Slab-derived boron isotope signatures in arc volcanic rocks from the Central Andes and evidence for boron isotope fractionation during progressive slab dehydration

[1] 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(11)B values (+ 4 to - 7parts per thousand) and boron concentrations ( 6 to 60 ppm). Positive delta(11)B values of samples from the volcanic front indicate involvement of a B-11-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(11)B values and boron concentrations in the lavas decrease with increasing depth of the Wadati-Benioff Zone. This across-arc variation in delta(11)B values and decreasing B/Nb ratios from the arc to the back-arc samples are attributed to the combined effects of boron-isotope fractionation during progressive dehydration in the slab and a steady decrease in slab-fluid flux toward 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. 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 boron composition of the fertile mantle and that the primary arc magmas were contaminated by an average addition of 15 to 30% crustal material. Modeling of fluid-mineral boron-isotope fractionation as a function of temperature shows that dehydration reactions liberate continuously changing fluid compositions from the slab during progressive subduction. A combination of a boron-isotope fractionation model and a temperature model for the Central Andean subduction zone fits the across-arc variation in delta(11)B and we conclude that the boron-isotope composition of arc volcanic rocks, especially in island arcs, is dominated by changing delta(11)B-composition of boron-rich slab-fluids during progressive dehydration. Owing to the decrease in slab-derived fluid flux crustal contamination becomes more important toward the back-arc. Because of the boron-isotope fractionation effect, across-arc variations in delta(11)B need not necessarily reflect different mixing proportions between boron derived from the slab-fluid and the mantle wedge.