Natural Garnet Research Articles

Metasomatic interaction of the eutectic Na- and K-bearing carbonate melts with natural garnet lherzolite at 6 GPa and 1100–1200°C: Toward carbonatite melt composition in SCLM

The range of carbonatite melts in equilibrium with the subcontinental lithospheric mantle (SCLM) under geothermal conditions is limited by alkali-rich near-eutectic compositions. Therefore, here we employed eutectic Na/K-Ca-Mg-Fe carbonate mixtures to model the interaction of a metasomatic carbonatite melt with natural garnet lherzolite. The experiments were performed at 1100 and 1200 °C and 6 GPa in graphite capsules using a multianvil press. The run duration was 111 and 86 h, respectively. To verify achieving an equilibrium, a synthetic mixture identical to natural lherzolite was also employed.We have found that both Na- and K-bearing carbonatite melts cause wehrlitization accompanying by the elimination of orthopyroxene and an increase of CaO in garnet at a constant Cr2O3. Interaction with the K‑carbonatite melt alters clinopyroxene composition toward lower Na2O (0.2–0.3 wt%), and higher K2O (0.5–1.0 wt%), whereas the Na‑carbonatite melt revealed the opposite effect. The resulting melts have a following approximate composition [40(Na, K)2CO3∙60Ca0.5Mg0.4Fe0.1CO2 + 0.6–1.4 wt% SiO2] displaying a decrease in Ca# at a nearly constant alkali content relative to the initial composition, where Ca# = 100∙Ca/(Ca + Mg + Fe). We have also found that alkali-poor (≤ 20 mol% (Na, K)2CO3) carbonate mixtures do not melt completely but yield magnesite and alkali- and Ca-rich melts like those in the systems with eutectic mixtures.Under SCLM P-T conditions the range of carbonatite melt compositions is restricted by the full melting field of alkali-rich carbonates in the corresponding Na/K-Ca-Mg carbonate systems. Infiltration of less alkaline higher-temperature carbonatite melt in SCLM and its subsequent cooling to the ambient mantle temperature, 1100–1200 °C at 6 GPa, should cause crystallization of magnesite and shift the melt composition to [30(Na, K)2CO3∙70Ca0.6Mg0.3Fe0.1CO3]. Owing to its high Ca#, this melt is not stable in equilibrium with orthopyroxene yielding its disappearance by CaMg exchange reaction producing clinopyroxene, magnesite, and shifting the melt composition toward higher alkali content. The melts containing 40–45 mol% of alkaline carbonates have no limitation in Ca# because the corresponding binary NaMg and KMg carbonate eutectics are located near 1200 °C. Therefore, these melts can achieve Ca# ≤ 30–40 and, be in equilibrium with garnet lherzolites and harzburgites under the geothermal condition of SCLM.Considering the present results and previous experimental data the following ranges of carbonatite melt compositions can be expected in equilibrium with garnet peridotites at the base of SCLM: Ca# < 30 and > 30 mol% (K, Na)2CO3 in equilibrium with harzburgite; Ca# 30–40, >25 mol% (K, Na)2CO3 in equilibrium with lherzolite; and Ca# 40–60 and >20 mol% (K, Na)2CO3 in equilibrium with wehrlite.

Elastic properties of majoritic garnet inclusions in diamonds and the seismic signature of pyroxenites in the Earth's upper mantle

AbstractMajoritic garnet has been predicted to be a major component of peridotite and eclogite in Earth's deep upper mantle (&amp;gt;250 km) and transition zone. The investigation of mineral inclusions in diamond confirms this prediction, but there is reported evidence of other majorite-bearing lithologies, intermediate between peridotitic and eclogitic, present in the mantle transition zone. If these lithologies are derived from olivine-free pyroxenites, then at mantle transition zone pressures majorite may form monomineralic or almost monomineralic garnetite layers. Since majoritic garnet is presumably the seismically fastest major phase in the lowermost upper mantle, the existence of such majorite layers might produce a detectable seismic signature. However, a test of this hypothesis is hampered by the absence of sound wave velocity measurements of majoritic garnets with relevant chemical compositions, since previous measurements have been mostly limited to synthetic majorite samples with relatively simple compositions. In an attempt to evaluate the seismic signature of a pyroxenitic garnet layer, we measured the sound wave velocities of three natural majoritic garnet inclusions in diamond by Brillouin spectroscopy at ambient conditions. The chosen natural garnets derive from depths between 220 and 470 km and are plausible candidates to have formed at the interface between peridotite and carbonated eclogite. They contain elevated amounts (12–30%) of ferric iron, possibly produced during redox reactions that form diamond from carbonate. Based on our data, we model the velocity and seismic impedance contrasts between a possible pyroxenitic garnet layer and the surrounding peridotitic mantle. For a mineral assemblage that would be stable at a depth of 350 km, the median formation depth of our samples, we found velocities in pyroxenite at ambient conditions to be higher by 1.9(6)% for shear waves and 3.3(5)% for compressional waves compared to peridotite (numbers in parentheses refer to uncertainties in the last given digit), and by 1.3(13)% for shear waves and 2.4(10)% for compressional waves compared to eclogite. As a result of increased density in the pyroxenitic layer, expected seismic impedance contrasts across the interface between the monomineralic majorite layer and the adjacent rocks are about 5–6% at the majorite-eclogite-interface and 10–12% at the majoriteperidotite-boundary. Given a large enough thickness of the garnetite layer, velocity and impedance differences of this magnitude could become seismologically detectable.

Multispecies Diffusion of Yttrium, Rare Earth Elements and Hafnium in Garnet

AbstractWe report experimental data for Y, La, Lu and Hf diffusion in garnet, in which diffusant concentrations and silica activity have been systematically varied. Experiments were conducted at 950 and 1050 °C, at 1 atm pressure and oxygen fugacity corresponding to the quartz–fayalite–magnetite buffer. At Y and REE concentrations below several hundred ppm we observe both slow and fast diffusion mechanisms, which operate simultaneously and correspond to relatively high and low concentrations, respectively. Diffusivity of Y and REE is independent of silica activity over the studied range. General formulae for REE diffusion in garnet, incorporating data from this and previous studies, are logDREE(f)(m2 s−1)=−10·24(±0·21)−221057(±4284)2·303RT(K) for the ‘fast’ REE diffusion mechanism at 1 atm pressure, and logDREE(s)(m2 s−1)=−9·28(±0·65)−265200(±38540)+10800(±2600)×P(GPa)2·303RT(K) for the ‘slow’ REE diffusion mechanism. These slow and fast diffusion mechanisms are in agreement with previous, apparently conflicting, datasets for REE diffusion in garnet. Comparison with high-pressure experiments suggests that at high pressures (&amp;gt;∼1 GPa minimum) the fast diffusion mechanism no longer operates to a significant degree. When Y and/or REE surface concentrations are greater than several hundred ppm, complex concentration profiles develop. These profiles are consistent with a multi-site diffusion–reaction model, whereby Y and REE cations diffuse through, and exchange between, different crystallographic sites. Diffusion profiles of Hf do not exhibit any of the complexities observed for Y and REE profiles, and can be modeled using a standard (i.e. single mechanism) solution to the diffusion equation. Hafnium diffusion in garnet shows a negative dependence on silica activity, and is described by logDHf(m2 s−1)=−8·85(±0·38)−299344(±15136)+12500(±900)×P(GPa)2·303RT(K)−0·52(±0·09)×log⁡10aSiO2. In many natural garnets, diffusion of both Lu and Hf would be sufficiently slow that the Lu–Hf system can be reliably used to date garnet growth. In cases in which significant Lu diffusion does occur, preferential retention of 176Hf/177Hf relative to 176Lu/177Hf will skew isochron relationships such that their apparent ages may not correspond to anything meaningful (e.g. garnet growth, peak temperature or the closure temperature of Lu or Hf). Late-stage reheating events are capable of causing larger degrees of preferential retention of 176Hf/177Hf relative to 176Lu/177Hf and partial to full resetting of the Sm–Nd system within garnet, thus increasing the separation between garnet Lu–Hf and Sm–Nd isochron dates, owing to the fact that these systems are more significantly disturbed through diffusion as more radiogenic 176Hf and 143Nd have accumulated.

Micro- and nano-size hydrogarnet clusters in calcium silicate garnet: Part II. Mineralogical, petrological, and geochemical aspects

AbstractThe nominally anhydrous, calcium-silicate garnets, grossular (Ca3Al2Si3O12), andradite (Ca3Fe23+Si3O12), schorlomite (Ca3Ti24+[Si,Fe23+]O12), and their solid solutions can incorporate structural OH-, often termed “water.” The IR single-crystal spectra of several calcium silicate garnets were recorded between 3000 and 4000 cm–1. Spectroscopic results are also taken from the literature. All spectra show various OH- stretching modes between 3500 and 3700 cm–1 and they are analyzed. Following the conclusions of Part I of this study, the garnets appear to contain local microscopic- and nano-size Ca3Al2H12O12- and Ca3Fe23+H12O12-like domains and/or clusters dispersed throughout an anhydrous “matrix.” The substitution mechanism is the hydrogarnet one, where (H4O4)4– ↔ (SiO4)4–, and various local configurations containing different numbers of (H4O4)4– groups define the cluster type. A single (H4O4) group is roughly 3 Å across and most (H4O4)-clusters are between this and 15 Å in size. This model can explain the IR spectra and also other experimental results. Various hypothetical “defect” and cation substitutional mechanisms are not needed to account for OH- incorporation and behavior in garnet. New understanding at the atomic level into published dehydration and H-species diffusion results, as well as H2O-concentration and IR absorption-coefficient determinations, is now possible for the first time. End-member synthetic and natural grossular crystals can show similar OH- “band patterns,” as can different natural garnets, indicating that chemical equilibrium could have operated during their crystallization. Under this assumption, the hydrogarnet-cluster types and their concentrations can potentially be used to decipher petrologic (i.e., P-T-X) conditions under which a garnet crystal, and the rock in which it occurs, formed. Schorlomites from phonolites contain no or very minor amounts of H2O (0.0 to 0.02 wt%), whereas Ti-bearing andradites from chlorite schists can contain more H2O (∼0.3 wt%). Different hydrogarnet clusters and concentrations can occur in metamorphic grossulars from Asbestos, Quebec, Canada. IR absorption coefficients for H2O held in hydrogrossularand hydroandradite-like clusters should be different in magnitude and this work lays out how they can be best determined. Hydrogen diffusion behavior in garnet crystals at high temperatures is primarily governed by the thermal stability of the different local hydrogarnet clusters at 1 atm.

Enrichment of manganese to spessartine saturation in granite-pegmatite systems

Abstract 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.

Raman discrimination of garnet inclusions in Siberian diamonds

AbstractThe composition of garnet inclusions in natural garnets is exceptionally useful for identification of the chemical and physical properties of mantle lithologies in which they occur. The results of Raman study of garnet inclusions in Siberian diamonds are presented. Most of studied garnets belong to either peridotitic (P type, n = 101) or eclogitic (E type, n = 63), and some rare garnets are websteritic (n = 5). Detected spectroscopic features of garnets in diamonds reveal a wide wave number ranges for rotational R[SiO4], bending ν2, and stretching ν1 modes. The peak positions of the Raman bands correlate with content of major components of garnets. Raman shift of stretching ν1 and bending ν2 vibrations reflects Ca–Mg and Al–Cr isomorphism. The found correlations have been applied for rough estimation of the garnet compositions. This method allows to discriminate E‐type and P‐type garnets marking diamondiferous mantle rocks.

