The volcanoes of the South Sandwich island arc follow three distinct series: low-K tholeiitic (followed by ^avodovski, Candlemas, Vindication, Montagu and Bristol), tholeiitic (followed by Visokoi, Sounders and Bellinghausen) and calcalkaline (followed by Ltskov, Freezland and part of Cook and Thule). Flux calculations indicate that the percentage contribution of the subduction component to the mantle source of all three series varies from undetectable (e.g. %r) through small (e.g. Nd-20%) and moderate (e.g. La, Ce, Sr = 5(h80%) to dominant (e.g. Pb, K, Ba, Rb, Cs >90%) with little change along the arc. Isotope systematics (Pb, Nd, Sr) show that this subduction component obtains a greater contribution from altered oceanic crust than from pelagic sediment. Elements for which the subduction contribution is small show that the mantle is already depleted relative to N-MORB mantle (equivalent to loss of an ~2-5% melt fraction) before melting beneath the arc. After addition of the subduction component, dynamic melting of this depleted mantle then causes the variations in K that distinguish the three series. The estimated degree of partial melting (~20%) is slightly greater than that beneath ocean ridges, though geothermometry suggests that the primary magma temperature (~1225°C) is similar to that of primary MORB. About half of the melting may be attributed to volatile addition, and half to decompression. Dynamic melting involving three-dimensional, two-phase flow may be needed to explain fully the inter-island variations.

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From the Middle Jurassic onwards persistent igneous activity in the southern Andes around 46 °S was controlled by easterly dipping subduction along the Pacific margin. Cogenetic plutonic rocks belonging to the Patagonian batholith, and calc-alkaline volcanics ranging from basaltic andesites to rhyolitic tuffs and ignimbrites are the principal products. Erosion of the primary volcanics has led at various times to the development of thick volcaniclastic sequences, for example in the Cretaceous-Lower Tertiary Divisadero formation. The Coyhaique region marks the northerly extension of a narrow back-arc basin in which the marine Neocomian successions accumulated. Volcaniclastics from the island arc, which presumably lay to the west, are intercalated with the sediments. Although the marine basin was short-lived a mildly extensional back-arc regime may have existed through much of Mesozoic-Recent times. Widespread basalt-rhyolite volcanism on the eastern side of the cordillera seems to have been associated with this tectonic environment. Remnants of the Patagonian basalt plateau at latitude 45-47 °S extend from the Argentine-Chile frontier to Lago Colhue Huapi. Four principal age and compositional groups have been distinguished in the lavas, (i) The oldest, which are about 80 Ma, occur in sections at Senguerr and Morro Negro. They are almost exclusively tholeiitic, but show some calc-alkaline affinities and resemble in other respects basalts from marginal basins, (ii) The second group (57-43 Ma) occur in the lower part of the Chile Chico section with a compositional spread from olivine tholeiites through alkali basalts to one occurrence of a basanite. (iii) The upper part of the main plateau sequence, where the flows are in the range 25-9 Ma, are dominantly of alkali basalt composition, (iv) Post-plateau flows from small cinder cones on the surface of the plateau range in age from ca. 4 Ma to 0.2 Ma or less. They are mostly highly undersaturated basanites, with occasional leucite basanites, enriched in incompatible elements. A few of the earlier tholeiites with calc-alkali traits may have been closely associated with subduction or marginal basin processes. The younger lavas are more alkalic intraplate types generated in the remote back-arc extensional zone.

Analytical instrumentation continues its amazing evolution, especially in regard to generating ever more sensitive, faster, and reliable measurements. Perhaps the most difficult challenges are making these instruments small enough to use in the field, equipping them with well-designed software that facilitates and simplifies their use by nonexperts while preserving enough of their analytical capabilities to render them useful for a wide variety of applications. Perhaps the most impressive and underappreciated example of instruments that meet these criteria are field-portable X-ray fluorescence (XRF) analyzers. In the past, these analyzers have been routinely used for environmental applications (lead in paint and soil, metal particulates in air samples collected onto filters), geology studies (ore and soil analysis, precious metal identification), and recycling industries (alloy identification). However, their use in the analysis of toxic elements in food, food ingredients, dietary supplements, and medicinal and herbal products, especially within the FDA and regulatory environments, has been surprisingly limited to date. Although XRF will not replace atomic spectrometry techniques such as ICP-MS for sub-parts per million level analyses, it offers a number of significant advantages including minimal sample preparation, high sample throughputs, rapid and definitive identification of many toxic elements, and accurate quantitative results. As should be obvious from many recent news reports on elevated levels of toxic elements in children's lunchboxes, toys, and supplements, field-portable XRF analyzers can fill a very important niche and are becoming increasingly popular for a wide variety of elemental analysis applications. This perspective begins with a brief review of the theory of XRF to highlight the underlying principle, instrumentation, and spectra. It includes a discussion of various analytical figures of merit of XRF to illustrate its strengths and limitations compared to existing methods such as ICP-MS. It concludes with a discussion of a number of different FDA applications and case studies in which XRF has been used to screen, identify, and in some cases quantify toxic elements in various products. This work clearly demonstrates that XRF analyzers are an exceedingly valuable tool for routine and nonroutine elemental analysis investigations, both in the laboratory and in the field. In the future, it is hoped that both field-portable and laboratory-grade XRF analyzers will see more widespread use for investigational and forensic-type applications of food and other regulated consumer products.