An empirical garnet (YAG) – xenotime thermometer

 A pronounced negative correlation between the yttrium concentration in garnet ([Y]_Grt) and temperature has been observed in xenotime (YPO₄)-bearing metapelites from central New England, USA. The [Y]_Grt decreases roughly two orders of magnitude (∼5500 to less than 100 ppm Y) over a 150 °C interval. A regression of ln([Y]_Grt) against estimated reciprocal temperature yields the following relationship:

The influence of Fe<Superscript>3+</Superscript> on garnet–orthopyroxene and garnet–olivine geothermometers

Applying Fe²⁺–Mg exchange geothermometers to natural samples may lead to incorrect temperature estimates if significant Fe³⁺ is present. In order to quantify this effect, high-pressure experiments were carried out in a belt apparatus in a natural system close to CFMAS at 5 GPa and 1,100–1,400 °C. The oxygen fugacity in the experiments was at or below the Re–ReO₂ buffer. This is at significantly more oxidized conditions than in previous experiments, and, as consequence, higher Fe³⁺/Fe²⁺ ratios were generated. The Fe³⁺ content of garnet in the experiments was quantified by electron microprobe using the flank method. Making the usual assumption that Fe_total = Fe²⁺, the two-pyroxene thermometer of Brey and Köhler (J Pet 31:1353–1378, 1990) reproduced the experimental temperature to ±35 °C and the garnet–clinopyroxene Fe²⁺–Mg exchange thermometer of Krogh (Contrib Miner Pet 99:44–48, 1988) overestimated the temperatures on average by only 25 °C. On the other hand, application of the garnet–olivine (O’Neill and Wood in Contrib Miner Pet 70:59–70, 1979) and garnet–orthopyroxene (Harley in Contrib Miner Pet 86:359–373, 1984) exchange geothermometers yielded an underestimation in calculated temperatures of >200 °C. However, making explicit accounting for Fe³⁺ in garnet (i.e. using only measured Fe²⁺) leads to a vast improvement in the agreement between calculated and experimental temperatures, generally to within ±70 °C for the garnet–orthopyroxene geothermometer as well as noticeable improvement of calculated temperatures for the garnet–olivine geothermometer. Our results demonstrate that the two-pyroxene and garnet–clinopyroxene thermometers are rather insensitive to the presence of Fe³⁺ whilst direct accounting of Fe³⁺ in garnet is essential when applying the garnet–olivine and garnet–orthopyroxene thermometers.     

Experimental evaluation of garnet–clinopyroxene geothermometry as applied to eclogites

This study performed equilibrium experiments in order to evaluate previously proposed formulations of the garnet (Grt)–clinopyroxene (Cpx) thermometer as applied to eclogites. The starting material is fine-grained powder of natural eclogite (<10 μm), whose main constituents are Grt (Fe:Mg:Ca∼44:28:28), Cpx (Na pfu∼0.55–0.60), phengite, quartz and rutile. Experimental conditions are 1,100–1,250°C at 2.5 GPa, and the run duration is 193–334 h. The experimental run products mainly consist of Grt, Cpx, and glass. In a preliminary experiment at 1,000°C for 144 h, Cpx grains are clearly zoned and most Grt grains maintain primary compositions. In the higher T (≥1,100°C) and longer run (≥193 h) experiments, Cpx in the run products becomes poorer in Na and higher in Fe/Mg compared with the starting material, and each grain does not show clear chemical zoning. Garnet compositions become poorer in Ca [Ca/(Fe+Mn+Mg+Ca)∼0.2–0.25] and lower in Fe/Mg compared with the starting material. The average composition of Cpx and the average of Ca-poor Grt compositions in each run product were used to evaluate previously proposed formulations of the Grt–Cpx thermometer. Temperatures calculated with formulations by Pattison and Newton (1989) and Berman et al. (1995) are much lower than the experimental temperatures, even though these formulations are based on the compositional bracketing-type experiment. One of the reasons for this discrepancy might be uncertainty of solid-solution properties of Al in Cpx, because the value of the excess interaction parameter for Al in the generally low-Al Cpx modeled by Berman et al. (1995) is much higher than those proposed by independent experiments, resulting in the estimated temperatures being significantly lower than the experimental temperatures.     

