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The ungrouped dunitic achondrite Northwest Africa (NWA) 12217 contains symplectic spinel-pyroxene veins that are mineralogically identical to symplectites in other ultramafic planetary materials. The morphology and amount of chromite present in these features relative to the Cr in their olivine hosts suggest an exogenous origin. Petrological experiments show that a Cr laden sulfide liquid reacts with olivine to produce pyroxene by scavenging Mg and Fe from olivine to crystallize chromite. The liquid infiltrates cracks and grain boundaries within the olivine and produces a vein-like symplectic chromite-pyroxene mineralogy similar to that observed in NWA 12217. This process is likely responsible for forming the symplectites in the related ultramafic achondrites NWA 12319, 12562, and 13954, along with many other achondrites. The nucleosynthetic Cr isotopic composition of chromites appears to be in disequilibrium with that of silicates in NWA 12217, suggesting that the liquids responsible for the symplectite forming reaction are at least partially sourced from a different parent body and result from an impact. [ABSTRACT FROM AUTHOR]

Zircon geochronology provides critical information on the rates and durations of geological processes and enables researchers to explore deep time. However, some zircon datasets show a continuum of concordant ages ('smear') without well‐defined age populations. These age smears are typically interpreted to represent variable loss of radiogenic Pb or protracted geological events lasting tens of millions of years. Coupled dissolution‐precipitation replacement of zircon has been suggested as one process that may produce these complex age datasets. Here, we react fragments of the well characterised Mud Tank zircon standard with natural intermediate and mafic melts (0.9 GPa, 1100–1180°C) to test if short‐term exposure to a melt can modify the geochronological patterns of zircon. Our observations show that within a short duration (18 h to 3.5 days), most Mud Tank zircon fragments display microstructural and/or chemical evidence for modification by dissolution at fragment boundaries along with partial replacement by coupled dissolution‐precipitation processes. The replaced zircon domains have U–Pb ages that smear over one hundred million years, between 764–647 Ma, illustrating variable mobility and redistribution of the U and Pb isotopes. Our experiments demonstrate that zircon modified by coupled dissolution‐precipitation replacement may not faithfully record the age or duration of geological events and that investigation of zircon microstructure in high‐resolution backscattered electron, cathodoluminescence imaging and/or Raman mapping is needed to better understand complex zircon geochronological datasets. [ABSTRACT FROM AUTHOR]

Bimodal magmatism is characteristic of the geodynamic evolution of the Main Ethiopian Rift (MER), which is a reference area for the study of the processes leading to continental break-up before seafloor spreading. There are abundant emissions of basalts and rhyolites, which are in possible parent–daughter relationships. However, the P – T –H2O conditions of production and storage of the basaltic end member remain unclear. Crystallization experiments have been conducted on an alkali basalt from the MER to define its pre-eruptive conditions and shed light on the compositional evolution of derivative liquids and source conditions. The experiments were performed at 100 to 200 MPa, 975°C to 1080°C, varying H2O/CO2 ratios, corresponding to melt water contents of 1 to 5 wt %, at f O2 slightly lower than the Fayalite-Magnetite-Quartz (FMQ) solid buffer. Comparison between the petrological attributes of the starting rock and the experiments shows that the basaltic magma was stored at 150 to 200 MPa, 1050 ± 10°C, with 1 to 2 wt % H2O in melt, with f O2 near FMQ prior to eruption. Geochemical modelling shows that the corresponding mantle source contained about 0.1 wt % H2O, reflecting a metasomatized source. Extensive crystallization of such basalts produces SiO2-rich liquids, which are not yet peralkaline, however. This underscores that extreme fractionation (>90 wt %) is required in order to produce peralkaline derivatives from mildly alkaline basalts. This extreme fractionation and the water-rich nature of the starting basalt readily explain the H2O-rich condition of peralkaline rhyolites that have fueled caldera forming eruptions in the Rift. [ABSTRACT FROM AUTHOR]

The processes responsible for the isotopic compositions and abundances of volatile elements in the early solar system remain highly debated. Orders of magnitude variation of (highly) volatile elements exist between different magmatic iron meteorite groups, but it is unclear to what extent their depletions can be explained by evaporation from metal melts during parent body accretion and/or subsequent break up. To this end, we present 86 new evaporation experiments with the aim of constraining the volatility of most volatile metals from metallic melts. The results confirm the previously proposed important effects of S in metal melt on the volatility of the elements of interest governed by their S-loving or S-phobic behavior. Nominally S-loving elements In, Sn, Te, Pb and Bi are significantly more volatile in Fe melt relative to FeS liquid, whereas nominally S-avoiding elements Ga and Sb are more volatile in FeS liquid relative to Fe melt, at a given pressure and temperature. The newly derived volatility sequences for S-free/poor and S-rich metallic melts were also compared with commonly used volatility models based on condensation temperatures. The results indicate significant differences between the latter, including the much more volatile behavior of Te, relative to Se, in both explored bulk compositions, which are traditionally assumed to be equally volatile. The (minimum) degree of volatile element depletion due to evaporation was quantified using the new experimental results and models. A comparison between the volatile element depletions in magmatic iron meteorites and the predicted depletions appropriate for evaporation from Fe melts shows that the latter depletions can be easily reconciled with (an) evaporation event(s). Altogether, the new data and models will provide an important framework when more accurate and precise estimates of magmatic iron meteorite bulk volatile element contents are available. [ABSTRACT FROM AUTHOR]

Thermodynamic, experimental and field studies have suggested that organic compounds could be stable, and in some cases predominate over inorganic carbon species, within subduction zones under high pressure and high temperature (HP-HT) conditions. Beyond sedimentary organic matter of biological origin, solid organics can be inherited from hydrothermal circulation at mid-ocean ridges or abiotically formed by carbonate destabilization in the slab. To assess the fate of solid organic compounds during subduction, HP-HT experiments using piston-cylinder and multi-anvil presses have been performed at 700–1000 °C and 3–7 GPa. Different starting solids were tested, including either synthetic polycyclic aromatic hydrocarbons (PAHs) alone, with (i.e., 1-hydroxypyrene, 1-pyrenebutyric acid) or without (pyrene) oxygen-bearing functional groups, or a mixing of pyrene and antigorite. Our results show that increasing P–T conditions lead to the formation of hydrogenated (±oxygenated) graphitic carbon preserving a high level of structural disorder, far from graphite structure. We also observe the formation of aqueous fluids during experiments at 700 °C and 3 GPa with oxygen-functionalized PAHs, suggesting quick water release from solid organic compounds at HP-HT in subduction zones. Pyrene-antigorite experiments reveal various mineral assemblages depending on redox conditions. Oxidizing conditions favor the formation of magnesite-enstatite-coesite while reducing conditions promote forsterite-enstatite-graphitic carbon assemblages. Our results finally highlight the limited reactivity of solid organic compounds when exposed to aqueous fluids derived from serpentinite under reducing conditions which could facilitate the recycling of organic carbon into the deep mantle. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Interpretation of calcite-dominated fossil carbonatite volcanoes is complicated by the instability of many igneous carbonatite materials on Earth’s surface. One hypothesis suggests that they originate by eruption of alkali-free calcic carbonatite lavas. However, liquid calcite is not thermodynamically stable at atmospheric pressure. A second hypothesis suggests that calcite is secondary and formed after primary nyerereite lost its alkalis to surface water. Here, we experimentally test a combined hypothesis in which solid calcite phenocrysts are suspended in natrocarbonatite lava that solidifies primarily to nyerereite, and determine calcite solubilities in sodic carbonate liquids. Then, we dissolve alkalis in water over several months to show formation of secondary calcite after nyerereite. Textural and geochemical observations from our experiments are consistent with many natural volcanic carbonatites, suggesting that alteration of initially calcite–nyerereite-bearing natrocarbonatites to pure calcite carbonatites was common in Earth’s geological past.

