MIXING OF HETEROGENEITIES IN THE MANTLE - EFFECT OF VISCOSITY DIFFERENCES

The effect of viscosity variations associated with large scale chemica l heterogeneities on mixing in the mantle is studied for a model two-d imensional problem. Low viscosity regions are rapidly deformed, develo p long tendrils, and are entrained towards regions of surface divergen ce. Very viscous regions are only slowly stretched; thus, geochemical reservoirs can persist relatively undisturbed for long periods of time if they are > O(10 - 100) times more viscous than the surrounding man tle. High viscosity ratio blobs have a tendency to aggregate, leading to the formation of large scale heterogeneities from smaller ones.