Explore the vast range of titles available.

Elastic geobarometry for host-inclusion systems can provide new constraints to assess the pressure and temperature conditions attained during metamorphism. Current experimental approaches and theory are developed only for crystals immersed in a hydrostatic stress field, whereas inclusions experience deviatoric stress. We have developed a method to determine the strains in quartz inclusions from Raman spectroscopy using the concept of the phonon-mode Gruneisen tensor. We used ab initio Hartree-Fock/Density Functional Theory to calculate the wavenumbers of the Raman-active modes as a function of different strain conditions. Least-squares fits of the phonon-wavenumber shifts against strains have been used to obtain the components of the mode Gruneisen tensor of quartz (γ1m and γ3m) that can be used to calculate the strains in inclusions directly from the measured Raman shifts. The concept is demonstrated with the example of a natural quartz inclusion in eclogitic garnet from Mir kimberlite and has been validated against direct X-ray diffraction measurement of the strains in the same inclusion.

Raman spectroscopy provides information on the residual strain state of host-inclusion systems that, coupled with the elastic geobarometry theory, can be used to retrieve the P-T conditions of inclusion entrapment. In situ Raman measurements of zircon and coesite inclusions in garnet from the ultrahigh-pressure Dora Maira Massif show that rounded inclusions exhibit constant Raman shifts throughout their entire volume. In contrast, we demonstrate that Raman shifts can vary from the center to the edges and corners of faceted inclusions. Step-by-step polishing of the garnet host shows that the strain in both rounded and prismatic inclusions is gradually released as the inclusion approaches the free surface of the host. More importantly, our experimental results coupled with selected numerical simulations demonstrate that the magnitude and the rate of the strain release also depend on the contrast in elastic properties between the host and the inclusion and on the inclusion crystallographic orientation with respect to the external surface. These results allowed us to give new methodological guidelines for determining the residual strain in host inclusion systems.

There has been much research undertaken on structural OH- in various nominally anhydrous minerals including the common silicate garnets (i.e., X3Y2Si3O12, where X = Mg, Fe2+, Mn2+, and Ca and Y = Al, Fe3+, and Cr3+). However, it is still largely not understood where small concentrations of H atoms are incorporated in the garnet crystal structure. In this work, the IR single-crystal spectra of end-member or approaching end-member composition andradite, pyrope, and almandine are measured. Both a natural and synthetic andradite sample show a broad, asymmetric OH--stretching mode at 3563 cm-1 that splits into two narrower modes at lower temperatures. They are located at 3575 and 3557 cm-1 at 80 K with the higher wavenumber mode being considerably more intense compared to that at lower energy. These results are analyzed together with published IR spectra of synthetic end-member katoite, pyrope, and almandine also recorded at low temperature. These garnets show similar IR behavior with a broad OH- band at room temperature that splits into two narrower bands at lower temperatures and with a similar intensity relationship as shown by andradite. This behavior is indicative of the hydrogarnet substitution. The measured IR spectra of natural almandine- and pyrope-rich (Dora Maira, Italy) crystals, on the other hand, show different spectroscopic features with several OH- modes that are not consistent with the hydrogarnet mechanism. An analysis of the energy of the OH--stretching mode is made for various composition hydrogarnet clusters [i.e., X3Y2(O4H4)3, where X = Mg, Fe2+, Mn2+ and Ca and Y = Al and Fe3+] in terms of crystal-chemical properties and local atomic configurations. The OH- mode energy, which lies roughly between 3660 and 3550 cm-1 at RT for various end-member garnets, is a function of the mass of the X- and Y-cations due to mode coupling and/or mixing. In addition, the strength of the chemical bonding between the X- and Y-cations and the O2- anion of the OH- dipole plays a role in affecting the wavenumber of the OH--stretching vibration. OH- mode broadening, observed in the spectra of end-member garnets, is primarily a result of thermal anharmonic disorder, especially with regard to the light H cation. OH mode broadening in intermediate solid-solution composition garnets is a function of both thermal effects and variations in local cation configurations around the OH- dipole(s). Published IR spectra of certain high-pressure pyrope-rich garnets, both synthetic and natural, are analyzed and arguments made that OH- can often be incorporated as the hydrogarnet or hydropyrope substitution. IR spectra similar in appearance, having multiple relatively narrow OH- modes that are distinct from those indicating the hydrogarnet substitution, can be observed for certain synthetic end-member and various composition natural pyropes from Dora Maira and some natural spessartines. This indicates that other common OH- substitution mechanisms, which have yet to be determined, can also occur in different silicate garnets.

