Inclusions of Amorphous and Crystalline SiO2 in Minerals from Itrongay (Madagascar) and Other Evidence for the Natural Occurrence of Hydrosilicate Fluids

Experimental studies increasingly often report low-temperature (200–800 °C) and low-pressure (0.05–3 kbar) hydrosilicate fluids with >40 wt.% of SiO2 and >10 wt.% of H2O. Compositionally similar fluids were long suggested to potentially exist in natural systems such as pegmatites and hydrothermal veins. However, they are rarely invoked in recent petrogenetic models, perhaps because of the scarcity of direct evidence for their natural occurrence. Here we review such evidence from previous works and add to this by documenting inclusions of hydrosilicate fluids in quartz and feldspar from Itrongay. The latter comprise opal-A, opal-CT, moganite and quartz inclusions that frequently contain H2O and have negative crystal shapes. They coexist with inclusions of CO2- and H2O-rich fluids and complex polycrystalline inclusions containing chlorides, sulphates, carbonates, arsenates, oxides, hydroxides and silicates, which we interpret as remnants of saline liquids. Collectively, previous studies and our new results indicate that hydrosilicate fluids may be common in the Earth’s crust, although their tendency to transform into quartz upon cooling and exhumation renders them difficult to recognise. These data warrant more comprehensive research into the nature of such hydrosilicate fluids and their distribution across a wide range of pressure and temperature conditions and geological systems.

Generation of highly silicic magmas at ultra-high temperature conditions : evidence from melt inclusions in peritectic garnet

<p>Metamorphism at ultra-high temperature (UHT) conditions (i.e., T >900°C and pressures from 7 to 13 kbar) is now recognized as a fundamental process of Earth’s crust, and although progress has been achieved on its understanding, constraining melt generation and fluid regime at such extreme conditions is still poorly explored.</p><p>In this study we use former melt inclusions found in peritectic garnet to investigate anatexis and fluid regime of metapelitic granulites in samples from the Rundvågshetta area, the thermal axis of the Lützow-Holm Complex (East Antarctica). Peak P-T estimates are 925-1039°C at 11.5-15 kbar. The studied rock is a coarse-grained heterogeneous metapelitic granulite with a predominant mafic residual domain and a relatively more felsic, melt-rich domain. The mineral association in the mafic domain typically contains orthopyroxene (Al<sub>2</sub>O<sub>3</sub>6-8.1 wt.%) + sillimanite + quartz + garnet (Prp<sub>42-55</sub>Alm<sub>40-52</sub>Grs<sub>3-4</sub>Sps<sub>0.2-1</sub>; X<sub>Mg</sub>0.5) + K-feldspar (Kfs) + cordierite (X<sub>Mg</sub>0.86) + rutile ± sapphirine ±biotite (X<sub>Mg</sub>0.75; TiO<sub>2</sub>3.7-5.8 wt.%) ±plagioclase (An<sub>35-46</sub>). Interstitial Kfs and quartz with low dihedral angles are often present, in particular as thin films between sillimanite and quartz; these features are interpreted as evidence for the presence of former melt along the grain boundaries. In contrast, the more felsic, melt-rich domain is composed of mesoperthite + quartz + garnet + sillimanite + brown biotite (X<sub>Mg</sub>0.7; TiO<sub>2</sub>3.7-5.4 wt.%) + rutile, but is free of orthopyroxene. Cores of garnet porphyroblasts (0.2-0.8 cm, Prp<sub>54-57</sub>Alm<sub>39-42</sub>Grs<sub>3-4</sub>Sps<sub>0.2-0.6</sub>, X<sub>Mg</sub>0.57) in the melt-rich domains contain clusters of primary glassy inclusions (GI) and crystallized melt inclusions (nanogranitoids; NI) together with multiphase fluid inclusions (MFI) and accessory phases (mainly rutile and apatite).</p><p>The GI (5-20 µm) have negative crystal shapes and contain shrinkage bubbles with or without CO<sub>2</sub>and N<sub>2</sub>. In some cases, GI may have trapped apatite and rutile. Micro-Raman investigation suggest that the H<sub>2</sub>O contents of these glasses range from 0 to 3.4 wt.%. Glasses are weakly peraluminous (ASI=1-1.1), have high SiO<sub>2</sub>(76-78 wt.%), very high K<sub>2</sub>O (6.5-10 wt.%) and extremely low CaO and FeO+MgO contents.</p><p>The NI have variable sizes (10-150 µm) and often contains intergrowth of plagioclase + quartz, K-feldspar (Kfs) and biotite (Bt). Less frequently NI may have euhedral to subhedral grains of Kfs and Bt. Trapped phases are apatite and rutile, except for one inclusion that contains the sapphirine + quartz pair indicating that melt inclusions were trapped at UHT conditions.</p><p>The MFI are composed of CO<sub>2</sub>(with densities from 0.23 to 0.93 g/cm<sup>3</sup>) and step-daughter magnesite, pyrophyllite. Methane, N<sub>2</sub>or H<sub>2</sub>O were not detected.</p><p>Our results show that anatexis of metapelites at extremely hot conditions occurred in the presence of COHfluids and generated highly silicic, weakly peraluminous, mildly to strongly potassic magmas with low H<sub>2</sub>O contents. Additional trace element data will be acquired to shed light on further geochemical fingerprints of these peculiar magmas.</p>

