The Watson meteorite was discovered near
Watson on the Nullarbor Plain in South Australia, Australia
during 1972. A single mass weighing 93kg was found which was
classified as a IIE iron meteorite.
During later slicing in
1990, a silicate inclusion was found in the meteorite. In fact
it is the largest single silicate (stony) rock mass in any iron
meteorite. It was found after study of this silicate, that it
has a composition that matches the H-chondrites minus the usual
metal and troilite content. It was also found that the oxygen
isotope composition matches the IIE iron meteorites but also
lies along the oxygen isotope fractionation line with the
H-chondrites. It is thought that the Watson silicate was an
H-chondrite which was engulfed by metal and melted at
temperatures higher than 1550oC in a relatively
closed system at or just below the asteroid surface. Metal and
sulfide from the silicate that were incapable of being mixed
were incorporated into the surrounding metal host. This helps to
explain the difference in metal and troilite content between the
silicate and normal H-chondrites.
Around 8 million years after the above
melting event, a shock event took place which liberated
the Watson material from the asteroid parent body. This time
frame also matches the peak frequency of exposure ages of the
H-chondrites which helps further link them with the IIE irons.
This shock event also caused some severe metal distortion and
some localised melt pockets of Schreibersite/metal. In fact,
along with the large silicate inclusion, one of the most
interesting and distinctive features of the Watson meteorite is
the presence of numerous dendritic melt pockets which have never
before been encountered in such a large volume of metal. The
photographs below show two distinct Schreibersite inclusions;
one of which is of the dark variety. This darker inclusion is an
example of one of the dendritic metal pockets where the
Widmanstätten pattern almost appears to swirl towards it.
The 8.29g partslice below has an interesting
origin and provenance which is listed as: Oscar E. Monnig
Collection (Texas Christian University) - David New - Al
Mitterling - Andrzej S. Pilski - Martin Altmann - and is now part of the Meteorites Australia Collection
Watson (IIE) - 8.29g Partslice.
Watson partslice showing part of the silicate inclusion.