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High-resolution EMPA images of biotite crystals included in andalusite from the andalusite-garnet grade sample 09DF067. a, b Are BSE images of two biotite inclusions. Image c–e are WDS X-ray maps of chlorine (c and d) and fluorine (e and f) for the same these biotite crystals. The only zoning visible is on topographic features, e.g. open grain boundaries and holes. The surface topography associated with the small hole (upper right, images a, c) and the small crack (center left, panels b, d) show increased concentrations of chlorine. They are likely the result of epoxy. No accumulation of halogens is seen on other grain boundaries with andalusite
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The hydroxyl (O(4)) site composition of biotite can in principle be used to retrieve information about fluid composition during fluid–rock interaction; however, due to low F and Cl content, as well as difficulties involved with analyzing the H2O content using in situ techniques, measuring these species in biotite has remained an elusive goal. Here...
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The Chinese Altay orogen is an important part of the giant Central Asian Orogenic Belt. However, its tectono–metamorphic evolution is still a matter of debate. Here, we report Devonian pelitic granulites at the Wuqiagou area in the southern Chinese Altay orogen. They are characterized by an assemblage of garnet + orthopyroxene + cordierite ± spinel...
Microscale oxygen isotope analysis (¹⁸O/¹⁶O) of minerals can identify distinct events of fluid-rock interaction. This method is, however, still limited to a few major and accessory minerals of which most are anhydrous minerals. We present the systematic study of oxygen isotope distribution in white mica by Secondary Ion Mass Spectrometry (SIMS). Te...
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... The positive correlation of XFe with Cl has also been documented from other mica occurrences as well as from amphiboles. This is mainly ascribed to the effect that incorporation of Cl is stimulated by the enlargement of the anion site of both minerals with increasing amounts of Fe 2+ and Al, (and K in the case of amphibole), resulting in a positive correlation of Cl with these elements (Henry and Daigle 2018;Oberti et al. 1993;Siron et al. 2018). ...
Ba-, Ti-, and Cl-rich micas associated with other Ba- and/or Cl-rich minerals in the rock matrix or in garnet and clinopyroxene hosted multiphase solid inclusions (MSI) are observed in mantle-derived garnet pyroxenites. The micas show extremely high variability in chemical composition ranging between Ba-rich phlogopite, chloroferrokinoshitalite, and oxykinoshitalite. Elemental covariation trends in mineral chemical data reveal the principal substitution mechanisms responsible for the observed chemical variability. The substitution Ba2+Al3+ ↔ K1+Si4+ associated with either OH1– ↔ Cl1– or Ti4+2O2– ↔ Mg2+2OH1 links phlogopite to chloroferrokinoshitalite and oxykinoshitalite, respectively, whereas the substitution Ti4+2O2– ↔ Fe2+2Cl1– links chloroferrokinoshitalite to oxykinoshitalite. The preferred incorporation of Cl in Fe-rich mica and of Ti+O in Mg-rich mica indicates that XFe (Fetot/Fetot+Mg) exerts an important control on mica composition. The positive correlation of XFe with Cl led to the formation of possibly the most Cl-rich mica so far described classified as chloroferrokinoshitalite (XFe0.88, Ba0.95K0.03Fe2.68Mg0.37Al1.91Si2.01Cl1.98) with 10.98 wt% Cl. Substantial substitution of OH– by Cl– and O2– in mica, and the presence of Cl-apatite, a rare Cl-rich phosphate goryainovite, and carbonates together with Cl-rich micas indicate high-Cl and -CO2 activity and low-H2O activity in metasomatizing fluids or melts that may be classified as Ba-Cl-rich silicocarbonatitic. The coexistence of two micas with distinct compositions close to chloroferrokinoshitalite (XFe0.57–0.77, K~0.1Ba0.6–0.8Mg0.7–1.3Fe1.7–2.3Ti0.0–0.1 Si2.2–2.3Al1.5–1.7Cl1.2–1.8) and oxykinoshitalite (XFe0.19–0.20, K~0.3Ba~0.5Mg2.0–2.1Fe~0.5Ti0.2–0.4Si2.4–2.6Al~1.8Cl~0.3) suggests that a miscibility gap exists between these two compositions. The exotic mineral assemblage was formed by interaction with metasomatizing fluids or melts whose origin cannot be defined with certainty. They may be derived from crustal or mantle lithologies or from the host garnet pyroxenites. The textural position of the MSI in garnet and their characteristic mineral assemblages indicate that they have been introduced into the garnet crystals under post-peak conditions, possibly during decompression. With this research we document substitution mechanisms in Ba-, Ti-, and Cl-rich micas and shed light on the behavior and composition of fluids or melts at the upper mantle/lower crust interface.
