ALTERATION PROCESSES AT DEEP-SEA DRILLING PROJECT OCEAN DRILLING PROGRAM HOLE 504B AT THE COSTA-RICA RIFT - IMPLICATIONS FOR MAGNETIZATION OF OCEANIC-CRUST

Magnetics properties and oxide petrography results are presented from the most recent penetration at hole 504B during Ocean Drilling Program leg 111. Our results, combined with those from previous studies, show abrupt first-order changes in magnetic properties at alteration boundaries. Within the 504B crustal section, changes in style and degree of alteration fall near the boundaries of the three well-defined lithologic units: the extrusive basalts, the transition zone, and the sheeted dike complex. This postemplacement alteration heavily influences magnetic properties and is observed to change, in both style and degree, with depth within each lithologic unit. Our results indicate that, at 504B, the extrusive section deeper than 600 m below seafloor has become magnetically less stable due to postemplacement reheating and alteration. The upper, more permeable basalts above this critical depth have not experienced this reheating. Within the sheeted dike complex, the subsolidus cooling rate and the degree of hydrothermal alteration of the opaque minerals both decrease with depth. The changes in alteration of the oxide minerals occur in parallel with the decrease in bulk permeability associated with increasing depth. Overall, these observations suggest that the effective penetration of water into layer 2C decreased with increasing depth and resulted in a lower rate of convective cooling. Magnetic properties of rock samples from the sheeted dike complex suggest that as a result of this gradient in hydrothermal alteration, the upper dikes have become magnetically more stable than the lower dikes. A review of all magnetic properties indicate that the dike section at 504B carries a lower remanent magnetization than its intrinsic rock magnetic properties and mineralogy would predict. We suggest that this lower remanent magnetization is a result of the long and complex thermal and alteration history which involves the acquisition of magnetic components in different directions. Despite a magnetization which is lower than expected, it appears that the sheeted dike complex at hole 504B is capable of making a substantial contribution to the overlying marine magnetic anomaly. Because of the systematic decrease in hydrothermal alteration of magnetic minerals, the ability of the dike section to contribute decreases as a function of depth.