2. Phase Relations, Reaction Sequences and Petrochronology

Abstract

At the core of petrochronology is the relationship between geochronology and the 22 petrological evolution of major mineral assemblages. The focus of this chapter is on outlining 23 some of the available strategies to link inferred reaction sequences and microstructures in 24 metamorphic rocks to the ages obtained from geochronology of accessory minerals and datable 25 major minerals. Reaction sequences and mineral assemblages in metamorphic rocks are 26 primarily a function of pressure (P), temperature (T) and bulk composition (X). Several of the 27 major rock-forming minerals are particularly sensitive to changes in P-T (e.g., garnet, staurolite, 28 biotite, plagioclase), but their direct geochronology is challenging and in many cases not 29 currently possible. One exception is garnet, which can be dated using Sm-Nd and Lu-Hf 30 geochronology (e.g., Baxter et al. 2013). Accessory mineral chronometers such as zircon, 31 monazite, xenotime, titanite and rutile are stable over a relatively wide range of P-T conditions 32 and can incorporate enough U and/or Th to be dated using U-Th-Pb geochronology. Therefore, 33Published in Reviews in Mineralogy, Yakymchuk, C., Clark, C., & White, R. W. (2017). Phase Relations, Reaction Sequences and Petrochronology. Reviews in Mineralogy and Geochemistry, 83(1), 13-53. https://doi.org/10.2138/rmg.2017.83.2 2 linking the growth of P-T sensitive major minerals to accessory and/or major mineral 34 chronometers is essential for determining a metamorphic P-T-t history, which is itself critical 35 for understanding metamorphic rocks and the geodynamic processes that produce them (e.g., 36England and McClelland and Lapen 2013;Brown, 2014). 37Linking the ages obtained from accessory and major minerals with the growth and 38 breakdown of the important P-T sensitive minerals requires an understanding of the 39 metamorphic reaction sequences for a particular bulk rock composition along a well-constrained 40 P-T evolution. Fortunately, the phase relations and reaction sequences for the most widely 41 studied metamorphic protoliths (e.g., pelites, greywackes, basalts) can be determined using 42 quantitative phase equilibria forward modelling (e.g., Powell and Holland 2008). Comprehensive 43 activity-composition models of the major metamorphic minerals in large chemical systems (e.g., 44White et al. 2014a) allow the calculation of phase proportions and compositions for a given rock 45 composition along a metamorphic P-T path. For accessory minerals, subsolidus growth and 46 breakdown can be modelled in some cases using phase equilibria modelling (e.g., Spear 2010; 47 Spear and Pyle 2010). Suprasolidus accessory mineral behaviour can be investigated by coupling 48 phase equilibria modelling with the experimental results of accessory mineral solubility in melt 49 (Kelsey et al. 2008;Yakymchuk and Brown 2014b). This technique provides a basic framework 50 for interpreting the geological significance of accessory mineral ages in suprasolidus 51 metamorphic rocks. 52In this chapter, we use pha...

Complex geochronological record of an emblematic Variscan eclogite (Haut‐Allier, French Massif Central)

“…The studied eclogites have been inferred to have undergone partial melting during the peak temperature part of their evolution (de Hoÿm de Marien et al, 2020; Figure 12). Additional arguments are presented here, because the presence of melt is important for the interpretation of the geochronological and geochemical data, as zircon or apatite might have interacted with or crystallized directly from the melt (e.g., Harley et al, 2007; Yakymchuk et al, 2019) and the geochemical signature of the samples may have been modified by partial melting (e.g., Palin et al, 2016).…”