Paper No. 299-12
Presentation Time: 9:00 AM-6:30 PM
MULTI-TECHNIQUE ELEMENTAL ANALYSIS OF THE SMITHSONIAN CASTLE SANDSTONE AND ROCK VARNISH
The Seneca sandstone of the Smithsonian Castle and an associated layer of rock varnish have been investigated using an extensive array of elemental analysis methods. The bulk composition of the sandstone was analyzed using both instrumental neutron activation analysis (INAA) and prompt gamma activation analysis (PGAA). The results were consistent with the composition of a typical arkosic, micaceous sandstone from the Triassic Newark Supergroup formation. The mineral assemblage was characterized using hyperspectral X-ray microanalysis in the scanning electron microscope. The stone consists primarily of quartz, feldspars and mica. Portable XRF analysis found that the surface layer of rock varnish was significantly enriched in Mn compared to the underlying sandstone. This was confirmed by femtosecond laser ablation-inductively coupled plasma-mass spectrometry (fs LA-ICP-MS) with a Mn/Fe ratio in the rock varnish of 1.40-1.63 compared to the ratio of 0.01 to 0.02 in the bulk sandstone as determined by INAA. X-ray microanalysis analysis has not yet revealed any Mn-rich minerals such as ilmenite or titanite in the sandstone that could be the source of the Mn in the varnish layer. Therefore the preliminary conclusion is that the source of Mn is airborne particulate matter. Based on the differential attenuation of the Fe Ka and Kb peaks in the portable XRF spectra, it was possible to estimate the thickness of a hypothetical MnO2 layer at 4 ± 1 µm, which implies an average growth rate of 26 ± 6.5 µm/kiloyear over the lifetime of the Castle. Both the portable XRF and fs LA-ICP-MS found elevated Pb and Cl concentrations in the rock varnish. The former is presumably from automobile pollution and the latter, a combination of road de-icing salts and sea salt particles.