Reproduction of melting behavior for vitrified hillforts based on amphibolite, granite, and basalt lithologies

John S Mccloy,Mostafa Ahmadzadeh,Erik Ogenhall,John A Wolff,Jack S Clarke,José Marcial,Mia Englund,Rolf Sjöblom,Albert A Kruger,Edward P Vicenzi,Carolyn I Pearce,David L Bollinger

doi:10.1038/s41598-020-80485-w

Abstract

European Bronze and Iron Age vitrified hillforts have been known since the 1700s, but archaeological interpretations regarding their function and use are still debated. We carried out a series of experiments to constrain conditions that led to the vitrification of the inner wall rocks in the hillfort at Broborg, Sweden. Potential source rocks were collected locally and heat treated in the laboratory, varying maximum temperature, cooling rate, and starting particle size. Crystalline and amorphous phases were quantified using X-ray diffraction both in situ, during heating and cooling, and ex situ, after heating and quenching. Textures, phases, and glass compositions obtained were compared with those for rock samples from the vitrified part of the wall, as well as with equilibrium crystallization calculations. ‘Dark glass’ and its associated minerals formed from amphibolite or dolerite rocks melted at 1000–1200 °C under reducing atmosphere then slow cooled. ‘Clear glass’ formed from non-equilibrium partial melting of feldspar in granitoid rocks. This study aids archaeological forensic investigation of vitrified hillforts and interpretation of source rock material by mapping mineralogical changes and glass production under various heating conditions.

Highlights

European Bronze and Iron Age vitrified hillforts have been known since the 1700s, but archaeological interpretations regarding their function and use are still debated
Several previous researchers have investigated the vitrified fort at Broborg, most notably that of Kresten et al.[2,7,21,29,30], with some recent preliminary and confirmatory studies looking at details of the vitrified material itself, as part of an ongoing project investigating vitrified hillfort glass stability over time[31,32,33]
For this study, vitrified rocks obtained from the Iron Age Broborg hillfort site were compared to laboratory-heated rocks of various types

Summary

Introduction

European Bronze and Iron Age vitrified hillforts have been known since the 1700s, but archaeological interpretations regarding their function and use are still debated. Laboratory experimental studies melting putative source rocks[8,20,22], geochemical and mineralogical investigation of archaeological materials[19], and large scale demonstrations[7,16,17,23,24,25] have attempted to settle the debates about vitrified hillforts in general, but much remains obscure and e nigmatic[20]. Based on these multiple lines of evidence, most researchers believe the necessary temperatures range from 1000 to 1250 °C6,8,18,21 and depend only weakly on lithology[26]. Several previous researchers have investigated the vitrified fort at Broborg, most notably that of Kresten et al.[2,7,21,29,30], with some recent preliminary and confirmatory studies looking at details of the vitrified material itself, as part of an ongoing project investigating vitrified hillfort glass stability over time[31,32,33]

Methods

Results

Discussion

Conclusion