Abstract Crystallisation-driven differentiation is one fundamental mechanism proposed to control the compositional evolution of magmas. In this experimental study, we simulated polybaric fractional crystallisation of mantle-derived arc magmas. Various pressure–temperature trajectories were explored to cover a range of potential magma ascent paths and to investigate the role of decompression on phase equilibria and liquid lines of descent (LLD). Fractional crystallisation was approached in a step-wise manner by repetitively synthesising new starting materials chemically corresponding to liquids formed in previous runs. Experiments were performed at temperatures ranging from 1140 to 870 °C with 30 °C steps, and pressure was varied between 0.8 and 0.2 GPa with 0.2 GPa steps. For most fractionation paths, oxygen fugacity (fO 2 ) was buffered close to the Ni-NiO equilibrium (NNO). An additional fractionation series was conducted at fO 2 corresponding to the Re-ReO 2 buffer (RRO ≈ NNO+2). High-pressure experiments (0.4–0.8 GPa) were run in piston cylinder apparatus while 0.2 GPa runs were conducted in externally heated pressure vessels. Resulting liquid lines of descent follow calc-alkaline differentiation trends where the onset of pronounced silica enrichment coincides with the saturation of amphibole and/or Fe–Ti–oxide. Both pressure and fO 2 exert crucial control on the stability fields of olivine, pyroxene, amphibole, plagioclase, and Fe–Ti–oxide phases and on the differentiation behaviour of arc magmas. Key observations are a shift of the olivine–clinopyroxene cotectic towards more clinopyroxene-rich liquid composition, an expansion of the plagioclase stability field and a decrease of amphibole stability with decreasing pressure. Decompression-dominated ascent trajectories result in liquid lines of descent approaching the metaluminous compositional range observed for typical arc volcanic rocks, while differentiation trends obtained for cooling-dominated trajectories evolve to peraluminous compositions, similar to isobaric liquid lines of descent at elevated pressures. Experiments buffered at RRO provide a closer match with natural calc-alkaline differentiation trends compared to fO 2 conditions close to NNO. We conclude that decompression-dominated fractionation at oxidising conditions represents one possible scenario for arc magma differentiation.
Abstract The depths of crustal reservoirs within volcanic systems may experience transitions over time. Here, we report the crystal and bulk rock compositions of the shield-forming basaltic lavas of the Tianchi composite volcano in the intraplate Changbaishan Volcanic Field, NE China to constrain the crustal magmatic evolution with time. We investigated samples covering the entire basaltic stratigraphic sequence, consisting of the Toudao (TD), Baishan (BS), and Laofangzixiaoshan (LFZ) units from bottom to top, respectively. The core compositions of olivine macrocrysts vary among the three units, i.e., the TD and BS olivine phenocrysts can both be divided into two populations: a high-Fo population (~Fo76-80) and a low-Fo population (~Fo72-74). The LFZ unit only exhibits a high-Fo population (~Fo77-80). Phase equilibria modelling using rhyolite-MELTS suggests that the high-Fo populations were stored at depths of ~20 km for the TD and BS units and ~15 km for the LFZ unit. The low-Fo populations crystallized at shallow depths, i.e., ≤15 km for the TD unit and ≤13 km for the BS unit. We employ a dynamic Fe-Mg interdiffusion modelling with constantly adapting boundary conditions in zoned olivine macrocrysts to constrain the magmatic environments and timescales during the pre-eruption and post-eruption, enabling clarify the magmatic histories recorded by two olivine populations. The dynamic Fe-Mg interdiffusion modelling considers the variable boundary condition caused by crystal growth and composition variation of melts during magma cooling. Calculated results suggest that the high-Fo populations from the TD and BS units recorded prolonged timescales ranging from six months to more than two years with lower cooling rates and slower crystal growth 37 rates. These characteristics reflect a relatively hot and slow-cooling magmatic environment; and the modelled timescales correspond to the sum time including shallow storage, magma ascent, and further cooling within the lava flows. Conversely, the high-Fo population from the LFZ unit and the low-Fo populations from the TD and BS units record shorter timescales (<140 days) with higher cooling rates and faster crystal growth rates. These results indicate relatively cold and highly undercooling magmatic environments; hence the timescales record magma ascent in the conduits and further cooling during lava emplacements. Our study demonstrates that the Tianchi basaltic plumbing system experienced a structural transition over time. In detail, the TD and BS magmas experienced multi-stage stalling and ascent, first accumulating in deep reservoirs and then transferring to shallow reservoirs for storage before the eruption. The LFZ magmas accumulated in a mid-crustal reservoir, followed by a direct ascent to the surface without additional residence.
