Contributions to Mineralogy and Petrology最新文献

We present a reconstruction of the physicochemical conditions of melts in the Pleistocene storage and plumbing system of the Doña Juana Volcanic Complex (SW Colombia): a poorly known, potentially active polygenetic volcano of dacitic composition comprising four major edifices and showing periods of long quiescence. Compositional data for plagioclase, amphibole, pyroxene, and Fe-Ti oxides were combined with new and existing whole-rock data from representative eruptive products, allowing for the implementation of equilibrium tests and geothermobarometry calculations within an established stratigraphic, petrographic, and geochronological framework. Textural and geochemical variabilities of all mineral phases suggest the existence of a trans-crustal magmatic system feeding the Pleistocene eruptions of Doña Juana, and cyclic rejuvenation of a crystal mush following each volcano edifice collapse. The assemblage of different crystal cargos before magma recharge and final eruption is attested by (i) the coexistence of equilibrium and disequilibrium textures and variable compositions in crystals of all studied species, (ii) felsic cores in antecrysts, (iii) mafic overgrowth rims, and (iv) significantly less differentiated microcrysts relative to the composition of meso- and macrocrysts. By integrating multiple mineral-only and mineral-liquid geothermobarometers, after careful textural analyses, we estimate the intensive parameters of the mush–melt interaction zone of the plumbing system in the middle crust, providing a preliminary view of the architecture of a trans-crustal magmatic system in a complex tectonic setting at a previously understudied area of the north-Andean volcanic zone.

The trace-element and isotope geochemistry of rutile are robust tools to determine metamorphic temperatures, age, and host-/source lithologies. The use of rutile as single grain indicator for pressure, temperature, time and composition (P–T–t–X) of the host rock, which is vital in the use of detrital rutile to trace plate-tectonic regimes throughout Earth’s history, requires the identification of a pressure dependent trace element in rutile. We investigate the pressure dependence of hydrogen in rutile using polarized in-situ Fourier Transform Infrared (FTIR) spectroscopy. H₂O contents in rutile vary between < 10–2500 μg/g H₂O with higher contents in samples with higher peak metamorphic pressures, making H₂O-in-rutile a viable pressure indicator. The highest H₂O contents at ~ 450–2000 μg/g are observed in mafic low temperature eclogite-facies rutile related to modern-style cold subduction conditions. Hydrogen zoning in FTIR maps indicates that H⁺ is retained at temperatures below 600–700 °C. Ratios of H₂O/Zr, using H₂O as pressure indicator and Zr as temperature proxy, are a proxy for thermal gradients of metamorphic rutile (i.e. P/T). Low temperature eclogite samples are also characterized by high Fe contents and therefore Fe/Zr-ratios might be used as a first order approximation for H₂O/Zr-ratios to identify mafic low temperature eclogite facies rutile. Based on common discrimination diagrams, Nb, W, and Sn can be used to distinguish different host/source rock lithologies of rutile. Combining both H₂O/Zr-ratios and Nb, W, and Sn contents can thus identify modern-style cold subduction signatures in rutile. The developed systematics can consequently be used to trace cold-subduction features in the (pre-Proterozoic) detrital record.

Lawsonite is important as a carrier of H₂O in subduction zones and as a petrological tracer. The trace-element content of lawsonite in mafic rocks has been used as a record of fluid–rock interactions but has received less attention in metamorphosed oceanic sediments. This study documents the major, and trace-element composition, together with ⁸⁷Sr/⁸⁶Sr isotopic ratios, of the different lawsonite types identified in the upper units of the Schistes Lustrés complex of the Western Alps, a paleo-accretionary prism of the Liguro-Piemont slow-spreading ocean subducted up to 40 km. The lawsonite-rich upper units of the Schistes Lustrés complex are principally composed of metamorphosed pelitic sediments and carbonates. Lawsonite content reaches 40 vol.% in the rock matrix and in veins. All lawsonite types originate from prograde metamorphic reactions which occurred up to peak metamorphism. Lawsonite compositions have been measured in situ with electron microscopy, microprobe, and laser-ablation mass spectrometry. Lawsonite separates have been measured for Sr content and ⁸⁷Sr/⁸⁶Sr isotopic ratios using TIMS. Bulk rock compositions have been measured with ICP-OES and ICP-MS. Analysis contamination by minute retrograde white mica inclusions in lawsonite crystals precluded using univalent cations for petrological interpretations. For other trace elements, the variability of lawsonite appears extremely high, with crystal compositions varying between samples and between crystals in individual samples. However, clear patterns emerge between lawsonite types, reflecting equilibrium and out-of-equilibrium processes unrelated to pressure–temperature conditions. At crystal scale, textural hourglass zoning predominates in lawsonite from the schist, while growth zoning is found in both schists and veins. The combination of both mechanisms results in spectacular zoning in Ti and in rare-earth element contents spanning four orders of magnitude. Over time, the La/Yb ratio decreases strongly (from ~100 to ~1), as La appears much more sensitive to surface effects leading to textural hourglass zoning. Interface-coupled dissolution–precipitation also contributes to decoupling of less mobile elements in the schist. This is best observed for Ti, but rare-earth element and chromium contents are affected too. These processes are considered to occur near closed-system conditions. Late lawsonite overgrowths with higher Sr contents are interpreted as reflecting system opening in the veins, yet sometimes observed in the schists. Strontium isotopic ratios measured on separated lawsonite crystals and on bulk rocks also show very large spreads. This indicates that the system did not equilibrate during blueschist-facies metamorphism, due to bulk rock heterogeneities during deposition and limited mineral reactivity at the local scale. Strontium isotopic ratios do not correlate with Sr content which rules out limestone devolatilizat

