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Journal of Petrology Volume 41 Number 6 Pages 845-866 2000
© Oxford University Press 2000
Magma Supply in Back-arc Spreading Centre Segment E2, East Scotia Ridge
1BRITISH ANTARCTIC SURVEY, HIGH CROSS, MADINGLEY ROAD, CAMBRIDGE CB3 0ET, UK
2DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF DURHAM, DURHAM DH1 3LE, UK
3c/o NERC ISOTOPE GEOSCIENCES LABORATORY, KINGSLEY DUNHAM CENTRE, KEYWORTH, NOTTINGHAM NG12 5GG, UK
Segment E2 is situated in the back-arc East Scotia Ridge. The segment is unusual in that it has an axial topographic high underlain by a seismically imaged melt lens. The axis of the segment, which is 70 km long, was sampled at 
2 km spacing. There is strong correlation between compositions and bathymetry, and there is no evidence that lateral flow of magmas along dykes within the segment was more than 25 km. Magmas are more evolved towards the summit, indicating that magma fractionation took place within the imaged melt lens. Na8·0 is roughly constant at 2·6, implying uniform degree of partial melting, except for some samples at the summit that have Na8·0 = 2·2. Compositions closest to N-MORB occur at the segment tips, and LREE/HREE ratios increase toward the summit. None of the magmas were derived from depleted sub-arc mantle. Nevertheless, most compositions in the segment were modified by slab-derived components. The low-Na8·0 samples have high Ba/Nb compared with the rest of the segment. Dredged lavas from the lateral flanks of the summit have the most extreme compositions, including ones derived from plume mantle, and are end-members for magma mixing processes that controlled compositions.
KEY WORDS: geochemistry; petrology; volcanism; back-arc; subduction
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