Petrogenesis of anomalous K-enriched MORB from the Southwest Indian Ridge: 11°53?E to 14°38?E
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Glassy pillow basalts with unusual geochemical characteristics for mid-ocean ridge basalt (MORB) have been dredge sampled from the Southwest Indian Ridge between 12 and 15-degrees-E during Leg ANT IV/4 of the F.S. POLARSTERN. avas from 4 of 6 dredges are moderately nepheline normative, highly K-enriched (0.5-1.77 wt% K2O) alkali basalts and hawaiites. Mg-numbers indicate that many of the lavas are fairly primitive (Mg No. = 63-67), yet show extreme enrichment in incompatible elements; e.g. Nb (24-60 ppm), Ba (170-470 ppm) and Sr (258-460 ppm). Incompatible-element ratios such as Zr/Nb (3-5) and Y/Nb (0.46-1.1) are extremely low even for E-type (enriched) MORB, whereas (La/Yb)n ratios are particularly high (3.4-7.8). Sr-87/Sr-86 (0.70290-0.70368), Nd-143/Nd-144 (0.51302-0.51284) and Pb-206/Pb-204 (18.708-19.564) isotopic ratios further indicate the geochemically ‘enriched’ nature of these lavas, which range from the compositional field for depleted N-type (normal) MORB towards the composition of Bouvet Island lavas. Mutually correlated incompatible-element and Sr-, Nd- and Pb-isotopic ratios allow a fairly well constrained model to be developed for the petrogenesis of these unusually alkalic mid-ocean ridge lavas. The alkalic nature and degree of enrichment in incompatible elements is ascribed to particularly low degrees of partial melting (3-5 wt%), at greater than usual depth, of a source region that has experienced prior geochemical enrichment (by veining) related to the upwelling Bouvet mantle plume. To account for the observed compositional variations, a model is proposed whereby mixing between partial melts derived from these geochemically enriched silicate veins, and an incipient to low percentage (+/- 2%) melt from the surrounding geochemically depleted suboceanic asthenosphere occurs as a consequence of increasing degree of melting with adiabatic upwelling. Eruption of these alkalic lavas in this spreading ridge environment is attributed to a temporary hiatus in tholeiitic volcanism and associated spreading along this section of the Southwest Indian Ridge, related to readjustment of spreading direction to a more stable plate geometry.