# Nanoscale distribution of Pb in monazite revealed by atom probe microscopy

D. Fougerouse, S.M. Reddy, D.W. Saxey, T.M. Erickson, C. Kirkland, W.D.A.

Rickard, Anne-Magali Seydoux-Guillaume, C. Clark, I.S. Buick

► To cite this version:

D. Fougerouse, S.M. Reddy, D.W. Saxey, T.M. Erickson, C. Kirkland, et al., Nanoscale distribution of Pb in monazite revealed by atom probe microscopy. Chemical Geology, 2018, 479, pp.251 - 258. 10.1016/j.chemgeo.2018.01.020. hal-01904668

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Nanoscale distribution of Pb in monazite revealed by atom probe
microscopy

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D. Fougerouse¹,², S.M. Reddy¹,², D.W. Saxey²,³, T. Erickson¹,⁴, C.L. Kirkland¹, W.D.A.

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Rickard²,³, A.-M. Seydoux-Guillaume⁵,⁶, C. Clark¹, I.S. Buick⁷

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¹School of Earth and Planetary Sciences, The Institute for Geoscience Research (TIGeR), Curtin University, GPO Box U1987, Perth, WA 6845, Australia.

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²Geoscience Atom Probe, Advanced Resource Characterisation Facility, John de Laeter Centre, Curtin University, GPO Box U1987, Perth, WA 6845, Australia

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³Department of Physics and Astronomy, Curtin University, GPO Box U1987, Perth, WA 6845, Australia

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⁴Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Boulevard, Houston, TX, 77058, USA

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⁵Université de Lyon, UJM-Saint-Etienne, UCA, CNRS, IRD, LMV UMR 6524, F-42023 Saint-Etienne, France

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⁶CNRS, Université Clermont Auvergne, IRD, OPGC, Laboratoire Magmas et Volcans, F-63000 Clermont-Ferrand, France

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⁷Department of Earth Sciences, Stellenbosch University, Matieland Private Bag X1, Stellenbosch, South Africa

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Abstract

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The widespread use of monazite (LREEPO₄) in U-Pb geochronology is underpinned by the assumption that it incorporates negligible amounts of Pb during initial growth, and that radiogenic Pb remains immobile after formation. We have investigated the nanoscale distribution of Pb in monazite from granulite facies rocks of the Sandmata Metamorphic Complex (Rajasthan, India) by atom probe microscopy to further understand the utility of monazite as a geochronometer. The studied monazite contains distinct 10 nm clusters,
enriched in Ca and with a bulk composition consistent with them being apatite (Ca₅(PO₄)₃(OH)), that are also enriched in Si and Pb relative to the monazite host. The ²⁰⁸Pb/²³²Th ratios of the clusters ranged from 1.1 ± 0.1 to 1.4 ± 0.2 (2σ), indicating that the clusters hold unsupported Pb. The ²⁰⁸Pb/²³²Th ratios of the whole specimen (including clusters) and the matrix alone are similar (<6% difference), indicating that the clusters formed shortly after monazite crystallisation by a phase exsolution mechanism that partitioned the initial common Pb and the minor radiogenic Pb into apatite. A volume-dependent analysis of the bulk monazite composition shows that a large variability in the Ca and, by proxy, Pb composition at small volumes (125 to 10,000 nm³) due to its heterogeneous distribution in the clusters, may have detrimental effects on radiometric dating with small analytical volumes. At larger volumes, including those used in EPMA and traditional isotopic dating methods (LA-ICPMS, SIMS), the variability of Pb content is negligible. However, the measured composition may result from the mixing of multiple reservoirs.

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## 42 Introduction

Monazite is a light rare earth element phosphate (LREEPO₄) widely used in the geosciences to date geological events. Monazite has been used to date the timing of diagenesis (Evans et al., 2002; Rasmussen et al., 2001), low-grade metamorphism (Del Río et al., 2009; Janots et al., 2008), high-grade metamorphism (e.g. (Ayers et al., 2002; Kelly et al., 2012; Laurent et al., 2016), stages of deformation during complex tectonic histories (Kirkland et al., 2016; Williams et al., 1999) and the timing of fluid movement on major structures (Kirkland et al., 2009).

