Mantle Samples Research Articles

Re-Os isotopic evidence for ancient melt depletion in refertilized Neo-Tethyan suboceanic mantle domain

Ancient mantle domains with unradiogenic Os isotopes have been well documented in the Yarlung-Tsangpo ophiolites (YTO), southern Tibet. Nonetheless, previous Os isotope studies have yielded relatively young Re-depletion model (TRD) ages of <1.0 Ga for mantle peridotites of the Zedong ophiolite in the eastern segment of the YTO. In this study, a comprehensive petrological and geochemical study has been conducted on a suite of mantle samples including harzburgites, lherzolites and one pyroxenite. Our results indicate that the Zedong harzburgites have been subjected to low to moderate degrees (~6–20 %) of partial melting and late-stage melt-rock interaction with basaltic melts. In contrast, both lherzolites and pyroxenite were generated by melt refertilization. The Zedong lherzolites display flat patterns of highly siderophile elements (HSE; Os, Ir, Ru, Pt, Pd, Re), whereas the harzburgites show depletion in Pt, Pd and Re relative to Os, Ir and Ru. The Re-Os isotopes of the Zedong lherzolites have been modified by melt refertilization and thus cannot provide reliable age information. Three Zedong harzburgites with the lowest PdN/IrN and ReN/IrN ratios have the least radiogenic 187Os/188Os of 0.11497–0.11537, giving the old TRD ages of 1.89–1.98 Ga. These ages are considerably older than previously reported TRD ages for the Zedong peridotites (< 1.0 Ga), but comparable to the oldest TRD age for the Luobusa peridotites. This supports the occurrence of similarly old mantle domains in both Zedong and Luobusa ophiolites in the eastern part of the YTO. The Zedong pyroxenite has a radiogenic 187Os/188Os ratio of 0.158 that is similar to the 187Os/188Os ratios (0.156–0.180) of previously reported for the YTO pyroxenites, which can be explained by selective assimilation of interstitial sulfides during the percolation of migrating melts through the mantle lithosphere. Grain boundary sulfides may play a significant role in the high 187Os/188Os of the YTO pyroxenites.

Thermal State and Thickness of the Lithospheric Mantle Beneath the Northern East-European Platform: Evidence from Clinopyroxene Xenocrysts in Kimberlite Pipes from the Arkhangelsk Region (NW Russia) and Its Applications in Diamond Exploration

This paper presents the reconstruction of the architecture of the lithospheric mantle, including its thermal state and thickness, as well as the scale and efficiency of its sampling by four kimberlites from the Arkhangelsk diamondiferous province: Arkhangelskaya, Lomonosovskaya, V. Grib, and TSNIGRI-Arkhangelskaya. These kimberlites differ in terms of their composition, diamond content, and location. Data presented include the major-element composition of clinopyroxene xenocrysts (&gt;2000 grains), P–T calculations from compositionally filtered Cr-diopside grains, and the reconstruction of local paleogeotherms. Additionally, we used available data on Ni content in peridotitic garnet xenocrysts to calculate their T values and project them onto local Cr-diopside-derived geotherms to reconstruct the vertical distribution of mantle xenocrysts and assess the efficiency of lithospheric mantle sampling by different kimberlites. We identified the presence of a &gt;200 km-thick lithospheric mantle beneath the region at the time of kimberlite emplacement. We also found that the diamond content of the studied pipes was, to some extent, dependent on the following set of factors: (1) the thermal state of the lithospheric mantle; (2) the width of the real “diamond window” marked by mantle xenocrysts, especially by diamond-associated garnets; and (3) the efficiency of lithospheric mantle sampling by kimberlite. The results of this study can be used to inform diamond exploration programs within the region.

Lunar mantle composition and timing of overturn indicated by Mg# and mineralogy distributions across the South Pole-Aitken basin

The South Pole-Aitken (SPA) basin, excavating more than 100 km depth, must have exposed extensive lunar mantle materials, making it a promising location for sampling mantle material. To investigate the distribution of potential mantle materials across the SPA basin, we mapped the distribution of Mg# and major minerals contents using Moon Mineralogy Mapping data and radiative transfer modeling. We found that the potential mantle material exposed by SPA is Mg-rich orthopyroxenite, and we identified the locations of seven mantle candidate sites. The Chang'E-6 sample return site is located near mantle candidate sites within the Apollo basin, making it promising to return the first unambiguous mantle sample. Our Mg# and mineral mapping results show that the SPA ejecta is enriched in low-Ca pyroxene (LCP) with Mg#≥85, consistent with a post-overturn upper mantle composition. The enrichment of LCP in the SPA upper mantle may result from a low content of dunite or incomplete overturn.

