Silicate Melts Research Articles

Trace element and isotope composition of calcite, apatite, and zircon associated with magmatic sulfide globules

AbstractThe formation of volatile-rich phases in magmatic sulfide systems has been interpreted at least in six different ways. The most popular model attributes their origin to secondary processes, mostly due to the presence of serpentine, chlorite, phlogopite, amphibole, and calcite. While chlorite and serpentine are likely to form as alteration products, the other volatile-rich minerals have the potential to originate in a range of ways, including by primary magmatic processes. Based on mineralogical and petrological studies, it was recently suggested that volatile- and incompatible element-rich halos around sulfide globules may form due to the interaction between three immiscible liquids: silicate, carbonate, and sulfide. This hypothesis was confirmed by experimental data revealing the systematic envelopment of sulfide globules by carbonate melt, indicating their mutual affinity. In this study, we present data on isotopic signatures and trace element distributions of three minerals commonly found in spatial association with sulfides—calcite, apatite, and zircon—to address the question of the source and nature of volatiles and other incompatible elements involved in the formation of the halos. Here we compare our new hypothesis with all the previously proposed explanations to show if they can be consistent with obtained results. Our findings indicate that both mantle and crustal sources play a role in the formation of volatile- and incompatible element-rich halos, strongly correlating with sulfur isotope data previously reported for the sulfide globules in the same intrusions. This correlation confirms the shared origin of sulfides, carbonate and fluids during ore-forming processes, ruling out the secondary origin of volatile-rich phases. The isotope and trace element signatures support the newly proposed hypothesis that volatile- and incompatible element-rich halos could have been formed due to the interaction of immiscible sulfide, carbonate, and silicate melts. The volatile-rich carbonate melt could be sourced from the mantle or it could be added from the crust. Regardless of the origin, carbonate melt and sulfide liquid both immiscible with mafic magma tend to stick to each other resulting in the formation of volatile- and incompatible element-rich halos commonly documented in magmatic sulfide deposits.

Role of metasomatism in formation of the Yichun rare-metal deposit, China

What role post-magmatic processes have played in the development and mineralization of rare-metal peraluminous granites and how important that role can be, are questions that researchers have been wrestling with for decades. The Yichun Li-Ta-Nb deposit is an example that has been the subject of such a debate. The granitic appearance of rocks, the inert nature of tantalum, and the scarcity of mineralization in country rocks have been taken to suggest that fluid overprints were limited to within the granites and were unimportant in rock- and ore-forming processes. In this study, the identification of two types of abundant Li- and Cs-rich pseudomorphs in both the topaz-lepidolite granite and overlying pegmatite, the only two mineralized units at Yichun, suggests that extensive metasomatism was involved in rock and ore formation. The textural and chemical similarities of lepidolite in comparable mineral assemblages from a variety of occurrences, including lepidolite in pseudomorphs, veins, and miarolitic cavities from both the topaz-lepidolite granite and pegmatite, suggest that all lepidolite at Yichun is metasomatic and largely inherited its chemical signatures from a magmatic-hydrothermal transitional fluid, rich in Li, Cs, and Ta, derived possibly from the Li-muscovite granite that lies beneath the topaz-lepidolite granite. We propose that this transitional liquid, composition of which lies between a silicate melt and aqueous fluid, was not in equilibrium with the original igneous mineralogy, thus bringing about significant metasomatism along its infiltration and evolution upwards. Variations in the trace-element composition of lepidolite likely reflect the influence on mineral compositions by precursor minerals. The ambiguous boundary between the topaz-lepidolite and Li-muscovite granites, combined with the intensely metasomatized nature of the former, is most consistent with the topaz-lepidolite granite being the extensively altered upper portion of the Li-muscovite granite, which is itself somewhat metasomatized. Although magmatic fractionation played a key role in the initial concentration of Nb, Li, and Ta in both granite and pegmatite formation, the Li-Ta-Nb-Cs mineralization and its host rocks were largely formed through magmatic-hydrothermal rejuvenation and re-enrichment.

