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Geochronologic data from the southern margins of the Colorado Plateau (western United States) show an inboard radial migration of Neogene basaltic magmatism. Nd and Sr isotopic data show that as basaltic volcanism migrates inboard it also becomes increasingly more asthenospheric. Strongly asthenospheric alkali basalt (ε Nd > 4) appeared on the western plateau margin ca. 5 Ma, on the southeastern margin at 7 Ma, and is lacking from the plateau's other margins. Tomographic data suggest that low-velocity mantle underlies almost all recent (younger than 1 Ma) basaltic volcanism in a ring around much of the Colorado Plateau at a depth of 80 km. The combined isotopic and tomographic data indicate that the low-velocity mantle is asthenosphere along the western and southeastern margins of the plateau, but modifi ed lithosphere around the remaining margins. Temporal and spatial patterns suggest a process by which upwelling asthenosphere is progressively infi ltrating and replacing lithospheric mantle, especially where Proterozoic boundaries exist. This model explains (1) the dramatic velocity contrast seen well inboard of the physiographic edge of the plateau, (2) the inboard sweep of Neogene magmatism, and (3) isotopic evidence that much (but not all) of the low-velocity mantle is asthenospheric. These data support models that ongoing uplift of the edges of the Colorado Plateau is driven by mantle processes.

Analytical procedures and results: U-Pb detrital zircon geochronologyDetrital zircons were analysed at the University of Alberta Radiogenic Isotope facility. Samples were crushed and zircons were separated and concentrated by standard techniques using a Wilfley table, heavy liquids and magnetic separator. The grains were ablated using a New Wave Research UP213 Nd:YAG with aperture imaging system. The wavelength was 213 nm, with a fluence of 3 Jcm -2 , a 4 Hz pulse rate and a spot size of 40 µm. Ablated ions were analysed with a Nuplasma multicollector ICP-MS with plasma power, gas flows, detector configuration and isotope measurements as described by Simonetti et al. (2005). Results from each sample site were recorded in 30 one-second integrations after a settling time of 3 s. Blanks and standards were recorded for the same duration as unknowns.For the Southern Uplands samples, analysed first, separated zircons were picked, avoiding cracked or altered grains, and ~200 grains were mounted in an epoxy mount and polished to a depth required to approximately expose grain centres. Ablation points were selected using a combination of electron backscatter images, reflected, and transmitted light, so as to avoid obvious inclusions, discontinuities and cracks. Grains were analysed in sequences of 10 or 12, preceded and followed by at least two analyses of standards. Results were normalized using the in-house standard LH94-15 with isotopic ratios determined by thermal ionization mass-spectrometry (TIMS) as quoted by Simonetti et al. (2005), yielding a concordant age of 1830 Ma.

[1] The ''laser ablation split stream'' (LASS) technique is a powerful tool for mineral-scale isotope analyses and in particular, for concurrent determination of age and Hf isotope composition of zircon. Because LASS utilizes two independent mass spectrometers, a large range of masses can be measured during a single ablation, and thus, the same sample volume can be analyzed for multiple geochemical systems. This paper describes a simple analytical setup using a laser ablation system coupled to a singlecollector (for U-Pb age determination) and a multicollector (for Hf isotope analyses) inductively coupled plasma mass spectrometer (MC-ICPMS). The ability of the LASS for concurrent Hf 1 age technique to extract meaningful Hf isotope compositions in isotopically zoned zircon is demonstrated using zircons from two Proterozoic gneisses from northern Idaho, USA. These samples illustrate the potential problems associated with inadvertently sampling multiple age and Hf components in zircons, as well as the potential of LASS to recover meaningful Hf isotope compositions. We suggest that such inadvertent sampling of differing age and Hf components can be a significant cause of excess scatter in Hf isotope analyses and demonstrate that the LASS approach offers a robust solution to these issues. The veracity of the approach is demonstrated by accurate analyses of 10 reference zircons with well-characterized age and Hf isotopic composition, using laser spot diameters of 30 and 40 mm. In order to expand the database of high-precision Lu-Hf isotope analyses of reference zircons, we present 27 new isotope dilution-MC-ICPMS Lu-Hf isotope measurements of five U-Pb zircon standards: FC1, Temora, R33, QGNG, and 91500.

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