Contributions to Mineralogy and Petrology最新文献

Kimberlites are volatile-rich, silica-undersaturated, mantle-derived magmas with an important role in studying mantle geochemistry, yet their compositional evolution during crystallization remains poorly constrained. This study defines the crystallization sequence and liquid line of descent of a reconstructed kimberlitic melt, using high-pressure experiments at 1–3 GPa and 1100–1400 °C. Early crystallization is dominated by olivine ± chromite, consistent with petrographic observations of natural samples, followed by clinopyroxene, ilmenite, perovskite, apatite, and phlogopite. The absence of clinopyroxene in natural kimberlitessuggests that kimberlitic melts remain above 1150 °C at 1 GPa, conditions at which clinopyroxene is not observed in our experiments. Substantial cooling of kimberlitic melt and related abundant crystallization likely occur in the crust, possibly linked to extensive degassing at shallow pressures. The experimental melts evolve continuously through decreasing SiO₂ and MgO, and enrichment in CaO, alkalis (Na, K), and volatiles (CO₂, H₂O), ultimately transitioning at ≤ 1150 °C to carbonatitic melts with 7–10 wt% SiO₂, 6.5–7.5 wt% Na₂O + K₂O, and up to 35 wt% volatiles. Olivine (± clinopyroxene) fractionation drives Si depletion at almost constant MgO + FeO + CaO and moderate alkali-enrichment, such that the carbonate-silicate miscibility gap is bypassed. This evolution is in sharp contrast to mafic alkaline silicate melts where olivine + clinopyroxene crystallization causes Si enrichment hence promoting melt evolution towards the carbonate-silicate miscibility gap. Overall, the experimental results demonstrate a petrogenetic continuum between kimberlitic and carbonatitic melts and provide constraints on the crystallization conditions of kimberlitic melts.

Many experimental studies have addressed the crystallisation of zircon, not least because zircon is of paramount importance as a geochronometer. Today, it is well established at what temperature, melt composition, and Zr concentration a silicate melt reaches zircon saturation. It remains unclear though what course a Zr bearing melt takes before zircon appears as a macroscopic phase. Does zircon nucleate directly from the melt, or does it use during nucleation and growth Zr-rich nanoparticles as fundamental building blocks? The question is relevant because Zr⁴⁺ is a high field strength (HFS) cation that should tend to polymerise in silicate melts to (ZrO₂)_n clusters. To fill that gap of knowledge, we performed experiments with a Zr-enriched phonolitic melt at 1200 (outside zircon stability) and 900° C (inside zircon stability). We investigated the glasses with Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) for Zr-rich nanoparticles, at a spatial resolution not achieved by previous experiments involving zircon. A wide range of nanoparticles is identified, including baddeleyite, zircon, zirconium titanate ZrTiO₄, and rutile. The diverse range of nanoparticles is probably owed to the fact that local-scale gradients in silica (aSiO₂) among silicate melt pools prevailed despite the high run temperatures. The smallest and presumably earliest phases are (ZrO₂)_n nanoparticles trapped by liquidus corundum. They reach diameters around 2 nm or ~ 200 unit cells if they are crystalline. The results support the assumption that Zr⁴⁺ dissolves in silicate melts as ZrO₂ monomers, then quickly polymerises to (ZrO₂)_n clusters. The same may be valid for other HFS cations. Possible applications to HFS element depletions in arc basalts are discussed. At 900° C inside zircon stability, many examples are noted where zircon grows by peritectic reaction of baddeleyite nanoparticles with SiO₂ of the melt although zircon nanoparticles also exist. Larger baddeleyite grains around 50 nm seem to grow by aggregation of smaller (ZrO₂)_n nanoparticles. That zircon also grows by particle attachment cannot be confirmed. Zircon can also crystallise directly from the melt when in melt pools the aSiO₂ and Zr contents were high enough to stabilise zircon.