MAGMATIC SOURCES OF THE EVOLVING CONTINENTAL TECTONOSPHERE IN INDIA: GENERATION OF ALKALINE IGNEOUS COMPLEXES WITH CARBONATITES IN THE SAMALPATTI (SOUTHERN INDIA) AND AMBA DONGAR (WESTERN INDIA) MASSIFS

Neoproterozoic Samalpatti and Cretaceous-Paleogene Amba Dongar massifs are considered to evaluate the change of sources of carbonatite magmatism in evolving tectonosphere of India. The inherited character of Ba and Sr distribution components is traced in heterogeneous carbonatites of India, whereas the independent trends in Ba and Sr enrichment are traced in carbonatites of North Asia. It is inferred that in the process of activity in the tectonosphere of the Indian subcontinent ~800 Ma ago, carbonatite melts were generated in the mantle whose photolith has an age of 4.26 Ga. This protomantle reservoir was remarkably different from the primordial mantle reservoir in the solidified magma ocean in terms of low initial μ and high initial Nd isotope ratio. After the Indian subcontinent had separated from Gondwana 130–100 Ma ago and began to join Asia ~66 Ma ago, ELMU source was activiated in the tectonosphere. The Amba Dongar carbonatites might have been derived from the 4.40 Ga mantle protoliths. Silicate melts of the Deccan LIP were simultaneously generated from the protoliths with Pb-Pb age of ~2 Ga.

Sulfide and Fe-Ti-P liquid immiscibility in the Ni-Cu-Co ovoid deposit of the Voisey’s Bay complex, Labrador, Canada

AbstractIn the Voisey’s Bay complex, sulfide-matrix breccias developed through the percolation of dense sulfide melt, leading to the displacement of the silicate melt within partially molten silicate-matrix breccias. In these sulfide matrix-breccias, hydrous silicate rims are commonly present at the interface between the sulfide matrix and the silicate framework. Multiple lines of evidence support a magmatic origin of these hornblende-biotite rims, which was largely coeval with the emplacement of the sulfide melt in the magmatic breccias. The formation of the hornblende-biotite rims required the addition of alkalis and water that could not have entirely been sourced from either the sulfide melt or the silicate framework. Through the integration of compositional maps with major and trace element analyses of the main accessory minerals, we propose that the critical components required for the development of the hydrous silicate rims in sulfide-matrix breccias originated from an immiscible Fe-Ti-P melt. Distinct textural and compositional features of apatite, hercynite, ilmenite and magnetite support the presence of small amounts of Fe-Ti-P melt in the sulfide melt. This Fe-Ti-P melt likely formed through melt immiscibility in the early stages of the development of the Voisey’s Bay complex, and was transported in the magma conduits together with the sulfide melt.

The behavior of nickel isotopes during mantle melting

Mantle-derived mafic rocks show relatively large variations in nickel (Ni) isotopes and mostly isotopically light compared to the bulk silicate Earth (BSE). Whether this signature is due to the source heterogeneity or controlled by melting processes has been a debatable issue. Here, we analyzed Ni isotopic compositions of 18 intraplate basalts from the Sabzevar region, northern Iran and 16 serpentinized peridotites from Cyprus and South China. The Sabzevar basalts were likely sourced from a sulfur-free/barren mantle domain with recycled pyroxenites involved, manifested by their high Cu contents (127–265 ppm), FeT/Mn (58.2–77.4) and Zn/FeT ratios (11.9–16.4). The Ni isotopic compositions of the Sabzevar basalts (average δ60/58Ni = +0.15 ± 0.08 ‰; 2SD) are slightly heavier than the BSE value (average δ60/58Ni = +0.11 ± 0.06 ‰; 2SD), consistent with equilibrium Ni isotope fractionation during mantle silicate melting as predicted by ionic model calculations. Our new data of serpentinized peridotites (average δ60/58Ni = +0.16 ± 0.06 ‰; 2SD), together with previously reported data for oceanic sediments and metabasalts, suggest that recycled lithologies have mantle-like or relatively heavy Ni isotopic compositions. Thus, the isotopically light Ni in mafic rocks is unlikely to be caused by crustal recycling-induced mantle heterogeneity. Rather, the light Ni isotopic signature is caused by the dissolution of recycled sulfides into the mantle melts. We suggest two sulfide dissolution models (“dissolve and go” and “dissolve and equilibrium”) to describe the δ60/58Ni and Cu systematics in terrestrial basalts. In both models, the initial sulfide content (Sinitial) and oxygen fugacity (fO2) exert a major control on Ni isotopic compositions of resulting melts. These two parameters vary among different geological settings. Basalts derived from mantle sources with high Sinitial and fO2 are characterized by light Ni isotopic compositions, whereas basalts sourced from sulfur-free/barren and reduced mantle domains are likely characterized by heavy Ni isotopic compositions, aligning with the characteristic observed in natural samples.

