body. We present the first data set for heavy halogens (Cl, Br, and I) and natural noble gas isotopes in bulk rock and mineral separates from the Peridotite Zone and the Olivine-Bearing Zone I of the Stillwater Complex, including samples from the J-M Reef and G Chromitite bodies. Our data reveal concentrations of 4 to 13 500 ppm for Cl, 26 ppb to 360 ppm for Br, and &amp;lt;1 ppb to 9 ppm I over the whole sample set. Cl, Br, and I correlate well with each other implying a shared process and/or distribution in mineral species. Br/Cl and I/Cl ratios span a range from 0.3 to 35 × 10−3 and 5 to 900 × 10−6 by weight, respectively, encompassing MORB-like to more enriched compositions, particularly for Br/Cl. High-Br/Cl ratios compared to MORB in some Stillwater samples suggest fractionation of halogens during the exsolution of a volatile-rich fluid to explain the most Br-enriched samples. More generally, the presence of minerals such as scapolite, hornblende, and apatite in the most halogen-enriched samples suggests that the halogen-bearing fluids were derived from the cooling of the intrusion rather than late-stage (low-temperature) metamorphism. The combined halogen abundance and noble gas isotope data set imply that crustal contamination may have played a limited role in the crystallization of pegmatoids and the G Chromitite but is not required to account for the halogen budget of the J-M Reef. High-halogen contents in the sulfide-bearing J-M Reef and associated lithologies are consistent with the influence of fluid-related activity during platinum-group element (PGE)-Reef formation, lending weight to the hydromagmatic model for mineralization in the Stillwater intrusion. Our new data also imply chalcophile tendencies of Br and I over Cl in sulfides in natural systems, hinting at the importance of sulfide liquid interaction with halogen-rich fluids in the formation of sulfide-hosted precious metal ore deposits.

Multi-stage fluid infiltration and metasomatism in supra-subduction zone mantle: evidence from halogens and noble gases in the Leka Ophiolite Complex, Norway

Subduction of hydrated oceanic lithosphere represents a profound coupling of the Earth’s surface and its interior, with significant implications for both the internal dynamics and chemistry, as well as the surface habitability, of our planet. Serpentinised lithospheric mantle is increasingly recognised as an important component in the geochemical cycling of many volatile species. However, our understanding of the extent of hydration and heterogeneity in volatile geochemistry at depth is limited by a lack of samples from in situ lithospheric mantle. In this study, we apply coupled analyses of halogen abundances and noble gas isotopes to a suite of mineral separates from ultramafic lithologies from the Leka Ophiolite Complex, Norway, in order to trace the sources of fluids and critically assess the relative timing of high- and low-temperature fluid infiltration episodes. Results indicate extreme halogen abundance heterogeneity in the mantle and lower crustal transition zone of the Leka ophiolite, covering much of the known global range of serpentinite compositions. The data extend from near seawater-like compositions (as wt/wt: Br/Cl = 3.47 × 10−3, I/Cl = 3.04 × 10−6) to very high values of Br/Cl (≤9.9 × 10−3) and I/Cl (≤6.7 × 10−3), and are characterised by a marked decoupling of halogens, both from one another and from the noble gases. Relative enrichments in I, 84Kr, 132Xe and radiogenic 40Ar in serpentinites and fresh minerals are attributed to supra-subduction zone fluid infiltration at high temperature, followed by serpentinisation involving variable mixtures of seawater, and fluid derived from terrigenous sediment and organic-rich sedimentary pore fluid. In addition, alteration by brine or assimilation of brine during hydrothermal circulation has resulted in enrichment of Br in some samples. These enrichments are absent, or much less pronounced, in harzburgite samples suggesting these rocks preserve an earlier episode of seawater dominated serpentinisation within a SSZ setting. Together, these observations allow deconvolution of a series of metasomatic events and indicate that hydration was asynchronous, resulting in low and high temperature events mutually overprinting one another. Although this complicates our understanding of Leka and other ophiolites as straightforward natural laboratories for oceanic serpentinisation, it demonstrates that complex multi-phase volatile histories can be preserved in ophiolitic materials over timescales of at least ∼ 0.5 Gyr. Ophiolites therefore have the potential to be employed as archives of a diverse range of fluid processes operating over multiple temporal and spatial scales during subduction.

