Evidence for continuous and discontinuous alteration in DSDP Hole 418A basalts and its significance to natural gamma-ray log readings

Abstract

During Ocean Drilling Program Leg 102, the Schlumberger natural gamma-ray spectrometry tool measured high potassium levels in one interval of DSDP Hole 418A basalts. Samples from the hole were examined by X-ray diffraction, SEM, and EDS analyses in order to identify mineral species responsible for the high readings. The results indicate that high K readings observed on wireline logs run in basement can be attributed to extensive low-temperature oxidative alteration resulting in formation of palagonite and K-rich clay. Further results of this study indicate a continous fluid evolution and secondary mineral formation in these basalts. In at least one interval with high permeability, a second stage of oxidative alteration resulting from the influx of a new fluid is demonstrated by the alteration of saponite to iddingsite (saponite + geothite). -from Author