Nuclear and Chemical Dating Techniques - ACS Publications

6

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 28, 2018 | https://pubs.acs.org Publication Date: January 29, 1982 | doi: 10.1021/bk-1982-0176.ch006

Ion Probe Magnesium Isotopic Measurements of Allende Inclusions IAN D. HUTCHEON Enrico Fermi Institute, University of Chicago, Chicago, IL 60637

The Mg isotopic compositions of eight Ca-Al-rich inclusions from the Allende meteorite have been measured with an ion microprobe. The microscopic spatial resolu tion of the ion probe and i t s ability to analyze isotopically femtogram quantities of Mg permitted exceptionally detailed studies of the distribution of Mg on a 10 26

μm s c a l e and r e v e a l e d l a r g e d i f f e r e n c e s i n Mg i s o t o p i c composition among p e t r o g r a p h i c a l l y d i s t i n c t Allende inclusions. Large M g excesses o f up t o 15 percent 26

found in anorthites from Bl inclusions correlate strictly with Al/ Mg ratios in anorthite and spinel and provide additional evidence for the i n situ decay of A1. The observed ( A1/ A1) = 4.6 x 1 0 i s 27

24

26

26

27

-5

0

consistent with other measurements. Ion probe measure ments establish the f i r s t single crystal Al-Mg isochron and show that a l l anorthites had i n i t i a l l y homogeneous Al isotopic ratios despite zoning of Mg and extensive mineralogical alteration. Anorthites from B2 inclusions also contain large Mg excesses but exhibit a heterogeneous isotopic pattern with no unique correlation between δ Mg and Al/Mg ratios. ( A1/ A1) ratios in B2 inclusions range from 4.6 χ 10 " to 3000) over t h e mass i n t e r v a l 28 t o 22 t o check f o r p o s s i b l e i n t e r f e r i n g s p e c i e s . The only i n t e r f e r e n c e detected a t a l e v e l g r e a t e r than 2

2

7

2 5

+

4 8

0.1 percent o f t h e M g s i g n a l i n a n o r t h i t e was C a a t mass 24. Figure 3 shows a high r e s o l u t i o n scan o f t h e Mg isotopes. I s o t o p i c data were c o l l e c t e d a t a mass r e s o l u t i o n o f * 300 f o r which

4 8

Ca

+

calculated

2 4

and M g

+

by m o n i t o r i n g 3

4 8

a r e not r e s o l v e d ; t h e C a 4 0

Ca

+ +

c o r r e c t i o n was

a t mass 20, using 2 4

4 8

4 0

Ca/ Ca =

1.9078 χ 1 0 ~ [ 1 8 ] . The maximum c o r r e c t i o n t o the M g was ^ 3 . 3 percent.

+

intensity

Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Ion Probe Mg Isotopic Measurements


HUTCHEON

Figure 1. SEM micrograph of a hole excavated in the surface of an anorthite crystal by the primary ion beam during approximately 4 h of isotopic analysis. The scale bar is 5 μ/η in length.


100

N U C L E A R A N D C H E M I C A L DATING

TECHNIQUES

Figure 2. Schematic drawing of the A EI IM-20 ion microprobe. The magnetic analyzer and ion counting system are linked to an on-line computer for automated analysis. 14300 c/s 1 intensi ty (c/s)

1

600

-

24

Mg

pi -

400

c condor]


10 - 25 Kev Ion Beam

-

-

200

a> CO

48

0

J u. 26

Ca

++

1 25

24

Moss

Figure 3. High-resolution (Μ/ΔΜ ~ 3500) scan of the Mg mass spectrum for a terrestrial anorthite. The scan is not continuous and shows only the region around each Mg isotope. The absence of any interferences other than Ca + at mass 24 and the 0.1 count/s background between peaks are evident. 48

2



6.

