Canadian meteorites: a brief review1

Transcription

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Canadian meteorites: a brief review1
Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIVERSITE QUEBEC A CHICOUTIMI on 11/25/13
For personal use only.
Graham C. Wilson and Phil J.A. McCausland
Abstract: We present a brief overview of Canadian meteorites with a focus on noting significant recent falls, finds, and research developments. To date, 60 Canadian meteorites have received official international recognition from the Nomenclature Committee of the Meteoritical Society, while at least 13 more are “in process” for submission to the Meteoritical
Bulletin, that organization’s official database of the world’s meteorites. The 60 meteorites (44 finds and 16 falls since the
recognition of the Madoc iron in 1854) include 25 irons, 3 pallasite stony-irons, and 32 stony meteorites. The latter include
14, 11 and 3 H, L and LL chondrites, 2 carbonaceous chondrites and 2 enstatite chondrites, but no achondrites. The most
intensively researched meteorites are Tagish Lake (C2 ungrouped) and Abee (EH5), followed by Bruderheim (L6) and
Springwater (pallasite). Bruderheim, a 1960 fall, is widely distributed, being the most massive reported Canadian meteorite
at 303 kg total known weight (TKW). Seven Canadian meteorites exceed 100 kg TKW, 36 are between 1 and 50 kg, and
17 are <1 kg. Recent years have seen the addition of the Tagish Lake, Buzzard Coulee and Grimsby meteorite falls, all of
which have well-determined fireball trajectories and therefore well-known orbits, a striking Canadian addition to the handful
that are known worldwide. The discovery of the Holocene Whitecourt iron impact crater is similarly a significant recent development in understanding the impactor flux. The lessons learned on meteorites can be applied to newly recovered samples
from the Moon, Mars, asteroids, and comets.
Résumé : Nous présentons une brève vue d’ensemble des météorites canadiennes tout en ciblant les plus récentes chutes
importantes, les trouvailles et les résultats de la recherche. À ce jour, 60 météorites canadiennes ont été officiellement reconnues par le comité de nomenclature de la Meteoritical Society, alors qu’au moins 13 autres sont « en traitement » pour être
soumises au Meteoritical Bulletin, la base de données officielle de cette organisation pour les météorites mondiales. Les
60 météorites (44 trouvailles et 16 chutes depuis la reconnaissance de la météorite ferreuse Madoc en 1854) comprennent
25 météorites ferreuses, 3 pallasites mixtes et 32 météorites pierreuses. Ces dernières comprennent 14, 11 et 3 chondrites H,
L et LL, 2 chondrites carbonées et 2 chondrites à enstatite, mais pas d’achondrites. Les météorites les plus étudiées sont
celles du lac Tagish (C2 non groupée) et d’Abee (EH5), suivies de Bruderheim (L6) et de Springwater (pallasite). Bruderheim, une pluie météorite à grande distribution, datant de 1960, est la météorite canadienne la plus massive, avec un poids
total connu de 303 kg. Sept météorites canadiennes ont un poids total connu de plus de 100 kg, 36 pèsent entre 1 et 50 kg
et 17 ont un poids inférieur à 1 kg. Au cours des dernières années, les chutes des météorites du lac Tagish, de Buzzard Coulee et de Grimsby, toutes avec des trajectoires bien déterminées de globes de feu et donc des orbites bien connus, ont été
des ajouts canadiens remarquables à la poignée de météorites connues mondialement. La découverte du cratère d’impact de
la météorite ferreuse Whitecourt (datant de l’Holocène) constitue aussi un développement important récent dans la compréhension du débit des impacteurs. Les leçons apprises sur les météorites peuvent être appliquées à des échantillons nouvellement récupérés de la lune, de Mars, des astéroïdes et des comètes.
[Traduit par la Rédaction]
Introduction
The history of modern meteorite research in Canada arguably dates to the time of recognition of the first meteorite in
the country 13 years prior to Confederation, near Madoc,
southern Ontario in 1854. The 167.5 kg single mass of iron
meteorite was acquired by William Logan, first director of
the fledgling Geological Survey of Canada (Hunt 1855). The
4th edition of the Natural History Museum catalogue of meteorites (Graham et al. 1985), lists only 46 authenticated meteorites for Canada, the world's second-largest country.
The official catalogue of the world’s meteorites is now
published on-line as the Meteoritical Bulletin, and (as of
early 2012) lists 60 officially recognized Canadian meteorites. These we focus on here, though it may be noted that
many years can pass between the first recovery of a meteorite
and its recognition as such by science. Even then, a proper
classification must be carried out, and a suitable amount of
material deposited with a recognized scientific depository
(such as a major museum or university department active in
meteorite research). The first author of this paper has main-
Received 27 January 2012. Accepted 16 May 2012. Published at www.nrcresearchpress.com/cjes on 12 December 2012.
