Silicic Magma Research Articles

Olivine Ni isotopes of Ni-Cu deposits in NW China: A potential sulfide segregation indicator in basaltic magmatic systems

Sulfide segregation from silicate magmas is essential in forming magmatic sulfide deposits, but a robust Ni-isotopic indicator has been lacking. This study investigates the Ni isotopic compositions of olivine and coexisting sulfide from three magmatic Ni-Cu deposits in East Tianshan, NW China, to reveal the behavior of Ni isotopes during magmatic differentiation and sulfide segregation processes. The δ60/58Ni values in olivine separates (+0.15 to +0.68 ‰, median 0.40 ‰) are notably higher than typical mantle and basalt values (+0.11 to +0.23 ‰). Decreased Ni at the rim of Fe-Mg unzoned olivine indicates certain degrees of post-crystallization Fe-Ni diffusion between sulfide and olivine. The intragrain diffusion modeling results show that the δ60/58Ni values in olivine can change up to 0.5 ‰ to 1 ‰ when the diffusional modification of Ni extends to the core. However, the observed thin Ni gradient at the rim and relatively flat Ni profiles in the cores of the olivine samples suggest limited kinetic modifications. Moreover, the δ60/58Ni values negatively correlate with the Ni content and Ni/Co ratio in olivine. The sulfide separates are isotopically lighter than the coexisting olivine grain, with δ60/58Niolivine – δ60/58Nisulfide values ranging from +0.10 to +0.40 ‰ (median +0.22 ‰). Given minimal Ni isotope fractionation between olivine and silicate melt, the observed Ni isotopic differences between olivine and sulfides are attributed to Ni isotope fractionation between silicate melt and sulfide melt at magmatic temperatures, efficiently incorporating isotopically light Ni into the sulfide melt. The sulfide segregation-driven Ni isotope fractionation aligns with the distinct Ni isotopic compositions observed in olivine grains from two highly differentiated mafic units of the Huangshandong deposit with different sulfide evolution histories. Olivine grains from mineralized gabbronorite have higher δ60/58Ni values (+0.68 ‰, Phase III) than those from sulfide-barren olivine gabbro (δ60/58Ni = +0.23 ‰, Phase I). Systematic stratigraphic Ni isotope variations in the Huangshanxi olivine from intervals with varying degrees of sulfide segregation and magma replenishment further support this interpretation. These findings demonstrate that Ni isotopes in olivine are reliable for tracing sulfide segregation, provided that diffusional modification is carefully assessed. With limited diffusional modification, the widespread isotopically heavy Ni signature in olivine (δ60/58Ni > 0.3 ‰) serves as a valuable indicator for sulfide segregation and exploring magmaticNi-Cu deposits.

Shape and Size Distribution of Sulfide Globules in the Raglan Magmatic Sulfide Deposit: Implications for Deposition and Exploration of Massive Sulfide Orebodies

Abstract Globular sulfide is the best-preserved textural representation of immiscible sulfide liquids in silicate magmas, containing valuable information about the mechanisms of their transport and deposition and the formation of magmatic sulfide deposits. Previous studies have indicated that sulfide globule textures may convey useful information about their proximity to massive sulfide accumulations. This study quantitatively evaluates the genetic and spatial relationships between globular sulfides and massive orebodies in Zones 8 and 14 of the Raglan Horizon of northern Quebec by investigating their geochemical characteristics and systematically measuring globule size distributions (GSDs) across this portion of the Raglan deposit group. Their compositions suggest that sulfide globules, disseminated sulfides, net-textured sulfides, and massive sulfides in Zones 8 and 14 are genetically related and geochemically indistinguishable. The sulfide GSDs show that most samples taken from locations distal to massive lenses exhibit a simple log-linear relationship resembling the result of a single homogeneous nucleation event with linear growth and relatively constant nucleation density, or disaggregation of sulfide droplets by ligament stretching. In contrast, most samples of the proximal population show kinked GSD shapes. The kinked profiles can be attributed to processes possibly including mechanical sorting, Ostwald ripening, coalescence of sulfide globules, or the mixing of two globule populations with different size distributions. Whereas mechanical sorting, Ostwald ripening, and coalescence between sulfide globules are considered as, at most, minor contributors to the GSD shapes observed in this region, the kinked GSDs are best explained as representations of a mixed population of large, transported globules deposited in an early stage and finer-grained sulfide droplets deposited in a latter stage. Based on these interpretations, an ore-forming mechanism is proposed which starts with (1) deposition of large sulfide globules in a footwall embayment during turbulent magma flow in which the finer globules remain in suspension, followed by (2) the deposition and entrapment of fine-grained sulfide microdroplets due to a transition from turbulent to a transitional or laminar flow regime, and eventually (3) the rapid downward percolation of sulfide microdroplets through the pore network of the cumulate pile to form massive sulfide pools on the hard substrate of the footwall embayment. Furthermore, we find a close spatial relationship between samples with kinked GSDs and high globule number densities and massive sulfide ores. We demonstrate the robustness of using globule number density as a pointer to massive sulfide accumulations by successfully predicting the actual locations of orebodies 8M, 14K, and 14J in a 3-D space. Overall, we suggest that the texture of large globules immersed in finely dispersed clouds of abundant disseminated small sulfide globules is a strong indicator for proximity to massive sulfide accumulations, and the recognition of this texture may provide a critical tool for future exploration for massive magmatic sulfide ore lenses. We present schematic illustrations of favorable and unfavorable GSDs to aid in the qualitative application of these concepts during logging of exploration drill core.

