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Friday, 10 February 2012

More about the Lake House Meteorite

 I found this as a PDF somewhere.  All very mysterious -- and serving as a nice reminder that the Boles Barrow bluestone was probably not the only "strange" or "interesting" stone to be picked up and built into a Neolithic or Bronze Age burial mound.......

I am intrigued by the idea that this meteorite landed somewhere in Wiltshire c 30,000 years ago and that it was then somehow embedded into snow and ice -- or sediments -- for 20,000 years before being exposed to the atmosphere c 10,000 years ago.  I haven't got a clue how this C14 date of c 10,000 years BP might have been arrived at.  You can only do C14 dating on organic materials -- so what was the organic material dated?  The piece below states very clearly that "samples of the meteorite" were dated, using Accelerator Mass Spectrometry terrestrial residence dating.  I am not sure why they would then want to convert that date to a radiocarbon date. Very strange indeed........


THE METEORITE FROM LAKE HOUSE.

C.T. Pillinger, J.M. Pillinger, R.C. Greenwood, D. Johnson,
A.G. Tindle, A.J.T. Jull and M. Ashcroft.

c.t.pillinger@open.ac.uk

Introduction: In 1991 a very big meteorite (>60kg, dimensions:
diameter 50cm x height 40cm) was brought to the Natural
History Museum by the then occupier of Lake House, a country
mansion in Wiltshire, UK, associated with a large estate of the
same name. The circumstances concerning the likely origins of
this ‘find’ are dealt with in a companion abstract [1]. Our attention
was attracted to it because of its proximity to Danebury Hill
(20 km east), where the only British meteorite ‘find’, collected
under controlled circumstances, was located in 1974 [2]. If the
large meteorite from Lake House turned out to be paired with the
much smaller Danebury find, then the mystery surrounding its
origin would be instantly solved.
The sample from Lake House was confirmed as a meteorite
by Robert Hutchison [3] and subsequently returned to its owners.
Our electron microprobe data from a PTS made from a chip
taken at the OU suggest it should be classified as type H5 similar
to our assignment for Danebury.
Petrography: The sample is a heavily weathered (W5) [4],
moderately shocked (S4) [5], equilibrated ordinary chondrite
(H5). Distinct chondrules are present, but these tend to have
poorly defined boundaries. Porphyritic types predominate, but
barred olivine and radial pyroxene textured chondrules are also
common. Chondrule mesostasis is recrystallised, with grain sizes
generally below 50μm. The sample is cut by a network of veins,
up to 2 mm thick, filled with secondary weathering products.
Discussion: In respect of weathering characteristics, the two
specimens are entirely different – areas in our Danebury PTS are
grade W1 and the whole specimen is no worse than W2, whereas
the meteorite from Lake House is W5. The thin section of the
latter was made from a near surface chip of the highly corroded
sample; the Danebury analyses were performed using a fresh interior
portion. Not wishing to have what appeared to be a very
delicate specimen disintegrate, we attempted to extract an interior
core from the larger meteorite using an experimental drill being
developed for robotic space missions. Much to our surprise prolonged
drilling was unable to penetrate more than a millimeter
into a location where the crust appeared to be absent. In earlier
performance tests the drill had no difficulty in obtaining 1.5 cm
long cores from basalt and concrete. We conclude that the interior
of the meteorite from Lake House may not be as weathered
as it appears from the outside.
Because we had obtained a very precise terrestrial weathering
age for Danebury, samples of the meteorite from Lake House
were removed for Accelerator Mass Spectrometry terrestrial residence
dating. The first sample studied has a 14C weathering age
of 10,600 +/- 1800 years BP. A second run with a better specimen
gave 9500 +/- 560 years. The large meteorite is clearly not
related to the small one authenticated from the archeological site
a few kilometers away. Given that it is a single stone, the meteorite
represents an unusually large ordinary chondrite find.

References: [1] C.T.Pillinger and J.M.Pillinger, this vol. [2]
C.T.Pillinger et al., this vol. [3] pers. comm. [4] Wlotzka F. 1993
Meteoritics 28:460. [5] Stöffler D. et al. 1991. GCA 55:3845-
3867.

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