Introduction

The Hopewell horizon extended over much of the Midwest and is known for the use of exotic material collected throughout North America during the Middle Woodland Period. At sites in southern Ohio and Havana, Illinois, meteoritic iron was fashioned into artifacts, including necklace beads. The source of the Ohio meteoritic iron is known to be the Brenham pallasite from Kansas. The source of the Havana beads has remained unknown. The purpose of this study is to determine the meteorite source and manufacturing techniques of the Havana beads and provide implications for the trade routes used by the Hopewell horizon.

Textural and geochemical data collected in previous studies suggest the source of the Havana beads may be an iron meteorite found near Anoka, MN. Trace element analysis has determined the Havana beads to be in a subgroup of iron meteorite in which few North American iron meteorites belong. Within the subgroup, Anoka has the closest composition to Havana. The Anoka, Carlton and Edmonton meteorites were originally found as single undamaged masses. Recently, Anoka was discovered to be a meteorite shower with many masses of varying size. This, along with trace element data, pointed to Anoka as the source of the Havana beads.

Methods

Sourcing: Microchemical Analysis

Manufacturing: Laboratory Simulation

Manufacturing conditions of the Havana beads were recreated using piece of Anoka iron. Anoka samples were deformed using a hammer and heated from 500-750°C in an furnace for 3 hours. Light microscopy was used to compare the microstructure of the heated sample to the Havana bead and the pristine Anoka iron.

Results

FE-SEM Results

• Backscattered electron image mosaics of the Anoka iron and Havana bead.
• Anoka exhibits a regular geometric pattern of elongate kamacite (low-Ni a iron).
• Havana kamacite lamellae are curvilinear, reflecting the deformation that occurred when the bead was manufactured.

EDS Results

• Composite (false-color) X-ray images of Fe, Ni and P for the Anoka iron and Havana bead.
• Schreibersite (Fe,Ni)3P is deformed into fragmented, elongated trails.
• Brittle schreibersite may have provided a weakness along which the meteorite was broken into smaller fragments.

EPMA Results

• Optical images of Anoka and Havana with the measured one dimensional Ni compositional profile overlain.
• Havana retains the M-shaped Ni concentration gradient, but taenite (high-Ni ? iron) has recrystallized from a plessitic structure to a fine-grained, uniform texture, resulting in a smoother central portion of the M-shaped profile.
• Havana kamacite has recrystallized into small domains as a result of cold-working followed by heating, producing greater scatter in the kamacite portion of the zoning profile.

LA-ICP-MS Results

• Composition of Havana and Anoka normalized to a primitive meteorite composition with elements arranged in order of increasing volatility.
• Anoka and Havana exhibit similar compositions for all elements except W, P and Ag.
• Differences due to heterogeneous distribution of schreibersite (incorporates Ag and W).
• Expanded scale of Havana/Anoka, most elements are similar to within 15% of each other. Similarity supports the idea that Anoka served as the material for the Havana beads.

Discussion

Laboratory Simulation

Evidence for Cold-Working:

• Kamacite has recrystallized into small domains.

Evidence for Heating at 700°C:

• M-shaped Ni composition profile is preserved in Anoka after substantial heating.

• Center portion of M-shaped profile is smoother that unheated Anoka, indicating the recrystallization of plessite into a uniform texture.

Hopewell Trade

The Anoka iron is located less than 40 km away from Indian Mounds Park where Hopewell artifacts have been found. Hopewell in Minnesota was small and lacked technology needed to work meteoritic iron. Local people may have found the Anoka iron, recognized it as a unique material and traded the iron through the Hopewell Interaction Sphere. Due to the malleability of iron, the Anoka iron was most likely worked into beads by someone with an extensive knowledge of metal working. Few meteoritic iron artifacts have been found in Illinois, suggesting the beads were produced else where. The Anoka iron may have been traded to a region with greater metal working technology, such as Northern Michigan or Ohio, where the raw material was turned into a finished good.

Conclusions

• The source of the Hopewell beads found in Havana, IL is very likely the Anoka iron meteorite.

• The Havana bead was likely manufactured by cold-working metal followed by heating around 700°C.

• The Anoka iron was likely traded by local people to a region of greater metal working technology where the iron was made into beads. The beads may have arrived in Havana as a finished good.

Acknowledgements

This research was conducted at the National Museum of Natural History as part of the Research Training Program and was funded by the Smithsonian Women’s Committee Internship Endowment. Prof. John T. Wasson of UCLA provided the Havana specimen used in this study. Tim Gooding of the Department of Mineral Sciences at NMNH assisted with sample preparation. LA-ICP-MS work was performed at the University of Maryland with the help of Dr. Richard Ash and Prof. William McDonough. Dr. Bruce Smith and Dr. James Krakker provided guidance to relevant references. Dr. R. Eric Holllinger and Lauren Sieg provided valuable insight in to the Hopewell horizon and many helpful suggestions.