Underpinning the widespread geochronological use of monazite is its ability to incorporate U and Th, but generally exclude Pb within its crystal structure during growth so there is little initial Pb (Parrish, 1990). In many circumstances, essentially all Pb in monazite can be related to radioactive decay of U and Th, which presents an ideal case for U-Pb geochronology. However, some studies have reported significant initial Pb fractions in monazite (Didier et al., 2013; Janots and Rubatto, 2014; Mottram et al., 2014; Seydoux-Guillaume et al., 2012)
indicating that a non-radiogenic Pb component can be present. The nature and the
mechanisms of incorporation of this initial Pb in monazite has not been extensively studied.

Previous studies have suggested that initial Pb in monazite is due to surface contamination
during sample preparation, the presence of Pb in cracks and inclusions or the mixing of
nanoscale Pb reservoirs (Grand'Homme et al., 2016; Seydoux-Guillaume et al., 2003).

Another assumption for the use of monazite in U-Th-Pb geochronology is that Pb produced
by the radioactive decay of U and Th remains immobile, or "trapped" in the crystal lattice and
that the isotopic parent-daughter pair remains coupled at the scale of the analytical volume.

However, discordance is has been observed in monazite from polymetamorphic terrains
indicating that Pb can be mobile at a scale of >10 µm (Bingen and van Breemen, 1998;
Cocherie et al., 1998). The mechanisms responsible for Pb mobility in minerals operate at the
nanoscale (Grand'Homme et al., 2017; Grand'Homme et al., 2016; Seydoux-Guillaume et al.,
2003). However, such mechanisms have been previously inferred only from comparatively
large scale analytical volumes (Crowley and Ghent, 1999; Goncalves et al., 2005; Paquette
and Tiepolo, 2007; Tilton, 1960). In this paper, atom probe microscopy (APM) is used to
investigate the nanoscale dissemination of Pb in a monazite grain to provide insight into
common Pb distribution and solid state Pb mobility. APM has the unique ability to accurately
measure the trace element and isotopic composition of minerals in three-dimensions and at
sub-nanometre resolution as demonstrated in zircon (Peterman et al., 2016; Valley et al.,
2014; Valley et al., 2015) and baddeleyite (White et al., 2017). This study focusses on the
distribution of Pb in two low strain (<0.5° misorientation; fig. 1) Paleoproterozoic domains of
the monazite studied by Erickson et al. (2015). These domains are characterised by low
fractions of initial ²⁰⁶Pb ranging between 0.08 and 0.73% (Erickson et al., 2015; their data
repository).

The studied monazite grain comes from granulite-facies rocks of the Sandmata Complex,
Rajasthan, India (Fig.1). The host rock has experienced two high-temperature metamorphic
events. M1<sub>sc</sub> records conditions of ~7–10 kbar and 800–900 °C and has been dated at ~1720
Ma, whilst the ~1000 Ma M2<sub>sc</sub> event took place at ~5–7 kbar and 600–750 °C (Buick et al.,
2006). Monazite grains within the matrix of the studied sample record a complex Mesoproterozoic age distribution, as determined by U-Th-Pb radiometric dating, with peaks at ca. 1700 Ma and 1200 Ma, and minor Neoproterozoic ages at ca. 900 Ma (Buick et al., 2010). Electron Backscattered Diffraction (EBSD) data and correlated secondary ion mass spectrometry (SIMS) U/Pb analyses reveal systematic Pb-loss with progressive lattice strain and complete age-resetting in neoblasts formed by dynamic recrystallization at 970 ± 14 Ma (2σ, n = 6, MSWD = 1.3; Erickson et al., 2015), within error of the age of amphibolite-facies metamorphism from the adjacent Mangalwar Complex (Buick et al., 2006).

