Garnet Components Research Articles

Highly stable lithium batteries enabled by composite solid electrolyte with synergistically enhanced in-built ion-conductive framework

The solid-state electrolytes (SSEs) are considered as a decent solution to rationally supersede the liquid one for the sake of dependable operation security and higher energy density when it comes to the next-generation lithium (Li) metal batteries. Herein, a sort of composite solid electrolyte with in-built ion-conductive 3D framework is designed to enable the stable performances of Li-metal batteries. The framework comprised of the Li+-conducting garnet components (Li6.75La3Zr1.75Nb0.25O12, LLZN) and polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) provides the vulnerable part with mechanical support and well-behaved electrochemical stability. Stemmed from this embedded framework, the whole electrolyte exhibits high Li+ conductivity (4.6×10−4 S cm−1), improved high-voltage endurance (4.4 V versus Li/Li+), better Li-anode compatibility and high-temperature feasibility. Additionally, the synergistic enhancements between LLZN and PVDF-HFP potentially facilitate the homogeneous ionic flux, thereby resulting in the persistence of a sound solid-electrolyte interphase. The solid-state batteries assembling composite solid electrolyte also deliver the preferable battery performance, featuring high-capacity output with stable cyclability. Application of this electrolyte in a pouch cell further demonstrates its feasibility and flexibility. This design concept is anticipated to give the way of exploring brand new solid electrolytes and boosting their overall electrochemical performances toward practicality.

P–T–t Path of Unusual Garnet–Kyanite–Staurolite– Amphibole Schists, Ellesmere Island, Canada—Quartz Inclusion in Garnet Barometry and Monazite Petrochronology

Abstract Garnet–kyanite–staurolite assemblages with large, late porphyroblasts of amphibole form garbenschists in Ordovician volcaniclastic rocks lying immediately south of the Pearya terrane on northernmost Ellesmere Island, Canada. The schist, which together with carbonate olistoliths makes up the Petersen Bay Assemblage (PBA), displays a series of parallel isograds that mark an increase in metamorphic grade over a distance of 10 km towards the contact with Pearya; however, a steep, brittle Cenozoic strike-slip fault with an unknown amount displacement disturbs the earlier accretionary relationship. The late amphibole growth, probably due to fluid ingress, is clear evidence of disequilibrium conditions in the garbenschist. In order to recover the P–T history of the schists, we construct isochemical phase equilibrium models for a nearby garnet–mica schist that escaped the fluid event and compare the results to quartz inclusion in garnet (QuiG) barometry for a garbenschist and the metapelitic garnet schist. Quartz inclusions are confined to garnet cores and the QuiG results, combined with Ti-in-biotite and garnet–biotite thermometry, delineate a prograde path from 480 to 600°C and 0.7 to 0.9 GPa. This path agrees with growth zoning in garnet deduced from X-ray maps of the spessartine component in garnet. The peak conditions obtained from pseudosection modelling using effective bulk composition and the intersection of garnet rim with matrix biotite and white mica isopleths in the metapelite are 665°C at ≤0.85 GPa. Three generations of monazite (I, II and III) were identified by textural characterization, geochemical composition (REE and Y concentrations) and U–Pb ages measured by ion microprobe. Monazite I occurs in the matrix and as inclusions in garnet rims and grew at peak P–T conditions at 397 ± 2 Ma (2σ) from the breakdown of allanite. Monazite II forms overgrowths on matrix Monazite I grains that are oriented parallel to the main schistosity and yield ages of 385 ± 2 Ma. Monazite III, found only in the garbenschist, is 374 ± 6 Ma, which is interpreted as the time of amphibole growth during fluid infiltration at lower temperature and pressure on a clockwise P–T path that remained in the kyanite stability field. These results point to a relatively short (≈12 Myr) Barrovian metamorphic event that affected the schists of the PBA. An obvious heat source is lacking in the adjacent Pearya terrane, but we speculate it was large Devonian plutons—similar to the 390 ± 10 Ma Cape Woods granite located 40 km across strike from the fault—that have been excised by strike-slip. Arc fragments that are correlative to the PBA are low grade; they never saw the heat and were not directly involved in Pearya accretion.

