Quantitative bulk and single-particle mineralogy of a thick Chinese loess–paleosol section: implications for loess provenance and weathering

Abstract

A 40 m loess–paleosol section (L1, S1, and upper L2) at the semi-arid western margin of the Chinese Loess Plateau was investigated by quantitative mineralogical, microscopic, and geochemical analyses of bulk samples and single particles. The average ( N=16) mineralogical composition of loess in the studied section is 34.8% quartz, 15.3% plagioclase, 5.0% K-feldspar, 25.1% phyllosilicates, 11.7% calcite, 3.3% dolomite, and 1.9% amphibole. Ratios of plagioclase, K-feldspar, and amphibole to quartz do not change throughout both the loess and paleosol units, while the ratio of total phyllosilicate to quartz is enhanced in the paleosol where there is significant variation in calcite and dolomite contents. Illite and interstratified illite–smectite are the most abundant (84±4%) clay minerals in the clay fraction of both loess and paleosol samples. The calcite/dolomite ratio, Mg/Si ratio, iron oxidation degree, and organic carbon contents are positively correlated to magnetic susceptibility. Weathering and elemental mobilization in paleosol samples was indicated by calcite–dolomite dissolution, nanofiber–calcite reprecipitation, and biotite–hydrobiotite transformation, while there was no notable evidence for weathering observed in loess samples. The essentially pristine state of loess samples with respect to weathering and the minimal alteration of minerals in the paleosol indicate relatively poor weathering conditions at both the depositional and source sites. Analysis of single particles show that most plagioclase is albitic, calcite occurs frequently in particles with dolomite but also with silicates, biotite is associated with barite–celestine inclusions, iron oxide occurs as a pseudomorph after framboidal pyrite, and rock fragments may be siliciclastic sedimentary, metasedimentary, and granitic. These observations indicate that the lithology of source regions must have been dominated by largely siliciclastic and carbonate sedimentary rocks and their low-grade metamorphic equivalents with contributions from granitoids partly affected by hydrothermal alterations and possibly high-grade metamorphic rocks. The mineralogical analysis of loess and paleosols opens new ground for refining the established climatic proxies, and for tracking source lithology and weathering in the source and depositional regions.