Roberts victor eclogites: Ancient oceanic crust

Abstract

The two groups of Roberts Victor eclogites may be further distinguished by their 18O/16O ratios; one above (I) and one below (II) the mantle value (5.7). In the group II suite systematic variation of the major elements with oxygen isotopes matches that calculated for oceanic volcanic rocks altered by circulating seawater in ridge crest hydrothermal systems. A negative linear correlation between 18O/16O and 87Sr/86Sr(WR) and an associated positive correlation with K2O (and Rb) of the anhydrous eclogite assemblage with 87Sr/86Sr(WR) are also products of hydrothermal alteration. The chemical and isotopic similarity of the eclogites to hydrothermally altered oceanic crust supports the conclusion that they are subducted fragments that have been metamorphosed from metabasalts to eclogites. During metamorphism the elements are fractionated into two components. One, enriched in the radiogenic isotopes of U, Th, K, and Rb, and light rare earth elements, is concentrated in a hydrous, interstitial, mobile component, and the other, enriched in Nd relative to Sm, is concentrated in the anhydrous, immobile, garnet and clinopyroxene host. The mobile component is comparable in chemistry to rocks of the MARID suite and may well play an important role in mantle metasomatism. Nd/Sm and Pb/Pb ages indicate an Archean age (approximately 2.47 b.y.) for the metamorphism. Depth estimates indicate a skewed distribution with most eclogites coming from the depth range 165–190 km. The concentration of eclogites correlates with the inflection in the Lesotho geotherm and appears to mark a disrupted layer of subducted Archean oceanic crust separating a shallower region of undeformed, refractory, Mg‐rich peridotites from deformed, Fe‐rich, more “fertile” pyrolite. This boundary may be interpreted as the “petrologic” base to the continental lithosphere.