Ultramafic Research Articles

A Seismic Tomography, Gravity, and Flexure Study of the Crust and Upper Mantle Structure of the Hawaiian Ridge: 1

AbstractThe Hawaiian Ridge has long been a focus site for studying lithospheric flexure due to intraplate volcano loading, but crucial load and flexure details remain unclear. We address this problem using wide‐angle seismic refraction and reflection data acquired along a ∼535‐km‐long profile that intersects the ridge between the islands of Maui and Hawai'i and crosses 80–95 Myr‐old lithosphere. A tomographic image constructed using travel time data of several seismic phases reveals broad flexure of Pacific oceanic crust extending up to ∼200–250 km either side of the Hawaiian Ridge, and vertically up to ∼6–7 km. The P‐wave velocity structure, verified by gravity modeling, reveals that the west flank of Hawaii is comprised of extrusive lavas overlain by volcanoclastic sediments and a carbonate platform. In contrast, the Hāna Ridge, southeast of Maui, contains a high‐velocity core consistent with mafic or ultramafic intrusive rocks. Magmatic underplating along the seismic line is not evident. Reflectors at the top and bottom of the pre‐existing oceanic crust suggest a ∼4.5–6 km crustal thickness. Simple three‐dimensional flexure modeling with an elastic plate thickness, Te, of 26.7 km shows that the depths to the reflectors beneath the western flank of Hawai'i can be explained by volcano loading in which Maui and the older islands in the ridge contribute ∼43% to the flexure and the island of Hawai'i ∼51%. Previous studies, however, revealed a higher Te beneath the eastern flank of Hawai'i suggesting that isostatic compensation may not yet be complete at the youngest end of the ridge.

Dynamic recrystallization study of the ultramafic rock of the Malili-Wasuponda mélange complex (South Sulawesi) and Wolo cape (Southeast Sulawesi), Indonesia

The ultramafic rock of the Wasuponda mélange complex and the Wolo cape areas, Sulawesi classified as lhezolite, harzburgite, dunite, olivine websterite, wherlite and serpentinite. The rocks have undergone dynamic recrystallization through bulging, subgrain rotation and grain boundary migration. The olivine neoblast grain size varies from a very fine size to a mean size of around 75 μm and a mean size range of 150, 225, 300 and 425 μm. The neoblast grain size indicates that dynamic recrystallization took place with various stress at about 13 MPa to 52 MPa. The optimum recrystallization of the Malili-Wasuponda peridotite occurs at 52 MPa, while rock peridotite of the Wolo cape formed at optimum stress reached 30-32 MPa.

Airborne geophysics, petrophysics, geochemistry and geology of Ni-Cu (Co-Au-PGE) footprints to define exploration targets in mafic-ultramafic rocks

The correlation between various prospecting methodologies plays a pivotal role in achieving substantial outcomes in the pursuit of mineral deposits. The use of these methodologies within prospecting programs is widely implemented across diverse geological settings. The purpose of this research was to develop a prospective model for Ni–Cu–Co–Au-PGE (Nickel–Copper–Cobalt–Gold–Platinum Group Elements) within a mafic-ultramafic intrusion known as the Barra do Gameleira Intrusion (BGI), located in Tocantins, Brazil. To achieve it, publicly available data were employed, including airborne geophysics and geochemical data obtained from stream sediments and pan concentrate. Additionally, newly obtained, and unpublished results from lithogeochemical sampling and semi-detailed geological mapping were used. According to what was found, three potential targets were identified based on geophysical and geochemical anomalies. The magnetic results proved a robust correlation between magnetization intensity and anomalous elemental contents in geochemistry. Radiometric data were employed to see the inverse correlation between the concentration of radioelements and geochemical anomalies, particularly with respect to potassium, which exhibited null values in petrophysical measurements. Furthermore, both anomalous potassium and anomalous uranium enhancements showed a strong correlation with the presence of ultramafic rocks. Finally, the geological mapping confirmed that mineralization occurrences are associated with lithologies composed of ultramafic rocks.

