Contributions to Mineralogy and Petrology最新文献

Magmas degas volatiles and metals into submarine arc-related hydrothermal systems, but the nature of this process and its role in seafloor massive sulfide (SMS) mineralisation are poorly understood, particularly in submarine arcs hosting Cu–Au-rich deposits. To address this, we studied glassy melt inclusions (MIs) and matrix glasses from Brothers submarine volcano in the Kermadec arc, where there is strong evidence for magmatic involvement in hydrothermal processes and Cu-Au-rich mineralisation. We present a new dataset of volatile contents (H₂O, CO₂, S, Cl), sulfur speciation, and trace element concentrations for these materials, including samples recovered from IODP drill cores. Primitive basaltic MIs from a nearby mafic ridge imply that the volatile contents and fO₂ (~ FMQ + 1.5) of parental Brothers melts are within the normal range for arc magmas, but they contain high concentrations of some chalcophile metals (notably, Cu). Silicic (dacite/low-SiO₂ rhyolite) MIs from the main Brothers edifice record degassing of an S-bearing vapour phase under oxidising conditions (fO₂ ~FMQ + 2) at very shallow depths (~ 1–3 km below seafloor). These vapours are Cl- and metal-poor, but trigger acid sulfate alteration and likely provide a dilute source of trace metalloids, plus some Au and Cu. The Cl contents (max = 6480 μg g⁻¹) of silicic Brothers MIs are among the highest known for any subalkaline arc magmas. Increasing Cl contents with differentiation indicate a progressive approach to brine saturation, inferred to be reached only at late stages of crystallisation when the magma is no longer eruptible. Such late, shallow degassing of Cl is traditionally viewed as unfavourable for metal extraction, contrasting with deeper and earlier degassing in subaerial porphyry systems. The unusually high Cu and Au grades of Brothers SMS mineralisation, however, imply that significant quantities of metal are mobilised into the hydrothermal system. We suggest that the adverse effects of late Cl degassing on metal extraction at Brothers are offset by (1) very shallow magma storage, providing circulating hydrothermal fluids direct access to dense metal-rich brines and magmatic sulfides, and (2) elevated metal contents in primitive parental Brothers melts, which enhance the overall magmatic metal budget. The Brothers example suggests that these factors may be more important controls on Cu–Au‑rich mineralisation in submarine arcs than in subaerial arcs built on thicker crust, where more favourable degassing regimes can develop. Ultimately, Brothers demonstrates that arc-related SMS-forming systems can be strongly magmatically influenced and share both commonalities and differences with Cu-Au mineralised subaerial systems.

Volcanism at continental arcs is ultimately driven by mantle-derived magmas; however, erupted compositions are typically overprinted by crustal interactions during ascent, obscuring deeper signatures. We target olivine and their melt inclusions (MI) in basaltic andesite scoria erupted from Red Crater (Tongariro, New Zealand) to capture primitive melt compositions prior to upper crustal processing. Olivine crystals comprise several, mostly antecrystic populations, with forsterite contents from Fo_70–91 and variable textures indicative of complex crystallisation histories within a dynamic, transcrustal magmatic system. MI in forsteritic (≥ Fo₈₄) olivine cores contain primitive basaltic melts and record early fractionation (~10–20%) of olivine, spinel ± clinopyroxene at sulfur-saturated, moderately oxidised conditions during entrapment. Incompatible trace elements display variable enrichments in fluid-mobile and light rare earth elements that can be explained by ~3–10% partial melting of a shallow mantle source with variable contributions of slab-derived fluid and/or sediment components and minor to no crustal assimilation. Similarities with caldera-associated olivine-hosted MI in the central Taupō Volcanic Zone (TVZ) suggest a common source, with differences such as higher calcium and chlorine at Red Crater and trace element variations potentially related to distinct slab-derived contributions, differences in mantle mineralogy, degree of melting or crustal interactions. Offsets in incompatible trace element abundances between Red Crater olivine-hosted MI, groundmass glass and whole-rock compositions imply 10–20% crustal contamination with metasedimentary basement rocks during magma ascent and shallow storage. Whole-rock data are considered less suitable for investigating primitive magma compositions and mantle sources, with olivine-hosted MI providing a more robust basis for interpretations.

