Mineralogical Association of Canada [MA]

 CC:713B  Monday  0800h

Manna From Heaven: Insights Into the Origin and Evolution of the Solar System From the Mineralogical and Physical Properties of Meteorites I

Presiding:  C Samson, Carleton University; K Tait, University of Toronto


The Whitecourt Meteorite Impact Crater: Observations and Interpretations at a Late Holocene Impact Structure

* Kofman, R S (rkofman@ualberta.ca), University of Alberta, Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, Edmonton, AB T6G 2E3, Canada
Herd, C D, University of Alberta, Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, Edmonton, AB T6G 2E3, Canada
Froese, D G, University of Alberta, Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, Edmonton, AB T6G 2E3, Canada
Walton, E L, University of Alberta, Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, Edmonton, AB T6G 2E3, Canada

Unlike most solid surfaces in our Solar System the surface of the Earth is significantly depleted in <100 m diameter craters formed by small impact events. Those present on Earth are typically heavily modified by subsequent erosion and often in remote locations. In contrast the Whitecourt Meteorite Impact Crater (WMIC), located several kilometres south of Whitecourt, Alberta, is both well-preserved and easily accessible. The WMIC also has a well-preserved ejecta blanket and has yielded over 150 meteorites to date. The impact crater is a bowl-shaped structure having a diameter and depth of 36 m and 6 m respectively. The target material consists of gently northeast ward dipping Quaternary deglacial sediments. The crater walls and floor show little evidence of subsequent erosion due primarily to age, stabilizing vegetation and to the observation that the local water table does not intersect any impact-affected materials. The ejecta blanket is presently observed to extend a minimum of about 17.5 m and maximum of about 37 m beyond the crater rim. The ejecta blanket was delineated by a thin buried soil horizon commonly hosting charcoal fragments which have been carbon-dated yielding a maximum age for the crater of 1.1 ka [1]. The meteorites collected at the WMIC primarily have the appearance of shrapnel. The meteorites all have rusty exteriors with no clear evidence of a fusion crust or regmaglypts. At present the largest meteorite collected has a mass of 1.2 kg though most are in the range of tens of grams. All samples collected outside the crater were found at the base of the modern soil. Several small fragments were also collected at a depth of about 2.9 m beneath the crater floor. The distribution of meteorites appears to mimic the shape of the ejecta blanket though extends beyond it in most directions. The most distal meteorite collected was found 70 m east of the crater rim. The WMIC's well-preserved structure and associated meteorites should provide new control points for current models. It is among the smallest and youngest terrestrial hypervelocity impact craters and also likely very near to the lowest energy end-point of such events. [1] Herd, C. D. K. et al. (2008) Geology, 36, 955-958.


The Importance of Porosity in the Characterization of Planetary Materials

* Strait, M M (straitm@alma.edu), Alma College, 614 W. Superior St., Alma College, MI 48801, United States

Current work investigating asteroids is beginning to recognize the importance of physical properties such as bulk density, grain density and porosity in modeling the character of these materials. Density and porosity are being factored into models that determine asteroid origin and history. Density is determined in classic fashion: measuring mass and volume. Porosity is classically measured using helium pycnometry, but newer methods involving image analysis of thin sections and X-ray microtomography are being attempted. The latter two methods allow for a 2-D and 3-D investigation of the nature and distribution of porosity in the samples and provides clues as to how the material will react to shock during an impact event. In addition to the recognition of the importance of porosity in modeling, experimental studies looking at the fragmentation of solid materials have also demonstrated the importance of physical properties in the mass-frequency distribution of particles during hypervelocity impact events. There is a correlation between the disruption energy of a hypervelocity impact event and the porosity of the sample. Terrestrial anhydrous basalts (porosity ∼2%) require about half the energy for disruption as meteorites used in hypervelocity impact experiments (∼6.5%). It has been found that porosity for ordinary chondrites fall in a surprisingly narrow range, with an average of slightly less than 10%. The porosity is present as microcracks in the fabric of the material. All the porosity data in this report are measured using thin section imaging. While there is concern that thin section preparation introduces porosity into the sample, measurements done using helium pycnometry for all meteorite classes measured thus far with the exception of the carbonaceous chondrites agree with thin section measurements. The porosity in the carbonaceous chondrites does not appear to be the same microcrack texture observed in the other classes of meteorites and is thus not measured using the imaging technique as it is not seen. Both the behavior of the material and the pycnometry measurements indicate that the porosity is present. This conundrum is being further explored.


