The trace element composition of basaltic glasses from the TAG segment of the MAR indicate that magmas erupted on the surface are derived from multiple primary magmas with Ce/Sm ratios varying from 2.2 to 3.9. In the pseudoquaternary system olivine-plagioclase-clinopyroxene-quartz, TAG glasses cluster between the 4 and 6 kb phase boundaries. This suggests that TAG primary magmas evolve by crystallization at moderate to high pressure, then ascend rapidly to the surface with very short residence times and limited degrees of crystallization in shallow chambers. Such a plumbing system implies a deep-seated heat source for hydrothermal circulation. ¿ American Geophysical Union 1996