Skarn formation and Cu[sbnd]Ag mineralization in the McKenzie Gulch area, northern New Brunswick, Canada: Implication for the applications of mineral chemistry in exploration for porphyry copper and skarn deposits

The McKenzie Gulch (MG) copper–silver skarn occurrences are associated with the Middle Devonian intermediate to felsic dyke swarms. Mineralization occurs as veins and stockwork of veinlets, disseminations, patchy, and locally as replacement of calc-silicate skarns in argillaceous limestone. This skarn is interpreted to have developed in three stages: the earliest contact metamorphic stage resulting in hornfels developed from calcareous mudstone and very fine-grained clastic sedimentary rocks (Stage I); this was followed by metasomatic replacement resulting in prograde anhydrous skarn containing grossular-andradite (grandite), diopsidic pyroxene, and wollastonite (Stage II); increasing fluid/rock interaction results in retrograde skarn dominated by epidote, calcite, green amphibole, chlorite, sulfide minerals, titanite, andradite and hedenbergite commonly along veins and veinlets (Stage III). Subordinate sphalerite and pyrite occur in late veins cross-cutting both porphyry dykes, skarns and hornfels zone.Prograde garnets in the study area belong to the grossular-andradite solid solution that ranges from Adr16Gr82 to almost pure andradite - Adr99Gr1, whereas other garnet end-members constitute less than 5% collectively. Pyroxenes form a diopside-hedenbergite solid solution with composition between Di31Hd60 and Di93Hd7, while other pyroxenes constitute less than 10% collectively. These garnets and pyroxenes exhibit chemical zonation patterns generally characterized by a rim of Fe enrichment relative to cores of grains, which may suggest possible change in salinity (or other factor that promotes Fe transport). This means grossular and diopsidic cores are rimmed by more andradite and hedenbergitic compositions for garnet and pyroxene, respectively. In general, there is a tendency for increasing Fe component in both garnet and pyroxene from the prograde to retrograde stages.Epidote formed during the retrograde stage exhibits oscillatory zoning similar to retrograde andradite and indicates Fe enrichment increasing from core to rim. Based on the major element composition, textural and optical characteristics of garnets, we conclude that grandites (Al-rich) formed under low water/rock ratios, in equilibrium with metasomatic fluids whose composition was locally buffered by the host rocks, whereas andradite (Fe-rich) resulted from relatively high water/rock ratios that were in equilibrium with a magmatic-derived fluid. The results of this study indicate that there is a relationship between the composition of pyroxenes and garnets of the skarn alteration facies and the dominant metal of the mineralized skarns. These facies plot in the compositional field of Cu-dominated skarns. As a result, this relationship may be applied in exploration for skarn and porphyry copper mineralization.

Effect of Thermoelastic Properties of the Pyrope-Almandine Solid Solutions on the Entrapment Pressure of Garnet-Related Elastic Geobarometer

The pyrope (Prp)–almandine (Alm) solid solutions are the most fundamental garnet components on the Earth, and both the quartz inclusions in garnet (QuiG) barometry and the garnet inclusions in diamond barometry need to be constrained by the thermoelastic parameters of Prp-Alm solid solution garnets. Here, we report the thermoelastic properties of a series of synthetic Prp-Alm solid solutions based on the high-pressure and high-temperature (HP–HT)in situsynchrotron single-crystal x-ray diffraction (SCXRD) experiments up to ∼20 GPa and 700 K, using diamond anvil cell (DAC). Fitting the SCXRD data by the Birch-Murnaghan equation of state (BM-EoS) and the thermal-pressure EoS, we obtain the thermoelastic parameters of Prp-Alm solid solution garnets, including bulk modulus (K0), its pressure derivative (K′0), and the thermal expansion coefficient (α0). TheK0along the Prp-Alm solid solution changes linearly with Prp content within their uncertainties and can be expressed byK0(GPa) = 181.0(8) – 0.11(1)Xprp(R2= 0.91,Xprpis the Prp mole fraction andK′0fixed at 4). Our result indicates that the compressibility of the Prp-Alm solid solution increases with the increasing Prp content. However, the thermal expansion coefficient of Prp-Alm solid solution at ambient pressure shows a non-linear trend with Prp content and can be expressed byα0(10−5K−1) = 2.7 (1) + 3.0 (5)XPrp−3.2 (4)X2Prp(R2= 0.985). It shows that the Prp-Alm solid solution with intermediate composition has a larger thermal expansion coefficient than those close to the endmembers at ambient conditions. Furthermore, we also evaluated the influence of thermoelastic properties of the Prp-Alm solid solution on the entrapment pressure (Pe) estimation for two types of elastic geobarometers. Our results indicate that the garnet component may significantly influence entrapment pressure, and among the thermoelastic parameters of garnet, the thermal expansion coefficient has the main effect on the estimation ofPe.

