Description: We evaluate the potential of ophiolites as archives of paleoseawater and hydrothermal fluid compositions by analysing the chemical and isotopic composition of abiogenic carbonates, precipitated from fluids within the oceanic crust of the 91 Ma Troodos Ophiolite, Cyprus. Calculated variations in fluid Mg/Ca, Sr/Ca and Sr-87/ Sr-86 with temperature within the upper sections of the ophiolite are similar to those from drilled oceanic crust, and yield literature values for late Cretaceous seawater Mg/Ca, Sr/Ca and Sr-87/ Sr-86. This indicates that carbonates from ophiolites could be used to estimate the composition of ancient seawater at times before the age of the oldest preserved in-situ oceanic crust. Whereas most carbonates recovered from in-situ oceanic crust were precipitated at temperatures 〈 60 degrees C, abiogenic carbonates from the Troodos Ophiolite formed over a temperature range of 7 degrees C to 218 degrees C. These provide unique insights into the chemical and mineralogical processes that transform seawater into a high temperature hydrothermal fluid within the oceanic crust. We use 'hydrothermal variation diagrams' of Mg/Ca, Sr/Ca, Sr-87/ Sr-86 and delta(44)/Ca-40 versus calculated temperature (delta O-18) to trace this fluid evolution within the Troodos oceanic crust. We find that successive fluid-crust-interaction, the precipitation of Mg- and Ca-bearing minerals and the early formation of anhydrite (〉 44 degrees C) gradually transform Cretaceous seawater into a Troodos hydrothermal fluid. Comparison of the Troodos data with a global dataset of abiogenic carbonates from in-situ oceanic crust shows that the chemical pathways of low-temperature fluid evolution are similar for all Cretaceous sites. These different sites represent varied geotectonic settings (midocean ridge vs. suprasubduction zone), with different basement composition (basalt, basaltic andesite/boninite) and situated in different ocean basins (Atlantic, Pacific, Mediterranean [Tethys]). The similarity in the carbonate record indicates that these differences do not significantly influence seafloor weathering and hydrothermal alteration at low temperatures. However, abiogenic carbonates from younger oceanic crust differ from the Cretaceous trends and follow different fluid evolution pathways. This indicates, that temporal variations in the composition of seawater may control the nature and the extent of seafloor weathering and hydrothermal alteration at low temperatures. A thermodynamic model of fluid-crust interaction, in which modern and Cretaceous seawater are heated to 200 degrees C while an average Troodos basaltic andesite is successively added under otherwise identical conditions predicts that fluid evolution and alteration of the oceanic crust were different in the Cretaceous than they are today, and that initial seawater chemistry affects the nature and the extent of seafloor alteration up to moderate fluid temperatures. For example, twice the amount of carbonate formed during alteration of the oceanic crust in the Cretaceous compared to modern times, indicating that the flux of CO2 from the hydrosphere-atmosphere system into the oceanic crust was greater in the Cretaceous than it is nowadays, and that it probably varied throughout geologic time.

Description: Detailed exploration with remotely operated and autonomous deep submergence vehicles has revealed, at 9 degrees 33'S, the presence of the southernmost active hydrothermal field known so far on the Mid-Atlantic Ridge. The size of the hydrothermal field, which we have named "Lilliput", is about 1000 m x 250 m. It lies in a water depth of 1500 m on a ridge segment (Segment A3) with considerably thickened crust of 11 km. Four relatively small diffuse vent sites occur on a large young (estimated 〈100 years old) lava flow, partly covering the flow with hydrothermal Fe-oxide/hydroxide sediments. Based on homogeneous major element compositions of ca. 25 lava samples, this flow covers an area of at least 5 km x 0.6 km. The lava flow erupted from a series of parallel fissures at the western edge of the flow and a volcanic ridge consisting of up to 30 m high pillow mounds. The volcanic ridge probably represents the surface expression of an underlying dike which fed the flow. Several drained lava pond structures were observed within the flow but only one shows hydrothermal activity. The hydrothermal venting and precipitation of abundant Fe-oxyhydroxides appear to be related to the young diking and eruption event and the four different hydrothermally active sites of the Lilliput field lie along and almost equidistant from the eastern flank of the supposed dike. Although a hydrothermal plume some 500 m above the seafloor was found in two consecutive years (2005 and 2006), no high-temperature venting associated with Lilliput has been found. in agreement with findings at other ridges with thick crust such as Reykjanes. High magma supply rate and frequent diking and eruption events may lead to hot hydrothermal vents being rare in slow-spreading segments with thick crust whereas diffuse venting is abundant. Interestingly, the fauna at the Lilliput vents largely consists of small and apparently juvenile mussels (Bathymodiolus sp.) and did not show any signs of growth during the four years of continuing observations possibly reflecting pulsing hydrothermal activity.

