Bland, PA

Caffee, MW

Denyer, I

Grady, MM

Jull, AJT

Lloyd, J

Masarik, J

Russell, SS

Welten, KC

Spiral - Imperial College Digital Repository

CHINGUETTI - TERRESTRIAL AGE AND PRE-ATMOSPHERIC RADIUS. , K.C. Welten1, P.A. Bland2, M. W. Caf-fee3, J. Masarik1, S.S. Russell2, A.J.T. Jull5, I. Denyer6, J. Lloyd7 and M.M. Grady2, 1Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450, USA; 2Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD; 3CAMS, Lawrence Livermore National Laboratory, Livermore, CA 94551, USA; 4NSF Arizona AMS Facility, Phys-ics Building, University of Arizona, Tucson, AZ 85721; 55 Statham Grove, Stoke Newington, London N16 9DP; 6Flat 2, 60 Brixton Road, London SW9 6BS.   Introduction:  Chinguetti is a 4.5 kg mesosiderite found in the Adra region of Mauretania, some 45 km from the oasis town of Chinguetti, in 1916. It was several years be-fore the sample was identified as a meteorite, however, when it was, and the description of the find was released in 1924, it generated considerable excitement. Gaston Ripert, the French officer who found the meteorite, maintained that it was a representative sample of a much larger mass. His description, as transcribed by Lacroix [1], states: “It was lying on top of an enormous metallic mass measuring about 100m on one side and about 40m in height, which stands up in the middle of the dunes...”. Ripert's position as a French officer, and his comparative lack of interest in the find (after turning it over to M.H. Hubert, a freind and a Doctor of Science in Dakar, he made no effort to follow up on any progress made in its analysis, and in fact never claimed that it was a meteorite) convinced many that his story was true. Numerous expeditions have attempted to find the larger mass, most notably those of Theodore Monod (e.g. [2]). Several explanations have been offered to explain their lack of success: the find location being somewhat uncertain, pos-sibly dunes had partially or completely covered the mass; Ripert may have mistaken desert varnished sandstone for a large metallic mass, and just happened by chance on the smaller sample; or, for his own reasons, he simply lied. In order to throw a little more light on this problem, we chose to analyse a portion of the recovered sample for cosmogenic radionuclides, to determine its terrestrial age, and its pre-atmospheric radius. Experimental procedures: We analyzed a chip taken about 5 cm from the fusion crust. We first separated the metal and stone fraction and treated the metal fraction with 0.2N HCl and concentrated HF to remove attached troilite and silicates, respectively. After adding carrier, containing Be, Al, Cl and Ca, the metal and stone fractions were dis-solved in 1.5N HNO3 and concentrated HF/HNO3, respec-tively. The Be, Al and Cl were separated and 10Be, and 36Cl concentrations were determined using the LLNL-AMS facil-ity [3]. Results and Discussion: The 10Be and 36Cl results are shown in Table 1. The 26Al measurements are still in pro-gress. Table 1. Cosmogenic radionuclide concentrations in Chinguetti mesosiderite.            Conc. (dpm/kg)* 10Be(stone)      21.2 ± 0.3 10Be(metal)      3.35 ± 0.05 36Cl(metal)       17.4 ± 0.2  *1σ-errors include all known AMS errors, but not the uncertainties in the AMS standards. Although the cosmic-ray exposure age of Chinguetti is unknown, the saturation activity level of 10Be in stone phase suggests that the exposure age of the meteorite is more than 5 My.  Furthermore, mesosiderites typically have exposure ages 10-300 My [4]. The 36Cl-10Be terrestrial age method [5] yields a terrestrial age <50 ky. Measurements of 41Ca in the metal phase and 14C in stone phase will further constrain the terrestrial age of Chinguetti.. Pre-atmospheric size. The low concentrations of 36Cl and 10Be in the metal phase compared to the saturation ac-tivities of those nuclides in typical size of meteorites suggest that Chinguetti experienced moderate shielding conditions.  The 10Be(stone)/ 10Be(metal) ratio of 6.3±0.2 is also consis-tent with moderate shielding conditions [6]. According to Monte-Carlo based calculations for the production rates of 10Be and 36Cl, we can constrain the pre-atmospheric radius to 50-80 cm and the shielding depth to 15-25 cm. Conclusions: Our analyses suggest that the most likely scenario is that our sample of Chinguetti was irradiated for >10 My at a depth of 15-25 cm in an object of about 50-80 cm in radius, and that it landed on Earth <50 ky ago. These data indicate that Chinguetti has a comparatively recent terrestrial age, at least, too recent to be consistent with Ripert's original description of the main mass having a heav-ily wind eroded base. More compelling is our estimate of a pre-atmospheric radius of <1m, suggesting that Ripert was either mistaken, or for some reason falsified his description of the find.  Acknowledgments: This work was supported by NASA grant NAG5-4992, and was performed under the auspices of the U.S. DOE by LLNL under contract W-7405-ENG-48. PAB acknowledges the support of PPARC.  References: [1] Lacroix A. (1924) Comptes Rendus 179, 303-313. [2] Monod Th. (translated by Brady L. F.) (1955) Meteoritics 1, 308-314. [3] Davis J. C. (1990) Nucl. Inst. Meth. B52, 269-272. [4] Terribilini D.et al. (2000), submitted to MAPS. [5] Lavielle B. et al. (1999) EPSL 170, 93-104. [6] Welten K. C. et al., MAPS 34, A121-122.  Lunar and Planetary Science XXXI 1483.pdf

