Contributions to Mineralogy and Petrology最新文献

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.

The bulk of the Icelandic crust is generated along the extensively studied neovolcanic zones. The subglacially formed Fjallgarðar Volcanic Ridge (FVR) in Northeast Iceland represents extensive near-rift volcanism and covers a large area east of the Northern Rift Zone, yet its magma plumbing system remains poorly understood. This study presents new geochemical data from basaltic pillow lavas and hyaloclastites sampled along the entire FVR, including major element compositions of whole-rock, groundmass glass, and minerals, trace element compositions of whole-rock and glass, and oxygen isotope data for whole-rocks and glasses. Our results suggest that two types of relatively evolved tholeiitic basalts (4.4 to 7.8 wt% MgO) are present along the entire, 200 km-long FVR: one group (Zr/Y = 2.5–4.3) is compositionally similar to basalts erupted in the Northern and Eastern Rift Zone, whereas a second group (Zr/Y = 4.7–5.4) is more enriched and overlaps compositionally with basalts from Kverkfjöll, a volcanic system adjacent to the southern tip of the FVR. These distinct magma types, which are generally similar to fissure basalts erupted in the rift axis, were stored at similar pressures and temperatures (2.2 ± 1.4 kbar, 1108 ± 45 °C), and experienced extensive crystal fractionation and some degree of crustal assimilation. We conclude that the crustal processes did not generate the compositional bimodality but instead, the two groups reflect the chemical variability of their parental mantle-derived melts. We propose that the high-Zr/Y magmas were injected in the FVR via long-distance (> 100 km) lateral dyke propagation from magma reservoir(s) from its southern distal end, and that the low-Zr/Y magmas were supplied from a series of more proximal magma reservoirs distributed along the FVR.

Serpentine minerals have received a lot of attention because of their unique crystal structures, their wide occurrence in orogenic belts and their potential role in contributing seismic anisotropy in subducting slabs. Several studies have investigated crystal preferred orientation (CPO) in high temperature antigorite serpentinites from Japan, the Alps, Spain, Cuba and Tibet, documenting significant crystal alignment. However, only a limited number of lower grade serpentines have been explored to date. Mainly because of submicroscopic microstructural heterogeneities CPO cannot be measured with conventional methods such as optical microscopy and EBSD. In this study 15 serpentinites from different tectonic settings in California, the Central Alps and Northern Spain have been investigated, mainly with high energy synchrotron X-ray diffraction, to quantify bulk crystal alignment. We find that CPO is strong on sheared surfaces of fractured blocks and secondary veins but the bulk of most serpentinite samples, except high-grade recrystallized antigorite serpentinite, show only weak crystal alignment. Correspondingly calculated seismic anisotropy based on CPO is not very significant. This is supported by very heterogeneous microstructures as documented with SEM and TEM analyses.