Oxygen isotopic composition of aerosol size quartz in shales

Abstract

In a suite of 12 Cretaceous shales of the northern Great Plains of the United States the whole-quartz oxygen isotopic delta values (δ18O) decrease from 22 to 15‰ from the central marine basin, where chert and biogenic silica are higher, to the westward shore near volcanic rock sources. The δ18O for the 1- to 10-μm size fractions showed the same trend. The δ18O values are still lower (12–14‰) for two nonmarine Tertiary Rocky Mountain outwash sediments of NB and CO. Thus the oxygen isotope ratios reflect the provenance of the quartz. The δ18O values of whole quartz and/or of the 1- to 10-μm size (the size commonly wind-transported over long distances) isolated from 36 Mesozoic and Paleozoic shales of the midcontinental United States range from 15.1 to 23.9‰. The mean δ18O values for 31 whole quartz (19.1±1.4‰; ±2 σ) and the 1- to 10-μm quartz fractions from 30 shales (20.7±1.20‰) differed by only 1.6‰ (a maximum of 3.1‰). The δ18O values of 1- to 10-μm quartz from the shales did not vary systematically with age through the Paleozoic and Mesozoic eras (Cambrian, 21–22‰; Ordovician, 18–22‰; Devonian, 18–22‰; Mississippian-Permian, 18–24‰; and Jurassic-Cretaceous, 17–l24‰). The variation for each period may reflect differences in provenance, such as distance from igneous sources versus localities of chert quartz deposition. The δ18O value of quartz from shales is intermediate between those values which characterize quartz of high- and low-temperature rock provenances; thus it is suggested that shales are reservoirs of quartz of mixed provenance. The range of δ18O values for the 1- to 10-μm (aerosol size) quartz of the shales and the abundance of the shales suggest that soils derived from shale bedrocks (via glacial, fluvial, eolian, and weathering processes) serve as a source of aerosolic minerals.