Eclogite-facies fluid infiltration: constraints from δ18O zoning in garnet

In situ analysis reveals that eclogite-facies garnets are zoned in δ¹⁸O with lower values in the core and rims that are ~1.5 to 2.5 ‰ higher. This pattern is present in 9 out of 12 garnets analyzed by SIMS from four orogenic eclogite terranes, and correlates with an increase in the mole fraction of pyrope and Mg/Fe ratio from core to rim, indicating prograde garnet growth. At the maximum temperatures and the time-scales experienced by these garnets, calculated intragranular diffusion distances for oxygen are small (<5 μm), indicating that δ¹⁸O records primary growth zoning and not diffusive exchange. The oxygen isotope gradients are larger than could form due to temperature changes during closed-system mineral growth. Thus, gradients reflect the compositions of fluids infiltrating during prograde metamorphism. Values of δ¹⁸O in garnet cores range from ?1 to 15 ‰, likely preserving the composition of the eclogite protoliths. Two garnet cores from the Almenningen eclogite in the Western Gneiss Region, Norway, have δ¹⁸O ~?1 ‰ and are the first negative δ¹⁸O eclogites identified in the region. In contrast with orogenic eclogites, seven high δ¹⁸O garnets (>5 ‰) from two kimberlites are homogeneous in δ¹⁸O, possibly due to diffusive exchange, which is possible for prolonged periods at higher mantle temperatures. Homogeneity of δ¹⁸O in garnets outside the normal mantle range (5–6 ‰) may be common in kimberlitic samples.     

Dating magmatic and hydrothermal processes using andradite-rich garnet U–Pb geochronometry

Andradite-rich garnet is a common U-bearing mineral in a variety of alkalic igneous rocks and skarn deposits, but has been largely neglected as a U–Pb chronometer. In situ laser ablation-inductively coupled plasma mass spectrometry U–Pb dates of andradite-rich garnet from a syenite pluton and two iron skarn deposits in the North China craton demonstrate the suitability and reliability of the mineral in accurately dating magmatic and hydrothermal processes. Two hydrothermal garnets from the iron skarn deposits have homogenous cores and zoned rims (Ad₈₆Gr₁₁ to Ad₉₈Gr₁) with 22–118 ppm U, whereas one magmatic garnet from the syenite is texturally and compositionally homogenous (Ad₇₀Gr₂₂ to Ad₇₇Gr₁₄) and has 0.1–20 ppm U. All three garnets have flat time-resolved signals obtained from depth profile analyses for U, indicating structurally bound U. Uranium is correlated with REE in both magmatic and hydrothermal garnets, indicating that the incorporation of U into the garnet is largely controlled by substitution mechanisms. Two hydrothermal garnets yielded U–Pb dates of 129 ± 2 (2σ; MSWD = 0.7) and 130 ± 1 Ma (2σ; MSWD = 0.5), indistinguishable from zircon U–Pb dates of 131 ± 1 and 129 ± 1 Ma for their respective ore-related intrusions. The magmatic garnet has a U–Pb age of 389 ± 3 Ma (2σ; MSWD = 0.6), consistent with a U–Pb zircon date of 388 ± 2 Ma for the syenite. The consistency between the garnet and zircon U–Pb dates confirms the reliability and accuracy of garnet U–Pb dating. Given the occurrence of andradite-rich garnet in alkaline and ultramafic magmatic rocks and hydrothermal ore deposits, our results highlight the potential utilization of garnet as a powerful U–Pb geochronometer for dating magmatism and skarn-related mineralization.     

