Natural Garnet Research Articles

Phase evolution of Nd3+-doped natural garnet minerals systems: A potential host phase for trivalent actinide immobilization

The immobilization of long-lived actinides produced from spent fuel reprocessing poses a significant environmental concern. In this work, we synthesized glass-ceramics at low temperatures using natural garnet minerals as precursors to immobilize trivalent actinides. Two primary metrics, the quantitative distribution of Nd3+ in glass-ceramics and its leaching rate, were employed to explore the mechanism of Nd3+ immobilization in glass-ceramic products. XRD results show that the primary crystalline phase of the glass-ceramics transitions from calcium feldspar and spinel to oxyapatite with increasing Nd2O3 content. Within a specific range, the proportion of Ca2Nd8(SiO4)6O2 in the samples increases from 37.9 % to 57.9 % with rising Nd2O3 content, while the glassy phase proportion decreases. The distribution of Nd3+ in Ca2Nd8(SiO4)6O2-based glass-ceramics was quantitatively investigated using a combination of TEM-EDX and Rietveld refinement calculations. Nearly 70 % of Nd was found to benefit from a double sealing barrier (Oxyapatite + glass) even under high Nd2O3 loading. A 42-day leaching experiment was conducted using deionized water as the medium. The data demonstrated that Ca2Nd8(SiO4)6O2-based glass-ceramics exhibited a lower Nd-normalized leaching rate(LRNd∼10−5 g m−2 d−1) in solution compared to products solely relying on the glass network for actinide immobilization. This work introduces a cost-effective natural solution for trivalent actinide immobilization.

Retentiveness of rare earth elements in garnet with implications for garnet Lu‐Hf chronology

AbstractIncorporation of rare earth elements (REE) in garnet enables garnet chronology (Sm‐Nd, Lu‐Hf), and imparts a garnet‐stable signature on cogenetic phases, which allows petrochronology and general petrogenetic tracing of garnet stability in minerals and melts. Constraints on the uptake and redistribution mechanisms, as well as on the diffusive behaviour of REE in garnet are required for allowing accurate interpretation of REE signatures and ages. Garnet REE profiles are often measured to gain insight into the nature and cause of REE zoning. Interpretation of such profiles is nevertheless complicated by poor constraints on the extent of diffusive relaxation. This is especially relevant for Lu, which, according to experiments, has a relatively high diffusivity and thus may re‐equilibrate with possible consequences for Lu‐Hf chronology. To provide new insight into the REE systematics of garnet, we applied quantitative trace‐element mapping of garnet grains from metamorphic rocks that record peak temperatures above 750°C and cooling rates as low as 1.5°C Ma−1. Garnet in all samples preserves Rayleigh‐type or oscillatory growth zoning with sharply defined interfacial angles that match the garnet habit. Re‐equilibration of REE compositions appears restricted to domains with nebulous and patchy zoning, which likely form by interface‐coupled dissolution and re‐precipitation reactions mediated by fluids or melts, rather than REE volume diffusion. The possible effect of Lu diffusion in the analysed grains was investigated by comparing the observations to the results from 2D numerical modelling using Lu diffusivities from recent diffusion experiments. This test indicates that Lu diffuses significantly slower in natural garnet than experiments predict. The retentiveness of REE in garnet demonstrates the reliability of REE signatures in magmatic tracing and petrochronology and establishes Lu‐Hf chronology as a robust means of dating garnet growth and recrystallization in metamorphic rocks, including those that underwent high‐ or ultrahigh‐temperature conditions.

The sound velocity of garnet at high pressure: Implications for high velocity anomalies in cold subducting slabs

The origin of the high velocity anomalies in cold subduction slabs revealed by seismological tomography remains unknown. Garnet is a major component of the pyrolitic mantle and subduction slabs and crucially affects their physical properties, such as densities and velocities. Based on previous studies, the predicted velocities of garnets with various compositions are much different from the measured values. Here, we measured the velocities of four hot-pressed natural garnet samples, namely, Alm65.1Grs17.5Prp8.5Sps2.0And6.9, Alm60.9Grs25.9Prp8.2Sps2.0And3.0, Alm49.6Grs5.3Prp37.8Sps1.7And5.6, and Grs89.0Prp0.4Sps6.3And4.3, at pressures up to 11 GPa using the ultrasonic interference method in conjunction with a Walker-type multi anvil, to further constrain the composition dependence on sound velocities. Our results show that the velocities and the bulk and shear moduli monotonically increase with pressure but display a complicated dependence on composition. Accordingly, we have determined the dependences of P- and S-wave velocities of garnets on pressure and composition and reconstructed the velocity profiles of pyrolite, eclogite, harzburgite, and a garnet layer at depths of 90–300 km in a cold subducted slab. Our model proposes that the presence of a garnet layer with a thickness of 5.9–15.9 km atop the slab could cause velocity contrasts of 4.0–6.0% for VP and 4.1–7.7% for VS between the slab and the overlying pyrolite. The presence of a layer with a high proportion of garnet could contribute to observed high-velocity anomalies in cold subduction zones.

