Mineralogy and Geochemistry of Alteration Products in Holes 417A and 417D Basement Samples (Deep Sea Drilling Project Leg 51)

Abstract

The striking difference between the basalts of Hole 417A (strongly altered) and those of Hole 417D (slightly altered) is one of the main discoveries of Legs 51-52. After a macroscopic description of the two basaltic sections, the authors describe the mineralogy of alteration in the two holes. In Hole 417D, the pillow margins exhibit a nice concentric zoning, with a hyaloclastic zone, a glassy zone, a variolitic zone, a spherolitic zone, and the pillow core. Fresh black and brilliant glass is abundant; it is partly altered mainly in the hyaloclastic zone (delicate concentric rims of brown palagonite) and in the variolitic zone (yellow fibropalagonite). X-ray diffraction patterns indicate a dioctahedral smectite (montmorillonite type) for the palagonite to a depth of 475 meters, and a trioctahedral smectite (saponite type) for deeper samples. SEM chemical profiles in the palagonite rims of hyaloclastic fragment indicate a strong enrichment in K and Fe in the outermost layers, suggesting a "protoceladonite" composition for the corresponding smectites. The glassy zone remains remarkably fresh: palagonitic layers developed along cracks are quite similar to the palagonite rims in the hyaloclastic zone. Clinopyroxene and plagioclase phenocrysts are generally devoid of alteration; olivine phenocrysts are completely replaced by calcite, iron oxides, and brown to green smectites. Vesicles and veinlets are filled by green smectites, brown smectites, and calcite, in order of abundance. No notable evolution with depth was observed.In Hole 417A, the pillow margins exhibit the same concentric structure. The glass is completely palagonitized, with two types of development, according to the density of fractures in the glassy zone. Olivine phenocrysts are completely replaced by iddingsite, calcite, and green and brown smectites. Plagioclase phenocrysts are strongly altered and replaced by analcite, calcite, potash feldspar, and/or chabazite. Clinopyroxenes are not affected by alteration. Vesicles and veinlets are filled by the same minerals described in Hole 417D, in the same order of succession. Green smectites are dioctahedral and brown smectites are trioctahedral. No notable variations with depth have been noticed, except a strong oxidation in the first 10 meters of the basaltic pile.Comparison between chemistry of fresh glasses and palagonites shows a complete depletion in CaO; loss in SiC 2, total FeO, MgO, CaO, Na2θ, and TiCh; strong enrichment in K2O and H2O; and a strong increase in the Fe2θ.j/FeO ratio. With these data and those available in previous studies of palagonites, the authors tried to estimate the balance of chemical exchange for the major oxides during the palagonitization process, and the average rates of chemical variation per year for each oxide. The comparison of the chemistry of fresh and altered basaltic pillow cores leads to similar conclusions concerning the behavior of major elements during alteration, but the quantities involved and rates of exchanges are quite smaller, especially for CaO.Rar...

Metamorphic Processes in Sediments in Contact with Young Oceanic Crust—East Pacific Rise, Leg 65

“…Earlier studies of submarine basalt alteration (e.g., Ijima and Harada, 1969;Bass, 1976) have concluded that di-and trioctahedral smectites are typical of low temperature alteration, or halmyrolysis (Melson and Thompson, 1973;Andrews et al, 1977;Seyfried et al, 1976;Robinson et al, 1977;Juteau et al, 1980) and that mixed-layer chlorite-smectites, associated with zeolites such as analcite, are index minerals for hydrothermal alteration (Humphris and Thompson, 1978). The samples studied here show trioctahedral smectites replaced by mixed-layer minerals.…”

Site Synthesis Report of DSDP Sites 417 and 418

“…A!teration products include, in decreasing order of volume, clay minerals, carbonates, K-feldspars, zeolites, silica, iron oxyhydroxide, opaque minerals, and chlorite (Pritchard 1980;Humphris et al, 1980;Rusinov, 1980, Scheidegger andStakes, 1980;Rusinov et al, 1980a;Juteau et al, 1980, Mevel, 1980Gitlin, 1985). 59 was altered to K-rich celadonite and/or nontronite by early low-temperature oxidative alteration.…”

“…Chemical budget implications of the alteration of basaltic glass to palagonite are discussed by Juteau et al (1980) andHart (1983). 180 is enriched in altered basalt increasing 5180 (Muehlenbachs, 1980;Javoy and Fouillac, 1980).…”

Site synthesis report of DSPP sites 417 and 418

“…Thompson (1983) recalculated elemental fluxes for 418A (his Table 47.14) and 417A (his Table 47.16). Chemical budget implications of the alteration of basaltic glass to palagonite are discussed by Juteau et al (1980) andHart (1983). 18Q is enriched in altered basalt increasing (518Q (Muehlenbachs, 1980;Javoy and Fouillac, 1980).…”

“…Alteration products include, in decreasing order of volume, clay minerals, carbonates, K-feldspars, zeolites, silica, iron oxyhydroxide, opaque minerals, and chlorite (Pritchard 1980;Humphris et al , 1980;Rusinov, 1980, Scheidegger andStakes, 1980;Rusinov et al ., 1980a;Juteau et al, 1980, Mevel, 1980Gitlin, 1985). ..…”