Late Cambrian-early Ordovician extensional magmatism, uplift and sedimentation of the N Gondwana margin: new field, geochemical and geochronological data from the peri-Gondwanan Central Sakarya Terrane, Sakarya Zone, NW Turkey

ABSTRACT The timing and mechanism of the crustal extension that caused blocks of continental crust to rift and drift northwards from the northern Gondwana margin during the Early Palaeozoic are currently debated. Here, we present new field, geochemical and U-Pb zircon data from peri-Gondwanan Central Sakarya terrane in NW Turkey to shed light on the timing and petrogenesis of extensional magmatism and subsequent passive margin development of the northern Gondwana margin. Our 1/25.000 scale mapping of a selected area (250 km2) has revealed a south-vergent thrust stack comprising three slices. The Middle Slice of the thrust stack consists of a gneiss-amphibolite complex that has a basement-cover type stratigraphy (before regional metamorphism). The basement unit consists of meta-granitoids, intruded by meta-syenite and banded amphibolites, with geochemical characteristics reflecting I&S-type, A-type and within plate alkaline/transitional-type magmatism, respectively. The cover meta-clastic sediments have amphibolite lenses with MORB and back-arc basin-type geochemistry. Meta-granitoid bodies yielded zircon U-Pb ages of 507 ± 16 Ma, 503 ± 12 Ma, and 488 ± 2.6 Ma (Late Cambrian-Early Ordovician), whereas the meta-syenites gave ages of 473.9 ± 5.7 Ma and 471.4 ± 2.5 Ma (late Early Ordovician); also, age of the banded amphibolite was dated as 473.3 ± 4 Ma. The youngest detrital zircon population in the pebbly quartzite sample within cover meta-sediments (458 ± 3 Ma) constrains the maximum depositional age to around early Late Ordovician. The late Early Ordovician bimodal magmatism, a key focus of our study, is interpreted to have formed in response to the Rheic Ocean rifting along the northern margin of Gondwana. The Late Cambrian granitic magmatism, a precursor to bimodal magmatism, can be explained by lithospheric thinning and upwelling of the asthenosphere to form anatectic silicic melts. The unconformity between the basement and cover units may represent the initiation of a new phase of extension at the N Gondwana margin that culminated in the northward drifting of the Central Sakarya and the opening of Palaeotethys, associated with back-arc basic magmatism in Late Ordovician-Silurian.

Dynamic evolution of the transcrustal plumbing system in Large Igneous Provinces: Geochemical and microstructural insights from glomerocrysts and melt inclusions

