High Degrees Of Partial Melting Research Articles

Constraints on the Genesis of Cobalt Deposits: Part II. Applications to Natural Systems

AbstractIn a companion paper in this issue, the authors reviewed the properties of cobalt, its mineralogy, and the processes that concentrate it to exploitable levels. Using this information and knowledge of the geology of the principal types of cobalt deposits, the present paper assesses the conditions and controls of cobalt transport and deposition and develops/refines plausible models for the genesis of these deposits. Economic cobalt deposits owe their origins to the compatible nature of Co2+, which causes it to concentrate in the mantle, mainly in olivine, and to be released to magmas only after high degrees of partial melting (i.e., to komatiitic and basaltic magmas). Thus, there is a very close association between cobalt deposits and mafic and ultramafic rocks. Magmatic deposits, in which Co is subordinate to Ni, develop through sulfide-silicate liquid immiscibility as a result of the very strong preference of these metals for the sulfide liquid. Predictably, these deposits reach their highest grades where hosted by olivine-rich ultramafic rocks. Approximately 60% of the world’s cobalt resource is of hydrothermal origin and is contained in sediment-hosted copper deposits in the Democratic Republic of the Congo. Using a combination of thermodynamic data and geologic information, we have refined a model in which Co is leached from mafic and ultramafic rocks by oxidized, chloride-rich hydrothermal fluids, derived from evaporation, and deposited in response to decreasing fO2 in carbonaceous sediments that accumulated in intracratonic rift basins. Economic Co deposits also develop as hydrothermal vein systems, in which Co is the primary ore metal. In the only deposits of this type that are currently being exploited (Bou Azzer, Morocco), the source of the Co was an adjacent serpentinized peridotite. The ore fluid was an oxidized, high-salinity brine derived from evaporites, and deposition occurred in response to pH neutralization by the felsic to intermediate igneous host. The final major class of Co deposits is laterite-hosted and develops on olivine-rich ultramafic rocks or their serpentinized equivalents. Our thermodynamic modeling shows that Co is leached from an ultramafic source by mildly acidic fluids as Co2+ and is transported down the laterite profile, eventually concentrating by a combination of adsorption on Mn oxides, incorporation in the structure of absolane (an Mn oxide), and precipitation as heterogenite (HCoO2). The dissolution of cobalt at the surface and its deposition at depth are controlled mainly by pH, which decreases downward; oxygen fugacity, which also decreases downward, has the opposite effect, inhibiting dissolution of cobalt at the surface and promoting it at depth. It is our hope that this introduction to the economic geology of cobalt and the processes responsible for the formation of cobalt-bearing ores will help guide future studies of cobalt ore genesis and strategies for the exploration of this critical metal.

Constraints on the Genesis of Cobalt Deposits: Part I. Theoretical Considerations

Abstract Cobalt is in high demand because of the key role that cobalt-lithium-ion batteries are playing in addressing the issue of global warming, particularly in facilitating the transition from the internal combustion engine to electrically driven vehicles. Here, we review the properties of cobalt and the history of its discovery, briefly describe its mineralogy, and explore the processes that concentrate it to potentially exploitable levels. Economic cobalt deposits owe their origin to the compatible nature of Co2+, its concentration in the mantle in olivine, and its release, after high degrees of partial melting, to komatiitic and (to a lesser extent) basaltic magmas. Primary magmatic deposits, in which Co is subordinate to Ni, develop through the separation of immiscible sulfide liquids from mafic and ultramafic magmas and the very strong partitioning of these metals into the sulfide liquid. We evaluate the factors that concentrate cobalt to economic levels by these processes. Cobalt is also concentrated by aqueous fluids, either at ambient temperature in laterites developed over ultramafic rocks or hydrothermally in sediment-hosted copper deposits and in cobalt-rich vein deposits, where it crystallizes mainly as sulfide and arsenic-bearing minerals, respectively. Using the available thermodynamic data for aqueous Co species, we evaluate cobalt speciation as a function of temperature and show that, whereas it is transported at ambient temperature in most environments as the simple ion (Co2+), it is most mobile in hydrothermal systems as chloride species. Based on thermodynamic data compiled from a variety of sources, we evaluate stability relationships among some of the principal cobalt sulfide and oxide minerals as a function of temperature, pH, fO2, and αH2S and, in conjunction with the aqueous speciation data, determine their solubility. This information is used, in turn, to predict the physicochemical conditions most favorable for cobalt transport and ore formation by hydrothermal fluids. As thermodynamic data are not available for the cobalt arsenide and sulfarsenide minerals that form the vein-type ore deposits, we use chemographic analysis to qualitatively evaluate their stability relationships and predict the physicochemical controls of ore formation. The data and interpretations of processes presented in this paper provide the theoretical basis for a companion paper in this issue in which we develop plausible models for the genesis of the principal cobalt deposit types.

