Geochemistry of lavas and dikes from the upper oceanic crust of Hess Deep Rift, eastern Pacific Ocean

Strontium isotopes are useful tracers of fluid-rock interaction in marine hydrothermal systems and provide a potential way to quantify the amount of seawater that passes through these systems. We have determined the whole-rock Sr-isotopic compositions of a section of upper oceanic crust that formed at the fast-spreading East Pacific Rise, now exposed at Hess Deep. This dataset provides the first detailed comparison for the much-studied Ocean Drilling Program (ODP) drill core from Site 504B. Whole-rock and mineral Sr concentrations indicate that Sr-exchange between hydrothermal fluids and the oceanic crust is complex, being dependent on the mineralogical reactions occurring; in particular, epidote formation takes up Sr from the fluid increasing the 87Sr/86Sr of the bulk-rock. Calculating the fluid-flux required to shift the Sr-isotopic composition of the Hess Deep sheeted-dike complex, using the approach of Bickle and Teagle (1992, doi:10.1016/0012-821X(92)90221-G) gives a fluid-flux similar to that determined for ODP Hole 504B. This suggests that the level of isotopic exchange observed in these two regions is probably typical for modern oceanic crust. Unfortunately, uncertainties in the modeling approach do not allow us to determine a fluid-flux that is directly comparable to fluxes calculated by other methods.

Rare earth elements in bottom sediments from the deeps of the Red Sea rift zone

Analysis of rare earth element (REE) distribution and behavior in ore-bearing hydrothermal-sedimentary deposits from the Red Sea is carried out. Geochemical patterns and mechanisms of REE accumulation in metalliferous sediments of the open ocean and in deposits adjoined to areas of hydrothermal discharge are shown. Main factors, which determine composition of REE and the level of their accumulation in hydrothermal occurrences of the Red Sea, are considered.

(Table 2) Layer-by-layer chemical composition of barite crusts from the Atlantis II Deep of the Red Sea rift zone

Results of studies of hydrothermal sulfide-sulfate rocks occurring in the Atlantis II Deep of the Red Sea are reported in the paper.

Cacao trees in tropical agroforestry landscapes of the Napu Valley in Central Sulawesi, Indonesia

All sample sites were situated at the northern tip of Napu Valley in Central Sulawesi, Indonesia. After an initial mapping of the study area, we selected 15 smallholder cacao plantations as sites for bird and bat exclosure experiments in March 2010. On each study site, we established 4 treatments for these exclosure experiments (bird exclosure - closed during daytime and open during night; bat exclosure - closed overnight and opened during daytime; full exclosure of both birds and bats - always closed and unmanipulated/open control treatments - always open). In each treatment, there were 2 cacao trees (total of 8 cacao trees per study site), surrounded by nylon filament (2x2 cm mesh size) that was opened and closed according to the activity period of day and night active flying vertebrates (05:00-06:00 am and 17:00-18:00 pm) on a daily basis. The mean tree height and diameter at breast height (dbh) result from two measures of all study trees at the beginning of the exclosure experiment (June 2010) and 6 months later (February 2011).

Lithium concentrations and isotope composition of basalt samples from ODP/DSDP Sites from Costa Rica Rift

