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.

Collisional orogens commonly include mono‐metamorphic and poly‐metamorphic units, and their different evolution can be difficult to recognize and reconcile. The Theodul Glacier Unit (TGU) in the Western Alps consists of an association of metasedimentary and metamafic rocks embedded within the Zermatt‐Saas tectonic unit. In spite of recent petrological studies, it remains unclear whether these rocks underwent one or multiple metamorphic cycles. In this study, different lithologies from the TGU unit (mafic schist, mafic granofels, and chloritoid schist) were investigated for petrography, quantitative compositional mapping of garnet, thermodynamic modelling, and Lu–Hf garnet dating. The data reveal a coherent mono‐metamorphic history with a β‐shaped pressure–temperature (P–T) path characteristic of oceanic subduction. Garnet Lu–Hf ages yield a restricted garnet crystallization time window between 50.3 and 48.8 Ma (±0.5%, 2SD). A prograde metamorphic stage recorded in garnet cores yields conditions of 490 ± 15℃ and 1.75 ± 0.05 GPa. Maximum pressure conditions of 2.65 ± 0.10 GPa and 580 ± 15℃ were reached at 50.3 ± 0.3 Ma. Initial exhumation was rapid and led to isothermal decompression to 1.50 ± 0.10 GPa within 1 Ma. This decompression was associated with lawsonite breakdown in mafic schist and in mafic granofels, causing intense fluid–rock interaction within and between different lithologies. This process is recorded in garnet textures and trace element patterns, and in the major element composition of K‐white mica. Initial exhumation was followed by re‐heating of ~30℃ at a pressure of 1.50 ± 0.10 GPa. Perturbation of the subduction‐zone thermal structure may be related to upwelling of hot asthenospheric mantle material and transient storage of the unit at the crust–mantle boundary.

Dehydration reactions in the subducting slab liberate fluids causing major changes in rock density, volume and permeability. Although it is well known that the fluids can migrate and interact with the surrounding rocks, fluid pathways remain challenging to track and the consequences of fluid-rock interaction processes are often overlooked. In this study, we investigate pervasive fluid-rock interaction in a sequence of schists and mafic felses exposed in the Theodul Glacier Unit (TGU), Western Alps. This unit is embedded within metaophiolites of the Zermatt-Saas Zone and reached eclogite-facies conditions during Alpine convergence. Chemical mapping and in situ oxygen isotope analyses of garnet from the schists reveal a sharp chemical zoning between a xenomorphic core and a euhedral rim, associated to a drop of ~ 8‰ in δ18O. Thermodynamic and δ18O models show that the large amount of low δ18O H2O required to change the reactive bulk δ18O composition cannot be produced by dehydration of the mafic fels from the TGU only, and requires a large contribution of the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is 1.1 × 105 cm3/cm2, which is above the open-system behaviour threshold and argues for pervasive fluid flow at kilometre-scale under high-pressure conditions. The transient rock volume variations caused by lawsonite breakdown is identified as a possible trigger for the pervasive fluid influx. The calculated schist permeability at eclogite-facies conditions (~ 2 × 10–20 m2) is comparable to the permeability determined experimentally for blueschist and serpentinites.

Secondary Ion Mass Spectrometry (SIMS) extracts chemical, elemental, or isotopic information about a localized area of a solid target by performing mass spectrometry on secondary ions sputtered from its surface by the impact of a beam of charged particles. This primary beam sputters ionized atoms and small molecules (as well as many neutral particles) from the upper few nanometers of the sample surface. The physical basis of SIMS has been applied to a large range of applications utilizing instruments optimized with different types of mass analyzer, either dynamic SIMS with a double focusing mass spectrometer or static SIMS with a Time of Flight (TOF) analyzer. Here, we present a short review of the principles and major applications of three different SIMS instruments located in Switzerland.

<p>Reconstructing the tectonic history of metamorphosed terranes is a key step towards establishing a comprehensive model for collisional orogens such as the Alps. Single chronometers tend to record one specific component of such history—be it inheritance, reactions or cooling—or record several of these, without a clear indication of what each age datum means. Resolving the complex evolution of such terranes requires chronometric data of different minerals, which on the basis of their chemistry, may be linked to distinct stages. Here we present a multi-mineral geochronology of the Theodul Gletscher Unit (TGU; Western Alps). The tectonic unit is a metamorphic sequence containing a variety of pelitic and mafic rocks that mainly record Alpine low-temperature, high-pressure metamorphism. In addition, however, the rocks are known to host age components related to events and processes in the Permian and Jurassic; these could be attributed to inherited components and pervasive fluid-rock interaction during oceanic alteration and subduction. To investigate this, we subjected pelitic schists and mafic rocks from the TGU to a multi-method analysis, involving thermometry, oxygen isotope analysis in garnet, and zircon U-Pb and garnet Lu-Hf dating.</p><p>Zircon crystals in all rock types are Permian in age and have no significant record of Alpine metamorphism; they are interpreted as dating the source of the felsic and mafic sediments. Complex garnet textures in the schists reveal multiple growth stages: whereas the garnet rim reflects the subduction stage, the relict nature of the garnet core allows for speculation of an older, perhaps Permian age (Bucher et al., 2019). A distinct and abrupt rim-ward drop in δ<sup>18</sup>O coherent with major-element zoning in garnet from the schists indicates open system fluid-rock interaction. Rutile included in the different garnet zones as well as in the matrix of the schists provided consistent Zr-in-rutile thermometry results of 520–560 °C (calculated at 2.5 GPa). Similarly, Raman spectroscopy of carbonaceous material in the same textural positions indicates 540–580 °C. These results indicate a single Alpine metamorphic cycle. To look back beyond that stage, Lu-Hf data will be presented for garnet with and without seemingly inherited cores, as well as for cores that were physically isolated from the sample material. The results, together, provide new insight into the petrological and tectonic processes that affected rocks in the TGU during and prior to their Alpine history.</p><p>REFERENCES:</p><p>Bucher, K., Weisenberger, T. B., Klemm, O., Weber, S. (2019). Decoding the complex internal chemical structure of garnet porphyroblasts from the Zermatt area, Western Alps. Journal of Metamorphic Petrology, 37, 1151-1169</p>