Microanalytical methods for in-situ high-resolution analysis of rock varnish at the micrometer to nanometer scale

Abstract

A wide range of analytical techniques were used to investigate rock varnish from different locations (Negev, Israel; Knersvlakte, South Africa; Death Valley and Mojave Desert, California): a 200nm-femtosecond laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS), an electron probe microanalyzer (EPMA), focused ion beam (FIB) slicing, and scanning transmission X-ray microscopy–near edge X-ray absorption fine structure spectroscopy (STXM–NEXAFS). This combination enables comprehensive high-spatial-resolution analysis of rock varnish. Femtosecond LA-ICP-MS and EPMA were used for quantitative determination of element concentrations. In-situ measurements were conducted on thick and thin sections with a resolution of 10–40μm and 2μm, respectively. The results demonstrate that some elements, such as Mn, Co, Pb, Ni, and Cu, are highly enriched in varnish relative to the upper continental crust (up to a factor of 100). The varnish composition is not influenced by the composition of the underlying rock, which is witnessed by plots of MnO2 vs. SiO2 contents. Furthermore, the Mn-free end members fall in the range of average dust compositions.The varnishes from the various locations show distinct differences in some elemental ratios, in particular Mn/Fe (0.3–25.1), Mn/Ba (4–170), Ni/Co (0.03–1.8) and Pb/Ni (0.4–23). The rare earth element (REE) patterns vary with LaN/YbN=3.5–12 and different degrees of Ce anomalies (Ce/Ce*=1.5–5.3).To study the internal structure of the varnish, 100–200nm thick FIB slices were prepared and mappings of Fe, Mn, N, CO32−, Ca, C, and Si at the nm scale performed. Banded internal structures of Mn, Fe and organic C were observed in the Israeli and Californian samples, however, no Fe-rich layers are present in the South African rock varnish samples. Furthermore, cavities were found that are partly filled by C, Fe, and Mn rich material. Internal structures are different for varnish from different locations, which might reflect different types of genesis. The results of the combined microanalytical techniques give important detailed insights towards unraveling the genesis of rock varnish.