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Alteration of the oceanic lithosphere
Halogens are critical elements for transporting metals in hydrothermal solution and tracing the sources of fluids and volatiles in subduction zones. This study tested the suitability of LA-ICPMS for simultaneous measurement of Cl, Br and I with selected trace elements in altered gabbros recovered from the Atlantis Bank core complex close to the SW Indian Ridge. The specific aim was to better understand the causes of I-enrichment in low temperature carbonate and Fe-oxyhydroxide alteration, which is intensely developed in core recovered from between ~100 and ~600 mbsf down Hole U1473A drilled during Expedition 360 of the International Ocean Discovery Program (IODP). The analysis of reference materials including scapolites, Durango apatite, USGS and NIST reference glasses and volcanic glasses demonstrate Cl, Br and I can be measured in most minerals, but accuracy is limited by matrix dependent laser backgrounds (false signals of Cl, Br and I generated during sample ablation). Silicate reference glasses define reproducible ‘laser backgrounds’ of ~320 μg/g Cl and ~1.7 μg/g Br that point to limits of quantification of ~200 μg/g Cl and ~1 μg/g Br in silicate glasses after correction for laser backgrounds. In contrast, laser backgrounds are much lower and possibly negligible for carbonate, suggesting limits of quantification as low as ~50 μg/g Cl and 0.1 μg/g for Br, dependent on instrument sensitivity. Laser backgrounds for I varied between minerals and analytical sessions, there was no laser background for I in apatite and under favourable conditions silicate glasses have laser backgrounds of <1 μg/g. In addition to the laser backgrounds, we report a correction for interference of doubly charged rare earth elements on Br measurement, which significantly improves data quality for some samples. The Atlantis Bank alteration minerals investigated include calcite veins and Fe-oxyhydroxide, which is intermixed with, and cut by veinlets of, calcite and smectite. The Fe-oxyhydroxide contains 100 s–1000s μg/g Cl, 10s μg/g Br and 5–30 μg/g I and it is strongly enriched in other trace elements including B and As. Most calcite veins contain <100–200 μg/g Cl, <0.4–0.9 μg/g Br and 5–26 μg/g I. Among the halogens, I is uniquely correlated with divalent cations in calcite including Mg and Sr, consistent with its incorporation into the carbonate lattice as iodate. In most cases smectite contains less I than coexisting calcite or Fe-oxyhydroxide, but three smectite grains in one sample are exceptionally enriched with 150–240 μg/g I. The in situ analyses support the attribution of exceptional iodine enrichment of Atlantis Bank lithologies to preferential adsorption of iodate over halides by Fe-oxyhydroxides and subsequent remobilisation with preferential fixing of iodate in carbonate-dominated alteration. The data demonstrate the utility of LA-ICPMS for in situ halogen studies. Iodine in carbonate veins can provide information about fluid oxidation state and fluid-rock reaction history. However, further work is required to test if Cl and Br in carbonate can provide information about fluid sources.
Serpentine in modern seafloor and ophiolitic environments incorporates and often retains high concentrations of atmospheric noble gases and seawater-derived halogens. Ancient serpentinites therefore provide the potential to trace the composition of early surface environments. Antigorite-serpentinites locally carbonated to talc-magnesite schist outcropping in a low strain zone within the Eoarchean Isua supracrustal belt (Greenland) are investigated here, to test the retention of paleo-atmospheric noble gases and Eoarchean seawater halogens, and to further determine the genetic setting and metamorphic history of some of Earth’s oldest serpentinites. Based on field relationships, whole rock major and trace element geochemistry, and mineral chemistry, the investigated serpentinites are shown to represent hydrated and variously carbonated magmatic olivine ± orthopyroxene + Cr-spinel cumulates emplaced at the base of a lava flow of boninitic affinity pillowed in its upper portion. In addition, rare zircons extracted from one of the serpentinised cumulates have distinct magmatic trace element signatures and a U-Pb age of 3721 ± 27 Ma indicating the pillowed lava flow erupted on the Eoarchean seafloor. The serpentinites have high concentrations of noble gases, but the presence of parentless radiogenic ‘excess’ ⁴⁰Ar, introduced by crustal-derived metamorphic fluids, obscures the ⁴⁰Ar/³⁶Ar ratio of Eoarchean seawater. Local carbonation of the serpentinites also caused halogen loss and fractionation. However, the least carbonated antigorite serpentinites preserve Br/Cl and I/Cl ratios within the range of modern seafloor serpentinites, which is interpreted as indicating Archaean serpentinising fluids were similar in composition to modern seawater-derived fluids. Importantly, the lowest measured I/Cl ratio of 29 (±2) × 10⁻⁶, taken as a maximum value for the Eoarchean ocean, is an order of magnitude lower than estimates for the primitive mantle I/Cl value. Iodine has a low concentration relative to Cl in modern seawater because it is sequestered by organic matter. If the inferred low I/Cl of Eoarchean seawater is correct, then similar I-sequestration was likely occurring in the Eoarchean, a process requiring the presence of significant biomass in Earth’s early oceans. Further constraining the Precambrian evolution of seawater I/Cl via serpentinites or other proxies may provide a novel method to explore the emergence and evolution of terrestrial biomass.
