The magnetic susceptibility of cherts: Archaeological and geochemical implications of source variation

Abstract

Recent exploratory studies have suggested the potential of magnetic susceptibility (MS) as a rapid and low-cost sourcing technique for lithic archaeological materials. Most commercially available susceptibility instruments, however, do not have the sensitivity to characterize weakly susceptible cherts and silicified woods. Comparative results from nine chert, two silicified wood, a porcellanite, and four obsidian sources using a highly sensitive, calibrated, and magnetically-shielded instrument allow exploration of MS variability within and between geological sources. Color, texture, grain size, and large inclusions of cherts and obsidians are not straightforward determinants of MS. Weathering rinds (patinas) and cortex from a variety of cherts yield reduced MS values, as does a recrystalized cobble of Rio Maior flint, indicating that magnetic mineral removal during chemical weathering and diagenesis is more prevalent than staining or other mineral penetration of cobbles. In situations of multiple overlapping MS ranges, analysis is limited to discriminating cherts with high or low MS range distributions, for example in an atypicality index. Some silicified woods, obsidians, and porcellanites possess a much greater range of intersource variability, and thus sourcing analysis is more likely to be broadly successful using the susceptibility of these materials. Unexpected results from an archaeological test using a historic lithic assemblage from Azinheira, Portugal, indicate that assemblage MS may be influenced by practices of raw material selection. Understanding variability in lithic MS has relevance for the design of most source geochemical sampling, as well as the behavioral interpretations that result from such investigations. ᭧

Magnetic Properties of Cherts from the Basque-Cantabrian Basin and Surrounding Regions: Archeological Implications

“…The exceptions are one sample from the continental chert of Ablitas and most samples from the flysch-derived chert of Artxilondo, whose magnetic susceptibility reaches up to 4 × 10 −8 m 3 /kg. Overall, the magnetic susceptibility of the studied cherts is much smaller than that of most chert types reported in the literature (see Thacker and Ellwood, 2002), with the exception of the Montmaneu (Spain, Ortega et al, 2016) and Ingoldmells (UK, Borradaile et al, 1998) cherts ( Figure 4B). IRM@1.2T values of up to 1 × 10 −4 Am 2 /kg indicate, despite the dominant diamagnetic behavior, measurable amounts of magnetic minerals ( Figure 4C).…”

“…The only exception is the flysch-derived chert of Artxilondo, which has a positive median magnetic susceptibility of 9.32 × 10 −9 m 3 /kg) that is within the lower range of other less pure, archeologically-relevant cherts from Portugal, USA, UK and Canada ( Figure 4A) (see Borradaile et al, 1998;Thacker and Ellwood, 2002). This indicates that all the studied BCB cherts, possibly including the Artxilondo type, can be considered as flint, which is the purest variety of chert (Borradaile et al, 1998).…”

“…Environmental magnetic parameters have been proposed to be useful for assessing chert quality, distinguishing source areas, and identifying heated samples in the archeological record (Borradaile et al, 1993(Borradaile et al, , 1998Thacker and Ellwood, 2002;Sternberg, 2008). We discuss our results from the BCB chert types in light of these three main issues, and consider first an additional aspect (type and origin of magnetic minerals in chert) that has not been addressed in previous archeo-magnetic studies.…”

“…They have been also applied in archeology for discriminating obsidian and other raw material of archeological interest such as chert and flint (McDougall et al, 1983;Tarling, 1990;Church and Caraveo, 1996;Borradaile et al, 1998;Thacker and Ellwood, 2002;Sternberg, 2008). Studies focused on chert samples have already shown the usefulness of rock magnetic properties to establish the degree of chert purity and to assess the provenance of some chert types (see Borradaile et al, 1993Borradaile et al, , 1998Thacker and Ellwood, 2002;Sternberg, 2008).…”

Going the distance: Mapping mobility in the Kalahari Desert during the Middle Stone Age through multi-site geochemical provenancing of silcrete artefacts

“…The most accurate means of determining source locations is through geochemical provenancing of lithic raw materials. A range of stone types have been used successfully in provenancing studies, including obsidian (e.g., Shackley, 1995;Roth, 2000;Negash and Shackley, 2006;Vogel et al, 2006;Eerkens et al, 2007;Negash et al, 2007;Morgan et al, 2009;Phillips and Speakman, 2009;Smith, 2010;Smith and Kielhofer, 2011;Ambrose, 2012;Freund, 2013), chert (e.g., Thacker and Ellwood, 2002;Evans et al, 2007;Milne et al, 2009;Parish, 2011;Gauthier et al, 2012;Speer, 2014;Boulanger et al, 2015), flint (e.g., Moroni and Petrelli, 2005;Navazo et al, 2008;Olofsson and Rodushkin, 2011;Ekshtain et al, 2014), dolerite (e.g., Gallello et al, 2016), quartzite (e.g., Pitblado et al, 2013), and, recently, silcrete (Nash et al, 2013a,b).…”