Mathematics – Logic
Scientific paper
Mar 1975
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975e%26psl..25..103s&link_type=abstract
Earth and Planetary Science Letters, Volume 25, Issue 2, p. 103-115.
Mathematics
Logic
52
Scientific paper
Rare earths (RE) in basalts erupted within the rift of the Mid-Atlantic Ridge show a progressive change from light-RE enriched to depleted patterns from the Azores Platform (40°N) down to 33°30'N. South, the pattern remains light-RE depleted as along other ``normal ridge'' segments. A progressive increase in chemical variability of the basalts towards the Azores is also noted. The latitudinal RE profile and corresponding ΣFeO/ΣFeO + MgO variations, together, indicate that the origin of these basalts cannot be accounted for simply by considering variable extents of partial melting of a single mantle source and subsequent fractional crystallization during the ascent of the magmas. These two processes produce only second-order effects on the RE patterns. The data requires the presence of a distinct, light-RE richer, mantle source beneath the Azores Platform relative to that of south of 33°30'N and an intermediate zone where both mantle types mix. The relative contribution of the Azores mantle source to the mix appears to decrease fairly regularly southward along the ridge and becomes negligible at 33°30'N. Increasing chemical variability of the basalts towards the Azores is probably caused by correspondingly larger extent of fractional crystallization at shallow depth, and/or greater variability in the extent of partial melting, apparently subsequent to, and superimposed on the mixing of the two mantle sources. The combined morphological, geophysical and RE evidence along the profile are consistent with a model suggesting upwelling of a major blob (plume) under the Azores Plateau; and reveal the present extent of the blob's overflow and mixing with the asthenosphere depleted in large ionic lithophile trace elements. The influence of the Azores blob is geochemically detectable up to 1000 km southwestward beneath the ridge axis.
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