Mantle Peridotite Research Articles

Gabbro of the mantle part of the ophiolite section in the Alapaevsky dunite-harzburgite-gabbro massif (Eastern zone of the Middle Urals)

Research subject. An intrusive body of gabbroids breaking through mantle peridotites (dunites and harzburgites) of the Alapaevsky ophiolite massif (Eastern zone of the Middle Urals). Methods. The contents of petrogenic elements were determined by the X-ray fluorescence method; the contents of rare and scattered elements were studied by the ICP-MS method. The age of gabbroids was determined by 147Sm-143Nd ID-TIMS by isotope dating. Results. The gabbro and their containing ultramafic rocks were established to be of almost the same age of about 580 Ма, which indicates their belonging to a single ophiolite association of the Vendian age. At the same time, the gabbroids of the Alapaevsky massif differ sharply from both isotropic and stratified gabbro of the crustal part of the ophiolite section, fragments of which are observed within the Eastern zone of the Middle Urals, in terms of a significantly reduced content of light REE, rare alkalis, barium, uranium and thorium, as well as the absence of Pb maxima on spider diagrams. Conclusions. The established features in the composition and intrusive occurrence of the gabbro of the Alapaevsky massif among mantle ultramafic rocks indicate that the studied rocks cannot be identified with gabbroids of the crustal part of the ophiolite association. This suggests that this rather large intrusion of gabbro may be analogous to small veins and dikes of gabbroids observed among mantle peridotites in a number of ophiolite massifs, such as Voikar-Synyinsky massif in the Polar Urals and the Samail ophiolite in Oman. The distribution specifics of rare elements on the spider diagrams of the gabbroids of the Alapaevsky massif, i.e., the presence of Sr and Ba maxima and Nd and Th minima, as well as the Harzburgite composition of the mantle restite containing the gabbro body under consideration, indicate that the Vendian ophiolite association of the Eastern zone of the Middle Urals formed in a suprasubduction (pre-arc) environment. A model for the formation of large masses of gabbro in the mantle part of the ophiolite section is proposed.

Geochemical signatures of forearc serpentinites from oceanic to continental subduction

In subduction systems, the forearc (FA) serpentinized mantle plays a key role as a reservoir for fluid-mobile elements, especially those prone to early mobilisation from the slab. FA serpentinites therefore provide an indication of the fluid characteristics that have been steamed out of the slab, and allow better quantification of elemental recycling and fluid flow. FA serpentinites outcropping along the Indus Suture Zone are formed by hydration of mantle peridotites by fluids derived from the Indian slab, providing a geochemical record of fluid-rock interactions during the India-Eurasia convergence. Systematic trace and multi-isotope (Sr and Pb) analyses of serpentinites from three major sites, namely Shergol-Tingdo, Kargil and Tso Morari, along the Indus Suture Zone in Ladakh (NW Himalaya) show that these rocks record a major change in the origin of metasomatic agents, from oceanic to continental subduction. The Shergol-Tingdo and Kargil FA serpentinites, formed under low temperature and pressure conditions, are characterised by high B enrichment and As and Sb depletion, high alkali/U ratios and relatively unradiogenic Sr and Pb isotope ratios. These geochemical characteristics are identical to those reported for modern oceanic subduction zones (e.g., Mariana forearc) and are consistent with input by fluids derived from subducted oceanic crust. In contrast, FA serpentinites from the Tso Morari ultra-high pressure unit show enrichment in fluid mobile elements such as As, Sb and U, low alkali/U ratios and radiogenic Sr and Pb isotopic compositions. These features suggest a change in the metasomatizing agent with a strong influence from subducted continental material. Consistent with this scenario, the Sr-Pb isotopic composition of the Kargil FA serpentinites can be reproduced by mixing between an Indian MORB-depleted mantle source and fluids derived from dewatering of blueschist facies oceanic metasediments, whereas in the case of Tso Morari a fluid end-member derived from various eclogitic continental metasediments is required. We propose that the observed geochemical signature of the Indus Suture Zone FA serpentinites can be used to reconstruct the geochemical exchanges between the slab and the overlying mantle from intra-oceanic to continental subduction. This successful systematic approach could be applied to other collisional contexts

Origin of the Oligocene–Miocene Sailipu ultrapotassic volcanic rocks in southern Tibet: Melting of Asian mantle pyroxenites triggered by eastward tearing of the subducting Indian continental slab

Widely distributed Oligocene−Miocene ultrapotassic volcanic rocks in the Lhasa terrane of southern Tibet have been associated with the melting of the lithospheric mantle, plateau uplift, and porphyry Cu-Au mineralization. This study presents the mineral chemistry of olivine and clinopyroxene phenocrysts, whole-rock major and trace element data, and zircon U-Pb geochronological and Hf isotopic data for the Sailipu primitive ultrapotassic volcanic rocks. The Sailipu volcanic rocks exhibit high MgO (5.6−11.4 wt%), Cr (386−981 ppm), Co (22−43 ppm), and Ni (95−423 ppm) concentrations and have highly fractionated rare earth elements [REEs; (La/Yb)N = 23−73] and high-Fo (89.1−90.8) olivine phenocrysts containing elevated NiO (up to 0.59 wt%), which suggests a pyroxenitic mantle source that partially melted in the garnet stability field. Their high K2O contents (4.8−8.0 wt%) and global subduction sediment-like trace element patterns suggest that the metasomatic agents, which reacted with mantle peridotites to form phlogopite-bearing pyroxenites, were dominantly derived from the melting of subducted continental sediments. Their high whole-rock Ba/La and Th/Nd ratios are consistent with this hypothesis. The Sailipu ultrapotassic volcanic rocks also exhibit low initial 176Hf/177Hf ratios that resemble those of Himalayan leucogranites, and high Ca contents in olivine phenocrysts, which is consistent with contributions from the subducted carbonate-rich sedimentary strata on top of the thinned Greater Indian continental crust. The zircon U-Pb chronological data yielded concordant ages of 24.33 ± 0.19 Ma, 21.20 ± 0.62 Ma, and 17.05 ± 0.31 Ma for different exposures of the Sailipu volcanic rocks, which establishes a maximum age of ca. 24 Ma for these rocks. The northwest−southeast spatial distribution and the southeastward decrease in age (80°E−90°E) suggest west-to-east tearing of the thinned Greater Indian slab, which caused asthenospheric upwelling and melting of the Tibetan lithospheric mantle. Geothermometric calculations show relatively high primary magma temperatures (∼1250 °C) that are consistent with asthenospheric upwelling. We propose a mechanism that could genetically link the coeval Cu-Au ore-forming granitoids with the ultrapotassic magmatism of the Gangdese belt. The ultrapotassic rocks supply a large-volume of external magmatic volatiles, particularly H2O, which could trigger melting of the Tibetan lower crust and lead to the generation of the ore-forming granitoids and the establishment of oxidizing conditions for porphyry deposits. The oxygen fugacity (log ƒO2 values of ΔFMQ) of the primitive Sailipu ultrapotassic volcanic rocks (ΔFMQ = 0.48 ± 0.51 based on the Dol/melt V oxybarometer and ΔFMQ = 0.33 ± 1.19 according to the magmatic zircon U-Ce-Ti oxybarometer) is slightly lower than that of porphyry Cu-Au ore-forming granitoids in the eastern Gangdese (ΔFMQ = +0.8 to +2.9), which suggests that the direct injection of ultrapotassic melts into ore-forming granitoids played a limited role in changing oxygen fugacity, but more oxidized fluids/volatiles exsolved from these ultrapotassic melts may have facilitated the remelting of sulfide-bearing lower crust and/or directly scavenged sulfides from the mush-state reservoirs of the ore-forming granitoids in the middle−upper crust.

Ore formation in mantle peridotites before and after emplacement: review and new perspective

Generation of ore deposits in and around mantle peridotite (serpentinite) massifs are reviewed and summarized, and their future studies are discussed as well. Podiform chromitites are essentially formed by magma-peridotite (mainly harzburgite) reaction and subsequent magma mixing at the upper mantle. The UHP chromitite is, however, possibly formed by deep recycling of the low-pressure one formed in the upper mantle. Seawater, which contains Cl, is potentially able to dissolve and transport Cr and precipitate chromian spinel. Chromitite formation in the mantle wedge by slab fluids is possible but has not been well examined. Olivine in peridotites is potentially an important source of some elements (iron and related transition metals), which are liberated upon its breakdown during serpentinization. They are sometimes in-situ precipitated as minute grains of oxides, sulfides or metals in serpentinite, but sometimes transported via hydrothermal solutions responsible for serpentinization and concentrated to form ores. Magnetite veins in serpentinite, possibly as well as Co-Ni arsenides along the serpentinite-diorite boundaries of the Bou-Azzer ophiolite, give us a good example. Precipitation of Mn nodules and other ores from seafloor are possibly related to circulated seawater through peridotite emplaced in the crust. This issue should be examined thoroughly in the future.

Volcanites of neoproterozoic ophiolites Kalgyn massif (Northeast Asia): new geochemical and isotopic data

The Kalgan ophiolite massif is a unit of the ophiolite association of the Chersky collision belt. It is part of the Verkhoyano-Kolyma folded system on Northeast Asia. The Kalgan massif includes a complex of mantle peridotites, a lower-crust complex represented by ultrabasic cumulates, cumulative gabbro-amphibolites and amphibolites, and an upper-crust complex represented by metabasalts. New geochemical data obtained by the ICP-MS method, as well as data on the isotopic composition (Sm, Nd), made it possible to significantly clarify the geodynamic conditions for the formation of volcanites and to determine the features of the sources of parent melts.

The importance of fault control on the geomorphological development of a deep karst system: the case of Sima GESM, a Z-supercave in southern Spain

The mountainous karst massif in the Sierra de las Nieves (southern Spain) and the adjacent large outcrops of subcontinental mantle peridotites provide this area with a distinctive geodiversity that has contributed to the recent declaration of the zone as the new Sierra de las Nieves National Park. In addition, the Alpine tectonic interaction between the peridotites and the carbonate formations originated an important fault, the Turquillas fault, that has conditioned the existence of two tectonic blocks in the karst massif, one uplifted with respect to the other. Also, in the uplifted block, the fault has contributed to the generation of Z-supercaves, which are defined in this paper as caves with more than 1000 m of development in the vertical or Z dimension. Sima GESM with 1100 m of depth is the only Z-supercave known in the giant geologic structure of the Gibraltar Arc, which speaks on the infrequent and exceptionality of the generation of these underground geomorphological systems. The task of this paper is to present a detailed study of the Turquillas fault and its influence in the surface and underground geomorphology and hydrogeology and in particular its influence and implications in the development of the deep karst system defined as the Z-supercave. It is hoped that this study will contribute to identify the necessary conditions for the development of these extraordinary geomorphic systems which are of great international relevance in the major mountain ranges of the world with high relief karst and deep karst systems.

