Age of Plutonic Rocks from the Vema Fracture Zone (Central Atlantic) and Nature of Their Mantle Sources

• Published in: Geochemistry international Vol. 56; no. 2; pp. 89 - 110
• Main Authors: Kostitsyn, Yu. A., Silantyev, S. A., Anosova, M. O., Shabykova, V. V., Skolotnev, S. G.
• Format: Journal Article
• Language: English
• Published: Moscow Pleiades Publishing 01.02.2018; Springer; Springer Nature B.V
• Subjects: Analysis; Chemical analysis; Composition; Cruises; Dredging; Earth and Environmental Science; Earth Sciences; Fracture zones; Gabbro; Gabbros; Genetic transformation; Geochemistry; Geochronology; Geochronometry; Geological time; Heterogeneity; Hydrothermal activity; Isotopes; Lead; Lithosphere; Magma; Mantle; Melting; Metamorphism; Metamorphism (Geology); Mid-ocean ridges; Radiometric dating; Rock; Rocks; Rocks, Metamorphic; Seafloor spreading; Seawater; Sediment samples; Spreading; Spreading centres; Substrata; Transform faults; Water analysis; Zircon; Mid-Atlantic ridge; magmatism; Sm–Nd; oceanic lithosphere; transform fault; age of seafloor; Rb–Sr; Vema; Atlantic; U–Pb; spreading rate
• ISSN: 0016-7029; 1556-1968
• DOI: 10.1134/S0016702918020039

This study presents the results of petrographic, geochemical, and isotope geochronological analyses of rock samples from the Southern flank of the Vema transform fault (Atlantic), which were dredged on cruises 19-th and 22-nd of the R/V Akademik Nikolai Strakhov . The sample suite includes both fresh and metamorphosed gabbros, dolerites, serpentinites, metapyroxenites. Zircons separated from three gabbro samples recovered at three different stations were used for in situ U–Pb dating by LA-ICP-MS. The ages reveal a strong linear relationship with a distance from the axis of the Mid-Atlantic ridge, which allowed us to estimate the rate of spreading in this segment of the Mid-Atlantic Ridge. It can be concluded that the estimated spreading rate of 16.2 ± 0.8 mm/yr was constant over the past 15 Myr. The mutual consistency of all U-Pb zircon and 39 Ar– 40 Ar amphibole ages (Cipriani et al., 2009) obtained from the sampled transect suggests the temporal continuity of magmatic events that led to the formation of the original gabbroic rocks and their transformation during subsequent metamorphism. Rb—Sr isotope data show that hydrothermal activity took place in the presence of seawater between 14.7 and 9 Ma in the spreading axis region. Variations in the Nd isotopic composition in the time sequence of magmatic events indicate a high degree of chemical and isotopic heterogeneity of the ascending mantle material which became later entrained in the melting region beneath a spreading zone. Melting of the sources with primitive mantle composition (ε Nd ~ + 8 to +9) as well as enriched sources took place in the time interval between ~ 17 and 14.7 Ma and at about 8 Ma. The enriched source material is most likely represented by ancient mafic substratum.