Critical reviews of an early version of the manuscript by J. Allan, J. Beard, and R. Batiza were very helpful. Dr. L.M. Larsen (Geological Survey of Greenland and Denmark) provided XRF analyses of starting materials and the whole-rock analyses of the Leg 152 and Leg 163 samples before publication. Support for this research was provided by NSF-94-19382 and JOI/USSAC (CEL).