Contributions to Mineralogy and Petrology最新文献

Oscillatory zoning — alternating high- and low-impurity (trace element) zones — is a hallmark of magmatic zircon from felsic systems and preserves the history of magmatic systems. Although commonly attributed to fluctuations in temperature, pressure, or melt composition, the mechanisms driving this zoning remain uncertain. Here, we show that high-impurity growth zones, which appear homogeneous when imaged with a scanning electron microscope (SEM), actually consist of finer-scale growth zones when viewed at the nanoscale - and still finer zones are revealed at the atomic scale. The apparent homogeneity in SEM images results from electron beam convolution, where features smaller than the beam’s interaction volume cannot be resolved. Backscattered electron images have higher spatial resolution than cathodoluminescent images, but high-impurity zones imaged with both are found to consist of finer zones at the atomic scale when imaged with atom probe tomography. Adjacent low-impurity zones are homogeneous across all scales. We interpret these observations as evidence of impurity poisoning during near-equilibrium zircon growth. Faceted crystal growth at low supersaturation leads to rejection of impurities, except for those allowed by equilibrium partitioning. Rejected impurities accumulate on the crystal surface, blocking normal incorporation of atoms and temporarily halting growth. When supersaturation exceeds a critical threshold, growth resumes, trapping the adsorbed impurities and forming a high-impurity zone. These findings not only help to resolve the origin of oscillatory zoning in zircon but also establish a generalizable mechanism of impurity poisoning during near-equilibrium crystal growth, redefining how mineral records are interpreted in igneous systems and beyond.

Replacement reactions progress to varying degrees depending on the P-T conditions, exhumation rates, and fluid availability. The preservation of reactants and retrogressed products allows for the reconstruction of microstructural and mineralogical progression, which we investigated using electron backscattered diffraction and microprobe analyses on the omphacite, amphibole-plagioclase symplectite, and matrix amphibole of the Tso Morari eclogite. Elliptical shapes, absence of chemical zonation, and scarce subgrains suggest that omphacite grains deformed via body diffusion creep. Because of the heterogeneous distribution of externally derived hydrous fluids in the eclogite, the omphacite is replaced by amphibole-plagioclase symplectite either partially along the peripheries (S1 symplectite) or completely (S2 symplectite). Strong omphacite CPOs, caused by growth anisotropy, are inherited by the symplectite constituents such that < 001 > _Omp//<001 > _Amp//<010 > _Plag, < 010 > _Omp//<010 > _Amp, and < 100 > _Omp//<100 > _Amp//<001 > _Plag. Both generations of amphibole grains, i.e., in S1 and S2, crystallised during exhumation. The S1 amphibole grains are poorer in Si (6.75–7.34 apfu) and crystallised earlier than those in S2 (Si = 7.29–7.79 apfu). Elevated stresses at the reaction interfaces deformed the plagioclase in S1 via dislocation creep. In contrast, due to fluid abundance, the plagioclase in S2 deformed via diffusion creep-accommodated grain boundary sliding. The misorientations across the subgrain boundaries in the amphibole grains constituting S1 and S2 are similar to those in the amphibole of the eclogite matrix and the garnet amphibolites. The amphibole grains in S1, eclogite matrix, and garnet amphibolites deformed via dislocation creep, whereas dislocation-accommodated grain boundary sliding deformed those in S2.