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## Methods

**Focused ion beam.** Site specific sample preparation for atom probe needle-shaped specimen was performed using the Tescan Lyra3 dual Focused Ion Beam Scanning Electron Microscope (FIB-SEM) at Curtin University using the lift-out method described in detail elsewhere (Thompson et al., 2007). The Ga⁺ beam was operated at 30kV for the lift out and atom probe needle shaping, with low voltage (5kV) being used for a final polishing stage to remove effects of surface knock-on damage created by the high energy Ga beam. The same instrument, fitted with an Oxford Instruments X-max EDS detector, was used for the energy dispersive x-ray (EDS) measurements of the composition of the monazite, using an accelerating voltage of 20 keV and beam current of 1 nA. Three EDS point analyses (500,000 counts each) were performed on homogenous flat polished regions adjacent to the atom probe lift-out. The EDS data was analysed using Oxford AZtec software. Quantification results were normalised to 100 at.% with oxygen calculated by stoichiometry (2 ions). The trace elements Tb, Li, Y, U and Pb were below the detection limit of EDS and as such were not included in the quantification. Uncertainties were determined as two standard deviations from the three analyses.

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**Atom probe microscopy (APM).** APM utilises field evaporation of atomic or molecular species from a needle-shaped specimen which are instantly ionised and accelerated to hit a
position sensitive detector. All species are identified by time-of-flight mass spectrometry by ranging of mass-to-charge ratio (Da) spectra (e.g., Fig. 2). Each peak in the mass spectrum is identified based on mass, charge state (1+ to 3+), and relative isotopic abundances. The original lateral (X, Y) location of the identified species within the specimen needle can be reconstructed from the detector impact location, whilst the depth (Z location) of the atom within the specimen is determined from the order in which the atoms field-evaporate.

Atom probe microscopy was undertaken on the Advanced Resource Characterisation Facility's Geoscience Atom Probe (Cameca LEAP4000X HR) housed in the John de Laeter Centre, Curtin University, Perth, Australia. The instrument was operated in laser mode with a UV laser (λ = 355 nm), pulse energy of 100 pJ, pulse rate of 125 kHz, base specimen temperature of 50 K and evaporation rate of 0.01 atoms/pulse. Two datasets, each of ~16 million ions, were collected before the specimens fractured. IVAS v3.6.12 processing software was used to reconstruct and analyse the data. Ca-rich clusters were observed in the reconstructed 3D data. In order to minimise cross-contamination between different compositional domains, a 2.5 at.% Ca isoconcentration surface (Hellman et al., 2000) was used to isolate the composition of the matrix, a 5 at.% Ca surface for the isotopic analysis of the clusters. Proximity histograms representing the composition of the monazite from the centre of the cluster to a maximum radius of 10 nm with a resolution of 0.5 nm increments (Hellman et al., 2000) were used to investigate the composition transition between the cluster and the host.

No instrumental drift standardisation has yet been established in atom probe microscopy. Data quality, in terms of background noise, mass peak overlaps and detector saturation, is assessed from the data sets themselves, with uncertainties typically comparable to those arising from the counting statistics for each atomic species (Peterman et al., 2016; White et al., 2017). Due to peak interferences on the thermal tail of the ThO<sup>++</sup> peak, each ion type used for isotopic quantification was measured using a narrow range of fixed width (0.1 Da). This procedure reduces the number of counts for each ion, but provides a better basis for quantitative comparison between isotopes. This method is equivalent to 'full-width' ranging
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applied at a specified fraction of the peak maximum, with the reasonable assumption that the
peak shapes are sufficiently similar between mass peaks within a single acquisition, and that
the peaks are well above the background noise (Hudson et al., 2011).

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Results

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The atom probe mass-to-charge spectrum of monazite is complex due to the presence of
single ion species and complicated molecular species of REEs with different charge states
(Fig. 2). The compositions of the monazite derived by atom probe data are deficient in O and
P and enriched in REE compared to the EDS data in the same domain (Table 1) and the
expected stoichiometry for monazite. Similar discrepancies in stoichiometry have been
reported in some other complex oxide phases and has been attributed to a number of potential
mechanisms, including the dissociation of complex molecules during the flight path, the
creation and evaporation of neutral species, and the ranging of the 16 Da peak as ¹⁶O⁺ despite
overlap with the dimer ¹⁶O₂⁺⁺ resulting in O underestimation (Bachhav et al., 2011; Gault et
al., 2016; Karahka et al., 2015; Saxey, 2011).

