Ferreira, P.L., Murton, B.J. and Boulter, C.
Mixing two enriched and distinct mantle sources beneath Lucky Strike segment, 37N on the Mid-Atlantic Ridge (abstract of paper presented at 15th Annual V.M. Goldschmidt Conference, Moscow, Idaho, , May 2005)
Geochimica et Cosmochimica Acta, 69, (10S), .
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New major and trace element, and Nd-Sr isotopic, data on samples from the Lucky Strike Segment are presented. All samples studied are enriched mid-ocean ridge basalts (E-MORB), but different degrees of enrichment are identified. Three distinct compositional groups can be established: - in Group 1, the lavas have the highest “more incompatible/less incompatible” (MI/LI) element ratios, 87Sr/86Sr and the lowest 143Nd/144Nd ratios. These lavas have a degree of enrichment between those of E-MORB and OIB and are spatially restricted to the central part of the axial volcano. Group 3 lavas have the smallest “MI/LI”, 87Sr/86Sr and the highest 143Nd/144Nd ratios. The lavas present REE and multi-element patterns typical of E-MORB and were collected throughout the Lucky Strike segment (from 37º12,0’ to 37º27,3’ in latitude). Group 2 lavas have chemical characteristics intermediate between those of groups 1 and 3 in terms of trace and isotope compositions, have the maximum dispersion in the different element or element ratios, and present an enrichment degree between those of group 1 and 3 lavas.
Relationships between various incompatible trace element ratios (e.g. La/Sm versus Nb/Zr), together with Sr and Nd isotopic ratios, or between both (e.g. La/Sm versus
143Nd/144Nd) show clearly a continuous chemical trend, defined by group 2 basalts, progressively connecting the chemical characteristics shown by groups 1 and 3. The justification for that trend being the result of mixing processes was tested, and a good fit of the mixing lines to the data was obtained. Moreover, since the mixing processes between a depleted mantle source (N-MORB type), and a highly enriched source (Plume type) are frequently postulated to explain the origin of E-MORB type basalts (identical to those of group 3), calculation of the mixing lines were extended to a N-MORB end-member, and good fits were also obtained.
These results show that mixing could be invoked to explain the chemical diversity among the Lucky Strike segment magmas, involving an N-MORB type source end-member and an enriched end-member similar to that assumed to generate the basalts from group 1. Hence, the existence of two distinct (both enriched) mantle sources beneath Lucky Strike segment could be postulated.
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