Subduction-zone contributions to axial volcanism in the Oman–U.A.E. ophiolite

Over four decades of research on the Semail ophiolite (Oman–U.A.E.) has greatly in uenced our understanding of processes occurring at fast-spreading ocean ridges. While the well-developed sheeted dike complex and comagmatic lower pillow lavas indicate that the early Semail crust formed at a spreading axis, the precise tectonic setting of this axis—whether true mid-ocean ridge, back-arc or “proto”-arc— is contentious. This is largely because the tectonic implications of the geochemistry of the main axial volcanic unit (Geotimes/V1) are disputed. We bypass this hurdle by focusing on intercalations of primitive lavas that are depleted relative to mid-ocean-ridge basalt and that are deeply intercalated within the early Geotimes axial volcanostratigraphy throughout the northern ophiolite. Our analyses of these intercalations show a clear trace-element in uence from a subducting slab. We interpret the depleted axial melts to have formed by localized, high-degree partial melting assisted by a high-Th/Nb slab uid. These results con rm a deep subduction in uence on the entire axial spreading phase of the world’s largest ophiolite. Considered in the context of later hydrous and boninitic Alley volcanism and of insight from modern tectonic environments, our observations support a proto-arc, subduction-initiation setting for the origin of the Semail ophiolite

Inherited structural controls on fault geometry, architecture and hydrothermal activity: an example from Grimsel Pass, Switzerland

Exhumed faults hosting hydrothermal systems provide direct insight into relationships between faulting and fluid flow, which in turn are valuable for making hydrogeological predictions in blind settings. The Grimsel Breccia Fault (Aar massif, Central Swiss Alps) is a late Neogene, exhumed dextral strike-slip fault with a maximum displacement of 25–45 m, and is associated with both fossil and active hydrothermal circulation. We mapped the fault system and modelled it in three dimensions, using the distinctive hydrothermal mineralisation as well as active thermal fluid discharge (the highest elevation documented in the Alps) to reveal the structural controls on fluid pathway extent and morphology. With progressive uplift and cooling, brittle deformation inherited the mylonitic shear zone network at Grimsel Pass; preconditioning fault geometry into segmented brittle reactivations of ductile shear zones and brittle inter-shear zone linkages. We describe ‘pipe’-like, vertically oriented fluid pathways: (1) within brittle fault linkage zones and (2) through alongstrike- restricted segments of formerly ductile shear zones reactivated by brittle deformation. In both cases, low-permeability mylonitic shear zones that escaped brittle reactivation provide important hydraulic seals. These observations show that fluid flow along brittle fault planes is not planar, but rather highly channelised into sub-vertical flow domains, with important implications for the exploration and exploitation of geothermal energy

Paleobathymetry of Submarine Lavas in the Samail and Troodos Ophiolites: Insights From Volatiles in Glasses and Implications for Hydrothermal Systems

Hydrostatic pressure exerted by the ocean water column fundamentally influences magmatic and hydrothermal processes in submarine volcanic settings and is therefore an important parameter to know when investigating such processes. Currently, there are few reliable methods for reconstructing past ocean depths for ancient volcanic terranes. Here, we develop and test an empirically calibrated statistical approach for determining paleodepths of eruption from the concentrations of H2O and CO2 dissolved in volcanic glasses, utilizing the well-defined pressure-dependent solubility of these volatiles in silicate melts. By comparing newly determined and published glass compositions from the Samail and Troodos ophiolites with sedimentary and fluid inclusion evidence, we propose that the Samail lavas erupted at ocean depths of ∼3.4 km, and the Troodos lavas at ∼4.1 km. These depths are 1–2 km deeper than those assumed in most previous studies of hydrothermal activity in the two ophiolites. These high depths imply high hydrostatic pressures within the underlying oceanic crust. Such pressures may have allowed convecting hydrothermal fluids to attain significantly higher temperatures (e.g., >450°C) than in typical modern ocean ridge hydrothermal systems during metal leaching in the crust and metal precipitation in seafloor sulfide deposits.ISSN:2169-9313ISSN:0148-0227ISSN:2169-935

Paleobathymetry of submarine lavas in the Samail and Troodos ophiolites: Insights from volatiles in glasses and implications for hydrothermal systems