Solidus of carbonated phlogopite eclogite at 3–6 GPa: Implications for mantle metasomatism and ultra-high pressure metamorphism

The interaction of natural eclogite (Ecl) with synthetic hydrous carbonate melts with Na:K = 0:1 (KH2) and 1:1 (NKH2) was studied in multianvil experiments at 3–6 GPa and 850–1250 °C. The interaction with KH2 consumes garnet and clinopyroxene producing phlogopite and calcite-dolomite solid solution. Besides, the interaction yields a decrease in the jadeite component of clinopyroxene, evolving eclogite toward pyroxenite. This is consistent with a metasomatic alteration of eclogite xenoliths, manifested as Na-poor “spongy” clinopyroxene, replacing primary omphacite, and kelyphitic rims around garnet, containing phlogopite and carbonates. The interaction with NKH2 also produces phlogopite and carbonate, but the latter is more magnesian and represented by magnesite, above the solidus, and magnesite + dolomite below the solidus. The interaction with NKH2 increases the jadeite component in clinopyroxene and grossular component in garnet, evolving eclogite Group A to eclogite Group B.The studied systems have H2O/K2O = 2, like that in phlogopite, and therefore correspond to carbonated phlogopite eclogite under fluid-absent conditions. Based on the obtained results its solidus is situated near 1050 °C at 3 GPa and decreases to 950 °C at 6 GPa. Thus, hydrous K- and Na-K-carbonatite melts can coexist with eclogite in SCLM at depths exceeding 120–170 km, and solidify as temperature decreases below 950–1050 °C according to the following solidus reactions: pyrope + diopside + melt → phlogopite + dolomite, below 6 GPa, and pyrope + diopside + melt → phlogopite + magnesite + grossular, at 6 GPa.The melting reaction, involving phlogopite and dolomite, suggests the partial melting at the peak of ultrahigh-pressure metamorphism (UHPM) during continent–continent plate collision. The prograde P-T path of UHPM crosses the solidus of clinopyroxene + garnet + phlogopite + dolomite assemblage at 4.7–5.2 GPa and 970–990 °C and yields the formation of hydrous K-carbonatite melt-fluid in situ. This melt could be responsible for the formation of K-bearing clinopyroxenes and microdiamonds in the UHPM marbles in the Kokchetav massif, Kazakhstan. The retrograde P-T path intersects the solidus that has a negative Clapeyron slope in the diamond stability field. Thus, the hydrous K-carbonatite melt should disappear soon after the peak of metamorphism reacting with garnet to produce Ca-Mg carbonates and phlogopite.

Sound Velocity of MgSiO3 Majorite Garnet up to 18 GPa and 2000 K

AbstractMgSiO3 majorite is the most significant endmember of the Al‐deficient majorite garnets that form at depths higher than ∼400 km, in the mantle transition zone (MTZ). Here, we report elastic wave velocity measurements on polycrystalline MgSiO3 majorite samples, up to 18.4 GPa and 2000 K by ultrasonic interferometry techniques combined with in situ X‐ray diffraction measurements in a multianvil apparatus. Our data show MgSiO3 majorite has the lowest elastic moduli among silicate garnet endmembers, under the pressure and temperature conditions of the MTZ. These new data combined with those of other garnet endmembers allowed to estimate VP and VS of majorite with compositions relevant to the MTZ. The results suggest that variation of MgSiO3 component in garnet may play a role for interpreting seismic gradients atop the 410‐km discontinuity while below ∼520 km, the velocity contrasts hold too low values to explain the discrepancy between mineral physics data and seismological observations.

Evolution of spinel-bearing ultrahigh-temperature granulite in the Jining complex, North China Craton: constrained by phase equilibria and Monte Carlo methods

The peak temperature and timescale of ultrahigh-temperature (UHT) metamorphism are significant for understanding its thermal budget and geodynamic evolution. The spinel-bearing (sapphirine-absent) UHT granulite at the Xuwujia area in the Jining complex, North China Craton was revealed to have undergone a metamorphic evolution that involves the pre-Tmax (maximum temperature) heating decompression to the Tmax stage with an extreme temperature of ~1125 °C, the post-Tmax cooling to the fluid-absent solidus (~860 °C) at 0.8–0.9 GPa, and sub-solidus decompression. The Tmax condition was recorded by the inferred feldspar-absent peak assemblage of spinel ± garnet. The post-Tmax cooling evolution was indicated by the sequential appearances of plagioclase, K-feldspar, sillimanite and biotite, as well as the core-to-rim ascending grossular component in garnet. Moreover, some texturally zoned spinel has exsolved lamellae of magnetite in the core and shows rim-ward increase of Mg and Al, which suggests a temperature decrease from >1100 °C in local domains isolated from quartz. Spinel with exsolved magnetite lamellae or low Al/(Al + Fe3+) is therefore proposed as an indicator for UHT conditions in metapelites. Probability simulation on the collected zircon ages yields a timescale of ~40 Myr (95% confidence; during 1.90–1.94 Ga) for the supra-solidus cooling stage of the UHT metamorphism in the Jining complex. This extreme UHT metamorphism has reached the rock’s dry solidus (~1125 °C) and undergone slow cooling, which is interpreted to result from a post-orogenic plume activity with sufficient advective heating from hyperthermal mafic intrusions.