Behaviour of bromine in Cl- and F-bearing alkali-rich felsic magmas at crustal depth: An experimental study at 800–1100 °C, 10–200 MPa

Bromine, although a minor component in volcanic gases, has received increasing interest in recent studies due to its high atmospheric ozone depletion potential, but its behaviour in alkali-rich felsic hydrous magmas remains unexplored. In this study, fluid-melt partitioning experiments were carried out using natural, Cl- and F-bearing silicate glasses with phonolitic, comenditic and pantelleritic compositions. For each composition, experiments were performed with a range of Br concentrations, at P-T conditions simulating isothermal decompression degassing or isobaric equilibrium cooling at shallow crustal depth (800–1000 °C, 10–200 MPa, oxidising and reducing conditions). The major element, Cl and F concentrations of the run-product glasses were determined by electron microprobe and the Br concentrations by LA-ICPMS. Volatile concentrations in the fluid were determined by mass balance calculations. The experimental results show that more Br partitions into the fluid phase with increasing bulk halogen concentration. The most Br-doped experiments are typically saturated with a vapor phase and a hydrosaline liquid. Experiments at the lowest Br concentrations, closest to natural systems, show that the pressure dependence of vapor-melt Br partitioning is complex, with a minimum vapor-melt partition coefficient observed at 50 MPa in the phonolitic composition (1.8 ± 0.9 at 1000 °C and oxidising conditions). Our results indicate that the eruptive degassing of Br from peralkaline felsic magmas is restricted to the shallowest levels of the magmatic plumbing system and is likely to occur at much lower pressures than in metaluminous magmas. This observation is consistent with the composition of melt inclusions preserved in alkaline silicic magmas. An important finding is that the vapor-melt partitioning of Br and Cl decreases with increasing temperature. In phonolite, at 200 MPa and oxidising conditions, the Br vapor-melt partition coefficient decreases from 18.5 ± 3.6 at 900 °C to 3.8 ± 1.5 at 1000 °C. As the ratio between the Br and Cl vapor-melt partition coefficients is not conservative, the Br/Cl ratio in the vapor phase is likely to increase during isobaric cooling and degassing. The vapor-melt partition coefficients of Br and, to a lesser extent Cl also increase with decreasing fO2 in the phonolitic system, and the Br vapor-melt partition coefficient for a reduced alkaline magma is close to that for an oxidised calc-alkaline magma. Our results also show that during protracted storage at shallow levels, oxidised alkali- and F-rich rhyolites coexist with vapor and brine. This suggests that high F concentration promotes unmixing of the halogen-bearing phase coexisting with such melts. The exsolution of immiscible vapor and brine efficiently removes Br from peralkaline magmas and probably limits the flux of Br to the atmosphere from such magmas.

Nucleation delay controlling the formation of mafic enclaves and banded pumice

The presence of mafic enclaves and banded pumice reveals key physical processes associated with volcanic eruptions. Here, through the textural and geochemical analyses of the 3550 B.P. Waimihia deposits in Taupō, New Zealand, we demonstrate how disequilibrium crystallization controls the way magmas mix. Andesitic enclaves in pyroclastic deposits from this predominantly rhyolitic eruption consist of microlites that crystallized rapidly during mafic injection into rhyolitic host magma. The variation of microlite textures depends on enclave size, implying that mafic enclaves crystallized as discrete blobs within a host rhyolitic magma. However, gray pumice and dark bands in banded pumice are characterized by a lack of or less plagioclase microlites that should be present if equilibrium crystallization occurred. Our textural and chemical data suggest that the lack of plagioclase in gray pumice and dark bands resulted from the nucleation delay arising from the mixing with rhyolitic magma. After mafic magma broke up in a magma chamber as discrete mafic blobs, the plagioclase-free rim of the blobs was disaggregated by shear flow. The eroded mafic blobs form a hybrid magma by mixing with rhyolitic magma, which further delays the plagioclase nucleation. This hybrid magma eventually erupted as gray pumice or banded pumice, depending on the intensity of magma mingling in the conduit. We use a plagioclase nucleation delay model to calculate residence times of hours to tens of hours prior to eruption. Our mixing model with nucleation delay enables small volumes of mafic magma to mix with large amounts of silicic magma.

