Emplacement of the Argyle diamond deposit into an ancient rift zone triggered by supercontinent breakup

The majority of economic diamond deposits are found in Archean cratons, with their old and thick continental lithosphere considered crucial for diamond formation. However, the Argyle mine in Western Australia is a significant exception, being one of the few economic deposits within a Paleoproterozoic orogen adjacent to cratonic regions. Argyle's host rock is olivine lamproite, not kimberlite, and it has yielded the vast majority of all pink diamonds. Its discovery revolutionized diamond exploration by expanding it to non-Archean terranes. Despite its importance, the geodynamic processes responsible for its formation remain unclear. The Argyle lamproite is situated in the Carr Boyd Basin, a Meso- to Neoproterozoic basin within the Paleoproterozoic to Paleozoic Halls Creek Orogen. This region experienced intracontinental rifting between 1910 and 1805 Ma due to various tectonic drivers, including the Hooper and Halls Creek Orogenies. Argyle consists of four merged diatremes, with the most diamond-rich one located at the southern end. Previous age estimations using K-Ar and Rb-Sr dating on phlogopite ranged from 1240 to 1110 Ma, but these methods are susceptible to alteration and are deemed unreliable. This study aims to determine a more precise age for the Argyle deposit using a combination of petrographic observations, U-Pb geochronology of titanite, and U-Pb and (U-Th)/He dating of zircon and apatite, in order to understand the timing and mechanisms of its formation.

Existing literature highlights the association of most economic diamond deposits with kimberlite diatremes in Archean cratons. The thick and old lithosphere of these cratons is believed to provide the necessary conditions for sustained diamond growth. However, the Argyle deposit's occurrence in a Paleoproterozoic orogen, hosted in lamproite rather than kimberlite, challenges this paradigm. Previous attempts to date Argyle using K-Ar and Rb-Sr methods yielded inconsistent results, attributed to the extensive alteration of K-bearing minerals. The lack of a clear understanding of the geodynamic drivers behind the emplacement of such an unusual diamondiferous pipe complex motivated this research. The Carr Boyd Basin, where Argyle is located, is a Meso- to Neoproterozoic intracontinental basin within the Paleoproterozoic Halls Creek Orogen. Earlier work has identified various tectonic events impacting this region, including rifting episodes between 1910 and 1805 Ma, but the relationship to Argyle's formation remained uncertain. This study sought to build on this existing research through improved dating techniques and a comprehensive analysis of the geological context.

The study employed a multi-faceted approach combining petrographic analysis, U-Pb geochronology of titanite, and U-Pb and (U-Th)/He dating of zircon and apatite. A representative drill core sample (AKI-Lh01) from the high-grade mineralized diatreme was analyzed. Petrographic analysis, using techniques such as automated mineral analysis and backscattered electron imaging, revealed the mineral composition and textural relationships within the sample. This revealed a sample composed of lamproite clasts, quartz clasts, calcite cement, and late-stage titanite that rims quartz-lamproite clusters. The textural evidence indicated that the titanite post-dates lamproite emplacement and carbonate alteration, suggesting it formed from deuteric alteration shortly after emplacement. U-Pb dating using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was performed on detrital zircon and apatite to determine a maximum depositional age. Hydrothermal titanite was also dated using LA-ICP-MS to obtain a minimum age of hydrothermal alteration, effectively bracketing the lamproite emplacement. Selected titanite grains were re-dated using high-precision U-Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS) to improve precision. (U-Th)/He dating of detrital apatite and zircon was also conducted to understand the low-temperature history and test for potential thermal resetting. Data reduction was performed using VizualAge UComPbine and Iolite software, with appropriate corrections for mass fractionation and instrumental drift applied.

Petrographic analysis of sample AKI-Lh01 revealed a complex assemblage of lamproite clasts, quartz clasts, calcite cement, and late-stage titanite. The titanite's textural relationship indicates its formation shortly after lamproite emplacement. Detrital apatite yielded U-Pb ages centered around 1900–1820 Ma, consistent with derivation from rocks formed during the Hooper and Halls Creek orogenies. The youngest detrital apatite analyses yielded a weighted mean age of 1828 ± 6 Ma. Detrital zircon analyses showed a major peak at ~1870 Ma, again consistent with the Hooper Orogeny, along with minor peaks at various older ages. Importantly, two zircon grains yielded a weighted mean age of 1311 ± 9 Ma, providing a robust maximum depositional age for the Argyle lamproite. Hydrothermal titanite yielded a minimum alteration age using LA-ICP-MS of 1268 ± 40 Ma, refined to 1257 ± 15 Ma using ID-TIMS data. The (U-Th)/He dating of detrital apatite yielded an age of 121 ± 10 Ma, consistent with Early Cretaceous exhumation, while zircon (U-Th)/He ages were more dispersed. Combining the zircon maximum age and titanite minimum age provides the best age constraint for the Argyle lamproite emplacement between 1311 ± 9 Ma and 1257 ± 15 Ma. The presence of a ~1 mm diamond fragment within the sample further confirms the diamond-bearing nature of the deposit.

The emplacement of the Argyle lamproite at ~1300 Ma coincides with the breakup of the supercontinent Nuna. Plate tectonic reconstructions place Argyle at the periphery of the McArthur-Yanliao Gulf, a Mesoproterozoic basin. The youngest detrital zircon grains in Argyle are likely derived from the Derim Derim-Galiwinku-Yanliao large igneous province (LIP), which formed around the same time (~1330–1295 Ma). The Halls Creek Orogen, where Argyle is located, is a rheologically weak rift zone prone to reactivation. Heat from the LIP and/or mechanical extension from Nuna's breakup may have reactivated mantle-to-crust pathways, facilitating the ascent of volatile-rich partial melts (lamproites). Lithospheric extension during continental rifting is a known mechanism for the emplacement of kimberlites and lamproites. Examples exist from other cratons during various supercontinent breakup events. While Argyle's emplacement seems synchronous with initial Nuna breakup, the peak of diatreme emplacement globally occurred later (1200–1100 Ma), suggesting a link between extension and plate velocities on volatile-rich ultramafic diatreme production. Preservation bias likely affects the global record of volatile-rich ultramafic diatremes, but continental extension is a primary control on their formation, as shown by the greater abundance during periods of continental breakup. The exceptional preservation of the Argyle pipe is noteworthy, highlighting the fortuitous combination of burial, exhumation, and erosion that allowed for its discovery.

This study presents the most accurate age constraints for the Argyle lamproite emplacement, between 1311 ± 9 Ma and 1257 ± 15 Ma, through a combination of high-precision geochronological techniques. The results strongly support a link between supercontinent breakup and the formation of this significant diamond deposit. Future research could focus on further refining the timing of Nuna breakup and investigating similar deposits in other ancient rift zones to test the model's general applicability. This will advance our understanding of diamond genesis in non-Archean settings and the global distribution of diamondiferous pipes.

While the multi-mineral geochronological approach provided robust age constraints, inherent uncertainties associated with geochronological dating techniques remain. The interpretation of detrital ages relies on assumptions regarding sediment provenance and transport pathways. Additionally, the (U-Th)/He ages provide some constraints but are susceptible to uncertainties caused by alpha-ejection effects and He diffusion. These factors could potentially affect the precision of the age estimations.