Microanalysis of Cl, Br and I in apatite, scapolite and silicate glass by LA-ICP-MS

Constraining the abundance and distribution of halogens in geological materials has the potential to provide novel insights into a broad range of earth system processes (e.g. metasomatism, melting, volatile cycling and ore formation). In this contribution we develop analytical protocols for the in situ measurement of Cl, Br and I in widely distributed standard reference materials (apatite, scapolite, silicate glass) using readily available laser ablation ICP-MS instrumentation. Ablations were performed at a range of square spot sizes (30–80 μm) using a high repetition rate (25 Hz) and extended analyte dwell times (up to 250 ms) to improve sensitivity and signal stability. A comparison of LA-ICP-MS results with published halogen data was used to calculate the following theoretical limits of quantification; Cl = 360 μg/g, Br = 8 μg/g, I = 0.75 μg/g. A detailed assessment of raw signal intensities for different matrices with known halogen contents, combined with high resolution mass scans, provides new constraints on the origin of apparent halogen signals: on mass 35Cl signal excesses are likely 16O18OH and/or 17O18O; 79Br is influenced by peak shoulder overlap from 40Ar40Ar (a diargon cation, Ar2+2) and a matrix-based interference (159Tb2+) for samples with Br/Tb < 0.6; 127I signals are similar for all but the highest I materials analysed here, suggesting the presence of ubiquitous gas-based interferences. The observation that false positive halogen signals only occur during sample ablation suggests that they are either matrix derived or related to the process of sample introduction. During ablation, matrix loading may reduce plasma energy, resulting in a greater proportion of polyatomic interferences in the system. For Cl, we provide a new time dependent excess apparent Cl spline correction defined by analysis of halogen-free olivine via a modified version of the Iolite Data Reduction Scheme ‘X_Trace_Elements_IS’. The correction improves the limit of linearity to ~100 μg/g for Cl in glasses down to a 38 μm spot size. We test our methodology on apatite from Permian alkaline lamprophyres in the Pyrenees (Spain) and quartz-hosted melt inclusions from rhyolitic deposits at the Taupo volcanic zone (New Zealand), obtaining results comparable to electron microprobe and SIMS data. We provide recommendations for analytical best practice and highlight the need for well characterised matrix matched SRMs spanning a broad range of concentrations to allow for the identification and removal of non-analyte related contributions to measured signals.

The distribution and abundance of halogens in eclogites: An in situ SIMS perspective of the Raspas Complex (Ecuador)