Geochemistry of garnet megacrysts from the Mir kimberlite pipe (Yakutia) and the nature of prothokimberlite melt

The paper presents the results of major and trace elements composition study of garnet megacrysts from Mir kimberlite pipe. On the major elements composition those garnets classified as low Cr and high Ti pyropes. Concentrations of TiO2 show a negative correlation with MgO и Cr2O3 contents in megacrysts composition. Fractional crystallization modeling indicates that the most appropriate melt to reproduce the garnet trace elements signatures is the melt of picritic composition. Composition of garnets crystallized from kimberlite melt do not correspond to observed natural garnets composition. Kimberlites contain less of Ti, Zr, Y and heavy REE (rare earth elements) but more of very incompatible elements such as light REE, Th, U, Nb, Ba then the model melt composition that necessary for garnet crystallization.

Geochemistry of Garnet Megacrysts from the Mir Kimberlite Pipe (Yakutia) and the Nature of Protokimberlite Melts

The chemical compositions of garnets from a megacryst association of the Mir kimberlite pipe have been studied. By petrogenic elements, the garnet megacrysts can be classified as high-Ti and low-Cr pyrope. The megacryst TiO2 contents of the Mir pipe correlate inversely with the MgO and Cr2O3 contents. Modeling of the composition of garnets through a fractional crystallization process showed that the most suitable composition of the melts parental for the garnets of the megacryst association is picrite. The composition of garnets crystallized from the kimberlite does not correspond to the composition of the natural garnets from the Mir pipe. The kimberlites contain less Ti, Zr, Y, and HREEs, but are more enriched with strongly incompatible elements (LREEs, Th, U, Nb, Ta, and Ba) than the model composition of the melt suitable for crystallization of the garnet megacrysts.

Magmatic Mn-rich garnets in volcanic settings: Age and longevity of the magmatic plumbing system of the Miocene Ramadas volcanism (NW Argentina)

The Miocene “Corte Blanco Tuff” rhyolite deposit is the product of a large volume and high intensity Plinian eruption from the solitary and monogenetic Ramadas Volcanic Centre (Central Andes, Province of Salta, NW Argentina). The “Corte Blanco Tuff” consists of vitreous tube pumices with rare euhedral sub-millimetric Mn-garnet phenocrysts, typically hosting inclusions of U-phases as zircon and monazite. Here, we present new textural, major and trace elemental analyses of garnet, zircon and glass that, combined with in situ U-(Th)-Pb zircon and monazite dating, are used to reconstruct the thermobaric environment of formation, age and longevity of the magmatic plumbing system of the Ramadas magma. The results indicate to a crystallization path of a peraluminous rhyolitic melt at shallow crustal levels (≤6 km), as sequentially tracked by the initial nucleation of zircon (780 °C at 9.16 Ma) and garnet (above or at ca. 700 °C), to the final monazite growth (660–670 °C, at 8.70 Ma) in a water-saturated (H2O = 3–5 wt%) environment, shortly before the eruption started. These data (1) define for the first time the primary magmatic origin of Mn-garnet in a rhyolitic volcanic setting; (2) provide new partition coefficients of rare earth elements (REE) between natural garnet, zircon and rhyolitic melts; and (3) permit reconstruction of the magmatic processes that resulted in the Ramadas eruption. On a wider scale, our results document the spatio-temporal (P-T conditions, timing and longevity) time scales involved in the petrogenesis of a shallow peraluminous water-saturated rhyolitic magmatic plumbing system that is able to generate the conditions for extremely explosive Plinian eruptions.

Natural garnet reinforced high density polyethylene composites for sustainable microwave substrates

A cost effective and ecofriendly alternative for commercially available microwave substrates is proposed in the present study. A polymer composite based on high-density polyethylene (HDPE) filled with garnet mineral was fabricated using sigma-blend technique with 0.1–0.5-volume fraction of filler. The atomic force microscopic images indicated the surface roughness of the substrate is very low and has the required degree of surface finish to avoid conduction loss. For 0.5 vol fraction of garnet, the composite showed a dielectric constant (εr) = 5.7 and dielectric loss (tan δ) = 0.00185 at 5 GHz. Moreover, coefficient of thermal expansion (CTE), thermal conductivity (kc) and tensile strength are 114 ppm/oC, 1.3 Wm−1K−1 and 14.3 MPa respectively for filler volume fraction, Vf = 0.5. The dielectric, thermal, moisture absorption and mechanical properties indicate that the garnet/HDPE composites are ideal candidate for cost effective and sustainable microwave substrates.