Toward Tectonic-Type and Global 1D Seismic Models of the Upper Mantle Constrained by Broadband Surface Waves

ABSTRACT The unprecedentedly dense current sampling of the upper mantle with seismic data offers an opportunity for determining representative seismic velocity models for the Earth’s main tectonic environments. Here, we use over 1.17 million Rayleigh- and 300,000 Love-wave, fundamental-mode, phase-velocity curves measured with multimode waveform inversion of data available since the 1990s, and compute phase-velocity maps in a 17–310 s period range. We then compute phase-velocity curves averaged over the globe and eight tectonic environments, and invert them for 1D seismic velocity profiles of the upper mantle. The averaged curves are smooth and fit by VS models with very small misfits, under 0.1%, at most periods. For phase-velocity curves extending up to 310 s, Rayleigh waves resolve VSV structure down to the shallow lower mantle. Love-wave sampling is shallower, and VSH and, thus, radial anisotropy profiles are resolved down to 375–400 km depth. The uncertainty of the VS models is dominated by the trade-offs of VS at neighboring depths. Using the model-space-projection approach, we quantify the uncertainty of VS in layers of different thickness and at different depths, and show how it decreases with the increasing thickness of the layers. Example 1D VS models that fit the data display the expected increase of the lithospheric seismic velocity with the age of the oceanic lithosphere and with the average age of the continental tectonic type. Radial anisotropy in the global and most tectonic-type models show a flip of the sign from positive (VSH&amp;gt;VSV) to negative at 200–300 km depth. Negative anisotropy is also observed in the shallow mantle lithosphere beneath oceans down to 45–55 km depth. We also compute a global model with the minimal structural complexity, which fits the data worse than the best-fitting one but does not include a sublithospheric low-velocity zone, providing a simple reference for seismic studies.

Distribution of REE between amphibole and pyroxenes in the lithospheric mantle: An assessment from the lattice strain model

Abstract Amphibole and pyroxenes are the main reservoirs of rare earth elements (REEs) in lithospheric mantle that has been affected by hydrous metasomatism. In this study, we developed semi-empirical models for REE partitioning between orthopyroxene and amphibole and between clinopyroxene and amphibole. These models were formulated on the basis of parameterized lattice strain models of mineral–melt REE partitioning for orthopyroxene, clinopyroxene, and amphibole, and they were calibrated using major element and REE data of amphibole and pyroxenes in natural mantle samples from intraplate setting. The mineral–melt REE partitioning models suggest that amphibole is not in equilibrium with coexisting pyroxenes in the mantle samples, and that the amphibole crystallized at a lower temperature than that of the pyroxenes. We estimated the apparent amphibole crystallization temperature using major element compositions of the amphibole and established temperature- and composition-dependent models that can be used to predict apparent pyroxene–amphibole REE partition coefficients for amphibole-bearing peridotite and pyroxenite from intraplate lithospheric mantle. Apparent pyroxene–amphibole REE partition coefficients predicted by the models can be used to infer REE contents of amphibole from REE contents of coexisting pyroxenes. This is especially useful when the grain size of amphibole is too small for trace element analysis.

Underplated melts control sulfide segregation at the continental crust-mantle transition

Exposures of the Earth’s crust-mantle transition are scarce, thus, limiting our knowledge about the formation of subcontinental underplate cumulates, and their significance for metal storage and migration. Here, we investigated chalcophile metals to track sulfide crystallization within the Contact Series, an <150-m-thick pyroxenite-gabbronorite sequence, formed by mantle-derived melts, highlighting the boundary between the Balmuccia mantle peridotite and gabbronoritic Mafic Complex of the Ivrea-Verbano Zone. Within the Contact Series, numerous sulfides crystallized in response to the differentiation of mantle-derived underplated melts. Such sulfide-controlled metal differentiation resulted in anomalous Cu contents (up to ~380 ppm), compared to reference mantle (~19 ppm) and crustal samples (~1 ppm). We propose that the assimilation of continental crust material is a critical mechanism driving sulfide segregation and sulfide-controlled metal storage. Our results evidence that sulfides are trapped in the underplated mafic-ultramafic cumulates and that their enrichment in Cu may provide essential implications for crustal metallogeny.