To develop new criteria to distinguish different crystal nucleation mechanisms in silicate melts, we performed crystallization experiments using a synthetic hydrous (2 wt% H2O) trachybasalt and combined three-dimensional information from synchrotron X-ray computed microtomography with two-dimensional mapping of crystallographic orientation relationships (CORs) using electron backscatter diffraction. Crystallization experiments were performed at 400 MPa by cooling the melt from 1300 °C to resting temperatures of 1150 and 1100 °C and maintaining isothermal conditions for 30 min and 8 h. Three distinct titanomagnetite (Tmt) populations formed: (1) skeletal crystals, isolated or partially embedded in clinopyroxene (Cpx); (2) anhedral crystals, always attached to Cpx; (3) flattened needle-shaped crystals, embedded in Cpx. These morphologically different Tmt populations formed in response to one cooling event, with varying nucleation mechanisms and at different undercooling conditions. The clustered three-dimensional distribution of population 2 and 3 Tmt grains and the high proportion of Tmt-Cpx interfaces sharing CORs indicate that these Tmt grains heterogeneously nucleated on Cpx. The near-random three-dimensional distribution of (often isolated) population 1 Tmt grains, together with the low proportion of Tmt-Cpx interfaces sharing CORs, imply their isolated, possibly homogeneous nucleation, potentially followed by heterogeneous nucleation of Cpx on population 1 Tmt. Heterogeneous nucleation in slightly to moderately undercooled magmas should affect the sequence of crystallization as well as morphology and clustering of crystals, which may actively contribute to the variation of rheological parameters like viscosity. Finally, observed intra- and inter-sample variations in Tmt-Cpx COR frequencies indicate the potential for this parameter to record further petrological information. [ABSTRACT FROM AUTHOR]

The thermal state of a magma reservoir controls its physical and rheological properties: at storage temperatures close to the liquidus, magmas are dominated by melt and therefore mobile, while at lower temperatures, magmas are stored as a rheologically locked crystal network with interstitial melt (crystal mush). Throughout the lifetime of a magmatic system, temperature fluctuations drive transitions between mush-dominated and melt-dominated conditions. For example, magma underplating or magma recharge into a crystal mush supplies heat, leading to mush disaggregation and an increase in melt fraction via crystal resorption, before subsequent cooling reinstates a crystal mush via crystal accumulation and recrystallisation. Here, we examine the textural effects of such temperature-driven mush reprocessing cycles on the crystal cargo. We conducted high-P-T resorption experiments during which we nucleated, grew, resorbed, and recrystallised plagioclase crystals in a rhyolitic melt, imposing temperature fluctuations typical for plumbing systems in intermediate arc volcanoes (20–40 °C). The experiments reproduce common resorption textures and show that plagioclase dissolution irreversibly reduces 3D crystal aspect ratios, leading to more equant shapes. Comparison of our experimental results with morphologies of resorbed and unresorbed plagioclase crystals from Mount St. Helens (MSH) (USA) reveals a consistent trend in natural rocks: unresorbed plagioclase crystals (found in MSH dacite, basalt and quenched magmatic inclusions [QMIs]) have tabular shapes, while plagioclase crystals with one or more resorption horizons (found in MSH dacite, QMIs, and mush inclusions) show more equant shapes. Plagioclase crystals showing pervasive resorption (found in the dacite and mush inclusions) have even lower aspect ratios. We therefore suggest that crystal mush maturation results in progressively more equant crystal shapes: the shapes of plagioclase crystals in a magma reservoir will become less tabular every time they are remobilised and resorbed. This has implications for magma rheology and, ultimately, eruptibility, as crystal shape controls the maximum packing fraction and permeability of a crystal mush. We hypothesise that a mature mush with more equant crystals due to multiple resorption–recrystallisation events will be more readily remobilised than an immature mush comprising unresorbed, tabular crystals. This implies that volcanic behaviour and pre-eruptive magmatic timescales may vary systematically during thermal maturation of a crustal magmatic system, with large eruptions due to rapid wholesale remobilisation of mushy reservoirs being more likely in thermally mature systems. [ABSTRACT FROM AUTHOR]

Natural carbonatite is associated with various types of deposits, including rare earth elements (REEs) and high-field strength elements (e.g., Nb). Although the spatial relationship between fluorine enrichment and the formation of carbonatite-type deposits is well-established, the underlying mechanism remains unclear. In this study, we investigate the influence of fluorine on the partitioning of trace elements in carbonatite melts (CM) from the mantle to the crust. Elemental partitioning experiments were conducted at 3 GPa and 1100–1400 °C, both with and without fluorine. We determined trace element partition coefficients for the mineral-carbonatitic melt pairs involving the following minerals: clinopyroxene, olivine, spinel, perovskite, magnetite, calcite solid solution, magnesio wüstite. The addition of fluorine influences the types and stability of Ti-rich oxides. Fluorine-bearing systems crystallize magnetite, while fluorine-free systems crystallize perovskite. Compared to fluorine-free compositions, systems with 1.55–2.40 wt% fluorine in the melt exhibited significantly lower (clinopyroxene, spinel, olivine)-melt partition coefficients for REEs and HFSEs by half to one order of magnitude. The decreased uptake of these elements by clinopyroxene, spinel, and olivine may be attributed to changes in their speciation within the melt phase, consistent with the formation of REE-F complexes. Using our partition coefficients, we simulated the differences in trace element characteristics between carbonatite melts derived from the mantle under fluorine-bearing and fluorine-free conditions. The fluorine-bearing carbonatitic melt extracted from mantle peridotite displayed significantly higher trace element compositions compared to fluorine-free carbonatitic melt, with trace element concentrations being 5–10 times higher. We simulated the trace element characteristics of carbonatitic melts released from subducted sediments and ocean crusts and found that these ocean crusts or sediments can release fluorine-rich carbonatite melts that are naturally enriched in REE due to their high fluorine and REE contents. Therefore, the subduction of sediments or the recycling of ocean crusts is highly favorable for the formation of carbonatite-hosted REE deposits. This can explain why the majority of carbonatite-hosted REE deposits are typically found in continental margins with subducted materials signals in source region. Furthermore, we modeled the impact of different mineral crystallization and fractionation in the shallow crust on the trace element compositions of carbonatitic melts. Late-stage magma crystallization and differentiation of silicate and carbonate minerals are favorable for the further enrichment of ore-forming materials (e.g., REEs). [ABSTRACT FROM AUTHOR]

Rhyolitic tuffs range widely in their crystal contents from nearly aphyric to crystal-rich, and their crystal cargoes inform concepts of upper crustal magma reservoirs. The Earthquake Flat pyroclastics (Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand) are 10 km3 of rhyolitic tuffs with abundant (~ 40 vol.%) plagioclase and quartz, minor biotite, hornblende, and orthopyroxene, and accessory Fe-Ti oxides, apatite, and zircon, set in high-silica rhyolitic glass. Major minerals form large, euhedral phenocrysts and abundant glomerocrysts with few disequilibrium textures excepting some faintly resorbed quartz. Plagioclase phenocrysts have thick rims of nearly constant composition near An30, and hornblende is weakly zoned or unzoned. The abundant and texturally complex mineral assemblage contrasts with the nearby (~ 25 km), nearly synchronous, but more voluminous and crystal-moderate rhyolite tuffs from Rotoiti caldera. New H2O-saturated phase-equilibria results on the erupted Earthquake Flat melt (glass) determine its co-saturation with the partial phenocryst assemblage of plagioclase, quartz, biotite, and Fe-Ti oxides at: 140 MPa, 755 ºC. These closely approximate the conditions of the pre-eruptive magma body assuming it was saturated with nearly pure H2O and at an fO2 of ~ Ni–NiO. Absence of hornblende and orthopyroxene from the synthesized assemblages may result from those minerals being in a peritectic reaction relation with melt to produce biotite, so they would not grow from the liquid used as starting material. Experimental results on Rotoiti rhyolite (Nicholls et al. 1992) show that the two bodies resided at similar pressures, temperatures, and fO2s. Lower crystal abundance of the Rotoiti tuffs may result from slight compositional differences. We interpret that the Earthquake Flat pyroclastics were sourced from the crystal-rich periphery of a mushy reservoir system with the Rotoiti occupying a more melt-rich central location. Uncertain is whether this was a single intrusion zoned continuously in crystallinity, or discrete adjacent intrusions, but our results illustrate and quantify complexities of magma storage across relatively short distances. [ABSTRACT FROM AUTHOR]

Abstract CO2 capture and storage in geological reservoirs have the potential to significantly mitigate the effects of anthropogenic gas emissions on global climate. Here, we report the results of the first laboratory experiments of CO2 injection in continental flood basalts of South America. The results show that the analyzed basalts have a mineral assemblage, texture and composition that efficiently allows a fast carbonate precipitation that starts 72 h after injection. Based on the availability of calcium, chemical monitoring indicates an estimated CO2 storage of ~ 75%. The carbonate precipitation led to the precipitation of aragonite (75.9%), dolomite (19.6%), and calcite (4.6%).