Description: Sediment cores containing up to twenty-five ash layers were taken at three sites close to Vesterisbanken Seamount in the Greenland Basin. These ash layers imply frequent eruptions of the volcano within the last 60 ka. The eruptions led to airborne transport and volcaniclastic turbidity flows which transported volcanic glassy and crystalline material from the volcano into the surrounding basin. During the eruption and the transport the glass and the crystal particles were mixed. The glasses range in composition between basanites and phonolites/benmoreites with MgO contents of 8 to 0.65%. The glass analyses follow a distinct trend of fractionation suggesting the crystallization of the phases olivine, clinopyroxene, plagioclase, kaersutite, Cr-spinel, Ti-magnetite and apatite. A strong zonation of clinopyroxene and kaersutite phenocrysts implies mixing processes in the magma system although the liquid compositions do not lie on mixing trends. A geochemical study of the bulk ashes shows that some ash layers possess distinct chemical compositions. The ashes are more evolved than the lavas of the volcano, suggesting fractionation of liquid from crystallized material during the eruption or transport of the ashes. Sixteen layers are statistically combined into four groups, of which several can be correlated from core to core reflecting individual eruptive events.

Description: Lavas from two young volcanic fields of the Easter Hotspot consist mainly of enriched tholeiites, with incompatible element and isotopic compositions similar to enriched MORB from the neighbouring spreading axis. The major element composition of these intraplate tholeiites suggests that they originate from melting at greater pressures, and REE models indicate slightly lower degrees of partial melting than beneath the ridge. This can be explained by the thickening of the lithosphere away from the spreading axis, accompanied by increasing mantle temperature as the Easter plume is approached. These processes combine to depress the melting zone to greater depth in the intraplate region. The relatively low degrees of partial melting and low volumes of melt compared, for example, to Galapagos imply a low excess temperature (∼ 100°C?) for the Easter plume. A few depleted tholeiites found on the Ahu volcanic field were generated by shallow melting of an extremely depleted (more depleted than MORB

Description: Subduction of oceanic lithosphere is a key feature of terrestrial plate tectonics. However, the effect of this recycled crustal material on mantle composition is debated. Ocean island basalts (OIB) provide direct insights into the composition of Earth's mantle. The distinct composition of the HIMU (high 238U/204Pb)- and EM (enriched mantle)-type OIB mantle sources may be due to either recycling of oceanic crust and sediment into the mantle or metasomatic processes within the mantle. Chlorine derived from seawater or crustal fluids potentially provides a tracer for recycled material. Previously reported δ37Cl values for mid-ocean ridge basalts (MORB) range from ca. −3.0 to near 0‰. In contrast to MORB, we find a larger variation in OIB glasses representing HIMU- and EM-type mantle sources based on replicate SIMS analyses with δ37Cl values ranging from −1.6 to +1.1‰ for HIMU-type and −0.4 to +2.9‰ for EM-type lavas. These δ37Cl values correlate positively with 87Sr/86Sr ratios for both the HIMU- and EM-type samples. The negative δ37Cl values of some HIMU-type lavas overlap with those of altered oceanic lithosphere, which is assumed to be present in the HIMU source. The EM lavas have high 87Sr/86Sr and primarily positive δ37Cl values. We hypothesize that subducting sediments may have developed high δ37Cl values by expelling 37Cl-depleted pore fluids, thus accounting for the positive δ37Cl values recorded in the EM-type lavas.