We evaluated the use of electron backscatter diffraction (EBSD) for microstructural quantification of silicate crystal- and glass-bearing (hemicrystalline) samples, focusing on the effects of EBSD post-processing (cleaning, grain reconstruction, boundary smoothing, re-gridding). Our case study reanalysed experimental samples of Pontesilli et al. (2019), comprising synthetic trachybasalt melts ± H2O, cooled to 1100°C and held for 30 minutes, producing a phase assemblage comprising dendritic (anhydrous sample) or skeletal (hydrous sample) clinopyroxene (Cpx), titanomagnetite (Tmt), and glass. EBSD indexing quality (Confidence Index and Fit) gradients occur across crystal-glass boundaries, causing a thresholding effect via the cutoff selected to identify glass. Standardising indexing quality to the grain maximum had the greatest impact on results, removing gradients but inflating grains. Phase proportions (Φ) and crystal sizes were influenced mainly by thresholding effects. Number density (NA) and surface area to volume ratio (${\mathrm{S}}_v^{\mathrm{P}}$) were also affected by boundary smoothing, minimum grain size threshold, and step size. Ideally, we recommend analysing differently processed scans simultaneously: non-standardised for Φ, grain sizes and growth rates, and standardised for NA, ${\mathrm{S}}_v^{\mathrm{P}}$, and crystal-crystal misorientations. Comparison with previous backscatter electron (BSE) imaging revealed that EBSD and BSE deliver similar results if spatial resolution is similar and few crystals of the same phase touch. EBSD is recommended if higher resolution is required, BSE contrast is poor, many crystals of one phase touch, and/or crystals are dendritic. Our case study revealed higher ΦCpx and total crystallinity but lower ΦTmt under anhydrous conditions, potentially due to undercooling-enhanced crystallisation of Al- and Ti-enriched Cpx.
Abstract The recycling of ultramafic–mafic cumulates formed at arc roots is widely regarded as a key mechanism for returning dense lower-crustal materials to the mantle during continental crust formation. However, direct petrological evidence constraining the nature and evolution of such recycled lithologies remains scarce. Here, we present an integrated petrographic, geochemical, and thermodynamic study of the Hujialin ultramafic body from the Sulu orogen. This body comprises refractory dunites alongside a coherent wehrlite-clinopyroxenite suite, offering insights into the formation and fate of arc lower-crustal cumulates. The dunites represent refractory mantle fragments of the North China Craton, characterized by depleted whole-rock and mineral major element compositions, including high olivine magnesium numbers (91.2–92.8) and spinel chromium numbers (66.1–79.1). In contrast, the Hujialin wehrlites and garnet clinopyroxenites exhibit enriched whole-rock signatures and mineral layering consistent with a cumulate origin. The decoupling between whole-rock incompatible element enrichment and Sr–Nd isotopic compositions, together with relatively low olivine magnesium numbers (86.7–88.3) and spinel characterized by high Al2O3 (58.2–66.6 wt.%) and low Cr2O3 contents (0.19–3.30 wt.%), also supports a cumulate origin. Forward fractional-crystallization modelling shows that a representative continental arc basaltic melt evolving at ~1 GPa can broadly reproduce the observed cumulate assemblages and major-element trends, supporting the interpretation of the wehrlite-clinopyroxenite suite as deep arc-related cumulates. Phase equilibrium modelling indicates that garnet was not part of the primary crystallizing assemblage but formed mainly during post-magmatic cooling at lower-crustal pressures. Early garnet coronas around spinel and abundant porphyroblastic and matrix garnet record this cooling evolution, whereas the spinel-free matrix assemblage, together with the low Al2O3 contents and consistently high magnesium numbers of matrix clinopyroxene, supports subsequent high-pressure re-equilibration during continental subduction. Notably, Cu contents in the wehrlite-clinopyroxenite suite are elevated relative to the bulk continental crust and locally reach 464 μg/g, while preserved sulfide inclusions provide petrographic evidence for sulfide retention in the deep cumulates. The Hujialin suite therefore provides petrological support for the formation of Cu-bearing arc-root cumulates and illustrates a plausible pathway by which dense cumulates may be removed from the arc root and subsequently undergo high-pressure metamorphism during continental subduction.