The enrichment of manganese in peraluminous (S-type) granitic melts beginning with the anatexis of metapelitic rock and ending with the crystallization of highly evolved pegmatites is explained using experimentally derived mineral-melt partition coefficients and solubility data for Mn-rich garnet. Mineral-melt partition coefficients for Fe, Mg, and Mn between garnet, cordierite, tourmaline, and peraluminous, B-bearing hydrous granitic melt were measured between 650 and 850°C at 200 MPaH2O. The compositions of garnet and tourmaline synthesized in these experiments are similar to those found in nature. Garnets evolve from Sps51Alm23Prp25 to Sps81Alm15Prp4 with decreasing temperature. The Mn content of cordierite increases with decreasing temperature. The composition of tourmaline does not vary with temperature. Partition coefficients, DMα/L, and exchange coefficients, KDα/L=DMα/L/DNα/L where α is a mineral, L is liquid (melt), and M and N are different elements, are presented for mineral-glass pairs. Partition coefficients for Mg, Fe, and Mn increase with decreasing temperature for garnet, tourmaline, and cordierite. The precipitation of garnet alone results in a progressive increase of MgO/FeO and a decrease of MnO/FeO in the melt. Crystallization of cordierite and tourmaline results in a decrease of MgO/FeO and an increase of MnO/FeO in melt. Tourmaline is most efficient at concentrating Mn in residual liquids. The trend toward increasing Mn/Fe in natural garnets in granites and pegmatites is not controlled by garnet itself, but instead by the crystallization of other mafic minerals in which Mg and Fe are more compatible than is Mn. A Rayleigh fractionation model constitutes a test of the partition coefficients reported in this manuscript. The starting composition for the model is that of a liquid (melt inclusions) from an anatectic S-type source. Normative modes of cordierite and biotite are calculated from that composition and are similar to modes of these minerals in natural occurrences. The model consists of crystallization of a cordierite-biotite granite from 850 to 650°C. The model predicts that ∼95% crystallization of the starting composition is required to reach saturation in spessartine-rich garnet at near-solidus temperatures. The model, therefore, is consistent with the occurrence of spessartine as restricted to highly fractionated granite-pegmatite systems at the end stages of magmatism.

