Significance of in situ SIMS chronometry of zoned monazite from the Lewisian granulites, northwest Scotland

,NCL”D,.w

ISOTOPE GEOSCIENCE

ELSEVIER

Chemical Geology

13.5 (1997) 35-53

Significance of in situ SIMS chronometry of zoned monazite from the Lewisian granulites, northwest Scotland X.K. Zhu a*by * , R.K. O’Nions a, N.S. Belshaw a, A.J. Gibb

a

” Department of EarthSciences, University of Oxford, Parks Road, Oxford, OXI SPR, UK b Deportment of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK Received

11 January

1996; accepted 5 July 1996

Abstract In situ studies of Lewisian metamorphic monazites show them to be strongly zoned in Th content, Th/U ratio and *07Pb/ *06Pb age. Traverses across compositional zones of individual monazites by SIMS and EMPA show that there are no resolvable shifts betwjeen 232Th/238U and *08Pb/ *OsPb ratios and between Th and Pb contents in any of the profiles obtained. Furthermore, tbe difference in 208Pb/232Th ages between two zones of monazites in the same thin section is

indistinguishable from the difference in 207Pb/206Pb ages determined by SIMS. These observations together with simple models suggest that transport of Pb relative to Th and U did not occur significantly during younger (e.g., the Laxfordian) events in the Lewisian and the ages preserved in these zoned monazites are close to concordant. Moreover, monazites having different compositions yield indistinguishable *07Pb/ ‘OsPb ages, which suggests that the closure temperature of the U-Pb system in these monazites is not significantly dependent upon their composition. Consequently, the pattern of age distribution observed in the monazites is taken to reflect the metamorphic history of the Lewisian. The results are consistent with the recent interpretations of the Lewisian thermal history which suggest two high-grade metamorphic events separated hy N 200 Ma. Keywords: Monazite;

SIMS chronometry;

Zonation;

Lewisian

1. Introduction Monazite is a common accessory mineral in intermediate- to high-grade metamorphic rocks, particularly metapelites and leucocratic gneisses (Overstreet, 1967). With the increasing understanding of the behaviour of monazite and its U-Th-Pb system during geological processes (Overstreet, 1967; Copeland et al., 1988; Parrish, 1990; Smith and

* Corresponding

author.

0009-2541/97/$17.00 Copyright PII SOOOS-2541(96)00103-9

Barreiro, 1990; Kinsbury et al., 1993), its use as a chronometer has increased in recent years. By virtue of its high Th-U content, rare inheritance (Overstreet, 1967; Parrish, 1990; Smith and Barreiro, 1990; Kinsbury et al., 1993) and its high closure temperature for the U-Th-Pb system (Copeland et al., 1988; Parrish, 19901, monazite U-Th-Pb chronology has become routine using conventional dissolution techniques. The use of secondary ion mass spectrometry (SIMS) techniques for in situ chronology of monazite in petrographic thin sections is relatively new

0 1997 Elsevier Science B.V. All rights reserved.

36

X.K. Zhu et al./ Chemical Geology 135 (1997135-53

(DeWolf et al., 1993; Zhu et al., in press). These first studies demonstrated the potential that exists for combining chronological information with textural information in metamorphic rocks. Zonation appears to be a common feature of metamorphic monazite, especially for large grains (Mohr, 1984; DeWolf et al., 1993; Zhu et al., 1993). Such zonation usually includes gradients in Th, U and Pb abundances and presumably reflects different stages of mineral growth. In this study of Lewisian monazites and those of Wind River monazites by DeWolf et al. (19931, “‘Pb/ ‘06Pb ages vary according to the compositional zoning pattern. However, in such studies the possibility of discordance within the U-Th-Pb system must be considered and evaluated. This paper is concerned with a detailed investigation of zoned monazites in two samples of Lewisian granulite from NW Scotland. The approach adopted here uses SIMS to obtain 207Pb/ 206Pb ages, and high resolution 208Pb/ 232Th age differences profiles of 232Th/ 238U and “*Pb/ 206Pb ratios across compositional zones of individual monazites. In this way, the amount of possible Pb transport that has occurred relative to U and Th is examined, and the degree of discordance and compositional dependence of the U-Pb closure temperature is evaluated.

2. Sample characteristics Two samples, 86004 and 86015, from the highgrade central region of the Lewisian granulites at Scourie have been studied in detail. A summary of

Fig. 1. Sample 86004. a: BSE images of monazites #3 and #4 showing simple zonation. The variations of brightness reflect the variations of average atomic number which is principally affected by Th abundance in the case of these monazites. The brighter the image, the higher the average atomic number. The locations of SIMS analyses are circled. b. BSE image of monazite #5 showing complex but somewhat concentric zoning. Note that some pits produced by the ion beam are just visible. c. Drawing of monazite #5 showing the locations of the SIMS analyses and the locations of EMP traverse (A B) D) shown in shown in Fig. 4a and SIMS traverse (CFig. 5a.

previous studies on the Lewisian thermal history and the petrographic details of these two samples have been given elsewhere (Zhu et al., in press).

