Composition of minerals and rocks from the Markov Deep, Central Atlantic

The paper presents materials on 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. Predominant harzburgites with protogranular and porphyroclastic textures contain two major generations of minerals: the first generation composes the bulk of rocks and consists of Ol_89.8-90.4 + En_90.2-90.8 + Di_91.8 + Chr (Cr#32.3-36.6, Mg#67.2-70.0), while the second generation composes very thin branching veinlets and consists of PlAn_32-47 + Ol_74.3-77.1 + Opx_55.7-71.9 + Cpx_67.5 + Amph_53.7-74.2 + Ilm. Syndeformational olivine neoblasts in recrystallization zones are highly magnesian. Concentrations and covariations of major elements in harzburgites indicate that these rocks are depleted in mantle residues (high Mg# of minerals and whole-rock samples and low in CaO, Al2O3, and TiO2) that are significantly enriched in trace HFSE and REE (Zr, Hf, Y, LREE, and all REE). Mineralogy and geochemistry of harzburgites were formed by interaction of mantle residues with hydrous, strongly fractionated melts that impregnated them. Mineral composition of veinlets in harzburgites and mineralogical-geochemical characteristics of related plagiogranites and gabbronorites suggest that these plagiogranites were produced by melt residuals after crystallization of gabbronorites. Modern characteristics of harzburgites were shaped by the following processes: (i) 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 maximal stress relief and interaction of this melt with magnesian mantle residue.

Composition of sulfides from the Sierra Leone Fracture Zone (Central Atlantic Ocean)

An analysis of data on the location of hydrothermal fields, seismicity, and satellite altimetry evidences that in mid-ocean ridges with low spreading rate hydrothermal fields tend to be grouped in areas with generally low seismic activity and at intersections of discontinuities and rift zones. Based on this assumption, the Sierra Leone Fracture Zone was studied in 2000 during Cruise 22 of R/V Akademik Nikolaj Strakhov. A study of gabbrodolerite and dolerite showed that sulfide ore minerals in them were formed both by hydrothermal and magmatic processes. An analysis of melt inclusions demonstrated that magmatic complexes formed from a high-temperature (1210-1255°C) low-potassium melt of the N-MORB type. Investigations of fluid inclusions revealed that gabbro and dolerite formed under influence of an active hydrothermal system at temperature 205-226°C. Thus, the Sierra Leone Fracture Zone is considered to be perspective for a discovery of a new hydrothermal field.

Chemical composition of ultrabasites and composing minerals from the Tonga Trench

A petrologic-geochemical study (petrochemistry, contents of siderophile and certain lithophile elements, composition of rock-forming silicates and accessory chrome spinels) of ultrabasic rocks dredged from the arc side in the northern end of the Tonga deep-sea trench has been carried out. The ultrabasites included harzburgites and dunites. Peridotites show clearly manifested material characteristics of ultrabasic relicts strongly depleted in low-temperature basaltic components. It is suggested that they have arose in the high degree of partial melting (about 30%) of a matrix mantle source of the lherzolite type. Great similarity of the rocks studied with ultrabasites of many ophiolites that are widespread in folded belts indicates that young island arcs are among the most likely geodynamic environments of ophiolite generation.