Local and regional variation of MORB parent magmas: evidence from melt inclusions from the Endeavour Segment of the Juan de Fuca Ridge

The development of petrogenetic models of igneous processes in the mantle is dependent on a detailed knowledge of the diversity of magmas produced in the melting regime. These primary magmas, however, undergo significant mixing and fractionation during transport to the surface, destroying much of the evidence of their primary diversity. To circumvent this problem and to determine the diversity of melts produced in the mantle, we used melt inclusions hosted in primitive plagioclase phenocrysts from eight mid-ocean ridge basalts from the axial and West Valleys of the Endeavour Segment, Juan de Fuca Ridge. This area was selected for study because of the demonstrated close association of enriched (E-MORB) lavas and incompatible element enriched depleted (N-MORB) lavas. Rehomogenized melt inclusions from E-MORB, T-MORB, and N-MORB lavas have been analyzed by electron and ion microprobe for major and trace elements. The depleted and enriched lavas, as well as their melt inclusions, have very similar compatible element concentrations (major elements, Sr, Ni and Cr). Inclusion compositions are more primitive than, yet collinear with, the host lava suites. In contrast, the minor and trace element characteristics of melt inclusions from depleted and enriched lavas are different both in range and absolute concentration. N-MORB lavas contain both depleted and enriched melt inclusions, and therefore exhibit the largest compositional range (K₂O: 0.01 to 0.4 oxide wt%, P₂O₅: <0.01 to 0.2 oxide wt%, La_N: 7 to 35, Yb_N: 1 to 13, and Ti/Zr: <100 to 1300). E-MORB lavas contain only enriched inclusions, and are therefore relatively homogeneous (K₂O: 0.32 to 0.9 oxide wt %, P₂O₅: 0.02 to 0.35 oxide wt%, La_N: 11 to 60, Yb_N: 4 to 21, and Ti/Zr: ∼100). In addition, the most primitive E-32 inclusions are similar in composition to the most enriched inclusions from the depleted hosts. Major element data for melt inclusions from both N-MORB and E-MORB lavas suggest that the magmas lie on a low pressure cotectic, consistent with a petrogenesis including fractional crystallization. However, the minor and trace element compositions in melt inclusions vary independently of the major element composition implying an alternative history. When fractionation-corrected, inclusion compositions correlate with their host glass composition. Hence, the degree of enrichment of the lavas is a function of the composition of aggregated melts, not of processing in the upper mantle or lower crust. Based on this fact, the lava suites are not produced from a single parent magma, but from a suite of primary magmas. The chemistry of the melt inclusions from the enriched lavas is consistent with a derivation from variable percentages of partial melting within the spinel stability field by a process of open system (continuous or critical) melting assuming a depleted lherzolite source veined with clinopyroxenite. The low percentage melts are dominantly enriched melts of the clinopyroxenite. In contrast, the depleted lavas were created by melting of a harzburgite source, possibly fluxed with a fluid enriched in K, Ba and the LREE. Such a source was likely melted up to or past the point at which all of its clinopyroxene was consumed. This set of characteristics is consistent with a scenario by which diverse melts produced at different depths travel through the melting regime to the base of the crust without homogenizing en route. The homogeneous major element characteristics are created in the lower crust by fractional crystallization and reaction with lower crustal gabbros. Therefore, the degree of decoupling between major and trace element characteristics of the melt inclusions (and lavas) is dictated by the reaction rate of the melts with the materials in the conduit walls, as well as the residence times and flux rate, in the upper mantle and lower crust. Received: 2 December 1997 / Accepted: 27 August 1998     

Field measurements of time-averaged suspended sediment concentrations under waves

A suction sediment sampler has been developed for sampling suspended sand in the field. It works by the pressure difference between seabed and water surface, and thus no pump is required. The fact that the sampler has no mechanical moving parts and no electronics makes it very dependable. Seven samples are taken simultaneously at different elevations starting at about 1 cm above the bed. The sampling time is about 3 min for water depths of about 1.5 m. Sixty-five concentration profiles, complete with hydraulic data and sediment characteristics are given in the appendix.     

Optical Changes in a Eutrophic Estuary During Reduced Nutrient Loadings

Loss of water clarity is one of the consequences of coastal eutrophication. Efforts have therefore been made to reduce external nutrient loadings of coastal waters. This paper documents improvements to water clarity between 1985 and 2008–2009 at four stations in the microtidal estuary Roskilde Fjord and find significant relationships to freshwater nutrient loadings. The paper then investigates to which extent changes in phytoplankton biomass (chlorophyll a (Chl a)), non-algal particulate organic matter (POM*), and residual attenuation in the water (K _b), respectively, can account for this optical improvement. Vertical light attenuation (K _d) declined, on average, by 34 %, accompanying a 71 % reduction of Chl a and an 80 % reduction of POM*. Residual attenuation declined by 26 % over the period in accordance with a measured 34 % decline of dissolved organic nitrogen. Analysis of simultaneous changes in light attenuation and Secchi depth also suggested a reduction of the scatter-to-absorption ratio over time. Considering the stronger reductions of particle concentrations than dissolved organic matter, the contribution of residual attenuation to vertical attenuation increased from 54 to 74 % in 1985 to 78 to 85 % in 2008–2009. Overall, efforts to reduce nutrient loading and improve water clarity appeared to have had a larger impact on POM* than on Chl a and colored dissolved organic matter concentrations in the estuary, which can account for the decrease in the scatter-to-absorption ratio. These optical changes lead to larger improvements of Secchi depth than of vertical light attenuation. The consequence of this is an overestimation (0.45–1.48 m) of the predicted increase of potential seagrass depth limits when based on Secchi depth rather than K _d.     

Factors affecting the electrification of wind-driven dust studied with laboratory simulations

The electrification of wind-blown dust grains was studied in a series of laboratory experiments to examine how grain electrification depends on grain size, grain mineralogy, atmospheric composition, atmospheric pressure, and the method of dust dispersal. This work is intended to contribute to a deeper physical understanding of particle electrification on both Mars and Earth. Findings indicate that the amount of electrification per suspended particle generally is independent of dust entrainment process and atmospheric composition. As expected, the electrification process is grain size-dependent, with smaller grains predominantly electrifying negatively. Although there appears to be a weak dependence upon dust mineralogy, this work supports the expectation that dust suspended in the Martian atmosphere will be significantly electrified.     

Governance arrangements targeting diversity in Europe: how New Public Management impacts working with social cohesion

This article analyses how policies to foster social cohesion within diverse and unequal urban contexts are affected by New Public Management and austerity policies. Based on the analysis of a handful of governance arrangements in three cities that differ in their institutional structure and diversity policy approaches (Copenhagen, Leipzig and Milan), it is shown that negative effects are quite widespread yet cushioned by a strong welfare state structure, solid local government and high priority given to the recognition of diversity. Nevertheless, the shift towards the application of market logic to social work reduces innovative potential, increases efforts spent on procedures and weakens public coordination.     

Meteorological Observations in 2000 at the Arctic Station,Qeqertarsuaq (69°15'N), Central West Greenland

An automatic meteorological station has been operating at the Arctic Station (69°15'N, 53°31'W) in West Greenland since 1990. This paper summarizes meteorological parameters during 2000 including snow and sea ice cover, ground temperatures and active layer development, and presents comments on the local permafrost thickness.