Phlogopite Formation in the Orthopyroxene–Garnet System in the Presence of H2O–KCl Fluid in Application to the Processes of Mantle Metasomatism

The results of experimental studies are presented for reactions in the orthopyroxene–garnet–phlogopite system in the presence of H2O–KCl fluid at 3–5 GPa and 900–1000°C, which model the processes of phlogopite formation in garnet peridotites and pyroxenites during alkaline metasomatism of the upper mantle. The experiments demonstrated regular variations in the composition of garnet, pyroxenes, and phlogopite depending on the KCl content of the fluid. With increasing KCl content of the fluid, enstatite and garnet become unstable, the Al2O3 content of enstatite decreases, and the amount of grossular and knorringite components in garnet are maximum at a KCl content of ~10 mol %. Our results illustrate well the regular variations in the compositions of the coexisting minerals and their zoning in phlogopite-bearing peridotites of the lithospheric mantle.

Ultramafic Carbonated Melt‐ and Auto‐Metasomatism in Mantle Eclogites: Compositional Effects and Geophysical Consequences

AbstractThe mineralogy, chemical composition, and physical properties of cratonic mantle eclogites with oceanic crustal protoliths can be modified by secondary processes involving interaction with fluids and melts, generated in various slab lithologies upon subduction (auto‐metasomatism) or mantle metasomatism after emplacement into the cratonic lithosphere. Here we combine new and published data to isolate these signatures and evaluate their effects on the chemical and physical properties of eclogite. Mantle metasomatism involving kimberlite‐like, ultramafic carbonated melts (UM carbonated melts) is ubiquitous though not pervasive, and affected between ~20% and 40% of the eclogite population at the various localities investigated here, predominantly at ~60–150 km depth, overlapping cratonic midlithospheric seismic discontinuities. Its hallmarks include lower jadeite component in clinopyroxene and grossular component in garnet, an increase in bulk‐rock MgO ± SiO2, and decrease in FeO and Al2O3 contents, and LREE‐enrichment accompanied by higher Sr, Pb, Th, U, and in part Zr and Nb, as well as lower Li, Cu ± Zn. This is mediated by addition of a high‐temperature pyroxene from a UM carbonated melt, followed by redistribution of this component into garnet and clinopyroxene. As clinopyroxene‐garnet trace‐element distribution coefficients increase with decreasing garnet grossular component, clinopyroxene is the main carrier of the metasomatic signatures. UM carbonated melt‐metasomatism at >130–150 km has destroyed the diamond inventory at some localities. These mineralogical and chemical changes contribute to low densities, with implications for eclogite gravitational stability, but negligible changes in shear‐wave velocities, and, if accompanied by H2O‐enrichment, will enhance electrical conductivities compared to unenriched eclogites.

Calibration of the garnet–biotite–Al2SiO5–quartz geobarometer for metapelites

AbstractA garnet–biotite–Al2SiO5–quartz (GBAQ) geobarometer was empirically calibrated using more than 700 natural metapelites with a broad compositional range of garnet and biotite under P–T conditions of 450–950°C and 1–17 kbar. In the calibration, activity models of garnet and biotite identical to those in the garnet–biotite (GB) geothermometer of Holdaway [American Mineralogist 2000, 85:881–892] were used. Therefore, the GBAQ geobarometer and the GB geothermometer can be simultaneously applied to iteratively estimate metamorphic P–T conditions. Successful applications of the GBAQ geobarometer to natural metapelites certify its validity. Most importantly, when plagioclase is absent or CaO components in garnet and/or plagioclase are deficient, this geobarometer may prove useful for estimating metamorphic pressures. The random error of the present GBAQ geobarometer is inferred to be around ±1.8 kbar. An electronic spreadsheet is available as Table S4 to apply the GBAQ geobarometer in combination with the GB geothermometer.