Eoarchean ultramafic rocks represent crustal cumulates: A case study of the Narssaq ultramafic body, southern West Greenland

Several ultramafic enclaves found within the Eoarchean Itsaq Gneiss Complex (IGC) in southern West Greenland have previously been interpreted as representing mantle relicts. However, as Archean ultramafic rocks are frequently overprinted by metamorphism and late-stage metasomatism, it can be difficult to distinguish between peridotites representing mantle residues or primitive crustal cumulates. Therefore, detailed evaluation of individual Eoarchean peridotite occurrences is required to better constrain their origin. Here, we present new petrological observations and geochemical data for the >3.8 Ga Narssaq Ultramafic Body (NUB) in the IGC of SW Greenland. The NUB ultramafic rocks have high FeOt and Cr contents with flat chondrite-normalized trace element patterns and positive Cr-Mg correlations, which distinguish them from mantle rocks. In addition, the variations in olivine and spinel compositions are consistent with a fractional crystallization process. The above geochemical features suggest that the ultramafic rocks of NUB cannot be explained as mantle residues. Instead, their major element compositions are related to the spatially associated tholeiitic amphibolites, indicating that these ultramafic rocks likely represent crustal cumulates derived from high-Mg magmas. This interpretation is supported by thermodynamic modeling using MELTS, which shows that the compositions of the ultramafic rocks in the NUB can be reproduced via fractional crystallization of local high-Mg tholeiitic basalts, involving the accumulation of olivine, spinel, and possibly clinopyroxene, in conjunction with trapped melt. The NUB ultramafic rocks can thus form as a natural consequence of fractional crystallization of regular highly magnesian tholeiitic magmas, making a cumulate origin the simplest explanation for Eoarchean peridotites in general. This also implies that the formation of the NUB did not involve refertilization of subduction-like components as previously assumed, and thus did not require subduction to have taken place in the Eoarchean.

Geospatial Insights into Ophiolitic Complexes in the Cimmerian Realm of the Afghan Central Block (Middle Afghanistan)

Ophiolites are remnants of oceanic crust that have been thrust onto continental crust due to tectonic processes. They are composed of mostly mafic and ultramafic rocks, which are genetically associated with gold, silver, platinum group element (PGE), chrome, manganese, titanium, cobalt, copper, and nickel deposits. The main objective of this research was to identify the spatial distribution of Mesozoic ophiolitic complexes within the Central Afghan Block in Middle Afghanistan using optical remote sensing data and spectral analyses. Distinct algorithms, such as false color composite (FCC), proposed band ratios (PBR), principal component analysis (PCA), and spectral angle mapper (SAM), were used to map the targeted ophiolitic complexes. New band ratios were proposed in this study based on the spectral properties of mafic-ultramafic minerals and rocks, which showed high efficiency. Based on the results, four different ophiolitic complexes were delineated within this study area. These complexes are consistent with previous studies. The accuracy assessment of this study showed an overall accuracy of 72.2%. The findings of this study can significantly contribute to further studies on the emplacement mechanism and paleo-Tethys history of Middle Afghanistan. Also, the spatial distribution of the ophiolitic complexes identified in this study can be used to constrain models of the tectonic evolution of the Central Afghan Block. Additionally, the identification of new band ratios for mapping ophiolitic complexes can be used in future studies of other ophiolite-bearing regions.

On the methodology of microstructural study of ultramafic rocks by electrone back scattered difraction (EBSD)

At the example of ultramafic rocks and chromitites of the Urals, the paper describes the possibilities of microstructural study of geological samples using one of the modern microstructural methods - electron backscattered diffraction (EBSD). This method has a number of advantages compared to previously used optical methods using a universal stage. The main advantages of the method: high accuracy, automation, objectivity, wide range of materials studied. However, there are some difficulties, which consist mainly in the susceptibility of geological materials to secondary changes and superimposed metamorphic processes. The article discusses various possibilities for processing primary materials and obtaining representative data on the microstructure of samples of ultramafic rocks and chromitites from various massifs of the Urals.

Characterization of Talcose Rocks Derived from Serpentinite—Pilot Study in the Ultramafic Complex of Cabo Ortegal (Spain) for Potential Use and Commercialization

Cabo Ortegal, in Spain, was declared a UNESCO Global Geopark in 2023. An ultramafic complex makes it a geologically exciting unit, where many research works have been carried out. Serpentinites, formed by the weathering of peridotites, dunites, and other ultramafic rocks, have been described in detail in previous works. Their role as a construction material has been elucidated, although due to their complexity and heterogeneity, the results of using these materials for construction are not always successful. Talcose rocks, related to serpentinites, can also be found in the complex, although it is difficult to distinguish them. Talcose rocks may be a resource to be explored and exploited in an unpopulated area. The use of talc in pharmaceuticals, cosmetics, and handicrafts makes it a material of growing importance. However, the asbestos and heavy metal content should be severely limited to prevent health issues. The goal of this work is to start the characterization of these talcose rocks before promoting their potential use in different contexts to support the economy of an underpopulated area of Spain.

