A MULTI-SYSTEM APPROACH TO DETERMINING THE AGE AND MELTING CONDITIONS OF VITRIFIED HILL FORTS IN SCOTLAND

Numerous remnants of vitrified hill forts have been documented in Scotland, France, and other parts of northern Europe. Determining the ages of vitrified hill forts, as well as the formation mechanism for the partially melted rocks present, is important for reconstructing the archaeology of these regions during prehistoric periods. However, existing constraints on the timing and melting histories of vitrified hill forts are difficult to reconcile with one another. Here, we pair ⁴⁰Ar/³⁹Ar geochronology with paleomagnetism on archival rock samples from 5 Scottish vitrified hill forts to address questions of hill fort age and melting conditions. These archival samples are unoriented, and therefore we were unable to determine paleomagnetic direction. Instead, archeointensity was determined using the Microwave IZZI technique carried out on the 14 GHz microwave paleointensity system with low-temperature SQUID magnetometer at the University of Liverpool. The vitrified fort samples proved to be excellent recorders of archeointensity, with all but one sample yielding results that pass commonly used reliability criteria. Archeointensity estimates fall into two groups, with values clustering around 80 and 50 μT. These are consistent with reference data from the Iron Age, and from Middle Age or possibly earlier prehistoric periods, respectively. ⁴⁰Ar/³⁹Ar geochronology, carried out that Scottish Universities Environmental Research Centre Argon Isotope Facility, proved challenging, with total fusion ages that range from 2 ka to 4 Ma. Subsequent step heating ⁴⁰Ar/³⁹Ar analyses confirmed that melted samples contain significant excess ⁴⁰Ar inherited from the protolith rock used to build the walls. The presence of inherited ⁴⁰Ar in the absence of inherited magnetization in the melted material suggests that temperatures were high enough to reset magnetite (> 585 °C), which happens instantaneously, but perhaps too brief to fully degas existing ⁴⁰Ar from the viscous partial melts examined. Additional paleomagnetic and noble gas observations could be used to quantify the thermal histories of other materials from these vitrified hill forts, and thereby better constrain the melting conditions that occurred.