Eagle Station pallasites
A tiny grouplet of pallasites with iron-rich olivine and nickel-rich metal, from a separate parent body.
- Classification
- Stony-iron meteorites › Pallasites › Eagle Station pallasites
- In our catalogue
- 4 meteorites
- Heaviest
- Karavannoe, 132 kg
- Type specimen
- Eagle Station, found in Kentucky in 1880
The Eagle Station pallasites (PES) look much like ordinary pallasites, but their olivine, their metal and above all their oxygen isotopes are different. Only about five are known, and they come from a different asteroid from the main group.
Mineralogy and chemistry
The Eagle Station pallasites (PES) have olivine that is more iron-rich than in the main group, around 18–20 mol% fayalite, and richer in calcium. Their metal is rich in nickel, about 14% to 15%, and in iridium and other refractory metals. Calcium-rich pyroxene and phosphate minerals are present in small amounts. Most strikingly, their oxygen isotopes plot far below the terrestrial line, in the region of the CV and CK carbonaceous chondrites, with a Δ17O of about −4 to −5 per mil.
Chromium and titanium isotopes also place them in the 'carbonaceous' family of meteorites thought to come from the outer Solar System. The metal resembles that of the IIF iron meteorites, and some researchers suggest the two share a parent body.
Parent body and formation
The Eagle Station pallasites came from a melted body built from carbonaceous chondrite-like material, unrelated to the main-group pallasite parent. Like the main group, they may have formed where molten metal mixed with olivine-rich mantle rock, but under more oxidising conditions, which explains their iron-rich olivine. That a body of carbonaceous make-up could melt and form metal and olivine shows that differentiation was not limited to the inner Solar System.
Abundance, type specimen and examples
Only about five Eagle Station pallasites are known, all finds. Eagle Station, the type specimen, was found in Carroll County, Kentucky, in 1880, about 36 kg. Cold Bay from Alaska, Itzawisis from Namibia and Karavannoe from Russia, a large mass of about 132 kg found in the 2010s, are other members. Choteau from Montana is also often counted.
How to recognise one
They look much like ordinary pallasites, but their olivine tends to be darker, greener or brownish, and the metal may weather differently. Only analysis of olivine composition, metal chemistry and oxygen isotopes can identify a member. Because so few exist, each specimen is scientifically important.
Why they matter
With only a handful of samples, the Eagle Station grouplet adds a whole extra melted asteroid, from the outer Solar System, to the family of bodies known to have formed cores and mantles. Its link with the IIF irons, if confirmed, would let core and mantle samples of one carbonaceous body be studied side by side.
Famous examples
- Eagle Station Pallasite, PES, United States, 1880 · 36 kg
- Cold Bay Pallasite, PES, United States, 1921 · 320 g
- Itzawisis Pallasite, PES, Namibia, 1946 · 350 g
- Karavannoe Pallasite, PES, Russia · 132 kg
Related groups
- IIF irons A small group of magmatic irons.
- Main-group pallasites Nine in ten pallasites: magnesium-rich olivine in metal related to the IIIAB irons.
- CV chondrites Large chondrules and big white inclusions, the oldest solids in the Solar System. Allende is the classic.
- Pallasites Gem-like olivine crystals set in iron-nickel metal.
Photos shared under Creative Commons licences (Wikimedia Commons).