Helium and lead isotope geochemistry of the Azores Archipelago

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doi: 10.1016/S0012-821X(99)00071-0
Authors:Moreira, Manuel; Doucelance, Régis; Kurz, Mark D.; Dupré, Bernard; Allègre, Claude Jean
Author Affiliations:Primary:
Institut de Physique du Globe de Paris, Laboratoire de Geochimie, Paris, France
Other:
Woods Hole Oceanographic Institution, United States
Volume Title:Earth and Planetary Science Letters
Source:Earth and Planetary Science Letters, 169(1-2), p.189-205. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X
Publication Date:1999
Note:In English. Inst. Phys. Globe de Paris, Contrib. No. 1574; Woods Hole Oceanogr. Inst., Contrib. No. 9893. 76 refs.; illus., incl. 2 tables, sketch maps
Summary: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 Sao Miguel and Terceira. The eastern part of Sao 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 Sao 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 Sao 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 Sao Miguel component at the ridge. Abstract Copyright (1999) Elsevier, B.V.
Sections:Geochemistry
Subsections:Igneous rocks
Subjects:Basalts; Geochemistry; He-4/He-3; Helium; Hot spots; Igneous rocks; Isotope ratios; Isotopes; Lead; Magmas; Mantle; Mantle plumes; Metals; Mid-ocean ridge basalts; Mixing; Noble gases; Ocean-island basalts; Pb-206/Pb-204; Pb-207/Pb-204; Pb-208/Pb-204; Plate tectonics; Radioactive isotopes; Stable isotopes; Volcanic rocks; Atlantic Ocean Islands; Azores
Abstract Numbers:99M/3926
Record ID:1999040243
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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