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Tourmaline nodules occurring in the Capo Bianco (Elba Island, Italy) aplitic rocks are here investigated by X-ray microtomography 3D imaging. This non-invasive technique provides 3D images of the tourmaline nodules, revealing an irregular morphology consisting of branches that extend radially from the cores. The nodules present scale-invariant feat...
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... Distinctive textures of the tourmaline-quartz nodules have been widely reported in peraluminous granites worldwide (e.g., Rozendaal and Bruwer, 1995;Balen and Broska, 2011;Drivenes et al., 2015;Yang et al., 2015;Hong et al., 2017;Harlaux et al., 2020). The origin of these nodules remains controversial, and at least four models have been proposed to explain the origin of them, i.e., (i) metasomatic pelitic xenoliths (Le Fort, 1991); (ii) post-magmatic metasomatism by external boronrich fluid (Rozendaal and Bruwer, 1995); (iii) magmatic crystallization from a boron-rich granitic melt via diffusion-limited aggregation (Perugini and Poli, 2007;Valentini et al., 2015;Harlaux et al., 2020); and (iv) immiscible hydrous borosilicate melt during the late stages of crystallization (Sinclair and Richardson, 1992;Trumbull et al., 2008;Drivenes et al., 2015;Zhao et al., 2021a). Pelitic xenoliths or residual pelitic texture are not observed in this study and thus a metasomatic pelitic xenolith origin can be excluded. ...
Neoproterozoic tin mineralization in South China was closely related to the highly-evolved peraluminous granites in the southeastern and western margins of the Yangtze Block. The Baotan (23 Mt @ 0.43 % Sn) is a largest tin deposit in the Jiuwandashan–Yuanbaoshan tin district, northern Guangxi, and the orebodies are mainly hosted in the Lower Neoproterozoic Sibao Group metasedimentary rocks and Early Neoproterozoic mafic rocks intruding the Sibao Group as well as the apex of the Pinying granitic pluton. This deposit shows widely important tourmaline alteration. Based on petrographic observations, four generations and eight occurrences of tourmalines are identified in the Baotan tin deposit. The first generation is late-magmatic tourmalines and occurs as disseminated (Tur1a) and tourmaline–quartz nodule (Tur1b) in the Pingying biotite granite. The pre-ore hydrothermal tourmalines (Tur2) occur as tourmaline–quartz veinlet in the biotite granite and can be subdivided into both the early Tur2a (second generation) and the late Tur2b (third generation) according to microscopic observation and BSE image. The fourth generation is syn-ore hydrothermal tourmalines (Tur3), and they occur as disseminated in mafic- (Tur3a) and metasedimentary (Tur3c) host rocks, or as vein-type tourmaline–quartz–cassiterite–chlorite ores in mafic- (Tur3b) and metasedimentary (Tur3d) host rocks.
The different generations of tourmalines have both similarities and distinct differences in their compositions. Both the late-magmatic Tur1a and Tur1b show similar chemical compositions, with high Fe/(Fe+Mg) (0.73 ∼ 0.96, avg. 0.89) and Na/(Na+Ca) (0.87 ∼ 0.98, avg. 0.95) ratios, and Li, Zn, Ga, Nb and Ta contents, and belong to alkali group and schorl tourmaline. Based on mineral textural and compositional features and Mössbauer spectroscopic analyses, it can be speculated that the Tur1b in the tourmaline–quartz nodules and Tur1a are mainly sourced from a late-magmatic immiscible B–Na–Fe-rich hydrous melt in a reduced environment. The pre-ore Tur2a has similar SiO2, Al2O3, TiO2, Na2O, Li, Be, V, Sn and Zr contents, Fe/(Fe+Mg) and Na/(Na+Ca) ratios and total Al (apfu) with the late-magmatic Tur1a and Tur1b, indicating that it also originates from the Pingying granitic magma. The lower compatible elements (e.g., Fe, Mn, Co, Ni and Zn) and higher LILEs (e.g., Sr, Rb, Ba and Cs) in the pre-ore Tur2a and Tur2b suggest that these tourmalines precipitate from the early evolved magmatic hydrothermal fluids. The wide variations of Fe/(Fe+Mg) and Na/(Na+Ca) ranges, elevated Sr and V contents for the syn-ore Tu3a to Tur3d are attributed to the compositional differences of the host rocks and different degrees of interaction between the ore-forming fluids and the host rocks. The increasing Sr content from the late-magmatic Tur1 and pre-ore Tur2 to the syn-ore Tur3 suggests an involvement of the meteoric water. The Mössbauer spectral features of tourmalines from the late-magmatic Tur1 and syn-ore Tur3 confirm that they form under the different redox environments. The early fluids are relatively reduced magma-derived fluids. The addition of recycled meteoric water causes the fluids at the Tur3 stage to become relatively oxidized and also to be Sr-rich. The significant increase of Sn content from the late-magmatic Tur1 and pre-ore Tur2 to the syn-ore Tur3 also reflect a change from the early reduced fluid to the late oxidized fluid. The textural and compositional changes of Tur1 to Tur3 reflect the evolution of the ore-forming fluids. Fluid mixing between an acidic, reduced, Sn-rich magmatic hydrothermal fluid and cooler, oxidizing meteoric water and as well intensive water-rock reaction are considered as the main processes that cause extensive cassiterite precipitation.
