Example: Does the Tool Match the Identified Source?
Above: A variety of lithic materials. Credit: University of Hawaii.
Last year I was brought a stone tool from a Paleolithic archaeological site as well as a sample of what geologists had identified as the source of the raw material. Details, names, etc. will be left out to avoid embarrassment. Suffice it to state that the archaeologist who brought me these materials doubted the source identification -- the two rocks looked similar macroscopically, but he thought that the suspected raw material just didn't feel right.
Backscattered electron (BSE) images, like those shown below, revealed immediately that the two rocks had different microtextures -- the rocks' mineral contents and their textural relationships clearly differed:
Below: BSE images of the tool (left) and the supposed raw material (right); the field of view is 5 x 5 mm.
I then used our energy-dispersive spectrometry (EDS) system to determine the bulk chemistry of these rocks. The beam was spread out, and X-rays were collected from a few square millimeters. The resulting EDS spectra for the rocks are below -- the horizontal axis is X-ray energy, and the vertical is X-ray counts:
The EDS spectra are clearly different too. This tool stone is mostly silica with small amounts of aluminum, calcium, potassium, and iron. The suspected raw material, on the other hand, has sodium, magnesium, and titanium, and it contains greater amounts of aluminum, potassium, calcium, and iron than the tool stone. More important than their overall elemental composition are the minerals in which these elements are distributed.
Electron imaging and X-ray analyses revealed that this tool is metaquartzite, a quartz-rich metamorphic rock. The original quartz grains have been fused by heat and pressure, giving the rock a metamorphic microtexture. The red regions in the first set of combination element maps below correspond to the fused quartz grains. The green areas correspond to apatite, and the red-green spots are sphene. The light blue corresponds to orthoclase feldspar. The purple is mica, muscovite in particular. In the second set of maps, the green is magnetite. The heavy minerals like apatite and magnetite in the rock are detrital in origin, that is, these minerals settled in the original sandstone which altered into this metaquartzite due to contact or regional metamorphism.
The raw material is a different type of rock. In the first set of combination element maps, green areas correspond to calcite. The edge of a calcite grain several millimeters in diameter appears on the right side of this map. No quartz occurs in this rock sample -- no red areas are present. In the second set of element maps, the green spots are tiny magnetite crystals. Small amounts of magnesiochromite, garnet, and barium-rich silicate are present. The rest, the purple areas in the first combination map, is comprised of feldspar (albite) and mica (muscovite). This rock appears largely igneous in origin, partially metamorphosed, with calcite deposited by water. Calcite is water-soluble and can dissolve in or be precipitated by groundwater depending on conditions. I didn't go any farther in classifying the rock because my question had already been answered: this was not the raw material.
Below: Combination element maps of the tool (left) and the supposed raw material (right). These are the same areas as above. For these maps, red represents the silicon concentration, green is calcium, and blue is aluminum.
Below: Combination element maps of the tool (left) and the supposed raw material (right). These are the same areas as above. For these maps, red represents the silicon concentration, green is iron, and blue is sodium.
The archaeologist was unfortunately proven right: the rock identified by the geologists wasn't the raw material of the stone tools. Locating the actual source would have to wait until another field season. Unfortunately, distance to the material source was a key piece of information for studying these stone tools -- it was important to know if the site's inhabitants were curating the tools because the raw material was far away or if these were expedient tools because additional material was just a short walk away. The actual raw-material source might not be known, but at least, the archaeologist knew that he wasn't accidentally using a wrong source in his analysis.
When I sent these results to the archaeologists, he replied: "Without being able to go back to sample the landscape for raw material sites, I am now up a certain creek without a fluid manipulating device."
11/29/07
Electron Microprobe Analysis in Archaeology
Electron microprobe analysis (EMPA), also known as electron probe microanalysis (EPMA), is an analytical technique that combines scanning electron microscopy (SEM) and compositional analysis using x-ray spectrometry. The ability to determine structure and chemistry of samples makes EMPA very versatile. This is a dominant analytical technique in geology, but it is not as commonly used in archaeology despite similar materials in studied both fields. Here I will post about topics in EMPA, artifacts I have analyzed, archaeological studies that use EMPA, etc. If there is a topic you'd like to see posted here, please let me know.
Ellery Frahm
Doctoral Candidate, Archaeology
Research Fellow, Geology & Geophysics
University of Minnesota - Twin Cities
