Journal of Petrology最新文献

{"title":"Apatite chemistry as a petrogenetic indicator for mafic layered intrusions","authors":"Marie A Kieffer, Sarah A S Dare, Olivier Namur, Eduardo T Mansur","doi":"10.1093/petrology/egae022","DOIUrl":"https://doi.org/10.1093/petrology/egae022","url":null,"abstract":"Mafic layered intrusions constitute a natural laboratory to investigate petrogenetic processes using trace element variations in apatite chemistry. Although these intrusions are related to large igneous provinces, there is a wide range of parameters that can affect the chemistry of the primary melt (i.e., composition of the source, pressure, temperature, oxygen fugacity), followed by possible crustal contamination. In this study, we use a comprehensive dataset of analyses of cumulus and intercumulus apatite from a variety of mafic layered intrusions to demonstrate the use of apatite as a powerful petrogenetic indicator. The dataset (determined in this study and compiled from the literature) comprises electron microprobe and LA-ICP-MS analyses, as well as in-situ LA-MC-ICP-MS analyses of Sr isotopes in apatite from well documented layered intrusions (Sept-Iles, Skaergaard, Bushveld, Panzhihua) and the Sudbury Igneous Complex. For the first time, we show that high values of (La/Nd)N, Th, U, Pb, and As in apatite correlate with high (87Sr/86Sr)initial and are related to contamination with continental crust. An elevated (Gd/Yb)N ratio might indicate melting of a mantle source deep enough to retain Yb in garnet. We also confirm that increasingly negative Eu anomaly and decreasing Sr/Y ratio in apatite are indicators of fractional crystallisation of plagioclase, and that high Sr/Y is indicative of early saturation of apatite and/or delayed crystallisation of plagioclase. The reversal to more primitive compositions caused by magma mixing is expressed by higher Sr, V, Mg and Sr/Y ratio, and lower REE+Y, As and Na concentrations in apatite following magma replenishment. Lastly, we show that apatite signature can efficiently distinguish a mafic from a felsic intrusion using its REE and Sr content coupled to its Eu anomaly. It is also possible to further identify the more primitive from the more evolved parts of a mafic layered intrusion, using the Lu, Th, V and volatile (F/Cl) content to distinguish intercumulus from cumulus apatite, respectively. Finally, identifying a mafic magmatic system using detrital apatite in till will prove useful for provenance and mineral exploration studies.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-02-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140007886","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Metamorphism of dolomitic and magnesitic rocks in collisional orogens and implications for orogenic CO2 degassing","authors":"Tamang Shashi, Groppo Chiara, Girault Frédéric, Perrier Frédéric, Rolfo Franco","doi":"10.1093/petrology/egae021","DOIUrl":"https://doi.org/10.1093/petrology/egae021","url":null,"abstract":"Carbonate-bearing sediments, containing calcite, dolomite or magnesite as major carbonate components, are important components of sedimentary sequences deposited on passive margins through Earth’s history. When involved in collisional orogenic processes, these sediments are metamorphosed at variable temperatures and pressures, and undergo decarbonation reactions. While the orogenic metamorphism of some of these lithologies (i.e., impure limestones and dolostones, marls sensu strictu and calcareous pelites) is relatively well understood, very little is known about the metamorphic evolution and decarbonation history of mixed carbonate-silicate rocks in which either dolomite or magnesite is the dominant carbonate component. Here we present the results of a petrologic study of representative samples of metasediments from Central Nepal, derived from Proterozoic dolomitic and magnesitic protoliths metamorphosed during the Himalayan orogeny. The main metamorphic assemblages developed in sediments originally containing different amounts of dolomite or magnesite are characterised in detail. Forward thermodynamic modelling applied to seven samples allows constraints to be placed on: (i) the main decarbonation reactions, (ii) the P-T conditions under which these reactions took place, (iii) the composition of the fluids, and (iv) the amounts of CO2 released. We conclude that the CO2 productivity of dolomitic and magnesitic pelites and marls originally containing 15-40% carbonate is significant (>5.5 ±1.0 CO2 wt% and up to 10.5 ±1.5 CO2 wt%), whereas for carbonate contents above 60-70%, CO2 productivity is negligible unless aqueous fluids infiltrate from the outside and trigger decarbonation reactions. Since the dolomitic and magnesitic protoliths are significantly abundant in the sedimentary sequences involved in the still active Himalayan orogen, the decarbonation processes described here could contribute to the diffuse CO2 degassing currently observed at the surface. Furthermore, we propose for the first time that the peculiar magnesium-rich assemblages investigated in this study may derive from evaporitic protoliths, and that the whole Upper Lesser Himalayan Sequence may therefore represent the metamorphic product of a Proterozoic sequence consisting of alternating layers of carbonatic, evaporitic and pelitic sediments.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-02-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140008075","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Comment on ‘The Generation of Eocene Mafic Dike Swarms During the Exhumation of a Core Complex, Biarjmand Area, NE Iran’ by Azizi et al. (2023), Journal of Petrology, 64, 1-18","authors":"Ahmadreza Malekpour-Alamdari","doi":"10.1093/petrology/egae011","DOIUrl":"https://doi.org/10.1093/petrology/egae011","url":null,"abstract":"Azizi et al. (2023) have attributed the E-W-oriented mafic dike swarm in the Biarjmand metamorphic core complex to an Eocene extensional event which is much younger than a previously suggested Late Jurassic-Early Cretaceous age (Malekpour-Alamdari et al., 2017). They proposed that the emplacement of these dikes occurred in a rapid extensional regime coeval with the exhumation of the core complex after gravitational instability in the Central Iran/Eurasia collision zone. I appreciate the opportunity this paper provides to shed light on specific aspects of the Late Mesozoic-Early Cenozoic continental extension within the Eurasian sector of the Neotethys subduction system. However, I here bring to attention certain discrepancies within Azizi et al.'s (2023) publication. Specifically, the assignment of an Eocene age to the emplacement of the E-W-oriented dike swarm, even though purportedly supported by U-Pb zircon dating, appears to be at odds with field observations and previously published geochronological data. Furthermore, the paper contains internal contradictions in its presentation of the core complex model for the study area. It is important to note that Malekpour-Alamdari (2017) and Malekpour-Alamdari et al. (2017) previously documented the geochronological-based metamorphic core complex model of the area. Regrettably, despite its direct relevance, these earlier works have not been acknowledged in Azizi et al.'s (2023) paper. In this comment, I outline the problems with the structural and regional geology, the zircon U-Pb age of the dike samples, the age of the dike swarm, and the geodynamic interpretations in Azizi et al.'s (2023) work.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-02-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139677647","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Fluid-induced metamorphism and deformation at the eastern boundary of the Sveconorwegian Province","authors":"C L Urueña, C Möller","doi":"10.1093/petrology/egae008","DOIUrl":"https://doi.org/10.1093/petrology/egae008","url":null,"abstract":"The Sveconorwegian orogen in Scandinavia and the Grenville orogen in Canada are both remnants of large and hot orogens that formed part of the supercontinent Rodinia around 1 billion years ago. Formerly deeply buried portions of crust in these orogens are exposed and offer insights into the tectonic dynamics of the basement within large orogens. The Eastern Segment of the Sveconorwegian Province hosts a ~30 000 km2 crustal portion that was buried to c. 40 km depth at a late stage of the orogeny, 980–960 Ma ago, and is bound towards the foreland in the east by a ~25 km wide zone of step anastomosing deformation, the Frontal wedge. This zone represents the outermost ductile deformation that developed within the crystalline basement in the orogen. We investigated a heterogeneously deformed and recrystallised syenodiorite with the aim to understand the character of the deformation-related metamorphism within the Frontal wedge. Field relations, microtextures, and mineral reactions show that the metamorphic recrystallisation was governed by hydrous fluid infiltration