Chemical Variation and MORB Generation on the Ultraslow-Spreading Southwest Indian Ridge (9°-25° E)
Abstract
Mid-ocean ridge basalt major element, trace element, and isotopic compositions range from depleted N-MORB to E-MORB along a single 1050-km first-order spreading segment on the ultraslow-spreading Southwest Indian Ridge. This segment consists of two supersegments with strikingly different geometry. To the east is a 630 km long orthogonal spreading (< 5° obliquity) supersegment that dominantly erupts N-MORB and T- MORB with progressive incompatible element enrichment from east to west. To the west is a 400 km long oblique spreading (up to 56°) supersegment with two robust volcanic centers erupting alkali basalt and E- MORB with three intervening amagmatic accretionary segments. In the latter, mantle peridotite is emplaced directly to the seafloor with only scattered N-MORB and E-MORB flows and minimal gabbro. At 16° E, where the easternmost amagmatic segment joins the orthogonal supersegment, there is a remarkable break in physiography, crustal structure, and chemistry, but not in axial linearity. We attribute the contrasts across this discontinuity to the enhanced and non-linear influence of mantle thermal structure on mantle and melt flow and melt generation at ultraslow-spreading rates. Along the orthogonal supersegment moderate and rather constant degrees of partial melting effectively sample the bulk mantle source, thus the enrichment to the west is largely a function of a changing mantle composition. On the oblique supersegment, however, suppression of mantle melting due to enhanced conductive cooling with slower mantle upwelling means that the bulk source is not uniformly sampled and thus "process" rather than "source" dominates basalt generation. Much of the local major element heterogeneity is explained by polybaric fractional crystallization with varying amounts of H{_2}O. The remaining major element variations, specifically elevated K{_2}O contents we attribute to mixing of depleted peridotite melt with enriched pyroxenite or eclogite melt. These compositionally different melts could be produced 1) by melting of two physically distinct components or 2) by deep melting of metasomatized peridotite plus melting of depleted peridotite. In either case basalt radiogenic isotope compositions lie on linear trends between DMM and Bouvet Island implying two possible end-member compositions. In addition, trace element signatures (i.e. (Nb/Ta){_n} > 1, (Hf/Sm){_n} < 1, inverse correlations between HREE and LREE, and extreme depletion of Sc correlated with elevated (Sm/Yb){_n}) suggest an important role for eclogite during mantle melting. Regardless of the exact nature of the enriched component, this model for MORB generation illustrates the dramatic influence of variable mantle thermal structure at spreading rates < 20 mm/yr. Simultaneously it provides insight into the mantle melting process and nature of the source, so often obscured at faster rates where higher extents of melting more directly sample the mantle bulk composition.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.V11G..05S
- Keywords:
-
- 1025 Composition of the mantle;
- 1032 Mid-oceanic ridge processes (3614;
- 8416);
- 3614 Mid-oceanic ridge processes (1032;
- 8416);
- 3619 Magma genesis and partial melting (1037);
- 8416 Mid-oceanic ridge processes (1032;
- 3614)