Volatile (Cl,F and S) and major element constraints on subduction-related mantle metasomatism along the alkaline basaltic backarc,Payenia, Argentina

We present data on volatile (S, F and Cl) and major element contents in olivine-hosted melt inclusions (MIs) from alkaline basaltic tephras along the Quaternary Payenia backarc volcanic province (~34°S–38°S) of the Andean Southern Volcanic Zone (SVZ). The composition of Cr-spinel inclusions and host olivines in Payenia are also included to constrain any variations in oxygen fugacity. The variation of potassium, fluorine and chlorine in MIs in Payenia can be modelled by partial melting (1–10%) of a variously metasomatised mantle. The high chlorine contents in MIs (up to 3200 ppm) from Northern Payenia require addition of subduction-related fluids to a mantle wedge, whereas volatile signatures in the southern Payenia are consistent with derivation from an enriched OIB source. Cl and Cl/K ratios define positive correlations with host olivine fosterite content (Fo_80-90) that cannot be explained by olivine fractionation, degassing and/or degree of mantle melting. Neither can the correlation between SiO₂ and TiO₂ in the MIs and host olivine Fo-content be explained by magmatic differentiation processes. Instead these correlations essentially require a south to north mantle source transition from a low Mg# pyroxenite (from recycled eclogite) to a high Mg# fluid metasomatised peridotite. The Cl/K and S/K ratios in Payenia MIs extend from enriched OIB-like signatures (south) to Andean SVZ arc like signatures (north). We show that the northward increase in S, Cl and S/K is coupled to a northward increase in melt oxidation states and thus in Fe³⁺/Fe^tot ratios in the magmas. The increase in oxidation state also correlates with an increase of Mn/Fe (olivine) ratios. We calculate that 25% of the apparent north–south pyroxenite–peridotite source variation in Payenia (based on olivine Mn/Fe ratios) can be explained by the south to north variation in melt oxidation states.     

Evidence for mantle metasomatism: an oxygen and strontium isotope study of the Vulsinian District, Central Italy

This paper presents new O and Sr isotope data for lavas from the northern part of the Roman perpotassic province. The samples comprise the tephritic leucititic to leucite phonolitic lavas and the saturated lavas from the Vulsinian District, the olivine leucite melilitite of San Venanzo, and the kalsilite diopside melilitite of Cupaello. Previous oxygen isotope work on the lavas of the Vulsinian District suggested crustal contamination of “normal” mantle-derived magmas. The new data cover the ranges previously found. O and Sr isotope ratios of evolved lavas of the undersaturated suite indicate assimilation in variable amounts of up to ca. 10% of continental crustal material. The saturated lavas probably assimilated large amounts (up to ca. 50%) of crust. Lavas chemically identified as corresponding to little modified mantle-derived liquids are high in both⁸⁷Sr/⁸⁶Sr andδ¹⁸O: 0.7103−0.7107, +7.8 to +9.4 (Vulsini), 0.7104, +12.3 (San Venanzo) and 0.7112, +14.4 (Cupaello). These high values are interpreted to have been inherited from a metasomatized parental mantle. Hydrous fluids enriched in large-ion lithophile elements and high inδ¹⁸O and⁸⁷Sr/⁸⁶Sr are thought to have mixed with mantle of “normal”δ¹⁸O and⁸⁷Sr/⁸⁶Sr. The fluids probably origi dehydration of continent-derived sediments, which were subducted beneath a mantle wedge in the continent-continent collision of the Corsica-Sardinia block and the Adriatic (Italian) plate. This hypothesis is supported by Pb and Nd isotopic evidence and is probably valid for the entire Roman Province.     

Danish seabird disasters in 1972

In recent years the Danish Game Biology Station has studied casualties to seabirds from oil pollution. 1972 was a particularly disastrous year and preliminary information on the events are given here. Very large losses of seabirds have been caused by quite small spillages of oil.     