Magmas contain crystals exhibiting diverse shapes and sizes, yet the relationship between crystal shape (specifically aspect ratio) and undercooling ((Delta T)), the driving force for crystallization, remains poorly constrained. Crystal shape should correlate with undercooling because undercooling governs the growth regime (interface-controlled versus diffusion-controlled) and thus the resulting crystal form. Prior experiments confirm that large nominal undercoolings drive transitions from polyhedral to hopper, skeletal, or dendritic forms. Large undercoolings reflect rapid decompression or cooling, differing from slower cooling rates typical of magmatic intrusions and storage systems. In such slowly cooled environments, crystals remain polyhedral, exhibiting subtle shape variations. Accurately quantifying crystal shape evolution at relatively low undercoolings could provide critical insights into crystallization histories, improving interpretations of the timescales and processes governing magma storage and eruption dynamics. Experimental verification of correlations between aspect ratios of polyhedral crystals and cooling rates remains inconclusive, possibly because nominal undercooling neglects the dynamic evolution of undercooling throughout crystallization. To address this, we introduce average instantaneous undercooling ((overline{{Delta T_{I} }})), a metric capturing dynamic undercooling history during crystallization. Through controlled cooling experiments and numerical modelling, we demonstrate that higher (overline{{Delta T_{I} }}) histories produce tabular, high aspect ratio plagioclase crystals, whereas lower (overline{{Delta T_{I} }}) produces more prismatic crystals with lower aspect ratios. These variations in shape reflect undercooling-driven shifts in the predominant growth mechanism operating on different crystal faces. By quantitatively linking crystal shape to (overline{{Delta T_{I} }}), our study provides a new approach for reconstructing crystallization histories in magmas under varying pH2O-T-t conditions.

In this contribution, the structural hydroxyl-defect (OH) content of quartz crystals from the 0.8 ± 0.1 Ma Tecoloquillo Ignimbrite (TI) from the Acoculco Caldera Complex (Puebla, Mexico) was analyzed to examine syn- and post-depositional thermal history of ignimbrites. Pumice-hosted quartz crystals were examined from 5 zones of the TI representing various heights above the basal contact of the ignimbrite. To separate quartz crystal populations, unpolarized micro-FTIR on unoriented crystal fragments was performed to determine the OH-speciation as well as the structural hydroxyl-defect content. Additionally, Raman spectroscopy was used to identify the mineral inclusions of quartz crystals. Two populations of quartz were identified based on the structural hydroxyl-defect concentrations and mineral inclusions: a “hydrous” group with boron-related OH-content (BOH) with evidence of hydrothermal origin (containing abundant rutile needles) and another “anhydrous” group with the absence of any structural hydroxyl-defect content (as well as lack of BOH). The “hydrous” group showed a diffusion profile of BOH with a maximum gradient of 2.4 ± 0.65 wt ppm between the core and the rim of the crystals. The pre-eruptive temperatures of the TI were calculated using the composition of Fe-Ti oxides found in the pumice fragments. Results indicate a 720 ± 20 °C and 790 ± 20 °C temperature for the non-welded main mass (middle level) and 840 ± 30 °C for the welded lower ~ 25 m thick basal level. Based on the pre-eruptive temperatures, the syn- and post-depositional cooling history of the TI was estimated based on thermal modelling. According to the cooling model, the cooling time can reach up to 50 years in the middle level (slow cooling rate: ~3^− 8 °C/s or ~ 1 °C/yr). In spite of such high and long-lasting temperature, the thermal stability of BOH defects (under 600 °C) compared to the common AlOH defects in quartz crystals implies that the ignimbrite emplacement temperature was high enough to cause the total AlOH content to be lost from the quartz crystals, but not as high as the OH-content associated with boron to be diffusively lost. Thus, the structural hydroxyl-defect content of quartz represents a function of the thermal history of the deposit they are embedded in and that of the structural hydroxyl-defect speciation. This is the first study showing the significantly higher thermal stability of BOH structural hydroxyl-defects over AlOH structural hydroxyl-defects within quartz crystals originated from silicic ignimbrites.

The crystallization conditions of orthopyroxene and fayalite-bearing granitoids remain critical for understanding the evolution of charnockitic magmas. This study integrates petrography, mineral chemistry, thermobarometry, and thermodynamic modeling to evaluate the P-T-fO₂-H₂O conditions during the crystallization of two contrasting Rhyacian igneous charnockites: the Maravilha (2.09 Ga) and Serra Azul (2.06 Ga) plutons, located in the Bacajá Domain, SE Amazonian Craton. The Maravilha Charnockite comprises (i) fayalite + quartz and (ii) pyroxene-bearing granites, whereas Serra Azul is dominated by pyroxene-bearing tonalites and granodiorites. Fayalite-bearing rocks evolved under reduced conditions (FMQ ± 0.5), with initial H₂O content of ≤ 3 wt%, while the pyroxene-bearing association evolved under more oxidizing conditions (NNO ± 0.5), with ~ 4 wt% initial H₂O. Both associations were emplaced at ~ 0.2–0.5 GPa. Serra Azul Charnockite crystallized under oxidizing conditions (NNO ± 0.7 to NNO + 2), with initial H₂O of ~ 2–3 wt% and pressures between 0.3 and 0.6 GPa. Thermodynamic modeling indicates that fayalite and orthopyroxene can crystallize in moderately hydrous melts, ranging from 2.3 to 6 wt% H₂O for fayalite, and ~ 5–6 wt% for orthopyroxene. Fayalite is restricted to reduced conditions and low pressures (≤ 0.3 GPa), whereas orthopyroxene is stable from reduced to moderately oxidizing conditions and over a wider pressure range. These findings highlight that crystallization of fayalite and orthopyroxene is not restricted to low-H₂O conditions, but instead reflects a complex interaction among H₂O content, melt pressure, redox state, and melt composition.