Abstract The nature of the magma plumbing system of Large Igneous Provinces is still poorly understood. Among these exceptional magmatic events from Earth’s past, the end-Triassic Central Atlantic Magmatic Province (CAMP) and the end-Cretaceous Deccan Traps (Deccan) coincided in time with two of the most catastrophic biotic crises during the Phanerozoic. In order to constrain the architecture of their magma plumbing system, glomerocrysts containing abundant bubble-bearing melt inclusions from basaltic lava flows of both CAMP and Deccan were investigated via in situ geochemical and microstructural analyses. The analysed glomerocrysts, dominated by augitic clinopyroxene crystals, represent fragments of a crystal mush entrained by basaltic magmas before eruption. The analysed melt inclusions, consisting of an intermediate to felsic composition glass and CO2-bearing bubbles, represent relics of interstitial melts and fluids within a porous crystal framework forming the crystal mush. The different volume proportions between bubbles and whole inclusions reveal that melt entrapment occurred after volatile exsolution. The minimum observed bubble/inclusion fraction indicates that the CO2 concentration in CAMP and Deccan melts was at least 0.3 wt.%, consistent with a maximum entrapment pressure of about 0.5 GPa at CO2–H2O fluid-saturated conditions. The MgO-rich composition of host clinopyroxene crystals and whole rocks is in contrast with the SiO2-rich composition of (trachy-) andesitic to rhyolitic glass of melt inclusions, pointing to disequilibrium conditions. Thermodynamic and geochemical modelling shows that fractional crystallization alone cannot explain the evolved composition of glass in melt inclusions starting from their whole rock composition. On one side, the oxygen isotope composition of clinopyroxene crystals in glomerocrysts ranges from + 3.9 (± 0.3) to + 5.8 (± 0.3) ‰ and their sample-averaged oxygen isotope composition spans from + 4.4 (N = 10) to + 5.6 (N = 10) ‰, implying that glomerocrysts crystallized from mafic melts with normal (i.e., mantle-like) to slightly low δ18O values. On the other side, the oxygen isotope composition of glass in melt inclusions ranges from + 5.5 (± 0.4) to + 22.1 (± 0.4) ‰, implying that melt inclusions entrapped intermediate to felsic melts with normal (i.e., mantle-like) to extremely high δ18O values, typical of (meta-) sedimentary rocks. Some melt inclusions are compatible with fractionation from the same mafic melts that crystallized their host mineral phase, but most melt inclusions are compatible with variable degrees of crustal assimilation and partial mixing, potentially followed by minor post-entrapment isotope re-equilibration. In the CAMP, where sedimentary basins are abundant, (meta-) pelites and occasionally granitoids were the most likely assimilants. On the contrary, in the Deccan, where sedimentary basins are rare, granitoids and metapelites were the most likely assimilants. Oxygen isotope compositions of glass in melt inclusions, spanning from mantle-like to crust-dominated signatures, suggest that the CO2 within their coexisting bubbles likely derived partly from the mantle and partly from assimilated crustal materials. The investigated glomerocrysts and their bubble-bearing melt inclusions are relics of a multiphase (i.e., solid + liquid + gas phases) crystal mush revealing a dynamic evolution for the magma plumbing system of both CAMP and Deccan, where crystals, silicate melts and exsolved fluids coexisted and interacted through most of the transcrustal section.

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.

Molecular H2 in silicate melts

A series of hydrothermal diamond anvil cell experiments was conducted to constrain the equilibrium distribution of molecular H2 between H2O-saturated sodium aluminosilicate melts and H2O at elevated temperatures (600–800 °C) and pressures (317–1265 MPa). The distribution of H2 between the silicate liquid and the aqueous fluid was achieved through real-time monitoring of the H-H stretching vibration under in situ conditions using Raman vibrational spectroscopy. Results show that the solubility of H2 in silicate melts saturated with H2O decreases as the temperature increases, with control exerted by the mole fraction of H2O in the melt. The dissolution of H2 in the hydrous silicate melts appears to follow Henrian behavior, resembling that of an inert, neutral non-polar species. To express species solubility as a function of temperature (T in K) an empirical equation was developed:ln Km/f = 11.4 (±1.3) *1000 / T (K) − 18.7 (±1.1)where Km/f is the equilibrium constant for the reaction H2(g) = H2(melt). This equation was derived by integrating data from the current and prior experimental studies that include silicate melts with varying H2O saturation levels. It should be deemed applicable within the temperature range of 600–1450 °C and pressures ranging from 0.3 to 3 GPa. The implications are extended into developing an understanding of the H partitioning between H2-rich atmospheres blanketing magma oceans in the early history of planetary bodies. For example, transferring H from primordial atmospheric envelopes to the interior of rocky exoplanets may be less efficient than previously believed, which should be considered in models of volatile retention. Experimental data also suggest that minimal amounts of solar nebula H2 are likely to dissolve in the molten surface of primitive objects in the protoplanetary disk (∼10−5 to 10−9 mol faction H2 in melt), contradicting the highly reducing conditions observed in chondrule mineral compositions.

