Highly Resolved Microanalyses in Varnish from Well-dated Meteorites Collected in the Libyan Desert for Paleoclimate Reconstruction
Abstract:Desert varnish is a dark and thin (< 250 µm) layer on rock surfaces mainly consisting of Mn and Fe oxides as well as clay minerals. Its unknown age information, however, is one of the major drawbacks for its use as a paleoclimate archive. In this respect, meteorites with known terrestrial ages fallen in deserts are helpful for paleoclimate reconstruction. Therefore, we investigated the structure at the nm - µm scale of varnish on two chondrites from the al Hamra desert (Libya), which have terrestrial ages of 4400 (DA011) and 35000 years (DA119), respectively. Thin 5 – 10 µm layers of varnish overlay parts of different degrees of weathering depending on the terrestrial age of these meteorites. To study major and trace element concentrations from the surface of the varnish coating to the underlying weathering layer and the fresh bulk meteorite, we generated 1 µm thin lines using a fast scan speed of 80 µm/s. Whereas - as expected - the fresh bulk meteorite shows a flat REE pattern, the varnish layers have a positive and the weathering parts a negative Ce anomaly.
Special settings of the femto- and nanosecond LA-ICP-MS systems have to be applied for high resolution Mn/Fe measurements, such as a small fluence of about 0.2 J cm-2 for low sample ablation and the Escan mode of the mass spectrometer for fast measurements, a mass resolution of 2000 for the separation of interferences from the ions of interest using a flat top peak. Using these settings it was possible to measure Mn/Fe ratios with a precision better than 1 - 3 % (RSD) at a resolution of 10 nm in the samples. Our results demonstrate significant variations of Mn/Fe ratios in varnish with depth (age) - in particular for the 35000 years old DA119 L4 chondrite (between less than 0.01 at the surface and 0.1 in the interior), probably caused by microlamination of Mn- and Fe-rich layers. The low and high Mn/Fe ratios can be related to dry and wet climate .
 Liu and Broecker (2013) Geomorphology 187, 38-60.