The solubility of molecular hydrogen (H₂) was measured in haplogranite, andesite, and basalt (MORB) melt. Experiments were carried out with rapid-quench TZM vessels and a piston cylinder apparatus at 0.2 GPa − 4 GPa, 1100 ˚C − 1400 ˚C, and iron-wüstite (Fe-FeO) buffer conditions. H₂ contents in quenched glasses were measured by infrared (FTIR) spectroscopy. For this purpose, the infrared extinction coefficient of the 4120 cm^− 1 band of H₂ in haplogranitic glass was re-calibrated by two independent methods. This yielded a linear molar extinction coefficient of (2.12 ± 0.05) liter mol^− 1cm^− 1, which is about one order of magnitude larger than a coefficient used in previous studies. The new extinction coefficient was used to quantify H₂ solubility in all glass samples of this study. The solubility of molecular hydrogen increases with increasing pressure, being higher in haplogranite than in andesitic or basaltic melt, as expected from ionic porosity considerations. The data at Fe-FeO buffer conditions can all be reproduced by a simple Henry style solubility law c_H2 = a_Henry P, with a_Henry = (206 ± 10) ppm/GPa for basalt, (362 ± 35) ppm/GPa for andesite, and (500 ± 62) ppm/GPa for haplogranite, where ppm is ppm H₂ by weight (µg/g). However, due to the use of an erroneous infrared extinction coefficient, previous studies may have overestimated H₂ solubility in silicate melts by about one order of magnitude. According to the new data presented here, H₂ dissolution in a magma ocean is not a very efficient mechanism for generating elevated hydrogen contents in planetary interiors. Equilibrium thermodynamic modelling shows that in an Earth with chondritic bulk composition, even at an oxygen fugacity six log units below the Fe-FeO buffer, the molar ratio of H₂/H₂O in the magma ocean is still below unity. At a more plausible oxygen fugacity two log units below Fe-FeO, the ratio is 0.06. However, the strong partitioning of hydrogen into the atmosphere under the very reducing conditions of early accretion may have enhanced hydrogen loss due to hydrodynamic escape and impact erosion. Possibly, this was a decisive mechanism for depleting the Earth in volatiles as compared to its chondritic building blocks.

The garnets in garnet pyroxenites from centimetre- to several-hundred-metre-sized mafic lenses embedded in felsic high-pressure granulites of the Gföhl Unit (Moldanubian Zone, Bohemian Massif) are relics of an early high-pressure–high-temperature metamorphic stage related to Variscan subduction and continental collision. Subsequent isothermal decompression to granulite-facies conditions led to the partial replacement of garnet by plagioclase-bearing assemblages. Associated with the partial replacement, a pronounced secondary compositional zoning developed in the relic garnets, which indicates relatively fast diffusion of Fe and Mg and comparatively slow diffusion of Ca. Based on inverse diffusion modelling, cooling rates in the range of 7–(1501 ^circ )C/Myr were estimated for the garnet pyroxenites, indicating rapid cooling and short-lived granulite-facies overprint after decompression. The petrological evidence is compatible with the extrusion of partially molten, buoyant felsic lithologies, which incorporated slivers of mafic lithologies en route. Through the heat they transported advectively, these lithologies produced perturbations of the thermal structure at mid-crustal levels, the decay times of which varied depending on the volumes of the hot material exhumed in different regions.

Crystallisation-differentiation drives arc magma evolution, yet discrepancies remain among field, geochemical and experimental evidence. Whereas other controls are better studied, the effect of fO₂, beyond oxide stability, remains less constrained. We investigate fO₂-pressure effects on olivine-clinopyroxene-spinel phase relations with implications for arc magmas. We conducted phase equilibria experiments at 200 MPa between 1010 and 1100 °C. We used basaltic compositions with different xMg* [MgO/(MgO + FeO^tot)] (0.5 to 0.7) at multiple fO₂ conditions (NNO-0.5 to NNO + 2.3), deconvolving the effects of Fe³⁺/Fe²⁺ and xMg^eff [MgO/(MgO + FeO)] on phase equilibria. Additionally, we ran 800 MPa experiments between NNO-0.4 and NNO + 2.5 to explore the combined effects of fO₂ and pressure. At 200 MPa, increasing fO₂ (1) stabilises Fe³⁺-rich spinel, leading to SiO₂-richer melts and, therefore, less pronounced ASI (alumina saturation index, ASI = Al₂O₃/(CaO + Na₂O + K₂O) molar) increase relative to SiO₂, and (2) expands olivine stability relative to clinopyroxene in ol-cpx cotectic melts, resulting in lower ASI melts (for a given SiO₂ content) that better match arc rocks. This is only observed under spinel-absent conditions. The 800 MPa experiments reveal decreasing spinel stability with increasing pressure, while fO₂ has a negligible effect on the ol-cpx cotectic. This suggests that the previously documented pressure effect on the olivine-clinopyroxene equilibrium is stronger than the effect of fO₂. Our results demonstrate that fO₂ increasingly influences the olivine-clinopyroxene cotectic equilibrium as pressure decreases. This supports models where decompression-driven polybaric crystallisation under oxidising conditions shapes arc magmatic compositions. The reported pressure-fO₂ interplay helps reconcile natural and experimental arc records.

