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The pressure dependence of the exchange of Cr between clinopyroxene and garnet in peridotite is applicable as a geobarometer for mantle-derived Cr-diopside xenocrysts and xenoliths. The most widely used calibration (Nimis and Taylor Contrib Miner Petrol 139: 541–554, 2000; herein NT00) performs well at pressures below 4.5 GPa, but has been shown to consistently underestimate pressures above 4.5 GPa. We have experimentally re-examined this exchange reaction over an extended pressure, temperature, and compositional range using multi-anvil, belt, and piston cylinder apparatuses. Twenty-nine experiments were completed between 3–7 GPa, and 1100–1400 °C in a variety of compositionally complex lherzolitic systems. These experiments are used in conjunction with several published experimental datasets to present a modified calibration of the widely-used NT00 Cr-in-clinopyroxene (Cr-in-cpx) single crystal geobarometer. Our updated calibration calculates P (GPa) as a function of T (K), CaCr Tschermak activity in clinopyroxene aCaCrTscpx, and Cr/(Cr + Al) (Cr#) in clinopyroxene. Rearranging experimental results into a 2n polynomial using multiple linear regression found the following expression for pressure:PGPa=11.03+-TKln(aCaCrTscpx)×0.001088+1.526×lnCr#cpxTKwhere Cr#cpx=CrCr+Al, aCaCrTscpx=Cr-0.81·Cr#cpx·Na+K, with all mineral components calculated assuming six oxygen anions per formula unit in clinopyroxene.Temperature (K) may be calculated through a variety of geothermometers, however, we recommend the NT00 single crystal, enstatite-in-clinopyroxene (en-in-cpx) geothermometer. The pressure uncertainty of our updated calibration has been propagated by incorporating all analytical and experimental uncertainties. We have found that pressure estimates below 4 GPa, between 4–6 GPa and above 6 GPa have associated uncertainties of 0.31, 0.35, and 0.41 GPa, respectively. Pressures calculated using our calibration of the Cr-in-cpx geobarometer are in good agreement between 2–7 GPa, and 900–1400 °C with those estimated from widely-used two-phase geobarometers based on the solubility of alumina in orthopyroxene coexisting with garnet. Application of our updated calibration to suites of well-equilibrated garnet lherzolite and garnet pyroxenite xenoliths and xenocrysts from the Diavik-Ekati kimberlite and the Argyle lamproite pipes confirm the accuracy and precision of our modified geobarometer, and show that PT estimates using our revised geobarometer result in systematically steeper paleogeotherms and higher estimates of the lithosphere‒asthenosphere boundary compared with the original NT00 calibration.

This work reports the first data on the Variscan metamorphic evolution of the Marmarosh/Maramuresh massif in the Outer Eastern Carpathians. Geothermobarometry determinations coupled with U-Th-Pb dating of monazite, apatite, titanite and rutile were used to construct P-T-t paths and refine the geodynamic evolution of the pre-Alpine crystalline basement. These clockwise P-T-t paths evolve from 560–630 MPa and 515–535 °C to c. 900–1180 MPa in the north (Ukraine), while in the southern nappe (Romania), the P-T-t conditions evolve from 455–620 MPa and 545–555 °C, through to 670–745 MPa and 540–560 °C, to 910–965 MPa and 645–660 °C. The northernmost nappes were likely structurally lower relative to the southern nappes. Variscan progressive metamorphism related to nappe stacking climaxed at 350–340 Ma, as documented by U-Pb rutile and U-Th-Pb monazite dating. In both regions, post-kinematic exhumation to 700–500 MPa, 550–630 °C MPa and then to the titanite stability field was dated at 317–327 Ma, using the U-Pb system on apatite and titanite. Subsequent Permian retrogression and exhumation was constrained to 280–290 Ma by U-Pb rutile and apatite and U-Th-Pb monazite dating. These data link the massif to the external zone of the Central European Variscides. We infer that all Variscan crystalline basement fragments in the Alps and Carpathians probably represent remnants of the same microcontinent, which was dismembered during Alpine orogenesis.