Experimental investigation of hydrogen isotope fractionation during hydration of olivine-hosted melt inclusions: Implications for D/H in Baffin Island picrites

Olivine-hosted melt inclusions are used to investigate the hydrogen isotope compositions (D/H) of Earth's mantle reservoirs. Studies have shown, however, that hydrogen in melt inclusion can equilibrate rapidly in response to changes to the external environment via the diffusion of protons (H+) and deuterons (D+) through the host olivine. Given that protons diffuse faster than deuterons, a kinetic fractionation of hydrogen isotopes is expected to accompany both the hydration and dehydration of melt inclusions. Other volatile species in the melt inclusion may also be affected by changes to the internal pressure and/or oxygen fugacity (fO2). Here we report results from experiments designed to investigate the behaviors of volatiles and hydrogen isotopes during the hydration of olivine-hosted melt inclusions. We show that the concentration of H2O in initially H2O-poor inclusions increases rapidly (up to ∼4 wt.% within 24 h) when the host olivine is in contact with aqueous fluid at 1200 °C and 300 MPa. The extent of hydration is controlled by time, temperature, melt inclusion volume, and H+ diffusion distance. Hydrogen isotopes initially become lighter (i.e., D/H decreases) as hydration proceeds, defining a negative correlation with H2O concentration. This trend reverses with increasing hydration as the inclusions must eventually equilibrate with the external fluid. These experimental results agree well with diffusion calculations carried out using a spherical geometry and a lattice diffusivity of 10–11.2±0.2 m2/s for H+ at 1200 °C. Therefore, anomalously light hydrogen isotopes in olivine-hosted melt inclusions from Baffin Island may not be taken as representative of the composition of the mantle source unless post-entrapment hydration can be excluded as a possibility. An increase in CO2 concentration and a significant drop in sulfur concentration accompany hydration of the melt inclusions in our experiments. The former is consistent with an observed decrease in vapor bubble size and results from a hydration-induced internal pressure increase. The latter is ascribed to the exsolution of molten sulfide from the silicate melt, which might be related to a lower fO2 in the experiments as compared to the starting materials. We find no evidence for exchange of F or Cl between the melt inclusion and external fluid.

The physical properties of silicate melts and the implications for the status and processes in the earth′s interior

The physical properties of silicate melts and the implications for the status and processes in the earth′s interior

High–precision determination of carbon stable isotope in silicate glasses by secondary ion mass spectrometry: Evaluation of international reference materials

Secondary ion mass spectrometry (SIMS) has been used for isotope analysis of volatile components dissolved in silicate melts for decades. However, carbon in situ stable isotope analysis in natural silicate glasses has remained particularly challenging, with the few published attempts yielding high uncertainties. In this context, we characterized 31 reference silicate glasses of basaltic and basanitic compositions, which we then used as reference materials to calibrate δ13C − value analyses in silicate glasses by SIMS. This set of reference materials covers a wide range of CO2 concentrations (380 ppm – 12,000 ppm) and δ13C values (−28.1 ± 0.2 to −1.1 ± 0.2 ‰, ±1σ). The sets of reference materials were analyzed using large−geometry SIMS at two ion microprobe facilities to test reproducibility across different instrumental setups. The instrumental mass fractionation (IMF) varied widely with two different large−geometry SIMS instruments as well as with different analytical parameters such as field aperture size and primary beam intensity. We found that a precision better than ±1.1 ‰ (both average internal and external precision, ±1σ) for CO2 content higher than 1800 ppm could be achieved using a primary beam intensity of less than 5 nA, resulting in a final spot size of 10–20 μm, allowing precise analysis of δ13C in mineral−hosted melt inclusions. This level of precision was achieved at CO2 concentrations as low as 1800 ppm. This advance opens a wide range of new possibilities for the study of δ13C − value in mafic melts and their mantle sources. The reference materials are now available at the CNRS–CRPG ion microprobe facility in Nancy, France and will be deposited at the Smithsonian National Museum of Natural History, Washington, USA where they will be freely available on loan to any researcher.