Delamination of dense mafic/ultramafic materials in arc roots has long been considered as the fundamental step in the paradigm of making an andesitic continental crust. However, the complexity in identifying ancient arc roots inherently with repeated modifications, poses a challenge in accurately determining the preservation time of the dense crustal materials and thus the delamination-driven model. Here, we conducted comprehensive petrographic, whole-rock, and mineral geochemical studies on 10 variably retrograded eclogite xenoliths from the ~ 35 Ma crustal-derived felsic porphyry in the Liuhe area, western Yangtze craton. Eclogite facies metamorphism is indicated by the fresh relic consisting of coarse-grained garnet, omphacite and rutile; the retrograde metamorphism is manifested by an additional assemblage of fine-grained diopside, amphibole, plagioclase, and biotite. Whole-rock element contents of the xenoliths generally display correlations with immobile Nb concentrations, suggestive of a dominant control from magmatic processes with negligible effects from the post-magmatic alteration. The protoliths of studied xenoliths are most likely accumulated garnet pyroxenites, where the negative correlation between heavy rare earth elements and Mg# (Mg/(Mg + Fe), atomic ratio), and the absence of positive Eu and Sr anomalies suggest the accumulation of garnet under high-pressure conditions. The parental magmas are inferred to be evolved and hydrous with arc-type trace-element patterns. Combined with studies on regional xenoliths, outcrops and tectonic history, the parental magmas likely record the melting of asthenospheric mantle wedge fluxed by recycled subducted slab in the Neoproterozoic (~ 800 Ma). The prolonged preservation (from ~ 800 Ma to at least 35 Ma) of the accumulated garnet pyroxenites with high densities in the deep continental crust can be ascribed to the support from the underlying refractory lithospheric mantle strengthened by plume head accretions. Therefore, we propose that the density-driven delamination of the arc root materials is more sluggish than previously expected and the longevity of dense crustal materials highlights the caution in understanding the role of arc root delamination in making an andesitic continental crust.

Groundmass textures of volcanic rocks provide valuable insights into the processes of magma ascent, crystallization, and eruption. The diktytaxitic texture, characterized by a lath-shaped arrangement of feldspar microlites forming glass-free and angular pores, is commonly observed in silicic dome-forming rocks and Vulcanian ashfall deposits. This texture has the potential to control the explosivity of volcanic eruptions because its micropore network allows pervasive degassing during the final stages of magma ascent and eruption. However, the exact conditions and kinetics of the formation of diktytaxitic textures, which are often accompanied by vapor-phase cristobalite, remain largely unknown. Here, we show that the diktytaxitic texture and vapor-phase minerals, cristobalite and alkali feldspar, can be produced from bulk-andesitic magma with rhyolitic glass under water-saturated, near-solidus conditions (± ~10 MPa and ± ~20 °C within the solidus; 10–20 MPa and 850 °C for our starting pumices). Such crystallization proceeds through the partial evaporation of the supercooled melt, followed by the deposition of cristobalite and alkali feldspar as a result of the system selecting the fastest crystallization pathway with the lowest activation energy. The previously proposed mechanisms of halogen-induced corrosion or melt segregation by gas-driven filter pressing are not particularly necessary, although they may occur concurrently. Diktytaxitic groundmass formation is completed within 4–8 days, irrespective of the presence or composition of the halogen. These findings constrain the outgassing of lava domes and shallow magma intrusions and provide new insights into the final stages of hydrous magma crystallization on Earth.