Paper handled by Associate Editor Richard Leveille.
G.C. Wilson. Turnstone Geological Services Limited, P.O. Box 1000, Campbellford, ON K0L 1L0, Canada.
P.J.A. McCausland. Department of Earth Sciences, Western University, London, ON N6A 3K7, Canada.
Corresponding author: Graham C. Wilson (e-mail: [email protected]).
1This
article is one of a series of papers published in this CJES Special Issue on the theme of Canadian contributions to planetary
geoscience.
Can. J. Earth Sci. 50: 4–13 (2013)
doi:10.1139/E2012-036
Published by NRC Research Press
Wilson and McCausland
Fig. 1. The Springwater main-group pallasite, showing coarse
rounded olivine in metal (kamacite) with minor iron–nickel phosphide (schreibersite) at the metal–silicate interface. Note metric scale
bar in centimetre, subdivided to millimetres.
Fig. 2. The Dresden (Ontario) H6 (S2) chondrite, displaying the
classic contrast, in a freshly recovered fragment, of black melted
glassy fusion crust ≤1 mm thick around the pale, metal-flecked,
silicate-dominated interior. Coin diameter = 23 mm.
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Fig. 3. A photomicrograph of the Red Deer Hill L6 (S3) chondrite.
A glassy shock melt vein, up to 0.25 mm thick, containing abundant
micron-scale troilite (FeS) globules. Coarse troilite, as a typical
aggregate of recrystallized polygonal domains (best seen in crosspolarized reflected light), occurs left of the vein, while a small
angular grain of white kamacite, and minor patches of secondary
oxide (goethite) are visible to the right. Nominal magnification 200×,
long-axis field of view 0.4 mm, in plane-polarized reflected light.
tained a modest database of these official meteorites plus
other finds “in process”, the latter currently numbering 13 to
15 (most of these are listed at: http://www.turnstone.ca/canamet4.pdf, 10 April 2012) — which we hope will be added to
the true tally before long.
Before considering the 60 meteorites, and describing the
significance of a few, we should first answer two questions,
namely: (1) why study meteorites? and (2) what does Canada
gain from its meteorites, and vice versa?
The importance of meteorites
Quite recently, meteorites were still rare. The total, historical worldwide haul of meteorites consisted of one or more
fragments from some 2000 distinct meteorite finds or witnessed meteorite falls (e.g., the 2611 “reasonably authenticated meteorites” of Graham et al. 1985). This last total did
not include the growing number of recoveries from the cold
deserts of Antarctica, and preceded the impressive recoveries
from the hot deserts of North Africa and the Middle East in
the past two decades.
The Meteoritical Bulletin (with 98 published editions
through 2010, and now available as on-line updates alone)
cited, as of 17 January 2012, some 41 714 valid entries, and
12 128 provisional names. This explosive growth of the meteorite inventory, roughly a 20-fold increase in a quarter-century,
has been accompanied by a steady growth in the science of
meteoritics, and associated fields of observational smallbody astronomy, cosmochemistry, and astrobiology. The
field has long been at the forefront of developments in geochemical and mineralogical analysis of small and valuable
samples, particularly in clean sample handling, mass spectrometry, and electron microscopy (e.g, Mason 1963; Clayton 1993; Bogard 1996).
A multidisciplinary approach to a well-defined problem,
such as the documentation and interpretation of a photoPublished by NRC Research Press
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Can. J. Earth Sci., Vol. 50, 2013
Fig. 4. Field photo of site location EG-06 in the Tagish Lake C2 chondrite strewnfield, recovered in April 2000 from the frozen lake surface
(Hildebrand et al. 2006). The dark carbonaceous chondrite was heated during the daytime and tended to melt and disaggregate into the ice
within distinctive melt holes. In this view, one of the meteorite-bearing melt holes has been chainsawed out of the ice as a keystone block and
tipped onto its side for viewing. The meteorite can be seen inside the ice block, having descended and formed a dark pancake of material
~10 cm below the snow-covered top of the block. Some dark debris can be seen to have continued downward along small holes much deeper
into the ice.
Fig. 5. The Grimsby H5 chondrite 21.9 g fragment “HP-1” as it was discovered two weeks after the 25 September 2009 fall, resting lightly on
a grassy field in the west end of Grimsby, Ontario. This is one of only 13 pieces recovered from the fall. Note the dark, millimetre thick
fusion crust and incipient rusting of FeNi metal on the fractured surfaces (McCausland et al. 2010). This fragment of Grimsby is evidently
part of a larger, post-ablation individual that survived the fireball, but its sister fragments have not to date been found. Coin diameter =
23 mm.
graphed meteorite fall, can provide interesting opportunities
for a diverse scientific team (e.g., Brown et al. 2000).