The U–Pb records of accessory minerals can become difficult to interpret when magmatic crystallization is overprinted by hydrothermal alteration and element redistribution. The Archibarca Pluton, in the shallow portion on the Paleozoic Famatinian Arc in northwestern Argentina, preserves a complex history of magma crystallization, magma mixing (hybridization), and autometasomatism that variably overprints the textures, chemistry and U–Pb record of zircon, titanite and apatite. We found that hydrothermal modification affects zircon U–Pb dates and trace element compositions, producing complex and apparently concordant date distributions. Nevertheless, after filtering analyses affected by metasomatic modification, zircon retains geochronological information that is consistent with the magmatic and hydrothermal history recorded by titanite and apatite. Thus zircon can preserve useful chronological information even in hydrothermally modified systems, but its U–Pb record should be evaluated together with textural and geochemical evidence and complemented by other accessory mineral petrochronometers to distinguish magmatic crystallization from subsequent fluid-mediated changes. Zircon displays a spectrum of preserved and modified domains, in which chemical alteration, expressed as Fe–Ti–P–Y-LREE enrichment or depletion, may be either coupled or decoupled from U–Pb dates. Although zircon yields apparently concordant LA–ICP–MS dates spanning from ~ 500 to 420 Ma, these dates should be interpreted with caution, as metasomatic modification can disturb the U–Pb system without producing evident chemical alteration. Therefore, trace-element-based filtering is insufficient to discriminate disturbed zircon dates. A SHRIMP date of 441 ± 5 Ma (MSWD = 0.5) obtained from a composite magmatic-metasomatic zircon overlaps with its LA–ICP–MS date (445 ± 1 Ma, MSWD = 11); however, the latter exhibits substantial excess scatter, indicating that part of the date dispersion reflects analytical sensitivity to disturbed domains rather than geological complexity alone. In contrast, titanite preserves a more robust magmatic U–Pb record, with most samples yielding consistent crystallization dates of ~ 460 to 450 Ma, overprinted by fluid-related Pb loss at ~ 420 to 390 Ma. Additional complexities include inheritance at ~ 611 Ma and partial Permian (~ 280 Ma) resetting linked to regional Choiyoi magmatism. Titanite exhibits dual petrochronological behaviour, recording both magmatic dates and hydrothermal overprint, whereas magmatic and hydrothermal apatite yield indistinguishable U–Pb dates (439 ± 13 Ma and 436 ± 13 Ma), interpreted to record cooling of the pluton below ~ 450 °C. In summary, integrating the textural, chemical and isotopic data from these three accessory phases help constrain the nature and timing of the magmatic-hydrothermal transition in plutonic rocks.

This study examines the microstructural, petrological, and geochemical processes that disrupt date relationships within biotite and apatite. We combine SEM-EBSD, cold/SEM-cathodoluminescence, EPMA and LA-ICP-MS geochemical mapping, with in situ multi-system geochronology (⁴⁰Ar/³⁹Ar and ⁸⁷Rb–⁸⁷Sr in biotite, apatite U–Pb) to investigate grain-scale microstructural controls on isotopic systematics in rocks from the south Rae craton, Canada. Our results show that intracrystalline defects (subgrains, kink bands, twin planes) and intercrystalline decompression-related fabrics (strain-softened and fluid-prone domains) promote fluid-mediated redistribution of major and trace elements (e.g., Rb, Sr, Ar, U, Th, Pb, La, Ce) in biotite and apatite, enabling recrystallization and/or open system behaviour. This results in both older and younger ⁸⁷Rb–⁸⁷Sr dates relative to ⁴⁰Ar/³⁹Ar and U–Pb systems within the same samples, independently of tectonic domain, reflecting spatially variable isotopic resetting, inheritance, and recycling. These findings underscore how age distributions in biotite and apatite are controlled by spatially variable recrystallization, inheritance, and fluid-mediated reworking rather than simple cooling through closure temperatures. ⁴⁰Ar/³⁹Ar, ⁸⁷Rb–⁸⁷Sr and U–Pb systems are best interpreted in terms of grain-scale processes rather than cooling paths. Regionally, our youngest 1.73–1.76 Ga ages reflect a late overprint and intracratonic reworking, suggesting that apparent cooling trends in the Trans-Hudson Orogen may sometimes record isotopic disturbance rather than exhumation. Integration of microstructural and geochemical mapping with micro-spatially resolved geochronology provides a more robust framework for interpreting legacy datasets and guiding in situ isotopic dating.

Molten albite (NaAlSi₃O₈) is a useful analog for aluminosilicate-rich magmas which produce continental crust due to their broadly similar compositions. The albite melt density constrains its buoyancy relative to surrounding rock, allowing us to evaluate the ascent of comparable magmas from deep below. Previous studies reported a wide range of values for compressibility and its pressure derivative of anhydrous albite melt, yielding significantly different equations of state (EoS) to calculate density at depth. Here, we further constrained the elastic properties of anhydrous albite melt via in-situ density measurements at ≤ 5.2 GPa and ≤ 2152 K using X-ray absorption. The melt density across these conditions is best described by a modified Birch–Murnaghan EoS and indicates a positive buoyancy throughout the crust and upper mantle. To further evaluate factors influencing the ascent of albite-like magmas, we also investigated the melt chemical structure using X-ray diffraction. At these conditions, compression densifies the melt primarily by bending connections between (Si, Al)O₄ units in its structure, which also decreases its viscosity by promoting depolymerization. Notably, natural magmas typically contain wt% H₂O. To assess how water influences the magma ascent, we combined our experimental constraints with previous simulations on hydrous albite melt. The albite melt density decreases by ~1% per wt% H₂O. In contrast, previous studies show that H₂O decreases the magma viscosity exponentially, exerting a stronger control on its movement. Emplacement of albite-rich magmas at depth is therefore promoted by increasing viscosity via decompression and potential H₂O loss during ascent, alongside crystallization, cooling, and rheology barriers.

