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, “josephinite” and “awaruite” 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 Earth’s 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.

 


 

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