along the ductile deformation zones. Equilibrium was attained on a millimetre scale only and metamorphic recrystallisation was dependent on the introduction of hydrous fluid. The metamorphism reached high-pressure epidote-amphibolite-facies; geothermobarometric estimates suggest 540–600 °C and 9–12 kbar. Metamorphic zircon formed during the breakdown of Zr-bearing igneous phases, primarily baddeleyite. SIMS U–Pb analyses of igneous zircon and baddeleyite date the igneous crystallisation of the syenodiorite at 1230 ± 6 Ma. Metamorphic zircon grains are <20 μm and too small for precise dating, but yielded ages around 1 Ga. Collectively, the metamorphic data indicate that subvertical movements along steep planes within the Frontal wedge allowed for the regional-scale tectonic burial to ~40 km depth of the Eastern Segment to the west. Some of the same steep deformation structures were re-utilised as discrete movement planes during later exhumation.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-02-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139680102","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The role of peritectic biotite for the chemical and mechanical differentiation of felsic plutonic rocks (Western Adamello, Italy)","authors":"Thomas Grocolas, Othmar Müntener","doi":"10.1093/petrology/egae009","DOIUrl":"https://doi.org/10.1093/petrology/egae009","url":null,"abstract":"The volcanic–plutonic connection plays a fundamental role for magmatic systems, linking crystallising plutons, volcanic activity, volatile exsolution and ore deposits. Nonetheless, our understanding of the nature of these links is limited by the scarcity of continuous outcrops exhibiting clear relationships between the plutonic roots that feed its volcanic counterpart. One way to better characterise the volcanic–plutonic connection is to quantify the amount of melt segregation within crystallising tonalitic to granodioritic plutonic rocks, and to compare those with recent silicic eruptions. Here we investigate the processes of interstitial melt segregation in the calc-alkaline Western Adamello pluton (Italy). The Western Adamello tonalite exhibits a coarse-grained, equigranular texture and is mainly composed of hornblende partially replaced by biotite, plagioclase, quartz and alkali feldspar. Within the tonalites, several types of schlieren textures, crystal accumulation zones and dikes are found, comprising: (i) hornblende-biotite-gabbros, spatially-related to (ii) plagioclase- and quartz-rich leucotonalites; and (iii) quartz-, albite- and alkali-feldspar-rich domains forming aplitic to pegmatitic dikes indicative of melt segregation and extraction. Hornblende, biotite and plagioclase phenocrysts have essentially the same compositional range in the tonalites, gabbros and leucotonalites. Together with field observations, this indicates that deformation-driven crystal–melt segregation controls the modal variation within the host tonalite. The calculated melt in equilibrium with the primitive amphiboles has the same trace element composition as the host tonalite to within 5–10 %, indicating that the tonalite did not experience substantial melt loss. Quantitative modal compositions and crystallisation–differentiation calculations suggest that the evolution of the tonalite is controlled by plagioclase and hornblende crystallisation followed by a biotite-forming peritectic reaction. This peritectic reaction can be written as melt1 + amphibole = melt2 + biotite + quartz + plagioclase and decreases the remaining interstitial melt fraction from 40 to 15 % in a small temperature interval (~50 °C), therefore reducing the temperature window for large-scale melt segregation. The biotite-forming reaction initiates in weakly corundum-normative compositions in low to intermediate K calc-alkaline differentiation (e.g., Western Adamello and Peninsular Ranges batholith, California), whereas it seems absent in intermediate to high K, clinopyroxene-normative melts (e.g., Tuolumne intrusive suite, California). This difference is likely controlled by the initial aluminium saturation index and the differentiation path of the parental melt within the middle to lower crust. Textural observations and mass balance models indicate that 75–88 % plagioclase and quartz and 30–70 % interstitial melt was mechanically removed from the Western Adamello tonalite to form hornblende-","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-02-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139677761","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The