Glass-bearing crustal xenoliths (buchites) erupted during the recent activity of Stromboli (Aeolian Islands)

Three xenoliths erupted as ejecta during recent violent explosion of Stromboli volcano (Aeolian Islands) were investigated in this paper. They consist of high-temperature mineral association (cordierite, hercynite spinels, sillimanite, ±plagioclase, ±mullite, ±corundum) and abundant glass (10–70 vol.%), and may be classified as buchites. The peraluminous composition of the xenoliths, their trace element distribution and REE patterns support their origin from granulite- and amphibolite-facies metapelites of the Calabrian continental crust, which is considered the crystalline basement beneath Stromboli. Buchites have an isotopic composition comparable to that of Stromboli extrusives and significantly different from that of the Calabrian basement.

The glass is generally colourless and has a Si–Al–alkali-rich composition, except for one sample where a Fe–Mg–Ca-rich reddish-brown glass also occurs. These two kinds of glass show complicated textures where patches of reddish-brown glass are often surrounded by plagioclase and/or cordierite or forms streaks and swirls with the colourless glass. Cordierite, plagioclase and oxides have different compositions according to their position in the xenoliths. Ca-rich plagioclase (An_72–95), Mg-poor cordierite (Mg-values 47–66) and Al-rich spinels are in the inner portions of the xenoliths and associated with colourless glass; on the contrary, close to the contact with the host lava or associated with coloured glass, cordierite shows higher Mg-values, Ti–Fe-bearing oxides occur and plagioclase is chemically similar to the basalt phenocrysts (An_66–71).

The abundant and fresh glass and the idiomorphic shape of the high-temperature minerals suggest that the xenoliths were hold in the basaltic magma, before its extrusion, for a significant time to allow their partial or nearly complete melting and subsequent nucleation and growth of new phases. During this stage, the interaction between the anatectic liquid and the basaltic magma affected the original isotopic composition of the xenoliths and, in some cases, produced glass and mineral phases (cordierite, plagioclase and oxides) with different composition.     

Loss of patch-scale heterogeneity on secondary productivity in the arid shrubland of Western Australia

General models of degradation suggest soil and nutrients are lost and conversion of rainfall into primary productivity is diminished when rangeland is degraded. These models are supported by studies on non-resilient landscapes, where loss of primary productivity also translated into loss of secondary productivity, but have not been tested on resilient landscapes. Elsewhere we showed that loss of chenopod shrubs from a landscape characterized as resilient was associated with declines in plant productivity and efficiency of conversion of rainfall into plant mass. To explore whether these differences in primary productivity translated into differences in secondary productivity, we grazed sheep at five rates of stocking for 10 years on 2000 ha of this landscape. The experiment was necessarily replicated in time not space (i.e. pseudo-replicated), which limits confident extrapolation of results to other landscapes.Productivity of sheep at all except highest rates of stocking varied little between sites where shrubs were abundant or scarce. From an animal production point of view, greater rates of stocking were unsustainable through dry years on the degraded site, but animal performance was generally unaffected on the non-degraded site, where shrubs were abundant. While these results provide evidence of economic penalties associated with degrading a resilient landscape, important ecological penalties were only partially explored.     

Replenishment episodes and crustal assimilation in the development of an early Tertiary magma chamber, East Greenland: evidence from layered cumulates of the Kælvegletscher ultramafic complex, Kangerlussuaq

Summary  From the early Tertiary K?lvegletscher ultramafic cumulate complex, emplaced into the Archaean basement on the west side of the Kangerlussuaq Fjord, East Greenland, we present geochemical and isotopic data from an outcrop of finely layered dunitic adcumulates. The dunites were deposited in a trough structure, interpreted to be a fossilized feeder channel to the K?lvegletscher magma chamber. Detailed geochemical sampling of the trough reveals subtle cryptic compositional variations of olivines (Fo = 86.5–89.3; Ni = 2000–2700 ppm) and chromites (Cr# = 66–80) in a stratigraphical profile perpendicular to the layering as well as relatively large simultaneous variations of whole-rock ⁸⁷Sr/⁸⁶Sr ratios. The dunites are separable into sequences of normal and reverse cryptic zonations which are interpreted as resulting from fractional crystallisation and magma chamber recharge, respectively. Up to 20% of magma chamber replenishment by high-Mg melts is suggested. Sr-isotope compositions of the cumulates correlate with olivine compositions and suggest mixing of fractionated and unfractionated parental melts assimilating up to 8% local basement. We propose the existence of a vigorous volcanic system at the K?lvegletscher site in early Tertiary times, where plume-derived magmas incorporated minor amounts of local basement and underwent fractionation and mixing in crustal chambers. Estimates of processed magma volumes during deposition of the trough cumulates are in the range of 100–130 km³. Received January 18, 2000;/revised version accepted December 19, 2000     

