A Bright Multiple Fragmentation Fireball and Meteorite Fall at Buzzard Coulee, Saskatchewan, Canada, November 20, 2008
A bright fireball was widely observed across Alberta, Saskatchewan, Manitoba, and Montana during late twilight on November 20, 2008. The fireball and subsequent dust trail, or shadows cast by the fireball, were widely recorded. The meteoroid had a below average initial velocity and sufficient data exist to constrain the orbit. The fireball fragmented multiple times over ~ 3 seconds with significant fragmentation continuing deep within the atmosphere to ~ 12 km altitude. After the initial meteorite recovery in Buzzard Coulee, SK, Nov. 27, 2008, a strewn field ~ 10 kilometers long and ~ 3 km wide with a wind drift tail of an additional ~ 3 km eastwards has been crudely outlined. Buzzard Coulee is an H4 chondrite at the low end of the thermal range and may be transitional to type 3. Two lithologies are contrasted by variation in the chondrule sizes; the finer grained phase has a abundance of > 200 micron cryptocrystalline chondrules. The meteorite is also distinguished by the presence of light coloured igneous-textured inclusions up to /sim 5 mm in size. Brecciation is only rarely visible in hand specimen, but where observable the inclusions are found only in the matrix. The meteorites are distinguished by the large number of specimens with immature surfaces (angular shapes with numerous small piezoglypts) presumably reflecting the meteoroids' late stage fragmentation low in the atmosphere. The general lack of veins and brecciation is consistent with the relatively low S2 shock state. Densities measured on six specimens span 3.26 gcm-3 to 3.45 gcm-3 with some evidence for a bimodal distribution. Abundant sonic phenomena were reported by witnesses including anomalous sounds, explosion booms, staccato cracks, and late-stage whirring sounds. The staccato cracks are interpreted as closely spaced arrivals of discrete sonic booms from individual fragments while the latter were still supersonic in the early portion of their dark flight; this type of sound was reported only within ~ 50 km of the fall. The whirring sounds were reported only within ~ 20 km of the fall and are interpreted as produced by individual rotating fragments falling to ground while in subsonic dark flight. At least six North American infrasound stations detected signals from the fireball. The rich waveforms from many stations offer the prospect for the first time of resolving which fragmentation events produced signals at which stations with detailed ray trace modeling. The stratospherically ducted peak frequency measured at each of the three calibrated IMS stations are consistently near 0.32 Hz. Using this measurement yields an infrasonic energy estimate for the source of 0.32 ± 0.09 kilotons; this energy yield implies an entry mass of ~ 15 tonnes for the meteoroid.
Mineralogy and petrology of the Buzzard Coulee H4 chondrite
The Buzzard Coulee meteorite was collected as fragments from a fireball witnessed at 17:26.43 MST on November 20, 2008 by thousands of residents across the Canadian prairies. Three samples were made available to this study, weighing 34, 37 and 151.7 g. In hand specimen, the stones are partially to completely covered by black fusion crust. The interior is light grey, with chondrules and metal grains readily visible on the broken surface. Prior to sample processing, a NextEngine Desktop 3D laser scanner was used to capture and preserve the meteorite morphology. This method also provides an estimate of the sample volume, which was used to determine a bulk density of 3.5 g/cm3. Microtextures were then characterized using a JEOL 6301F Field Emission SEM at the University of Alberta. Mineral and glass compositions were collected using a Cameca SX- 100 electron microprobe at the same institution. The overall texture is massive, with chondrules embedded in a fine-grained matrix with coarser Fe-Ni metals (500 to 800 microns). Major minerals / phases include low-Ca pyroxene (Fs15.8Wo1.0), pigeonite (Fs12.9Wo14.6), olivine (Fa17.7), devitrified alkali-rich glass, troilite, kamacite and taenite with minor chromite, merrillite, pentlandite, augite (Fs5.2Wo46.0), and rare spinel (s.s.) and silica glass. The matrix is texturally heterogeneous on a cm-sale; minor recrystallization and small mineral fragments (5 to 15 microns) are typical; dendritic textures of low-Ca pyroxene in glass have also been observed. A variety of chondrule types are present in the thin section; barred olivine, porphyritic olivine, porphyritic pyroxene, porphyritic olivine-pyroxene, cryptocrystalline pyroxene, radial pyroxene, metal-rich and Al-rich. The Al- rich chondrule contains skeletal grains of olivine in devitrified alkali-rich glass with euhedral spinel (s.s.) zoned to chromite near the chondrule rim. In general, the cryptocrystalline pyroxene chondrules are the best preserved with sharply defined edges. The sample has been weakly affected by shock (S2 / S3); olivine exhibits straight to undulose extinction with irregular fractures. Troilite exhibits local melting and infilling of cracks and fractures in neighboring minerals. These melts are restricted to grain boundaries and we note that preferential melting of sulfides is well documented from ordinary chrondites as a result of its compressible crystal structure, thereby absorbing shock wave energy.