Abstract 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 SiO 2 (43–46 wt. %), MgO (5.0–7.2 wt. %), Ni (18–40 μg/g), and high Al 2 O 3 (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 fO 2 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 Magma mixing is a widespread magmagenic process. However, its significance in the formation of ultrapotassic magmas has been largely overlooked so far as they are commonly thought to originate directly from the mantle and ascend rapidly through the crust. The Hezhong ultrapotassic lavas in Western Yunnan (SW China) are (basaltic) trachy-andesitic in composition. These rocks display porphyritic textures with olivine, clinopyroxene (Cpx), spinel, and phlogopite occurring as both phenocryst and glomerocryst. Disequilibrium textures and complex zonation of crystals are commonly observed. Specifically, based on the textural and compositional characteristics, olivines can be classified into three different populations: two populations are characterized by highly to moderately magnesian olivines with normal chemical core-rim zonation (Fo~94–86 to Fo~89–79 and Fo~91–89 to Fo~86–84, respectively). The third population lacks obvious crystal zonation, but individual crystals exhibit some compositional variety at lower Fo contents (Fo83–76). Similarly, four populations of Cpx and two populations of spinel phenocrysts are recognized in terms of texture and composition. Notably, Cpx with reverse zoning contains a ‘green-core’ surrounded by a colourless mantle and rim. Hence, based on the variations of mineral assemblage, types of inclusions, and chemical compositions, all phenocryst/glomerocryst minerals can be divided into three groups. Mineral Group I (MG I) consists of high Fo cores of olivine, cores of the zoned spinel, and phlogopite. MG II only includes the green cores of reversed zoned Cpx (green-core Cpx), and MG III is composed of micro phenocrysts without obvious zoning and rims of large phenocrysts. Comparing these mineral groups with relevant minerals occurring in typical temporally and spatially associated igneous rocks, we suggest that the MG I and II could have been derived from magmas with compositions resembling an olivine lamproite and a trachyte, respectively. The overall bulk-rock geochemical and isotopic features of Hezhong lavas also agree with a mixing process between these two endmembers. Hence, we infer that mixing between these two magmas played a key role in the petrogenesis of the ultrapotassic Hezhong lavas and that the MG III crystallized from the mixed magmas. Our study highlights the complex formation of ultrapotassic magmas inferring that caution must be taken when using bulk chemical magma compositions are to deduce source signatures.
Abstract. The growing interest in Li diffusion as a tool to determine timescales of short-time magmatic events, such as magma ascent during eruption, increases the necessity to better understand Li diffusion in common mineral phases. In this context, well-constrained diffusion coefficients and understanding of kinetic processes specific to mineral phases are of crucial importance. To gain further insight especially into the kinetic processes in plagioclase, we investigated the diffusion of Li between natural An61 plagioclase crystals and synthetic glasses of An80 plagioclase composition. Experiments were conducted at 200 MPa in rapid-heat/rapid-quench cold-seal pressure vessels (RH/RQ CSPVs) and internally heated pressure vessels (IHPVs) at temperatures between 606 and 1114 °C. Concentration and isotope profiles of Li were measured using femtosecond laser ablation multicollector inductively coupled plasma mass spectrometry (fs-LA-MC-ICP-MS). We adopted a multispecies diffusion model and specified boundary conditions for plagioclase of labradoritic composition. Using this model, we were able to distinguish between an interstitial (DLii) and a vacancy process (DLiA), with the interstitial process being 0.2–1 orders of magnitude faster than the vacancy process, depending on temperature. DLii=10-3.76±0.58exp-180.0±12.0kJmol-1RTm2s-1DLiA=10-5.53±0.16exp-151.7±3.2kJmol-1RTm2s-1 Our data indicate charge compensation of Li by Na in both the crystal and the glass. Chemical Li diffusion coefficients in An80 glass are up to 3 orders of magnitude slower compared to Li tracer diffusion in silicate and aluminosilicate glasses and melts, which is attributed to slow Na diffusion at high An content. Our results for chemical diffusion of Li in plagioclase crystals are 1.5–2 orders of magnitude slower than Li tracer diffusion in An- and Ab-rich plagioclase determined in previous studies. This indicates that earlier studies on natural intermediate plagioclase compositions have underestimated timescales by up to 2 orders of magnitude. For accurate determination of timescales from Li diffusion in plagioclase we suggest further exploring the role of Na and a possible dependence on An content.
Magmas readily react with their wall-rocks forming metamorphic contact aureoles. Sulphur and possibly metal mobilization within these contact aureoles is essential in the formation of economic magmatic sulphide deposits. We performed heating and partial melting experiments on a black shale sample from the Paleoproterozoic Virginia Formation, which is the main source of sulphur for the world-class Cu-Ni sulphide deposits of the 1.1 Ga Duluth Complex, Minnesota. These experiments show that an autochthonous devolatilization fluid effectively mobilizes carbon, sulphur, and copper in the black shale within subsolidus conditions (≤ 700 °C). Further mobilization occurs when the black shale melts and droplets of Cu-rich sulphide melt and pyrrhotite form at ∼1000 °C. The sulphide droplets attach to bubbles of devolatilization fluid, which promotes buoyancy-driven transportation in silicate melt. Our study shows that devolatilization fluids can supply large proportions of sulphur and copper in mafic-ultramafic layered intrusion-hosted Cu-Ni sulphide deposits.
