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We use published and new trace element data to identify element ratios which discriminate between arc magmas from the supra-subduction zone mantle wedge and those formed by direct melting of subducted crust (i.e. adakites). The clearest distinction is obtained with those element ratios which are strongly fractionated during refertilisation of the depleted mantle wedge, ultimately reflecting slab dehydration. Hence, adakites have significantly lower Pb/Nd and B/Be but higher Nb/ Ta than typical arc magmas and continental crust as a whole. Although Li and Be are also overenriched in continental crust, behaviour of Li/Yb and Be/Nd is more complex and these ratios do not provide unique signatures of slab melting. Archaean tonalite-trondhjemite-granodiorites (TTGs) strongly resemble ordinary mantle wedge-derived arc magmas in terms of fluidmobile trace element content, implying that they did not form by slab melting but that they originated from mantle which was hydrated and enriched in elements lost from slabs during prograde dehydration. We suggest that Archaean TTGs formed by extensive fractional crystallisation from a mafic precursor. It is widely claimed that the time between the creation and subduction of oceanic lithosphere was significantly shorter in the Archaean (i.e. 20 Ma) than it is today. This difference was seen as an attractive explanation for the presumed preponderance of adakitic magmas during the first half of Earth's history. However, when we consider the effects of a higher potential mantle temperature on the thickness of oceanic crust, it follows that the mean age of oceanic lithosphere has remained virtually constant. Formation of adakites has therefore always depended on local plate geometry and not on potential mantle temperature.

A one-step preconcentration method using two columns in series was used to partition trace metals in natural waters into several operationally defined fractions. The water passed through a Chelex-100 cation exchange column to collect dissolved labile trace metals and then through an AG MP-1 macroporous resin column to collect trace metals complexed with natural organic matter. A third fraction (inert) was obtained from the difference between total dissolved trace metal concentrations, determined after IN irradiation of a separate aliquot, and the sum of the fractions retained on the two columns. This analytical scheme was successfully applied to the determination of Al, Cd, Co, Cu, Fe, Mn, Ni, Pb, V, and Zn in river waters collected from across the state of Texas. Trace metal partitioning between fractions was found to be stable for at least 8 days after the addition of ammonium acetate buffer (pH 5.5). In most samples collected from relatively pristine sections of Texas rivers, the sum of fractions retained on the two columns accounted for better than 80% of the total dissolved trace metal concentrations. In rivers where anthropogenic inputs were more evident (San Antonio and Trinity Rivers), the inert fraction became pronounced, in some cases accounting for up to 95% of the total dissolved concentrations. The preconcentrate is free of matrix interference and can be easily analyzed by atomic absorption spectrometer or inductively coupled plasma mass spectrometer. Using a 1-L sample allows preconcentration factors of 150 or higher for both labile and organic fractions, and therefore, trace metal concentrations and speciation in natural waters at ambient levels can be accurately and precisely determined.

Variability in responses to trace elements and nutrients and transmission of stressor effects have been studied in an estuarine food web. Multiple stressors may increase or may dampen the temporal and spatial variability in aquatic systems, dependent on interactions among stressor and influences of background conditions and sensitive species on stressor effect expression. Experiments were carried out four times spring-autumn, 1996, in 20 1-m3 mesocosms. A range of population-, system-, and individual-level parameters were evaluated to measure responses of various organisms. The experiment was intended to deal with part of the overall objective of the Complexity and Stressors in Estuarine Systems (COASTES) project, focused on understanding how multiple stressors affect estuarine environments and how the effects are modified by complexity of the system.

Unusual amounts of trace elements were shown by ion probe investigations of Al-Di pyroxenites from the Zabargad peridotite of Italy. This characteristic, not exhibited by pyroxenes in spinel websterite, indicates the presence of a precursor or mantle garnet and emphasizes the importance of metamorphic processes in the redistribution of trace elements through mantle upwelling. Thestudy also shows that the Zabargad peridotite had been formed prior to lithospheric rifting and thinning of the area.

A workshop on metal speciation was held in conjuction with the 23rd Annual International Symposium on Environmental Analytical Chemistry held at Jekyll Island, GA, in Jun 1993. The workshop examined the present knowledge on metal speciation, local differences in speciation and regulatory approaches for managing present problems. The discussions focused on analytical techniques, biomonitoring methods, modeling studies, bioavailability and sediment management.