X. K. Zhu et al. / Chemical Geology 135 (1997) 35-53

2.1. Sample

86004

Sample 86004 was collected from a layer of brown weathering gneiss, possibly metasediment (Ordnance Survey map grid reference, NC 15 174576). Monazite occurs in this sample mainly as large ( > 100 pm across) discrete matrix grains in association with feldspar, lbiotite, quartz and sulphide, and as very small (< 10 km diameter) inclusions in biotite. The overall morphology of the discrete monazite grains varies from euhedral to anhedral. All the

37

discrete monazite grains are zoned, when imaged using back-scattered electrons (BSE), due principally to variations in Th content. Some grains show relatively simple zoning with a thin high-Th (bright in BSE image) rim (Fig. la), whereas others are more complexly zoned (Fig. lb). The boundaries between the zones are sharp in the BSE images and some grains show cuspate embayments (Fig. lb). 2.2. Sample 86015 This is a sulphide-rich felsic gneiss and probably a metasediment (NC 15 154520). Monazite occurs as discrete matrix grains in association with feldspar, quartz and sulphide, or as micro-inclusions in garnet, biotite and sulphide. The overall morphology of discrete monazite grains varies from irregular to nearly euhedral. Most of the large (> 100 pm) discrete monazite grains show zoning in BSE image, which is either irregular or concentric. As with sample 86004, some zoned monazite grains in this sample also show cuspate embayments (Fig. 2a and b). Most of the concentrically zoned monazite grains have low-Th (dark in BSE image) cores and high-Th (bright in BSE image) rims (Fig. 2a), although some monazite grains show a reversed pattern of zoning (Fig. 2b).

3. Analytical techniques

Fig. 2. Sample 86015. a. BSE image of monazite #2 showing low-Th bulk core and high-Th rim. The locations of SIMS analysis are circled. The location of the EMP traverse shown in Fig. 4b is shown as line A--B, and the location of the SIMS traverse shown in D. Fig. 5c is show as line c’p b. BSE image of monazite #5 showing an overall reversed zoning pattern of high-Th core and low-Th rim. The locations of the SIMS analyses are circled.

Pb isotope ratio measurements of monazites have been made at a mass resolving power of 6500. The procedures and techniques used for in situ SIMS Pb-isotope chronometry of monazites using ISOLAB” have been reported elsewhere (DeWolf et al., 1993; Belshaw et al., 1994; Zhu et al., 1993). An important aim of this study was to obtain 232Th/ 238U and *08Pb/ *06Pb prohigh-resolution files across zones within individual monazites. To reduce possible molecular effects arising from the compositional variations that exist within the zoned monazite (see Tables 1 and 2) an energy-filtering technique which uses the 2 70-eV portion of the secondary ion energy distribution (cf. Shimizu and Hart, 1982; Hinton, 1990) was used. In this case, ISOLAB” is operated at a resolving power of only

38 Table 1 Represensative

X.K. Zhu et al. /Chemical

EMPA results of monazites

Geology 13.5 (1997135-53

in sample 86004

Monazite # 1

Monazite #2

Monazite #3

core (a)

core (b)

core

rim

core

y2°3

0.17 0.67 1.16 0.55 28.05 14.32 30.81 3.57 12.16 1.25 0.05 0.40 0.06

6.36 0.20 0.77 1.18 0.26 27.84 14.69 31.42 3.42 12.03 1.25 < d.1. 0.44 0.07

5.95 0.18 0.72 1.13 0.22 28.04 14.93 31.32 3.44 11.96 1.23 < d.1. 0.41 0.05

8.69 0.36 0.96 1.35 0.61 27.26 13.28 29.82 3.44 12.22 1.27 < d.1. 0.47 0.1

Total

98.75

99.96

99.6

B(LREE),O,

62.11

62.81

62.88

ThO,

uo,PbO CaO SiO, p205 La203 Ce203 Pr203 Nd203 Sm203 E”203 Gd203

5.53

d.1. = detect limit which is _ 0.05% for Eu,O, two separate parts of the core of monazite # 1.

Monazite #4

Monazite #5

rim

core

rim

core

intermediate

rim

6.18 0.16 0.77 1.17 0.39 28.63 15.15 32.02 3.28 11.19 1.19 < d.1. 0.43 0.08

11.44 0.42 1.28 1.59 1.17 26.19 12.51 28.82 3.13 11.28 1.25 0.05 0.44 0.07

5.68 0.18 0.67 1.18 0.23 28.82 15.81 30.91 3.27 11.10 1.16 0.09 0.50 0.08

11.87 0.37 1.30 1.64 2.00 26.7 1 12.74 27.26 3.09 11.48 1.27 0.11 0.49 0.10

5.46 0.16 0.65 0.97 0.34 28.64 16.51 31.39 3.25 10.88 1.05 < d.1. 0.43 0.07

7.82 0.15 0.88 1.37 0.69 28.19 13.67 29.68 3.58 12.23 1.13 0.09 0.43 0.07

14.16 0.5 1.55 1.94 1.46 26.44 12.26 25.81 3.08 11.01 1.24 < d.1. 0.52 0.08

99.85

100.66

99.64

99.68

99.63

99.82

99.98

100.09

60.03

62.83

56.99

62.25

55.84

63.08

60.29

53.40

and Y203. X(LREE),O,

2000 because molecular species are suppressed to negligible level by energy filtering. An assessment of 232Th/ 238U ratio reproducibilTable 2 Representative

EMPA results of monazites

= La,O,

+ Ce,O,

+ Pr,O,

+ Nd,O,

+ Sm,O,.

