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Author(s): Patricia L. Clay (corresponding author) [1]; Ray Burgess [1]; Henner Busemann [2]; Lorraine Ruzi-Hamilton [1]; Bastian Joachim [3]; James M. D. Day [4]; Christopher J. Ballentine [5] Volatile element [...]

Recycling of material at subduction zones has fundamental implications for melt composition and mantle rheology. Ocean island basalts (OIBs) sample parts of the mantle from variable depths that have been diversely affected by subduction zone processes and materials, including the subducted slab, metasomatising melts and fluids. Resultant geochemical differences are preserved at a variety of scales from melt inclusions to whole rocks, from individual islands to chains of islands. Here we examine a global dataset of ocean island basalt compositions with a view to understanding the connection between silica-saturation, olivine compositions, and halogens in glass and olivine-hosted melt inclusions to reveal information regarding the mantle sources of intraplate magmatism. We find that minor elements incorporated into olivine, although informative, cannot unambiguously discriminate between different source contributions, but indicate that none of the OIB analysed here are derived solely from dry peridotite melting. Nor can differences in lithospheric thickness explain trace element variability in olivine between different ocean islands. We present new halogen (F, Cl, Br/Cl, I/Cl) data along with incompatible trace element data for the global array and encourage measurement of fluorine along with heavier halogens to obtain better insight into halogen cycling. We suggest that Ti-rich silica-undersaturated melts require a contribution from carbonated lithosphere, either peridotite or eclogite and are an important component sampled by ocean island basalts, together with altered oceanic crust. These results provide new insights into our understanding of mantle-scale geochemical cycles, and also lead to the potential for the mantle transition zone as an underestimated source for observed volatile and trace-element enrichment in ocean island basalts. [ABSTRACT FROM AUTHOR]

Understanding the growth rate of the continental crust through time is a fundamental issue in Earth sciences (1-8). The isotopic signatures of noble gases in the silicate Earth (mantle, crust) [...]

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 <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. [ABSTRACT FROM AUTHOR]

Hydrothermal alteration of oceanic crust represents a globally significant geochemical exchange between the crust and oceans, introducing volatile elements into the oceanic lithosphere and removing latent heat from crystallization. The extent to which hydrothermal systems penetrate the lower oceanic crust is debated and is significant for competing models of lower oceanic crust formation. In this study, we apply halogen and noble gas geochemistry in order to better understand the fluid sources and conditions during a hydrothermal alteration in a suite of gabbros sampled from drill cores in the mid‐ (hole GT3A) and lower‐ (hole GT1A) crust of the Oman Ophiolite. Low Br/Cl in mid‐crustal samples is controlled by amphibole formed at relatively high fluid/rock ratios. Lower crustal samples, meanwhile, have Br/Cl ranging from seawater to much higher values (<5 × 10−3). These require alteration at highly variable fluid/rock ratios from ∼1 to <0.01 in the lower crust. Noble gas isotopes also suggest alteration at relatively high and low fluid/rock ratios in the mid‐ and lower‐crust, respectively. Indicators of alteration and proxies for fluid/rock ratios (loss on ignition, Pb/Ce, Br/Cl, 40Ar/36Ar96Ma) vary systematically with distance from major fault zones in the lower crust with the strongest correlation in the least altered minerals. Together with age constraints derived from K‐Ar decay, these data indicate faults were major conduits, supplying seawater to the lower crust roughly contemporaneous with the formation of the ophiolite, and commencing at the magmatic stage. These results suggest that hydrothermal alteration of the lower crust is primarily controlled by major fault structures. Plain Language Summary: After a new oceanic crust is formed by volcanic activity at mid‐ocean ridges, it cools and reacts with circulating seawater. However, there is controversy over whether water penetrates to the lower oceanic crust, or whether this is cooled by conduction alone. This is important for theories of how the lower oceanic crust is formed. Investigating the lower crust is difficult since few samples have been recovered by deep seafloor drilling. However, ophiolites–sections of oceanic crust, uplifted onto continental margins during tectonic collisions–offer a potential window into processes in the deep oceanic crust. In this study, we use halogens and noble gases as chemical fingerprints for the origin of water in rocks from the Oman Ophiolite. We show that rocks from the mid‐oceanic crust and from the lower‐oceanic crust interacted with water under markedly different conditions and that the supply of water to the lower crust was very variable. Rocks close to large fractures cutting through the lower crust were altered by seawater ∼96 Myr ago. Further from these fractures, compositions are increasingly different from seawater and water was scarce. This provides important evidence that large fractures delivered water to the lower crust and may be important for cooling new oceanic crust globally. Key Points: Gabbros from the Oman Ophiolite record hydrothermal circulation in both the upper and lower oceanic crustHalogen and noble gas systematics suggest alteration under high and low fluid/rock ratiosFluid supply to the lower crust is variable and controlled by major fault zones commencing at the magmatic stage [ABSTRACT FROM AUTHOR]