N. S. Bortnikov

Finds of young and ancient zircons in gabbroids of the Markov Deep, Mid-Atlantic Ridge, 5°54′–5°02.2′ N (Results of SHRIMP-II U-Pb Dating): Implication for deep geodynamics of modern oceans

Unusually ancient (ranging from ~100 to 330 and even ~2230 Ma [1, 2]) and young (~1.2‐1.4 Ma) [3]) zircons were discovered in the axial Mid-Atlantic Ridge (MAR) zone using U‐Pb dating. At first glance, finds of ancient zircons are inconsistent with the generally accepted plate tectonic model, which suggests the formation of new oceanic crust in the spreading zone of the World Ocean. However, they can be explained by specific geodynamic processes in the oceanic mantle. This paper presents results of zircon dating, which identified both ancient and young zircons in the same samples of MAR rocks.

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

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.

Modern problems of geochemical and U-Pb geochronological studies of zircon in oceanic rocks

We present results of zircon LA-ICP-MS U–Pb, Lu–Hf, and trace-element study in combination with whole-rock Sm-Nd and Rb-Sr isotope data on the magmatic rocks of the Markov Deep and Ashadze hydrothermal field (Mid-Atlantic Ridge). Zircon from three gabbronorite samples in the Markov Deep defined an U–Pb ages between 0.90 ± 0.02 and 2.00 ± 0.05 Ma, with the youngest age found in the deepest sample. Zircons from four samples of gabbros and trondhjemites of the Ashadze Field have identical ages: from 1.04 ± 0.07 to 1.12 ± 0.09 Ma. Plagioclase troctolite from the Markov Deep (sample I-1069/19) contains exotic zircon grains with ages widely ranging from 90 Ma to 3.2 Ga, which is inconsistent with age of the rocks in the Mid-Atlantic Ridge. Several hypotheses are discussed to explain the origin of such exotic grains, in particular, their formation at mantle depths, or reaching these depths with subducted crust, and others. Experimental study of zirconium solubility shows that the mafic and ultramafic melts could be oversaturated with respect to zirconium only at unrealistically high contents, which usually do not occur in the corresponding rocks. Entrapped xenogenic zircon must be dissolved in the mafic and ultramafic melts and its finds in these rocks presumably indicate its disequilibrium precipitation. Zircon could be formed in the intrusive mafic rocks at the final stages of fractional crystallization, which explains the presence of own zircon in gabbroids. Zircon is very stable in crustal magmatic processes, especially at lowered activity of alkalis, but almost instantly (on geological scale) loses radiogenic lead by diffusion way under upper mantle conditions (1300–1500°C). While applying REE distribution for interpreting zircon origin, as many as possible elements should be analyzed to discriminate between intrinsic zircon element distribution and anomalies caused by defects in its structure.

Time of the formation of the oceanic core complex of the Ashadze hydrothermal field in the Mid-Atlantic Ridge (12°58′ N): Evidence from zircon study

1301 A plutonic association of the axial zone of the Mid Atlantic Ridge (MAR) composed of peridotite, gab bro, and trondhjemite is typical for geodynamically crucial components of the lithosphere section of slow spreading mid oceanic ridges indicated in modern publications as oceanic core complexes (OCCs). According to the existing ideas, OCCs are represented by blocks of deep rocks moved to the surface of the oceanic bottom by the system of low angle detach ment faults (for example, [1, 2]). During the last 15 years, a huge volume of petrological and geochem ical data on rocks composing OCCs has been accumu lated and petrogenetic schemes of their formation have been suggested [1–4]. In addition, the data on the age and duration of OCC formation are still scarce and have been discussed in a limited number of publi cations (for example, [5, 6]). The Ashadze hydrother mal field is located in the axial zone of the MAR and is related to a large cluster of active hydrothermal fields: Ashadze–Semenov–Logachev discovered during the expeditions of the Russian R/V Professor Logachev. These fields occur in blocks of deep rocks moved to the oceanic bottom surface and mainly represented by peridotite and gabbro. As a result of investigation of the petrological peculiarities of rocks from the OCC hosting the Anshadze field, it was established that hydrothermal alteration of rocks of the gabbro–peri dotite association resulted not only in mobilization of ore material, but in the formation of veins of acid rocks (trondhjemite) as well [3]. In this study the U–Pb isotope dating of zircon was performed in order to determine the age of the forma tion of the proper (OCC) hosting the Ashadze hydro thermal field, as well as the age of the formation of vein trondhjemite, i.e., the age of the paleohydrothermal system. We also investigated the hafnium isotope com position and the distribution of minor elements in zir con, gabbro, and trondhjemite for estimation of their relationships. Plutonic rocks collected during the Russian–French expedition Serpentine by the French R/V Pourquoi pas? in 2007 [7] were studied.

Disseminated sulfide mineralization in oceanic restitic ultramafics from rift valley of the Mid-Atlantic Ridge, 5°-7°10′ N

Several occurrences of present-day hydrothermal activity and sulfide ore formation associated with mafic and ultramafic rocks were found at the end of the last century at the slow-spreading Mid-Atlantic Ridge (MAR) [1–3]. However, it remains unclear whether these rocks are sources of fluids and metals that precipitated later as sulfide ores on the ocean floor. The problem of redistribution and segregation of metals in the course of interaction of fluids or melts with rocks might be solved by studying ore minerals disseminated in the mantle rocks of the MAR. For this purpose, we examined sulfides disseminated in the restitic ultramafic rocks dredged near the Sierra Leone Fracture Zone, where stringer–disseminated sulfide mineralization was found in metasomatically altered gabbro. Hydrophysical and hydrochemical anomalies of bottom water detected in this area suggest that active hydrothermal vents occur in this MAR segment [3]. The disseminated sulfide mineralization in oceanic ultramafics has been pointed out by many researchers [4, 5, and others]. We carried out for the first time comprehensive study of disseminated sulfide mineralization in various ultramafic rocks of a single oceanic structure.