The Early Miocene Kestanbol Magmatic Complex (KMC) in the SW Biga Peninsula, Western Anatolia, is an unusual, high-K post-collisional plutonic-volcanic system comprising coexisting plutonic, hypabyssal, and volcanic lithologies. These lithologies show pronounced enrichments in rare earth elements (REE), Th, and U relative to adjacent Oligo-Miocene plutons and regional high-K suites. Whether this system-wide enrichment primarily reflects progressive magmatic differentiation or inheritance from an enriched mantle source remains debated. Here, we integrate whole-rock geochemistry, mineral chemistry, zircon trace-element thermometry, zircon O-Hf isotopes, and available geochronological constraints to evaluate the origin and evolution of the KMC and the associated Th–U–REE enrichments. Comparatively elevated Th–U–REE abundances occur across compositionally distinct KMC lithologies, including silica-undersaturated high-K and locally ultrapotassic rocks, and therefore cannot be attributed solely to late-stage differentiation of felsic melts. Thermodynamically constrained FC and AFC models reproduce divergent evolutionary paths toward silica-undersaturated and silica-saturated to silica-oversaturated compositions and show that differentiation can further concentrate Th and U. However, neither process reproduces the highest observed concentrations or the overall enrichment pattern. Whole-rock Sr–Nd and zircon O–Hf isotope systematics further argue against progressive crustal assimilation as the principal control on isotopic variability, although localized crustal modification cannot be excluded. Together, these results indicate that the elevated Th–U–REE budget was largely inherited from enriched mantle-derived high-K magmas and subsequently redistributed by fractional crystallization, variable crustal assimilation, and magma interaction within a shallow reservoir. Thermometric, petrographic, and field relations indicate that the Kestanbol pluton formed as an epizonally emplaced, dynamically replenished crystal mush closely linked to coeval subvolcanic and volcanic activity. The KMC therefore provides an important example of how enriched mantle domains and upper-crustal hybridization together generate potassic and actinide-rich post-collisional magmatic systems in the northern Aegean region.

To assess the magma plumbing architecture of Mount Ruapehu, an active stratovolcano in the Taupō Volcanic Zone (New Zealand), we combine whole rock, groundmass and crystal geochemistry of lava samples from the south-west flank, focusing on the Mangawhero (50–15 ka) and Whakapapa (< 15 ka) formations. Carrier melts show nearly identical compositions, suggesting homogenous melt production and differentiation through time. Variations in whole rock MgO compositions are attributed to accumulation of mafic crystals (ortho- and clinopyroxene). The crystal cargo (plagioclase and pyroxene) is relatively small (typically < 100 μm, maximum 2 mm), however compositional variability, crystal zoning and disequilibrium with carrier liquids suggest transcrustal differentiation involving fractional crystallization, magma mixing and interaction with crystalline bodies. The most recent lavas of Mount Ruapehu (< 26 ka), exhibit SiO₂ versus K₂O reverse trends overlapping with earlier Mangawhero Formation compositions. We interpret this trend, observed in whole rock and groundmass compositions, as the result of variable crustal assimilation and fractional crystallization, consistent with previous studies based on trace elements, isotope data, and crustal xenoliths. We propose a plumbing system of stacked, narrow magma bodies (likely dike- or sill-geometry) connected by a dike transport network, where ascending melts incorporate glomerocrysts and crystal fragments from previously emplaced crystalline bodies within the crust, modulating bulk rock MgO signatures. Rapid transport of melts and crystal cargo is favored by re-injection through the dike network, enhanced by rifting geodynamics.

