EMERALD DATING THROUGH AR-40 AR-39 STEP-HEATING AND LASER SPOT ANALYSIS OF SYNGENETIC PHLOGOPITE/

Emerald, occurring in K-metasomatic rocks developed at the contact of the Carnaiba leucogranite with serpentinite (Bahia State, Brazil), has been dated using an original Ar-40/Ar-39 procedure. It combines step heating and spot fusion experiments on two types of phlogopite crystal s: (1) bulk samples and individual grains extracted from the enclosing K-metasomatic host rocks; and (2) syngenetic solid inclusions precipi tated along growing zones of the emerald host crystals. The second pro cedure uses in situ laser probe experiments on rock sections. In spite of the huge amounts of excess Ar-40 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 K-Ar system during the cooling of t he whole pluton and associated hydrothermal halo. These accurate measu rements lead to the following conclusions: (1) direct emerald dating i s possible; (2) in spite of a polyphase history during the Transamazon ian orogenesis (2 Ga), combined step heating and spot fusion experimen ts give a better precision for granite-related emerald mineralization than the scattered ages obtained by Rb-Sr and K-Ar methods; (3) the la te-Transamazonian tectonothermal retrograde event which probably cause d the dispersion of previous Rb-Sr and K-Ar data is not revealed by ou r procedure; (4) the emerald mineralization and K-metamorphism appear to be linked with the thermal history of the leucogranite; (5) in addi tion to its use in polyphase crustal domains, accurate Ar-40/Ar-39 dat ing is of major interest in the field of metallogenic models, even, fo r instance, for mineralizations characterized by disturbed isotopic sy stems, which record effects as excess argon.