Hydrostatic pressure exerted by the ocean water column fundamentally influences magmatic and hydrothermal processes in submarine volcanic settings and is therefore an important parameter to know when investigating such processes. Currently, there are few reliable methods for reconstructing past ocean depths for ancient volcanic terranes. Here, we develop and test an empirically calibrated statistical approach for determining paleodepths of eruption from the concentrations of H 2O and CO 2 dissolved in volcanic glasses, utilizing the well-defined pressure-dependent solubility of these volatiles in silicate melts. By comparing newly determined and published glass compositions from the Samail and Troodos ophiolites with sedimentary and fluid inclusion evidence, we propose that the Samail lavas erupted at ocean depths of ∼3.4 km, and the Troodos lavas at ∼4.1 km. These depths are 1–2 km deeper than those assumed in most previous studies of hydrothermal activity in the two ophiolites. These high depths imply high hydrostatic pressures within the underlying oceanic crust. Such pressures may have allowed convecting hydrothermal fluids to attain significantly higher temperatures (e.g., &gt;450°C) than in typical modern ocean ridge hydrothermal systems during metal leaching in the crust and metal precipitation in seafloor sulfide deposits. </p

Paleobathymetry of submarine lavas in the Samail and Troodos ophiolites: Insights from volatiles in glasses and implications for hydrothermal systems

Hydrostatic pressure exerted by the ocean water column fundamentally influences magmatic and hydrothermal processes in submarine volcanic settings and is therefore an important parameter to know when investigating such processes. Currently, there are few reliable methods for reconstructing past ocean depths for ancient volcanic terranes. Here, we develop and test an empirically calibrated statistical approach for determining paleodepths of eruption from the concentrations of H 2O and CO 2 dissolved in volcanic glasses, utilizing the well-defined pressure-dependent solubility of these volatiles in silicate melts. By comparing newly determined and published glass compositions from the Samail and Troodos ophiolites with sedimentary and fluid inclusion evidence, we propose that the Samail lavas erupted at ocean depths of ∼3.4 km, and the Troodos lavas at ∼4.1 km. These depths are 1–2 km deeper than those assumed in most previous studies of hydrothermal activity in the two ophiolites. These high depths imply high hydrostatic pressures within the underlying oceanic crust. Such pressures may have allowed convecting hydrothermal fluids to attain significantly higher temperatures (e.g., &gt;450°C) than in typical modern ocean ridge hydrothermal systems during metal leaching in the crust and metal precipitation in seafloor sulfide deposits. </p

Determination of ultra-trace Au, Ag, As, Pt and Re mass fractions in volcanic glasses and rock powders by LA-ICP-MS

Practical methods for determining the ultra-trace abundances of precious metals in geological materials are needed for research into magmatic and hydrothermal processes and to expand the geochemical footprints of concealed ore deposits. This study presents a new protocol for determining Au, Ag, As, Pt and Re mass fractions in both volcanic glasses and in rock powders prepared as nano-powder pellets, through the synthesis and refinement of published LA-ICP-MS methods. This matrix flexibility allows the method and its limitations to be rigorously assessed for the first time using different volcanic materials. High-yield laser parameters, interference corrections and low oxide production rates facilitated by laser ablation sampling enabled accurate measurements without chemical pre-separation. A key finding is that ablation-remobilised system contamination must be quantified and corrected to make accurate ng g−1-level Au determinations by LA-ICP-MS, resulting in a mean + 2s quantification limit for Au of 0.38 ng g−1. This approach is likely necessary for other ultra-trace LA-ICP-MS analyses of certain elements. Following this correction, the protocol can be usefully applied to both in situ analysis of volcanic materials and efficiently integrated into methods for the determination of major and trace elements in nano-powder pellets

A revised map of volcanic units in the Oman ophiolite: insights into the architecture of an oceanic proto-arc volcanic sequence

