Boron geochemistry of the lower crust: Evidence from granulite terranes and deep crustal xenoliths

Abstract

Boron contents are uniformly low in more than 100 granulites from exposed terranes in India, Norway, and Scotland and from xenolith suites in the western USA. Averages for the terranes (2.5 ppm) and for xenoliths (1.2 ppm) suggest maximum B contents of about 2 ± 1 ppm for the lower crust. Abundance distributions from exposed terranes are skewed to higher values as B has been added to some samples via retrograde fluids during decompression. The samples studied include mafic to felsic lithologies of both igneous and sedimentary origin. There is no correlation of B content with bulk composition or with protolith type in any of the suites studied. Boron is apparently depleted in all granulite protoliths during prograde metamorphism and dehydration. Similar depletions of B and other fluid-mobile elements (e.g., U, Cs) with respect to rare-earth elements (REE) Zr, Ba, Rb and K 2O are seemingly inconsistent with origin of granulites primarily via extraction of silicate melts. These distinctive geochemical features are attributed to selective element transport in fluids released by devolatilization reactions. Anatexis is not necessarily precluded, but dehydration of the subsolidus protolith normally would precede melting and lead to depletion of the fluid-mobile elements. Compositions of any melts and restites eventually formed would reflect the effects of this antecedent process. The systematic depletion of B (and Cs) in granulites contrasts with the highly variable contents of most other incompatible elements determined. For this reason, the estimated lower crust abundance for B is relatively well constrained, whereas abundance estimates for many other elements are model dependent, have large uncertainties and are unlikely to be globally representative.