Understanding 21Ne inventories in Precambrian basement below the Great Unconformity in Estonia

Stable cosmogenic Ne isotopes are widely used to determine the erosion rate of slowly-eroding land surfaces through the Cenozoic. Constraining erosion and surface exposure back in Earth history remains a challenge largely due to the presence of Ne isotopes generated by natural decay processes over the lifetime of rocks.  Prospects are best when cosmogenic nuclide production has been significant and nucleogenic Ne production is low and can be quantified.  We have explored the limits of palaeo-cosmogenic Ne in one of the Earth’s most extensive erosion surfaces, the late Precambrian Great Unconformity in Estonia. Here deep kaolinitic saprolites formed on Baltica prior to the deposition of Late Ediacaran quartz sandstones. On the basis of geochemical mass balance the duration of saprolite development is estimated to be of the order of a few Myr.

Borehole F163 samples a section through still-buried weathered unconformity that includes a saprolite surface consistent with negligible erosion during the marine transgression. Samples from the unconformity have ²¹Ne concentrations (>10⁸ atoms/g) that are significantly higher than shielded samples from >20 m below the unconformity. This difference is borne out by Ne isotope composition, and leads to the tanatalising prospect that Precambrian cosmogenic Ne is present in the saprolite. Using modern ²¹Ne production rates the palaeosols appear to record a few million years irradiation. This is broadly consistent with geochemical estimates of saprolite development.  Samples from the uppermost preserved part of the weathering profile in borehole F231 have low ²¹Ne concentrations that are indistinguishable from deeper in the rock profile. This would require profile truncation or the redeposition of weathered material.  The borehole is located on the western flank of an uplifted basement block rising ca 130 m above the typical Precambrian basement level in the area and likely that the thick regolith contains material eroded from the uplifted basement units. Clearly these are early days and quantifying surface exposure in deep time will require effort in field as well as the lab.