Laser-Assisted APT of Deformed Minerals 407
we can use for dating. Table 1 shows the ions and molecular ions detected in the mass spectra. The main elements detected were, as expected, Zr, Si, and O. The trace elements detected were U, Pb, Al, Y, and Hf. The following elements were also detected in some of the samples: Mg, P, Yb, Gd. The three mass spectra from the different zircon samples (DN5, Krossey, and DN77) are shown in Figure 4. The high laser energy necessary to thermally ionized
zircon could explain the large number of molecular ions detected in the mass spectra (Affatigato, 2015). For the most prominent radiogenic elements (O, Si, and Zr), we used isotopic ratios to validate the identification of ions and molecular ions. No major isobaric interference was encountered. Although there is always H present in an APT data set, it is found to be reasonably low in all the data sets analyzed, which is indicative of a clean vacuum system. The presence of H during an APT experiment may lead to hydrides after each peak, which has an impact on the com- position measurement accuracy. The O content was around 66.5 at% for all samples, very close to the nominal compo- sition of 66.7 at%. A small deficit of measured Zr (~16 at% instead of 16.7 at%) and Si (~15.5 at% instead of 16.7 at%) was noted. Some of the key trace elements for dating zircons are
U, Pb, Th, and Hf. Th was not detected in our data sets. Hf was only detected as HfO2+. Although it is possible to isolate the relative abundance for each HfO isotopes, most of them
Figure 3. Effect of laser pulse frequency and sample base tem- perature. Normalized mass spectrum of ZrO2+ and SiO2
+ (a) for
different laser pulse frequencies (50–625 kHz) (b) for different specimen base temperatures (20–120 K).
change in thermal tail with increasing base temperature up to 120 K. A higher temperature is usually beneficial for a decrease of tip fracture as it lowers the field and the electrostatic pressure (Wilkes et al., 1972).
Ion Identification in Zircon
In APT the mass-to-charge ratios of ions are recorded by time-of-flight mass spectroscopy (Gault et al., 2012). Correct identification of the peaks in the mass spectrum is essential for accurate elemental analysis. High resistivity materials, such as complex oxides, can lead to particularly challenging mass spectra with a higher probability of molecular ion evaporation (Cerezo et al., 1985; Gorman et al., 2007; Müller et al., 2011). The large number of elements also increases the potential for peak overlaps. In order to discriminate peaks when they overlap, the isotopic ratio is generally used to deconvolve those particular peaks, resulting in a more accurate composition measurement (Kunicki et al., 2006). However, this method cannot be used with radiogenic ele- ments such as Pb or U in zircons, which limits the isotopes
overlap. Only 174Hf16O at 190 Da, 174Hf17O at 191Da and 180Hf18O at 198Da have no overlaps. However, geologists use 176Hf/177Hf ratio (Scherer et al., 2001), so it is impossible to use the Lutetium–Hafnium clock with APT analysis. The
only Pb isotopes detected with no interference are 206Pb2+ and 207Pb2+ at 103 and 103.5 Da, respectively, whereas 208Pb2+ overlaps with 104Si2O3
+ at 104Da (Valley
et al., 2014, 2015). 238U and 235U decay to 206Pb and 207Pb, respectively, allowing geologists to use the 206Pb/207Pb ratios to provide accurate dating of zircons. In the case where both Pb isotope counts can be measured with good confidence, their ratio can be used to date an APT sample.
Key Trace Element Detection and Quantification Limits The minimum detection limit of a specific element mainly depends on the position of the peak in the mass spectrum and its associated background or noise level. The detection limit is also function of the number of ions collected to achieve sampling statistics, and whether the element is homogenously distributed within the material. Overlapping peaks are also an important factor, as it is not always possible to deconvolve them. Generally, where conditions are optimal, the APT detection limit is ~10ppma (Gault et al., 2012). Figure 5 contains the mass spectra from 102 to 105Da
and 126 to 136Da for sample DN5, containing a Pb-rich precipitate and the Krossey sample, containing Al-rich dislocations.
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
