Subduction controls of Hf and Nd isotopes in lavas of the Aleutian island arc

The Hf and Nd isotopic compositions of 71 Quaternary lavas collected from locations along the full length of the Aleutian island arc are used to constrain the sources of Aleutian magmas and to provide insight into the geochemical behavior of Nd and Hf and related elements in the Aleutian subduction-magmatic system. Isotopic compositions of Aleutian lavas fall approximately at the center of, and form a trend parallel to, the terrestrial Hf-Nd isotopic array with {var_epsilon}{sub Hf} of +12.0 to +15.5 and {var_epsilon}{sub Nd} of +6.5 to +10.5. Basalts, andesites, and dacites within volcanic centers or in nearby volcanoes generally all have similar isotopic compositions, indicating that there is little measurable effect of crustal or other lithospheric assimilation within the volcanic plumbing systems of Aleutian volcanoes. Hafnium isotopic compositions have a clear pattern of along-arc increase that is continuous from the eastern-most locations near Cold Bay to Piip Seamount in the western-most part of the arc. This pattern is interpreted to reflect a westward decrease in the subducted sediment component present in Aleutian lavas, reflecting progressively lower rates of subduction westward as well as decreasing availability of trench sediment. Binary bulk mixing models (sediment + peridotite) demonstrate that 1-2% of the Hf in Aleutian lavas is derived from subducted sediment, indicating that Hf is mobilized out of the subducted sediment with an efficiency that is similar to that of Sr, Pb and Nd. Low published solubility for Hf and Nd in aqueous subduction fluids lead us to conclude that these elements are mobilized out of the subducted component and transferred to the mantle wedge as bulk sediment or as a silicate melt. Neodymium isotopes also generally increase from east to west, but the pattern is absent in the eastern third of the arc, where the sediment flux is high and increases from east to west, due to the presence of abundant terrigenous sediment in the trench east of the Amlia Fracture Zone, which is being subducting beneath the arc at Seguam Island. Mixing trends between mantle wedge and sediment end members become flatter in Hf-Nd isotope space at locations further west along the arc, indicating that the sediment end member in the west has either higher Nd/Hf or is more radiogenic in Hf compared to Nd. This pattern is interpreted to reflect an increase in pelagic clay relative to the terrigenous subducted sedimentary component westward along the arc. Results of this study imply that Hf does not behave as a conservative element in the Aleutian subduction system, as has been proposed for some other arcs

University-industry relationships and open innovation: Towards a research agenda

Accepted versio

Schmidt-hammer exposure ages from periglacial patterned ground (sorted circles) in Jotunheimen, Norway, and their interpretative problems

© 2016 Swedish Society for Anthropology and Geography Periglacial patterned ground (sorted circles and polygons) along an altitudinal profile at Juvflya in central Jotunheimen, southern Norway, is investigated using Schmidt-hammer exposure-age dating (SHD). The patterned ground surfaces exhibit R-value distributions with platycurtic modes, broad plateaus, narrow tails, and a negative skew. Sample sites located between 1500 and 1925 m a.s.l. indicate a distinct altitudinal gradient of increasing mean R-values towards higher altitudes interpreted as a chronological function. An established regional SHD calibration curve for Jotunheimen yielded mean boulder exposure ages in the range 6910 ± 510 to 8240 ± 495 years ago. These SHD ages are indicative of the timing of patterned ground formation, representing minimum ages for active boulder upfreezing and maximum ages for the stabilization of boulders in the encircling gutters. Despite uncertainties associated with the calibration curve and the age distribution of the boulders, the early-Holocene age of the patterned ground surfaces, the apparent cessation of major activity during the Holocene Thermal Maximum (HTM) and continuing lack of late-Holocene activity clarify existing understanding of the process dynamics and palaeoclimatic significance of large-scale sorted patterned ground as an indicator of a permafrost environment. The interpretation of SHD ages from patterned ground surfaces remains challenging, however, owing to their diachronous nature, the potential for a complex history of formation, and the influence of local, non-climatic factors

Timing of colonoscopy after resection for colorectal cancer: Are we looking too soon?

BACKGROUND: Based on current National Comprehensive Cancer Network guidelines, colonoscopic surveillance after colorectal cancer resection should begin at 1 year

The Paleogene Intertropical Convergence Zone (abstract of paper presented at AGU Fall Meeting, San Francisco, 6-10 Dec 2002)

Latitudinal transects of eolian deposition can help define patterns of equatorial zonal winds during Paleogene warm periods and their movement in response to global climatic transitions of the Cenozoic. We examined geochemical and mineralogical data from a number of piston cores and ODP drill sites in the tropical and subtropical Pacific Ocean. In the modern Pacific, dust deposited beneath the northeast trade winds reflects Asian provenance and is likely transported back west through the trade wind system via input from the mid-latitude westerly winds. The amount of dust supplied from American source regions is an order of magnitude lower and has limited influence on the dust record in the offshore pelagic realm of the Pacific. The inter-tropical convergence zone (ITCZ) forms an effective barrier to inter-hemispheric dust transport and marks the southern boundary of the Asian dust component. Just south of the ITCZ, dust is transported by southern trade winds predominantly from andesitic source regions of Central and South America.During warm periods of the early Paleogene, andesitic sources appear to dominate eolian deposition throughout the central and equatorial Pacific. Two hypotheses are offered to explain this observation. First, increased andesitic input is associated with trade wind transport from a North American provenance because Asian dust flux, which overwhelms this component in the modern Pacific, was significantly reduced at this time. Mineralogical data collected by light-absorption spectroscopy techniques on bulk sediments recovered during ODP Leg 199 supports this scenario showing increased illite/smectite ratios during the late Cenozoic near the time when Asian dust flux increased. Alternatively, the increased andesitic component during the Paleogene may reflect eolian deposition beneath the southern trade winds with the position of the ITCZ at a latitude as far north as perhaps 25°N. Data from clay mineralogy (from XRD), elemental geochemistry and Nd isotopic ratios from piston cores EW9709-01 and LL44-GPC3 support this hypothesis and show a transition to Asian-like dust provenance occurring by the early Miocene