Heterogeneous alteration of the upper oceanic crust: Correlation of rock chemistry, magnetic properties, and O isotope ratios with alteration patterns in basalts from site 396B, DSDP

Abstract

Mineralogy, chemistry, and some magnetic properties of altered basaltic rocks from DSDP Site 396B are rather simply related to one another but differ dramatically with the nature of the igneous material and with the sharp alteration zonation within individual basalt fragments. Low‐temperature alteration of the major igneous minerals appears to have been primarily a function of oxidation, which affects titano‐magnetite pervasively (gray interiors) and olivine locally (brown zones). Destruction of olivine in the pillow basalts released Mg++, SiO2, Co, and Ni to the circulating solution and resulted in a calculated 9–10% decrease in density of the most altered (brown) zones in the rock. Seawater phosphate was incorporated in secondary ferric oxides replacing the olivine. In the massive basalts, all of the SiO2 and much of the Mg from olivine recombined as smectite, and the inferred rock density is almost unchanged. In almost all of the crystalline rocks, primary Ca, Na, Fe, Ti, and Al were immobile, owing to the metastability of plagioclase and clinopyroxene. The filling of cracks by smectite, Fe‐Mn oxides, phillipsite, and calcite consumed elements from various sources and was probably more important than basalt alteration for some chemical fluxes. Values of δ18O for the altered rocks are roughly 1–3‰ higher than their fresh equivalents, owing to the addition of 18O‐rich secondary oxides, smectites, and lesser carbonates and phillipsite. The highest values are associated with the smectite‐rich samples. However δ18O is not a good indicator of the degree of alteration because of strong leaching in the most altered zones of the pillow basalts. Pervasive oxidation of titanomagnetite to titanomaghemite has caused all samples to have high Curie temperatures, indicating up to about 70% oxidation. With the destruction of olivine in the brown zones, strong oxidation has caused breakdown of titanomaghemite to more stable nonmagnetic oxides, hence a drop in saturation intensity. These reactions have proceeded further in the pillow basalts than in the massive basalts. Freshly created oceanic crust consisting of various basaltic materials with abundant void space and (or) interstitial sediments is clearly heterogeneous; this primary heterogeneity is accentuated by alteration.

Alteration of Upper Ocean Crust in a Ridge-Flank Hydrothermal Upflow Zone: Mineral, Chemical, and Isotopic Constraints from Hole 896A

“…Altered oceanic volcanic rocks drilled from other localities are generally similar in mineralogy and sequence of alteration zonations to those sampled in Hole 896A (e.g., Andrews, 1977;Böhlke et al, 1981;Alt and Honnorez, 1984;Alt et al, 1986a;. Basalts from oceanic crust younger than Hole 896A provide some constraints on the absolute timing of seafloor alteration.…”

“…Similarly, MgO contents decrease with increasing LOI, which is consistent with the accumulation of plagioclase phenocrysts in the rocks and with more intense alteration of rocks of more primitive compositions. But some loss of Mg may have occurred during alteration of the rocks as well (Böhlke et al, 1981;Alt and Honnorez, 1984;Alt, in press). Other trends in Figure 8A confirm the K 2 O and P 2 O 5 gains apparent in Figure 6A, and suggest the addition of Na 2 O to and the loss of MnO from these rocks.…”

Correlation among the Changes with Alteration in the Mineralogical, Chemical, and Magnetic Properties of Upper Ocean Crust, Hole 896A

“…Potassium content is thought to be one of the good indicators of low-temperature alteration of oceanic basalts by seawater (see discussion in Honnorez, 1981). In the study samples, K 2 O generally ranges from 0.02% to 0.16%, and only four samples have K 2 O values markedly higher.…”

“…= interstitial. Honnorez's (1981) observation that no single chemical or mineralogical criteria can be used for measuring the degree of low-temperature alteration of seafloor basalts because of the heterogeneity of the processes involved, and that of the resulting secondary mineralogy. In oceanic basalts altered at low temperature, K mainly resides in celadonite, nontronite, and phillipsite.…”

“…Low-temperature alteration is defined (Honnorez, 1981) as "ranging from the bottom water temperature up to, but not including, the crystallization temperature of unequivocally metamorphic minerals . .…”