Venture Grant: Lunar and Planetary Science Conference
- First Student – Bencubbin Meteorite
- Second Student – Mars: Stereography of Debris Aprons and Related Flow Features
- Budget Request
First Student – Bencubbin Meteorite
To: Venture Grant Committee
From: ___________
Date: 6 November 1997
RE: Funding to attend the 29th annual Lunar and Planetary Science Conference, Houston, TX, March 16-20
Over the past three years at Colorado College, I have pursued many interests. Deciding in my sophomore year to be geology major, I set out to gain a diverse knowledge of this scientific field. Beginning the summer of my sophomore year I participated in a Keck Consortium research project in Massachusetts. Confronted with an option to contribute to an additional Keck project my junior year summer, I made the decision to expand my horizons in geology. I applied, and was accepted, to an internship in the natural sciences at the Smithsonian’s National Museum of Natural History.
At the Museum of Natural History, I was able to work with Dr. Glenn MacPherson and Dr. Timothy McCoy, both leading scientists in the Department of Mineral Science at the Museum of Natural History. Our research focused on the detailed macroscopic analysis of the Bencubbin meteorite, a meteorite which fell in Bencubbin, Australia. I have attached a copy of our extended abstract to provide additional detailed information on our research.
The 29th Lunar and Planetary Science Conference is an annual conference held in order to share cutting edge knowledge in the fields of planetary and lunar science. I would use the opportunity to attend this conference to present my research in either a poster session or talk. Currently, my plans are to display my results in the form of a poster. In addition to providing me with the opportunity to present my research at a professional scientific conference, I will have the chance to explore research being undertaken by leading scientists in these fields. I feel that the opportunity to present research at the 29th Lunar and Planetary Science Conference will provide valuable professional experience in a field in which I hope to make my career.
Thank you for your time and consideration.
The History of the Bencubbin Meteorite: A Macroscopic Analysis
The Colorado College, Colorado Springs, CO
National Museum of Natural History Research Training Program
Smithsonian Institution, Washington, D.C. 20560
Department of Mineral Sciences, National Museum of Natural History
Smithsonian Institution, Washington, D.C. 20560
Department of Mineral Sciences, National Museum of Natural
History
Smithsonian Institution, Washington, D.C. 20560
Abstract
Bencubbin is a stony-iron meteorite that has, despite attempts, remained largely unclassified. Two scenarios are offered for the evolution of Bencubbin: formation as a result of processes in the solar nebula, and formation as a result of impact melting on a chondritic parent body. Previous researchers have proposed sorting mechanisms that may have existed in the solar nebula resulting inadvertently from processes responsible for aggregation of nebular particles. Quantifying evidence of such a mechanism in Bencubbin may aid in unraveling the place and processes of its origin (Nebular Origin vs. Impact Origin). No previous detailed study has been completed in order to clarify and quantify these mechanisms, or apparent foliation.
A macroscopic study of Bencubbin was completed by tracing — 1300 metal and silicate chondrule-like clasts. Metal and silicate clasts exhibited size sorting, but there was no evidence of size equivalency. Results do, however, indicate mass equivalency implying an aerodynamically driven sorting mechanism. In addition, we quantified evidence for foliation. Most metal and silicate clasts are aligned at an angle of 85-90e above the horizontal. Although the presence of a sorting mechanism alone cannot determine a nebular origin or classification for this meteorite, we conclude that the results reinforce a current hypothesis classifying Bencubbin as a member of the CR chondrite clan. Measured foliation verifies a later impact in this enigmatic meteorite’s formation.
Introduction
Described first by Simpson and Murray (1932), Bencubbin was initially classified as a mesosiderite. However, recent studies (Weisberg et al., 1990, 1995) indicate that this classification should have been made with some reservation. In fact, Weisberg et al. (1990) proposed two alternate scenarios for the formation of Bencubbin: formation as a result of processes in the solar nebula, and formation as a result of impact melting on a chondritic parent body. Neither of which defend this enigmatic meteorite’s original classification as a mesosiderite, but rather, imply Bencubbin’s classification as a chondrite. Further evidence defending Bencubbin’s formation either in the solar nebula, or by extensive impacting may exist within what is known about nebular processes.
