the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Oct 2026
A magnetite reference material for LA-ICP-MS U–Pb geochronology from the Troodos Ophiolite, Cyprus
Abstract. Magnetite is an increasingly important target material for in situ geochemical analysis and has recently emerged as a promising U–Pb geochronometer. However, the wider application of the method is hindered by the lack of a widely available matrix-matched reference material. Here we propose TR41, a hydrothermal magnetite from the Troodos ophiolite (Cyprus), as a candidate reference material for LA-ICP-MS U–Pb magnetite geochronology. Petrographic, geochemical, and isotopic data demonstrate that TR41 is sufficiently homogeneous for laser ablation analyses and yields clearly defined regressions in Tera–Wasserburg diagram despite U concentrations below 1 µg g-1. As independent ID-TIMS characterisation of magnetite is currently not feasible, the age of TR41 was constrained using complementary geochronological approaches. U–Pb dating of coexisting hydrothermal andradite yields a lower-intercept age of 92.04 ± 2.23 Ma. Assuming that the andradite and magnetite formed during the same hydrothermal event, four blind-test samples containing coexisting magnetite and garnet or apatite were analysed. In all cases, magnetite ages obtained using TR41 as the primary reference material agree, within uncertainty, with independently determined ages of the associated minerals. These results demonstrate that TR41 provides an appropriate matrix correction for LA-ICP-MS U–Pb analyses of magnetite and represents a practical reference material for expanding the magnetite U–Pb geochronology. TR41 is available for distribution to interested laboratories upon request from the corresponding author.
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- RC1: 'Comment on egusphere-2026-4975', Liam Courtney-Davies, 23 Oct 2026 reply
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Comment on egusphere-2026-4975 Beranoaguirre et al. for GChron.
Beranoaguirre et al. propose a new matrix-matched reference material for LA-ICP-MS U-Pb geochronology of magnetite.
I am supportive of the manuscript overall, and empathise with the authors endeavor to identify and characterize a suitable natural iron oxide RM. There is still a lot more work to be done as the authors state, and this is a start. The manuscript is lacking in parts which can be addressed. I think a major revision is necessary to address missing details. The sentence on line 65 - “We propose that this material may serve as a reference material for method development and inter-laboratory comparison until additional magnetite reference materials become available” should be stated in the abstract.
It should also be noted that another magnetite U-Pb RM paper has just been published (Jun-Duan et al. 2026, https://doi.org/10.1016/j.gr.2026.09.012). This material is also limited.
The hydrothermal magnetite RM contender from Cyprus is a mineralogically and chemically heterogenous massive aggregate dominated by magnetite with abundant intergrown mineralogy. This RM candidate differs from typical single grain-type zircon or apatite RMs, however, this is characteristic of iron oxides (particularly those from magmatic-hydrothermal systems where U is sometimes found). The magnetite RM is less than 1 ppm U; the range is not stated in manuscript but is shown to average 0.24 ppm for the RM in the appendix tables.
The authors state that it is unusual to not provide ID-TIMS data for a proposed RM, and provide some fair rationale for why they have not acquired that data, largely because such low-U contents in magnetite seemingly preclude ID-TIMS (it would still be worthwhile to try, has anyone successfully dated magnetite via TIMS this centuary? not that I’m aware). I don’t doubt that the magnetite candidate presented here is near coeval in age to dated coexisting garnet/apatite and this is further supported by magnetite (U-Th)/He data (despite overdispersion as expected), but is that enough for a RM? different reviewers will have differing opinions.
Does this present a circular challenge whereby there can never be a ‘true’ magnetite RM, because magnetite does not incorporate much U (if any? there is no mention of how U is hosted in this magnetite - possibly as inclusions given the formation environment). There is no outline or discussion of ablation behavior in these magnetite’s, which almost certainly do not ablate ‘cleanly’ and will differ between formation environment based on the huge variations in magnetite minor and trace element chemistries (see recent in-situ [U-Th]/He papers on different ablation behavior of oxide minerals). Your unknown magnetite’s CB1/CB2 have very high Th/U – are these all just apatite inclusions in magnetite? I don’t think Th should substitute in magnetite(?). You mention some spots being rejected for high Th, maybe some discussion on whether U and Th is lattice bound in magnetite at all is required and what are the ramifications for future magnetite U-Pb.
From the probe data, this magnetite looks silician (‘silician magnetite’, See Ciobanu et al. 2019 or Courtney-Davies et al. 2020) – whereby Si is hosted in nano inclusions. This effects ablation behavior and merits further study. This magnetite is zoned; however, your presented images are not high resolution enough to show this – what is the zonation effect? I think more detailed petrography on such heterogenous samples is required in the form of additional figures in the manuscript. You state in the text “High-contrast BSE images reveal oscillatory rims within magnetite grains, which are accompanied by a systematic decrease in Si”. Why are these high contrast images not shown in the manuscript? The very large variations in minor to trace element chemistry from core to rim will create highly variable ablation conditions, unlikely to be matched to an unknown magnetite matrix of a different environment (e.g. BIF v serpentinite). This raises a further question about trying to match one type of magnetite to all magnetite matrices – they simply might be too heterogenous intra-sample/intra-environment.
However, the data from magnetite unknowns is compelling as it gives the ‘correct’ date when normalized to the candidate RM, although no information as far as I can tell is provided for these magnetite unknowns identity apart from ‘skarn’ labelling for one. It should be noted that zircon RMs in Courtney-Davies et al 2020 also gave seemingly ‘correct’ dates for magnetite unknowns when compared with solution standards which gave ‘correct’ dates. The newly published magnetite RM in Jun-Duan et al (2026) also gets ‘correct’ dates using a zircon RM to calibrate magnetite. So, for the level of accuracy/precision required, maybe this RM is enough? hard to tell for sure. To be unequivocal, we need TIMS data as you state.
I believe the thermobarometer has been applied incorrectly, is this not strictly meant to be for magmatic magnetite, not this regressed hydrothermal type (?) – in any case, I don’t think this adds much to the manuscript.
To this end, this material should only be considered a ‘secondary’ standard, as the authors more or less state. Finding a suitable ‘true’ natural material is unlikely to work out until the ID-TIMS chemistry is carried out by a willing party.
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