Numerous studies have revealed genetic similarities between Tethyan ophiolites and oceanic “proto-arc” sequences formed above nascent subduction zones. The Semail ophiolite (Oman–U.A.E.) in particular can be viewed as an analogue for this proto-arc crust. Though proto-arc magmatism and the mechanisms of subduction initiation are of great interest, insight is difficult to gain from drilling and limited surface outcrops in marine settings. In contrast, the 3–5 km thick upper-crustal succession of the Semail ophiolite, which is exposed in an oblique cross section, presents an opportunity to assess the architecture and volumes of different volcanic rocks that form during the proto-arc stage. To determine the distribution of the volcanic rocks and to aid exploration for the volcanogenic massive sulfide (VMS) deposits that they host, we have remapped the volcanic units of the Semail ophiolite by integrating new field observations, geochemical analyses, and geophysical interpretations with pre-existing geological maps. By linking the major-element compositions of the volcanic units to rock magnetic properties, we were able to use aeromagnetic data to infer the extension of each outcropping unit below sedimentary cover, resulting in a new map showing 2100 km2 of upper-crustal bedrock. Whereas earlier maps distinguished two main volcanostratigraphic units, we have distinguished four, recording the progression from early spreading-axis basalts (Geotimes), through axial to off-axial depleted basalts (Lasail), to post-axial tholeiites (Tholeiitic Alley), and finally boninites (Boninitic Alley). Geotimes (“Phase 1”) axial dykes and lavas make up ∼55 vol % of the Semail upper crust, whereas post-axial (“Phase 2”) lavas constitute the remaining ∼45 vol % and ubiquitously cover the underlying axial crust. Highly depleted boninitic members of the Lasail unit locally occur within and directly atop the axial sequence, marking an earlier onset of boninitic magmatism than previously known for the ophiolite. The vast majority of the Semail boninites, however, belong to the Boninitic Alley unit and occur as discontinuous accumulations up to 2 km thick at the top of the ophiolite sequence and constitute ∼15 vol % of the upper crust. The new map provides a basis for targeted exploration of the gold-bearing VMS deposits hosted by these boninites. The thickest boninite accumulations occur in the Fizh block, where magma ascent occurred along crustal-scale faults that are connected to shear zones in the underlying mantle rocks, which in turn are associated with economic chromitite deposits. Locating major boninite feeder zones may thus be an indirect means to explore for chromitites in the underlying mantle

Major element and volatile compositons of volcanic glasses and related datasets for paleobathymetry of the Samail &amp;amp; Troodos ophiolites

This archived dataset accompanies the article &quot;Paleobathymetry of submarine lavas in the Samail and Troodos ophiolites: insights from volatiles in glasses and implications for hydrothermal systems&quot; in the Journal of Geophysical Research: Solid Earth (Belgrano et al. 2021). Comprehensive details on the aquisition and selection of this data are given in the accompanying article. This dataset archive consists of a multi-sheet Excel file containing: (1) Newly measured major element and H2O (&plusmn; CO2) compositions for volcanic glasses recovered from the Samail ophiolite, respectively determined by electron microprobe analysis (EMPA) and Fourier Transform infrared spectroscopy (FTIR). (2) The raw Beer-Lambert equation parameters as used to determine these H2O (&plusmn; CO2) compositions. (3) The H2O (&plusmn; CO2) compositions of volcanic glasses from the Troodos ophiolite reproduced from Woelki et al. (2020) together with newly calculated volatile saturation pressures and their depth equivalents. (4) A compilation of previously published volcanic glass data used to calibrate and test the paleobathymetric approach at the centre of the related publication. (5) A reference list for previously published data.</span

Geochemical compositions (XRF, LA-ICP-MS, EMPA) and bulk magnetic properties of volcanic rocks used for mapping in the northern Semail ophiolite (Oman)

Lava, dyke and volcanic glass samples from across the northern Semail ophiolite were collected to provide references for geological mapping of the Semail volcanic units. These samples were analysed by various geochemical means to assign them to a volcanic unit. In addition, the bulk rock magnetic properties of a subset of these samples were determined to aid interpretation of existing aeromagnetic data. The whole-rock major element and select trace element (Ba, Sr, Zr, Y, Zn, Cu, Ni, Cr, V, Sc) composition of the majority of these samples was determined by X-Ray fluorescence (XRF) at ETH Zürich using a PANalyticalTM Axios wavelength-dispersive instrument. Trace elements in a subset of these samples were further analysed by pressed-powder-pellet laser-ablation inductively-coupled plasma spectrometry (PPP-LA-ICP-MS) using a GeoLas-Pro 193 nm ArF Excimer™ laser system in combination with an ELAN DRC-e™ quadrupole mass spectrometer at the University of Bern. Where necessary for unit assignment, igneous clinopyroxenes in another subset of samples were measured by Electron microprobe (EMP) on a JeolTM JXA-8200 EMP at the University of Bern. A set of volcanic glasses were also analysed by EMP on the same instrument. Bulk magnetic susceptibility was determined by two methods: a handheld Exploranium KT-5 kappameter and a desktop Magnon kappameter at the Institute for Rock Magnetism (IRM), University of Minnesota. Duplicate analyses of the same samples with both instruments indicates good comparability between the two datasets. Natural remanent magnetization (NRM) was determined on a 2G Enterprises 760 RF TM SQUID superconducting rock magnetometer at the IRM