Gillespie, Jack1, Kinny, Pete1, Martin, Laure2, Kirkland, Christopher1, Nemchin, Alexander1, Cavosie, Aaron1
1The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, Perth, WA 6845, Australia, 2Center for Microscopy, Characterisation and Analysis (CMCA), University of Western Australia, 35 Stirling Highway, Perth, WA 6009, Australia
Rb-Sr isotopes in geological materials provide a system for tracing crustal differentiation processes and insights into the evolution of planetary bodies. The ingrowth of radiogenic 87Sr from the decay of 87Rb leads to increased 87Sr/86Sr over time, and due to the strong relationship between the Rb/Sr and SiO2 contents of igneous rocks, this provides a time-integrated window into the evolution of geochemical reservoirs. However, the high geological mobility of both Rb and Sr in whole rocks during metamorphism and fluid alteration means that this record becomes progressively less reliable in older rocks that have experienced post-crystallization geological events.
Strontium is easily substituted into the crystal lattice of apatite, occurring as a trace element in concentrations ranging from less than a hundred parts per million to several weight percent. In contrast, apatite nearly entirely excludes Rb (<1 ppm) resulting in negligible radiogenic ingrowth of 87Sr, and consequently the initial 87Sr/86Sr ratio of the melt from which an apatite crystallizes is faithfully recorded by the mineral. Inclusions of apatite within magmatic zircons are particularly valuable as they are armoured by the more robust host mineral, allowing them to survive subsequent events that might otherwise cause isotopic reset or recrystallization. However, the typically very small size of apatite inclusions in zircon and the complex isobaric interferences on the isotopes of Sr during in-situ mass spectrometry have previously limited the information that can be obtained from this archive.
Our recent development of a method to measure the 87Sr/86Sr ratio in apatite by SIMS with a spot size appropriate for accessing typical mineral inclusions in zircon (<15 µm) makes it possible to routinely analyse the commonly occurring inclusions of apatite in zircon. We have applied this method to determine the initial 87Sr/86Sr ratios of various Eo-Meso Archean igneous rocks by analysing the Sr isotope composition of apatite inclusions. High resolution SEM imaging and EPMA analysis illustrate the primary nature of these inclusions. Combining the measured 87Sr/86Sr of apatite inclusions with the U-Pb age and Hf isotopic composition of the co-genetic zircon host allows for the ‘triangulation’ of the Rb/Sr necessary for the ingrowth of radiogenic strontium over the crustal residence interval calculated from the crystallization and Hf model ages. Examples from SW Greenland and the Narryer Gneiss Terrane of Western Australia suggest that these rocks were derived from the melting of ancient crustal material that was on average of intermediate-felsic rather than mafic composition.
Jack Gillespie is a post-doctoral researcher at Curtin University working on developing new methods for understanding the evolution of the early earth