Although there is now an almost unlimited supply of the
most abundant classes of chondritic meteorite, the recognition of rare classes of igneous (achondritic) meteorites, some
of them attributed to parent bodies such as the Earth’s moon,
Mars, and asteroid 4 Vesta, means that the sample-conservative approach to analysis of small samples remains valid. The
study of meteorites, particularly with the parallel evolution of
geochemistry to handle the requirements of the Apollo and
Luna sample-return programs in the late 1960s and early
1970s, has had concrete benefits, in terms of widely applicable technologies and also specific contributions to materials
science, discovery of new minerals, and a refinement of our
picture of the evolution of the solar system (McSween 1999;
Hutchison 2004).
Since the productive 1969 fall of the Allende carbonaceous
chondrite in Mexico, it has become clear that the most “primitive” early constituents of the solar nebula are preserved in
certain meteorite classes that have escaped significant metamorphism or aqueous alteration, casting light onto an epoch
that predates the Earth’s oldest known rocks by hundreds of
millions of years. Presolar grains of phases such as graphite,
diamond, SiC, and corundum offer glimpses into a remote
past, right back to the origin of the chemical elements (nucleosynthesis), in a generation of stars that predated our Sun.
Meteorites provide also a wealth of clues to the origins and
evolution of their parent bodies, ejection of the meteoroid
from the parent body, and cosmic ray interactions with the
meteoroid prior to its fall through Earth’s atmosphere (Hutchison 2004).
Meteorites in Canadian science
The 60 officially-sanctioned meteorites in Canada that
were found or seen to fall before being recovered are listed
in order of class and name in Appendix A Table A1. Note
that one of the Canadian attributions of Graham et al.
(1985), the Leeds iron, has been discredited, recognized by
scientific detective work as a synonym of the large Toluca
iron from Mexico (Kissin et al. 1999).
The largest public collections in Canada, as of 2012, include: the National Collection, curated at the Geological Survey of Canada in Ottawa; the University of Alberta at
Edmonton; and the Royal Ontario Museum in Toronto. The
latter may be smaller in numerical terms, but has a very active acquisitions program and a remarkable diversity of the
rarer achondritic classes of meteorite, including many specimens from the hot deserts of North West Africa (the numbered NWA series) and some notable Canadiana, including
the recently unearthed main mass of the Springwater pallasite. The National Collection (Herd 2002; Herd et al. 2010)
has the widest selection of Canadian meteorites, and more
than 2700 samples of over 1100 meteorites. It has some
iconic samples (e.g., the main masses of Madoc, Abee, and
St-Robert). Other collections include: the University of Calgary
(Tagish Lake and Buzzard Coulee falls); Planétarium de Montréal; University of Western Ontario; and smaller collections at
the University of British Columbia, Queens and elsewhere.
The evolution of meteorite research in Canada since 1950
has proceeded in part via a number of small, very modestlyfunded academic groupings with a soupcon of government
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cachet, under various acronyms such as ACOM, MIAC, and
ADWG. The Associate Committee on Meteorites, and its
successors the Meteorites and Impacts Advisory Committee
(to the Canadian Space Agency) and Astromaterials Discipline Working Group, have all benefitted from a range of scientific expertise, in fields of physics and astronomy,
mineralogy, geology and geochemistry (e.g., Millman and
McKinley 1967). Peter Millman suggested establishing an archive of mainly-Canadian fireball events at the first ACOM
meeting in 1960. The archive recounts 2129 Canadian fireballs reported from 1962 to 1989, plus 410 US and six other
fireball reports (Beech 2005, 2006). Another product of
ACOM was the MORP (Meteorite Observation and Recovery
Project from 1971 to 1985) that focused on the three Prairie
provinces and recorded 795 fireball events. Based on MORP
data, ACOM undertook some epic adventures, notably recovery of the Innisfree fall of 1977 (Halliday et al. 1978, 1996).
Within its modest financial bounds, MIAC also sponsored
unsuccessful attempts to locate meteorites in sub-Arctic and
Arctic settings such as Devon and Baffin islands. A much
greater degree of success with finds has attended the Prairie
Meteorite Search initiative, run by the University of Calgary,
Campion College at the University of Regina, and the University of Western Ontario, which has sent summer students
into communities to solicit samples from the public. The initiative brought to light substantial additional material from
the Red Deer Hill find, and as many as 13 previously unknown finds, although most of these have yet to reach official status in the Meteoritical Bulletin.