The grossular-andradite solid solutions in garnet from skarn deposits in relation to hydrothermal processes and physicochemical conditions of ore formation remain controversial. Here we investigate garnet occurring in association with calcic and magnesian skarn rocks in the Cuihongshan polymetallic skarn deposit of NE China. The calcic skarn rocks contain three types of garnets. (1) Prograde type I Al-rich anisotropic garnets display polysynthetic twinning and a compositional range of Grs18-80Adr10-75. This type of garnet shows markedly low rare earth element (REE) contents (3.27-78.26 ppm) and is strongly depleted in light rare earth elements (LREE, 0.57-44.65 ppm) relative to heavy rare earth elements (HREE, 2.31-59.19 ppm). They also display a significantly negative Eu anomaly (Eu/Eu* of 0.03-0.90). (2) Fe-rich retrograde type II garnets are anisotropic with oscillatory zoning and own wide compositional variations (Grs1-47Adr30-95) with flat REE (13.73-377.08 ppm) patterns. (3) Fe-rich retrograde type III isotropic garnets display oscillatory zoning and morphological transition from planar dodecahedral {110} crystal faces to {211} crystal faces in the margin. Types III garnets exhibit relatively narrow compositional variations of Grs0.1-12Adr85-97 with LREE-enrichment (0.80-51.87 ppm), flat HREE patterns (0.15-2.46 ppm) and strong positive Eu anomalies (Eu/Eu* of 0.93-27.07 with almost all >1). The magnesian skarn rocks contain euhedral isotropic type IV Mn-rich garnet veins with a composition of Grs10-23Sps48-62Alm14-29. All calcic garnets contain considerable Sn and W contents. Type II garnet containing intermediate compositions of andradite and grossular shows the highest Sn contents (64.36-2778.92 ppm), albeit the lowest W range (1.11-468.44 ppm). Birefringence of garnet is probably caused by strain from lattice mismatch at a twinning boundary or ion substitution near intermediate compositions of grossular-andradite. The fine-scale, sharp, and straight garnet zones are probably caused by self-organization, but the compositional variations of zones from core to rim are probably caused by external factors. The zoning is likely driven by external factors such as composition of the hydrothermal fluid. REE concentrations are probably influenced by the relative proportion and temperature of the system. Moreover, the LREE-HREE fractionation of garnet can be attributed to relative compositions of grossular-andradite system. The W and Sn concentrations in garnet can be used as indicators for the exploration of W-Sn skarn deposits.

Subcalcic Cr-rich pyrope is a typical inclusion in natural diamond and considered as the main indicator mineral in diamond exploration. This article presents the results of experiments using a model garnet-spinel-harzburgite with the objective of determining the maximum Cr content of the garnet. This study supplements the existing CaO vs Cr2O3 diagram with new experimentally obtained data on Cr-rich garnets. A high-pressure apparatus (BARS) was used to conduct experiments at a pressure of 5.5 GPa and temperature of 1300°C, which corresponds to the stability field of both garnet and diamond. A model harzburgite was obtained using natural Mg-serpentine, which decomposes at the pressures and temperatures of this experiment into olivine and orthopyroxene. Natural chromite and carbonatite were used as the sources of Cr and Ca, respectively. The samples formed are composed of forsterite, enstatite, Cr-pyrope and Cr-spinel. The maximum Cr2O3 content, 16.23 wt.%, was detected in grains which grew in contact with chromite. The addition of 1-2 wt.% carbonatite resulted in the crystallisation of garnets with varying Ca contents (2.83-7.49 wt.% CaO). The experiments confirmed the boundary at 16 wt.% Cr2O3 for subcalcic pyrope associated with diamond. It is concluded that the origin of natural samples of Ca-rich lherzolitic/wherlitic and Ca-poor harzburgitic garnets with >16 wt.% Cr2O3 can be attributed to the specific Ca/Cr/Al ratios of the host medium.

We present U-Pb dates from peridotitic pyrope-rich garnet from four mantle xenoliths entrained in a kimberlite from Bultfontein, South Africa. Garnet dates magmatic emplacement due to the high mantle residence temperatures of the source material prior to eruption, which were most likely above the closure temperature for the pyrope U-Pb system. We determine a U-Pb date of 84.0 ± 8.1 Ma for the emplacement of the Bultfontein kimberlite from garnet in our four xenolith samples. The date reproduces previous dates obtained from other mineral-isotope systems (chiefly Rb-Sr in phlogopite). Garnet can be dated despite extremely low concentrations of U (median ∼0.05 µg/g), because concentrations of common Pb are often low or non-detectable. This means that sub-concordant garnets can be dated with moderate precision using very large laser-ablation spots (130 µm) measured by quadrupole inductively coupled plasma - mass spectrometry (LA-Q-ICP-MS). Our strategy demonstrates successful U-Pb dating of a U-poor mineral due to high initial ratios of U to common Pb in some grains, and the wide spread of isotopic compositions of grains on a concordia diagram. In addition, the analytical protocol is not complex and uses widely available analytical methods and strategies. This new methodology has some advantages and disadvantages for dating kimberlite emplacement versus established methods (U-based decay systems in perovskite and zircon, or Rb- or K-based systems in phlogopite). However, this method has unique promise for its potential application to detrital diamond prospecting and, more speculatively, to the dating of pyrope inclusions in diamond.

