Quantifying geological uncertainty in metamorphic phase equilibria modelling; a Monte Carlo assessment and implications for tectonic interpretations

Pseudosection modelling is rapidly becoming an essential part of a petrologist’s toolkit and often forms the basis of interpreting the tectonothermal evolution of a rock sample, outcrop, or geological region. Of the several factors that can affect the accuracy and precision of such calculated phase diagrams, “geological” uncertainty related to natural petrographic variation at the hand sample- and/or thin section-scale is rarely considered. Such uncertainty influences the sample’s bulk composition, which is the primary control on its equilibrium phase relationships and thus the interpreted pressure–temperature (P–T) conditions of formation. Two case study examples—a garnet–cordierite granofels and a garnet–staurolite–kyanite schist—are used to compare the relative importance that geological uncertainty has on bulk compositions determined via (1) X-ray fluorescence (XRF) or (2) point counting techniques. We show that only minor mineralogical variation at the thin-section scale propagates through the phase equilibria modelling procedure and affects the absolute P–T conditions at which key assemblages are stable. Absolute displacements of equilibria can approach ±1 kbar for only a moderate degree of modal proportion uncertainty, thus being essentially similar to the magnitudes reported for analytical uncertainties in conventional thermobarometry. Bulk compositions determined from multiple thin sections of a heterogeneous garnet–staurolite–kyanite schist show a wide range in major-element oxides, owing to notable variation in mineral proportions. Pseudosections constructed for individual point count-derived bulks accurately reproduce this variability on a case-by-case basis, though averaged proportions do not correlate with those calculated at equivalent peak P–T conditions for a whole-rock XRF-derived bulk composition. The main discrepancies relate to varying proportions of matrix phases (primarily mica) relative to porphyroblasts (primarily staurolite and kyanite), indicating that point counting preserves small-scale petrographic features that are otherwise averaged out in XRF analysis of a larger sample. Careful consideration of the size of the equilibration volume, the constituents that comprise the effective bulk composition, and the best technique to employ for its determination based on rock type and petrographic character, offer the best chance to produce trustworthy data from pseudosection analysis.RMP acknowledges a NERC postgraduate grant (reference number NE/H524781/1) for funding analytical work performed at the University of Oxford, UK

Quantifying geological uncertainty in metamorphic phase equilibria modelling; a Monte Carlo assessment and implications for tectonic interpretations

Pseudosection modelling is rapidly becoming an essential part of a petrologist's toolkit and often forms the basis of interpreting the tectonothermal evolution of a rock sample, outcrop, or geological region. Of the several factors that can affect the accuracy and precision of such calculated phase diagrams, “geological” uncertainty related to natural petrographic variation at the hand sample- and/or thin section-scale is rarely considered. Such uncertainty influences the sample's bulk composition, which is the primary control on its equilibrium phase relationships and thus the interpreted pressure–temperature (P–T) conditions of formation. Two case study examples—a garnet–cordierite granofels and a garnet–staurolite–kyanite schist—are used to compare the relative importance that geological uncertainty has on bulk compositions determined via (1) X-ray fluorescence (XRF) or (2) point counting techniques. We show that only minor mineralogical variation at the thin-section scale propagates through the phase equilibria modelling procedure and affects the absolute P–T conditions at which key assemblages are stable. Absolute displacements of equilibria can approach ±1 kbar for only a moderate degree of modal proportion uncertainty, thus being essentially similar to the magnitudes reported for analytical uncertainties in conventional thermobarometry. Bulk compositions determined from multiple thin sections of a heterogeneous garnet–staurolite–kyanite schist show a wide range in major-element oxides, owing to notable variation in mineral proportions. Pseudosections constructed for individual point count-derived bulks accurately reproduce this variability on a case-by-case basis, though averaged proportions do not correlate with those calculated at equivalent peak P–T conditions for a whole-rock XRF-derived bulk composition. The main discrepancies relate to varying proportions of matrix phases (primarily mica) relative to porphyroblasts (primarily staurolite and kyanite), indicating that point counting preserves small-scale petrographic features that are otherwise averaged out in XRF analysis of a larger sample. Careful consideration of the size of the equilibration volume, the constituents that comprise the effective bulk composition, and the best technique to employ for its determination based on rock type and petrographic character, offer the best chance to produce trustworthy data from pseudosection analysis