Diverse intrusion modes during the construction of a high-silica magma reservoir: Evidence from La Obra–Cerro Blanco intrusive suite (central Chile)

Several conceptual models have been proposed for the amalgamation of granitoid plutons, which range from incremental growth to single-stage emplacement of these systems. This diversity of views has led to intense debate about the thermomechanical state of silicic intrusions and the magma differentiation paths within the crust. In this contribution, we present a comprehensive petrologic, geochronologic, and magnetic fabric data set from the La Obra–Cerro Blanco intrusive suite, which allows us to explore the petrogenesis and magma emplacement processes in the upper crust. This intrusive suite is composed of (1) a vertically zoned granitoid intrusion in spatial association with mafic layers and stocks and (2) a cupola-like high- silica granite. We interpret this intrusive suite as assembled by diverse but coexisting intrusion mechanisms over a time span of ~1.4 m.y. from 21.4 to 19.9 Ma. As indicated by the subhorizontal magnetic lineation, the first stage was dominated by horizontal emplacement of sheet-like intrusions of intermediate compositions, which became increasingly silicic after plagioclase and amphibole fractionation throughout the crustal column. The latest stage was instead dominated by cooling, crystallization, and differentiation of a thickened granitoid body and the formation of a high-silica magma chamber. The steep magnetic lineation and the abundance of aplite and rhyolitic dikes observed in the cupola-like, high-silica granites suggest that this portion acted as an evacuation channel of high-silica magma toward shallower levels, offering a rare opportunity to understand not only silicic magma accumulation and storage in the upper crust, but also the processes connecting the plutonic and volcanic environments.

A Trachytic Monogenetic Volcano in the Changbaishan Volcanic Field, Xitudingzi Volcano: Petrogenesis, Magma Plumbing System and Implications for Non-basaltic Monogenetic Volcanism

Abstract The Changbaishan volcanic field is the largest Quaternary volcanic field in China and includes the most dangerous polygenetic volcano, Tianchi volcano. In addition to the Tianchi volcano, the Changbaishan volcanic field contains two other polygenetic volcanoes (Wangtian’e volcano and Namphothe volcano) and at least 150 monogenetic volcanoes. The compositions of these monogenetic volcanoes range from basalt to rhyolite. Basaltic monogenetic volcanoes are the most abundant and spread throughout the volcanic field, whereas intermediate to evolved monogenetic volcanoes are found only within the Touxi subfield. We know very little about these non-basaltic monogenetic volcanoes, especially the magmatic petrogenesis, volcanological origins and structure of the magma plumbing system beneath them. Therefore, in this study, the Xitudingzi volcano within the Touxi subfield is used as an example to study the petrogenesis of silicic magma and explore the structure of the magma system by combining new 40Ar/39Ar dating, whole-rock geochemistry, Sr-Nd isotopes, and mineral chemistry. The 40Ar/39Ar chronology results showed that the Xitudingzi volcano erupted at 1.17 Ma; this volcano is younger than the other monogenetic volcanoes in the Touxi subfield. The geochemical results indicated that the eruption products of the Xitudingzi volcano are trachytic and formed from the assimilation fractional crystallization (AFC) process of an alkaline basaltic parental magma. The results from the Rhyolite-MELTS and thermobarometry indicated that the magma plumbing system of the Xitudingzi volcano contained two magma chambers at depths of 8 km and 19 km, respectively. The trachytic magma of Xitudingzi was produced by the AFC process from the alkaline basaltic magmas in the deep magma chamber and then transported to the shallow magma chamber. Furthermore, differences in the trace element concentrations between the different units of the Xitudingzi volcano indicated that the shallow pre-eruptive magma chamber was compositionally zoned. Based on the results from our study, the upper crustal magma plumbing system of the Xitutingzi volcano existed independently of the magma plumbing system of the cone-construction stage of the Tianchi volcano, even though the two volcanoes had similar ages of activity as well as the same chemical composition of eruptive products. Based on these results and other existing studies, a relatively thick crust and the developed faults are likely the key factors for the formation of intermediate to evolved monogenetic volcanos in the Touxi volcanic field.

The Origin of Carbonatites—Combining the Rock Record with Available Experimental Constraints

