Laser-Assisted APT of Deformed Minerals 405
the sample that is analyzed. This may be achieved by corre- lating electron backscattered diffraction (EBSD) and/or transmission Kikuchi diffraction (TKD) (Trimby, 2012) with APT. In this paper, we focus on the influence of APT analysis
parameters (laser pulse energy and frequency, base tem- perature) on the overall data quality and mass spectrum thermal tail. We discuss the important topic of ion identifi- cation in the APT mass spectrum and assess the Pb and U detection and quantification limits for two different zircons. Finally, we present a correlative EBSD, TKD, and APT approach for the study of trace element mobility in deformed zircons.
MATERIALS ANDMETHODS
The zircons used in this study come from two different regions: plastically deformed Archean zircons (~2.5Ga old) from the Napier complex in Antarctica and younger zircons from a decimeter wide high strain zone developed on the island of Krossey within the Bergen Arc of southwest Norway (age range ~760–405 Ma). The Archean zircons, namely DN5 and DN77 contain ~2,000ppma U, ~600ppma Pb, and ~150ppma U, ~60ppma Pb, respectively. In con- trast, those from the Bergen Arc contain ~10ppma U and ~1 ppma Pb. Supplementary Table 1 presents U, Pb, and Th composition measured by SIMS for the Krossey sample and by SHRIMP for DN5 and DN77.
Supplementary Table 1
Supplementary Table 1 can be found online. Please visit
journals.cambridge.org/jid_MAM The general microstructure of the zircons was char-
acterized using combined EBSD and energy dispersive X-ray spectroscopy (Oxford Instrument Aztec) in a Zeiss Ultra Plus field emission gun (FEG; Zeiss) scanning electron microscope (SEM). Thin sections of the samples (DN5, Krossey, and DN77) were mechanically polished down to 1 μm, and subsequently mechano-chemically polished using colloidal silica to allowEBSD analysis. The thin sections were coated with ~5–10nm carbon. Regions of interest (low-angle boundaries with small misorientations, <4°) were then selected based on the EBSD deformation maps. APT samples were prepared using a Zeiss-Auriga focused ion beam SEM (FIB-SEM) equipped with a Kleindiek micromanipulator system (Kleindiek). Bars of selected areas were lifted-out from milled cross-sections using the micromanipulator. Samples were milled from these bars, attached to electro- polished molybdenum grids and finally milled to form APT tips with a typical diameter of around 60nm (Felfer et al., 2012). The final ion beam milling step was carried out at low energies (typically 5 kV) in order to minimize Ga-implantation and damage. In cases where carbon contamination was an issue, the SEM chamber was plasma
cleaned for ~5min before TKD analyses using an on-chamber plasma cleaner (Evactron Zephyr; XEI Scientific). The defor- mation structures within each APT tip were analyzed in the FIB-SEM using TKD after the final annular milling. The APT experiments were conducted on a Cameca LEAP 4000× SiTM (Cameca) atom probe equipped with a picosecond-pulse ultraviolet laser (355nm, spot size <4 μm). The data were reconstructed using the Cameca IVAS 3.6.6 software.
RESULTS AND DISCUSSION
Effect of Laser Energy The influence of laser energy on the mass spectra quality was investigated to determine the optimum laser energy for zircon, as an example of a typicalmineral. A single tip of the Krossey zircon was used and the laser energy was progres- sively increased from 25 to 700 pJ. This experiment was carried out at a base temperature of 60 K, a laser pulse repetition rate of 250 kHz and an evaporation rate of 1%. A total of 1 million atoms per laser energy condition were collected. It is important to note that several factors such as tip radius and voltage are changing with laser energy increase. The tip radius is increasing slightly after each million atoms collected. The interaction volume with the laser becomes greater improving the thermal conductivity at the apex. However, this effect is thought to be minimal due to the small volume evaporated between each laser energy. The thermal tails are equally important for most of the spe- cies in the mass spectra. Their evolution with changing analysis parameters also follows exactly the same trend. We have arbitrarily chosen to show the thermal tails evolution for ZrO and SiO2. Figure 1a shows the mass spectra of the ZrO2+ and SiO2
+
peaks for data collected at different laser pulse energies. Laser-assisted field ionization is a thermal process inducing local heating of the tip, which in turn provokes a broadening of the peak tail (Vurpillot et al., 2009). The extent of the tail broadening (thermal tail) is function of the local heating intensity and tip cooling rate. The thermal tail after each peak only slightly increases
with increasing laser energy. We therefore conclude that the local heating of the tip due to the laser pulse is minimal, with only a limited effect on the extent of the tail behind each peak. It shows that mineral zircon has a sufficiently high thermal diffusivity and fast cooling rate when exposed to the picosecondUVlaser. The temperature conductivity of zircon was reported to have only a minimal dependency on temperature above 300–400K (Kingery et al., 1954). This explains the relatively fast thermal diffusivity of the zircons at high laser energy. The mass resolving power (mass resolution of a specific peak) in APT is usually measured as full-width half maximum (FWHM) and full-width tenth maximum (FWTM) on a peak around 30Da (Larson et al., 2013). The FWHM and FWTM measured at 32Da (O2
+)
increases with increasing laser energy and reaches a plateau from ~400 pJ with FWHM ~780 and FWTM ~320.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200 |
Page 201 |
Page 202 |
Page 203 |
Page 204 |
Page 205 |
Page 206 |
Page 207 |
Page 208 |
Page 209 |
Page 210 |
Page 211 |
Page 212 |
Page 213 |
Page 214 |
Page 215 |
Page 216 |
Page 217 |
Page 218 |
Page 219 |
Page 220 |
Page 221 |
Page 222 |
Page 223 |
Page 224 |
Page 225 |
Page 226 |
Page 227 |
Page 228 |
Page 229 |
Page 230 |
Page 231 |
Page 232 |
Page 233 |
Page 234 |
Page 235 |
Page 236 |
Page 237 |
Page 238 |
Page 239 |
Page 240 |
Page 241 |
Page 242 |
Page 243 |
Page 244 |
Page 245 |
Page 246 |
Page 247 |
Page 248 |
Page 249 |
Page 250 |
Page 251 |
Page 252 |
Page 253 |
Page 254 |
Page 255 |
Page 256 |
Page 257 |
Page 258 |
Page 259 |
Page 260 |
Page 261 |
Page 262 |
Page 263 |
Page 264 |
Page 265 |
Page 266 |
Page 267 |
Page 268 |
Page 269 |
Page 270 |
Page 271 |
Page 272 |
Page 273 |
Page 274 |
Page 275 |
Page 276 |
Page 277 |
Page 278 |
Page 279 |
Page 280 |
Page 281 |
Page 282 |
Page 283 |
Page 284
