Primitive neon from the center of the Galapagos hotspot

We present new helium and neon measurements in dredged basaltic glasses from the Western Galapagos. 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; He-3/He-4 ratios vary from 18 to 29 times atmospheric (Ra), which spans the range previously observed at subaerial Fernandina. Samples from north of Fernandina have He-3/He-4 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 He-3/Ne-22 ratios, coupled with helium and neon isotopic systematics show that He-3/Ne-22 values are extremely low in the Fernandina samples compared to the northern dredges. Coupled crushing and melting experiments show that vesicles often have lower Ne-20/Ne-22 and higher He-3/Ne-22, 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 Galapagos volcanoes, and in major and trace elements, the large He-3/Ne-22 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. (C) 2009 Elsevier B.V. All rights reserved.