Compositional variations of volcanics along segments of recent spreading ridges |
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| Authors: | J M Morel R Hekinian |
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| Institution: | (1) Geology Department, Université de Bretagne Occidentale, Avenue Le Gorgeu, F-29283 Brest, France;(2) Centre Océanologique de Bretagne, B.P. 337., F-29273 Brest, France |
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| Abstract: | An investigation of glassy volcanics erupted within the last ten-million years along various segments of the mid-Atlantic Ridge and the East Pacific Rise has revealed major crustal compositional changes. The available data from the mid-Atlantic Ridge shows the existence of two petrological provinces: One, located between latitudes 33° and 53° N, is characterized by volcanics which have a tendency to be oversaturated ocean ridge basalts (OSORB) with respect to normative quartz; the second group of rocks, found between 25° S and 33° N, is generally composed of saturated ocean ridge basalts (SORB). In addition, the SORB volcanics have higher TiO2 (1.7±0.3%), higher Na2O (2.8±0.2%) and higher FeO*/MgO (1.36±0.2) values than do the OSORB types (with 1.1±0.2%, 2.2±0.2% and 1.22±0.2 for the TiO2, Na2O, and FeO*/MgO respectively), There is a correlation between the rate of crustal spreading and the compositional changes observed on the volcanics erupted along various segments of oceanic ridges. Slow-accreting plate boundaries having a total spreading rate of 2–3 cm/year are characterized by a low TiO2 content (1.1±0.2%), low FeO*/ MgO ratio (1.22±0.2) and a high an/an+ab ratio (0.62±0.05). Segments of fast-spreading ridges (total rate 11–13 cm/year) show a higher range of TiO2 (2.1±0.4%) and FeO*/MgO (1.6±0.4) and a lower range of the an/an + ab ratio (0.5±0.07). Ridge segments with a total spreading rate of 5–9 cm/year con sist of volcanics having intermediate values for the above parameters. Different degrees of partial melting of rising mantle material are suggested as a possible mechanism for explaining the compositional diversities encountered along oceanic ridge systems.Contribution n 677 du Département de Géophysique, Géologie, Géochimie Marines du C.O.B. |
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