Contributions to Mineralogy and Petrology最新文献

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.

Magmatic activity in young oceanic sedimentary basins impacts the transfer of life-essential volatiles, such as C, S, H and Cl between the Earth’s interior and the surface. The intrusion of magmas into sediments leads to significant remobilization and/or sequestration of carbon and sulphur in the metamorphic aureoles. Core samples from the Guaymas Basin, collected during the IODP 385 expedition include Site U1546, where a tholeiitic sill was encountered. The metamorphic aureole below the sill is characterized by a pyrrhotite-pyroxene-calcite-siderite assemblage in a quartz-bearing matrix. Combining petrological and geochemical analyses, coupled with thermodynamic modelling, we show that cooling under a quasi-closed system conditions controls the precipitation of carbonates in equilibrium with sulphides. In contrast, the upper aureole remains unaffected by retrograde phase precipitation, suggesting that fluids have been totally evacuated from the system. While degassing toward the surface is not in question, we estimate that up to 38% of the carbon (equivalent to 1.26 Mt) may have been trapped in the metamorphic aureoles of the sill U1546 in the Guaymas Basin. At the basin scale, the volume of these metamorphic reservoirs, that are directly correlated to the magmatic activity, and their ability to trap volatiles, are important considerations for global cycles of the volatile elements. These findings contrast with the common view that metamorphic aureoles are always a source of volatiles, which has implications for estimating the volatile release fluxes during past episodes of increased magma-sediment interaction in analogous basin settings and their consequences on the atmospheric carbon cycle.

Geophysical observations have revealed the existence of low-velocity zones that potentially coincide with the ongoing downward flow of mantle material through the globally recognized sharp discontinuity at a depth of 670 km. The leading interpretation for these heterogeneities is related to dehydration induced incipient partial melting. Here, we have employed an iterative method to achieve equilibrium between melt pools and undifferentiated pyrolite at geotherm conditions, allowing for a precise determination of partitioning of the major components (SiO₂-MgO-Al₂O₃-FeO-CaO) between the melt and minerals. The melt holds 5.5 (0.4) wt% H₂O and is relatively mafic compared to its source material, with considerable enrichment in CaO and FeO. For the (shallow) lower mantle in hydrous conditions, the solidus phase is davemaoite, followed by ferropericlase, and bridgmanite becomes the liquidus phase. Density models show that such magma is positively buoyant in the shallow lower mantle. Thus, it is expected to travel upward and possibly freeze in the transition zone. This process implies that, over time, continuous dehydration of the downgoing slab and subsequent hydrous melt extraction from the lower mantle wedge limit the deep water cycle to the upper 670 km depth of the mantle.