Search

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 a 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-labeled 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 the exhumation of mafic eclogites. The model predicts dissolution rates that are three to five 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. [ABSTRACT FROM AUTHOR]

Understanding the interplay between bulk composition and metamorphic grade underpins our interpretations of metamorphism in orogenic belts. The focus of this study is the regional garnet–staurolite–kyanite–sillimanite metamorphic sequence of the Whetstone Lake area, southeastern Ontario. In the kyanite and lower sillimanite zones of this area, there is exceptional diversity in metapelitic mineral assemblages that cannot be accounted for by differences in metamorphic grade. We present a data set of petrographic observations, phase proportions, whole-rock geochemical compositions, and mineral compositions, from thirty-two samples that encapsulate the range of assemblages found in these zones. Differences in bulk composition are the primary control on mineral assemblage development. Whole-rock XMg = molar MgO/(MgO + FeO) and |${\textrm{X}}_{\textrm{Fe}^{3+}}=\kern0.5em \textrm{molar}\ 2\times{\textrm{Fe}}_2{\textrm{O}}_3/\left(2\times{\textrm{Fe}}_2{\textrm{O}}_3+\textrm{FeO}\right)$| exert the greatest control on the observed mineral assemblages, whilst variation in MnO, K2O, and Al2O3 have a secondary influence. We use a set of quality factors (Duesterhoeft & Lanari, 2020) to test the ability of thermodynamic models to reproduce the observed mineral assemblages, modal abundances, and mineral compositions in the diverse bulk compositions at Whetstone Lake. Eight samples were selected for phase equilibrium modelling, for which two bulk compositions were calculated for each sample: (1) a whole-rock bulk composition based on an X-ray fluorescence analysis and (2) a carefully considered local bulk composition based on combining mineral proportions with representative mineral compositions, as obtained from a single thin section. Our modelling uses thermodynamic data set 6.2 (Holland & Powell, 2011) and the solution models of White et al. (2014a , 2014b) that incorporate several Fe3+ end members needed to model the natural data. Modelling in both types of bulk composition broadly predicted mineral assemblages that match those observed. In addition, predicted mineral assemblage fields overlap within uncertainty between 620°C and 675°C and between 6.5 and 7.5 kbar, consistent with the limited range of grade represented by the natural rocks. Predicted modal abundances better match those observed when phase diagrams are constructed using local bulk compositions compared to whole-rock bulk compositions. Despite the acceptable agreement between predicted and observed mineral assemblages, consistent discrepancies are found between predicted and observed mineral compositions. These include overestimation of |${\textrm{X}}_{\textrm{Mg}}^{\ast }$| in garnet, staurolite, and cordierite, overestimation of Ti in staurolite and biotite, underestimation of Si in biotite, and overestimation of Al and underestimation of Fe3+, Fe2+, and Mg in muscovite. The Whetstone Lake suite of this study will be useful to test the predictive capability of future thermodynamic models. [ABSTRACT FROM AUTHOR]

Pelites (shales and mudstones) are arguably the most important rock type for interpreting metamorphism. Their significance derives from their widespread occurrence and the range of mineral assemblages they develop at different conditions of pressure and temperature. We compiled a global database of 5729 major-element whole-rock analyses of pelites from different metamorphic grades (shales to granulite-facies paragneisses) to (1) determine an average composition, (2) examine the range and variability in their composition, and (3) assess if there is evidence for grade-related geochemical changes. Median values are given instead of average values to eliminate the effect of extremes. The median worldwide pelite is as follows (anhydrous, values in wt%): SiO2 = 64.13, TiO2 = 0.91, Al2O3 = 19.63, FeOtotal = 6.85, MnO = 0.08, MgO = 2.41, CaO = 0.65, Na2O = 1.38, and K2O = 3.95. The median XMg = MgO/(MgO + FeOtotal) in moles is 0.39. The median XFe3+ = 2 Fe2O3/(2 x Fe2O3 + FeO) in moles was measured in 1964 samples and is 0.23. On an Al2O3-FeO-MgO (AFM) diagram, the median worldwide pelite plots within a strong clustering of analyses between XMgproj = projected molar MgO/(MgO + FeOtotal) = 0.30--0.55 (median = 0.42) and AMs = molar [Al2O3 -- (3 x K2O)]/[Al2O3 -- (3 x K2O) + FeOtotal + MgO] = 0.0--0.4 (median = 0.19). Pelites show a continuous decrease in volatile content with increasing metamorphic grade and a decrease in XFe3+ from the diagenetic to biotite zone. Lower median SiO2 values and higher median Al2O3 and AMs values in the porphyroblast and subsolidus sillimanite or K-feldspar zones, as well as higher median MnO values in the garnet zone, may reflect sampling bias or metasomatism. [ABSTRACT FROM AUTHOR]

