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 200 nm-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 10-40 mu m and 2 mu m, 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 La-N/Yb-N = 3.5-12 and different degrees of Ce anomalies (Ce/Ce* = 1.5-5.3). To study the internal structure of the varnish, 100-200 nm 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. (C) 2015 Elsevier B.V. All rights reserved

Rock varnish on architectural stone: microscopy and analysis of nanoscale manganese oxide deposits on the Smithsonian Castle, Washington, DC

“…These results are consistent with experiments that have determined divalent metal preferential adsorption for Pb, Zn, and Cu on interlayer crystallographic sites in birnessite [25]. Analytical studies also show a significant enrichment of Mn, Co, Pb, Ni, and Cu in desert varnishes [24,26].…”

“…Step 1 was recently bolstered by the observation that anticorrelated Mn and Si electron microprobe analyses for desert varnish have a common Mn-free end-member regardless of the composition of the varnished rock [24]. Regarding step 2, one may question whether Mn is derived locally from the host rock or externally via atmospheric transportation [8].…”

Suitability of Mn‐ and Fe‐Rich Reference Materials for Microanalytical Research

“…, Macholdt et al . ) and a New Wave UP213 Nd:YAG nanosecond laser ablation system with a wavelength of 213 nm. Important parameters of the ICP‐MS and lasers are listed in Tables and .…”

“…Analyses were performed at the Max Planck Institute for Chemistry, Mainz, Germany, using a combination of a ThermoFisher Element2 single-collector sector-field ICP-MS and two different types of lasers: an Electro Scientific Industries (ESI) femtosecond laser ablation system, NWR-Femto200 operating at a wavelength of 200 nm (Jochum et al 2014, Macholdt et al 2015) and a New Wave UP213 Nd:YAG nanosecond laser ablation system with a wavelength of 213 nm. Important parameters of the ICP-MS and lasers are listed in Tables 1 and 2.…”

“…The RMs were then used as microanalytical RMs for LA-ICP-MS investigations of Mn-rich materials (Garbe-Sch€ onberg and McMurtry 1994, Axelsson et al 2002, Hlawatsch et al 2002, assuming a sufficient homogeneity. Furthermore, the homogeneous NIST SRM 61X and USGS GSE-1G glasses were used for calibration of Mn-rich materials (Hlawatsch et al 2002, Huelin et al 2006, Wayne et al 2006, Xu et al 2007, Nowinski et al 2010, Macholdt et al 2015, although these samples have low Mn and/or Fe contents.…”