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 Fo70–91 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
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.
Ilmenite is a ubiquitous accessory mineral in igneous and metamorphic rocks, controlling behavior of trace elements during generation of anatectic granitic melts and fractionation of magmas. However experimental data on reactions involved in ilmenite dissolution in felsic melts are limited. We performed twenty two experiments in felsic melts of a wide compositional range (Al/(Na + K) from 0.7 to 1.3) at pressure 0.1 and 1 GPa, and at temperature from 800 to 1000 °C. In the piston-cylinder experiments, boron nitride (BN) and MgO assemblies were employed to exert different redox conditions. The oxygen fugacities estimated from ilmenite composition indicate substantial overlap between the 0.1 GPa cold-seal pressure vessels (CSPV) and 1 GPa MgO piston-cylinder experiments, whereas in BN piston-cylinder experiments conditions were more reducing. Elevated temperatures and oxygen fugacity promoted ilmenite dissolution, while melt water content and pressure exhibited relatively minor effects. Ilmenite solubility is lowest when the A/NK ratio close to 1.1 and increases toward both more alkaline and more peraluminous melt compositions. Furthermore, our experiments demonstrate that ilmenite dissolution in Ca-free systems can produce rutile at pressures as low as 0.1 GPa and melts coexisting with ilmenite commonly are close to rutile saturation. The experiments produced homogeneous ilmenite, sponge-textured ilmenite, and rutile-ilmenite intergrowths with oriented rutile lamellae. Homogeneous and exsolved ilmenite can occur under similar oxygen fugacities, and the development of the exsolution textures was determined by the combination of the elevated oxygen fugacity and melt composition. The more common development of rutile–ilmenite intergrowths at 1 GPa suggests an additional pressure control on textural re-equilibration. The experimental textures show similarities to natural textures, but the evolution of those in more stable conditions results in coarser-grained assemblages.
In this study, we investigate the effect of the availability of sites for heterogeneous nucleation on the microstructure of moderately to highly undercooled synthetic trachybasalts under anhydrous and hydrous (2 wt