Active near-surface mobilisation of slab-derived geochemical signatures by hyperalkaline waters in brecciated serpentinites

Unusual hyperalkaline meteoric groundwaters on Troodos massif, Cyprus, issue from highly deformed and completely serpentinized ultramafic brecciated rock masses of the Artemis Diapir and have high salinity (25–30% seawater total dissolved solids) and some of the highest recorded pH values (11−13) for natural waters. These waters have elevated dissolved Na, K, Li, B, Ba, Rb, Cs, Cl and SO4 with ion/chloride substantially above seawater ratios, overprinting minor contributions from marine aerosols. For example, K concentrations are similar to seawater values. Water stable isotope ratios imply extensive water-rock interaction and 87Sr/86Sr is rock-dominated (0.705) and significantly lower than seawater. These rare fluids contrast with surface and shallow ground waters draining the ultramafic Troodos Mantle Sequence that have pH of 8.5–9, meteoric stable isotope ratios, 87Sr/86Sr similar to early Miocene seawater (0.7085) and low dissolved salts albeit with ion/chloride ratios also greater than seawater. The combination of high pH and salinity is unusual in ophiolites and these hyperalkaline waters are most similar to end-member fluids emanating from serpentine mud volcanoes in the Mariana forearc. Rainwater rapidly transiting terraces of asbestos mine tailings on Troodos mostly resemble the surface waters but show slight contamination by a saline component. We propose that inclusions within the serpentinite that contain highly soluble salts have been made accessible by tectonic deformation during uplift and diapirism, or comminution during mining activity. A multistage process is proposed whereby alkali and other signatures have been released from the stalled subducting slab beneath the Troodos massif and have infused the mantle wedge. Some of these rocks have been carried to the surface by diapiric uplift and erosion, where tectonic deformation or mining activities have enabled the near-surface mobilisation of slab-derived geochemical signatures by modern meteoric waters.

Theoretical evaluation on CO2 removal potential of enhanced weathering based on shrinking core model

The discrepancy between current CO2 emission trend and the targeted 1.5 °C warming requires the implementation of carbon dioxide removal (CDR) technologies. Among the engineered CDRs, enhanced weathering (EW) is expected to exhibit substantial potential for CO2 removal, owing to the availability of abundant reserves of ultramafic rocks and demonstration of worldwide liming practice. While the shrinking core model (SCM) has been commonly adopted in previous theoretical and experimental studies, there still lacks a comprehensive assessment on the impacts of model parameters, such as rock particle size, size distribution, weathering rate and time length on the weathering kinetics and the resultant CDR potential. Herein, this study incorporates particle size distribution of rock powder into the surface reaction-controlled SCM, and conducts sensitivity analysis on EW’s CDR potential quantitatively. Even fully powered by low-carbon energy in the optimistic case, the application of EW with olivine only achieves maximum CDR per unit of rock and energy consumption of 0.01 kg CO2 per kg rock and 19 g per kWh at size of 8 and 22 μm respectively, indicating the limitations of EW. The derived optimal application parameters with olivine powers within 3.7–79 μm provide valuable insights into the practical real-world applications to achieve net CO2 removal.

