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

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doi: 10.1029/JB086iB09p07935
Author(s): Boehlke, J. K.; Honnorez, J.; Honnorez-Guerstein, B. M.; Muehlenbachs, K.; Petersen, N.
Author Affiliation(s): Primary:
Univ. Miami, Rosenstiel Sch. Mar. and Atmos. Sci., Miami, FL, United States
Univ. Alberta, Canada
Ludwig-Maximilians Univ. Muench., Federal Republic of Germany
Volume Title: Journal of Geophysical Research
Source: Journal of Geophysical Research, 86(B9), p.7935-7950. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227
Note: In English. 50 refs.; illus. incl. 27 anals., 6 tables
Summary: 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 pil-low 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 incorpo-rated 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 al-most all of the crystalline rocks, primary Ca, Na, Fe, Ti, and Al were immobile, owing to the metastabi-lity 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%o 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 brownzones, 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 accentu-ated by alteration.
Year of Publication: 1981
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
Key Words: 18 Geophysics, Solid-Earth; Alteration; Atlantic Ocean; Basalts; Chemical composition; Crust; DSDP Site 396; Data; Deep Sea Drilling Project; Geochemistry; Halmyrolysis; Heterogeneity; IPOD; Igneous rocks; Isotopes; Lava; Leg 45; Leg 46; Low temperature; Magnetic properties; North Atlantic Ridge; O-18/O-16; Oceanic crust; Oxygen; Paleomagnetism; Physical properties; Pillow lava; Ratios; Stable isotopes; Tectonophysics; Temperature; Trace elements; Upper crust; Volcanic rocks
Coordinates: N225853 N225853 W0433057 W0433057
Record ID: 1982015062
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute.