# Low-temperature dissociation of zircon in highly alkaline conditions: A cautionary note for studies on natural glasses of debated origin

A. Musolino, B. Devouard, P. Rochette, P. Roperch, P M Zanetta, Alexis

Licht, D. Ferry, A. Campos

► To cite this version:

A. Musolino, B. Devouard, P. Rochette, P. Roperch, P M Zanetta, et al.. Low-temperature dissociation of zircon in highly alkaline conditions: A cautionary note for studies on natural glasses of debated origin. Meteoritics and Planetary Science, 2025, 60 (7), pp.1573-1586. 10.1111/maps.14372. hal-05142549

HAL Id: hal-05142549

https://hal.science/hal-05142549v1

Submitted on 3 Jul 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Low-temperature dissociation of zircon in highly alkaline conditions: A cautionary note for studies on natural glasses of debated origin

A. MUSOLINO<sup>1*</sup>, B. DEVOUARD<sup>1</sup>, P. ROCHETTE<sup>1</sup>, P. ROPERCH<sup>2</sup>, P. M. ZANETTA<sup>3</sup>, A.-M. SEYDOUX-GUILLAUME<sup>3</sup>, A. LICHT<sup>1</sup>, D. FERRY<sup>4</sup>, and A. CAMPOS<sup>5</sup>

*Correspondence

A. Musolino, Aix Marseille Univ, CNRS, IRD, INRAE, CEREGE, Aix-en-Provence, France.

Email: musolino@cerege.fr

*(Received 26 April 2024; revision accepted 15 May 2025)*

**Abstract**—Dissociated zircon is largely used as a robust indicator of glasses generated by impact cratering and airbursts. The reaction of zircon dissociation, i.e. 'ZrSiO₄ → ZrO₂ + SiO₂', requires high temperatures (>1670°C) only reached by extreme geological processes. Using high-temperature experiments, this study shows that zircon can dissociate and form ZrO₂-rich coronitic rims at temperatures of 900–1000°C (P = 1 bar), in the presence of a specific chemical environment made of NaCl or a mixture of NaCl and caliche soil (Ca-sulfates). The use of silica glass vessels provides a SiO₂-rich environment during the experiments. We observe that the dissociation is strongly related to the complexity of the surrounding system (e.g. the presence of other minerals that act as a flux) in which the reaction occurs. For these reasons, we suggest considering a more careful approach in using dissociated zircon as indicative of very high temperatures in glass-forming processes.

## INTRODUCTION

Impact melts generated by hypervelocity impacts have experienced temperatures well above the maximum temperatures recorded in volcanic or ancient anthropogenic glasses (<1200°C; Dressler & Reimold, 2001; Osinski et al., 2018; Cicconi et al., 2022). Evidence for such high temperatures may be gained from the following observations: (1) loss of volatiles including water, (2) presence of inclusions of a pure SiO₂ glass, also known as lechatelierite, (3) dissociation of zircon (ZrSiO₄) (e.g., Glass, 1990; El Goresy, 1965; Glass & Simonson, 2012). The ZrO₂-SiO₂ phase diagram determines that crystalline zircon dissociates at ~1670°C into tetragonal or cubic zirconia and liquid silica, depending on temperature conditions. During cooling, due to martensitic phase transformation from

high-temperature zirconia polymorphs, monoclinic ZrO₂ forms, i.e., baddeleyite (e.g., Kaiser et al., 2008; Timms, Erickson, Pearce, et al., 2017). The identification of preexisting cubic zirconia, which can be achieved using EBSD phase heritage/reconstruction (Cayron, 2007; Cayron et al., 2010), is an important geothermometer as cubic zirconia is stable at temperatures >2370°C, indicating the formation of superheated melts (Chinchalkar et al., 2024; Timms, Erickson, Zanetti, et al., 2017; Tolometti et al., 2022). Under reflected light and backscattered electrons (BSE) imaging in a scanning electron microscope (SEM), dissociated zircon in glasses is easily recognized by the presence of bright coronitic rims of baddeleyite surrounding the primary crystal core of zircon (e.g., Timms, Erickson, Pearce, et al., 2017, their figures 4 and 9). Dissociation can also locally affect the border and fractures of the crystal. Dissociated zircon

<footnote n="1"><sup>1</sup>Aix Marseille Univ, CNRS, IRD, INRAE, CEREGE, Aix-en-Provence, France</footnote>

<footnote n="2"><sup>2</sup>Géosciences Rennes, CNRS, Univ Rennes1, Rennes, France</footnote>

<footnote n="3"><sup>3</sup>CNRS, Université Jean Monnet (UJM) Saint-Étienne, LGL-TPE, UMR5276, Saint-Étienne, France</footnote>

<footnote n="4"><sup>4</sup>Aix Marseille Univ, CNRS, CINaM, Marseille, France</footnote>

<footnote n="5"><sup>5</sup>Aix Marseille Univ, CNRS, Centrale Med, FSCM, CP2M, Marseille, France</footnote>
crystals have been observed in tektites (Australites [Cavosie, Timms, Erickson, et al., 2018] and Georgiaites [Glass et al., 1995]), Libyan desert glass (Cavosie & Koeberl, 2019; Kleinmann, 1968) and impact glasses and related rocks from the following impact structures: Acraman (Schmieder et al., 2015), Aouelloul (El Goresy, 1965), Chixculub (Wittmann et al., 2005), Luizi (Cavosie, Timms, Ferrière, et al., 2018), Meteor crater (Cavosie et al., 2016), Mien (Martell et al., 2021), Mistastin (Timms, Erickson, Zanetti, et al., 2017; Tolometti et al., 2022; Zanetti et al., 2015), Pantasma (Rochette et al., 2019), Ries (El Goresy, 1965; Wittmann et al., 2006), Yarrabubba (Erickson et al., 2020), Vredefort (Kovaleva et al., 2021), and West Clearwater Lake (Chinchalkar et al., 2024). Zircon dissociation is also observed in the lunar Apollo 14 and 15 breccias and soils (Crow et al., 2019), the lunar meteorite Dhofar 458 (Zhang et al., 2011), and Howardite-Eucrite-Diogenite (HED) meteorites from the asteroid Vesta (Barboni et al., 2024). Although it does not correspond to the stringent diagnostic criteria for impact (high pressure or extraterrestrial contamination evidence), the presence of dissociated zircon in a glass of disputed origin has been presented as evidence for impact or airburst (Bunch et al., 2021; Schultz et al., 2022a).

The case of airbursts differs from that of cratering impacts by their P–T conditions: both create very high temperatures (up to 10,000°C; French, 1998) but airbursts are characterized by lower pressures (~1–10 kPa, e.g., Aftosmis et al., 2019) than cratering impacts at the Earth’s surface (~7 to >100 GPa; Cavosie & Koeberl, 2019). However, the pressure generated by an airburst event is debated, with some models suggesting that pressures up to ~30 GPa can occur (West et al., 2024). The process of airburst is still not well documented, due to the lack of clear geological evidence (e.g. the presence of a crater) which makes their (eventual) products even more difficult to recognize (Boslough & Crawford, 2008). The only other known natural process that can cause zircon dissociation is lightning (Essene & Fisher, 1986; Kenny & Pasek, 2021). However, the distinct shape and internal structure of fulgurites usually make them readily identifiable.

