Peridotite Heterogeneity Along the Ultra-Slow Spreading SWIR Oblique Supersegment
Abstract
The ultra-slow spreading Oblique 9°-16°E Supersegment of the SW Indian Ridge provides an opportunity to address length scales, timing and origins of mantle chemical and isotopic heterogeneities. Due to extremely slow mantle upwelling, conductive cooling limits mantle melting. Thus, peridotites at the seafloor are expected to represent the closest approach to asthenospheric mantle composition of anywhere in the oceans. In this environment, however, conductive cooling should also lead to trapping of variable quantities of partial melt at high pressures in the peridotites. We undertook detailed geochemical studies on peridotites and pyroxenites from two dredges that each represent sampling length scales of <1 km. Dredge 96, from the inside-corner high of the ridge with the Shaka Fracture Zone, contains lherzolites and harzburgites associated with abundant pyroxenite veins. In contrast, Dredge 85, from an amagmatic region of the Oblique Supersegment, was selected for its lack of veins in any of the peridotites. Trace element and modal characteristics of these rocks indicate variable degrees of melting, melt-rock reaction, and melt extraction. Nd and Sr isotopic compositions of Cpx and Opx separates, determined by TIMS, overlap those of Indian MORB. Effects of seawater alteration on Sr isotopes were minimized by acid leaching - as indicated by isotopic values lower than MORB - but not eliminated in all samples. The isotopic range for Dredge 96 (ɛNd= 6-13; 87Sr/86Sr= 0.702117-0.703753) covers 32% of the entire Indian MORB Nd dataset, and extends to more depleted values than any previously observed in SW Indian Ridge peridotites. In contrast, Dredge 85 has a more restricted isotopic range (ɛNd= 12-16; 87Sr/86Sr= 0.702197-0.704322), but still extends to more depleted ɛNd values than DMM. Dredge 96 pyroxenites and peridotites with signatures of melt-rock reaction have affinities to basalts from Bouvet Hotspot, which was in the vicinity of this ridge segment at ~20 Ma. Nd and Sr isotopic values range from depleted compositions similar to DMM to enriched compositions similar to Bouvet Hotspot. Peridotite Cpx Pb isotopes are almost identical to Bouvet basalts (e.g., 206Pb/204Pb= 19.039- 19.606). However, Nd isotopic variations do not correlate with Nd concentrations, indicating that these systems where decoupled. We suggest that the large local-scale isotopic heterogeneities observed among Dredge 96 peridotites are the result of asthenospheric veining by Bouvet melts. More recently, during melting beneath the mid-ocean ridge, interactions between migrating vein-dominated melt and residual peridotites resulted in the observed trace element characteristics of the Dredge 96 peridotites. In contrast, isotopic compositions and trace element abundances indicate that Dredge 85 peridotites are unaffected by the Bouvet Hotspot.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.V11G..06W
- Keywords:
-
- 1025 Composition of the mantle;
- 1040 Radiogenic isotope geochemistry;
- 1065 Major and trace element geochemistry;
- 3621 Mantle processes (1038);
- 8416 Mid-oceanic ridge processes (1032;
- 3614)