27 The Galápagos: A Natural Laboratory for the Earth Sciences, Geophysical Monograph 204. First Edition. Edited by Karen S. Harpp, Eric Mittelstaedt, Noémi d’Ozouville, and David W. Graham. © 2014 American Geophysical Union. Published 2014 by John Wiley & Sons, Inc. ABSTRACT Spatial asymmetry in the isotopic composition of volcanic rocks has been identified at several Pacific hotspots, including Hawai‘i, the Marquesas, Samoa, the Societies, and the Galápagos. At each hotspot, the volcanoes are arranged in two sub-parallel chains that also define distinct fields in isotopic space. Here we present interpreta- tions of data from two additional ocean island systems that exhibit spatial isotopic asymmetry: the Galápagos and Easter hotspots. Both systems display geochemical asymmetry despite being near-ridge hotspots, suggest- ing that compositional zonation in plumes originates at depths greater than the plumbing systems supplying mid-ocean ridges. Furthermore, the correspondence of the compositional boundaries of the Galápagos and Easter hotspots with those of the Large Low Shear Velocity Province (LLSVP) is consistent with the assertion that spatial patterns of isotopic enrichment at hotspots may reflect the distribution of compositional heteroge- neity within the thermal boundary layer at the base of the mantle that gives rise to the plumes. At the Galápagos hotspot, which is located along the northern side of the LLSVP, the southern side of the chain exhibits geo- chemical enrichment, whereas at the Easter hotspot, located along the southern side of the LLSVP, it is the northern side of the chain that is enriched. Consequently, spatial variations in the geochemistry of hotspot lavas may provide a method for mapping the geochemical structure of the lower mantle. 3 Galápagos and Easter: A Tale of Two Hotspots Karen S. Harpp 1 , Paul S. Hall 2 , and Matthew G. Jackson 3 1 Geology Department, Colgate University 2 Department of Earth and Environment, Boston University 3 Department of Earth Sciences, UC Santa Barbara 3.1. INTRODUCTION Morgan [1971] attributed hotspot volcanism to thermally buoyant plumes that rise from the deep mantle. If this hypothesis is valid, then the lavas erupted at hotspot volcanoes provide a glimpse into the composition of the otherwise inaccessible deep mantle. Archipelago-scale geochemical zoning at several Pacific hotspots, including Hawaiʻi, the Marquesas, the Societies, and possibly Samoa [Abouchami et al., 2005; Chauvel et al., 2012; Huang et al., 2011; Payne et al., 2013; Stille et al., 1983; Weis et al., 2011] suggests that the mantle plumes feeding these hotspots are themselves compositionally zoned. At each of these hotspots, volcanoes are distributed geographically along two sub-parallel chains that are geochemically distinct, with the southern trend enriched relative to the northern trend. Geodynamical studies suggest that the spatial distribution of heterogeneities within the thermal boundary layer that gives rise to man- tle plumes may be preserved within the plume conduit as material ascends from the core-mantle boundary (CMB) to the surface [Farnetani and Hofmann, 2009, 2010; Farnetani and Samuel, 2005; Kerr and Mériaux, 2004; Lohmann et al., 2009]. Consequently, the observed bilat- eral geochemical asymmetry at hotspots may reflect the geometry of geochemical reservoirs in the lower mantle. Weis et al. [2011] and Huang et al. [2011] proposed that geochemical zoning along individual hotspot tracks results from the plumes being located on the northern periphery of the Pacific Large Low Shear Velocity Province (LLSVP), a 0002098021.INDD 27 6/26/2014 11:23:24 PM