The use of zircon dissociation as a strong criterion to identify impact glasses (in the following, the term will be used as comprehensive of impacts and airburst products) fails to acknowledge that the temperature prediction from the zircon phase diagram is valid only in the absence of chemical reactivity with other compounds than ZrO₂ and SiO₂. In the mineral processing industry, it is known that there are ways to obtain the dissociation of zircon at much lower temperatures, e.g. using alkaline fusion (Abdelkader et al., 2008; West, 1954). Zircon dissociation to baddeleyite (and other Ca-rich phases) can be achieved

at temperatures as low as 600°C–900°C at pressures of 400–1000 MPa (Lewerentz, 2011; Lewerentz et al., 2019). The dissociation of zircon is also strongly dependent on its structural state, as it often suffers structural damage induced by radiation (due to the decay of U and Th included in the crystals). Amorphous (metamict) zircon has been experimentally shown to decompose into ZrO₂ + SiO₂ at ~1000°C at room pressure (Nasdala et al., 2002). Both hydrothermal alteration experiments (Geisler et al., 2001, 2003, 2004, 2007) and natural samples case studies (e.g., Nasdala et al., 2009, 2010; Seydoux-Guillaume et al., 2015) have also shown that metamict zircon is more easily subjected to alteration (e.g., decreased stability when subjected to hydrothermal solutions, forming micrometer-thick reaction rims, nanoscale pores, accumulating solvent element in the structure [Ca, Al, Fe]).

This work investigates the possibility that crystalline zircon dissociation to baddeleyite can occur at atmospheric pressure and under dry conditions at T ≤ 1000°C, suggesting that this reaction should not be considered, alone, as indisputable evidence of extreme geological conditions. The study is primarily intended for glasses of debated origin, where there is no other clear evidence of shock and high-temperature conditions. Specifically, we explored highly alkaline conditions, such as those that could occur in the presence of evaporitic soils. Our aim is also to emphasize how crucial it is to constrain the conditions under which this reaction occurs, as it is fundamental for understanding the formation of certain natural glasses whose origins are still debated. One case is that of Pica glass, whose geological context was considered for the design of our experiments. Pica glass is an unusual surface glass layer found in different locations of the Atacama Desert (Chile), whose origin has been attributed to different processes: surface vitrification due to biomass fires in soils (Roperch et al., 2017, 2022) and cometary airburst (Schultz et al., 2022a, 2022b). The extended areal distribution of Pica glass and the lack of evidence of remagnetization by strong fields excluded the hypothesis of lightning strikes for their formation (Roperch et al., 2017). The presence of dissociated zircon in Pica glass is seemingly robust evidence for impact produced by Schultz et al. (2022a). Whatever the source of the high temperatures, Pica glass has been generated from the melting of the Atacama Desert surface soil, which is rich in *caliche*, i.e. crusts made of salts, mostly NaCl and Ca-sulfates (Voigt et al., 2020). The soil itself may thus provide the necessary reagents to lower the temperature of zircon dissociation. To demonstrate this, we conducted an experimental study to determine the temperature range for zircon dissociation in the presence of salts at atmospheric pressure.
# MATERIALS AND METHODS

## Experimental Technique

The zircon crystals used for the experiments come from a homogeneous batch of crystals extracted from granitoids from the Kaapvaal Craton, South Africa (Poujol et al., 2021), with grain sizes of ~100 µm (Figure 1a). The experiments were conducted inside silica glass vessels (V ~ 1.5 cm³) covered with a silica glass lid. The zircon crystals were laid at the bottom of the vessel, in direct contact with the glass. The reagents used to investigate the dissociation temperature are (i) caliche soil collected in the Atacama Desert, composed of Ca sulfates—primarily gypsum CaSO₄2H₂O with minor anhydrite CaSO₄ (Figure S1), and grain size of 10–20 µm, and (ii) NaCl crystals, with grain sizes between 100–500 µm (Figure 1b,c). Some experiments were conducted using a mixture of halite and caliche. The addition of NaCl is justified by its widespread presence in the Atacama Desert, where it is often found mixed with gypsum (Voigt et al., 2020). In the case of impact or airburst in hot deserts, zircon can be present as single crystals surrounded by sand. To ensure that the zircon crystals would be completely surrounded by the reagents, and to have a sufficient number of zircon crystals to investigate after the experiments, we chose the following proportion of zircon to reagent. For each portion of zircon crystals (~1–2 mg), about ten portions of reagents are used (10–20 mg). Since gypsum and anhydrite are the most common salts in the Atacama Desert, the ratio of caliche to NaCl chosen for experiments where both are present is approximately 5:1. Precise proportions of the reagents used are arbitrary, due to the wide variation in the distribution and concentration of salts in the desert (Voigt et al., 2020). Using silica glass vessels ensures the presence of an SiO₂-rich environment during the experiments, which is necessary to simulate zircon dissociation in a silicate glass. A droplet of pure H₂O is added to each experiment to make the components adhere to each other. Once filled, the vessel is covered by the lid (not sealed, used to limit the escape of volatile elements) and put in a WiseTherm oven for six hours at temperatures from 800°C to 1000°C, at atmospheric pressure. A preliminary experiment has been conducted at 1000°C for 30 min, which is reported among the other results in Table 1. At the end of the six hours, after cooling, the vessels are embedded in resin and polished down to expose zircon interiors and obtain a section of numerous zircon crystals. The area analyzed for each experiment is ~0.1 cm².

## Analytical Techniques

The zircon crystals were analyzed using a Hitachi S-3000N SEM with a Bruker Energy Dispersive X-ray

Spectrometer (EDS) at the Centre de Recherche et d'Enseignement de Geosciences de l'Environnement (CEREGE) and a Zeiss Gemini 500 Field-Emission Gun Scanning Electron Microscope (FEG-SEM) with an EDAX EDS system at Centre Pluridisciplinaire de Microscopie Electronique et de Microanalyse (CP2M) in Marseille, France. BSE images were acquired at a voltage of 15 kV.

The age of the zircon crystals has been determined to better characterize their properties. Forty zircon crystals were dated via U-Pb at the Envitop analytical facility at CEREGE using an Element XR ICP-MS connected to a NWR193 laser (ArF 193 nm) ablation system. Zircon crystals were ablated with a 25-micron spot diameter, a 15 Hz pulse repetition rate, an energy fluence of 1.5 J/cm², and a carrier gas flow of 0.975 L/min. Data reduction, date, and date uncertainty calculations were conducted with an in-house MATLAB script. Details about our U-Pb dating workflow, data reduction steps, and discordance filter are given in Licht et al. (2024). The three zircon validation reference materials used during these sessions yielded offsets around TIMS ages <1% in most cases, and <2% otherwise. Out of the 40 analyzed zircon crystals, 27 yield concordant U-Pb ages (see Table S1). The final Concordia age was calculated with concordant ages only using IsoPlotR (Vermeesch, 2018).

X-ray diffraction on a powdered sample of caliche was conducted with an X'Pert Pro (Panalytical) equipped with a ray X cobalt tube ($\lambda$ = 1.79 Å), operating at 40 kV and 40 mA. The analysis was conducted between 5° (2θ) et 75° (2θ) with a step size of 0.026° and a total counting time of 3 h. The sample was powdered in an agate mortar and deposited with ethanol on a low-background silicon support. The sample rotates on a horizontal plane at a velocity of 15 rotations per minute to improve the statistics. The phase identification has been conducted using the software X'Pert Highscore Plus, using the database ICDD PDF2.

For Raman spectroscopy, we used a Renishaw inVia Raman Microscope with a 100x objective at Centre Interdisciplinaire de Nanoscience de Marseille (CINaM), France. The samples were excited by a 633 nm laser at 100% power or attenuated to 50% to 5%. The spectra were collected with multiple acquisitions (n = 6) with counting times of 10 s and with a 1200 lines/mm grating (therefore a resolution of 1.35 cm⁻¹). The spectrum range was 50–1200 Raman/cm⁻¹. Raman spectroscopy was used for the identification of phases in the rims (after the experiment) and to investigate the crystallinity of the zircon crystals used (before the experiments). In both cases, the analysis was performed on polished samples embedded in resin.

Two electron-transparent lamellae of the mineral interface (zircon/rim/matrix) were prepared using a Thermo Fisher Scientific FEI 125 Helios Nanolab 600i
<figure><img src="image_2.png" /><figcaption>FIGURE 1. (a) Optical images of the zircon crystals used for the experiments (granulometry ~100 µm). (b, c) Optical image of NaCl (100–500 µm) and caliche (gypsum, 10–20 µm) crystals used as reagents for the experiments. (d) Picture of the SiO₂ vessel and lid used for the experiments. (e) Vessel (after removal of the lid) after one experiment, ready for the embedding in resin and polishing (red dashed line is a typical section obtained after the polishing). (f) Samples embedded in resin uncoated and C-coated. (g) BSE image of the whole section from the experiment at 900°C where zircon crystals are exposed to high temperature with no reagents (yellow box indicates the position of the zircon in h). Minor apatite and quartz grains are unintentionally present (darker crystals). (h) BSE image of typical unreacted zircon: fractured, often filled with bright phases (Zr-rich), no signs of dissociation. The fractures observed could be due to thermal breakdown or they may have already been present.</figcaption></figure>

Focused-Ion Beam Scanning Electron Microscope (FIB/SEM) at MANUTECH USD platform in Saint-Etienne, France. A lamella was taken from each experiment showing dissociated rims. One lamella, where zircon has a very porous rim, was kept thicker to avoid its destruction. Nanoscale characterization was performed at 200 kV with a NeoARM200F (scanning) Transmission Electron Microscope (TEM/STEM) operated by the Consortium Lyon Saint-Etienne de Microscopie (CLYM) and hosted within the Hubert Curien Laboratory, Saint-Etienne, France. The NeoARM200F is equipped with a cold field-emission gun and a sixth-order aberration corrector (ASCOR). Images were obtained using a bright field and dark field detector for TEM and a high-angle annular dark field (HAADF) in STEM mode. A JEOL large angle silicon drift EDS

detector (0.96 sr) was used to acquire X-ray maps with 0.1 µs dwell time and a real time of 15 min to 2 h. Quantification was achieved using theoretical k-factors provided by JEOL®, the Cliff-Lorimer method (Cliff & Lorimer, 1975), and an absorption correction procedure (Horita et al., 1987; Morris, 1980; Zanetta et al., 2021, 2022). EDS data were processed using Hyperspy library (De La Peña et al., 2024).

# RESULTS

## Pre-Experiment Characterization

The zircon crystals yield a homogenous 2748.3 ± 6.5 Ma (2 s) Concordia age (MSWD = 1.1 with 25 out of the 27 zircons, see Figure S2). Their chemical composition is
<table><caption>TABLE 1. All experiments conducted are listed along with their running conditions: duration, temperature (T), and reagents.</caption><thead><tr><th>Time (h)</th><th>T(°C)</th><th>Reagents</th><th>Reacted grains (%)</th><th>Rims</th><th>Bdy</th><th>Fig. (ref)</th></tr></thead><tbody><tr><td rowspan="7">6</td><td>800</td><td>NaCl</td><td>None</td><td>n</td><td>n</td><td>3a</td></tr><tr><td>800</td><td>Caliche + NaCl</td><td>None</td><td>n</td><td>n</td><td>3b</td></tr><tr><td>900</td><td>None (blank)</td><td>None</td><td>n</td><td>n</td><td>1g</td></tr><tr><td>900</td><td>Caliche</td><td>None</td><td>n</td><td>n</td><td>—</td></tr><tr><td>900</td><td>NaCl</td><td>None</td><td>n</td><td>n</td><td>3c</td></tr><tr><td>900</td><td>Caliche + NaCl</td><td>&gt;50%</td><td>y</td><td>n</td><td>3d</td></tr><tr><td>1000</td><td>NaCl</td><td>&lt;50%</td><td>n</td><td>y</td><td>3e</td></tr><tr><td rowspan="2">0.5</td><td>1000</td><td>Caliche + NaCl</td><td>100%</td><td>y</td><td>y</td><td>3f</td></tr><tr><td>1000</td><td>NaCl</td><td>&lt;50%</td><td>n</td><td>y</td><td>Inset 3e</td></tr></tbody></table>

*Note:* All experiments were run at atmospheric pressure. The last four columns report the percentage of reacted grains, the occurrence of rims and baddeleyite (Bdy), and the figure references (Fig. [ref]).

mostly pure ZrSiO$_{4}$, with trace Hf present (averaging 0.7 ± 0.1 wt% when it is present). The crystals are fractured and some have apatite inclusions, but they do not show zoning. The contents of U and Th are 215 ± 90 and 508 ± 229 ppm, respectively (Table S1). Overall, the population is very homogeneous in visual aspect, age, and composition. Their structural state has been inferred using Raman spectroscopy and the calculation of α-doses (Figure S3; Murakami et al., 1991; Nasdala et al., 2001). The α-dose damage can be determined from the U and Th concentrations in zircon and their age (Nasdala et al., 2001). For the zircon crystals used in the experiments, the α-dose damage calculated is equal to $3.84 \times 10^{18}$ α-doses events/g. Concerning Raman spectroscopy, the peaks are well-defined and show only slight deviations from the $v_3(\text{SiO}_4)$ band (from 1005 to 1007 cm$^{-1}$), with a FWHM of 8.8–13.5 cm$^{-1}$ (N = 10) (Figure 2b; Table S2).

## Post-Experiment Characterization

One experiment that served as a blank was run with no reagents at T = 900°C, which did not show signs of dissociation in the grains (Figure 1g,h). Three out of nine experiments successfully reproduced coronitic rims, i.e., borders surrounding zircon with symplectic texture made of micrometer-sized Zr-rich crystals (Figure 3). In all the experiments in which caliche is present, skeletal secondary anhydrite and wollastonite are formed (from 800°C to 1000°C; right column of Figure 3). Instead, when NaCl is the only reagent, a matrix of Na-rich silicate glass embeds the zircon crystals, progressively more abundant going from 800°C to 1000°C (left column of Figure 3). We obtained the following results (Table 1):

* At T = 800°C, for all the experiments, no zircon crystals showed dissociation (Figure 3a,b).

* At T = 900°C, in the presence of NaCl only, the zircon crystals do not show dissociation (Figure 3c).

* At T = 900°C, in the presence of caliche and NaCl, >50% of the zircon crystals have dissociation rims (Figure 3d).

* At T = 1000°C, in the presence of NaCl, >50% of zircon crystals do not show dissociation. Some grains (<50%) show dissociation in fractures and inclusions, but they do not have dissociated rims (Figure 3e). In the same conditions, but for a duration of half an hour, dissociation occurred in inclusions and fractures, but not forming rims (inset in Figure 3e).

* At T = 1000°C, in the presence of caliche and NaCl, all the zircon crystals are dissociated with rims (Figure 3f).

The rims formed differ by texture and mineralogical composition. At 1000°C with caliche plus NaCl (Figures 3f and 4), all the zircon crystals have dissociation rims, with an average thickness of ~15 µm. In this experiment, the rim is characterized by the alternate presence of two textures: (a) highly porous, made only of anhedral $ZrO_2$ crystals; (b) $ZrO_2$ crystals cemented by a crystalline Ca-silicate phase. The $ZrO_2$ phase is identified as baddeleyite by Raman spectroscopy, selected area electron diffraction (SAED) patterns (Figure 4c), and atomic lattice imaging in aberration-corrected STEM mode (Figure 4b, inset). EDS quantification (spot analysis) of the Ca-silicate phase gives Ca = 23.5, Fe = 0.6, O = 56.9, Si = 14.9, and Zr = 4.1 atom%, close to a $Ca_2SiO_4$ stoichiometry. SAED patterns are best indexed as calcio-olivine, rather than its polymorph larnite (Figure 4d–g). However, the size of the crystals and their superposition with baddeleyite do not allow us to constrain this phase with certainty, either structurally (could correspond to other Ca-, Si- and Zr-rich phases) or chemically (superposition of crystals gives variable elemental ratios depending on sample locality that could account for the low Zr content). This is because the lamella used for TEM/STEM analysis had been kept thick due to the porous nature of the rim which limited the thinning with FIB, thus increasing uncertainties in EDS analyses and SAED indexing.

In the experiment at 900°C (Figures 3d and 5), with caliche plus NaCl, the majority of zircon crystals are dissociated. The rims are ~5 µm thick on average and are finer-grained compared to the experiment at 1000°C. The contact between the rim and the unaltered zircon is sharp, and generally, there is an increase in granulometry from the inner to the outer part of the rim. TEM reveals the presence of a single $ZrO_2$ phase enriched in Ca. Standardless EDS analyses yield a composition of Ca = 8.57, Zr = 33.9, and O = 54.6 atom% which
<figure><img src="image_3.png" /><figcaption>FIGURE 2. Raman spectra of a random selection of zircon crystals used in experiments. The spectra (n = 8) are compared to the excitation modes expected for non-metamictic zircon (dashed lines, Zhang et al., 2000).</figcaption></figure>

converts approximately to 90 wt% ZrO₂ and 10 wt% CaO. Selected area electron diffraction (SAED) patterns are consistent with cubic ZrO₂, rather than baddeleyite (Figure 5d). In this experiment, Raman spectroscopy did not detect baddeleyite in the rim either (orange spectrum in Figure 6a).

DISCUSSION

Pre-Experiment Characterization

The zircon crystals used can be defined as crystalline
from their spectroscopical properties (intermediate
FWHM and well-defined peaks). Based on spectroscopy
results, the zircon crystals used have intermediate
characteristics between perfectly crystalline and partly
metamict crystals: deviation from 1008 cm⁻¹ of the
v₃(SiO₄) band, and values of FWHM of 10.9 ± 1.4 (1σ)
(when it is <3 cm⁻¹ for very well-ordered ZrSiO₄ and
>30 cm⁻¹ for metamict zircon [Zhang et al., 2000;
Nasdala et al., 2001]). From the theoretical α-dose
calculated for the zircon crystals (3.84 × 10¹⁸ α/g) it is
expected that they exceed the first percolation threshold
(around 2.2 × 10¹⁸ α/g; Salje et al., 1999 and
Pidgeon, 2014). However, plotting the global theoretical
α-dose of zircon crystals (3.84 × 10¹⁸ α/g) against the
measured FWHM values revealed a deviation from
the expected curve for unannealed zircon (Palenik
et al., 2003). This indicates they underwent annealing and
confirms they are not metamict (Figure S3). We thus infer

that the observed low-temperature decomposition cannot
be explained by metamictization.

Post-Experiment Characterization

The grain-to-grain variations in observed
decomposition can be explained by variable local
chemical conditions or by variable individual properties
of the crystals. We surmised that the latter did not play a
major role in rim formation in our experiments. This
conclusion is supported by the fact that no preferential
dissociation was observed for inclusion-bearing crystals
compared to others, and whenever alteration rims occur,
they affect either all the crystals or all those in the same
area of the sample. However, the individual properties
must be considered whenever dissociation is observed in
isolated crystals. Additionally, any possible temperature
variation in the same experiment likely did not play a
significant role, as the volume of the vessels used is very
small (~0.03 cm³), limiting temperature gradients within
the sample. In these experiments, the key factor for
dissociation is considered to be the variable distribution
of reagents in the volume surrounding zircon.

The dissociation of zircon, forming coronitic rims, is
observed starting from a temperature of 900°C when
caliche and NaCl are present. With these reagents and an
increase in temperature to 1000°C, the reaction observed
is more extensive, leading to the formation of thicker and
more common rims. Instead, when NaCl is the only
reagent, dissociation occurs starting from 1000°C, in
<figure><img src="image_4.png" /><figcaption>FIGURE 3. Backscattered electron images of the experiments conducted with NaCl (a, c, e) or NaCl + caliche (b, d, f), at T = 800°C (a, b), 900°C (c, d), and 1000°C (e, f). In the inset in (d), there is a zircon that underwent severe dissociation in the same experiment. Zircon dissociation is seen in d, e, and f. Red arrows point to dissociation in cracks and inclusions in the experiment with NaCl at 1000°C (with heating duration of 6 h, and 30 min for the inset image; the greater extent of dissociation at 30 min is not representative of all the grains in the same experiment and could be due to local more favorable conditions). Abbreviations: zircon (Zrn), skeletal crystals of anhydrite and wollastonite mixed (Anh/Wo). All the scale bars indicate 50 µm (unless otherwise stated).</figcaption></figure>

inclusions, fractures, or partly along the border of
crystals. Therefore, the dissociation of zircon is highly
favored by the occurrence of a more complex reaction
environment (caliche + NaCl), even if the favorable
conditions for the dissociation reaction can be
heterogeneously distributed in the reaction vessel (i.e.,
not all the zircon crystals behave in different regions of
the same experiment and the total area analyzed for each
experiment is only ~0.1 cm²). Finally, the dissociation of
ZrSiO₄ in rims is not necessarily associated with the

presence of baddeleyite (ZrO₂). For the experiments
where the dissociation of ZrSiO₄ occurs, we propose the
following interpretation:

1. **T = 900–1000°C, caliche + NaCl.** Gypsum and NaCl (with melting points of 200–300°C [after a starting release of H₂O at 163°C] and 800°C, respectively) act as fluxes, lowering the temperature at which zircon dissociates. As documented in hydrothermal experiments (Lewerentz et al., 2019), the importance
<figure><img src="image_5.png" /><figcaption>FIGURE 4. (a, b) STEM-HAADF image of a local part of the FIB sampled in the zircon rim in the experiment conducted at 1000°C, with caliche + NaCl. The atomic model of baddeleyite (Bdy) on top of the STEM-HAADF image matches the atomic lattice. (c) Indexed SAED pattern of baddeleyite along the [100] direction. (d) Indexed SAED pattern of calcio-olivine along the [012] direction. Distortion in the SAED pattern is visible. The first-order Laue zone reflections confirm the zone axis. (e–g) Elemental composition of phases in the rim (a): Si, Zr, and Ca, respectively. Two different textures are present: (i) only baddeleyite (Bdy) and (ii) baddeleyite intermixed with the Ca₂SiO₄ phase that potentially contains traces of Zr.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>FIGURE 5. (a, b) STEM-HAADF image of dissociated rims in the experiment conducted at 900°C, with caliche + NaCl. (c, d) Diffraction patterns of identified zircon crystal and cubic zirconia (CZ). (e–g) Elemental composition of phases in the rim: Si, Zr, and Ca, respectively. Cubic zirconia (ZrO₂) contains a significant proportion of CaO in its structure (~10%). The zircon rim is homogeneous (only composed of one phase).</figcaption></figure>

of Ca is that it keeps the Si-activity low, forming calcium silicates in the matrix; this way restoring an undersaturated environment in the melt—a condition necessary for ZrO₂ to form. However, in this

experiment, we observe that the ZrO₂ phase in the dissociation rims is not baddeleyite, but rather a cubic CaO-bearing ZrO₂ polymorph. This is due to the high Ca activity in the system, in accordance with
<figure><img src="image_7.png" /><figcaption>FIGURE 6. (a) Raman spectra of (i) zircon (Zrn) from the RRUFF Database (#R050034)—in green; (ii) cubic ZrO₂ from the RRUFF database (#R040142)—in red; (iii) zircon rim in the experiment at 900°C, caliche + NaCl—in orange; (iv) zircon rim in the experiment at 1000°C, NaCl, 30 min—in yellow; (v) zircon rim of the experiment at 1000°C, caliche + NaCl—in pink; (vi) dissociated zircon in an impact glass (Aouelloul, from Pellegrino, 2021)—in dark blue; and (vii) baddeleyite (Bdy) from the RRUFF Database (#R060078)—in light blue. (b) BSE image of dissociated zircon experimentally formed (1000°C, caliche + NaCl); (c) STEM-HAADF image of baddeleyite crystals in (b). (d, e) BSE images of naturally dissociated zircon in Aouelloul impact glass (Pellegrino, 2021). The colored boxes in (b) and (d) are indicative of the zoomed image in (c) and (e), and on the position in which the Raman spectra were acquired.</figcaption></figure>

the ZrO₂-CaO phase diagram (Duwez et al., 1952; Szczerba & Pędzich, 2010). At this temperature, since less SiO₂ is released from the dissociation of zircon

(and from the vessel), the Ca activity remains high in the melt, destabilizing the formation of baddeleyite at the rim of zircon. Increasing temperatures at 1000°C, more Ca reacts with the melted SiO₂ in the matrix, lowering Ca activity: baddeleyite is stable, and the remaining Ca in proximity to the zircon is retained in the Ca₂SiO₄ phase included between baddeleyite crystals in the rim (Figure 4).

2. **T = 1000°C, NaCl.** In experiments with only NaCl, the melting temperature of the SiO₂ glass of the vessel is also lowered, causing the inner part of the vessel to melt and form a layer of Na-rich glass that embeds zircon crystals, whose abundance is proportional to temperatures—(see Figure 3, left column). For these experiments, dissociation and formation of baddeleyite occur at 1000°C, but to a lesser extent. Baddeleyite forms just in fractures and inclusions of zircon (also documented in natural cases like Pica glass, see figure 3d in Schultz et al., 2022a), which may be due to the high Si activity of the melt (SiO₂ oversaturated system) or facilitated by the presence of weak points in the crystals, i.e., where there are inclusions and fractures.

Experimental and Natural Zircon Dissociation

No phases other than ZrO₂ and SiO₂ are documented in the dissociated rims of zircon found in impact glasses (Cavosie, Timms, Ferrière, et al., 2018; Timms, Erickson, Pearce, et al., 2017), presumed airbursts (e.g., Schultz et al., 2022a) and fulgurites (Kenny & Pasek, 2021). However, a positive identification of baddeleyite is not always presented, e.g. using Raman spectroscopy or diffraction patterns, and the zircon dissociation product is only evidenced as a heavier and Zr-rich compound (e.g., Bunch et al., 2021; Schultz et al., 1994, 2022a). Instead, in hydrothermal experiments (P = 130–1000 MPa, T = 400–900°C), when zircon is reacting with Ca-rich fluids (mainly CaCl₂ and Ca(OH)₂ diluted in H₂O) other Ca- or Ca/Zr-rich phases form together with baddeleyite (Geisler et al., 2001; Lewerentz et al., 2019), similarly to what we observed in our dry experiments (rims of baddeleyite plus Ca₂SiO₄ or cubic ZrO₂ incorporating ~10% CaO). The occurrence of different ZrO₂ polymorphs in zircon rims is documented in naturally dissociated zircon (Chinchalkar et al., 2024; Timms, Erickson, Zanetti, et al., 2017; Tolometti et al., 2022). The significance of cubic zirconia lies in its use as an indicator of temperatures >2370°C, providing evidence of a superheated melt during glass formation, and its identification is achieved through EBSD phase heritage/reconstruction from monoclinic zirconia (baddeleyite) (Cayron, 2007; Cayron et al., 2010). In our experiments, cubic zirconia crystallization at lower
temperatures is possible due to the stabilizing effect of CaO (as documented also for industrial applications by Swab, 2001; Duwez et al., 1952).

Dissociated zircon is documented in different cases of alleged impact or airburst glasses as evidence of extreme temperature conditions, and for this reason, deserves to be revisited, based on re-evaluation of the temperature estimate: Bronze Age archaeological sites (Bunch et al., 2021 and related papers, refuted by Jaret & Scott Harris, 2022); the various Argentinian Pampa glasses, including Rio Cuarto glass (Schultz et al., 1994, 2004); and Pica glass from the Atacama Desert in Chile (Schultz et al., 2022a, 2022b). Alternative explanations for these observations may be combustion in plant-rich soils (Melson & Potts, 2002; Svensen et al., 2003), dry vegetation fires (Baker & Baker, 1964; Vélain, 1878), or past human activity, including metallurgy (Chirikure et al., 2010; Thy et al., 1995), as reviewed by Cicconi et al. (2022).

The case of Pica glass has been used as a reference for our experiments (i.e., caliche soil from the Atacama Desert was used as a reagent). As for other cases, the identification of dissociated zircon was used as evidence of a very high-temperature process (~1700°C), only reachable on a large surface by the kinetic energy dissipation of a high-velocity extraterrestrial body or by a lightning strike. The lack of an impact crater and glass transport as ejecta was explained by an atmospheric airburst (e.g., Boslough & Crawford, 2008). However, we propose that lower-temperature thermal events affecting the Atacama Desert surface, where caliche and salt-rich soils are ubiquitous (Voigt et al., 2020), could produce dissociated zircon at temperatures as low as ~900°C, that do not necessarily imply events like meteoritic impacts and airbursts. The baddeleyite ($ZrO_2$) formed at 1000°C in our experiments is texturally and spectroscopically similar to rims formed around zircon dissociated by impact events (Figure 6b,d). Our experiments, however, differ from dissociated zircon reported in the literature by the presence of an interstitial Ca-rich olivine between baddeleyite crystals and the finer grain size of baddeleyite (coarser grains would be likely produced with longer duration or higher temperature). One experiment showed a Zr-rich rim composed not of baddeleyite but of a cubic $ZrO_2$ with 10% CaO, which could have been easily mistaken for baddeleyite due to its small grain size if diffraction analysis had not been performed.

For this reason, we suggest that a more careful approach is needed in the use of dissociated zircon and the presence of baddeleyite to establish the origin of unknown glasses (when other diagnostic shock features are not present). Moreover, a detailed characterization of rims for naturally dissociated zircon is not always done, and the diversity between natural and experimental cases

could be due to the lack of in-depth investigation of the variety of natural samples documented.

Finally, it is necessary to recognize that this experimental study is an oversimplified first attempt to explore the question of zircon dissociation in the presence of salts, focusing on very specific, extremely alkaline conditions where zircon crystals are not embedded in the host rock but are in direct contact with the sediments or soil. In the future, further investigations should explore more heterogeneous and common compositional conditions to assess the behavior of zircon crystals under varying circumstances.

## CONCLUSIONS

The detection of dissociated zircon in glasses, alone, cannot be used as an unequivocal geothermometer for the identification of impact or airburst glasses. Experiments conducted at atmospheric pressure have shown that the dissociation of zircon, forming symplectic $ZrO_2$ rims, can occur at a much lower temperature ($T \ge 900°C$) than the ones previously assumed (~1670°C) if zircon reacts in a more complex chemical system, e.g., in the presence of evaporite crusts rich in Ca-sulfates and NaCl acting as flux. Since the occurrence, abundance, thickness, and composition of these rims might be related to different variables (e.g., temperature, heating duration, matrix composition, presence of impurities, etc.), the minimal P–T conditions required for the dissociation of zircon should be used carefully to assess the origin of glasses. At the same time, since concentrations of Na and Ca can be very variable at a small scale in a heterogeneous sample, glass samples formed during the same geological event can show dissociated zircon or not.

