Sm-Nd and Rb-Sr isotopic systems and captured He and hydrocarbon gases as markers of melt sources and fluid regime under which the oceanic crust of the Mid-Atlantic Ridge was formed at 5°–6° N

Autor: G. N. Savelieva, N. S. Bortnikov, Tamara B. Bayanova, S. V. Ikorskii, I. L. Kamenskii
Rok vydání: 2008
Předmět:
Zdroj: Geochemistry International. 46:745-758
ISSN: 1556-1968
0016-7029
Popis: The paper presents characteristics of the Nd and Sr isotopic systems of ultrabasic rocks, gabbroids, plagiogranites, and their minerals and data on He and hydrocarbons in fluid inclusions in the same samples. The materials presented in this publication were obtained by studying samples dredged from the MAR crest zone at 5°–6° N (U/Pb zircon dating, geochemical and petrological-mineralogical studies). It was demonstrated that the variations in the isotopic composition of He entrapped in the rocks and minerals were controlled by variable degrees of mixing of juvenile He, which is typical of basaltic glass of MAR (DM source) and atmospheric He. An increase in the fraction of atmospheric He in the plutonic rocks and, to a lesser degree, in their minerals reflects the involvement of seawater or the hydrated material of the oceanic crust in the magmatic and postmagmatic processes. This conclusion finds further support in a positive correlation between the fraction of mantle He (R) and the 87Sr/86Sr ratio. The high-temperature hydration of ultrabasic rocks (amphibolization) was associated with an increase in the fraction of mantle He, while their low-temperature hydration (serpentinization) was accompanied by a drastic decrease in this fraction and a significant increase in the 87Sr/86Sr ratio. The insignificant variations in the 143Nd/144Nd (close to 0.5130) and 87Sr/86Sr (0.7035) in most of the gabbroids and plagiogranites, as well as the fraction of mantle He in these rocks, amphibolites, and their ore minerals, indicate that the melts were derived from the depleted mantle. SimilarɛNd values of the gabbroids, plagiogranites, and fresh harzburgites (6.77–8.39) suggest that these rocks were genetically related to a single mantle source. The value of ɛNd = 2.62 of the serpentinized lherzolites likely reflects relations of these relatively weakly depleted mantle residues to another source. The aforementioned characteristics of the rocks generally reflect the various degrees of mixing of components of the depleted mantle with crustal components (seawater) during the metamorphic and hydrothermal processes that accompanied the formation of the oceanic crust.
Databáze: OpenAIRE
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