... Texturally homogeneous nodules and disseminations of Tur 1 intergrown with the quartz-feldspar granitic groundmass (Figs. 3, 4) are typical of magmatic tourmaline in evolved peraluminous granites (e.g., London and Manning, 1995;London et al., 1996;Balen and Broska, 2011;Balen and Petrinec, 2011;Drivenes et al., 2015). The formation of tourmaline nodules has been widely discussed in the literature, and three main hypotheses have been proposed for their origin: (1) postmagmatic hydrothermal alteration of granitic bodies by externally derived boron-rich fluids (e.g., Rozendaal and Bruwer, 1995), (2) crystallization from immiscible, hydrous, boron-aluminosilicate melts or boron-rich aqueous fluids that separated from coexisting silicate melt (e.g., Dini et al., 2007;Balen and Broska, 2011;Drivenes et al., 2015;Burianek et al., 2016), and (3) products of magmatic crystallization on the liquid line of descent of boron-rich granitic melts (e.g., Perugini and Poli, 2007;Balen and Petrinec, 2011;Valentini et al., 2015). The Tur 1 nodules observed in the San Rafael granites are devoid of alteration features (i.e., absence of veins, dissolution-reprecipitation texture, and pervasive alteration), thus arguing against a hydrothermal origin related to postmagmatic alteration. ...
The world-class San Rafael tin (-copper) deposit (central Andean tin belt, southeast Peru) is an exceptionally large and rich (>1 million metric tons Sn; grades typically >2% Sn) cassiterite-bearing hydrothermal vein system hosted by a late Oligocene (ca. 24 Ma) peraluminous K-feldspar-megacrystic granitic complex and surrounding Ordovician shales affected by deformation and low-grade metamorphism. The mineralization consists of NW-trending, quartz-cassiterite-sulfide veins and fault-controlled breccia bodies (>1.4 km in vertical and horizontal extension). They show volumetrically important tourmaline alteration that principally formed prior to the main ore stage, similar to other granite-related Sn deposits worldwide. We present here a detailed textural and geochemical study of tourmaline, aiming to trace fluid evolution of the San Rafael magmatic-hydrothermal system that led to the deposition of tin mineralization. Based on previous works and new petrographic observations, three main generations of tourmaline of both magmatic and hydrothermal origin were distinguished and were analyzed in situ for their major, minor, and trace element composition by electron microprobe analyzer and laser ablation-inductively coupled plasma-mass spectrometry, as well as for their bulk Sr, Nd, and Pb isotope compositions by multicollector-inductively coupled plasma-mass spectrometry. A first late-magmatic tourmaline generation (Tur 1) occurs in peraluminous granitic rocks as nodules and disseminations, which do not show evidence of alteration. This early Tur 1 is texturally and compositionally homogeneous; it has a dravitic composition, with Fe/(Fe + Mg) = 0.36 to 0.52, close to the schorl-dravite limit, and relatively high contents (10s to 100s ppm) of Li, K, Mn, light rare earth elements, and Zn. The second generation (Tur 2)—the most important volumetrically—is pre-ore, high-temperature (>500°C), hydrothermal tourmaline occurring as phenocryst replacement (Tur 2a) and open-space fillings in veins and breccias (Tur 2b) and microbreccias (Tur 2c) emplaced in the host granites and shales. Pre-ore Tur 2 typically shows oscillatory zoning, possibly reflecting rapid changes in the hydrothermal system, and has a large compositional range that spans the schorl to dravite fields, with Fe/(Fe + Mg) = 0.02 to 0.83. Trace element contents of Tur 2 are similar to those of Tur 1. Compositional variations within Tur 2 may be explained by the different