Deep origin of mantle peridotites from the Aladağ ophiolite, Turkey: Implication from trace element geochemistry of pyroxenes and mineralogy of ophiolitic diamonds

The Aladağ peridotites mainly consist of harzburgites with subordinary dunites. High olivine Fo [100 × Mg/(Mg + Fe2+) = 90.8–92.0] and spinel Cr# [100 × Cr/(Cr + Al) = 61.5–71.6] values indicate the Aladağ harzburgites have experienced high degrees of partial melting. Significant depletion of middle rare earth element in the clinopyroxene and orthopyroxene implies that the partial melting of mantle source initiated in the garnet stability field. Occurrences of the interstitial clinopyroxene and spinel-hosted hydrous mineral inclusions, as well as variable enrichments of light rare earth element (LREE) and large-ion lithophile element in the clinopyroxene and orthopyroxene, suggest that the harzburgites suffered pervasive melt-rock interaction. The modeled interacting melts show enrichments of LREE and have a boninite-like affinity. In the dunites, olivines have higher contents of Fo (92.3–92.7), Ca (476–1830 ppm), and Ti (3.4–9.8 ppm) than those in the harzburgites (90.8–92.0, 50–319 ppm, and 0.6–6.4 ppm, respectively). Spinels in dunites have high Cr# (73.4–77.4) values and Ti (569–722 ppm) concentrations. Such mineral chemical characteristics imply extensive melt-rock interaction. We propose that the dunite was produced by early-stage interaction of harzburgite with migrating Si-undersaturated melt in asthenosphere. Subsequently, dunite and harzburgite together experienced late-stage interaction with boninite-like melt as incorporated into thermal boundary layer above a supra-subduction zone. An unusual mineral assemblage of ophiolitic diamond, moissanite, zircon, corundum and rutile, separated from the Aladağ harzburgites, indicates that relatively deep processes involved in the formation of the Aladağ mantle peridotites.

RETRACTED: Fe3+ partitioning between clinopyroxene and silicate melt at 1–2.5 GPa: Implications for Fe3+ content of MORB and OIB source mantle

Pyroxene is the chief reservoir of Fe 3+ in upper mantle peridotite, but experiments exploring pyroxene/melt Fe 3+ partitioning ( D Fe3+ pyx/liq ) have been restricted to 100 kPa and pyroxene with low alumina. Here we present Fe 3+ partitioning experiments between clinopyroxenes (cpx) and mafic melt at elevated pressures (1–2.5 GPa). Experiments were conducted with f O2 buffered and modulated by Ru + RuO 2 and Fe-Pt alloy capsules, respectively, between ΔQFM −2.7 and +5.1. Fe 3+ /Fe T of both cpx and melt were determined by Fe K -edge X-ray absorption near edge structure spectroscopy. The experimentally synthesized cpx compositions (Al 2 O 3 = 2.36–6.01 wt.%, CaO = 19.33–22.21 wt.%) approximate those expected in basalt source regions. We find that Fe 3+ is moderately incompatible in cpx and D Fe3+ cpx/liq correlates with cpx Al 2 O 3 content, increasing from 0.05 ± 0.09 to 0.81 ± 0.04. D Fe3+ cpx/liq also increases with increasing f O2 . Comparison between experimentally synthesized cpx with those from natural peridotites indicates influences of both temperature and composition on Fe 3+ /Fe T for cpx in spinel and garnet peridotites. The combined effects of decreased pyroxene Al 2 O 3 concentration and pyroxene mode with progressive partial melting of peridotite diminishes the bulk partition coefficients of Fe 3+ , leading to greater Fe 2 O 3 contents in high degree partial melts, and this accounts for an inverse relationship between Na 2 O and Fe 2 O 3 observed in mid-ocean ridge basalts (MORB). Comparison to numerical experiments with pMELTS and the model of Jennings and Holland (2015) show that these models overpredict D Fe3+ cpx/liq for partial melting of the mantle, and so they do not accurately determine the relationship between the f O2 and Fe 2 O 3 of peridotite in basalt source regions. To estimate the Fe 3+ /Fe T ratio of the mantle source of MORB, we modeled liquid Fe 2 O 3 during isentropic batch melting of peridotite at three potential temperatures (1320 °C, 1400 °C, and 1440 °C) for peridotitic sources with Fe 3+ /Fe T ratios between 0.02–0.06. A source with an Fe 3+ /Fe T ratio of 0.038 ± 0.007 matches most of the span of natural MORB. This ratio is similar to that typical of continental lithospheric mantle sampled by xenoliths, but lower than that surmised by several recent experimental and thermodynamic studies. Considering this source Fe 3+ /Fe T but extending the partial melting calculations to higher pressures (2.5–4 GPa) reveals that bulk D Fe3+ perid/liq significantly decreases for garnet peridotite relative to spinel peridotite because the cpx become significantly less aluminous with increasing pressure. This results in high pressure partial melts with greater liquid Fe 3+ /Fe T ratios. Therefore, elevated Fe 3+ /Fe T ratios observed from some oceanic island basalts (OIB), such as those from Hawaii and Iceland, reflect in part the differences in conditions of melting and may not require mantle source regions more oxidized than those that produce MORB.