Dating high-grade metamorphism—constraints from rare-earth elements in zircon and garnet

We integrate petrography and SIMS REE analyses of garnet and polyphase zircon from a pelitic granulite adjacent to the Ronda peridotite, Betic Cordillera, southern Spain to constrain the significance of zircon U-Pb geochronology. Sillimanite inclusions in garnet rims suggest that they grew during decompression, and Ca enrichment in their rims records initiation of partial melting. Chondrite-normalised REE profiles of zircon cores are typically magmatic (positive La to Lu slope and Ce anomaly), whereas overgrowths have flat or negatively sloping heavy-REE profiles (Gd-Lu). The presence of rimmed zircon grains only in the garnet rims and the matrix suggests that this zircon phase grew after garnet had already sequestered heavy REEs, a process documented here by progressive depletion of heavy REE in the garnets from centre to rim. Combined with the textural evidence, we suggest that the U-Pb age of 21.3ǂ.3 Ma obtained from the zircon rims dates a point on this decompression path rather than the peak metamorphic pressure.     

Cr-rich megacrysts of clinopyroxene and garnet from Lac de Gras kimberlites,Slave Craton,Canada – implications for the origin of clinopyroxene and garnet in cratonic lherzolites

Kimberlites from the Diavik and Ekati diamond mines in the Lac de Gras kimberlite field contain abundant large (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) crystals. We present the first extensive mineral chemical dataset for these megacrysts from Diavik and Ekati and compare their compositions to cratonic peridotites and megacrysts from the Slave and other cratons. The Diavik and Ekati Cr-diopside and Cr-pyrope megacrysts are interpreted to belong to the Cr-rich megacryst suite. Evidence for textural, compositional, and isotopic disequilibrium suggests that they constitute xenocrysts in their host kimberlites. Nevertheless, their formation may be linked to extensive kimberlite magmatism and accompanying mantle metasomatism preceding the eruption of their host kimberlites. It is proposed that the formation of megacrysts may be linked to failed kimberlites. In this scheme, the Cr-rich megacrysts are formed by progressive interaction of percolating melts with the surrounding depleted mantle (originally harzburgite). As these melts percolate outwards, they may contribute to the introduction of clinopyroxene and garnet into the depleted mantle, thereby forming lherzolite. This model hinges on the observation that lherzolitic clinopyroxenes and garnets at Lac de Gras have compositions that are strikingly similar to those of the Cr-rich megacrysts, in terms of major and trace elements, as well as Sr isotopes. As such, the Cr-rich megacrysts may have implications for the origin of clinopyroxene and garnet in cratonic lherzolites worldwide.

     

Monomineral universal clinopyroxene and garnet barometers for peridotitic,eclogitic and basaltic systems

New versions of the universal Jd-Di exchange clinopyroxene barometer for peridotites,pyroxenites and eclogites,and also garnet barometer for eclogites and peridotites were developed.They were checked using large experimental data sets for eciogitic(~530) and peridotitic systems(650).The precision of the universal Cpx barometer for peridotites based on Jd-Di exchange is close to Cr-Tschermalite method produced by Nimis and Taylor(2000).Cpx barometer was transformed by the substitution of major multiplier for K_D by the equations dependent from Al-Na-Fe.Obtained equation in combination with the thermometer of Nimis and Taylor(2000) allow to reconstruct position of the magma feeder systems of the alkali basaltic magma within the mantle diapirs in modern platforms like in Vitim plateau and other Southern Siberia localities and several localities worldwide showing good agreement of pressure ranges for black and green suites.These equations allow construct PTX diagrams for the kimberlite localities in Siberia and worldwide calculating simultaneously the PT parameters for different groups of mantle rocks.They give very good results for the concentrates from kimberlite lamproites and placers with mantle minerals.They are useful for PT estimates for diamond inclusions.The positions of eclogite groups in mantle sections are similar to those determined with new Gar—Cpx barometer produced by C.Beyer et al.(2015).The Fe rich eclogites commonly trace the boundary between the lower upper parts of subcontinental lithospheric mantle(SCLM) at 3-4 CPa marking pyroxenite eclogites layer.Ca-rich eclogites and especially grospydires in SCLM beneath Precambrian kimberlites occurs near pyroxenite layer but in younger mantle sections they became common in the lower parts.The diamondiferous Mg Cr-less group eclogites referring to the ancient island arc complexes are also common in the middle part of mantle sections and near 5-6 GPa.Commonly eclogites in lower apart of mantle sections are remelted and trace the high temperature convective branch.The Mg-and Fe-rich pyroxenites also show the extending in pressure trends which suggest the anatexic melting under the influence of volatiles or under the interaction with plums.     