The emplacement, alteration, subduction and metamorphism of metagranites from the Tso Morari Complex, Ladakh Himalaya

Eclogite-facies mineral assemblages are commonly preserved in mafic protoliths within continental terranes. It is widely accepted that the entirety of these continental terrains must also have been subducted to eclogite-facies conditions. However, evidence that the felsic material transformed at eclogite-facies conditions is lacking. Low-strain metagranites of the ultrahigh-pressure metamorphic Tso Morari Complex in Ladakh, Himalaya, are host to eclogite-facies mafic sills and preserve evidence of subduction to eclogite-facies conditions. Following the eclogite-facies metamorphism, the granites and their gneissic equivalents were overprinted by amphibolite-facies Barrovian metamorphism, obscuring their earlier metamorphic history. We present evidence that the Tso Morari metagranites preserve a complex magmatic, hydrothermal and polymetamorphic history that involved four stages. Stage 1 was magmatic crystallisation, a record of which is preserved in the primary igneous mineralogy and relict igneous microstructures. Monazite grains record a U–Pb age of 474.0 ± 11.6 Ma, concurrent with a published zircon crystallisation age. Stage 2 represents pervasive late-magmatic hydrothermal alteration of the granite during emplacement and is evident in the mineral composition, particularly in the white micas preserved in the igneous domains. Stage 3 involved the (ultra)high-pressure metamorphism of these granite bodies during the Himalayan subduction of continental material. The high-pressure stage of the metamorphic history (>25 kbar at 550–650°C) is preserved as thin coronas of garnet and phengite around igneous biotite, garnet with kyanite inclusions in pseudomorphs after cordierite, and rare palisade quartz textures after coesite. Stage 4 was a result of Barrovian metamorphism of the Tso Morari Complex and is evident in the replacement of garnet by biotite. Many of these features are preserved in localised textural domains in the rock, where local equilibrium was important and the anhydrous conditions limited reaction progress, though aided preservation potential. Collectively, these four stages record a 480 Myr history of metamorphism and reworking of the northernmost Indian plate

Metamorphic evolution of the Great Slave Lake shear zone

Funder: Natural Environment Research Council; Id: http://dx.doi.org/10.13039/501100000270Funder: Natural Sciences and Engineering Research Council of Canada; Id: http://dx.doi.org/10.13039/501100000038Abstract: The Palaeoproterozoic Great Slave Lake shear zone (GSLsz) is a crustal‐scale strike‐slip structure, with a total length of >1,000 km and a width of ~25 km, that separates the Archean Rae and Slave cratons. The range of metamorphic rocks now exposed at the surface encompasses granulite facies mylonite through to lower‐greenschist facies ultramylonite and cataclasite, providing a potential type example of fault‐zone structure in the middle and lower crust. However, the metamorphic evolution of the units remains poorly quantified, hindering detailed structural and tectonic interpretations. Here, we use phase equilibria modelling and thermobarometry to determine the metamorphic conditions recorded by pelitic, mafic and felsic GSLsz mylonites. Samples from the entire range of granulite–greenschist facies units preserve evidence for nested clockwise pressure–temperature paths that are consistent with a single orogenic cycle. Our findings indicate that the northern Rae margin underwent pervasive crustal thickening with peak pressures in metasedimentary rocks reaching ~1.1 GPa. The crustal thermal gradient at the onset of thickening was ~650°C/GPa, whereas the final stages of equilibrium recorded by fine‐grained matrix minerals in all samples collectively define a metamorphic field gradient of ~1,000°C/GPa. Deformation microstructures are consistent with the main phase of dextral shear having been synchronous with or following peak metamorphism. The history of metamorphism and exhumation of the GSLsz is consistent with the Sibson–Scholz model for shear zones, with a narrowing of the deforming zone and the progressive overprinting of higher‐grade assemblages during exhumation through shallower crustal levels

