ISPY SAN CRISTOBAL'S MAGMA SOURCE: THE GALAPAGOS ISLAND EDITION

San Cristóbal is the easternmost island of the Galápagos Archipelago, with an area of 570 km². On the basis of magnetic stratigraphy and K-Ar ages, both parts of the island emerged at least 2.3 Ma m.y. ago (Geist et al., 1986). The SW volcano is a 730 m high, heavily weathered and vegetated shield, with cliffs along the south coast. The NE volcano is lower in elevation and dominated by fissure-fed flows. Two large palagonite cones are preserved along the coast, and at least 4 are located several km inland. Either a water source existed during the formation of the inland tuff cones, or the coastline differed from its current configuration at the time the cones were active. On the western flanks of the NE volcano, there are at least 5 preserved, sparsely vegetated flows that are significantly younger than the rest of the island (X-Y ka, based on new cosmogenic helium ages), abundant with spatter cones and ENE-aligned fissures.

Incompatible trace element ratios indicate that the mantle source for San Cristóbal is a mixture of plume material plus ancient recycled oceanic crust, with minor contributions from the depleted mantle. Furthermore, San Cristóbal lavas are produced by variable extents of melting of the source. While both shields display flat to light-REE enrichment, lower Sm/Yb ratios from the NE volcano suggest melts were derived at shallower depths than those from the SW shield. Relatively primitive Mg# values (49-74) indicate that melts did not experience extensive crystallization prior to eruption, mostly olivine and minor cpx crystallization, with some plagioclase in the SW. San Cristóbal, as well as the other eastern Galápagos shields, constitute a volcanic province that differs from the younger western islands in three notable ways: a) there are no calderas (or positive gravity signals) in the east, in contrast to the western shields; b) eastern island lava compositions are more variable than in the west; and c) eastern lavas are more primitive than those in the west. We propose that eastern Galápagos volcanoes may have formed by a mechanism distinct from that responsible for the younger western shields, possibly related to the archipelago’s interaction with the nearby Galápagos Spreading Center. Ongoing SEM analysis of phenocrysts in San Cristóbal lavas will provide insight into how volcanoes evolve in a near-ridge setting.