... For minerals containing significant concentrations of these elements, both these halogens are considered as 1/2 oxygen (2F = 1O; 2Cl = 1O) and should be corrected before adding to the total concentrations. Fluorine and chlorine occur on anionic positions in structures of silicates, and the oxygen equivalent of F and Cl atoms must be subtracted from the total of analyses (Li et al., 2015;Siron et al., 2018). ...
The migmatites of Chandrapur area near Guwahati, which forms a part of the Assam-Meghalaya Gneissic Complex (AMGC) in Northeast India, have preserved magnetite ocelli. Petrographic observations have revealed that magnetite crystals occurring in the ocellar rock are surrounded by partial to complete thin rims of biotite. Moreover, mineral chemical analyses, backscattered electron images, and quantitative elemental mapping have revealed that magnetite crystals occurring in the ocellar rock have been extensively replaced by manganoan ilmenite at the periphery. From field observations, textural, and mineral chemical analyses, it has been inferred that invasion of hot, volatile-bearing pegmatitic magmas into thermally rejuvenated basement gneisses of the AMGC during the late Pan-African tectonothermal episode (ca. 530–450 Ma) induced very restricted partial melting in the gneissic rocks producing neosomes. The newly formed neosomes were then infiltrated by the nearby pegmatitic melts leading to mixing between the two melts. The mixing event facilitated transformation of magnetite to manganoan ilmenite owing to diffusion of elements like Mn and Ti from the neosome to the pegmatitic domains. Moreover, formation of ilmenite released surplus Fe hosted in the magnetite that combined with in situ K and Al, and diffusing Mg from the neosome to form biotite crystals around magnetite. From this study, it can be concluded that magnetite-manganoan ilmenite transformation may be considered as one of the petrogenetic indicators to decipher magma mixing events.
... The positive correlation of XFe with Cl has also been documented from other mica occurrences as well as from amphiboles. This is mainly ascribed to the effect that incorporation of Cl is stimulated by the enlargement of the anion site of both minerals with increasing amounts of Fe 2+ and Al, (and K in the case of amphibole), resulting in a positive correlation of Cl with these elements (Henry and Daigle 2018;Oberti et al. 1993;Siron et al. 2018). ...
