Inherited [formula omitted] isotope components preserved in monazite inclusions within garnets in leucogneiss from East Antarctica and implications for closure temperature studies

Abstract

Application of the SmNd dating technique using garnet can be hampered by the presence of minor microscopic to submicroscopic REE-rich inclusions, in particular monazite and, to a lesser extent, zircon. An acid leaching method using a mixture of HNO 3 + HCl has been used to tackle the monazite contamination problem. With this leaching technique, we demonstrate that inherited monazite inclusions have been preserved in garnet formed under high-grade metamorphic conditions. Even very low levels of monazite contamination ( > 0.001 wt%) have a profound effect on the SmNd system in the garnet. Analysis of a garnet from a leucogneiss dyke in the northern Prince Charles Mountains, eastern Antarctica, reveals that unleached garnet concentrates and the monazite inclusions (as represented by the leachates) define an apparent isochron of 1150 ± 30 Ma (MSWD = 1.88), which represents a mixing age of inclusions preserving their inherited SmNd isotopic systematics. In contrast, analyses of the leached garnet concentrates yield much younger ages of 630 to 555 Ma. Model calculations show that the presence of as little as 0.01 wt% of monazite inclusions would swamp the SmNd isotope signature of the garnet concentrates. This very low level of contamination is beyond the reach of physical purification processes and, without leaching, contamination is inevitable in inclusion-bearing garnets. Such contamination can lead to older apparent ages than those of garnet formation, and thus has profound implications for SmNd, as well as UPb, garnet chronometry.