AbstractWe present in situ secondary ion mass spectrometry (SIMS) and electron microprobe analyses of coexisting garnet, omphacite, phengite, amphibole, and apatite, combined with pyrohydrolysis bulk-rock analyses to constrain the distribution, abundance, and behavior of halogens (F and Cl) in six MORB-like eclogites from the Raspas Complex (Southern Ecuador). In all cases concerning lattice-hosted halogens, F compatibility decreases from apatite (1.47–3.25 wt%), to amphibole (563–4727 μg/g), phengite (610–1822 μg/g), omphacite (6.5–54.1 μg/g), and garnet (1.7–8.9 μg/g). The relative compatibility of Cl in the assemblage is greatest for apatite (192–515 μg/g), followed by amphibole (0.64–82.7 μg/g), phengite (1.2–2.1 μg/g), omphacite (&amp;lt;0.05–1.0 μg/g), and garnet (&amp;lt;0.05 μg/g). Congruence between SIMS-reconstructed F bulk abundances and yield-corrected bulk pyrohydrolysis analyses indicates that F is primarily hosted within the crystal lattice of eclogitic minerals. However, SIMS-reconstructed Cl abundances are a factor of five lower, on average, than pyrohydrolysis-derived bulk concentrations. This discrepancy results from the contribution of fluid inclusions, which may host at least 80% of the bulk rock Cl. The combination of SIMS and pyrohydrolysis is highly complementary. Whereas SIMS is well suited to determine bulk F abundances, pyrohydrolysis better quantifies bulk Cl concentrations, which include the contribution of fluid inclusion-hosted Cl. Raspas eclogites contain 145–258 μg/g F and at least 7–11 μg/g Cl. We estimate that ~95% of F is retained in the slab through eclogitization and returned to the upper mantle during subduction, whereas at least 95% of subducted Cl is removed from the rock by the time the slab equilibrates at eclogite facies conditions. Our calculations provide further evidence for the fractionation of F from Cl during high-pressure metamorphism in subduction zones. Although the HIMU (high U/Pb) mantle source (dehydrated oceanic crust) is often associated with enrichments in Cl/K and F/Nd, Raspas eclogites show relatively low halogen ratios identical within uncertainty to depleted MORB mantle (DMM). Thus, the observed halogen enrichments in HIMU ocean island basalts require either further fractionation during mantle processing or recycling of a halogen-enriched carrier lithology such as serpentinite into the mantle.

Heavy halogen geochemistry of martian shergottite meteorites and implications for the halogen composition of the depleted shergottite mantle source

AbstractVolatile elements (e.g., H, C, N) have a strong influence on the physical and chemical evolution of planets and are essential for the development of habitable conditions. Measurement of the volatile and incompatible heavy halogens, Cl, Br, and I, can provide insight into volatile distribution and transport processes, due to their hydrophilic nature. However, information on the bulk halogen composition of martian meteorites is limited, particularly for Br and I, largely due to the difficulty in measuring ppb-level Br and I abundances in small samples. In this study, we address this challenge by using the neutron irradiation noble gas mass spectrometry (NI-NGMS) method to measure the heavy halogen composition of five olivine-phyric shergottite meteorites, including the enriched (Larkman Nunatak LAR 06319 and LAR 12011) and depleted (LAR 12095, LAR 12240, and Tissint) compositional end-members. Distinct differences in the absolute abundances and halogen ratios exist between enriched (74 to136 ppm Cl, 1303 to 3061 ppb Br, and 4 to 1423 ppb I) and depleted (10 to 26 ppm Cl, 46 to 136 ppb Br, and 3 to 329 ppb I) samples. All halogen measurements are within the ranges previously reported for martian shergottite, nakhlite, and chassignite (SNC) meteorites. Enriched shergottites show variable and generally high Br and I absolute abundances. Halogen ratios (Br/Cl and I/Cl) are in proportions that exceed those of both carbonaceous chondrites and the martian surface. This may be linked to a volatile-rich martian mantle source, be related to shock processes or could represent a small degree of heavy halogen contamination (a feature of some Antarctic meteorites, for example). The differences observed in halogen abundances and ratios between enriched and depleted compositions, however, are consistent with previous suggestions of a heterogeneous distribution of volatiles in the martian mantle. Depleted shergottites have lower halogen abundances and Br and Cl in similar proportions to bulk silicate Earth and carbonaceous chondrites. Tissint in particular, as an uncontaminated fall, allows an estimate of the depleted shergottite mantle source composition to be made: 1.2 ppm Cl, 7.0 ppb Br, and 0.2 ppb I. The resultant bulk silicate Mars (BSM) estimate (22 ppm Cl, 74 ppb Br, and 6 ppb I), including the martian crust and depleted shergottite mantle, is similar to estimates of the bulk silicate earth (BSE) halogen composition.