Measurement of Meteorite Density, Porosity and Magnetic Susceptibility: Fast, Non- destructive, Non-contaminating and Very Informative

* Macke, R J (macke@alum.mit.edu), University of Central Florida, 4000 Central Florida Blvd., Orlando, FL 32816, United States
Britt, D T
EM: , University of Central Florida, 4000 Central Florida Blvd., Orlando, FL 32816, United States
Consolmagno, G J
EM: , Specola Vaticana, V-00120 Citta del Vaticano, Italy

The development of the "glass bead" method [1] for measuring bulk density, coupled with other fast, non- destructive and non-contaminating methods for measuring grain density and magnetic susceptibility, has enabled broad surveys of large meteorite collections. We have employed these methods extensively on meteorites in numerous collections, including those at the Vatican, the American Museum of Natural History (New York), the National Museum of Natural History (Washington, DC), Texas Christian University, University of New Mexico, and Arizona State University. We present here a summary of some of the findings to date. Using the glass bead method, the meteorite is placed into a container which is then filled entirely with small (sub- millimeter) glass beads. The beads behave collectively as an Archimedean fluid, flowing around the sample to fill the empty space in the container. Through mass measurement, the volume displaced by the sample can be determined. Grain density is determined via helium ideal-gas pycnometry. Magnetic susceptibility is determined using a commercially available hand-held device [2]. Among notable findings to date, grain density and magnetic susceptibility together can distinguish H, L and LL ordinary chondrite falls into clearly distinct groupings [3]. On the other hand, enstatite chondrites of EH and EL subgroups are indistinguishable in these properties, indicating that EH and EL do not differ significantly in iron content [4]. Carbonaceous chondrites can have porosities that are significantly higher than ordinary chondrites and (especially for aqueously altered meteorites) lower density, though these also vary according to subgroups [5]. References: [1] Consolmagno and Britt, 1998. M&PS 33, 1231-1240. [2] Gattacceca et al., 2004. GJI 158, 42-49. [3] Consolmagno et al., 2006. M&PS 41, 331-342. [4] Macke et al., 2009. LPSC 40, 1598. [5] Consolmagno et al., 2008. MetSoc 71, 5038.


Small Meteorite Fragment Bulk Density via Visible Light 3-D Laser Imaging

* McCausland, P J (pmccausl@uwo.ca), University of Western Ontario, Dept. of Earth Sciences, 1151 Richmond St., London, ON N6A 5B7, Canada
Samson, C (csamson@earthsci.carleton.ca), Carleton University, Dept. of Earth Sciences, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada
DesLauriers, A (adeslauriers@neptec.com), Neptec, 302 Legget Drive,, Kanata, ON K2K 1Y5, Canada

Bulk density is an important intrinsic property of meteorites, reflecting their formation and subsequent history. Bulk density requires the measurement of bulk volume as well as mass, but meteorite volume is difficult to measure in a truly non-destructive way. Archimedean bulk volume methods involving the displacement of a fluid such as 40 micrometre sized glass beads are most commonly applied. These methods are, however, overwhelmed by container volume error at meteorite volumes less than ~8 cm3 and also are not suitable for the measurement of more fragile meteorites. Visible light laser imaging and 3D modeling can be used to measure bulk volumes for fragments which are too friable or too small for conventional methods. Laser imaging also provides an archival record of a meteorite's shape and distinctive surficial features prior to its storage, manipulation or subsampling. In this study, we have used the Laser Metrology System (LMS) of Neptec to measure bulk volumes for meteorite fragments ranging in mass from 100g to 3g. We compared these new results with 'beads' Archimedean bulk volume measurements on the same fragments where possible, or with the bulk densities of sister fragments reported in the literature. Bulk densities determined by the laser imaging method tend to be greater than those obtained by conventional methods by 1 to 5 per cent across the mass range. This difference may be due to systematic bulk volume overestimation by the 'beads' Archimedean method, bulk volume underestimation by the laser imaging method, or both. Realistic small fragment volumes can nevertheless be obtained by LMS 3D imaging, but further study is needed to assess the source of the systematic difference between the bulk volume methods.


The unusual Lovina Ataxite: Examination of Meteoritic Microstructures and Terrestrial Weathering by μXRD, Petrography, SEM, INAA and sXRF.

