Reliability of petrogenetic information obtained from the distribution of trace elements in zircon in light of the complex infusion by foreign-element-bearing low temperature solutions

The increasing application of minor elements such as REE, Ti, Fe in zircon as petrogenetic indicators depends on zircon remaining a closed chemical system over a history that can include metamorphic events, erosion, transport, and sedimentation. Whereas examples have been described of UPb loss and element exchange during metamorphic reactions, for over 50 years there have been reports of the accumulation of a range of non-formula elements such as Ca, Al, Fe, in zircons with no metamorphic history that have been tentatively related to hydrothermal action or some other process such as weathering. This open system behaviour of zircons needs further explanation and in this contribution we present data from three zircons from a late Archaean granite from Western Australia that confirm previous observations on the distribution of non-formula elements in radiation damaged zircon and support our earlier conclusion that elements are added from percolating low temperature weathering solutions and provide new insights into the mechanisms and processes controlling access of weathering fluids into the zircon. These include radiation damage accumulation over time from the decay of U and Th to produce domains of porous amorphous material that vary in porosity and the presence of weaknesses along the boundaries of oscillatory zones which allow the penetration of low temperature groundwater fluids carrying solute elements. Also important is the size of particles of the solute elements such as Al and Ca, which depends on Eh - pH of the groundwater and porous domains within the zircon structure. This low temperature fluid interaction is distinguished from hydrothermal activity by a lack of any reaction fronts or annealing of the radiation damage and a lack of any modification of existing zircon structures. Capillary action is proposed as the mechanism driving the penetration of weathering solutions into radiation damage expansion fractures and the radiation damaged zonal structure. The susceptibility of radiation damaged zircon to infusion by low temperature fluids and deposition of accompanying solute elements has implications for the application of zircon trace element chemistry as a petrogenetic and temperature indicator as well as a geochronometer and host for nuclear waste.