Sierra Negra volcano is a voluminous, active basaltic shield situated in the western Gal¿pagos archipelago, a hotspot-related chain adjacent to the Gal¿pagos spreading center. The volcano has erupted Fe-rich tholeiitic basalts of very limited compositional range. Comparison of lavas from each of five stratigraphic units covering the entire volcano indicates that the lavas are mineralogically similar and compositionally restricted (4.4 to 6.9 wt % MgO). The lavas are Fe-rich and hypersthene-normative and plot in both the alkaline and tholeiite fields of the alkali-silica diagram. Rare earth element (REE) patterns are steep and parallel. The major and trace element data indicate a comagmatic relationship by fractional crystallization of the observed phenocryst phases. Projections into pseudoternary phase diagrams suggest that the magmas cooled and crystallized at pressures between 1 and 3 kbar. Calculated melt densities range from 2.73 to 2.77 g/cm3, which are higher than those of typical mid-ocean ridge basalt (MORB), Hawaiian tholeiite, and other Gal¿pagos lavas.

The high calculated melt densities may be caused by fractionation at greater depths than is ordinary for the other basaltic magmas. Suppression of plagioclase and olivine crystallization in favor of augite may eliminate the minimum density ''window'' that has been proposed for MORB fractionation. Sierra Negra lavas have the most radiogenic lead and strontium isotopic ratios in the western Gal¿pagos, indicating that the magmas have a relatively large contribution of plume material and have been minimally contaminated by entrainment of MORB-producing mantle. Magmatic 3He/4He isotopic ratios from Sierra Negra are approximately 15 times the atmospheric ratio; although these ratios clearly indicate plume helium, they are not the highest 3He/4He in the archipelago, suggesting that helium is decoupled from the heavier isotopes. High Sm/Yb ratios, light rare earth element (LREE) enrichment, and a steep REE slope are consistent with an origin by moderate extents (5--15%) of partial melting of a garnet-lherzolite source with REE characteristics that are between chondrite and depleted Earth mantle sources. Sierra Negra lavas are compositionally monotonous because homogeneous plume-rich material has experienced long-term exposure to a well-regulated sublithospheric thermal environment with additional compositional restrictions imposed in the lithosphere by high magma flux. ¿ American Geophysical Union 1995