Sediment generation in modern glacial settings: Grain-size and source-rock control on sediment composition

Clastic sediment generation is controlled by physical and chemical processes acting in concert in most geological settings. In glacial settings, however, it is possible investigating the sole impact of mechanical processes such as comminution on sediment composition, as chemical processes are thought to be negligible in this environment. Comminution is a selective process in the sense that minerals behave differently under mechanical forcing and has yet not been thoroughly investigated under strict grain-size control. We sampled sediment from modern front and side moraines from six retreating glaciers in the Alps, that drain and erode either pure felsic crystalline rocks (granites, granodiorites, orthogneisses) or largely pure metamafic rocks (amphibolites and hornblende-rich gneisses). Samples were split in up to eleven grain-size fractions from very coarse sand to clay. Grain-size fractions were analysed for major and trace elements using X-ray fluorescence. Mineralogical composition was determined by X-ray diffraction and endmember modelling of geochemical data.Results reveal in general strong grain-size control on sediment composition and strikingly similar patterns for both source lithologies. Significant influence of chemical weathering and hydrodynamic sorting is ruled out. Zr/Zn ratio is found as a valuable proxy for grain size while Cr/Rb constitutes one of the rare discriminants between the two cases over the entire grain-size range. Most trace elements, however, are not suitable for source rock discrimination across grain size grades even in glacial environment and extreme proximity to the source. Consequently, bulk sediment geochemistry has only limited benefit in provenance studies unless the samples were analysed under strict grain-size control.The data can be modelled by linear regression with two components: (i) a linear trend describing preferential enrichment of phyllosilicates at the expense of quartz and feldspar towards finer fractions, and (ii) some breaks at certain grain-size thresholds. Due to the observed step functions the model describes a four-step enrichment-depletion pattern that is largely similar for the two source-rock cases: feldspar is highest in the very coarse to medium sand fraction; quartz is highest in very fine sand; epidote, garnet, hornblende, apatite are highest and plagioclase is relatively high in the silt range; sheet silicates (chlorite, biotite, muscovite) are highest in the clay fraction. The observed pattern describes the process of comminution, i.e. the impact of mechanical forces on minerals with contrasting durability: the most durable minerals like quartz are concentrated close to their inherited grain-sizes while less durable minerals are enriched in silt fractions, and least durable minerals (i.e. sheet silicates) are enriched in the very fine silt to clay fractions. The latter, not chemical weathering causes an increase in chemical index of alteration (CIA) values up to ~ 63 at the finest grain-size grades. The model provides a quantitative description of the composition to grain-size relations and is thought to form a valuable module for building comprehensive sediment generation models that describe the entire network of sediment production processes from source to sink.