Multiple Mantle Sources of High‐Magnesium Terranes on Mercury

AbstractThe composition and interior structure of Mercury's mantle are poorly known but critical to understanding its magma ocean differentiation, mantle dynamics, and crustal petrogenesis. Various crustal geochemical terranes are identified on Mercury based on their Mg/Si and Al/Si values, but element abundances exhibit a weak correlation with surface reflectances. The high‐magnesium region (HMR), which includes end‐members for both reflectance spectral units (high‐reflectance plains and low‐reflectance materials (HRP and LRM)) and geochemical terranes (Mg and Al contents), contains important information about Mercury's mantle composition and dynamics. We propose that the northwestern boundary of HMR can be extended to the Sobkou basin and its ejecta deposits. Within the resolution limit of element maps, we notice that mantle materials beneath the Sobkou basin have an elevated content of magnesium, and a large area of HRP‐like materials in HMR exhibit Mg/Si greater than 0.5. Our results suggest that HMR was formed by partial melting of heterogeneous mantle sources in terms of compositions, supporting the existence of stratified and/or lateral geochemical diversities in the mantle. While typical HRP on Mercury, including the Borealis Planitia and Caloris interior plains formed by effusive volcanism around 3.7 Ga, exhibit a lower Mg/Si than the global average, the Rachmaninoff interior plains emplaced by volcanism at 1.7 Ga exhibit a Mg/Si greater than 0.5. This observation indicates that local thermal anomalies may be prolonged in the mantle, inducing a higher degree of partial melting. We also report a weak correlation between the global LRM and magnesium content in the crust.

Lithospheric Structure underneath the Archean Tanzania Craton and Adjacent Regions from a Joint Inversion of Receiver Functions and Rayleigh-Wave Phase Velocity Dispersion

Abstract Lithospheric structure beneath the Archean Tanzania craton and adjacent regions, including segments of the East African rift system (EARS) and the Proterozoic–early Paleozoic orogenic belts between the EARS and the craton, is imaged by a joint inversion of receiver functions and Rayleigh wave dispersion measurements derived from ambient seismic noise for shorter periods and teleseismic data for longer periods. Our resulting crustal thickness, crustal VP/VS measurements and 3D shear-wave velocity model for the upper 120 km show a clear spatial correspondence with major surficial geological features. The new results suggest the presence of a mafic layer in the bottom of the crust of the entire Archean craton, which is previously only identified beneath the southern portion of the craton. High crustal VP/VS values measured in the Rungwe Volcanic Province and most areas of the Kenya and Tanganyika rift segments can be attributed to a combined result of basaltic sediments atop the crust, magmatic intrusion, and crustal partial melting. The Kivu Volcanic Province and parts of the Kenya rift segment are characterized by localized lower-than-normal crustal VP/VS values and shear velocities in the lower crust and uppermost mantle, which, given the presence of large volume of CO2 from surficial observations, can be best interpreted by CO2-filled fractures or conduits. Lower-than-normal shear velocities in the uppermost mantle are revealed beneath almost the entire study region with the lowest values found in all the three volcanic provinces. The low velocities are indicative of an underplated layer formed by mantle-derived magmatic materials trapped below the Moho. The relatively low velocities beneath the volcanic provinces might be caused by a higher degree of partial melting in the uppermost mantle.

New U[sbnd]Pb zircon and geochemical constraints on Late Devonian Back-arc basin origin of eclogite protoliths from northeastern Hainan Island, South China

Geochronological, geochemical, and SrNd isotopic data are used to constrain the age, origin, and tectonic setting of the protoliths of newly discovered eclogites from Chaotanbi, northeastern Hainan Island, South China. New sensitive high-resolution ion microprobe (SHRIMP) UPb zircon dating reveals that the protoliths of the Chaotanbi eclogites formed at 364 ± 2 Ma and were metamorphosed at 297 ± 1 Ma. All samples collected from a coherent area of ~1.8 km2 in the intertidal and subtidal zones on the island exhibit tholeiitic compositions, with initial 87Sr/86Sr and εNd values of 0.705–0.707 and − 1.1 to +7.4, respectively. The samples can be further categorized into three groups based on incompatible trace elements. Group #1 samples have a normal mid-ocean ridge basalt (N-MORB) affinity and were originated from relatively high-degree partial melting of a depleted spinel lherzolite mantle source slightly modified by slab-derived fluids. Group #2 samples have an enriched mid-ocean ridge basalt (E-MORB) affinity and were derived from an enriched spinel and spinel–garnet lherzolite transition mantle source at depths greater than that of Group #1. Group #3 samples have an island arc basalt (IAB) affinity and were generated by partial melting of an enriched spinel–garnet lherzolite mantle source modified by fluid- and sediment-related recycled components. The association of these diverse basalt types, with MORB-type samples accounting for ~90% of all samples, suggest a Late Devonian mature back-arc basin existed on northeastern Hainan Island, as a part of the Jinshajiang–Ailaoshan–Song Ma–Hainan back-arc basin system. Therefore, the Chaotanbi eclogites are a fingerprint of the opening and closure of the eastern Paleo-Tethyan Ocean.