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Despite the complexity of the spectrum, the ²⁰⁶Pb⁺⁺ and ²⁰⁸Pb⁺⁺ peaks are clearly identifiable above the background noise at 103 Da and 104 Da respectively (Fig. 2, inlay). However, the minor ²⁰⁶Pb signal and relatively high background levels introduce a large uncertainty in its quantification and for this reason we have not reported ²⁰⁶Pb values. No peaks were identifiable for ²⁰⁴Pb and ²⁰⁷Pb and therefore are not quantified in this study. Th has been quantified using four different peaks, primarily as ThO⁺⁺ (~120 Da), but also as ThPO₃⁺⁺⁺ (~103.7 Da), ThPO₄⁺⁺ (~163.5 Da) and as ThP₂O₆⁺⁺⁺ (~195 Da). Interference of the ThO⁺⁺ peak tail is caused by P₅O₅REE molecules (Fig. 2) altering the precise quantification of the Th concentration, and inevitably leading to the underestimation of the total Th content. ²³⁸UO₂⁺⁺ has been identified at mass 135 Da, however ²³⁵UO₂⁺⁺ is below the detection limit of our analysis and cannot be quantified. The ²⁰⁸Pb/²³²Th ratios measured from the whole specimen and matrix domain are elevated when compared to the expected age of the studied grain

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167 (Erickson et al., 2015) because the APM measurement appears to have underestimated the
168 Th content of the monazite. The presence of initial Pb would also increase this ratio.

169 The atom probe results reveal the presence of ~10 nm diameter clusters enriched in Ca, Si
170 and Pb in a homogeneous matrix for both datasets (Fig. 3, 4). The proximity histogram profile
171 for the largest cluster defined by a 1.3 at.% Ca isoconcentration surface (Fig. 4) shows a
172 gradual compositional change over several nanometres with an increase of Ca, Si and Pb and
173 decrease of O, P and REE. As shown on the proxigram (Fig. 4C), the Ca concentration varies
174 from 1 at.% outside of the cluster to ~10 at.% in the centremost domain of the cluster. Pb and
175 Si exhibit the same trends with an increase superior to one order of magnitude in the cluster
176 compared to the matrix. Conversely, all other element abundances decrease in the clusters
177 (Fig. 4, table 1). Notably, the O content decreases by ~3 at % and P by ~2 at.%. Th does not
178 show any clear trend with values in the same range between the different domains. No F or
179 Cl was measured in the clusters or matrix and the presence of background H in the atom
180 probe analysis chamber precludes the discrimination of oxygen isotope peaks and overlapping
181 hydroxyl and water molecules.

182 In specimen 1 the Isotopic composition of the clusters defined by a 2.5 at.% isosurface has a
183 ²⁰⁸Pb/²³²Th ratio of 1.1 ± 0.1 (1σ) In contrast, the matrix has a ²⁰⁸Pb/²³²Th ratio of 0.1304 ±
184 0.0016 and the whole specimen, including the matrix and clusters, yields a ²⁰⁸Pb/²³²Th ratio of
185 0.1384 ± 0.0016. In specimen 1 the difference between the ²⁰⁸Pb/²³²Th ratio in the whole
186 specimen and the matrix is 6 ± 2 %.

187 In specimen 2 the ²⁰⁸Pb/²³²Th ratio is 1.4 ± 0.2 (1σ) for the clusters, 0.1152 ± 0.0015 for the
188 matrix and 0.1178 ± 0.0015 for the whole specimen. In this specimen the ²⁰⁸Pb/²³²Th ratios
189 are within error between the whole specimen and the matrix, but the difference between
190 absolute values is 2 ± 2 %.

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192 Discussion

193 Nanoscale compositional heterogeneities in monazite
The reconstructed atom probe data show variations in major and trace element distribution. These variations are manifest as clusters that are depleted in O, P and REEs and enriched in Ca, Pb and Si. Similar nanoscale clusters were observed in monazite by transmission electron microscopy (TEM) and were suggested to be a Pb-Ca-Si-O phase such as margarosanite (Ca₂PbSi₃O₉), represent material that crystallized in fluid inclusions, or be a local domain in the monazite structure, high in Pb, like the brabantite (CaTh(PO₄)₂) end member (Seydoux-Guillaume et al., 2003). The chemical composition of the clusters measured by APM with ~10 at.% Ca, ~1 at.% Si and ~0.7 at.% Pb is however inconsistent with this interpretation.

The composition of the clusters is significantly different than the monazite host (O, P and REE decrease by a few at.% whereas Ca increases by ~9 at.%), and is more consistent with apatite Ca₅(PO₄)₃(OH, Cl, F). Pb and Si can substitute for Ca in apatite (Harrison et al., 2002; Rønsbo, 1989) and can account for the increased Si and Pb content of the clusters as observed in our APM data. The gradual compositional change between the two domains is also consistent with the interpretation that the cluster represent nano-scale inclusions (Fougerouse et al., 2016; Vurpillot et al., 2000).

## Timing of inclusion formation

Different models have been proposed for the formation of nano-inclusions. These include formation during annealing of radiation damages (Valley et al., 2014), dissolution-reprecipitation (Harlov et al., 2011), fluid supersaturation at the crystal/fluid interface (Gebauer et al., 2014; Markov, 2003) or phase immiscibility (Ferraris et al., 2005; Putnis, 1992). To discriminate between these potential models it is useful to consider the mobility of Pb associated with cluster formation and the timing of this mobility.

Previous studies investigating Pb mobility in zircons have concluded that radiation damage and metamictization of the crystal structure is a prerequisite for Pb mobility and formation of Pb-enriched clusters during high temperature metamorphism (Peterman et al., 2016; Valley et al., 2014; Valley et al., 2015). However, the fast annealing of radiation damage in monazite precludes the formation of metamict domains (Meldrum et al., 1998; Seydoux-Guillaume et
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al., 2002b) which could have enhanced Pb mobility. Pb diffusion in monazite is extremely slow and almost negligible at temperatures of ~1050 °C (Cherniak and Pyle, 2008; Cherniak et al., 2004; Gardés et al., 2006; Gardés et al., 2007; Harrison et al., 2002). Hence, volume diffusion is unlikely to be responsible for the segregation of Pb into already existing apatite nanoinclusions and therefore, Pb mobility and formation of apatite nanoinclusions are likely to be coupled.

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The Sandmata rock sample studied here does not record a significant hydrous retrograde metamorphic assemblage and has remained in a dry environment since granulite facies metamorphism and initial crystallisation of the monazite (Buick et al., 2006; Buick et al., 2010; Erickson et al., 2015). The chemical zonation of the monazite (Erickson et al., 2015, their Fig. 2), including preservation of epitaxial growth zoning, is therefore not consistent with either dissolution-reprecipitation (Erickson et al., 2016) or supersaturation of the fluid at the crystal/liquid interface (Gebauer et al., 2014; Markov, 2003).

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The phase immiscibility model is based on a potential miscibility gap of Ca in monazite similar to the miscibility gap between monazite and xenotime (Andrehs and Heinrich, 1998; Gratz and Heinrich, 1997; Heinrich et al., 1997; Seydoux-Guillaume et al., 2002a). Ca can be incorporated in the monazite structure by the brabantite/cheralite coupled substitution ($Ca^{2+}$ or $Pb^{2+}$ + $Th^{4+}$ or $U^{4+}$ = 2REE$^{3+}$; (Clavier et al., 2011; Spear and Pyle, 2002)). Excess Ca impurities can be assimilated in large quantities during rapid crystal growth favouring exsolution (unmixing) during slow cooling (Ferraris et al., 2005; Putnis, 1992; Watson, 1996; Watson and Liang, 1995), or during subsequent heating after a fast cooling (Putnis, 1978).

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In both of our samples, the $^{208}$Pb/$^{232}$Th ratio of the whole specimen and the matrix alone has a slight, but measurable difference of 6 ± 2 % in specimen 1 but are within uncertainty in specimen 2. This similarity between the two domains indicates that only a short time elapsed before the formation of the clusters. Our isotopic data are therefore in agreement with the formation of the inclusions by exsolution during the slow cooling of the Sandmata rocks, shortly after monazite crystallisation.

