Contributions to Mineralogy and Petrology最新文献

Paragonite is an important high-pressure (HP) indicator in epidote-amphibolites; however, its occurrence in these rocks is notably rare. Paragonite-bearing and paragonite-free epidote-amphibolites from Taiwan offer a great opportunity to understand the influence of bulk-rock chemistry on paragonite stability by phase equilibrium modelling. The investigated epidote-amphibolites have basaltic compositions with elevated Al₂O₃ content (16.2–19.7 wt%). The paragonite-bearing (PEA) type is characterised by pargasitic hornblende + epidote + paragonite + rutile + quartz ± garnet assemblages, while the paragonite-free (EA) type contains pargasitic hornblende + epidote + rutile + quartz ± chlorite assemblages. The mineralogical difference between the two types is attributed to a variation in bulk-rock Mg# (PEA: 28–54; EA: 63–67). Both types experienced similar peak pressure–temperature (P–T) conditions at 1.2–1.6 GPa and 575–625 °C (M1 stage). Although the P–T estimates are comparable to that of some eclogites, these rocks do not exhibit eclogite facies mineral assemblages. The corresponding paleo-geothermal gradients of 12–16 °C km^− 1 indicate a warm subduction environment, likely reflecting the thermal structure of a young intra-oceanic subduction zone. Although these rocks show little or no retrogression, we still identified two post-peak metamorphic stages, including a blueschist facies overprint at 0.7–0.8 GPa and 440–475 °C (M2 stage), and a greenschist facies overprint at 0.4–0.6 GPa and 400–475 °C (M3 stage). These P–T estimates suggest a cooling and decompression from M1 to M2, followed by a near isothermal decompression from M2 to M3. P–T–X modelling in the MnNCFMASHTO (K-free) systems shows that paragonite preferentially stabilises in high-Al, high-Na/(Na + Ca), high-Fe³⁺/ΣFe, or low-Mg# metabasic compositions. H₂O saturation, or near H₂O saturation, is also essential for paragonite stability. Using the median worldwide metabasite composition in Forshaw et al. (2024) as a global reference, the P–X modelling in the NCFMASHTO (K-free) system predicts that paragonite is stable when any one compositional parameter meets the following approximate thresholds: Al₂O₃ > ~17.0 wt%, Na/(Na + Ca) > ~ 0.45, or Fe³⁺/ΣFe > ~ 0.50. However, these values should be considered as a first-order approximation, rather than strict criteria. Our results emphasise the pivotal role of bulk-rock chemistry in controlling the occurrence of paragonite in metabasites.

Mineral deposits with Au − Sb − W metal association are rare worldwide, but the Zhaishang deposit in the western Qinling Orogen, central China, is one of such deposits. This deposit formed through complex ore-forming processes comprising two main ore stages, stage II marked by Au − As mineralization and stage III by Au − Sb − W mineralization. Four types of pyrite (i.e., Py0, Py1, Py2, Py3) and two types of arsenopyrite (i.e., Apy1, Apy2) were recognized based on their texture, geochemistry, and sulfur isotopic signature. The Py0 has diagenetic origin with the lowest As compared to Py2 and Py3. The syn-ore Py2 in stage II can be subdivided into three sub-types, which have the highest Au and As (up to 57.7 ppm and 72,380 ppm, respectively) and show similar positive correlations among Pb, Zn, Ag, Sb and/or Cu. The values of δ³⁴S were similar in all three Py2 sub-types namely 14.2–16.7‰ (Py2a), 10.0–14.4‰ (Py2b), and 13.6–16.6‰ (Py2c). The core–rim textures of arsenopyrite and the dissolution − reprecipitation textures of Py2 imply that the deposition of Au − As was driven by fluid–rock interaction, and the dissolution–reprecipitation reactions resulted from multiple pulses of hydrothermal fluids that originated from a common sedimentary sulfur source. Gold in Stage III occurs as native Au and gold tellurides. Py3 is enriched only in Sb and depleted in other ore-forming elements, such as Au, Ag, As, Cu, Pb, and Zn. Sulfur dominantly originates from a sedimentary sequence and only small fraction has a magmatic origin. Based on thermodynamic modeling, assuming a fluid pH of 4 to 6.6, fluid processes such as oxidation and cooling of fluid induced the formation of the Au − Sb mineralization, while the addition of Ca²⁺ promoted the W mineralization, due to boiling, fluid mixing and fluid–rock interaction. This study highlights that the unique Au − Sb − W metal association reflect diverse fluid sources and dynamic precipitation mechanisms.