Sublithospheric Diamonds: Plate Tectonics from Earth's Deepest Mantle Samples

Sublithospheric diamonds and the inclusions they may carry crystallize in the asthenosphere, transition zone, or uppermost lower mantle (from 300 to ∼800 km), and are the deepest minerals so far recognized to form by plate tectonics. These diamonds are distinctive in their deformation features, low nitrogen content, and inclusions of these major mantle minerals: majoritic garnet, clinopyroxene, ringwoodite, CaSi perovskite, ferropericlase, and bridgmanite or their retrograde equivalents. The stable isotopic compositions of elements within these diamonds (δ11B, δ13C, δ15N) and their inclusions (δ18O, δ56Fe) are typically well outside normal mantle ranges, showing that these elements were either organic (C) or modified by seawater alteration (B, O, Fe) at relatively low temperatures. Metamorphic minerals in cold slabs are effective hosts that transport C as CO3 and H as H2O, OH, or CH4 below the island arc and mantle wedge. Warming of the slab generates carbonatitic melts, supercritical aqueous fluids, or metallic liquids, forming three types of sublithospheric diamonds. Diamond crystallization occurs by movement and reduction of mobile fluids as they pass through host mantle via fractures—a process that creates chemical heterogeneity and may promote deep focus earthquakes. Geobarometry of majoritic garnet inclusions and diamond ages suggest upward transport, perhaps to the base of mantle lithosphere. From there, diamonds are carried to Earth's surface by eruptions of kimberlite magma. Mineral assemblages in sublithospheric diamonds directly trace Earth's deep volatile cycle, demonstrating how the hydrosphere of a rocky planet can connect to its solid interior. ▪Sublithospheric diamonds from the deep upper mantle, transition zone, and lower mantle host Earth's deepest obtainable mineral samples.▪Low-temperature seawater alteration of the ocean floor captures organic and inorganic carbon at the surface eventually to become some of the most precious gem diamonds.▪Subduction transports fluids in metamorphic minerals to great depth. Fluids released by slab heating migrate, react with host mantle to induce diamond crystallization, and may trigger earthquakes.▪Sublithospheric diamonds are powerful tracers of subduction—a plate tectonic process that deeply recycles part of Earth's planetary volatile budget.

Sublithospheric diamond ages and the supercontinent cycle

Subduction related to the ancient supercontinent cycle is poorly constrained by mantle samples. Sublithospheric diamond crystallization records the release of melts from subducting oceanic lithosphere at 300–700 km depths1,2 and is especially suited to tracking the timing and effects of deep mantle processes on supercontinents. Here we show that four isotope systems (Rb–Sr, Sm–Nd, U–Pb and Re–Os) applied to Fe-sulfide and CaSiO3 inclusions within 13 sublithospheric diamonds from Juína (Brazil) and Kankan (Guinea) give broadly overlapping crystallization ages from around 450 to 650 million years ago. The intracratonic location of the diamond deposits on Gondwana and the ages, initial isotopic ratios, and trace element content of the inclusions indicate formation from a peri-Gondwanan subduction system. Preservation of these Neoproterozoic–Palaeozoic sublithospheric diamonds beneath Gondwana until its Cretaceous breakup, coupled with majorite geobarometry3,4, suggests that they accreted to and were retained in the lithospheric keel for more than 300 Myr during supercontinent migration. We propose that this process of lithosphere growth—with diamonds attached to the supercontinent keel by the diapiric uprise of depleted buoyant material and pieces of slab crust—could have enhanced supercontinent stability.

Tribological study of fourth-stage crusher coatings applied in an iron ore processing plant

