RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 2, NO. 3, PAGES 227–242, doi:10.2205/2000ES000042, 2000

Mantle substrate evolution of the Mid-Atlantic Ridge 14-16^oN and 33^o40' N: time and matter limitations of accretion processes of the oceanic lithosphere from the data of Sm-Nd research of isotope system

¹GEOKHI RAS, Moscow, Russia
²Institute of geology and geochronology RAS, Sankt-Peterburg, Russia

Abstract

[1]  The objects of this study were the ultrabasic rocks the samples of which were collected from two areas of the crustal zone of the Mid-Atlantic Ridge (MAR): (1) 13-17^o N (near the intersection of the ridge axis with the 15^o 20 fracture zone); and (2) 33^o 40 N (the western intersection of the MAR crest with the Heis fracture zone). 24 samples of peridotite and of the plutonic and volcanic rocks associated with it were used to measure their Sm/Nd, ¹⁴³Nd/¹⁴⁴Nd, and ¹⁴⁷Sm/¹⁴⁴Nd ratios, which allowed to test the time and genetic relationships between the evolution of the mantle material under the ridge crest and the products of its magmatic activity. The results of this work proved the ubiquitous discrepancy between the melting degree values of the extremely depleted mantle peridotites in the MAR area between 14^o N and 16^o N, obtained using petrologic and geochemical methods. This discrepancy suggests the large-scale interaction between the mantle material and magmatic melts and fluids, enriched in the incompatible elements or fluids. The results obtained suggest that the repeated melting of the mantle under the axial MAR zone is the universal characteristic of magmatism in low-velocity spreading centers. The results of this study also proved the crestal MAR zone in the Central Atlantic region show distinct indications of the isotope-geochemical segmentation of the mantle. It is suggested that the geochemically anomalous MAR mantle peridotite in the zone of the MAR intersection with the 15^o 20 fracture zone can be interpreted as the fragments of the mantle substrate, foreign for the Atlantic mantle north of the equator.

Received 16 November 2000; published 5 January 2001.

Keywords: Mid-Atlantic Ridge, mantle substrate evolution, accretion processes, oceanic lithosphere.