16-46 Application of Olivine-melt Thermometry and Hygrometry to Basalts from Neoproterozoic Dikes crosscutting the Marcy Anorthosite Massif

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

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This project focuses on five basaltic dikes exposed in the Adirondack Highlands of New York, particularly those exposed by rockslides triggered during Hurricane Irene (2011). These rockslides provide a three-dimensional view into the Marcy Anorthosite Massif, part of a region dominated by high-grade metamorphic rocks that record deep crustal processes associated with the Ottawan phase of the Grenville orogenic cycle (~1090–1035 Ma), which involved significant crustal thickening and thermal overprinting (McLelland et al., 1996, 2010). Crosscutting basaltic dikes are observed throughout the Adirondack Highlands and Lowlands. These intrusions are interpreted to reflect extensional tectonics that followed compression during the Grenville cycle and may be linked to the early stages of Iapetan basin formation, as suggested by regional geochronology and geochemistry (Isachsen and Fisher, 1970; Chiarenzelli et al., 2010). The objective of this study is the application of olivine-melt thermometry and hygrometry to Neoproterozoic basaltic dikes in the Adirondack Highlands to estimate magmatic temperatures and dissolved water content at the onset of crystallization. This study evaluates whether equilibrium mineral compositions are preserved and whether temperature–water variations reflect multiple magmatic intrusion events across the Marcy Anorthosite Massif. Application of new olivine-melt thermometry/hygrometry methods (Pu et al., 2017, 2021, 2024; Brehm and Lange, 2020, 2023, 2025) on Precambrian rocks assesses the broader applicability of the method and will contribute to understanding mantle conditions during post-Grenville extension. Petrographic analysis indicates a subset of basaltic dikes are dominated by plagioclase with subordinate pyroxene, olivine, and Fe-Ti oxides. An additional subset is olivine-bearing with subordinate clinopyroxene and plagioclase largely restricted to the groundmass. Olivine phenocrysts in these samples display diffusion-limited growth textures (e.g. hopper, dendritic) consistent with rapid phenocryst growth, which allows for application of olivine-melt thermometry/hygrometry at the liquidus. Preliminary microprobe results from Twin Slide (Santanoni Peak) show a bimodal distribution of olivine with populations clustered at Fo₇₉ and Fo₇₇, which may indicate magma mixing.  Alternatively, microprobe results for Bennies Brook show a single population with a peak at Fo₇₇. Application of olivine-melt thermometry will be done using the most Mg-rich olivine in each sample paired with the whole-rock composition. Pairs will be evaluated for chemical equilibrium using ^Fe-MgK_D^{(olivine-melt)} and ^Mg-MnK_D^{(olivine-melt)} liquidus tests outlined in Brehm and Lange (2023, 2025).   

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