New Thermodynamic Models for Anhydrous Alkaline-Silicate Magmatic Systems

Abstract A new thermodynamic model for silicate melt in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–TiO2–Fe2O3–Cr2O3 model system is presented, building on the tholeiitic through to granitic melt model of Holland et al. (2018) [Journal of Petrology, 59, 881–900] but extending for the first time into anhydrous alkaline systems. The new melt model is accompanied by new thermodynamic models for nepheline, kalsilite, leucite, melilite and ilmenite. Collectively these models enable pseudosection modelling of alkaline-silicate magmatic systems, providing a new tool for investigating this geologically- and economically-important compositional space. The models are calibrated with respect to experimental data on phase relations among minerals and melt, and the fit is benchmarked here via detailed comparison with seven experimental datasets, which encompass a range of pressure (0–22 kbar), temperature (680–1350°C), oxygen fugacity (log fO2 ΔFMQ-3 to +1), total alkali (3–16 wt %) and silica (37–70 wt %) conditions. The calculated pseudosections successfully reproduce experimental crystallisation sequences and phase compositions, indicating that the thermodynamic models are well calibrated across this spectrum of conditions. Redox buffered experimental conditions are simulated using oxygen buffered pseudosections. Contouring of oxygen buffered pseudosections with XFe3+ (mol. Fe3+/Fetotal), or pseudosections of varying XFe3+ with ΔFMQ, reveals (i) often complex and non-intuitive relationships between these two representations of oxidation state, and (ii) substantial variation in ferric iron over narrow temperature intervals in some oxygen buffered sets of experiments. An implication is that simulating oxygen buffering is vital when benchmarking thermodynamic models using experimental results. Furthermore, because natural igneous systems likely feature a near-constant XFe3+, it is important to assess experimental results in this framework when making inferences about natural systems, recognising that oxygen fugacity is a consequence not a control of phase equilibria in nature. Overall, our new models provide a novel tool to explore the role of variables, such as pressure, fractional crystallisation and crustal assimilation in the petrogenesis of alkaline-silicate magmatic systems and their associated mineralisation.

The Petrogenesis and Geological Implications of the Sanggeda Gabbros, Southern Tibet: Insights from the Amphibole Crystal Population

Amphibole is an important mineral during the differentiation of arc magmas but rarely as a phenocryst in arc lavas or eruptive pyroclastic rocks. The Sanggeda complex, intruded into the ophiolite of the Indus–Yarlung Zangbo Suture Zone (IYZSZ), Zedong, southern Tibet, mainly consists of amphibole-rich, fine-grained, and porphyritic gabbros. The complex provides an opportunity to study the differentiation of arc magmas through amphibole crystals. Four distinct amphibole crystal populations can be recognized according to petrographic observations, EMPA, and LA–ICP–MS analysis. The first ones (Type 1) are fined-grained and euhedral, are crystallized during ascent, and are the product of the shallow emplacement of host magma. The second ones (Type 2) are euhedral, with slight negative Eu and Sr anomalies, and crystallize from an evolved magma that previously experienced plagioclase fractionation. Type 3 amphiboles have similar morphological characteristics to Type 2 but are without Eu and Sr anomalies. Type 4 crystals are shown as pseudomorphs, formed by the reaction–replacement between the clinopyroxene and melt. Type 1 crystals are autocrysts. Other amphiboles within host magma, whether presented as phenocrysts or cumulate nodules, are antecrysts. Based on the amphibole crystal population developed in the complex, in this study, a trans-crustal magma plumbing system is proposed, containing at least three magma reservoirs located at different crust depths: the shallow emplaced crust (~4.8 km), the mid-crust (~12.9 km), and the lower crust (~21.8–24.9 km). Early amphibole crystallization is an effective process to generate silicic residual melts. Gravity could help in that sense. Precursor amphibole and clinopyroxene can efficiently delaminate back into the mantle and promote the generation of silicic continental crust.

Trace element partitioning between minerals and melt: parameterization of experimental data for olivine, pyroxenes, and feldspars

Experimental data were analyzed on the partitioning of trace elements between minerals (olivine, orthopyroxene, clinopyroxene, and feldspars) and silicate melts. The data span over a wide range of conditions (from 1 atm to 10 GPa and ~1000–2000 °C) and compositions of melt (from ultrabasic to ultrasilicic) and minerals. The dependence of logarithms of partition coefficients (lnDi ) on conditions and compositions was approximated by linear functions of 1/T, P/T (P is pressure, and T is absolute temperature) and compositional parameters of mineral and melt. It was found that Di /Dj values for a number of element pairs are independent of experimental parameters and vary within narrow ranges. The coefficients of Di dependencies on conditions and compositions were estimated by minimization of quadratic deviations of model Di and Di /Dj values from experimental values. The obtained equations can be used to calculate Di values for a number of elements with an accuracy of approximately a factor of 1.2–2.0. The obtained values were applied as an example for a model of formation of mafic melts of mid-ocean ridges through melting of a peridotite source and crystallization of primary magmas under crustal conditions.

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.

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).