Again About the "Magmatic" Nature of Topaz Crystals From Chamber Pegmatites of Volyn (Ukrainian Shield)

Various aspects of the genesis of primary fluid inclusions (0.01-1.0 sometimes up to 2 mm) with a large number of mineral inclusions in topaz crystals from chamber pegmatites of Volyn were analyzed. The data could be interpreted in two fundamentally different ways. The first argues for crystals grown in a magmatic melt; the second for an aqueous solution, with a density close to critical. The essence of the discrepancy is the reliability of the identification of the nature of mineral phases in the primary inclusions, if they are crystals captured during growth (xenogenic) or daughter crystals from the fluid. The xenogenic origin of the phases is indicated by the following observations: 1) The location of the mineral inclusions on the growing faces of the topaz crystals depends on the orientation of the crystal’s axis [001] relative to the horizontal plane. It determines the faces on which small mineral phases could be deposited from an aqueous suspension during the growth of topaz crystals. The studied crystals are dominated by individuals in which the mineral inclusions are located on the growing faces {011}, {021}, (001) (and others) of the crystal head. During growth, they were approximately in an upright position. 2) The filling of primary fluid inclusions is not constant. The volume of mineral phases in the inclusions varies from 40 to 95%, often 70-75%, the rest of the volume is gas and aqueous solution. Liquid-gas (liquids ˂ 40%) inclusions without or with < 5% solid phases are very rare. In addition, the ratio between the volumes of different mineral phases in the inclusions is not constant. 3) Light rims (Becke lines) around the inclusions record a change in the refractive indices (caused by a different chemical composition) of topaz when inclusions are acquiring the equilibrium form of the negative crystal. 4) The xenogenic nature of the mineral phases of the primary fluid inclusions in topaz is indirectly confirmed by the value of the fluid pressure (260-300 MPa)of the magmatic melt (determined by the method of homogenization of these inclusions), as it denies the possibility of chamber pegmatite formation at depths of 9-11 km. Thus, the peculiar mineral inclusions were deposited on the face of growing topaz crystals of small mineral phases from a turbid aqueous suspension, which boiled violently. We conclude that topaz crystals in chamber pegmatites of Volyn grew in aqueous solution at a temperature of 380-415ºС and a pressure of 30-40 MPa.

Preliminary data for melt inclusions in garnet porphyroblasts from Ograzhden metapelites, SW Bulgaria

Micro-inclusions in garnet porphyroblasts from high-grade Ograzhden metapelites, SW Bulgaria, have been studied by SEM and micro-Raman Spectroscopy. Micro-inclusions are presented by single grains with facetted outlines parallel to rational crystallographic orientations of the host garnet or by multiphase aggregates with negative crystal shape. Many of studied micro-inclusions can be formed by the presence of melt. The morphology of some of them suggests formation under high pressure metamorphism.