Notable orbital information has been reconstructed after
the fact for a number of documented falls in Canada, including Shelburne (van Drongelen et al. 2010), Dresden
(McCausland et al. 2006), Abee (Marti 1983), and St-Robert
(Brown et al. 1996). Detailed contemporaneous follow-up
analyses of public photographic and video records from fireball events have allowed for the determination of useful orbits for Tagish Lake (Brown et al. 2000) and Buzzard
Coulee (Milley et al. 2010; Brown et al. 2011). Increasingly
sophisticated dedicated camera networks have permitted orbital reconstructions with minimal observer error (Halliday et al.
1996; Weryk et al. 2008). Canadian fireball network meteorite
recoveries so far include Innisfree (Halliday et al. 1978, 1981)
and Grimsby (Brown et al. 2011). Worldwide, several fireball
camera networks, along with the careful analysis of fortuitously
recorded meteorite-dropping fireball events, have to date led to
the reliable reconstruction of just 14 pre-atmospheric meteoroid orbits (Brown et al. 2011), of which four are from
Canadian falls.
Recently highlighted Canadian meteorites
Here is a quick chronological survey of a dozen selected
Canadian meteorites, including some of the most extensively-researched examples, with emphasis on the subjects of
recent research in Canada. This is a vade mecum, a basic introduction, and much more could be said. Indeed, Whyte
(2009) has provided an excellent historical overview of 14 of
the meteorites recovered in the province of Alberta, plus
notes on two more recent finds.
Shelburne (1904 fall) provides a classic case of a fireball
event, witnessed on a summer evening, with numerous observers and the rapid recovery of two multi-kilogram fragPublished by NRC Research Press
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ments in farmland in southern Ontario (see recent synthesis
by McCausland and Plotkin 2009; van Drongelen et al.
2010). It illustrates the vastly greater likelihood of meteorite
recovery in the well-tended farmlands and populous areas of
Canada, as in the Prairie provinces and southern Ontario and
Quebec, versus the sparsely settled regions of the western
mountains, boreal forest, and sub-Arctic to Arctic regions.
Shelburne is a veined and brecciated L5 chondrite, a fine representative of one of the most abundant meteorite classes,
well-distributed in public and private meteorite collections
worldwide (McCausland and Plotkin 2009).
Springwater (1931 find) was the first (Nininger 1932) and
by far the largest of three pallasite finds in Canada in the
20th century. Recent searching has yielded more material,
and the main mass (53 kg) is now in the collection of the
Royal Ontario Museum. It is the fourth-most-cited Canadian
meteorite, in part because pallasites are an uncommon class,
with less than 100 representatives, and because the original
find was substantial (67.6 kg). Besides olivine and kamacite
(a relatively Ni-poor Ni–Fe alloy), Springwater is notable for
its content of phosphate minerals. Like Brenham, an historic
Kansas find, Springwater is famed for its rounded olivine
grains (Fig. 1). See Yang et al. (2010) for a recent synthesis
of ideas on pallasite petrogenesis.
Dresden (Ontario) (1939 fall), like Shelburne before it, is a
fine example of the fall and recovery of an ordinary chondrite. The fascinating human history of the meteorite is recounted by Plotkin (2006). This historic stone (Fig. 2) is
affirmed as a brecciated H6 (S2) chondrite by a recent reexamination of the 40 kg main mass and several other small
individuals, with a measured bulk specific gravity (3.48) and
aspects of mineral chemistry and bulk composition (total Fe
content is 28.0% and elemental abundances are quintessential
H-chondrite, except for low S, with 1.90 wt.% Ni, 0.36 wt.%
Cr, and 0.10 wt.% Co; McCausland et al. 2006).
Abee (1952 fall) is an enstatite chondrite, a grouping that
includes some of the most chemically reduced stony meteorites. It has seen a level of research unrivalled until the arrival
of the Tagish Lake carbonaceous chondrite, 48 years later,
being described in ∼130 articles. The single large mass, an
impressive 107 kg egg-shaped body, was retrieved from a
deep hole in a farmer’s field following a widely observed
evening twilight fireball (Dawson et al. 1960; Whyte 2009).
The metal- and enstatite-dominated, dense and brecciated
meteorite is the largest enstatite chondrite known, and so
came to be widely distributed and the subject of a multidisciplinary consortium effort (Marti 1983 and references therein).
The mineralogy is very exotic by terrestrial standards, including graphite and diamond, and other reduced phases scarcely
ever found on Earth, such as the sulphides oldhamite, niningerite, and keilite (Shimizu et al. 2002).
