Bedrock controls on the mineralogy and chemistry of PM10 extracted from Australian desert sediments

Given the relevance of desert aerosols to environmental issues such as dust storms, climate change and human health effects, we provide a demonstration of how the bedrock geology of an arid area influences the mineralogy and geochemistry of even the finest particulate matter (i.e., the inhalable fraction < 10 mu m in size: PM10). PM10 samples extracted from desert sediments at geologically contrasting off-road sites in central and southeastern Australia (granitic, high grade metamorphic, quartzitic sandstone) were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). The granitic PM10 are highly alkali feldspathic and illitic, with a wide range of accessory minerals including rutile (TiO2), monazite [(Ce, La, Nd, Th, Y) PO4], xenotime (YPO4), apatite [Ca-5(PO4)(3) (F, OH, Cl)], hematite (Fe3O4), zircon (ZrSiO4) and thorite (ThSiO4). This mineralogy is reflected in the geochemistry which shows notable enrichments in rare earth elements (REE) and most high field strength elements (both held in the accessory minerals), and higher than normal levels of low (< 2.0) ionic potential elements (Na, K, Li, Cs, Rb: held in alkali feldspar and illite). The metamorphic resuspended PM10 define a mineralogy clearly influenced by local exposures of pelitic and calc-silicate schists (sillimanite, muscovite, calcite, Ca-amphibole), a dominance of monazite over other REE-bearing phases, and a geochemistry distinguished by enrichments in alkaline earth metals (Ca, Mg, Ba, Sr) and depletion in heavy REE. The quartzite PM10, derived from rocks already recycled by Precambrian erosion and sedimentary transport, show a sedimentologically mature mineralogy of mostly quartz and kaolinite, detrital accessory ilmenite, rutile, monazite and hematite, and the strongest geochemical depletion (especially K, Rb, Cs, Na, Ca, Mg, Ba).