Experimental Modeling of Decarbonation Reactions, Resulting in the Formation of CO2 Fluid and Garnets of Model Carbonated Eclogites under Lithospheric Mantle P,T-Parameters

First experimental modeling of decarbonation reactions resulting in the formation of CO2-fluid and Mg, Fe, Ca, and Mn garnets, with composition corresponding to the garnets of carbonated eclogites of types I and II (ECI and ECII), was carried out at a wide range of lithospheric mantle pressures and temperatures. Experimental studies were performed on a multi-anvil high-pressure apparatus of a “split sphere” type (BARS), in (Mg, Fe, Ca, Mn)CO3-Al2O3-SiO2 systems (with compositional variations according to those in ECI and ECII), in the pressure interval of 3.0–7.5 GPa and temperatures of 1050–1450 °C (t = 10–60 h). A specially designed high-pressure cell with a hematite buffering container—preventing the diffusion of hydrogen into the platinum capsule—was used, in order to control the fluid composition. Using the mass spectrometry method, it was proven that in all experiments, the fluid composition was pure CO2. The resulting ECI garnet compositions were Prp48Alm35Grs15Sps02–Prp44Alm40Grs14Sps02, and compositions of the ECII garnet were Prp57Alm34Grs08Sps01–Prp68Alm23Grs08Sps01. We established that the composition of the synthesized garnets corresponds strongly to natural garnets of carbonated eclogites of types I and II, as well as to garnets from xenoliths of diamondiferous eclogites from the Robert Victor kimberlite pipe; according to the Raman characteristics, the best match was found with garnets from inclusions in diamonds of eclogitic paragenesis. In this study, we demonstrated that the lower temperature boundary of the stability of natural garnets from carbonated eclogites in the presence of a CO2 fluid is 1000 (±20) °C at depths of ~90 km, 1150–1250 (±20) °C at 190 km, and 1400 (±20) °C at depths of about 225 km. The results make a significant contribution to the reconstruction of the fluid regime and processes of CO2/carbonate-related mantle metasomatism in the lithospheric mantle.

Thermal conductivity of aluminous garnets in Earth’s deep interior

Abstract Aluminous garnets [(Mg,Fe,Ca)3Al2(SiO4)3] are a key mineral group in Earth’s interior. Their thermal conductivity with relevant chemical compositions and at high-pressure-temperature (P-T) conditions plays a crucial role in affecting the thermal states of pyrolytic mantle and subducted basaltic crust over the depth range they are present. Using ultrafast optical pump-probe spectroscopy combined with an externally-heated diamond-anvil cell, we have precisely determined the high-P-T thermal conductivity of aluminous garnets, including pyrope, grossular, and pyrope-almandine solid solution. We find that the variable chemical composition has minor effects on the thermal conductivity of these garnets over the P-T range studied. Combined with previous results, we provide new depth-dependent thermal conductivity profiles for a pyrolytic mantle and a subducted basaltic crust. These results significantly benefit geodynamics simulations and advance our understanding of the thermal structure and evolution dynamics in Earth’s upper mantle and transition zone. In addition, as garnets are also a key, useful material family for modern technology, our results on the thermal property of natural garnets also shed light on the novel design of optical and electronic devices based on various synthetic nonsilicate garnets.

Synthesis of Pyrope Garnet with High Samarium Content at 5 GPa and 1300 °C

In natural garnets associated with diamonds, an increased content of "light" rare-earth elements, especially samarium, is often noticed. There are various estimates of the composition of the crystallization medium. Experimental data on the synthesis of Cr-rich garnets containing rare-earth elements are still scarce. The experiments were carried out on a multi-anvil apparatus of the “split-sphere” type (BARS) at a pressure of 5 GPa and a temperature of 1300 °С. The high pressure cell was made of Zr02 and CaO. A tubular graphite element was used as a heater. The measurement error of pressure and temperature was ± 0.2 GPa and ± 25 °С. The starting materials for the experiments were natural serpentine-antigorite, chromium spinel, and corundum. As a result of the experiments, pyrope crystals with a high content of samarium were synthesized in the range of 0.77 to 2.34 wt% Sm2O3. The study demonstrates that the interaction of components in the serpentine-chromite-corundum-Sm system in the presence of fluid leads to crystallization of pyrope garnet with samarium content exceeding the values known for natural garnets associated with diamond.

Calibration of Infrared Absorption Coefficients for Structural Hydroxyl in Garnet by a Combined Study of Isotope Ratio Spectrometry and Vibrational Spectroscopy

An accurate determination of water content in garnet is critical to quantify the transport of water to the deep mantle by the subducted oceanic crust beyond the breakdown of hydrous phases. Fourier transform infrared spectroscopy (FTIR) is the most widely used approach to determine the species and contents of water in garnet. Accurate quantification of OH in garnet requires independent calibration using an external method, as OH absorbance is mineral and composition specific. To obtain the infrared absorption coefficients of structural hydroxyl in garnet, a combined study of spectrometric analyses by FTIR and a method combining a thermal conversion elemental analyser with isotope ratio mass spectrometry (TC/EA‐MS) was carried out for fourteen gem‐quality natural garnet crystals with variable compositions. The obtained molar absorption coefficients were 9322 ± 338 and 240 ± 26 l mol−1 cm−2 for grossular‐ and spessartine‐rich garnet and pyrope‐almandine garnet, respectively. These results are within the range of previous studies. A new molar absorption coefficient of 689 ± 177 l mol−1 cm−2 was obtained for pyrope‐spessartine garnet. The large variation in the absorption coefficient indicates it is controlled by both garnet composition and OH‐absorption bands. The obtained absorption coefficients are only appropriate for certain types of eclogitic garnet, and more studies should be carried out on eclogitic garnets.

Dielectric Properties of Plasma-Sprayed Fully Natural Garnets

Various kinds of natural garnets belonging to the almandine type (3FeO·Al2O3·3SiO2) were sprayed by plasma spray technique to build coatings on metallic substrates. The experimental garnet powders came from different mines in the Czech Republic and Mongolia. After coating and cooling the substrates were removed. In this way, self-supporting plates were obtained and further studied with microscopy, X-ray diffraction, and dielectric spectroscopy. Mechanical properties were in our focus as well. Microhardness was measured on cross sections dedicated to microstructure observation. Wear resistance in wet conditions was tested in a slurry. Reflectance was measured applying visible and infrared (VIS-NIR) radiation. Dielectric properties of coatings were studied at low voltage capacitance, loss tangent and also under direct current (DC) resistance. The results show that garnet minerals are interesting candidates for various optical and electronic applications; they have similar dielectric behavior as, for example, aluminum oxide or similar high-purity synthetic oxides, and, simultaneously, they have extraordinarily low reflectance in VIS-NIR radiation. The differences between natural powders and resulting coatings are discussed in connection with their chemical and phase compositions.

Morphostructure of garnets from coastal-marine placers
 as a key feature for determining their processing technology

One of the most popular abrasive materials at the
 moment is natural garnet sand. Interest in it increased
 significantly after the collapse of the USSR, as many
 industrially significant geological objects were preserved
 within the borders of the former Soviet republics. Many
 primary deposits of raw garnet materials have been
 explored in Russia, but almost all of them are not of
 industrial interest for economic reasons. Currently, the
 coastal zone of the White Sea is considered prospective
 for the discovery of industrially significant placers and
 placer occurrences of garnet sand. The range of application
 of garnet sand is very wide – it is waterjet cutting,
 cleaning of various surfaces and removal of corrosion
 defects, free grinding of glass and porcelain and
 the production of grinding powders. In 2021, the volume
 of government purchases of garnet sand in physical
 terms increased by 16.3 %, and the volume of commercial
 purchases increased by 64.2 % compared to the same
 period of the previous year. Based on the study of the
 composition of the garnet mineral group, we used a set
 of research methods, including precision (X-ray spectral
 and electron microscopy) in order to determine the
 technology of obtaining industrial concentrates.
 The development of new occurrences of coastal garnet-
 containing sand in the Arkhangelsk region can provide
 the country with highly liquid, strategic raw materials.