It is widely accepted that chondrules form as a result of processes attributable to the accretion of silicates, irons, and sulfides within the solar nebula. Early on, Dodd (1967, 1976) made the observation of possible size sorting of chondrules due to aerodynamic drag resulting inadvertently from these processes. More recently, researchers have noted similar mechanisms for size sorting and aerodynamic equivalency, or mass sorting (Kuebler et al., 1996, 1997; Skinner and Leenhouts, 1993) in silicate and metal chondrules. Skinner (1993) concluded that mean diameters of metal and silicate chondrules are, in fact, aerodynamically equivalent for different densities in the CR2 chondrite Acfer 059. This reinforces the suggestion that sorting in the nebula involved an aerodynamic component, as postulated by Shu et al. (1996). The question is: Can evidence of such a component in Bencubbin’s history aid in efforts to explain this problematic meteorite’s formational history?
The purpose of this paper is to present the results of our macroscopic analysis of Bencubbin. As mentioned, evidence of a sorting mechanism in Bencubbin may aid in unraveling the place and processes of its origin. Despite access to such large samples of this meteorite, no previous detailed study has been completed to clarify these mechanisms or quantify observations of apparent foliation (McCall, 1968). It is our goal to show that metal and silicate clasts in Bencubbin exhibit aerodynamic equivalency and to quantify evidence for foliation.
Analytical Procedures
Two polished thin sections of Bencubbin (USNM 5717-1 and 5717-2) were used to aid in clast identification on a macroscopic level using transmitted and reflected light microscopy. Chondrule diameters were determined by direct measurements of the major and minor axis of chondrule-like clasts on a cut slab of the Bencubbin meteorite (USNM 5625). A sheet of Mylar was used to protect the sample while each clast was hand-traced. Actual measurements were calculated by digitizing four points; the first two defining the major axis, and the second two defining the minor axis. The average diameter was calculated by averaging the two orthogonal dimensions defined by the digitizer.
Once the diameters of each of the 273 silicate and 1007 metal clasts were determined, a volume was calculated assuming the original geometry of a sphere. In order to accurately measure dimensions of these clasts, disagregation is necessary. However, disagregation is not always an option in a study such as this one. Therefore, it is important to note that data is subject to sampling bias in thin section as well as hand sample. Hughes (1977) and Eisenhour (1996) have proposed equations which likely correct for errors due to sampling bias. However, the proposed methods of correction are in disagreement. The data in this study has not been corrected using either one of these equations. Nonetheless, it is assumed that the metal and silicate chondrule-like clasts were originally spherical, and were flattened to display the observed elliptical geometry. From this calculation, densities of 3.2 g/cm’ for predominantly pyroxene silicates (Deer, Howie, and Zussman, 1978) and 7.6 g/cm’ for Fe,Ni metal (Keil, 1960) were used to calculate mass(g) for each chondrule-like clast.
Apparent lineations were quantified in a similar fashion. Again, the digitizer was used, selecting the same major and minor axis defined for the previous calculations. In order to quantify preferred orientations of each clast, the angle of the major axis above a defined horizontal was calculated. In addition, the extent of clast deformation was determined by calculating the aspect ratio (major axis/minor axis).
Results
The data represent a general macroscopic description of the Bencubbin meteorite based on four major clast characteristics: 1) average diameter, 2) mass, 3) aspect ratio and 4) angle of major axis above defined horizontal. Bencubbin is composed of -40% silicates and -60% metal (Simpson and Murray, 1932) occurring as sub-angular and rounded clasts. Newsom and Drake (1978) proposed a nebular condensation model for the metal clasts, and inferred accretion of the silicate clasts as solids, perhaps originating in the solar nebula as well. Weisberg et al., (1990) speculated the silicate clasts did, in fact, form in the solar nebula. For the remainder of this paper, silicate and metal clasts in Bencubbin will be identified as chondrules.
Average Diameter
Figures 1 and 2 illustrate histograms of the average diameter of silicate and metal chondrules, respectively. It is evident that there is some extent of sorting due to size. However, the size distribution, as seen by the `bell curve’ in each diagram, is not identical for silicate and metal chondrules. There is no size equivalency. Dashed lines represent the mode of the average diameter of silicate chondrules. The same dashed line appears on the diagram of metal chondrules for comparison. This comparison reveals the silicate chondrules are – 2 times the diameter of metal chondrules. The mode, or greatest number of samples falling within a certain bin range, for silicate and metal chondrules defends this affinity:
Mode of Silicate = 0.65 – 0.70 cm
Mode of Metal = 0.25 – 0.30 cm
Figures 3 and 4 illustrate histograms of the average mass of silicate and metal chondrules composing Bencubbin. It is again evident that there is some type of mass distribution. Recognizing whether or not this distribution is isolated to silicate and metal chondrules individually is difficult. However, noting the similarity of the modes of these chondrules may provide some clues:
Mode of silicate = 0.04 – 0.06 g
Mode of metal = 0.06 – 0.08 g
Aspect Ratio
Figures 5 and 6 illustrate histograms of the aspect ratio of silicate and metal clasts in Bencubbin, respectively. The long axis of both silicate and metal chondrules has been elongated to a length – 2 times the length of the minor axis. The mode for silicate and metal is equal:
Mode of silicate = 1.8 – 2.0
Mode of metal = 1.8 – 2.0
Lineations
Figures 7 and 8 illustrate histograms of the preferred orientations of silicate and metal chondrules. Metal and silicate chondrules demonstrate a preferred orientation, 85-90 degrees above the horizontal. As is the case with the aspect ratios, both metal and silicate clasts have equal modes.