182W and 187Os constraints on the origin of siderophile isotopic heterogeneity in the mantle

Major and trace element abundances, including highly siderophile elements, and 187Os and 182W isotopic compositions were determined for ca. 89 Ma mafic and ultramafic rocks from the islands of Gorgona (Colombia) and Curaçao (Dutch Caribbean). The volcanic systems of both islands were likely associated with a mantle plume that generated the Caribbean Large Igneous Provence. The major and lithophile trace element characteristics of the rocks examined are consistent with the results of prior studies, and indicate derivation from both a chemically highly-depleted mantle component, and an enriched, or less highly-depleted mantle component. Highly siderophile element abundances for these rocks are generally similar to rocks with comparable MgO globally, indicating that the major source components were not substantially enriched or depleted in these elements. Rhenium-Os isotopic systematics of most rocks of both islands indicate derivation from a mantle source with an initial 187Os/188Os ratio between that of the contemporaneous average depleted mid-ocean ridge mantle and bulk silicate Earth. The composition may reflect either an average lower mantle signature, or global-scale Os isotopic heterogeneity in the upper mantle. Some of the basalts, as well as two of the komatiites, are characterized by calculated initial 187Os/188Os ratios 10–15 % higher than the chondritic reference. These more radiogenic Os isotopic compositions do not correlate with major or trace element systematics, and indicate a mantle source component that was most likely produced by either sulfide metasomatism or ancient Re/Os fractionation. Tungsten-182 isotopic compositions measured for rocks from both islands are characterized by variable μ182W values ranging from modern bulk silicate Earth-like to strongly negative values. The μ182W values do not correlate with major/trace element abundances or initial 187Os/188Os compositions. As with some modern ocean island basalt systems, however, the lowest μ182W value (−53) measured, for a Gorgona olivine gabbro, corresponds with the highest 3He/4He previously measured from the suite (15.8 R/RA). Given the lack of correlation with other chemical/isotopic compositions, the mantle component characterized by negative μ182W and possibly high 3He/4He is most parsimoniously explained to have formed as a result of isotopic equilibration between the mantle and core at the core-mantle boundary.

Microstructural Relationship between Olivine and Clinopyroxene in Ultramafic Rocks from the Red Hills Massif, Dun Mountain Ophiolite

The microstructural relationship between olivine and clinopyroxene is significant in recovering the mantle evolution under clinopyroxene-saturated melting conditions. This study focuses on olivine/clinopyroxene-related ultramafic rocks (dunite, wehrlite, olivine clinopyroxenite, and clinopyroxenite) in the Ells Stream Complex of the Red Hills Massif. (Olivine) clinopyroxenites have an A/D-type olivine crystallographic preferred orientation (CPO) whereas peridotites have various olivine CPO types. B-type olivine CPO was newly discovered, which may have been generated under hydrous conditions. The discovery of B-type CPO means that all six olivine CPO types could exist in a single research area. Clinopyroxene CPOs also vary and have weaker deformation characteristics (e.g., lower M index and weaker intracrystalline deformation) than olivine; thus, they probably melted and the clinopyroxene-rich ultramafic bands existed as melt veins. Irregular clinopyroxene shapes in the peridotites and incoherent olivine and clinopyroxene CPOs ([100]OL and [001]CPX are not parallel) also indicate a melted state. The dominant orthorhombic and LS-type CPOs in olivine and clinopyroxene imply that simple shear was the main deformation mechanism. Such complicated microstructural characteristics result from the overprinted simple shear under high temperatures (>1000 °C) and hydrous melting environments until the melt-frozen period. This case study is helpful to better understand the olivine and clinopyroxene relationship.

Are Placer Platinum Nuggets Formed During Lateritization? The Verdict from the Owendale Alaskan-Uralian Complex in Australia Is an Emphatic No!

Abstract The thick laterite developed over platinum group element (PGE)-bearing ultramafic rocks of the Owendale Alaskan-Uralian Complex in New South Wales, Australia, provides an ideal environment in which to address the question of whether Pt-Fe nuggets are formed during lateritization. This is an important issue to settle because Pt-Fe nuggets in alluvial placers and eluvial deposits associated with the Alaskan-Uralian complexes were the world’s major source of Pt prior to the commencement of Pt production from the Bushveld Complex and continue to produce a significant amount of Pt. Some of the Owendale laterites have high Pt but low Cu contents, while others have both high Pt and high Cu contents. Heavy mineral concentrates were prepared from about 1 kg of drill chips from both types of laterites. Only five of the 61 samples processed contained any platinum group minerals (PGMs) greater than 5 μm in size, even though many of the samples contained more than 1 g/t Pt. The largest PGM found was about 100 μm long, and the majority were <20 μm. The bulk of the PGMs recovered were zoned PGMs consisting of a core of isoferroplatinum mantled by tetraferroplatinum with an outer rim of tulameenite; many of these zoned PGMs are encased in hematite grains that often have high Pt contents and appear to be pseudomorphs after the PGMs. The textural evidence indicates that at least half of the primary PGMs in the ultramafic protoliths to the laterites were destroyed during weathering and that the liberated PGEs could have been available for the formation of PGM nuggets. However, despite the large amount of PGEs liberated during the destruction of the primary PGMs, no evidence was found for the neogenic growth of PGE nuggets. Rather, the Pt liberated during the destruction of the PGMs appears to have only traveled distances of micrometers to tens of centimeters to form Pt nanoparticles or Pt oxides or to be absorbed/adsorbed by the Fe oxide hosts.