An experimental assessment of the chalcophile behavior of F, Cl, Br and I: Implications for the fate of halogens during planetary accretion and the formation of magmatic ore deposits

The elemental and isotopic abundances of halogens (F, Cl, Br, I) are used to constrain planetary volatile loss and volatile delivery processes, but their behavior during magmatic differentiation in general, and sulfide liquid segregation in particular, is currently not well constrained. To test whether sulfide liquid segregation could affect halogen behavior during magmatic processes, we performed high-pressure experiments to systematically quantify the sulfide liquid – silicate melt partition coefficients (Dsulliq-silmelt values, defined as the ratio between the wt.% concentration of the halogen in the sulfide liquid and silicate melt, respectively) of F, Cl, Br and I at a pressure of 1 GPa and temperatures of 1683–1883 K.Results show that dry-polishing target surfaces is crucial for obtaining representative halogen concentrations of sulfide liquids. The results also show that no appreciable amounts of F partition into sulfide liquids, whereas Cl, Br and I behave increasingly chalcophile with increasing atomic radius (i.e., DFsulliq-silmelt <DClsulliq-silmelt <DBrsulliq-silmelt < DIsulliq-silmelt), presumably as a result of an increasingly covalent nature of Fe-halogen bonds with increasing radius. This results in I behaving chalcophile (DIsulliq-silmelt > 1) in several experiments. In contrast to previous observations, DCl/Brsulliq-silmelt was found to be <1. The DCl,Br,Isulliq-silmelt predominantly vary with sulfide liquid melt composition, showing an increase with increasing O in the sulfide liquid, which itself is correlated with more oxidizing conditions (i.e., higher fO2) or silicate melt FeO contents. The DCl,Br,Isulliq-silmelt values remain constant and/or potentially decrease again at the highest O concentrations of the sulfide liquids in this study (∼2.5 wt.% O).Results indicate that the magnitude of halogen depletions in the terrestrial, martian and lunar mantle are not strongly affected by segregation of sulfide liquids during their accretion, given the expected low modal abundance of sulfide liquids and/or relatively low DCl,Br,Isulliq-silmelt values. Core formation remains the most important process in establishing iodine depletion in the terrestrial mantle, whereas volatility-related loss seems most likely for F, Cl, Br and I, in case of the martian mantle. However, segregation of sulfide liquids during accretion could have resulted in a relative increase of the offset between the mantle depletions of the lighter and heavier halogens. The experimental results confirm the previously proposed feasibility of sulfide liquids as reservoirs for halogens in magmatic sulfide ore environments. As proposed by Mungall and Brenan (2003), fractional crystallization of these sulfide liquids in the absence of a silicate melt can lead to the formation of halide melts or fluids, consistent with the association between halide minerals and magmatic sulfide ores in some localities.

Crystallographic and fluid compositional effects on the halogen (Cl, F, Br, I) incorporation in pyromorphite-group minerals

Abstract Pyromorphite-group minerals (PyGM), mainly pyromorphite [Pb5(PO4)3Cl], mimetite [Pb5(AsO4)3Cl], and vanadinite [Pb5(VO4)3Cl], are common phases that form by supergene weathering of galena. Their formation is strongly influenced by processes at the Earth's surface and in the soil overlying a lead deposit, and they incorporate high amounts of halogens, mostly Cl and, in some cases, F. The abundance of Br and I in natural PyGM and their potential as process tracers during surface and sub-surface fluid-rock interaction processes has not been investigated in detail due to analytical difficulties. We, therefore, developed methods for the simultaneous determination of Cl, F, Br, and I in PyGM for (1) powdered bulk samples via combustion ion chromatography (CIC) and (2) compositionally zoned crystals by means of secondary ion mass spectrometry (SIMS). Our study is based on well-characterized samples of pyromorphite (N = 38), mimetite (N = 16), and vanadinite (N = 2) from Schwarzwald (Germany). Natural pyromorphite incorporates more I (up to 26 μg/g) than mimetite (up to 2 μg/g) and vanadinite (up to 1 μg/g), while Br contents are higher in mimetite (up to 20 μg/g) and vanadinite (up to 13 μg/g) compared to pyromorphite (less than 4 μg/g). These results are unexpected, as mimetite and vanadinite have longer As/V-O bonds giving them larger unit cells and larger polyhedral volumes for the Cl site in the Pb26 octahedron than pyromorphite. Accordingly, pyromorphite was expected to preferentially incorporate Br rather than I, but the opposite is observed. Hence, halogen chemistry of PyGM is probably not governed by a crystal-chemical control (alone) but by fluid composition. However, the exact reasons remain enigmatic. This idea is corroborated by spatially resolved SIMS analyses that show that many pyromorphite-group minerals are strongly zoned with respect to their halogen mass ratios (e.g., Br/Cl, Br/I mass ratios). Furthermore, variations in halogen abundance ratios do not correlate with Ca/Pb, P/As, or P/V ratios and therefore may record alternating and season-dependent environmental parameters including biological activity, vegetation density, physico-chemical soil properties, and rainfall rate. We suggest that the zonation reflects multiple single fluid flow episodes and, hence, records surface processes. However, further experiments concerning the fractionation of halogens between fluid and PyGM are needed before halogen ratios in pyromorphite-group minerals can be used as reliable monitors of fluid-driven processes.