Abstract Carbonatites are rare igneous rocks that host the largest resources of REE and Nb, yet, their genesis and evolution are far from clear. The leading models of carbonatite formation are the direct melting of carbonate-bearing peridotites, silicate-carbonatite liquid immiscibility, and fractionation of carbonated silicate melts. The validity of these models has never been robustly addressed through combining the available experimental results with the natural rock record. We thus re-evaluate the presently 633 alleged carbonatite occurrences including carbonatite type, bulk composition, mineralogy, and field exposure, followed by a review of experimental data pertinent to carbonatite genesis and evolution. Based on the available data, 454 carbonatite occurrences are magmatic, of which 87 without and 338 with spatially associated alkaline magma, 9 with kimberlites, and 20 with ultramafic cumulates only. Eighty-four percent of the magmatic occurrences contain calcite carbonatite (of which 1/3 also contain dolomite carbonatite), only 9% have dolomite but not calcite carbonatite, the incidence of dolomite carbonatite being similar for occurrences with or without associated silicate magmas. Available experimental data show that crystallization of calcite, dolomite, ankerite, and siderite at crustal conditions requires moderately alkaline and/or hydrous carbonate melts with ≥20 to 25 wt % (Na,K)2CO3 + H2O. It follows that carbonatite rocks, poor in these elements, are at best magmatic cumulates (if not carbo- or hydrothermal) that lost these ephemeral components. Carbonatitic melts could form in the lithospheric mantle, but these are always dolomitic and cannot deviate from close-to-minimum compositions when rising, their strong adiabatic cooling keeping them on the solidus until they decompose to olivine, clinopyroxene (cpx), and CO2 when reaching <2.1 GPa, i.e. the carbonated peridotite solidus ledge, which renders their extraction from the mantle highly unlikely. Furthermore, dolomitic carbonate melts crystallize periclase + calcite at crustal conditions. Only when containing ≥15 wt % (Na,K)2O + H2O they may crystallize dolomite and form dolomitic carbonatites. This value is far above the 2 to 5 wt % (Na,K)2O of mantle-derived carbonatitic melts. Liquid immiscibility from CO2-bearing close-to-natural melilititic, nephelinitic, and phonolitic melts requires 10 to 15 wt % Na2O + K2O in the silicate melt, increasing with SiO2. Extensive differentiation of primitive alkaline melilititic or basanitic parents is hence required to achieve immiscibility. The experimental data show that evolved nephelinites and phonolites unmix calcic carbonatitic melts, while melilitites and undifferentiated nephelinites with >4 wt % MgO may also unmix dolomitic carbonatitic melts. The latter may hence arise from liquid immiscibility or develop through fractionation from calcic carbonatitic melts. Finally, carbonatites may also derive through fractionation of CO2-rich ultramafic melts, but a continuous increase in dissolved CO2 from a carbonated silicate melt to a carbonatitic melt requires ≥3 GPa. We conclude that the combination of the natural rock record with melting and crystallization phase relations excludes a direct mantle origin for almost all carbonatites found in the crust. Instead, their vast majority forms through immiscibility from an alkali-rich differentiated silicate melt that stems from a mantle-derived alkaline parent, consistent with the common spatial association with alkaline complexes and similar isotopic compositions of carbonatite and alkaline silicate rocks. Direct fractionation from silicate melts may occur for kimberlitic or ultramafic lamprophyric melts, but only at ≥3 GPa, i.e. within the lithospheric mantle. To make progress in this field, we suggest a more rigorous distinction of magmatic and carbo- or hydrothermal carbonatite rocks in each occurrence, and to focus on mineral compositions in the carbonatite and associated silicate magmas, as bulk rocks are at best cumulative in nature. Additional experimental work to understand the role of alkalis and H2O in the formation and evolution of carbonatites, in particular crystallization and fluid saturation at crustal conditions, will be essential to provide a more complete understanding of carbonatite petrogenesis.

Zircon U-Pb Age and Hf-O Isotope Evidence of the Taihua Complex on the Southern Margin of the North China Craton: Implications for Its Magmatic Process During the Early Paleoproterozoic

The Xiaoqinling is the largest terrane of the Taihua Complex and records Paleoproterozoic silicic magmatism, which is one of the most potential areas for understanding secular changes in tectonic processes during the Tectono-Magmatic Lull (TML). In this study, we use zircon U–Pb dating, Lu–Hf and O isotopic analysis to constrain the age, provenance, and magmatic processes of the protoliths for the Paleoproterozoic granitic gneiss, tonalitic gneisses, and migmatitized tonalitic gneiss from the Xiaoqinling area. The isotopic composition of zircon provides a key proxy for querying the crustal record. Integration of our new U–Pb and Hf–O isotope data with available published data from other regions of the North China Craton (NCC), allows us to better assess the secular changes of the North China Craton during the magmatic lull at ∼2.3 Ga. This study shows that the protolith of the tonalitic gneisses in the study area have crust-like Hf–O isotopic compositions with slightly negative to positive εHf(t) (−3.0− +2.1) and low to moderately elevated δ18O values (2.71–7.87‰), indicating a mixed origin of continental crust assimilated by mantle-derived magmas.

The Effect of Nitrogen on the Dihedral Angle Between Fe−Ni Melt and Ringwoodite: Implications for the Nitrogen Deficit in the Bulk Silicate Earth

AbstractNitrogen (N) is extremely depleted in the bulk silicate Earth (BSE). However, whether the silicate magma ocean was as N‐poor as the present‐day BSE is unknown. We performed multi‐anvil experiments at 20 GPa and 1,673−2,073 K to determine the dihedral angle of Fe−Ni−N alloy melt in ringwoodite matrix to investigate whether percolation of Fe‐rich alloy melt in the solid mantle can explain N depletion in the BSE. The dihedral angles ranged from 112° to 137°, surpassing the wetting boundary. Our experiments suggest that N removal from the mantle by percolation of Fe‐rich alloy melt to the Earth's core is unlikely. Therefore, besides N loss to space during planetesimal and planetary differentiation, as well as its segregation into the Earth core, the stranded Fe‐rich metal in the deep mantle could be a hidden N reservoir, contributing to the anomalous depletion of N in the observable BSE.