Abstract Porphyry copper deposits tend to be associated with differentiated arc magmas characterized by high Sr/Y ratios, which feature is thought to result from magma fractionation at high pressure. However, significant debate persists regarding the question which specific magma property/properties rendered them so fertile. Potential candidates are: (1) higher Cu content, (2) higher S content, (3) higher Cl content, (4) higher H2O content. To understand how pressure affects these factors during magma fractionation we conducted crystallization experiments on a natural arc basalt doped with 3.5 wt% H2O, 3200 μg/g CO2, 2200 μg/g S, 1000 μg/g Cl, and 200 μg/g Cu, which concentrations are geologically realistic. The experiments were performed at 750-1250 °C and 0.4, 0.7 and 1.0 GPa using piston cylinder presses. In order to obtain minerals and melt pools that are large enough to be analyzed by LA-ICP-MS and FTIR the samples were first heated to 1100-1250 °C and then slowly cooled to the target temperature. Loss of sulfur, iron and carbon to the PtRh capsule was minimized by using capsule liners made of gem-quality zircon crystals. Oxygen fugacity was buffered internally at ~2.3±0.7 log units above the fayalite-quartz-magnetite fO2 buffer through the coexistence of anhydrite and monosulfide solid solution. To verify the results, a set of isothermal experiments was performed without zircon liners in Au capsules. Modal mineral abundances were quantified via SEM-EDS mapping, and minerals and quenched silicate melt were analyzed by EPMA, LA-ICP-MS and FTIR. The slow-cooling experiments produced crystals that are up to several hundred micrometers in size and show textures resembling those of natural samples. The crystallization sequence at 0.7 and 1.0 GPa was spinel + clinopyroxene → amphibole + magnetite + anhydrite + sulfide → plagioclase + apatite, whereas at 0.4 GPa plagioclase started to crystallize before amphibole. Correspondingly, evolved residual melts reached high Sr/Y ratios at 1.0 GPa, but retained low Sr/Y ratios at 0.4 GPa, confirming previous predictions. The CO2 and S concentrations in the residual melts increased until the melt reached ~55 wt% SiO2, after which point they decreased rapidly. In the case of CO2 the decline is due to exsolution of CO2-rich fluid, whereas in the case of S it is due to the precipitation of anhydrite + sulfide. Copper concentrations declined already earlier due to Cu-loss to the noble capsule despite the use of zircon liners. In contrast, H2O and Cl concentrations in the melt increased until ~65-70 wt% SiO2, after which point they remained more or less stable (H2O) or declined (Cl). The highest Cl concentrations were reached at low pressure, whereas the highest H2O concentrations were reached at high pressure. We conclude that the high mineralization potential of high-Sr/Y magmas is primarily a consequence of unusually high H2O contents, which causes the ascending magmas to stop and accumulate at the optimal depth level of ~10-15 km. This depth range seems to be required for the generation of fluid-focusing structures such as porphyry fingers and breccia pipes without resulting in catastrophic volcanic eruptions.
Supercritical phenomena arising from the closure of miscibility gaps between vapor/gas and liquid/melt represent an intriguing aspect of deep Earth processes. While corresponding phase relationships have been discussed predominantly in binary systems, these relationships remain inadequately explored in higher-order systems. In this study, we analyse the phase relationships of fluid miscibility in three geologically relevant ternary systems (CaO-CO2-H2O, MgO-SiO2-H2O, SiO2-NaAlSiO4-H2O). This analysis clarifies the definitions of second critical endpoint (SCE) and ordinary critical point by highlighting their differences. We also introduce a new related concept-critical pressure-to underscore the importance of distinguishing between these terms. It is demonstrated that SCEs are invariant points, independent of system composition, whereas both ordinary critical points and critical pressures vary with system composition. A systematic examination of the metastability of certain SCEs and critical curves is also provided, along with a discussion of the geological implications. The extension of our considerations to the quaternary system SiO2-KAlSi3O8-NaAlSi2O6-H2O further reveals that SCEs and critical curves are hierarchically organized in higher-order systems, with critical curves linking SCEs across systems of different orders. Although the phase relationships presented here are largely schematic, the general topologies, together with the conceptual framework developed, provide a useful foundation for future experimental and natural observations.
Abstract The Jiaodong gold province, with proven resources of more than 5,700 tonnes, is the largest gold-producing region in China. Owing to the close spatial and temporal relationships between mantle-derived intrusions and gold deposits, the role of mantle magmatism in Jiaodong gold mineralization has long been debated. Integration of field observations, petrography, and major and trace element geochemistry of mantle-derived lamprophyres, monzodiorites, and mafic microgranular enclaves is required to address this issue. The Jiaodong lamprophyres, monzodiorites, and mafic microgranular enclaves were derived from a common metasomatized mantle lithosphere source but followed distinct evolutionary pathways. Lamprophyres represent relatively primitive mantle-derived magmas that mainly experienced fractional crystallization, whereas the monzodiorites and MMEs represent more evolved magmas that underwent variable degrees of crustal contamination and magma mixing. Chalcophile element geochemistry indicates that sulfide segregation during lamprophyre magma evolution was limited. Consequently, the Au concentrations of the lamprophyres closely reflect the primary Au contents of their parent magmas (0.24–2.34 ng/g). In contrast, the evolved monzodiorites and MMEs experienced significant sulfide segregation and contain lower Au concentrations (0.05–0.60 ng/g). Overall, the Jiaodong mantle-derived magmas did not possess an unusually large magmatic Au budget compared to global magmas genetically associated with magmatic-hydrothermal Au systems (1.30–5.50 ng/g). Mineral–melt thermobarometry, together with previous estimates of regional exhumation, indicate that these mantle-derived magmas were emplaced at depths that precluded extensive H2O–CO2 fluid exsolution. Increasing X(Cl/F) and X(OH/F) ratios from lamprophyres to monzodiorites and mafic microgranular enclaves further demonstrate that little fluid exsolved during the late stages of magma crystallization. Combined with the limited volume of mantle-derived intrusions, these factors suggest that mantle-derived magmas were unlikely to have directly generated the ore-forming fluids responsible for the giant Jiaodong gold deposits.