Silica-rich igneous rims (SIRs) occur commonly as an outer rim layer on porphyritic chondrules in CR (Renazzo-like) chondrites, as well as more rarely in other chondrite groups. Formation conditions for SIRs can provide insight into chemical and physical conditions in the chondrule-forming region of the protoplanetary disk, as well as helping to understand temporal changes to successive thermal events within the CR and other chondrule formation regions. Both accretion and condensation models have been proposed as formation mechanisms of SIRs, but a lack of robust thermal constraints prevents a detailed interpretation. We have carried out an experimental study aimed to define the conditions of SIR formation by reproducing the texture, mineralogy, mineral chemistry, and silica polymorphs present in natural SIRs. Experiments conducted on an SIR bulk composition, at peak temperatures of 1310–1507 °C with linear cooling rates between 30 and 90 °C/hr successfully reproduce natural SIRs that contain cristobalite, low-Ca pyroxene, Ca-rich pyroxene, and glass. Microcrystalline mesostasis was formed in an experiment with lower cooling rates at low temperatures (6 °C/hr from <1200 °C). All silica in the experiments was cristobalite, including in experiments with maximum temperatures as low as 1310 °C. Since the cristobalite stability field in the silica phase diagram is >1470 °C, it is clear that silica polymorphs are not robust temperature indicators and that the presence of cristobalite in natural SIRs does not necessarily indicate high peak temperatures. We suggest that accretion of SIR precursors onto solidified chondrules, followed by melting, is the most likely scenario for their formation. Our constraints on SIR formation are similar to those that are usually discussed for chondrules. Therefore, in the CR chondrule-forming region, a repeatable heating mechanism is required that does not change significantly during the time in which condensing material evolved to highly silica-rich compositions. The need for recurring heating events is a general constraint for modeling chondrule formation. [ABSTRACT FROM AUTHOR]

The surface of Venus is in contact with a hot (~470 °C), high pressure (92 bars), and caustic (CO2 with S, but little H2O) atmosphere, which should cause progressive alteration of the crust in the form of sulfate and iron-oxide coatings; however, the exact rate of alteration and mineral species are not well constrained. Different experimental approaches, each with its own limitations, are currently being used to constrain mineralogy and alteration rates. One note is that no experimental approach has been able to fully replicate the necessary conditions and sustain them for a significant length of time. Furthermore, geochemical modeling studies can also constrain surface alteration mineralogy, again with different assumptions and limitations. Here, we review recent geochemical modeling and experimental studies to constrain the state of the art for alteration mineralogy, rate of alteration, open questions about the surface mineralogy of Venus, and what can be constrained before the fleet of missions arrives later this decade. Combining the new results confirms that basalt on the surface of Venus should react quickly and form coatings of sulfates and iron-oxides; however, the mineralogy and rate of alteration are dependent on physical properties of the protolith (including bulk composition, mineralogy, and crystallinity), as well as atmospheric composition, and surface temperature. Importantly, the geochemical modeling results show that the mineralogy is largely controlled by atmospheric oxygen fugacity, which is not well constrained for the near-surface environment on Venus. Therefore, alteration experiments run over a range of oxygen and sulfur fugacities are needed across a wide range of Venus analog materials with varying mineralogy and crystallinity. [ABSTRACT FROM AUTHOR]

Modeling the behavior of trace elements during lunar magma ocean solidification is important to further our understanding of the chemical evolution of the Moon. Lunar magma ocean evolution models rely on consistent datasets on how trace elements partition between a lunar silicate melt and coexisting minerals at different pressures, temperatures, and redox conditions. Here we report new experimental trace element partition coefficients (D) between clinopyroxene (cpx), pigeonite, orthopyroxene, plagioclase, olivine (ol), and silicate melt at conditions relevant for the lunar magma ocean. The data include Dcpx−melt at ambient and high pressures (1.5 GPa and 1310 °C), and partition coefficients at ambient pressure for pig, opx, ol, and pl. Overall, clinopyroxene is a phase that may control the fractionation of key geochemical trace element ratios, such as Lu/Hf and Sm/Nd, during the evolution of the lunar magma ocean. We explore the impact of the new silicate Dmineral−melt on the trace element evolution of the lunar magma ocean and we find that accessory phosphate minerals, such as apatite or whitlockite are of critical importance to explain the observed trace element and isotopic signature of the KREEP reservoir on the Moon. The new partition coefficients were applied to calculate the trace element evolution of the residual melts of the crystallizing lunar magma ocean and we propose a new trace element composition for the urKREEP reservoir. The new data will be useful for future thermo-chemical models in order to adequately predict the duration of the lunar magma ocean and the age of the Moon. [ABSTRACT FROM AUTHOR]

CO2 capture and storage in geological reservoirs have the potential to significantly mitigate the effects of anthropogenic gas emissions on global climate. Here, we report the results of the first laboratory experiments of CO2 injection in continental flood basalts of South America. The results show that the analyzed basalts have a mineral assemblage, texture and composition that efficiently allows a fast carbonate precipitation that starts 72 h after injection. Based on the availability of calcium, chemical monitoring indicates an estimated CO2 storage of ~ 75%. The carbonate precipitation led to the precipitation of aragonite (75.9%), dolomite (19.6%), and calcite (4.6%). [ABSTRACT FROM AUTHOR]

Along with temperature, pressure and melt chemistry, magmatic oxygen fugacity (fO2) has an important influence on liquid and solid differentiation trends and melt structure. To explore the effect of redox conditions on mineral stability and mineral-melt partitioning in basaltic systems we performed equilibrium, one-atmosphere experiments on a picrite at 1200–1110 °C with fO2 ranging from NNO-4 log units to air. Clinopyroxene crystallizes from 1180 °C to near-solidus, along with plagioclase, olivine and spinel. Olivine Mg# increases with increasing fO2, eventually reacting to pigeonite. Spinel is absent under strongly reducing conditions. Mineral-melt partition coefficients (D) of redox-sensitive elements (Cr, Eu, V, Fe) vary systematically with fO2 and, in some cases, temperature (e.g. DCr in clinopyroxene). Clinopyroxene sector zoning is common; sectors along a- and b-axes have higher AlIV, AlVI, Cr and Ti and lower Mg than c-axis sectors. In terms of coupled substitutions, clinopyroxene CaTs (MgSi = AlVIAlIV) prevails under oxidized conditions (≥ NNO), where Fe3+ balances the charge, but is limited under reduced conditions. Overall, AlIV is maximised under high temperature, oxidizing conditions and in slowly grown (a–b) sectors. High AlIV facilitates incorporation of REE (REEAlIV = CaSi), but DREE (except DEu) show no systematic dependence on fO2 across the experimental suite. In sector zoned clinopyroxenes enrichment in REE3+ in Al-rich sectors is quantitatively consistent with the greater availability of suitably-charged M2 lattice sites and the electrostatic energy penalty required to insert REE3+ onto unsuitably-charged M2 sites. By combining our experimental results with published data, we explore the potential for trace element oxybarometry. We show that olivine-melt DV, clinopyroxene-melt DV/DSc and plagioclase-melt DEu/DSr all have potential as oxybarometers and we present expressions for these as a function of fO2 relative to NNO. The crystal chemical sensitivity of heterovalent cation incorporation into clinopyroxene and the melt compositional sensitivity of the Eu2+–Eu3+ redox potential limit the use of clinopyroxene-melt and plagioclase-melt, however, olivine-melt DV affords considerable precision and accuracy as an oxybarometer that is independent of temperature, and crystal and melt composition. Variation of DV and DV/DSc with fO2 for olivine and clinopyroxene contains information on redox speciation of V in coexisting melt. By comparing the redox speciation constraints from partitioning to data from Fe-free synthetic systems and XANES spectroscopy of quenched glasses, we show that homogenous equilibria involving Fe and V species modify V speciation on quench, leading to a net overall reduction in the average vanadium valence. Mineral-melt partitioning of polyvalent species can be a useful probe of redox speciation in Fe-bearing systems that is unaffected by quench effects. [ABSTRACT FROM AUTHOR]

We experimentally investigated the phase relations of a primary basaltic glass erupted on the 10–15°E ultraslow spreading (∼0.4 cm/yr) oblique amagmatic segment of the Southwest Indian Ridge (SWIR). Piston‐cylinder experiments were conducted at 9–13 kbar and 1270–1310°C, in accordance with the predicted pressure and temperature (9.5 ± 0.8 kbar and 1290 ± 12°C) of saturation with a mantle lherzolite assemblage using the recently developed multiphase reverse fractional crystallization model ReversePetrogen (RevPet, Krein et al., 2021, https://doi.org/10.1029/2020JB021292). The experiments were multiply saturated with a plagioclase lherzolite assemblage (olivine + augite + orthopyroxene + plagioclase) at 10 kbar and 1290°C, within the uncertainty of the RevPet prediction. We use the forward model Petrogen (Krein et al., 2020, https://doi.org/10.1029/2020JB019612) to find the best fitting mantle composition and melting conditions that could produce glass KN162‐9 48‐21. This model independently finds a melting pressure and temperature of 9.5 kbar and 1285°C, which agree remarkably well with the RevPet predictions and our experiments. This sample is also significantly more depleted in incompatible trace elements than other glasses from the same ridge segment, suggesting its mantle source experienced prior episodes of depletion. This is consistent with previous work suggesting that ultraslow spreading at the SWIR preserves heterogeneities on a local scale. The variability of erupted compositions largely reflects aggregation and mixing of heterogeneous near‐fractional melts produced in the spinel and plagioclase stability fields along similar melting paths and their fractional crystallization products. More generally, plagioclase‐field mantle melting does occur at mid‐ocean ridges and is preserved in some melting environments. Plain Language Summary: The mantle melting conditions that produce seafloor basalts are important for understanding the Earth's temperature structure. The two most important parameters to constrain are the pressure and temperature at which primary magmas are produced, yet different lines of evidence point to different conclusions, in part because the composition of these magmas are modified en route to the surface. In this work, we investigate a very primitive basalt erupted at the Southwest Indian Ridge, one of the slowest spreading ridges which also displays extreme geochemical variability. We perform experiments that allow us to constrain the pressure and temperature of melting, which match almost perfectly with predictions from the recently developed model ReversePetrogen. These point to shallow melting of a depleted mantle, which is likely a common end‐member at the Southwest Indian ridge as well as other mid‐ocean ridges globally. Our analysis suggests that while a geochemically heterogeneous mantle is required, slow spreading conditions favor the preservation and eruption of these depleted primitive melts. Key Points: A near‐primary basalt from the Southwest Indian Ridge originates from shallow mantle melting in the plagioclase stability fieldShallow melting at slow and ultraslow spreading ridges may explain the preservation of primitive melts with low apparent mantle potential temperaturesMixing of deep, enriched melts, and shallow depleted melts can explain most of the geochemical variability at the Southwest Indian Ridge [ABSTRACT FROM AUTHOR]

