Lead-strontium isotopic variations along the East Pacific Rise and the Mid-Atlantic Ridge: a comparative study |
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| Authors: | Bruno Hamelin Bernard Dupré Claude J Allègre |
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| Institution: | Laboratoire de Géochimie et Cosmochimie, Institut de Physique du Globe et Département des Sciences de la Terre, Universités de Paris VI et VII, 4, Place Jussieu, 75230 Paris Cedex 05 France |
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| Abstract: | We have determined the Pb and Sr isotopic compositions in a number of fresh young oceanic basalts from the East Pacific Rise (between 20°N and 21°S latitudes), and from the Mid-Atlantic Ridge (between 65°N and 10°N).The results confirm the Pb Sr isotopic correlation for mid-ocean ridges basalts obtained by Allègre et al. 1], Cohen et al. 2], Dupréand Allègre 3], and the correlation between isotopic variation and the compatible trace elements ratios variation 1].A comparison between the Atlantic and Pacific results reveals that there is a wider range of values for the Atlantic than for the Pacific. After filtering the short wavelengths, a good correlation is obtained between long-wavelength bathymetric and isotopic variations for the Atlantic.The preferred model proposed to explain these differences involves the constant presence of hot spots under ridges. On slow-spreading ridges like the Atlantic, the hot spots signature is clearly visible in both bathymetry and isotopic ratios. On fast-spreading centers, the hot spot signature in both the bathymetry and isotopic signature may be diluted by the rapid supply of material coming from the asthenosphere.However, an alternative explanation for which no hot spot influence is found on the East Pacific Rise cannot be definitely ruled out.In two occurrences, south of the Hayes fracture zone (Atlantic), large isotopic heterogeneities are observed within a single dredge. This does not contradict the concept of regional isotopic regularities, but suggests that blob injection and source mixing may be observed at very different scales under the ridges. |
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