Compositional mapping has greatly impacted mineralogical and petrological studies over the past half-century with increasing use of the electron probe micro-analyser. Many technical and analytical developments have benefited from the synergies of physicists and geologists and they have greatly contributed to the success of this analytical technique. Large-area compositional mapping has become routine practice in many laboratories worldwide, improving our ability to measure the compositional variability of minerals in natural geological samples and reducing the operator bias as to where to locate single spot analyses. This chapter aims to provide an overview of existing quantitative techniques for the evaluation of rock and mineral compositions and to present various examples of applications. A new advanced method for compositional map standardization that relies on internal standards and accurately corrects the X-ray intensities for continuum background is also presented. This technique has been implemented into the computer software XMapTools. The improved workflow defines the appropriate practice of accurate standardization and provides data-reporting standards to help improve petrological interpretations.

The knowledge of the fractionation behaviour between phases in isotopic equilibrium and its evolution with temperature is fundamental to assist the petrological interpretation of measured oxygen isotope compositions. We report a comprehensive and updated internally consistent database for oxygen isotope fractionation. Internal consistency is of particular importance for applications of oxygen isotope fractionation that consider mineral assemblages rather than individual mineral couples. The database DBOxygen is constructed from a large dataset of published experimental, semi-empirical and natural data, which were weighted according to type. It includes fractionation factors for 153 major and accessory mineral phases and a pure H2O fluid phase in the temperature range of 0–900°C, with application recommended for temperatures of 200–900°C. Multiple primary data for each mineral couple were discretized and fitted to a model fractionation function. Consistency between the models for each mineral couple was achieved by simultaneous least square regression. Minimum absolute uncertainties based on the spread of the available data were calculated for each fractionation factor using a Monte Carlo sampling technique. The accuracy of the derived database is assessed by comparisons with previous oxygen isotope fractionation calculations based on selected mineral/mineral couples. This database provides an updated internally consistent tool for geochemical modelling based on a large set of primary data and including uncertainties. For an effective use of the database for thermometry and uncertainty calculation we provide a MATLAB©-based software ThermoOx. The new database supports isotopic modelling in a thermodynamic framework to predict the evolution of δ18O in minerals during metamorphism.

Garnet is one of the most robust and ubiquitous minerals that record element zoning during crustal metamorphism. In addition to major elements, zoning in trace elements can provide a wealth of information to document the changing conditions of garnet growth and modification. However, mapping trace elements at low concentrations, over large areas and with high resolution has remained a major challenge.We present a comprehensive investigation of the TE distribution in garnet from three Alpine samples that underwent a complex evolution at different metamorphic conditions. The TE distribution in garnet grains is mapped in 2D in thin section with a novel approach using laser ablation inductively coupled plasma time of flight mass spectrometry (LA-ICP-TOFMS) to achieve a lateral resolution of 5 µm and limits of detection for the heavy rare earth elements (REE) down to 0.2 µg/g. Comparison with major element zoning measured by electron probe micro-analysis (EPMA) and trace elements measured by conventional LA-ICPMS spot analysis testifies to the accuracy of the measurements.Garnet in an amphibolite-facies metapelite from Campolungo, Central Alps that recorded metamorphism to 600°C preserves Y+REE trace element zoning that closely matches that of Ca. In this sample, there is no notable diffusive modification for trace elements. Y+REE zoning is dominated by Rayleigh fractionation in the core and by the sporadic breakdown of accessory phases producing annuli in the rim of the garnet.A granulite-facies garnet from Malenco, Eastern Central Alps, formed during subsolidus heating, followed by peritectic melting reactions up to temperatures of 800-850°C. Major and trace element zoning are decoupled indicating diffusional resetting of major elements, whereas trace elements still largely document the growth history. Enrichment of trace elements in the garnet mantle may be related to the consumption of biotite (V, Cr) and the dissolution of zircon (Zr) and monazite (Y+REE) in the melt. Diffusion of Y+HREE at the core-mantle boundary occurred over a length scale of ~200 µm.Garnet in an eclogite from the Sesia Zone, Western Alps (P~2 GPa, T~600°C) displays pronounced fluid-related veinlets, visible in FeO, MgO and MnO, which cross-cut the primary growth zoning. Surprisingly, complex Y+REE and Cr zoning is not affected by the veinlets, indicating that they did not form by a crack-seal mechanism but are rather related to a selective replacement process.The trace element maps provide a detailed insight into the growth and modification of garnet and thus allow assessment of equilibrium versus disequilibrium processes, and assist in determination of P-T conditions, garnet dating, diffusion modelling as well as documenting fluid-induced modifications.