Depletion and refertilisation of the lithospheric mantle below the Kapsiki plateau (Northern Cameroon Volcanic Line) deduced from trace element and H2O systematics in mantle xenoliths

Clinopyroxene bearing spinel harzburgites recovered from Cenozoic alkaline basalts from the Kapsiki plateau were investigated with the aim of understanding depletion and enrichment processes in the subcontinental mantle below a major tectonic line. Concentrations of major and trace elements (including H) were determined in-situ in olivine, orthopyroxene, clinopyroxene and spinel by EPMA, LA-ICP-MS and FTIR. The xenoliths are refractory with high olivine (0.91–0.92) and orthopyroxene (0.92–0.93) Mg# coupled with relatively high modal proportions of clinopyroxene (5–8%) and a remarkably wide range in clinopyroxene Mg# (0.91–0.94), interpreted to reflect high degrees of partial melting followed by modal and chemical metasomatic refertilization. A fractional partial melting model of orthopyroxene trace elements in a depleted mantle indicates 20–30% of melting. Clinopyroxenes show two groups based on their CaO contents (18.7–18.9 and 20.6–22.6 wt %) suggesting a secondary origin and modal metasomatism. The low Ca clinopyroxenes were the latest to be formed, and the wide range of clinopyroxene Mg# shows a lack of equilibrium with the coexisting minerals. The high Ca clinopyroxenes record ancient metasomatic events that are also documented in orthopyroxene. Cryptic metasomatism is shown by strong progressive enrichments in LREEs and MREEs of orthopyroxene and clinopyroxene. Th vs La and Sr vs Nd of clinopyroxene are used to identify different generations of metasomatism. Ti/Eu vs (La/Yb)N and Zr/Hf systematics indicate that the metasomatising agent has both a carbonatite and a silicate component. The structural hydroxyl contents of the different mineral phases are very low (<1 ppm for olivine, 40–48 ppm for orthopyroxene and 164–277 ppm for clinopyroxene). The very low water contents compared to typical sub-continental peridotites could be attributed to the depleted nature of the xenoliths or reflect lower water activities of carbonatitic melts relative to alkaline melts. Estimated fO2 values expressed as ΔlogFMQ, (where FMQ corresponds to the fayalite magnetite-quartz oxygen buffer) range from -0.42 to -0.26 and are consistent with minor OH stretching regions of 3520 cm-1 in orthopyroxene, related to a ferric iron defect.The combination of textural investigations, major, and trace element and H2O data suggests that a refractory subcontinental mantle was affected by several episodes of intraplate metasomatism dominated by silicate-carbonatite melts above the North-Eastern end of the Cameroon Volcanic Line.

Rh, Ir, and Ru Partitioning in the Cu-Poor IPGE Massive Ores, Talnakh Intrusion, Skalisty Mine, Russia

Pyrrhotite (or Cu-poor) massive ores of the Skalisty mine located in Siberia, Russia, are unique in terms of their geochemical features. These ores are Ni-rich with Ni/Cu ratios in the range 1.3–1.9 and contain up to 12.25 ppm Ir + Rh + Ru in bulk composition, one of the highest IPGE contents for the Norilsk–Talnakh ore camp. The reasons behind such significant IPGE Contents cannot simply be explained by the influence of discrete platinum-group minerals on the final bulk composition of IPGE because only inclusions of Pd minerals such as menshikovite, majakite, and mertieite II in Pd-maucherite were observed. According to LA-ICP-MS data obtained, base metal sulfides such as pyrrhotite, pentlandite, and pyrite contain IPGE as the trace elements. The most significant IPGE concentrator being Py, which occurs only in the least fractionated ores, and contains Os up to 4.8 ppm, Ir about 6.9 ppm, Ru about 38.3 ppm, Rh about 36 ppm, and Pt about 62.6 ppm. High IPGE contents in the sulfide melt may be due to high degrees of partial melting of the mantle, interaction with several low-grade IPGE impulses of magma, and (or) fractionation of the sulfide melt in the magma chamber.

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.

