퇴적학/층서학/고생물학/석유지질학 10월29일(금)
Characteristics of recycled feldspars from sedimentary rocks
Feldspars have been generally believed to be easily weathered under Earth's surface conditions because they are more susceptible to weathering compared with quartz.
Accordingly, Feldspar/Quartz ratio in sedimentary rocks is lower than that of igneous and metamorphic rocks. Even though there have been many studies dealing with weathering of feldspars in granites, few results have been reported regarding the weathering phenomenon of feldspars in feldspathic sandstones. Feldspars in sedimentary rocks commonly experience compositional alteration during diagenesis such as albitization, which could affect the rate and mechanism of their weathering after exposure. In this study, we have examined sediments collected from three small watersheds, composed of Cretaceous feldspathic sandstones in southern and eastern Korea. Each watershed has single lithology with limited compositions so that the compositional variations found in sediments would reflect weathering processes instead of changes in source rock lithology. Source rocks and sediment samples were collected along streams of each watershed, and medium and fine sand fractions were seperated and impregnated with epoxy. Petrographic and geochemical analyses such as X-ray fluorescence and electron microprobe analysis were carried out to examine the sand samples. Source rocks are mainly composed of monocrystalline quartz, plagioclase, K-feldspar, and rock fragments with minor amount of micas and opaque minerals. Most plagioclases are albitized, and show dusty and inclusion-rich features.
Compositional variation in plagioclase is quite limited with an average of 97% albite.
Na-contents of K-feldspar in the sand-sized sediments are significantly smaller compared with those of source rocks, suggestive of selective removal of Na-containing K-feldspars during weathering. Plagioclase contents in sediments and source rocks are rather similar although plagioclase content variation is greater for sand samples. On the other hand, K-feldspar content in the sand samples is smaller than that of sandstones. The ratios of Plagioclase/K-feldspar increase with increasing transport distances. The result indicates that weathering rate of K-feldspar is higher than that of plagioclase in the studied watersheds.
The result of this study shows deviation from common thought that plagioclase is more labile than K-feldspar during weathering.
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