Contributions to Mineralogy and Petrology最新文献

Crystal mush systems, often referenced in the context of large silicic magma bodies, involve the reactivation of a near-solidus crystal mush by heat input from mafic injections. This model suggests that interstitial melt is extracted from the mush, leading to the generation of high-silica rhyolites and granites. Such processes have been well-documented in various tectonic settings and contribute to both large-scale eruptions and the formation of granitic plutons. However, in the Mineral Mountains, Utah, the zircon and whole rock geochemical record indicate a different scenario. The presence of sector-zoned zircons and the absence of highly evolved central domains indicative of extraction from a mush suggest rapid magma generation from partial melting of solid granitoids rather than from a long-lived crystal mush. Fractional crystallization and equilibrium partial melting models support derivation from the granitoid bodies, rather than from a common shared parental rhyolitic magma or from coeval basalts. The proposed model, presented here, for rhyolite formation in the Mineral Mountains involves episodic injections of mafic magma into the crust, leading to localized partial melting of different granitoid lithologies. Partial melting up to 30% can produce isolated, ephemeral pools of high-silica melt, which crystallize zircons rapidly and ascend to form rhyolitic domes. This process is distinct from the long-lived crystal mush model, explains the lack of intermediate compositions, and the confinement of mafic eruptions to lower elevations. By integrating geochemical data, zircon morphology, and fractionation modeling, this study provides a comprehensive framework for understanding the magmatic processes at play in the Mineral Mountains.

The minerals zircon, monazite, xenotime and rutile commonly occur in metasedimentary rocks as detrital but also authigenic grains, which can result from different fluid-driven processes. In this study such processes are investigated on the basis of detailed petrographic observations, mineral-geochemical data, and results of in situ U-Pb dating and Nd isotope analyzes in a kaolinitized micaschist. The data provide evidence for the preservation of detrital grains of zircon, rutile and monazite crystallized between ~ 3100 and 1150 Ma, and for authigenic zircon, xenotime, rutile and monazite formed at ca. 530 Ma. The authigenic character is indicated by zircon outgrowths closely intergrown with xenotime and rutile crystals. The outgrowths occur where detrital zircon faces are intensely dissolved in contact with Fe-Mg-rich phengitic muscovite, suggesting the involvement of an aqueous fluid enriched in K-Mg-Fe-Al-Ti-P near the thermal peak at 510 °C and > 0.8 GPa. In contrast, authigenic monazite rims overgrowing rounded cores were formed during the retrograde evolution, as indicated by their occurrence in assemblage with kaolinite, and results of geothermobarometry (T = 280 °C, P < 0.3 GPa). The monazite rims show very low U contents (4–14 µg/g), extremely high Th/U (up to 1670), and nearly identical ¹⁴³Nd/¹⁴⁴Nd_t (0.51184 ± 0.00006) values that markedly differ from those of the detrital cores (¹⁴³Nd/¹⁴⁴Nd_t = 0.51000 ± 0.00050; U = 162–16418 µg/g; Th/U = 2.6–153). The high Th/U points to the involvement of an oxidizing fluid, in line with late goethite formation. Monazite microstructures, and Nd isotope characteristics require a chain of processes, from partial dissolution of detrital monazite, through REE transport and Nd isotope homogenization in an aqueous fluid, to new monazite growth.