Compressional origin of the Naxos metamorphic core complex, Greece: structure, petrography, and thermobarometry

The island of Naxos, Greece, has been previously considered to represent a Cordilleran-style metamorphic core complex that formed during Cenozoic extension of the Aegean Sea. Although lithospheric extension has undoubtedly occurred in the region since 10 Ma, the geodynamic history of older, regional-scale, kyanite- and sillimanite-grade metamorphic rocks exposed within the core of the Naxos dome is controversial. Specifically, little is known about the pre-extensional prograde evolution and the relative timing of peak metamorphism in relation to the onset of extension. In this work, new structural mapping is presented and integrated with petrographic analyses and phase equilibrium modeling of blueschists, kyanite gneisses, and anatectic sillimanite migmatites. The kyanite-sillimanite−grade rocks within the core complex record a complex history of burial and compression and did not form under crustal extension. Deformation and metamorphism were diachronous and advanced down the structural section, resulting in the juxtaposition of several distinct tectono-stratigraphic nappes that experienced contrasting metamorphic histories. The Cycladic Blueschists attained ∼14.5 kbar and 470 °C during attempted northeast-directed subduction of the continental margin. These were subsequently thrusted onto the more proximal continental margin, resulting in crustal thickening and regional metamorphism associated with kyanite-grade conditions of ∼10 kbar and 600−670 °C. With continued shortening, the deepest structural levels underwent kyanite-grade hydrous melting at ∼8−10 kbar and 680−750 °C, followed by isothermal decompression through the muscovite dehydration melting reaction to sillimanite-grade conditions of ∼5−6 kbar and 730 °C. This decompression process was associated with top-to-the-NNE shearing along passive-roof faults that formed because of SW-directed extrusion. These shear zones predated crustal extension, because they are folded around the migmatite dome and are crosscut by leucogranites and low-angle normal faults. The migmatite dome formed at lower-pressure conditions under horizontal constriction that caused vertical boudinage and upright isoclinal folds. The switch from compression to extension occurred immediately following doming and was associated with NNE-SSW horizontal boudinage and top-to-the-NNE brittle-ductile normal faults that truncate the internal shear zones and earlier collisional features. The Naxos metamorphic core complex is interpreted to have formed via crustal thickening, regional metamorphism, and partial melting in a compressional setting, here termed the Aegean orogeny, and it was exhumed from the midcrust due to the switch from compression to extension at ca. 15 Ma

Conversion Discriminative Analysis on Mild Cognitive Impairment Using Multiple Cortical Features from MR Images

Neuroimaging measurements derived from magnetic resonance imaging provide important information required for detecting changes related to the progression of mild cognitive impairment (MCI). Cortical features and changes play a crucial role in revealing unique anatomical patterns of brain regions, and further differentiate MCI patients from normal states. Four cortical features, namely, gray matter volume, cortical thickness, surface area, and mean curvature, were explored for discriminative analysis among three groups including the stable MCI (sMCI), the converted MCI (cMCI), and the normal control (NC) groups. In this study, 158 subjects (72 NC, 46 sMCI, and 40 cMCI) were selected from the Alzheimer's Disease Neuroimaging Initiative. A sparse-constrained regression model based on the l2-1-norm was introduced to reduce the feature dimensionality and retrieve essential features for the discrimination of the three groups by using a support vector machine (SVM). An optimized strategy of feature addition based on the weight of each feature was adopted for the SVM classifier in order to achieve the best classification performance. The baseline cortical features combined with the longitudinal measurements for 2 years of follow-up data yielded prominent classification results. In particular, the cortical thickness produced a classification with 98.84% accuracy, 97.5% sensitivity, and 100% specificity for the sMCI–cMCI comparison; 92.37% accuracy, 84.78% sensitivity, and 97.22% specificity for the cMCI–NC comparison; and 93.75% accuracy, 92.5% sensitivity, and 94.44% specificity for the sMCI–NC comparison. The best performances obtained by the SVM classifier using the essential features were 5–40% more than those using all of the retained features. The feasibility of the cortical features for the recognition of anatomical patterns was certified; thus, the proposed method has the potential to improve the clinical diagnosis of sub-types of MCI and predict the risk of its conversion to Alzheimer's disease

