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

Most primary economic diamond deposits occur in kimberlite diatremes within Archean cratons, where thick, old lithosphere is thought to support diamond growth. Argyle, however, is hosted in olivine lamproite within a Paleoproterozoic orogen adjacent to Archean basement and has produced over 90% of all known pink diamonds. Its discovery shifted exploration to non-Archean terranes, yet the geodynamic drivers for this unusual diamondiferous complex remained unclear. Argyle lies in the Carr Boyd Basin within the Halls Creek Orogen at the southeastern margin of the Kimberley Craton, an intracontinental rift system influenced by Paleoproterozoic orogenies. The study aims to resolve the emplacement age of the Argyle lamproite and to identify the geodynamic context and trigger for its emplacement, testing whether supercontinent breakup-related extension could have facilitated volatile-rich ultramafic magmatism in this rifted orogenic zone.

Previous age estimates for the Argyle lamproite relied on whole-rock and altered phlogopite K-Ar and Rb-Sr dates ranging from ~1240 to 1110 Ma, but these systems are susceptible to alteration; prior work noted anomalously low K contents and partial chloritization in phlogopite, undermining reliability. Attempts at high-precision 40Ar/39Ar dating were complicated by excess argon, and even in situ Rb-Sr approaches are challenged by minor alteration and uncertainty about dyke-diatreme coevality. Regionally, detrital zircon spectra from the Carr Boyd Group indicate sources from the Hooper and Halls Creek Orogenies. Geodynamically, emplacement of volatile-rich ultramafic diatremes has been linked elsewhere to lithospheric extension during continental breakup, suggesting Argyle may also relate to breakup processes around the Mesoproterozoic Nuna supercontinent.

- Petrography: A high-grade drill core sample from the Argyle diatreme (AK1-Lh01) was studied. Two polished thin sections were imaged optically and with SEM (TIMA for automated mineralogy; BSE and CL imaging). The sample contains ~45% lamproite clasts (chlorite and serpentine after olivine and phlogopite), ~40% quartz clasts, ~10% calcite cement, and ~4% late-stage titanite rimming lamproite-detritus clusters. Textures indicate titanite formed shortly after emplacement via deuteric alteration, replacing perovskite with added silica from meteoric waters. - Mineral separation and mounts: ~2 kg of rock was disaggregated (SelFrag), concentrated by shaker table and heavy liquids, magnetically separated, and mounted/polished to expose interiors for imaging and in situ analyses. - U-Pb geochronology via LA-ICP-MS: Detrital apatite and detrital zircon were dated to provide maximum depositional ages; hydrothermal titanite was dated to constrain minimum alteration/emplacement age. Analytical setups used 193 nm excimer lasers and Agilent 8900 triple quadrupole ICP-MS, with appropriate reference materials and data reduction in Iolite. For apatite and titanite, common Pb was significant; 207Pb-corrected 206Pb/238U model dates and free-regressed discordia were used. Zircon analyses emphasized concordant data with common Pb negligible. - Titanite U-Pb via ID-TIMS: Ten titanite grains (previously analyzed by LA-ICP-MS) were leached and dated with isotope dilution TIMS (ET535 tracer), allowing higher-precision constraints and combined regression with LA-ICP-MS data. - Low-temperature thermochronology: Detrital apatite and zircon (U-Th)/He ages were obtained to assess exhumation timing relative to hydrothermal alteration, using Alphachron II extraction, He measurements, alpha-ejection correction, and ICP-MS for U, Th, Sm. - Plate reconstructions: GPlates with a full-plate model was used to position Argyle relative to the McArthur-Yanliao Gulf through time and to contextualize magmatic and sediment transport pathways.

