Multiple stages of growth in snowball garnets revealed by EBSD analyses

Electron backscattered diffraction (EBSD) analysis, chemical mapping, and computed tomography (CT) imaging have been performed on snowball garnet prophyroblasts from the Lukmanier pass area (central Swiss Alps). The geometry of most garnets is characterized by a core region, around which two spiral arms are wound over 360 degrees. The EBSD maps reveal that the snowball garnets are composed of several grains with distinct crystallographic orientations. The core region and the base of the spiral arms are generally formed by one single, large grain, whereas the arm terminations are made of smaller and often box-shaped crystals separated by straight grain boundaries. The central sector domain exhibits approximately 270 degrees of rotation. Crenulated internal foliations observed near the rim of the garnets indicate a change in the stress field during garnet growth. The distribution of equal Mn concentration domains, considered as time markers, reveals that the change in the growth regime coincides with the occurrence of the first subgrains at the end of the spiral. A similar approach was applied on a second population of snowball garnets collected from a neighboring outcrop. These latter garnets display an apparent rotation of 270 degrees and are formed by one single crystallographic domain for the whole spiral. No evidence of modification in the stress field during garnet growth is observed in these snowball garnets suggesting that the whole spiral grew under a rotational regime. Quantitative microprobe analyses reveal that the end of the spiral for those garnet contain the same amount of Mn as the end of the central sector domain in garnet displaying 360 degrees of apparent rotation. The similarity in geometry between the central sector domains and the geometry acquired by the snowball garnets under the rotational regime strongly suggests that as long as the growth is accompanied by rotation, the primary core orientation is preserved, but once the rotation stops the crystallographic orientation may change. EBSD data also indicate that the central domain displays a crystallographic orientation characterized by a [001] pole oriented sub-parallel to the rotation axis of the snowball garnet. Moreover, in most crystallographic sectors, one of the two other [100] poles is (sub)parallel to the orientation of the internal foliation. This feature suggests that the crystallographic orientation across the garnet spiral is not random and that a relation between rotation axis, internal foliation and crystallographic orientation does exist. In this view, EBSD data can potentially be used to distinguish between the rotational and non-rotational models.