The study of recent eruptions in Ocean Islands (OIs) provides a unique window into the magma dynamics governing their plumbing systems and the mechanisms leading to eruptions. Here we present an integrated approach to unravel the dynamics of magmatic plumbing systems through detailed spatial, petrological, and geochemical characterisation of volcanic products ranging from crystal-rich ankaramitic lavas to trachytic tephras. We focus on the textural and geochemical spatial variations of 42 Holocene subaerial eruptions at the OI of El Hierro (Canary Islands), as well as on their petrogenetic significance for magmatic evolution and plumbing system architecture. Integrating geochemical data within fractional crystallisation modelling and mass balance calculations reveals that ankaramitic and porphyritic lavas with phenocryst modal abundances > 10 vol% result from melt extraction and crystal accumulation. Aphyric to sub-aphyric eruption products and porphyritic lavas with phenocryst modal abundances < 10 vol% usually follow fractional crystallisation trajectories that start at ~ 10 wt% MgO. Periodic extraction of evolved melt from crystal mushes likely led to the occurrence of minor trachytic eruptions, which are difficult to reconcile with simple closed system fractional crystallisation trends. A complex, heterogeneous crustal mush system beneath El Hierro is, in fact, the most reliable scenario to explain the wide range of textures, whole-rock and mineral compositions, and the overall surface distribution of vents and eruptive styles displayed by the Holocene volcanism on the island. Our integrated findings highlight the importance of a combined field, petrological, and geochemical study to decipher plumbing system dynamics of OI magmatism. The results allow us to put forward an updated conceptual model of the current plumbing architecture of El Hierro’s volcanic system during the Holocene.

Metamorphic zircon is commonly assumed to record growth during prograde or peak metamorphic conditions. However, numerical models of zircon predict growth during cooling. Linking the relative timing of zircon growth to a metamorphic evolution requires determining the potential zircon forming reactions during protracted metamorphic cycles. Meta-granitoids from the Grenville Province in Ontario contain a high proportion of igneous zircon with metamorphic rims that provide a rare opportunity to study zircon forming reactions in situ; acquiring U–Pb dates while maintaining their petrographic context and textural setting in thin section. Textures and trace element composition of major minerals indicate that metamorphic zircon grew as a result of melt crystallization and breakdown of titanomagnetite. Phase equilibrium modelling shows that these reactions occurred during retrograde metamorphism. Measured U–Pb dates of these retrograde metamorphic zircons are between ca. 1100 and 1070 Ma. This contradicts previous interpretations of regional geology that suggest prograde and peak metamorphism occurred between 1080 and 1050 Ma. These results highlight the need to carefully analyze the metamorphic textures of zircon to provide the necessary context to assess the zircon-forming reaction and its link to the pressure–temperature history of the rock. Without this context, pressure–temperature–time paths determined with zircon dates should be treated with caution and could be incorrectly linked to disparate stages of orogenic cycles.