Analysis of pigments and mortars from the wall paintings of the Roman archaeological site of Las Dunas (San Pedro de Alcántara, Malaga S Spain)

This study aims to describe the composition, texture, and arrangement of mortars and pictorial layers from a newly excavated Roman archaeological site in the province of Baetica, southern Spain. Representative fragments with well-preserved pictorial layers were analyzed using optical imaging, spectrophotometry, Scanning Electron Microscopy with Energy-Dispersive Spectroscopy (SEM-EDS) mapping, and X-ray diffraction. The results reveal similarities to other well-studied Roman archaeological sites, particularly in the use of mixtures of natural (hematite, goethite, aragonite, phyllosilicates, and quartz) and synthetic minerals (cuprorivaite) used in classical Roman pigments. However, the mortar from the investigated site shows a distinctive use of ferromagnesian silicates (olivine and pyroxenes from local ultramafic rocks) as aggregate material, making this site unique among all Roman provinces. Additionally, the common occurrence of Mg in the binder of the mortars under the form of brucite, hydromagnesite and magnesium silicate hydrates, indicates the use of a magnesian (dolomitic) lime that, according to the chemical analyses, was obtained by adding up to 15 wt% of powdered dolomitic marbles from nearby Roman quarries to the limestone. We speculate that this addition might be intended to improve the workability and mechanical resistances of the mortar in combination with the abundant ferromagnesian silicates used as aggregates, some of them showing chemically reactive rims in contact with the binder. In conclusion, this study provides further evidence of the advanced knowledge and skills achieved by Roman craftsmen who were able to adapt traditional recipes to suit local resources, obtaining materials of excellent quality and durability.

Geochemistry, mineral chemistry and Re–Os isotopes of refractory peridotites of the North Balkhash ophiolite zone in the West Central Asian Orogenic Belt (Central Kazakhstan): multi-stage melt evolution of a late Precambrian forearc mantle

In this study we report on the petrography, major and trace element and mineral chemistry, platinum-group elements and Re–Os isotope systematics of depleted ultramafic rock suites from the Itmurundy Block in the North Balkhash ophiolite zone in Kazakhstan. Represented mainly by variably serpentinized harzburgites and dunites, our samples are characterized by low whole-rock Al 2 O 3 (0.33–0.86 wt%), CaO (0.51–0.86 wt%) and Na 2 O (0.07–0.25 wt%) concentrations, and high-Mg olivine (Fo = 91–92) and orthopyroxene (Mg# = 92–93) contents, together with moderately high spinel Cr-numbers (Cr# = 63–68). They are depleted in incompatible elements ( Σ REE, Nb, Sc) and enriched in compatible elements, such as Cr (up to 2817 ppm) and Ni (up to 2327 ppm), representing highly refractory mantle residues derived from a forearc mantle wedge. They underwent 19–23% hydrous partial melting to produce boninitic melts. 187 Os/ 188 Os values vary from 0.1202 to 0.12599, and 187 Re/ 188 Os ratios from 0.230 to 0.316. The Re–Os model ages (T MA ) and maximum Re depletion model age (T RD ) were calculated based on the obtained data. Re–Os isotope systematics suggests that the analysed peridotites formed in two stages: a first stage around 1.5 Ga and a later stage around 668–589 Ma. Supplementary material: A supplementary figure is available at https://doi.org/10.6084/m9.figshare.c.6845683 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Amphibole peridotite and hornblendite of the Schrisheimite Hill intrusion of the Revdinskiy massif (Ural Platinum Belt)