We report new experimental data from ultrabasic basanite and ultrabasic tephrite as starting material compositions in the 1350°C to 1000°C temperature range. Crystallization experiments under low- to high-pressure (0.5–2.0 GPa) were carried out under reduced conditions (≈CCO buffer), while one-atmosphere, anhydrous, experiments were performed ranging from reduced to oxidized conditions (−2 ≤ ∆QFM ≤ +2). The results highlight the ƒO2 role on the silica saturation of the alkali liquids differentiated from these primary ultrabasic magmas, on the mineral assemblage, and its composition. The liquid lines of descent (LLDs) from basanite are sodic and strongly SiO2 undersaturated, whereas from tephrite, the LLDs are sodic-potassic/potassic for both weakly SiO2-undersaturated and SiO2-saturated compositions, being more silica saturated under oxidized conditions. At the lowest temperature experiments, the percentage of liquid remaining is significantly higher in the basanite-derived products (ca. 35 wt.%) than in tephrite, indicating that the equivalent magmas are more prone to produce larger quantities of evolved melts. The best obtained Fe–Mg olivine/melt and clinopyroxene/melt exchange coefficients for these alkali compositions considering the new and available data are |${K_D}_{Fe^{2+}- Mg}^{Ol- Alkali\ melt}=0.285\pm 0.014$| and |${K_D}_{Fe^{2+}- Mg}^{Cpx- Alkali\ melt}=0.245\pm 0.008$|⁠ , slightly lower than those observed in tholeiitic melts. Clinopyroxene compositions are Ti–Al-rich and Si-poor as compared with common clinopyroxenes in subalkali systems. We suggest that Ti should be allocated in the tetrahedral sites substituting for Si and that its contents are inversely correlated with pressure. Our results allow a simple new barometer based on clinopyroxene-only compositions, as follows: $$ P\left(\pm 0.16\ GPa,1\sigma \right)=16.028\left(\pm 1.042\right) Na-6.715\left(\pm 0.727\right)\frac{Ti}{Ti+ Al(t)}+0.494\left(\pm 0.441\right) Si+0.144\left(\pm 0.079\right) $$ where Na, Ti, Al(t), and Si are molar proportions relative to 6O. This formulation accounts for the jadeite (NaAlSi2O6) component, herein computed from the Na contents, corrected for the Ti-diopside (CaMgTi2O6) component in clinopyroxene and also considers the evolutionary trend from Mg-augite to ferroan diopside. It applies to alkali ultrabasic to intermediate compositions in the examined P–T–ƒO2 range, resulting in more accurate estimates than the available calibrations. The MgO-in-melt thermometer was optimized for the studied compositions at one-atmosphere pressure and anhydrous conditions, as follows: $$ T\ \left(\pm 5{}^{\circ}C,1\sigma \right)=27.35\left(\pm 0.65\right)\ \big({MgO}^{liq}\big)+984\left(\pm 4\right) $$ which provides much more reliable liquidus temperatures for these alkali systems. Given data restriction, this formulation may be expanded to include the pressure effects for relatively low-H2O (< 3 wt.%) systems as: $$ T\ \left(\pm 20{}^{\circ}C,1\sigma \right)=27.35\left(\pm 0.65\right)\big({MgO}^{liq}\big)+80.20\left(\pm 5.50\right)P(GPa)+981\left(\pm 4\right) $$ [ABSTRACT FROM AUTHOR]

Drilled cores of Macusani green obsidian pebbles from Peru were hydrated and melted above their liquidus temperatures at 200 MPa to a single column ~4 cm in length and then undercooled in a thermal gradient >150 °C along the length of the cores. Despite thousands of hours at subliquidus temperatures, the crystallinity of the products ranged from ~30 to ~5 vol%. Mineral assemblages varied along the length of the cores but not in relation to the thermal gradients in all cases. Oscillations in the abundances of plagioclase, K-feldspar, and quartz were observed across the crystallization fronts and along the lengths of the cores. The originally homogeneous melts became heterogeneous in response to crystallization and to thermal gradients. Chemical gradients in the glasses included sharp boundary layer pile-up of F and B adjacent to the crystallization fronts, as well as linear gradients among several of the elements spanning the entire lengths of the melt columns. Values of molar K/(K+Na) in the glasses, plotted as K*, varied positively with Si, inversely with Al and F, and positively with the domains of K-feldspar that formed at maximum distance from the plagioclase-rich regions. Overall, the results are marked by sharply bounded textural domains, by the spatial segregation of mineral assemblages, by oscillations in mineral assemblages at multiple scales, and by monomineralic crystal aggregates that are hallmarks of pegmatite bodies. Temperatures recorded by feldspars closely approached the actual temperature gradient down to ~500 °C, and the solvus on the alkali feldspar join has been delineated for the first time by the simultaneous crystallization of feldspars from an undercooled melt. [ABSTRACT FROM AUTHOR]

Boron nitride (BN) is a commonly used pressure-transmitting material in experimental petrology. It is often considered to be as inert as MgO or Al2O3, and its redox potential is seldomly discussed. It is generally implied that, when used as a capsule sleeve, BN may impose relatively reduced conditions, similar to the effect of the fayalite-magnetite-quartz (FMQ) buffer. However, sediment melting experiments performed at 1050 °C and 3 GPa with BN as the capsule sleeve, produced a hydrous rhyolitic melt with dissolved H2S and CH4 (Li et al. 2021). The resulting fO2 estimate is significantly more reduced than that for the magnetite-wüstite (MW)-buffered experiment where H2S and CH4 were undetected (Li et al. 2021), possibly to the extent of the quartz-iron-fayalite (QIF) buffered conditions produced when BN is used as a capsule or crucible (Wendlandt et al. 1982). To establish an explanation for such a discrepancy, we have conducted further investigation to better constrain the fO2 imposed by BN, when used as a capsule sleeve. Here we report results on analyses of Fe content in Au capsules, a comparative experiment using a QIF buffer and an experiment with an Fe-(Mg,Fe) O sensor for direct analysis of fO2. The calibration of the equilibrium between FeO in melt and Fe in the Au capsule, from Ratajeski and Sisson (1999) appears to be inadequate in constraining fO2 for our experiments. However, we were able to obtain Fe diffusion coefficients in Au from the Fe diffusion profiles observed in the capsule of the Fe-(Mg,Fe)O sensor experiment, and both the inner and outer capsules of the MW-buffered experiment, with resulting values of 1 × 10–13 m2/s, 3 × 10–14 m2/s, and 5 × 10–14 m2/s, respectively. The QIF-buffered and Fe-(Mg,Fe)O sensor experiments provide several lines of evidence supporting the observation that BN imposes QIF-like redox conditions. First, the Fe-(Mg,Fe)O sensor returned an fO2 value of QIF. Second, the "apparent" partition coefficients between FeO content in melt and Fe in the Au capsules are similar between the BN experiment and the QIF-buffered experiment. Third, we observe CH4 and H2O peaks with similar intensities in the Raman spectra of melts from these two experiments, suggesting similar H2 and thus O2 fugacity. As our experiments were performed on a cubic press with the experimental assembly encased in a pyrophyllite cube, we interpret that the significantly reduced conditions imposed by BN are likely due to high H2O activity maintained by dehydration of pyrophyllite, which can be explained using the reaction 2BN + 3H2O = B2O3 + N2 + 3H2. Lower H2O activity will reduce or inhibit the oxidation of BN and its fO2 buffering ability. If heat-treated, BN acts as a highly efficient H2 barrier, as shown by Truckenbrodt et al. (1997). Through our efforts to determine the fO2 imposed by using BN as a capsule sleeve in our experimental assembly, we are able to demonstrate the reducing ability of BN as an assembly component and, furthermore, shed light on the process by which BN imposes such reduced fO2. We hereby present what we have learned during the course of this investigation in the hope that the effect of BN on fO2 control is both recognized and further exploited in future experimental studies. [ABSTRACT FROM AUTHOR]