* Flemming, R L (rflemmin@uwo.ca), Department of Earth Sciences, University of Western Ontario, London, ON N6A 5B7, Canada
McCausland, P J (pmccausl@uwo.ca), Department of Earth Sciences, University of Western Ontario, London, ON N6A 5B7, Canada
Kissin, S A (sakissin@lakeheadu.ca), Department of Geology, Lakehead University, Thunder Bay, ON P7B 5E1, Canada
Corcoran, P L (pcorcor@uwo.ca), Department of Earth Sciences, University of Western Ontario, London, ON N6A 5B7, Canada
Biesinger, M C (biesingr@uwo.ca), Surface Science Western, University of Western Ontario, London, ON N6A 5B7, Canada
McIntyre, N S (smcintyr@uwo.ca), Department of Chemistry, University of Western Ontario, London, ON N6A 5B7, Canada
Fuller, M L (mfuller@uwo.ca), Department of Chemistry, University of Western Ontario, London, ON N6A 5B7, Canada
Feng, R (Renfei.Feng@lightsource.ca), Candian Light Source, University of Saskatchewan, Saskatoon, SK S7N 0X4, Canada

The football-sized 8.2 kg Lovina ataxite is a newly classified iron meteorite that was found in Bali, Indonesia in 1981. Its unusual appearance and strong weathering have, over the years, precluded its being identified as a meteorite. Remarkable features include cm-sized pyramidal projections, or ziggurats, with mm-spaced ribs on its top surface (orientation as discovered) and deep vugs in its lower surfaces. In situ examination of Lovina's weathered ziggurats by micro X-ray diffraction (μXRD revealed that they consisted of two iron alloys: Ni-rich taenite and very Ni-rich awaruite (Ni3Fe). Although this texture is reminiscent of Widmanstätten pattern, kamacite was not observed. Magnetite was frequently observed in association with awaruite, indicating very intense weathering. Micro-XRD of several locations on a polished thin section cut near the weathered surface and a freshly polished surface of Lovina, free of weathering, revealed primarily taenite with minor troilite. Measurement of bulk grain density by He-pycnometry for the 32.5 g type specimen (cut end piece) of Lovina and other iron meteorites found that Lovina's grain density of 7.00+/- 0.02 g/cm3 was significantly less than those found for Canyon Diablo (7.37+/-0.01) and a slightly weathered Mundrabilla fragment (7.20+/-0.01), measurably reflecting the presence of the lower-density weathering products in Lovina. The presence of taenite and troilite suggested that Lovina was an ataxite, as confirmed by correlated SEM X-ray maps, petrographic and bulk INAA analysis. X-ray maps of the thin section confirmed the identities of magnetite, troilite, massive taenite, and located Ni enrichment (awaruite) in the alloy surrounding magnetite in severely weathered areas. Petrographic observations indicated the taenite to be massive, lacking exsolved kamacite spindles, daubreelite and Neumann bands, which are commonly present in ataxites. Abundant globular troilite nodules up to 0.8 mm in diameter are present. Many of the nodules are partially or totally oxidized to Fe oxides. Analysis by INAA revealed Lovina to have a composition outside the range of most grouped ataxites in group IVB. Thus, Lovina is an ungrouped ataxite. Lovina resembles other ungrouped ataxites, e.g. N'Goureyma, in its abundance of troilite nodules with a very low abundance of kamacite spindles and daubreelite, but differs in composition. Lovina's high Ni- and low Ir-content is similar to that of some ungrouped ataxites, but it differs in its relatively high Ge and Ga contents. The ziggurat structure is attributed to differential weathering within a taenite microstructure. In an effort to identify microstructures, synchrotron X-ray Fluorescence (sXRF) data have been collected using the Very sensitive Elemental and Structural Probe Employing Radiation by a Synchrotron (VESPERS) beamline at CLS. Synchrotron X-ray maps have revealed non-uniform Ni distribution across the taenite, which had appeared to be massive by petrography and SEM. This may correlate with the differential weathering behaviour of the Lovina ataxite.


Mineralogical and spectroscopic investigation of enstatite chondrites by X-ray diffraction and infrared reflectance spectroscopy

* Izawa, M R (matthew.izawa@gmail.com), Dept. of Earth Sciences, The University of Western Ontario, 1151 Richmond St., London, ON N6A5B7, Canada
King, P L (penking@unm.edu), Institute of Meteoritics, The University of New Mexico, 200 Yale Blvd. NE, Albuquerque, NM 87131, United States
Flemming, R L (rflemmin@uwo.ca), Dept. of Earth Sciences, The University of Western Ontario, 1151 Richmond St., London, ON N6A5B7, Canada
Peterson, R C (peterson@geol.queensu.ca), Dept. of Geological Sciences and Geological Engineering, Queens University, 99 University Ave, Kingston, ON K7L3N6, Canada
McCausland, P J (pmccausl@uwo.ca), Dept. of Earth Sciences, The University of Western Ontario, 1151 Richmond St., London, ON N6A5B7, Canada