HUTCHEON

Ion

Probe Mg

Isotopic Measurements

101

A n a l y s i s of t e r r e s t r i a l samples demonstrated t h a t the t u n i n g of the i o n e x t r a c t i o n system, and consequently the amount of i n s t r u mental mass f r a c t i o n a t i o n , was very s e n s i t i v e to charge b u i l d - u p on the sample, the p o s i t i o n of the primary beam r e l a t i v e t o the spectrometer o p t i c a x i s , and the p o s i t i o n of the sample r e l a t i v e t o the e x t r a c t i o n l e n s . To optimize r e p r o d u c i b l e t u n i n g of the e x t r a c t i o n system from sample to sample, we developed the f o l l o w i n g criteria: (1) r e s i s t a n c e of the sample (Au-coated) to ground l e s s than ΙΟ Ω; (2) alignment of the primary beam to w i t h i n 10 pm using 6

2 4

+

an o p t i c a l microscope; (3) maximum M g secondary i o n i n t e n s i t y ; (4) f l a t - t o p p e d peaks (±1%) a t a mass r e s o l u t i o n of * 300; (5) uniform t u n i n g (no changes i n lens v o l t a g e s t o maintain maximum i n t e n s i t y ) over the mass range 48 to 20. By f o l l o w i n g t h i s procedure i t was p o s s i b l e to c o n t r o l the magnitude of instrumental mass f r a c t i o n a t i o n by a d j u s t i n g only the d e f l e c t i o n v o l t a g e s to maintain maximum s i g n a l voltages ( l e n s voltages constant) when moving from one sample t o another. The o p e r a t i n g c o n d i t i o n s remained r e l a t i v e l y unchanged throughout the m a j o r i t y of the analyses discussed below. A l l of the samples analyzed were standard one-inch diameter p o l i s h e d t h i n s e c t i o n s . Whenever f e a s i b l e the samples r e c e i v e d a f i n a l , c l e a n s i n g p o l i s h w i t h 1 pm diamond compound made from commercial graded diamonds embedded i n " v a s e l i n e " . Commercial diamond paste has proved u n s a t i s f a c t o r y due to high l e v e l s of K, Na, C l , S i , F, and Ca. Samples are then cleaned w i t h carbon t e t r a c h l o r i d e , r i n s e d i n e t h a n o l , and coated w i t h * 100 A of gold i n a vacuum evaporator. T h i s sample p r e p a r a t i o n technique was developed during our s t u d i e s of minor elements [16,17] and has proved t o produce c o n s i s t e n t l y contamination-free samples. M e t e o r i t i c Samples I n i t i a l l y , our i o n probe study concentrated on Type Β i n c l u sions from the A l l e n d e m e t e o r i t e , s i n c e previous s t u d i e s [6] had shown t h a t Mg i s o t o p i c e f f e c t s were l a r g e s t i n a n o r t h i t e c r y s t a l s from these i n c l u s i o n s . As d i s c u s s e d e x t e n s i v e l y by Grossman [ 1 , 1 9 ] , A l l e n d e Type Β i n c l u s i o n s are s u b s p h e r i c a l , centimeters i z e d o b j e c t s composed predominantly of coarse-grained T i - A l pyroxene, s p i n e l , m e l i l i t e , and a n o r t h i t e . C h a r a c t e r i s t i c of many Type Β i n c l u s i o n s i s a t h i c k (y 1 mm wide) outer mantle of g e h l e n i t i c m e l i l i t e surrounding the 4-phase core (see, e.g., f i g u r e 4 and [ 2 0 ] ) . Wark and Lovering [21] subdivided Type Β i n c l u s i o n s i n t o Bl and B2 based on the presence or absence of t h i s m e l i l i t e mantle. We examined s i x A l l e n d e Type Β i n c l u s i o n s , of which f o u r are B l ' s (TS-23, TS-33, TS-34, and Al 3529G) and two are B2's (TS-8 and TS-21). (The nomenclature used i s given i n [19]. I n c l u s i o n s are referenced by t h i n s e c t i o n number except Al 3529G and Al 3529-45, which were provided by Mason [22].) Anor t h i t e c r y s t a l s are l a r g e (up t o 500 pm i n length) and r e l a t i v e l y


102

TECHNIQUES



Figure 4.