The Lewisian Complex in northwest Scotland presents a record of the transition from the Neo-Archean to the Paleoproterozoic. However, this record is complicated by a long and varied history after peak metamorphism that has erased and/or partially reset much of the early history of the rocks. Such overprinting is a common feature of Archean granulites and poses a substantial problem when trying to understand the tectonic processes that were active prior to the onset of modern plate tectonics. By combining careful petrography with phase diagram modeling and a range of exchange thermometers we obtain the peak and retrograde temperature history of the Lewisian Complex from a single, well-preserved, representative sample of garnet-bearing mafic granulite. We present the application of high-resolution electron probe microanalysis (HR-EPMA) to characterize sub-micrometer orthopyroxene exsolution lamellae in clinopyroxene. We discuss ways to mitigate issues associated with HR-EPMA including surface contamination, beam drift, standards, and the need to correct for secondary fluorescence effects. The resulting compositions from our HR-EPMA analyses provide an independent measure of the retrograde temperature conditions and can also be used to back-calculate the compositions of clinopyroxene in the peak assemblage. We obtain peak metamorphic conditions for the Lewisian of >11 kbar and >1025 °C, and constrain subsequent metamorphic overprints to 850 °C (Grt-Cpx), 590 °C (Opx-Cpx), and 460 °C (Mag-Ilm). These peak and retrograde temperatures span the range of those found in the literature. Whereas recent phase equilibrium studies assume equilibrium among all preserved high-T minerals, this study considers microstructural and mineral-chemical evidence for corona formation that reflects post-peak decompression with partial equilibration at ~850 °C, as recognized in some earlier studies. [ABSTRACT FROM AUTHOR]

The Kohistan island arc represents a >50 km thick and completely exposed Late Cretaceous–Eocene Neo‐Tethyan island arc sequence between the Indian and Eurasian plates in the western Himalaya. At the base of the Kohistan Arc, the Jijal Complex exposes a continuous sequence from ultramafic upper mantle rocks to mafic rocks of the lower crust across the Moho transition zone. In the shallower mafic section of the Jijal Complex, widespread garnet–clinopyroxene gabbros are uniform in assemblage but show a spectrum of microstructural variation involving two distinguishable styles of garnet growth. Uncertainty surrounds the genetic origin of these equilibria across the sequence which have been variably proposed to derive either from (a) dehydration melting of amphibole‐bearing precursor rocks, (b) subsolidus recrystallization of gabbronorite, or (c) primary magmatic crystallization. Owing to extensive high‐T re‐equilibration, compositional records of magmatic precursors and the pre‐peak evolution across the complex have been erased. However, quantitative textural analysis evidences a progressively increasing departure from microstructural randomness from deep to shallow levels of the upper Jijal Complex. This is interpreted to reflect the increased propensity for shallow garnet–clinopyroxene gabbros to have been derived from solid‐state recrystallization of a cumulate gabbronorite precursor, resulting in the development of coronitic garnet microstructures. In the deepest portions of the mafic sequence, both cumulate textures and thermodynamic modelling attest to the basal levels of the Kohistan arc crust at ~10–14 kbar crystallizing primary magmatic garnet and clinopyroxene from low H2O (<6 wt%) basaltic–andesitic melts. Granulite facies metamorphism and re‐equilibration across the whole upper Jijal Complex—shown via phase equilibrium modelling to be associated with over‐accretion in the mid‐arc crust and a pressure increase of 2–3 kbar to 850–1,050℃ and 11–14 kbar—was followed by a period of approximately isobaric cooling across ~200℃ and amphibolitization. Compositions of low‐volume leucosome‐hosted garnet indicate melting occurred prior to, or during, this re‐equilibration period and melt reintegration simulations suggest that dehydration melting of amphibole‐bearing precursors played only a minor role in the genesis of the Jijal garnet–clinopyroxene gabbros. By combining field relationships, petrological and textural analysis, interpretations of mineral compositions, and phase equilibrium modelling, we therefore provide detailed constraints on crustal thickening and the thermal evolution of the lowermost arc crust. [ABSTRACT FROM AUTHOR]