Combined in-situ determination of halogen (F, Cl) content in igneous and detrital apatite by SEM-EDS and LA-Q-ICPMS: A potential new provenance tool

Apatite is an accessory mineral that is widely present in most igneous and metamorphic rocks, and is a common detrital component in clastic rocks. As apatite typically incorporates U at sufficient concentrations to enable thermochronometry (e.g., U-Pb, fission track, and (UTh)/He), it is commonly used in detrital provenance studies. Apatite also incorporates a wide range of trace elements at concentrations readily detectable by routine laser ablation-quadrupole-inductively coupled plasma-mass spectrometry (LA-Q-ICPMS) analysis, which can also yield diagnostic provenance information (in particular Sr-Y-REE). However, detrital thermochronometry is of limited use in the case of source terrane(s) with similar crystallisation or cooling histories, while trace elements may yield non-diagnostic provenance information when their source rocks are lithologically similar (e.g., co-genetic granitoids). Here we test a complementary provenance approach which exploits F and Cl substitution into the stoichiometric Z-site in the apatite lattice. Halogen abundance in igneous apatite typically reflects the source melt composition and its subsequent differentiation history, offering potential for provenance discrimination. We apply this method to apatite from bedrock samples of five late- to post-tectonic granitoids in the Grampian and Moine terranes of the Scottish Caledonides, which yield zircon UPb ages of ca. 425–398 Ma. Emplacement over this relatively short time window makes it difficult to discriminate between sediment sourced from these plutons using radiometric dating alone. Apatite F content was measured using an energy dispersive X-ray spectrometer coupled to a scanning electron microscope (SEM-EDS), while Cl and trace element (Sr-Y-REE) abundances were determined by LA-Q-ICPMS. We show that the sampled granitoids can be divided into three groups based on their apatite trace element contents and into three separate groupings based on their F and Cl abundances; all five plutons can thus be discriminated from each other using their integrated trace elements and F-Cl systematics. The granitoid groups identified by their apatite F and Cl contents correlate with those depicted by their modal mineralogy as defined in Quartz-Alkali Feldspar-Plagioclase (QAP) space, with granitoids richer in K-Feldspar (and thus more evolved) yielding apatites with higher F contents. As a pilot study, we also analysed apatite from modern river sediment in the Spey River catchment of the Grampian terrane, for which UPb age data were also acquired. The Spey detrital apatites yield UPb ages that are indistinguishable at the 2σ-level, but their trace element abundances allow three bedrock types to be distinguished: I-type granitoids, S-type granitoids and medium- to high-grade metamorphic rocks. The detrital apatite F and Cl data do not permit such straightforward discrimination, as apatites from S-type granitoid and metamorphic rocks have comparably low Cl contents. However, the detrital apatites with I-type granitoid affinity display significant dispersion in halogen contents (yielding the lowest F abundances with correspondingly the highest Cl contents), which identifies the presence of several more “mafic” I-type granitoid bodies in the Spey catchment. Apatite F and Cl abundances thus have potential for detailed characterisation of the individual components of granitoid suites, including delineating I-type granitoids or the presence of less evolved magmatic bodies among detrital apatites.

