Sector-zoned zircon: differential trace element incorporation and implications for petrogenetic indices

Abstract

Zircon is a chemically and physically resistant accessory mineral that is commonly used to date and characterise intermediate to felsic rocks and associated metal ore deposits. The development of robust petrogenetic vectors, however, relies on understanding the distribution of trace elements in zircon, which often show complexity such as sector zoning. Sector-zoned grains comprise cathodoluminescence (CL)-dark and CL-light prism sectors growing perpendicular to the c-axis, making the sides of the crystal, and CL-intermediate pyramid sectors growing along the c-axis, building crystal tips.

To assess the chemical impact of sector zoning in zircon, here we investigate texturally constrained Andean detrital zircon data (n = 585), demonstrating that: (1) the REE + Y (except Ce) and Th are enriched in dark prisms relative to light prisms and pyramids; (2) the extent of dark prism enrichment increases from heavy to light REE (up to >300 %) as ionic potential decreases, reflecting the preferential substitution of low charge density cations in protosites accompanied by in-plane oxygen atoms; (3) Al is enriched in light prisms, where increased bond spacing within the zircon atomic lattice is interpreted to more easily accommodate the large Al cation in the Si site; (4) Ti, Zr, Hf, U and Ce are sector-independent; and (5) the highly charged cations P⁺⁵, Nb⁺⁵ and Ta⁺⁵ show limited inter-sector dependence, but show clear sector trends versus ΣREE + Y (molar). These trends suggest sector-specific incorporation: light prisms follow the xenotime substitution mechanism whereas dark prisms and pyramids require additional charge balancing configurations. We show that distinct atomic arrangements on discrete zircon sector growth surfaces result in differential uptake of cations from the melt, leading to sector specific partition coefficients.

Assessment of the role of sector zoning on zircon petrogenetic indices shows that indicators dependent on Ti, Zr, Hf, U and Ce only (e.g. Ce/√(U × Ti)) remain robust as they are sector-independent. Similarly, indices that ratio neighbouring REE with similar ionic potential show minimal sector zoning effects (e.g. Eu/Eu*). In contrast, we caution the use of petrogenetic indicators incorporating elements that show variable sensitivity to sector zoning (e.g. Ce/Ce*, Dy/Yb, (10³ × Ce/Nd)/Y, (10⁴ × Eu/Eu*)/Y), which may generate misleading trends. Intriguingly, the utility of sector zoning combined with key geochemical characteristics of zircon (e.g. low P) shows promise as a recorder of the dynamic crystallisation conditions associated with the development of porphyry copper mineralisation.