degree of interaction of the magmatic-hydrothermal fluid with the host rocks (granites and shales), in part because of the effect of replacement versus open-space filling. The third generation is syn-ore hydrothermal tourmaline (Tur 3). It forms microscopic veinlets and overgrowths, partly cutting previous tourmaline generations, and is locally intergrown with cassiterite, chlorite, quartz, and minor pyrrhotite and arsenopyrite from the main ore assemblage. Syn-ore Tur 3 has schorl-foititic compositions, with Fe/(Fe + Mg) = 0.48 to 0.94, that partly differ from those of late-magmatic Tur 1 and pre-ore hydrothermal Tur 2. Relative to Tur 1 and Tur 2, syn-ore Tur 3 has higher contents of Sr and heavy rare earth elements (10s to 100s ppm) and unusually high contents of Sn (up to >1,000 ppm). Existence of these three main tourmaline generations, each having specific textural and compositional characteristics, reflects a boron-rich protracted magmatic-hydrothermal system with repeated episodes of hydrofracturing and fluid-assisted reopening, generating veins and breccias. Most trace elements in the San Rafael tourmaline do not correlate with Fe/(Fe + Mg) ratios, suggesting that their incorporation was likely controlled by the melt/fluid composition and local fluid-rock interactions. The initial radiogenic Sr and Nd isotope compositions of the three aforementioned tourmaline generations (0.7160–0.7276 for 87Sr/86Sr(i) and 0.5119–0.5124 for 143Nd/144Nd(i)) mostly overlap those of the San Rafael granites (87Sr/86Sr(i) = 0.7131–0.7202 and 143Nd/144Nd(i) = 0.5121–0.5122) and support a dominantly magmatic origin of the hydrothermal fluids. These compositions also overlap the initial Nd isotope values of Bolivian tin porphyries. The initial Pb isotope compositions of tourmaline show larger variations, with 206Pb/204Pb(i), 207Pb/204Pb(i), and 208Pb/204Pb(i) ratios mostly falling in the range of 18.6 to 19.3, 15.6 to 16.0, and 38.6 to 39.7, respectively. These compositions partly overlap the initial Pb isotope values of the San Rafael granites (206Pb/204Pb(i) = 18.6–18.8, 207Pb/204Pb(i) = 15.6–15.7, and 208Pb/204Pb(i) = 38.9–39.0) and are also similar to those of other Oligocene to Miocene Sn-W ± Cu-Zn-Pb-Ag deposits in southeast Peru. Rare earth element patterns of tourmaline are characterized, from Tur 1 to Tur 3, by decreasing (Eu/Eu*)N ratios (from 20 to 2) that correlate with increasing Sn contents (from 10s to >1,000 ppm). These variations are interpreted to reflect evolution of the hydrothermal system from reducing toward relatively more oxidizing conditions, still in a low-sulfidation environment, as indicated by the pyrrhotite-arsenopyrite assemblage. The changing textural and compositional features of Tur 1 to Tur 3 reflect the evolution of the San Rafael magmatic-hydrothermal system and support the model of fluid mixing between reduced, Sn-rich magmatic fluids and cooler, oxidizing meteoric waters as the main process that caused cassiterite precipitation.
... Although X-ray computed microtomography (X-CT) is becoming increasingly valuable in the geosciences (Ketcham and Carlson, 2001;Mees et al., 2003;Ketcham, 2005;Carlson, 2006;Gualda et al., 2010;Polacci et al., 2010;Voltolini et al., 2011;Baker et al., 2012a, b;Iglauer et al., 2012;Blunt et al., 2013;Fusseis et al., 2014;Wiesmaier et al., 2015;Valentini et al., 2015;Arzilli et al., 2016;Morgavi et al., 2016;Paredes-Mariño et al., 2017), so far the application of this method to the study of accretionary lapilli is limited to a single study (Van Eaton et., 2012). In this study, X-CT scans were performed using a Skyscan 1172 (Bruker, Billerica, US) at the Geosciences Department (University of Padova, Italy) equipped with a W X-ray source. ...
