30 Applications of Igneous Petrology, Geochemistry, and Geochronology to Understanding Earth and Other Planetary Systems

This session invites contributions that explore igneous systems through traditional and nontraditional methods. Studies that link crustal evolution, planetary formation, resource potential, and comparative planetology across temporal and spatial scales are especially encouraged.

Tuesday, 10 March 2026
1:30 PM - 3:30 PM
RCC, 104

Carbonate Mineralization and REE Distribution in Altered Ultramafic Lamprophyres of the Illinois–Kentucky Fluorspar District

Paper number: 30-6
Program: Technical Sessions
Time: 3:00 PM – 3:15 PM

Authors

Abstract

The ultramafic lamprophyres (UMLs) of the Illinois–Kentucky Fluorspar District (IKFD) have been identified as a potential domestic resource for rare earth elements (REEs), yet their strong alteration and carbonate-rich character complicate interpretations of carbonate genesis and REE distribution. This study evaluates the relative roles of mantle source heterogeneity and post-emplacement alteration in controlling carbonate mineralization and REE enrichment in the IKFD UMLs, integrating isotopic, trace element, and petrographic observations.

Nd–Sr isotope systematics indicate a mantle-derived magma, while variations in isotopic ratios are consistent with derivation from a metasomatized subcontinental lithospheric mantle source. Enrichment trends in Nb/Ta and Zr/Hf further suggest involvement of metasomatic components such as phlogopite-bearing melts and possibly carbonate-rich (carbonatite-like) agents within the mantle source, particularly for the most REE-enriched UMLs. These data collectively support generation of moderately REE-enriched lamprophyric melts from compositionally heterogeneous mantle domains prior to emplacement.

Petrographic observations demonstrate that carbonate in the IKFD UMLs occurs in two texturally and genetically distinct forms. Rounded carbonate-rich ocellar domains surrounded by phlogopite and Fe–Ti oxide reaction rims are interpreted as primary magmatic features formed by late-stage segregation of a carbonate-rich melt or volatile-rich liquid during magma ascent and shallow emplacement. In contrast, the volumetrically dominant carbonate occurs as pseudomorphic replacements of mafic phenocrysts, reflecting multistage post-magmatic hydrothermal alteration. Primary olivine and clinopyroxene were first serpentinized, generating Mg-rich fluids that promoted early Mg-carbonate replacement, followed by progressive evolution toward Ca-rich carbonate assemblages and late-stage quartz precipitation, likely involving interaction with external fluids such as basin brines or meteoric waters.

Although these alteration processes may locally redistribute REEs, the overall REE abundances and distribution patterns are interpreted to primarily reflect inherited mantle source characteristics. The preservation of mantle-like REE signatures despite intense alteration highlights the dominant control of subcontinental lithospheric mantle metasomatism on REE enrichment in the IKFD UMLs, with post-emplacement processes playing a secondary modifying role.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1

doi: 10.1130/abs/2026SE-411599

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