Composition of rocks and minerals from the transition zone of the Reykjanes Ridge

Investigations of petrography, mineralogy, and chemical composition of gases and fluids in tuffs and lavas were carried out on samples dredged in the transition zone from the shelf and slope of Iceland to the Reykjanes Ridge. The samples were collected from the depths of 950-720 m during different expeditions of R/V Akademik Kurchatov and Mikhail Lomonosov. Mantle ultrabasite inclusions were first recognized in the region of Iceland. It can be assumed that they are related to eruptive structures formed on the ocean floor during Pliocene and are associated with the Iceland hot spot.

Benthic fauna associated with mussel beds and shrimp swarms within hydrothermal fields on the Mid-Atlantic Ridge

Macrofaunal assemblages with prevalence of Bresiliidae shrimps and Mytilidae mussels are abundant in at hydrothermal vents along the Mid-Atlantic Ridge. Mussels inhabit zones of diffuse seeps of hydrothermal fluids with temperature abnormalities up to several degrees. Shrimps inhabit an extreme biotope in a mixed interface between seawater and hydrothermal fluids at temperature up to 20-30°C. We studied the mussel and shrimp assemblages in three hydrothermal vent fields: Rainbow, Broken Spur, and Snake Pit. Species richness of the mussel assemblages within at least two fields (Broken Spur and Snake Pit) is higher as compared with shrimps from the same hydrothermal vent fields. Fauna inhibiting shrimp swarms lack almost any taxa specific for particular assemblages: almost all the taxa are also present in the mussel beds. Structure of the shrimp assemblage is less homogeneous as compared with that of the mussel assemblage. Population prevalence of one taxon (Copepoda) in the shrimp assemblage is most likely connected with extreme and unstable conditions of the biotope occupied by the shrimps in a hydrothermal field. Taxonomic similarity between the mussel and shrimp assemblages within one hydrothermal vent field is higher as compared with similarity between the mussel (or shrimp) assemblages from different fields.

The early Eocene global warming events: Implications from ODP Leg 208 Walvis Ridge depth transect

Five sections drilled in multiple holes over a depth transect of more than 2200 m at the Walvis Ridge (SE Atlantic) during Ocean Drilling Program (ODP) Leg 208 resulted in the first complete early Paleogene deep-sea record. Here we present high-resolution stratigraphic records spanning a ~4.3 million yearlong interval of the late Paleocene to early Eocene. This interval includes the Paleocene-Eocene thermal maximum (PETM) as well as the Eocene thermal maximum (ETM) 2 event. A detailed chronology was developed with nondestructive X-ray fluorescence (XRF) core scanning records and shipboard color data. These records were used to refine the shipboard-derived spliced composite depth for each site and with a record from ODP Site 1051 were then used to establish a continuous time series over this interval. Extensive spectral analysis reveals that the early Paleogene sedimentary cyclicity is dominated by precession modulated by the short (100 kyr) and long (405 kyr) eccentricity cycles. Counting of precession-related cycles at multiple sites results in revised estimates for the duration of magnetochrons C24r and C25n. Direct comparison between the amplitude modulation of the precession component derived from XRF data and recent models of Earth's orbital eccentricity suggests that the onset of the PETM and ETM2 are related to a 100-kyr eccentricity maximum. Both events are approximately a quarter of a period offset from a maximum in the 405-kyr eccentricity cycle, with the major difference that the PETM is lagging and ETM2 is leading a 405-kyr eccentricity maximum. Absolute age estimates for the PETM, ETM2, and the magnetochron boundaries that are consistent with recalibrated radiometric ages and recent models of Earth's orbital eccentricity cannot be precisely determined at present because of too large uncertainties in these methods. Nevertheless, we provide two possible tuning options, which demonstrate the potential for the development of a cyclostratigraphic framework based on the stable 405-kyr eccentricity cycle for the entire Paleogene.

Sm-Nd, Rb-Sr, and He isotopic composition and hydrocarbon gases in rocks and minerals from the Markov Deep, Mid-Atlantic Ridge rift valley

The paper presents characteristics of the Nd and Sr isotopic systems of ultrabasic rocks, gabbroids, plagiogranites, and their minerals as well as data on helium and hydrocarbons in fluid inclusions of the same samples. 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 variations in the isotopic composition of He entrapped in rocks and minerals were controlled by variable degrees of mixing of juvenile He, which is typical of basaltic glass for MAR (DM source), and atmospheric He. Increase in the atmospheric He fraction in plutonic rocks and, to a lesser degree, in their minerals reflects involvement of seawater or hydrated material of the oceanic crust in magmatic and postmagmatic processes. This conclusion finds further support in positive correlation between the fraction of mantle He (R ratio) and 87Sr/86Sr ratio. High-temperature hydration of ultrabasic rocks (amphibolization) was associated with increase in the fraction of mantle He, while their low-temperature hydration (serpentinization) was accompanied by drastic decrease in this fraction and significant increase in 87Sr/86Sr ratio. Insignificant variations in 143Nd/144Nd (close to 0.5130) and 87Sr/86Sr (0.7035) in most of 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. Similar e-Nd values of gabbroids, plagiogranites, and fresh harzburgites (6.77-8.39) suggest that these rocks were genetically related to a single mantle source. e-Nd value of serpentinized lherzolites (2.62) likely reflects relations of these relatively weakly depleted mantle residues to another source. Aforementioned characteristics of the rocks generally reflect various degrees of mixing of depleted mantle components with crustal components (seawater) during metamorphic and hydrothermal processes that accompanied formation of the oceanic crust.