Onset and tempo of ignimbrite flare-up volcanism in the eastern and central Mogollon-Datil volcanic field, southern New Mexico, USA

The Cenozoic ignimbrite flare-up (40–18 Ma) generated multiple volcanic fields in the southwestern United States and northern Mexico resulting from asthenospheric mantle upwelling after removal of the Farallon slab. The correlation of tuffs to one another and to source calderas within these volcanic fields is essential for determining spatiotemporal patterns in volcanism and magma geochemistry, which have been used to deduce migration of the Farallon slab at depth and associated mantle melting. However, the correlation of Eocene–Oligocene tuffs in the southwestern U.S. is difficult because of post-emplacement erosion and faulting. This study focuses on spatiotemporal patterns of the initial episode of ignimbrite flare-up activity (ca. 36.5–33.8 Ma) in the Mogollon-Datil volcanic field in south-central New Mexico, USA. We show that alkali feldspar major and trace element geochemistry is an effective tool for correlating tuffs when combined with high-precision, single-crystal 40Ar/39Ar geochronology and bulk-rock geochemistry. Using these data, we correlate several tuff units and differentiate other tuffs that have the same eruption age but very different geochemistry, and we conclude that there was a broadly northwestward migration in volcanism over time. The new tuff correlations are used to investigate spatiotemporal variations in magma geochemistry, erupted volumes, and recurrence intervals during the initial episode of Mogollon-Datil volcanic field volcanism. Early-erupted tuffs restricted to the eastern Mogollon-Datil volcanic field share similarities with western U.S. topaz rhyolites, which suggests that the silicic magmas were generated by partial melting of mafic lower crustal rocks. We also find differences in the compositions, crystallinities, and mineral assemblages between the early- and late-erupted tuffs. The early-erupted tuffs tend to have single-feldspar mineralogies, lower feldspar Or contents, large negative Eu anomalies, and low-whole–rock Ba concentrations. Conversely, late-erupted tuffs have two feldspar plus quartz assemblages, lesser Eu anomalies, higher whole-rock Ba concentrations, and feldspars have higher Or contents. Thus, we suggest that for some of the early eruptions, after magmas underwent crystal fractionation in the crust, the silicic melt largely separated from the crystalline mush prior to eruption, whereas late-erupted tuff magmas underwent crystal fractionation at near the eutectic minimum and were remobilized and erupted with a larger proportion of their crystalline mush. Using our new ages, correlations, and previously published data, we find that the initial phase of Mogollon-Datil volcanic field volcanism produced at least 15 eruptions between 36.5 Ma and 33.8 Ma, with a minimum total erupted volume of ~1350 km3 and an average recurrence interval of 170 k.y. However, eruptions were generally smaller in volume (most <15 km3) than in other coeval fields, and most eruptions (n = 11) occurred in the first 1.2 m.y. (ca. 36.5–35.3 Ma) of activity. Altogether, our work sheds new light on variations in the composition, timing, and migration of volcanism during the initial phase of Mogollon-Datil volcanic field activity and highlights the utility of feldspar geochemistry in both “fingerprinting” tuffs and elucidating magma evolution.

Anatomy of a Lunar Silicic Construct—The Wolf Crater Complex, Mare Nubium and Implications for Early Silicic Magmatism on the Moon

AbstractSilicic lithologies on planetary surfaces indicate magmatic evolutionary processes in their interiors. The Wolf crater complex within Mare Nubium on the Moon is one such silicic construct associated with a high thorium anomaly. This study integrates morphological, compositional, chronological and gravity anomaly analyses of high‐resolution data from various lunar missions to establish this construct as a silicic volcanic caldera. Lobate flows with steeply sloping fronts indicate that the crater rims comprise high‐viscosity silicic lavas, while the structurally controlled inner crater walls suggest caldera collapse triggered by magma depletion. In the crater rims, low Christiansen Feature position values reaffirm the presence of silicic lithologies, consistent with the low gravity anomaly signature beneath the complex, while spectroscopic data reveal low mafic mineral abundances and negligible hydration features. Chronological analyses yield silicic volcanism ages coeval with surrounding mare basalts (3.8–3.6 Ga), while intra‐caldera basalts have 2.36–2.02 Ga ages, indicating prolonged magmatism in this region. Melting of suitable crustal protoliths like alkali gabbronorite/monzogabbro/troctolite by basaltic underplating is inferred to have generated silicic magmas that formed the Wolf volcanic complex, instead of basaltic magma fractionation or silicate‐liquid immiscibility processes. Large impacts during the Late Heavy Bombardment may have enhanced partial melting of the mantle and created crustal fractures that facilitated the ascent of viscous silicic melts through the lunar crust. Contemporaneous existence of suitable protoliths and adequate crustal pathways for magma ascent may have controlled silicic volcanism on the Moon, and can explain the sporadic occurrence and overlapping ages of the lunar silicic constructs.

