Preliminary XPEEM Analysis of Rock Varnish

We have performed preliminary mapping of the manganese and iron present in a cross-section of a rock varnish from the Cima volcanic flows in the Mojave Desert, California. Rock varnish is a nanostratigraphic deposit found on typically fine-grained rocks in hyper-arid to semi-arid deserts (Figure 1a) [1]. Rock varnish is defined as consisting of about 70% clay particles held together by about 30% iron and manganese oxides [2]. The mechanism(s) by which rock varnish nucleates and grows remains unknown despite more than 30 years of study using electron microscopy. The dating of rock varnish layers and a possible microbiological contribution to the growth of varnish have both been exceedingly controversial during recent years. Microbiology is often invoked as the mechanism of varnish growth due to the significant enhancement of manganese in varnish compared to the concentration in the host rock, but its involvement remains a mystery.