Melt inclusions in olivines from phoscorites and olivinites of the Kovdor massif.

&lt;p&gt;The Kovdor massif is a part of the Paleozoic Kola alkaline province and located in the eastern part of the Baltic Shield. Kovdor carbonatites host a unique complex baddeleyite-apatite-magnetite deposit from which iron ores and zirconium have been mined. New data on melt inclusions in olivine crystals from phoscorites and olivinites of the ore complex are presented in this contribution. Daughter minerals in crystallized melt inclusions were identified by Raman spectroscopy and scanning electron microscopy. The trace element composition of inclusions was determined using LA-ICP-MS.&lt;/p&gt;&lt;p&gt;Melt inclusions in olivine from Kovdor phoscorites are negative crystal or round in shape, with sizes ranging from 5 to 50 microns. They form groups or line up. According to the mineral composition, two types of melt inclusions can be distinguished: carbonate and silicate-carbonate. In the first type, Ca-Na-Mg- (Sr?) - REE carbonates are dominant among daughter phases. In the second one, silicate phases (phlogopite, monticellite, diopside), Ca-Na-Mg carbonates and magnetite are found together. Melt inclusions in olivine from olivinites are isometric or elongated, 5&amp;#8211;25 &amp;#956;m in size. They form groups or occur as isolated inclusions. Benstoneite, geylussit, ankerite, calcite and hydroxyl-bastnesite along with phyllosilicates (phlogopite, paragonite?) were identified among daughter minerals.&lt;/p&gt;&lt;p&gt;The rare earth elements composition of melt inclusions from both types of rocks is characterized by the predominance of light REE. The content of REE, especially light ones, in inclusions from phoscorites is higher. Strontium and barium contents in most melt inclusions have negative correlations with niobium and zirconium concentrations.&lt;/p&gt;&lt;p&gt;Melt inclusions from phoscorites and olivinites contain carbonate and silicate mineral phases in various proportions, which may imply heterogeneous trapping of crystalline phases and two immiscible melts, silicate and carbonatite. Inclusions from phoscorite represent a more evolved magma with higher concentrations of rare metals.&lt;/p&gt;&lt;p&gt;This work was supported by the Russian Science Foundation, grant No 19-17-00013.&lt;/p&gt;

Quartz inclusions in garnet from high-temperature metamorphic rocks change their shape

&lt;p&gt;Trapped and sheltered inside other crystals, mineral inclusions preserve fundamental and otherwise lost information on the geological history of our planet. In the last decade, quartz inclusions in garnet have become a fundamental tool to estimate pressure and temperature of metamorphic rocks at the time of inclusion entrapment. In these approaches, as well as in all other applications, inclusions are regarded as immutable objects and the possibility of a change in their shape has never been considered.&lt;/p&gt;&lt;p&gt;With a detailed characterization of samples from greenschist and granulite facies, performed by optical and electron microscopy, EBSD, X-ray tomographic microscopy, laser Raman spectroscopy and FIB serial slicing, we show that after being trapped with irregular (&amp;#8220;scalloped&amp;#8221;) shape in low-temperature rocks, quartz inclusions in garnet from granulites formed at 750-900 &amp;#176;C and various pressures acquired a polyhedral &amp;#8220;negative crystal&amp;#8221; shape imposed by the host garnet, and almost exclusively defined by the facets of dodecahedron and icositetrahedron. A similar behaviour is also observed in biotite inclusions. The 3-fold and 4-fold morphological symmetry axes of the polyhedral negative crystals are parallel to corresponding crystallographic axes in the host garnet.&lt;/p&gt;&lt;p&gt;The systematic presence of a fluid film at the quartz-garnet boundary is not supported by Raman and FIB investigation.&lt;/p&gt;&lt;p&gt;Strengthened by microstructures indicating the process of &amp;#8220;necking down&amp;#8221; of polycrystalline quartz inclusions, our data support that - like in fluid inclusions changing shape to negative crystals - shape maturation of mineral inclusions occurs by temperature-assisted dissolution-precipitation via grain boundary diffusion. This process tends to minimize the surface free energy of the host-inclusion system by forming energetically favored facets and by decreasing the inclusion surface/volume and aspect ratios.&lt;/p&gt;&lt;p&gt;Optical investigation of numerous samples of worldwide provenance suggests that the negative crystal shape of quartz inclusions in garnet from granulites is a widespread microstructure that underpins a systematic phenomenon so far overlooked.&lt;/p&gt;