Photomicrograph of TS-23, a coarse-grained Type Bl inclusion from Allende. The outer 1.5 mm is a mantle of polycrystalline melilite whose Mg content increases radially inwards, as does the abundance of included spinel. The interior of the inclusion is predominantly coarsely crystalline Ti-Al-pyroxene, melilite, and anorthite, all containing euhedral crystals of spinel. The entire inclusion is bounded by a fine-grained rim of complex mineralogy. The inclusion is ~ 1.5 cm in diameter.



6.


HUTCHEON

103

abundant (5 t o 25% modal abundance) i n Type Β i n c l u s i o n s but c h a r a c t e r i s t i c a l l y a r e intergrown w i t h high-Mg phases making s e p a r a t i o n o f high p u r i t y , low-Mg phases f o r i s o t o p i c a n a l y s i s by conventional techniques d i f f i c u l t . Type Β a n o r t h i t e s a l s o e x h i b i t l a r g e c o r r e l a t e d v a r i a t i o n s o f t h e i r Mg and Na contents (y 100 t o > 1000 ppmw Mg [12]) such t h a t t h e i s o t o p i c a n a l y s i s o f even a s i n g l e c r y s t a l [23] averages over several d i s t i n c t compositional zones (see f o l l o w i n g d i s c u s s i o n ) . The oxygen i s o t o p i c composi t i o n s o f i n c l u s i o n s TS-8, TS-23, and TS-34, measured by Clayton e t a l . [2 and 4 2 ] , e x h i b i t 0 excesses t y p i c a l o f Allende coarse grained i n c l u s i o n s . We a l s o analyzed one Allende Type A i n c l u s i o n , an i r r e g u l a r l y shaped o b j e c t c o n s i s t i n g predominantly o f m e l i l i t e (Ikermanite content 0 t o 33%) and s p i n e l . The i n c l u s i o n s t u d i e d , A l 3529-45, was p a r t i c u l a r l y chosen because i t a l s o contains hibonite ( C a A l 0 ) , a h i g h l y r e f r a c t o r y m i n e r a l , which on thermodynamic grounds, i s expected t o be one o f the f i r s t major-element bearing phases t o condense i n the s o l a r nebula [ 1 ] . The f i n a l i n c l u s i o n examined i n t h i s study was A l 3510, a small (y 3 mm diameter) c i r c u l a r o b j e c t w i t h an i g n e o u s - l i k e t e x t u r e composed predomi n a n t l y o f intergrown l a t h s o f a n o r t h i t e (An 95) and o l i v i n e (Fo 88). R e l a t i v e t o most Allende coarse-grained i n c l u s i o n s , A l 3510 i s enriched i n Na, Mg, S i , and Fe and depleted i n the r e f r a c t o r y elements A l , Ca, and T i . A l l o f the Type A and Β i n c l u s i o n s s t u d i e d are surrounded by a layered r i m sequence o f complex mineralogy [21] which c l e a r l y d e f i n e s the i n c l u s i o n - m a t r i x boundary. Secondary a l t e r a t i o n phases ( g r o s s u l a r and nepheline, e s p e c i a l l y ) are a l s o a common f e a t u r e o f these i n c l u s i o n s , suggesting t h a t vapor phase r e a c t i o n s w i t h a r e l a t i v e l y cool nebula occurred a f t e r formation o f i n c l u s i o n s . A n o r t h i t e , i n p a r t i c u l a r , i s u s u a l l y one o f t h e most h e a v i l y a l t e r e d phases; t h e r e l a t i o n s h i p between Mg i s o t o p i c composition and a l t e r a t i o n i s discussed below. (See [ 1 2 ] f o r s t r i k i n g cathodoluminesce photographs o f t y p i c a l Allende a l t e r a t i o n miner alogy.) I n c l u s i o n A l 3510 does not f i t the normal p a t t e r n as i t has no Wark-rim and does not c o n t a i n the usual a r r a y o f secondary minerals. 1 6