U-Pb garnet chronometry in high-grade rocks—case studies from the central Damara orogen (Namibia) and implications for the interpretation of Sm-Nd garnet ages and the role of high U-Th inclusions

Garnets from different migmatites and granites from the Damara orogen (Namibia) were dated with the U-Pb technique after bulk dissolution of the material. Measured ²⁰⁶Pb/²⁰⁴Pb ratios are highly variable and range from ca. 21 to 613. Variations in isotope (²⁰⁸Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁴Pb) and trace element (Th/U, U/Nd, Sm/Nd) ratios of the different garnets show that some garnets contain significant amounts of monazite and zircon inclusions. Due to their very low ²⁰⁶Pb/²⁰⁴Pb ratios, garnets from pelitic migmatites from the Khan area yield Pb-Pb ages with large errors precluding a detailed evaluation. However, the ²⁰⁷Pb/²⁰⁶Pb ages (ca. 550–500 Ma) appear to be similar to or older than U-Pb monazite ages (530±1–517±1 Ma) and Sm-Nd garnet ages (523±4–512±3 Ma) from the same sample. It is reasonable to assume that the Pb-Pb garnet ages define growth ages because previous studies are consistent with a higher closure temperature for the U-Pb system in garnet relative to the U-Pb system in monazite and the Sm-Nd system in garnet. For igneous migmatites from Oetmoed, Pb-Pb garnet ages (483±15–492±16 Ma) and one Sm-Nd garnet whole rock age (487±8 Ma) are similar whereas the monazite from the same sample is ca. 30–40 Ma older (528±1 Ma). These monazite ages are, however, similar to monazite ages from nearby unmigmatized granite samples and constrain precisely the intrusion of the precursor granite in this area. Although there is a notable difference in closure temperature for the U-Pb and Sm-Nd system in garnet, the similarity of both ages indicate that both garnet ages record garnet growth in a migmatitic environment. Restitic garnet from an unmigmatized granite from Omaruru yields similar U-Pb (493±30–506±30 Ma) and Sm-Nd (493±6–488±7 Ma) garnet ages whereas the monazite from this rock is ca. 15–25 Ma older (516±1–514±1 Ma). Whereas the monazite ages define probably the peak of regional metamorphism in the source of the granite, the garnet ages may indicate the time of melt extraction. For igneous garnets from granites at Oetmoed, the similarity between Pb-Pb (483±34–474±17 Ma) and Sm-Nd (492±5–484±13 Ma) garnet ages is consistent with fast cooling rates of granitic dykes in the lower crust. Differences between garnet and monazite U-Pb ages can be explained by different reactions that produced these minerals at different times and by the empirical observation that monazite seems resistant to later thermal re-equilibration in the temperature range between 750 and 900 °C (e.g. Braun et al. 1998). For garnet analyses that have low ²⁰⁶Pb/²⁰⁴Pb ratios, the influence of high- inclusions is small. However, the relatively large errors preclude a detailed evaluation of the relationship between the different chronometers. For garnet with higher ²⁰⁶Pb/²⁰⁴Pb ratios, the overall similarity between the Pb-Pb and Sm-Nd garnet ages implies that the inclusions are not significantly older than the garnet and therefore do not induce a premetamorphic Pb signature upon the garnet. The results presented here show that garnet with low ²³⁸U/²⁰⁴Pb ratios together with Sm-Nd garnet data and U-Pb monazite ages from the same rock can be used to extract geologically meaningful ages that can help to better understand tectonometamorphic processes in high-grade terranes.Editorial responsibility: J. Hoefs     

Timing of high-pressure metamorphic events in the Bulgarian Rhodopes from Lu–Hf garnet geochronology

