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.
This study is focused on a specific outcrop in the Bergen Arcs, Norway where the transition between dry granulite and the hydrated eclogite and amphibolite is exposed. In this outcrop the foliation in the granulite is continuous as it passes through eclogite- and amphibolite-facies rocks, presenting a challenge to understanding the nature of these spatial relationships. Although there is no major change in the bulk chemical composition of all three metamorphic-facies rocks, the loss of ignition (LOI) content increases from granulite to the eclogite and to the amphibolite. During hydration and metamorphism, the density changes from c. 3 g cm−3 for the anorthositic granulite to 3.2 g cm−3 for the eclogite, and 2.75 g cm−3 for the amphibolite. Based on the mass balance equation, eclogitization of the granulite shows a reduction of volume of c. 3% whereas amphibolitization of the granulite gains c. 5% in volume. By assuming equilibrium, modelling the phase equilibria provides estimates of the amount of fluid necessary to form the eclogite and the amphibolite assemblages. Results show that both assemblages can be stable at similar temperature and a similar fluid composition but differ in pressure by c. 10 kbar. This study suggests that the stress generated during hydration of the granulite may influence the local mineral assemblage equilibrium.
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