Ultrabasic Liquids

Abstract

THERE is a considerable body of geological evidence suggesting that, in a submarine environment, extrusion of silicate liquids with a composition comparable to peridotite occurs. Later, this material is hydrated to form serpentine. In part, this evidence comes from repeated association of serpentines with marine sediments such as greywackes and cherts, and repeated association with volcanics including pillow lavas. Further, the ultramafics of this environment show features to be expected from rocks which have flowed. An excellent description of such a situation is provided by Bailey and McCallien¹. Hess² has summarized some of the data from field evidence suggesting that many ultrabasic rocks were formed by intrusion of liquids. But laboratory investigation has consistently heaped difficulties on an extrusive explanation of such rocks. Known surface lava temperatures do not exceed 1,200° C. while the melting temperatures of the anhydrous equivalents of serpentine are of the order of 1,700° C. Clearly there are difficulties in moving such materials to the surface of the Earth, or even in forming liquids at reasonable depths in the crust. The following statement from Hess² bears repeating : “For most of this century a magnificent argument has gone on between field geologists who have worked on the peridotites of alpine mountains and laboratory investigators of their chemistry (particularly Bowen). The former stoutly maintains that the evidence indicates that they were intruded in a fluid state, as magmas; and the latter equally forcefully has proved to his satisfaction that such magmas are not possible… At the other extreme we find Bailey and McCallien suggesting that the serpentines of Turkey are submarine lava flows.”