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.
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 H 2 O. 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 H 2 O 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 CO 2-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 − > IO − 3 > 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.nH 2 O) 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.
Ophiolitic serpentinites and secondary peridotites formed by serpentinite dehydration were investigated to improve constraints on the fates of noble gases and halogens during subduction zone metamorphism. The work extends previous studies to encompass F and four stages of serpentinization and serpentinite dehydration including: (i) oceanic serpentinites preserving the features of seafloor serpentinization; (ii) subducted high grade (olivine bearing) antigorite-serpentinites; (iii) spinifex and granofels textured chlorite harzburgites; and (iv) a garnet peridotite. Serpentinites and secondary peridotites from different ophiolites are shown to have characteristic ranges of ⁴⁰Ar/³⁶Ar: chrysotile and antigorite serpentinites from Erro Tobbio (Western Alps) have ⁴⁰Ar/³⁶Ar of ∼296-390; antigorite serpentinites and chlorite harzburgites from Cerro del Almirez (Betic Cordillera) have ⁴⁰Ar/³⁶Ar of ∼340-600, and chlorite harzburgites and garnet peridotites from Cima di Gagnone (Swiss Alps) have ⁴⁰Ar/³⁶Ar of ∼600-1100. The variation of ⁴⁰Ar/³⁶Ar is unrelated to metamorphic grade at each locality but is broadly correlated with variation in other radiogenic isotopes (²⁰⁶Pb/²⁰⁴Pb and ⁸⁷Sr/⁸⁶Sr) between localities. This suggests excess ⁴⁰Ar was derived from terrigenous sediments with characteristic ranges of ⁴⁰Ar/³⁶Ar and ⁸⁷Sr/⁸⁶Sr in different subduction zones. The secondary chlorite harzburgites have ²⁰Ne/³⁶Ar ratios of greater than seawater, contain parentless (or excess) ⁴He, and have higher F concentrations than any of the serpentinites investigated. The ²⁰Ne/³⁶Ar is broadly correlated with ⁴⁰Ar/³⁶Ar in samples from Cerro del Almirez suggesting derivation of excess ⁴⁰Ar and atmospheric ²⁰Ne from a common source. The chlorite is shown to have higher concentrations of F, Ne and other noble gases than coexisting olivine and enstatite, which contain abundant desiccated fluid inclusions. The high F content and high ²⁰Ne/³⁶Ar ratios of the chlorite harzburgites are ascribed to fluxing of dehydrating serpentinites with F-, ⁴⁰Ar-, ⁴He- and ²⁰Ne-rich fluids derived from metasediments in the subducting slab, and the inferred high compatibility of F and Ne in chlorite. The garnet peridotite from Cima di Gagnone records the final and complete dehydration of serpentinite. Based on the analysis of mineral separates minimally affected by retrogression (marked by garnet breakdown and the appearance of Cl-rich hornblende), nominally anhydrous garnet peridotite retains Cl, Br, I and non-radiogenic noble gas concentrations up to an order of magnitude higher than average depleted mantle. The data are consistent with serpentinised lithosphere and related secondary peridotites as major sources of deeply subducted seawater-derived volatiles in the Earth’s mantle. The data also demonstrate that the relative abundances of volatiles subducted into the mantle are controlled by multiple factors including: original seafloor alteration, the relative compatibilities of different noble gases and halogens in minerals forming during different stages of subduction and chemical exchange between different lithologies during subduction. The combination of these processes has produced elevated ²⁰Ne/³⁶Ar in chlorite harzburgites from two unrelated localities. This suggests that subduction of atmospheric Ne could be significantly more efficient than previously realised, which has implications for interpretation of the mantles primordial ²⁰Ne/²²Ne ratio and how the Earth accreted.