Ba-, Ti-, and Cl-rich micas associated with other Ba- and/or Cl-rich minerals in the rock matrix or in garnet and clinopyroxene hosted multiphase solid inclusions (MSIs) are observed in mantle-derived garnet pyroxenites. The micas show extremely high variation in chemical composition ranging between Ba-rich phlogopite, chloroferrokinoshitalite, and oxykinoshitalite. Elemental covariation trends in mineral chemical data revealed the principal substitution mechanisms responsible for the observed chemical variability. Ba2+Al3+↔K1+Si4+ associated with either OH-1↔Cl-1 or Ti4+2O2-↔Mg2+2OH-1 substitutions links phlogopite to chloroferrokinoshitalite and oxykinoshitalite, respectively, whereas the Ti4+2O2-↔Fe2+2C-1 substitution links chloroferrokinoshitalite to oxykinoshitalite. The preferred incorporation of Cl in Fe-rich mica and of Ti+O in Mg-rich mica indicates that XFe exerts an important control on mica composition. The positive correlation of XFe with Cl led to the formation of possibly the most Cl-rich mica so far described classified as chloroferrokinoshitalite with 10.98 wt% Cl. Substantial substitution of OH- by Cl- and O2- in mica, and the presence of Cl-apatite, a rare Cl-rich phosphate goryainovite, and carbonates together with Cl-rich micas indicate high Cl and CO2 activity and low H2O activity in metasomatizing fluids or melts that may be classified as Cl-rich silicocarbonatitic. The coexistence of two micas with distinct compositions close to chloroferrokinoshitalite and oxykinoshitalite suggests that a miscibility gap exists between these two compositions. The exotic mineral assemblage was formed by interaction with metasomatizing fluids or melts whose origin cannot be defined with certainty. They may be derived from crustal or mantle lithologies or from the host garnet pyroxenites. The textural position of the MSIs in garnet and their characteristic mineral assemblages indicate that they have been introduced into the garnet crystals under post-peak conditions, possibly during decompression. This research extends information on substitution mechanisms in Ba-, Ti-, and Cl-rich micas and also contributes to knowledge of the behavior and composition of fluids or melts at the upper mantle/lower crust interface.
... In general, the high concentrations of volatile elements are thought to play a key role in modifying melt structure and determining the physico-chemical properties of silicate melt (London, 1987), and the effects of these elements are thought to be additive (Dingwell, 1988;Linnen, 1998). Micas are one of the most common volatile-bearing minerals in rare-metal granites and their major and trace element compositions are useful petrogenetic indicators for magmatic and magmatic-hydrothermal processes (e.g., Munoz, 1984;Loferski and Ayuso, 1995;Roda et al., 2007;Siahcheshm et al., 2012;Rasmussen and Mortensen, 2013;Legros et al., 2018;Siron et al., 2018;Villaros and Pichavant, 2019). The mica geochemistry has been long used to track the consolidation history of rare-metal granites and pegmatites (e.g., Stepanov et al., 2014;Legros et al., 2018;Xie et al., 2018;Zhu et al., 2018;Wang et al., 2018). ...
The Lingshan granite complex, located in the northeast part of the Qin-Hang Belt, Jiangxi Province, southeast China, has produced the largest Ta-Nb deposits in China, i.e., the Huangshan Nb-Ta and Songshugang Ta-Nb-(W-Sn) deposits. This complex is spatially a ring-like multiphase batholith, and comprises a series of granitic rocks from the central to marginal units. Mafic microgranular enclaves (MMEs) occur only in the central unit. Geochronologic results show that the magmatic activities at Lingshan district took place at Early Cretaceous, with similar emplacement ages (133-134 Ma) between the central and marginal units. The Lingshan granites are metaluminous to weakly peraluminous (most A/CNK <1.1), and display typical A-type affinity. However, the whole-rock REE contents and Zr/Hf and (La/Yb)N ratios exhibit an abnormal increase from the central to marginal units. The Li, Rb and Cs elements of biotite as well as the Zr/Hf ratios of zircon do not exhibit a continuous enrichment trend from the central towards marginal granites. All these features suggest that the generation of Lingshan granites is more complex than the fractional crystallization from a single parental magma. Alternatively, a crystal mush model could be applied to better interpret the geochemical and isotopic contrasts between different granite phases. The marginal granites were likely formed by later crystallization events after the rejuvenation and extraction from the crystal mush reservoirs, while the central granites were formed from the residual crystal-rich mush with a relatively low terminal porosity. Moreover, the Nb and Ta contents of biotite exhibit a gradual enrichment with increasing fluorine contents in the marginal granites. The enrichment of lithium and fluorine in the granitic melt can significantly promote the proportion of non-bridging oxygens (NBOs) for coordinating Nb and Ta in the melt, further enhancing the solubility of columbite and tantalite, and eventually causing the Nb and Ta elements either incorporate into the mineral lattice of micas in the early marginal granites or form columbite-group minerals in the pegmatite/altered granite. In summary, we propose that the Ta-Nb enrichment event in the Lingshan district is spatially and genetically related to the extensive fractionation evolution from central to marginal units.
