Primitive neon from the center of the Galápagos hotspot Mark D. Kurz a, ⁎, Joshua Curtice a , Dan Fornari b , Dennis Geist c , Manuel Moreira d a Marine Chemistry and Geochemistry, MS #25, Clark 421, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States b Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States c Department of Geological Sciences, University of Idaho, Moscow, Idaho, United States d Institut de Physique du Globe, Tour 14-24, 3eme, 75005 Paris, France abstract article info Article history: Received 16 January 2009 Received in revised form 28 May 2009 Accepted 3 June 2009 Available online 22 July 2009 Editor: R.W. Carlson Keywords: Galápagos neon isotope helium basalt glass hotspot We present new helium and neon measurements in dredged basaltic glasses from the Western Galápagos, with an emphasis on the submarine flanks of Fernandina volcano, but including the adjacent flanks of Darwin, Ecuador, Wolf, and Roca Redonda. The samples from the submarine flanks of Fernandina volcano have the least radiogenic helium and neon; 3 He/ 4 He ratios vary from 18 to 29 times atmospheric (Ra), which spans the range previously observed at subaerial Fernandina. Samples from north of Fernandina have 3 He/ 4 He ratios closer to Mid-ocean ridge basalt (MORB) values (7.6 to 11.8 Ra), and are similar to subaerial lava flows from Darwin, Ecuador, Wolf, and Roca Redonda volcanoes. On a three-isotope neon diagram, the new submarine Fernandina data define a line that is closer to “solar” than data from Hawaii and Iceland, and therefore are among the least radiogenic, most primitive, neon isotopic compositions found on Earth. In contrast, the northern dredges have neon isotopic compositions similar to MORB. This sharp isotopic contrast between Fernandina and the adjacent volcanoes, whose summit calderas are only 35 to 40 km apart, is consistent with the hypothesis that Fernandina lies over the hotspot center. Measured 3 He/ 22 Ne ratios, coupled with helium and neon isotopic systematics show that 3 He/ 22 Ne values are extremely low in the Fernandina samples compared to the northern dredges. Coupled crushing and melting experiments show that vesicles often have lower 20 Ne/ 22 Ne and higher 3 He/ 22 Ne, suggesting that vesicle formation is not necessarily in solubility equilibrium, and that vesicles are preferentially affected by atmospheric contamination (as compared to host glass). There is a crude correlation between Mg# and total helium content in the glasses, suggesting that fractionation/degassing in a shallow magma reservoir is a primary control on noble gas contents. Due to similarities in volcanic plumbing between the Galápagos volcanoes, and in major and trace elements, the large 3 He/ 22 Ne variations (extrapolated to solar neon) between Fernandina (1.5) and the northern dredges (15) may be related to mantle source characteristics rather than the effects of recent degassing or melting. Simple closed-system evolution models show that the unradiogenic neon isotopic compositions require preservation since the first few hundred million years of Earth history, which is consistent with undegassed material in the lower mantle. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Noble gas isotopes provide unique constraints on early Earth degassing, the evolution of the mantle and atmosphere, and the nature of present-day mantle heterogeneity. Helium in some ocean island basalts, such as those found at Hawaii, Iceland, Galápagos, and Samoa is much less radiogenic than mid-ocean ridge basalts (MORB). This reflects lower time-integrated (Th + U)/He; samples from each of these islands have 3 He/ 4 He ratios greater than 25 times atmospheric (Ra), as compared to 8±1 Ra for MORB (e.g., Kurz et al., 1982; Kurz and Geist, 1999; Stuart et al., 2003; Jackson et al., 2007). The traditional interpretation of the difference between these high 3 He/ 4 He ocean islands and MORB, is that the unradiogenic noble gases are derived from a relatively undegassed lower mantle, perhaps left over from the early Earth (e.g., Kurz et al., 1982; Allègre et al., 1983). Geochemical and geophysical data have increasingly implicated whole-mantle circulation (e.g., Hofmann, 1997; Van der Hilst and Karason, 1999), making storage of such primitive material in the lower mantle controversial, and the “standard model” is under attack. A competing model for unradiogenic helium in ocean island basalts is based on the possibility that helium may be more compatible than Th and U during silicate melting (e.g., Parman et al., 2005; Heber et al., 2007). If so, melting could leave behind a residue of depleted mantle with lower (Th + U)/He, and high present-day 3 He/ 4 He would then indicate ancient depleted residue, rather than undegassed primitive mantle. However, noble gas partitioning in mantle mineral phases is poorly known and such models are speculative. The origin of unradiogenic helium is therefore of fundamental importance to models of the deep Earth. There are few neon, argon, and xenon isotopic data from high 3 He/ 4 He islands, because submarine glasses that trap magmatic gases are Earth and Planetary Science Letters 286 (2009) 23–34 ⁎ Corresponding author. E-mail address: mkurz@whoi.edu (M.D. Kurz). 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.06.008 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl