BORON AND BERYLLIUM MINERAL DIVERSITY AND BORON ISOTOPES: WHAT CAN THEY TELL US ABOUT RECYCLING OF EARTH’S CRUST AND CRUSTAL EVOLUTION?

Boron and beryllium are rare in the Cosmos because their nuclei are fragile, and thus scarce in Earth’s primitive mantle (0.19-0.26 µg/g B, 0.062 µg/g Be), yet both are potential tracers for the growth of continental crust. Despite their different geochemical behavior, B and Be were both concentrated by the processes that led to the formation of continental crust, probably because both elements are mobilized by fluids with high SiO₂, Cl or F. With the evolution of the continental crust, the possibility for B and Be to be concentrated to levels well above the 17 µg/g B and 2.1 µg/g Be upper crustal averages also increases, resulting in more opportunities for mineralogical diversity. Increase in diversity with geologic time can be illustrated either with curves showing the cumulative increase in the total number of species inferred to have formed by a given time in Earth’s history or with histograms showing the variation in the number of species inferred to have been present during 50 Myr intervals. Comparing the proportion of exposed continental crust at a given geologic period with cumulative diversity suggests at first that mineral diversity simply reflects exposed area. A closer look reveals that the increase in cumulative diversity is punctuated by steps of which the three oldest correspond to steps in the formation of juvenile crust and to the collisional phases of the supercontinent cycles of Kenorland, Nuna, and Rodinia. Li-Cs-Ta pegmatites, typical for settings of crustal thickening associated with subduction and continental collision, host a variety of B and Be minerals. The histograms show spikes at 2.7-2.5 Ga and 1.85 Ga followed by relative leveling off in diversity from 1.85 to 0.55 Ga, resulting in a diversity trend through the Precambrian resembling more the curve for growth of crust with recycling. A model relating growth of continental crust to seawater δ¹¹B (proportion of the heavier ¹¹B isotope) suggests that seawater δ¹¹B approached the present value of +40‰ by the time precursors of the Central Metasedimentary Belt (Grenville Province) were deposited at ca. 1.25 Ga. Boron isotopic data on metamorphic tourmaline from marble and metapelite in this belt (Quebec, Ontario, New York) are consistent with a scenario involving marine incursion into rift segments resulting from back-arc spreading at a continental margin.