We present a new experimental dataset for reaction experiments between natural amphibole-clinopyroxene metasomes (hornblendite) and synthetic lherzolite that produced Na-rich alkaline melts. Experiments were conducted at 1, 3 and 4 GPa and 1000–1300 °C. The generated melts range from foidite over basanite to phonotephrite. At 1 GPa between 1000 and 1100 °C amphibole decompression-breakdown products generate a phonotephritic melt. Among the breakdown components rhönite was found to be stable up to 1100 °C and 1 GPa. At 3 and 4 GPa the melt compositions are affected by phlogopite melting and shift to more foiditic compositions. We find that the melting of hornblendites and the reaction of the melt with the lherzolite produce wehrlitic residues with different olivine/clinopyroxene ratios. Wehrlite formation does not always require separate metasomatic processes but can be a direct by-product of alkaline volcanism. We applied a metasome melting model to the magmas of the Kula volcanic province, Turkey, and show that at 1 GPa basanite melts and phonotephrite melts cover the whole range of known Kula lava compositions. The Kula lava compositional trend can be therefore generated by basanite-phonotephrite melt mixing. A comparison of high-pressure (3–4 GPa) melts with natural nephelinite data shows overlap with many major, minor, and trace elements but differences in SiO2, FeO, and TiO2 argue that the natural nephelinite data do not represent primary metasome melts. [ABSTRACT FROM AUTHOR]

The electrical properties of isolated graphene established precedents for studies of electrical superconducting materials at room temperature. After the discovery of stabilized graphene and graphite nanoplatelets in a geological context, the interest in characterizing the properties of these minerals arose. This work evaluates the electrical resistance evolution of mineral graphite and talc heterostructures under progressive metamorphism simulated in the laboratory. The experiments were conducted on an end-loaded piston-cylinder apparatus. This equipment allows for the application of equal pressure in all sample directions (lithostatic pressure) and heating, simulating geological phenomena. The behavior of two sets of mineral samples were compared: graphite and talc in billets and powder. Samples in billets were submitted to treatments at 400 °C and 4 kbar; 400 °C and 6 kbar; and 700 °C and 9 kbar. The powder samples were subjected to 700 °C and 9 kbar, with two ways of disposing the mineral powders (mixed and in adjacent contact) beyond 900 °C and 9 kbar (in adjacent contact). The results show that the samples in billets had lower electrical resistance when compared to the powder samples. The lowest electrical resistance was observed in the sample treated at 400 °C and 6 kbar, conditions that are consistent with metamorphic mineral assemblage observed in the field. Powdered samples showed better cleavage efficiency during the experiment, resulting in thinner flakes and even graphene, as pointed out by Raman spectroscopy. However, these flakes were not communicating, which resulted in high electrical resistance, due to the need for an electrical current to pass through the talc, resulting in a Joule effect. The maximum electrical resistance obtained in the experiment was obtained in the sample submitted to 900 °C, in which talc decomposed into other mineral phases that were even more electrically insulating. This work demonstrates that electrical resistance prospecting can be an efficient tool to identify potential target rocks with preserved mineral nanometric heterostructures that can form an important raw material for the nanotechnology industry. [ABSTRACT FROM AUTHOR]

Despite the first-order importance of crystallisation–differentiation for arc magma evolution, several other processes contribute to their compositional diversity. Among them is the remelting of partly crystallised magmas, also known as cumulate melting or 'petrological cannibalism'. The impact of this process on the plutonic record is poorly constrained. We investigate a nepheline-normative dyke suite close to the Blumone gabbros, a large amphibole-gabbro unit of the Tertiary Southern Alpine Adamello igneous complex. The compositions of the studied dykes are characterised by low SiO2 (43–46 wt. %), MgO (5.0–7.2 wt. %), Ni (18–40 μg/g), and high Al2O3 (20.2–22.0 wt. %) contents. Phenocrystic plagioclase in these dykes exhibits major, trace, and Sr isotope compositions similar to Blumone cumulate plagioclase, suggesting a genetic link between the nepheline-normative dykes and the amphibole-gabbro cumulates. We tested this hypothesis by performing saturation experiments on a nepheline-normative dyke composition in an externally heated pressure vessel at 200 MPa between 975 and 1100 °C at fO2 conditions close to the Ni–NiO buffer. Plagioclase and spinel are near-liquidus phases at and above 1050 °C, contrasting with the typical near-liquidus olivine ± spinel assemblage in hydrous calc-alkaline basalts. The alkaline nature of the dykes results from the abundance of amphibole in the protolith, consistent with melting of amphibole-gabbro cumulates. We modelled the heat budget from the repeated injection of basaltic andesite into a partly crystallised amphibole-gabbro cumulate. The results of this model show that no more than 7% of the cumulate pile reaches temperatures high enough to produce nepheline-normative melts. We propose that such nepheline-normative dykes are a hallmark of hydrous cumulate melting in subvolcanic plumbing systems. Therefore, ne-normative dykes in arc batholiths may indicate periods with high magma fluxes. [ABSTRACT FROM AUTHOR]

Liquid immiscibility has become the preferred mode of genesis for the carbonatite rocks, which commonly, but not exclusively, accompany silicate rocks in alkaline-rock complexes. This concept has been universally based on the presumption that nephelinitic and phonolitic magmas can evolve to a stage where two conjugate immiscible liquids separate. It is assumed that these two liquids separate quickly, or even instantaneously, into discrete bodies of magma capable of being intruded or extruded with subsequent independent crystallization. Supporting evidence generally given is: alleged consanguinity as discrete occurrence of the two rock types; similarity of radiogenic isotope ratios; trace element contents similar to those predicted from experimentally derived partition coefficients. We do not accept that a general case for liquid immiscibility has been demonstrated; although we do accept that silicate and carbonate liquids are inherently immiscible, we maintain that they are not conjugate in a petrogenetic context. We have reviewed and critically examined the experimental data purporting to establish liquid immiscibility and find that when applied to natural rocks, they are based on inappropriate experimental designs, which are not relevant to the genesis of calcite or dolomite carbonatites, although they might have some relevance to Oldoinyo Lengai nyerereite–gregoryite lavas. The design of these experiments guarantees immiscibility and ensures that the carbonate liquids formed will be calcitic or sodium-rich. We dispute the validity of comparing the trace element contents of natural rocks, which in many instances do not represent liquid compositions, to experimentally determine partition coefficients. We consider that experimental design inadequacies, principally assuming but not proving, that the liquids involved are conjugate, indicate that these coefficients are merely an expression of the preference of certain elements for particular liquids, regardless of how the liquids formed. Proof of consanguinity in alkaline complexes requires more accurate age determinations on the relevant rock types than has generally been the case, and in most complexes, consanguinity can be discounted. We dispute the contention that melt inclusions represent parental melts, although they might elucidate the character of magmas undergoing fractional crystallization from magmatic to carbothermal stages. Radiogenic isotope data are shown to be too widely variable to support a case for liquid immiscibility. We address the contention that calcite cannot crystallize from a dolomitic liquid formed by direct mantle melting, and must therefore have crystallized from a calcite carbonate liquid generated by liquid immiscibility, and demonstrate that it is an unsupported hypothesis as calcite can readily crystallize from dolomitic liquids. We observe that, because immiscible dolomite liquids have never been produced experimentally, the liquid immiscibility proposition could at best be applied only to calcite carbonatites, thus leaving unexplained the large number of dolomite carbonatites and those of either type, which are not accompanied by alkaline silicate rocks. The assumed bimodality of alkaline-rock carbonatite complexes is considered to be fallacious and no actual geological or petrographic evidence for immiscibility processes is evident in these complexes. Several examples of alkaline rock carbonatite complexes for which immiscibility has been proposed are evaluated critically and shown to fail in attempts to establish them as exemplifying immiscibility. We conclude that no actual geological or experimental data exist to establish liquid immiscibility being involved in the genesis of calcite or dolomite carbonatite-forming magmas. [ABSTRACT FROM AUTHOR]