Formation of new minerals in rocks of specific composition during prograde metamorphism depends mostly on pressure and/or temperature changes. However, some of these minerals are very sensitive to reverse reactions when the rocks are subject to decompression and/or cooling. This is well known from high- or ultrahigh-pressure rocks which poorly preserve or even lack a number of minerals, like lawsonite or phengite, whose former presence is expected based on the results of experimental data or thermodynamic modelling applied to a given rock composition. Another such mineral is K-cymrite that is stable at UHP conditions but has not been observed in UHP rocks returned to and exposed at the surface. It likely decomposes to K-feldspar or K-mica during temperature increase or pressure decrease. This study examines the textural and compositional relationships of pseudomorphs in blueschist and eclogite facies felsic and mafic lithologies from the Meliata unit (Western Carpathians) and the Bohemian Massif, concluding that they are best interpreted as alteration products of former K-cymrite. The calculated P-T conditions for the host rocks plot at or very near the experimentally constrained stability field of K-cymrite. The occurrence of these pseudomorphs across a range of lithologies suggests that K-cymrite may have formed abundantly and in a diverse range of bulk rock compositions during subduction under a low-temperature geothermal gradient of ~ 7 °C/km. However, due to its hydrous nature and narrow P-T stability field, it is most commonly transformed into other phases during exhumation and is rarely preserved as shape relics (pseudomorphs) because of ongoing deformation.

The prograde pressure (P)-temperature (T) path of eclogites reflects important information on the geodynamic evolution and the thermal structure of subduction zones. The robustness of garnet allows its compositions to be commonly utilized in conjunction with thermodynamic equilibrium modelling to trace the prograde P-T paths of eclogites. Nevertheless, some recent studies have demonstrated that garnet may undergo disequilibrium nucleation and growth, which can lead to an inaccurate reconstruction of the prograde evolution by using phase equilibrium modelling. This suggests that the prograde P-T conditions determined by phase equilibrium modelling should be verified using other independent thermobarometric methods. In this study, the quartz-in-garnet (QuiG) barometry was combined with the Ti-in-calcic amphibole (TiCA) thermometry to re-evaluate the prograde P-T evolution of eclogites from the Tongbai orogen in central China, which has previously been constrained by phase equilibrium modelling. The results show that the Tongbai eclogites experienced an early blueschist facies prograde stage under 380–520 °C and 1.5 GPa and a late epidote eclogite facies prograde stage under 520–600 °C and 1.8 GPa. These pressure conditions are significantly lower than those constrained by phase equilibrium modelling (2.3–2.6 GPa), but are within the stability fields of mineral inclusions in garnet porphyroblasts. This suggests that the results of this study provide a reliable constraint on the prograde evolution experienced by the Tongbai eclogites. T-composition modelling indicates that the discrepancy of CaO between the effective equilibrium compositions for garnet growth and the whole-rock compositions of eclogite can lead to an overestimation of prograde pressures when using phase equilibrium modelling based on whole-rock compositions of eclogites. The combination of the prograde P-T conditions determined in this study with the well-established peak P-T conditions from previous research (~ 590 °C and 2.7 GPa) suggests that the Tongbai eclogites underwent a prograde evolution characterized by an initial heating followed by a later compression, with continuously decreasing geothermal gradient from ~ 8–11 °C/km at depth of ~ 50–60 km to ~ 6–7 °C/km at depth of ~ 80–90 km. This is comparable with the reported thermal structure of other continental subduction zones, indicating that the continental subduction zones may have a common thermal structure characterized by strongly concave upward geothermal gradients.