Fe3+ and XFe3+, defined as Fe3+/(Fe2+ + Fe3+) on a molar basis, are now recognised as key parameters in phase equilibrium modelling. A hindrance is that it is only possible to routinely measure total Fe, and not Fe3+ and Fe2+, in minerals using the electron microprobe. Charge balance techniques can be used to estimate Fe3+ and Fe2+ for some minerals, but not for those that contain vacancies. Whilst other analytical techniques can determine XFe3+ in minerals, these are not commonly applied by metamorphic petrologists. Therefore, researchers must rely on estimates. This study collates wet chemical, Mössbauer spectroscopy, and X-ray absorption near edge structure (XANES) spectroscopy analyses of XFe3+ in metapelitic minerals and rocks from the literature. The resulting database of 77 studies contains 591 samples, of which 261 have XFe3+ determined for the whole rock. There are XFe3+ measurements for 483 biotites, 192 white micas, 78 chlorites, and 32 staurolites. Average (± 1σ) XFe3+ values in whole rock, biotite, white mica, chlorite, and staurolite are 0.23 ± 0.16, 0.11 ± 0.08, 0.55 ± 0.18, 0.08 ± 0.07, and 0.07 ± 0.06, respectively. The average (± 1σ) number of Fe3+ cations in biotite, white mica, chlorite, and staurolite is 0.28 ± 0.19 (22 O + Ti cations per formula unit, pfu), 0.17 ± 0.13 (22 O pfu), 0.31 ± 0.27 (28 O pfu), and 0.20 ± 0.17 (46 O pfu), respectively. The mean whole rock XFe3+ is similar for metapelites containing ilmenite and magnetite, as well as those that report no Fe-oxide, but is considerably higher for hematite-bearing rocks. Whilst there is little variation with pressure and temperature, there is an increase in the number of Fe3+ cations and XFe3+ of both white mica and biotite with the type of Fe-oxide present. Our observations are compared with the predictions of phase equilibrium modelling using thermodynamic dataset 6.2 (Holland and Powell, J Metamorph Geol 29:333–383, 2011) and the solution models of White et al. (J Metamorph Geol 32:261–286, 2014a) for Fe3+ and XFe3+ in these minerals. The predicted XFe3+ and number of Fe3+ cations in biotite, chlorite, and staurolite broadly match natural observations, but for white mica the predicted mean XFe3+ is underestimated by 0.2–0.4 and the number of Fe3+ cations by 0.05–0.2. Whilst modelling correctly predicted increases in the XFe3+ of white mica and biotite with whole rock XFe3+, it also predicted variations in mineral XFe3+ as a function of pressure and temperature which are not observed in the natural samples. [ABSTRACT FROM AUTHOR]

Recently published activity–composition (a–x) relations for minerals in upper amphibolite‐ and granulite facies intermediate and basic rocks have expanded our ability to interpret the petrological evolution of these important components of the lower continental crust. If such petrological modelling is to be reliable, the abundances and compositions of phases calculated at the interpreted conditions of metamorphic equilibration should resemble those in the sample under study. Here, petrological modelling was applied to six granulite facies rocks that formed in different tectonic environments and reached different peak metamorphic pressure–temperature (P–T) conditions. While phase assemblages matching those observed in each sample can generally be calculated at P–T conditions that approximate those of peak metamorphism, a consistent discrepancy was found between the calculated and observed compositions of amphibole and clinopyroxene. In amphibole, Si, Ca and A‐site K are underestimated by the model, while Al and A‐site Na are overestimated; comparatively, in clinopyroxene, Mg and Si are generally underestimated, while Fe2+ and Al are typically overestimated, compared to observed values. One consequence is a reversal in the Fe–Mg distribution coefficient (KD) between amphibole and clinopyroxene compared to observations. Some of these mismatches are attributed to the incorrect partitioning of elements between the predicted amphibole and clinopyroxene compositions; however, other discrepancies are the result of the incorrect prediction of major substitution vectors in amphibole and clinopyroxene. These compositional irregularities affect mineral modal abundance estimates and in turn the position and size (in P–T space) of mineral assemblage fields, the effect becoming progressively more marked as the modal abundance of hornblende increases; hence, this study carries implications for estimating P–T conditions of high‐temperature metabasites using these new a–x relations. [ABSTRACT FROM AUTHOR]