Veins composed of glaucophane + phengite + quartz cross-cut the high pressure-low temperature (HP-LT) Cycladic Blueschist Unit (CBU) of southern Evia, Greece. The veins exhibit a rheology-dependent distribution within layered metamorphic rock comprising cm-scale intercalations of albite-clinopyroxene metabasalt and schistose quartzite. Strain was accommodated by ductile processes in the quartzite, whereas brittle deformation produced four sets of crack-seal syntaxial veins in the coarser-grained metabasalt. All vein sets are subvertical to steeply-dipping and are oriented at high angles to one another. The geometry of the planar vein walls suggests the veins are mode-I (opening mode) fractures, whose sub-vertical orientations indicate formation during extension. Oxygen isotope thermometry using phengite-quartz pairs provides crystallization temperatures of 315–335 °C. Combined ⁴⁰Ar/³⁹Ar and in-situ ⁸⁷Rb/⁸⁷Sr geochronology of vein-hosted phengite and glaucophane indicate crystallization and vein sealing at c. 22–23 Ma when the CBU is predicted to be undergoing greenschist facies metamorphism coincident with regional extension. The structural and stable isotope data are likewise consistent with a syn-exhumation extensional setting, and easily reconciled with existing petrological data indicating the CBU sustained prolonged residence near the greenschist-blueschist facies boundary. We propose a model whereby phengite and glaucophane were stabilized at greenschist facies conditions by the elevated α_SiO2 and fO₂ in the fluid parental to the veins. Our data provide strong new evidence for the sensitivity of nominally blueschist facies minerals to bulk system chemistry, supported by thermodynamic modelling evidence from other orogens that such HP-LT minerals may exhibit stability that spans multiple stages of orogenesis.

Modelling plumbing systems dynamics of active volcanoes through textural and chemical studies of mineral phases is crucial to unravel their eruptive behaviour, but it is rarely applied in ancient volcanic and volcano-plutonic systems. Here, we present an investigation of the architecture, magma dynamics and pre-eruptive timing of Middle Triassic plumbing systems in the Dolomites area (Southern Alps) through a detailed investigation of textures and compositional zoning of clinopyroxenes in lavas and dykes from Predazzo, Mt. Monzoni, Cima Pape and Sciliar volcano-plutonic complexes. The clinopyroxene composition varies between low-Mg# (67–78), low-Cr augite and high-Mg# (77–91), Cr-rich diopside. Diopside is less frequent and appears as variably thick single or multiple bands between augitic cores and rims or as resorbed homogeneous, or patchy-zoned and mottled cores. Rims are homogeneous or oscillatory zoned, with augitic composition. The mid- to low-crustal plumbing systems of all volcanic centres were characterised by the presence of a mildly evolved trachyandesitic magma (Mg# 45; T = 1044–1118 °C), where augitic clinopyroxene formed. Periodic mafic injections of more primitive and hotter trachybasaltic magma (Mg# 56; T = 1056–1170 °C) caused frequent crystallisation of diopsidic bands around augitic cores. The presence of resorbed or patchy-zoned mottled diopsidic cores in clinopyroxene phenocrysts, as well as of rare clinopyroxenitic xenoliths with analogous diopsidic composition, indicates the recycling and remobilisation of antecrysts from the deeper part of the plumbing system, which was probably located at a depth of 10–17 km. Diffusion chronometry models based on Fe–Mg interdiffusion in clinopyroxene revealed that the time elapsed from the mafic injection into the shallow portion of the plumbing systems to the eruption ranges from decades to < 1 year. Our findings enabled us to resolve the different plumbing system dynamics acting at the local scale beneath each volcanic centre. Our data indicate a striking similarity with magma dynamics and timing of pre-eruptive processes at active volcanoes: therefore, we posit that this work sets a starting point to deepen our knowledge of the volcano-plutonic links and, in turn improves our ability to interpret the main processes acting in active plumbing systems.

