Abstract
Positive identification of the origin of fine-grained quartz in highly weathered soils is not proven by routine microscopic examination. The oxygen isotope composition (δ 18O vs SMOW) of quartz was determined in the albic horizon of three soils developed on the Cretaceous Nohhi rhyolite from Kiso, central Japan. Differences in the isotopic composition of quartz as a function of particle size were observed and attributed to the degree of hydrothermal alteration. One soil sample developed nearby hornblende porphyrite intrusion showed systematic increases in the δ 18O values with increasing particle size, from +3.6‰ (1–10 μm) to +9.5‰ (500–2000 μm). Such a trend is indicative of the higher contribution of secondary quartz precipitated from hydrothermal meteoric fluids in fine size fractions in contrast to primary magmatic quartz in coarse size fractions. Hydrothermal activity was inferred for another soil where no lateral intrusion is mapped. The variation in the oxygen isotopic composition of quartz in different particle sizes precludes both eolian and authigenic origins for the soils. The stable isotope technology provides the only rationale for positive identification of the origin of fine-grained quartz in soils.
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