Primary Geology of the Paleoproterozoic MT Weld Carbonatite Complex, Western Australia","authors":"Ross Chandler, Ganesh Bhat, John Mavrogenes, Brad Knell, Rhiannon David, Thomas Leggo","doi":"10.1093/petrology/egae007","DOIUrl":"https://doi.org/10.1093/petrology/egae007","url":null,"abstract":"The paleoregolith overlying the c. 2.06 Ga Mt Weld carbonatite (eastern Yilgarn Craton, Western Australia) hosts one of the largest Rare Earth Element (REE) deposits globally. Historic exploration and research has focussed on this weathered material, with a lack of unweathered samples preventing a thorough investigation into the nature of the underlying carbonatite. Recent deep drilling has allowed this first in-depth study into the primary geology, mineralogy and geochemistry of the carbonatite complex. Mt Weld shares a similar lithological architecture to other global carbonatite complexes such as Ngualla (Tanzania, Bonga (Angola), Chilwa Island (Malawi) and Mirima Hill (Malawi), displaying a central (~600 m diameter) unit of magnesio- to ferrocarbonatite is surrounded by a broad (~1.2 km) annulus of calciocarbonatite, itself surrounded by a fenitic halo. Primary REE mineralisation occurs within the central magnesio- to ferrocarbonatites (~2% Total Rare Earth Oxides in bulk rock) and is dominated by fine-grained monazite hosted within fractures and voids, as well as late magmatic synchysite/bastnäsite. Both high and low phosphourous ferrocarbonatites occur within this central magnesio- to ferrocarbonatite unit that display (respectively) monazite or synchysite/bastnäsite mineralisation with textural evidence for these REE-bearing carbonates occasionally forming as polycrystalline pseudomorphs of earlier burbankite group minerals. Magnesio- to ferrocarbonatite dykes with zhonghuacerite/olekminskite/ancylite assemblages occur throughout the otherwise REE-poor calciocarbonatites (~0.2% Total Rare Earth Oxides in bulk rock). Late hydrothermal events strongly influenced the modern-day mineral assemblages with much of the existing ore mineral textures interpreted as hydrothermal reworkings of pre-existing REE-bearing minerals. A comparison of the fresh carbonatite and the paleoregolith geochemistry suggest minimal horizontal migration of ore elements during paleoregolith formation, with the overlying paleoregolith material broadly reflecting the underlying carbonatite trace and minor element signatures. This allows the inference of an approximately 5x upgrade in REE (and Nb) concentrations from the primary carbonatite to the overlying paleoregolith. Mt Weld shows distinct geological, mineralogical and isotopic differences to other currently mined carbonatite-associated REE deposits such as Bayan Obo (China), Mountain Pass (USA) and the Mianning-Dechang belt (China), which suggests that fundamentally different carbonatite mantle sources and evolutionary paths can form world-class REE ore bodies.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-02-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139677766","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Estimates of Crystallinity utilizing Differential Scanning Calorimetry: Application to the Kīlauea 2018 lower East Rift Zone Eruption","authors":"B A Halverson, A Emerson, J Hammer, J Lira, A Whittington","doi":"10.1093/petrology/egae010","DOIUrl":"https://doi.org/10.1093/petrology/egae010","url":null,"abstract":"Rocks produced by diverse processes, from condensation in space to impacts on planetary surfaces to volcanism, contain both crystals and amorphous material. Crystallinity provides information on the thermal history of the sample and is especially important in characterizing volcanic rocks and pyroclasts because lava rheology is profoundly influenced by the crystal content. Crystallinity is typically quantified via microscopy, using transmitted light or backscattered electrons. However, many samples present visibly ambiguous textures such as intimate intergrowth of crystal phases, and/or crystal sizes extending down to the nanometer scale. Here we apply calorimetric methods involving heat capacity and enthalpy to assess the crystallinity of a series of volcanic samples. We tested three different approaches, using differential scanning calorimetry, on 30-40 mg aliquots of powdered basalts from the 2018 Kīlauea lower East Rift Zone. The first approach involves determining the magnitude of