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ABSTRACT: Bulk Chemistry and 7Li/6Li Variations of Hydrothermally Altered Rocks from Taiwan

Chen Ju-Chin, Hua Sheng Tseng

The bulk chemistry and 7Li/6Li variations of hydrothermally altered rocks recovered from drill cores in Lutao, an islet about 34 km off southeastern Taiwan, and Chingshui of northeastern Taiwan have been studied. 7Li/6Li ratios were determined by Perkin-Elmer Model 703 atomic absorption spectrophotometer using a monoisotopic 6Li source lamp and a normal Li lamp. In the hydrothermally altered argillites and slates recovered from drill core at Chingshui there is a negative correlation between the rock formation temperature and 7Li/6Li ratios indicating that hydrothermal alteration may become more intense at relatively higher temperature (around 220°C). It should be noted that 7Li/6Li ratios show no correlation with the absolute Li concentrations in the samples recovered from the two drill cores indicating that equilibrium may have been reached at hydrothermal temperature. Hydrothermal alteration has caused relative enrichment of Mg, Ca, and Sr but depletion of Na, K, and Rb in the andesites from Lutao. The major factors that may effect the lithium isotope ratio as a result of hydrothermal alteration include the lithium contents of the hydrothermal solution and the wall rock and the temperature of equilibrium of the reaction. Unlike the oxygen system lithium isotopes may not be usable as a "calibrated" geothermometer since equilibrium may not be always established at hydrothermal temperature and lithium isotope ratios can be employed as an empirical guide o hydrothermal alteration.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990