FY 1984

Geochemical studies of abyssal lavas recovered by DSRV Alvin from eastern Galapagos Rift, Inca Transform, and Ecuador Rift. 1. Major element variations in natural glasses and spacial distribution of lavas

Fornari, D.J., M.R. Perfit, A. Malahoff, and R. Embley

J. Geophys. Res., 88(B12), doi: 10.1029/JB088iB12p10519, 10,519–10,529 (1983)

Fifty-three major element chemical analyses of volcanic glasses, collected by ALVIN, from the eastern Galápagos rift-Inca transform-Ecuador rift plate boundary are used to assess tectonic controls on petrogenesis and subrift magma chamber evolution. These lavas span a compositional range that includes mid-ocean ridge basalts (MORB), FeTi basalts, basaltic andesites, and andesites. This entire chemical spectrum of rocks was recovered on each of several dives within the eastern Galápagos rift along short (~2 km) traverses of the seafloor at the axis of accretion. This juxtaposition of chemically diverse lavas supports the idea that very small, shallow-level isolated magma bodies are the eruptive sources for fractionated accretionary lavas in this rift segment. A distinct difference in chemical diversity occurs between two of the dives (1002 and 998) in the eastern rift. Lavas recovered east of the break exhibit high degrees of fractionation, while rocks from west of this divide have only MORB compositions. This break occurs approximately 60 km west of the Galápagos rift-Inca transform intersection, and it is believed to mark an important east-to-west transition in subrift accretionary dynamics. East of this break, accretion is perturbed by transform ("cold edge") plus attempted rift propagation effects, while west of it a more stable accretionary environment (large, buffered magma chambers which erupt only MORB spectrum lavas) prevails. Only extremely Fe- and Ti-enriched lavas were recovered from the southern intersection of the Inca transform with the Galápagos rift. The abundance of FeTi basalts at this locale is considered to be an effect of attempted rift propagation through thick, cold Nazca plate lithosphere. Rocks from the Ecuador rift-northern Inca transform intersection have similar MORB chemistries and are among the least fractionated basalts recovered from the Cocos-Nazca plate boundary.