Mechanisms of Co enrichment in the Zhubu Ni–Co–PGE deposit in the Emeishan LIP, SW China: Constraints from mineral geochemistry and Fe–S isotopes of sulfides

The Zhubu Ni–Co–PGE sulfide deposit in the Permian Emeishan Large Igneous Province (LIP), SW China is hosted by a mafic–ultramafic intrusion that comprises two morphologically distinct ore-forming spaces: a central, layered sequence that represents a magma chamber surrounded by a marginal sub-vertical conduit zone. The marginal conduit zone contains significant Ni–Co–PGE sulfide mineralization, suggesting that high volumes of sulfide-bearing silicate melt flowed through the magma conduit system. Determining the hosts of Co is key to understanding the Co enrichment process in the Zhubu deposit, as well as in the Emeishan LIP as a whole. We assess this here using the geochemistry and Fe–S isotope compositions of sulfides in the Zhubu deposit. Cobalt is mainly concentrated in pentlandite sulfide in the websterite, with Co contents increasing in the order pyrrhotite < chalcopyrite < pyrite < pentlandite < violarite; the abundance of violarite, however, is notably lower than that of the other sulfides. The Co contents of pentlandite, pyrite, and chalcopyrite are negatively correlated with their Ni and Fe contents, suggesting that Co substitutes isomorphically for Ni and Fe in the sulfide structure. The δ56Fe values of sulfides increase in the order pyrrhotite < chalcopyrite < pentlandite < pyrite resulted from kinetic fractionation of Fe isotopes. The δ56Fe values of pentlandite and chalcopyrite in lherzolite are higher than those in websterite, implying that lherzolite and websterite formed in compositionally distinct magmas, Fe isotope compositions become heavier with crystal fractionation. The δ34S values of sulfides increase in the order pyrrhotite < pentlandite < chalcopyrite < pyrite, similar to the fractionation sequence of sulfide melts, suggesting that externally derived S was added to the magma and contributed to its sulfide saturation. The Co contents of chalcopyrite and pyrite vary positively with δ56Fe values, and negatively with δ34S values, indicating that the addition of externally derived S and resulting sulfide segregation were critical to Co enrichment.

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 fate of nitrogen during early silicate differentiation of rocky bodies constrained by experimental mineral-melt partitioning

Nitrogen (N) is an essential element for life. Yet the processes of planet formation and early planetary evolution through which rocky planets like Earth obtained their atmospheric and surface nitrogen inventory are poorly understood. In order to understand the effect of early silicate differentiation of the rocky bodies on N inventory, here we study the elemental partitioning of N between the silicate minerals and melts. We conducted laboratory experiments using tholeiitic basalts and Fe + Si alloy mixtures at 1.5 – 4.0 GPa and 1300 to 1550 °C under graphite saturation at an oxygen fugacity range of IW–1.1 to IW–3.0. The experiments yielded an assemblage of Fe-rich alloy melt (am) + silicate melt (sm) + clinopyroxene (cpx) ± garnet (grt) ± orthopyroxene (opx) ± plagioclase (plag). Using electron microprobe, we determine that under the experimental conditions, N act as an incompatible element with DNcpx/sm (0.11 – 0.47) >DNplag/sm (0.41) >DNopx/sm (0.25) >DNgrt/sm(0.06 – 0.21). The DNmineral/sm do not show any strong dependence on temperature, pressure, and melt composition. However, through comparison with previous estimates, it appears that with decreasing fO2, N becomes less incompatible. Under our experimental conditions of alloy melt-mineral equilibria, N behaves as a siderophile element (DNam/mineral ranging from 4.1 to 60.6) with fO2 playing the strongest control on DNam/mineral. Our data suggest that under reducing conditions, in the early stages of a magma ocean (MO) and/or deeper mantle, silicate minerals would hold a non-negligible fraction of N as N becomes less atmophile and siderophile. Therefore, reduced parent bodies could also retain substantial N in the residual mantle during partial melting. The extraction of N from an internal MO or a solid planetary mantle is thus enhanced only as the system becomes more oxidizing, enriching the surficial reservoirs in N. Thus, Earth’s N2-rich atmosphere may be intrinsically linked to its mantle oxidation, whereas other rocky planets of the Solar System, such as Mars and Mercury, may have retained a significant portion of their N inventory in nominally N-free mantle silicates through episodes of MO crystallization and mantle melting.