Petrography and Geochemistry of Gabbroic Rock from the Penjwin Ophiolite, Kurdistan Region, Northeastern Iraq

The gabbroic rocks as a part of Zagros ophiolite are exposed in northeastern Iraq, Penjwin area. These rocks with granular to ophitic textures are widely distributed without metamorphic halos. The main minerals are plagioclase (An90-99), olivine, clinopyroxene (Wo27-47 En 45-64 Fs8-14) and orthopyroxene (Wo2 En78 Fs20) respectively based on the abundances. The major elements show a broad range of compositional variations, with SiO2 (46.2–50.9 wt. %), and low concentrations Na2O (0.15–0.62 wt. %), K2O (0.01–0.03 wt. %) and TiO2 (0.06–0.2) and high concentrations, Al2O3 (6.4–19.75 wt. %), total Fe2O3 (6.29–11.6 wt. %), MgO (9.63–24.5 wt. %), CaO (8.02–18 wt. %) and low alkali contents (Na2O + K2O = 0.16–0.65 wt. %). On Ti-V diagram, all of the gabbroic samples have Ti/V less than 10 and consequently fall in the low Ti- Island arc tholeiitic. Whole rocks chemistry shows a depletion of High field strength elements in comparison with the primitive mantle with an arched upward rare earth elements pattern, characterized by light rare earth elements depletion (La N/Sm N = 0.05–0.8) and enrichment in the High field strength elements. Whole rocks chemistry, mineral paragenesis and chemistry of these rocks are more consistent with tholeiitic magma series. Based on our findings in this research, the primary magma has been produced from the depleted mantle with a high degree of partial melting.

Deserpentinization and high-pressure (eclogite-facies) metamorphic features in the Eoarchean ultramafic body from Isua, Greenland

Discontinuous chains of ultramafic rock bodies form part of the 3800–3700 Ma Isua Supracrustal Belt (ISB), hosted in the Itsaq Gneiss Complex of southwestern Greenland. These bodies are among the world’s oldest outcrops of ultramafic rocks and hence an invaluable geologic record. Ultramafic rocks from Lens B in the northwestern limb of ISB show characteristics of several stages of serpentinization and deserpentinization forming prograde and retrograde mineral assemblages. Ti-rich humite-group minerals such as titanian chondrodite (Ti-Chn) and titanian clinohumite (Ti-Chu) often occur as accessory phases in the metamorphosed ultramafic rocks. The Ti-rich humite minerals are associated with metamorphic olivine. The host olivine is highly forsteritic (Fo96-98) with variable MnO and NiO contents. The concentrations of the rare-earth elements (REE) and high-field strength elements (HFSE) of the metamorphic olivine are higher than typical mantle olivine. The textural and chemical characteristics of the olivine indicate metamorphic origin as a result of deserpentinization of a serpentinized ultramafic protolith rather than primary assemblage reflecting mantle residues from high-degrees of partial melting. The close association of olivine, antigorite and intergrown Ti-Chn and Ti-Chu suggests pressure condition between ∼1.3–2.6 GPa within the antigorite stability field (<700 °C). The overall petrological and geochemical features of Lens B ultramafic body within the Eoarchean ISB indicate that these are allochthonous ultramafic rocks that recorded serpentine dehydration at relatively lower temperature and reached eclogite facies condition during their complex metamorphic history similar to exhumed UHP ultramafic rocks in modern subduction zone channels.

Origin of the basal lherzolite of the Muslim Bagh Ophiolite, Pakistan, deduced from the trace element characteristics of clinopyroxene

The origin of basal lherzolites in the mantle section of harzburgite‐dominated ophiolites is enigmatic. The basal part of the mantle section is well exposed in the Muslim Bagh Ophiolite, Pakistan, which is one of the harzburgite‐dominated ophiolites of the Tethys Ophiolite Belt. In this contribution, we describe the basal lherzolite of the Muslim Bagh Ophiolite, Pakistan, and discuss its origin based on the trace‐element characteristics of its clinopyroxenes. The basal lherzolite exhibits porphyroclastic to mylonitic textures. Primitive mantle‐normalized trace‐element patterns of the porphyroclastic clinopyroxenes were characterized by low ratios of light rare‐earth elements (LREEs) to heavy rare‐earth elements (HREEs), low abundances of HREEs, and positive Sr anomalies. These geochemical characteristics are not consistent with a lherzolite and its clinopyroxenes, which is formed by a residue after a low degree of partial melting and melt extraction from peridotites in a mid‐ocean ridge setting. Instead, the compositions of the clinopyroxenes are consistent with open‐system melting induced by the infiltration of slab‐derived fluids into residual peridotites that had been depleted in REEs. The compositions of chromian spinels in the chromitites of the basal peridotite sequence are also consistent with their formation in an arc setting. We conclude that the basal lherzolites of the Muslim Bagh Ophiolite represent a residue after a relatively high degree of partial melting, and that the clinopyroxenes were added as metasomatic crystallization from slab‐derived arc‐related melts to this residual depleted peridotite in a subduction setting.

