Ultramafic rocks from the Markov Deep in the rift valley of the Mid-Atlantic Ridge

Abstract

The paper presents materials on the composition and texture of weakly serpentinized ultrabasic rocks from the western and eastern walls of the Markov Deep (5°30.6′–5°32.4′N) in the rift valley of the Mid-Atlantic Ridge (MAR). The predominant harzburgites with protogranular and porphyroclastic textures contain two major generations of minerals: the first generation composes the bulk of the rocks and consists of Ol89.8–90.4 + En90.2–90.8 + Di91.8 + Chr (Cr#32.3–36.6, Mg#67.2–70.0), while the second generation composes very thin branching veinlets and consists of PlAn32–47 + Ol74.3–77.1 + Opx55.7–71.9 + Cpx67.5 + Amph53.7–74.2 + Ilm. The syndeformational olivine neoblasts in recrystallization zones are highly magnesian. The concentrations and covariations of major elements in the harzburgites indicate that these rocks are depleted mantle residues (the high Mg# of minerals and whole-rock samples and the low CaO, Al2O3, and TiO2 concentrations) that are significantly enriched in trace HFSE and REE (Zr, Hf, Y, LREE, and all REE). The mineralogy and geochemistry of the harzburgites were formed by the interaction of mantle residues with hydrous, strongly fractionated melts that impregnated them. The mineralogical composition of veinlets in the harzburgites and the mineralogical-geochemical characteristics of the related plagiogranites and gabbronorites suggest that these plagiogranites were produced by melts residual after the crystallization of gabbronorites. The modern characteristics of the harzburgites were shaped by the following processes: (i) the partial melting of mantle material simultaneously with its subsolidus deformations, (ii) brittle-plastic deformations associated with cataclastic flow and recrystallization, and (iii) melt percolation along zones of the maximum stress relief and the interaction of this melt with the magnesian mantle residue.