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Distinguishing Meteoritic Nanodiamonds from Disordered Carbon Using Atom-Probe Tomography


J. B. Lewis , 1 * D. Isheim , 2 C. Floss , 1 and D. N. Seidman 2 1 Laboratory for Space Sciences , Physics Department , Campus Box 1105 , One Brookings Drive , Washington University , St. Louis ,


MO 63130 2 Center for Atom-Probe Tomography , and Dept. of Materials Science and Engineering , 2200 North Campus Drive , Northwestern


University , Evanston , IL 60208 * jlewis@physics.wustl.edu


Abstract: We have analyzed atom probe tomography reconstructions of disaggregated meteoritic material containing nanodiamonds and disordered carbon to determine whether these phases formed in the solar system or whether they predate the solar system and were formed in supernovae or the interstellar medium. We developed a method to distinguish between these two carbonaceous phases in < 100 nm diameter aggregates using the ratios of various native and contaminant molecular species. We fi nd variations in measured 12 C/ 13 C ratios between the two phases that suggest hydrides form more readily during fi eld evaporation of the disordered C than the nanodiamonds.


Keywords: meteoritic nanodiamonds, atom probe tomography (APT), element ratios, isotope ratios, presolar grains


Introduction


Nanodiamonds are found in primitive meteorites by dissolving the host material in acid and conducting size separa- tions on the residue [ 1 ]. T e nanodiamonds account for only about half of the carbon in the residue [ 2 ], and the remainder is a disordered carbon phase with primarily sp2 bonding, probably glassy carbon [ 3 ]. T ese nanodiamond acid residues also contain an anomalous mixture of xenon isotopes, Xe-HL, enriched in heavy and light isotopes, believed to be produced only in Type II (core collapse) supernova explosions [ 1 ]. T ese data suggest that at least some of the acid residue material is composed of presolar grains—particles that condensed around supernovae and late-type stars and survived the formation of the sun and solar system to be incorporated into primitive solar system material such as chondrites, which are stony meteorites that have not experienced signifi cant alteration by melting or icing and contain chondrules, that is, round grains of diff erent minerals [ 4 ]. However, the ratios of stable carbon and nitrogen isotopes in bulk nanodiamond acid residues are consistent with terrestrial values [ 5 ], a puzzling observation since the Xe appears to be of supernova origin. T e ratio of stable isotopes is the best indicator of whether material is presolar, as any material formed from solar gas will have a 12 C/ 13 C ratio within a small range of values near 90/1, whereas presolar materials, including material from diff erent supernovae, have 12 C/ 13 C ratios with


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ranging orders of magnitude from 1/1 to 1000/1, depending on the source. Transmission electron microscopy studies of the nanodiamonds show that they are on average less than 3 nm across, each containing around 2000 atoms [ 6 ]. Detailed studies of various types of presolar grains, which correlate with the observation and modeling of late-type stars, supernovae, and the interstellar medium, have yielded new information about these environments. However, meteoritic nanodiamonds and fragments of disordered carbon are so small that understanding their formation and alteration histories is limited by technical capabilities. Prior studies of the nanodiamond acid residues have measured millions or billions of nanodiamonds at a time. It is possible that a subset of the acid residue is presolar, and the rest formed in the solar system with the terrestrial carbon and nitrogen from the solar material diluting anomalous signals from the presolar fraction to such a degree that it goes undetected in bulk measurements of millions of nanodiamonds. In this article we use atom probe tomography (APT) to measure the 12 C/ 13 C ratio of smaller fractions of acid residue than previously charac- terized [ 7 – 9 ]. We cannot study Xe with this approach because its concentration is too low for us to expect detection of even a single meteoritic Xe atom in samples of our size. An important advance


Figure 1 : Sample preparation for APT. (1) A multilayer containing a circular deposit of acid residue (arrows), which presses up the covering Pt layer. The residue contains nanodiamonds and disordered carbon separated from the carbonaceous chondrite Allende by acid dissolution and size and density separations. A small droplet of water with suspended acid residue is placed on a Pt-coated substrate to make the deposit, which is then covered in a second layer of sputter-deposited Pt. (2) A FIB liftout of a 25 μ m long, 5 μ m wide undercut region of the multilayer. (3) A slice of the rotated multilayer attached to a 2 μ m diameter micropost using Pt deposition. (4) Annular milling by FIB produces a conical shape. (5) Additional FIB milling from four different angles produces a pyramid shape while preserving the Pt patch between the slice of multilayer and the micropost. (6) Final annular milling produces a nanotip with an apex of well under 100 nm in radius.


doi: 10.1017/S1551929518000226 www.microscopy-today.com • 2018 March


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