Alkali-carbonate melts in the cratonic mantle evidenced by a wehrlite xenolith from the Majuagaa kimberlite, West Greenland

Carbonate melts are critically important for the deep carbon cycle, mantle melting, redox reactions, and transport of highly incompatible elements. The presence of carbonate melts in the cratonic mantle has been inferred from experimental studies, metasomatic transformations, and melt/fluid inclusions in xenoliths, kimberlites, and diamonds. However, the exact composition of such melts is difficult to determine due to their ephemeral nature and highly reactive properties. Once formed, they migrate away from the source and react with silicate mantle minerals, especially orthopyroxene, causing mantle metasomatism. Wehrlite is one of the products of interaction between the carbonate melt and peridotitic mantle and hence is an excellent candidate for locating in situ carbonate melts. Here, we report petrological, geochemical, and melt inclusion data for a garnet wehrlite xenolith in the Majuagaa kimberlite dike, West Greenland. The xenolith, which last equilibrated with the mantle at 4.5 GPa and 1000 °C, contains abundant melt pools composed of dolomite, calcite, serpentine, spinel, apatite, and phlogopite. Although the original magmatic mineralogy was largely destroyed by low-temperature alteration, remnants of the crystallized carbonatitic melt are preserved as primary melt inclusions in the liquidus Ti-Mg-Fe spinel. These melt inclusions, composed of carbonates, alkali carbonates, periclase/brucite, and minor halides, K-sulfide, apatite, and phlogopite, are the first direct evidence for in situ alkali-carbonate melt in the deep cratonic mantle. Compositionally, they are very similar to primary Na-dolomite melt found in experiments and in fluid inclusions within diamonds.

Upper mantle flow and crustal deformation patterns beneath the Dangerous Grounds and Borneo where multiple plates converge in South China Sea revealed by 3-D anisotropic magnetotelluric imaging

SUMMARY A large-scale magnetotelluric (MT) study was carried out in offshore Borneo to understand the lithospheric structure in this geologically complex region where three tectonic plates converge and past crustal studies generated long-standing debates. Marine MT data were acquired at 1416 stations with 3 km spacing along 13 regional lines (covering 4677 line-km) with periodicities of 0.1 to 10 000 s. These were inverted in 3-D incorporating electrical anisotropy with cross-gradients constraints between vertical and horizontal resistivities and the results were validated with resistivity well-logs from several exploration wells. The models reveal widespread presence of electrically resistive upper crustal and uppermost mantle layers, each underlain by a laterally varying conductive and anisotropic layer. The geometries of the anisotropic layers suggest large-scale ductile flow, thrusting and folding forming belts of alternating deep roots and thin lithosphere, consistent with multiple underthrusting/subduction. Our results are in agreement with the seismologically detected lithospheric variations in northern Borneo suggesting onshore–offshore continuity and a common lithospheric–asthenospheric origin for the deformation patterns. Over thinned lithosphere, we found consistent low resistivity and high anisotropy anomalies in the mantle above the spatial locations of fast shear-wave bodies imaged by recent seismological workers which we interpret to indicate post-subduction lithospheric–asthenospheric ductile flow in response to multidirectional regional compression. We demonstrate that these zones are spatially correlated with the distribution of mainly Cretaceous ophiolitic rocks and melanges exposed onshore and suggest that serpentinization of mantle peridotite can explain some of the anisotropic conductivity anomalies. The taper zones of these deep anisotropic conductors are also associated with Neogene sub-basins, high thermal gradients, and intrasedimentary magmatic bodies indicating a link between lithospheric thinning and magmatism. We propose that such anomalies could be important pathfinders for geothermal and natural hydrogen systems in the ongoing global drive for carbon-free energy resources.

Tracking subduction-related metasomatism of the subcontinental lithospheric mantle using Ca-, O-, and H-isotopes

Mantle xenoliths provide effective records of the metasomatic processes that affect continental lithosphere evolution, such as interaction with subducted components or modification via small-degree melts. Correlations between major/trace element geochemistry with stable and radiogenic isotope compositions can help constrain the source and timing of this metasomatism. We report new δ18O, δ44/40Ca, and δD values for twelve kimberlite-hosted mantle xenoliths from the Slave Craton (NWT, Canada), which show varying degrees of metasomatism. The δ18O values of olivine (δ18Ool = +5.33 ± 0.13‰; 1σ; n = 12) overlap average mantle values. Clinopyroxene and garnet δ18O values (δ18Ocpx = +5.31 ± 0.10‰; δ18Ogrt = +5.37 ± 0.23‰; 1σ) extend below those reported in most mantle peridotites and are strongly correlated with clinopyroxene δ44/40Ca (avg. = +1.00 ± 0.10‰; 1σ) and garnet δ44/40Ca (avg. = +1.18 ± 0.19‰; 1σ) respectively, extending from typical mantle values to low δ18O and high δ44/40Ca values. In general, Δ18Ocpx-ol and Δ18Ogrt-ol (ranging from −0.19‰ to +0.19‰ and from −0.56‰ to +0.35‰, respectively) are lower than expected equilibrium values at mantle temperatures. Strong negative correlations are found between δ18Ogrt and Δ18Ogrt-ol and garnet major and trace element composition (Na2O, H2O, La/YbN). Furthermore, phlogopite-bearing kelyphitic rims have δD values (avg. = −126 ± 13‰; 1σ) lower than typical mantle values. Whole rock Sm-Nd model ages and oxygen isotope diffusion modeling suggest that metasomatism occurred during the Mesozoic, shortly before kimberlite entrainment, consistent with indications from diamond-forming fluids from the Slave craton. The combined low δ18O, δD, and high δ44/40Ca signature of the mantle peridotite xenoliths, along with the age constraints, suggest the metasomatic fluid/melt is sourced from a recycled oceanic crust component related to Mesozoic subduction in western North America.