Mode of silicate = 85 – 90 degrees
Mode of metal = 85 – 90 degrees
Discussion
Metal and silicate particles in Bencubbin exhibit distinct peaks in their size distribution, suggesting that each of these groups has been well sorted. The lack of equivalency in diameter between metal and silicate particles suggests, however, that sorting did not occur by size during a common event. In contrast, metal and silicate particle mass histograms demonstrate mass equivalency, suggesting sorting by mass during a common event for both types of particles. Mass equivalency as a result of preferential sorting in the solar nebula has been proposed by numerous previous workers. The presence of mass equivalency in Bencubbin supports a nebular origin for the particles in this rock, although this rests on the assumptions that Bencubbin particles are in fact chondrules and that mass equivalence and sorting is a uniquely nebular feature. The latter assumption has been tested by Klint Cowan during a parallel project.
Common aspect ratios and lineations for metal and silicate particles suggests a common deformation event for Bencubbin after accretion of the individual particles into a parent body. Abundant petrographic evidence for shock in the form of fracturing and deformation of the silicate "chondrules", finely-dispersed troilite within Fe,Ni metal, and intimately mixed and sheared metal and silicate suggests that this deformation event was a shock event due to the impact of another asteroid with the parent body of Bencubbin.
Conclusions
The Bencubbin meteorite is one of the most intriguing meteorites. High ‘SN/14N ratios (Prombo and Clayton, 1985), reduced, Mg-rich silicates (Weisber et al., 1995), high modal abundances of Fe,Ni metal, a solar Ni : Co ratio (Newsom and Drake, 1978) in Fe,Ni metal, and similar oxygen isotope compositions on or near the CR chondrite mixing line (Clayton and Mayeda, 1978), defend the classification of Bencubbin as a member of the CR chondrite clan. Weisberg et al. (1995) acknowledges the fact that members of the CR chondrite clan must have formed in the same region of the solar nebula. The results of our macroscopic analysis of chondrule-like clasts in Bencubbin support this classification, as well as an aerodynamic sorting mechanism acting within the solar nebula. Although the data alone does not indicate a definite nebular origin for Bencubbin, our results point in this direction. In addition, foliation is indicative of an impact event after accretion of individual particles into a parent body.
Future work
Although this theory is plausible, there are a few concerns. Some of which are: 1) sampling bias, and 2) interpretation of clasts as chondrules. Kuebler et al. (1996) proposed Eisenhour’s equation (Eisenhour, 1996) as the more accurate method of correcting for sampling bias. Further research will include this correction, although it is still unclear if the new results will prove to be significantly different for interpretive purposes. In addition, dissagregation, resulting in a 3D analysis, would prove to eliminate many assumptions and inherently improve accuracy. The interpretation of metal and silicate clasts poses a problem. If these clasts are, in fact, chondrules, how do you explain their anomalous size (approximately 6 times the normal diameter of chondrules)?
Acknowledgments
The authors would like to thank Ralph Chapman (Smithsonian Institution, Morphometrics Lab) for providing computer expertise. He saved us many days of data collection by writing a digitizing program. We would also like to thank the Office of the Director, Smithsonian Institution (NMNH) for providing financial support for this research.
References
Clayton R.N. and Mayeda T.K. (1978) Multiple parent bodies of polymict brecciated meteorites. Geochim. Cosmochim. Acta 41, 1777-1790.
Deer W.A., Howie R.A., and Zussman, J. (1978) The Rock Forming Minerals: Single Chain Silicates. Volume 2A, p. 52.
Dodd R.T. (1967) Chondrites. EOS 52, 447-453
Dodd R.T. (1976) Accretion of the ordinary chondrites. Earth Planet. Sci. Lett. 30,281-291
Eisenhour D.D. (1996) Determining chondrule size distributions from thin-section measurements Meteoritics 31, 243-248.