Accurate ion microprobe analysis of oxygen isotope ratios in garnet requires appropriate reference materials to correct for instrumental mass fractionation that partly depends on the garnet chemistry (matrix effect). The matrix effect correlated with grossular, spessartine and andradite components was characterised for the Cameca IMS 1280HR at the SwissSIMS laboratory based on 17 reference garnet samples. The correlations fit a second degree polynomial with maximum bias of ca. 4‰, 2‰ and 8‰ respectively. While the grossular composition range 0 -25% is adequately covered by available reference materials, there is a paucity of them for intermediate compositions. We characterize three new garnet reference materials GRS2, GRS-JH2 and CAP02 with a grossular content of 88.3 ± 1.2% (2sd), 83.3 ± 0.8% and 32.5 ± 3.0% respectively. Their microscale homogeneity in oxygen isotope composition was evaluated by multiple SIMS sessions. The reference δ 18 O value was determined by CO2 laser fluorination (δ 18 OLF). GRS2 has δ 18 OLF = 8.01 ± 0.10‰ (2sd) and repeatability within each SIMS session of 0.30 -0.60‰ (2sd), GRS-JH2 has δ 18 OLF

Abstract. Oxygen isotope geochemistry is a powerful tool for investigating rocks that interacted with fluids, to assess fluid sources and quantify the conditions of fluid–rock interaction. We present an integrated modelling approach and the computer program PTLoop that combine thermodynamic and oxygen isotope fractionation modelling for multi-rock open systems. The strategy involves a robust petrological model performing on-the-fly Gibbs energy minimizations coupled to an oxygen fractionation model for a given chemical and isotopic bulk rock composition; both models are based on internally consistent databases. This approach is applied to subduction zone metamorphism to predict the possible range of δ18O values for stable phases and aqueous fluids at various pressure (P) and temperature (T) conditions in the subducting slab. The modelled system is composed of a mafic oceanic crust with a sedimentary cover of known initial chemical composition and bulk δ18O. The evolution of mineral assemblages and δ18O values of each phase is calculated along a defined P–T path for two typical compositions of basalts and sediments. In a closed system, the dehydration reactions, fluid loss and mineral fractionation produce minor to negligible variations (i.e. within 1 ‰) in the bulk δ18O values of the rocks, which are likely to remain representative of the protolith composition. In an open system, fluid–rock interaction may occur (1) in the metasediment, as a consequence of infiltration of the fluid liberated by dehydration reactions occurring in the metamorphosed mafic oceanic crust, and (2) in the metabasalt, as a consequence of infiltration of an external fluid originated by dehydration of underlying serpentinites. In each rock type, the interaction with external fluids may lead to shifts in δ18O up to 1 order of magnitude larger than those calculated for closed systems. Such variations can be detected by analysing in situ oxygen isotopes in key metamorphic minerals such as garnet, white mica and quartz. The simulations show that when the water released by the slab infiltrates the forearc mantle wedge, it can cause extensive serpentinization within fractions of 1 Myr and significant oxygen isotope variation at the interface. The approach presented here opens new perspectives for tracking fluid pathways in subduction zones, to distinguish porous from channelled fluid flows, and to determine the P–T conditions and the extent of fluid–rock interaction.