... Presence of halogens may increase the stability of biotite and amphibole, enhance the transport of metallic elements via complexation (Seward et al. 2014) and alter the pH of subduction fluids (Galvez et al. 2016). In this context, understanding and quantifying the incorporation of halogens in minerals is paramount to modelling their geochemical cycle (see Siron et al. 2018; Barnes et al. 2019) and its effects. Halogens are commonly major components of accessory phases such as apatite and tourmaline, and they are hosted in minor to trace amounts in hydrous silicates such as micas, amphibole and serpentine (e.g. ...
... Correlations and anticorrelations between major elements with F and Cl have been found in several studies, leading to the hypothesis of Fe-F avoidance and Mg-Cl avoidance (e.g. Chevychelov et al. 2008;Aranovich and Safonov 2018), although some authors obtained discordant results (Siron et al. 2018;Heckel et al. 2020), possibly due to threshold effects in halogen concentration (Daigle 2015). The low abundance of halogens in silicates makes in situ analysis difficult as detection limits are often too high (Hammerli and Rubenach 2018). ...
Understanding how halogens are distributed among usual hydrous silicates in the lithosphere is important to constrain their deep geochemical cycle and fluid-rock interactions in subduction zones. This article presents first-principles modelling of halogen (F−, Cl−, Br−) incorporation in hydrous silicates including mica, chlorite, serpentine, amphibole, epidote and carpholite. The approach allows studying the impact of crystal chemistry on halogen partitioning by quantification of the energetic cost of halogen incorporation in minerals. Calculations are carried out in large systems where halogens are in minor to trace concentrations. Estimations show that F-bearing defects must be separated at least 9 Å from one another to reproduce trace element behaviour, this value increasing to at least 10 Å for Cl and Br. Results highlight the competition between the effects of electrostatic interactions and steric hindrance for incorporation of halogens, where steric hindrance has greater importance for heavy halogens, in particular for Br. Interaction with alkalis is a major control for F incorporation, especially in mica. Other parameters such as octahedral site occupancy, Si/Al ratio of tetrahedral sites and the nature of alkalis in amphibole and mica (K or Na) appear to play subordinate roles. Partition coefficients have been estimated in mineral assemblages in an effort to be representative of subduction zone metamorphism. Results show that pargasite, biotite and lizardite are favoured hosts for all three halogens, followed by clinochlore, tremolite and carpholite. The energetic cost of incorporating halogens into dioctahedral phyllosilicates and epidote is comparatively higher, and partitioning is predicted as unfavourable to these minerals. Fractionation between halogens in subduction zones is predicted by the evolution of mineral assemblages and partition coefficients, a consequence of the influence of crystal chemistry over halogen incorporation in hydrous silicates.
... It is noteworthy that the calculated OH − content of the phengites from metagranite samples are considerably more variable than in the whiteschists, opening up the possibility that an actual fluid phase might have been absent in metagranites, and suggests that the H 2 O content in phengite is indeed dependant on the H 2 O activity on the grain boundaries. Such thin-section scale disequilibrium has previously been observed for OH − content in biotite in a contact aureole environment with similar temperatures 31 . Hence, the increase of Si in phengite from the farthest to the closest metagranite from the whiteschist represents an increase in pressure and not an artefact due to H 2 O activity variations. ...
