C.P. Gravenor Lecture Series - Dr. Phil McCausland

• Fri, 02/17/2012 - 2:30pm - 3:30pm

Speaker: Dr. Phil McCausland Title: Fall of the Grimsby meteorite; Arrival of
an (extra) ordinary chondrite Abstract: On the early evening of Sept 25,
2009, a brilliant fireball with three major bursts was seen and heard widely
over southwestern Ontario and adjacent regions. The event was recorded by a
network of automated cameras, radar and infrasound sensors operated by the
University of Western Ontario, making the Grimsby fireball the best
instrumentally-observed meteorite delivery to date. The fireball trajectory
and peak brightness greater than that of the full moon indicates that a ~40
kg object on a 27° inclined Apollo-type orbit collided with the upper
atmosphere at 20.9 km/s. Camera records, Doppler weather radar as well as
visual reports suggest that a number of sub-kg mass fragments and other
debris survived to reach the ground to the west and south of Grimsby,
Ontario.  During the autumn of 2009, a field search was undertaken in the
projected meteorite strewnfield, along with an effort to raise public
awareness of the event. The first recovered meteorite hit the windshield of a
parked vehicle and was collected as five fragments on the morning after the
fall, but remained unrecognized until the likelihood of the meteorite fall
was publicized. A total of 13 meteorites were found by search parties and
individuals within a 4 by 8 km strewnfield west of Grimsby, giving a total
collected mass of 215 g. More remain to be found to the southeast on the
escarpment and to the east, in Grimsby itself. Non-destructive examination of
Grimsby fragments by X-ray micro-CT, micro-XRD along with petrographic and
SEM examination of sections shows the meteorite to be an H4-5 chondrite
breccia, bearing occasional large 3 mm diameter chondrules, set in a variably
recrystallized sub-mm chondrule rich matrix with abundant small flecks of
Fe,Ni metal and sulphide. Coarse olivines and pyroxenes show sharp optical
extinction that imply only low shock metamorphism, but other textures such as
the presence of chromite veins and a heterogeneous distribution of metal and
sulphide amongst breccia blocks point to a more complex history of shock and
post-shock annealing. Noble gas and AMS study of short-lived radionuclides
indicate that Grimsby has a complex cosmic ray exposure history, and
surprisingly may have been a larger object than was expected from analysis of
the fireball entry. Grimsby, and future well documented fall events like it,
will provide an essential “ground truth” for estimating the source
regions, size distribution and type of objects impacting the Earth.