The Ca-in-olivine geohygrometer, first calibrated in 2016 (Gavrilenko et al. J Petrol, 57(9):1811–1832, 2016a), has since been widely applied to diverse datasets, providing significant insights into magmatic H₂O contents. Building on extensive experience with this method, this study reviews the application of this petrological tool, summarizing its key features, strengths, and limitations. Using a large dataset of olivine-hosted melt inclusions (MIs) from Klyuchevskoy volcano, we highlight the method's advantages and challenges, propose strategies for optimizing its use, and suggest potential improvements for Ca-in-olivine hygrometry. Applying the Ca-in-olivine geohygrometer to extensive MI datasets for a given arc volcano can reveal the H₂O content variation during this magma evolution, showing magmatic H₂O accumulation at greater depth due to incompatible H₂O behavior, and then a degassing trend at shallow depth when H₂O saturation is reached. While effective for evolved compositions (Fo <  ~ 85), the method underestimates magmatic H₂O content in primitive compositions (Fo >  ~ 85). Based on the 1-atm and high-pressure experiments with Klyuchevskoy compositions, combined with secondary fluorescence modeling around olivine-hosted MIs, we suggest that refining current Ca partitioning models (olivine/melt) and routinely measuring CaO in host olivine for reported MIs can improve the method's accuracy and broaden its applicability in magmatic studies. These findings aim to enhance the accuracy and applicability of this technique in studying magmatic processes.

Understanding the extent by which layered intrusions have been modified by post-cumulus processes is important for unravelling primary magmatic histories. This study focusses on how upward migrating late-stage fluids or melts may have affected the bases of Bushveld magnetitite layers and their underlying anorthosites. Key observations include dramatic enrichments in the An-contents of plagioclase grains at the magnetitite-anorthosite contact, from An₅₉ to An₉₀, depletion of the lowermost few mm of the magnetitite layer in Cr, and an increase in the extent of ilmenite exsolution in the magnetitite, locally enriching the surrounding magnetite in Cr in some areas. Sr-isotopes from plagioclase are consistent with those recorded for the Upper Zone of the Bushveld Complex, suggesting that the fluids or melts were internally derived. Late-stage melts are unlikely to be responsible for the formation of Cr-rich domal structures at the bases of magnetitite layers because (a) cumulus magnetite underneath magnetitite layers are very poor in Cr, suggesting that late-stage melts were not Cr-rich, (b) where a large xenolith obstructs liquid migration from below, Cr contents within the magnetitite on top and adjacent to the xenolith are indistinguishable, and (c) a small scale protrusion of magnetitite into the underlying anorthosite that would have been submerged in late stage melt are depleted in Cr. While metasomatism at the base of magnetitite layers may have caused some minor redistribution and depletion of Cr, the macroscale Cr-distribution features are inferred as being of primary magmatic origin.

The Okinawa Trough, a continental-margin back-arc basin in the northwestern Pacific Ocean, is characterized by numerous hydrothermal fields and spatially consistent submarine volcanoes, indicating the presence of active, shallow magma chambers. The potential for volcanic eruptions, coupled with its proximity to populated areas, underscores the need for comprehensive studies of the underlying magmatic processes. In this study, we perform a detailed investigation of cathodoluminescence imaging, trace elements (Ti), and oxygen isotopes in quartz from two rhyolite samples collected from the middle and southwestern Okinawa Trough (MOT and SWOT). We observe varying proportions and CL patterns of rapid growth features in quartz from SWOT (40%) and MOT (4%), which imply different magmatic processes. Step zoning, involving Ti-poor and Ti-rich overgrowths, is prevalent in both regions and may reflect temperature variations rather than changes in TiO₂ activity or pressure. Oscillatory and step zoning represent the products of near-equilibrium crystallization, and their Ti contents can, therefore, be used to reconstruct the P-T conditions of the magma. Based on the estimated crystallization temperatures of 750 °C and 770 °C for SWOT and MOT quartz, respectively, the depths of the shallow silicic reservoirs for SWOT and MOT are inferred to be ~ 9 km. Jagged, bright cores (with high Ti content and variable δ¹⁸O values) in MOT quartz are interpreted as xenocrysts, providing mineralogical evidence for the assimilation of country rocks. The δ¹⁸O values, corrected for fractional crystallization and representing primary magmatic signatures, suggest that the SWOT magma has undergone greater contamination by upper crustal material (15–25%) than the MOT magma (< 18%). Moreover, our study reveals a slight negative correlation between δ¹⁸O values and grayscale values (or Ti content). This relationship can be readily explained by a near-wall crystallization mechanism, which also accounts for the common occurrence of Ti-poor overgrowths. Additionally, we attempted to constrain the crystal residence time using the diffusion of Ti in quartz, but the uncertainties of available calibrations largely hampered the interpretation. Nonetheless, if long-term near-wall crystallization is accepted, the mush model would imply that the magma chamber is primarily melt-dominated. Collectively, the distinctive textures and chemistry support the robustness of quartz as a reliable tracer for magmatic processes in shallow silicic reservoirs beneath the seafloor.

