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

Abstract

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 δ11B values (+4 to −7 ‰) and boron concentrations (6 to 60 ppm). Positive δ11B 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 δ11B values and boron concentrations in the lavas decrease with increasing depth of the Wadati‐Benioff Zone. This across‐arc variation in δ11B 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 δ11B and we conclude that the boron‐isotope composition of arc volcanic rocks, especially in island arcs, is dominated by changing δ11B‐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 δ11B need not necessarily reflect different mixing proportions between boron derived from the slab‐fluid and the mantle wedge.