The reliability of eight Fe–Mg exchange geothermobarometers for garnet-bearing peridotites, pyroxenites and eclogites has been examined using a database comprised of more than 300 published peridotite, pyroxenite and eclogite experiments conducted from 10 to 70 kbar and 850 to > 1650 ∘ C . We have tested Fe–Mg exchange geothermometers suitable for a range of mantle lithologies, including websterite, harzburgite, wehrlite and eclogite. All geothermometers maintained an average difference in experimental and calculated temperature (T) Δ T = T exp - T calc of less than ± 50 °C with a standard deviation of Δ T between ± 50 to 150 °C. Most geothermometers performed well across a narrow range in ln Kd Fe - Mg A - B (where Kd Fe - Mg A - B = Fe A × Mg B ( Fe B × Mg A ) ), however, systematic overestimation and underestimation of T were observed outside of the optimal range of lnKd Fe - Mg A - B . Increases in experimental pressure (P) adversely affected several geothermometers, particularly those calibrated empirically using natural samples. All previously published calibrations of the garnet-clinopyroxene geothermometer were unable to reliably reproduce the experimental T for both peridotite and eclogite experimental compositions, which hinders their confident application to natural datasets. To improve the state of mantle geothermobarometry we have used our experimental database to recalibrate the (1) garnet-clinopyroxene Fe–Mg exchange geothermometer, and (2) garnet-orthopyroxene Fe–Mg exchange geothermometer. Each geothermometer has been recalibrated across an extended P, T, and compositional range. The inclusion of eclogitic experiments in the calibration for the garnet-clinopyroxene geothermometer permits application to both eclogitic and peridotitic/pyroxenitic assemblages equilibrated under a wide range of PT conditions in the upper mantle. Using multiple linear regression to solve for lnKd, we found the following expressions best reproduced the experimental T (℃) of our dataset: T Fe - Mg grt - cpx ( ∘ C) = 3356.34 - 0.008 × P kbar + 0.259 × X Ca grt + 0.914 × X Mg grt + - 0.159 × Jd cpx + ln Kd Fe - Mg grt - cpx + 1.265 - 273 T Fe - Mg grt - opx ( ∘ C) = 1851.85 - 0.007 × P kbar + - 1.83 × X Ca grt + ln Kd Fe - Mg grt - cpx + 1.08 - 273 . where, X Ca grt = Ca Ca + Fe + Mg , Kd Fe - Mg grt - opx = Fe grt × Mg opx ( Fe opx × Mg grt ) , X Mg grt = Mg Ca + Fe + Mg , Jd cpx = Na - Cr - 2 × Ti , Kd Fe - Mg grt - cpx = Fe grt × Mg cpx ( Fe cpx × Mg grt ) , with all elements calculated on the basis of 12 oxygen anions in garnet and 6 oxygen anions in clino- and orthopyroxene. Fe 2+  = total Fe. Our updated calibrations resolve several issues with earlier calibrations, including a poor performance at elevated P and compositional limitations. An improvement in precision and accuracy has been demonstrated through application to the experimental calibration dataset, a second independent set of published experimental data, and to natural peridotites, pyroxenites and eclogites from on and off craton settings. Iterative PT estimates on natural datasets calculated using our updated calibrations compare well with estimates from widely used calibrations such as the Taylor (1998) two-pyroxene solvus geothermometer. We anticipate that this contribution will provide an important reference for the reliability of mantle geothermometers and that our updated calibrations will be used in future studies on peridotite, pyroxenite and eclogite inclusions in diamond and mantle-derived xenoliths.

