150-4 U-Pb, Trace Element, and Textual Characterization of Zircon from Western Maine LCT Pegmatites

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

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Abstract:

Pegmatites enriched in lithium, cesium, and tantalum (LCT) are exploration targets for these critical elements that are essential to the energy transition. The enrichment of pegmatites in these elements, and others, is generally thought to form from fluids during late stage magmatic-to-hydrothermal transition. LCT pegmatites, specifically, are emplaced during the late stages of collisional orogeny, although many aspects of their genesis remain unclear. 

Zircon is the preeminent geochronometer in granitic rocks due to its prevalence, durability, and longevity. Its use in probing the origins of LCT pegmatites has been limited, however, by factors including xenocrystic zircons, radiation damage, hydrothermal alteration, and fluid-induced U-Pb discordance. Problematic zircons from several western Maine LCT pegmatites of known or probable Mississippian to Permian ages were investigated with detailed microscopy, mapping, and laser ablation depth profiling transects. These analyses aim to extract age information retained in these grains, and the results have implications for the thermochemical history of this storied pegmatite district. 

The zircons are heavily altered, having complex internal textures, chemistry, and inclusions, which together yield a dense dataset. We use split stream laser ablation inductively coupled mass spectrometry (LASS-ICP-MS) depth profiling to characterize the zircon grain’s U-Pb isotopes and trace elements. After depth profiling, the grains are rotated, mounted, and polished to midsection to expose the depth profile transect, after which in situ microanalytical tools are applied. Many of the zircons exhibit typical igneous zircon chemistry, but a significant portion contain widely varying major and trace element chemistries. Imaging reveals a variety of textures, including oscillatory zoning patterns, amorphous recrystallization, and an inclusion-heavy “patchy” texture. We present the results from our paired U-Pb, trace element, major element, and structural evaluations to assess the causes of complex zircon characteristics in LCT pegmatites. We evaluate the potential of complex, possibly inherited, zircon in LCT pegmatites in providing previously inaccessible age information regarding the thermal and fluid histories of these pegmatites. 

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

doi: 10.1130/abs/2025AM-10117

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