- Emplacement age bracket: Detrital zircon yields a youngest coherent age of 1311 ± 9 Ma (206Pb/238U weighted mean of the two youngest grains), providing a robust maximum depositional/emplacement age for the Argyle lamproite. Hydrothermal titanite defines a combined LA-ICP-MS + ID-TIMS free-regressed lower concordia intercept of 1257 ± 15 Ma (MSWD = 1.2, n = 64 of 68), recording pervasive hydrothermal alteration shortly after emplacement and thus a conservative minimum emplacement age. Together, emplacement is bracketed between 1311 ± 9 Ma and 1257 ± 15 Ma (2σ), older than previously recognized. - Supporting geochronology: LA-ICP-MS titanite alone gives 1268 ± 40 Ma (MSWD = 1.3, n = 58); ID-TIMS subset gives 1271 ± 40 Ma (MSWD = 0.75, n = 6 of 10). Detrital apatite shows youngest coherent 207Pb-corrected model age of 1828 ± 6 Ma, consistent with derivation from Proterozoic sources and not reset post-orogeny. Apatite (U-Th)/He yields a weighted mean age of 121 ± 10 Ma (n = 7 of 8), linking to Early Cretaceous exhumation; zircon (U-Th)/He dates are overdispersed (~1150–185 Ma), providing only a broad minimum constraint and indicating partial retention/complex thermal history. - Petrography: Titanite is late-stage, anhedral, common-Pb rich, and rims mixed lamproite-detritus clasts, consistent with deuteric hydrothermal formation shortly after eruption. - Geodynamic interpretation: Timing of emplacement coincides with the onset of Nuna supercontinent breakup and the Derim Derim–Galiwinku–Yanliao LIP activity (ca. 1330–1295 Ma). Argyle’s position in the weak, rifted Halls Creek Orogen implies lithospheric extension enabled generation and ascent of low-degree volatile-rich melts (lamproites) along transcrustal pathways. - Broader context: Diamondiferous diatreme emplacement during continental breakup is likely widespread but under-recognized in rift zones bordering ancient cratons; global diatreme production peaks tend to lag breakup and correlate with elevated plate velocities (notably 1200–1100 Ma).

The integrated geochronology resolves Argyle’s emplacement to a narrow Mesoproterozoic interval contemporaneous with the initial stages of Nuna breakup. Detrital zircon maximum age (1311 ± 9 Ma) indicates rapid post-depositional volcanism or syn-sedimentary eruption, while hydrothermal titanite (1257 ± 15 Ma) captures early post-emplacement alteration, bracketing eruption. These constraints align with extensive dyking and volcanism of the Derim Derim–Galiwinku–Yanliao LIP around the McArthur–Yanliao Gulf and with expected sediment pathways delivering rare young zircons to the basin. In a rheologically weak rift zone adjacent to an Archean craton, modest additional heat and/or extension during breakup would have reactivated deep conduits, enabling rapid ascent of volatile-rich, low-degree melts. Comparisons with other cratonic margins during breakup episodes show that kimberlite/lamproite emplacement commonly accompanies extension, with global production peaks lagging breakup and coinciding with faster plate motions. Although preservation bias affects very old diatremes, the observed association with breakup surpasses preservation effects, underscoring continental extension as a first-order control on volatile mobilization and ultramafic diatreme magmatism.

This study tightly constrains the Argyle lamproite emplacement to 1311 ± 9 Ma (maximum) to 1257 ± 15 Ma (minimum), demonstrating that Argyle formed during the nascent breakup of the Nuna supercontinent. Petrographic and geochronological evidence indicates rapid emplacement followed by pervasive deuteric hydrothermal alteration recorded by titanite. The findings highlight lithospheric extension in a reactivated rift zone adjacent to an Archean craton as the principal trigger for ascent of volatile-rich ultramafic melts. More broadly, the work supports a strong temporal and mechanistic link between (super)continental breakup, plate velocity changes, and global kimberlite/lamproite magmatism. Future research should obtain similarly integrated geochronologic datasets from other rifted craton margins to test the breakup-emplacement linkage, refine paleogeographic reconstructions and breakup timing, and reassess exploration models for diamondiferous diatremes in under-explored ancient rift zones.

- Titanite LA-ICP-MS dates have low precision due to small grain size, low U, and high common Pb; ID-TIMS data show scatter possibly from Pb loss, introducing uncertainty despite combined regressions. - Detrital apatite U-Pb dates are non-concordant and treated as 207Pb-corrected model ages dependent on assumed common Pb composition and closed-system behavior. - Zircon (U-Th)/He dates are overdispersed owing to partial retention/complex thermal histories, limiting tight eruption constraints from He thermochronology. - The exact timing of Nuna breakup remains imprecise due to sparse high-quality paleomagnetic data between 1320 and 1220 Ma, affecting correlation detail. - Preservation bias likely affects the global pre-Pangean diatreme record, complicating comparisons of emplacement frequency through deep time.