150-5 A Potential Garnet Reference Material for U-Pb Dating from the Magnet Cove Igneous Complex, Arkansas

Session: Geochronology of Critical Mineral Deposits with Special Reference to U-Th-Pb Dating of Common-Pb-Rich Minerals (Posters)

Presenting Author:

Authors:

Abstract:

The Magnet Cove Igneous Complex (MCIC) is a concentric structure of intrusive and extrusive, mainly silica-undersaturated, rocks in Central Arkansas.  It is mined for vanadium mineralization in the adjacent sedimentary rocks, was previously mined for titanium, and is potentially of economic interest again as a source of critical mineral elements such as niobium.  The MCIC is also well known by collectors for its unique mineral specimen. It hosts rare zirconium- and titanium-rich minerals, e.g. perovskite, titanite, and schorlomite garnet, and is the kimzeyite garnet type locality.  This study examines the potential of schorlomite garnets from the magnetite rich portion of a garnet-biotite ijolite as a potential new reference material (RM) for laser ablation or TIMS U-Pb geochronology. In an earlier part of this project, we dated garnet, perovskite, apatite and calcite from the Kimzey Calcite Pit in the carbonatite unit of the MCIC, and from surrounding ijolite and syenites units (Möller & Pell, 2023; Pryor et al. 2025).  We defined at least a two-phase magmatic emplacement history between ca. 102 Ma and 98 Ma.  Schorlomite garnets from the grt-bt ijolite gave precise, repeatable and very radiogenic results, and we pursue garnet from this unit as a potential reference material.  In thin section, the garnets show strong color zoning from honey brown to almost non-transparent black.  Some overgrow perovskite.

Several pounds of schorlomite garnet larger than 5 mm diameter were separated from loose heavy mineral residue surface material collected adjacent to the defunct Kimzey magnetite pit.  Close inspection of the individual subhedral and euhedral crystals and crystal clusters shows a range of colors from light brown to black, as in thin section.  Smaller, bright green euhedral garnets overgrow the larger schorlomite.  Representative crystals of all different colors and sizes have been mounted in epoxy and sectioned for compositional and isotopic analyses to test whether all these groups yield the same age and may be used as a primary or secondary U-Pb RM.  With plenty of material available from a well-defined source that may be distributed to the geochronology community this could make an excellent RM.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10577

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