The Effect of Lithospheric Discontinuities on the Composition of Lavas From the Northern Galápagos Platform Extension

The platform supporting the Galápagos Archipelago extends ~50 km NW toward the Galápagos Spreading Center (GSC) into the Northern Galapagos Volcanic Province (NGVP), where it underlies Pinta, Marchena, and Genovesa Islands. Approximately 45 km north of Pinta is the southern termination of a ~100-km long transform fault, at 90^o50'W. The seafloor surrounding the NGVP was surveyed and dredged during the 2010 MV1007 and 2001 DRIFT04 cruises. All the large volcanoes, including the islands, have elongate bathymetric footprints with extensive submarine ridges. Lavas from this small region (<150 km in diameter) exhibit isotopic and trace element signatures that span the compositional range of the entire Galápagos Archipelago. Pinta and its submarine extension have the most enriched signatures, whereas at the eastern end of the platform extension, Genovesa is indistinguishable from normal MORB. Samples dredged around Pinta and Marchena have intermediate compositions. In contrast, lavas from the adjacent transform fault are more depleted than those from Genovesa and anywhere along the GSC for hundreds of km in both directions. Lavas from this region exhibit a range of ³He/⁴He (6.5-9.5 R_a), significantly lower than the high ³He/⁴He signature of material erupted by the plume-rich western Galápagos shield volcanoes (>25 R_a). Whereas the compositions of lavas erupted on the Nazca Plate in the NGVP require 3 or more distinct mantle endmembers to explain their compositions, our data indicate that the platform extension region only requires components previously described for the Galápagos Islands. In addition, Sm/Yb decreases abruptly along an E-W transect from Pinta to Genovesa. Gibson and Geist (2010) concluded that Sm/Yb ratios reflect variations in surface wave velocities (Villagomez et al., 2007), enabling them to predict lithospheric thickness. We apply the relationship defined by Gibson and Geist to map lithospheric thickness across our study area. Our results suggest that the lithosphere (and thus minimum melt generation depth) gets progressively shallower from Pinta to Genovesa; this phenomenon may explain unexpected Holocene volcanism in the NGVP. The wide range in Sm/Yb further suggests a complex lithospheric structure underlying the platform extension, which we propose reflects formation of the adjacent GSC transform fault within the last ~3 Ma, possibly through a series of ridge jumps as suggested by modeling of new magnetic data. We propose a model in which heterogeneous plume material with enriched and depleted components (Ito and Mahoney, 2005) is migrating toward the GSC along the base of the lithosphere, having previously lost its primordial helium signature during deep melting near the western shield volcanoes. As the material crosses the complex lithospheric thickness variations in the NGVP, melt generation becomes progressively shallower. The enriched compositions at Pinta thus result from limited, deep melting of enriched compositions owing to a thick lithospheric cap. East of Pinta, the plume melts more extensively and to shallower depths, producing progressively depleted signatures toward Genovesa.