Dynamics of the Galapagos hotspot from helium isotope geochemistry
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We have measured the isotopes of He, Sr, Nd and-Pb in a number of lava flows from the Galapagos Archipelago; the main goal is to use magmatic helium as a tracer of plume influence in the western volcanoes. Because the Galapagos lava flows are so well preserved, it is also possible to measure surface exposure ages using in situ cosmic-ray-produced He-3. The exposure ages range from <0.1 to 580 Ka, are consistent with other chronological constraints, and provide the first direct dating of these lava flows. The new age data demonstrate the utility of the technique in this important age range and show that the western Galapagos volcanoes have been erupting distinct compositions simultaneously: for the last similar to 10 Ka. The magmatic He-3/He-4 ratios range from 6.9 to 27 times atmospheric (R-a), with the highest values found on the islands of Isabela (16.8 R-a for Vulcan Sierra Negra) and Fernandina (23 to 27 R-a). Values from Santa Cruz are dose to typical mid-ocean ridge basalt values (MORB, of similar to 9 R-a) and Pinta: has a He-3/He-4 ratio lower than MORE (6.9 R-a). These data confirm that the plume is centered beneath Fernandina which is the most active volcano in the archipelago and is at the leading edge of plate motion. The data are consistent with previous isotopic studies, confirming extensive contributions from depleted asthenospheric or lithospheric mantle sources, especially to the central islands. The most striking aspect of the helium isotopic data is that the He-3/He-4 ratios decrease systematically in all directions from Fernandina, This spatial variability is assumed to reflect the contribution of the purest plume component to Fernandina magmatism, and shows that helium is a sensitive indicator of plume influence. The highest He-3/He-4 ratios are found in volcanoes with lowest Na2O(8) and FeO(8), which may relate to source composition as well as degree and depth of melting. An excellent correlation is observed between He-3/He-4 and Nb/La, suggesting that the Galapagos plume source is characterized by high concentrations of Nb (and Ta). The major and trace element correlations demonstrate that helium is controlled by silicate melting and source variations rather than degassing and/or metasomatic processes. Although lavas with radiogenic isotopic compositions tend to have higher He-3/He-4, the island-wide isotopic variability cannot be explained by simple two components mixing alone. The preferred model to explain the isotopic data includes a heterogeneous plume, centered at Fernandina, which undergoes polybaric melting, and spatial divergence and mixing with asthenospheric material at shallower depths. The unique regional pattern of the helium isotopic data suggests that helium is extracted more efficiently than other elements during the early stages of melting in the ascending plume. Copyright (C) 1999 Elsevier Science Ltd.