Altered Magnetic Mineralogy Of Crescent Formation Basalt Flows

It has been very difficult to extract reliable paleomagnetic data from basalt flows of the Crescent Formation of the Olympic Peninsula, WA. Warnock et al. (1993) managed to squeeze out some useful paleomagnetic directions from the northern Crescent, but the authors of this study have attempted several locations in the southern Olympic Mountains without success. NRM intensities obtained from these studies are commonly less than 50 mA/m and extremely variable from sample to sample within a single basalt flow. Magnetic directions from a single flow also exhibit great variability, with alpha95 values commonly greater than 30 degrees and occasionally in excess of 100 degrees . Polished sections of samples from studied sites were examined using reflected light microscopy and scanning electron microscopy (SEM). Optical and SEM studies reveal the cause of magnetic intensity and direction variability to be low-temperature alteration which affects silicate, sulfide, and oxide minerals. Primary titanomagnetite and ilmentite have been altered to hematite, maghemite, goethite, and, in some cases, rutile. Degree of alteration ranges from minimal to extensive, and rock volumes along fractures and veins show the greatest effects. Coarse-grained portions of the samples containing large oxide crystals appear to be more highly altered than fine-grained pillow rims. It is hoped that more detailed examination of coarse- vs. fine-grained samples will allow development of sampling and analytical techniques which can provide reliable paleomagnetic data from these rocks