Reflectance spectroscopy of well-characterized meteorites provides an important means of linking meteorites to potential parent objects; an important objective in meteoritics research. There is a lack of such sample- correlated spectroscopic and mineralogical data sets in the literature to date. In an effort to improve this situation, the bulk mineralogy and infrared reflectance spectra of 13 enstatite chondrite meteorite finds, spanning the full range of textural alteration grades in both EL and EH classes have been investigated, including eleven recovered from the Antarctic and one from Northwest Africa. Rietveld refinement of high- resolution powder X-ray diffraction (XRD) data was used to identify the major mineral phases and quantify their modal abundances. The mineralogy and modes agree well with those of well-documented enstatite chondrites. Terrestrial weathering products such as Fe-oxyhydroxides, gypsum, and carbonates also occur in most of the meteorites from Antarctica. The mineral abundances determined via Rietveld refinement have been used to calculate model grain densities for each meteorite (i.e. density of the solid phases). Bulk magnetic susceptibility measurements combined with modal mineralogy reveal that as terrestrial weathering increases, both grain density and bulk susceptibility decrease. Sample-correlated thermal infrared (400-4500 cm-1, 2-25 μm) biconical (Diffuse) Reflectance Infrared Fourier Transform Spectroscopy data were collected for each meteorite to facilitate comparison with remote sensing data. The meteorite spectra are dominated by features corresponding to enstatite. Terrestrial weathering manifests itself as a broad, asymmetric H2O band centered near ~3400 cm-1, analogous to the "3 μm water of hydration feature" recognized in asteroid spectra, particularly from the enigmatic W-type asteroids. Additional sharp features superimposed on this band, as well as the sharpness of an asymmetric feature related to bound molecular water at ~1625 cm-1 correlate with the degree of weathering of the meteorites. These reflectance IR data provide an analog for remotely-sensed IR spectra from water-bearing asteroid regoliths and point to the need for instruments with much higher spectral resolution to identify structurally- bound water in asteroid regolith.


Towards a Novel Classification of Chondrules: Examples From the L4 Ordinary Chondrite Saratov

* Herd, R K (herd@nrcan.gc.ca), Natural Resources Canada, 601 Booth Street, Ottawa, ON K1A 0E8, Canada
* Herd, R K (herd@nrcan.gc.ca), Department of Earth Sciences, Carleton University, Ottawa, ON K1S 5B6, Canada
Dixon, L (ldixon@connect.carleton.ca), Department of Earth Sciences, Carleton University, Ottawa, ON K1S 5B6, Canada
Samson, C (csamson@earthsci.carleton.ca), Department of Earth Sciences, Carleton University, Ottawa, ON K1S 5B6, Canada
Hunt, P A (phunt@nrcan.gc.ca), Natural Resources Canada, 601 Booth Street, Ottawa, ON K1A 0E8, Canada

In order to classify and thoroughly investigate the origin of chondrites, the most primitive class of meteorites, exhaustive and systematic textural and mineralogical observations of chondrules are required. Long-standing mineralogical-textural classifications for chondrules obscure relevant data. Accepted classification schemes place chondrules in relatively few categories (e.g. radiating, cryptocrystalline, granular, porphyritic, barred) but ignore abundant micron-scale features within chondrules (relict crystals, overgrowths, zonations, quench textures) that provide invaluable evidence of chondrule history. Chondrule mineralogy and textures in polished thin sections and slabs of Saratov (L4) have been recorded using a scanning electron microscope (SEM). The most detailed work so far, on a single section, has mapped the size, sorting, packing, mineralogy and textures of 370 chondrules greater than 100 microns in diameter. Using back-scatter electron (BSE) images, a photomosaic of the entire thin section was created, and overlain with a grid system to locate and map specific chondrules. Textures and mineral phases were documented with BSE images, energy-dispersive spectrometry (EDS) and cathodoluminescence (CL). Chondrule textures are akin to those of igneous and metamorphic Earth rocks, and the processes forming them can be likewise interpreted. Many different heating, cooling and annealing histories, for chondrules now found associated in the same chondrite, are implied. Groups of chondrules with similar provenance, different from other groups, may be recognized by their analogous textural histories. Each chondrule has had at least a two-stage origin. Many show the effects of multi-stage processing. We anticipate that our conclusions will contribute to a new comprehensive classification scheme for chondrules and chondrites, and encourage others to examine the petrology of these complex and fascinating rocks.