Trace element geochemistry of carbonatites and associated alkaline rocks of Mundwara Igneous complex, Sirohi district, Rajasthan, India

Carbonatites are widely recognized for their potential to host significant deposits of rare earth elements (REEs) and other rare metals. These igneous rocks, characterized by their high carbonate mineral content, are often rich in REEs, niobium, tantalum, and other valuable elements. This makes carbonatites key targets for mining and exploration. Carbonatites are believed to form from either primary carbonate magma derived directly from the mantle or from secondary melts. These secondary melts can originate from carbonated silicate magmas through liquid immiscibility or from the residual melts resulting from the fractional crystallization of silicate magmas. The coexistence of carbonatites and alkaline silicate rocks in most complexes, their coeval emplacement in many, and overlapping initial 87Sr/86Sr ratios supports the theory of their cogenesis. In this study, we aim to investigate the trace element geochemistry of carbonatites and coeval alkaline silicate rocks from the Mundwara Igneous Complex, Sirohi district, Rajasthan in India. Our trace element data indicate that the carbonatites and alkaline silicate rocks in this complex are products of fractional crystallization from two separate parental melts, formed through liquid immiscibility of a carbonated alkaline silicate magma.

Pulsatile extractions of Lower Cretaceous silicic volcanic plumbing system beneath the Baiyunzhang and Lianhuashan basins, eastern Guangdong: silicic magma evolution and related mineralization

Highly differentiated magmas are closely related to the formation of tin deposits. The Lianhuashan Basin and Baiyunzhang Basin, surrounded by multiple coeval tin deposits, developed various types of volcanic and subvolcanic rocks. The diagenetic connections between different types of rocks within the two basins provide new insight into the evolution of the silicic magma systems and the impact of highly differentiated magmas on tin mineralization. Both basins have consistent zircon U–Pb ages (143–138 Ma) and similar whole-rock Nd isotopes ( ε Nd ( t ) = −5.7 to −3.4) and zircon Hf–O isotope ranges ( ε Hf ( t ) = −9.0 to −2.5; δ 18 O = 5.1–7.9), suggesting that both basins originate from the same deep-level magma reservoir, followed by different degrees of crystal differentiation and several episodes of crystal–melt separation in their respective shallow-level magma reservoirs. The multiple pulses of magma extraction eventually produced different volcanic rocks (granite porphyry and rhyolite porphyry), whereas the remaining crystal mush consolidated in situ to form quartz monzonite porphyry. Further studies show that the tectonic regime changed from a compressive to an extensional environment at c . 140 Ma. Consequently, mantle-derived magmas with low oxygen fugacity injected shallow magma reservoirs that were able to evolve to a high degree of differentiation through multiple recharge, thus favouring the formation of the Sn deposit.

Magmatic (Cr-Ni-PGE) and secondary/hydrothermal (emerald-peridot-rodingite-nephrite jade) mineralization associated with mafic-ultramafic rock complexes of Pakistan

Mafic magmatism has repeatedly taken place in Pakistan throughout its geologic history from the earliest Proterozoic to Recent. Much of it, however, does not have associated metallic mineralization. This paper concerns mineralization associated with mafic-ultramafic plutonic rock occurrences in the western- and northern suture zones of Pakistan, which demarcate the collision boundary between the northwestern Indian plate and Eurasian blocks. We discuss the petrology of the ophiolites and basaltic magma-related chromite deposits, showings of PGE and Ni minerals, and secondary mineral deposits (emerald and peridot gems, rodingite, and nephrite jade) associated with mafic-ultramafic complexes of the two suture zones. Chromite with high Cr#s (68 to 85) occurs in fair quantity and is mostly considered to be genetically related to basaltic magma of subduction-related environments. PGE and nickeliferous minerals are insignificant in quantity; PGE's are formed by a higher degree of partial melting and melt-rock reaction, whereas Ni-sulphides are the product of serpentinization of the ultramafic rocks. Emerald occurs in several localities in the ISZ, whereas the peridot has so far been reported from only one locality. Emerald is associated with Mg-carbonate ± quartz ± fuchsite ± talc ± serpentine, and peridot (Fo 89-92 ) is associated with chrysotile ± magnesite ± talc ± magnetite ± ludwigite. Both are formed via hydrothermal alteration of ultramafic rocks through the introduction of hydrous fluids incorporating CO 2 , Be, B, K, Al, and possibly Na. In view of the undeformed nature of the emerald and peridot and the 23 Ma age of emerald formation, the mineralization appears to post-date deformation and may be of Late Oligocene-Early Miocene age. The metasomatising fluids were either derived from the subducting Indian plate crust underneath the Kohistan arc or were related to 23-26 Ma silicic magmatism in this area of the NW Indian plate. The nephrite jade in the ophiolite mélange is hosted in serpentinites and has disseminated grains of chrome spinel with elevated Fe/(Mg + Fe) ratios (0.10–0.18), implying low-grade metamorphism of ultramafic rocks (protolith of serpentinite) with loss of Al and introduction of SiO 2 during accompanying metasomatism. The rodingite in Dargai ophiolite could have formed by the action of seawater channeled through fractures and cracks in rocks. However, their mineral assemblages indicate that they formed as a result of the influx of Ca, Al, and Na-bearing hydrothermal fluids related to the intrusion of gabbros/sheeted dykes in the serpentinized ultramafic rocks, resulting in Ca-metasomatism.

