Emerald dating through Ar/Ar step-heating and laser spot analysis of syngenetic phlogopite

Emerald, occurring in K-metasomatic rocks developed at the contact of the Carnaíba leucogranite with serpentinite (Bahia State, Brazil), has been dated using an original ⁴⁰Ar/³⁹Ar procedure. It combines step heating and spot fusion experiments on two types of phlogopite crystals: (1) bulk samples and individual grains extracted from the enclosing K-metasomatic host rocks; and (2) syngenetic solid inclusions precipitated along growing zones of the emerald host crystals. The second procedure uses in situ laser probe experiments on rock sections. In spite of the huge amounts of excess ⁴⁰Ar detected in adjacent emerald, we could measure reliable ages of 1951 ± 8 Ma and 1934 ± 8 Ma for the Trecho Velho and Braulia occurrences, respectively. Spot fusion data had higher discrepancy than the step heating data, but minute crystals of phlogopite included in emeralds bearing excess argon do not reveal excess argon. A muscovite belonging to the same granite hydrothermal complex gave a plateau age of 1976 ± 8 Ma, which may correspond to a higher closure temperature of the KAr system during the cooling of the whole pluton and associated hydrothermal halo.

These accurate measurements lead to the following conclusions: (1) direct emerald dating is possible; (2) in spite of a polyphase history during the Transamazonian orogenesis (2 Ga), combined step heating and spot fusion experiments give a better precision for granite-related emerald mineralization than the scattered ages obtained by Rb-Sr and K-Ar methods; (3) the late-Transamazonian tectonothermal retrograde event which probably caused the dispersion of previous Rb-Sr and K-Ar data is not revealed by our procedure; (4) the emerald mineralization and K-metamorphism appear to be linked with the thermal history of the leucogranite; (5) in addition to its use in polyphase crustal domains, accurate ⁴⁰Ar/³⁹Ar dating is of major interest in the field of metallogenic models, even, for instance, for mineralizations characterized by disturbed isotopic systems, which record effects as excess argon.