The thermo-elastic behavior of synthetic single crystals of grossular garnet (Ca3Al2Si3O12) has been studied in situ as a function of pressure and temperature separately. The same data collection protocol has been adopted to collect both the pressure-volume (P-V) and temperature-volume (T-V) data sets to make the measurements consistent with one another. The consistency between the two data sets allows simultaneous fitting to a single pressure-volume-temperature Equation of State (EoS), which was performed with a new fitting utility implemented in the latest version of the program EoSFit7c. The new utility performs fully weighted simultaneous fits of the P-V-T and P-K-T data using a thermal pressure EoS combined with any P-V EoS. Simultaneous refinement of our P-V-T data combined with that of KT as a function of T allowed us to produce a single P-V-T-KT equation of state with the following coefficients: V0=1664.46(5)°A3, KT0=166.57(17) GPa and K'=4.96(7)α(300K,1bar)=2.09(2)×10-5K-1 with a refined Einstein temperature (θE) of 512 K for a Holland-Powell-type thermal pressure model and a Tait third-order EoS. Additionally, thermodynamic properties of grossular have been calculated for the first time from crystal Helmholtz and Gibbs energies, including the contribution from phonons, using density functional theory within the framework of the quasi-harmonic approximation.

The single-crystal elastic moduli of minerals and related materials with cubic symmetry have been collected and evaluated. The compiled data set covers measurements made over an approximately 70 year period and consists of 206 compositions. More than 80% of the database is comprised of silicates, oxides, and halides, and approximately 90% of the entries correspond to one of six crystal structures (garnet, rocksalt, spinel, perovskite, sphalerite, and fluorite). Primary data recorded are the composition of each material, its crystal structure, density, and the three independent nonzero adiabatic elastic moduli (C11, C12, and C44). From these, a variety of additional elastic and acoustic properties are calculated and compiled, including polycrystalline aggregate elastic properties, sound velocities, and anisotropy factors. The database is used to evaluate trends in cubic mineral elasticity through consideration of normalized elastic moduli (Blackman diagrams) and the Cauchy pressure. The elastic anisotropy and auxetic behavior of these materials are also examined. Compilations of single-crystal elastic moduli provide a useful tool for investigation structure-property relationships of minerals.

Garnet is a common and important mineral in metamorphic systems, but the mechanisms by which it incorporates Ti-one of the major elements in the crust-are not well constrained. This study draws upon garnets synthesized at a range of temperatures and pressures to understand Ti solubility and the substitution mechanisms that govern its incorporation into garnet at eclogite and granulite facies conditions. Garnets from these synthesis experiments can incorporate up to several wt% TiO2 Comparison of Ti content with deficits in Al and Si in garnet indicates that Ti is incorporated by at least two substitution mechanisms (VITi4++VIM2+ ⇌ 2VIAl3+, and VITi4++IVAl3+ ⇌ VIAl3++IVSi4+). Increasing Ti solubility is correlated with increasing Ca and Fe/Mg ratios in garnet, clinopyroxene and melt. The complexity of the substitution mechanisms involved in Ti solubility in garnet makes practical Ti-in-garnet thermobarometry infeasible at present. However, a model fit to Ti partitioning between garnet and melt can be used to predict melt compositions in high-grade metamorphic systems. Additionally, the solubility and substitution mechanisms described here can help explain the presence of crystallographically aligned rutile needles in high-grade metamorphic systems.