36 Rates, Dates, and Plates: Petrochronological Approaches to Unraveling Tectonometamorphic Histories

This session welcomes contributions  applying modern approaches in metamorphic petrology, geochronology, and structural geology to unravel the dynamics of convergent margin processes from the Appalachians and beyond. We encourage submission from Early Career participants.

Tuesday, 24 March 2026
8:00 AM - 12:00 PM
CCC, Room 22/23

Geological Provenance of Archeological Quartz Ecofacts Revealed Using O-isotopes and Fluid Inclusions at James Madison’s Montpelier, VA

Paper number: 36-12
Program: Technical Sessions
Time: 11:40 AM – 12:00 PM

Authors

Abstract

Quartz crystal ecofacts were discovered during archeological excavation of the Burial Ground of the Enslaved at James Madison’s Montpelier (north-central Virginia). Investigating the ecofacts’ origin provides data for ongoing research into the regional geology of Virginia’s Blue Ridge and anchors their anthropologic story in the local geologic context. The ecofacts are clusters of singly-terminated quartz crystals excavated from the topsoil down to the subsoil. Montpelier plantation is located on the southeastern limb of the Blue Ridge Anticlinorium (central Appalachians) and is underlain by two primary geologic units: greenstone of the Catoctin Formation (Ediacaran flood basalt metamorphosed in the Neoacadian) and sedimentary rocks of the Mesozoic Barboursville basin. Both units contain quartz veins which represent potential ecofact sources. Our investigation focuses on the origin of the ecofacts in relation to these units. Quartz in samples from the Catoctin greenstone and Barboursville Basin were analyzed using cathodoluminescence (CL) imaging and oxygen isotope analysis to be compared to the quartz crystal ecofacts. A comparative study of fluid inclusion petrography (populations, size, morphology, assemblages) was also conducted.

Panchromatic CL imaging of representative ecofact, Catoctin, and Barboursville Basin quartz vein samples were done using the JEOL JXA-iHP200F Microprobe at Virginia Tech. No major quartz zoning was found. One Barboursville Basin sample showed recrystallized vein quartz in apparent fractures through the sandstone. Oxygen isotope analysis was done in the High Temp Stable Isotope Lab at UT Austin. Quartz ecofacts yielded δ18O values of 10.7 ± 0.6 (geometric mean, 2SD (n=11)). These values overlap with δ18O values from quartz veins from the Catoctin greenstone (δ18O = 11.2 ± 0.8 (n=6)) and are lower than O-isotope values from the Barboursville Basin vein quartz (δ18O = 14.5 ± 0.5 (n=4)). Petrographic analysis through quartz recrystallization textures indicates that the veins formed during zeolite to greenschist facies metamorphism (<450°C). We interpret the values for the Catoctin greenstone and ecofacts as being consistent with the equilibration of quartz with a magmatic fluid (~5.3± 0.6‰ Valley et al. 2003) at 339 °C, or ocean water at 211°C, fitting with metamorphic conditions of greenstone formation. Multiple fluid generations have been identified based on morphology and trail orientations. Further chemical analysis will provide insight into similarities and differences between fluids involved in the formation of these rock structures.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411489

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