In situ U-Pb apatite ages combined with Sr-Sr and Sm-Nd for petrochronology

AIR-G is currently improving new methods to obtain in situ isotope information of apatite.

The mineral apatite is a key mineral for thermochronology studies via U/Th–He, fission-track and U-Pb dating. It may contain tens to thousands of ppm of U, and it is present in a wide range of rock types, being particularly common in syenitic/granitic magmatic rocks and metamorphic/hydrothermal rocks (e.g., Cherniak et al., 1991; Corfu and Stone, 1998; Sha and Chappel, 1999; Chamberlain and Bowring, 2000; Poitrasson et al., 2002; Pan and Fleet, 2002; Cherniak, 2005). It is often stable at amphibolite-facies metamorphic conditions, and, because of its lower closure temperature for Pb diffusion relative to zircon, it is an ideal mineral to date the cooling histories of metamorphic process and magmatic rocks (e.g., Schoene and Bowring, 2007; Henrichs et al., 2018). Contrary to other metamorphic U-bearing minerals (e.g., monazite), apatite seems to be essentially inert to recrystallization during metamorphism, making the U-Pb systematic largely a function of Pb diffusion (Willigers et al. 2002; Schoene and Bowring, 2007; Cochrane et al., 2014); however, in some cases, apatite is affected by dissolution and regrowth (Kirkland et al., 2018). Thus, once details of the grain growth/alteration history are well characterized, apatite may offer important insights into the timing of metamorphism, hydrothermal fluid crystallization and/or the cooling history of magmatic and metamorphic systems (e.g., Chew et al., 2014; Yang et al., 2014; Chew and Spikings, 2015; Kirkland et al., 2017; Mark et al., 2016; Weisberg et al., 2018).

Apatite is also an ideal mineral for petrochronology. It accommodates variable concentrations of the REE and isotope pairs such as Sm-Nd and Sr-Sr (e.g., Bizzarro et al., 2003; Schmidberger et al., 2003; Yang et al., 2009; Foster and Vance 2006; Foster and Carter, 2007, Wu et al., 2010), and its chemistry has significant implications for understanding the trace element/isotopic evolution of magmatic reservoirs (Belousova et al., 2001, 2002; Chu et al., 2009; Cao et al., 2012; Mao et al., 2016). Some trace element partition coefficients in apatite are sensitive to changes in magma composition, meaning that this phosphate mineral can yield information about different petrogenetic processes that are invisible at the whole-rock scale. A recent compilation by Bruand et al. (2017) demonstrated that in situ analyses of matrix apatite and apatite inclusions can be used to reliably estimate the original magma composition, providing a new way to look into magmatic petrogenesis. For instance, the concentrations of F, Mn, Sr, and rare earth elements in apatite can reflect the composition of the host magma and thus have high potential as petrogenetic tracers (Chu et al., 2009). Apatite is also sensitive to fluid-induced metasomatism and, given a well-constrained U-Pb age, its trace element chemistry and isotope composition make powerful tools for tracking fluid-rock interactions processes in the lower, upper crust, and possibly the mantle.

Apatite can be directly dated in thin sections, via LA ICP MS. Together with the U-Pb ages, in situ Sm-Nd and Sr Sr can be directly obtained from the apatite themselves or other minerals in the thin secitons such as plagioclase.

This method is ideal for investigating the magmatic evolution of mafic rocks.