Dynamics of the Galapagos hotspot from helium isotope geochemistry

We have measured the isotopes of He, Sr, Nd and-Pb in a number of lava flow s from the Galapagos Archipelago; the main goal is to use magmatic helium a s a tracer of plume influence in the western volcanoes. Because the Galapag os 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 ra nge from <0.1 to 580 Ka, are consistent with other chronological constraint s, and provide the first direct dating of these lava flows. The new age dat a demonstrate the utility of the technique in this important age range and show that the western Galapagos volcanoes have been erupting distinct compo sitions 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 va lues 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 archip elago and is at the leading edge of plate motion. The data are consistent w ith previous isotopic studies, confirming extensive contributions from depl eted asthenospheric or lithospheric mantle sources, especially to the centr al islands. The most striking aspect of the helium isotopic data is that th e He-3/He-4 ratios decrease systematically in all directions from Fernandin a, This spatial variability is assumed to reflect the contribution of the p urest plume component to Fernandina magmatism, and shows that helium is a s ensitive indicator of plume influence. The highest He-3/He-4 ratios are fou nd 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 correlatio n is observed between He-3/He-4 and Nb/La, suggesting that the Galapagos pl ume source is characterized by high concentrations of Nb (and Ta). The majo r and trace element correlations demonstrate that helium is controlled by s ilicate melting and source variations rather than degassing and/or metasoma tic processes. Although lavas with radiogenic isotopic compositions tend to have higher He-3/He-4, the island-wide isotopic variability cannot be expl ained 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 a sthenospheric 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 pl ume. Copyright (C) 1999 Elsevier Science Ltd.