biotite (FM7025) with the following results.
Calculate dates for each fraction and plot them versus the
number of the heating step. Estimate the age of this biotite and
outline a history for it consistent with the spectrum of dates.
I am having trouble with this problem. I am having trouble
finding the age of biotite. Please be as descriptive as possible
and if you can explain the steps and why they are used. Thank
you.
- The Same Authors Analyzed Several Argon Fractions From Another Biotite Fm7025 With The Following Results Calculate Da 1 (84.04 KiB) Viewed 26 times
- The Same Authors Analyzed Several Argon Fractions From Another Biotite Fm7025 With The Following Results Calculate Da 2 (472.03 KiB) Viewed 26 times
112 Chapter 7 / THE Ar/>Ar METHOD OF DATING REFERENCES 1 5 Podosek and Huneke (1973) measured the fol lowing Arm "Ar ratios in argon fractions released at increasing temperatures from the achondrite me- teorite Pasamonte. isochron is the Ar/Ar ratio of argon that is not associated with potassium. In cases where this contaminant argon is not of at- mospheric origin, the initial "Ar/6Ar ratio should be used to determine the radiogenic Ar component in the sample. The versatility of the "Ar/Ar method of dating has been enhanced by the use of lasers to release Ar from individual mineral grains and by demonstrations that low-grade met- amorphic rocks and sulfide minerals may be dateable under favorable circumstances. In addition, the data collected routinely during stepwise outgassing of minerals can be used to derive apparent diffusion coefficients and activation energies from which the temper- ature at the time recorded by the plateau date can be calculated. This extension of the Ar/ 29 Ar method of dating enables us to construct cooling curves for rocks that contain several K-bearing minerals with different closure temperatures Their flux monitor had an age of 1.062 x 10 years and its Ar*/**Ar ratio was 14.01. Calculate a date for each fraction of argon released and plot them versus the extraction temperature. Observe the re sulting spectrum and estimate the age of this me- teorite as indicated by these data. Extraction Temperature 545 630 745 850 935 1040 1195 1515 Art/Ar 106.0 1280 141.2 141.5 146.9 145.8 168,6 183.8 3 A specimen of basalt (15555, 33) from Hadley Rille on the Moon (Apollo 15 was analyzed by Hu- sain et al. (1972) with the following results: Problems Use the constants listed in the Problems section of Chapter 6. 1 The Ar*/*Ar ratio of a fraction of argon re- 1 leased from a biotite (FMX-25, Bryhni et al., 1971) is 12.31. The value is 1.925 x 10- Calculate the corresponding "Ar*/***Ar date. Answer: 383.7 Ma.) 2 The same authors analyzed several argon frac- tions from another biotite (FM7025) with the fol- lowing results: Cumulative *Ar, 3 *Ar*/Ar 58.14 61.34 72.77 80.15 83.32 79.80 10 27 61 79 100 Heating Step 1 2 3 40Art/Ar 2.27 4.97 6.68 9.58 10.25 10.10 10.26 References Adams, C. J. D., C. T. Harper, and M. G. Laird (1975) K-Ar ages of low grade melasediments of the Greenland and Waiuta Groups in Westland and Buller, New Zealand New Zealand J. Geol. Geophys., 18, 39-48 Albarede, F. (1978) The recovery of spatial isotope distributions from stepwise degassing data, Earth Planet. Sci. Letters, 39, 387-397 Ashkinadze, G. Sh., B. M. Gorokhovskiy, and Yu A. Shukolyukov (1978) Ar/Ar dating of bio- tite containing excess "Ar. Geochemistry Inter- national, 1977, 172-176. Berger, G. W., and D. York (1981) Geothermometry from Ar/Ar dating experiments. Geochim. Cosmochim. Acta, 45, 795-811. Bogard, D.D., L. Husain, and L. E. Nyquist (1979) "Ar-"Ar age of the Shergotty achondrite and implications for its post-shock thermal his- tory. Geochim. Cosmochim. Acta, 43, 1047-1055. Bogard, D.D., and W. C. Hirsch (1980) "Ar/Ar dating, Ar diffusion properties, and cooling rate determinations of severely shocked chondrites. Geochem. Cosmochim. Acta, 44, 1667-1682 Brereton, N. R. (1970) Corrections for interfering isotopes in the Ar/Ar dating method. Earth Planet. Sci. Letters, 8, 427-433 Brereton, N. R. (1972) A reappraisal of the Ar Ar” stepwise degassing technique. Geophys. J. Roy Astron. Soc., 27, 449-478. Brereton, N. R., P. J. Hooker, and J. A. Miller (1976) Some conventional potassium-argon and "Ar/Ar age studies of glauconite. Geol. Mag., 113, 329-340. Bryhni, L, EJ. Fitch, and J. A. Miller (1971) Ary Ar" dates from recycled Precambrian rocks in the Gneiss region of the Norwegian Caledonides. Norsk Geologisk Tidsskrift, 51, 391-406. Claesson, S., and J. C. Roddick (1983) Ar/Ar data on the age and metamorphism of the Ottfjal. let dolerites, Sarv Nappe, Swedish Caledonides. Lithos, 16, 61-73 Czamanske, G. K., M. A. Lanphere, R. C. Erd, and M. C. Blake, Jr. (1978) Age measurements of potassium-bearing minerals by the "Ar/Ar technique. Earth Planet. Sci. Letters, 40, 107- 110. 4 5 10 15 Calculate "Ar*/"Ar dates for each fraction (J = 9.83 x 10-) and plot them versus the percent of "Ar released. What is the age of this basalt? (Answer: 3.28 x 10 y.) Calculate dates for each fraction (J = 2.90 x 102) and plot them versus the number of the heating step. Estimate the age of this biotite and outline a history for it consistent with the spectrum of dates. Answer: about 489 Ma.. 4 The monitor irradiated by Turner et al. (1971) was a hornblende whose ago is 1.062 x 10 years based on a conventional K-Ar date. Its Ar*/***Ar ratio after irradiation was 29.33. Calculate the value of J. (Answer: 2.580 X 10)