Co-Ni distribution and Re-Os isotopic age of the Wuxing complex in the Central Asian orogenic Belt: Implications for sulfide mineralization in Alaskan-type complexes

Alaskan-type mafic–ultramafic complexes have been recognized as potential hosts of magmatic Ni-Cu sulfide deposits. However, the distribution and controlling factors of ore-forming metal elements like Co and Ni within these complexes remain poorly understood. This study offers comprehensive petrological observations and quantifies the Co and Ni contents of silicate minerals and sulfides in the Wuxing complex, the oldest Alaskan-type complex hosting a distinctive PGE-rich Ni-Cu sulfide deposit in the Central Asian Orogenic Belt. The Co-rich minerals, including cobaltite and pentlandite, occur as inclusions or interstitial grains in other mineral phases. Among the sulfide assemblages (pyrrhotite, pentlandite and chalcopyrite), pentlandite exhibits the highest 3.59 ∼ 5.98 wt.% of Co and 28.7 ∼ 32.7 wt.% of Ni contents, with the Co contents being 2 to 10 times higher than those in magmatic Ni-Cu sulfide deposits. Clinopyroxene and hornblende in the Wuxing complex display lower Co and Ni contents compared to those in sulfide-free rocks of typical Alaskan-type complexes. This could be ascribed to the reduction in Co and Ni contents in evolved magmas following sulfide segregation. Additionally, lack of olivine and chromite crystallization would elevate Co and Ni contents in silicate magma and promote the sulfide mineralization in the Wuxing complex. The Re-Os isotopic dating in ore samples at 596 Ma, combined with previously zircon U-Pb ages of 510 ∼ 517 Ma, reveals that long-term magmatism would facilitate mineralization in the Wuxing complex. Our investigations provide a new insight of mineral crystallization for understanding of magmatic Ni-Cu-PGE sulfide mineralization in Alaskan-type complexes.

Micron-to-nanoscale investigation of Cu-Fe-Ni sulfide inclusions within laurite (Ru, Os)S2 from chromitites

AbstractThis paper provides a top-down nanoscale analysis of Cu-Ni-Fe sulfide inclusions in laurite from the Taitao ophiolite (Chile) and the Kevitsa mafic-ultramafic igneous intrusion (Finland). High-resolution transmission electron microscopy (HRTEM) reveal that Cu-Ni-Fe sulfide inclusions are euhedral to (sub)-anhedral (i.e., droplet-like) and form single, biphasic or polyphasic grains, made up of different polymorphs, polytypes and polysomes even within a single sulfide crystal. Tetragonal (I4$$\stackrel{-}{2}$$d) and cubic (F$$\stackrel{-}{4}$$3m) chalcopyrite (CuFeS2) host frequent fringes of bornite (Cu5FeS4; cubic F$$\stackrel{-}{4}$$3m and/or orthorhombic Pbca) ± talnakhite (Cu9(Fe, Ni)8S16; cubic I$$\stackrel{-}{4}$$3m) ± pyrrhotite (Fe1 − xS; monoclinic C2/c polytype 4C and orthorhombic Cmca polytype 11C) ± pentlandite ((Ni, Fe)9S8; cubic Fm3m). Pentlandite hosts fringes of pyrrhotite, bornite and/or talnakhite. Laurite and Cu-Fe-Ni sulfide inclusions display coherent, semi-coherent and incoherent crystallographic orientation relationships (COR), defined by perfect edge-to-edge matching, as well as slight (2–4º) to significant (45º) lattice misfit. These COR suggest diverse mechanisms of crystal growth of Cu-Fe-Ni sulfide melt mechanically trapped by growing laurite. Meanwhile, the mutual COR within the sulfide inclusions discloses: (1) Fe-Ni-S melt solidified into MSS re-equilibrated after cooling into pyrrhotite ± pentlandite, (2) Cu-Ni-Fe-S melts crystallized into the quaternary solid solution spanning the compositional range between heazlewoodite [(Ni, Fe)3±xS2] (Hzss) and ISS [(Cu1±x, Fe1±y)S2]. Additionally, nanocrystallites (50–100 nm) of Pt-S and iridarsenite (IrAsS) accompanying the sulfide inclusions spotlight the segregation of PGE-rich sulfide and arsenide melt earlier and/or contemporarily to laurite crystallization from the silicate magmas. Cobaltite (CoAsS)-gersdorffite (NiAsS) epitaxially overgrown on laurite further supports the segregation of arsenide melts at early stages of chromitite formation.

