Abstract
A new approach is presented for the simultaneous analysis of lead isotope ratios and gold, lead, and bismuth concentrations in metallic silver using nanosecond laser ablation multi-collector inductively-coupled plasma mass spectrometry (LA-MC-ICP-MS). Corrections for both isotope and concentration analyses are performed using an in-house matrix matched silver reference material RM3834. Accuracy and external reproducibility are demonstrated by repeat analyses of a further seven silver reference materials all characterised by solution (MC)-ICP-MS approaches. Typical internal precisions, expressed as two relative standard errors (S.E.) of the mean of the cycles comprising one analysis, are <0.5% for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb, and <0.03% for 207Pb/206Pb and 208Pb/206Pb. External reproducibilities, based on repeat analyses over a 10 month period and expressed as two relative standard deviations (S.D.) of the mean, are <0.4% for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb and <0.2% for 207Pb/206Pb and 208Pb/206Pb. Internal precisions and external reproducibilities of gold, lead and bismuth concentration analyses are typically <25%. Data are consistent with solution-based approaches. Methods are further demonstrated through analyses of a set of Islamic silver dirhams from the mint of al-Muhammadiyya, highlighting their applicability to geochemical studies of archaeological artefacts.
Highlights
The isotopic and elemental signature of archaeological objects are important archives of information regarding the societies who created and used them, shedding light on aspects including: location of raw material source, technological processing, circulation of metal pools, and patterns of trade and exchange (e.g. Bevins et al, 2012; Bray and Pollard 2012; Standish et al, 2015; Merkel 2016)
Considerations of artefact conservation are especially pertinent when studies focus on objects made from precious metals, and a number of studies have begun to investigate the suitability of laser ablation (LA)-MCICP-MS techniques with regards to Pb isotope analysis of silver arte facts (Ponting et al, 2003; Baker et al, 2006; García de Madinabeitia et al, 2017)
NIST SRM610 was trialled as a Pb isotope standard for mass bias corrections (Baker et al, 2004), whilst silver reference materials 133X AGA1 and 133X AGA3 were used for both Pb isotope and concentration analyses (MBH Analytical Ltd., Barnet, UK)
Summary
The isotopic and elemental signature of archaeological objects are important archives of information regarding the societies who created and used them, shedding light on aspects including: location of raw material source, technological processing, circulation of metal pools, and patterns of trade and exchange (e.g. Bevins et al, 2012; Bray and Pollard 2012; Standish et al, 2015; Merkel 2016). Laser ablation (LA)-MC-ICP-MS can allow minimally destructive analysis of objects, with the added benefit of faster sample through-put due to the absence of time-consuming sample digestion and purification procedures (Standish et al, 2013). With artefact conservation being a key consideration for curators of culturally important artefacts worldwide, this technique is increasingly becoming a key method in the field of archaeological sci ence despite typically lower precisions (Dussubieux et al, 2016). Considerations of artefact conservation are especially pertinent when studies focus on objects made from precious metals, and a number of studies have begun to investigate the suitability of LA-MCICP-MS techniques with regards to Pb isotope analysis of silver arte facts (Ponting et al, 2003; Baker et al, 2006; García de Madinabeitia et al, 2017).
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