In the present work, a tribological study was carried out on concave and mantle linings of cone crushers used in the fourth-stage crushing process of an iron ore processing plant. Initially, an analysis of the characteristics over the lifetime of the applied coatings was carried out, and samples of the parts that suffered the most critical wear were selected to carry out the tribological study. Investigation of the wear mechanism and deformation was performed via microhardness analysis, optical microscopy (OM), and scanning electron microscopy (SEM) measurements. The mantle and concave were composed of the same alloy, austenitic manganese steel with 12% Mn. Abrasion tests in the rubber wheel configuration were performed on the mantle and concave samples, and the results were compared with those of two other commercial alloys of manganese steel (one with 18% Mn and one with 18% Mn and 2.5% Cr). It was observed that concave wear occurs differently from mantle wear. While the concave suffers more indentation, the mantle has a more abrasion-dominant profile. This analysis is of paramount importance for conducting a review of the maintenance strategy. The mantle presented less work hardening and a higher wear rate than the concave. The predominant wear mechanism is abrasion. Austenitic manganese steel with 18% Mn and 2.5% Cr presented high abrasive wear resistance, with a wear rate 18% lower than that of the alloy used in the manufacture of cone crusher linings. Higher hardness values result in a lower wear rate, and the use of austenitic manganese steel with 18% Mn and 2.5% Cr represents an opportunity to increase the useful life of cone crusher linings.

Comparison of physicochemical and volatile flavor properties of neon flying squid (Ommastrephes bartramii), jumbo squid (Dosidicus gigas), and Argentine shortfin squid (Illex argentinus) during chilled storage.

The difference and similarities in the physicochemical and volatile flavor properties were determined in neon flying squid (OB), jumbo squid (DG), and Argentine squid (IA) mantles during 8 days of chilled storage. Physicochemical analysis indicated the chilled conditions induced rapid increases in pH value, total volatile basic nitrogen (TVBN), and carbonyl and malondialdehyde (MDA) content of the three squid species. In addition, myofibrillar protein (MP) content decreased and springiness in the OB, DG, and IA mantle samples declined with the extension of storage time. Importantly, OB mantles presented less chemical stability than the other two squid samples during 8 days of chilled storage. In addition, histological observations suggest DG mantle tissues presented more compact structures than those of the other two samples. Volatile flavor analysis showed propionaldehyde, 3-pentanone, trimethylamine, 3-furanmethanol, 2-methyl butyric acid, and 2-butanone were highly abundant in the squid mantles after storage, likely resulting from decomposition, oxidation, and degradation of proteins and lipids in the squid mantle, which varied with different squid species. The findings provide insight into the performance of three squid species during chilled storage.

Haplosporidium pinnae Parasite Detection in Seawater Samples.

In this study, we investigated the presence of the parasite Haplosporidium pinnae, which is a pathogen for the bivalve Pinna nobilis, in water samples from different environments. Fifteen mantle samples of P. nobilis infected by H. pinnae were used to characterize the ribosomal unit of this parasite. The obtained sequences were employed to develop a method for eDNA detection of H. pinnae. We collected 56 water samples (from aquaria, open sea and sanctuaries) for testing the methodology. In this work, we developed three different PCRs generating amplicons of different lengths to determine the level of degradation of the DNA, since the status of H. pinnae in water and, therefore, its infectious capacity are unknown. The results showed the ability of the method to detect H. pinnae in sea waters from different areas persistent in the environment but with different degrees of DNA fragmentation. This developed method offers a new tool for preventive analysis for monitoring areas and to better understand the life cycle and the spread of this parasite.

Frontier of Understanding Earth's Dynamics

Frontier of Understanding Earth's Dynamics

Rejuvenation of the lithospheric mantle beneath the orogens: Constraints from elemental geochemistry and Os isotopes in mantle xenoliths