Petrology, geochemistry and geochronology of a metapelite (sillimanite-garnet-biotite-plagioclase-quartz) from the vicinity of the Archean Mercara Shear Zone in Coorg, S. India show that metamorphism at temperatures > 850 °C occurred between 2700–3300 Ma (Phase equilibria, thermobarometry, U-Pb dating of zircons and Lu-Hf dating of garnets). Subsequently, the rocks experienced thermal events at lower temperatures at 2400-2600 Ma as well as at 600-640 Ma (U-Pb dates from rutile). There are indications of multiple episodes of metasomatic/ (high temperature) hydrothermal activity during the Archean events. Residence of the rocks at lower temperatures between the high temperature events is indicated by the kinetics of dissolution of zircon in melt. Taken together, this history shows that (a) P-T-t evolution in this Archean collisional setting happened along an overall clockwise path but not in a single continuous loop - episodes at high temperatures were interspersed with residence at cooler temperatures in between, (b) subtle effects of metamorphism that occurred at temperatures below the peak temperature could help to resolve some controversies related to tectonothermal reconstructions in the region (e.g. whether signatures of both - amalgamation of Dharwar and Coorg cratons and activity along an equivalent of the Betsimisaraka suture zone in east-central Madagascar may be present in the region), and (c) the duration of high-temperature events (several 100 million years at ~ 800 °C) are consistent with an early Earth peel-back style of plate tectonics, rather than modern day plate tectonics, operating in the region at the time.

The 156 ka, crystal-rich (~ 40 vol%) Haramul Mic dacite represents eruption of a crystal mush. It marks the first eruptive product of the Ciomadul Volcanic Complex, East-Central Europe, following at least 100 kyr of dormancy. The mineral phases (plagioclase, amphibole, biotite, apatite, titanite and zircon) and the interstitial glass have relatively restricted major and trace element composition. Thermobarometric and hygrometric calculations indicate a low-temperature (~ 720 °C), low-pressure (~ 300 MPa), water-saturated (dissolved H₂O ~ 6.5 wt%) and oxidizing silicic magmatic system. A comprehensive set of amphibole-melt trace element partition coefficient data is provided, applicable to low-temperature, near-eutectic silicic volcanic and plutonic systems. The maximum partition coefficient (D₀ = 8.7 ± 0.1) and the optimal ionic radius (r₀ = 1.0494 ± 0.0008 Å) for trivalent elements, as determined from Onuma plots, are consistent with an evolved silicic magmatic system and a low-magnesian amphibole composition. Additionally, mineral-melt trace element partition coefficients are calculated for coexistent titanite, zircon, plagioclase and biotite. We note that in highly evolved silicic volcanic systems, partition coefficients remain consistently similar, even with slight variations in magma composition and conditions near the thermal minimum. In contrast to the subsequent eruptions, there is no evidence for mafic magma recharge and mixing as eruption initiation for the Haramul Mic. Instead, effective thermomechanical reactivation of a portion of the long-standing crystal mush, followed by rapid magma ascent is envisaged. Ciomadul provides an example where multiple processes can contribute to eruption initiation, as reflected in the textural and compositional characteristics of the crystal cargo, despite the relatively uniform composition of the erupted magma.

The isotopic ratio of boron in metamorphic and metasomatic olivine can shed light on its origin and fluid sources, but this data cannot be interpreted without understanding the isotopic fractionation of boron between different phases as a function of physical conditions. In this work we use ab-initio Density Functional Theory to calculate relative isotopic fractionation factors for ¹¹B and ¹⁰B in a variety of phases (β_olivine, β_antigorite and β_fluid) at a variety of pressures and temperatures relevant to dehydration of serpentinites, and derive values for Δ¹¹B_{olivine-fluid} and Δ¹¹B_{olivine-antigorite}. We consider new variables (pressure and boron concentration [B]) and new defect sites in olivine and antigorite. We show that fractionation in olivine and serpentine is complex and cannot be considered solely with trigonal/tetrahedral sites and that the effects of pressure and [B] cannot be discarded. For olivine produced by subduction-related antigorite serpentine dehydration we predict that it largely preserves the original B isotopic composition of the solid and that Δ¹¹B_Ol-Srp is generally close to 0. Δ¹¹B_Ol-fluid is large (> −3 to −17‰) and so exposure to metasomatic fluids can overwrite the B isotopic signature of olivine. Variations in these values are controlled primarily by two interrelated key parameters, fluid pH and depth. Boron isotopic systematics in metamorphic olivine are thus predicted to largely be records of internal and external fluid exposures, and the depth of this exposure. Serpentinite material will largely preserve its B isotope signature during subduction dehydration and potentially introduce anomalous material into the deeper mantle, whilst initial isotopic heterogeneities may be reduced by fluid circulation accompanying serpentinite dehydration.