This chapter aims to provide a framework for understanding the distribution of halogens in the oceanic lithosphere. It reviews the concentrations of F, Cl, Br and I in seawater, marine sediment pore waters, hydrothermal vent fluids, fluid inclusions from deeper in the crust, and the complementary solid-phase reservoirs of organic matter and minerals present in sediments and crustal/mantle rocks from varying depths. Seawater (3.4–3.5 wt% salt) is depleted in F, weakly enriched in I and strongly enriched in Cl and Br compared to the primitive mantle. Sequestration of I and Br by phytoplankton lead to the storage of these elements in marine sediments, which are the Earths dominant I reservoir. Regeneration of organic matter during diagenesis releases I⁻ and Br⁻ to marine sediment pore waters, which acquire Br/Cl and I/Cl ratios of higher than seawater and can be advected into the underlying crust and lithosphere. In contrast, Cl is usually assumed to behave conservatively in pore waters and F is precipitated in authigenic sedimentary minerals meaning it is not significantly advected into the underlying basement. Vent fluids have salinities of 0.1–6 wt% salts, which provide evidence for phase separation and segregation of vapours and brines in hydrothermal systems. The majority of vent fluids have Br/Cl ratios within 10% of the seawater value. However, elevated Br/Cl and I/Cl ratios indicate that some vent fluids interact with organic-rich sediments, and low Br/Cl ratios suggest some vent fluids leach Cl from glassy volcanic rocks or halite. Vent fluids have F/Cl ratios scattered around the seawater value which reflects the generally low mobility of F during diagenesis and hydrothermal alteration. In comparison to vent fluids, fluid inclusions also provide evidence for phase separation but preserve a much greater range of salinity including brines with salinities as high as ~50 wt% salt. The altered ocean crust has a F concentration of close to its initial value. In contrast, Cl is mobilised within layer 2 pillows and dykes and strongly enriched in layer 3 gabbros subjected to high temperature alteration. Amphibole is the dominant Cl host in the oceanic crust, with Cl concentrations of <500 ppm under greenschist conditions and up to wt% levels under amphibolite conditions. The increasing Cl content of amphibole as a function of metamorphic grade most likely reflects a decreasing water/rock ratio and a general increase in fluid salinity as a function of depth in the crust. Amphibole preferentially incorporates Cl relative to Br and I; however, I is enriched in absolute terms, and relative to Cl, in clay-rich alteration and biogenic alteration of glassy rocks in the upper crust. Serpentinites formed in the oceanic lithosphere can contain thousands of ppm Cl and some serpentinites preserve Br/Cl and I/Cl signatures very similar to sedimentary pore waters, indicating that all halogens have high compatibilities in serpentine. Fluorine is slightly enriched in some serpentinites compared to peridotites, which may indicate minor mobilisation of F from igneous lithologies in the overlying crust. The altered oceanic crust and mantle lithosphere reaching subduction zones have poorly defined halogen concentrations. However, the average Cl concentration could be as high as ~400 ppm. And it may have a F/Cl ratios as low as ~0.25 compared to ~2 in pristine crust. It is estimated that approximately 90% of the Cl present in altered oceanic lithosphere is introduced during seawater alteration.
Six variably amphibolitised metagabbros cut by quartz-epidote veins containing high salinity brine and vapour fluid inclusions were investigated for halogen (Cl, Br, I) and noble gas (He, Ne, Ar, Kr, Xe) concentrations. The primary aims were to investigate fluid sources and interactions in hydrothermal root zones and determine the concentrations and behaviours of these elements in altered oceanic crust, which is poorly known, but has important implications for global volatile (re)cycling. Amphiboles in each sample have average concentrations of 0.1-0.5 wt. % Cl, 0.5-3 ppm Br and 5-68 ppb I. Amphibole has Br/Cl of ~0.0004 that is about ten times lower than coexisting fluid inclusions and seawater, and I/Cl of 2-44×10-6 that is 3-5 times lower than coexisting fluid inclusions but higher than seawater. The amphibole and fluid compositions are attributed to mixing halogens introduced by seawater with a large halogen component remobilised from mafic lithologies in the crust and fractionation of halogens between fluids and metamorphic amphibole formed at low water-rock ratios. The metamorphic amphibole and hydrothermal quartz are dominated by seawater-derived atmospheric Ne, Ar, Kr and Xe and mantle-derived He, with 3He/4He of ~9 R/Ra (Ra = atmospheric ratio). The amphibole and quartz preserve high 4He concentrations that are similar to MORB glasses, and have noble gas abundance ratios with high 4He/36Ar and 22Ne/36Ar that are greater than seawater and air. These characteristics result from the high solubility of light noble gases in amphibole and suggest that all noble gases behave similarly to ‘excess 40Ar’ in metamorphic hydrothermal root zones. All noble gases are therefore trapped in hydrous minerals to some extent and can be inefficiently lost during metamorphism implying that even the lightest noble gases (He and Ne) can potentially be subducted into the Earth’s mantle.