*Acknowledgments*—We thank Philippe Pengrech for his help in the early stages of this study. Marc Poujol kindly provided the zircon crystals. We benefited from the experience of Simonpietro Di Pierro with industrial glasses. This work was supported by ANR ET-Megafire contract (ANR-21-CE49-0014-03). We thank Daniel Borschneck for XRD data acquisition. We thank Pierre Deschamps and Abel Guihou from the CEREGE Envitop analytical facility for their analytical and administrative support. Envitop has received funding from “Excellence Initiative” of Aix Marseille University A*MIDEX—DATCARB project, a French “Investissement d'avenir” program. PMZ and AMSG thank the consortium Lyon/Saint Etienne de Microscopies (CLYM) for access to the TEM. Two reviewers (W. Hyde and N. Timms) as well as the associate editor (G. Osinski) are thanked for their in-depth efforts toward improving the first version of our manuscript.
*Data Availability Statement*—The data that support the findings of this study are available from the corresponding author upon reasonable request.

*Editorial Handling*—Dr. Osinski Gordon

## REFERENCES

Abdelkader, A. M., Daher, A. M., and El-Kashef, E. 2008. Novel Decomposition Method for Zircon. *Journal of Alloys and Compounds* 460: 577–580.

Aftosmis, M. J., Mathias, D. L., and Tarano, A. M. 2019. Simulation-Based Height of Burst Map for Asteroid Airburst Damage Prediction. *Acta Astronautica* 156: 278–283.

Baker, G., and Baker, A. A. 1964. Hay-silica glass from Gnarkeet, Western Victoria. *Memoirs of the National Museum of Victoria* 26: 21–35.

Barboni, M., Marquardt, M., Timms, N. E., and Bell, E. A. 2024. HED Zircons as a Window into the Solar system's First Crust: Decoupling Primordial Differentiation, Metamorphism and Impact Events through Textural and Chemical Studies. *Geochimica et Cosmochimica Acta* 376: 113–133.

Boslough, M. B. E., and Crawford, D. A. 2008. Low-Altitude Airbursts and the Impact Threat. *International Journal of Impact Engineering* 35: 1441–48.

Bunch, T. E., LeCompte, M. A., Adedeji, A. V., Wittke, J. H., Burleigh, T. D., Hermes, R. E., Mooney, C., et al. 2021. A Tunguska Sized Airburst Destroyed Tall el-Hammam a Middle Bronze Age City in the Jordan Valley near the Dead Sea. *Scientific Reports* 11: 1–64.

Cavosie, A. J., and Koeberl, C. 2019. Overestimation of Threat from 100 Mt-Class Airbursts? High-Pressure Evidence from Zircon in Libyan Desert Glass. *Geology* 47: 609–612.

Cavosie, A. J., Timms, N. E., Erickson, T. M., Hagerty, J. J., and Hörz, F. 2016. Transformations to Granular Zircon Revealed: Twinning, Reidite, and ZrO₂ in Shocked Zircon from Meteor Crater (Arizona, USA). *Geology* 44: 703–6.

Cavosie, A. J., Timms, N. E., Erickson, T. M., and Koeberl, C. 2018. New Clues from Earth's most Elusive Impact Crater: Evidence of Reidite in Australasian Tektites from Thailand. *Geology* 46: 203–6.

Cavosie, A. J., Timms, N. E., Ferrière, L., and Rochette, R. 2018. FRIGN Zircon—The Only Terrestrial Mineral Diagnostic of High-Pressure and High Temperature Shock Deformation. *Geology* 46: 891–94.

Cayron, C. 2007. ARPGE: A Computer Program to Automatically Reconstruct the Parent Grains from Electron Backscatter Diffraction Data. *Journal of Applied Crystallography* 40: 1183–88.

Cayron, C., Douillard, T., Sibil, A., Fantozzi, G., and Sao-Jao, S. 2010. Reconstruction of the Cubic and Tetragonal Parent Grains from Electron Backscatter Diffraction Maps of Monoclinic Zirconia. *Journal of the American Ceramic Society* 93: 2541–44.

Chinchalkar, N. S., Osinski, G. R., Erickson, T. M., and Cayron, C. 2024. Zircon Microstructures Record High Temperature and Pressure Conditions during Impact Melt Evolution at the West Clearwater Lake Impact Structure, Canada. *Earth and Planetary Science Letters* 636: 118714.

Chirikure, S., Heimann, R. B., and Killick, D. 2010. The Technology of Tin Smelting in the Rooiberg Valley, Limpopo Province, South Africa, ca. 1650–1850 CE. *Journal of Archaeological Science* 37: 1656–69.

Cicconi, M. R., McCloy, J. S., and Neuville, D. R. 2022. Non-Magmatic Glasses. *Reviews in Mineralogy and Geochemistry* 87: 965–1014.

Cliff, G., and Lorimer, G. W. 1975. The Quantitative Analysis of Thin Specimens. *Journal of Microscopy* 103: 203–7.

Crow, C. A., Moser, D. E., and McKeegan, K. D. 2019. Shock Metamorphic History of >4 Ga Apollo 14 and 15 Zircons. *Meteoritics & Planetary Science* 54: 181–201.

De La Peña, F., Prestat, E., Tonaas Fauske, V., Burdet, P., Lähemann, J., Jokubauskas, P., Furnival, T., et al. 2024. *hyperspy/hyperspy*: v2.0.1 (v2.0.1). Switzerland: Zenodo.

Dressler, B. O., and Reimold, W. U. 2001. Terrestrial Impact Melt Rocks and Glasses. *Earth-Science Reviews* 56: 205–284.

Duwez, P., Odell, F., and Brown, F. H., Jr. 1952. Stabilization of Zirconia with Calcia and Magnesia. *Journal of the American Ceramic Society* 35: 107–113.

El Goresy, A. 1965. Baddeleyite and its Significance in Impact Glasses. *Journal of Geophysical Research* 70: 3453–56.

Erickson, T. M., Kirkland, C. L., Timms, N. E., Cavosie, A. J., and Davison, T. M. 2020. Precise Radiometric Age Establishes Yarrabubba, Western Australia, as Earth's Oldest Recognised Meteorite Impact Structure. *Nature Communications* 11: 300.

Essene, E. J., and Fisher, D. C. 1986. Lightning Strike Fusion: Extreme Reduction and Metal-Silicate Liquid Immiscibility. *Science* 234: 189–193.

French, B. M. 1998. *Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures* No. LPI Contribution 954, Houston, Texas: Lunar and Planetary Institute.

Geisler, T., Pidgeon, R. T., Kurtz, R., Van Bronswijk, W., and Schleicher, H. 2003. Experimental Hydrothermal Alteration of Partially Metamict Zircon. *American Mineralogist* 88: 1496–1513.

Geisler, T., Schaltegger, U., and Tomaschek, F. 2007. Re-Equilibration of Zircon in Aqueous Fluids and Melts. *Elements* 3: 43–50.

Geisler, T., Seydoux-Guillaume, A. M., Wiedenbeck, M., Wirth, R., Berndt, J., Zhang, M., Mihailova, B., Putnis, A., Salje, E. K., and Schlüter, J. 2004. Periodic Precipitation Pattern Formation in Hydrothermally Treated Metamict Zircon. *American Mineralogist* 89: 1341–47.

Geisler, T., Ulonska, M., Schleicher, H., Pidgeon, R. T., and van Bronswijk, W. 2001. Leaching and Differential Recrystallization of Metamict Zircon under Experimental Hydrothermal Conditions. *Contributions to Mineralogy and Petrology* 141: 53–65.

Glass, B. P. 1990. Tektites and Microtektites: Key Facts and Inferences. *Tectonophysics* 171: 393–404.

