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During Ocean Drilling Program Leg 102, the Schlumberger natural gamma-ray spectrometry tool measured high po­ tassium levels in one interval of DSDP Hole 418A basalts. Samples from that 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 al­ teration resulting in formation of palagonite and K-rich clays (celadonite, potassic nontronite, and mixed-layer celadonite/nontronite). Where these readings are accompanied by high porosity, low density, and low velocity readings, a brec­ cia with a once-glassy matrix, now altered to K-rich clay, may be inferred. Where porosity, density, and velocity readings indicate no breccia, high K readings may be attributed to potassium feldspar, an indicator of a second stage of oxidative alteration. In either case, there is a significant contribution to the K readings from palagonite. Further results of this study indicate a continuous fluid evolution and secondary mineral formation in these basalts. An initial stage of oxidation and formation of iron oxide-hydroxides was followed by precipitation of K-rich clay miner­ als. The first clays to form may have been celadonite, mixed-layer celadonite/nontronite, or potassic nontronite. As the fluid evolved, the minerals formed were increasingly depleted in K and ferric iron and increasingly enriched in Mg, Al, Ca, and ferrous iron. The final phase was an iron-rich saponite. 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 + goethite). Potassium feldspar is associated with this zone and is believed to have formed with the second stage of oxidative alteration.

Evidence for Continuous and Discontinuous Alteration in DSDP Hole 418A Basalts and its Significance to Natural Gamma-Ray Log Readings