科研之友
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摘要
Kaolinite-rich fractions of <45, <2 and <1 mu m size from deposits of Galicia (NW Spain) at Ramon-Fazouro and Nuevo Montecastelo to the north, Vimianzo, Barilongo and Lendo in the center, and Santa Tecla to the south show significant ranges in delta O-18 from 16.5 to 22.4 parts per thousand and in delta D from -99 to -40 parts per thousand. These widely spread values question a single crystallization process as no statistically significant overall correlation between delta O-18 and delta D can be obtained. The analytical spread is such that 11 of the 43 analyzed separates plot below the kaolinite weathering line, and one at the line separating the supergene from hypogene evolution field. Noteworthy for this wide scatter is the fact that some separated size fractions, especially from Ramon-Fazouro deposit contain also up to 65% halloysite, as well as varied amounts of contaminating minerals generally considered as relicts from parental igneous rocks, such as quartz, illite (=muscovite) and feldspars. However, the addition of these minerals does not appear to significantly bias the isotopic data of the <2 mu m size fractions. Mean values for the <1 mu m kaolin fractions of the five deposits for which there is more than a single sample have linearly correlated delta O-18 and delta D values with a best-fit line: delta D = 8.75 delta O-18 - 248 (R = 0.896, n = 5, >95%) rather similar to the kaolinite weathering line of delta D = 7.5 delta O-18-220. However, the overall spread of values and particularly those for coarser material, together with occurrences of halloysite, strongly suggest that higher temperature process(es) also occurred. Kaolinite, and also likely halloysite, crystallized from waters, whose delta O-18 values were of meteoric signature from about -5 to -1 parts per thousand, at temperatures ranging from about 20 degrees C to 50 degrees C. The higher crystallization temperatures, especially at the sedimentary Lendo deposit, imply fluids at higher temperatures than during a climatic-controlled weathering process, probably of a low-temperate hydrothermal character with a fluid isotopically close to meteoric origin. However, interaction with fluids at higher temperatures and with higher delta O-18 cannot be ruled out, because the relatively wide ranges of kaolin delta O-18 and delta D values are difficult to reconcile with a narrow range of delta O-18 and delta D for the interacting parental fluids. The most likely scenario for the kaolinite and halloysite precipitation is then a two-stage process of interaction with meteoric waters at close Earth-surface temperature, but with one of the two episodes occurring at temperatures slightly above those of weathering processes even under tropical conditions. There is no compelling evidence for an episode of significantly high temperature, such as in most hydrothermal fluid-rock interactions, but a low-temperature episode of about 50 degrees C cannot be ruled out unequivocally, especially for the <2 mu m material. Following local continental weathering, percolation of meteoric waters at temperatures up to similar to 50 degrees C, and even higher, through the granitoids and felsites could have favored halloysite crystallization and affected the isotopic composition of earlier precipitated kaolinite.
- 出版日期2015-12-6
