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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 896

Last Page: 896

Title: Feldspar Dissolution Before Advent of Land Plants on Earth: ABSTRACT

Author(s): Abhijit Basu

Article Type: Meeting abstract


The sedimentary record throughout geologic time shows that, in general, plagioclases alter faster than K-feldspars. Examination of detrital feldspars in Cambrian and Ordovician arenites shows that most K-feldspars are altered very little, whereas plagioclases are either absent or severely altered. This is in sharp contrast to the varying degrees of alteration suffered by all feldspars found in post-Silurian sediments. It is possible that regolithic processes of Cambrian-Ordovician times preferentially destroyed plagioclases while preserving K-feldspars.

Weak organic acids in soils, commonly due to humus, can dissolve all feldspars. In addition, continued uptake of potassium by land plants depletes the K+ ion concentration in soil waters and favors the dissolution of potassium-bearing phases. During Cambrian-Ordovician times, in the absence of vascular land plants (green algae and lichen notwithstanding), there would be no potassium-chelating agent, and the K+ ion concentration in soil waters would not have been depleted. Therefore, the composition of Cambrian-Ordovician soil waters could have been within the stability field of K-feldspars but beyond that of plagioclase (assuming that the silicon, aluminum and hydrogen activities did not make all feldspars unstable). Under such circumstances, K-feldspars would no be altered but the dissolution of plagioclases would continue. If so, weathering processes before landplant times must have been responsible for the unusual detrital mineral assemblage (quartz + fresh K-feldspars + no plagioclases) observed in Cambrian-Ordovician arenites.

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