NANO-DIAMONDS IN CHONDRITIC INTERPLANETARY DUST PARTICLES. Z. R. Dai1,D. J. Joswiak2 , J. P. Bradley1 , D. E. Brownlee2 , H.G.M. Hill3. 1School of Materials Science and Engineering,Georgia Institute of Technology, Atlanta, GA 30332-0245, 2Dept. of Astronomy, University of Washington, SeattleWA 98195. 3Extraterrestrial Physics, NASA GSFC Greenbelt, MD 20071. john.bradley@mse.gatech.edu

Introduction: Chondritic IDPs from the anhydrouschondritic porous (CP) subset are believed to beamong the most primitive meteoritic materialsavailable for laboratory investigations. Their silicatemineralogy resembles those of silicates in theinterstellar medium and around some young stars [1,2].High D/H ratios measured in some IDPs are consistentwith the preservation of carbonaceous material from apresolar interstellar molecular cloud [3]. ChondriticIDPs may also contain “presolar” nano-diamonds likethose found in primitive meteorites. In principle,nano-diamonds should be easy to detect in IDPsbecause the bulk carbon content of chondritic IDPs istypically several times higher than that of (CI)chondrites [4]. In practice, 2-5 nm diameter nano-diamonds have been difficult to unambiguouslyidentify in IDPs because, in contrast to chondriticmeteorites, the fine-grained matrices of IDPs aremineralogically heterogeneous on a scale of 2-5 nmand nano-diamonds scatter electrons weakly relative toother higher atomic-number nanophase minerals (e.g.FeNi metal and sulfides). The goal of this work is toestablish the abundance of nano-diamonds in IDPs,compare their properties with those found in meteorites(e.g. size distribution and defect structures), anddetermine their petrographic context in IDPs (i.e. whatminerals they are associated with). Answering thesequestions might clarify the relationship betweenchondritic IDPs and meteorites and provide an estimateof the relative abundance of nano-diamonds in small,primitive solar system bodies as a function ofheliocentric distance. Methods: Five chondritic IDPs and fragments offine-grained matrix from two chondritic meteoriteswere selected for this study. The IDPs are W7027A-8D, U2-20GCA, U2-17B19, U2073B-1F, andU2073B-4C, and the meteorites are Murchison (CM)and Tagish Lake (CMI). In order to optimize thechances of detecting nano-diamonds we used an in-situacid dissolution procedure [5]. The IDPs and severalmeteorite fragments were mounted in sulfur beads thatwere in turn embedded in epoxy and ultramicrotomed[6]. The ~100 nm thick sections were collected oncarbon-film/gold-mesh TEM substrates and the sulfurwas sublimed leaving the IDP or meteorite matrix sliceat the center of a ~50µm hole in the epoxy. Thesections were then individually characterized usinganalytical TEM prior to the etching procedure.Etching is done by placing the grids on on apolyethylene frit which is then saturated with triple

distilled concentrated HF. After a room temperatureetch for an hour, the grid is rinsed (5 times) withnanopure water and dried. We examined the etched sections using transmissionelectron microscopes (TEM’s). Lattice fringe imageswere acquired using a 400 keV JEOL 4000EX, a 200keV analytical JEOL 2010 TEM, and a 200 keV fieldemission Hitachi HF2000 STEM. Grain compositionswere measured (in the JEOL 2010 & HF2000) usingenergy-dispersive x-ray spectroscopy and electronenergy loss spectroscopy (EELS). Results: The simple HF etching works remarkablywell for chondritic IDPs and meteorite fragments. Inless than an hour all silicates and metals are removedleaving only Fe-rich sulfides and carbonaceousmaterials, including organic compounds [5]. Imagestaken before and after etching confirm that, despitecontact with aqueous solutions, the spatial distributionof structure remaining within the sections is preservedat the nanometer scale. Figure 1 is a darkfield image of residue remainingafter after acid-etching a fragment of Murchisonmatrix. Most of the bright spots are 2-5 nm diameternano-diamonds. Some of the larger grains are Fe,Nisulfides. The measured nano-diamond concentrationon the carbon substrate is ~3.1 x 1015 meter2.Assuming the thickness of the pre-etched section was100 nm, there are ~1.6 X 1023/meter3, which translatesinto an abundance of ~2000 ppm nano-diamonds in the(pre-etched) thin section. The nano-diamonds arerelatively uniformly distributed over the surface of thecarbon support substrate, suggesting that theirdistribution was not highly localized in the fragment offine-grained matrix of Murchison Figure 2 is a lattice-fringe image of a nano-diamondcrystal in the etched Murchison thin section. Thecrystal is multiply-twinned with pseudo-pentagonal (orstar-twin) morphology. This striking diamond growthfeature is relatively common in nano-diamonds inmeteorite acid residues and in CVD diamond thin-films [3]. Packets of turbostratic graphite-like fringesaround the edge of the crystal suggest that the hostcarrier of at least some of the nano-diamonds inMurchison is carbonaceous, in accordance withprevious in-situ observations of nano-diamonds in theAllende meteorite [7]. Figure 3 is a lattice-fringe image of a nano-diamondin an etched thin section of IDP U2-20GCA. The grainsize and twin structures are similar to those of nano-