Research subject. We studied the concentric-zonal massif (≈400 × 400 m) of amphibole-rich ultramafic rocks composed of amphibole peridotite (schriesheimite and amphibole wehrlite) with olivinite schlieren, pyroxenite and hornblendite located among gabbro at the Schrisheimite Hill in the Revdinskiy massif of the Ural Platinum Belt. The scope is characteristic of both of amphibole peridotites, which are extremely rare in the massifs of the Ural Platinum Belt, and the associated amphibole-rich ultramafic rocks. Aim. To expand the understanding of aqueous ultramafic magmatism in Ural Platinum Belt massifs. Materials and methods. To determine the age, we selected monofractions of high-alumina amphibole (pargasite-magnesiohastingsite) from two samples. Amphibole is not significantly altered by the regional metamorphism. 40Ar/39Ar dating was carried out according to the method described in (Travin et al., 2009; Yudin et al., 2021). The argon isotope composition was measured using a Micromass 5400 mass spectrometer (IGM SB RAS). Results. Amphibole peridotite has a hypidiomorphic (subhedral) or poikilitic texture. It is typical of igneous rocks, which indicates the crystallization of amphibole peridotite was by H2O-saturated magmas. Schlieren segregations of fine-grained schriesheimites and fine-grained olivinites are among coarse-grained schriesheimites. Amphibole peridotite, pyroxenite and hornblendite are deficient in high field strength elements (HFSE) and enriched in large ion lithophile elements (LILE) (with contrast concentration of Cs and Rb), which is typical of supra-subduction igneous formations. We obtained a 40Ar/39Ar age of 437.2 ± 6.7 Ma for an amphibole from hornblendite, which differs significantly from the ages of hornblendite determined earlier. Conclusions. Our new data indicate that hornblendite dikes in different UPB massifs formed over a fairly long time interval from the Early Silurian to the Early Devonian. These data expand modern ideas about the timing and nature of ultramafic water magmatism in the massifs of the Ural Platinum Belt.

Mineralogy and Conditions of Formation Genesis of Aggregates of Natural and Sulfide Minerals of the Poldnevskoe Demantoid Deposit (Middle Urals)

Polymineral aggregates of rounded shapes (“nodules”) composed of native and sulfide minerals of Cu, Ni, Fe, Ag, and other elements from vein magnetite–calcite–chrysotile rocks with jewelry demantoid in the Korkodinskoe hypermafic massif are described. A common feature of the six identified types of native sulfide nodules, composed of native copper, heazlewoodite, pentlandite, cuprite, and other native sulfide minerals, is their spheroidal shape, which makes them similar to individual grains of other gangue minerals (calcite, magnetite, etc.). In heazlewoodite–pentlandite nodules, specific symplectites of mercuric silver and nickel copper in heazlewoodite, as well as awaruite in Co–pentlandite, were found. The matching set of ore minerals in the host serpentinite vein mass (native copper, mercuric silver, heazlewoodite, pentlandite, awaruite) and nodules from the vein material indicates their genetic connection and the conjugation of demantoid mineralization with the evolving processes of serpentinization. It was established that the nodules formed at temperatures below 380°C under reducing conditions at very low sulfur fugacity values (10–17–10–27 bar) and oxygen (10–30 bar at 200°C to 10–21 bar at 350°C). For heazlewoodite–pentlandite nodules, such conditions persisted throughout the entire time of their formation, while, for other nodules, the reducing conditions of early parageneses were replaced by oxidative conditions in late parageneses, which is recorded by the replacement of native copper with cuprite. It is assumed that the features of the morphology and structure of native sulfide nodules and the presence of symplectite intergrowths of ore minerals in them are associated with specific conditions created during the decompression of the crust-mantle mixture rising to the surface in the fault zone. The source of the metals was a deep, high-temperature fluid interacting with mafic and ultramafic rocks under reducing conditions at a low water-to-rock ratio.