Continental orogens that involve subduction and collision are important target areas for studying the growth, stabilization and transformation of Earth’s continents. In particular, the nature of the lithospheric mantle and its relationship to the overlying crust in these active subduction-collision zones are key factors to be constrained. Mantle xenoliths entrained by Cenozoic basalts from the Indochina Block, the Sukhothai Arc and the Inthanon Zone (IB-SA-IZ) provide direct deep-seated samples of the lithospheric mantle derived from part of the Tethyan orogenic belt, offering a unique opportunity to study mantle processes during orogenesis in response to the closure of the Paleo-Tethys Ocean. The studied mantle xenoliths comprise 32 spinel lherzolites, six harzburgites, and one olivine websterite. We present a comprehensive study of the elemental geochemistry and Os isotopes of this sample suite. Their lithophile element systematics indicate that they represent mantle residues after varying degrees of melt extraction (5–24%), followed by melt refertilization. Although the highly siderophile element abundances have been affected to some extent by melt refertilization through removal of sulfides, the Os isotopic compositions of the studied xenoliths were minimally disturbed. Lherzolites and the sole olivine websterite from the IB-SA-IZ share similar Re-Os isotope systematics (187Os/188Os: 0.121–0.137) to global abyssal peridotites, representing juvenile lithospheric mantle accreted from the upwelling asthenosphere, most likely associated with arc-continent collision in response to the closure of the Paleo-Tethys Ocean and back-arc basin during the Middle-Late Triassic. In contrast, the refractory harzburgites from the Indochina Block and Sukhothai Arc are characterized by substantially lower 187Os/188Os ratios (187Os/188Os: 0.117–0.122) than the lherzolites, yielding Mesoproterozoic Re-depletion Os model ages, which can be interpreted as either ancient lithosphere relics beneath these regions or accreted ancient mantle debris from the upwelling asthenosphere. Although the interpretation for the origin of the older harzburgites is ambiguous, we can conclude that rejuvenation of the lithospheric mantle commonly occurred beneath different tectonic units of the Tethyan orogenic belt: i.e., the pre-existing lithospheric mantle beneath different tectonic units of the Tethyan orogenic belt has been largely replaced by upwelling asthenospheric mantle during the Middle-Late Triassic orogenesis. This mantle was then further refertilized to varying degrees by asthenosphere-derived melts. Both processes play key roles in defining the nature of the lithospheric mantle in orogenic settings. This study also highlights that the geodynamic processes in the lithospheric mantle beneath orogenic belts can be well constrained by integrated evidence from lithophile and highly siderophile elements and their isotopes (e.g., Os isotopes) of mantle xenoliths.

Diamonds, dunites, and metasomatic rocks formed by melt/rock reaction in craton roots

The thick mantle lithosphere beneath cratons consists of strongly reduced rocks that have reacted with oxidized melts. These low-silica, incipient melts are rich in CO2 and H2O and react with surrounding rocks forming an enriched zone at the base of the lithosphere, which is the source region for many diamonds. Here, we reproduce these reactions in novel experiments in which oxidised, hydrous carbonate-rich melts are reacted with reduced, depleted peridotite at 5 GPa pressure. Results explain several key features of the mantle sample in kimberlites as products of a single process, namely the formation of diamonds, phlogopite and alkali-amphibole bearing rocks, iron-rich dunites, and garnets and clinopyroxenes with pyroxenitic compositions. Initially, redox freezing occurs where melts meet the reduced peridotite, depositing diamonds and associated garnet and clinopyroxenes. The spreading reaction front leaves behind Fe-rich dunite, and crystallizing phlogopite and amphibole when the melt solidifies at the reaction front.

Incipient metal and sulfur extraction during melting of metasomatised mantle

A long-standing debate on the genesis of magmatic-hydrothermal Cu-Au deposits is whether the elevated oxygen fugacity (fO2) and metal endowment of their parental magmas are inherited from the mantle source or acquired during fractionation within the crust. This debate is built mainly on the composition of magmas emplaced into the upper crust that underwent varying degrees of differentiation and assimilation, which inevitably obscure their relationship to mantle processes. Here, we approach this debate from a new perspective by studying melt generation directly in mantle samples. This opportunity is provided by xenoliths from Lake Bullen Merri (Australia), which record partial melting of a pre-existing metasomatic assemblage. Melting occurred when the xenoliths were incorporated in hot basalts and carried towards the surface, where quenching upon eruption froze-in the melt-restite relationships. These samples preserve patches and interconnected networks of silicate glass along grain boundaries, caught in the act of undergoing melting and melt extraction. Melting consumed mainly metasomatic hornblende, phlogopite, and clinopyroxene, generating shoshonitic compositions that can be sulfide-rich, or have negligible sulfide content. We use four independent approaches to estimate the fO2 of glass, including sulfide content, oxybarometry based on spinel-olivine pairs, Fe3+/Fetot of glass determined by synchrotron X-ray absorption near-edge structure (XANES) spectroscopy, and Cu/Fe ratio of the sulfide assemblage. Sulfide-rich melt had higher fO2 compared to the sulfide-poor melt, and these characteristics were inherited from the metasomatic assemblage; the oxidation state of the melts controlled their sulfide content. Subduction-modified mantle can thus store elevated oxidation state, sulfur, and metal content for hundreds of millions of years before low-degree partial melting generates oxidised, sulfur- and metal-enriched high-K/shoshonitic melts in post-collisional settings.

Detection of Haplosporidium pinnae from Pinna nobilis Faeces

Pinna nobilis (Linnaeus, 1758) is the largest bivalve endemic to the Mediterranean. It is distributed in a wide range of coastal environments, including estuaries. Pinna nobilis has recently become a critically endangered species (with almost 100% mortality) along the entire Spanish Mediterranean coast. This may be due to coinfections caused by Haplosporidium pinnae and bacterial pathogens such as Mycobacterium spp. We extensively sampled P. nobilis from Mar Menor lagoon (SE Spain), a site where individuals still survive. Using conventional PCR, we found Haplosporidium spp. in 7.1% of mantle and faecal DNA samples in different individuals of P. nobilis. We identified and quantified Haplosporidium pinnae in P. nobilis using Sanger sequencing and qPCR. Faecal H. pinnae detection is non-invasive, unlike biopsies. Therefore, this non-lethal and non-invasive sampling method could contribute to the welfare of living populations, particularly in eutrophic environments, where they are prone to septicaemia. The use of faecal DNA analysis could be a major advance in epidemiology and recovery assessment studies of P. nobilis.

Evidence for basalt enrichment in the mantle transition zone from inversion of triplicated P- and S-waveforms

Recycling of chemically stratified oceanic lithosphere at subduction zones and mantle mixing is the main source for the production and long-term maintenance of heterogeneities as evidenced through mantle samples and the presence of seismic scatterers. The mantle transition zone (MTZ), which is bound by seismic discontinuities around 410 and 660 km depth that reflect mantle mineral phase transformations in the olivine system, is considered to play a significant role in reprocessing and remixing of subducted oceanic plates. However, the compositional nature and mixing state of the MTZ is generally unconstrained with models ranging from a fully-equilibrated pyrolitic to a mechanically-mixed MTZ enriched in basalt. Here, we consider regional P- and S-waveforms from different regions that have interacted extensively with the MTZ and are triplicated at the seismic discontinuities. Because of the strong interaction with the MTZ and the sensitivity to mantle temperature and composition, triplicated waveforms can be used as probes of the thermochemical conditions in and around the MTZ. Here, we model the MTZ by considering two-endmember models: a fully-equilibrated and a mechanical mixture of depleted (harzburgite) and fertile (basalt) lithologies, whose properties are computed self-consistently using petrologic phase equilibria in combination with equation-of-state modeling as a function of pressure, temperature, and composition. Based on this method, we invert carefully selected triplicated P- and S-waveforms for local one-dimensional profiles of MTZ structure. In some of the studied regions, we find both radial and lateral heterogeneities in composition that show a trend from more fertile lithologies in the mid-MTZ to more pyrolitic compositions below the MTZ. This is in line with evidence provided by global geodynamic models that support the segregation and accumulation of basalt toward the mid-MTZ. Because a number of the regions studied here encompass subduction zone settings, we also observe lower-than-average mantle temperatures (∼1200∘C) and, as a consequence, thicker-than-average MTZ (∼240–280 km). While some areas appear to be better fit with a particular mixing model, rigorous statistical analysis of the datafit shows that on average, and at the level of the resolving power of the data, an equilibrated mantle across a large part of the studied subduction zone settings is favored.

Adaptive mitochondrial genome functioning in ecologically different farm-impacted natural seedbeds of the endemic blue mussel Mytilus chilensis

We assessed the adaptive contribution of the mitochondrial genes involved with the respiratory chain and oxidative phosphorylation of the blue mussel Mytilus chilensis, a native and heavily exploited species in the inner sea of Chiloé Island, southern Chile. The assembled mitochondrial transcriptome of individuals from two ecologically different farm-impacted natural seedbeds, Cochamó (41°S) and Yaldad (42°S), represented about 4.5% of the whole de novo transcriptome of the species and showed location and tissue (gills, mantle) specific expression differences in 13 protein-coding mitochondrial genes. The RNA-Seq analysis detected differences in the number of up-regulated mitogenes between individuals from Cochamó (7) and Yaldad (11), some being tissue-specific (ND4L and COX2). However, the analysis did not detect transcripts-per-million (TPM = 0) of ND2 and ND5 in gills and ATP6 in mantle samples from Cochamó. Likewise, for ND6 and ATP8 in any sample. Several monomorphic location-specific mitochondrial genetic variants were detected in samples from Cochamó (78) and Yaldad (207), representing standing genetic variability to optimize mitochondrial functioning under local habitats. Overall, these mitochondrial transcriptomic differences reflect the impact of environmental conditions on the mitochondrial genome functioning and offer new markers to assess the effects on mussel fitness of habitat translocations, a routine industry practice. Likewise, these mitochondrial markers should help monitor and maintain adaptive population differences in this keystone and heavily exploited native species.

