Radiation damage allows identification of truly inherited zircon

The presence of inherited zircon (ZrSiO₄) – zircon grains with U-Pb dates older than their host rock – is a common feature in felsic rocks and increasingly reported in mafic and ultramafic rocks from oceanic and continental settings. The survival and retention of U-Pb dates in zircon under certain mantle conditions suggests zircon recycling through the mantle, potentially appearing in mantle peridotites or mantle-derived igneous rocks. However, experimental studies indicate that zircon readily dissolves in mafic melts, raising questions about the origin of zircon in these settings. This study investigates a Pliocene gabbro from a Mohns Ridge peridotite-gabbro complex, revealing Precambrian zircon grains significantly older than the expected 3-5 Ma age. The presence of such ancient zircon in young oceanic lithosphere has significant geodynamic implications, suggesting recycled continental material in the mantle source. However, contamination during sample handling remains a concern. While methods like fission tracks and (U-Th)/He thermochronology can shed light on zircon's thermal history, they are time-consuming and destructive. Raman spectroscopy offers an alternative, allowing in situ characterization of radiation damage to the zircon lattice. Experiments show that moderately radiation-damaged zircon restructures at >700 °C over hours to days. This means zircon incorporated into a melt will anneal, showing crystallinity similar to the host rock's zircon, while preserving the original U-Pb dates due to slow Pb diffusion. This paper aims to demonstrate how radiation damage measured via Raman spectroscopy can distinguish between contamination and truly inherited zircon, offering a rapid and non-destructive method to verify zircon origin and potentially revisit previous findings.

Previous research has extensively documented the occurrence of inherited zircon in various geological settings. Studies on mid-ocean ridges, oceanic hotspots, and island arcs have reported the presence of zircon grains significantly older than their host rocks, suggesting the recycling of continental material into the mantle. However, the interpretation of these findings has been challenged by the potential for contamination during sample collection and processing. While some studies have proposed that zircon can survive and retain its U-Pb dates even under high-temperature conditions in the mantle, others have demonstrated its susceptibility to dissolution in mafic melts. The existing methods for identifying inherited zircon, such as luminescence features and trace element compositions, are often inconclusive due to the overlap between zircon from different sources. The need for a robust and reliable method to distinguish between truly inherited zircon and contaminants has been highlighted by the uncertainties surrounding the interpretation of existing data.

The study involved two cases: Case 1, a Pliocene mid-ocean ridge gabbro from the Mohns Ridge, and Case 2, Cenozoic dacitic-trachydacitic sills from the Gjallar Ridge intruding Cretaceous sedimentary rocks. For Case 1, approximately 10 kg of gabbro underwent mineral separation to isolate zircon grains. 18 of 44 recovered grains were selected for U-Pb isotope analysis using LA-ICP-MS. Seven grains yielded Pliocene U-Pb ages (mean weighted average ²⁰⁶Pb/²³⁸U age of 4.15 ± 0.13 Ma), while ten grains displayed Precambrian ages (2724 to 554 Ma). Raman spectroscopy was employed to analyze radiation damage. For Case 2, 215 zircon grains from two samples were analyzed for U-Pb by LA-ICP-MS, resulting in 448 analytical spots. The samples yielded mean weighted average ²⁰⁶Pb/²³⁸U ages of 54.21 ± 0.13 Ma and 55.57 ± 0.12 Ma, consistent with magmatic sills in the Vøring basin. 79 analyses showed ages between 80 and 2200 Ma. Raman spectroscopy examined 87 grains (both young magmatic and old inherited), analyzing both rims and cores in 37 composite grains for a total of 116 measurements. Radiation damage was assessed by measuring the full-width half maximum (FWHM) of the ν₃ (SiO₄) peak position in the Raman spectra and calculating alpha doses using U and Th concentrations. A calibration curve of unannealed zircon grains allowed calculation of zircon radiation damage ages, representing the time needed for a specific zircon to develop its radiation damage. Calculations considered U-Th concentrations and FWHM values, assuming they were representative of the entire zircon grain or section, while acknowledging potential variations due to zoning and spot size differences between LA-ICP-MS and Raman analyses. The evolution of FWHM at different eU content was calculated using a refined calibration by Váczi.

Case 1 (Pliocene gabbro) revealed that Precambrian zircon grains exhibited significantly more radiation damage than would be expected if they were truly inherited, indicating contamination. Their ν₃ (SiO₄) peak positions and FWHM values did not align with the expected trajectory for unannealed grains. Case 2 (Cenozoic sills) demonstrated that older-than-host-rock zircon showed crystallinity comparable to young magmatic zircon, suggesting genuine inheritance. The inherited grains exhibited ν₃ (SiO₄) peak positions and FWHM values similar to the young magmatic zircon, and their radiation doses plotted below the calibration curve for unannealed grains, implying annealing after formation. The range of observed FWHM values for both young and old zircon in Case 2 fitted well with the expected range for radiation damage accumulation since approximately 55 Ma, confirming their shared thermal history. In contrast, grains not experiencing annealing would show higher FWHM. The study confirmed that the annealing of zircon in natural magmas provides evidence for inherited materials. This approach is particularly valuable for young magmatic rocks with a large age contrast between inherited and magmatic zircon. It also allows revisiting previously dated samples, potentially verifying disputed hypotheses. The lack of inherited zircon indicating continental material in the shallow upper mantle beneath the Mohns Ridge raises questions about mantle dynamics, suggesting either localized recycling, variable incorporation rates, or previous misinterpretations due to contamination.

The findings demonstrate that combining U-Pb dating with Raman spectroscopy analysis of radiation damage provides a powerful tool for distinguishing between contamination and truly inherited zircon. The ability to determine whether zircon has experienced annealing following incorporation into a melt allows for a more reliable interpretation of inherited zircon in various geological settings, including mid-ocean ridges, oceanic hotspots, and island arcs. The results challenge previous interpretations of inherited zircon, particularly in oceanic settings where contamination may have been overlooked. The observed absence of inherited zircon in the Mohns Ridge sample suggests that the recycling of continental material in the oceanic mantle may be less widespread or occur in discrete pulses than previously thought. This highlights the importance of considering the low-temperature history of zircon grains and using multiple analytical techniques for a comprehensive assessment of their origin.

This study introduces a two-step approach combining U-Pb dating and Raman spectroscopy to reliably identify truly inherited zircon, differentiating it from contaminants. The method proves particularly useful for young magmatic rocks, offering a means to revisit and verify past interpretations. The findings have implications for our understanding of crustal recycling and continental-influenced magmatism, potentially leading to revisions of existing geodynamic models. Future research should focus on expanding the application of this technique to a wider range of geological contexts and on developing more sophisticated models to account for the complex interplay between time, temperature, and eU concentrations in zircon annealing.

The study acknowledges some limitations. The assumption that U and Th concentrations and FWHM values are representative of the entire zircon grain or section may not always hold true due to zoning variations. The spot size differences between LA-ICP-MS and Raman analyses could also introduce uncertainties. Moreover, the zircon annealing process is influenced by both time and temperature, and variations in these factors could complicate the interpretation of results in some geological settings, especially continental rocks with complex histories. At low eU concentrations, slow FWHM evolution and uncertainties in U and Th concentrations can affect the reliability of radiation damage age estimates.