Journal of Conference Abstracts

Petrological and Geochemical Variability of Basalts from Spreading Ridges in the Indian Ocean

J. H. Natland RSMAS, University of Miami, Miami, FL 33149, USA

jnatland@rsmas.miami.edu

R. L. Fisher Scripps Institute of Oceanography, University of California, San Diego, La Jolla, CA 92093, USA

H. J. B. Dick Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

J. J. Mahoney SOEST, University of Hawaii, Honolulu, HI 96822, USA

S. H. Bloomer Oregon State University, Corvallis, OR 97331, USA

We have compiled glass-group (mainly) or whole-rock (less commonly) averages, and correlative XRF trace-elements, REE and Sr-Nd-Pb isotopic systematics on 198 basalts from 128 locations on spreading segments and in fracture zones of the Central, Southeast and Southwest Indian Ridges (CIR, SEIR and SWIR). The locations span 62 degrees of latitude, the west Indian Ocean triple junction, and ridge segments adjoining the Reunion-Rodrigues, Amsterdam-St. Paul and Marion-Prince Edward hot spots on the CIR, SEIR and SWIR, respectively.

Most basalts are moderately fractionated abyssal tholeiites (Mg# = 0.62 ± 0.06, 1 sigma) with significant fluctuations to lower parental Na₂O and higher parental SiO₂ at shoal ridge segments near each of the three hot spots, and an abrupt transition to very high parental Na₂O and great axial depths (>6 km) across the triple junction to the SWIR. These trends are the expression in the Indian Ocean of global systematics for ridge basalts. Indeed, the extremes of parental Na₂O and SiO₂ are found in the Indian Ocean. However, the match to global correlations is not exact since basalts of both the CIR and SEIR are displaced toward higher parental SiO₂ at given parental Na₂O than their counterparts in the North Atlantic. A third regional characteristic - high parental FeO* (total iron as FeO), which was previously identified in the eastern Pacific by Klein and Langmuir (1987) - is even more strongly developed along segments of the SWIR spanning the Marion-Prince Edward hot spot.

Plume-related trace-element and isotopic characteristics occur in particular basalts nearest each of the three hot spots. These are superimposed on an overall background, present both near and away from plumes, of somewhat higher alkalis, higher ⁸⁷Sr/⁸⁶Sr at given ¹⁴³Nd/¹⁴⁴Nd, and lower ²⁰⁶Pb/²⁰⁴Pb than in the North Atlantic and eastern Pacific, which together comprise the characteristics of "Indian-Ocean-type" mantle. There is an abrupt transition to more depleted compositions and least ²⁰⁶Pb/²⁰⁴Pb from the CIR and SEIR near the triple junction to the high-soda basalts of the SWIR. From there, ²⁰⁶Pb/²⁰⁴Pb increases steadily, and parental Na₂O decreases along the SWIR, until "Atlantic-type" values are reached south of Marion-Prince Edward. The systematic shifts in composition of parental Indian Ocean basalts away from the "global trend" and the abrupt compositional transition across the triple junction to the SWIR, suggest that regional variations in average extents of partial melting are here modified by complex patterns of both major-oxide (lithologic) and isotopic heterogeneity in mantle sources. This may reflect involvement of refractory, perhaps hydrous, subcontinental mantle components left from the separation of the Indian subcontinent from Gondwana.

References

Klein, E.M. & Langmuir, C.H., J. Geophys. Res. 92, 8089-8115 (1987).