Influence of Particle Size and Calcination Temperature on the Abrasiveness of Garnet for Water Jet Cutting of Tough Materials

This study shows the influence of calcinations temperature and particle size of natural garnet mineral on its hardness and abrasiveness (H/A) for use in water jet cutting of tough materials. A standard commercial sample of the mineral was characterized in terms of the hardness and abrasiveness properties. The sample was sized to 4 fractions having the size of &lt;60 um, &gt; 60 &lt; 100 um, &gt; 100 &lt; 200 um and &gt; 200 &lt; 250 um designated the symbols, FF, MF, MC and C respectively. Each fraction was separately heated in controlled conditions at temperatures up to 1000°C at a heating rate of 10°C/min in an electrically heated tube furnace. Soaking time at the maximum temperature was 60 minutes. Hardness and abrasiveness properties of the heat treated samples were tested to cut marble having a thickness of 30 mm. Results revealed that H/A of the garnet mineral increased by heating at temperatures up to 750°C and exhibited gradual decrease with higher temperatures up to 1000°C. Results were explained in the light of a structural irreversible metamorphism (SM) of the garnet crystals; almandine Fe2+ 3Al2Si3O12and propel Mg3Al2Si3O12. Metamorphism involved reorientation of the crystals followed by coherence to form dense grouping. At higher temperature, irreversible thermal expansion of the structure sets in. Characterization of the mineral was carried out with the help of XRD, SEM and FT-IR measurements.

Surface-Modified Garnet Particles for Reinforcing Epoxy Composites

The present study investigated the tribological performance of epoxy (EP) matrix composites enhanced with natural garnet. The garnet was surface-modified with sodium stearate for optimal performance. Composites comprising different contents and particle sizes of modified garnet (MG) were prepared with a mixture of EP and MG. The sodium stearate-bonded garnet and EP formed a stable structure. Tribological performance was measured by a ball-on-plate apparatus under permanent dry sliding conditions and a wear track was obtained by an optical profilometer. The wear mechanism was explored by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) images. Wear test results showed that the coefficient of friction for all EP/MG composites decreased compared with that for neat epoxy. The results also indicated that the addition of MG can evidently improve the tribological properties of EP matrix composites.

Application of microprobe-based flank method analysis of Fe3+ in garnet of North Qilian eclogite and its geological implication

A newly developed microprobe-based methodology (the Flank Method) for Fe3+/ΣFe quantification has been successfully applied to some natural garnets from the North Qilian eclogites by JEOL JXA-8100 microprobe at Peking University. The results demonstrated an obvious discrepancy in comparison with the outcomes by conventional stoichiometric calculations. This methodology allows to measure the Fe3+/ΣFe ratio and perform elemental analyses simultaneously in the same condition. Accurate in-situ measurement of Fe3+ content in garnet may bring certain impact on the garnet-based P-T estimation. According to the compositional zonation displayed in the studied eclogitic garnets from North Qilian, a prograde metamorphic PT path from 19.5 kbar, 520 °C to 22 kbar, 600 °C was reconstructed. More interestingly, the measured Fe3+/ΣFe ratios in garnets decreasing from core to rim may probably imply that the oxygen fugacity (fO2) declines with the depth of the subduction zone.

Spectral reflectance (0.35–2.5 µm) properties of garnets: Implications for remote sensing detection and characterization