The Ötztal Nappe (Austroalpine Superunit, Eastern Alps) is volumetrically the largest polymetamorphic nappe of the western Austroalpine nappe stack and consists of paragneisses and micaschists with various intercalations of orthogneisses, amphibolites and rare marbles. The main foliation of these rocks is cross-cut by numerous dikes of basaltic to andesitic composition. Field- and textural investigations reveal that these dikes underwent only Eoalpine metamorphism and deformation and are not part of the earlier Variscan history of the region. While quantitative Eoalpine data are only available from the central and southeastern Ötztal Nappe, almost no such data is available from the northwestern part of the nappe. The aim of this study therefore is to refine the constraints of the northwest to southeast trending Eoalpine metamorphic gradient in the western Austroalpine nappe stack especially in the central and northwestern Ötztal Nappe using metamorphic mafic dikes. For this purpose, multi-equilibrium geothermobarometry paired with classical geother- mobarometry was performed on 19 dike samples from the Ötztal Nappe, the Texel Nappe and the Silvretta Nappe from the investigations of Rammlmair (1980). The metamorphic mafic dikes contain the mineral assemblage plagioclase + amphibole + quartz + chlorite + epidote + titanite ± biotite ± muscovite ± garnet. The anorthite content in plagioclase and the tschermakite and edenite component in Ca-amphiboles are clear mineral chemical indicators for increasing conditions from the northwest to the southeast of the Ötztal Nappe. The thermobarometric calculations with multi-equilibrium geothermobarometry (THERMOCALC v.3.21) yield conditions of 250–300 °C and 2–4 kbar for the northwestern Ötztal Nappe, reaching 550–600 °C and 8–10 kbar in the southeastern Ötztal Nappe, adjacent to the Schneeberg Nappe. The metamorphic mafic dike from the western Texel Nappe yields conditions of 540±41 °C and 8.9±1.6 kbar. Geother-mobarometric calculations using THERMOCALC v.3.45 yield similar results. Our results represent the first quantitative estimates of the complete northwest to southeast trending Eoalpine metamorphic field gradient in the Ötztal Nappe above and below the chloritoid isograd.

The study area (Rampur domain) is situated to the east of the Eastern Ghats Boundary Shear Zone (EGBSZ) and encompasses portions of the granulite facies rocks of the exhumed Proterozoic Eastern Ghats Province (EGP), India. The EGP is characterized by a diverse array of rock types, featuring a wide variety of mineral parageneses and chemical compositions, including charnockite, mafic granulite, Mg-Al granulite, felsic granulites, amphibolite, khondalite and anorthosite. In this study, we report for the first time evidence of ultra-high temperature (UHT) metamorphism within the mafic granulites of the relatively unexplored Rampur domain of the Eastern Ghats Province, using the two-pyroxene assemblage. The stable mineral assemblage present during peak metamorphism typically includes garnet, orthopyroxene 1 , clinopyroxene, hornblende 1 , quartz, and plagioclase 1 . The consumption of garnet observed in different reaction textures, alongside the formation of striking orthopyroxene 2 –plagioclase 2 and hornblende 2 –plagioclase 2 symplectites, represent the later phases of metamorphism. By applying TWQ calculation procedures to the mineral core compositions, we have determined peak metamorphic conditions of approximately 970 °C at a pressure of 10.5 kbar. Zircon dating results from LA-HR-ICP-MS indicate upper intercept ages of 2509.9 ± 21.7 Ma and 2479.9 ± 21.0 Ma for the protolith, while lower intercept ages of 965.7 ± 40.7 Ma and 979.8 ± 18.1 Ma correspond to the metamorphic age of the analyzed samples E-185 and E-186, respectively. Based on the textural relationship, derived zircon ages, fluid-P-T constraints, and P-T pseudosection model, we propose a decompressional evolutionary P-T-t path that supports the Neo-Proterozoic assembly of the Indo-Antarctic region.

A detailed mineral-scale study was conducted on pumices of the latest, dominantly explosive eruption epoch (56–30 ka) of Ciomadul, the youngest, long-dormant volcano in eastern-central Europe for characterizing the magma storage system and for understanding better the changes in eruption style from effusive to explosive. The mineral cargo of dacitic pumices enables us to constrain the conditions of the pre-recharge crystal mush, the recharge magmas and the post-recharge magma prior to eruptions. A careful evaluation of the results yielded by various thermometers, barometers, oxybarometers, chemometers and hygrometers as well as direct comparison with experimental data were necessary to select the appropriate techniques and therefore to constrain the conditions for the Ciomadul magmatic system. Beneath the volcano, a felsic crystal mush body is inferred at 8–12 km depth comprising slightly oxidized (0.5–1.6 ∆NNO), low-temperature (680–750 °C), highly crystalline magma. This zone is underlain by a deep magma storage zone with less evolved, hot (> 900 °C) magma at 16–40 km depth. The dominantly explosive volcanism after the effusive eruptions (160–90 ka) can be explained by the ascent of distinct recharge magmas. They contained high-Mg (MgO > 18 wt%) amphibole, which could have crystallized from ultrahydrous (H2O > 8 wt%) magma at near-liquidus conditions. The rates of amphibole overgrowth and microphenocryst formation require weeks to months for the magma mixing and the eruption events. The hybridized melt became more oxidized and contained dissolved water in around 5.5 wt% at temperature of 790–830 °C calculated from the re-equilibrated Fe-Ti oxides. These magma properties along with the degree of crystallinity (27–38 vol% crystals) favored rapid magma ascent and an explosive style eruption. Thus, the strongly hydrous nature of the recharge magma in addition to the crystallinity and H2O content of the pre-eruption magma plays an important role in controlling the eruption style.