Natural alteration of zircon takes place in melts or fluids either via dissolution coupled with overgrowth or via a coupled dissolution-reprecipitation process. The latter results in the zircon being partially or totally replaced by new, compositionally re-equilibrated zircon or a new mineral phase or both. In this study, fragments (50–300 μm) from a large, inclusion-free, clear, 520–530 Ma euhedral zircon with light radiation damage from a nepheline syenite pegmatite, Seiland Igneous Province, northern Norway, were experimentally reacted in 20 mg batches with 5 mg of ThO 2 + ThSiO 2 + SiO 2 and a series of alkali-bearing fluids in sealed Pt capsules at 900 °C and 1000 MPa for 6–11 days in the piston cylinder press using a CaF 2 setup with a cylindrical graphite oven. ThO 2 + ThSiO 2 + SiO 2 was present at the end of the experiment. In experiments involving H 2 O, H 2 O + NaCl, H 2 O + KCl, and 2 N KOH, no reaction textures formed other than a slight dissolution of the zircon grain fragments. Experiments involving 2 N NaOH, Na 2 Si 2 O 5 + H 2 O, and NaF + H 2 O resulted in zircon reaction textures with varying degrees of intensity, which took the form of partial replacement by compositionally modified zircon via a coupled dissolution-reprecipitation process. In the NaF + H 2 O experiment some overgrowth also occurred. Altered zircon is separated by sharp compositional boundaries from unaltered zircon. Secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis indicates that, relative to the unaltered zircon, the altered zircon is strongly enriched in Th, and heavily to moderately depleted in U and (Y + REE). In all the experiments, 206Pb (3–5 ppm in unaltered zircon) is depleted in the altered zircon to below the SIMS detection limit and to at or below the LA-ICP-MS detection limit. Hafnium and Ti concentrations in the altered zircon retained the same approximate value (within error) as the original zircon. The results from these experiments demonstrate that zircon can be compositionally modified by alkali-bearing and alkali-F-bearing fluids via a coupled dissolution-reprecipitation process. Near to total loss of radiogenic Pb via such processes under high-grade conditions resets the internal zircon geochronometer. Although the end result is the same as with zircon overgrowth, i.e. the production of new generation zircon at the time of a metamorphic/metasomatic event, such replacement processes can explain incomplete isotopic 'resetting'; inclusion production through unmixing of solid solutions in metastable zircon compositions; and 'ghost' textures that preserve initial growth features but with isotopic disturbance. Diagnostic replacement features produced in experiments, such as interface geometries between altered and unaltered zircon, provide markers of the mechanism and aid in zircon interpretation. A major implication from this study is that if zircon with low radiation damage can be metasomatically altered under high-grade conditions, this would have important consequences with respect to zircons presumed role as an impregnable container for mineral inclusions. Namely the mineral inclusions contained within zircon could also be altered, reset as a geochronometer, or even replaced by another mineral. [ABSTRACT FROM AUTHOR]

Apatite containing 14 wt% TREO (total rare earth oxide) and coexisting with calciobritholite with 37.2 wt% TREO has been synthesized at 800 °C and 10 kbar from a felsic melt with the addition of NaCl. The analysis of the experimental products with regression analysis of time-resolved (RATR) laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) data allowed to estimate the composition of the coexisting phases. The results suggest that equilibrium has been established during the run and both apatite and calciobritholite contained REE in [Si4+REE3+] to [Ca2+P5+] solid solution, whereas the coupled substitution [Na1+REE3+] to [2Ca2+] was insignificant despite crystallization from an alkaline, Na-rich melt. The coexistence of the apatite and calciobritholite and available experimental data allowed the miscibility gap to be constrained between apatite and calciobritholite, and suggest complete miscibility between apatite and britholite above 950 °C. The melt that produced coexisting apatite and calciobritholite was characterized by a significant Cl content of (0.51 wt%) and elevated REE (526 ± 19 ppm Ce) and low-P content (112 ± 49 ppm). The change of the accessory mineral association from monazite to apatite and calciobritholite with the addition of NaCl illustrates the importance of halogens for mineral associations. The partition coefficients of britholite are similar to those of apatite and are distinguished mainly by a higher preference for REE and Th. Henry's law was not acting for the total REE content in the melt because of the buffered system; however the partition coeficients could still be used for the prediction of the relative REE patterns for melts that generated high-REE apatite and/or calciobritholite. These results have implications for the interpretation of the phosphate associations in alkaline volcanic and plutonic rocks. [ABSTRACT FROM AUTHOR]

The diffusion coefficient associated with the 3,612 cm−1 infrared absorbance band in H‐bearing olivine, attributed to a defect involving a vacant Si site balanced by four H+ (i.e., (4H)Si× ${(4\mathrm{H})}_{\mathrm{S}\mathrm{i}}^{\times }$), was determined in H in‐diffusion experiments at 1200°C–1400°C and 1.5–3.0 GPa in piston‐cylinder and multi‐anvil apparatuses. The retrieved values are in good agreement with experimentally determined diffusion coefficients ascribed to the same process from previous experiments, despite those being conducted at atmospheric pressure and representing out‐diffusion rather than in‐diffusion. Overall, the diffusivity of the Si vacancy is several orders of magnitude lower than that of the M‐site vacancy (∼4 orders of magnitude at 1000°C), with a higher activation energy (∼450–500 vs. 200–250 kJ mol−1), when both are charge‐compensated solely by H+. From these data, the diffusion coefficient of Si in H‐bearing olivine can be estimated using the relationship between vacancy concentration, vacancy diffusivity, and diffusivity of an ion moving by a vacancy mechanism. However, these diffusivities are inconsistent with published results for Si diffusivity, with a considerable discrepancy of around two orders of magnitude. This suggests that simple relationships between diffusion coefficients of vacancies and species moving by vacancy diffusion mechanisms may be inappropriate, highlighting again the complexity of H incorporation and diffusion mechanisms in olivine. Key Points: Hydrogen diffusion associated with Si vacancies in olivine is confirmed to be slowH‐bearing defects form by complex processes involving inter‐site reactionsSimple relationships between vacancy diffusivity and vacancy concentration cannot explain Si diffusion in H‐bearing olivine [ABSTRACT FROM AUTHOR]

Alkaline magmatism is an important chemical end-member of magmatic activity that typically occurs in response to small volume melting of asthenospheric- and/or lithospheric mantle material in intra-continental settings. Understanding trace element partitioning and phase equilibria during alkaline magmatism can therefore provide constraints on intra-continental geodynamic settings. However, the partitioning of trace elements between alkaline melts and their dominant equilibrium mineral phases remains poorly constrained. Feldspathoids in particular have received limited attention with regards to their trace element contents, hampering our ability to interpret geochemical trends in alkaline magmatic systems. In this study, we performed a series of 1 atmosphere experiments in a gas-mixing furnace using a variety of highly alkaline (Na 2 O + K 2 O = 4.15–14.97 wt%) and silica-undersaturated (SiO 2 = 36.73–45.96 wt%) lava compositions from Nyiragongo, Democratic Republic of Congo, in order to investigate the partitioning behaviour of trace elements in minerals from alkaline magmas. Experimental runs were performed with oxygen fugacity buffered at both QFM (quartz-fayalite-magnetite equilibrium) and QFM + 1 and cover a range of geologically-relevant temperatures (1025–1200 °C). The quenched products of these experiments contained leucite, nepheline, melilite, clinopyroxene, olivine, and rhönite crystals, of which glass-crystal pairs were analysed for rare earth elements, large-ion lithophile elements, and high-field-strength elements. Leucite and nepheline host considerable quantities of large-ion lithophile elements but take up negligible amounts of more highly charged cations. Åkermanitic melilite readily incorporates mono- to trivalent cations with a preference for light over heavy rare earth elements, but incorporates only select divalent cations. Rhönite and clinopyroxene have analogous partitioning behaviours, with a strong preference for heavy over light rare earth elements. Fractionation modelling using the reported partitioning behaviours reproduces the 2021 eruption products of Nyiragongo, with 48% fractionation from an olivine-melilitic parental melt composition. Crystallization of trace-element poor feldspathoid amplifies pre-existing high LREE/MREE ratios of the parental magma and progressively increase trace element abundances for all but monovalent cations. [ABSTRACT FROM AUTHOR]

Anhydrite has become increasingly recognized as a primary igneous phase since its discovery in pumices from the 1982 eruption of El Chichón, Mexico. Recent work has provided evidence that immiscible sulfate melts may also be present in high-temperature, sulfur-rich, arc magmas. In this study we present partition coefficients for 37 trace elements between anhydrite, sulfate melt and silicate melt based on experiments at 0.2–1 GPa, 800–1200 °C, and fO2 > NNO+2.5. Sulfate melt–silicate melt partition coefficients are shown to vary consistently with ionic potential (the ratio of nominal charge to ionic radius, Z/r) and show peaks in compatibility close to the ionic potential of Ca and S. Partition coefficients for many elements, particularly REE, are more than an order of magnitude lower than previously published data, likely related to differences in silicate melt composition between the studies. Several highly charged cations, including V, W, and Mo are somewhat compatible in sulfate melt but are strongly incompatible in anhydrite. Their concentrations in quench material from natural samples may help to fingerprint the original presence of sulfate melt. Partition coefficients for 2+ and 3+ cations between anhydrite and silicate melt vary primarily as a function of the calcium partition coefficients (DCaAnh-Sil) and can be described in terms of exchange reactions involving the Ca2+ site in anhydrite. Trivalent cations are dominantly charge-balanced by Na1+. Most data are well fit using a simple lattice-strain model, although some features of the partitioning data, including DLaAnh-Sil > DCaAnh-Sil, suggest the occurrence of two distinct anhydrite Ca-sites with slightly different optimum radii at the experimental conditions. The ratio DSrAnh-Sil / DCaAnh-Sil is shown to be relatively insensitive to silicate melt composition and should vary from 0.63–0.53 between 1200–800 °C, based on a simple, "one-site" lattice strain model. Comparison to DSrAnh-Sil and DCaAnh-Sil calculated for natural anhydrite suggests that in most cases, including the S-rich eruptions of Pinatubo and El Chichón, the composition of anhydrite is consistent with early crystallization of anhydrite close to the liquidus of silicate melt with a composition approximately that of the bulk erupted material. This illustrates how anhydrite (and perhaps sulfate melt) provides a mechanism to transport large quantities of sulfur from significant depth to the eruptive environment. [ABSTRACT FROM AUTHOR]