Quantitative 18F-AV1451 Brain Tau PET Imaging in Cognitively Normal Older Adults, Mild Cognitive Impairment, and Alzheimer's Disease Patients

Recent developments of tau Positron Emission Tomography (PET) allows assessment of regional neurofibrillary tangles (NFTs) deposition in human brain. Among the tau PET molecular probes, 18F-AV1451 is characterized by high selectivity for pathologic tau aggregates over amyloid plaques, limited non-specific binding in white and gray matter, and confined off-target binding. The objectives of the study are (1) to quantitatively characterize regional brain tau deposition measured by 18F-AV1451 PET in cognitively normal older adults (CN), mild cognitive impairment (MCI), and AD participants; (2) to evaluate the correlations between cerebrospinal fluid (CSF) biomarkers or Mini-Mental State Examination (MMSE) and 18F-AV1451 PET standardized uptake value ratio (SUVR); and (3) to evaluate the partial volume effects on 18F-AV1451 brain uptake.Methods: The study included total 115 participants (CN = 49, MCI = 58, and AD = 8) from the Alzheimer's Disease Neuroimaging Initiative (ADNI). Preprocessed 18F-AV1451 PET images, structural MRIs, and demographic and clinical assessments were downloaded from the ADNI database. A reblurred Van Cittertiteration method was used for voxelwise partial volume correction (PVC) on PET images. Structural MRIs were used for PET spatial normalization and region of interest (ROI) definition in standard space. The parametric images of 18F-AV1451 SUVR relative to cerebellum were calculated. The ROI SUVR measurements from PVC and non-PVC SUVR images were compared. The correlation between ROI 18F-AV1451 SUVR and the measurements of MMSE, CSF total tau (t-tau), and phosphorylated tau (p-tau) were also assessed.Results:18F-AV1451 prominently specific binding was found in the amygdala, entorhinal cortex, parahippocampus, fusiform, posterior cingulate, temporal, parietal, and frontal brain regions. Most regional SUVRs showed significantly higher uptake of 18F-AV1451 in AD than MCI and CN participants. SUVRs of small regions like amygdala, entorhinal cortex and parahippocampus were statistically improved by PVC in all groups (p < 0.01). Although there was an increasing tendency of 18F-AV-1451 SUVRs in MCI group compared with CN group, no significant difference of 18F-AV1451 deposition was found between CN and MCI brains with or without PVC (p > 0.05). Declined MMSE score was observed with increasing 18F-AV1451 binding in amygdala, entorhinal cortex, parahippocampus, and fusiform. CSF p-tau was positively correlated with 18F-AV1451 deposition. PVC improved the results of 18F-AV-1451 tau deposition and correlation studies in small brain regions.Conclusion: The typical deposition of 18F-AV1451 tau PET imaging in AD brain was found in amygdala, entorhinal cortex, fusiform and parahippocampus, and these regions were strongly associated with cognitive impairment and CSF biomarkers. Although more deposition was observed in MCI group, the 18F-AV-1451 PET imaging could not differentiate the MCI patients from CN population. More tau deposition related to decreased MMSE score and increased level of CSF p-tau, especially in ROIs of amygdala, entorhinal cortex and parahippocampus. PVC did improve the results of tau deposition and correlation studies in small brain regions and suggest to be routinely used in 18F-AV1451 tau PET quantification