1 2

1 9

T e r r e s t r i a l Samples and S t a n d a r d i z a t i o n Since t h e IM-20 had not p r e v i o u s l y been used f o r high p r e c i s i o n i s o t o p i c measurements and s i n c e previous i s o t o p i c measurements w i t h other ion microprobes [24,25] were c h a r a c t e r i z e d by percent l e v e l r e l a t i v e e r r o r s , we c a r r i e d out an extensive s e r i e s o f Mg i s o t o p i c analyses o f t e r r e s t r i a l samples. T e r r e s t r i a l standards were e s p e c i a l l y emphasized p r i o r t o t h e study o f m e t e o r i t i c sam p l e s , but were a l s o p e r i o d i c a l l y i n t e r s p e r s e d w i t h l a t e r m e t e o r i t i c analyses as a check on the performance o f the ion probe. Terres trial standards i n c l u d e d Ceylon s p i n e l ( M g A l 0 ) , Madagascar h i b o n i t e ( C a A l 0 ) , a s u i t e o f o l i v i n e s (Fo 100 t o Fo 5) used i n 2

1 2

4

1 9


104

N U C L E A R A N D C H E M I C A L DATING T E C H N I Q U E S

a previous i o n probe study [ 2 6 ] , and numerous p l a g i o c l a s e f e l d spars (An 98 t o An 50) whose Mg c o n c e n t r a t i o n s overlapped those o f anorthite from A l l e n d e i n c l u s i o n s . Analyses o f t e r r e s t r i a l plagioclase were p a r t i c u l a r l y important f o r e s t a b l i s h i n g the 4 8

+ +

v a l i d i t y o f the C a c o r r e c t i o n , f o r t e s t i n g the l i m i t s o f p r e c i s i o n o f Mg i s o t o p i c measurements i n samples w i t h low Mg Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 28, 2018 | https://pubs.acs.org Publication Date: January 29, 1982 | doi: 10.1021/bk-1982-0176.ch006

2 7

+

2 4

+

l e v e l s (< 300 ppmw), and f o r c a l i b r a t i n g i o n probe Al / Mg r a t i o s a g a i n s t Al/Mg elemental r a t i o s measured w i t h the e l e c t r o n probe. A unique f e a t u r e o f the i o n microprobe i s the p o t e n t i a l t o measure both elemental c o n c e n t r a t i o n s and i s o t o p i c r a t i o s i n the same spot. This c a p a b i l i t y was p a r t i c u l a r l y v a l u a b l e i n the present study since the p o s s i b l e correlations between the Mg/ Mg and Al/Mg r a t i o s a r e c e n t r a l t o the i n t e r p r e t a t i o n o f Mg i s o t o p i c anomalies. The Al/Mg r a t i o i s c a l c u l a t e d from the 26

27

24

24

A 1 / M g secondary i o n r a t i o measured every twenty scans. The wide range i n s e n s i t i v i t y f o r the d e t e c t i o n o f d i f f e r e n t elements w i t h the i o n probe i s w e l l documented (e.g., [16,27,28]), r e q u i r ing c a l i b r a t i o n o f secondary ion i n t e n s i t i e s a g a i n s t e l e c t r o n probe measurements. Figure 5 shows the data f o r A l and Mg i n p l a g i o c l a s e . The e x c e l l e n t l i n e a r c o r r e l a t i o n i n d i c a t e s t h a t the y i e l d of Mg i s independent both o f Mg content and o f p l a g i o c l a s e mineral chemistry, a t l e a s t over the range An 100 t o An 50. The slope o f the Williamson l e a s t squares f i t [ 2 9 ] , 0.81 ± 0.05, 2 4

+

2 7

+

r e f l e c t s the higher y i e l d o f M g r e l a t i v e t o A 1 . For a more complete d i s c u s s i o n o f q u a n t i t a t i v e elemental a n a l y s i s w i t h the ion probe and i o n p r o b e - e l e c t r o n probe i n t e r c a l i b r a t i o n s see [16,17]. A r e p r e s e n t a t i v e sample o f the i s o t o p i c data from t e r r e s t r i a l standards i s given i n Table 1 and p l o t t e d i n f i g u r e 6 together w i t h data from Mg-rich minerals ( s p i n e l and Ti-pyroxene) from Allende inclusions. Raw i s o t o p i c data are presented, c o r r e c t e d 4 8