Petrogenetic links between rare metal-bearing pegmatites and TTG gneisses in the West African Craton: The Mangodara district of SW Burkina Faso

We describe the geological context of rare metal-bearing pegmatites from the Mangodara district (South-West Burkina Faso, Paleoproterozoic West African Craton) and discuss their petrogenesis and links with the host rocks. The Mangodara district exposes a gneiss-granitoid complex structured in a regional-scale dome, mantled by granodioritic gneiss enclosing rafts of amphibolite, micaschist and paragneiss, and cored by tonalitic to trondhjemitic gneisses. These gneisses enclose granitoid plutons, and four populations of rare metal-bearing pegmatites: titanite-allanite-, apatite-zircon-, garnet-columbite (Li, Nb) and garnet-REE (Ti, Y, HREE)-bearing varieties.Rafts of migmatitic amphibolite, micaschist and paragneiss are chemically equivalent to nearby Birimian greenstone belts. An origin of the gneiss-granitoid complex by partial melting is suggested by diffuse contacts of rafts with the gneisses as well as the presence of garnet-bearing or hornblende-bearing leucosome in paragneiss and in amphibolites, respectively. Textural continuity between pegmatites and granitic veins concordant to the foliation of the gneisses point to syntectonic segregation of the pegmatite-forming magmas.The compositions of gneisses and plutonic rocks spread between a Na-rich pole and a K-rich pole. Granodioritic gneiss, hornblende-biotite granodiorite and potassic biotite granitoids define a K-rich series attributed to relatively low-pressure high-degree partial melting of dominantly paragneiss, which accounts for low to absent LILE and HFSE fractionation compared to the paragneiss. Tonalitic-trondhjemitic gneiss, a biotite trondhjemite and a peraluminous two-mica trondhjemite belong to a Na-rich series characterized by low K content and strong depletion in REE and HFSE, which might reflect plagioclase clustering after partial melting at a relatively high pressure of amphibolite, and fractionation of HFSE-REE-bearing minerals. Thermodynamic and mass balance modeling further corroborate these propositions implying that partial melting of paragneiss and amphibolite at about 5 to 10 kbars for a temperature of 780 to 850 °C followed by melt/solid segregation and fractional crystallization are susceptible to generate the Na- and K-rich series.U-Pb dating of zircon from a titanite-allanite-bearing pegmatite at 2094.3 ± 8.8 Ma and of apatite from tonalitic-trondhjemitic gneiss, apatite-zircon-bearing pegmatite and granodioritic gneiss at 2094 ± 21 Ma, 2055 ± 20 Ma, and 2041 ± 33 Ma respectively, confirm that partial melting, melt segregation and crystallization-cooling occurred during the Eburnean orogeny.Based on these data, we propose that (i) titanite-allanite- and apatite-zircon-bearing pegmatites result from syntectonic segregation of residual melt during crystallization of the tonalitic-trondhjemitic gneiss, (ii) garnet-columbite-bearing pegmatites originate from syntectonic melt segregation from the migmatitic paragneiss, and (iii) garnet-REE-bearing pegmatites derive from segregation of a residual melt within the granodioritic gneiss.

Petrographic and geochemical constraints on the evolution of the Matarazzo Sequence, Arroio Grande Ophiolite, Brazil: Evidence from migmatites and marbles

This study comprises the amphibolites, marbles, granitoids, metadiabases, and skarns of the Matarazzo Sequence (Arroio Grande Ophiolite, Dom Feliciano Belt, Brazil), which are here, for the first time, discussed from a migmatite perspective. The contribution presents novel petrographic and elemental geochemical data for the Sequence. Petrographic evidence demonstrates that high degrees of partial melting of the paleosomes (amphibolites) led to the development of millimeter-to meter-sized networks of mesocratic neosome veins (diorite to quartz-monzodiorite). The partial melting of the neosomes, in turn, led to the formation of micrometer-to meter-sized veins and pods of leucosomes (quartz-syenite to monzogranite). Garnet- and epidote-rich skarn layers were generated by the marble/melt interaction. The synkinematic nature of the partial melting event is evidenced by macrostructures such as rotated amphibolite schollen, neosome/leucosome veins concordant to the NE-SW-trending regional shear structures, and accumulation of melt in the hinge of folds. The minimum crystallization temperature of the neosomes and leucosomes are constrained, respectively, at 709 ± 31 and 732 ± 63 °C (Zr saturation thermometry). The geochemical data show that the trace element behavior of the paleosomes, neosomes and leucosomes are similar, strengthening the hypothesis of a genetic relationship and the interpretation that the metadiabases and amphibolites are the protoliths of the diatexites. The geochemical signature of the metadiabases and amphibolites suggest magmatic protoliths developed in an oceanic back-arc setting. The composition of the associated marbles is also consistent with this tectonic setting, reinforcing previous hypotheses that suggest that the studied ophiolite represents metamorphic remnants of an oceanic back-arc basin (South Adamastor paleo-ocean). The minimum metamorphic and obduction age of the Arroio Grande Ophiolite is, so far, marked by the U–Pb crystallization age of the studied leucosomes at 640 Ma. Since the same age is recorded in migmatitic orthogneisses from the southernmost sector of the Punta del Este Terrane in Uruguay, we suggest that the Punta del Este Terrane reached, synchronically, partial melting conditions at 640 Ma.

