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The editorial discusses advances in low-temperature mineralogy and geochemistry, focusing on processes related to weathering, diagenesis, and metamorphism. Isotope ratios and mineralogical compositions are used as tracers to study fluid-mineral interactions and element recycling. The document highlights studies on ore-forming processes, mineralogical reactions, and hydrothermal systems in various geological settings, including Portugal and China. Additionally, the text explores the conversion mechanisms of minerals like kaolinite to chlorite and describes active acid-sulfate environments associated with volcanic activity. [Extracted from the article]

Estimation of [Fe.sup.3+]/∑Fe ratios in materials at the submicrometre scale has been a long-standing challenge in the Earth and environmental sciences because of the usefulness of this ratio in estimating redox conditions as well as for geothermometry. To date, few quantitative methods with submicrometric resolution have been developed for this purpose, and most of them have used electron energy-loss spectroscopy carried out in the ultra-high vacuum environment of a transmission electron microscope (TEM). Scanning transmission X-ray microscopy (STXM) is a relatively new technique complementary to TEM and is increasingly being used in the Earth sciences. Here, we detail an analytical procedure to quantify the [Fe.sup.3+]/[summation[Fe ratio in silicates using Fe [L.sub.2,3]-edge X-ray absorption near edge structure (XANES) spectra obtained by STXM, and we discuss its advantages and limitations. Two different methods for retrieving [Fe.sup.3+]/∑Fe ratios from XANES spectra are calibrated using reference samples with known [Fe.sup.3+] content by independent approaches. The first method uses the intensity ratio of the two major peaks at the L3-edge. This method allows mapping of [Fe.sup.3+]/∑Fe ratios at a spatial scale better than 50 nm by the acquisition of 5 images only. The second method employs a 2-eV-wide integration window centred on the [L.sub.2] maximum for [Fe.sup.3+], which is compared to the total integral intensity of the Fe [L.sub.2]-edge. These two approaches are applied to metapelites from the Glarus massif (Switzerland), containing micrometre-sized chlorite and illite grains and prepared as ultrathin foils by focused ion beam milling. Nanometrescale mapping of iron redox in these samples is presented and shows evidence of compositional zonation. The existence of such zonation has crucial implications for geothermometry and illustrates the importance of being able to measure [Fe.sup.3+]/∑Fe ratios at the submicrometre scale in geological samples. Keywords Ferric/ferrous iron * STXM * XANES spectroscopy * [L.sub.2,3]-edge * Redox mapping * Silicate, Introduction Determination of the redox state of iron and its spatial variations in sediments and rocks is of critical importance in both geosciences and environmental sciences, because of the need [...]

The evolution of chlorite composition with temperature (and pressure) serves as basis to a number of chlorite chemical thermometers, for which the oxidation state of iron has been recognised as a recurrent issue, especially at low temperature (T). A new chlorite geothermometer that does not require prior [Fe.sup.3+] knowledge is formulated, calibrated on 161 analyses with well-constrained T data covering a wide range of geological contexts and tested here for low-T chlorites (T < 350°C and pressures below 4 kbar). The new solid-solution model used involves six end-member components (the Mg and Fe end-members of 'Al-free chlorite S', sudoite and amesite) and so accounts for all low-T chlorite compositions; ideal mixing on site is assumed, with an ordered cationic distribution in tetrahedral and octahedral sites. Applied to chlorite analyses from three distinct low-T environments for which independent T data are available (Gulf Coast, Texas; Saint Martin, Lesser Antilles; Toyoha, Hokkaido), the new pure-[Fe.sup.2+] thermometer performs at least as well as the recent models, which require an estimate of [Fe.sup.2+] content. This relief from the ferric iron issue, combined with the simple formulation of the semi-empirical approach, makes the present thermometer a very practical tool, well suited for, for example, the handling of large analytical datasets--provided it is used in the calibration range (T < 350°C, P < 4 kbar). Keywords Chlorite * Geothermometry * Diagenesis * Low-grade metamorphism * Solid solution, Introduction Chlorites are ubiquitous in most diagenetic and metamorphic rocks (Foster 1962; Cathelineau 1988; Laird 1988; De Caritat et al. 1993; Vidal et al. 2001). Their wide compositional variations, through [...]