Bruderheim (1960 fall) remains, at 303 kg, the largest
documented fall or find on Canadian soil (Folinsbee and
Bayrock 1961; Whyte 2009). An L6 chondrite, it was recovered rapidly from a very productive strewnfield 50 km northeast of Edmonton (Folinsbee and Bayrock 1961), and was
distributed widely for research. Bruderheim has thus been
well studied as a freshly-recovered representative of an abundant class of stony meteorite. Rare gases, halogens, rare-earth
elements, the light elements Li and B, and radionuclides such
as 14C are all well-documented in the case of Bruderheim
(Jull et al. 2000), making the meteorite an oft-used reference
standard. The 1960 Bruderheim fall was seminal for Canadian meteoritics, being instrumental in the growth of the University of Alberta’s diverse meteorite collection (by trades)
and in the establishment of the National Research Council’s
Associate Committee on Meteorites (ACOM) as a coordinating scientific body for the study of fireball reports and meteorite falls in Canada (Millman and McKinley 1967; Whyte
2009).
Peace River (1963 fall) is an L6 chondrite with shock melt
veins, and rare achondritic clasts (see Herd 2012). It is especially noted for its evidence of high-pressure extraterrestrial
shock events (Price et al. 1983), generating high-pressure
polymorphs of otherwise familiar silicate phases, such as
wadsleyite and ringwoodite (after olivine) and majorite (after
orthopyroxene). The mineral wadsleyite was first discovered
in a Peace River shock vein (Price et al. 1983). Brief optical
study reveals that other Canadian meteorites like Red Deer
Hill (L6, Fig. 3) may show similar textural or mineralogical
evidence of impact events.
St-Robert (1994 fall) arrived in spectacular style, northeast
of Montreal, depositing fragments of crusted H5 chondrite
across an 8 km × 3.5 km strewnfield (Brown et al. 1996;
Hildebrand et al. 1997). Several fragments of St-Robert were
studied for noble gas isotopic ratios and short-lived cosmogenic radionuclides, formed by cosmic ray irradiation of the
parent meteoroid during its journey to Earth (Leya et al.
2001). From these data, St-Robert is ascertained to have had
a relatively simple cosmic ray exposure (CRE) history, having been liberated as a ∼90 cm diameter meteoroid from a
larger body at 7.8 Ma, a common CRE age amongst H chondrites that likely represents a H chondrite small body breakup
event (Leya et al. 2001). Poirier et al. (2004) reported a precise Pb–Pb age of 4566 ± 7 Ma (2s) for St-Robert chondrules, and a mineral–whole-rock Pb–Pb age of 4565 ±
23 Ma (2s), indicating that the Pb–Pb system was undisturbed in the early history of the H chondrite parent body.
St-Robert is a particularly useful example of a ‘typical’ H5
chondrite with a relatively simple, well known history from
its early history on the H chondrite parent body through its
delivery to the Earth. There is sufficient St-Robert in Canadian collections to permit fresh study and re-evaluations,
such as of physical properties, density, and porosity
(McCausland et al. 2011).
Tagish Lake (2000 fall) is a C2 ungrouped carbonaceous
chondrite that appears to be unique; within 12 years it has
become arguably the most-researched Canadian meteorite. Its
recovery is a remarkable story of the arrival of friable primitive material that landed fortuitously on the frozen surface of
Tagish Lake in northern British Columbia (Brown et al.
2000; Hildebrand et al. 2006). Its recovery is eerily similar
to that of the 1965 Revelstoke meteorite, the smallest known
Canadian fall, also a fragile primitive carbonaceous chondrite
(CI1). Revelstoke was discovered some two weeks after its
fall, when two beaver trappers, crossing a frozen lake on
snowshoes, noted blackened snow (Folinsbee et al. 1967).
Multiple, pristine frozen fragments of Tagish Lake were
found on its eponymous, frozen lake (Fig. 4) ten days after a
brilliant fireball was witnessed over a huge region. Later
dedicated searching of the ice surface before the spring 2000
breakup defined a strewnfield at least 16 km long, consisting
of more than 420 fall locations (Brown et al. 2000; Hildebrand et al. 2006). The remarkable fall was soon awarded its
own consortium study (Brown et al. 2002 and references
therein). Tagish Lake ranks among the most primitive and
friable meteorites ever studied, a heterogeneous accretionary
breccia with high microporosity of ∼40% and a bulk specific
gravity of just 1.66, a large range of unequilibrated olivine
compositions (Fo71–100), magnetite, carbonates, and organic
compounds (Zolensky et al. 2002; Izawa et al. 2010a,
2010b). Tagish Lake is a repository of prebiotic organic matter with origins in the solar nebula as well as from parent
body aqueous alteration processes, and has become a signature pristine planetary material for developing sample handling protocols (Zolensky et al. 2002; Herd et al. 2011).
Infrared reflectance spectra of Tagish Lake (which contains
4%–5% carbon and has been subject to aqueous alteration on
its parent body) are consistent with its derivation from an
outer main belt D-type asteroidal parent body (Hiroi et al.