From phosphates to silicates and back: An experimental study on the transport and storage of phosphorus in eclogites during uplift and exhumation

High P-T experiments have shown that in major rock types such as MORBs, peridotites, and pelitic sediments, increasing P (and T ) leads to a gradual transfer of P from phosphates mostly represented by apatite to silicates with garnet as most important silicate P carrier. This is due to the formation of a Na 3 Al 2 (PO 4 ) 3 phase component in garnet via [ 8 ] Na [ 4 ] P [ 8 ] M − 1 2 + [ 4 ] Si − 1 that is strongly P -, and to a lesser extent T -dependent and creates garnets with significant P and Na at P as low as 2–3 GPa. Based on this experimental evidence, one would expect to routinely find P-Na-rich garnets in UH P -rocks with a wide range in composition. With very few exceptions, however, this is not the case. This discrepancy indicates that both P and Na are effectively released from garnet and re-distributed within the garnet matrix during uplift and exhumation. To explore the mechanisms of this P-Na release, P-Na-rich garnet pre-synthesized at 7 GPa and 1200 °C, containing 0.7 wt% P 2 O 5 and 0.3 wt% Na 2 O, was exposed to P-T conditions of 2 GPa and 800–1000 °C in a simplified, H 2 O-bearing, model eclogitic bulk composition. The experiments show that at subsolidus temperatures of 850–975 °C, and in the presence of a hydrous fluid, apatite quickly forms from garnet breakdown involving consumption of coexisting quartz and clinopyroxene. The apatites usually appear as rounded to lath-shaped isolated grains scattered in the garnet + clinopyroxene ± orthopyroxene + quartz + rutile matrix. More rarely, single apatite inclusions, or clusters of inclusions, may form in clinopyoxene or garnet. The observed apatite grain size is in the range ≤ ~1 × 1 to 24 × 6 μm with the largest grains occasionally containing clinopyroxene inclusions. Combined garnet breakdown and neo-formation, using pre-existing garnet as a nucleation site, may form zoned garnets with Na-P-depleted and Ti-enriched rims that represent an approach to a garnet composition typical for mid- to shallow crustal P-T conditions. Partial melting experiments indicate that eclogites containing P-rich garnet may produce P-rich and apatite-undersaturated melts for moderately SiO 2 -rich melt compositions. These melts can crystallize abundant apatite during solidification and, thus, would be effective agents for P-extraction during partial melting. Due to their very low apatite saturation concentration, the P-transport and storage capacity of granitic melts would be much more limited. An unexpected finding of this study are the substantial P and Mg contents in kyanite with 0.17–0.20 wt% P 2 O 5 and 0.20–0.56 wt% MgO, respectively. The combined P-Mg incorporation into kyanite is consistent with the coupled substitution [4] Si 4+ + [6] Al 3+ = [4] P 5+ + [6] Mg 2+ and with a strong preference of P for orthosilicate structures. The results of this study suggest that some to even all of the apatite now present in eclogites that underwent deep subduction formed by chemical adjustment of the eclogite garnet to decreasing Na 3 Al 2 (PO 4 ) 3 solubility during uplift and exhumation. This results in the appearance of apatite as a new phase in a hitherto apatite-free assemblage. Rapid transport to the surface and/or a lack of suitable reactants can suppress this re-equilibration and explain the occasionally high P- and Na-contents of eclogitic garnet-inclusions in diamond or garnets from diamondiferous eclogites sampled by kimberlites. Similarly, concurrent decreasing Ti-solubility in garnet may lead to rutile-saturation in eclogites that did not contain this phase under peak- P conditions as evidenced by the joint occurrence of (oriented) rutile and apatite inclusions in the eclogitic garnets. For the application of geothermobarometry, this delayed Ti (+P) saturation is important to be kept in mind.

An experimental investigation of the stability of majoritic garnet in the Earth’s mantle and an improved majorite geobarometer

The stability of the majorite component in garnet has been experimentally investigated at high pressure and high temperature, focusing on the effect of bulk composition and temperature. High-pressure experiments were performed in a multi-anvil apparatus, at pressures ranging from 6 to 14.5 GPa, and temperatures between 1400 and 1700 °C. Experiments were performed in a range of bulk compositions in the system SiO2–Al2O3–Cr2O3–CaO–MgO with varying Cr/(Cr + Al) ratios. The majorite content of garnet gradually increases with pressure, and the composition of the garnet, specifically the Cr/(Cr + Al) ratio, exerts a significant effect on the majorite substitution. We found no significant effect of temperature. We use the experimental results in combination with the literature data to derive two empirical geobarometers, which can be used to determine the equilibration pressure of natural majoritic garnets of peridotitic and eclogitic bulk compositions. The barometer for peridotitic compositions is $${\text{P}} = - 77.1 + 27.6 \times {\text{Si}} + 1.67 \times {\text{Cr}}$$ And the barometer for eclogitic compositions is $${\text{P}} = - 29.6 + 11.8 \times {\text{Si}} + 7.81 \times {\text{Na}} + 4.49 \times {\text{Ca}}.$$