Sticking together: Mechanisms of quartz synneusis in high-silica magma

The formation of crystal clusters by synneusis (magmatic sintering) affects a wide range of magmatic systems from olivine clusters in komatiite to quartz clusters in high-silica granite. A common feature of synneusis in any mineral phase is the alignment of neighbouring crystals in certain lower-energy orientation relationships. However, the underlying mechanisms involved with both the alignment of crystals in lower-energy orientations and the binding of crystal clusters are not well understood. In the absence of mechanisms that bind crystals together upon contact, the same hydrodynamic forces that may bring crystals together can in theory also serve to disaggregate clusters. Here I use cathodoluminescence imaging and crystal orientation data from quartz clusters in high-silica granite to show that i) rapid crystalline neck growth along attachment surfaces and ii) grain rotation are two mechanisms that reduce the grain boundary energy of crystal clusters while increasing clusters’ shear strength. The continued crystallization of sintered phases as the magmatic body cools further cements crystal pairs and resists cluster disaggregation. Together these mechanisms underpin both the formation and preservation of large crystal clusters in dynamic magmatic environments

Textural and petrological studies of anatexis and melt transfer in the Himalayan Orogen

Mineral textures, preserved in the metamorphosed sedimentary sequences that are exposed in orogenic hinterlands, are crucial to understanding the architecture and evolution of collisional mountain belts. In this thesis the textural record of anatexis and melt transfer in the Himalayan metamorphic core is decoded and the controls that these processes exert on the tectonic evolution of the Himalaya are explored. The problem is divided into two parts, corresponding to variations in protolith lithostratigraphy: melt source â the pelitic region where melt was first generated, and melt sink â the psammitic region where melt accumulated and crystallised. Dehydration melting of muscovite has long been recognized as a critical reaction for the generation of anatectic melt in the Himalaya, but a textural understanding of how this reaction progresses is limited by the inherent difficulties in identifying specific reaction products. Using samples collected from the Langtang area in central Nepal, a mechanistic model for muscovite dehydration melting was constructed, and a set of textural criteria were developed, which were used to distinguish peritectic K-feldspar from K-feldspar grains formed during melt crystallisation. Melt is transferred from the source to the sink in two stages: firstly along a pervasive network of mineral grain boundaries, and secondly via a channelised network of sills and dykes in the melt sink where it solidified as leucogranite. Variation in the primary mineral assemblage and appearance of leucogranite bodies reflect the degree of interaction that occurred between the melt and metasedimentary country rock, rather than a change in primary melt composition. The modal proportion of K-feldspar in the melt source requires vapour-absent conditions during muscovite dehydration melting and leucogranite formation, indicating that the generation of large volumes of granitic melts in orogenic belts is not necessarily contingent on an external source of fluids. The crystallisation of hydrous minerals in leucogranite consumes &LT;15.5 &percnt; of water released by the breakdown of muscovite. These results indicate that anatexis efficiently dehydrates the middle crust and suggests that the continents have limited potential to store water over geological time.</p

Exploring the metamorphic consequences of secular change in the siliciclastic compositions of continental margins

Shale and greywacke compositions from the Archean to Phanerozoic record a secular change in the siliciclastic material that comprises much of Earth's continental margins, past and present. This study explores the metamorphic consequence of these compositional changes, by comparing phase equilibrium models constructed for average Archean, Proterozoic, and Phanerozoic shale and greywacke compositions equilibrated along two Barrovian-type geotherms: 1330 °C/GPa (A) and 800 °C/GPa (B). Our models show that Archean siliciclastic rocks can retain up to 4 vol.% water at middle to lower crustal conditions, nearly twice that of Proterozoic and Phanerozoic compositions. The increased ferromagnesium content of Archean siliciclastic rocks stabilizes chlorite to higher temperatures and results in a biotite-rich assemblage at solidus temperatures. Accordingly, water-absent biotite dehydration melting is predicted to play a greater role in the generation of melt in the metamorphism of Archean aged units, and water-absent muscovite dehydration melting is of increasing importance through the Proterozoic and Phanerozoic. This secular variation in predicted mineral assemblages demonstrates the care with which metamorphic facies diagrams should be applied to Archean compositions. Moreover, secular changes in the composition of shale and greywacke is reflected in the evolution of anatectic melt towards an increasingly less viscous, Ca-rich, and Mg-poor monzogranite. Keywords: Secular change, Shale, Greywacke, Dehydration melting, Thermodynamic modelling, Perple_