English

Chinguetti – terrestrial age and pre-atmospheric radius.

Chinguetti is a 4.5 kg mesosiderite find recovered from the Adra region of Mauretania. In this paper the authors analyse a portion of the recovered sample for cosmogenic radionuclides to determine its terrestrial age, and to determine its pre-atmospheric radius. They determined the terrestrial age of Chinguetti to be &lt; 30 ky. They constrain the pre-atmospheric radius to 50--80 cm and the shielding depths of 15--25 cm. These data indicate that Chinguetti is a comparatively recent fall

Welten, K. C.

Masarik, J.

Bland, P. A.

Caffee, M. W.

Russell, S. S.

Grady, M. M.

Denyer, I.

Lloyd, J.

UNT Digital Library

Approved for public release; further dissemination unlimitedPreprintUCRL-JC-137140Chinguetti- Terrestrial Ageand Pre-Atmospheric SizeK.C. Welten, P.A. Bland, M.W. Caffee, J. Masarik, S.S.Russell, I. Denyer, J. Lloyd and M.M. GradyThis article was submitted to31st Lunar and Planetary Science ConferenceHouston, TXMarch 13-17, 2000January 14, 2000LawrenceLivermoreNationalLaboratoryU.S. Department of Energy DISCLAIMER  This document was prepared as an account of work sponsored by an agency of the United StatesGovernment.  Neither the United States Government nor the University of California nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately owned rights. Reference herein to any specificcommercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does notnecessarily constitute or imply its endorsement, recommendation, or favoring by the United StatesGovernment or the University of California.  The views and opinions of authors expressed herein do notnecessarily state or reflect those of the United States Government or the University of California, andshall not be used for advertising or product endorsement purposes.  This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may bemade before publication, this preprint is made available with the understanding that it will not be citedor reproduced without the permission of the author.   This report has been reproduced directly from the best available copy.  Available to DOE and DOE contractors from the Office of Scientific and Technical Information P.O. Box 62, Oak Ridge, TN  37831 Prices available from (423) 576-8401 http://apollo.osti.gov/bridge/  Available to the public from the National Technical Information Service U.S. Department of Commerce 5285 Port Royal Rd., Springfield, VA  22161 http://www.ntis.gov/  OR  Lawrence Livermore National Laboratory Technical Information Department’s Digital Library http://www.llnl.gov/tid/Library.html  CHINGUETTI - TERRESTRIAL AGE AND PRE-ATMOSPHERIC SIZE . K.C. Welten1,P.A. Bland2, M. W. Caffee3, J. Masarik1, S.S. Russell2, I. Denyer5, J. Lloyd6 and M.M. GRADY2,1Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450, USA; 2Department ofMineralogy, Natural History Museum, Cromwell Road, London SW7 5BD; 3CAMS, LawrenceLivermore National Laboratory, Livermore, CA 94551, USA;; 5I Denyer, 5 Statham Grove, StokeNewington, London N16 9DP; 6Flat 2, 60 Brixton Road, London SW9 6BS.Introduction . Chinguetti is a 4.5 kgmesosiderite find recovered from the Adraregion of Mauretania, some 45 km from theoasis town of Chinguetti, in 1916. It was,however, several years before the samplewas identified as a meteorite. When it was,and the description of the find was releasedin 1924, it generated considerableexcitement. Gaston Ripert, the Frenchofficer who found the meteorite, maintainedthat it was a representative sample of a muchlarger mass. His description, as transcribedby Lacroix (1924), states: "It was lying ontop of an enormous metallic mass measuringabout 100m on one side and about 40m inheight, which