Pressure-temperature-time paths obtained from minerals in metamorphic rocks allow the reconstruction of the geodynamic evolution of mountain ranges under the assumption that rock pressure is lithostatic. This lithostatic pressure paradigm enables converting the metamorphic pressure directly into the rock’s burial depth and, hence, quantifying the rock’s burial and exhumation history. In the coherent Monte Rosa tectonic unit, Western Alps, considerably different metamorphic pressures are determined in adjacent rocks. Here we show with field and microstructural observations, phase petrology and geochemistry that these pressure differences cannot be explained by tectonic mixing, retrogression of high-pressure minerals, or lack of equilibration of mineral assemblages. We propose that the determined pressure difference of 0.8 ± 0.3 GPa is due to deviation from lithostatic pressure. We show with two analytical solutions for compression- and reaction-induced stress in mechanically heterogeneous rock that such pressure differences are mechanically feasible, supporting our interpretation of significant outcrop-scale pressure gradients.
... H 2 O and halogen contents of the hydroxyl group of micas are controlled by fluid-rock interactions (Siron et al. 2018a) and serve as tracers to identify fluid composition. Fluorine mass fractions can be measured by electron probe microanalysis (EPMA) in white mica, while the chlorine mass fractions are often too low to be measured quantitatively. ...
Five new natural white mica reference materials (RMs) were developed for in situ H2O content analyses by secondary ion mass spectrometry at the SwissSIMS laboratory of Lausanne University, Switzerland. The white mica reference materials cover a large part of the natural muscovite–phengite compositional range and are therefore suitable as reference materials for analysis of natural rocks as well as individual minerals. The independent H2O content of the reference materials UNIL_WM1 to UNIL_WM5 was obtained by thermal conversion elemental analyser and corresponds to 4.35 ± 0.02, 4.33 ± 0.03, 4.30 ± 0.07, 4.50 ± 0.02, 4.42 ± 0.11 (% m/m, ± 1s) respectively. SIMS determinations of H2O content revealed a matrix effect correlated to the FeO content of white mica. The compositional range in FeO of the reference materials that were calibrated for H2O determination is from 1.13 to 3.67% m/m. No crystallographic orientation dependency was observed at the level of homogeneity of these reference materials. An analytical precision of 0.02 to 0.08% m/m (1SE) is expected for the final uncertainty on measurements of unknown white micas in natural samples. This article is protected by copyright. All rights reserved.
... The surficial behavior of halogens in the sedimentary cycle has been widely investigated, particularly for the Cl and Br geochemistry of sea water as well as formation and diagenetic fluids (e.g., Kendrick, 2018;Worden, 2018aWorden, , 2018b. Several recent studies focused on the halogen transfer from the mantle to the crust and atmosphere by subduction related magmatic processes and metamorphism (e.g., Teiber et al., 2014;Page et al., 2016;Bénard et al., 2017;Fink and Thomkins, 2017;Li and Hermann, 2017;Urann et al., 2017;Abersteiner et al., 2018;Hammerli and Rubenach, 2018;Hanley and Koga, 2018;Siron et al., 2018). In this study we investigated the geochemical behavior of F and Cl during the weathering-diagenesis-metamorphism-anatexis cycle using the Amazon River mouth clays and metapelititic rocks from the Seridó Belt, Brazil. ...
... This suggests an anhydrous condition for the Acari pluton with its metamorphic aureole (Cunha de Souza, 1996) being generated only by devolatilization of the Seridó schist (Sallet et al., 2015). Similarly, at the contact with the Adamello pluton, the halogen content of metamorphic biotite is invariable and does not follow zonation of the contact metamorphism aureole (Siron et al., 2018). In contrast, the F and Cl contents of the regional Seridó schist biotite that is in contact with small Cambrian pegmatitic granite intrusions is enriched by infiltration of exsolved fluids. ...