the increase in heat capacity at the glass transition , which can determine crystallinity to a 1𝜎 precision of ±3%. The second approach is based on the enthalpy of fusion, which requires a longer more complex procedure with results that are typically more uncertain than for the heat capacity method, with a 1𝜎 of ±6%. A final method utilizing differences in enthalpies calculated from the heat capacities required the most complex procedure, and has the greatest uncertainty of ±18%. Preliminary results for lavas with microscopically determined crystallinities ranging from 11-98% indicate that crystallinity based on calorimetric data can be tens of percent higher than the average value identified using microscopy and petrographic analysis. Image-based methodologies applied to sections of samples reveal spatial heterogeneity and details in texture and crystallinity, whereas calorimetry-based methodologies capture the overall \"bulk sample\" properties, unbiased by section effects or imaging resolution limits. These techniques are a powerful combination that can present complementary views of crystallinity.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-02-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139762600","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The origin and differentiation of CO2-rich primary melts in Ocean Island volcanoes: Integrating 3D X-ray tomography with chemical microanalysis of olivine-hosted melt inclusions from Pico (Azores)","authors":"Thomas D van Gerve, David A Neave, Penny Wieser, Hector Lamadrid, Niels Hulsbosch, Olivier Namur","doi":"10.1093/petrology/egae006","DOIUrl":"https://doi.org/10.1093/petrology/egae006","url":null,"abstract":"Constraining the initial differentiation of primary mantle melts is vital for understanding magmatic systems as a whole. Chemical compositions of olivine-hosted melt inclusions preserve unique information about the mantle sources, crystallisation behaviour and volatile budgets of such melts. Crucially, melt inclusion CO2 contents can be linked to mantle CO2 budgets and inform us on Earth’s carbon fluxes and cycles. However, determining total inclusion CO2 contents is not straightforward, as they often need to be reconstructed from CO2 dissolved in melts and CO2 stored in a vapour bubble. Here, we improve upon existing reconstruction methods by combining 3D X-Ray Computed Tomography (CT) with geochemical microanalyses of major, trace and volatile elements. We show that in comparison to CT data, traditional reconstruction methods using 2D photomicrographs can underestimate CO2 budgets by more than 40%. We applied our improved methods to basaltic olivine-hosted melt inclusions from Pico volcano (Azores) in order constrain the formation and differentiation of volatile-rich primary melts in the context of a mantle plume. Results for these inclusions yielded 1935–9275 μg/g reconstructed total CO2, some of the highest values reported for ocean island volcanoes to date. Using these CO2 concentrations, we calculate entrapment pressures of 105–754 MPa that indicate a magma reservoir comprising stacked sills straddling the crust-mantle boundary. In the magma reservoir, crystallisation of volatile saturated melts drives extensive degassing, leading to fractionated CO2/Ba ratios of 3.5–62.2 and a loss of over 79% of primary mantle-derived CO2. Variabilities in trace elements (La, Y) show that differentiation occurred by concurrent mixing and crystallisation of two endmember melts, respectively depleted and enriched in trace elements. Geochemical models show that enriched endmember melts constitute 33 wt.% of all melts supplied to the crust at Pico and that primary melts underwent 60% crystallisation prior to eruption. Mantle melting models indicate that the enriched and depleted primary melt endmembers are low- and high-degree melts of carbon-poor lherzolite and carbon-rich pyroxenite respectively. Moreover, since deep magmas at Pico island are dominantly pyroxenite derived, their CO2-enrichement is mainly controlled by mantle source carbon content. Overall, our study illustrates that by combining 3D imaging, geochemical microanalyses and numerical modelling, melt inclusions provide a unique record of differentiation and storage of deep magmas, as well as mantle melting.