54. THE CHEMISTRY OF ALTERED BASALTS AT SITE 417,

A chemical study of 200 meters of highly altered basalt from Hole 417A shows that potassium is strikingly enriched in these rocks, and that calcium, magnesium, and sodium are all depleted. Manganese is very slightly depleted and phosphorus slightly enriched. A comparison of these rocks with fresher examples from nearby Hole 417D suggests that they were originally the same materials. Silicon, titanium, total iron, and aluminum cannot be shown to have moved during alteration; a small difference in estimated original aluminum composition is explained on the basis of original variation in plagioclase content. Of the minor elements, lithium, boron, and barium are strongly enriched, and none of the elements can be shown to be generally depleted, although several (yttrium, strontium, nickel) are slightly redistributed during alteration. A calculation of the budget of alteration shows that about 690 grams of potassium have been introduced for a 200-meter column of rock that is 1 cm2 in cross-section. Because hydrogen ion is needed to maintain elec-trical balance, bicarbonate is probably consumed, yielding carbonate ion. If sea water is the altering fluid, then differing assumptions as to the original porosity show that an amount equal to 700 to 1600 volumes of the original void space must have been supplied, if potassium is the limiting chemical species and if it is completely removed from the water. Assuming a low temperature of alteration (on the basis of mineralogy and oxygen isotopes), the amount of heat necessary to drive the required volume of water through the rock must come from a crustal section in excess of what was drilled