Effects of hydrothermal alteration on shear localization and weakening in the mantle lithosphere

Hydrothermal alteration occurs in upper-mantle fault zones and significantly modifies the rheological properties of ultramafic rocks. To understand how hydrothermal alteration affects shear localization and the strength of the fault zones, we conducted simple-shear deformation experiments on water-saturated harzburgite (70 wt% olivine +30 wt% orthopyroxene) gouges sandwiched between three types of shear pistons (tungsten, yttria-stabilized zirconia, and corundum) using two deformation-DIA apparatuses at confining pressures of 1–4 GPa, temperatures of 500–580 °C, average shear strain rates of 6.1 × 10−6 s−1 to 1.0 × 10−4 s−1, and water contents of 4–30 wt%. The harzburgitic gouges with water contents of 10 and 30 wt% exhibited steady-state sliding or strain-hardening behavior, and the shear strength of the latter sample was less than one-third of that of the former sample, possibly reflecting the difference in pore fluid pressure. Olivine/orthopyroxene grains exhibit distributed microfracturing and grain rotation towards P foliation, indicating the operation of cataclastic flow. Shear displacement is also accommodated by the development of serpentine- or talc-bearing multiple shear zones including B, R1, and/or Y shears, where intense cataclasis-related grain size reduction and dissolution/precipitation occur. For experimental runs using tungsten and zirconia shear pistons, the serpentine or talc blades that wrap finer-grained and dissolved olivine/orthopyroxene grains form an interconnected network, indicative of frictional sliding or dislocation glide on their basal planes as a dominant deformation mechanism. Raman spectra of the serpentine minerals indicate that they represent antigorite with low crystallinity. These findings suggest that when deep lithospheric fault zones undergo hydrothermal alteration, deformation is controlled by competition between cataclastic flow in olivine/orthopyroxene matrix and slip or glide of antigorite or talc in semi-brittle fault zones, the predominance of which depends on the amount of the hydrous phyllosilicates and the magnitude of pore fluid pressure.

Re-Os geochronology of the Middle to Upper Jurassic marine black shales in the Agardhfjellet Formation, Central Spitsbergen, Svalbard: A cornerstone for global faunal correlation and Os isotopic change

Ammonite provincialism makes it difficult to link the Boreal and Tethyan realms during the Late Jurassic to Early Cretaceous time. Absolute ages through radiometric dating offer new age information that is independent and complementary to relative ages based on biostratigraphy. In this study, we report seven new Re-Os ages for black shales from the Agardhfjellet Formation, Svalbard. We also report pyrolysis results, carbon and sulfur stable isotopic data, and initial 187Os/188Os ratios to evaluate the paleoenvironment during organic-rich shale deposition. Re-Os ages are derived from three Boreal ammonite zones: (1) 159.2 and 160.1 Ma from the shale intervals containing Cadoceras sp.; (2) 149.6, 149.9, 150.8, and 151.9 Ma from the Rasenia cymodoce Zone in the Early Kimmeridgian, which is the equivalent of the Sutneria platynota and Ataxioceras hypselocyclum zones in the Tethyan realm; (3) 146.8 Ma from an upper Volgian interval above the occurrence of Laugeites sp.. Within this 14.5 Myr interval, the initial Os profile shows a gradually increasing trend from 0.335 (one of the lowest shale values throughout the Phanerozoic) to 0.529, consistent with previous initial Os studies and Sr isotopic ratio chemostratigraphic studies. The trend of increasing initial Os ratios indicates an increase in continental runoff relative to unradiogenic Os input from mantle sources, such as seafloor hydrothermal activity or ultramafic source rocks. We provide new Re-Os geochronological anchors in the Late Jurassic with new Re-Os ages and propose correlations of specific Late Jurassic ammonite zones between the Tethyan and Boreal realms. In turn, the gradual increase in seawater Os isotopic ratios may signal climate change in the Late Jurassic, indicating the increase of radiogenic Os or decrease of unradiogenic Os, induced by high continental runoff or less hydrothermal/volcanic activity.

The Neoarchean mafic-ultramafic São José do Jacuípe Suite in the Itabuna-Salvador-Curaçá orogen, Brazil: New U-Pb ages and geochemical data

