8 Laser Microprobe Argon-39-Argon-40 Dating of Individual Mineral Grains O. A . SCHAEFFER
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State University of New York—Stony Brook, Department of Earth and Space Sciences, Stony Brook, N Y 11794
The use of a ruby laser to obtain K-Ar ages by releasing the argon from selected sites, 50 μm in diameter, representing about 0.2 pg of a mineral, is described. The ages so obtained when used in conjunc tion with a conventional Ar- Ar thermal release study yield chronologically significant ages, often in cases where the Ar- Ar thermal release study is disturbed. The method is illustrated by examples of studies of meteorites, lunar rocks, and terrestrial rocks. 39
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The K-Ar isotopic system is useful in dating both terrestrial and extraterrestrial samples. The strength of the method lies in the widespread occurrence of Κ coupled with the fact that the daughter, A r , is a rare gas and quite often absent in most mineral systems. Because of the ability to detect extremely minute amounts of argon, coupled with the long half-life of K , the method gives a wide age range of applicability from thousands to billions of years. The K-Ar system has two problems: 1) the diffusive loss of the daughter, A r , during metamorphic events, and 2) the presence of extraneous A r not from the in situ decay of K , especially in the case of low-Κ minerals. The application of the A r - A r thermal release method has gone far to eliminate these difficulties. In this method, the sample is irradiated by fast neutrons which produce A r from K . The age determination then reduces to an isotope ratio measurement of the daughter to the parent: A r to K , viz. A r [I] . By releasing the argon in temperature steps, and by determining the A r / A r ratio for each temperature step, it is possible to 40
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Figures in brackets indicate the literature references at the end of this paper.
0097-6156/82/0176-0139$05.00/0 © 1982 American Chemical Society Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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o b t a i n a s o - c a l l e d p l a t e a u age. That i s , a t the lower tempera t u r e s , the argon i s r e l e a s e d from mineral s i t e s which have s u f f e r e d d i f f u s i v e argon l o s s and show younger ages. A f t e r a high enough temperature i s reached, the argon i s r e l e a s e d only from mineral s i t e s which have not s u f f e r e d d i f f u s i v e argon l o s s e s , and f o r subsequent temperatures the A r / A r r a t i o remains constant and i s a measure of the age of the sample. In c e r t a i n i n s t a n c e s , extraneous argon i s r e l e a s e d a t the lower temperatures. This i s e s p e c i a l l y the case f o r implanted argon i n lunar rocks or atmo s p h e r i c argon i n t e r r e s t r i a l samples. In other cases, however, the temperature r e l e a s e p a t t e r n of A r / A r values i s not a simple i n c r e a s e t o a steady value but may show a drop a t high temperatures, show a saddle shape w i t h intermediate temperature r e l e a s e s g i v i n g lower ages, or g i v e a more or l e s s v a r i a b l e age w i t h temperature r e l e a s e . Samples which have s u f f e r e d a metamorphic event or events o f t e n do not develop a w e l l d e f i n e d p l a t e a u age. Non-ideal behavior i s more evident i n t e r r e s t r i a l samples than l u n a r or meteorite samples; however, understanding of non-ideal behavior i s a l s o important f o r the e a r l y chronology of events on the l u n a r s u r f a c e . In some cases, the complicated r e l e a s e p a t t e r n i s due t o the e x i s t e n c e of minerals whose ages have been a l t e r e d but which a l s o r e l e a s e argon a t a l l tempera tures. In many cases, i t i s not p o s s i b l e t o make a mineral separation. The r e l e a s e of argon by a focused l a s e r beam can be of great a s s i s t a n c e i n i n t e r p r e t i n g the complicated r e l e a s e patterns. 4 0
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C H E M I C A L DATING T E C H N I Q U E S
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Laser Probe Mass Spectrometry A focused l a s e r beam of 0.2 j o u l e s per pulse i s capable of r e l e a s i n g r a r e gases from w e l l d e f i n e d 10-100 pm s i z e spots on a p o l i s h e d s u r f a c e . As a r e s u l t , i t i s p o s s i b l e t o extend r a r e gas mass spectrometry t o the r e g i o n of l e s s than 1 pg samples. The technique was f i r s t a p p l i e d t o the study of complex l u n a r samples by George Megrue [ 2 ] . I f the samples are i r r a d i a t e d by f a s t neutrons, A r i s pro duced from Κ by the r e a c t i o n : K ( n , p) A r . The Κ can be determined by neutron a c t i v a t i o n on the i d e n t i c a l sample f o r which the A r i s determined, r e s u l t i n g i n a K-Ar age. The p r i n c i p a l advantages of the l a s e r r e l e a s e over the thermal Ar- Ar age determination are: 1) The a b i l i t y t o p r e c i s e l y d e f i n e the minerals s t u d i e d , i . e . , i t i s p o s s i b l e t o a v o i d i n c l u d i n g m a t e r i a l from g r a i n boundaries or microscopic i n c l u s i o n s w i t h i n minerals g r a i n s . 2) The s i z e of the sample being i n v e s t i g a t e d i s approximately two orders of magnitude s m a l l e r than t h a t p o s s i b l e f o r a thermal r e l e a s e study. The lower sample s i z e i s t o a l a r g e extent p o s s i b l e because of the much s m a l l e r values f o r the blanks. In the case of the thermal r e l e a s e , the argon blank due t o the heated m a t e r i a l s surrounding the sample i s higher than the blank argon a s s o c i a t e d w i t h a l a s e r 3 9
