FLUID AND SOLID INCLUSIONS IN EAST TENNESSEE FIELD DIAMONDS (DOUBLY TERMINATED QUARTZ CRYSTALS)

Doubly terminated “Field Diamond” or “Herkimer” type euhedral quartz crystals occur in various localities globally, commonly observed in fissures in metamorphic and sedimentary rocks associated with orogenic belts, and are thought to have grown from fluids mobilized during orogenesis. Interpretation of solid and liquid inclusions within these crystals should provide insight into their formation histories giving a unique understanding of tectonic processes. Many contain hydrocarbons, offering insight into the paleo-maturation and migration of oil and gas. Moreover, brines captured in these crystals may be associated with regional Mississippi Valley-type deposits. East Tennessee Field Diamonds occur in a variety of locations in the Valley and Ridge portion of the Appalachian tectonic belt, primarily in Cambrian to Ordovician dolomite breccias, and zones of folding and faulting within overlying Ordovician shales. Crystals display a variety of morphologies from bipyramidal to elongate-prismatic, with combinations suggesting multiple stages of growth.

Sulfate and carbonate solid inclusions have been identified using laser Raman. These solid inclusions (~1-500 μm size) at times contain opaque and translucent mineral inclusions and/or fluid inclusions, either of which may fluoresce under UV illumination. Narrow growth zones of the host quartz are decorated by solid inclusions, typically carbonates, and commonly a distinct solid-rich zone marks the change in host morphology.

Primary fluid inclusion assemblages (FIA) are observed in solid inclusions. Secondary FIA dominate, with some Pseudosecondary FIA cross-cut by solid inclusion defined growth zones. Some paragenesis of FIA can be determined by cross-cutting relationships of secondary FIA trails. Hydrocarbon bearing inclusions range in maturity from blue-fluorescing colorless oil to gas-rich. Initial results suggest crystals from more northeasterly areas contain only gas-rich inclusions. Crystals from multiple locations contain light oil with a high amount of paraffin wax. Secondary H₂O-CH₄­ inclusions have ice melting temperatures from -17 up to -3 °C, indicating a large range in salinities. Many FIA exhibit hook and annular decrepitation textures, consistent with isothermal decompression during uplift.