Stepwise crushing analyses of He, Ne and Ar isotopes in magmatic and magmatic–carbothermal carbonatites and associated alkaline-ultramafic rocks from the Seblyavr complex (Kola Peninsula, Russia) provide compelling evidence for a mantle plume source throughout the entire magmatic evolution of the complex. In contrast to carbonatite and ultramafic complexes from Polar Siberia and India, noble gases in all investigated lithologies and paragenetic stages are dominated by a plume-derived component exhibiting isotopic characteristics that are more primitive than those of the present-day Réunion mantle plume. The mantle endmember is characterized by ⁴He/³He = 34,211 ± 1688, ²¹Ne/²²Ne_(mantle) = 0.0384 ± 0.0007, and ⁴⁰Ar/³⁶Ar_(mantle) = 4481 ± 403 at ²⁰Ne/²²Ne_(mantle) = 12.9. One calcite sample (97–11 Cal) records a more radiogenic and nucleogenic mantle signature, with ⁴⁰Ar/³⁶Ar_(12.9) = 6180 ± 35, and ²¹Ne/²²Ne_(12.9) = 0.0446 ± 0.0008. These compositions are best explained by minor addition of crust-derived fluids to the mantle source prior to calcite crystallization, followed by trapping of a well-mixed fluid within the mineral. The Seblyavr rocks contain exceptionally high concentrations of mantle-derived noble gases (e.g., up to 1.2×10^–9 ccm/g of ³He), exceeding those reported for most carbonatite complexes worldwide. We attribute this exceptional preservation to the unique geological setting of the complex. As the deepest-emplaced and oldest Paleozoic intrusion within the Kola Alkaline Province, Seblyavr likely retained abundant mantle volatiles, because magmas ascended rapidly from depth and experienced only limited pre-emplacement degassing. Rapid ascent also minimized interaction with crustal reservoirs. These results provide new constraints on the composition of the subcontinental mantle beneath the Kola Alkaline Province and on the evolution of magmatic and hydrothermal fluids during Paleozoic carbonatite magmatism.

The lower oceanic crust beneath fast-spreading ridges is commonly attributed to fractional crystallization of basaltic melts, but reactive melt transport and crystal-mush reorganization may also be important. We investigated spinel-bearing layered gabbros from Hole U1415P, Hess Deep Rift (IODP Expedition 345), focusing on Cr-spinel-hosted silicate inclusions. Heterogeneous spinel-bearing olivine gabbros are sharply juxtaposed against relatively homogeneous, spinel-poor troctolite. They contain plagioclase chadacrysts enclosed by olivine and clinopyroxene oikocrysts, orthopyroxene rims and veinlets associated with olivine, and locally abundant Cr-spinel, indicating localized melt-mush interaction. Cr-spinel hosts fully crystallized inclusions with contrasting mineral assemblages. Cpx + Pl-type multiphase solid inclusions (MSIs) have host-gabbro-like assemblages and basaltic to locally picritic bulk compositions, consistent with a resident basaltic component. In contrast, rounded to subrounded Opx-type MSIs contain voids and accessory phases absent from the host gabbro and define a distinct SiO₂-rich, Mg-rich bulk-compositional group. Opx-bearing MSIs and single-phase Opx inclusions indicate that orthopyroxene-bearing components were present during Cr-spinel growth. We infer localized interaction between a troctolitic mush and an infiltrating SiO₂-rich reactive melt. Reaction with olivine may have generated locally Mg-enriched reaction-zone melts, whereas interaction or mixing with resident basaltic interstitial melt likely promoted Cr-spinel saturation. The U1415P rocks therefore record melt infiltration, hybridization, and reactive reworking of preexisting troctolitic cumulates, processes that may contribute to chemical and textural heterogeneity within the lower oceanic crust.

Fourier-transform infrared (FTIR) spectroscopy is the standard technique for quantifying trace hydroxyl (OH) in nominally anhydrous minerals because it is accessible, fast, and can distinguish hydrous defects by the orientation and number of OH dipoles in the defect complex. Although most minerals absorb infrared light anisotropically, most studies approximate the total absorbance (the sum over the three principal directions) from unpolarised spectra of randomly oriented crystals, discarding the OH dipole orientation information held by birefringent crystals. Polarised, spatially resolved FTIR (μ-FTIR) recovers the OH bond orientation of multiple grains relative to the crystal axes, provided the crystal orientations are known. We present a method that recovers the full crystallographic orientation of an anisotropic crystal in specimen coordinates, expressed as Euler angles, from polarised μ-FTIR spectra collected over a range of polariser angles along with reference spectra measured along three orthogonal directions. The method requires only a single diagnostic wavelength, making it more flexible than spectrum-range approaches because the user can avoid wavenumbers degraded by detector non-linearity or baseline issues. We validate it on the orientation dependence of the silica overtone region in olivine, using Monte Carlo simulations and crystals of known orientation measured by Electron Backscatter Diffraction (EBSD). We also provide an open-source Python implementation (FTIRkit). By recovering crystal orientation and OH content from a single FTIR measurement, the method provides a powerful tool for constraining water budgets, metasomatism, and magma sources in the mantle and lower crust, testing DFT predictions of OH-group orientation, and correcting CPO bias in deformation studies. The approach applies to any birefringent crystalline phase and can be extended to other polarised techniques (Raman, UV–Vis, and laser-direct IR) where principal-direction spectra and a model relating spectrum to orientation are available.