OXYGEN-ISOTOPE GEOCHEMISTRY OF KAOLINITE IN LATERITE-FORMING PROCESSES, MANAUS, AMAZONAS, BRAZIL

Mineralogic, crystallographic, and oxygen isotopic analyses were made of kaolinites from a 20 m thick lateritic profile developed on the det rital sands and clays of the mid-Cretaceous to Tertiary Alter-do-Chao formation, central Amazon Basin, Brazil. A detailed delta(18)O profile of soil water from the upper 4 m of the same section was also measure d. Kaolinite separates were prepared from different pedogenic facies o f small hand specimens. Most of these kaolinite separates exhibit poly modal size distributions. The separates were further divided into indi vidual particle size modes for subsequent isotopic and crystallographi c analysis. In general, Hinckley Index and delta(18)O values of differ ent size fractions of the same sample differ. Hinckley Index values de crease strongly, and Fe content increases in the upward direction in t he profile. We conclude that (1) the kaolinite in the profile has prog ressively evolved in response to changing microenvironments as weather ing fronts have moved progressively downward, (2) different kaolinite fractions, even within a single facies of a single sample a few cm(3) in size, typically formed under somewhat different conditions, (3) the bulk of the fine-grained kaolinite in the lateritic profile does not constitute a continuous, inverted time record; that is, lower levels d o not represent more recent kaolinite formation, and higher levels do not represent older kaolinite formation, and (4) isotopic compositions of kaolinites from lateritic paleosols should provide an indication o f mean climatic conditions and some measure of seasonality during the interval preceding the removal of the laterite from the active weather ing environment. We speculate that the different conditions under whic h the different kaolinite fractions have formed may be related, at lea st in part, to variations in soil microenvironments that are ultimatel y derived from seasonal and year-to-year variations in the chemistry a nd isotopic composition of infiltrating rainfall. Copyright (C) 1998 E lsevier Science Ltd.