Naturally derived, fine-grained olivine ceramics were synthesized by an evacuated hot-pressing method that yielded samples with porosities of < 0.1%, a marked reduction compared to samples fabricated by conventional hot pressing with porosities of ≳1%. Evacuated hot pressing yields bright-green olivine samples that are transparent at the mm-scale, resembling the appearance of the single crystals from which they were synthesized, while conventional hot pressing produces milky-green aggregates that are opaque. This contrast in macro-scale transparency reflects the difference in micro-scale porosity. Annealing experiments on samples synthesized by the two different methods, some at 1 atm and others at 300 MPa confining pressure, reveal contrasting styles of grain and pore growth. During high-temperature annealing at both low and high pressures, evacuated hot-pressed samples underwent rapid, abnormal grain growth, while conventionally hot-pressed samples remained fine grained over long annealing times with very limited abnormal grain growth. During annealing at 1 atm, evacuated hot-pressed samples exhibited very little pore growth compared to conventionally hot-pressed samples that disaggregated due to pervasive pore growth. These experiments demonstrate the primary influence that porosity plays in grain growth in olivine aggregates. Further, the methods presented in this study provide a means to produce low-porosity olivine aggregates from naturally derived powders that can be used for high-temperature experiments at low pressures, as well as a method to make dense, coarse-grained olivine aggregates for laboratory studies.

Aso volcano in southwest Japan has produced four repeated caldera-forming eruptions over the last 270,000 years, each generating compositionally zoned ignimbrites that transition from silicic to more mafic magmas. To understand the magmatic processes behind these chemical and thermal zonations, we analyzed major and trace element compositions of melt inclusions and groundmass glasses. Our results reveal three distinct melt types: high-K silicic (HK-S), high-K mafic (HK-M), and medium-K (MK) melts. The HK-S and HK-M melts dominate the silicic and mafic units, respectively, while the MK melt is a minor component in the mafic units. Combining experimental petrology and mass balance modeling (mostly focusing on rare earth element compositions), we propose the following magmatic evolution: (1) The HK-M magma formed in a mid-lower crustal MASH zone through crystallization of basaltic magma and/or partial melting of basaltic rock; (2) this magma ascended and differentiated in a shallow upper crustal reservoir, generating the HK-S melt; (3) subsequent melt extraction from crystal mush, coupled with HK-M magma recharge, created a compositionally zoned shallow reservoir. The heat from the recharge also triggered partial melting and remobilization of the cumulate mush, producing the MK melt. These processes collectively explain the systematic zonation observed in Aso’s ignimbrites.

Garnet crystals in metapelites from the Goshen Formation, western Massachusetts, have experienced replacement by muscovite + biotite + quartz + plagioclase following decompression from ca. 1.0 to 0.35 GPa. The whole-rock reaction was garnet + muscovite = biotite + plagioclase + quartz; however, the phases replacing garnet (herein called the replacement mantles) always include muscovite as well as biotite, plagioclase, and quartz even though muscovite is a reactant. Numerical models are presented in which reactions only occur on a very local scale with adjacent phases and material in the grain boundaries and the progress of each local reaction is determined by the amount of available chemical affinity. Local reactions change the grain boundary composition, which sets up chemical potential gradients across the grid driving diffusive flux through the grain boundaries. This diffusion changes the grain boundary compositions elsewhere in the rock which changes the local chemical affinity and drives additional reactions in these localities. Thus, the local grain boundary composition drives muscovite dissolution in the rock matrix and precipitation in the replacement mantle surrounding garnet. Models are presented in which the amount of diffusion is varied from very little at one extreme to the other extreme with very long diffusion times such that the grain boundary composition remains homogeneous. The model results reveal that for very short diffusion times, the replacement mantle surrounding garnet is comprised largely of muscovite whereas with very long diffusion times the mantle is mostly biotite. Therefore, the ratio of muscovite to biotite in the replacement mantles may be interpreted to reflect the relative efficacy of grain boundary diffusion. This texture in which muscovite locally grows even though the whole rock reaction consumes muscovite is thus not the result of K-metasomatism but rather the consequence of grain boundary diffusion-limited metamorphic recrystallization.

