Large U loss during weathering of upper continental crust; the sedimentary record

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doi: 10.1016/j.chemgeo.2012.12.016
Author(s): Carpentier, Marion; Weis, Dominique; Chauvel, Catherine
Author Affiliation(s): Primary:
University of British Columbia, Department of Earth and Ocean Sciences, Vancouver, BC, Canada
Other:
Université Joseph Fourier, France
Volume Title: Chemical Geology
Source: Chemical Geology, Vol.340, p.91-104. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0009-2541 CODEN: CHGEAD
Note: In English. Includes appendices. 67 refs.; illus., incl. 2 tables, sketch map
Summary: Oceanic sediments deposited at high rate close to continents are dominated by terrigenous material. Aside from dilution by biogenic components, their chemical compositions reflect those of nearby continental masses. This study focuses on oceanic sediments coming from the juvenile Canadian Cordillera and highlights systematic differences between detritus deriving from juvenile crust and detritus from old and mature crust. We report major and trace element concentrations for 68 sediments from the northernmost part of the Cascade forearc, drilled at ODP Sites 888 and 1027. The calculated weighted averages for each site can then be used in the future to quantify the contribution of subducted sediments to Cascades volcanism. The two sites have similar compositions but Site 888, located closer to the continent, has higher sandy turbidite contents and displays higher bulk SiO2/Al2O3 with lower bulk Nb/Zr, attributed to the presence of zircons in the coarse sands. Comparison with published data for other oceanic sedimentary piles demonstrates the existence of systematic differences between modern sediments deriving from juvenile terranes (juvenile sediments) and modern sediments derived from mature continental areas (cratonic sediments). The most striking systematic difference is for Th/Nb, Th/U, Nb/U and Th/Rb ratios: juvenile sediments have much lower ratios than cratonic sediments. The small enrichment of Th over Nb in cratonic sediments may be explained by intracrustal magmatic and metamorphic differentiation processes. In contrast, their elevated Th/U and Nb/U ratios (average values of 6.87 and 7.95, respectively) in comparison to juvenile sediments (Th/U∼3.09, Nb/U∼5.15) suggest extensive U and Rb losses on old cratons. Uranium and Rb losses are attributed to long-term leaching by rain and river water during exposure of the continental crust at the surface. Over geological times, the weathering effects create a slow but systematic increase of Th/U with exposure time. Abstract Copyright (2013) Elsevier, B.V.
Year of Publication: 2013
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 06 Petrology, Sedimentary; Actinides; Alkali metals; Basins; British Columbia; Canada; Cascadia Basin; Cascadia subduction zone; Chemical composition; Chemical ratios; Clastic sediments; Coast plutonic complex; Continental crust; Crust; East Pacific; Fore-arc basins; Geochemistry; ICP mass spectra; Leg 146; Major elements; Marine sediments; Mass spectra; Metals; Niobium; North America; North Pacific; Northeast Pacific; ODP Site 1027; ODP Site 888; Ocean Drilling Program; Pacific Ocean; Provenance; Rubidium; Sediments; Spectra; Thorium; Trace elements; Turbidite; Upper crust; Uranium; Weathering; Western Canada
Coordinates: N480959 N481001 W1263943 W1263948
N474524 N474524 W1274351 W1274351
Record ID: 2013051937
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands