61:. Ar/Ar dating relies on neutron irradiation from a nuclear reactor to convert a stable form of potassium (K) into the radioactive Ar. As long as a standard of known age is co-irradiated with unknown samples, it is possible to use a single measurement of argon isotopes to calculate the K/Ar* ratio, and thus to calculate the age of the unknown sample. Ar* refers to the
174:
The Ar/Ar method only measures relative dates. In order for an age to be calculated by the Ar/Ar technique, the J parameter must be determined by irradiating the unknown sample along with a sample of known age for a standard. Because this (primary) standard ultimately cannot be determined by Ar/Ar,
215:
has a closure temperature of ~550°C. Thus, a granite containing all three minerals will record three different "ages" of emplacement as it cools down through these closure temperatures. Thus, although a crystallization age is not recorded, the information is still useful in constructing the thermal
82:
via a laser or resistance furnace. Heating causes the crystal structure of the mineral (or minerals) to degrade, and, as the sample melts, trapped gases are released. The gas may include atmospheric gases, such as carbon dioxide, water, nitrogen, and radiogenic gases like argon and helium, generated
235:
This technique allows the errors involved in K-Ar dating to be checked. Argon–argon dating has the advantage of not requiring determinations of potassium. Modern methods of analysis allow individual regions of crystals to be investigated. This method is important as it allows crystals forming and
65:
Ar, i.e. the Ar produced from radioactive decay of K. Ar* does not include atmospheric argon adsorbed to the surface or inherited through diffusion and its calculated value is derived from measuring the Ar (which is assumed to be of atmospheric origin) and assuming that Ar is found in a constant
227:
they cooled down below the closure temperature, and this may not represent all of the events which the rock has undergone, and may not match the age of intrusion. Thus, discretion and interpretation of age dating is essential. Ar/Ar geochronology assumes that a rock retains all of its Ar after
161:
of K (approximately 5.5 x 10 year, corresponding to a half-life of approximately 1.25 billion years), J is the J-factor (parameter associated with the irradiation process), and R is the Ar*/Ar ratio. The J factor relates to the
83:
from regular radioactive decay over geologic time. The abundance of Ar* increases with the age of the sample, though the rate of increase decays exponentially with the half-life of K, which is 1.248 billion years.
152:
244:
One problem with argon-argon dating has been a slight discrepancy with other methods of dating. Work by Kuiper et al. reports that a correction of 0.65% is needed. Thus the
74:
The sample is generally crushed and single crystals of a mineral or fragments of rock are hand-selected for analysis. These are then irradiated to produce Ar from K via the
199:. However, in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral. Dating of movement on
462:
245:
166:
of the neutron bombardment during the irradiation process; a denser flow of neutron particles will convert more atoms of K to Ar than a less dense one.
385:
Kuiper, K. F.; Deino, A.; Hilgen, F. J.; Krijgsman, W.; Renne, P. R.; Wijbrans, J. R. (2008). "Synchronizing Rock Clocks of Earth
History".
191:
The primary use for Ar/Ar geochronology is dating metamorphic and igneous minerals. Ar/Ar is unlikely to provide the age of intrusions of
283:
97:
248:(when the dinosaurs died out)—previously dated at 65.0 or 65.5 million years ago—is more accurately dated to 66.0-66.1 Ma.
263:
175:
it must be first determined by another dating method. The method most commonly used to date the primary standard is the
310:"Ar/Ar ages of tephras intercalated in astronomically tuned Neogene sedimentary sequences in the eastern Mediterranean"
57:
measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of
492:
449:
Precise dating of the destruction of
Pompeii proves argon-argon method can reliably date rocks as young as 2,000 years
459:
176:
46:
203:
systems is also possible with the Ar/Ar method. Different minerals have different closure temperatures;
394:
324:
448:
196:
223:
provide age information on a rock, but assumptions must be made. Minerals usually only record the
418:
367:
42:
179:. An alternative method of calibrating the used standard is astronomical tuning (also known as
410:
79:
58:
402:
359:
332:
328:
257:
472:
466:
309:
398:
497:
200:
180:
158:
75:
287:
486:
371:
422:
363:
17:
444:
454:
336:
49:
in accuracy. The older method required splitting samples into two for separate
212:
62:
439:
406:
208:
50:
414:
195:
as the age typically reflects the time when a mineral cooled through its
204:
192:
163:
308:
Kuiper, K. F.; Hilgen, F. J.; Steenbrink, J.; Wijbrans, J. R. (2004).
284:"New Mexico Geochronology Research Laboratory: K/Ar and Ar/Ar Methods"
286:. New Mexico Bureau of Geology and Mineral Resources. Archived from
54:
440:
477:
469:
of the
Scottish Universities Environmental Research Council
147:{\displaystyle t={\frac {1}{\lambda }}\ln(J\times R+1)}
78:
K(n,p)Ar. The sample is then degassed in a high-vacuum
350:
Renne, P. R. (1998). "Absolute Ages Aren't
Exactly".
