CHARACTERIZATION OF SECONDARY ALTERATION IN THE COLUMBIA RIVER BASALT BY BACKSCATTERED ELECTRON IMAGING AND ENERGY-DISPERSIVE X-RAY SPECTROSCOPY.

The thick sequences of flood basalts which underlie the Columbia River basin are important aquifiers, providing water for both agricultural and domestic use. Secondary alteration in these rocks occurs primarily as coatings or fillings in fractures and vesicles; alteration is generally believed to have occurred at low temperatures ( less than 100 C) by reaction with meteroic waters. The distribution and compositional variation of secondary minerals are therefore of major interest. This paper focuses on the compositional variation of the major alteration products and on present formulas based on the mean composition of these phases, and suggests possible reactions for the observed sequence of alteration.