Accretional Impact Melt From the L-Chondrite Parent Body
MIL 05029, a unique achondritic Antarctic meteorite with L-chondritic affinity, has a medium-grained, well equilibrated texture of large poikilitic low-Ca pyroxenes that overgrew smaller, euhedral olivines. Plagioclase filled interstitial spaces and has an abundance that is twice that typical for L-chondrites, while Fe-Ni metal and troilite are strongly depleted in that respect. No relic clasts or shock features were found in the thin section analyzed. However, based on its chemical affinity to L-chondrites, MIL 05029 was classified as an impact melt. This is confirmed by its olivine and low-Ca pyroxene compositions, the Co content in Fe-Ni metal, and its oxygen isotopic composition that lies very close to that of L-chondrites. An igneous origin of MIL 05029 cannot be ruled out but would have to be reconciled with thermochronometric constraints for the formation of the ordinary chondrite parent bodies. These studies infer delayed accretion of the parent asteroids of the ordinary chondrites and, thus, insufficient heating from short-lived radiogenic isotopes to produce endogenic magmatism. Metallographic cooling rates of ∼2-22 °C/Ma in the temperature range between ∼700-400°C were determined on five zoned metal particles of MIL 05029. Thermal modeling showed that such cooling rates relate to metamorphic conditions at depths of 5-12 km on the L-chondrite parent body. For an impact to deposit material at this depth, scaling relationships for an impact event on the 100-200 km diameter parent asteroid require a 15 to 60 km diameter simple crater that produced a basal melt pool, in which MIL 05029 crystallized. Further constraints for the formation conditions of MIL 05029 were derived from three whole-rock samples that gave well-defined Ar-Ar plateau ages of 4.53±0.02 Ga. This age indicates the time at which MIL 05029 cooled below ∼180°C, the Ar-closure temperature of plagioclase. Considering its slow metallographic cooling, the impact event that formed MIL 05029 then becomes indistinguishable from the initial accretion of its parent body.
The Known and the Unknown: Petrographic and Geochemical Analysis of the Shelburne Meteorite and an Unknown L6 Ordinary Chondrite
Two meteorites have been studied by the author, the first was a known L5 ordinary chondrite. The Shelburne meteorite had been included in several studies, but no comprehensive study of this meteorite had been done previously. The current subject of study is an L6 ordinary chondrite from the Royal Ontario Museum (ROM). The initial study was of the Shelburne meteorite, is an L5 ordinary chondrite, which was observed to fall in August, 1904 near Shelburne, Ontario, Canada. The study of a 30 g sample provided by the ROM includes petrographic work to describe mineral occurrence and textures, Scanning Electron Microscope (SEM) imaging, X-ray diffraction (XRD) analysis, and Electron Probe Microanalysis (EMPA). Several other Shelburne fragments have been investigated to determine bulk physical properties such as density, porosity, and magnetic susceptibility. Through petrographic work, textures characteristic of L chondrites are apparent, as well as a variation in chondrule types. The chondrules are fairly well delineated but not unaltered, suggesting a moderate degree of thermal metamorphism. Back-scattered Electron (BSE) images and petrographic work reveals troilite (FeS) and Ni-Fe metal (kamacite and taenite) occurring as independent subhedral grains, veins, and within chondrules. XRD analysis of the bulk meteorite has identified the main mineral phases as Mg-rich olivine, low-Ca pyroxene, high-Ca pyroxene, plagioclase, troilite, and Ni-Fe metals. EPMA was used to examine the chemical variation in mineral species through the sample to determine the degree of equilibration. Olivine plots in a discrete area of Fa content, indicating a high degree of equilibration in the Shelburne meteorite, supported by the fairly uniform compositions of pyroxene. The current study of the undescribed L6 ordinary chondrite began with petrographic work to examine the textures and mineralogy of this meteorite, with particular attention to the shock veins. Using the low-loss wire saw at the ROM, two slices from the main mass of this meteorite were removed, one of which was used to make five thin sections. This was followed by EMPA work to determine the degree of thermal equilibration, defined by the compositions of the olivine and pyroxene. The shock veins are the focus of a micro-XRD study, to better characterize the high pressure mineralogy. The undescribed L6 ordinary chondrite was purchased under the designation NWA 869 in 2001 and may or may not be a component of that large strewn field (totalling approximately 2 metric tonnes). Individual masses of NWA 869 range in size from < 1 g to > 20 kg. Studies of other samples with this name have ranged from L4 to L6, which suggests a heterogeneous meteor, which makes determining NWA 869 links difficult. Once the instrumental work has been completed, the undescribed meteorite will be compared to other meteorites with the NWA 869 designation at the ROM.
The Tagish Lake C2 Carbonaceous Chondrite: Arrival Circumstances of a Unique, Primitive Messenger from the Early Solar System
The Tagish Lake meteorite is a unique carbonaceous chondrite which fell on January 18 of 2000 in northwestern British Columbia. The fireball event was widely observed; instrumental and eyewitness records determined its preatmospheric orbit and fragmentation events during it atmospheric entry, placing the study of Tagish Lake in a dynamical and meteor physics context as well. Fragments of the meteorite were first collected in pristine condition from the frozen surface of Tagish Lake a week after the fall, before the area was covered by snow for the season. A subsequent collection in April and May as the snow cover diminished yielded a further ~10 kg of the meteorite, defining a large 16 km x 3 km strewnfield of more than 400 fragment locations which straddled the frozen lake surface. Fragments from the latter fieldwork were recovered in a variety of settings, ranging from apparently frozen intact fragments on the ice surface to severely disaggregated fragments sunk into melt holes within the lake ice. A recent survey of 42 Tagish Lake non-pristine fragments showed a striking subdivision into "inclusion rich" and dark, "inclusion free" fragments which may represent intrinsic lithological variations in the meteorite, or may represent some degree of terrestrial water-rock alteration.