How reliable are lunar phosphate and zircon as geochronometers?

Introduction:

All models of Solar System formation indicate that all planetary bodies were exposed to intensive asteroid and comet bombardment during the first 700 myr after their formation. However, our current understanding of the early impact flux throughout the early Solar System is mainly based on studying lunar samples, because impact structures are much better preserved on the Moon than on Earth, and there is a lack of ancient samples from other planetary bodies. One of the fundamental research questions in planetary formation remains the exact timing and chronology of these large impact events.

Uranium-bearing materials, such as the minerals zircon, baddeleyite and phosphate, occur widely as accessory minerals in planetary materials. This fact, together with advances in ion probe dating techniques within the last decades, has made U-Pb dating of these minerals an important tool to understand the magmatic and impact history of the Moon. Several impact events were previously dated within zircon and phosphate grains from different Apollo landing sites, and an apparent cluster of ages around 3.94 to 3.92 Ga was noticed in the phosphate age distribution [e.g., 1, 2, 3]. However, in some cases, it remains uncertain whether the determined phosphate U-Pb ages represent actual impact events or are disturbed due to incomplete resetting of their U-Pb system during an impact event [e.g., 2, 3].

A solution to this problem lies in the thorough investigation of microstructures within the grains, which provide important information when interpreting U-Pb ages. Shock-recrystallized domains have been reported for several terrestrial and extraterrestrial zircon and baddeleyite grains [e.g., 4, 5], but little is known about microtextures and shock deformation in phosphate, despite of its wide application in geochronology. A few recent studies have examined shock-induced microstructures in phosphate in detail [e.g., 6, 7], but the shocked domains have not yet been linked to radiometric ages. Moreover, how shock deformation as the result of impact events leads to incomplete resetting of the U-Pb system in phosphate and zircon remains poorly understood.

Therefore, in this study detailed microstructural, isotopic and geochemical analyses of Ca-phosphates and zircon are combined to establish a precise chronology of impact events. Understanding the timing and intensity of impact events during the infancy of the Solar System is not only important for reconstructing the early crustal evolution of the Moon, but also to determine the formation, evolution, and habitability of planetary bodies.

Methods: For the purpose of this study, distinct impact breccias from the Apollo 14 landing site were selected (samples 14303, 14169, 14305, 14314, 14321, 14051, 14311). The Apollo 14 landing site is especially interesting, since it is still debated whether these samples originated from a single dominant impact event (i.e. the Imbrium basin) or several impact events.

The µXRF (Bruker M4 Tornado X-ray spectrometer), located at the Analytical, Environmental and Geochemistry (AMGC) department at the Vrije Universiteit Brussel (VUB), was used to obtain element maps of the thin sections with a resolution of 25 µm. These analyses yielded a first impression of the sample lithology and mineralogy. The thin sections were then carbon-coated and high-resolution backscatter electron (BSE) images (Fig. 1) and element maps were obtained of the entire thin section, as well as BSE images of the grains of interest (Fig. 2). These analyses were performed on a Zeiss Merlin Gemini II Fe-SEM (Scanning Electron Microscope) at the European Space Research and Technology Centre (ESTEC).

Figure 1.: Backscatter electron image of thin section 14311,9.

Preliminary results: As of the time of writing, several phosphate and zircon grains (with sizes ranging from app. 10 to 50 µm) were identified within the thin sections. As a next step, Cathodoluminescence (CL) images of the phosphate and zircon grains will be obtained at Utrecht University on a JEOL JXA-8530F Hyperprobe. Following this, Electron Backscatter Diffraction (EBSD) analyses will be performed at the University of Portsmouth with an Oxford Instruments Nordlys EBSD mounted on a Zeiss EVO MA 10 LaB6-SEM. The detection of microstructures will guide the selection of domains of the grains for U-Pb analyses.

U-Pb dating of the thin section in combination with the microtextural analyses will constrain the relationships between impact-induced Pb-loss, microtextural and chemical changes and allow us to unequivocally interpret the new U-Pb ages to date primary crystallization, impact events or to reflect partially reset ages.        

Figure 2. Ca-phosphate-grain intergrown with pyroxene and plagioclase. P – Phosphate, Pl – Plagioclase, Px – Pyroxene.

References:

[1] Snape J. F., Nemchin A. A., Grange M. L., Bellucci J. J., Thiessen F., and Whitehouse M. J. (2016). Phosphate ages in Apollo 14 breccias: resolving multiple impact events with high precision U–Pb SIMS analyses. Geochimica et Cosmochimica Acta, 174, 13-29.

[2] Thiessen F., Nemchin A. A., Snape J. F., Whitehouse M. J., and Bellucci J. J. (2017). Impact history of the Apollo 17 landing site revealed by U‐Pb SIMS ages. Meteoritics & Planetary Science, 52(4), 584-611.

[3] Merle R. E., Nemchin A. A., Grange M. L., Whitehouse M. J., and Pidgeon R. T. (2014). High resolution U‐Pb ages of Ca‐phosphates in Apollo 14 breccias: Implications for the age of the Imbrium impact. Meteoritics & Planetary Science, 49(12), 2241-2251.

[4] Cavosie A. J., Timms N. E., Ferrière L., and Rochette P. (2018). FRIGN zircon—The only terrestrial mineral diagnostic of high-pressure and high-temperature shock deformation. Geology, 46(10), 891-894.

[5] White L. F., Darling J. R., Moser D. E., Reinhard D. A., Prosa T. J., Bullen D., Olson D., Larson D. J., Lawrence D., and Martin, I. (2017). Atomic-scale age resolution of planetary events. Nature Communications, 8(1), 1-6.

[6] Černok A., White L. F., Darling J., Dunlop J., and Anand M. (2019). Shock‐induced microtextures in lunar apatite and merrillite. Meteoritics & Planetary Science, 54(6), 1262-1282.

[7] Kenny G. G., Karlsson A., Schmieder M., Whitehouse M. J., Nemchin A. A., and Bellucci J. J. (2020). Recrystallization and chemical changes in apatite in response to hypervelocity impact. Geology, 48(1), 19-23.