Glass, B. P., Koeberl, C., Blum, J. D., Senftle, F. E., Izett, G. A., Evans, B. W., Thorpe, A. N., Povenmire, H., and Strange, R. 1995. A Muong Nong-Type Georgia Tektite. *Geochimica et Cosmochimica Acta* 59: 4071–82.

Glass, B. P., and Simonson, B. M. 2012. *Distal Impact Ejecta Layers: A Record of Large Impacts in Sedimentary Deposits*. Berlin-Heidelberg, Germany: Springer-Verlag.

Horita, Z., Sano, T., and Nemoto, M. 1987. Simplification of X-Ray Absorption Correction in Thin-Sample Quantitative Microanalysis. *Ultramicroscopy* 21: 271–76.
Jaret, S. J., and Scott Harris. R. 2022. No Mineralogic or Geochemical Evidence of Impact at Tall el-Hammam, a Middle Bronze Age City in the Jordan Valley near the Dead Sea. *Scientific Reports* 12: 5189.

Kaiser, A., Lobert, M., and Telle, R. 2008. Thermal Stability of Zircon (ZrSiO₄). *Journal of the European Ceramic Society* 28: 2199–2211.

Kenny, G. G., and Pasek, M. A. 2021. The Response of Zircon to the Extreme Pressures and Temperatures of a Lightning Strike. *Scientific Reports* 11: 1560.

Kleinmann, B. 1968. The Breakdown of Zircon Observed in the Libyan Desert Glass as Evidence of its Impact Origin. *Earth and Planetary Science Letters* 5: 497–501.

Kovaleva, E., Kusiak, M. A., Kenny, G. G., Whitehouse, M. J., Habler, G., Schreiber, A., and Wirth, R. 2021. Nano-Scale Investigation of Granular Neoblastic Zircon, Vredefort Impact Structure, South Africa: Evidence for Complete Shock Melting. *Earth and Planetary Science Letters* 565: 116948.

Lewerentz, A. 2011. “Experimental Zircon Alteration and Baddeleyite Formation in Silica Saturated Systems: Implications for Dating Hydrothermal Events.” MS thesis, Lund University.

Lewerentz, A., Harlov, D. E., Scherstén, A., and Whitehouse, M. J. 2019. Baddeleyite Formation in Zircon by Ca-Bearing Fluids in Silica-Saturated Systems in Nature and Experiment: Resetting of the U–Pb Geochronometer. *Contributions to Mineralogy and Petrology* 174: 1–25.

Licht, A., Folch, A., Sylvestre, F., Yacoub, A. N., Cogné, N., Abderamane, M., Guihou, A., et al. 2024. Provenance of Aeolian Sands from the Southeastern Sahara from a Detrital Zircon Perspective. *Quaternary Science Reviews* 328: 108539.

Martell, J., Alwmark, C., Holm-Alwmark, S., and Lindgren, P. 2021. Shock Deformation in Zircon Grains from the Mien Impact Structure, Sweden. *Meteoritics & Planetary Science* 56: 362–378.

Melson, W. G., and Potts, R. 2002. Origin of Reddened and Melted Zones in Pleistocene Sediments of the Olorgesailie Basin, Southern Kenya Rift. *Journal of Archaeological Science* 29: 307–316.

Morris, P. L. 1980. The Correction of Thin Foil Microanalysis Data for X-Ray Absorption Effects. *Electron Microscopy and Analysis* 1979. *Institute of Physics Conference Series* 52: 413–16.

Murakami, T., Chakoumakos, B. C., Ewing, R. C., Lumpkin, G. R., and Weber, W. J. 1991. Alpha-decay event damage in zircon. *American Mineralogist* 76: 1510–32.

Nasdala, L., Hanchar, J. M., Rhede, D., Kennedy, A. K., and Váczi, T. 2010. Retention of Uranium in Complexly Altered Zircon: An Example from Bancroft, Ontario. *Chemical Geology* 269: 290–300.

Nasdala, L., Kronz, A., Wirth, R., Váczi, T., Perez-Soba, C., Willner, A., and Kennedy, A. K. 2009. The Phenomenon of Deficient Electron Microprobe Totals in Radiation-Damaged and Altered Zircon. *Geochimica et Cosmochimica Acta* 73: 1637–50.

Nasdala, L., Lengauer, C. L., Hanchar, J. M., Kronz, A., Wirth, R., Blanc, P., Kennedy, A. K., and Seydoux-Guillaume, A. M. 2002. Annealing Radiation Damage and the Recovery of Cathodoluminescence. *Chemical Geology* 191: 121–140.

Nasdala, L., Wenzel, M., Vavra, G., Irmer, G., Wenzel, T., and Kober, B. 2001. Metamictisation of Natural Zircon:

Accumulation Versus Thermal Annealing of Radioactivity-Induced Damage. *Contributions to Mineralogy and Petrology* 141: 125–144.

Osinski, G. R., Grieve, R. A., Bleacher, J. E., Neish, C. D., Pilles, E. A., and Tornabene, L. L. 2018. Igneous Rocks Formed by Hypervelocity Impact. *Journal of Volcanology and Geothermal Research* 353: 25–54.

Palenik, C. S., Nasdala, L., and Ewing, R. C. 2003. Radiation damage in zircon. *American Mineralogist* 88: 770–781.

Pellegrino, V. 2021. “Conditions de formation des verres d’impact d’Aouelloul (Mauritanie) et de Darwin (Australie), et comparaison aux tectites.” Master’s thesis, Aix-Marseille Université.

Pidgeon, R. T. 2014. Zircon radiation damage ages. *Chemical Geology* 367: 13–22.

Poujol, M., Jaguin, J., Moyen, J. F., Boulvais, P., and Paquette, J. L. 2021. Archaean S-Type Granites: Petrology, Geochemistry and Geochronology of the Lekkersmaak and Willie Plutons, Kaapvaal Craton, South Africa. *South African Journal of Geology* 124: 87–110.

Rochette, P., Alaç, R., Beck, P., Brocard, G., Cavosie, A. J., Debaille, V., Devouard, B., et al. 2019. Pantasma: Evidence for a Pleistocene circa 14 km Diameter Impact Crater in Nicaragua. *Meteoritics & Planetary Science* 54: 880–901.

Roperch, P., Gattacceca, J., Valenzuela, M., Devouard, B., Lorand, J.-P., Arriagada, C., Rochette, P., Latorre, C., and Beck, P. 2017. Surface Vitrification Caused by Natural Fires in Late Pleistocene Wetlands of the Atacama Desert. *Earth and Planetary Science Letters* 469: 15–26.

Roperch, P., Gattacceca, J., Valenzuela, M., Devouard, B., Lorand, J.-P., Rochette, P., Latorre, C., and Beck, P. 2022. Widespread Glasses Generated by Cometary Fireballs during the Late Pleistocene in the Atacama Desert, Chile: COMMENT. *Geology* 50: e550.

Salje, E. K. H., Chrosch, J., and Ewing, R. C. 1999. Is “Metamictization” of Zircon a Phase Transition? *American Mineralogist* 84: 1107–16.

Schmieder, M., Tohver, E., Jourdan, F., Denyszyn, S. W., and Haines, P. W. 2015. Zircons from the Acraman Impact Melt Rock (South Australia): Shock Metamorphism, U–Pb and 40 Ar/39 Ar Systematics, and Implications for the Isotopic Dating of Impact Events. *Geochimica et Cosmochimica Acta* 161: 71–100.

Schultz, P. H., Harris, R. S., Perroud, S., Blanco, N., and Tomlinson, A. J. 2022a. Widespread Glasses Generated by Cometary Fireballs during the Late Pleistocene in the Atacama Desert, Chile. *Geology* 50: 205–9.

Schultz, P. H., Harris, R. S., Perroud, S., Blanco, N., and Tomlinson, A. J. 2022b. Widespread Glasses Generated by Cometary Fireballs during the Late Pleistocene in the Atacama Desert, Chile: REPLY. *Geology* 50: e551.

