Absolute Paleointensity Study of Miocene Tiva Canyon Tuff, Yucca Mountain, Nevada: Role of Fine‐Particle Grain‐Size Variations

Abstract

Fine-grained, Ti-poor titanomagnetite in the~12.7 Ma Tiva Canyon (TC) Tuff systematically increases in grain size from superparamagnetic (SP) at the flow base to single domain (SD) at a few meters height. This allows us to examine the role of grain-size variation on paleointensity, within the transition from SP to stable SD. We present magnetic properties from two previously unreported sections of the TC Tuff, as well as Thellier-type paleointensity estimates from the lowermost~7.0 m of the flow. Magnetic hysteresis, frequency-dependent susceptibility, and thermomagnetic data show that sample grain-size distribution is dominated by SP in the lower~3.6 m, transitioning upwards to mostly stable SD. Paleointensity results are closely tied to stratigraphic height and to magnetic properties linked to domain state. SD samples have consistent absolute paleointensity values of 28.5 ± 1.94 μT (VADM of 51.3 ZAm 2 ) and behaved ideally during paleointensity experiments. The samples including a significant SP fraction have consistently higher paleointensities and less ideal behavior but would likely pass many traditional quality-control tests. We interpret the SD remanence to be a primary thermal remanent magnetization but discuss the possibility of a partial thermal-chemical remanent magnetization if microcrystal growth continued at T < T c and/or the section is affected by post-emplacement vapor-phase alteration. The link between paleointensity and domain state is stronger than correlations with water content or other evidence of alteration and suggests that the presence of a significant SP population may adversely impact paleointensity results, even in the presence of a stable SD fraction. Plain Language SummaryThe strength-or intensity-of Earth's magnetic field changes over geologic time, and these changes are important in learning about the internal evolution and processes of Earth's core. These "paleointensity" variations are recorded in many Earth materials, including volcanic ash flows, sometimes called "tuffs." Magnetic minerals in these flows acquire a magnetization proportional to Earth's field strength when they cool. The approximately 12 million-year-old Tiva Canyon Tuff in the western United States contains very fine-grained magnetite particles, the size of which increases upwards from the base of the flow. This small size is typically considered ideal for a paleointensity recorder, and the systematic size variation allows us to test effect of grain size on the paleointensity estimation across the lower end of the size spectrum. Samples collected from the lowermost 7 m of the Tiva Canyon Tuff were subjected to laboratory experiments designed to recover paleointensity. We find that the smallest particles perform poorly in the experiments and do not produce consistent results, while the slightly larger particles perform ideally and provide a consistent answer. This tells us that smaller is not always better, and there is a very narrow grain size range that will produce accurate results.