stands up in the middle of thedunes...".Ripert's position as a Frenchofficer, and his comparative lack of interestin the find (he turned the sample over toM.H. Hubert, a freind and a Doctor ofScience in Dakar, and in the interveningyears between the find and theannouncement in 1924, made no effort tofollow up on any progress made in itsanalysis, and in fact never claimed that itwas a meteorite) convinced many that hisstory was true. Numerous expeditions haveattempted to find the larger mass, andseveral explanations offered to explain theirfailure: the find location being somewhatuncertain, possibly dunes had partially orcompletely covered the mass; Ripert mayhave mistaken desert varnished sandstonefor a large metallic mass, and just happenedby chance on the smaller sample; or, for hisown reasons, he simply lied. To elucidatethis problem, we chose to analyse a portionof the recovered sample for cosmogenicradionuclides, to determine its terrestrialage, and its pre-atmospheric radius.Experimental Procedures.  W eanalyzed a chip taken about 5 cm from thefusion crust. We separated both metal andstone fraction. After adding carrier,containing Be, Al, Cl and Ca, the metal andstone fractions were dissolved in 1.5NHNO3 and concentrated HF/HNO3,respectively. The Be, Al and Cl wereseparated and 10Be, and 36Cl concentrationswere determined using the LLNL-AMSfacility(Davis et al., 1990).Results and discussion. The 10Be and36Cl results are shown in Table 1, whereasthe 26Al measurements are still in progress.Table 1. Cosmogenic radionuclideconcentrations in Chinguetti mesosiderite.Conc. (dpm/kg)*10Be(stone) 21.2 ± 0.310Be(metal) 3.35 ± 0.0536Cl(metal) 17.4 ± 0.2* 1σ-errors include all known AMS errors, butnot theuncertainties in the AMS standardsAlthough the cosmic-ray exposure ageof Chinguetti is unknown, the saturationactivity level of 10Be in stone phase suggeststhat the exposure age of the meteorite ismore than 5 My. Mesosiderites typicallyhave exposure ages 10-300 My [5]. The36Cl-10Be terrestrial age method [6] yields aterrestrial age <30 kyMeasurements of 41Cain the metal phase and 14C in the stone phasewill further constrain the terrestrial age ofChinguetti.Pre-atmospheric size.  The lowconcentration of 36Cl and 10Be in the metalphase relative to the saturation activities ofthose nuclides in meteorites of typical sizesuggests that Chinguetti was exposed toenergetic particles while moderatelyshielded. According to Monte-Carlo basedcalculations for the production rates of 10Beand 3 6Cl, we can constrain the pre-atmospheric radius to 50-80 cm and theshielding depths of 15-25 cmConclusions. These data indicate thatChinguetti is a comparatively recent fall, atleast, too recent to be consistent withRipert's original description of the mainmass having a heavily wind eroded base.More compelling is our estimate of a pre-atmospheric radius of <1m, suggesting thatRipert was either mistaken, or for somereason falsified his description of the find.Acknowledgments. This work wassupported by NASA grant NAG5-4992, andwas performed under the auspices of theU.S. DOE by LLNL under contract W-7405-ENG-48.References: [1] Lacroix A. (1924)Comptes Rendus 179, 303-313. [2] Weltenet al. (1999) NIPR Ant. Met. Res. 12, 94-107. [3] Davis J. C. (1990) Nucl. Inst. Meth.B52, 269-272. [4] Lavielle B. et al. (1999)EPSL 170, 93-104. [5] Terribilini D.et al.(2000), submitted to MAPS. [6] NishiizumiK. et al. (1997) MAPS 32, A100. [7] WeltenK.C. et al. (1999) MAPS 34, A121-122.

Chinguetti - Terrestrial Age and Pre-Atmospheric Size

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Chinguetti – terrestrial age and pre-atmospheric radius.

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