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Pressure is one of the key variables that controls magmatic phase equilibria. However, estimating magma storage pressures from erupted products can be challenging. Various barometers have been developed over the past two decades that exploit the pressure-sensitive incorporation of jadeite (Jd) into clinopyroxene. These Jd-in-clinopyroxene barometers have been applied to rift zone magmas from Iceland, where published estimates of magma storage depths span the full thickness of the crust, and extend into the mantle. However, tests performed on commonly used clinopyroxene-liquid barometers with data from experiments on H2O-poor tholeiites in the 1 atm to 10 kbar range reveal substantial pressure-dependent inaccuracies, with some models overestimating pressures of experimental products equilibrated at 1 atm by up to 3 kbar. The pressures of closed-capsule experiments in the 1-5 kbar range are also overestimated, and such errors cannot be attributed to Na loss, as is the case in open furnace experiments. The following barometer was calibrated from experimental data in the 1 atm to 20 kbar range to improve the accuracy of Jd-in-clinopyroxene barometry at pressures relevant to magma storage in the crust: P(kbar) = -26.27+39.16T(K)/104ln [XJdCpx/XNaO0.5liq XAlO1.5liq (XSiO2liq)2]-4.22ln(XDiHdCpx)+78.43XAlO1.5liq+393.81(X NaO0.5liq XKO0.5liq)2. This new barometer accurately reproduces its calibration data with a standard error of estimate (SEE) of ±1.4 kbar, and is suitable for use on hydrous and anhydrous samples that are ultramafic to intermediate in composition, but should be used with caution below 1100 °C and at oxygen fugacities greater than one log unit above the QFM buffer. Tests performed using with data from experiments on H2O-poor tholeiites reveal that 1 atm runs were overestimated by less than the model precision (1.2 kbar); the new calibration is significantly more accurate than previous formulations. Many current estimates of magma storage pressures may therefore need to be reassessed. To this end, the new barometer was applied to numerous published clinopyroxene analyses from Icelandic rift zone tholeiites that were filtered to exclude compositions affected by poor analytical precision or collected from disequilibrium sector zones. Pressures and temperatures were then calculated using the new barometer in concert with Equation 33 from Putirka (2008) Putative equilibrium liquids were selected from a large database of Icelandic glass and whole-rock compositions using an iterative scheme because most clinopyroxene analyses were too primitive to be in equilibrium with their host glasses. High-Mg# clinopyroxenes from the highly primitive Borgarhraun eruption in north Iceland record a mean storage pressure in the lower crust (5.7 kbar). All other eruptions considered record mean pressures in the mid-crust, with primitive clinopyroxene populations recording slightly higher pressures (3.1-3.6 kbar) than evolved populations (2.6-2.8 kbar). Thus, while some magma processing takes place in the shallow crust immediately beneath Iceland's central volcanoes, magma evolution under the island's neovolcanic rift zones is dominated by mid-crustal processes.

Elastic geobarometry for host-inclusion systems can provide new constraints to assess the pressure and temperature conditions attained during metamorphism. Current experimental approaches and theory are developed only for crystals immersed in a hydrostatic stress field, whereas inclusions experience deviatoric stress. We have developed a method to determine the strains in quartz inclusions from Raman spectroscopy using the concept of the phonon-mode Gruneisen tensor. We used ab initio Hartree-Fock/Density Functional Theory to calculate the wavenumbers of the Raman-active modes as a function of different strain conditions. Least-squares fits of the phonon-wavenumber shifts against strains have been used to obtain the components of the mode Gruneisen tensor of quartz (γ1m and γ3m) that can be used to calculate the strains in inclusions directly from the measured Raman shifts. The concept is demonstrated with the example of a natural quartz inclusion in eclogitic garnet from Mir kimberlite and has been validated against direct X-ray diffraction measurement of the strains in the same inclusion.

Elastic geothermobarometry is a method of determining metamorphic conditions from the excess pressures exhibited by mineral inclusions trapped inside host minerals. An exact solution to the problem of combining non-linear Equations of State (EoS) with the elastic relaxation problem for elastically isotropic spherical host-inclusion systems without any approximations of linear elasticity is presented. The solution is encoded into a Windows GUI program EosFit-Pinc. The program performs host-inclusion calculations for spherical inclusions in elastically isotropic systems with full P-V-T EoS for both phases, with a wide variety of EoS types. The EoS values of any minerals can be loaded into the program for calculations. EosFit-Pinc calculates the isomeke of possible entrapment conditions from the pressure of an inclusion measured when the host is at any external pressure and temperature (including room conditions), and it can calculate final inclusion pressures from known entrapment conditions. It also calculates isomekes and isochors of the two phases.

