Comparison of observed intrusive to extrusive ratios in Iceland and the Troodos Massif with results of experimental emplacement mode analysis of DSDP igneous rocks

Earlier work has shown that measurements of the anisotropy of magnetic susceptibility (AMS) of Deep-Sea Drilling Project (DSDP) igneous rocks can contribute to resolution of the mode of emplacement with 80% confidence. In order to clarify the reason for this 20% failure rate, the authors have made detailed AMS analyses on traverses through pillow basalts collected during cruises of the R/V Trident. Although extrusive AMS characteristics dominate, occasional pillows posses intrusive AMS featurs towards the pillow center. This can be explained by magnetic crystal elongation (growth or realignment) occurring in response to a directed thermal stress in the cooling pillow, such as could occur during implosions. The authors have analyzed 182 DSDP igneous samples and found that the ratio of extrusive to intrusive samples in the entire collection is about 2 to 1. It is concluded that the DSDP basalts which have been sampled, particularly prior to leg 34, are to some extent not true basement but represent intrusives into layer 1 during later phases of igneous activity outside the midocean ridge.