[S-CG57] Hybrid Geochronology from Date to Age

[SCG57-05] Zircon Formation Mechanisms in Mafic Rocks and Interpretation of Their Ages

*Keita Itano¹, Mami Takehara², Kenji Horie², Tsuyoshi Iizuka³, Ikuya Nishio⁴, Tomoaki Morishita⁵ (1.Akita University, 2.National Institute of Polar Research, 3.The University of Tokyo, 4.Hiroshima University, 5.Kanazawa University)

Keywords:U–Pb age, Hf isotope, Trace element

Zircon U-Pb dating is a crucial tool for understanding the timescales of magmatic activity. The complex age distributions observed in zircons from granites reflect the longevity of granitic magma systems and their diverse processes of magma accumulation, mixing, and solidification. In recent years, zircon ages from mafic plutonic rocks have been increasingly reported. The crystallization of such zircons was traditionally attributed to the solidification and differentiation of interstitial melts in cumulates, accompanied by an increase in SiO₂ concentration and zircon saturation. However, the relationship between lithology and zircon content in gabbros at mid-ocean ridges suggests that crystallization from the "last drop" of interstitial melt alone cannot adequately explain the observed zircon abundance. Numerical simulations have demonstrated that during the crystallization of rock-forming minerals with extremely low partition coefficients for Zr, such as olivine and orthopyroxene, Zr can become concentrated at the mineral-melt interface due to slow Zr diffusion, leading to zircon saturation (Bea et al., 2022, Chemical Geology). Thus, zircon crystallization in mafic magmas is a more complex process than previously understood, and its formation mechanism must be considered carefully when interpreting zircon ages.
In this study, we investigated zircon in hornblende peridotite from the Hida Belt (Itano et al., 2024, Geology), integrating Hf isotope ratios and trace element analyses to distinguish these crystallization mechanisms. The analyzed zircons show distinct dark and bright zones in cathodoluminescence images. These zones exhibit different trace element characteristics and show an age difference of approximately 10 million years. The dark zones, primarily found in the zircon cores, are enriched in trace elements and are interpreted as having crystallized due to local Zr enrichment during olivine crystallization. In contrast, the bright zones are depleted in trace elements and are likely to have formed from melts with increased SiO₂ content during amphibole crystallization. The Hf isotope data show that both zones have identical initial values within measurement error (εHf = 10.3‰ ± 1.7), suggesting that the mafic magmatic system originated from a common mantle source and remained active for ca. 10 million years.