The subsurface anatomy of a mid-upper crustal magmatic intrusion zone beneath the Boku volcanic complex, Main Ethiopian Rift inferred from gravity data

Strain accommodation in the Main Ethiopian Rift has been localized since the Quaternary in axial magmatic segments that contain magma intrusion, volcanic complexes, and fault zones. However, the crustal structure and magmatic plumbing features of the individual volcanic complexes within these magmatic segments are poorly constrained. In this study, gravity data from the Global Gravity Model plus2013 was used to interpret the crustal structure and subsurface volcanic network at and near the Boku Volcanic Complex (Boku VC). Two-dimensional gravity models and an upward continuation map analysis of the upper crust reveal a gravity maximum that is interpreted as mafic intrusion at depths between 5 and 10 km beneath the Boku VC. A circular gravity maximum on the upward continued and residual gravity anomaly maps over the Boku VC and adjacent segments suggest the shallow plumbing systems beneath the segments are discrete, but that they merge into the deeper crust. The gravity models suggest that below 5 km beneath the center of magmatic segments nearly all the extension over the last 2 My can be accounted for by magmatic intrusion. Our models require faults in the uppermost crust which likely contribute to extension and may serve locally as conduits for the conveying melts or hydrothermal fluids. Our gravity analysis supports petrological studies that indicate a two-level magmatic plumbing system beneath the Wonji fault belts in which a melt supply from the upper mantle moves to mid-crust and then to shallow upper crust where the magma fractionates into more siliceous magma within smaller magma chambers.

Heat pulse−dominated magmatic storage: The 33 ka dacite dome eruption at Taápaca volcano (Central Andes)

We present 40Ar/39Ar ages of dacite domes and rare volcanic sanidine megacrysts from Taápaca volcano (northern Chile) that record 1.3 m.y. of activity. Our focus is on 20 megacrysts from a single 32.9 ka eruption. We interpret that their surprisingly correlated Ba-rich and Ba-poor growth zones separated by resorption surfaces reflect frequent heat pulses with a uniform thermal history over >300 k.y. of growth. We infer extended storage in small (<400 m), shallow “hot” domains within a larger magma system. Our findings bear on the origin of K-feldspar megacrysts in plutonic rocks, thus linking volcanic and plutonic processes in shallow silicic magma systems, and support protracted residence of hot magma in small batches at upper-crustal levels to produce megacryst-bearing granitoid intrusive complexes.

Granitic mush compaction by impingement of a large felsic enclave: Evidence from the Aztec Wash pluton (Nevada, USA)

Abstract Compaction and melt extraction are key processes in the generation of melt-dominated magma bodies. We analyze magmatic fabrics in the Aztec Wash pluton (15.7 Ma; Nevada, USA) to understand magma dynamics during emplacement of a large (2 m diameter) felsic enclave (LFE) in a silicic magma body. We hypothesize that the LFE moved through crystal-poor magma, settling into crystal-rich magma. We analyze four granite samples collected near the enclave: two 0.25 m below the enclave (E-5 and E-2); one 0.5 m below (E-6); and one 2.5 m to the side (E-3, far field). We use the scanning electron microscope (SEM) to gather data using backscattered electron (BSE) microscopy, energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD) imaging. BSE maps show large (up to 10 mm) euhedral alkali feldspar grains in E-5 and E-2, while E-6 and E-3 have large (up to 6 mm) alkali feldspar with distinct irregular and wavy rims. Dark BSE alkali feldspar rims correlate with Ba depletion in EDS maps. Broken feldspars in E-5 and E-2 have mismatched zones in BSE and orientation differences in EBSD. Grain orientation spread (GOS) maps show that alkali feldspar has the highest degree of intragrain deformation, followed by plagioclase, then quartz. Histograms of the angle between the vertical plane and the pole to the (010) plane demonstrate clustering in E-5, suggesting an enhanced fabric. Differences in rim texture suggest melt extraction beneath the enclave and melt retained in the far field. EBSD data reveal an enhanced foliation beneath the LFE. We conclude that mush under the LFE was deformed during emplacement of the LFE, expelling melt and reorienting grains.

Fracturing around magma reservoirs can explain variations in surface uplift rates even at constant volumetric flux

Many volcanoes show continuous but variable deformation over timescales of years to decades. Variations in uplift rate are typically interpreted as changes in magma supply rate and/or a viscoelastic response of the host rock. Here we conduct analogue experiments in the laboratory to represent the inflation of a silicic magma body at a constant volumetric flux, and measure the chamber pressure and resulting surface displacement field. We observe that dyke intrusions radiating from the magma body cause a decrease in the peak uplift rate, but do not significantly affect the spatial pattern of deformation or spatially averaged uplift rate. We identify 4 distinct phases: 1) elastic inflation of the chamber, 2) a gradual decrease in the rate of uplift and pressurisation, associated with the formation of visible cracks 3) propagation of a dyke by mode 1 failure at the crack tip and 4) a pressure decrease within the chamber. Phase 2 can be explained by either a) crack damage, which reduces the elastic moduli of the surrounding rock or b) magma filling pre-existing cracks. Thus these experiments provide alternative mechanisms to explain observed variations in uplift rate, with important implications for the interpretation of deformation patterns at volcanoes around the world.