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Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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study. The l a t t e r argon i s only due t o the mass spectrometer r e s i d u a l s t a t i c vacuum. For a w e l l baked system t h i s can be 4.11 G.y.) i s a t the centers of the l a r g e s t g r a i n s , m a t e r i a l showing the youngest age (3.81 - 3.88 G.y.) i s near g r a i n margins, and m a t e r i a l showing intermediate age (3.99 - 4.05 G.y.) i s i n intermediate zones i n the l a r g e s t g r a i n s and a t the centers of i n t e r m e d i a t e - s i z e d g r a i n s . These r e s u l t s are confirmed by previous s t u d i e s of another c l a s t of s i m i l a r a n o r t h o s i t i c gabbro from 73215. The age p a t t e r n i s i n t e r p r e t e d as the r e s u l t of p a r t i a l outgassing of the c l a s t s when they were incorporated i n the 73215 b r e c c i a . The combined data f o r the two a n o r t h o s i t i c gabbros s e t a lower l i m i t of 4.26 G.y. on the date of an episode of high-temperature m e l t i n g / r e c r y s t a l l i z a t i o n t h a t a f f e c t e d the parent rocks of the c l a s t s . Rb-Sr data (Compston e t a l . , 1977) provide an upper l i m i t of 4.45 G.y. on the date of t h i s high-temperature event. The l a s e r A r - A r r e s u l t s f o r a b l a c k aphanite c l a s t from 73215 demonstrate t h a t t h i s rock i s cogenetic w i t h the aphanite t h a t forms the matrix of the b r e c c i a . Ages determined f o r f e l s i c g l a s s fragments i n the two types of aphanite, and f o r groundmass i n 3 9
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Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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the black aphanite, are identical within error with each other and with the data of the 73215 breccia-forming event, ^3.87 G.y. The time of crystallization of a lunar "granite" has also been determined by the laser method. This "granite" is the K- and S i rich felsite that forms clasts in 73215, 73255, and Boulder 1 at Station 2. The laser results set a lower limit of 4.00 G.y. on crystallization of the parent body of this felsite, and the Rb-Sr data set an upper limit of * 4.05 G.y. [6].
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Meteorite Ages The application of the laser probe to meteorite chronology is illustrated by a study of Ca-Al-rich inclusions from the Allende meteorite [7]. This study was able to show that the K i n the inclu sions studied mainly concentrated in veins and rims with very l i t t l e , i f any, Κ in the major minerals. The limit obtained i s something of the order of 10 ppm. On the other hand, the major minerals do contain appreciable Ar. Individual chondrules and the matrix were also studied in the Allende meteorite from places adjacent to the Ca-Al-rich inclusions. For these samples the ages varied from 3.3 to 4.4 G.y. There appears to be evidence that the Allende meteorite has been subjected to numerous metamorphic events, presumably of a collisional origin. 40
Terrestrial Ages We have shown that for terrestrial samples with ideal pla teaus i t is possible by preheating the samples to obtain reliable and precise laser ages [8]. The study was made on granites and blue schists from the Alpine area for samples in the age 40 - 2000 M.y. For a Precambrian granite from the Ivory Coast, under the microscope, a polished section i s seen to contain two kinds of biotites: dark biotites with an average size of 50 pm and light coarse biotites with a maximum length of 200 pm. For the analysis by A r - A r stepwise heating, these two kinds of biotites were not separated due to a very similar density and the same magnetic susceptibility. The plateau age, resulting for the mixture of these two components was 1850 M.y. ± 80 M.y. The laser results show clearly two different ages: the dark biotites give ages of 2025 to 2100 ± 100 M.y. and the light ones 1550 and 1600 ± 100 M.y. The difference can be understood as related to a late thermal resetting event affecting only one kind of the biotites. 40
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References 39
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[1] Turner, G., Thermal histories of meteorities by the A r - A r method. In: Meteorite Research, ed. P. M. Millman, SpringerVerlag, New York, 1969, p. 407-417.
Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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[2] Megrue, G. Η., Distribution and Origin of Helium, Neon, and Argon Isotopes in Apollo 12 Samples by In Situ Analysis with a Laser-Probe Mass Spectrometer, J. Geophys. Res. 76, 4956 (1971). [3] Hartung, J. Β., Plieninger, T., Müller, H. W., and Schaeffer, O. Α., Helium, Neon, and Argon on Sunlit and Shaded Surfaces of Lunar Rock 12054, Proc. Lunar Sci. Conf. 8th, 1977, p. 865. [4] Schaeffer, O. Α., Müller, H. W., and Grove, T. L., Laser Ar- Ar Study of Apollo 17 Basalts, Proc. Lunar Sci. Conf. 8th, 1977, p. 1489. [5] Eichhorn, G., James, O. Β., Schaeffer, O. Α., and Müller, H. W., Laser Ar- Ar Dating of Two Clasts From Consortium Breccia 73215, Proc. Lunar Planet. Conf. 9th, 1978, p. 855. [6] Compston, W., Foster, J. J., and Gray, C. Μ., Rb-Sr Syste matics in Clasts and Aphanites from Consortium Breccia 73215, Proc. Lunar Sci. Conf. 8th, 1977, p. 2525. [7] Herzog, G. F., Bence, Α. Ε., Bender, J., Eichhorn, G., Maluski, Η., and Schaeffer, O. Α., Ar/ Ar Systematics of Allende Inclusions, Proc. Lunar Planet, Sci. Conf. 11th, 1980, p. 959. [8] Maluski, M. and Schaeffer, O. Α., manuscript in preparation. 39
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