Halogens in altered ocean crust from the East Pacific Rise (ODP/IODP Hole 1256D)

The halogens (F, Cl, Br and I), H2O and CO2 were investigated in eighteen representative lavas, dykes and a gabbro spanning depths 750–1450 mbsf in the International Ocean Discovery Project (IODP) Hole 1256D. Whole rock analyses of halogens, H2O and CO2, and in situ F and Cl electron microprobe analyses were combined to provide new information about hydrothermal alteration of the ocean crust and the mineral controls on the abundances of all four halogens in altered ocean crust that is subducted into the mantle. Whole rock concentrations of halogens, H2O and CO2 are heterogeneous at all crustal levels with maxima in highly altered zones of fluid infiltration. Nonetheless, Cl and Br show a general increase down the hole from minima of 130 ppm Cl and 330 ppb Br in the lavas, with dominant saponite-chlorite alteration, to maxima of 1620 ppm Cl and 2720 ppb Br in amphibolite facies granoblastic dykes and the gabbros near the base of the hole. In contrast, H2O concentrations of 1.5–2 wt% were common in clay-rich (saponite) alteration with lower values in the deeper amphibole-rich samples. The concentrations of F (130–300 ppm), I (6–25 ppb) and CO2 (0.13–0.43 wt.%) do not show any obvious relationship to depth. The altered lavas, dykes and gabbros are enriched in Cl, Br, I and H2O by ∼2–20 times, but have F concentrations similar to uncontaminated fresh N-MORB glasses from the East Pacific Rise. The majority of amphiboles analysed in this and previous studies have low F/Cl ratios that are typical of hydrothermal amphiboles formed from F-poor seawater-derived fluids. The most Cl-rich amphiboles in the deepest gabbro typically contain ∼2000–4000 ppm Cl, but a patch of amphibole with 1.6 wt.% Cl was found close to a chlorapatite. Together the Cl-rich amphibole and hydrothermal chlorapatite, with up to 5.4 wt.% Cl, provide evidence for high salinity seawater-derived brines (∼50 wt.% salts) at the base of the hole. In addition a population of F-rich amphiboles (>1000 ppm F) with elevated F/Cl ratios (F/Cl ∼ 6–8) provide evidence for the passage of F-rich magmatic fluids in the granoblastic dykes and gabbros. Hydrous silicates including saponite-chlorite at the top of the hole and amphibole below 1300 mbsf, typically accommodate most of the F and Cl (e.g. 20–60%) in whole rock samples. Apatite accommodates a further <5–40% of the total F and Cl, with non-structural sites including grain boundaries and fluid inclusions estimated to accommodate 30–50% of the halogens in many samples. Bromine and I are preferentially hosted by phyllosilicates and non-structural sites, and excluded from amphibole relative to Cl. As a result, saponite-chlorite alteration is characterised by a range of Br/Cl and I/Cl ratios overlapping those typical of the Earth’s mantle, whereas deeper amphibolite facies alteration has lower Br/Cl and I/Cl ratios. Halogen abundance ratios therefore indicate that if subduction of oceanic crust significantly contributes to the halogen inventory of the Earth’s mantle, low-temperature (<150 °C) alteration could be an important reservoir of halogens in the subducting slab.