Thermal annealing of zircons prior to uranium-lead dating by laser ablation inductively coupled plasma mass spectrometry is a commonly-implemented procedure which improves data accuracy and precision by partially repairing radiation damage from the decay of uranium and thorium. However, it also leads to significantly higher concentrations of lithium in the zircon lattice, which become positively correlated with trivalent yttrium and rare earth elements. Prior to such treatments, zircons typically contain lithium below detection limits (typically <0.55 μg g−1), unless correlated with lanthanum and aluminum (i.e., melt/mineral inclusion tracer elements). This suggests that lithium in zircon is primarily sequestered within inclusions, and is able to permeate the crystal lattice to couple with yttrium and rare earth elements during the thermal annealing procedure. This process occurs 2–3 orders of magnitude faster than diffusion experiments have previously determined, indicating that another diffusion mechanism may apply. A model is proposed, whereby: (i) charge compensation of stoichiometrically over-abundant trivalent cations under water-rich magmatic conditions is likely accomplished by hydrogen, given the incompatibility of lithium in zircon and the abundance of hydrogen. However, (ii) conditions that are high temperature and low pressure (characteristic of both thermal annealing and the syn-eruptive environment), drive silicate melt inclusions to exsolve water, generating a motive force for both hydrogen and lithium from inclusions to permeate the lattice in order to reestablish electrochemical equilibrium between the interior and exterior of the zircon. To test the pressure dependency of lithium migration in the zircon lattice, thermal annealing experiments were performed at 850 °C and 1 bar, 2 kbar and 6 kbar using zircons from the Fish Canyon Tuff. The experiments demonstrate that thermal annealing at 2 and 6 kbar inhibits lithium mobility, with zircons registering lithium concentrations below detection limits similar to controls. The experimental results suggest that lithium concentrations in zircon are vulnerable to rapid perturbation by decompression (concurrent with high temperatures), which further indicate that lithium-in-zircon diffusion data should be interpreted with caution.
Abstract. The diffusive exchange of major elements in Na-series tephrite–phonolite diffusion couples with compositions relevant to the Canary Islands magmatism was determined at 300 MPa and variable H2O concentrations (0.3 wt % to 3.3 wt %), temperatures (1150 to 1300 °C), and fO2 levels (NNO−1.5 to NNO+1.7). Composition-dependent effective binary diffusion coefficients were determined from concentration–distance profiles. Results show a wide range of diffusivities for different cations, consistently following the sequence Na ≫ Al ≫ K ≥ Mg = Fe = Ca > Si > Ti, with a mild diffusivity contrast (0.2–0.8 log units) between tephritic and phonolitic melts. Na is the fastest component, with diffusivities falling ∼1.0 log units above those of Si for any given condition. An anomalously fast Al diffusion is observed, with DAl falling ∼0.4 log units above Si and ∼0.6 log units below Na, suggesting a prevalence of Al–alkali coupling across our range of run conditions. The relationships between log D and H2O content in melt for all cations in an intermediate composition are strongly nonlinear and can be fitted using an exponential function with a convergence in diffusion coefficients for different temperatures with increasing H2O content. Thus, Arrhenius analyses result in a decrease in activation energies from 222–293 kJ mol−1 at 1.7 wt % H2O to 48–112 kJ mol−1 at 3.0 wt % H2O. These results provide new data on chemical interdiffusion in highly alkaline Na-rich melts and suggest that H2O content plays a key role in increasing the chemical efficiency of magma mixing at low temperatures. The obtained dataset is used to test chemical controls of magma mixing in the El Abrigo ignimbrite, Tenerife, where banded pumices involving basanitic–tephritic to phonolitic magmas are common in several compositionally bimodal ignimbrite units.
Abstract. Clinopyroxene-only thermobarometry is one of the most practical tools to reconstruct crystallization pressures and temperatures of clinopyroxenes. Because it does not require any information of coexisting silicate melt or other co-crystallized mineral phases, it has been widely used to elucidate the physiochemical conditions of crystallizing magmas. However, previously calibrated clinopyroxene-only thermobarometers display low accuracy when being applied to mafic and intermediate magmatic systems. Hence, in this study, we present new empirical nonlinear barometric and thermometric models, which were formulated to improve the performance of clinopyroxene-only thermobarometry. Particularly, a total of 559 experimental runs conducted in the pressure range of 1 bar to 12 kbar have been used for calibration and validation of the new barometric and thermometric formulation. The superiority of our new models with respect to previous ones was confirmed by comparing their performance on 100 replications of calibration and validation, and the standard error of estimate (SEE) of the new barometer and thermometer are 1.66 kbar and 36.6 ∘C, respectively. Although our new barometer and thermometer fail to reproduce the entire test dataset, which has not been used for calibration and validation, they still perform well on clinopyroxenes crystallized from subalkaline basic to intermediate magmas (i.e., basaltic, basalt-andesitic, dacitic magma systems). Thus, their applicability should be limited to basaltic, basalt-andesitic and dacitic magma systems. In a last step, we applied our new thermobarometer to several tholeiitic Icelandic eruptions and established magma storage conditions exhibiting a general consistency with phase equilibria experiments. Therefore, we propose that our new thermobarometer represents a powerful tool to reveal the crystallization conditions of clinopyroxene in mafic to intermediate magmas.
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