Complete recycling of a magmatic arc evidence fro

Authors: Friedrich Lucassen Michael Wiedicke Gerhard Franz

Publish Date: 2009/01/10

Volume: 99, Issue: 3, Pages: 687-701

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Abstract

Marine Quaternary trench and slope sediments were sampled along the margin of the Southern Andes Chile between 36° and 40°S Major and trace element contents indicate only minor influence of weathering and transport fractionation The whole rock composition of the sediments is similar to the average rock of the Cretaceous to Holocene magmatic arc of this section of the southern volcanic zone Sr Nd and Pb isotope composition of the sediments also resembles closely the average composition of the magmatic arc The contribution of compositionally distinct Palaeozoic crust which makes up most of the volume of the forearc is ~0–20 crustal Sr Nd and Pb according to the isotope record of the trench and slope sediments Input of sediments from the continent into the subduction system was dominated by detritus from the magmatic arc at least for the last 20 My on the basis of the Oligocene to Holocene exhumation history of the marginWe thank Maren Lewerenz for performing XRF analyses at the Technische Universität Berlin Peter Dulski and Birgit Zander for ICPMS analyses at GFZPotsdam Cathrin Schulz for support in the GFZ radiogenic isotope laboratory Rosemarie Geffe at TUBerlin for polishing the Figures We thank Rolf L Romer for reading a previous draft of the manuscript and Christoph Breitkreuz and Diego Morata for their journal reviews which all improved the manuscript This study was funded by DFG Deutsche Forschungsgemeinschaft in the frame of Sonderforschungsbereich 267 ‘Deformation processes in the Andes’The initial sample size was ca 50 g which is representative even for the sand fraction Semisolidified rocks from the dredge samples were carefully disaggregated in a PEbag using a hammer All samples where transferred into Teflon beakers washed several times in the ultrasonic in ultra clean water and dried The water was preserved with few drops HCl added The dried samples were ground in an agate mill Major elements were determined by XRF at TUBerlin Trace elements were determined by ICPMS at GFZPotsdam for the procedure and reproducibility of international standards see Dulski 2001For Nd Pb and Sr isotope analyses by TIMS at GFZ Potsdam ~200 mg of sample were weighted into Savillex beakers Prior to dissolution in HF–HNO3 mixture the samples were treated with warm 1 N HCl followed by washing in ultra clean water in order to remove possible organic carbonates All isotope ratios were determined using TIMS Nd and Sr isotope ratios were measured using dynamic multicollection on a MAT 262 mass spectrometer and on a VG 54 Sector mass spectrometer at the GFZPotsdam respectively Nd isotopic ratios were normalized to 146Nd/144Nd = 07219 Sr isotopic ratios to 86Sr/88Sr = 01194 During the analytical work at GFZPotsdam the NBS 987 Sr standard yielded 87Sr/86Sr = 0710265 ± 28 2σ n = 47 and the La Jolla Nd standard yielded 143Nd/144Nd = 0511850 ± 8 2σ n = 46 Pb isotope ratios were measured at GFZPotsdam using static multicollection on a MAT 262 mass spectrometer at controlled temperatures between 1220 and 1250°C The 2σ reproducibility of all Pb isotope ratios of the NBS SRM 981 standard measured ratios uncorrected for fractionation 206Pb/204Pb = 16906 ± 10 207Pb/204Pb = 14454 ± 10 208Pb/204Pb = 36583 ± 20 n = 28 is better than 01 and a 2σ error of 01 is assumed for the measured samples considering the uncertainty of correction for mass fractionation Instrumental massfractionation has been corrected using 01 per amu atomic mass unit on the base of the NBS SRM 981 values Procedural blanks were 30 pg for Pb 50 pg Nd and 100 pg for Sr No blank corrections have been applied to the measured ratios because blank contribution was insignificant in comparison to the amount of the respective elements in the sample

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