Phase evolution and chemical stability of Nd-doped natural garnet waste form by microwave sintering

Advanced materials and processes are needed to immobilize fission products in the form of chemically durable waste. Nd-doped Ca3Zr2Fe2SiO12 (GZG) ceramics with different sintering temperatures (1100–1300 °C), sintering times (0.5–6 h) and solid solutions (0 ≤ x ≤ 2.5) were prepared by microwave sintering. The results show that by adjusting a series of key parameters, natural garnet structures with excellent chemical stability are obtained. XRD shows that a single phase of solid solution can be obtained by increasing the sintering temperature and controlling the actinide solid solution below the solid solution limit. The sintering temperature only affects the compactness of the solid solution. Thermodynamic calculations show that high temperature is favorable for the synthesis of GZG, and it is positive and irreversible. The normalized leaching rate of Nd3+ in the GZG structure is approximately ∼ 10−6 g m−1 d−1. The release form of Nd in aqueous solution is inner diffusion. The experimental results show that the naturally occurring garnet structure is an excellent waste form.

Towards composition of carbonatite melts in peridotitic mantle

It is generally accepted that carbonatite metasomatism in the subcontinental lithospheric mantle (SCLM) inevitably causes wehrlitization of the primary lherzolite substrate. However, the K-rich carbonatite inclusions in kimberlitic diamonds containing orthopyroxene indicate that this is not always the case. In the present study, we equilibrated natural garnet lherzolite with carbonate melts containing 33–38 wt% K2O with various Ca# = 10, 20, 30, and 40 at 6 GPa and 1200–1500°C, where Ca# = 100⋅Ca/(Ca+Mg+Fe). The original ratio of peridotite to carbonate was 58 to 42 by weight. In the studied temperature range, the melt retains essentially carbonate composition with silica content increasing from 1 to 11–12 wt%. The melt with Ca# 10 alters lherzolite to harzburgite, replacing clinopyroxene by orthopyroxene and decreasing CaO content in garnet below 4 wt%. The melts with Ca# 20-30 also consume clinopyroxene; although CaO content in garnet remains in the range of lherzolitic compositions. The melt with Ca# 40 yields wehrlitization, consuming orthopyroxene, increasing clinopyroxene fraction, and increasing CaO content in garnet above 6 wt%. After the interaction, the Ca# of the melt changes as follows 10 → 16–28, 20 → 20–33, 30 → 27–34, and 40 → 30–34. The olivine + orthopyroxene + clinopyroxene + garnet assemblage was found in equilibrium with carbonatite melt with Ca# 34 at 1200°C and Ca# 30 at 1400°C. Thus, K-rich (26–35 wt% K2O) carbonatite melts with Ca# = 30-34 can appear in equilibrium with garnet lherzolite, while the melts with Ca# < 30 and > 34 can be in equilibrium with harzburgite and wehrlite, respectively, at 6 GPa and 1200–1400°C. Considering that Ca-Mg-Fe carbonates do not melt at the geothermal conditions of the SCLM, while sodic, dolomitic melt causes wehrlitization, high-Mg (Ca# < 35) K-rich dolomitic melt is the only possible carbonatite fluids that are thermodynamically stable in equilibrium with garnet harzburgites and lherzolites in the SCLM at a depth of about 200 km. At higher temperatures corresponding to the underlying asthenosphere, the high alkalinity ceases to be a requirement for the stability of the carbonate melt. Nevertheless, the regularities established here for the K-rich melts remain valid for less alkaline (4–15 wt% Na2O+K2O) primary kimberlite (i.e., mantle carbonatite) melts in the sublithospheric mantle.

Slab-derived melts interacting with peridotite: Toward the origin of diamond-forming melts

It was recently shown that partial melting of carbonated metapelites, subducted to a depth of 200 km, yields the formation of two immiscible melts, CO2-bearing phonolitic and K-rich carbonate. These melts resemble silicic and low-Mg carbonatitic melt inclusions in diamonds from kimberlites and placers worldwide. Here we studied the interaction of these melts with natural garnet lherzolite at 6 GPa. We found that the CO2-bearing phonolite melt reacts with peridotite consuming olivine to produce orthopyroxene and garnet, while K2O and CO2 enter carbonate melt. The latter has Ca# 24–29 and appears in equilibrium with garnet lherzolite. The SiO2 content in the carbonate melt varies from 2 to 18 wt% as temperature increases from 1200 to 1500 °C. Our results imply that the slab-derived immiscible silicic and low-Mg carbonatitic melts react with peridotitic mantle producing the high-Mg carbonatitic melt, which makes up the majority of carbonatitic inclusions in diamonds. Thus, the melt entrapped by diamonds may decipher genetic signatures of different mantle lithologies: silicic and low-Mg carbonatitic inclusions correspond to eclogite or recycled pelite, while high-Mg carbonatitic inclusions correspond to peridotite.