+ +

only f o r any C a interference. Figure 6 u t i l i z e s ô-notation which expresses the d e v i a t i o n o f an isotope r a t i o from the standard r a t i o i n parts-per-thousand; i . e . , m

2

( Mg/ *Mg)

0B$

m

ô Mg =

^ χ 1000.

m

2 4

[( Mg/ Mg)

S T D

I t i s c l e a r from f i g u r e 6 t h a t the t e r r e s t r i a l data do not c l u s t e r about a s i n g l e p o i n t but i n s t e a d l i e along a l i n e o f slope * 0.5 on the t h r e e - i s o t o p e diagram, i n d i c a t i n g i s o t o p i c v a r i a t i o n due t o mass-dependent f r a c t i o n a t i o n . Since mass f r a c t i o n a t i o n e f f e c t s i n Mg have not been observed i n t e r r e s t r i a l m a t e r i a l s [30,31], t h i s d i s t r i b u t i o n o f observed i s o t o p e r a t i o s must be due t o f r a c t i o n a t i o n i n the i o n probe. The p h y s i c a l process which produces the



6.


HUTCHEON

1

800

1

'

1

105

1

1

1

L

y „

•

600

Ο CL Ο CP

-

400

CM

CM

200

ι,

__J1

1I

200

400 27

27

Al/ Mg 24

ι

I 1

ι

600

1 I

,

1

800

1LJ 1000

Electron Probe

24

27

2A

Figure 5. Correlation of Al*/ Mg* secondary ion intensity ratios vs. Al/ Mg elemental ratio measured with the electron probe for Allende anorthite (Φ) and terrestrial plagioclase 27

Thefitof the data to the correlation line indicates that the relative yields of Al* and Mg+ secondary ions are constant over the compositional range anorthite 100 to anor thite 50. The slope of the line, 0.81, reflects the higher sensitivity of the ion probe fo Mg, and is used to convert measured secondary ion ratios to elemental ratios.

24


106


Table 1.

Unnormalized Mg Isotope Data from T e r r e s t r i a l Samples and Mg-rich M i n e r a l s from A l l e n d e Sample


TECHNIQUES

25

ô Mg (permi1)

26

6 Mg (permi1)

T e r r e s t r i a l Minerals Olivine

2.1 -4.3 -3.6 -3.5 -5.4

± ± ± ± ±

1.5 2.5 1.5 1.3 1.8

4.8 -8.0 -3.2 -8.0 -10.4

± ± ± ± ±

2.0 3.0 2.2 1.8 2.0

Spinel

3.8 0.2 -2.4 1.9 0.7

± ± ± ± ±

1.2 1.0 1.1 1.3 1.5

6.6 0.8 -4.5 3.5 1.8

± ± ± ± ±

1.9 1.4 1.7 1.3 1.0

Pyroxene

- 1 . 5 ± 1.2 3 . 4 ± 1.1 - 2 . 4 ± 1.3 - 0 . 6 ± 1.0

-2.7 6.4 -4.1 -1.4

± ± ± ±

1.3 1.7 1.5 1.2

Plagioclase

1.2 0 0.5 -2.3 2.6 -2.4 -2.2 2.1

± ± ± ± ± ± ± ±

2.0 1.8 2.0 2.2 1.8 1.8 2.0 1.5

0.5 -0.3 -0.5 -5.2 4.8 -4.7 -4.0 1.5

± ± ± ± ± ± ± ±

3.0 2.0 2.2 2.5 2.0 1.8 1.8 2.0

2.4 -1.3 -0.1 4.4 2.9 0.2 1.3

± ± ± ± ± ± ±

1.0 1.2 1.0 1. 4 1. 0 1.2 1.0

4.8 -3.0 1.0 9.5 6.0 0.6 2.7

± ± ± ± ± ± ±

1. 2 1.2 1.0 1. 6 1. 0 1. 4 1.0

Allende Minerals TS-21

pyroxene


6.