There are several models to account for the compositional effect of mafic silicate melts on ferric/ferrous ratio. However, felsic melts have been poorly investigated. In this study, both synthetic and natural silicic melts with a large compositional range (eighteen experimental series) were equilibrated at oxidizing (atmospheric conditions) and more reducing conditions in the temperature range 1401–1533 °C at 1 bar total pressure. The bulk glass composition was determined with electron probe microanalysis (EPMA) and the ferric/ferrous ratio in experimental glasses was analyzed using a wet-chemical technique. The experimental data were combined with previous data in order to formulate an empirical equation describing ferric/ferrous ratio in silicic melts as a function of oxygen fugacity, temperature and melt composition. The proposed equation predicts the log(X_FeO1.5/X_FeO) ratio of melts with a standard error of 0.077. The mean and standard deviation of the residuals for back-calculations of logfO₂ are 0.00 and 0.36 and are − 1 and 39 °C for the temperature.

The quartz + feldspar rhyolite-MELTS phase-equilibrium geobarometer is a useful tool for calculating equilibration pressures of rhyolitic magmas. However, it is somewhat limited by typically requiring quartz saturation in magma. Here, we employ the principles from Parts 1–4 to move beyond modeling a specific mineral assemblage. We demonstrate methods for carefully interpreting rhyolite-MELTS geobarometry results to constrain multiple-phase equilibration pressure in quartz-undersaturated dacites to rhyolites, and where quartz saturation is uncertain. We show examples of storage pressure calculations from quartz-absent rhyodacites to rhyolites from Puyehue-Cordón Caulle (PCC), Chile and examples of equilibration between extracted rhyolitic melt compositions and unknown mush mineral assemblages from the Taupō Volcanic Zone, New Zealand. In these cases, orthopyroxene + plagioclase pressures can be used. However, orthopyroxene saturation pressure results are higher at lower modelled oxygen fugacity (f_O2). This can be resolved by modelling at independently constrained f_O2, or by modelling at a range of f_O2 to search for orthopyroxene + magnetite + feldspar co-saturation. We show that orthopyroxene + magnetite + feldspar pressures for PCC are consistent with results from other geobarometers and occur at f_O2 values within error of the f_O2 calculated from Fe-Ti oxides. If quartz saturation is uncertain, quartz + feldspar pressures are a maximum and pyroxene-bearing pressures at low f_O2 are a minimum. For uncertain mineral assemblages, the coincidence of multiple phases (≥3) saturating together at reasonable f_O2 could be used to infer the equilibrium mineral assemblage. Careful inspection of rhyolite-MELTS geobarometry results therefore gives nuanced information about equilibration pressure, mineral assemblage, and f_O2.

Mineral hydration has fundamental geological and technological implications. Despite extensive research, its reaction mechanism(s) and effects on the progress of key metamorphic reactions such as serpentinization, the damage of ceramics, or the setting/hardening of cements are not fully understood. Here, we studied the hydration of periclase (MgO) single crystals and powders forming brucite (Mg(OH)₂) as an analogue for key mineral hydration reactions. Our results show that hydration occurs through an intermediate amorphous phase, as in other oxide and silicate minerals, and results in an epitaxial relationship between periclase and brucite. Mg(OH)₂ precipitates on MgO despite the bulk solution remaining undersaturated with respect to both the amorphous precursor and brucite. This is related to the development of strong concentration gradients at the periclase-solution interface. Remarkably, the transformation of the amorphous precursor into brucite within periclase etch pits and precritical microcracks generates enough supersaturation and crystallization pressure to fracture MgO crystals, enabling further progress of the hydration reaction. This previously unrecognized mechanism for reaction-induced fracturing has relevant natural and technological implications.

