Halogens and noble gases in modern and ancient oceanic lithosphere: Implications for subduction budgets and evolution of Earth's surface reservoirs

Abstract

Noble gases and halogens are key tracers of volatile transport on Earth that can be incorporated into alteration minerals in the oceanic lithosphere and subducted to some degree back into Earth's mantle. The minerals and sites hosting these volatiles and the extent they are present in average oceanic lithosphere are subject to high uncertainties. Combined noble gases and halogen measurements were undertaken on samples of modern and ancient oceanic lithosphere, to study the incorporation and preservation of these volatiles in the oceanic lithosphere. The first part investigates the role of hydrothermal alteration in the repartition of noble gases and halogens from two complete transects through intact sections of fossil oceanic lithosphere preserved in the Oman ophiolite. Variability in Ar, Kr, Xe, Cl, Br and I concentrations in vein minerals from seven major vein systems and altered wall rocks increases from the upper lava to the upper mantle, but with no clear trends. The noble gas isotopic signatures of alteration minerals are dominated by an atmospheric component indicating formation by seawater-derived fluids. Halogens are incorporated in the mineral structure of amphibole, the main Cl reservoir, and serpentine, but significant variations in Cl concentrations at the micro-scale and depletion of halogens in serpentine veins, show a range of factors influence halogen uptake by these minerals. Epidote veins are ubiquitous and epidote rare earth element patterns variable in the crust, with hydrothermal signatures mostly developed in the lower crust. Titanite U-Pb dating at ~86(6) Ma, at least 2 Ma after crustal accretion, confirms some of the epidote veins formed off-axis. Halogens in quartz, epidote, zoisite/clinozoisite, diopside and prehnite veins with iodine concentrations of up to 290 ng/g, reside primarily in fluid inclusions. The halogen composition of the fluid phase is explained by two-component mixing, between a hydrothermal brine component, similar to that identified in previous studies, and a fluid with I/Cl of up to ~0.024 and Br/Cl of ~0.001, significantly enriched in I/Cl and depleted in Br/Cl relative to seawater. The Oman fluid can be explained by the introduction of a subducted slab component in the hydrothermal system, suggesting halogens might support the supra-subduction zone origin of the Oman ophiolite. In a second part, well-preserved antigorite-serpentinites and talc-magnesite schists sampled in a low strain zone of the Eoarchean Isua supracrustal belt were investigated to test the retention of paleo-atmospheric noble gases and Eoarchean seawater halogens. The lithologies formed by progressive alteration of olivine-rich magmatic cumulates that developed at the base of a lava flow erupted on the Eoarchean seafloor. U-Pb geochronology and trace element concentrations of rare zircons extracted from one of the samples record a crystallisation age of 3721(27) Ma. The serpentinites have high concentrations of noble gases, but parentless 'excess' 40Ar, introduced by crustal-derived metamorphic fluids, obscures the initial Ar isotopes of Eoarchean seawater. The least carbonated antigorite serpentinites preserve Br/Cl and I/Cl ratios within the range of modern serpentinites implying that Archean serpentinising fluids were similar in composition to modern day seawater-derived fluids. Importantly, the lowest I/Cl weight ratio of 0.000029, interpreted as a maximum value for the Eoarchean Ocean, is an order of magnitude lower than estimated for the primitive mantle. Iodine in the modern ocean is sequestered by organic matter and has low concentration relative to Cl, so that low I/Cl in Eoarchean seawater suggests similar I-sequestration was already occurring, which favours the presence of abundant life in Earth's early oceans. The evolution of seawater I/Cl on the early Earth might be traceable via seafloor serpentinites and is a novel new proxy with which to explore the emergence and evolution of life.