Paleointensity estimates from ignimbrites: An evaluation of the Bishop Tuff

Ash flow tuffs, or ignimbrites, typically contain fine‐grained magnetite, spanning the superparamagnetic to single‐domain size range that should be suitable for estimating geomagnetic field intensity. However, ignimbrites may have a remanence of thermal and chemical origin as a result of the complex magnetic mineralogy and variations in the thermal and alteration history. We examined three stratigraphic sections through the ∼0.76 Ma Bishop Tuff, where independent information on postemplacement cooling and alteration is available, as a test of the suitability of ignimbrites for paleointensity studies. Thermomagnetic curves suggest that low‐Ti titanomagnetite (T c = 560°C–580°C) is the dominant phase, with a minor contribution from a higher T c phase(s). Significant remanence unblocking above 580°C suggests that maghemite and/or (titano)maghemite is an important contributor to the remanence in most samples. We obtained successful paleofield estimates from remanence unblocked between 440°C and 580°C for 46 of 89 specimens (15 sites at two of three total localities). These specimens represent a range of degrees of welding and have variable alteration histories and yet provide a consistent paleofield estimate of 43.0 μ T (±3.2), equivalent to a VADM of 7.8 × 10 22 Am 2 . The most densely welded sections of the tuff have emplacement temperatures inferred to be as high as ∼660°C, suggesting that the remanence may be primarily thermal in origin, though a contribution from thermochemical remanence cannot be excluded. These results suggest that ignimbrites may constitute a viable material for reliable paleointensity determinations.