Garnet: A Rock-Forming Mineral Petrochronometer

Garnet could be the ultimate petrochronometer. Not only can you date it directly (with an accuracy and precision that may surprise some), but it is also a common rock-forming and porphyroblast-forming mineral, with wide ranging—yet thermodynamically well understood—solid solution that provides direct and quantitative petrologic context. While accessory phase petrochronology is based largely upon establishing links to the growth or breakdown of key rock-forming pressure–temperature–composition ( P–T–X ) indicators (e.g., Rubatto 2002; Williams et al. 2007), garnet is one of those key indicator minerals. Garnet occurs in a great variety of rock types (see Baxter et al. 2013) and is frequently zoned (texturally, chemically) meaning that it contains more than just a snapshot of metamorphic conditions, but rather a semi-continuous history of evolving tectonometamorphic conditions during its often prolonged growth. In this way, garnet and its growth zonation have been likened to dendrochronology: garnet as the tree rings of evolving tectonometamorphic conditions (e.g., Pollington and Baxter 2010). In some ways, the dream of ‘petrochronology’ all started with garnet (Fig. 1). When Atherton and Edmunds (1965) or Hollister (1966) recognized the chemical zonation in garnet, when Rosenfeld (1968) noted the spiral ‘snowball’ of inclusions in rotated garnet, or when Tracy et al. (1976) drew the first 2-D map of garnet chemical zonation, illuminating those ‘tree-rings’ for the first time, they could only imagine what is now a reality decades later—direct zoned garnet geochronology of those concentric rings of growth. Geoscientists soon thereafter attempted the first garnet geochronology (van Breemen and Hawkesworth 1980), though several factors severely limited the development and wider-spread use of garnet geochronology from that point. These factors included 1) contamination of garnet by micro-mineral inclusions, 2) analytical limitations of small sample size, 3) the requirement of anchoring a garnet age analysis with another point on an isochron, and …