Ti- and Ba-rich phlogopitic micas in alkaline basic and upper mantle igneous rocks; stoichiometry, stability, and Fe valence estimation reassessed and rationalised

Ti- and Ba-rich tri-octahedral micas occur in fractionated basic igneous rocks, metasomatized mantle peridotites, metamorphosed pelites/carbonates, and hydrothermally altered mineral deposits. Electron microprobe analyses (EMP), with all iron reported as FeO, were widely used in the 1970/80s to interpret Ti and Ba substitution mechanisms, based on 22 O2– unit cell calculations, implying that cation vacancies occur in octahedral and/or intersheet sites. In 1996 EMP with chemical and physical analyses for ferric and total Fe, H2O, (OH), and element-specific Fe X-ray Absorption Spectroscopy (both K and L-edges) established valence states for Fe and Ti and cation site occupancies, that ∼50 % O replaces (OH) molecules, and that 24 anion cell formulae show the absence of cation vacancies. Cell formula calculation protocol for phlogopitic micas is refined here and results tested against the stoichiometric formula for vacancy-free phlogopite, XIIK2VIMg6IV[Si6Al2]O20(OH)4. Hypothetical sheet silicate compositions, calculated with fixed contents of vacancies linked to particular mixed-valence element substitutions, confirm that reliable unit cell formulae for natural mica solids require that each stoichiometric vacancy must be accounted for. If reliable estimates for ‘excess O’ (denoted WO2−) are assigned to EMP analyses, the proportion of the oxy-mica component in a mica solid solution can be defined. This approach is tested using published analyses for Ti- and Ba-rich biotites from fractionated basic and ultramafic volcanic igneous rocks (oxymica range 2.5–45 %; TiO2 up to 14 %; BaO up to 23 %), upper mantle peridotites (equivalent values 7–18 %; 6 %; 0.7 %), and metasomatised upper mantle (2–37 %; 9 %; 23 %). Enrichments of Ti and Ba in micas are clearly linked to the extra oxygen charge required to neutralise the more highly charged Ba2+ and Ti4+ replacing K+ and Mg2+.Substitution mechanisms involving Ba, Ti, and Fe3+ in ideal phlogopite involve coupled inter-site / inter-valence interactions so both site-ordering and valence-balance between the different cation sites must be accounted for. The cation exchange 2XIIK+ + 4VI(Mg2+ + Fe2+) + 4IVSi4+ ↔ XIIBa2+ + 3VITi4+ + 4(Altotal + Fe3+ + Crtotal)3+ is used here so that valence and site order can be considered together. Compositional variations show well-defined trends towards high-oxymica content and vacancy-free Ti and Ba mica end-members, rather than to oxy-free end-members with essential vacancies. Average compositions for different source rock micas are used to assess compositional trends; molecular WO2− values are refined by relating initial estimates directly to the wt.% TiO2 contents; and refined WO2− and stoichiometrically calculated Fe3+/Fetotal ratios ranging ∼0.1–0.5) are correlated with possible primary magmatic values.

Do we really need to drill through the intact ocean crust?

We must persevere to drill through the intact ocean crust to fully address fundamental questions towards completion of the plate tectonics theory. The primary questions include: what is the ocean crust made up of, how thick is it and what is the petrological nature of the crust-mantle boundary (i.e., Mohorovičić discontinuity or Moho)? These questions may sound naive because they are widely believed to be well-understood facts, but they are not. Correctly, our current knowledge remains incomplete, and some popular misperceptions come from interpretations based on convenient assumptions. One assumption is that the ocean crust inferred from seismic data is of magmatic origin. Testing this assumption is a principal motivation of Project Mohole (1957–1966), attempting to drill intact ocean crust across the Moho into the mantle. Project Mohole failed because of its high cost, engineering challenges and insufficient tries, but the technologies developed made subsequent ocean drilling successful. However, answers to the original questions remain unsatisfactory. For example, seismic crust interpreted to be of magmatic origin is shown to have globally uniform thickness of 6.0 ± 1.0 km, but crust with such thickness at many slow-spreading ridge segments is dominated by serpentinized mantle peridotites exposed on seafloors. Therefore, the popular view on ocean ridge magmatism must be re-examined, which needs intact ocean crust drilling into the mantle. Drilling at geologically simple sites in the fast-spreading Pacific seafloor is most promising.The US-led D/V JOIDES Resolution that has well served the scientific ocean drilling since 1985 is to retire by the end of 2024, but timely the Chinese geoscience community wishes to continue this international endeavor using the purpose-built D/V Meng Xiang to be in service in 2025. The international community is to gather in November 24–27, 2024, Guangzhou, China, to discuss strategies on where and how to successfully drill intact ocean crust across the Moho in coming years