The utility of spectral reflectance for identification of the main end-member garnets: almandine (Fe2+3Al2Si3O12), andradite (Ca3Fe3+2Si3O12), grossular (Ca3Al2Si3O12), pyrope (Mg3Al2Si3O12), spessartine (Mn2+3Al2Si3O12), and uvarovite (Ca3Cr3+2Si3O12) was studied using a suite of 60 garnet samples. Compositional and structural data for the samples, along with previous studies, were used to elucidate the mechanisms that control their spectral reflectance properties. Various cation substitutions result in different spectral properties that can be determine the presence of various optically-active cations and help differentiate between garnet types. It was found that different wavelength regions are sensitive to different compositional and structural properties of garnets. Crystal-field absorptions involving Fe2+ and/or Fe3+ are responsible for the majority of spectral features in the garnet minerals examined here. There can also be spectral features associated with other cations and mechanisms, such as Fe2+–Fe3+ and Fe2+–Ti4+ intervalence charge transfers. The visible wavelength region is useful for identifying the presence of various cations, in particular, Fe (and its oxidation state), Ti4+, Mn2+, and Cr3+. In the case of andradite, spessartine and uvarovite, the visible region absorption bands are characteristic of these garnets in the sense that they are associated with the major cation that distinguishes each: [6]Fe3+ for andradite, [8]Mn2+ for spessartine, and [6]Cr3+ for uvarovite. For grossular, the presence of small amounts of Fe3+ leads to absorption bands near 0.370 and 0.435 µm. These bands are also seen in pyrope–almandine spectra, which also commonly have additional absorption bands, due to the presence of Fe2+. The common presence of Fe2+ in the dodecahedral site of natural garnets gives rise to three Fe2+ spin-allowed absorption bands in the 1.3, 1.7, and 2.3 µm regions, providing a strong spectral fingerprint for all Fe2+-bearing garnets studied here. Garnets containing Mn2+ have additional visible (∼0.41 µm) spectral features due to [8]Mn2+. Garnets containing Cr3+, exhibits two strong absorption bands near ∼0.7 µm due to spin-forbidden [6]Cr3+ transitions, as well as [6]Cr3+ spin-allowed features near 0.4–0.41 µm and 0.56–0.62 µm, and [6]Cr3+ spin-allowed transitions between 0.41 and 0.68 µm. Common silicate garnet spectra, in summary, are distinct from many other rock-forming silicates and can be spectrally distinct from one garnet species to another. Iron dominates the spectral properties of garnets, and the crystallographic site and oxidation state of the iron both affect garnet reflectance spectra.

XAFS spectroscopic study of Ti coordination in garnet

Titanium can be incorporated either tetrahedrally ( IV Ti) or octahedrally ( VI Ti) in most silicate minerals. Ti K -edge X-ray absorption fine structure (XAFS) spectroscopy enables observation of Ti coordination in minerals and melts. In this study, XAFS is used to determine the coordination of Ti in synthetic and natural garnets. Garnets grown synthetically at eclogite- and granulite-facies conditions can contain several wt% TiO 2 , most of which is incorporated as VI Ti. This observation aligns with major element trends in these garnets. In natural garnets grown at lower temperatures and pressures, on the other hand, Ti is observed to occupy both the octahedral and tetrahedral sites in garnet—in some cases Ti is almost entirely fourfold coordinated. Combined with previous research (see Ackerson et al. 2017, this issue) on substitution mechanisms for VI Ti, the results of this study demonstrate that Ti is incorporated on two crystallographic sites in garnet by at least three primary substitution mechanisms. In both natural and synthetic garnets, there is a discernible increase in VI Ti content in garnet with increasing temperature and pressure, suggesting a significant role for these two parameters in determining Ti solubility. However, a continuous increase in VI Ti with increasing grossular content also suggests that the Ca content of the garnet plays a critical role.

Accurate determination of ferric iron in garnets

Numerous techniques are available to determine the amount of Fe 2+ and Fe 3 + in minerals. Calculating Fe 2+ and Fe 3 + by charge-balance using electron probe microanalysis (EPMA) data is the most common method, but several studies question the usefulness and accuracy of this approach (Canil and O’Neill 1996; Dyar et al. 1993, 2012; Lalonde et al. 1998; Li et al. 2005; McGuire et al. 1989; Schingaro et al. 2016; Schmid et al. 2003; Sobolev et al. 2011). We compile and compare data for natural garnets that have been analyzed by both EPMA and Mossbauer spectroscopy. Comparison of Fe 3 + / ∑Fe determined by charge-balance vs. Mossbauer spectroscopy shows an approximate 1:1 correlation. The EPMA data set of Dyar et al. (2012) is reexamined and it is shown that disagreement between EPMA and Mossbauer for their data is not nearly as bad as reported. Data for charge-balance vs. Mossbauer spectroscopy are compared and show that the EPMA/charge-balance approach provides a suitable alternative when other methods are not practical.

