Boron Isotopes in the Continental Crust: Granites, Pegmatites, Felsic Volcanic Rocks, and Related Ore Deposits

Boron is an incompatible lithophile element that is readily transported by granitic melts and hydrous fluids and therefore is concentrated in the continental crust relative to the mantle. The isotopic composition of boron in crystalline rocks of the continental crust (e.g., metamorphic and igneous lithologies) varies over a wide range of −20 to +10‰, depending on the B-isotope composition of the protoliths and on fractionation effects caused by phase transitions (metamorphic devolatilization reactions, fluid exsolution from magmas). Studies of progressive metamorphism and anatexis show that the behavior of boron and its isotopes depends heavily on the presence or absence of B-retentive minerals like tourmaline. In general, boron is prone to loss during devolatilization reactions, and metamorphic fluid preferentially removes the heavier isotope, but growth of tourmaline can minimize or prevent these effects. A new compilation of over 250 B-isotope analyses from about 90 localities of felsic igneous rocks in the continental crust shows a first-order distinction in composition between I-type magmas (subduction-related having meta-igneous sources) and S-type magmas (derived from metasedimentary rocks). Boron in I-type magmas is isotopically heavy (mean δ11B = −2‰, s.d. = 5) relative to unaltered MORB (mean δ11B = −7‰, s.d. = 1), presumably because of a greater contribution by subducted oceanic crust and pelagic sediments. Boron in S-type granitic rocks has a much lighter isotopic signature (mean δ11B = −11‰, s.d. = 4). The latter corresponds to the commonly cited B-isotope value of −10‰ for continental crust, but because much of Earth’s crust is derived from I-type magmas, its average B-isotope value is probably higher than previously thought. The dichotomy of B-isotope compositions in I- and S-type granitoids is also observed in their genetically related magmatic-hydrothermal ore deposits, as we demonstrate in a review of data from porphyry and Iron Oxide-Copper-Gold (IOCG) systems (I-type) and from Sn-W veins and granitic pegmatites (S-type). However, it is important to note that in all of these systems, there are significant and locally complex effects of isotopic fractionation due to magmatic fluid exsolution and to mixing of boron sourced from externally derived fluids.