Aqueous fluids released by metamorphic dehydration of serpentinites are a key component for seismicity, creep, and geochemical cycling in subduction zones. How these fluids drain and migrate towards the mantle wedge has yet to be fully understood. Here we address the influence of pre-existing structural and mineralogical heterogeneities in serpentinites on dehydration and fluid migration at forearc conditions. We partially dehydrated natural serpentinite containing brucite veins in a piston-cylinder apparatus with a temperature gradient across the conditions of the brucite + antigorite = olivine + fluid reaction (485–520 °C; 1.5 GPa). Micro-tomography, electron microscopy and microstructural analysis of the experimental results, coupled with thermodynamic modelling, show that temperature, mineralogical heterogeneity and variable ingress of external H₂ controlled the dehydration extent. Experimentally formed olivine indicates a topotactic relationship between [100]_Ol and [0001]_Brc, although the resultant fabric is overall random because brucite was randomly oriented. Olivine forms mono-mineralic aggregates along the walls of brucite veins, displaying very high porosity (up to 32%) and permeability (10^–13–10^–14 m²). Tracing the pre-existing brucite vein network, these aggregates can form a transient network of interconnected, highly permeable fluid channels that allows drainage and may enhance open-system exchange with neighboring lithologies. Infiltration of reduced external fluids can trigger redox dehydration of magnetite + antigorite to Fe-rich olivine, which renews porosity and propagates focused fluid flow. The distribution of brucite and magnetite, especially as vein networks, therefore has a first-order control on how focused fluid drainage and flow paths develop during subduction of serpentinites.

Heterogeneous impact glass beads are abundant in lunar soils and have been extensively used to study the geological processes that shaped the Moon’s surface. In this study, we examine the compositional complexity of three heterogeneous glass beads containing undigested zirconolite and zircon, using EPMA, Nano-SIMS mapping, and SIMS U–Pb isotope analyses. The undigested zircon and zirconolite crystals document three key volcanic events in the lunar history: at ~ 4.31 Ga the formation of alkali-suite rocks from the highlands, and at ~ 3.92 Ga, and ~ 2.04 Ga mare basalts, indicating that the U–Pb system in these zirconium-bearing crystals remains undisturbed during the ultra-high-temperature, short-duration impact melting events. EPMA and Nano-SIMS mapping reveal significant compositional inhomogeneity in the glass matrices, which complicates accurate provenance determination based on in-situ analysis. Bulk composition calculated from quantitative maps, however, provides a more reliable reference for inferring the origins of these beads. The high proportions of common Pb in the heterogeneous glass matrices, originating from diffusion-controlled processes during partial melting of impact involved minerals, introduce substantial uncertainties in U–Pb dating, complicating the interpretation of impact event ages. These findings highlight the challenges of U–Pb dating in heterogeneous glass beads and provide new insights into the preservation of pristine age information in lunar impact materials.

Eclogite and omphacitite xenoliths of the Navajo Volcanic Field (NVF) provide a unique opportunity to study processes in an oceanic slab down to subarc depths. However, ambiguities remained about protolith origin, prograde metamorphic conditions, and metasomatic processes, which we address here with new geothermobarometric and in-situ U-Pb data. The earliest garnet (53.4 ± 4.8 Ma), omphacite, and phengite generations yielded conditions of 2.4–3.3 GPa, 400–540 °C. Zircon cores (87−37 Ma ²⁰⁶Pb/²³⁸U dates) yielded higher Th/U ratios (0.1–0.6) than zircon mantles (65−30 Ma and Th/U < 0.1). Rutile dates cluster at 32.3 ± 1.2 to 28.4 ± 1.9 Ma. The Th/U ratios suggest magmatic growth of the zircon cores, which we consider a strong argument that the NVF eclogites and omphacitites were at least partly derived from Cretaceous Farallon oceanic crust. Rare relic Proterozoic zircon can be explained by inheritance, or by derivation of some xenoliths from the Proterozoic North American lithosphere. Regardless of protolith origin, the rocks were brought to depth by the Farallon slab and resided at lawsonite eclogite facies conditions for a ca. 20–25 Myr interval bracketed by the garnet and rutile dates, during which they experienced two (likely ultra-high pressure) metasomatic events. Firstly, Na-Si-rich fluids, likely derived from metasedimentary rocks, caused growth of Na-rich omphacite. A second metasomatic episode through serpentinite-derived fluids happened just prior to ~ 30 Ma xenolith exhumation as part of the “Great Hydration Event” that affected the Colorado Plateau.

