Dynamics of the Galapagos hotspot from helium isotope geochemistry
The authors 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 {sup 3}He. The exposure ages range from {lt}0.1 to 580 Ka, are consistent with other chronological constrains, 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 distance compositions simultaneously for the last {approximately}10 Ka. The magmatic {sup 3}He/{sup 4}He ratios range from 6.0 to 27 times atmospheric (R{sub a}), with the highest values found on the islands of Isabella (16.8 R{sub a} for Vulcan Sierra Negra) and Fernandina (23 to 27 R{sub a}). Values from Santa Cruz are close to typical mid-ocean ridge basalt values (MORB, of {approximately}9 R{sub 2}) and Pinta has a {sup 3}He/{sup 4}He ratio lower than MORB (6.9 R{sub 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 {sup 3}He/{sup 4}He 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 {sup 3}He/{sup 4}He ratios are found in volcanoes with lowest Na{sub 2}O(8) and FeO(8), which may relate to source composition as well as degree and depth of melting. An excellent correlation is observed between {sup 3}He/{sup 4}He 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 {sup 3}He/{sup 4}He, 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.
- Research Organization:
- Woods Hole Oceanographic Institution, MA (US)
- OSTI ID:
- 20006272
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 63, Issue 23-24; Other Information: PBD: Dec 1999; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
Similar Records
He, Ne, Ar, and C isotope systematics of geothermal emanations in the Lesser Antilles Islands Arc
Magma sources of the Cape Verdes archipelago: Isotopic and trace element constraints