Petrogenesis and an Evaluation of the Melting Conditions of the Late Permian ELIP Picrites, SW China: Constraints Due to Primary Magma and Olivine Composition

The late Permian Emeishan large igneous province (ELIP) in SW China is a melting product of the Emeishan mantle plume. Recently, it has been debated whether peridotite or pyroxenite is the dominant lithology of the mantle source in the ELIP. To address this, systematic analyses of bulk-rock and coexisting spinel and olivine compositions were conducted on picrites from Lijiang–Yongsheng, Dali–Binchuan, Yumen, Muli, and Ertan. The ELIP picrites exhibit positive TiO2–CaO and negative MgO–CaO correlations, as well as low FC3MS values (−0.24–0.1), supporting a peridotite-dominated mantle source. This lithology of the mantle source is also supported by the high 100 × Mn–Fe (1.43–1.73) and Mn–Zn (13.6–18.4) values but low 10,000 × Zn–Fe (8.0–12.7) ratios of the olivine phenocrysts. The estimated mantle potential temperature for Lijiang, Yongsheng, Yumen–Ertan, Muli, and Dali–Binchuan picrites decreased away from Lijiang and Yongsheng, suggesting that the Lijiang and Yongsheng areas were the center of the ELIP. The Lijiang–Yongsheng primary magma shows similar SiO2 content but lower Al2O3 contents (average of 8.24 wt.%) and higher MgO contents (average of 21.42 wt.%) than those of Dali–Binchuan primary magma (Al2O3: 9.86 wt.%; MgO: 19.02 wt.%). Also considering the high Gd–Yb (average of 3.05) and La–Yb (average of 14.61) ratios and mantle potential temperature (average of 1599 °C), we proposed that Lijiang–Yongsheng lavas are produced via the melting of a garnet–peridotitic mantle. In contrast, the Dali–Binchuan lavas with low Gd–Yb (average of 1.91) and La–Yb (average of 5.88) ratios can be explained by their formation in the garnet–spinel transition zone of a peridotitic mantle. The Yumen–Ertan primary magma displays similar mantle potential temperature (average of 1600 °C), Al2O3 and FeO content, and Gd–Yb ratios to those of Lijiang–Yongsheng lavas, indicating that YumenvErtan primary magma may be attributed to the partial melting of garnet with minor peridotite. Therefore, heterogeneous plume-head mantle sources lead to the evaluation of melting conditions of the late Permian ELIP picrites.

Li enrichment in peridotites and chromitites tracks mantle-crust interaction

The ultramafic massifs of the Serranía de Ronda in southern Spain are the Earth's largest exposures of subcontinental lithospheric mantle (SCLM) peridotites (∼450 km2). These ultramafic massifs experienced asthenosphere melt percolation during their crustal emplacement. Mixing of these mafic melts with anatectic melts and fluids led to the formation of a world's unique Ni-arsenide-rich chromitite ores (hereafter CrNi ores) associated with orthopyroxenite and/or cordieritite (i.e., > 90 % volume of cordierite) hosted within the peridotites. This study uses laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to investigate the Li in rock-forming minerals of peridotite and CrNi ores to evaluate the role of Li as crustal tracer. Clinopyroxene crystallized from asthenospheric melts exhibits high Li contents (up to 8.5 ppm), exceeding the average values of the upper mantle (∼ 0.7 ppm), whereas orthopyroxene, olivine, and Cr-spinel from peridotite are mostly Li-depleted. In contrast, all rock-forming minerals of CrNi ores have abnormally high Li contents, displaying an overall Li enrichment trend toward the external parts of ultramafic massifs, on the way to the crustal rocks. This trend is evident in Cr-spinel from the CrNi ores, which display 6.9–7.9 ppm Li in the deepest portions of the massif (Arroyo de la Cala CrNi ore) up to 1.4–8.5 ppm in the shallowest part (La Gallega CrNi ore), as well as in orthopyroxenes that have 31.3–44.7 ppm Li in Arroyo de la Cala, and 45.1–51.4 ppm Li in La Gallega. Cordierite is present only in the CrNi ores situated in the external part of the ultramafic massifs, exhibiting 113.15–160.82 ppm Li in the Barranco de las Acedías CrNi ore and 36.5–60.5 ppm Li in La Gallega CrNi ore. Similarly to Li, LREE, fluid-mobile elements (K, Rb, Ba), and Sr in orthopyroxenes from the CrNi ores display enrichment from the inner to the outer parts of the ultramafic massif. These geochemical variations suggest that Li enrichment in CrNi ores and host peridotites was a twofold process: (1) asthenospheric melt percolation slightly increased Li abundances in the SCLM peridotites by modal and cryptic metasomatism involving clinopyroxene; (2) additional infiltration of Li-bearing crustally-derived fluids during the intracrustal emplacement of the mantle section boosted the Li contents of minerals in the CrNi ores. Our results highlight that Li may effectively track the interaction of the SCLM with crustal components.

