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Table 15. Chemical composition (wt. percent) and minimum mineral ages of various generations of selected alkali amphiboles of Bayan Obo.

[Analysts: J.M. Back and J.A. Minkin. Analysis by electron microprobe (EPMA). *, below minimum detection level (MDL); MDL for TiO2=0.04. Minimum 40Ar/39Ar mineral ages determined by J.E. Conrad, E.H. McKee, and B.D. Turrin; n.d., not determined]

Sample no. .

Drew 1

7B13C

40-14

8B24-1

7B25-2

8B42

7B34-1P

7B34-1F

796-48,252

796-48,366

7B30-30

8B7

796-48,277

SiO2

55.07

56.01

56.55

55.31

57.19

55.96

57.54

57.24

55.98

53.53

55.90

55.75

53.85

TiO2

.07

.25

.04

*

.12

*

*

.05

.06

*

*

*

.04

Al2O3

.55

.26

.59

.33

.46

.48

.37

.26

.66

.70

.58

.47

.44

1 FeO

22.74

14.15

13.24

8.84

9.29

15.48

12.80

10.81

17.18

14.30

14.45

9.55

12.59

MnO

.30

.62

.13

.13

.04

2.82

.08

.13

.91

4.91

.57

.90

5.68

MgO

10.28

14.95

15.96

17.96

18.55

12.88

17.12

18.34

14.01

11.84

15.44

19.30

12.47

CaO

.48

.65

.61

2.01

1.06

1.77

.72

4.15

2.33

2.56

2.69

3.28

1.92

Na2O

7.33

7.69

7.03

7.74

7.18

7.19

8.98

6.13

6.74

6.74

7.17

6.91

7.64

K2O

1.03

1.95

2.93

2.00

2.26

2.21

1.18

1.18

1.02

2.14

1.57

.67

1.90

F

.98

2.03

2.51

2.20

2.52

2.87

2.56

1.38

2.04

3.08

2.81

3.40

3.41

Total

98.83

98.56

99.59

96.52

98.67

101.66

101.35

99.67

100.93

99.80

101.18

100.23

99.94

-O=F

0.41

0.85

1.06

0.93

1.06

1.21

1.08

0.58

0.86

1.30

1.18

1.43

1.44

Total

98.42

97.71

98.53

95.59

97.61

100.45

100.27

99.09

100.07

98.50

100.00

98.80

98.50

Minimum mineral age (Ma)

1,260

820

890

890

n.d.

n.d.

424.8+-2.4

428.0+-7.3

419.5+-2.0

n.d.

395.8+-6.2

395.5+-4.4

343.1+-3.0
1Total Fe as FeO.

 

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Last updated 12.17.97