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-01-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139644794","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Incomplete hydration during ‘retrograde’ metamorphism: ‘Barrovian’ kyanite-, staurolite-, chloritoid-bearing pseudomorphs after andalusite (Cap de Creus, E Pyrenees, Spain)","authors":"Pavel Pitra, Francisco José Martínez","doi":"10.1093/petrology/egae004","DOIUrl":"https://doi.org/10.1093/petrology/egae004","url":null,"abstract":"Metapelitic rocks from Cap de Creus (Eastern Pyrenees, Spain) have developed andalusite±cordierite-bearing assemblages during the Variscan low-pressure high-temperature metamorphism. Andalusite crystals are commonly transformed to very fine-grained assemblages containing white micas with various combinations of staurolite, kyanite, chloritoid, and/or biotite, chlorite, corundum. The hydrous character of the replacement minerals (pseudomorphing anhydrous andalusite) and the fact that transformation locally occurs along fractures, suggests that it was enhanced by fluid circulation. Using calculated equilibrium-assemblage phase diagrams (pseudosections) for the analysed whole-rock compositions, the equilibration of the primary andalusite-bearing assemblage is constrained at ~3±1 kbar and ~550±30°C. The presence in the pseudomorphs of kyanite and staurolite, typical of Barrovian-type metamorphism, could suggest a pressure increase. However, P–T–M(H2O) modelling of phase equilibria for local bulk compositions of the pseudomorphed andalusite porphyroblasts points to the crystallisation of the observed pseudomorphs at low pressures (~2.5 kbar) and variable, but relatively low temperatures (~370–500°C), in an H2O-undersaturated environment due to the incomplete character of the hydration. In other words, the fluid incursion triggered the crystallisation of the hydrous pseudomorphing assemblages, but the fluid was consumed during this process, leading finally to fluid-absent, H2O-undersaturated conditions. This highlights the critical importance of considering the effects of H2O undersaturation during metamorphism. The partial hydration can be tentatively attributed to a prograde hydrothermal overprint associated with the development of shear zones during Jurassic stretching of a cooled Variscan crust, rather than ‘simple’ late-Variscan retrogression.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-01-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139645068","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Tracking garnet dissolution kinetics in 3D using deep learning grain shape classification","authors":"Philip Hartmeier, Pierre Lanari, Jacob B Forshaw, Thorsten A Markmann","doi":"10.1093/petrology/egae005","DOIUrl":"https://doi.org/10.1093/petrology/egae005","url":null,"abstract":"The kinetics of fluid-driven metamorphic reactions are challenging to study in nature because of the tendency of metamorphic systems to converge towards chemical equilibrium. However, in cases where mineral textures that reflect incomplete reactions are preserved, kinetic processes may be investigated. Atoll garnet, a texture formed by the dissolution of a garnet’s core, has been described in 2D from thin sections of rocks worldwide. Quantifying the extent of this dissolution reaction requires sample-wide examination of hundreds of individual grains in 3D. In this study, we quantified the distribution of atoll garnet using micro-computed tomography and grain shape analysis. A convolutional neural network was trained on human-labelled garnet grains for automated garnet classification. This approach was applied to a retrogressed mafic eclogite from the Zermatt-Saas Zone (Western Alps). Pervasive atoll-like resorption preferentially affected the larger porphyroblasts, suggesting that compositional zoning patterns exert a first-order control on dissolution rates. A kinetic model shows that the reactivity of metastable garnet to form atolls is favored at pressure-temperature conditions of 560±30 °C and 1.6±0.2 GPa. These conditions coincide with the release of water when lawsonite breaks down during exhumation of mafic eclogites. The model predicts dissolution rates that are 3–5 times faster for the garnet core than for the rim. This study shows that deep learning algorithms can perform automated textural analysis of crystal shapes in 3D and that these datasets have the potential to elucidate petrological processes, such as the kinetics of fluid-driven metamorphic reactions.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-01-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139644647","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}