Combining fs LA-ICP-MS, FIB and STXM-NEXAFS Methods for in-situ High-Spatial-Resolution Rock Varnish Analyses at the nm to µm Scale

Abstract:

Since over 200 years a number of studies debate the origin of the black or red, up to 250 µm thick coatings on desert rocks, referred to as rock varnish. The main components of varnish are poorly crystallized Mn and Fe oxides and clay minerals. Both biological and geological processes have been suggested for the genesis.

We used a combination of 200 nm-femtosecond LA-ICP-MS, focused ion beam (FIB) slicing and scanning transmission X-ray microscopy – near edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS) for the high-spatial-resolution analysis of rock varnish from different locations (Negev, Israel; Knersvlakte, South Africa; Death Valley and Mojave Desert, California). Femtosecond LA-ICP-MS was used for the quantitative determination of major and trace element concentrations. In-situ measurements were conducted on thick sections with the advantages of non-matrix matched calibration, low ablation depth of ~10 µm for laser spot sizes of 10-40 µm and low detection limits of trace elements (< 0.01 – 1 µg g^-1). Our results demonstrate that 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 1000). Differences between varnishes from the various locations can be observed especially for the Mn/Fe (0.4-25.6), Mn/Ba (3-363) and Ni/Co (0.03-1.8) ratios. The REE patterns differ with La_N/Yb_N = 2-14 and positive Ce anomalies (Ce/Ce* = 1.1 – 9.1).

To study the internal structures of varnish, 100 – 200 nm thick FIB slices were prepared to perform mappings of Fe, Mn, O, N, CO₃, K, Ca, and C at the nm scale. In addition, the oxidation stages of Mn and Fe were identified. Banded internal structures of Mn and organic C can be observed in some samples, and cavities that are partly filled by C and Mn rich material. Some coatings tend to incorporate bigger dust grains (> 1 µm), while others only show very fine grained material (< 0.1 µm). The results of the combined microanalytical techniques give us detailed insights that can be a further step to unravel the genesis of rock varnish.