Hughes D.W. (1977) A disaggregation and thin section analysis of the size and mass distribution of the chondrules in the Bjurbole and Chainpur meteorites. Earth Planet. Sci. Lett. 38, 391-400.
Keil K. (1968) Mineralogical and chemical relationships among enstatite chondrites. Geophys. Res. 73, num. 22.
Kuebler K., McSween Jr. H.Y., and Carlson W.D. (1996) The chondrule size distribution of Bjurbole. LPSC XXVII, 715-716. [Abstract]
Kuebler K., McSween Jr. H.Y., and Carlson W.D. (1997) Size distributions and the mass equivalence of chondrules and metal grains in Bjurbole. LPSC XXVIII, 773-774. [Abstract]
Shu F.H., Shang H., and Lee T. (1996) Toward an Astrophysical Theory of Chondrites. Science 271, 1545-1552.
Simpson E.S. and Murray D.G. (1932) A new siderolite from Bencubbin, Western Australia. Mineral Mag. 23, 33-37.
Skinner W.R. and Leenhouts J.M..(1993) Size distributions and aerodynamic equivalence of metal chondrules and silicate chondrules in ACFER 059. LPSC XXIV, 1315-1316. [Abstract]
McCall G.J.H. (1968) The Bencubbin meteorite: further details, including microscopic character of host material and two chondrite enclaves. Min. Mag. 36, 726-739.
Newsom H.E. and Drake M.J. (1979) The origin of metal clasts in the Bencubbin meteoritic breccia. Geochim. Cosmochim. Acta 43, 689-707.
Prombo C.A. and Clayton, R.N. (1985) A striking nitrogen isotope anomoly in the Bencubbin and Weatherford meteorites. Science 230, 935-937.
Weisberg M.K., Prinz M. and Nehru C.E. (1990) The Bencubbin chondrite breccia and its relationship to CR chondrites and the ALH85085 chondrite. Meteoritics 25, 269-279.
Weisberg M.K., Prinz M., Clayton R.N., Mayeda T.K., Grady M.M., and Pillinger C.T. (1995) The CR chondrite clan. Meteoritics 8, 11-32.
Second Student – Mars: Stereography of Debris Aprons and Related Flow Features
TO: Venture Grant Committee
FROM: ___________
DATE: 3 November 1997
RE: to present research at the 29th Lunar and Planetary Science Conference in Houston, Texas, March 16-20
As a geology major at Colorado College I find myself inspired to pursue many aspects of the scientific world. Last year I began following an interest in planetary geology, which led me to a summer internship at the Lunar and Planetary Institute and NASA Johnson Space Center in Houston. I worked with Dr. Paul Schenk, a staff scientist at the Lunar and Planetary Institute (LPI). We produced topographic images of lobate debris aprons on the surface of Mars, using a computer program developed at LPI. Lobate debris aprons are geologic features found in the mid-latitudes on the surface of Mars that exhibit flow-like characteristics, similar to rockglaciers and landslides found on Earth. Debris aprons are thought to have flowed or be flowing by the slow movement of ice entrained in the rock. The production of these images allowed me to collect data on these features that has not been observed before by this method. My goal is to determine a best fit descriptive model of debris aprons and how they formed. I am using my summer research to produce a senior thesis for distinctions in the geology department and also plan to present this original research at the 29th Lunar and Planetary Science Conference in Houston in March of 1998.
The Lunar and Planetary Science Conference is one of the largest scientific conferences in the United States and attending will allow me an opportunity to show my research and discuss it with the top geologist and planetary scientists from around the nation and world. Currently, I am planning to present a poster or give a talk about my research. This four and one-half day conference will not only allow me to present my research, but I will also be exposed to many new areas of geology and planetary science which I may choose to pursue in graduate school. I consider presenting at this conference a project which will culminate my career as liberal arts science major at The Colorado College. I am requesting that the Venture Grant Committee fund my experience of presenting research at the 29th Lunar and Planetary Science Conference.
Thank you fo your consideration.
Also included: photocopy of article as published
Budget Request
Budget Request
for _____ and _____ to present research at the 29th Lunar and Planetary Conference in Houston, TX. March 16-20 1998
| Travel: | Airfare to Houston on Delta Airlines 297.00 Shuttle or Taxi 25.00 |
| Lodging: | 5 nights @ 79.00 dollars (double occupancy) per night at hotel suggested by conference. (Room to be shared with ________ to cut down on cost) 197.50 |
| Food: | 15.00 per day X 6 days 90.00 |
| Poster/slide preparation: | Film, paper, poster board,etc 50.00 |
| Registration fee: | Student fee 30.00 |
| Total requested funds $689.50 |