Age variation in the physical properties of oceanic basalts: Implications for crustal formation and evolution

Abstract

The time‐dependent variability of the physical properties of upper oceanic crustal rocks have been characterized by compiling acoustic velocity, density, porosity, iron oxidation state, and H2O+ data from the upper 50 m of all Deep Sea Drilling Project and Ocean Drilling Program drill holes which recovered basalt samples from normal oceanic basement. This data compilation is interpreted in terms of the geological processes responsible for these changes as the seafloor ages. Drill site averages of both acoustic velocity and bulk density show a decrease with increasing age for crustal ages less than 100 Ma. Similarly, chemical data from the same drill cores indicate the presence of two distinct alteration processes. Oxidation of ferrous to ferric ions within the rock‐forming minerals proceeds rapidly within the first 5 Ma after formation, slows in the subsequent 5‐ to 20‐m.y. period, and then appears to stop. Hydration of the silicate minerals also appears to generally increase with age over the full 150‐m.y. age range of the samples, although at a slower rate than oxidation. The observed distribution of the physical properties of the upper crustal rocks for the first 100 Ma can be explained by three processes: (1) a large initial variability of Vp and density, caused by the range of original porosity of the rocks, (2) an age‐progressive alteration associated with oxidation that systematically reduces both Vp and density with increasing age for the first 5 m.y., and (3) an additional, nonoxidative alteration that increases the hydration of the silicate minerals, with a resulting major impact on the physical properties. Still unexplained, however, is the lack of an expected trend of decreasing porosity with increasing age and alteration and the large, nonsystematic scatter in the physical property data for samples older than 100 Ma.

Advanced geophysical studies of accretion of oceanic lithosphere in Mid-Ocean Ridges characterized by contrasting tectono-magmatic settings

“…Although the elastic P wave velocities of intact, relatively unaltered, saturated samples of oceanic basalts range from approximately 5 to 6 km s À1 [Johnson and Semyan, 1994], P wave velocities of the uppermost oceanic crust measured in situ by seismic refraction are as low as 2.2 km s À1 at near-zero age [Purdy and Detrick, 1986;Harding et al, 1989;Vera et al, 1990]. Our V1 velocity class, assigned to basalts (±dikes), falls well within the range noted above.…”

Geomicrobiology of the ocean crust: the phylogenetic diversity, abundance, and distribution of microbial communities inhabiting basalt and implications for rock alteration processes

“…There is a significant decrease in the permeability of upper ocean crust within the first several million years of crustal evolution, suggesting the sealing of voids and fractures due to the development of secondary minerals. Age dating of secondary minerals , as well as Fe3+÷/Fe measurements of basalt samples from numerous drill cores Johnson and Semyan, 1994), further supports this observation by demonstrating that oxidative alteration occurs primarily in the first 10-15 million years. A petrographic study compared different alteration textures, considered to be either biologically or abiotically induced, in basalt glass from a wide variety of drill cores.…”

Physical properties of young (3.5 Ma) oceanic crust from the eastern flank of the Juan de Fuca Ridge: Comparison of wireline and core measurements with global data

“…P wave velocities, electrical resistivities and total gamma ray measured in situ using downhole wireline measurements also tend to increase with basement age [Bartetzko, 2005]. However, velocities and densities measured on drill cores often decrease with age while intergranular porosity increases [e.g., Johnson and Semyan, 1994;Johnston and Christensen, 1997;Jarrard et al, 2003]. These changes in physical properties with the age of the crust are interpreted to result mainly from the interaction of water and rock during the ridge-flank hydrothermal circulation.…”

“…The increase in P wave velocity and electrical resistivity represents a decrease in porosity, whereas an increase in total gamma ray results from the incorporation of potassium from seawater into secondary minerals. On the scale of core samples, basalt alteration is expressed as an increase in intergranular porosity and replacement of the original mineralogy by secondary minerals [e.g., Johnson and Semyan, 1994].…”