Fig. 3
From: Weathering in a world without terrestrial life recorded in the Mesoproterozoic Velkerri Formation
SEM backscatter image with Nanomin mineral map overlay from a dark fine-grained lamina in Urapunga 4 core. a A mineral map showing the distribution and composition of grains with quantified percentage of weight percent calculated from area showing quartz and illite to be the dominant mineral phases. Scale bar = 100 μm. b Zoomed in image from the box in a provides detail of the detrital silt grains, replacement minerals and intergranular micro quartz cement with minerals quantified for this area as in a. Scale bar = 10 μm. c Primary detrital constituent map of image b with replaced areas returned to likely original phases and intergranular cement left as background BSE image. Correction is based on relict crystal forms evident in BSE. Quantified normalized values for the area show feldspar is dominant with quartz, mica, and only a small component of detrital illite. Illite present in replaced mica and feldspar is 32% of the 39% in image b leaving a maximum of 7% that could have been deposited as DCM. C is a micaceous arkosic siltstone with a calculated CIA value of 58% indicative of low chemical weathering. The original sample from the core was taken from the finest-grained laminae, has a significant illite fraction (32%) and a CIA index of 73% suggestive of intense weathering resulting from illite replacement of feldspar and mica after deposition. Scale bar = 10 μm