Schultz, P. H., Koeberl, C., Bunch, T., Grant, J., and Collins, W. 1994. Ground Truth for Oblique Impact Processes: New Insight from the Rio Cuarto, Argentina, Crater Field. *Geology* 22: 71–100.

Schultz, P. H., Zárate, M., Hames, B., Koeberl, C., Bunch, T. E., Storzer, D., Renne, P. R., and Wittke, J. H. 2004. The Quaternary Impact Record from the Pampas, Argentina. *Earth and Planetary Science Letters* 219: 221–238.

Seydoux-Guillaume, A. M., Bingen, B., Paquette, J. L., and Bosse, V. 2015. Nanoscale Evidence for Uranium Mobility in Zircon and the Discordance of U–Pb Chronometers. *Earth and Planetary Science Letters* 409: 43–48.
Svensen, H., Dysthe, D. K., Bandlien, E. H., Sacko, S., Coulibaly, H., and Planke, S. 2003. Subsurface Combustion in Mali: Refutation of the Active Volcanism Hypothesis in West Africa. *Geology* 31: 581-84.

Swab, J. J. 2001. *Role of Oxide Additives in Stabilizing Zirconia for Coating Applications*. Rep. ARL TR-2591. Aberdeen, MD: Army Research Laboratory, Aberdeen Proving Ground.

Szczerba, J., and Pędzich, Z. 2010. The Effect of Natural Dolomite Admixtures on Calcium Zirconate–Periclase Materials Microstructure Evolution. *Ceramics International* 36: 535–547.

Thy, P., Segobye, A. K., and Ming, D. W. 1995. Implications of Prehistoric Glassy Biomass Slag from East-Central Botswana. *Journal of Archaeological Science* 22: 629–637.

Timms, N. E., Erickson, T. M., Pearce, M. A., Cavosie, A. J., Schmieder, M., Tohver, E., Reddy, S. M., Zanetti, M. R., Nemchin, A. A., and Wittmann, A. 2017. A Pressure-Temperature Phase Diagram for Zircon at Extreme Conditions. *Earth-Science Reviews* 165: 185–202.

Timms, N. E., Erickson, T. M., Zanetti, M. R., Pearce, M. A., Cayron, C., Cavosie, A. J., Reddy, S. M., Wittmann, A., and Carpenter, P. K. 2017. Cubic Zirconia in >2370°C Impact Melt Records Earth's Hottest Crust. *Earth and Planetary Science Letters* 477: 52–58.

Tolometti, G. D., Erickson, T. M., Osinski, G. R., Cayron, C., and Neish, C. D. 2022. Hot Rocks: Constraining the Thermal Conditions of the Mistastin Lake Impact Melt Deposits Using Zircon Grain Microstructures. *Earth and Planetary Science Letters* 584: 117523.

Vélain, C. 1878. Etude microscopique des verres résultant de la fusion des cendres de graminées. Production artificielle de la tridymite, de l'anorthite, de la Wollastonite et de l'augite. *Bulletin de la Société Minéralogique de France* 1: 113-124.

Vermeesch, P. 2018. IsoplotR: A Free and Open Toolbox for Geochronology. *Geoscience Frontiers* 9: 1479-93.

Voigt, C., Klipsch, S., Herwartz, D., Chong, G., and Staubwasser, M. 2020. The Spatial Distribution of Soluble Salts in the Surface Soil of the Atacama Desert and their Relationship to Hyperaridity. *Global and Planetary Change* 184: 103077.

## SUPPORTING INFORMATION

Additional supporting information may be found in the online version of this article.

**Fig S1.** XRD results for caliche soils. Gypsum (light green) and anhydrite (dark green) are the minerals present. The plot shows a semi-quantitative composition of 96% gypsum and 4% anhydrite based on the XRD spectrum. The reference peaks used for the identification are 96-901-3166 for gypsum and 01-072-0503 for anhydrite (intensities are not to scale). Peaks identifying anhydrite are marked with a capital 'A'.

**Fig S2.** Pb/U Concordia diagram of the 27 single zircon grains screened as concordant, with their 2 s uncertainty (circles). The calculated Concordia age and its 2 s uncertainty are also displayed. The two outliers that are not included in the Concordia age calculation are

West, J. B. 1954. *Caustic Treatment of Zircon Sand*. Ames, Iowa: Iowa State University.

West, A., Young, M., Costa, L., Kennett, J. P., Moore, C. R., LeCompte, M. A., Kletetschka, G., and Hermes, R. E. 2024. Modeling Airbursts by Comets, Asteroids, and Nuclear Detonations: Shock Metamorphism, Meltglass, and Microspherules. *Airbursts and Cratering Impacts* 2: e20240004.

Wittmann, A., Kenkmann, T., Schmitt, R. T., Hecht, L., and Stöffler, D. 2005. Shock metamorphism and thermal annealing of zircon in the ICDP-Chicxulub drill core Yaxcopoil-1 (abstract) Integrated Ocean Drilling Program/International Continental Drilling Program Joint Meeting. CD-ROM.

Wittmann, A., Kenkmann, T., Schmitt, R. T., and Stöffler, D. 2006. Shock-Metamorphosed Zircon in Terrestrial Impact Craters. *Meteoritics & Planetary Science* 41: 433–454.

Zanetta, P. M., Le Guillou, C., Leroux, H., Zanda, B., Hewins, R. H., and Bellino, G. 2022. Processes and Temperatures of FGR Formation in Chondrites. *Geochimica et Cosmochimica Acta* 319: 94–117.

Zanetta, P. M., Leroux, H., Le Guillou, C., Zanda, B., and Hewins, R. H. 2021. Nebular Thermal Processing of Accretionary Fine-Grained Rims in the Paris CM Chondrite. *Geochimica et Cosmochimica Acta* 295: 135–154.

Zanetti, M., Wittmann, A., Carpenter, P. K., Jolliff, B. L., Vicenzi, E. P., Nemchin, A. A., and Timms, N. E. 2015. Using EPMA, Raman LS, Hyperspectral CL, SIMS, and EBSD to Study Impact-Melt-Induced Decomposition of Zircon. *Microscopy and Microanalysis* 21: 1447–48.

Zhang, A. C., Hsu, W. B., Li, X. H., Ming, H. L., Li, Q. L., Liu, Y., and Tang, G. Q. 2011. Impact Melting of Lunar Meteorite Dhofar 458: Evidence from Polycrystalline Texture and Decomposition of Zircon. *Meteoritics & Planetary Science* 46: 103–115.

Zhang, M., Salje, E. K., Farnan, I., Graeme-Barber, A., Daniel, P., Ewing, R. C., Clark, A. M., and Leroux, H. 2000. Metamictization of Zircon: Raman Spectroscopic Study. *Journal of Physics. Condensed Matter* 12: 1915.

shown with empty circles. The plot and calculations were generated with IsoPlotR.

**Fig S3.** FMHM values of the $v_3(\text{SiO}_4)$ band in a random set of zircon crystals versus their global $\alpha$-dose radiation damage. The calibration curve for unannealed zircon is derived from Equation 4 in Palenik et al. (2003). The red squares indicate the global dose age of zircon crystals ($3.84 \times 10^{18} \ \text{\AA}/\text{g}$) whose FWHM has been measured (Table S2). The global $\alpha$-dose age of the zircon grains used in the present study deviates from the expected trend for unannealed zircon, indicating that they were subjected to annealing.

**Table S1.** Summary of U-Pb dating results of the zircon crystals used in the experiments.

**Table S2.** FWHM values of the $v_3(\text{SiO}_4)$ band and the corresponding frequencies in a random set of zircon crystals.