HUTCHEON

Table 1.

26

Ô Mg

TS-23 pyroxene


(continued) 25

Sample

107

-5.2 2.8 3.3 2.1 0.3 0.1

+ + + + + +

ô Mg 1 1 1 1 1 1

4 0 2 0 0 2

-9.9 5.3 6.0 4.0 -0.3 -0.4

+ 1. 5

+ + + + +

1 0 1. 3 1 0 1 2 1. 4

3529G pyroxene

-2.2 + 1 2 -2.6 + 1 3 0.3 + 1 0

-4.8 + 1 5 -4.7 + 1 5 -0.7 + 1 5

3529-45 s p i n e l

-0.8 + 1 0 -3.3 + 1 2

-0.4 + 1 0 -6.5 + 1 2

3510

olivine

-3.6 + 1 .0 -4.8 + 1 .2 -1.0 + 1 .0

-7.2 + 1 .2 -10.5 + 1 .4 -2.5 + 1 .5

TS-34 s p i n e l

-1.6 + 1 .2 0.6 + 1 .0

-3.6 + 1 .5 1.6 + 1 .4

TS-34 pyroxene

1.0 + 1 .2 1.7 + 1 .2

1.9 + 1 .4 3.3 + 1 .5

TS-8 pyroxene

+ 1 .0 0 1.3 + 1 .0 4.0 + 1 .0

1.2 + 1 .4 3.0 + 1 .2 7.9 + 1 .1

^Corrected f o r

4 8

Ca

T T

interference.

f r a c t i o n a t i o n i s p o o r l y understood and f r a c t i o n a t i o n may occur e i t h e r d u r i n g s p u t t e r i n g (generation o f secondary i o n s ) o r d u r i n g t r a n s f e r o f secondary ions from the v i c i n i t y o f the sample t o the mass spectrometer. The range o f f r a c t i o n a t i o n , ± 7 permi1/amu, r e f l e c t s t h e s e n s i t i v i t y o f t h e i o n probe t o small changes i n e x t r a c t i o n c o n d i t i o n s and i t was only by r i g i d l y adhering t o t h e t u n i n g c r i t e r i a d i s c u s s e d above t h a t we l i m i t e d the f r a c t i o n a t i o n w i t h i n t h i s range. S i m i l a r instrumental mass f r a c t i o n a t i o n has a l s o been reported i n other i o n probe s t u d i e s [24] and i n s o l i d source mass s p e c t r o m e t r i c measurements using t h e d i r e c t l o a d i n g technique [ 3 2 ] . The slope o f t h e f r a c t i o n a t i o n l i n e on t h e ô M g versus ô M g diagram i n f i g u r e 6 i s 0.51 ± 0.03, c o n s i s t e n t w i t h the slope expected f o r simple mass-dependent f r a c t i o n a t i o n . I t i s important t o recognize t h a t a l l o f t h e t e r r e s t r i a l i s o t o p i c data l i e along t h e b e s t - f i t l i n e ( n e a r l y always w i t h i n 2σ) independent o f sample mineralogy and Mg content. We observed no systematic d e v i a t i o n s from t h i s l i n e due t o molecular 25

26



108


TECHNIQUES

Τ

8h

—ι

-12

"

«

-8

1

1

-4

ι

I

ι

0

I

4

ι

L

8

8 Mg (permiI) 26

Figure 6. Three-isotope correlation diagram for Mg using B-notation (see text) for unnormalized data from terrestrial samples and Mg-rich Allende minerals. The data plot along a line of slope ~ 0.5, reflecting instrumental mass fractionation of ±7%/amu. The proper correction for ^Ca * is indicated by the terrestrial plagioclase data which, if uncorrected, would plot far below the mass fractionation line off the dia gram. Data from Allende Mg-rich minerals overlap those of terrestrial samples, denoting the absence of fractionation indigenous to the Allende material. Key: O, terrestrial olivine + pyroxene; •, terrestrial plagioclase; · , Allende Ti-pyroxene; and •, Allende spinel. 2


6.