Compositions of hydrothermal silicates and carbonates as indicators of physicochemical conditions in the Reykjanes geothermal system, Iceland

This paper presents results of a quantitative investigation of mineral chemistry in the Reykjanes geothermal system, Iceland, designed to assess the relative influence of various hydrothermal minerals on the observed bulk-rock mass changes of elements. Detailed petrography and electron microprobe analyses (n=∼1400) were performed on samples of rock cuttings from eleven wells that intersect different parts of the hydrothermal system. An important finding was the strong partitioning of barium into adularia, which becomes incorporated through “capture” substitution into the 12-fold coordination site, coupled with Al3+ substitution into the tetrahedral site. Adularia has average and maximum Ba contents of 0.8 and 3.5wt.%, respectively. In addition, there is no relationship between Ba concentration in adularia and crystal size, occurrence type (replacement versus vein/vug), or rock composition. There is also no correlation between the Ba content of replacement adularia with that of the igneous plagioclase being altered, demonstrating that Ba concentration in the altered rocks of the Reykjanes geothermal system is being controlled by large-scale hydrothermal redistribution processes and not primary chemostratigraphic variability. Calcite exerts a major control on strontium proportions in the bulk rock, and contains up to 1wt.% Sr. Concentrations of many minor and trace elements and some major elements in hydrothermal minerals at Reykjanes correlate positively with measured downhole temperature (to varying degrees), e.g.: Ba and Rb in adularia; Al(iv) in chlorite; Fe, Mg, Mn, and Sr in calcite; and the andradite component in garnet. The Fe/(Fe+Mg) of chlorite is unrelated to the precursor host rock ratio. Chlorite geothermometry, based on Al(iv) substitution and Fe/(Fe+Mg) content, predicts temperatures of 94–313°C (average=236°C). These temperatures are similar to or less than the present-day temperatures measured for corresponding sample depths in all wells (with the exception of a shallow interval from RN10). This may indicate a prolonged and ongoing heating trend in the deep liquid-phase region of the Reykjanes reservoir.

Early Variscan P-T evolution of an eclogite body and adjacent orthogneiss from the northern Malpica-Tuy shear-zone in NW Spain

An eclogite and its surrounding gneiss from the Malpica-Tuy zone, northwestern Spain, were studied. In addition to garnet and omphacite, the eclogite contains barroisitic amphibole, quartz, phengite with Si contents between 3.27 and 3.45 per formula unit (pfu), rutile, and accessory talc and (clino)zoisite–epidote. Garnet exhibits a chemical zonation with Alm 59 Prp 13 Grs 24 Sps 4 , Alm 57 Prp 13 Grs 27 Sps 3 , Alm 56.6 Prp 12.5 Grs 30.2 Sps 0.7 , and Alm 54 Prp 20 Grs 25 Sps 1 as inner core, outer core, inner rim, and outermost rim compositions, respectively. The gneiss is a former granodiorite now mainly composed of quartz, plagioclase, K-feldspar, biotite, garnet, (clino)zoisite–epidote, and phengite with Si contents between 3.27 and 3.38 pfu. Garnet shows inner core, outer core, and rim compositions of Alm 26.9 Prp 0.2 Grs 70 Sps 2.9 , Alm 33.8 Prp 0.7 Grs 64 Sps 1.5 , and Alm 42.6 Prp 1.4 Grs 54 Sps 2 , respectively. A series of P–T pseudosections, calculated with PERPLE_X for the bulk-rock compositions of the studied eclogite and gneiss, were contoured by isopleths of various parameters, including molar fractions of garnet components. Using mainly the variable garnet and phengite compositions and these isopleths, P–T paths were derived. After an isothermal burial path the eclogite reached peak pressures of 22.5 kbar at 540°C. The subsequent exhumation path passed through P–T conditions of 21 kbar and 575°C and 13.5 kbar and 625°C. The latter data represent the peak P–T conditions of the gneiss. These P-T data suggest that the protolith of the eclogite, a basic rock of an island-arc setting, underwent high-pressure metamorphism in a subduction zone. During its exhumation it came in contact with the buried metagranodiorite, representing the tip of a descending continental plate, at depths of 45–50 km probably 350–355 Ma ago.