Unraveling the Protolith Origin of Serpentinite Rocks from the Tengnoupal Area, Manipur Ophiolite Complex, Indo-Myanmar Range

Objective: This article mainly focuses on investigating the protolith origin of serpentinite rocks in and around Tengnoupal, within the Manipur Ophiolite Complex (MOC), using a combined petrographic and geochemical approach. Methods: During fieldwork, samples were collected and cut into thin sections to reveal their mineralogical and textural relationships. Major oxides and trace elements were determined by XRF, and rare earth elements analysis was carried out by ICP-MS to determine the nature and origin of the rocks. Findings: The studied rock samples exhibit varying degrees of serpentinization, ranging from partial to complete, and exhibit a pseudomorphic texture including mesh and hour structures replacing the original olivine and bastite texture replacing pyroxene. Geochemically, the analyzed rock types exhibit komatiitic affinity and are characterized by high magnesium numbers (Mg# average 84.4). The studied serpentinite rocks have lower Al/Si ratios (Average=0.05) and higher Mg/Si ratios (average= 0.95) in bulk rock compositions indicating a refractory abyssal mantle peridotite protolith when compared to the primitive mantle. The high Ti (range 300-480 ppm) and Cr (628-3186 ppm) contents indicate a subduction environment. In addition, the rock samples exhibit slight depletion chondrite-normalized Rare Earth Elements (REE) patterns ((La/Yb) N = average 0.87). The rocks exhibit spoon-shaped normalized Rare Earth Element patterns and primitive mantle are similar to mid-oceanic ridge abyssal mantle rocks. Novelty: The origins of the rocks studied suggest a transitional phase between abyssal and subduction zone-related serpentinites, forming after harzburgite-peridotite. As subduction began, dehydrated orogenic peridotite from the abyssal serpentinites was thrust or obducted, forming the MOC serpentinites. This indicates that the serpentinite rocks in MOC share similarities with those found in mid-oceanic ridge environments, which is characteristic of a supra-subduction zone setting. Keywords: Ultramafic rocks, Serpentinization, Supra­subduction zone, Abyssal, REE

Deformation and melt–rock interaction in the upper mantle: Insights from the layered structure of the Horoman peridotite, Japan

To obtain a better understanding of melt–rock interactions in the upper mantle, microstructural and petrological analyses were conducted on deformed mantle peridotites from the Horoman peridotite complex, Hokkaido, Japan. The Horoman peridotite complex is lithologically heterogeneous and contains various kinds of ultramafic and mafic rocks. We studied an outcrop of 3 × 70 m in size that contains layered spinel harzburgite, plagioclase lherzolite, and mafic rocks. The results indicate that reactive melts migrated preferentially along the foliation in the already deformed peridotite, and that these melt-rich zones became especially prone to further deformation. This inference is supported by (1) the parallelism of the boundaries of rock layers and foliation in the deformed peridotite, and the shape and crystallographic preferred orientations (SPOs and CPOs) of olivine in the peridotites; (2) the diffusive trends of magnesium and modal compositions of pargasite grains near the boundaries between peridotite and mafic layers; (3) variations in the NiO content of olivine crystals; (4) variations in olivine CPOs with orthorhombic (010)[100] slip system patterns and weak fiber-[010] patterns; and (5) the strong pargasite SPOs, the cuspate shapes of the pargasites, and the absence of intercrystallite deformation. The results, combined with previously reported P–T conditions for the Horoman peridotite complex, indicate that the deformed peridotites and mafic rocks with a layered structure represent temperatures of 1050–1150 °C and pressures of 0.7–1.5 GPa. Our results suggest that a decrease in pressure led to the transition from a melt-free to a melt-bearing system with a consequent change in the deformation mechanism, from dislocation creep in the melt-free system to diffusion creep in the melt-bearing system, with strain localization in the fine-grained melt-rich layers. The change in deformation mechanism is likely to have occurred in the uppermost mantle beneath a mid-ocean ridge, where strong rheological contrasts are controlled by spatial variations in the melt fraction.

Heavy halogen compositions of peridotite massifs in the Ivrea-Verbano Zone and implications for strong modification of mantle rocks

The lithospheric mantle affected by metasomatism is an important reservoir for halogens and often preserves significant heterogeneity in halogen abundances. However, the halogen compositions of the non-metasomatized mantle rocks with depleted geochemical features have not been well studied, impeding a comprehensive understanding of the halogen distribution in the mantle. The Balmuccia (BM) and Baldissero (BD) peridotite massifs in the Ivrea-Verbano Zone, Italian Alps, represent fragments of the sub-continental lithospheric mantle (SCLM) that are fresh and devoid of subduction-related mantle metasomatism. Here we examine the heavy halogen contents of well-characterized spinel-facies mantle peridotites (n = 20) and pyroxenites (n = 16) from the BM and BD massifs. The results indicate significant variation in whole rock halogen contents of peridotites and pyroxenites, with chlorine (Cl) ranging from <30 to 244 μg/g, bromine (Br) from <0.10 to 0.94 μg/g, and iodine (I) from <0.015 to 0.12 μg/g. Some peridotites and pyroxenites show rather low halogen contents (below the detection limit), consistent with their depleted features. However, a large number of samples display halogen contents far higher than the depleted MORB mantle (i.e., Cl: 3–10 μg/g) and peridotite xenoliths from the strongly metasomatized mantle, although ∼30 % of the halogens were leached out by ultrapure water or dilute nitric acid. The data suggest variable halogen enrichment in many mantle rocks. The halogen contents of different types of mantle rocks do not vary with major elements and incompatible trace elements (e.g., Nb), indicating that halogen compositions are not controlled by magmatic processes such as melting and melt-peridotite reaction. Instead, secondary fluid inclusions are abundant and mainly distributed as trails within olivine and pyroxene in both peridotites and pyroxenites, which were likely trapped during the exhumation of these mantle rocks. Their elevated halogen/Nb ratios compared with MORB mantle and strong correlations with Ba/Nb support the effect of fluids. Micro-Raman analyses identify hydrous serpentines within fluid inclusions. Moreover, mass balance calculation indicates that halogen enrichment is mainly caused by secondary fluids (> 90 %). Together, our findings highlight that the fluid percolation in the mantle rocks during exhumation can intensely modify the halogens, to a similar degree as mantle metasomatism, suggesting the sensitivity of halogens to secondary modification.