Volcanic products are widely present on Mercury: they occur as low-viscosity lava flows, but traces of ash deriving from explosive volcanism are also observed. Silicate glasses represent a major component in volcanic products, and it is likely that the fine-powdered regolith on Mercury contains a non-negligible fraction of glassy material. In the laboratory, we have reproduced a Mercury-like silicate glass, from which we have obtained 14 powdered samples with different granulometric characteristics: 8 samples are extremely sorted with grain sizes ranging from 25 to 425 µm, and 6 samples consist of less sorted powders with normal distributions, varying mean values (30, 95, and 160 µm) and standard deviation (40 and 80 µm). The reflectance of samples was investigated in the mid-infrared (MIR) region: we observe how the reflectance intensity increases with grain size, and the presence of extremely fine material defines the emergence of the transparency feature (TF). We provide reference data with qualitative observations and quantitative parameterization of spectral characteristics; in particular, we observe how a small fraction of fine material can greatly influence the spectral response of coarser powders. Results of this work will be crucial for the interpretation of data collected by the BepiColombo mission, but need to be integrated with other possible Mercurian compositions. [ABSTRACT FROM AUTHOR]

During a series of high P-T experiments aimed at understanding the hydrothermal alteration of the REE,Ti silicate chevkinite-(Ce), three ThSiO 4 phases (thorite, huttonite, and an amorphous ThSiO 4 variety) were formed, showing a wide range of textures from small discrete grains to large-scale replacement of the chevkinite-(Ce). All three phases have compositions close to ThSiO 4 , except that the amorphous type contains small amounts of LREE (≤ 0.025 apfu) and inferred H 2 O. The three different forms have been confirmed by electron backscatter diffraction, which has also provided information on the microstructure of the crystals. The earliest phase to form was huttonite, composed of many randomly oriented microcrystals. This was later replaced by thorite via fluid-aided coupled dissolution-reprecipitation. The amorphous type formed both by apparent hydrous alteration of the original crystalline thorite and huttonite. The inferred sequence of events is compatible with thermodynamic constraints on the relative stability of thorite and huttonite. [Display omitted] • Thorium is highly soluble and mobile in high pH, transported as hydroxyl or F complexes. • Huttonite to thorite conversion: at constant P and T, due to Ostwald's kinetic effects. • ThSiO 4 formation sequence: huttonite, thorite, then an amorphous phase from hydrated thorite or huttonite. • ThSiO 4 rapidly turns amorphous, revealing a fluid-driven geochemical process distinct from metamictization. • REE arrangement in chevkinite influences thorite crystal orientation. [ABSTRACT FROM AUTHOR]

Eudialyte-group minerals (EGM) are unique tracers of peralkaline silica-undersaturated melts. They receive global interest as potential resources for high-field-strength elements (HFSE, e.g., Zr, Nb, Ta) and rare-earth elements (REE), which are critical materials for modern technologies. The main condition for magmatic crystallization of EGM in general is that the concentration of Zr in the parental melt should reach saturation. The solubility of EGM in peralkaline melts from the system Na 2 O–CaO–Al 2 O 3 –SiO 2 ± H 2 O at temperatures between 750 and 1000 °C and pressures of 100 and 200 MPa was investigated. Newly formed crystalline phases in the run products are EGM grains, parakeldyshite and albite. EGM are stable between 750 and 900 °C, and melt incongruently to parakeldyshite between 900 and 1000 °C. EGM crystallization from peralkaline silica-undersaturated melts at nominally dry conditions and 750–850 °C requires a minimum of 0.2–0.22 wt% ZrO 2 in the melt. In experiments with the addition of H 2 O, saturation is attained at much higher ZrO 2 concentrations (1.1–2.85 wt%) in the same temperature interval. REE and HFSE are strongly compatible with eudialyte-group minerals as the EGM-melt distribution coefficients (D) vary from 2 to 90 with falling temperature. The lowest D values are observed in experiments with the highest Zr solubility, i.e., at high temperature and in hydrated compositions. Light REE and especially La tend to have lower D values than the heavy REE. • EGM are stable below 900 °C. • Crystallization of eudialyte requires minimum 0.2–0.22 wt% ZrO 2 in the melt. • The addition of H 2 O increases the solubility of EGM significantly. • Eudialyte-melt distribution coefficients for REE and HFSE vary from 2 to 90. [ABSTRACT FROM AUTHOR]

Biogenic carbonaceous material (CM) is the main carrier of organic carbon in the subduction zone and contributes to COH fluid production and volcanic arc gaseous emissions. Here we investigated the effect of the structural, textural and chemical heterogeneity of CM on its reactivity and redox dissolution by conducting short-lived (1 h) experiments, where synthetic analogues of CM [ordered graphite, graphite oxide (GO), mesoporous carbon (MC), Vulcan® carbon (VC) and glass-like carbon (GC)], are reacted with water at p = 1GPa and T = 550°C–conditions typical of a warm forearc subduction–and fO2 buffered from ▵FMQ ≈ +4 to −7. We show that the amount of dissolved CM (CMdissolved) and the proportion of volatile carbon species (Cvolatile) in the fluid is related both to the structure and the peculiar surficial properties of the carbon forms, such as carbon sp2-and sp3-hybridization, amount of oxygen heteroatoms, presence of oxygenated functional groups (OFGs) and of active sites. MC and graphite (C(sp2) > 94 at%, O < 1 at%, OFGs < 2.2 at %, high proportion of active sites) are relatively inert (CMdissolved < 0.4 mol%) but the former reacts more extensively at extreme redox conditions (producing CO2 + CO and CO2 + CH4 Cvolatile mixtures at ▵FMQ ≈ +4 and −7, respectively), while the latter has a maximum of Cvolatile production (CO2 + CH4) at ▵FMQ ≈ 0, which is not observed in a 10-day long run; partly-ordered GO (C(sp3) ∼ 92 at%, O ∼31 at%, OFGs ∼41 at%) is the most reactive material at all redox conditions (CMdissolved < 2.6 mol%) and produces CO2 as the dominant Cvolatile species; disordered GC, and VC (C(sp3) < 18 at%, O < 8 at%, OFGs < 30 at%) are more reactive at ▵FMQ ≈ +4 (CMdissolved ∼ 1mol%) and ▵FMQ ≈ −7 (CMdissolved < 1 mol%), where Cvolatile is dominantly CO2 and CH4, respectively. Besides the significant deviations from thermodynamically predicted graphite-saturated COH fluid composition and speciation, our results suggests that: 1) immature CM [disordered, rich in C(sp3), O, OFGs] is preferentially dissolved under high fluid fluxes and may buffer fluids to rather oxidizing conditions; 2) a descending flux of oxygen (and hydrogen) bond to CM may exist.

