Computer Science
Scientific paper
May 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999e%26psl.169..189m&link_type=abstract
Earth and Planetary Science Letters, Volume 169, Issue 1-2, p. 189-205.
Computer Science
29
Scientific paper
New helium and lead isotopic data for basalts from the Azores archipelago (North Atlantic) show that the Azores have 4He/3He ratios both higher and lower than MORB values. Good covariations of helium and lead isotopes are observed at the scale of the archipelago, and suggest the coexistence of two mantle components in the Azores which are identified by data from Sa~o Miguel and Terceira. The eastern part of Sa~o Miguel island displays radiogenic helium (4He/3He > 140,000, R/Ra<5.1) and lead (20.00, 15.75 and 40.33 for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb). The 207Pb/204Pb and 208Pb/204Pb ratios for Sa~o Miguel are unusually radiogenic for oceanic basalts. Terceira basalts contain relatively unradiogenic/primitive 4He/3He ratios, with a minimum value of 64,000 (R/Ra=11.3), and relatively high lead isotopic ratios (206Pb/204Pb = 20.02, 207Pb/204Pb = 15.64 and 208Pb/204Pb = 39.35). We propose that the Terceira source has a composition produced by a mixing between recycled oceanic crust (high 206Pb/204Pb) and entrained lower mantle (high 3He) material. The Sa~o Miguel island isotopic signature may be due to sampling of local (km-size) heterogeneity located at relatively shallow depth. The preferred origin of this heterogeneity is the Jurassic delamination of subcontinental lithosphere, which occurred during rifting and opening of the North Atlantic. The primitive helium ratios were also observed on the Mid Atlantic ridge at 38.5°N, reflecting plume-ridge interaction, whereas radiogenic ratios (>100,000) were observed at latitude higher than 40°N and may reflect the influence of the Sa~o Miguel component at the ridge.
Allègre Claude J.
Doucelance Régis
Dupré Bernard
Kurz Mark D.
Moreira Manuel
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