Within the Mediterranean realm, the Rhodopes represent a nappe stack of oceanic and continental fragments assembled along the Eurasian continental margin during the Alpine orogeny. The timing of the high-pressure (HP) metamorphism has long been ambiguous, lacking detailed geochronological and geochemical control on subduction-exhumation and nappe stacking processes. Here we apply the Lu–Hf and Sm–Nd chronometers to a suite of representative eclogite samples covering two different key units of the Rhodopean nappe stack: (1) the Kimi Complex (Upper Allochthon) and (2) the Middle Allochthon. In addition to geochronology, we also determined whole rock Hf and Nd isotope compositions as well as major and trace element concentrations in order to constrain the nature of the eclogite protoliths. Two HP metamorphic events were revealed by Lu–Hf geochronology: (1) a Lower Cretaceous event in the Upper Allochthon (126.0 ± 1.7 Ma) and (2) an Eocene event in the Middle Allochthon (44.6 ± 0.7 Ma; 43.5 ± 0.4 Ma; 42.8 ± 0.5 Ma), at conditions of ca. 700°C/20–25 kbar. Our new data provide direct evidence for multiple subduction events in the Rhodopes. Exhumation and subsequent thrusting of the Middle Allochthon on the Lower Allochthon can be narrowed down to the time span between 42 and 34 Ma. In a broader tectonic context, the Eocene ages for the HP metamorphism support the view that the Rhodopes represent a large-scale tectonic window, exposing the deepest nappe units of the Hellenides.     

Compositional zoning of garnet porphyroblasts from the polymetamorphic Wölz Complex, Eastern Alps

We employ garnet isopleth thermobarometry to derive the P–T conditions of Permian and Cretaceous metamorphism in the Wölz crystalline Complex of the Eastern Alps. The successive growth increments of two distinct growth zones of the garnet porphyroblasts from the Wölz Complex indicate garnet growth in the temperature interval of 540°C to 560°C at pressures of 400 to 500 MPa during the Permian and temperatures ranging from 550°C to 570°C at pressures in the range of 700 to 800 MPa during the Cretaceous Eo-Alpine event. Based on diffusion modelling of secondary compositional zoning within the outermost portion of the first garnet growth zone constraints on the timing of the Permian and the Eo-Alpine metamorphic events are derived. We infer that the rocks remained in a temperature interval between 570°C and 610°C over about 10 to 20 Ma during the Permian, whereas the high temperature stage of the Eo-Alpine event only lasted for about 0.2 Ma. Although peak metamorphic temperatures never exceeded 620°C, the prolonged thermal annealing during the Permian produced several 100 µm wide alteration halos in the garnet porphyroblasts and partially erased their thermobarometric memory. Short diffusion profiles which evolved around late stage cracks within the first garnet growth zone constrain the crack formation to have occurred during cooling below about 450°C after the Eo-Alpine event.     

Spinel to garnet lherzolite transition in relation to high temperature palaeogeotherms,eastern Australia∗

Eastern Australian xenolith suites and lithospheric transition zones are re‐evaluated using new mineral analyses and thermo‐barometry. Some suites, including that defining the southeastern Australian geotherm, are not fully equilibrated. New pressure‐temperature estimates, based on experimental calibrations that allow for Cr and Ti in pyroxenes, differ from earlier results by up to 0.6 GPa and 250°C. The preferred Brey and Köhler 1990 thermo‐barometer indicates a shallower cooler garnet lherzolite transition under Mesozoic New South Wales (50 km depth at 980° C) than for Tertiary Tasmania (60 km depth at 1090°C).

Deviations between palaeogeotherms may reflect: (i) higher temperature gradients for Tasmania and New South Wales (by 100°C/0.1 GPa) related to abnormally hot mantle; (ii) higher temperature gradients linked to more voluminous magmatism, largely Cenozoic in age; and (iii) complex temperature perturbations linked to different levels of magmatic intrusion.

These deviations blur reconstructions of lithospheric assemblages, where temperature is determinable and pressure comes from an assumed geotherm. Potential errors in locating spinel lherzolite and crust‐mantle transition assemblages may reach 15 km in depth. The highest Tertiary geothermal gradients in Tasmania and northeastern New South Wales match those from regions of active lithospheric extension. The young southeastern Australian geotherm is decaying from a higher temperature equilibration, based on experimental work, and Mesozoic New South Wales geotherms trend towards the lower gradients of bounding cratons.     

