Freeze-thaw cycles investigate weathering properties of different lithologies used in 10Be exposure age dating in the Eastern Alps

During the Last Glacial Maximum (LGM), many glaciers in the Eastern Alps terminated in narrow inneralpine valleys resulting in a limited amount of datable landforms. At the former Enns and Mur Glaciers, boulders, at or close to laterofrontal moraine ridges were dated with cosmogenic ¹⁰Be. Boulders at the Mur Glacier are composed of weathering-resistant pegmatite gneiss, whereas quartzbreccia/greywacke was the only suitable boulder lithology in the Enns Glacier region. This lithology consists of large quartz components within a fine matrix and is more easily affected by weathering. Ages inferred from ¹⁰Be concentrations in pegmatite gneisses (Mur Glacier) are around 20 ka, in accordance with other ages around the Alps. In contrast, Enns Glacier boulders yielded surprisingly young ages between 14-17 ka. In order to obtain plausible LGM ages via erosion corrections from the ¹⁰Be concentrations, a 30 cm thick surface layer would need to be removed (if the boulder was at the surface since deposition). In order to understand, if such a large amount of material can be removed from the quartzbreccia, we applied freeze-thaw cycle experiments to both lithologies. For the experiment, we produced four cubes, roughly 2x2x4 cm in size, three of quartzbreccia and one of pegmatite gneiss. Each of the quartzbreccia cubes has a different thickness of weathering crust (0-4 cm) with abundant holes and cracks. Their mineralogical composition is quartz, carbonate, and phyllosilicates with a preferential orientation. All cubes were dried, weighted and their volume determined and afterwards they were subjected to over 100 freeze-thaw cycles. To simulate moisture and rain, the cubes were thawed in a water bath at a constant temperature of 20 °C.  All cubes were photodocumented before the test, roughly every three weeks over a 4 months period, and after the test. CT scans were made of one quartzbreccia cube before and after the test to better visualise structural changes within the cube. The freeze-thaw cycles show that the quartzbreccia cubes weather much more intense than the pegmatite gneiss. No visual changes were detected in the latter, whereas quartzbreccia cubes constantly changed. Water seems to enter the cubes following pathways along the aligned sheet silicates. These delicate minerals are then destroyed and once enough matrix is removed, larger quartz crystals fall out, which is nicely seen on the photos. Overall, this process seems to result in a rather continuous loss of material, but varies in different cubes and even on different planes of the cubes. As a result, cosmogenic ¹⁰Be is removed constantly. Additionally, the nature of discontinuous weathering results in an inhomogeneous ¹⁰Be production due to edge effects resulting in loss of ¹⁰Be at the rim of a raised quartz pebble. In summary, the freeze-thaw cycles show that the quartzbreccia weathers much faster than the pegmatite gneiss and therefore the age difference could at least partly be explained. Therefore, caution is required when dating conglomerates, breccias or similar lithologies with cosmogenic ¹⁰Be.