2001; Izawa et al. 2010a), possibly representing material
more primitive than is found in other classes of meteorite.
Southampton (2001 find) is the most recent Canadian pallasite find, discovered by an observant passer-by on a beach
on the shore of Lake Huron, west of Owen Sound. The main
mass of this beautiful meteorite has been preserved at the
Royal Ontario Museum, and the pallasite has been the subject of recent research (see Kissin et al. 2012). At the time
of its discovery it was just the 52nd pallasite known to science.
Whitecourt (2007 find) is a IIIAB iron meteorite that has
the distinction of being one of very few meteorites in the
world known to be associated with its own impact crater,
some 36 m wide and 6 m deep in glacial till (Herd et al.
2008; Kofman et al. 2010). Whitecourt was discovered by
hunters who were curious about the closed bowl structure
and explored the local ground with metal detectors, correctly
suspecting it to be an impact crater. A follow-up systematic
study found innumerable mostly small (<5 cm) shrapnel-like
shards, similar to fragments from a minority of other irons,
such as Sikhote-Alin and Gebel Kamil (Kofman et al. 2010).
The “total known mass” recovered is necessarily an approximation. The structure, and thus the impact of the iron meteoroid, is established to be less than 1100 years old, and it
represents a size of impactor that is usually missed in the terrestrial impact record (Herd et al. 2008).
Buzzard Coulee (2008 fall) arrived as the climax to a spectacular (magnitude –20) fireball event on 20 November 2008.
The meteoroid, estimated mass 10 tonnes, entered the atmosphere at a low velocity of 14 km s–1 (an average velocity
would be 20 km s–1), enabling it to penetrate to an altitude
of just 12 km above surface before shattering to produce a
fine meteorite shower of black crusted fragments, the largest
found to date weighing 13 kg (Hildebrand et al. 2009). Initial
classification of Buzzard Coulee is H4 (S2, W0). More than
40 kg (>130 fragments) were recovered before the first serious seasonal snow cover interfered with recovery on December 6th (Kulyk 2009; Weisberg et al. 2009) and there are
most likely some thousands of fragments, TKW (total known
weight) >200 kg.
Grimsby (2009 fall) is an H5 chondrite (McCausland et al.
2010), the 14th meteorite fall with an instrumentally measured pre-atmospheric orbit indicating an origin in the main
9
belt asteroids (Brown et al. 2011). This fall event is notably
the first to incorporate Doppler weather radar as an essential
scientific component in the analysis of the behaviour of falling meteorite fragments. Grimsby is a fall with small recovered mass (215 g recovered from 13 fragments as of early
2012) in a mixed urban and agricultural area at the west end
of Lake Ontario (Fig. 5). Grimsby made headline news for
human interest as a “hammer” that struck, amongst other
anthropogenic targets, a vehicle windshield and a garage
(McCausland et al. 2010).
Conclusions
Vast by land mass but small by population, the recovery
rate per square kilometre in Canada is very low (Beech
2003), yet some very special falls and finds have been recovered. With such a small number of meteorite recoveries, it is
not surprising that some of the rarer meteorite classes, including all the achondrites (a broad chemical and textural variety of primary igneous and derived brecciated lithologies)
are not, as yet, represented (Appendix A Table A1).
Future meteoritical research in Canada will likely include
some advanced projects as well as some inevitable associated
spadework. The advanced programs will include continued
optimization of fireball tracking networks to aid in impactor
flux determination and possible meteorite recovery (Weryk
et al. 2008; Brown et al. 2011), and additional advances in
techniques for materials characterization of the payloads of
sample-return missions (Herd et al. 2011; McCausland et al.
2011). The lessons learned on meteorites will in the next
generation be applied, in all probability, to newly recovered
samples from the moon, Mars, asteroids, and comets.
Less glamourous, but nevertheless of great educational importance, the spadework includes public interaction, a feature
of the Canadian meteorite community since ACOM days.
Public education via presentations, Web sites and social media, museum displays, and field visits to current and historical meteorite strewnfields are all important (Plotkin 2006;
McCausland and Plotkin 2009). It is a common perception
that meteorites are “manna from heaven”, and every researcher soon develops a repertoire of stories concerning
“meteorwrongs” and members of the public who occasionally
are suspicious of any demythologizing of their precious
finds. Much more frequently, finders of prospective meteorites are genuinely curious about their finds and open to learning about meteorites.
We consider the best approach to be to engage the curious
public, and (in most cases) explain not only that their material is not a meteorite, but what it actually is, and discuss the
specific features that attracted their attention in the first place.