Systematic behaviour of 3He/4He in Earth’s continental mantle

Helium isotopes are unrivalled tracers of the origins of melts in the Earth’s convecting mantle but their role in determining melt contributions from the shallower and rigid lithospheric mantle is more ambiguous. We have acquired new 3He/4He data for olivine and pyroxene separates from 47 well-characterised mantle xenoliths from global on- and off-craton settings. When combined with existing data they demonstrate a new systematic relationship between fluid-hosted 3He/4He and major and trace element composition of host minerals and whole rock. We show that a significant proportion (>70 %) of mantle peridotites from continental off-craton settings with depleted major element compositions (e.g., olivine Mg# ≥ 89.5) have 3He/4He in the range of modern-day mid-ocean ridge basalt (MORB) source mantle (7–9 Ra) and we propose that they represent underplated melt residues, which initially formed in the convecting upper mantle. Furthermore, we observe that off-craton mantle xenoliths with signatures often attributed to enrichment by melts or fluids from ‘ancient’ subducted oceanic lithosphere have lower 3He/4He (<7 Ra). Modest correlations between 3He/4He and whole rock incompatible trace element signatures commonly used as proxies for metasomatism by small-fraction carbonatite and silicate melts or C-O-H fluids characterise lithospheric mantle with 3He/4He ranging from 5 to 8 Ra.Using a numerical model that integrates temperature-dependent melt extraction from the upper mantle with in-situ radiogenic ingrowth of 4He in the continental mantle we show that the initial 3He/4He of continental lithosphere mantle has decreased over time. This is consistent with previous observations demonstrating that ancient (2.5–3.5 Ga) cratonic mantle has a depleted mineral chemistry (e.g., olivine Mg# = 91–94) and low 3He/4He (0.5–6.7 Ra), while continental off-craton mantle (<2.5 Ga) is more fertile (olivine Mg# = 88–92) and has less radiogenic 3He/4He (4–8.8 Ra). This relationship defines a ‘global lithospheric mantle array’ for intraplate peridotites on plots of 3He/4He vs olivine Mg#. Peridotites influenced by past and present subduction fluids, including those that contain amphibole, plot off this array. Our findings have broad implications for the 3He/4He signatures observed in continental magmas. Many of Earth’s deepest melts, i.e. proto-kimberlites, are characterised by relatively low 3He/4He. We attribute this to assimilation and incorporation of low 3He/4He cratonic mantle material during ascent of carbonate-rich melts through thick lithosphere, which overprints the original signatures. Moreover, our findings suggests that the lithospheric mantle acts as a long-term reservoir for other fluid-hosted volatiles (e.g., CO2, CH4, H2O), and in some cases able to sequester these over billion-year timescales until physio-chemical perturbation (e.g., during major rifting or heating events).

Crystals and melt inclusions record deep storage of superhydrous magma prior to the largest known eruption of Cerro Machín volcano, Colombia

Abstract Cerro Machín, a volcano located in the northern segment of the Andes, is considered one of the most dangerous volcanoes in Colombia with an explosive record that involves at least five plinian events. Prior studies focused on the last dome-building eruption have suggested the presence of a water-rich mid-crustal magma reservoir. However, no direct volatile measurements have been published and little work has been completed on the explosive products of the volcano. Here, we study the largest known eruption of Cerro Machín volcano which occurred 3600 yr BP producing dacitic pyroclastic fall deposits that can be traced up to 40 km from the vent. Lapilli pumice clasts have a mineral assemblage of plagioclase, amphibole, quartz, and biotite phenocrysts, with accessory olivine, Fe-Ti oxides, and apatite. The occurrence of Fo89-92 olivine rimmed by high Mg# amphibole and the established high-water contents in the magma imply the presence of magma near or at water saturation at pressures &amp;gt;~500 MPa. Measurements of up to 10.7 wt% H2O in melt inclusions hosted in plagioclase and quartz in the 3600 years BP eruption products support the idea that Cerro Machín is a remarkably water-rich volcanic system. Moreover, this is supported by measurements of ~103 – 161 ppm H2O in plagioclase phenocrysts. The application of two parameterizations of water partitioning between plagioclase and silicate melt allows us to use our water in plagioclase measurements to estimate equilibrium melt water contents of 5 ± 1 – 11 ± 2 wt% H2O, which are in good agreement with the water contents we measured in melt inclusions. Results of amphibole geobarometry are consistent with a magma reservoir stored in the mid-to-lower crust at a modal pressure of 700 ± 250 MPa, corresponding to a depth of ~25 km. Minor crystallization in the shallow crust is also recorded by amphibole barometry and calculated entrapment pressures in melt inclusions. Amphibole is present as unzoned and zoned crystals. Two populations of unzoned amphibole crystals are present, the most abundant indicate crystallization conditions of 853 ± 26 °C (1 se; standard error), and the less abundant crystallized at an average temperature of 944 ± 24 °C (1 se). Approximately 18% of the amphibole crystals are normally or reversely zoned, providing evidence for a minor recharge event that could have been the trigger mechanism for the explosive eruption. Plagioclase crystals also show normal and reverse zoning. The moderate Ni concentrations (&amp;lt;1600 μg/g) in the high-Fo olivine xenocrysts suggest that Cerro Machín primary magmas are generated by inefficient interaction of mantle peridotite with a high-silica melt produced by slab melting of basaltic material. Some sediment input is also suggested by the high Pb/Th (&amp;gt;2.2), Th/La (0.3 – 0.4), and low La/Th (&amp;lt;13; relative to mantle array) ratios. Whole rock chemistry reveals heavy rare earth element (HREE) depletion and Sr enrichment that likely formed during the crystallization of garnet and amphibole in the upper part of the mantle or lower portion of the crust, promoting the formation of water-rich dacitic magma that was then injected into the middle-to-lower crust. Textural and compositional differences in the crystal cargo that erupted during dome-building and plinian events support the idea that large volumes of magma recharge lead to effusive eruptions, while only small recharge events are needed to trigger plinian eruptions at Cerro Machín.

