A volcanic chronosequence as a time-resolved paleo-detector array to study the cosmic-ray flux in the Late Pleistocene and Holocene

We present a phenomenological study demonstrating the feasibility of using olivine xenoliths from the Chaîne des Puys as a time-resolved paleo-detector array to probe the cosmic-ray flux over the last 40,000 years. This volcanic region provides a unique chronosequence of samples brought to the surface by well-dated eruptions. By modeling the expected density of nuclear recoil tracks induced by cosmic-ray muons in olivine, we show that the signal is detectable and above backgrounds from natural radioactivity. We demonstrate that by analyzing samples with different exposure ages, it is possible to construct a time-differential measurement of the cosmic-ray flux. This method shows sensitivity to historical variations, such as the enhanced flux expected during the Laschamp geomagnetic excursion ($\sim$41~kyr) and the potential contribution from nearby supernovae, for which we use the Antlia supernova remnant precursor as a benchmark. This work establishes a new application of the paleo-detector technique for long-scale time-domain high-energy astrophysics and provides the direct scientific motivation for experimental efforts to measure these track records.

💡 Research Summary

The paper presents a phenomenological feasibility study for using olivine xenoliths from the Chaîne des Puys volcanic field as a time‑resolved paleo‑detector array to probe the cosmic‑ray flux over the last ~40 kyr. The authors exploit the unique geological setting: each eruption brings mantle‑derived olivine to the surface, and the high temperature of the magma anneals any pre‑existing damage tracks, effectively resetting the detector at a known time. By selecting a series of well‑dated eruptions spanning from ~7 kyr to ~41 kyr ago, they obtain a chronosequence of independent detectors, each with a distinct exposure window.

The detection principle relies on muons produced in the atmosphere by primary cosmic rays. When a muon traverses olivine it can induce spallation or capture reactions, creating energetic recoil nuclei that leave nanometre‑to‑micrometre scale damage tracks. The density of these tracks encodes the integrated muon flux, while the length distribution reflects the recoil energy spectrum. The authors model the primary cosmic‑ray spectrum with the crflux package, propagate it through the atmosphere using MCEq to obtain sea‑level muon fluxes, and then simulate muon‑matter interactions in Geant4. They generate recoil spectra for a wide energy range (1 MeV–10 TeV) and convert recoil energies into track lengths using SRIM stopping‑power tables.

Two astrophysical scenarios are considered: a “normal” flux consistent with present‑day measurements, and an enhanced flux (“SN250”) that adds a contribution from a hypothetical supernova (age ~50 kyr, distance ~250 pc) modeled after the Antlia remnant. Additionally, the authors incorporate a modest increase in low‑energy cosmic rays during the Las Champ geomagnetic excursion (≈41 kyr ago) by boosting the GeV‑scale primary flux. Background tracks from radiogenic neutrons and spontaneous fission of ^238U are evaluated using tabulated neutron spectra and the WIMPy‑NREFT framework, with SRIM again providing range information.

The simulations show that for track lengths above ~2 µm the muon‑induced signal dominates over radiogenic backgrounds, while fission fragments contribute at a comparable level in the same length interval. The total number of tracks per gram scales linearly with exposure time, as expected for a constant flux, with slight deviations reflecting the imposed astrophysical variations. Assuming 0.2 g of olivine per eruption (≈1.6 g total), the cumulative track count distinguishes the SN250 scenario from the normal one at the ~1 σ level up to ~15 µm track length, even when Poissonian statistics and a 10 % counting systematic are included.

For the Las Champ interval, the predicted enhancement yields a signal difference that sits near the 1 σ confidence band, making detection challenging with current optical microscopy resolution and etching biases. The authors note that more advanced imaging (e.g., electron microscopy) and careful calibration of systematic effects could improve sensitivity.

In conclusion, the study demonstrates that olivine xenoliths from a well‑dated volcanic chronosequence can serve as a natural, time‑resolved detector array for high‑energy astrophysics. The method provides a novel avenue to investigate long‑term variations in the cosmic‑ray flux, geomagnetic field excursions, and possible nearby supernova events, and it can be extended to other volcanic regions worldwide where mantle xenoliths are present. Experimental validation will be required, but the work lays a solid theoretical foundation for using geological archives as high‑energy astrophysical observatories.