Lithium and boron distributions in geological samples

The concentration of elements in whole rocks and maps of their distribution in petrographic thin sections were determined using ion beam microprobe an alysis. Lithium and B contents in rocks and minerals are measured using the Li-7(p, alpha)He-4 and B-11(p, alpha)Be-8 reactions. X-rays are simultaneo usly detected for elements heavier than Na, including Cl, to corroborate mi croscopic mineral identification. The ion beam analysis data are integrated with observations under the petrographic and scanning electron microscopes , as well as analysis using X-ray fluorescence and the electron microprobe. Lithium, B and Cl can be used to assess volcanic and hydrothermal processes . In a study of 62 samples, Li, B and Cl increase proportionally with incre asing silica content in fresh volcanic rocks from the Taupo Volcanic Zone i n New Zealand. The median values for Li, B and Cl in rhyolites (SiO2% = 70- 76%) are 35, 20 and 800 ppm (wt), respectively, and 19, 11 and 340 ppm (wt) , respectively, for andesites (SiO2 = 56-62%). Boron and Cl preferentially partition into the glassy matrix of rhyolite and andesite. In rhyolites, Li occurs mainly in minerals such as hornblende and biotite but resides in th e glassy matrix of andesites. During hydrothermal alteration of volcanic ro cks, CI always partitions into hydrothermal solutions while Li and B are pr eferentially redistributed in the rock. As hydrothermal alteration proceeds , Cl in the rock decreases and B and Li increase proportionally, depending on the type of mineralization present and the temperature of alteration. (C ) 1999 Elsevier Science B.V. All rights reserved.