Almost invariably, possible meteorite enquiries become excellent opportunities to educate the very people who are
most curious about their surroundings. A small investment
of meteorite education can pay large dividends in elevating
interest in the real wonder of science and possibly lead to
the recovery of future meteorites. Good illustrated reference
books to have on hand are those of Norton (1994) and Norton and Chitwood (2008).
Once a meteorite has been recovered, classification using
standard techniques can be conducted (see, e.g., Dodd 1981;
Hutchison 2004), with a view to submitting a type specimen
10
to a meteorite research institution as soon as possible to enable long-term availability of research sample from the meteorite, and submitting a brief descriptive report for the
consideration and approval of the Nomenclature Committee
of the Meteoritical Society.
Acknowledgements
We thank past and present members of the Meteorite Impact Advisory Committee (MIAC) and the Astromaterials
Discipline Working Group (ADWG) for stimulating discussion, and all those sample preparators who struggle to provide excellent polished thin sections for meteorite research!
Reviews by Martin Beech and Michael Higgins were most
helpful in improving this work. The second author gratefully
acknowledges financial support from the Centre for Planetary
Science and Exploration (CPSX) at Western University.
References
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Appendix A
Appendix A begins on the following page.
12
Table A1. Canadian meteorites — approved (60) and listed in Meteoritical Bulletin.
Meteorite
Chondrites (32)
Revelstoke
Tagish Lake
Blithfield
Abee
Beaver Creek
Buzzard Couleea
Redwater
Skiff
Wood Lake
Grimsby
Riverton
St-Robert
Wynyard
Belly River
De Cewsville
Dresden (Ontario)
Great Bear Lake
Vulcan
Shelburne
Vilna
Blaine Lake
Bruderheim
Catherwood
Ferintosh
Homewood
Kinley
Kitchener
Peace River
Red Deer Hill
Holman Island
Innisfree
Benton
Achondrites (0)
Irons (25)
Fillmore
Mayerthorpe
Midland
Osseo
Annaheim
Bernic Lake
Burstall
Hagersville
Lac Dodon
Penouille
Torontob
Bruno
Edmonton (Canada)
Chambord
Iron Creek
Madoc
Manitouwabing
Welland
Whitecourt
Kinsella
Thurlow
Millarville
Class
Region
Ni%Metal
Fa%Oliv
History
Date
TKW (kg)
RefsMINLIB
Earliest
CI1
C2 ungrouped
EL6
EH5
H4
H4
H4
H4
H4
H5
H5
H5
H5
H6
H6
H6
H6
H6
L5
L5
L6
L6
L6
L6
L6
L6
L6
L6
L6
LL(?)
LL5
LL6
BC
BC
Ont
Alta
BC
Sask
Alta
Alta
Ont
Ont
Man
Que
Sask
Alta
Ont
Ont
NWT
Alta
Ont
Alta
Sask
Alta
Sask
Alta
Man
Sask
Ont
Alta
Sask
NWT
Alta
NB
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
0–29
—
—
19
17.8
19
18
19
18
20
19
18
20
18
20
19
20
24
25
26
24
25
26
25
—
26
23
26
29
27
31
Fall
Fall
Find
Fall
Fall
Fall
Find
Find
Find
Fall
Find
Fall
Find
Find
Fall
Fall
Find
Find
Fall
Fall
Find
Fall
Find
Find
Find
Find
Fall
Fall
Find
Find
Fall
Fall
1965
2000
1910
1952
1893
2008
2009
1966
2003
2009
1960
1994
1968
1943
1887
1939
1936
1962
1904
1967
1974
1960
1965
1965
1970
1965
1998
1963
1975
1951
1977
1949
0.001
11.0
1.83
107
14
200
0.230
3.54
0.35
0.215
0.103
25.4
3.479
7.9
0.340
47.7
0.04
19
18.6
0.00014
1.896
303
3.92
2.201
0.325
2.44
0.202
45.76
25.0
0.552
4.58
2.84
15
132
20
131
22
11
0
7
3
10
2
26
4
11
3
21
2
9
20
5
5
89
10
4
5
6
12
39
6
8
30
7
1967
2000
1922
1960
1963
2008
2010
1980
2004
2009
1976
1994
1980
1953
1900
1939
1963
1967
1904
1973
1978
1961
1973
1984
1976
1971
1998
1967
1978
1963
1978
1964
IA
IA
IA
IA
IA-ANOM
IAB
IAB
IAB
IAB
IAB
IAB
IIA
IIA
IIIA
IIIA
IIIA
IIIA
IIIA
IIIAB
IIIB
IIIB
IVA-ANOM
Sask
Alta
Ont
Ont
Sask
Man
Sask
Ont
Que
Que
Ont
Sask
Alta
Que
Alta
Ont
Ont
Ont
Alta
Alta
Ont
Alta
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
Find
Find
Find
Find
Find
Find
Find
Find
Find
Find
Find
Find
Find
Find
Find
Find
Find
Find
Find
Find
Find
Find
1916
1964
1960
1931
1916
2002
1992
1999
1993
1984
1997
1931
1939
1904
1866
1854
1962
1888
2007
1946
1888
1977
0.200
12.61
0.034
46.3
11.84
9.8
0.359
30.0
0.800
0.072
2.715
13
17.34
6.6
145.93
167.5
38.6
8.16
200
3.72
5.5
15.636
3
6
4
14
13
5
5
5
4
3
5
9
6
5
15
26
16
23
7
4
5
6
1971
1971
1971
1938
1921
2004
1998
2001
1995
1995
1997
1936
1969
1963
1886
1855
1964
1891
2008
1978
1900
1979
7.18
7.19
8.37
6.51
7.74
6.53
6.57
6.89
8.64
9.40
7.04
5.79
5.37
7.53
7.72
7.52
7.34
8.77
8.11
8.78
9.92
9.78
13
Table A1 (concluded).
