Albion, A New Iron Meteorite

by Russell Kempton, New England Meteoritical Services

Basic Truths

"My tomato field burned last night. The ground is scorched." relates a farmer calling from North Dakota. "it must have been a meteor" he says. Patiently, I explain a basic truth: "meteors" do not land in a ball of flame, and suggest that he look for other explanations. Another caller tells of finding many "perfectly round little meteorites" in a river stream. I ask if they attract a magnet. "Well, no," he says, "but they look like meteorites!" Again, I explain a basic truth: meteorites are not found as perfectly shaped spheres and all will attract a magnet to some degree. A caller from South Carolina tells of finding a 220 gram "iron meteorite." He describes "holes" and little pockets of "bubbles" in the interior. "No," I explain, "it's probably a piece of slag." A basic truth of iron meteorites is that they do not have "holes" or vacuoles throughout their interior. Well, we can't say that any longer. Unbelievably, an iron meteorite with vacuoles has been found!

Whitman County, Washington, USA

During the winter of 1966 - 1967, a single 12.28 kg iron mass was found in a wheat field adjacent to the Palouse River in Albion, Washington. The mass was in the possession of its finder, Kenneth Oliphant, until sometime in 1991 when it was sectioned to determine if it was a meteorite. Based upon the initial classification by Dr. John Wasson of UCLA as a Fine Octahedrite (IVA), it was recently submitted to the Meteoritical Society with the proposed name of Albion, Washington.

The interior of Albion is remarkable! Occurring homogeneously throughout Albion are irregular shaped vacuoles -- holes ranging in diameter from 4mm to 9mm. In one 620 gram slice (174 mm x 134 mm x 6 mm), 9 vacuoles were observed. The interior of these voids is lined with what appears to be solid spherical blebs (bubbles) covered with well developed, highly intergrown cubic crystals of almost pure iron. Of the more than 800 known iron meteorites, none have exhibited any form of vugs or "holes" within their Ni/Fe structure. To find them in an iron meteorite appears to "rock the boat" on the way we think of asteroid cores -- the parent bodies of iron meteorites.

Asteroids

It is generally accepted that the solar nebula was formed by the gravitational collapse of an extended mass of interstellar gas. Accretion led to the creation of bodies of differing composition and to their location at varying distances from the early Sun. The decay of radioactive elements within these bodies was the dominant source of heat that led to differentiation -- the separation into layers. As temperatures rose to around 1500° C, large quantities of material melted. Most of the dense elements, i.e. nickel and iron, gravitated toward the center of the core where pressures in the smaller asteroidal bodies increased to levels around 1 to 2 kilobars -- one to two thousand atmospheres. The unmelted outer areas formed a mantle of rock and surface crust that was to act over time as an insulative blanket, thus allowing for the extraordinary slow cooling rates of the Ni/Fe core.

There is substantial evidence that iron meteorites are the shattered remains of differentiated asteroidal cores 10 to 800 km in diameter violently disrupted through impact. Most appear to have gone through a liquid state about 4.5 billion years ago, slowly cooling at the rate of 0.4° C to 500° C per million years, to the region in which the Widmanstatten Pattern forms through diffusion at 700° to 450° C. Dr. John Wood of the Smithsonian Astrophysical Observatory estimated that an asteroidal body larger than 850 km would not cool to below 500° C at the center within the age of our solar system. Consequently, any proposed parent body for iron meteorites seems to be restricted to asteroidal-sized bodies rather than planetary size. The high temperature and intense pressures of one to two thousand atmospheres within the core during their molten core phase should preclude the existence of any type of voids or vacuoles that we see in Albion. Therefore, it seems very unlikely that a void could form in molten nickel-iron under such high pressures. Perhaps we should look further for an explanation of Albion's vacuoles.

Secondary Structures

Our solar system is bombarded from the outside by cosmic rays from galactic sources (109 electron volts per nucleon) and from within by solar radiation (107 electron volts). The iron cores of asteroids are shielded from these high energy cosmic rays prior to breakup. However when the parent asteroid is disintegrated by one or more violent collisions, the broken core fragments become exposed to this intense radiation. The period of time that a meteoroid is exposed to these particles is referred to as their cosmic ray exposure age.

The cosmic ray exposure ages of iron meteorites indicate that most fragmented as solidified, cool bodies about 500 million years ago -- approximately 4 billion years after their initial molten state. Plenty of time to cool below 450° C. In Albion, microscopic examination of the Widmanstatten Pattern reveals no distortion of the kamacite and taenite lamellae indicating normal cooling throughout this period. However, the granular structure of the solid blebs seems to indicate rapid cooling after an intense melting event. This implies that the vacuoles in Albion are secondary structures.

Unlike the primary structures in iron meteorites i.e.: kamacite, taenite, etc. that formed through a process of solid state diffusion, secondary structures are, geologically speaking, metamorphic. They are the product of shock and heating events. Buchwald in the HANDBOOK OF IRON METEORITES defines four main categories of secondary structures: shock-induced plastic deformation, shock-induced solid state transformation, shock melting, and thermal annealing. Of the four, shock melting seems the most likely candidate for the structures in Albion.

Diamond, troilite, kamacite, cohenite, and schreibersite are all shock indicators in iron meteorites. Of these, the mineral troilite (FeS) is a particularly sensitive mineral to shock and will undoubtedly play a major role in the research of Albion. For now, while research is ongoing, let's speculate on the origins of these vacuoles using the following shock-melting model.

After being repeatedly hit by other celestial bodies, a fractured 100 km diameter asteroid breaks apart; its iron core exposed. Ultimately, a particularly violent "hypervelocity" impact shatters the core. The impacting projectile is completely vaporized while thousands of fragments from the impacted asteroid survive, some of them hundreds of meters in diameter. Created is a range of meteoroids, shock-altered to varying degrees according to their distance from the impact point.

One of these meteoroids, with silicate (olivine) and graphite inclusions throughout its nickel-iron structure, reacted to the temperature rise of the hypervelocity impact by vaporizing its inclusions. As the internal pressure of the core decreased from breakup, the released oxygen from the vaporized inclusions resulted in the smelting of iron from the silicates (olivine) and, through a vapor phase, deposited the iron back upon the spherical blebs that we now see several hundred million years later. Smelting is the melting process that separates pure metal from extraneous substances. Ferrous silicates i.e. olivine-rich inclusions, are vulnerable to smelting in the presence of graphite. This smelting is suppressed at high pressures but promoted as pressure falls. The entire process probably occurred within minutes, leaving the voids that we now see in Albion.

Again, this is only speculation, but think how fortunate we are to find mysteries that keep us searching for new answers. "Holes" in the internal structure of iron meteorites have never before been seen. They were not thought to be possible. But then, my next phone call may be a verified report of a scorched vegetable garden littered with Ureilites.

Acknowledgments - The author expresses his sincere thanks to Dr. Carl Francis, Curator, Mineralogical Museum, Harvard University and Dr. Timothy Grove, Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, for their valuable discussions.

Russell W. Kempton is the Director of New England Meteoritical Services based in Mendon, Massachusetts, USA.

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