To improve the understanding of the formation and evolution of the sub‐continental lithospheric mantle (SCLM) underlying the South Australian Craton we have conducted a detailed petrological study on >3,000 mantle xenocrysts from 13 kimberlites emplaced across the craton. Pressure (P) and temperature (T) estimates on Cr diopside and garnet have been coupled with their chemical concentrations to constrain lithospheric thickness and chemo‐lithostratigraphy. We show that lithospheric thickness is greatest beneath the Gawler Craton, whereas thinner lithosphere occurs beneath the Adelaide Fold Belt. Mineral compositions highlight two litho‐chemical domains within the shallow and deep SCLM that are separated by a mid‐lithosphere discontinuity (MLD). The shallow SCLM (60–130 km) comprises low Cr2O3 lherzolite and wehrlite. Shallow SCLM xenocrysts record depleted and refertilized compositions enriched in light rare earth elements related to metasomatism by kimberlite or related melts. The mid‐lithosphere (130–160 km) is depleted in garnet and Cr diopside which may relate to a layer of pargasite lherzolite. The deep SCLM (>160 km) comprises high Cr2O3 lherzolite with elevated TiO2 and FeO. We interpret the litho‐chemical stratification of the SCLM to reflect a multi‐stage top‐down growth. The shallow SCLM reflects an amalgamation of Precambrian cratonic nuclei characterized by heterogeneity in geochemical enrichment and depletion. Interaction of the shallow SCLM with mantle plumes accreted melts along the paleo‐lithosphere‐asthenosphere boundary, which now occurs as a MLD. The deep SCLM represents depleted mantle residue formed during mantle plume impingement and thickened during orogenesis. This domain has been metasomatized and refertilized by high‐T melts from the asthenosphere. Key Points The South Australian Craton comprises three litho‐chemical layers within the shallow, middle and deep lithospheric mantle Xenocrysts define a heterogeneously depleted‐kimberlite melt metasomatized shallow sub‐continental lithospheric mantle (SCLM) and fertile‐melt metasomatized deep SCLM The mid‐lithosphere is a seismic and geochemical discontinuity marked by negative Vp and lower modal proportions of garnet and diopside