Bastnäsite [REE(CO3)F] is the main mineral of REE ore deposits in carbonatites. Synthetic bastnäsite-like compounds were precipitated from aqueous solutions by many different methods, but previous attempts to model magmatic crystallization of bastnäsite from hydrous calciocarbonatite melts were unsuccessful. Here we present the first experimental evidence that bastnäsite and two other REE carbonates, burbankite, and lukechangite, can crystallize from carbonatite melt in the synthetic system La(CO3)F-CaCO3-Na2CO3 at temperatures between 580 and 850 °C and a pressure 100 MPa. The experiments on starting mixtures of reagent-grade CaCO3, Na2CO3, La2(CO3)3, and LaF3 were carried out in cold-seal rapid-quench pressure vessels. The studied system is an isobaric pseudoternary join of a quinary system where CO2 and fluorides act as independent components. Liquidus phases in the run products are calcite, nyerereite, Na carbonate, bastnäsite, burbankite solid solution (Na,Ca)3(Ca,La)3(CO3)5, and lukechangite Na3La2(CO3)4F. Calcite and bastnäsite form a eutectic in the boundary join La(CO3)F-CaCO3 at 780 ± 20 °C and 58 wt% La(CO3)F. Phase equilibria in the boundary join La(CO3)F-Na2CO3 are complicated by peritectic reaction between Ca-free end-member of burbankite solid solution petersenite (Pet) and lukechangite (Luk) with liquid (L): N a 4 L a 2 C O 3 5 (P e t) + N a F (L) = N a 3 L a 2 C O 3 4 F (L u k) + N a 2 C O 3 (N c). $$ \begin{equation} \mathrm{Na}_4 \mathrm{La}_2\left(\mathrm{CO}_3\right)_5(\mathrm{Pet})+\mathrm{NaF}(\mathrm{L})=\mathrm{Na}_3 \mathrm{La}_2\left(\mathrm{CO}_3\right)_4 \mathrm{~F}(\mathrm{Luk})+\mathrm{Na}_2 \mathrm{CO}_3(\mathrm{Nc}). \end{equation}$$ The right-hand-side assemblage becomes stable below 600 ± 20 °C. In ternary mixtures, bastnäsite (Bst), burbankite (Bur), and calcite (Cc) are involved in another peritectic reaction: 2 L a C O 3 F (B s t) + C a C O 3 (C c) + 2 N a 2 C O 3 ( L) = N a 2 C a L a 2 C O 3 5 (B u r) + 2 N a F (L). $$\begin{equation} 2 \mathrm{La}\left(\mathrm{CO}_3\right) \mathrm{F}(\mathrm{Bst})+\mathrm{CaCO}_3(\mathrm{Cc})+2 \mathrm{Na}_2 \mathrm{CO}_3(\mathrm{~L})=\mathrm{Na}_2 \mathrm{CaLa}_2\left(\mathrm{CO}_3\right)_5(\mathrm{Bur})+2 \mathrm{NaF}(\mathrm{L}). \end{equation}$$ Burbankite in equilibrium with calcite replaces bastnäsite below 730 ± 20 °C. Stable solidus assemblages in the pseudoternary system are: basnäsite-burbankite-fluorite-calcite, basnäsite-burbankite-fluorite-lukechangite, bastnäsite-burbankite-lukechangite, burbankite-lukechangite-nyerereitecalcite, and burbankite-lukechangite-nyerereite-natrite. Addition of 10 wt% Ca3(PO4)2 to one of the ternary mixtures resulted in massive crystallization of La-bearing apatite and monazite and complete disappearance of bastnäsite and burbankite. Our results confirm that REE-bearing phosphates are much more stable than carbonates and fluorocarbonates. Therefore, primary crystallization of the latter from common carbonatite magmas is unlikely. Possible exceptions are carbonatites at Mountain Pass that are characterized by very low P2O5 concentrations (usually at or below 0.5 wt%) and extremely high REE contents in the order of a few weight percent or more. In other carbonatites, bastnäsite and burbankite probably crystallized from highly concentrated alkaline carbonate-chloride brines that were found in melt inclusions and are thought to be responsible for widespread fenitization around carbonatite bodies. [ABSTRACT FROM AUTHOR]

Complex multiphase reaction rims that form during garnet breakdown are known as kelyphite coronae and are common amongst exhumed mantle xenoliths. It has long been established that a reaction of garnet and olivine produces kelyphite corona consisting of spinel and pyroxenes, and that preservation of high-pressure garnet cores requires sufficiently rapid uplift of material through the spinel lherzolite stability field from depths of at least 60 km. We present new high-pressure, high-temperature experiments of garnet breakdown in the spinel–lherzolite stability field demonstrating that a series of cascading reactions can reproduce the multilayer, multiphase kelyphites seen in nature. In all experiments where breakdown occurred, a melt appears to have moderated the reactions towards equilibrium; we believe this to be the first experimental confirmation of the importance of such melts in garnet breakdown reactions. In our experiments at least three distinct zones of concentric kelyphite growth can occur at a single pressure, temperature condition; we suggest, therefore, that such kelyphites seen in natural samples do not have to be caused by a multistage uplift path as is often assumed. Kelyphitic coronae surrounding garnet have previously been used to estimate uplift rates; however, the lack of kinetic data for relevant exhumation reactions has limited their use for PTt pathway estimations and the understanding of emplacement mechanisms. In order to constrain accurate PTt pathways we use reaction rim thickness as a proxy for reaction progress and present preliminary results for the kinetics of garnet breakdown. [ABSTRACT FROM AUTHOR]

Oxygen fugacity is an important but difficult parameter to constrain for primitive arc magmas. In this study, the partitioning behavior of Fe3+/Fe2+ between amphibole and glass synthesized in piston-cylinder and cold-seal apparatus experiments is developed as an oxybarometer, applicable to magmas ranging from basaltic to dacitic composition. The partitioning of Fe2+ is strongly dependent on melt polymerization; the relative compatibility of Fe2+ in amphibole decreases with increasing polymerization. The Fe2+/Mg distribution coefficient between amphibole and melt is a relatively constant value across all compositions and is, on average, 0.27. The amphibole oxybarometer is applied to amphibole in mafic enclaves, cumulates, and basaltic tephra erupted from Shiveluch volcano in Kamchatka with measured Fe3+/FeTotal. An average Fe3+/Fe2+ amphibole-glass distribution coeficient for basalt is used to convert the Fe3+/FeTotal of amphibole in samples from Shiveluch to magmatic oxygen fugacity relative to NNO. The fO2 of primitive melts at the volcano is approximately NNO+2 and is faithfully recorded in amphibole from an amphibole-rich cumulate and the basaltic tephra. Apparently, higher fO2 recorded by amphibole in mafic enclaves likely results from partial dehydrogenation of amphibole during residence in a shallow andesite storage region. We identify three pulses of mafic magma recharge within two weeks of, a month before, and two to three months before the eruption and find that, at each of these times, the host andesite was recharged by at least two magmas at varying stages of differentiation. Application of the amphibole oxybarometer not only gives insight into magmatic fO2 but also potentially details of shallow magmatic processes. [ABSTRACT FROM AUTHOR]

Large silicic eruptions can be preceded by small eruptions of different styles and volumes. The Holocene Llao Rock, Cleetwood, and climactic eruptions of Mt. Mazama, OR, USA, were sourced from the same magma and followed this pattern. The Llao Rock and Cleetwood eruptions are both relatively small, have pyroclasts with microlites and a wide range of vesicularities, and each consisted of an explosive phase followed by an effusive phase. The climactic eruption had no effusive phase and created highly vesicular pyroclasts with no microlites. We analyzed microlite crystallization using phase equilibria and decompression experiments. Comparing the results to the pyroclasts from the Llao Rock and Cleetwood eruptions, we find that the differences between the small and climactic eruptions are likely caused by different magma ascent dynamics. Our experiments show that plagioclase and pyroxene microlites crystallize only during decompressions that are most likely too slow to result in explosive eruptions. The Llao Rock magma likely stalled at shallow depths before continuing fast ascent, which allowed for microlite crystallization that might have also caused explosive eruptions. The Cleetwood magma likely took two separate ascent paths, a majority fraction that ascended quickly from high pressure without stalling and a minority fraction that stalled at shallow depths before continuing ascent along with the majority fraction. These ascent dynamics of the Llao Rock and Cleetwood magmas led to the creation of obsidian pyroclasts from sidewall sintering of fragmented majority-fraction ash. The climactic magma did not stall at shallow depths and instead ascended from depth quickly to the surface, creating the conditions necessary for caldera collapse. [ABSTRACT FROM AUTHOR]

We present the results of high pressure, high temperature multiple saturation experiments at variable oxygen fugacity (f O 2 ) conditions (IW+1.5 and IW−2.1) on three lunar high titanium ultramafic glasses: the Apollo 17 Orange glass (A17O, 9.1 wt% TiO 2), the Apollo 15 Red glass (A15R, 13.8 wt% TiO 2), and the Apollo 14 Black glass (A14B, 16.4 wt% TiO 2). We performed experiments in graphite (f O 2 = IW+1.5) and iron (f O 2 = IW−2.1) capsules. The experimentally determined multiple saturation points (MSPs) in graphite capsules are 2.5 GPa and ∼1530 °C (A17O), 1.3 GPa and ∼1350 °C (A15R), and 1.55 GPa and ∼1425 °C (A14B). In iron, we found MSPs of 3.3 GPa and ∼1565 °C (A17O), 2.8 GPa and ∼1490 °C (A15R), and 4.0 GPa and ∼1540 °C (A14B). These results, when combined with previous experiments on the lunar ultramafic glasses, indicate that the increase in the pressure of multiple saturation is linearly proportional to the TiO 2 content of the melt (Δ M S P G P a = 0.14 × T i O 2 (w t %) - 0.15 , R2 = 0.93, RMSE = 0.2 GPa). The high depths of melting correlated with the lowest f O 2 conditions are hard to reconcile with buoyancy constraints on these iron and titanium rich magmas. In addition, measurements of f O 2 on the orange glass as well as the presence of iron blebs in the glasses suggest that the glasses were reduced during eruption. To reconcile buoyancy constraints with f O 2 estimates, we present a model in which the high titanium magmas experienced higher f O 2 conditions at their source, but underwent subsequent reduction at shallow depths (4–52 km) just prior to their eruption. In this model, we can then further bracket the depth of melting to be from the minimum multiple saturation pressure in graphite to the deepest depth at which the magmas are buoyant: assuming the Hess and Parmentier (1995) post overturn cumulate mantle, the depths of melting range from ∼550–770 km for the A17O glass, ∼260–490 km for the A15R glass, and ∼320–350 km for the A14B glass. [ABSTRACT FROM AUTHOR]