Identification and Study of Superparamagnetic Grains from the Tiva Canyon, NV
The magnetic properties of superparamagnetic particles are difficult
to study due to their rare occurence in nature, and the inability to produce
them synthetically without particle interactions. The Tiva Canyon member
of the Paintbrush Tuff, a Miocene-aged rhyolitic ash-flow sheet located
near Yucca Mt, Nevada contains an ideal spatial distribution of magnetic
grains, as well as a narrow grain size distribution, providing a perfect
material in which to identify and study superparamagnetic grains. The magentic
materials in this member are believed to consist primarily of magnetite
with some titanium. A 500 cm. section of this unit was sampled and tested
for remanent magnetization, bulk magnetic susceptibility, and hysteresis
properties. The combined data revealed that magnetic grain sizes vary with
height throughout the sequence, from superparamagnetic grains on the order
of a few nanometers near the base of the sequence, to near-micron sized
pseudo-single domain particles near the top of the sampled section. This
distribution has implications for the cooling history of the flow unit,
as well as more comprehensive knowledge about the behavior of superparamagnetic
particles and their occurence in nature.