Paleoproterozoic migmatitic gneisses from the Tandilia belt (Argentina), Río de la Plata craton, record cooling at deep crustal levels

We studied high-grade metamorphic rocks of the El Cristo hill area of the Tandilia belt. Mineral analyses and thermodynamic calculations were carried out for two adjacent rock samples: an amphibole–biotite gneiss and a garnet–biotite-bearing migmatite. Peritectic garnets in the migmatite show core compositions of pyr4.5(gro + andr)10spes6alm79.5 changing to pyr3.5(gro + andr)17spes6alm73.5 at their thin rims. Garnet compositions in the gneiss are pyr6.5(gro + andr)26spes12alm55.5 and pyr4.5(gro + andr)34spes12alm49.5 for core and rim, respectively. A P–T path was constructed by calculating pseudosections in the 11-component system Si–Ti–Al–Fe–Mn–Mg–Ca–Na–K–O–H and contouring them by isopleths for garnet components using the PERPLE_X software package. Supra-solidus crystallization of garnet cores in the migmatite began at 5.8 kbar and 660 °C. Garnet rims equilibrated at 7.0 kbar and 640 °C compatible with garnet cores in the amphibole–biotite gneiss (7.6 kbar and 660 °C). The further chemical development of garnet in this rock points to P–T conditions of 11.6 kbar and 620 °C and 12.2 kbar and 595 °C (outermost garnet rim). At this high-pressure stage Ca-amphibole was not stable. Most biotite formed during exhumation whereas the high-pressure accessory minerals, titanite and epidote, persisted. According to the obtained anti-clockwise P–T path the originally partly melted material was tectonically transported from ∼22 km (middle crust) to ∼40 km (lower crust) depths reaching a geothermal gradient as low as 15 °C km−1. This transport probably occurred along a major suture zone, which was active during the Paleoproterozoic (2.25–2.10 Ga), before a terminating collision of terranes near the SW boundary of the Rio de la Plata craton.

Calculation of the energetics of water incorporation in majorite garnet

Interpretation of lateral variations in upper mantle seismic wave speeds requires constraints on the relationship between elasticity and water concentration at high pressure for all major mantle minerals, including the garnet component. We have calculated the structure and energetics of charge-balanced hydrogen substitution into tetragonal MgSiO 3 majorite up to P = 25 GPa using both classical atomistic simulations and complementary first-principles calculations. At the pressure conditions of Earth’s transition zone, hydroxyl groups are predicted to be bound to Si vacancies (□) as the hydrogarnet defect, [□ Si +4OH O ] X , at the Si2 tetrahedral site or as the [□ Mg +2OH O ] X defect at the octahedral Mg3 site. The hydrogarnet defect is more favorable than the [□ Mg +2OH O ] X defect by 0.8-1.4 eV/H at 20 GPa. The presence of 0.4 wt% Al 2 O 3 substituted into the octahedral sites further increases the likelihood of the hydrogarnet defect by 2.2-2.4 eV/H relative to the [□ Mg +2OH O ] X defect at the Mg3 site. OH defects affect the seismic ratio, R = dlnvs/dlnvp, in MgSiO 3 majorite (ΔR = 0.9-1.2 at 20 GPa for 1400 ppm wt H 2 O) differently than ringwoodite at high pressure, yet may be indistinguishable from the thermal dlnvs/dlnvp for ringwoodite. The incorporation of 3.2 wt% Al 2 O 3 also decreases R(H 2 O) by ~0.2-0.4. Therefore, to accurately estimate transition zone compositional and thermal anomalies, hydrous majorite needs to be considered when interpreting seismic body wave anomalies in the transition zone.

Petrogenesis and chemogenesis of oceanic and continental orogens in Asia: Current topics, Part I

Petrogenesis and chemogenesis of oceanic and continental orogens in <scp>A</scp>sia: Current topics, <scp>Part I</scp>

Zoning of garnets as an indicator of metamorphic evolution in metapelites of the Yenisei Ridge

The geochemical patterns of major and trace elements in zonal garnets and the mineral inclusions in them formed by progressive and regressive metamorphism of pelites are established. It is shown that an increase in temperature and pressure led to a decrease in the Y and HREE contents in garnets, and the increase in their contents is related to a decrease in the PT-parameters of their formation. A negative correlation between the CaO and REE contents in garnet indicates their isomorphism. The main reason for the sharp increase in the CaO content in garnets during collision metamorphism is mass transfer between the garnet and the plagioclase. The deviations from this situaiton, which are expressed in simultaneous increase in the grossular component in garnet and the anorthite component in plagioclase, are caused by metamorphic reactions related to the epidote decomposition. The mass transfer of major and trace elements between the reacting phases in metamorphic reactions mostly occurred with preservation of the balance of matter. The mirror shape and the character of the REE patterns of the rock-forming minerals relative to the composition of the rock indicate the equilibration of the HREE and Y contents between garnet, the major concentrator of these elements in the rock, and other phases. The balance between the LREEs and HREEs in the rock is achieved by the presence of variable amounts of monazite.

