Changes in detrital garnet grain morphology and microtextures during fluvial transport in the Western Carpathians revealed by scanning electron microscopy and 3D model analysis: Implication for paleoenvironmental reconstruction

Abstract

Siliciclastic grain surfaces preserve an information about mechanical transport effects and/or various chemical processes that the detritus has undergone. Weathering, transport, and diagenesis leave the clear traces that may be used to interpret the paleo-environmental conditions. We aimed to detail surface analysis of detrital garnet by the proven scanning electron microscopy (SEM), X-ray micro-tomography (XRT) and subsequent morphometric analysis of derived 3D surface model. We tracked garnet morphological and surface microtextural changes during fluvial transport in an active Beňatinská voda and Sobranecký potok streams in the Western Carpathians from the place of garnet removal from the host igneous rocks to the last appearance of its fragments in the fluvial sediments. Analysed garnet grains show a range of almost complete solid solution between almandine and grossular (Alm_70-73Grs_15-19Prp_5-6Sps_2-3Adr_0-3). Precise SEM analysis showed that conchoidal fractures, straight and arcuate steps, crescentic percussion marks and V-shaped cracks are main mechanical fluvial microtextures, whilst 3D surface models derived from XRT provided the information that no relationship between mechanical surface-microtextural development and mineral inclusion distribution exists. Thus, the presence of inclusions and their proximity to the grain surface have no influence on garnet destruction during its fluvial transport. Morphometric variables such as surface roughness and curvatures derived from 3D garnet model can be used to distinguish between mechanical impacts and corrosion-induced depressions. The results related to detritus behaviour in the modern streams are applicable to the ancient fluvial sediment research and can help to correctly reconstruct the paleoenvironmental and paleogeographic conditions in various areas.