The São José do Jacuípe Suite (SSJJ) comprises an association of mafic and ultramafic rocks that crops out in the northern portion of the Itabuna Salvador-Curaçá Orogen (ISCO), São Francisco Craton. They are represented by small NW-SE elongated bodies comprising metamorphosed anorthosites, leucogabbros, gabbronorites, gabbros, ferrogabbros, and subordinate pyroxenites and serpentinites. The rocks are part of an orogenic belt that comprises orthogranulites and orthogneisses of the Caraíba complex, and metasedimentary rocks of the Tanque Novo-Ipirá complex, which appear to be amalgamated into a folded mélange. Petrographical analysis of the SSJJ rocks revealed relict cumulus textures in the pyroxenites. Moreover, inclusion relationships allowed the identification of two metamorphic phases; the first phase likely occurred at amphibolite facies conditions and was overprinted by granulite facies metamorphism that represents the peak metamorphic conditions recorded by the SSJJ rocks. Analytical results for major and trace elements indicate cogeneticity between the mafic rocks, and arc-related mafic cumulate field compositions. In the geotectonic environment diagrams the rocks plot in the MORB and IAT fields, indicating an origin involving melting of different sources or magma mixing. The characteristic signature of the studied rocks is represented by negative HFSE anomalies (Nb, Ti, Zr and Hf), depletion in HREE and enrichment in LILE (Ba, Rb, and Cs), indicating their derivation from metasomatic mantle in a suprasubduction setting. These data corroborate previous isotope data that point to mildly enriched mantle sources. Incompatible trace elements signatures are similar to ophiolites generated in supra-subduction zone (SSZ) environments worldwide. New U/Pb LA-ICP-MS ages indicate igneous crystallization at 2549 ± 23 Ma and 2563 ± 13 Ma for SSJJ gabbronorites, thus indicating that they postdate the other members of the magmatic arc that formed the ISCO by 20–30 Ma.

Combining methane clumped and bulk isotopes, temporal variations in molecular and isotopic composition, and hydrochemical and geological proxies to understand methane's origin in the Ronda peridotite massifs (Spain)

In serpentinised peridotite and ultramafic rock systems, methane (CH4) origin is frequently considered abiotic, but variable microbial and thermogenic components can also exist. Typically, the origin of CH4 is studied using bulk, 13C/12C and 2H/H isotopic composition, molecular gas composition, occasionally radiocarbon (14C), microbiology and geological context. Recent advances in CH4-clumped isotope methods have yielded novel insights into the formation of CH4: nonetheless, their interpretation in natural gas samples is often uncertain and requires additional research. Here, we study the origin of the gas released in hyperalkaline (pH > 10) springs in the Ronda Peridotite Massifs (southern Spain), combining bulk and clumped CH4 isotopes with molecular gas composition, hydrochemical (Total Organic Carbon and Platinum Group Elements in water), geothermal and geo-structural data. Five springs analysed in 2014 have been re-examined for changes in gas chemistry over time, and three newly discovered gas-bearing springs are analysed for the first time. Regardless of whether springs have microbial or abiotic isotopic fingerprints, we find that bulk CH4 isotopes are fairly stable over a seven-year period. This suggests that the CH4 source(s) or postgenetic processes (such as oxidation and diffusion) have not undergone significant temporal changes. Major variations in H2 and CH4 concentrations in certain springs may be the result of changes in gas pressure and migration intensity. Paired CH4 clumped isotopes (Δ12CH2D2 - Δ13CH3D) were analysed in two bubbling springs, where the presence of CH4 can be interpreted as non-microbial based on 13C enrichment, absence of 14C, and the presence of ethane and propane. However, these isotopes are in disequilibrium, which prevents the quantification of the gas formation temperature. Within the Δ12CH2D2 - Δ13CH3D diagram, the data lie within both the microbialgenic zone, suggested by previous authors, and the abiotic zone that results combining data from laboratory gas synthesis and other natural gas samples. Therefore, attributing a microbial origin to CH4 based only on clumped isotopes is less definite than previously assumed. The amount of Total Organic Carbon appears to be correlated with the origin of CH4, as it is higher in 13C-depleted CH4 samples and lower in 13C-enriched samples. Palladium (Pd) and Rhodium (Rh) dissolved in water (the more soluble Platinum Group Elements) can be a proxy for the chromitite ore deposits contained in plagioclase tectonite layers throughout the investigated area, which may act as catalysts for abiotic CO2 hydrogenation. Clumped isotope disequilibrium and the reported absence of diffuse CH4-bearing fluid inclusions in the peridotites appear to rule out high temperature gas genesis in post-magmatic inclusions. These observations, along with the moderate temperatures at the base of the peridotite massifs and the consistent occurrence of gas along tectonic contacts between serpentinised (H2-bearing) peridotite and carbon-bearing rocks, are compatible with the theory of low-temperature CO2 hydrogenation.