Halogens in Atlantis Bank gabbros, SW Indian Ridge: Implications for styles of seafloor alteration

The processes controlling halogen (F, Cl, Br, I) abundances in gabbroic ocean crust recovered from the 809-m deep Hole U1473A drilled on the Atlantis Bank during International Ocean Discovery Program (IODP) Expedition 360 were investigated. The aims were to provide new constraints on hydrothermal alteration and the abundances of halogens potentially transported to subduction zones in oceanic crust produced on a slow spreading ridge.Halogens in 51 gabbros and felsic veins have concentrations of ∼20-260 ppm F, 15-840 ppm Cl, 44-1230 ppb Br and 1-2490 ppb I. On average the gabbros retain a melt-derived magmatic F component of 58 ± 26% but are dominated by ∼96% hydrothermal Cl, Br, I and H2O. The abundances of hydrothermal Cl, Br and I are consistent with alteration at a seawater/rock ratio of <1. However, hydrothermal F is more enriched than expected and some amphiboles have high F/Cl ratios of 10-30 that provide evidence for the minor additional involvement of F-rich magmatic fluids. The abundant late-stage felsic veins that transect the gabbros and account for 1.5 vol.% of Hole U1473A lithologies are suggested as the most likely source of F-rich magmatic fluids.Downhole variations show F, Cl, Br and H2O are most abundant in amphibole- and clay-rich alteration zones at the top of the hole. The highest I concentrations of 1-2.5 ppm delineate an oxidised CO2-rich alteration zone in which seawater iodate was incorporated into carbonate and Fe-oxyhydroxide alteration. The role of iodate, which is more reactive than other halides, in generating I-rich alteration can be distinguished from alteration by I-rich sedimentary pore waters because the oxidised alteration is characterised by high I/Cl together with low Br/Cl, whereas organic matter in pore waters is enriched in both Br/Cl and I/Cl.The halogens have inferred compatibilities of F− > IO3− > OH− > Cl− ≥ Br− ∼ ≥ I− in the investigated alteration assemblage. EPMA and SHRIMP analyses indicate amphibole contains 1000-3000 ppm Cl in amphibolite facies alteration at the top of the Hole. Amphibole, chlorite and talc in greenschist facies alteration have much lower concentrations, typically in the range of 20-800 ppm Cl (median ∼100 ppm Cl), and F/Cl ratios of <4. In comparison, low temperature limonite (FeOOH.nH2O) has 160-8500 ppm Cl (median ∼840 ppm Cl). Amphibole is the dominant host of Cl and F in amphibolite facies alteration, but chlorite and limonite are important at lower grades. The samples have fairly constant Br/Cl ratios suggesting that Br excluded from the amphibole lattice is retained in minerals such as chlorite and/or non-structural sites. High I concentrations of 10's of ppm are inferred for some carbonate and limonite. Overall amphibole is a less dominant host of halogens than has been suggested previously, which has important implications for the eventual release and availability of halogens during subduction zone metamorphism.

Halogen Heterogeneity in the Lithosphere and Evolution of Mantle Halogen Abundances Inferred From Intraplate Mantle Xenoliths

AbstractWe present halogen, noble gas, and major and trace element compositions of mantle xenoliths from intraplate settings (Eifel, Kilbourne Hole, San Carlos, and Hawaii). The xenoliths show a wide range of halogen elemental ratios, which form two arrays centered on the halogen composition of mid‐ocean ridge basalts. The samples on the array toward high I/Cl value have relatively low Cl concentration and low ratios of highly incompatible elements relative to heavy rare earth elements, whereas the samples on the array toward low Br/Cl value have higher Cl concentration and trace elements ratios. The detailed mechanisms to account for these signatures are equivocal at present. However, they are most likely to be related to secondary processes of volatile loss during partial melting and secondary phase formation during interaction with melts. The common primary mid‐ocean ridge basalt‐like halogen ratios in mantle xenoliths from different parts of the globe indicate that the mantle itself must have a relatively uniform composition over a wide scale. The mantle has maintained its halogen composition over billion year timescales without being affected by I‐rich halogens being transported into the mantle. Mass balance calculations suggest that, in order to maintain the I/Cl ratio of the convecting mantle over 2 Gyr, the I/Cl ratio of the subducted halogens must be no more than several times higher than the present‐day mantle value.