Raman spectroscopy provides information on the residual strain state of host-inclusion systems that, coupled with the elastic geobarometry theory, can be used to retrieve the P-T conditions of inclusion entrapment. In situ Raman measurements of zircon and coesite inclusions in garnet from the ultrahigh-pressure Dora Maira Massif show that rounded inclusions exhibit constant Raman shifts throughout their entire volume. In contrast, we demonstrate that Raman shifts can vary from the center to the edges and corners of faceted inclusions. Step-by-step polishing of the garnet host shows that the strain in both rounded and prismatic inclusions is gradually released as the inclusion approaches the free surface of the host. More importantly, our experimental results coupled with selected numerical simulations demonstrate that the magnitude and the rate of the strain release also depend on the contrast in elastic properties between the host and the inclusion and on the inclusion crystallographic orientation with respect to the external surface. These results allowed us to give new methodological guidelines for determining the residual strain in host inclusion systems.

Igneous rock forming minerals carry valuable information from the deep earth that is not directly accessible at the surface. Each mineral represents the physico-chemical conditions at which various magmatic processes have occured over a wide range of depths from upper mantle to shallow crustal levels. These processes are cryptically inscribed in the whole-rock and mineral compositions (e.g. major elements, trace elements and isotopic ratios) and textures (equilibrium vs. disequilibrium features), together with intensive variables (e.g. pressure, P; temperature, T). Therefore, particular attention should be given to igneous minerals to understand better the processes that took place during their journey from the source through magma chambers and conduit systems to the Earth's surface. MagMin_PT is an Excel based user-friendly program, designed to calculate mineral formulae and end-members, and to estimate pressure and temperature (e.g. geothermobarometry) from electron microprobe analytical data. The program operates using the most common igneous rock-forming minerals (olivine, pyroxene, amphibole, biotite, feldspar, magnetite, ilmenite, apatite and zircon), resulting in various classification diagrams and P-T diagrams. The program allows for whole-rock or glass composition to be entered together with the EPMA data to evalaute the equilibration status for most P-T calculation models. Fe2+ and Fe3+ estimation is routinely performed in MagMin_PT based on stoichiometric constraints, and to some extent using machine learning methods for different iron-bearing minerals. MagMin_PT is also able to carry calculations of fugacity, magmatic water content and saturation temperature. Graphical and numerical outputs produced by the program can be easily copied to other media for further processing.