The average contents F = 788 ppm and Cl < 45 ppm of the clay fraction from Amazon River mouth sediments represent the composition of a large portion of weathered South American continental crust prior to any contact with high salinity formation waters. The data show a large fractionation between F and Cl with the latter being strongly leached during weathering along with Na and Ca. The one-order-of-magnitude fractionation is comparable to Cl fractionation from degassing during early Earth accretion. In contrast, F is strongly retained in the hydroxyl sites of neoformed clay minerals, mainly illite, as well as in detrital apatite and muscovite. The high average ratio F/Cl >10 of the clay fraction from Amazon River sediments are comparable only to Cl-poor MORB and evolved granite and rhyolite. The high content Cl = 180 ppm and low F/Cl = 3 of average shale is due to interaction of the sediments with high salinity diagenetic fluids. The Seridó schist belt is used to assess the behavior of F and Cl during shale metamorphism and anatexis. Its ratio F/Cl >10 ppm is high compared to average shale and is related to the strong partitioning of Cl into metamorphic fluids. The obtained data suggest that the Cl partitioned into shales during diageneses is released back into fluids produced during metamorphic conditions. The experimental data indicate that anatexis of mica schist significantly fractionates F/Cl ratios producing schist residue with F/Cl > 38 and peraluminous melts with F/Cl = 6.
The metamorphic rocks from the Torres del Paine contact aureole (Patagonia, Chile) show field, petrographic, and geochemical evidences for small amounts of igneous fluid infiltration due to the solidification of the granite complex. Hydrogen isotope ratios (D/H) in the contact aureole first decrease while approaching the intrusion and subsequently increase toward the granite contact. Initial decrease with metamorphic grade is due to preferential loss of the 2H isotopes due to Rayleigh fractionation during prograde dehydration reactions. The infiltration of high δD fluids from the intrusion increases δD within the last 150 m. In contrast, (18O/16O) ratios show no systematic changes, indicating that neither oxygen loss by Rayleigh fractionation, nor oxygen exchange by fluid infiltration was significant enough to dominate original variations seen in the oxygen isotope ratio of the protolith. Calculated volume of fluid using the position of the hydrogen isotope exchange front gives a relatively low time integrated fluid flux of about 4m3/m2 at the contact for the igneous fluid. These small amounts of fluid flux are in agreement with whole rock oxygen isotope data that are not affected in the contact aureole. Chlorine content of metamorphic biotite crystals, in contrast to oxygen isotopes, supports infiltration of igneous fluids. Indeed, relatively high Cl concentrations in biotite were measured in some samples close to the intrusion (up to 0.2 wt%), while chlorine concentrations in biotite are constant everywhere else in the entire contact aureole, having low concentrations (0.01-0.06 wt%). The absence of a well-marked Rayleigh fractionation trend in Cl concentrations with increasing metamorphism is surprising, since chlorine strongly fractionates into the fluid. This is best explained by slow diffusive exchange of chlorine in biotite in the cooler outer aureole. Hence recrystallization of biotite would be required to modify its Cl composition. Biotite grains from samples close to the intrusion with high Cl content also have lower Ti content (0.4 p.f.u.) than biotite (0.5 p.f.u.) from other samples containing biotite with lower Cl content located at the same distance from the contact. Since Ti content in biotite is a function of temperature, this is a good indication that magmatic fluid infiltration started post peak, early during cooling of the metamorphic rocks. Episodes of fluid-flow seemed to be nearly continuous during cooling as evidenced by numerous retrogression textures such as secondary muscovite (above 470 °C) or chlorite + muscovite intergrowth after cordierite or biotite (slightly below 470 °C). This might be related to crystallization of subsequent batches of granites, or the onset of minor fluid convection during cooling of the aureole. Nevertheless, only minor secondary muscovite has been found and fresh cordierite is present throughout the aureole confirming small amounts of fluid infiltration.
The time integrated fluid flux computed from the hydrogen isotope exchange front is two orders of magnitude lower than values computed for metacarbonate in many other contact aureoles, suggesting low permeability of pelitic rocks. In conclusion, Cl content and hydrogen isotope composition of hydrous minerals provide a sensitive tool to identify small fluid-rock interaction events, much more sensitive than oxygen isotope compositions of the whole rock or minerals.
