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 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 alteration 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 breccia 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 minerals. 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.

Petrophysical Results from Logging in DSDP Hole 395A, ODP Leg 109

“…An increase in the potassium content in these basalts is generally recognized to be the result of alteration by interaction with seawater at low temperatures (as discussed by Holmes, 1988). The profiles of potassium content with depth in the phyric units suggest that each of these units was extruded over a relatively short period of time, and remained accessible to the downward flow of seawater for the period between the deposition of each unit.…”

“…The profiles of potassium content with depth in the phyric units suggest that each of these units was extruded over a relatively short period of time, and remained accessible to the downward flow of seawater for the period between the deposition of each unit. However, the very low concentration of potassium is in sharp contrast to the basalts at similar intervals within the basement at DSDP Hole 418A (Broglia and Moos, 1988;Holmes, 1988). In Hole 418A potassium-rich clays, potassium feldspars, and palagonite contribute the majority of the signal; these formed at low temperatures during a second phase of alteration.…”

Elastic wave velocities within oceanic layer 2 from sonic full waveform logs in Deep Sea Drilling Project Holes 395A, 418A, and 504B

“…Alteration is predominantly due to low-temperature interaction with seawater and increases the potassium and hydroxyl content, adding smectite, calcite, pyrite, and zeolites primarily as veins and vesicle fillings but also replacing olivine and interstitial groundmass [Holmes, 1988]. Alteration filling voids between pillows in the uppermost 190 m of basement (units 1, 5, and 6A) reaches as much as 25% by volume; infilling decreases abruptly below about 514 mbsf (the base of breccia unit 6A), and thus the position of the ancestral layer 2A/2B boundary is chosen at that point [e.g., Broglia .…”

“…Again, these differences may be a consequence of emplacement and/or alteration history. For example, the breccia in hole 418A is completely infilled with alteration materials [Broglia and Moos, 1988;Holmes, 1988]. In contrast, breccias in hole 395A have considerably less alteration infilling, on the basis of both log response and core descriptions [see Moos, 1990].…”

Morphology of extrusive basalts and its relationship to seismic velocities in the shallow oceanic crust

“…The primary infilling minerals are clays, zeolites and carbonates, although accessory minerals are sometimes present in small amounts [Alt et al, 1986;Lawrence et al, 1978]. For relatively young crust considerable free pore space persists, although under certain conditions this can be completely filled with clay minerals in older basalts [Holmes, 1988]. Alteration products infilling oceanic basalts include clays, which are quite soft and porous when hydrated, and calcite, which is quite stiff and nonporous.…”