The Role of Mafic Intrusions in Producing Alkaline Vent Fluids at the Lost City Hydrothermal Field: Evidence from Stable Potassium Isotopes

The alkaline, H2-rich vent fluids of the Lost City Hydrothermal Field (LCHF) have generally been attributed to serpentinization of tectonically uplifted mantle peridotite. However, elevated concentrations of highly incompatible and fluid mobile trace alkali metals, Rb and Cs, indicate reaction with relatively unaltered mafic components (Seyfried et al., 2015). Here, we present high-precision stable-K isotope analyses of LCHF vent fluids, which provide new constraints on source-rock lithology and support more quantitative estimates of fluid:rock ratio, providing a new constraint on heat and mass transfer processes. We find that δ41K values of LCHF vent fluids (0.03–0.07 ‰) are distinct from local seawater (0.13 ± 0.02 ‰). In combination with near-seawater concentrations of K (10.4 ± 0.1 mmol/kg), these data are incompatible with hydrothermal alteration of peridotite alone, which is highly depleted in K. Instead, K-isotope and concentration data are consistent with hydrothermal alteration of mafic rocks (e.g., gabbro or diabase) at a fluid:rock ratio of 6–26, a range that agrees well with Rb, Cs, and 87Sr/86Sr-based estimates of fluid:rock ratio, updated here to reflect derivation from mafic source rocks.Geochemical modeling of hydrothermal fluid-rock reactions indicates that LCHF vent fluids can be effectively reproduced by a two-stage reaction in which seawater initially reacts at 200–300 °C with mafic rocks at in a fluid:rock ratio of ∼ 10 —as constrained by the above isotopic and trace-element systematics— and subsequently reacts with peridotite at a fluid:rock ratio of ∼ 30 at roughly the same temperature, pressure conditions. We also note that LCHF vent fluids are ∼ 2 % depleted in Cl compared to seawater, which may reflect admixture of a vapor component derived from ongoing phase separation at higher (> 450 °C) temperatures. We thus propose that the LCHF represents waning-stage hydrothermal activity initiated by a discrete off-axis mafic intrusion that has subsequently cooled to intermediate temperatures conducive to olivine hydrolysis and production of alkaline vent fluids. In this interpretation, alkaline H2-rich vent fields occupy a narrow thermal window in the expected evolution of hydrothermal activity associated with episodic off-axis mafic intrusions into tectonically exposed ultramafic host rocks and are thus likely to be rare on the modern seafloor.

Textural and chemical variations in peridotite induced by hydrous melt migration in mantle under the back-arc spreading

This study presents the structural and petrological characteristics of mantle peridotites in relation to the rock-hydrous melt reaction found in the Hayachine ultramafic complex, NE Japan. We conducted sampling, microstructural observations, crystal-orientation analyses, and major element composition analyses of the major constituent minerals in the peridotites. We used 17 serpentinized peridotites that preserved better mantle textures. The peridotites are composed of lherzolite to harzburgite, consisting of olivine, orthopyroxene, clinopyroxene, amphibole, and spinel. The peridotites were classified into two textures according to olivine grain size: coarse-grained (ca. 2–3 mm) and fine-grained (ca. 0.3–0.5 mm). Fine-grained peridotites were newly found in this study and were characterized by aggregates of orthopyroxene and amphibole with a large number of spinel inclusions. Based on olivine crystal-preferred orientation (CPO), we found that the coarse-grained peridotites were further classified into Group 1 (A type CPO) and 2 (AG type CPO) and the fine-grained peridotites were accordingly classified into Group 3 (weak CPO). The systematic continuity in the chemical compositions of the minerals suggests that Group 1 peridotites partially melted to form Group 2 peridotites, followed by Group 3 peridotites, due to further reaction of Group 2 peridotites with hydrous melts. These textural and chemical variations in the peridotites could have resulted from rock-hydrous melt reactions under back-arc spreading and subsequent processes.