Zircon – Tiny but Telling: A Petrochronological Study
Authors:   , Streicher Linus Bijan 
Published: February 2025

This thesis concerns the field of petrochronology, where time is linked to the physical
conditions (e.g., pressure P and temperature T) of rock-forming events, to provide better
constraints on key geological processes such as mountain building, metamorphism, and
magmatism. The goal of this work is to enhance applications using the chemical composition of
the mineral zircon in petrochronological studies of high- and ultrahigh-pressure ((U)HP)
metamorphic rocks. The primary use of zircon in petrochronology is to determine rock ages
through U-Pb dating. Zircon dating is considered particularly reliable because zircon is stable over
a wide PT range in the Earth's interior and elemental and isotopic composition undergo little or
no change during a variety of geological processes. A major challenge of zircon petrochronological
studies, however, is to determine PT conditions of zircon growth.
The methodical focus of this work is to investigate the extent to which equilibrium rare
earth element (REE) distribution between zircon and melt, which is used here as an analogy to
the REE partitioning between zircon and co-existing metamorphic minerals, is sensitive to
pressure and temperature. By expanding a previously developed latÝce strain model-based
prediction function for zircon-melt REE partition coefÏcients (DREE) – a function enabling the
prediction of the overall REE partitioning behavior from only a small number of DREE values – with
an empirically determined constraint, the function is made applicable to existing zircon-melt/bulk
rock DREE data. Evaluation of literature data with this expanded prediction function confirms the
previously discovered temperature dependence of zircon-melt DREE and allows the calibration of
a zircon-melt DREE based geothermometer. Zircon synthesized in high-pressure-temperature
experiments in equilibrium with magma reveal that, in addition to temperature, pressure and the
major element composition of the coexisting magma affect zircon-melt DREE. The finding that
pressure, alongside temperature, affects the partitioning behavior of REE between zircon and
melt establishes a foundation for developing a mineral-mineral geothermobarometer based on
zircon-melt DREE for (U)HP metamorphic rocks, offering a potential new method to link zircon ages
with the PT conditions of zircon formation. Quantifying trace element contents in tiny zircon
crystals from high-pressure-temperature experiments is challenging. To improve trace element
determination in zircon using high-resolution techniques like NanoSIMS, the synthesis of a high-quality zircon standard from a Li-Mo flux was attempted. Although the synthesis of a suitable
standard for quantitative trace element determinations was unsuccessful, it provides insights into
the challenges for future synthesis attempts.
In the applied part of the thesis, a detailed petrochronological study is conducted, reevaluating
the zircon ages of the Earth's youngest known UHP coesite eclogite from the Papua
New Guinea UHP terrane. Zircon ages of the coesite eclogite obtained in previous studies using
different analytical techniques to determine U and Pb contents are inconsistent. Based on zircon
REE contents, determined at the same spots as the U-Pb isotopic data for zircon dating, along
with zircon textures, two distinct zircon generations are now identified: an older and a younger
generation with ages ranging from 7.0±0.2 to 7.9±0.3 Ma and from 4.4±0.3 to 5.5±0.4 Ma,
respectively. A thorough petrological analysis of the coesite eclogite identifies a primary mineral
assemblage of garnet, omphacite, amphibole, coesite, phengite, rutile, and zircon. The middle to
heavy REE patterns of the older zircon indicate it was part of the primary mineral assemblage. PT
pseudosection modeling suggests that this assemblage formed under UHP conditions of
P=3.1±0.2 GPa and T=765±30°C.
The findings of this study underscore the unique role of zircon in petrochronology and
pave the way toward a comprehensive tool for determining the PT conditions of zircon formation,
to integrate it with age information obtained on the same zircon grain.