JOSEPHINITE
Natural iron-nickel alloys are very rare on Earth,
with the exception of iron meteorite material. Terrestrial Fe-Ni is so
rare it's practically unheard of. One area that does have terrestrial
Fe-Ni alloy is southwestern Oregon. Some streams draining certain
portions of the Josephine Peridotite (Josephine Ophiolite) in southwestern
Oregon have (usually) small nuggets dominated by nickel-iron metal. These
nuggets are called josephinite.
Josephinite (weathered nugget) (13 mm across); a magnet will stick to this.
Josephinite rocks contain an unusual mineral
assemblage consisting of awaruite (Ni2Fe to Ni3Fe),
andradite garnet (ideally Ca3Fe2Si3O12),
wairuite (FeCo), minor pyrrhotite (Fe1-xS), minor pentlandite
((Fe,Ni)9S8), minor Ni6Fe4,
etc. Despite statements to the contrary, ÒjosephiniteÓ and ÒawaruiteÓ are
not synonyms. Josephinite is a rock name. Awaruite is
a mineral name. Josephinite is a crystalline-textured,
polymineralic rock that is dominantly composed of the mineral awaruite.
Josephinite is often found in stream gravels, but it
is also found in-situ within rocks of the Josephine Peridotite.
The Josephine Peridotite (Josephine Ophiolite) represents ancient oceanic
lithosphere of Late Paleozoic or early Mesozoic age that was obducted onto the
western edge of North America during the Nevadan Orogeny (Jurassic).
The origin of josephinite & its Fe-Ni metal has
been very controversial in the geologic literature for many years. The
most extreme interpretation of josephinite considers the Fe-Ni metal to
represent samples from the deep interior of Earth, from the lower mantle, the
mantle-core boundary interval, or the outer core itself. In this
scenario, the iron-nickel metal reached the EarthÕs near-surface by being
entrained in a mantle plume (hotspot plume) generated at or near the
core-mantle boundary.
Petrologic studies of josephinite nuggets and in-situ
josephinite have shown that the Fe-Ni metal alloys persistently embay
the iron and iron-nickel sulfides (pyrrhotite, pentlandite) commonly found in
the nugget interiors. In other words, it appears that the Fe-Ni metal is derived
from the sulfide minerals. The host rocks have been serpentinized,
and this metamorphism likely is responsible for the generation of
josphinite. How? Josephinite is apparently the result of a reducing
desulfurization metamorphic process.
However, some workers have continued to present new
evidence & arguments for an outer core or core-mantle origin for
josephinite.
Most info. from Dan Leavell & John Bird.