Lunar and Planetary Science XXXII (2001) 1631.pdf

Nano-diamonds in chondritic IDPs. Dai et al.

diamonds observed in meteorite residues [3]. Incontrast to those in the Murchison residue (e.g. Fig. 1),the nano-diamonds in IDP U2-20GCA appear to bemore highly localized in clumps of carbonaceousmaterial at a concentration ~3X higher than those weobserve in the Murchison thin section. Discussion: Using in-situ acid-etching, lattice-fringe imaging, defect structure analysis, and energy-dispersive x-ray spectroscopy we have identifiedpopulations of face centered cubic (FCC) carbon-richnanocrystals in ultramicrotomed thin sections of achondritic IDP and meteorite matrix fragments. Wetentatively identify these crystals as nano-diamonds.At the same time we recognize that unambiguousidentification of nano-diamonds using the TEM shouldinclude electron diffraction ring patterns and, moreimportantly, EELS spectra showing clear evidence ofsp3-bonded carbon. Unfortunately, these options arepresently not available for isolated (non-agglomerated) nano-diamonds resting on (sp2-bonded)carbon substrates because of insufficient signal-to-noise. Our estimate of the abundance of nanodiamonds inthe Murchison thin-section (~2000 ppm) is a factor of~ 5 higher than their abundance in the bulk meteorite[8]. Either the selected matrix fragment contained alocally higher concentration than the meteorite as awhole, our assumptions about the nano-diamond sizedistribution are inaccurate, or some of the nanocrystalswe observe are not nano-diamonds. Furthermore, wehave so far detected nano-diamonds in only one of fivechondritic IDPs. Either they are not present in mostchondritic IDPs, some of them may have beendestroyed by heating and oxidation during atmosphericentry, or their abundance in chondritic IDPs issignificantly lower than in chondritic meteorites.Alternatively, the carrier of nano-diamonds may be acarbonaceous phase that, like the carrier of the D/Henrichments [2], is heterogeneously distributed amongchondritic IDPs.

Acknowledgement: This research is supported byNASA grants NAG5-7450 and NAG5-9797.

References: [1] F. J. Molster et al. (2001) LPSXXXII, (this volume). [2] S. Messenger (2000) Nature404, 968. [3] T. L. Daulton et al. (1996) GCA, 60,4853. [4] K. L. Thomas et al. (1993) GCA, 57, 1551.[5] D. E. Brownlee et al. (2000) LPS XXXI, 1920.[6] J. P. Bradley et al., LPS XXIV, 173. [7] F.Banhart et al. (1998) Meteoritics & Planet. Sci., 33,A12. [8] P. Hoppe and E. Zinner (2000) J. Geophys.Res. 105 No. A5, 10,387.

Figure 1. Darkfield micrograph of nano-diamond-richresidue in etched thin section of Murchison fine-grained matrix.

Figure 2. Star-twinned nano-diamond in etched thinsection of Murchison matrix.

Figure 3. Twinned nano-diamond in etched thinsection of chondritic IDP U2-20GCA.

Lunar and Planetary Science XXXII (2001) 1631.pdf