QUANTIFYING RATES OF SURFACE PROCESSES IN THE DEEP GEOLOGICAL PAST (>107 YR) USING STABLE COSMOGENIC NUCLIDES (3HE AND 21NE) – POSSIBILITIES AND LIMITATIONS

Cosmogenic nuclides have long been an important tool to evaluate rates of surface processes, to record the time sediments spent at the surface and to quantify transport durations and erosion rates. Unlike their radioactive counterparts, stable cosmogenic nuclides (mostly ²¹Ne and ³He) can theoretically help to decipher landscape evolution and continental tectonics in the deep geological past (>10⁷ yr), yet to date this aspect was mostly overlooked.

This study explores the pertinence of using stable cosmogenic nuclides (²¹Ne and ³He) in order to quantify rates of surface processes in the deep geological past (>10⁷ yr). We focus on ²¹Ne in quartz, consider its various sources and examine potential limitations and pitfalls associated with old sediments that were often buried by overburden to a certain depth.

We utilize clastic sediments deposited along the northern passive margin of Gondwana during the Precambrian to Lower Cretaceous, sampled from four deep boreholes at depths ranging from around 250 to 2100 m. We constrain three potential sources of ²¹Ne in the sediments: (1) non-cosmogenic, nucleogenic ²¹Ne produced indirectly by radioactive decay of U and Th; (2) Post-burial cosmogenic ²¹Ne produced by muons and (3) Pre-burial cosmogenic ²¹Ne. Losses of ²¹Ne due to thermally-activated diffusion are quantified as a function of time and temperature (burial depth).

We discuss the limitations of the methodology and demonstrate that the ²¹Ne signal in the Lower Cretaceous samples can be interpreted in terms of paleo surface exposure age and erosion rate. In contrast, the ²¹Ne signal in the Cambrian and Precambrian samples is limited as a tool for quantifying paleo rates but can still yield valuable information.

This novel application of in situ cosmogenic nuclides holds a promising potential as a tool for quantifying surface processes and understanding landscape evolution during the deep geological past. It can thus provide valuable insight into macro-scale processes that have shaped Earth over the past hundreds of millions years.