(a) and (b) are

ity has been made using NBS 610 glass which has 457 ppm Th and 461 ppm U (Hinton, 1990). The external reproducibility of 28 separate analyses is

in sample 86015

Monazite # 1

Monazite #2

core

core

intermediate

rim

main core

y2°3

9.71 0.28 1.07 1.44 0.78 26.48 14.03 29.75 3.33 11.8 0.72 0.05 0.11 0.05

2.40 0.54 0.46 0.44 0.27 26.72 15.87 32.20 3.60 13.33 1.36 < d.1. 0.24 < d.1.

10.84 0.24 1.12 1.61 0.75 26.04 14.07 29.72 3.28 10.94 0.72 0.06 0.13 < d.1.

17.50 0.84 1.98 1.67 2.21 22.52 10.59 24.74 3.21 12.42 0.92 < d.1. 0.15 < d.1.

14.72 1.11 1.91 2.65 0.96 26.15 8.95 24.72 3.14 13.21 1.10 < d.1. 0.37 0.06

9.26 0.61 1.12 1.49 0.84 25.29 13.34 28.82 3.18 12.23 1.11 0.13 3.14 0.05

Total

99.6

97.47

99.54

98.79

99.05

100.61

Z(LRH$,O3

59.63

66.36

58.73

51.88

51.12

58.68

no2 uo2

PbO CaO SiO, p205 La203

Ce203 Pr203 Nd,O, Sm203 E”203

Gd,O,

See footnote of Table 1

Monazite #5 rim

X.K. Zhu et al./Chemical

f 2% (2 (T) (Fig. .3). This is comparable to the precision ( + 1%, 2 17) for 208Pb/ 206Pb ratio measurements by SIMS on the same glass at high mass resolution (Belshaw et al., 1994). Techniques employed here for monazite 208Pb/ 232Th chronology are analogous to those used for zircon U-Pb chronology (Compston et al., 1984) and for monazite as reported by Harrison et al. (1995). The working conditions used for analyses presented here are 2000 mass resolution power and a 50-eV energy window with a 40-eV offset for Th+. Under these operating conditions and due to correlated discrimination of Pbf and ThOf secondary ions, repeated Th--Pb isotopic measurements of monazite yield a linear relationship between 264ThO:/ 232Th+ and 208Pb+/ 232Th+ which passes through the origin, and: 2o*pb+ -= 232n+

39

Geology 135 (1997) 35-53

where (208Pbf/ 232Th+ ) is the SIMS. Assume that the measured ratio for a sample is related to the actual by a factor which is the same for the identical operating conditions.

measured

ratio by

(208Pb+/ 232Thf) ratio 208Pb/ 232Th the standard under Therefore:

(3)

Then:

(4)

264ThO?+

b -5232

(1)

Th -I-

where b is the slope. If measured ratios are relative to the same 264Th0;/ 232Th+ ratio which varies only with operating conditions, then:

standard

(5)

The primary beams used for 207Pb/ 206Pb and 208Pb/ 232Th measurements were typically N 20 km in diameter. However, in order to obtain high spatial resolution profiles, the beam diameter used with the energy-filtering technique was N 10 pm.

: 0.9

0.9 : Mcau = 0.9929 +/- 0.0039 (Zsigms) STD = 0.0103 n=28

0.8 .-

0

* *.

* ‘* 5

* *.

’* ”

10

0.8

s I.‘. 15

* ” 20

. * ‘I.. 25

Analysis Number Fig. 3. 232Th/ 238U ratio repeatability

in NBS-610 glass.

‘.

xl

0.7

X.K. Zhu et al. /Chemical

40

Geology 135 (1997) 35-53

15

-2 !.O -C-lk -

Pb

-1 ..5

10

? 's -1 1.0g PC

3 & I2 :

5

-C I.5

0

40

80

A

120

160

200

240

280

-C

I.0 B

Distance @m)

25

- 2.0 > '- 1.5

10

- 1.0

5

- 0.5

~,,,.,,,,,,,,,,,,,,,,,, 0

A

20

40

~,_....I 60

60

Distance (pm)

0.0 100

120

140 B

Fig. 4. Chemical features of monazites. a. EMPA traverse of monazite #5 in sample 86004. Both the step size and beam size are 2 pm. Note that there are no relative shifts in the concentration profiles of Th and Pb, and that the boundaries are sharp. See Fig. lc for the traverse location. b. EMPA traverse of monazite #2 in sample 86015. The step size is 2 p,m and the beam size is 3 pm. See Fig. 2a for the traverse location.