Melt inclusions in metamorphic rocks: how localized melting promoted the formation of the Gore Mountains mega garnets (Adirondacks, US)

&lt;p&gt;Melt inclusions have been for almost 150 years an exclusive feature of magmatic rocks. However, intensive research activity in the last decade has shown that melt inclusions, or nanogranitoids, are also a widespread feature of high grade metamorphic rocks. Such inclusions rapidly became fundamental tools to unravel partial melting and melt-related processes taking place during orogenesis.&lt;/p&gt;&lt;p&gt;One of the latest discoveries in this field has been the identification of nanogranitoids and glass inside the mega almandine-pyrope garnets of Barton Mine (Gore Mountain, NY State, US). These crystals are arguably the world&amp;#8217;s largest garnets and occur within garnet hornblendite. Their size is ca. 35 cm in average, while garnet diameters up to 1 m were reported in historical record. Fluid is often invoked in the formation of large crystals, but so far no study has identified clear witnesses for the presence of fluid during garnet formation, e.g. primary fluid inclusions.&lt;/p&gt;&lt;p&gt;Polycrystalline inclusions of primary nature were instead reported by Darling et al. (1997) to occur inside the garnet: such inclusions are the main target of our study. Their shape ranges from tubular (2-100 &amp;#181;m in length) to negative crystal shape (2-50 &amp;#181;m). They mainly contain cristobalite/quartz, kumdykolite and amphibole. Minor phases such as biotite/phlogopite, enstatite, rutile, ilmenite and a second, Ca-richer plagioclase (or its rare polymorphs dmisteinbergite and svyatoslavite) may be also present. The inclusions were re-homogenized to a silicate-rich glass via piston cylinder experiments at 1.0-1.5 GPa and 925-940&amp;#176;C. Experimental results prove that such inclusions are former droplets of melt, in agreement with the finding of preserved residual glass in one single inclusion before the experimental runs. The melt composition measured in situ via electron microprobe is tonalitic-trondhjemitic with 5-6 wt% H&lt;sub&gt;2&lt;/sub&gt;O.&lt;/p&gt;&lt;p&gt;The identification of melt inclusions points toward a melt rather than a fluid as the medium which favored extreme garnet growth under low nucleation rate conditions. The elements necessary to grow garnets &amp;#8211; mainly Fe, Al, Si, Mg- are indeed far more effectively transported by a silicate melt rather than simple aqueous fluid, at least at the limited depth envisioned for this process. In conclusion, the finding of melt inclusions in metamorphic rocks brought us forward along the path toward the solution of the enigma represented by the formation of these giant garnets.&lt;/p&gt;&lt;p&gt;References&lt;br&gt;Darling, R.S., Chou, I.M., Bodnar, R.J., 1997. An Occurrence of Metastable Cristobalite in High-Pressure Garnet Granulite. Science 276, 91.&lt;/p&gt;

THE SPIN-1 BLUME–CAPEL MODEL WITH ALTERNATINGLY CHANGING BILINEAR EXCHANGE INTERACTIONS BETWEEN SUBLATTICES

The spin-1 Blume–Capel model is studied on a Bethe lattice which is divided into two sublattices A and B. Alternatingly changing bilinear exchange interactions, JAB and JBA, between the sublattices, i.e., between the nearest-neighbor shell spins, are assumed. The phase diagrams of the model are studied on the (JAB, T) planes for given values of JBA, crystal fields D and the coordination numbers q = 3, 4 and 6. It was found that the model either displays only second-order phase transition lines at higher crystal field values or second- and first-order phase transitions lines combined at tricritical points at lower negative crystal fields. It was also found that the tricritical points move to higher temperatures and to higher values of JAB as the crystal field becomes more negative.