Mineral chemistry and geochemistry of serpentinites from the Bianmagou ophiolite in the North Qilian Belt, NW China: Implications for protoliths, melt extractions, and melt/fluid metasomatism

The North Qilian Belt in northwestern China contains a large number of ophiolites. Most ultramafic rocks from the ophiolites have been entirely transformed into serpentinites because of extensive serpentinization, making it difficult to determine their origin and tectonic significance and further hindering advancements in the understanding of the evolutionary history of the Palaeozoic North Qilian Ocean. In this study, we present new mineral chemistry and whole‐rock geochemistry analyses (including rare earth elements, [REE]) for the serpentinites from the Bianmagou ophiolite to discuss their protoliths, geodynamic environment, melt extractions, and melt/fluid interactions. The mineral chemistry of the relic minerals (e.g., olivine, orthopyroxene, and spinel) and the whole‐rock geochemical compositions (e.g., Mg#, Ni, etc.) strongly demonstrate that the protoliths of the serpentinites were refractory mantle residues after melt extractions and probably emplaced into a tectonic regime of mid‐ocean ridge akin to abyssal peridotites. They experienced high degrees of partial melting (15–20%) recorded by spinel Cr# values and bulk incompatible elements (e.g., Ti, heavy rare‐earth element), resulting from multiple episodes of melt depletion in the lithospheric mantle. Moreover, the serpentinites commonly have U‐shaped chondrite‐normalized REE patterns. The significantly elevated LREEs probably reflect the early partial‐melting processes overprinted by later melt‐dominated metasomatism. The medium to strong enrichment of certain fluid‐mobile elements (e.g., U, Pb, Ba, Sr, As, etc.) in the studied serpentinites was a consequence of the fluid/rock interactions during serpentinization. Overall, despite the variable serpentinization they experienced, this study indicates that the mineral chemical and geochemical proxies of serpentinites provide reliable constraints on the petrogenesis and mantle metasomatism of ancient ophiolites.

Late Ordovician fore-arc ophiolitic mélange in the southern margin of the Bainaimiao arc: constraints from zircon U–Pb–Hf isotopes and geochemical analyses

AbstractThe Harihada–Chegendalai ophiolitic mélange, which is located between the Bainaimiao arc and the North China Craton, holds significant clues regarding the tectonic setting of the southern margin of the Central Asian Orogenic Belt. The ophiolitic mélange is mainly composed of gabbroic and serpentinized ultramafic rocks. Here, zircon U–Pb dating, in situ zircon Hf isotopic, whole-rock geochemical and in situ mineral chemical data from the ophiolitic mélange are reported. The zircons in the gabbroic rocks yielded concordia U–Pb ages of 450–448 Ma and exhibited slightly positive ϵHf(t) values (0.87–4.34). The geochemical characteristics of the gabbroic rocks indicate that they were generated from a mantle wedge metasomatized by subduction-derived melts from sediments with continental crust contamination, in a fore-arc tectonic setting. These rocks also experienced the accumulation of plagioclase. The geochemical characteristics of the ultramafic rocks and their Cr-spinels indicate that they may constitute part of residual mantle that has experienced a high degree of partial melting and has interacted with fluids/melts released from the subducted slab in the same fore-arc tectonic setting. The ophiolitic mélange may therefore have formed in this fore-arc tectonic setting, resulting from the northward subduction of the South Bainaimiao Ocean beneath the Bainaimiao arc during Late Ordovician time, prior to the collision between the Bainaimiao arc and the North China Craton during the Silurian to Carboniferous periods.

Platinum-group element geochemistry of the Paraná flood basalts – Modelling metallogenesis in rifting continental plume environments

The 135 Ma Paraná-Etendeka Large Igneous Province (PELIP) is one of the largest areas of continental flood basalt (CFB) volcanism in the world and is widely agreed to be a product of intracontinental melts related to thermal anomalies from the Tristan mantle plume. The province rifted during the break-up of Gondwana, as the plume transitioned into an oceanic geodynamic environment. This study reports analyses of plume-derived basalts from the Brazilian side of the PELIP (the Serra Geral Group) to investigate major, trace and platinum-group element (PGE) abundances in an evolving plume-rift metallogenic setting, with the aim of contextualising metallogenic controls alongside existing magmatic interpretations of the region. The chalcophile geochemistry of these basalts defines three distinct metallogenic groupings that fit with three modern multi-element magma classifications for Serra Geral lavas. In this scheme, Type 4 lavas have a distinctive PGE-poor signature, Type 1 (Central-Northern) lavas are enriched in Pd, Au and Cu, and Type 1 (Southern) lavas are enriched in Ru and Rh. Our trace element melt modelling indicates that the compositional variations result from changes in the melting regime between the garnet and spinel stability fields, in response to the thinning and ‘unlidding’ of the rifting continent above. This process imposes progressively shallower melting depths and higher degrees of partial melting. Accordingly, Type 4 magmas formed from small degree melts, reducing the likelihood of sulphide exhaustion/chalcophile acquisition at source. Type 1 (Central-Northern) magmas incorporated components of the sub-continental lithospheric mantle (SCLM) in higher-degree partial melts; the SCLM was heterogeneously enriched via metasomatism prior to plume melting, and this produced enrichment in volatile metals (Pd, Cu, and Au) in these magmas. In contrast, the Ru-Rh enrichment in Type 1 (Southern) lavas is attributed to increased spinel-group mineral and sulphide incorporation from the mantle into higher degree partial melts close to the continental rift zone. Our models confirm the importance of contributions from SCLM melts in precious metal mineral systems within CFB provinces, and reinforce the role of heterogeneous metasomatic enrichment underneath cratons in boosting intracontinental prospectivity with respect to ore deposits.