Correlations between Garnet Species and Vibration Spectroscopy: Isomorphous Substitution Implications

Garnet has many species because of its common isomorphism. In this study, a suite of 25 natural gem-quality garnets, including pyrope, almandine, spessartine, grossular, and andradite, were examined by standard gemological testing, LA-ICP-MS, FTIR, and Raman analysis. Internal stretching and bending vibrations of the SiO4-tetrahedra of garnet exhibit correlate with the type of cations in garnet’s dodecahedral position (A site) and octahedral position (B site). FTIR and Raman spectra showed that with the increase of the radius of Mg2+, Fe2+, Mn2+, and Ca2+ in A site, or the unit cell volumes of pyrope, almandine, spessartine, and grossular, the spectral peaks of Si–Ostr and Si–Obend modes shift to low wavenumber. Because of the largest cations both in A site (Ca2+) and in B site (Fe3+), andradite exhibited the lowest wavenumber of Si–Ostr and Si–Obend modes of the five garnet species. Therefore, garnet has correlations between chemical composition and vibration spectroscopy, and Raman or IR spectroscopy can be used to precisely identify garnet species.

Dynamic Metasomatism Experiments Investigating the Interaction between Migrating Potassic Melt and Garnet Peridotite

Dynamic metasomatism experiments were performed by reacting a lamproite melt with garnet peridotite by drawing melt through the peridotite into a vitreous carbon melt trap, ensuring the flow of melt through the peridotite and facilitating analysis of the melt. Pressure (2–3 GPa) and temperature (1050–1125 °C) conditions were chosen where the lamproite was molten but the peridotite was not. Phlogopite was formed and garnet and orthopyroxene reacted out, resulting in phlogopite wehrlite (2 GPa) and phlogopite harzburgite (3 GPa). Phlogopites in the peridotite have higher Mg/(Mg + Fe) and Cr2O3 and lower TiO2 than in the lamproite due to buffering by peridotite minerals, with Cr2O3 from the elimination of garnet. Compositional trends in phlogopites in the peridotite are similar to those in natural garnet peridotite xenoliths in kimberlites. Changes in melt composition resulting from the reaction show decreased TiO2 and increased Cr2O3 and Mg/(Mg + Fe). The loss of phlogopite components during migration through the peridotite results in low K2O/Na2O and K/Al in melts, indicating that chemical characteristics of lamproites are lost through reaction with peridotite so that emerging melts would be less extreme in composition. This indicates that lamproites are unlikely to be derived from a source rich in peridotite, and more likely originate in a source dominated by phlogopite-rich hydrous pyroxenites. Phlogopites from an experiment in which lamproite and peridotite were intimately mixed before the experiment did not produce the same phlogopite compositions, showing that care must be taken in the design of reaction experiments.

Solidus and melting of carbonated phlogopite peridotite at 3–6.5 GPa: Implications for mantle metasomatism