No mantle residues in the Isua Supracrustal Belt

A critical component of our understanding of the evolution of Earth's mantle comes from rocks identified as direct mantle samples. Eoarchaean dunite lenses from the Isua Supracrustal Belt (ISB), North Atlantic Craton, Greenland, have been previously interpreted as depleted mantle wedge residues, complementary to arc-like volcanic rocks in the supracrustal sequence. This would place the ISB dunites among Earth's oldest mantle samples. We present new major element, platinum-group element (PGE) and Re-Os isotopic data for the ISB dunites, and critically assess the criteria previously used to invoke a mantle origin for the dunites. We find no evidence that uniquely supports a mantle origin. Instead, evidence of chromite and Os-Ir alloy fractionation, consistent Pt and Pd depletion, elevated Ni contents, and trace element systematics indicate that the dunites formed as olivine ± chromite cumulates with varying amounts of intercumulus melt. Their compositions indicate crystallisation from magmas represented by ISB volcanic rocks, and their Re-Os model ages overlap the ∼3720 Ma age of the volcanic sequence, consistent with the dunites representing magma chambers or conduits that fed the volcanic eruptions. Formation of the Isua dunites as cumulates removes an important line of evidence used to interpret the ISB as an ophiolite, and highlights the risks of using criteria that do not discriminate mantle residues from olivine-rich cumulates. Extending this reasoning to other Eoarchaean crustal peridotites previously identified as mantle rocks suggests there may be no mantle residues anywhere in the Itsaq Gneiss Complex, and that the oldest mantle samples may only be found as xenoliths in volcanic rocks.

The kabr El-Bonaya peridotites, Southeastern Sinai, Egypt: petrology, geochemistry, and metamorphism of Neoproterozoic arc ultramafic cumulates

Two small, isolated ultramafic masses in the northeastern part of the Wadi Kid area, southeast Sinai, are composed of variably serpentinized harzburgite and lherzolite with minor talc-anthophyllite rock. The primary phases are dominantly olivine, orthopyroxene and Cr-spinel; clinopyroxene, amphibole, and phlogopite are also found in lherzolite samples. The whole-rock Mg# of harzburgite samples (89–91) is higher than that of lherzolite (average 82). The harzburgite samples contain olivine with higher Mg and Ni contents, orthopyroxene with higher Mg#, and Cr-spinel with higher Cr content than do the lherzolite samples. The REE patterns of clinopyroxene and amphibole in lherzolite are most consistent with a cumulate origin. Although several compositional characteristics of the harzburgites resemble those of residual mantle, in detail the Cr₂O₃ and Al₂O₃ contents of fresh Cr-spinel in harzburgite are different from those found in mantle samples or in any of the Neoproterozoic ophiolitic peridotites throughout the Arabian-Nubian Shield. Thus, all the ultramafic rocks at Kabr El-Bonaya are best explained as ultramafic cumulates, with harzburgite consisting of early-formed cumulate phases and lherzolite containing later-formed cumulate phases with higher REE abundances, primary hydrous minerals, evolved primary silicates, and high TiO₂ (0.77 wt.%) and Al₂O₃ (18 wt.%) contents in Cr-spinel. The trace-element characteristics of the rocks indicate a subduction-related parental magma: whole-rock chondrite-normalized REE patterns are LREE-enriched; calculated <i>f</i>O₂ values are elevated (+2.47 to +3.39 log units above the fayalite-magnetite-quartz buffer); and computed N-MORB-normalized trace element patterns for melts in equilibrium with clinopyroxene and amphibole have negative Nb-Ta anomalies and enrichment in large-ion lithophile elements. The low Al₂O₃/SiO₂ ratios (0.007–0.040) of harzburgite samples and the low TiO₂ contents and high Cr# of their Cr-spinel indicate derivation from a mantle source that experienced high-degree partial melting. From these characteristics, we infer a boninitic parental melt for the harzburgite. We offer an illustrative quantitative fractionation model that can explain the successive derivation of harzburgite and lherzolite cumulates along a single equilibrium, polybaric cooling path. We conclude that the Kabr El-Bonaya ultramafic cumulates represent the exposed roots of a Neoproterozoic island arc that was caught in the collision between East and West Gondwana.