Mineralogy and Source of High Altitude Glacial Deposits in the Western Alps: Clay Minerals as Saharan Dust Tracers

Variable amounts of Saharan dust deposit on the glaciers of the Alps, forming thin layers or mixing with local pre-existent sediments. To investigate the mineralogical composition and the source of the deposits, samples of Saharan dust, fine-grained tills and cryoconite accumulations were collected on the surface of the glaciers and/or in ice-cores. The fractionation of the samples and the bulk mineralogy allow a sharp differentiation among the different deposits. Only the clay mineralogy, however, points out the fundamental characteristics of the finest fractions of the deposited materials. Typical clay mineral assemblages were recognised in the Alpine glacial deposits, giving evidence of the different provenance of the materials. Reddish-yellow Saharan dust exhibits prevailing amounts of kaolinite and poorly-crystallised illite, with lower quantities of palygorskite and chlorite. Olive-greyish tills, connected to the local morainic deposits, are mainly composed by high percentages of well-crystallised mica associated with lesser chlorite and serpentine. Blackish cryoconite accumulations are mixture, in variable proportions, of both Saharan dust and local tills. The mineralogical data clearly differentiate distal and proximal sources of mineral particles deposited on Alpine glaciers.