X.K. Zhu et al. / Chemical Geology 135 (1997) 35-53

4. Results 4. I. Electron probe results All monazite grams analysed by SIMS in sample 86004 have been characterised by EMPA, and representative results are presented in Table 1. Th and Pb line scans for a cotnplex, concentrically zoned monazite grain (#5) are shown in Fig. 4a. Th in this grain reaches a maximum of 12 wt% at the rim and varies between 5 and 7 wt% in the bulk core region. The boundaries between these portions of the grain are sharp and there are no resolvable relative shifts between the Th and Pb profiles (Fig. 4a). The monazite grains identified in sample 86015 have Th abundances. varying from below the EMPA detection limit of - 0.1 wt% up to - 20 wt%. Representative EMPA results are presented in Table 2. A traverse across monazite #2, which is also complexly zoned (Fig. 2a), is shown in Fig. 4b. The Th abundance in this grain varies from - 2 wt% in the core to N 15 wt% in the rim and the Pb profile mirrors that of Th with sharp boundaries between different portions of the grain (Fig. 4b). 4.2. SIMS results 4.2.1. Profiles of 232Th/ 238V and “‘Pb/ ‘06Pb Two complexly zoned monazite grains, monazite #5 in 86004 and monazite #2 in 86015, have been analysed by SIMS using the energy-filtering technique described above. The results are shown in Fig. 5a-d. For monazite #5 in 86004, a traverse across the boundary between the intermediate portion and highTh rim is shown in Fig. 5a. The 232Th/ 238U ratio and *‘*Pb/ *06Pb ratios vary from - 110 and N 27, respectively in the intermediate portion of the grain

41

to - 40 and - 10 in the rim. The 208Pb/ 206Pb profile shows no resolvable shift relative to the 232Th/ 238U profile (Fig. 5a) and the 208Pb/ *06Pb ratio is linearly correlated with the 232Th/ 238U ratio (Fig. 5b). A second traverse has been obtained across monazite #2 in 86015 from the low-Th core to the high-Th rim (Fig. 5~). The 232Th/ 238U and 208Pb/ 206Pb ratios range from - 5 and - 1.2 in the core to a maximum of - 100 and N 24 in the intermediate portion, respectively. As in the previous case, there is no resolvable shift between the profiles of 232Th/ 238U and *08Pb/ *06Pb (Fig. 5d).

4.2.2. Monazite 207Pb/ 206Pb chronology Five grains in sample 86004, either simply or complexly zoned, have been analysed. The results are shown in Table 3. A total of five analyses from the low-Th core regions of four monazite grains (monazite #l to #4) yield 207Pb/ 206Pb ages in the range 2740 f 12 Ma (2 cr > to 2746 f 18 Ma. In contrast, 207Pb/ *06Pb ages for four points from the rims of monazite #5 and monazite #l are in the range of 2525 * 12 to 2542 IfI 12 Ma. These are statistically indistinguishable from one another and - 200 Ma younger than ages obtained from the cores. An even younger age of 2439 f 17 Ma has been obtained from the grain tip of monazite #5 where the Th content is significantly lower than the rest of the rim. Ages obtained from the intermediate portions of monazites #l and #5 are - 2710 Ma, and intermediate between ages obtained from the cores and rims of monazites (Table 3). However, the core of monazite #5 gives a range of ages from 2623 f 29 to 2706 f 11 Ma. The results for three zoned monazite grains analysed from sample 86015 are presented in Table 4.

Fig. 5. SIMS traverses of monazites. a. Profiles of 232Th/238U and *‘*Pb/ *06Pb in monazite #5 in sample 86004. Note that there is no detectable shift between the two profiles. See Fig. lc for the traverse location. b. Plot of *“Pb/ ‘06Pb vs. z3’Th/ 238U for the same traverse shown in (a). c. Profiles of 232Th/23sU and *08Pb/ *“Pb of monazite #2 in sample 86015 made by SIMS analysis with energy filtering. There is no detectable shift between the two profiles. See Fig. 2a for the traverse location. d. Plot of *“Pb/ *06Pb vs. 232Th/ 238U of the same traverse shown in (cl. Note that the apparent gradient in the boundary between zones results from overlap of the 10 km in diameter ion beam when using a 2+m step size. The “*Pb/ *“Pb ratios plotted are as measured - common Pb contributions are negligible (see Tables 3 and 4).

42

. .I.. -

232T,/238U

: 120

208Pb/LwPb

1 100 : 80 : 60 ':40 : 20 0’

I

I

0

5

C

I

10

I

15

Distance (pm)

p5-l 25 -

232~238~

I

I

I

20

25

30

D

'0

X.K. Zhu et al. / Chemical Geology 135 (1997) 35-53

43

J

-o-

120

232 ~h/238 U ??

208pb/206Pb

40

20

0

20

10

C

30

40

so

Distance (pm)

40

60

0

D

80

100

120

232Th/238u Fig. 5 (continued).