Experimental constraints on formation of hematite in olivine at high pressures and temperatures

Iron-rich oxides, such as magnetite or hematite, have been reported in olivine grains in many orogenic garnet peridotites from continental collision zones. Whether these iron-rich minerals originate from dry oxidation, dehydrogenation-oxidation or exsolution from a precursor wadsleyite phase is debatable. This paper explores high-pressure and high-temperature experiments in a hydrous harzburgite system, by taking advantage of electron backscattered diffraction (EBSD) analyses, to examine the formation of hematite in olivine. Experimental results show that hematite can be formed within olivine grains at pressures >6 GPa and temperatures in the 1073–1473 K range. EBSD analysis suggests that hematite rods (not associated with clinopyroxene) and host olivine have the following crystallographic relations: \( \langle 0001 \rangle _{\text{Hem}} // [100]_{\text{Ol}} , \langle 10{-}10\rangle _{\text{Hem}} //[001]_{\text{Ol}} , \langle 11{-}20\rangle _{\text{Hem}} //[010]_{\text{Ol}} \), which are consistent with those observed in natural garnet peridotite from the Dabie–Sulu ultra-high-pressure (UHP) metamorphic terrane. It is postulated that both hydroxide (OH−) and hydrogen (H+) ions have the potential to oxidize Fe2+ to Fe3+, followed by rapid dehydrogenation and slow Fe diffusion, thus forming hematite within the olivine grains. It is proposed that dehydrogenation-oxidation is the most likely formation mechanism of hematite inclusions within olivine, with the following two requirements: an ample amount of H2O and specific P–T conditions (>6 GPa, at 1073 K). Such conditions are consistent with those calculated in natural garnet peridotites from the Dabie–Sulu UHP metamorphic terranes. The present study also indicates that hematite (or magnetite?) inclusions in olivine contain important clues about the tectonic evolution of UHP rocks in continental crust collision zones.

Diamond–garnet geobarometry: The role of garnet compressibility and expansivity

We report P–V and T–V Equations of State (EoS) for synthetic single crystal pyrope (Py, Mg3Al2Si3O12) and the P–V EoS for synthetic single crystals of almandine (Alm, Fe3Al2Si3O12) as well as an intermediate composition (Py60Alm40) as measured by in-situ high-pressure and high-temperature X-ray diffraction experiments. The unit-cell volumes of the three samples were measured at room temperature and different pressures and up to about 8GPa in a diamond-anvil cell. The high-temperature experiment was carried out using a micro-furnace. The pressure-volume data were fitted to a third order Birch–Murnaghan EoS giving the following coefficients: V0=1506.15(16)Å3, KT0=163.7(1.7)GPa and K′=6.4(4) for pyrope, V0=1533.52(10)Å3, KT0=172.6(1.5)GPa and K′=5.8(5) for almandine and V0=1516.32(13)Å3, KT0=167.2(1.7)GPa and K′=5.6(5) for the intermediate Py60Alm40 composition. The unit-cell volume along the pyrope–almandine join changes linearly within the error of measurement indicating ideality in the volume of mixing behaviour for the solid solution. The first pressure derivative, K′, for all three garnets is similar within experimental uncertainty with an average value of K′=6.0(4). The thermal expansion parameters for end-member pyrope, as described using to the Kroll-type EoS, with the Einstein temperature, θE, fixed to 320K, are α(303K, 1bar)=2.543(5)∗10−5K−1 and V0=1504.64(4)Å3. The determined thermoelastic parameters were combined in a thermal-pressure type EoS (thermal-P type EoS) to calculate the entrapment pressures, Pe, for natural garnet inclusions in diamonds at mantle temperatures. A calculated pressure of 5.8GPa at T=1500K using our newly determined thermoelastic parameters appears to be more consistent with diamond-forming conditions compared to the higher pressures that range up to 6.8GPa at T=1500K, which are obtained using thermoelastic data in the literature.