Phase equilibrium experiments were used to determine conditions of melt evolution and phenocryst growth in high-Mg andesite magmas that were erupted at Whakaari (White Island) in New Zealand between 1976 and 2000. The high-Mg andesites are both mafic (7.21–10.3 wt% MgO) and silica-rich (55.3–58.6 wt% SiO₂) with phenocrysts of plagioclase, orthopyroxene, clinopyroxene, olivine, Cr-spinel and Fe–Ti oxides contained in a glassy to fine-grained matrix of mostly dacitic composition. Experiments were conducted on one of the most primitive samples available (the high-Mg andesite TRW34) at conditions ranged from 1 atm to 500 MPa at temperatures of 950 to 1200 °C with total water concentrations of 0 to 10 wt%. Except for the 500 MPa experiments, ƒO₂ was buffered at 1 or 2 log units above Ni–NiO. Consistent with earlier thermodynamic modelling, our results demonstrate that residual Whakaari melts (now represented by matrix glasses) evolved along a plagioclase + two-pyroxene cotectic (± magnetite ± ilmenite) under comparatively low-pressure, shallow conditions (< 200 MPa or < 6 km) and were relatively hot (> 950 °C) and dry (≤ 3 wt% melt-H₂O), with oxygen fugacities either at, or slightly above Ni–NiO + 1 log unit. Although the bulk-rock trends of Whakaari volcanic rocks are clearly calc-alkaline, those of the residual matrix glasses are only weakly so. A likely explanation for this contrast is that the primary magmas were relatively hydrous, but became dehydrated when intruded at shallow depths. The effectiveness of water in this role, combined with the demonstrable presence of primitive calc-alkaline magmas in the upper-crust, highlights the importance of magmatic water, in place of deep crustal fractionation, for shaping the calc-alkaline evolutionary trend.

The Fui Norte and Fui Sur small eruptive centres, together with the nearby Mocho-Choshuenco Volcanic Complex, are all located within what is potentially the most hazardous segment of the Southern Volcanic Zone of Chile. Developing comprehensive knowledge on the origin of evolution of these systems is not only important to better understanding of small eruptive centres, but also contributes to improved volcanic hazard prediction and mitigation. Using Sr–Nd isotopes, we determined that Fui Norte cluster has an independent plumbing system, while Fui Sur would be genetically related to the Mocho-Choshuenco stratovolcano. Through mixing models, we determined that the isotopic signatures of Fui Norte are closer to a MORB mantle isotopic composition, whereas the products from the Fui Sur cluster and Mocho-Choshuenco volcano exhibit a greater influence from slab components. This result shows that even in spatially constrained areas, magmas can record significant source differences. Using petrographic information and diffusion chronometry, we determined crustal timescales for the Fui Norte and Fui Sur SECs, from 1 month up to 4.5 years. This unexpectedly large time-scale range is interpreted as the lifespan of the crustal reservoir for these small eruptive centres. The significant differences in their source origin and the petrologic approaches reveal that both Fui Norte and Fui Sur have independent magmatic histories at the mid to upper crustal environment under similar timescales. Understanding that these systems operate independently from mantle to crust is relevant for future hazard assessment in the Southern Andes.