Hornblende–garnet–plagioclase thermobarometry: a natural assemblage calibration of the thermodynamics of hornblende

Calibrations are presented for an independent set of four equilibria between end-members of garnet, hornblende, plagioclase and quartz. Thermodynamic data from a large internally-consistent thermodynamic dataset are used to determine the ΔG° of the equilibria. Then, with the known mixing properties of garnet and plagioclase, the non-ideal mixing in amphibole is derived from a set of 74 natural garnet–amphibole–plagioclase–quartz assemblages crystallised in the range 4–13 kbar and 500–800 °C. The advantage of using known thermodynamic data to calculate ΔG° is that correlated variations of composition with temperature and pressure are not manifested in fictive derived entropies and volumes, but are accounted for with non-ideal mixing terms. The amphibole is modelled using a set of ten independent end-members whose mixing parameters are in good agreement with the small amount of data available in the literature. The equilibria used to calibrate the amphibole non-ideal mixing reproduce pressures and temperatures with average absolute deviations of 1.1 kbar and 35 °C using an average pressure–temperature approach, and 0.8 kbar with an average pressure approach. The mixing data provide not only a basis for thermobarometry involving additional phases, but also for calculation of phase diagrams in complex amphibole-bearing systems. Received: 8 November 1999 / Accepted: 7 July 2000     

A refined garnet - biotite Fe−Mg exchange geothermometer and its application in amphibolites and granulites

A new formulation of garnet-biotite Fe–Mg exchange thermometer has been developed through statistical regression of the reversed experimental data of Ferry and Spear. Input parameters include available thermo-chemical data for quaternary Fe–Mg–Ca–Mn garnet solid solution and for excess free energy terms, associated with mixing of Al and Ti, in octahedral sites, in biotite solid solution. The regression indicates that Fe–Mg mixing in biotite approximates a symmetrical regular solution model showing positive deviation from ideality withW _FeMg ^bi =1073±490 cal/mol. H _r and S _r for the garnet-biotite exchange equilibrium were derived to be 4301 cal and 1.85 cal respectively. The resultant thermometer gives consistent results for rocks with a much wider compositional range than can be accommodated by earlier formulations.     

Anisotropic mean-squared-displacement tensor in cubic almandine garnet: a single crystal 57Fe M?ssbauer study

This paper describes single-crystal measurements on a crystal plate cut from a naturally-occurring almandine-rich single crystal (Alm₆₉Pyr₁₉Spe₈Gro₄) from Wrangell Alaska. The objective was to measure the mean-squared-displacement (msd) tensor precisely using M?ssbauer spectroscopy. Parallel quantum-mechanical calculations based on X-ray determined atomic displacement parameters and M?ssbauer parameters from polycrystalline measurements indicated that the msd tensor should display significant anisotropy, easily measurable within the precision of the M?ssbauer experiment. For each single-crystal orientation the observed M?ssbauer spectrum represents a macroscopic quantity that is the average over six symmetry-related (local) dodecahedral sites in which the high-spin Fe²⁺ ions reside. The anisotropy in the measured msd tensor is, nevertheless, unequivocal. Furthermore, the magnitudes of the M?ssbauer-determined msd principal values exceed those of the corresponding X-ray-determined quantities by a factor 3.7. The equivalent recoilless fractions are also anisotropic and consequently one observes the Gol’danskii–Karyagin Effect (GKE), as manifested by an asymmetric quadrupole doublet in polycrystalline absorbers. Moreover, the line widths of the two quadrupole lines are markedly different but angle invariant. This is interpreted as implying that, in addition to anisotropy in the msd tensor, differential spin–spin relaxation is present in the $ m = \pm 3/2 \leftrightarrow \pm 1/2 $ and $ m = \pm 1/2 \leftrightarrow \pm 1/2,\, \mp 1/2 $ nuclear transitions. While both effects contribute to the quadrupole asymmetries observed in M?ssbauer spectra of polycrystalline almandine, the GKE is apparently predominant.     