206 The 207pb/ Pb ages range from 2549 AC13 to 2406 f 22 Ma and show a close relationship with the internal micro-structures of the grains. Monazite #l

is a zoned grain, 140+m diameter, with a low-Th core and a narrow high-Th rim. The core of this grain yields a *07Pb/ “‘Pb age of 2546 + 27 Ma,

Spot position

1 2 3 4 5

core

core

core

2 3 4 5 6 I 8 9 10 11 12 13

intermediate zone intermediate zone high-Th rim core high-Th rim high-Th rim rim, lower-Th corner core core mixture of core and intermediate core intermediate portion intermediate portion 3500 f 200 6100+500 82OOk 300 4400 * 300 9400 + 500 22000 + 4500 8400 * 600 7sOO+ 1500 3600 + 700 3300 + 200 5800 + 700 9700 + 700 5900 + 500

5400 f 800

4900 + 600

3700+600

5200 k 400 22000* 1200 4ooo~400 6ooo&- 1800 85OOf 100

‘06 Pb/ ‘04 Pb

34.247 + 0.43 1 29.999 + 0.269 9.4694 + 0.095 18.046 + 0.267 10.056 + 0.085 7.4159kO.053 9.9030 + 0.400 19.517*0.102 21.605kO.106 31.156*0.087 20.12 1 + 0.050 25.246kO.146 26.123 + 0.203

22.022 kO.150

18.668 + 0.104

15.744 + 0.282

18.5301bO.179 10.682 f 0.085 26.872 + 0.044 25.220+ 0.146 20.20 1 f 0.132

206Pb)C =

34.368 f 0.433 30.059 * 0.270 9.48 1 1 + 0.0953 18.094 f 0.268 10.067 + 0.085 7.4190+0.053 9.9153 *0.4007 19.546 + 0.102 21.677 kO.107 3 1.273 + 0.088 20.162 f 0.050 25.278kO.146 26.177 + 0.204

22.070+0.151

15.792 + 0.283

18.571 kO.180 10.687 + 0.085 26.95 l* 0.045 25.272kO.147 20.229 + 0.132

(*“‘Pb/

206Pb),

0.1899 + 0.0018 0.1890 + 0.0018 0.1682+0.0017 0.1795 f 0.003 1 0.1699+0.0015 0.1675 + 0.0008 0.1599~0.0016 0.1822*0.0010 0.1861 kO.0016 0.1843+0.0013 0.1879 f 0.0012 0.1772*0.0014 0.1862 + 0.0022

0.1924t0.0016

0.1921+0.0014

0.1937 + 0.0020

0.1926&0.0016 0.1905*0.0019 0.1770 + 0.0008 0.1688+0.0013 0.1867+0.0012

(‘“‘Pb/ 206Pb), =

0.1864~0.0018 0.1860 + 0.0018 0.1667+0.0012 0.1767 + 0.003 1 0.1686+0.0015 0.1669 + 0.0008 0.1584+0.0016 0.1807+0.0011 0.1827+0.0017 0.1806 + 0.0013 0.1858 k 0.0012 0.1759*0.0014 0.1848 + 0.0022

0.1902+0.0016

0.1904+0.0021

0.1903f0.0016 0.1900*0.0019 0.1746 k 0.0009 0.1667~0.0015 0.1853+0.0012

(‘“‘Pb/

18

2711&16 2707t 16 2525 f 12 2623 f 29 2544* 15 2527 & 8 2439+ 17 2659 f 10 2678k 16 2658+ 12 2706&11 2645+ 13 2697 f 20

2744 + 14

274Ok 12

2746+

2745+ 14 2742+ 16 2603 + 9 2525 + 5 2701+11

Age (Ma)

All errors are quoted at 2~7 level. m = measured; c = corrected for common Pb based on initial Pb composition estimated using Pb single-stage evolution model. a Uncertainty estimated with an error propagation procedure that takes into account measurement errors of ‘“‘Pb/ ‘OnPb, ‘07Pb/ ‘“‘Pb and ‘“‘Pb/ ‘04Pb and the effect of an uncertainty of + 2% on the initial Pb composition.

spot spot spot spot Spot spot Spot spot spot spot spot spot

spot 1

Monazite #5:

spot 1

Monazite #4:

spot 1

Monazite #3:

spot 1

:

core (a) core (b) mixture of rim and intermediate rim intermediate portion

Monazite #2

spot spot spot spot spot

Monazite #l :

Label

Table 3 SIMS results of sample 86004

Spot position

2.0684 + 0.0576 1.1820*0.0108 5.5309 f 0.0587 13.250k0.1174

2.0686 f 0.0576 1.1824&-0.0108 5.5270+0.0586 13.213+0.1170

31800+2600 45OOOk 1200 11000*600 4300 + 280

core core mixture of thin and intermid. intermediatezone

spot-4 spot-5 Spot-6 spot-7

high-Th core low-Th rim low-Th rim

See footnotes of Table 3.

0.1600+0.0006 0.1603f0.0011 0.1614~0.0012 0.1606 f 0.0020

4.1407+0.0012 4.3926 f 0.03 18 7.2619~0.1004 8.6621k0.1117

4.1393~0.0012 4.3910+0.0318 7.2321 f 0.0998 8.626kO.1110

16300+ 1900 16700~2400 2250+ 140 2400+ 120

high-Th core

spot-2 spot-3 spot-4

0.1680+0.0011 0.1692f0.0016 0.1617+0.0015 0.1712~0.0021

0.1698 + 0.0005 0.1577*0.0017 0.1724f0.0016

0.1743~0.0026 0.1597+0.0018

Spot-l

Monazite #5:

2.7380+0.1096 7.4819+0.0770 12.964+0.1456

2.7378 f 0.1095 7.4716 kO.0768 12.913&0.1432

17.409+0.179 7.2870 k 0.0404

22000 * 3800 6800 + 440 2800 + 400

17.319kO.1772 7.2691+0.0402

low-Th core high-Th rim intermediatezone

226Ok 190 3750+230

‘06Pb/ ‘04Pb

spot- 1 spot-2 spot-3

:

mixture of core and rim

COP.?

