Factors involved in discordant ages of euxenite from a Grenville pegmatite

Metamict euxenite crystals from a pegmatite in Grenville rocks near St. Pierre de Wakefield, Quebec, gave the following ages: Pb 2O7 /Pb 2O6 1000 m.y., Pb 2O7 /U 235 710 m.y., Pb 2O6 /U 238 620 m.y., and Pb 208 / Th 232 550 m.y. Age of coeval microcline from this pegmatite by Rb/Sr is 1050+ or -50 m.y. Total Pb, U and Th analyses and Pb isotope analyses were made of different parts of the euxenite crystals and of the minerals enveloping them. Results of this work suggest that discordance in ages is due primarily to the following factors : 1) differential loss of U, relative to Th from the outer zones of the euxenite, 2) over-all loss of Pb from the euxenite with some evidence of greater loss of Pb 2O8 than of Pb 2O7 and Pb 2O6 . Experiments on driving off Pb from metamict uranothorite and crystalline thorianite, both of Grenville age, by heating under vacuum, showed that Pb from the metamict mineral is easily driven off but that from the crystalline mineral is much more difficult to move.