Meteorite
Class
Region
Ni%Metal
Fa%Oliv
History
Date
TKW (kg)
Skookum
Garden Head
Gay Gulch
Stony irons (3)
Giroux
Southampton
Springwaterc
IVB
IRANOM
IRANOM
YT
Sask
YT
17.13
16.96
15.06
—
—
—
Find
Find
Find
1905
1944
1901
15.88
1.296
0.483
19
6
8
1915
1971
1915
Pallasite
Pallasite
Pallasite
Man
Ont
Sask
10.3
9.47
12.6
11
12.5
18
Find
Find
Find
1954
2001
1931
4.275
3.58
167.6
12
4
62
1967
2002
1932
RefsMINLIB
Earliest
Note: All 60 meteorites are listed in the Meteoritical Bulletin, but individual details may be drawn from the wider meteoritic literature. The various falls
and finds are listed by the province or territory where they were recovered: Alberta, British Columbia, Manitoba, New Brunswick, Northwest Territories,
Ontario, Quebec, Saskatchewan, and Yukon Territory. No recoveries have yet been reported for Newfoundland, Nova Scotia, Prince Edward Island, and the
Territory of Nunavut. Classification: two key factors are included: the percentage of nickel in bulk metal (irons and stony irons) and the mole proportion of
fayalite (100 – Mg #) in olivine. Total known weight (TKW) is reported using the best available data, but in the newer falls (e.g., Buzzard Coulee) and
finds (e.g., Whitecourt) the quoted values should be taken as minima. As an indication of the extent of research and general interest, the number of records
citing each meteorite in Graham C. Wilson’s unpublished MINLIB bibliographic database are quoted. Published titles on Canadian meteorites, appearing as
records in the MINLIB database, may be viewed in an on-line chronological–alphabetical bibliography at http://www.turnstone.ca/canmetbib.htm
a
The Buzzard Coulee TKW is now believed to be in excess of the nominal 200 kg noted here, in >1000 fragments.
b
The Toronto iron (Fig. A1) was identified in that city and classified at the University of Toronto and Lakehead University, but the true provenance may
never be known — an earlier find in rural Quebec is suspected but unverifiable.
c
The TKW includes the original 67.6 kg from the 1930s and a nominal 100 kg from recent finds.
Fig. A1. The Toronto iron, in the uncut 2.7 kg mass, a find of uncertain but probable Quebec provenance (Kissin and Wilson 2006).
Note added to proof
1. A 61st Canadian meteorite was accepted in Meteoritical
Bulletin 101, on 23 August 2012: this is the Lone Island
Lake IAB-sLL iron, Manitoba, 7.62% Ni in metal, a find
in 2005, TKW 4.8 kg, 3 references, 2005 onwards.
2. As of Met. Bull. 101, 23 August 2012, the official world
tally of approved meteorite names rose to 43 973. In judging the qualities of a meteorite display, it may be helpful
to remember the approximate proportions of the major
classes. In round figures, seven out of eight meteorites
are the most-common, ordinary chondrites (H, L and LL,
88%); 5% are achondrites (of which 55% are in the "vestoid" HED clan); 4% are less-common to rare chondrites,
primarily eight subclasses of carbonaceous chondrite, plus
the enstatite chondrites; >2% are irons; and <1% are pallasite and mesosiderite stony-irons.
Reference
Kissin, S.A., and Wilson, G.C. 2006. Toronto, a new Canadian
meteorite. Meteoritics & Planetary Science, 41(S8): A243–A246.
doi:10.1111/j.1945-5100.2006.tb01001.x.

Canadian meteorites: a brief review1

Transcription

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