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This finding is different to that reported in previous studies in zircon were Pb mobility was linked to a metamorphic event several hundred million years after crystallisation (Peterman et al., 2016; Valley et al., 2014; Valley et al., 2015). In our sample, this similar isotopic ratio between whole specimen and matrix indicates that the $M_{2_{sc}}$ 600-750 °C metamorphic event at 1 Ga, some 700 Ma after monazite crystallisation cannot be responsible for the cluster formation. Hence the Pb mobility associated with neoblast formation during deformation, as documented by Erickson et al. (2015), represents a temporally distinct process from the segregation of Pb-enriched apatite inclusions.

The studied monazite grain hosts up to 0.73% initial $^{206}$Pb (Erickson et al., 2015). This initial Pb value is lower than the ~6% difference between the $^{208}$Pb/$^{232}$Th ratio of the whole specimen and matrix alone indicating that the Pb in the inclusions is a combination of initial Pb and radiogenic Pb produced before they formed. Additional radiogenic $^{208}$Pb, would also be produced within the inclusion from the decay of inclusion-hosted Th (Table 1).

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## Geochronological Implications of Nanocluster in Monazite

The presence of initial Pb in nanoscale inclusions in monazite has the potential to be detrimental for geochronology depending on their concentration and the scale of the analyses. If uncorrected, the presence of unsupported Pb would increase the calculated age. There are, for instance, no common Pb correction protocols for U-Th-Pb chemical dating using electron microprobe (EPMA; Montel et al., 1996). Common Pb correction protocols exist for some dating methods based on isotopic analyses (Andersen, 2002; Ludwig, 2003), but these methods assume a homogeneous common Pb distribution and the common Pb composition at the time of incorporation. The isotopic composition of common Pb is frequently estimated using the expected composition at the age of crystallization, based on a Pb evolution model for the Earth (Stacey and Kramers, 1975), or projected using a free regression through the uncorrected data in a Tera Wasserburg diagram (Williams, 1998).

The atom probe data reveals the presence of Pb clusters composed of initial Pb and radiogenic Pb. The geochronological significance of the Pb bearing nanoclusters identified by

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277 APM has been investigated using a spatial and statistical analysis of the 3D APM data set. In
278 this analysis the Ca content has been used as a proxy for Pb as it provides a better signal-to-
279 noise ratio and a more robust estimation of the effect of the nanoinclusions. Cubic volumes of
280 varying size (125 nm³ to 150,000 nm³) were sampled and analysed at evenly-spaced locations
281 throughout the three-dimensional atom probe data. Volumes composed of Ca concentrations
282 greater than the average Ca content of the monazite indicate the presence of a cluster (or
283 portion of one) within this volume, and hence also, a higher fraction of Pb. The Ca
284 concentration measurements, as a function of the cubic sampling volume (Fig. 5) indicate a
285 large variability in the Ca concentration for small sampling volumes, with values up to 20 at.%
286 Ca. This highlights the heterogeneity of Ca throughout the analysed material and the
287 dependence of measured values on the size of the analysis volume. By proxy, the Pb content
288 is also highly variable for small volumes and becomes decoupled from radioactive parents.
289 This variability decreases and is analytically negligible for sampling volumes greater than
290 100,000 nm³ (10⁻⁴ µm³).

291 Based on the size and the distribution of the clusters in the Sandmata monazite, our study
292 predicts that for analytical volumes used for commonplace dating methods such as Electron
293 microprobe (EPMA; ~0.5 µm³), Secondary Ion Mass Spectrometry (SIMS; ~250 µm³) and
294 Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS; 20,000 µm³), the
295 variability in the Pb fraction is negligible, as a sufficient quantity of clusters will have been
296 incorporated into the analytical volume to result in an average Pb concentration. However,
297 these analyses represent the mixing of multiple, discrete Pb isotopic reservoirs that do not
298 permit the initial Pb component present in the clusters to be accurately measured or corrected.
299