High-precision Pb and Hf isotope and highly siderophile element abundance systematics of high-MgO Icelandic lavas

Ocean island basalts (OIB) exhibit significant variations in their 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf, 186,187Os/188Os, and 206,207,208Pb/204Pb compositions that indicate contributions from multiple mantle sources. To further constrain the relationships between diverse long-lived isotopic systems in OIB, new double-spike Pb and Hf isotope data, as well as isotope dilution highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundances are reported for nineteen Icelandic picrites and basalts for which 3He/4He, 87Sr/86Sr, 143Nd/144Nd, and 186,187Os/188Os data have been reported previously (Brandon et al., 2007. GCA). Hafnium-Nd-He isotope systematics for the sample set reveal the presence of an ancient refractory source with low-3He/4He, while He-Pb isotope correlations indicate that recycled oceanic crustal components with high-3He/4He added from a less degassed mantle source are present in the Iceland plume. The highly depleted, low-3He/4He signature is sampled in the Northern and Western Volcanic Zones, likely due to higher degrees of partial melting at those localities enabling sampling of this refractory endmember. The 187Os/188Os ratios show correlations with Pb, Hf, and Nd isotope ratios indicating binary mixing between depleted and enriched sources, with no evidence for decoupling due to sulfide metasomatism. The spread of 187Os/188Os- 206,207,208Pb/204Pb data can be explained by addition of 1–11% young recycled oceanic crust to a depleted mantle source distinct from the source of Atlantic MORB. Abundances of the HSE in Northern and Western Volcanic Zone lavas overlap with those from other OIB with similar MgO contents, showing fractionated patterns that are enriched in the more incompatible Re, Pd, and Pt over the compatible Ru, Ir, and Os. Calculated parental melt HSE abundances of those lavas also overlap with those of other OIB. Eastern Volcanic Zone lavas are highly depleted in Pt and Pd relative to other Icelandic lavas. Abundances of Pt and Pd correlate with isotopic parameters, such as 206Pb/204Pb, in Icelandic lavas, indicating that the isotopically enriched endmember sampled in the Eastern Volcanic Zone is depleted in the HSE, consistent with it being mafic in nature. The abundances of MgO in the lavas also roughly correlate with isotopic parameters, supporting the identity of the depleted endmember as ultramafic and the enriched endmember as mafic. New and previously published double-spike Pb isotope data show chiefly binary mixing with an additional component present in the vicinity of the volcano Öræfajökull. Icelandic 207Pb/204Pb and 208Pb/204Pb data plot below and above the majority of Mid-Atlantic Ridge basalt data, respectively, indicating that both depleted and enriched end-members in the plume have a lower U depletion age and a higher Th/U than ambient mantle. This is consistent with the enriched and depleted end-members within the Iceland plume being <2 Ga recycled oceanic crust and lithospheric mantle, respectively.

Petrogenesis and tectonic settings of Proterozoic mafic magmatism from the northern Indian Shield and the Himalaya: possible role for interaction of mantle plume with the subcontinental lithospheric mantle

Abstract Northern Indian Shield and the western Himalaya have an impressive record of mafic magmatism. The Aravalli Craton preserved 2.3 Ga komatiitic (picritic) and 2.1–1.8 Ga tholeiities. Gwalior and Betul belts preserved 2.1 and 1.5–1.2 Ga tholeiites, respectively. Western Himalaya has preserved 2.1–1.8 Ga tholeiites in Garhwal and Himachal regions. Studied rocks depict enriched rare earth elements, large ion lithophile elements and depleted high field strength elements. Whereas komatiites/picrites represent higher degrees of partial melting ( c. 35–40%) at higher temperatures ( c. 1500°C), tholeiites represent lower degrees of partial melting ( c. 10%) at lower temperatures ( c. 1200°C). Our results indicate interaction of mantle plume with variably enriched subcontinental lithospheric mantle sources, causing generation of these varied magmatic suites of rocks. Whereas the higher temperature komatiitic/picritic melts from the Aravalli region appear to have been generated closer to the plume head, the lower temperature tholeiitic melts from the shield region and western Himalaya were generated towards the plume margins. Different terrains of the study have undergone plume tectonics causing the development of the rift valleys. The majority of these developed into aulacogens, except for the Aravalli basin, which developed into deeper marine facies.