100-6 What Can Martite Tell Us About Unconformity Development in Deep Time? A Comparison of Two Martite (U-Th)/He Data Sets from the Colorado Front Range (USA)

Session: Snowballs, Unconformities, BIFs and Beyond: Navigating the Neoproterozoic Rock and Climate Records Using Geochronology

Presenting Author:

Authors:

Abstract:

Magnetite (Fe₃O₄), a common accessory mineral in magmatic rocks, can be replaced by hematite (Fe₂O₃) during exhumation to and residence in near-surface conditions. The pseudomorphic replacement product, martite, is amenable to (U-Th)/He dating, which can potentially constrain the timing of unconformity development in deep time. To evaluate the extent of martitization and impact of the long-term thermal history on martite (U-Th)/He dates and interpretations, we compare martite (U-Th)/He results from two locations (Morrison and Boulder Canyon, CO) that sample Paleoproterozoic crystalline basement below a nonconformity with the Pennsylvanian Fountain Formation. Both martite (U-Th)/He data sets exhibit intrasample scatter, with individual dates ranging from 1040 to 127 Ma (Morrison, n = 52) and 1720 to 227 Ma (Boulder Canyon, n = 11). Reflected light microscopy and electron backscattered diffraction analyses reveal Morrison martite is fully converted to hematite (i.e., displays negligible remnant magnetite), contrasting with Boulder Canyon martite, which has been variably transformed into hematite with lower U content. Intrasample scatter in both data sets is in part controlled by variable He loss from each aliquot over the long-term thermal history. We interpret the oldest martite (U-Th)/He dates from Morrison to record the minimum timing of martitization, suggesting ancient crystalline basement rocks in this locality were exhumed to the near-surface environment by the latest Mesoproterozoic. We cannot make these same interpretations for the Boulder Canyon data set, as variable martitization implies martite aliquots were not completely reset during recrystallization and may include a pre-martitization He ”memory.” Although we air abraded martite to mitigate the effects of alpha ejection and implantation, the low U concentrations of Boulder Canyon martite also make these aliquots more susceptible to the implantation of parentless He from adjacent U-hosting phases, making the martite appear older. We interpret that these two processes are responsible for Boulder Canyon martite (U-Th)/He dates approaching the known crystallization age of the host rock (~1720 Ma). Our findings highlight the promise of martite (U-Th)/He dating for investigating deep-time processes including the development of unconformities and paleotopography, while also illustrating its limitations associated with low U and incomplete martitization.

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

doi: 10.1130/abs/2025AM-9400

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