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Boron as a tracer for material transfer in subduction zones

Urheber*innen

Rosner,  Martin Siegfried
Scientific Technical Report STR, Deutsches GeoForschungsZentrum;
4.2 Inorganic and Isotope Geochemistry, 4.0 Chemistry and Material Cycles, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;

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Zitation

Rosner, M. S. (2004): Boron as a tracer for material transfer in subduction zones, PhD Thesis, (Scientific Technical Report STR ; 04/09), Potsdam : Deutsches GeoForschungsZentrum GFZ, 104 S.  p.
URN: http://nbn-resolving.de/urn/resolver.pl?urn=urn:nbn:de:kobv:b103-04091


Zitierlink: https://gfzpublic.gfz-potsdam.de/pubman/item/item_8638
Zusammenfassung
Late Miocene to Quaternary volcanic rocks from the frontal arc to the back-arc region of the Central Volcanic Zone in the Andes show a wide range of delta 11B values (+4 to -7 ‰) and boron concentrations (6 to 60 ppm). Positive delta 11B values of samples from the volcanic front indicate involvement of a 11B-enriched slab component, most likely derived from altered oceanic crust, despite the thick Andean continental lithosphere, and rule out a pure crust-mantle origin for these lavas. The delta 11B values and B concentrations in the lavas decrease systematically with increasing depth of the Wadati-Benioff Zone. This across-arc variation in delta 11B values and decreasing B/Nb ratios from the arc to the back-arc samples are attributed to the combined effects of B-isotope fractionation during progressive dehydration in the slab and a steady decrease in slab-fluid flux towards the back arc, coupled with a relatively constant degree of crustal contamination as indicated by similar Sr, Nd and Pb isotope ratios in all samples. Modelling of fluid-mineral B-isotope fractionation as a function of temperature fits the across-arc variation in delta 11B and we conclude that the B-isotope composition of arc volcanics is dominated by changing delta 11B composition of B-rich slab-fluids during progressive dehydration. Crustal contamination becomes more important towards the back-arc due to the decrease in slab-derived fluid flux. Because of this isotope fractionation effect, high delta 11B signatures in volcanic arcs need not necessarily reflect differences in the initial composition of the subducting slab. Three-component mixing calculations for slab-derived fluid, the mantle wedge and the continental crust based on B, Sr and Nd isotope data indicate that the slab-fluid component dominates the B composition of the fertile mantle and that the primary arc magmas were contaminated by an average addition of 15 to 30 % crustal material.