Monazite #2

spot- 1 spot-2

Monazite #I:

Label

Table 4 SIMS results of zoned monazites in sample 86015

0.1592 k 0.0006 0.1596 + 0.0011 0.1558+0.0013 0.1553*0.0020

0.1692~0.0005 0.1557+0.0017 0.1680+0.0017 0.1676+0.0011 0.1687~0.0016 0.1606~0.0015 0.1683+0.0021

0.1688 +0.0027 0.1563+0.0018

2447+7 2451+ 12 241Ok 14 2406 f 22

2549 f 5 241Ok 18 2538k 17 2534+ 11 2545& 16 2462+ 16 2541 k21

2546 f 27 2416+20

Age (Ma)

X.K. Zhu et al. / Chemical Geology 135 (lYY7135-53

46

0.5

Ia

0.4

0.3 +c 2 5

w

0.2

0.1

0.2

0.4

0.6

0.8

1.0

2a Th02+/232Th+ 0.5

0.4

0.3 5 E

0.2

+i :

0.1

0.0

IA

0.0

0.2

0.4 m$232~r

0.8

1.0

1.2

264 Fig. 6. Results of *32Tk-zo8Pb isotopic measurements by SIMS. Common Pb corrections are negligible for these monazites a. Plot of 264ThO;/ 232Th+ vs. *“Pb+/ 232Th+ for the core of monazite #4 in 86004. Slope b,,,, = 0.389 5~0.002. b. Plot of 264Th0i/ 232Th+ vs. “‘Pb+/ 232Th+. Slope b,,, = 0.354 k 0.003.

(see Table 3).

X.K. Zhu et al. /Chemical

and a second spot which straddles the boundary between the core and rim gives a younger age of 2416 5 22 Ma. Monazite #2 is a more complexly zoned grain (Fig. 2al. Five separate analyses from its core and intermedi,ate portions yield ‘07Pb/ ‘06Pb ages between 2534 :t 11 and 2549 L- 13 Ma, in good agreement and consistent with the ages obtained from the core of monazite #l in the same sample and with the ages obtained from the rims of monazites in sample 86004. A younger age of 2410 f 10 Ma is obtained from the high-Th rim of the same grain. Monazite #5 in this sample is another complexly zoned grain but with a reverse pattern of high-Th core and low-Th rim (Fig. 2b). Two separate analyses on the low-Th rim give indistinguishable 207Pb/ 206Pb ages of 2410 + 14 and 2406 f 22 Ma, consistent with the ,ages obtained from the high-Th rim of monazite #2 in the same sample. Two further separate analyses on the high-Th core of this grain yield older ages of 2447 * 7 and 2451 k 12 Ma. 4.2.3. Monazite ‘O’Pb/ 232Th chronology The aim in this ,study is to obtain ‘“*Pb/ 232Th ages between mo’nazite zones with different 207Pb/ 206Pb ages. While use of an “external” standard with in situ studies is made difficult by poor reproducibility upon repositioning of the standard and sample, it is straightforward however to calibrate one monazite zone against another. The core of monazite #4 in sample 86004 has *07Pb/ 206Pb age of - 2740 Ma, and the rim of monazite #5 in the same thin section has 207Pb/206Pb age of - 2530 Ma, a difference of 210 Ma between the two. The Th-Pb isotopic measurements have been carried out

Geology 135 (1997) 35-53

47

on these two zones by SIMS and the results are plotted in Fig. 6a and b. The rim which has an average 207Pb/ 206Pb age of 2532 5 7 Ma or the core which has a ‘07Pb/ 206Pb age of 2742 + 14 (Table 3) may be considered as the standard, in either case the time interval CT,,,, - Tim) of 234 &-46 Ma (2 (+) is obtained by 208Pb/ 232Th dating. This time interval is indistinguishable from that of the 212 * 17 Ma difference between the 207Pb/ 206Pb ages (Table 3).

5. Discussion 5.1. U-Th-Pb

transport and age concordance

For zoned monazite grains analysed in this study, both the contents of Th and U and the ratios of Th/U and 208Pb/ *06Pb vary by more than an order of magnitude. Such differences, in principle, make it possible to detect any relative movement of these species between zones. The 208Pb/ 206Pb ratio at any oint in a monazite grain depends upon time and its P 32Th/ 238U ratio. However, because the half-lives of 232Th and 238U are similar, 208Pb/ *06Pb production ratios between 2800 and 2500 Ma only differ by 1.7%. The traverses across monazite #5 in 86004 and monazite #2 in 86015 (Fig. 5a-d) show no significant shifts between the 208Pb/ ‘06Pb and 232Th/ 238U profiles. There is therefore no resolvable transport of lh relative to U, or Pb relative to Th/U in these monazite grains on the scale of the - 2-pm step size used. The sharp boundaries in the EMP Th line

Fig. 7. Results of model calculations. a. The effect of Pb loss on ao7Pb/ ao6Pb ages. Starting time: 2710 Ma. AT= ttrue - t2”‘pb,~~mpt,. For AT= 200 Ma, - 40% Pb loss is required at 1750 Ma (Laxfordian event). Note that 2% Pb loss at that same time corresponds to AT< 10 Ma. b. The effect of Pb loss on *” Pb/ ‘06Pb ages. As (a), but with a starting time 2540 Ma. c. The effect of Pb transport relative to U and Th on *“Pb/ ‘06Pb ratios at different times in the intermediate portion of monazite #5 in sample 86004. s’“‘,, zohr&%b)= [(*“Pb/ 206Pb)m,xture/(208Pb/ Zo6Pb)unmixed- I] x 100. d. Ditto for *OSPb/ ‘&Pb ratio in the rim of monazite #5 in sample 86004. e. Ditto for *“Pb/ *06Pb ratio in the core of monazite #2 in sample 86015. f. Ditto for “‘Pb/ ao6Pb ratio in the intermediate portion of monazite #2 in sample 86015. Parameters used for monazite #5 are Th,,,/Th,,ie, = 2, (*s*Th/ 23*U),i,/(232Th/ 238U)intrr = (*“Pb/ zo6Pb)r,,,,/(208Pb/ 206Pb),nter = 0.37; and for monazite #2, Th,,,,,/Th,,,, = 4, (*s’Th/ 238U)inter/(232Th/ 238U)C0rC= c208Pb/ 206Pb)lnt,,/(208Pb/ 206Pb)C0re = 20; start time (t) is 2.71 Ga for the intermediate portion of monazite #5, and 2.54 Ga for the rest. See Appendix A for the equations and further details.

X.K. Zhu et al. /Chemical

2500

2000

Geology 135 (1997) 35-53

1500

1000

0

500

1000

10

1

0.1

2500

2000

1500

1000

500

X.K. Zhu et al./ Chemical Geology I35 (1997) 35-53

2500

2000

1500

1000

49

500

0

.. . . . . . .. . . .. . .. . .. .. . . . . . .. . . .. . . . . . . 1”. n ’ * * ’ 2500

2500

2000

2ooo

1500

1500 T=(Ma)

Fig. 7 (continued).

1000

loo0

500

500

0

0

50

X.K. Zhu et al. /Clzemical

scans (Fig. 4a and b) further limit the extent of any Th transport across the zones. Together, SIMS and EMPA traverses fail to provide any evidence for relative transport of Pb and Th/U on the N 2-pm scale since the formation of the high-Th rim of monazite #5 in 86004 and the formation of the intermediate portion of monazite #2 in 86015. It is worth recalling that these yield indistinguishable ‘07Pb/ *06Pb ages of - 2525 + 12 to - 2549 + 13 Ma (Tables 3 and 4) with an average of 2537 $- 5 Ma. The possibility that the - 2540 Ma *07Pb/ ‘06Pb ages are the result of U-Pb discordance and that the true age is - 27 lo-2740 Ma similar to that obtained from the bulk cores of monazites in 86004 may be considered further. Such possible discordance could be due to Pb loss during the Laxfordian ( N 1750 Ma), or Scourie dyke emplacement (- 2400-2000 Ma). Fig. 7a shows the dependence of AT, the difference between true age and measured ‘07Pb/ ‘06Pb age, on the time of Pb loss for different amount of Pb removal. For example, if AT = 200 Ma, then 40% Pb loss is required at 1750 Ma, or 45% at 2000 Ma, or 70% at 2400 Ma. Similarly the high-Th rim of monazite #2 in 86015 with a 207Pb/ *06Pb age of 2410 Ma could be produced by 30% Pb loss at 1750 Ma from a monazite with a true age of 2540 Ma (Fig. 7b). The loss of such large amounts of Pb would inevitably disrupt any Th-UPb zoning profile that existed. Whether such disruption in zoning profiles is evident today depends on the amount of radiogenic Pb produced subsequently and the precision of *“Pb/ 206Pb and 232Th/ 238U ratio measurement. These possibilities are explored with a simple model in Fig. 7c-f for the case of monazite #5 in 86004 and monazite #2 in 86015 (see captions and Appendix A for details). For a given percentage of Pb exchange between two monthe difference between measured azite zones, “‘Pb/ 206Pb ratio and that predicted for the case of no Pb transport is expressed as i?8pb,2~6Pb (%). 208Pb/ *06Pb ratios are measured to a precision of f l-2% (2 (T), which makes shifts in the zoning profile resolvable at pXpb, ~04~~2 2% on the scale of - lo-p,rn spot size. Fig. 7c and d relates to the intermediate and rim portions of monazite #5 in 86004. For the intermediate zone with Pb transport at 1750 Ma, then 2% Pb exchange would produce a

