PETROLOGY OF BASALTIC GLASSES FROM THE TAG SEGMENT - IMPLICATIONS FORA DEEP HYDROTHERMAL HEAT-SOURCE

The trace element composition of basaltic glasses from the TAG segment of the MAR indicate that magmas erupted on the surface are derived fr om multiple primary magmas with Ce/Sm ratios varying from 2.2 to 3.9. In the pseudoquaternary system olivine - plagioclase clinopyroxene - q uartz, TAG glasses cluster between the 4 and 6 kb phase boundaries. Th is suggests that TAG primary magmas evolve by crystallization at moder ate to high pressure, then ascend rapidly to the surface with very sho rt residence times and limited degrees of crystallization in shallow c hambers. Such a plumbing system implies a deep-seated heat source for hydrothermal circulation.