Boron and arsenic in highly evolved Archean felsic rocks: Implications for Archean subduction processes

Abstract

The importance of subduction zone magmatism for Archean crustal evolution is highly debated, particularly with regard to the direct melting of subducted slabs. The debate centers largely around trace element geochemical observations. Among the fluid mobile elements, boron is widely accepted to be one of the most sensitive tracers of subduction zone processes. In modern subduction zone magmas, the B/Be ratio can be used to constrain supra-subduction zone mantle metasomatism. We present the first systematic database for concentrations of fluid mobile elements (Li, Be, B, and As) in 60 Archean granitoids spanning ∼ 3.8 to ∼ 2.5 Ga in age. The behavior of the very fluid mobile B and As relative to the similarly incompatible Be and Pr, respectively, demonstrates that two end-member types of Archean felsic magmas existed. Those with low fluid mobile element abundances have the steepest La/Yb, lowest W/Th and highest Nb/Ta and Sc concentrations, features compatible with an origin as direct slab melts that ascended through the mantle. On the other end of the spectrum are TTG with strong B and As enrichment, which in all regards other than their high La/Yb cannot be distinguished from modern arc melts. In between these extremes are TTG that share some characteristics with each end-member. These could have formed by melting of underplated basaltic eclogite at the base of the crust, in fluid-mobile element poor mantle above (hot) subduction zones where B and As were lost at shallow depth, or they could represent differentiates from very hydrous basaltic precursors in which garnet and amphibole replace olivine and pyroxene as the earliest crystallizing phases. The diversity of geochemical signatures of Archean granitoids suggests that they have formed in a wide range of geodynamic environments.