Resetting monazite ages during fluid-related alteration

Fluid-related coupled dissolution–reprecipitation processes can modify the composition of monazite and other orthophosphate minerals at temperatures well below diffusional closure temperatures. It is critical to evaluate the effects of alteration processes on Th–U–Pb dates, especially because incomplete removal of Pb during alteration could lead to spurious dates, either older or younger than the actual age of the monazite formation. Experimental alteration of monazite at 450 MPa and 450 °C has resulted in the production of altered domains in the vicinity of the grain rim as well as in the vicinity of inclusions and microfractures without any obvious structural control within grains. The composition of the altered domains is relatively constant from domain to domain and from grain to grain. Thorium was reduced from ca. 7 wt.% (ThO2) to ca. 2 wt.%. Uranium and Pb were almost entirely removed from the altered domains. While unaltered areas of the monazite grains as well as the original monazite grains before the experiment yield very consistent dates of ca. 350 Ma, dates from the altered domains are consistently near zero. The implication is that fluid-aided alteration of monazite, for the conditions of this experiment, has resulted in a near complete resetting of the monazite “age” at temperatures well below the “closure” temperature. Because alteration textures in monazite are relatively common in nature, careful analysis of such textures offers a means of recognizing and potentially dating fluid–rock interactions.

Research Highlights
► Monazite grains of known age and composition have been altered during an experimental run under amphibolites facies conditions.
► Altered domains have a specific composition with essentially no Pb.
► The age of altered domains has been reset to zero.
► Monazite alteration by a dissolution–reprecipitation process can reset monazite ages at temperatures well below closure temperatures.