Export 3 results:
Sort by: Author Title Type [ Year  (Desc)]
Avery, MS, Gee JS, Bowles JA, Jackson MJ.  2018.  Paleointensity estimates from ignimbrites: The Bishop Tuff Revisited. Geochemistry Geophysics Geosystems. 19:3811-3831.   10.1029/2018gc007665   AbstractWebsite

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 similar to 767ka Bishop Tuff for measuring paleointensity found an internally consistent estimate of 43.03.2T. 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 41.911.8T; 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, 39.69.9T, 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. Understanding past variations of Earth's magnetic field help us understand processes in Earth's core and help us to better understand current field behavior, which is important to life on Earth. Earth's field is recorded by magnetic-minerals in rocks as they form. Variations in the strength of the magnetic field (paleointensity) are less well known than large variations in direction. This is partially due to the difficulty in identifying rocks that are suitable for paleointensity experiments. Rocks made of volcanic ash (ignimbrites) have been shown to successfully record the field strength during recent volcanic eruptions. However, we show evidence that ignimbrites may not all be suitable for paleointensity studies. The Bishop Tuff, located in eastern California, erupted about 767 thousand years ago, emplacing a large volume (similar to 200km(3), i.e., about 80 million Olympic swimming pools or slightly bigger than Lake Tahoe) of ash and lava over a few days. With samples from the Bishop Tuff we test variations in magnetic-mineralogy that may be related to venting volcanic gas, interaction with water, eruption temperatures, or the degree to which the ash compacted and solidified into rock. These factors affect the magnetic-minerals' ability to record paleointensity and the success rate of our experiments.

Selkin, PA, Gee JS, Meurer WP.  2014.  Magnetic anisotropy as a tracer of crystal accumulation and transport, Middle Banded Series, Stillwater Complex, Montana. Tectonophysics. 629:123-137.   10.1016/j.tecto.2014.03.028   AbstractWebsite

Fabric studies of layered mafic intrusions have led to improved understanding of the mechanical processes operating in large magma chambers, including crystal accumulation and crystal mush deformation. Such studies, however, are typically limited by a tradeoff between breadth (number of sites studied, characteristic of field-focused work) and sensitivity (ability to discern subtle fabric elements, characteristic of laboratory fabric analyses). Magnetic anisotropy, if analyzed in a systematic way and supported by single-crystal and petrofabric measurements, permits relatively rapid characterization of magmatic fabrics for large numbers of samples. Here we present the results of a study of remanence and susceptibility anisotropy from three transects through the Middle Banded Series of the Stillwater Complex, Montana. All three transects exhibit a magnetic foliation that increases with stratigraphic height up to the top of Olivine Bearing Zone III, consistent with crystal mush compaction. Perhaps more importantly, each tansect is characterized by a subtle lineation in the anisotropy of magnetic susceptibility with a consistent direction within that transect The magnetic lineation directions, which generally coincide with crystallographic preferred orientations of silicate minerals, likely record a pre-compaction fabric. Lineation directions differ from one transect to another, implying that the process generating the lineation - either slumping of a semiconsolidated crystal mush or magma transport - acted on length scales of at most a few km. These results demonstrate the sensitivity of magnetic anisotropy to petrofabric in mafic rocks. (C) 2014 Elsevier B.V. All rights reserved.

Selkin, PA, Gee JS, Tauxe L, Meurer WP, Newell AJ.  2000.  The effect of remanence anisotropy on paleointensity estimates: a case study from the Archean Stillwater Complex. Earth and Planetary Science Letters. 183:403-416.   10.1016/s0012-821x(00)00292-2   AbstractWebsite

Paleomagnetism of Archean rocks potentially provides information about the early development of the Earth and of the geodynamo. Precambrian layered intrusive rocks are good candidates for paleomagnetic studies: such complexes are commonly relatively unaltered and may contain some single-domain magnetite 'armored' by silicate mineral grains. However, layered intrusives often have a strong petrofabric that may result in a strong remanence anisotropy. Magnetic anisotropy can have particularly disastrous consequences for paleointensity experiments if the anisotropy is unrecognized and if its effects remain uncorrected. Here we examine the magnetic anisotropy of an anorthosite sample with a well-developed magmatic foliation. The effect of the sample's remanence fabric on paleointensity determinations is significant: paleointensities estimated by the method of Thellier and Thellier range from 17 to 55 muT for specimens magnetized in a field of 25 muT. We describe a technique based on the remanence anisotropy tensor to correct paleointensity estimates for the effects of magnetic fabric and use it to estimate a paleointensity for the Stillwater Complex (MT, USA) of similar to 32 muT (adjusted for the effects of slow cooling). (C) 2000 Elsevier Science B.V. All rights reserved.