The Pulo do Lobo belt is one of the units related to the orogenic suture between the Ossa-Morena and the South Portuguese zones in the SW Iberian Variscides. This metasedimentary unit has been classically interpreted as a Rheic subduction-related accretionary prism formed during the pre-Carboniferous convergence and eventual collision between the South Portuguese Zone (part of Avalonia) and the Ossa-Morena Zone (peri-Gondwanan terrane). Discrete mafic intrusions also occur in the dominant Pulo do Lobo metapelites, related to an intraorogenic Mississippian transtensional and magmatic event that had a significant thermal input. Three different approaches have been applied to the Devonian/Carboniferous phyllites and slates of the Pulo do Lobo belt in order to study their poorly known low-grade metamorphic evolution. X-Ray diffraction (XRD) was used to unravel the mineralogy and measure crystallographic parameters (illite "crystallinity" and K-white mica b-cell dimension). Compositional maps of selected samples were obtained from electron probe microanalysis, which allowed processing with XmapTools software, and chlorite semi-empirical and thermodynamic geothermometry was performed. Thermometry based on Raman spectroscopy of carbonaceous material (RSCM) was used to obtain peak temperatures. The microstructural study shows the existence of two phyllosilicate growth events at the chlorite zone, the main one (M1) related to the development of a Devonian foliation S1, and a minor one (M2) associated with a crenulation cleavage (S2) developed at middle/upper Carboniferous time. M1 entered well into epizone (greenschist facies) conditions. M2 conditions were at lower temperature, reaching the anchizone/epizone boundary. These data accord well with the unconformity that separates the Devonian and Carboniferous formations of the Pulo do Lobo belt. The varied results obtained by the different approaches followed, combined with microstructural analysis, are indicative of different snapshots of the metamorphic history. Thus, RSCM temperatures are higher in comparison with the other methods applied, which is interpreted as reflecting a faster reequilibration during the short-lived thermal Mississippian event. Regarding the metamorphic pressure, the data are very homogeneous (very low celadonite content in muscovite and low values of K-white mica b-cell dimension), indicating a low-pressure gradient, which is unexpected in a subduction-related accretionary prism. [ABSTRACT FROM AUTHOR]

The degree of recrystallization of carbonaceous material (CM), as monitored by Raman microspectroscopy, was examined as a function of metamorphic grade in two well‐studied contact aureoles containing carbonaceous pelites: the Nelson aureole, British Columbia and the Ballachulish aureole, Scotland. Here, we use (a) the R2 ratio extracted from the Raman spectrum of CM as a proxy for the degree of graphitization (0.0 in perfect graphite then increasing with structural defects) and (b) the second‐order S1 band (~2,700 cm−1) as a marker for the tridimensional ordering of CM. The Nelson aureole (garnet–staurolite–andalusite–sillimanite–K‐feldspar sequence, ~550–650°C, 3.5–4.0 kbar) was developed in rocks that were unmetamorphosed prior to contact metamorphism, whereas the Ballachulish aureole (cordierite–andalusite–K‐feldspar–sillimanite sequence, ~550–700°C, ~3.0 kbar) was developed in rocks that had been metamorphosed to garnet grade conditions (~7 kbar, ~500°C) c. 45 Ma before contact metamorphism. Thirty‐one samples were examined from Nelson and 29 samples from Ballachulish. At Nelson, the R2 ratio steadily decreases from ~0.25 to 0.0 as the igneous contact is approached, whereas at Ballachulish, the R2 ratio remains largely unchanged from regional values (~0.20–0.25) until less than 100 m from the igneous contact. The second‐order S1 band reveals that carbonaceous material (CM) was transformed to highly "ordered" locally tridimensional graphitic carbon at Ballachulish by regional metamorphism prior to contact metamorphism, whereas CM was still a disordered turbostratic (bidimensional) material before contact metamorphism in the case of Nelson. Pretexturation of CM likely induced sluggish recrystallization of CM and delayed graphitization in the Ballachulish aureole. Temperatures of recrystallization of the CM in the two aureoles were estimated using different published calibrations of the thermometry based on Raman Spectroscopy of Carbonaceous Material (RSCM), with differences among the calibrations being minor. In the Nelson aureole, temperatures are in reasonable agreement with those indicated by the metapelitic phase equilibria (all within 50°C, most within 25°C). In the Ballachulish aureole, the retarded crystallization noted above results in increasing underestimates of temperatures compared to the metapelitic phase equilibria (up to ~75°C too low within 200 m of the igneous contact). Our study calls for careful attention when using RSCM thermometry in complexly polymetamorphosed rocks to assess properly the meaning of the calculated temperature. [ABSTRACT FROM AUTHOR]

Phyllosilicates are among the most important stable minerals within the Earth's crust. Their use as geo-thermometer bears great potential for application to the thermal history of rocks within the stability range of layered silicates and was tested here. A high-resolution analytical technique combining focused ion beam (FIB) milling and analytical electron microscopy (AEM) analysis has been applied to a series of sandstone core samples from the Gulf Coast (Texas, U.S.A.). The nanoscale compositional variations of K-deficient mica and chlorite flakes show that rim compositions are the most likely to approach equilibrium compositions, whereas core compositions may be relict, especially for illite-like phases. These rim analyses were used to test existing empirical or thermodynamically formulated thermo(baro)meters against maximum temperatures, which are perfectly constrained for the selected samples as they were measured in situ during drilling (100-230 °C and 300-1200 bars). The results show that most of the empirical models overestimate the temperature, while thermodynamic models yields reasonable estimates for diagenetic to anchizonal conditions, especially if the Fe3+ content is taken into account. This study clearly shows that phyllosilicates thermometry is reliable when combined with an analytical technique giving access to the fine-scale compositional variations that may represent local equilibration, whereas using micrometric compositional analysis precludes trustworthy application of such thermometers. [ABSTRACT FROM AUTHOR]