Ocean Drilling Program (ODP) Hole 504B near the Costa Rica Rift is the deepest hole drilled in the ocean crust, penetrating a volcanic section, a transition zone and a sheeted dike complex. The distribution of Li and its isotopes through this 1.8-km section of oceanic crust reflects the varying conditions of seawater alteration with depth. The upper volcanic rocks, altered at low temperatures, are enriched in Li (5.6-27.3 ppm) and have heavier isotopic compositions (delta7Li=6.6-20.8?) relative to fresh mid-ocean ridge basalt (MORB) due to uptake of seawater Li into alteration clays. The Li content and isotopic compositions of the deeper volcanic rocks are similar to MORB, reflecting restricted seawater circulation in this section. The transition zone is a region of mixing of seawater with upwelling hydrothermal fluids and sulfide mineralization. Li enrichment in this zone is accompanied by relatively light isotopic compositions (-0.8-2.1?) which signify influence of basalt-derived Li during mineralization and alteration. Li decreases with depth to 0.6 ppm in the sheeted dike complex as a result of increasing hydrothermal extraction in the high-temperature reaction zone. Rocks in the dike complex have variable isotopic values that range from -1.7 to 7.9?, depending on the extent of hydrothermal recrystallization and off-axis low-temperature alteration. Hydrothermally altered rocks are isotopically light because 6Li is preferentially retained in greenschist and amphibolite facies minerals. The delta7Li values of the highly altered rocks of the dike complex are complementary to those of high-temperature mid-ocean ridge vent fluids and compatible to equilibrium control by the alteration mineral assemblage. The inventory of Li in basement rocks permits a reevaluation of the role of oceanic crust in the budget of Li in the ocean. On balance, the upper 1.8 km of oceanic crusts remains a sink for oceanic Li. The observations at 504B and an estimated flux from the underlying 0.5 km of gabbro suggest that the global hydrothermal flux is at most 8*10**9 mol/yr, compatible with geophysical thermal models. This work defines the distribution of Li and its isotopes in the upper ocean crust and provides a basis to interpret the contribution of subducted lithosphere to arc magmas and cycling of crustal material in the deep mantle.

Composition of dikes and lavas from the Pito Deep Rift

The northwest trending walls of the Pito Deep Rift (PDR), a tectonic window in the southeast Pacific, expose in situ oceanic crust generated ?3 Ma at the superfast spreading southern East Pacific Rise (SEPR). Whole rock analyses were performed on over 200 samples of dikes and lavas recovered from two ~8 km**2 study areas. Most of the PDR samples are incompatible-element-depleted normal mid-ocean ridge basalts (NMORB; (La/Sm)N < 1.0) that show typical tholeiitic fractionation trends. Correlated variations in Pb isotope ratios, rare earth element patterns, and ratios of incompatible elements (e.g., (Ce/Yb)N) are best explained by mixing curves between two enriched and one depleted mantle sources. Pb isotope compositions of most PDR NMORB are offset from SEPR data toward higher values of 207Pb/204Pb, suggesting that an enriched component of the mantle was present in this region in the past ?3 Ma but is not evident today. Overall, the PDR crust is highly variable in composition over long and short spatial scales, demonstrating that chemically distinct lavas and dikes can be emplaced within the same segment over short timescales. However, the limited spatial distribution of high 206Pb/204Pb samples and the occurrence of relatively homogeneous MgO compositions (ranging <2.5 wt %) within a few of the individual dive transects (over distances of ~1 km) suggests that the mantle source composition evolved and magmatic temperatures persisted over timescales of tens of thousands of years. The high degree of chemical variability between pairs of adjacent dikes is interpreted as evidence for along-axis transport of magma from chemically distinct portions of the melt lens. Our findings suggest that lateral dike propagation occurs to a significant degree at superfast spreading centers.

(Table 1) Geochemical composition of surface waters obtained aboard the R/V Rift along the Volga-Caspian canal

The Volga River discharge consists of the waters transferred by fast currents through channels and the waters which are passing through the shallow areas of the delta overgrown by cane. Using the hydrochemical data, it is possible to track distribution of the waters modified by 'biofilters' of macrophytes in the delta shallows starting from the external edge of the delta. The main distinctive features of these waters are the high content of dissolved oxygen, the abnormally high values of the pH, and the low content of dissolved inorganic carbon (both total and as CO2). These waters extend in the shape of 1 to 3-km-wide strips at a distance of 20-40 km from the outer border of the delta. The analysis of the data obtained during the expeditions run by the Institute of Oceanology of the Russian Academy of Sciences in 2003-2009, along with archived and published data, show that such 'modified' waters occur almost constantly along the outer edge of the Volga River delta.