Search

In this paper, new main and trace elements and isotopic data are presented for 14 coarse-grained eclogite xenoliths from the V. Grib kimberlite pipe in the central part of the Arkhangelsk Diamondiferous Province. Based on reconstructed whole rock MgO content, this suite is divided into high-MgO and low-MgO varieties. Eclogitic groups have a similar range of variations in the trace element compositions of garnet, clinopyroxene and reconstructed whole rock. All eclogites show positive Eu anomalies in garnet and Sr anomalies in the whole rock. The negative correlation between the Mg#, Sr/Lu ratio and HREE in a whole rock points to upper and lower oceanic crustal rocks as a protolith for eclogites with high and low whole rock HREEs, respectively. Low-MgO eclogites with higher whole rock HREEs have the basaltic upper oceanic crustal protolith, whereas the protoliths of eclogites with lower whole rock HREEs could be of gabbroic composition from the lower oceanic crust. High-MgO eclogites could represent MgO-rich portions of oceanic crustal rocks: picritic/MgO basalt portions in the upper oceanic crust and troctolite portions in the lower oceanic crust. The Sr and Nd isotope compositions suggest a complex history of eclogites during their residence in the lithospheric mantle. Similarities in the Nd isotope compositions and two-point Sm-Nd isochron ages are evidence for re-equilibration of the Sm-Nd isotope system between the eclogite garnet and clinopyroxene via a pre-kimberlite thermal event at 396 ± 24 Ma. The subset of clinopyroxenes from four eclogites has a Sr isotope composition that plots on the isochron at an age of 2.84 Ga, which reflects the time of the subduction event and emplacement into the lithosphere and corresponds to the time of the Belomorian Eclogite Province of Baltic Shield formation. [ABSTRACT FROM AUTHOR]

The Hawaiian–Emperor Seamount Chain is composed of seamounts capped with fossil coral reef deposits that originally formed close to sea level but are now covered by hundreds of meters of water owing to prolonged subsidence. These fossil reef deposits are important archives of paleoenvironmental change and retain possible traces of Aeolian input from continental Asian dust. We studied the trace and rare earth element composition of Oligocene-Miocene coral reef limestone of Koko guyot aged 25–6 Ma to assess the influence and possible increase of continental allothigenic material input that could persist in marine carbonates. The composition of trace and rare earth elements of modern coral and ancient limestones demonstrated that they had inherited the composition of sea water. Positive europium anomaly observed in modern coral and calcified echinoid spines aged 0–50 thousand years was absent in ancient limestones. This effect could be associated with a change in the oxidation state during diagenesis and recrystallization, which probably led to the loss of europium. The distribution of trace and rare earth elements was constant in carbonate rocks of different ages despite the increase of aeolian input, which confirmed the earlier conclusions about the restriction of incorporation of certain elements into the crystal lattice of carbonate minerals, and demonstrated the regulating role of seawater. In addition, trace and rare earth elements, even if they were adsorbed on the thin films of biological objects during their life, obviously didn't persist there after burial. • The trace and rare earth elements composition of warm-water corals aged 25 million years was obtained. • The influence of aeolian input of Asian deserts in the Pacific Ocean has been determined. • The rare earth composition of ancient corals is completely controlled by the composition of the seawater where they formed. [ABSTRACT FROM AUTHOR]

The Hawaiian–Emperor Seamount Chain, a major topographic feature of the Pacific Ocean floor, is composed of seamounts capped with fossil coral reef deposits that originally formed close to sea level but are now covered by hundreds of meters of water owing to prolonged subsidence. These fossil reef deposits are important archives of paleoenvironmental change and yield information on the subsidence history of the seamounts. We studied the Sr isotope compositions of Oligocene–Miocene coral reef limestone from Koko Guyot in the southern Emperor Seamount Chain to assess the dynamics of the subsidence. The ages of the studied samples containing coral fragments established by Sr isotope stratigraphy vary from 26.3 to 20.1 Ma. In contrast, the youngest samples (15.3 Ma), which were deposited in water depths of >120 m, are barren of corals and are composed exclusively of bryozoans and coralline algae. The subsidence rate of the Koko Guyot volcanic structure was not constant over time. Integration of our new data with the results of previous studies reveals that the subsidence rate was 0.046 ± 0.005 mm/yr during the first 25–30 Myr (from 49–44 to 20 Ma). During this period, Koko Guyot was in a bathymetric interval favorable for coral reef development, and its subsidence was compensated by rapid vertical growth of the reef. Subsequently, the subsidence rate decreased to an average value of 0.019 ± 0.003 mm/yr from 20 to 15 Ma. The decrease in the rate of bottom subsidence coincided with unfavorable environmental conditions for coral reef development, leading to the disappearance of corals. The average subsidence rate has been 0.015 ± 0.002 mm/yr since 15 Ma, comparable to the present-day subsidence rate. We also analyzed the stable Sr isotope ratios (δ88/86Sr) of warm-water coral samples formed at 25–20 Ma (0.32‰ ± 0.1‰), as well as carbonate of large benthic foraminifera, coralline algae, and other non-coral species for the period 20–15 Ma (0.10‰ ± 0.09‰). We suggest that the large difference in carbonate δ88/86Sr between 25–20 and 20–15 Ma corresponds to a difference in the fractionation factor caused by environmental and benthic community change. [Display omitted] • The subsidence rate of Koko Guyot was not uniform. • The age of corals and larger foraminiferal species was determined by strontium isotope stratigraphy. • The study of strontium isotope composition showed that the least altered limestones containing the fragments of reef corals were formed in the early Miocene. [ABSTRACT FROM AUTHOR]