Garnet oxygen analysis by SHRIMP-SI: Matrix corrections and application to high-pressure metasomatic rocks from Alpine Corsica

Garnet is a key mineral used to constrain pressure, temperature and age of metamorphic rocks. This contribution reports oxygen isotope measurements in garnet using the SHRIMP-SI ion microprobe. The reproducibility of oxygen isotope analyses on garnet standard UWG2 is ~0.3–0.4‰ (2σ) within and across sessions. The correlation between oxygen isotope measurements and the grossular and andradite components in garnet fits a second-degree polynomial with a maximum bias in δ18O of 2.4 and 8.3‰, respectively. This bias is similar to that determined for other large ion microprobes. Analysis of two additional Mn-rich garnet crystals allowed identification of a separate bias caused by the spessartine component, which can reach a maximum of 2.3‰.The standardisation and correction scheme proposed in this study are applied to garnet crystals from two samples from Alpine Corsica in order to link fluid evolution with the pressure–temperature–time path. The samples have experienced a polyphase metamorphic history, which includes Permian high temperature metamorphism, followed by late However, 92-W2 was re-analysed because of the variability observed in the previously published δ18O values (-0.29 and 0.53‰, Kohn and Valley, 1998). The δ18O value found in this study (0.81 ± 0.44‰, 2s, Table 1) agrees within error with the highest value previously published for this garnet. Eocene high pressure–low temperature metamorphism. Permian Ca-poor garnet cores have high δ18O values (9.9±0.6‰ and 11.1±0.5‰ 2σ, in two samples) with respect to garnet mantles (7.2±0.4‰) and rims (5.4±0.5‰ and 2.2±0.4‰). The dramatic decrease in δ18O from Permian garnet cores to Alpine rims in both samples reflects a combination of external fluid influx and change in sample mineralogy. The low δ18O of the garnet rims that formed at eclogite facies conditions indicates that the metasomatic fluid equilibrated with mafic or ultramafic rocks. This study illustrates that fluid-mediated mass transfer during subduction occurred at lithological contacts between felsic and ultramafic rocks.

Evolution of a very deeply subducted metasediment from As Sifah, northeastern coast of Oman

Near the coastal village of As Sifah, NE Oman, eclogite-facies rocks occur in the Saih Hatat window. We investigated a metapelite from this area, which is composed of mm-sized garnet and greenish phengite and minor epidote, blue amphibole, paragonite, albite, quartz, rutile, opaque phases, barite, and carbonate. Garnet exhibits a chemical zonation with Gro17Alm66Pyr6Spe11, Gro22Alm72.5Pyr5Spe0.5, and Gro25Alm65Pyr8Spe2 as inner core, mantle and outermost rim compositions. Inner portions of phengite have Si contents of up to 3.6 per formula unit (pfu), whereas rims are poorer in Si (3.2–3.4 pfu). We constructed a P-T pseudosection in the system Na2O-K2O-CaO-FeO-O2-MnO-MgO-Al2O3-SiO2-TiO2-H2O for the bulk-rock composition of the studied metapelite and contoured it by isopleths of various parameters such as the molar fractions of garnet components. Based on this contouring a P-T path was derived that starts at ultrahigh-pressure conditions. Garnet began to form at 25kbar and 490°C. Subsequently, temperatures increased and pressures decreased to finally reach P-T conditions of 13kbar and 565°C at which low-Si phengite, the outermost rim of garnet, Na-amphibole, epidote, quartz, magnetite, and rutile were in equilibrium. The P-T path is related to events in a subduction channel where the top of subducted oceanic crust, including the studied metasediments, was involved in an upwards-directed mass flow, resulting in the release of about 3wt.% H2O by garnet formation from hydrous minerals such as chlorite and lawsonite. In order to get hints at the interaction of such hydrous fluids, we have analyzed the trace and minor elements in phengite. The contents of B, Rb, Cs, and Tl (20, 397, 6.7 and 1.7ppm, respectively) are nearly constant over the entire Si range of potassic white mica. In contrast, the contents of Ba and Sr increase from 900 and 0.5 to 10500 and 14ppm, respectively, with decreasing Si content in phengite. We hypothesize that this result reflects early leaching of mobile elements during subduction and the later approach of the rock to a barite deposit within a melange ascending in the subduction channel.