What happens to zircon during subduction?

Daniela Rubatto 1 , Jörg Hermann 1

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Rocks that underwent high-pressure metamorphism and are now exhumed to the surface are the main geological record of subduction, the process by which continents are buried to depth by plate tectonic and mantle convection. The only way to constrain rates and duration of this process is to date accessory minerals in high-pressure rocks. An important step in this process is to understand what happens to datable minerals, like zircon, during high-pressure metamorphism. Metamorphic zircon that forms during subduction-exhumation is texturally distinctive. Two main features are observed: partial or complete replacement of a zircon crystal by a zircon of different composition (also called recrystallization) and new growth of zircon, often on a relict (inherited) grain. Generally replacement is common in sub-solidus conditions, whereas new growth is virtually ubiquitous when melt is present. The figure illustrates increasing degrees of structural modification of zircon in response to HP metamorphism. Zircon in equilibrated eclogite-facies rocks may be unaffected by metamorphism and represent the only magmatic relict in the mineral assemblage. The preservation of older zircon grains (inheritance) is the rule, particularly in HP rocks that experienced relatively low temperatures (<650°C). Metamorphic zircon first occurs along fractures, and likely formed in the presence of fluids ( A ). Commonly, inherited magmatic crystals have irregular domains where the original zoning is replaced by chaotic, patchy zircon (B and E). The altered zircon is often porous, rich in micro-inclusions, shows signs of corrosion (C), and is isotopically disturbed, i.e. ages measured in altered zones are geologically meaningless. An insight into zircon recrystallization is provided in D: a magmatic zircon has been replaced by an aggregate of small zircon crystals, intergrown with HP minerals. Common features in subducted rocks are discrete zircon rims or domains. These rims form on inherited magmatic (E) or detrital cores (F) and often provide reliable ages for the metamorphism. Occasionally, completely new zircon grains are found in HP metamorphic veins. This requires dissolution of Zr from other sources (most likely magmatic zircon in the country rock) and very high fluid/rock ratios. These hydrothermal zircon crystals lack inheritance, are euhedral and polygonally zoned, and may contain inclusions of HP minerals (G). In subducted rocks that reached partial melting (T > 650°C), inherited zircon can be completely lost to new metamorphic zircon (H and I), which tends to be euhedral and to exhibit regular zoning. Recent detailed studies reveal that zircon in HP rocks in fact forms over a wide range of conditions from subduction to exhumation. This is why zircon often preserves multiple growth zones formed at different stages of metamorphism (E, H and I).

Figure 1. Internal structure of zircon crystals from subducted rocks. Sources are own work and Tomasheck et al. 2003.

References: Tomaschek F., Kennedy A.K., Villa I.M., Lagos M., and Ballhaus, C. (2003) Zircons from Syros , Cyclades , Greece - recrystallization and mobilisation of zircon during high pressure metamorphism. Journal of Petrology 44:1977-2002.