Paleointensity Estimates From Ignimbrites: The Bishop Tuff Revisited

Abstract

Volcanic ash flow tuffs (ignimbrites) may contain single domain‐sized (titano) magnetite that should be good for recording geomagnetic field intensity, but due to their complex thermal histories also contain other magnetic grains, which can complicate and obscure paleointensity determination. An initial study of the suitability of the ~767 ka Bishop Tuff for measuring paleointensity found an internally consistent estimate of 43.0 ± 3.2 μT. This initial study also showed a spatial heterogeneity in reliable paleointensity estimates that is possibly associated with vapor‐phase alteration and fumarolic activity, which motivated resampling of the Bishop Tuff to examine spatial changes in magnetic properties. Three new stratigraphic sections of the Bishop Tuff within the Owens River gorge were sampled, and the paleointensity results from the initial study in the same locality were reinterpreted. The mean of all sites is 41.9 ± 11.8 μT; this agrees with the initial study's finding but with substantially greater scatter. Two sections show evidence of vapor‐phase alteration where the presence of titanohematite, likely carrying a thermochemical remanence, produces nonideal behavior. This thermochemical remanence in the upper portion of the section also produces some paleointensity estimates of technically high quality that have significantly higher intensity than the rest of the tuff. Our best estimate for paleointensity, 39.6 ± 9.9 μT, comes from the densely welded ignimbrite that was emplaced above the Curie temperature of magnetite. The low permeability of this unit likely shielded it from vapor‐phase alteration. Our results suggest that care must be taken in interpreting paleointensity data from large tuffs as nonthermal remanence may be present.

Characteristics and properties of Bitlis ignimbrites and their environmental implications

“…Pyroclastics are volcanic materials coming from a volcanic chimney to the surface through volcanic mechanisms and wind by means of a sedimentation to bring to gather with the temperature of more than 500-600 °C in boiling or fusion form of glass or glassy parts as a result of the rotation of the product with minerals (zeolite) that are from the rocks formed. Ignimbrite is a pyroclastic flow unit that contains large amounts of pumice, volcanic glass and a small amount of lithic particles, with a flowing in high temperature, laminar flow system and gravity control (1)(2)(3)(4)(5)(6)(7)(8)(9)(10). Locally, Bitlis stone is found in the volcanic tuff group.…”

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

“…By contrast, dissolution of the magnetominerals and/or low‐temperature oxidation after TRM acquisition can lead to a paleointensity overestimation (Fabian, ). Theoretically, TCRM in both cases have straight Arai plots (Fabian, ), and both Ferk et al () and Avery et al () have shown that a likely TCRM can result in paleointensity results of high technical quality.…”

“…High LOI values in our samples are associated with higher paleointensity estimates of somewhat lower quality, suggesting there may be a link to secondary alteration. In the Bishop Tuff, Avery et al () find higher paleointensity associated with vapor‐phase alteration, the added presence of titanohematite, and a TCRM. An opposite trend is observed by Bowles et al () who reported less reliable and lower paleointensity results from the 1912 Novarupta ash flow tuffs containing >0.75 wt% water.…”

“…Geochemistry, Geophysics, Geosystems cases have straight Arai plots (Fabian, 2009), and both Ferk et al (2011) and Avery et al (2018) have shown that a likely TCRM can result in paleointensity results of high technical quality.…”

Differential Thermal Analysis Using High Temperature Susceptibility Instruments

“…Then, the tube is used to measure the sample of interest. Classically, magnetic susceptibility is plotted as a function of temperature and the derivative is often used to determine critical temperatures, such as Curie and/or Néel temperature (Avery et al., 2018; Fabian et al., 2013; Petrovskỳ and Kapička, 2006).…”