The Secche di Lazzaro formation (ca. 6.2-7 kys BP) is a phreatomagmatic deposit
situated in the southwestern part of the island of Stromboli (Aeolian Archipelago, Italy). The volcanic sequence is comprised of three main units. In the lower unit accretionary lapilli are particularly abundant and are characterized by strong cementation between the particles and an uncommon resistance to breakage. To understand the processes behind the formation of the Secche di Lazzaro (SdL) accretionary lapilli a multi-analytical approach was used on the lapilli Aggregate Tuff (AT), and on single Accretionary Lapilli (AL). We carried out granulometric analysis, Field Emission – Scanning Electron Microscopy (FE-SEM), Electron Microprobe Analysis (EMPA), X-ray powder diffraction (XRPD) and 3D imaging by X-ray micro-tomography (X-mCT). The granulometric data shows that most particles in the AT have a diameter equal to phi -1 corresponding to 2 mm. The EMPA, FE-SEM and XRPD analyses reveal the presence of different mineral phases, mainly plagioclase, K-feldspar, halite, and clinopyroxene, together with volcanic glass. From the X-mCT analysis, we constrained the particle distribution and estimated the porosity of AL. The results of the FE-SEM images provided the chemical distribution within individual lapilli allowing the identification of rim and core zoning as well as the presence of halite (NaCl) located both on the border of single lapilli and on the juncture between different lapilli. Moreover, halite occurs among different aggregates in single AL, thus acting as a binding agent, as well as within rim pores. The results of this work shed new light into the formation of accretionary lapilli in phreatomagmatic eruption at volcanic island involving marine water.
... In the geosciences, one of the first reported application of computed X-ray tomography is the pioneering work by Carlson and Denison (1992) focused on porphyroblasts crystallization. Since then, X-μCT has become increasingly popular, in particular during the last decade, with many successful applications, including e.g., 3D microstructural characterization of garnet porphyroblasts (Huddlestone-Holmes and Ketcham 2005, 2010; George and Gaidies 2017), analysis of 3D distribution and shape of vesicles in volcanic rocks (Polacci et al. 2006Voltolini et al. 2011;Giachetti et al. 2011;Baker et al. 2012), evaluation of porosity in reservoir rocks (Van Geet et al. 2000;Blunt et al. 2013;Zambrano et al. 2017), study of crack formation mechanisms in sedimentary rocks (Zabler et al. 2008), microstructural analysis of ore-bearing rocks (Godel 2013), identification of mineral inclusions in diamonds (Nestola et al. 2012), and morphological analysis of mineral nodules (Valentini et al. 2015). ...
X‑ray computed microtomography (X-μCT) is applied here to investigate in a non-invasive way the three-dimensional (3D) spatial distribution of primary melt and fluid inclusions in garnets from the metapelitic enclaves of El Hoyazo and from the migmatites of Sierra Alpujata, Spain. Attention is focused on a particular case of inhomogeneous distribution of inclusions, characterized by inclusion-rich cores and almost inclusion-free rims (i.e., zonal arrangement), that has been previously investigated in detail only by means of 2D conventional methods. Different experimental X-μCT configurations, both synchrotron radiation- and X‑ray tube-based, are employed to explore the limits of the technique. The internal features of the samples are successfully imaged, with spatial resolution down to a few micrometers.
By means of dedicated image processing protocols, the lighter melt and fluid inclusions can be separated from the heavier host garnet and from other non-relevant features (e.g., other mineral phases or large voids). This allows evaluating the volumetric density of inclusions within spherical shells as a function of the radial distance from the center of the host garnets. The 3D spatial distribution of heavy mineral inclusions is investigated as well and compared with that of melt inclusions.
Data analysis reveals the occurrence of a clear peak of melt and fluid inclusions density, ranging approximately from 1⁄3 to 1⁄2 of the radial distance from the center of the distribution and a gradual decrease from the peak outward. Heavy mineral inclusions appear to be almost absent in the central portion of the garnets and more randomly arranged, showing no correlation with the distribution of melt and fluid inclusions. To reduce the effect of geometric artifacts arising from the non-spherical shape of the distribution, the inclusion density was calculated also along narrow prisms with different orientations, obtaining plots of pseudo-linear distributions. The results show that the core-rim transition is characterized by a rapid (but not step-like) decrease in inclusion density, occurring in a continuous mode. X‑ray tomographic data, combined with electron microprobe chemical profiles of selected elements, suggest that despite the inhomogeneous distribution of inclusions, the investigated garnets have grown in one single progressive episode in the presence of anatectic melt. The continuous drop of inclusion density suggests a similar decline in (radial) garnet growth, which is a natural consequence in the case of a constant reaction rate.
