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.
Bibliographical noteFunding Information:
We thank C. Sprain and R. Fu for their thoughtful reviews that helped improve the manuscript. This work was supported by National Science Foundation (NSF) grant EAR0943999. This research constituted a portion of the doctoral dissertation of MSA. The Institute for Rock Magnetism is supported by the NSF Instruments and Facilities program. The data from this study are available in the Magnetics Information Consortium (MagIC) database [https://earthref.org/MagIC/16504/224339fa-c543-477c-8ef5-38947455dc6c].
- Bishop tuff
- geomagnetic intensity
- pyroclastic density currents