Pervasive horizontal fabric and rapid vertical extrusion: Lateral overturning and margin sub-parallel flow of deep crustal migmatites, northeastern Bohemian Massif

In the West Sudetes, northeastern Bohemia Massif, geochronometry provides evidence for repeated episodes of rapid cooling that contrasts sharply with an absence of structural evidence for significant tectonic exhumation by crustal extension. Instead, high-grade assemblages of the Orlica–Snieznik Complex have a regional sub-horizontal foliation and sub-horizontal lineations that trend parallel to narrow sub-vertical shear zones containing exhumed high-pressure assemblages. Mesoscopic petrofabrics combined with anisotropy of magnetic susceptibility (AMS) data from amphibolite facies to migmatitic meta-sedimentary and meta-igneous rocks reveal remarkably consistent average lineations that plunge shallowly to the SSW on both steep and sub-horizontal NNE-trending planar fabrics. The dominant SSW–NNE fabric orientation is parallel to the Bohemia–Brunia suture, which marks a major boundary along the eastern margin of the massif. The shape of the AMS ellipsoid is predominantly oblate, revealing flattened fabrics, with only local prolate ellipsoids. We envisage that the continental Brunian indentor operated as a rigid backstop and allowed the migmatized lower crustal orogenic root to be exhumed along the Bohemian margin shortly following terminal arc collision. Sub-vertical extrusion of the orogenic root was arrested in the mid-crust, where the lower ductile crust was laterally overturned at the base of rigid upper crustal blocks. Upon reaching the crustal high-strength lid the exhumed ductile mass of continental material laterally spread sub-parallel to the margin, underwent subsequent supra-Barrovian metamorphism, and quickly cooled. The application of AMS techniques to high-grade metamorphic rocks in concert with macroscopic structural observations is a powerful approach for resolving the deformation history of a terrane where visible rock fabrics can be tenuous.     

Sonication of bacteria, phytoplankton and zooplankton: Application to treatment of ballast water

We investigated the effect of high power ultrasound, at a frequency of 19 kHz, on the survival of bacteria, phytoplankton and zooplankton, in order to obtain estimates of effective exposure times and energy densities that could be applied to design of ultrasonic treatment systems for ballast water. Efficacy of ultrasonic treatment varied with the size of the test organism. Zooplankton required only 3-9s of exposure time and 6-19 J/mL of ultrasonic energy to realize a 90% reduction in survival. In contrast, decimal reduction times for bacteria and phytoplankton ranged from 1 to 22 min, and decimal reduction energy densities from 31 to 1240 J/mL. Our results suggest that stand-alone ultrasonic treatment systems for ballast water, operating at 19-20 kHz, may be effective for planktonic organisms >100 microm in size, but smaller planktonic organisms such as phytoplankton and bacteria will require treatment by an additional or alternative system.     

Near-source particulate emissions and plume dynamics from agricultural field operations

Particulate matter emissions generated by agricultural field preparation and harvesting operations were measured remotely via aerosol lidar and sampled simultaneously with a variety of aerosol point samplers in order to quantify dust plume space and time dynamics and particulate mass and number concentrations. Data for two cotton operations (disking, harvesting) in a flood-irrigated field in New Mexico are presented. Dust plume dynamics varied with boundary layer meteorological conditions, especially atmospheric stability, with plume maximum height significantly lower under stable conditions. Plume tracking indicated little change in plume area with height under unstable conditions and plume movement depended on wind speed and direction. Particle mass distributions indicate approximately 50% of the measured PM₁₀ mass was PM_2.5, significantly higher than previously reported values, possibly due to the near-source nature of the samples collected here. Variability in plume movement matched the variability in short-term wind fluctuations and this variability helps explain why models that utilize long-term averages perform poorly when trying to capture plume dynamics for nonsteady sources such as tractor operations.