Kimberlites are volatile-rich, silica-undersaturated, mantle-derived magmas with an important role in studying mantle geochemistry, yet their compositional evolution during crystallization remains poorly constrained. This study defines the crystallization sequence and liquid line of descent of a reconstructed kimberlitic melt, using high-pressure experiments at 1–3 GPa and 1100–1400 °C. Early crystallization is dominated by olivine ± chromite, consistent with petrographic observations of natural samples, followed by clinopyroxene, ilmenite, perovskite, apatite, and phlogopite. The absence of clinopyroxene in natural kimberlitessuggests that kimberlitic melts remain above 1150 °C at 1 GPa, conditions at which clinopyroxene is not observed in our experiments. Substantial cooling of kimberlitic melt and related abundant crystallization likely occur in the crust, possibly linked to extensive degassing at shallow pressures. The experimental melts evolve continuously through decreasing SiO₂ and MgO, and enrichment in CaO, alkalis (Na, K), and volatiles (CO₂, H₂O), ultimately transitioning at ≤ 1150 °C to carbonatitic melts with 7–10 wt% SiO₂, 6.5–7.5 wt% Na₂O + K₂O, and up to 35 wt% volatiles. Olivine (± clinopyroxene) fractionation drives Si depletion at almost constant MgO + FeO + CaO and moderate alkali-enrichment, such that the carbonate-silicate miscibility gap is bypassed. This evolution is in sharp contrast to mafic alkaline silicate melts where olivine + clinopyroxene crystallization causes Si enrichment hence promoting melt evolution towards the carbonate-silicate miscibility gap. Overall, the experimental results demonstrate a petrogenetic continuum between kimberlitic and carbonatitic melts and provide constraints on the crystallization conditions of kimberlitic melts.

Many experimental studies have addressed the crystallisation of zircon, not least because zircon is of paramount importance as a geochronometer. Today, it is well established at what temperature, melt composition, and Zr concentration a silicate melt reaches zircon saturation. It remains unclear though what course a Zr bearing melt takes before zircon appears as a macroscopic phase. Does zircon nucleate directly from the melt, or does it use during nucleation and growth Zr-rich nanoparticles as fundamental building blocks? The question is relevant because Zr⁴⁺ is a high field strength (HFS) cation that should tend to polymerise in silicate melts to (ZrO₂)_n clusters. To fill that gap of knowledge, we performed experiments with a Zr-enriched phonolitic melt at 1200 (outside zircon stability) and 900° C (inside zircon stability). We investigated the glasses with Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) for Zr-rich nanoparticles, at a spatial resolution not achieved by previous experiments involving zircon. A wide range of nanoparticles is identified, including baddeleyite, zircon, zirconium titanate ZrTiO₄, and rutile. The diverse range of nanoparticles is probably owed to the fact that local-scale gradients in silica (aSiO₂) among silicate melt pools prevailed despite the high run temperatures. The smallest and presumably earliest phases are (ZrO₂)_n nanoparticles trapped by liquidus corundum. They reach diameters around 2 nm or ~ 200 unit cells if they are crystalline. The results support the assumption that Zr⁴⁺ dissolves in silicate melts as ZrO₂ monomers, then quickly polymerises to (ZrO₂)_n clusters. The same may be valid for other HFS cations. Possible applications to HFS element depletions in arc basalts are discussed. At 900° C inside zircon stability, many examples are noted where zircon grows by peritectic reaction of baddeleyite nanoparticles with SiO₂ of the melt although zircon nanoparticles also exist. Larger baddeleyite grains around 50 nm seem to grow by aggregation of smaller (ZrO₂)_n nanoparticles. That zircon also grows by particle attachment cannot be confirmed. Zircon can also crystallise directly from the melt when in melt pools the aSiO₂ and Zr contents were high enough to stabilise zircon.