Processes taking place within the magma plumbing system can exert an important control on the composition of mid-ocean ridge basalts (MORB). Plagioclase ultraphyric basalts (PUBs) found at magma-poor mid-ocean ridges exhibit diverse disequilibrium characteristics, which can provide vital insights for distinguishing the complex effects of melt transport from those of source heterogeneity on the compositions of MORBs. Here, we present new insights into magmatic processes using integrated petrologic and geochemical studies of the PUBs from two zones (~ 50° and ~ 64°E longitude) along the ultraslow-spreading southwest Indian ridge (SWIR). The studied PUBs have complex mineral morphologies, including skeletal and acicular crystals, glomerocrysts with open and closed structure, reverse and normally zoned crystals and external and internal resorption even in single samples. Both low- and high-Fo olivine and An plagioclase crystals are in disequilibrium with their matrix glasses. Some plagioclase phenocrysts have repeated oscillatory zoning (An_77–86) going from their core to rim and an abrupt decrease in An content toward the rim. Disequilibrium Sr isotopic compositions are present at several scales: between cores and rims of plagioclase crystals, between different plagioclase crystals and between plagioclase and their host lavas. Inferred pressures of magma storage range from 0.3 to 11.3 kbar. The textural and compositional diversity of crystals together with the variability in melt compositions reflect the combined influences of source heterogeneity and magmatic processes (e.g. crystallization, assimilation and magma mixing processes) taking place within crystal mushes. Our data combined with previous studies suggest that the magmatic processes within the SWIR magma plumbing system involve formation, disaggregation and juxtaposition of crystal-rich mush zones.

The forsterite zone of the Ubehebe Peak contact aureole, Death Valley, USA consists of an outer zone of tabular/jack-straw olivine and an inner zone of subequant polyhedral olivine. Subequant polyhedral forsterite crystals close to the intrusion are small and tabular forsterite crystals farther away are larger. To investigate the formation of the two morphologies, forsterite growth experiments were conducted in cold seal pressure vessels in the CaO-MgO-SiO₂-CO₂-H₂O system. Forsterite precipitation follows a disequilibrium reaction pathway made of three reactions: [1] tabular forsterite growth from quartz and dolomite, [2] forsterite growth from tremolite dissolution, and [3] subequant polyhedral forsterite growth from tabular forsterite dissolution. Initially, quartz reacts with dolomite to simultaneously form twinned tabular forsterite and tremolite. As quartz reacts away, forsterite precipitation continues at a slower rate through tremolite dissolution. A second generation of forsterite then precipitates on top of some tabular forsterite but has different habit and tracht. Once all the tremolite reacts away, subequant polyhedral forsterite precipitation continues at an even slower rate through dissolution of tabular forsterite. The tabular morphology of jack-straw olivine is a consequence of twin-mediated unidirectional growth; the abundance of twins being due to rapid nucleation and growth at initially high reaction affinities. Twin junctions are preferential nucleation centers for steps, so faceted growth is enhanced on {100}. This phenomenon is the twin plane re-entrant effect. Subequant polyhedral forsterite in the Ubehebe Peak inner contact aureole recrystallized and ripened from tabular forsterite. In the outer contact aureole, conditions were not conducive to recrystallization and ripening so well-developed tabular forsterite persists.

Garnet is a prominent mineral in skarn deposits and its rare earth elements (REE) geochemistry is pivotal for understanding skarn mineralization and fluid evolution. In contrast to magmatic and metamorphic garnets, skarn garnets are mainly grossular-andradite in composition. They exhibit variable REE patterns, spanning from notable heavy (H)-REE enrichment to significant light (L)-REE enrichment, accompanied by negative to positive europium (Eu) anomalies. However, the key factors governing REE fractionation in skarn garnets remain uncertain. This study applies the lattice-strain theory (LST) to investigate the influence of crystal chemistry and structure on REE fractionation in garnets from the Lazhushan Fe skarn deposit in eastern China. Our results demonstrate that the garnet-liquid partition coefficient ratios of D_La/D_Yb significantly increase (up to 5–7 orders of magnitude) with rising andradite content in garnet. This variation underscores the pivotal role of garnet structure in controlling LREE/HREE fractionation. The results further show that partition coefficient ratios of D_La/D_Sm are strongly dependent on andradite content in garnets, whereas the D_Gd/D_Yb ratios only show a weak correlation to the garnet composition. This contrast suggests that fractionation of LREE in garnet is more sensitive to variations of andradite content than HREE. Data compilation of major elements and REE for garnet from the Lazhushan Fe skarn deposit and other skarn deposits worldwide shows that the garnet REE patterns vary from positive through concave to negative shapes with the garnet ranging from grossularitic to andraditic compositions. Such variations in garnet REE patterns are consistent with the results of geochemical modeling based on the LST. This study demonstrates that, through LST equations, the shape of fluid REE patterns can be predicted from garnet REE patterns, and vice versa. Furthermore, the Eu anomaly (Eu/Eu*_Grt) in skarn garnet depends mainly on fluid Eu anomaly (Eu/Eu*_fluid) and garnet-fluid partition coefficient ratio of D(Eu²⁺)/D(Eu³⁺) with the latter being influenced by garnet composition. These findings highlight the critical role of crystal chemistry and structure in garnet REE fractionation, enhancing our ability to utilize garnet REE in tracing the origin and evolution of skarn-forming fluids.