It is well known that water significantly lowers mantle solidi. But it turns out this paradigm is not always true. Here, we studied the interaction of K-rich carbonate melts with the natural garnet lherzolite from the Udachnaya kimberlite (Russia) in the presence of water at 3.0–6.5 GPa, corresponding to depths of 100–200 km. We found that at ≤ 1100 °C, the metasomatic interaction consumes garnet, orthopyroxene, and melt to produce phlogopite ± K-richterite + magnesite ± dolomite. Besides, primary clinopyroxene is replaced by one with a lower amount of jadeite component. Thus, the addition of water to the K-rich carbonate melt, infiltrating the subcontinental lithospheric mantle, should yield its partial or complete disappearance accompanied by phlogopitization and carbonation.The studied systems have H2O/K2O = 2, like that in phlogopite, and therefore correspond to carbonated phlogopite peridotite under fluid-absent conditions. At 4.0–6.5 GPa, the solidus of carbonated phlogopite peridotite is controlled by the following reaction: phlogopite + clinopyroxene + magnesite = garnet + orthopyroxene + olivine + hydrous K-carbonatite melt, which is bracketed between 1100 and 1200 °C. At 3 GPa, the solidus temperature decreases to about 1050 °C presumably owing to the Ca-Mg exchange reaction, clinopyroxene + magnesite = orthopyroxene + dolomite, which stabilizes dolomite reacting with phlogopite at a lower temperature than magnesite.Our results suggest that the phlogopite- and carbonate-rich metasomatic vein networks, weakening rigid lithosphere and promoting continental rifting, could be formed as a result of infiltration of hydrous K-carbonatite melt at the base of subcontinental lithospheric mantle. Stretching and thinning of the cratonic lithosphere make geotherms warmer and shifts their intersections with the solidus of carbonated phlogopite peridotite to shallower depths. Consequently, the successive erosion of the continental lithosphere and ascent of the lithosphere-asthenosphere boundary during continental rifting should be accompanied by remelting of phlogopite-carbonate metasomes, upward percolation of K-rich melt, and its solidification at the front of the magmatic-metasomatic mantle system.

Deformation and strength of mantle relevant garnets: Implications for the subduction of basaltic-rich crust

Abstract Garnet is an important mineral phase in the upper mantle as it is both a key component in bulk mantle rocks, and a primary phase at high pressure within subducted basalt. Here, we focus on the strength of garnet and the texture that develops within garnet during accommodation of differential deformational strain. We use X-ray diffraction in a radial geometry to analyze texture development in situ in three garnet compositions under pressure at 300 K: a natural garnet (Prp60Alm37) to 30 GPa, and two synthetic majorite-bearing compositions (Prp59Maj41 and Prp42Maj58) to 44 GPa. All three garnets develop a modest (100) texture at elevated pressure under axial compression. Elasto-visco-plastic self-consistent (EVPSC) modeling suggests that two slip systems are active in the three garnet compositions at all pressures studied: {110}&amp;lt;111&amp;gt; and {001}&amp;lt;110&amp;gt;. We determine a flow strength of ~5 GPa at pressures between 10 to 15 GPa for all three garnets; these values are higher than previously reported yield strengths measured on natural and majoritic garnets. Strengths calculated using the experimental lattice strain differ from the strength generated from those calculated using EVPSC. Prp67Alm33, Prp59Maj41, and Prp42Maj58 are of comparable strength to each other at room temperature, which indicates that majorite substitution does not greatly affect the strength of garnets. Additionally, all three garnets are of similar strength as lower mantle phases such as bridgmanite and ferropericlase, suggesting that garnet may not be notably stronger than the surrounding lower mantle/deep upper mantle phases at the base of the upper mantle.

A Method for Secondary Ion Mass Spectrometry Measurement of Lithium Isotopes in Garnet: The Utility of Glass Reference Materials

We present a novel method for the measurement of lithium isotopes in garnet utilising glass reference materials and secondary ion mass spectrometry (SIMS). Measured lithium isotopic compositions of natural garnets are heterogeneous, making them unreliable reference materials for in situ determination. However, SIMS lithium isotope measurements of glasses derived from these natural garnets are isotopically identical to their parent garnets and more homogeneous, demonstrating that they can be used as reliable reference materials. To characterise the composition dependence of instrumental mass fractionation (IMF), oxide and silicate powders were used to synthesise custom‐made glass reference materials (CGRMs) with garnet‐equivalent compositions. Results for six CGRMs measured by SIMS show a significant linear relationship between IMF and FeO and MnO contents. Corrections for this compositional IMF result in changes of up to 12‰ within the compositional range explored. Uncertainty in IMF‐corrected SIMS analyses with a 20 μm spot is in the range of 2.5 to 4.5‰, depending on the garnet composition and reference materials used. The method for in situ lithium isotope measurement in garnet by SIMS presented here is highly adaptable, valid across a range of Al‐rich garnet compositions, and yields spatial resolution and precision necessary to address a range of geological applications.

