Petrology and Geochemistry of MORB from 25°E to 46°E along the Southwest Indian Ridge: Evidence for Contrasting Styles of Mantle Enrichment

• Published in: Journal of Petrology
• Main Authors: LE ROEX, ANTON P., DICK, HENRY J. B., FISHER, ROBERT L.
• Format: Text
• Language: English
• Published: Oxford University Press 1989
• Subjects: Articles; Indian; Marion Island; Ocean Island
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/30/4/947; https://doi.org/10.1093/petrology/30.4.947

The 1984 PROTEA expedition, leg 5, to the central Southwest Indian Ridge recovered basaltic lavas from fracture zones and ridge segments between 25�E and 48�E. In terms of petrography and major element variations the samples are unremarkable for ocean ridge basalts and range from aphyric to highly plagioclase phyric and from primitive ( mg -number = 70) to moderately evolved ( mg -number = 40) in composition. Multiply saturated (i.e., olivine, plagioclase, and clinopyroxene) basalts are common within this region. There is no systematic difference in compositional characteristics between basalts dredged from fracture zone walls and those dredged from ridge segments, and fractional crystallization has played an important role in controlling the overall range in lava composition in both tectonic environments. Incompatible element abundance ratios in the basalts are more notable and distinguish between geochemically depleted (N-type) MORB with high Zr/Nb (16→68) and Y/Nb (4�7→23) ratios and low (La/Sm)m, ratios (0-�76–1�00), and geochemically enriched (E-type) MORB with low Zr/Nb (3�4–15�8) and Y/Nb (0�5–8�8) and high (La/Sm). ratios (1�07–3�8). N-type MORB appears to be absent in the immediate vicinity of Marion Island, but occurs further along the ridge to the northeast and southwest. Geochemically enriched MORB occurs at scattered localities along the ridge but is particularly abundant along the section of the ridge closest to the Marion hotspot. In detail, two distinct varieties of E-type MORB can be recognized. The one type has incompatible element and isotopic ratios similar to, although slightly less enriched than, those characteristic of the Marion hotspot (Zr/Nb=5�8–8�6; Y/Nb=0�5–0�8; Ba/Nb=5�1–9�0). The second type can be distinguished by having high Ba/Nb ratios (9–22), unlike any lavas directly associated with the Marion hotspot, but similar to those characteristic of DUPAL ocean island basalts (OIB). A single sample from this group for which there are isotopic data indicates derivation from an ...