Natural peridotite samples containing olivine, orthopyroxene, and spinel can be used to assess the oxygen fugacity fO2 of the upper mantle. The calculation requires accurate and precise quantification of spinel Fe3+/ΣFe ratios. Wood and Virgo (1989) presented a correction procedure for electron microprobe (EPMA) measurements of spinel Fe3+/ΣFe ratios that relies on a reported correlation between the difference in Fe3+/ΣFe ratio by Mossbauer spectroscopy and by electron microprobe (ΔFe3+/ΣFeMoss-EPMA) and the Cr# [Cr/(Al+Cr)] of spinel. This procedure has not been universally adopted, in part, because of debate as to the necessity and effectiveness of the correction. We have performed a series of replicate EPMA analyses of several spinels, previously characterized by Mossbauer spectroscopy, to test the accuracy and precision of the Wood and Virgo correction. While we do not consistently observe a correlation between Cr# and ΔFe3+/ΣFeMoss-EPMA in measurements of the correction standards, we nonetheless find that accuracy of Fe3+/ΣFe ratios determined for spinel samples treated as unknowns improves when the correction is applied. Uncorrected measurements have a mean ΔFe3+/ΣFeMoss-EPMA = 0.031 and corrected measurements have a mean ΔFe3+/ΣFeMoss-EPMA = -0.004. We explain how the reliance of the correction on a global correlation between Cr# and MgO concentration in peridotitic spinels improves the accuracy of Fe3+/ΣFe ratios despite the absence of a correlation between ΔFe3+/ΣFeMoss-EPMA and Cr# in some analytical sessions. Precision of corrected Fe3+/ΣFe ratios depends on the total concentration of Fe, and varies from ±0.012 to ±0.032 (1σ) in the samples analyzed; precision of uncorrected analyses is poorer by approximately a factor of two. We also present an examination of the uncertainties in the calculation contributed by the other variables used to derive FO2. Because there is a logarithmic relationship between the activity of magnetite and LogfO2, the uncertainty in fO2 relative to the QFM buffer contributed by the electron microprobe analysis of spinel is asymmetrical and larger at low ferric Fe concentrations (+0.3/-0.4 log units, 1σ, at Fe3+/ΣFe = 0.10) than at higher ferric Fe concentrations (±0.1 log units, 1σ, at Fe3+/ΣFe = 0.40). Electron microprobe analysis of olivine and orthopyroxene together contribute another ±0.1 to ±0.2 log units of uncertainty (1σ). Uncertainty in the temperature and pressure of equilibration introduce additional errors on the order of tenths of log units to the calculation of relative fO2. We also document and correct errors that appear in the literature when formulating fO2 that, combined, could yield errors in absolute fO2 of greater than 0.75 log units-even with perfectly accurate Fe3+/ΣFe ratios. Finally, we propose a strategy for calculating the activity of magnetite in spinel that preserves information gained during analysis about the ferric iron content of the spinel. This study demonstrates the superior accuracy and precision of corrected EPMA measurements of spinel Fe3+/ΣFe ratios compared to uncorrected measurements. It also provides an objective method for quantifying uncertainties in the calculation of fO2 from spinel peridotite mineral compositions.

Unveiling the pressure-temperature path of low-grade metamorphic rocks is challenging because of the occurrence of detrital minerals and high-variance mineral assemblages (i.e. chlorite-white mica-quartz). This paper is an attempt to reconstruct the pressure-temperature history on metapelites from a low-grade metamorphic unit, i.e. the Cabanaira Unit, located in the Marguareis Massif (Western Ligurian Alps, Italy). In order to obtain the most robust result possible, multi-equilibrium thermobarometry, forward modelling and crystallochemical index measurements are used together to reconstruct a pressure-temperature path, with consideration of the strengths and weaknesses of these methods. This multidisciplinary approach allowed us to reconstruct the metamorphic evolution of the unit of interest, characterised by a pressure peak reached under low-temperature conditions (0.85-0.68 GPa and 250-285°C) followed by decompressional warming (low pressure-high temperature, 0.4-0.6 GPa and 300-335°C). This pressure-temperature path is consistent with the tectonic evolution of the investigated area proposed by previous studies, where a geological scenario in which the Cabanaira Unit experienced subduction-related processes was postulated, even if the reasons for warming remain unclear. Multi-equilibrium thermobarometry is considered to be the most suitable method to unravel the metamorphic history of low-grade rocks, whereas forward thermodynamic modelling and the calculation of crystallochemical indexes seem to resolve only some segments of the pressure-temperature path.

High-silica rhyolites and granites (>75 wt% SiO2, anhydrous basis) are common features of the crust as part of both the volcanic and the plutonic records. While low crystallization pressure (<250 MPa) is typically inferred, it has been suggested that they form via polybaric evolution, with initial crystallization at relatively high pressures (>500 MPa). We use glass compositions derived from the EarthChem portal, selected natural examples from the literature, and rhyolite-MELTS calculations to show that the phase relations in the quartz-albite-orthoclase ternary dictate the silica content of silicic melts. In particular, we show that silica content of melts increases with decreasing pressure as a result of the displacement of the quartz-sanidine cotectic toward the Qz apex with decreasing pressure. It follows from our analysis that (1) the crust is expected to be stratified in terms of the silica content of residing melts; (2) high-silica glasses form at low pressure, requiring shallow-level crystallization, and preclude a polybaric evolution for many systems (e.g., Bishop Tuff); (3) the existence of high-silica pumice requires fractionation (or melting) at low pressure, showing that high-silica rhyolites are intrinsic to the shallow crust; and (4) low-pressure cumulates (or melting residues) must exist in the shallow crust, weighing in favor of the cumulatic nature of many granitoids found in plutons.