Geologists appreciate the presence of garnet in a rock, because they can use it to reconstruct the metamorphic history of a geologic terrain. Garnet's physical strength protects early mineral inclusions from being destroyed during intense deformation of the rock. Plastic deformation of garnet has been recognized to occur in granulite facies rocks (e.g. Martelat et al., 2012). Thus, the preservation of any early information in garnet may be restricted by the temperature and pressure conditions during deformation.
We have conducted a combined EDS/EBSD analysis of garnet in a granulite facies mafic boudin that occurs enclosed in quartzite layers within amphibolite facies gneiss of the Lindås nappe, SW-Norway.
A thin section of the sample was polished using colloidal silica and then coated with carbon. Data were acquired using a scanning electron microscope (with field emission gun) in combination with a Bruker e-FlashHR+ EBSD detector and a Bruker XFlash® 6|30 EDS SD detector. EBSD and EDS data were obtained simultaneously.
In order to optimize the SEM occupation time, only the presence of garnet and orthopyroxene was considered for the EBSD results at the time of the measurement. The other phases were identified afterwards (offline) using the Advanced Phase ID (watch a Webinar about “Advanced Phase ID”). The final map hit rate is above 82% (strong erosion of the feldspar grains lead to very poor pattern, and >10% of the map represents holes and cracks and are therefore not indexed). (see EBSD results in figure 3a and 3b, and EDS results in figure 4).
Additional EDS maps and point analyses were collected to study the chemistry of the inclusions in the garnet.
From EBSD/EDS results, we can infer four geological events during the evolution of this rock:
- The first mineral assemblage (garnet, clinopyroxene, Fe-Ti-oxide and ternary feldspar of few mm grain size) was formed during emplacement and crystallization of a mafic-ultramafic melt in the lower continental crust. Microstructural observations within this assemblage, such as orthopyroxene lamellae in clinopyroxene, perthitic feldspar and exsolution textures of Fe-Ti-oxides, indicate cooling from temperatures of 1000-1050°C at pressures of ~14 kbar (Fuhrman and Lindlsey 1988, Spencer and Lindsley 1981, Bacon and Hirschmann 1988, De Capitani and Petrakakis 2010). Tiny ilmenite inclusions are preserved in some garnets (see figure 5a and figure 5b) that additionally may be evidence for solid-state exsolution during cooling from a former HT garnet precursor.
- Further cooling was accompanied by a chemical reaction of garnet + clinopyroxene to orthopyroxene + plagioclase + oxides that formed a coronitic reaction texture around the garnets (see figure 2 and figure 6). This indicates that the stability field of garnet was clearly left (T<800°C, p<14 kbar, Brey and Köhler 1990, De Capitani and Petrakakis 2010), but the rock did not completely re-equilibrate outside the garnet stability field.
- A subsequent deformation event during prograde metamorphism caused:
A) Plastic deformation of the large crystals and especially the garnets (now porphyroclastic). One of the large garnet is highly deformed with a strong misorientation (angle higher than 10°, see figure 7 and figure 8)
B) Recrystallization of (formerly porphyroclastic) garnet, e.g. part of the "arm " on the left side of the big deformed grain (see figure 9).
C) Growth of small, rounded garnet grains as a reaction product of matrix phases (including oxides). These small garnets do not show internal lattice misorientation (dark blue with misorientation angle smaller than 2°, below the big grain, see figure 9).
The presence and chemistry of recrystallized (B) and newly grown matrix garnets (C) indicates that deformation occurred back in the garnet stability field (but the temperature was lower than in 1.). The deformation event caused an overall grain size reduction of the garnet, feldspar and pyroxene – from an initial grain size at the mm-scale to a grain size of 100 µm and less (see figure 7).
- Retrogression: From the EDS results (See figure 6), we can see that the large porphyroclastic garnet grains have some minor Fe-enrichment towards the edges, while the recrystallized and matrix garnet grains are homogeneous in Fe/Mg and generally have a higher Fe-content than the cores of the large grains. Regarding the Fe/Mg partitioning between garnet and pyroxene, we know that at high temperature (and at a given pressure) Mg prefers garnet and Fe prefers pyroxene. When the temperature decreases, Fe diffuses into garnet.
The combined EBSD/EDS analysis of a granulite facies mafic boudin from the Lindås nappe (SW-Norway) implies that most of the deformation has been accommodated by the embedding quartzite, partially preserving the old mineral textures within the ultrabasic rock. In this specific sample studied here, high temperature plastic deformation leaves the major element chemistry of the garnet and its old inclusions nearly unaffected, thus in chemical disequilibrium with matrix grains.
Bacon and Hirschman, 1988, American Mineralogist 73:57-61.
Brey and Köhler, 1990, JPET 31:1353-1378.
De Capitani and Petrakakis, 2010, American Mineralogist 95:1006-1016.
Fuhrman and Lindsley, 1988, American Mineralogist 73:201-215.
Martelat, Malamoud, Cordier, Randrianasolo and Lardeaux, 2012, JMG 30:435-452.