Serpentinites form by hydration of ultramafic lithologies in a range of seafloor and shallow subduction zone settings. Serpentinites are recognised as major reservoirs of fluid mobile elements and H2O in subducting oceanic lithosphere, and together with forearc serpentinites formed in the mantle wedge, provide critical information about shallow-level volatile fluxes during subduction. The current study provides new Cl, as well as the first comprehensive Br, I and noble gas analyses reported for seafloor and forearc chrysotile–lizardite serpentinites. The samples were recovered from IODP drilling campaigns of mid-ocean ridge, passive margin and forearc settings (n=17), and ophiolites in the Italian Alps and Apennines (n=10). The aims of this study were to determine the compositional variability of noble gases and halogens in serpentinites entering subduction zones and evaluate the efficiency of gas loss during the early stages of serpentinite subduction.
Volatiles are critically important in controlling the chemical and physical properties of the mantle. However, determining mantle volatile abundances via the preferred proxy of submarine volcanic glass can be hampered by seawater assimilation. This study shows how combined Cl, Br, I, K and H2O abundances can be used to unambiguously constrain the dominant mechanism by which melts assimilate seawater-derived components, and provide an improved method for determining mantle H2O and Cl abundances. We demonstrate that melts from the northwest part of the Lau Basin, the Galápagos Spreading Centre and melts from other locations previously shown to have anomalously high Cl contents, all assimilated excess Cl and H2O from ultra-saline brines with estimated salinities of 55 ± 15 wt.% salts. Assimilation probably occurs at depths of ˜3–6 km in the crust when seawater-derived fluids come into direct contact with deep magmas. In addition to their ultra-high salinity, the brines are characterised by K/Cl of <0.2, I/Cl of close to the seawater value (˜3 × 10‑6) and distinctive Br/Cl ratios of 3.7–3.9 × 10‑3, that are higher than both the seawater value of 3.5 × 10‑3 and the range of Br/Cl in 43 pristine E-MORB and OIB glasses that are considered representative of diverse mantle reservoirs [Br/Clmantle = (2.8 ± 0.6) × 10‑3 and I/Clmantle = (60 ± 30) × 10‑6 (2σ)]. The ultra-saline brines, with characteristically elevated Br/Cl ratios, are produced by a combination of fluid-rock reactions during crustal hydration and hydrothermal boiling. The relative importance of these processes is unknown; however, it is envisaged that a vapour phase will be boiled off when crustal fluids are heated to magmatic temperatures during assimilation. Furthermore, the ultra-high salinity of the residual brine that is assimilated may be partly determined by the relative solubilities of H2O and Cl in basaltic melts. The most contaminated glasses from the Galápagos Spreading Centre and Lau Basin have assimilated ˜95% of their total Cl and up to 35–40% of their total H2O, equivalent to the melts assimilating 1000–2000 ppm brine at an early stage of their evolution. Dacite glasses from Galapagos contain even higher concentrations of brine components (e.g. 12,000 ppm), but the H2O and Cl in these melts was probably concentrated by fractional crystallisation after assimilation. The Cl, Br, I and K data presented here confirm the proportion of seawater-derived volatiles assimilated by submarine magmas can vary from zero to nearly 100%, and that assimilation is closely related to hydrothermal activity. Assimilation of seawater components has previously been recognised as a possible source of atmospheric noble gases in basalt glasses. However, hydrothermal brines have metal and helium concentrations up to hundreds of times greater than seawater, and brine assimilation could also influence the helium isotope systematics of some submarine glasses.