The temperature-dependent exchange of Ni and Mg between garnet and olivine in mantle peridotite is an important geothermometer for determining temperature variations in the upper mantle and the diamond potential of kimberlites. Existing calibrations of the Ni-in-garnet geothermometer show considerable differences in estimated temperature above and below 1100 °C hindering its confident application. In this study, we present the results from new synthesis experiments conducted on a piston cylinder apparatus at 2.25–4.5 GPa and 1100–1325 °C. Our experimental approach was to equilibrate a Ni-free Cr-pyrope-rich garnet starting mixture made from sintered oxides with natural olivine capsules (Niolv ≅ 3000 ppm) to produce an experimental charge comprised entirely of peridotitic pyrope garnet with trace abundances of Ni (10–100 s of ppm). Experimental runs products were analysed by wave-length dispersive electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We use the partition coefficient for the distribution of Ni between our garnet experimental charge and the olivine capsule lnDgrt/olvNi;NigrtNiolv, the Ca mole fraction in garnet (XgrtCa; Ca/(Ca + Fe + Mg)), and the Cr mole fraction in garnet (XgrtCr; Cr/(Cr + Al)) to develop a new formulation of the Ni-in-garnet geothermometer that performs more reliably on experimental and natural datasets than existing calibrations. Our updated Ni-in-garnet geothermometer is defined here as:T∘C=-8254.568XgrtCa×3.023+XgrtCr×2.307+lnDgrtolvNi-2.639-273±55where Dgrt/olvNi=NigrtNiolv, Ni is in ppm, XgrtCa = Ca/(Ca + Fe + Mg) in garnet, and XgrtCr= Cr/(Cr + Al) in garnet. Our updated Ni-in-garnet geothermometer can be applied to garnet peridotite xenoliths or monomineralic garnet xenocrysts derived from disaggregation of a peridotite source. Our calibration can be used as a single grain geothermometer by assuming an average mantle olivine Ni concentration of 3000 ppm. To maximise the reliability of temperature estimates made from our Ni-in-garnet geothermometer, we provide users with a data quality protocol method which can be applied to all garnet EPMA and LA-ICP-MS analyses prior to Ni-in-garnet geothermometry. The temperature uncertainty of our updated calibration has been rigorously propagated by incorporating all analytical and experimental uncertainties. We have found that our Ni-in-garnet temperature estimates have a maximum associated uncertainty of ± 55 °C. The improved performance of our updated calibration is demonstrated through its application to previously published experimental datasets and on natural, well-characterised garnet peridotite xenoliths from a variety of published datasets, including the diamondiferous Diavik and Ekati kimberlite pipes from the Lac de Gras kimberlite field, Canada. Our new calibration better aligns temperature estimates using the Ni-in-garnet geothermometer with those estimated by the widely used (Nimis and Taylor, Contrib Mineral Petrol 139:541–554, 2000) enstatite-in-clinopyroxene geothermometer, and confirms an improvement in performance of the new calibration relative to existing versions of the Ni-in-garnet geothermometer.

We present a reconstruction of the physicochemical conditions of melts in the Pleistocene storage and plumbing system of the Doña Juana Volcanic Complex (SW Colombia): a poorly known, potentially active polygenetic volcano of dacitic composition comprising four major edifices and showing periods of long quiescence. Compositional data for plagioclase, amphibole, pyroxene, and Fe-Ti oxides were combined with new and existing whole-rock data from representative eruptive products, allowing for the implementation of equilibrium tests and geothermobarometry calculations within an established stratigraphic, petrographic, and geochronological framework. Textural and geochemical variabilities of all mineral phases suggest the existence of a trans-crustal magmatic system feeding the Pleistocene eruptions of Doña Juana, and cyclic rejuvenation of a crystal mush following each volcano edifice collapse. The assemblage of different crystal cargos before magma recharge and final eruption is attested by (i) the coexistence of equilibrium and disequilibrium textures and variable compositions in crystals of all studied species, (ii) felsic cores in antecrysts, (iii) mafic overgrowth rims, and (iv) significantly less differentiated microcrysts relative to the composition of meso- and macrocrysts. By integrating multiple mineral-only and mineral-liquid geothermobarometers, after careful textural analyses, we estimate the intensive parameters of the mush–melt interaction zone of the plumbing system in the middle crust, providing a preliminary view of the architecture of a trans-crustal magmatic system in a complex tectonic setting at a previously understudied area of the north-Andean volcanic zone.

Mantle xenoliths (lherzolites and a wehrlite) found in Cenozoic alkaline lavas from Azrou–Timahdite (Middle Atlas, Morocco) were petrographically studied with a particular emphasis on mineral chemistry. The most striking feature of these rocks is that plagioclase formed as a result of Al-rich spinel destabilization. This reaction is recorded in different steps and determined a variety of microstructures that allowed to identify four groups of xenoliths. Spinel chemistry generally becomes more refractory approaching the magma-xenolith interface. Geothermobarometry of both xenoliths and host lavas based on pyroxene equilibria and Forsterite—Anorthite—Ca Tschermak—Enstatite (FACE) geobarometer is consistent with a scenario in which spinel to plagioclase reaction was triggered at 0.7–1 GPa by hot (> 1100 °C) upraising magma in a mantle already characterized by a high heat flow (> 80 mW/m 2 ). The entrapment of the xenoliths by the host magma en route to the surface caused a chemical interaction resulting in orthopyroxene destabilization with the formation of anorthoclase. The high geothermal gradient outlined by the mineral chemistry of the xenoliths is consistent with the geodynamic setting of the area, where the lithosphere has been thinned due to the upraising of a mantle plume active since Mesozoic and producing the Mesozoic Central High Atlas basalts. Graphical abstract