300 **Conclusion**

301 This research presents detailed trace element and isotopic compositional measurements of
302 monazite at the nanoscale to yield unique information about this widely used geochronometer.
303 We observed 10 nm apatite nano-inclusions in monazite which formed shortly after monazite
304 crystallisation and contain both initial Pb and radiogenic Pb. The inclusions formed by
exsolution of a Ca-bearing phase during the cooling of the rock and gathered excess Pb and Si from the monazite hosts. The inclusions spatial distribution and size control the amount of Pb signal that may end up in a given analytical volume. In the case of this monazite a negligible variability of inclusion load is found at analytical volumes >100,000 nm³. Atom probe microscopy provides a unique and complementary tool for characterising sub-micron elemental and isotopic variations in Pb distribution within monazite.

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312 Acknowledgements

The authors would like to acknowledge constructive reviews by Fermando Corfu, an anonymous reviewer and editorial handling by Catherine Chauvel. The Australian Resource Characterisation Facility (ARCF), under the auspices of the National Resource Sciences Precinct (NRSP) – a collaboration between CSIRO, Curtin University and The University of Western Australia – is supported by the Science and Industry Endowment Fund (SIEF RI13-01). ISB acknowledges funding under the NRF (South Africa) Incentives for Rated Researchers Scheme.

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517 Figures:
<figure><img src="image_2.png" /></figure>

Figure 1: A) Simplified geological map of the Sandmata granulite complex indicating the location of sample Raj59 modified from Buick et al. (2010). Abbreviations: AS - Aravalli Supergroup; BGC - Banded Gneiss Complex; DS - Delhi Supergroup. B) Crystallographic orientation electron backscattered diffraction map. Misorientation in each of the host domain grains is colour coded red from a reference point "+" to maximum of 16°; Neoblasts formed by dynamic recrystallization are colour coded yellow for misorientation ranging between 0 and 10° from the reference point "+"; Low-angle grain boundaries (1°) are coloured yellow, while (001), (100) and (122) deformation twins are plotted green, modified from Erickson et al. (2015). The atom probe microscopy (APM) samples were extracted from two weakly deformed domains (<0.5°) of the crystal, marked as open circles on the figure.

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<figure><img src="image_3.png" /><figcaption>Figure 2: Atom probe mass spectrum. Peaks are color-coded by atomic or molecular species.</figcaption></figure>

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<figure><img src="image_4.png" /><figcaption>Figure 3: Specimen 1: Reconstructed three-dimensional atom probe image of Ca, Si and Pb ($^{206}$Pb + $^{208}$Pb) distribution. Each sphere represents one atom. The maps show the presence of Ca-Si-Pb clusters.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 4: Specimen 2: A) Reconstructed three-dimensional atom probe image of Ca, Si and Pb ($^{206}$Pb + $^{208}$Pb) distribution. Each sphere represents one atom. The maps show the presence of three Ca-Si-Pb clusters. B) Close up of the biggest cluster. C) Proximity histogram profile based on a 1.3 at.% Ca isosurface (distance 0 represent the edge of the isosurface). The histogram shows the decrease of O, P and REEs but the increase of Ca, Si and Pb.</figcaption></figure>

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<figure><img src="image_6.png" /><figcaption>Figure 5: Box and whisker plot indicating the range of Ca concentration measurements that could be obtained depending on the scale of the cubic sampling volume applied to an APM dataset (specimen 2). Ca concentrations for volumes from 125 nm³ to 150,000 nm³ were generated from the three-dimensional atom probe data, by sampling cubic volumes at evenly-spaced locations throughout the data. Boxes indicate 95th percentile limits (approximately 2-sigma for Gaussian distributed data), and whiskers 99th percentile. Outliers beyond the 99.9th percentile are plotted in red for volumes up to 150,000 nm³. Whiskers and outliers are only shown if their percentile cut-offs are compatible with the sample size. The dashed line indicates the bulk composition, calculated from the full atom probe dataset.</figcaption></figure>

Table 1: Composition (major, trace and isotopic) of the different domains given by atom probe and EDX. The composition of a stoichiometric apatite is also given for reference.