VSF 2010 Report - Nabo

and second, to ensure the most accurate geochemical data, as microtephra pretreatment would
lead to the widest sample range (rather than sampling from only larger shards which might
fall into a particular chemical composition). The samples were extracted according to the
procedures outlined in Blockley et al. (2005), which uses stepped heavy liquid flotation to
separate microtephra from other elements within the sediment. During each extraction, a
blank was also prepared in order to test against cross-contamination of samples. Once
floated, a proportion of each sample fraction less dense than the heavy flotation liquid is
mounted in a welled slide with distilled water. From this, individual microtephra were
extracted using a syringe. Where possible, approximately sixty shards were extracted per
sample to ensure at least thirty shards large enough for geochemical analysis. These shards
were mounted on 25mm diameter stubs of polished heat-setting epoxy resin, ground, and then
polished before they are probed for geochemistry.
All analysis documented here was carried out on a wavelength-dispersive JEOL8600
electron microprobe (WDS-EPMA) at the Research Lab for Archaeology and the History of
Art, University of Oxford. The instrument was calibrated using a suite of mineral standards,
and accuracy and precision during each analytica run was assessed using MPI-DING glass
standards (see Jochum et al., 2000; Jochum et al., 2006).
Geochemical correlation
During the initial extraction, the sample VSF09 Area 32 did not yield any tephra;
however, the other three samples contained large quantities of tephra, and these samples were
probed for their geochemistry. In total, there were over 150 data points from these three
samples. Figures 1 and 2 show the results of WDS-EPMA after the data had been filtered for
quality (i.e., removing mineral analyses and those with totals < 95%). As shown in Figure 1,
the majority of the tephra falls into the intermediate andesitic and trachy-andesitic range,
similar to the results from Sigurgeirsson (2006). Data from OxT-5165 (n=60) indicates it was
from an evolved eruption. The distinct outliers may represent the product of a mixed
eruption, or contamination from soil movement during deposition.
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