HUTCHEON

109

i n t e r f e r e n c e s o r u n s p e c i f i e d instrumental e f f e c t s o f any kind. Data from some mineral groups c l u s t e r e d i n c e r t a i n areas along t h e l i n e r a t h e r than being u n i f o r m l y d i s t r i b u t e d ; data from o l i v i n e and h i b o n i t e , f o r example, t y p i c a l l y had ô M g l e s s than -4 permi1. T e r r e s t r i a l p l a g i o c l a s e data a l l l i e w i t h i n 2σ o f t h e 26


4 8

+ +

l i n e , demonstrating a v a l i d c o r r e c t i o n f o r C a . Without t h i s c o r r e c t i o n these data would be d i s p l a c e d from the f r a c t i o n a t i o n l i n e by up t o n e a r l y 30 permi 1 along a l i n e o f slope one on t h e three-isotope plot. The i o n probe i s o t o p i c data were c o r r e c t e d f o r f r a c t i o n a t i o n using the measured slope of the Mg/ Mg versus Mg/ Mg c o r r e l a t i o n l i n e from t e r r e s t r i a l samples, 0.454 ± 0.022, a f t e r normaliz i n g t h e observed Mg/ Mg r a t i o s t o the NBS standard Mg/ Mg r a t i o , 0.12663 [33]: r- i 25

25

26

24

( Mg/ Mg)

26

24

24

2 6

25

2 4

= ( Mg/ Mg)

N

24

2 5

+

0 B S

24

2 4

0.12663 - ( M g / M g )

0 B S

I

With t h i s n o r m a l i z a t i o n t h e t e r r e s t r i a l Mg i s o t o p i c data d e f i n e a "normal" Mg/ Mg r a t i o o f 0.13938 (corresponding t o Mg/ Mg = 0.12663). V a r i a t i o n s i n the M g content o f samples were then 26

24

25

24

26

26

c a l c u l a t e d as ô M g i n permi1 (°/oo) r e l a t i v e normal value ( Mg/ Mg) = 0.13938: 26

°r 26 (

δ

All

t o the measured

24

Ν

6 Μ

9=

[

M

G

/

24

M

G

1

)

0.13938

· U

X

1

0

0

0

26

of the t e r r e s t r i a l

samples y i e l d e d ô M g = 0 w i t h i n 2 a . mean i s c a l c u l a t e d from t h e standard d e v i a t i o n o f the mean

where

m û

mean of the i s o t o p e r a t i o measurement, propagated t o account f o r t h e f r a c t i o n a t i o n c o r r e c t i o n . This f r a c t i o n a t i o n c o r r e c t i o n removes the e f f e c t o f mass-dependent f r a c t i o n a t i o n both i n the i o n probe and i n nature. The c o r r e c t e d values ( 6 M g ) thus r e f l e c t o n l y Ν i s o t o p i c e f f e c t s other than those due t o mass-dependent f r a c t i o n a tion. 26

Results Mg I s o t o p i c Composition o f I n c l u s i o n s The Mg i s o t o p i c data f o r the A l l e n d e i n c l u s i o n s a r e sum marized i n Table 2 where we present the raw, unnormalized i s o t o p e r a t i o s , t h e M g excess expressed as 6 fMg ( p e r m i l ) , and t h e Ν A l / M g r a t i o f o r each sample. Data f o r A l l e n d e Ti-Al-pyroxene and s p i n e l f a l l w i t h i n the range o f t h e t e r r e s t r i a l standards: ô M g = 0 ± 7 p e r m i l . ô M g = 0 ± 14 p e r m i l . I t i s important t o recognize t h a t the spread i n the data f o r A l l e n d e Mg-rich minerals due t o instrumental f r a c t i o n a t i o n i s the same as t h a t observed f o r 26

2 7

2

2 4

25

26




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Nuclear and Chemical Dating Techniques - ACS Publications

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