Discovery of Ordovician–Silurian metamorphic monazite in garnet metapelites of the Alpine External Aiguilles Rouges Massif

The pre-Mesozoic, mainly Variscan metamorphic basement of the Col de Bérard area (Aiguilles Rouges Massif, External domain) consists of paragneisses and micaschists together with various orthogneisses and metabasites. Monazite in metapelites was analysed by the electron microprobe (EMPA-CHIME) age dating method. The monazites in garnet micaschists are dominantly of Variscan age (330–300 Ma). Garnet in these rocks displays well developed growth zonations in Fe–Mg–Ca–Mn and crystallized at maximal temperatures of 670°C/7 kbar to the west and 600°C/7–8 kbar to the east. In consequence the monazite is interpreted to date a slightly pressure-dominated Variscan amphibolite-facies evolution. In mylonitic garnet gneisses, large metamorphic monazite grains of Ordovician–Silurian (~440 Ma) age but also small monazite grains of Variscan (~300 Ma) age were discovered. Garnets in the mylonitic garnet gneisses display high-temperature homogenized Mg-rich profiles in their cores and crystallized near to ~800°C/6 kbar. The Ordovician–Silurian-age monazites can be assigned to a pre-Variscan high-temperature event recorded by the homogenised garnets. These monazite age data confirm Ordovician–Silurian and Devonian–Carboniferous metamorphic cycles which were already reported from other Alpine domains and further regions in the internal Variscides.     

Constraints on the early cooling rates of Mt. Dabie from diffusion modeling of chemical zoning of garnet

Diffusion modeling of zoning profiles in eclogite garnets from three different tectonic units of Mt. Dabie, UHPM unit, HPM unit and northern Dabie, was used to estimate the relative time span and cooling rates of these rocks. Modeling result for the Huangzhen eclogite garnet shows that the maximal time span for the diffusion-adjustment process is about 22 Ma since the peak-temperature metamorphism, which is the maximum time span from amphibolite facies metamorphism to greenschist facies metamorphism. The Bixiling eclogites had subjected to a cooling process at a rate of - 10℃/Ma from 750℃ to 560℃ during 20 Ma. The second cooling stage of the Raobazhai eclogite following granulite-facies metamorphism is an initial fast cooling process at a rate of about 25℃/Ma and then slowed down gradually. All these belong to a coherent Dabie collision orogen with differences in subduction depth and exhumation/uplifting path.     

The CaGeO3–Ca3Fe2Ge3O12 garnet join: an experimental study

Germanate garnets are often used as isostructural analogues of silicate garnets to provide insight into the crystal chemistry and symmetry of the less accessible natural garnet solid solutions. We synthesised two series of germanate garnets at 3 GPa along the join^VIIICa₃^VI(CaGe)^IVGe₃O₁₂–^VIIICa₃^VIFe₂^IVGe₃O₁₂ at 900 °C and 1,100 °C. Samples with compositions close to the CaGeO₃ end-member consist of tetragonal garnet with a small amount of triclinic CaGe₂O₅. Samples with nominal compositions between X_Fe=0.4 and 1.0 consist of a mixture of tetragonal and cubic garnets; whereas, single-phase cubic garnets were obtained for compositions with X_Fe>1.2 (X_Fe gives the iron content expressed in atoms per formula unit, and varies between 0 and 2 along the join). Run products which were primarily single-phase garnet were investigated using Mössbauer spectroscopy. Spectra from samples synthesised at 1,100°C consist of one well-resolved doublet that can be assigned to Fe³⁺ in the octahedral site of the garnet structure. A second doublet, present primarily in samples synthesised at 900°C, can be assigned to Fe²⁺ at the octahedral sites of the garnet structure. The relative abundance of Fe²⁺ decreases with increasing iron content. Transmission electron microscopy analyses confirm this tendency and show that the garnets are essentially defect-free. The unit-cell parameters of tetragonal ^VIIICa₃^VI(CaGe)^IVGe₃O₃ garnet decrease with increasing synthesis temperature, and the deviation from cubic symmetry becomes smaller. Cubic garnets show a linear decrease of unit-cell parameter with increasing iron content. The results are discussed in the context of iron incorporation into ^VIIIMg₃^VI(MgSi)^IVSi₃O₃ majorite.