100:
232:
and that this was properly sampled during analysis.
236:cooling during different events to be identified.
146:
91:The age of a sample is given by the age equation:
478:Argon Laboratory / Australian National University
473:Open University Ar/Ar and Noble Gas Laboratory
183:), which arrives at a slightly different age.
8:
455:New Mexico Geochronology Research Laboratory
107:
99:
275:
7:
317:Earth and Planetary Science Letters
66:ratio to Ar in atmospheric gases.
25:
246:Cretaceous–Paleogene extinction
141:
123:
1:
364:10.1126/science.282.5395.1840
264:Berkeley Geochronology Center
47:potassium–argon (K/Ar) dating
45:method invented to supersede
260:, inventor of the technique
177:conventional K/Ar technique
157:where λ is the radioactive
514:
337:10.1016/j.epsl.2004.03.005
27:Radiometric dating method
407:10.1126/science.1154339
329:2004E&PSL.222..583K
460:Argon Isotope Facility
148:
216:history of the rock.
149:
170:Relative dating only
98:
399:2008Sci...320..500K
358:(5395): 1840–1841.
230:closing temperature
211:is about 400°C and
197:closure temperature
493:Radiometric dating
465:2010-05-10 at the
144:
43:radiometric dating
18:Argon-argon dating
393:(5875): 500–504.
228:cooling past the
115:
80:mass spectrometer
16:(Redirected from
505:
447:press release: "
427:
426:
382:
376:
375:
347:
341:
340:
314:
305:
299:
298:
296:
295:
280:
258:Grenville Turner
219:Dating minerals
153:
151:
150:
145:
116:
108:
21:
513:
512:
508:
507:
506:
504:
503:
502:
483:
482:
467:Wayback Machine
436:
431:
430:
384:
383:
379:
349:
348:
344:
312:
307:
306:
302:
293:
291:
282:
281:
277:
272:
254:
242:
189:
172:
96:
95:
89:
72:
28:
23:
22:
15:
12:
11:
5:
511:
509:
501:
500:
495:
485:
484:
481:
480:
475:
470:
457:
452:
442:
435:
434:External links
432:
429:
428:
377:
342:
323:(2): 583–597.
300:
274:
273:
271:
268:
267:
266:
261:
253:
250:
241:
238:
188:
185:
181:orbital tuning
171:
168:
159:decay constant
155:
154:
143:
140:
137:
134:
131:
128:
125:
122:
119:
114:
111:
106:
103:
88:
85:
76:(n-p) reaction
71:
68:
59:argon isotopes
26:
24:
14:
13:
10:
9:
6:
4:
3:
2:
510:
499:
496:
494:
491:
490:
488:
479:
476:
474:
471:
468:
464:
461:
458:
456:
453:
450:
446:
443:
441:
438:
437:
433:
424:
420:
416:
412:
408:
404:
400:
396:
392:
388:
381:
378:
373:
369:
365:
361:
357:
353:
346:
343:
338:
334:
330:
326:
322:
318:
311:
304:
301:
290:on 2017-08-03
289:
285:
279:
276:
269:
265:
262:
259:
256:
255:
251:
249:
247:
240:Recalibration
239:
237:
233:
231:
226:
222:
217:
214:
210:
206:
202:
198:
194:
186:
184:
182:
178:
169:
167:
165:
160:
138:
135:
132:
129:
126:
120:
117:
112:
109:
104:
101:
94:
93:
92:
86:
84:
81:
77:
69:
67:
64:
60:
56:
52:
48:
44:
40:
36:
32:
19:
390:
386:
380:
355:
351:
345:
320:
316:
303:
292:. Retrieved
288:the original
278:
243:
234:
229:
224:
220:
218:
190:
187:Applications
173:
156:
90:
87:Age equation
73:
38:
34:
30:
29:
445:UC Berkeley
207:is ~300°C,
31:Argon–argon
487:Categories
294:2008-09-16
270:References
213:hornblende
63:radiogenic
372:129857264
225:last time
209:muscovite
130:×
121:
113:λ
51:potassium
463:Archived
423:11959349
415:18436783
252:See also
395:Bibcode
387:Science
352:Science
325:Bibcode
205:biotite
193:granite
164:fluence
421:
413:
370:
70:Method
39:dating
498:Argon
419:S2CID
368:S2CID
313:(PDF)
201:fault
55:argon
41:is a
35:Ar/Ar
411:PMID
53:and
33:(or
403:doi
391:320
360:doi
356:282
333:doi
321:222
221:may
489::
417:.
409:.
401:.
389:.
366:.
354:.
331:.
319:.
315:.
118:ln
37:)
451:"
425:.
405::
397::
374:.
362::
339:.
335::
327::
297:.
142:)
139:1
136:+
133:R
127:J
124:(
110:1
105:=
102:t
20:)
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.