We experimentally investigated plagioclase nucleation and growth in anhydrous arc basaltic andesite at 1 atm and Ni-NiO equilibrium. After equilibration at 1190 °C (15 °C above the liquidus) for 24 h, experiments were cooled at 1, 3, or 9 °C/h and quenched at 1175–1000 °C. New plagioclase grains nucleated near the liquidus, followed by minor amounts of Fe–Ti oxides and pyroxene below 1120 and 1050 °C, respectively. Plagioclase shapes varied from 2D tabular/elongated (1 and 3 °C/h) to hopper and swallowtail textures (9 °C/h), suggesting a transition from interface- to diffusion-controlled growth. Crystal shapes and sizes were correlated, with the smallest and largest having equant/elongated and tabular/bladed 3D shapes, respectively. To identify the most suitable method for inferring storage timescales in natural magmas, we calculated nucleation (J) and growth rates (G) with different methods: G_max from the average size of the 10 biggest crystals, G_mean from the entire crystal population, J_batch and G_batch from the number and proportion of plagioclase estimated by point counting, and J_CSD and G_CSD from the crystal size distribution (CSD). J and G were greatest near the liquidus and decreased during cooling; the decrease was minimal at slow cooling rates, making G nearly constant. G decreased with decreasing cooling rates (from 10⁻⁷ to 10⁻⁹ cm/s at 9 and 1 °C/h, respectively), stabilizing after ~ 20 h of cooling. These variations of G principally resulted from differences in experimental conditions, more than the calculation method considered. Given the uncertainties of CSD theory in closed systems and the size and crystallographic axis-dependence of growth rates, combining Gₘₑₐₙ and Gₘₐₓ appears to be the most effective method for experimentally determining growth rates. However, the batch method (J_Batch) still provides a good estimate of J.

The Lower Critical (LCZ)–Upper Critical Zone (UCZ) boundary of the Rustenburg Layered Suite is an intrusion-wide, major stratigraphic transition from intercumulus plagioclase in the LCZ to cumulus plagioclase in the UCZ. No consensus exists regarding the nature of this boundary, with some regarding the attainment of cumulus status by plagioclase at this level of the intrusion due to continued fractionation of the resident magma, whilst others argue for the addition of compositionally distinct magma(s) at this level of the intrusion. Here we report in-situ Sr-isotopic compositions for plagioclase along with whole-rock major- and trace element geochemical and mineral chemical data across the LCZ–UCZ boundary as intersected by borehole BH6958 on the farm Forest Hill in the eastern Bushveld Complex. Major and trace element data across the LCZ–UCZ boundary (e.g. the Cr content of orthopyroxene) support the notion that no compositionally distinct magma was added at this level of the intrusion. Sr- and Nd-isotopic data, however, point to open-system behaviour. The isotopic excursion cannot be explained through mixing between resident (B1) magma and other proposed parental magmas (e.g. B2 or B3 magmas). Modelling suggests that the observed isotopic excursion may be explained through mixing of resident (B1) magma with small amounts of lower crustal melts. Whether such mixing would have resulted in plagioclase stabilisation remains unclear. The observed isotopic excursion can also be explained through mixing of resident (B1) magma with small amounts of crustal fluids. In this case, the introduction of these fluids appears to have happened gradually, with ⁸⁷Sr/⁸⁶Sr_i in plagioclase being higher in LCZ rims than cores, and higher yet in the lower UCZ. We argue on the basis of thermodynamic modelling that when plagioclase joined the crystallising assemblage, the system contracted at a rate higher than that preceding plagioclase stabilisation, with fluids from the surrounding hydrothermal system entering the magma chamber to counter the volume loss experienced by the cooling system.