Mineralogical controls on garnet composition in the cratonic mantle

Garnet concentrates are a rich source of geochemical information on the mantle, but the mineralogical implications of wide ranging garnet compositions are poorly understood. We model chemical reactions between mantle minerals that may buffer the Ca–Cr lherzolitic garnet trend common in the lithospheric mantle. A harzburgitic trend of garnet compositions featuring a lower increase in Cr with Ca relative to the conventional lherzolitic trend is reported for the first time. Representation of garnet chemistry in terms of additive and exchange components in the Thompson space shows that the lherzolitic and harzburgitic trends are controlled by the cation exchanges MgFeAl ↔ Ca2Cr and MgFeAl4 ↔ Ca2Cr4, respectively. Various equilibrium reactions are presented to explain the trends assuming a closed or open system mantle. The compositional variability of the natural garnets from the Canastra 8 kimberlite (Brazil) is modeled by a linear system of mass balance equations. The solution returns the reaction coefficients of products (positive values) and reactants (negative values), which are then evaluated against the observed mantle mineralogy. In the isochemical mantle, the lherzolitic trend can form in the absence of clinopyroxene, but requires the presence of spinel and reflects the thickness of the spinel–garnet transition zone. This requirement contradicts observations on natural occurrences of the trend and the thermobarometry of the host peridotites. In the preferred model of a variably depleted mantle, the lherzolitic trend critically depends on the presence of clinopyroxene. The occurrence of lherzolitic garnet compositions in harzburgite can be explained by exhaustion of clinopyroxene as a result of garnet buffering. The open system behavior of the peridotitic mantle also provides a better explanation for the harzburgitic trend in garnet compositions. In an isochemical mantle, the trend can be controlled by many possible reactions, and no single mineral is essential. In the variably depleted mantle, spinel is required to make the harzburgitic trend garnet.

P–V–T equation of state of spessartine–almandine solid solution measured using a diamond anvil cell and in situ synchrotron X-ray diffraction

The pressure–volume–temperature (P–V–T) equation of state (EoS) of two natural garnet samples along spessartine–almandine (Spe–Alm) join has been measured at high temperature up to 800 K and high pressures up to 15.46 and 16.17 GPa for Spe64Alm36 and Spe38Alm62, respectively, using in situ angle-dispersive X-ray diffraction and diamond anvil cell. Analysis of room-temperature P–V data to a third-order Birch–Murnaghan EoS yields: V 0 = 1,544.4 ± 0.4 A3, K 0 = 180 ± 3 GPa and $$ K_{0}^{{\prime }} $$ = 4.0 ± 0.4 for Spe38Alm62, and V 0 = 1,557.5 ± 0.3 A3, K 0 = 176 ± 2 GPa and $$ K_{0}^{{\prime }} $$ = 4.0 ± 0.3 for Spe64Alm36. Fitting of our P–V–T data by means of the high-temperature third-order Birch–Murnaghan EoS gives the thermoelastic parameters: V 0 = 1,544.6 ± 0.6 A3, K 0 = 180 ± 4 GPa, $$ K_{0}^{{\prime }} $$ = 4.0 ± 0.4, (∂K/∂T) P = −0.028 ± 0.005 GPa K−1 and α 0 = (3.16 ± 0.14) × 10−5 K−1 for Spe38Alm62, and V 0 = 1,557.7 ± 0.9 A3, K 0 = 176 ± 4 GPa, $$ K_{0}^{{\prime }} $$ = 4.0 ± 0.5, (∂K/∂T) P = −0.029 ± 0.005 GPa K−1 and α 0 = (3.04 ± 0.16) × 10−5 K−1 for Spe64Alm36. The results confirm that almandine content increases the bulk modulus of the spessartine–almandine join following a nearly ideal mixing model. The relation between bulk modulus and almandine mole fraction (X Alm) in this garnet join is derived to be K 0(GPa) = 171.6(±2.6) + 10.9(±1.8)X Alm. Present results are also compared with previously studies determined the thermoelastic properties of other garnets.