Our results confirm the advantages of high-resolution X-μCT compared to conventional destructive 2D observations for the analysis of the spatial distribution of micrometer-scale inclusions in minerals, owing to its non-invasive 3D capabilities. The same approach can be extended to the study of different microstructural features in samples from a wide variety of geological settings.
Keywords: Garnet, anatexis, melt inclusions, X‑ray computed microtomography, El Hoyazo; High-grade Metamorphism, Anatexis, and Granite Magmatism
... In the geosciences, one of the first reported application of computed X-ray tomography is the pioneering work by Carlson and Denison (1992) focused on porphyroblasts crystallization. Since then, X-μCT has become increasingly popular, in particular during the last decade, with many successful applications, including e.g., 3D microstructural characterization of garnet porphyroblasts (Huddlestone-Holmes and Ketcham 2005, 2010; George and Gaidies 2017), analysis of 3D distribution and shape of vesicles in volcanic rocks (Polacci et al. 2006Voltolini et al. 2011;Giachetti et al. 2011;Baker et al. 2012), evaluation of porosity in reservoir rocks (Van Geet et al. 2000;Blunt et al. 2013;Zambrano et al. 2017), study of crack formation mechanisms in sedimentary rocks (Zabler et al. 2008), microstructural analysis of ore-bearing rocks (Godel 2013), identification of mineral inclusions in diamonds (Nestola et al. 2012), and morphological analysis of mineral nodules (Valentini et al. 2015). ...
X-ray computed microtomography (X-μCT) is applied here to investigate in a non-invasive way the three-dimensional (3D) spatial distribution of primary melt and fluid inclusions in garnets from the metapelitic enclaves of El Hoyazo and from the migmatites of Sierra Alpujata, Spain. Attention is focused on a particular case of inhomogeneous distribution of inclusions, characterized by inclusion-rich cores and almost inclusion-free rims (i.e., zonal arrangement), that has been previously investigated in detail only by means of 2D conventional methods. Different experimental X-μCT configurations, both synchrotron radiation- and X-ray tube-based, are employed to explore the limits of the technique. The internal features of the samples are successfully imaged, with spatial resolution down to a few micrometers.
By means of dedicated image processing protocols, the lighter melt and fluid inclusions can be separated from the heavier host garnet and from other non-relevant features (e.g., other mineral phases or large voids). This allows evaluating the volumetric density of inclusions within spherical shells as a function of the radial distance from the center of the host garnets. The 3D spatial distribution of heavy mineral inclusions is investigated as well and compared with that of melt inclusions.
Data analysis reveals the occurrence of a clear peak of melt and fluid inclusions density, ranging approximately from ⅓ to ½ of the radial distance from the center of the distribution and a gradual decrease from the peak outward. Heavy mineral inclusions appear to be almost absent in the central portion of the garnets and more randomly arranged, showing no correlation with the distribution of melt and fluid inclusions. To reduce the effect of geometric artifacts arising from the non-spherical shape of the distribution, the inclusion density was calculated also along narrow prisms with different orientations, obtaining plots of pseudo-linear distributions. The results show that the core-rim transition is characterized by a rapid (but not step-like) decrease in inclusion density, occurring in a continuous mode. X-ray tomographic data, combined with electron microprobe chemical profiles of selected elements, suggest that despite the inhomogeneous distribution of inclusions, the investigated garnets have grown in one single progressive episode in the presence of anatectic melt. The continuous drop of inclusion density suggests a similar decline in (radial) garnet growth, which is a natural consequence in the case of a constant reaction rate.
Our results confirm the advantages of high-resolution X-μCT compared to conventional destructive 2D observations for the analysis of the spatial distribution of micrometer-scale inclusions in minerals, owing to its non-invasive 3D capabilities. The same approach can be extended to the study of different microstructural features in samples from a wide variety of geological settings.