Magnetodielectric response of composites based on a natural garnet and spinel ferrites for sub-GHz wireless applications

A natural garnet with excellent dielectric properties was mixed with different weight percentages of two spinel ferrites, Ni0.5Zn0.5Fe2O4 (NZO) and LiFe5O8 (LFO) to tailor its magnetodilectric properties. 3 wt% B2O3 was added to enhance the density of the composites. X-ray diffraction study revealed the decomposition of the mineral into hematite and cordierite and vibrational spectroscopic analysis confirmed the non-reactivity of decomposed mineral with spinel ferrites. Microstructural analysis shows well densified and almost tightly packed grains for garnet-Ni0.5Zn0.5Fe2O4 (G-N) and garnet-LiFe5O8 (G-L) composites. The optimised dielectric and magnetic properties of 0.5 G-0.5 NZO are εr = 4.1, μr = 1.8, tan δε = 0.02, tan δμ = 0.49, whereas that of 0.5 G–0.5 LFO are εr = 4.4, μr = 1.4, tan δε = 0.001, tan δμ = 0.05 at 1 GHz. Due to the moderate permittivity of garnet, a better impedance matching compared to magnetodielectric composites based on high-permittivity dielectric counterparts is observed. Hence, the present study indicates that G-N and G-L composites are potential candidates for sub-gigahertz wireless applications.

Residence time of igneous garnet in Si-rich magmatic systems: Insights from diffusion modeling of major and trace elements

Although garnet is an important accessory phase in many granitic rocks, the petrogenetic history of such garnets is often debated. Explanations range from crystallization from (mostly) peraluminous melts to entrainment of peritectic or xenocrystic garnets originating from country rocks. Here we present a detailed microchemical study of a mid-crustal granodiorite from the Ivrea-Zone (N-Italy), which contains metapelitic enclaves and composite metamorphic-magmatic xeno-phenocryst garnet. Garnets from this locality exhibit high-phosphorus (∼800 ppm) metamorphic cores and multiple igneous overgrowth rims. The high-phosphorus cores appear to originate from granulite facies country-rock, while low-phosphorus rims indicate magmatic overgrowth in two main episodes. Zircon-bearing nanogranitoid inclusions are trapped along embayments between garnet cores and the first magmatic growth zones. Zirconium saturation thermometry indicates a temperature of 820 ± 5°C during the first magmatic overgrowth, and a temperature of 778 ± 10°C for the second overgrowth. The latter temperature agrees with garnet-biotite thermometry using the garnet rim composition, which yields a temperature of 773 ± 29 °C. In order to quantify the duration of the overgrowth episodes, we have numerically modeled Cr, Y, REEs and Hf trace element diffusion, as well as multicomponent major divalent cation diffusion within garnet using available experimental diffusion data and Cr diffusion data retrieved from natural garnets. All modeled diffusants conform to a single temperature-time path, in which the temperatures associated with the first and second magmatic overgrowths persisted for 5.4 and 6.3 kyr, respectively. Composite garnets in Si-rich magmatic systems have the potential to constrain magma transit rates in the continental crust.

Ferric-ferrous iron ratios of experimental majoritic garnet and clinopyroxene as a function of oxygen fugacity

Abstract The oxidation state of iron in upper mantle minerals is widely used to constrain the Earth mantle's oxidation state. Previous studies showed high levels of ferric iron in high-pressure majoritic garnets and pyroxenes despite reducing conditions. To disentangle the effects of pressure and increasing oxygen fugacity on the Fe3+/ΣFe ratios of garnet and clinopyroxene, we performed high-pressure experiments at a pressure of 10 GPa in a 1000-ton Walker-type multi-anvil apparatus at the University of Münster. We synthesized majoritic garnets and clinopyroxenes with a total iron content close to natural mantle values at different oxygen fugacities, ranging from IW+4.7 to metal saturation at IW+0.9. We analyzed the iron oxidation state in garnets with the electron microprobe “flank method.” Furthermore, we investigated the oxidation state of iron in garnets and clinopyroxenes with transmission electron microscopy (TEM) electron energy loss spectroscopy (EELS). Although the flank method measurements are systematically lower than the EELS measurements, Fe3+/ΣFe obtained with both methods agree well within 2σ errors. The “flank method” has the advantage of being much faster and more easily to set up, whereas TEM-EELS has a much higher spatial resolution and can be applied to various non-cubic minerals such as orthopyroxenes and clinopyroxenes. We used our experimental results to compare two geobarometers that contain a term for ferric iron in garnet (Beyer and Frost 2017; Tao et al. 2018) with two geobarometers that do not account for ferric iron (Collerson et al. 2010; Wijbrans et al. 2016). We found that for garnets with low total Fe and Fe3+ (like many natural garnets), the pressures can be calculated without including the ferric iron content.