Geology 135 (1997135-53

resolvable shift in 6””rb,?~hPb. The exchange of 10% of the Pb at 2400 Ma also would give a resolvable shift in 8’“’rb, Z”h rb. Whereas the rim of this grain is less sensitive and - 10% Pb exchange is required to produce a resolvable shift in ~8pt,,~“~Pb at 1750 Ma. Fig. 7e and f refers to the core and intermediate portions of monazite #2 in 86015. Again at 1750 Ma, exchange of 2% Pb will produce a resolvable shift in both the core and the intermediate zones of the monazite. These observations suggest that the age patterns of the monazites are not the result of major Pb loss at either 1750 Ma (Laxfordian) or an earlier time. Indeed it seems unlikely from the *‘*Pb/ *06Pb and 232Th/ 238U profiles for these grains that the discordance for ages of - 2540 and N 2410 Ma will much exceed N 10 Ma, assuming that the amount of Pb loss is similar to the amount of Pb exchange. A further constraint on the age concordance comes from monazite Th-Pb dating. By using the core of monazite #4 against the rim of monazite # 5 in the same thin section of sample 86004, a time interval of 232 h 46 Ma (2 (T) has been obtained, which is indistinguishable to the time interval of 212 f 17 Ma defined by 207Pb/ *06Pb ages for these two portions of monazites (Table 3). The similarity of the time intervals between the core and rim defined by 208Pb/ 232Th dating and *07Pb/ 206 Pb chronometry shows that the age group of - 2740 Ma preserved in monazite cores in sample 86004 cannot be related to the age of protolith formation of the Lewisian complex, which Friend and Kinny (1995) suggested to be - 2950 Ma. It shows that the ages of N 2740 and - 2530 Ma, preserved in monazite cores and rims, respectively, are concordant within error. Whilst the ages of - 2740, - 2540-2530 and - 2410 Ma have been constrained to be close to concordant, the variation in *07Pb/ *06Pb ages from the core of monazite #5 in sample 84004 could be due to partial recystallisation or Pb loss occurred for that portion during subsequent thermal events 5.2. Monazite

closure

The *07Pb/ *06Pb age patterns preserved in these zoned monazites are unlikely to result from major Pb loss. The relationship between the compositional structures of zoned monazite grains and the

X. K. Zhu et al. /Chemical

‘07Pb/ *06Pb ages therefore has two possible interpretations. The ages could correspond to the time of zone formation, or possibly reflect a compositional dependence of the closure temperature (Parrish, 1994). The monazite zones differ principally in their Th abundance, and other elements vary either positively (e.g., Ca and Si) or negatively (e.g., CLREE) with Th (Tables 1 and 2). The following discussions therefore centres upon the relationship between age and Th abundance. Both of the low-Th cores of monazite #5 in 86004 and monazite #2 in 86015 give older ages than the high-Th rims. However, the rims despite having similar Th contents (Tables 1 and 2) differ by more than 100 Ma. in ‘07Pb/ *06Pb age, and the high-Th rim of monazite #5 in 86004 is indistinguishable from the age obtained from the low-Th core of monazite #2 in 86015 (Tables 3 and 4). Such relationships would not be expected if the age structure was related in a straightforward manner to a dependence of the closure temperature on composition. Furthermore, although monazite #5 in 86015 has reverse zoning with a high-Th core and low-Th rim (Fig. 2b; Table 21, the age pattern is similar to that obtained from monazite grains with low-Th cores and high-Th rims. Together, these show that the closure temperature for the U-Th-Pb system in monazite is not strongly dependent upon composition. The preservation of age information for an early thermal event ( > 2.7 Gal in the cores of large ( > 100 km> monazites implies that their closure temperature is not lower than the temperatures associated with subsequent thermal (events. The ages of N 2540 Ma obtained from the rim of monazite #5 in 86004 and the core and intermediate regions of monazite #2 in 86015 therefore appear to record a time close to the peak of a younger thermal event.

5.3. The significance

of zoned monazite chronometry

compositional zonation of the The concentric monazites apparent1.y records discontinuous growth in response to metamorphic episodes. Therefore, both the age and compositional zonation preserved in monazites from the two Lewisian samples have geological significance.

Geology I35 (1997) 35-53

51

Despite extensive investigations, the Lewisian thermal history remains poorly understood. The long standing view that granulite facies metamorphism peaked at N 2660 Ma as recorded by U-PI, zircon population (Pidgeon and Bowes, 1972) and Pb-Pb whole rocks (Chapman and Moorbath, 1977; Cohen et al., 1991) is in doubt. Two episodes of high-grade metamorphism ( > 2710 and = 2480 Ma, respectively) have been suggested on the basis of U-Pb single-zircon and intra-grain dating (Corfu et al., 1994). More recently, however, based on the SHRIMP zircon dating, Friend and Kinny (1995) claimed that both of the zircon ages of 2660 Ma (Pidgeon and Bowes, 1972) and 2 2710 Ma (Corfu et al., 1994) are mixed ages without geological significance. In this study, the ages obtained from zoned monMa from the azite grains group at N 2740-2710 bulk cores of monazites in 86004, N 2540 Ma from the rims of monazites in 86004 and the bulk cores of monazites in 86015, N 2410 Ma from the rims of monazite in 86015, and N 2450 Ma from a monazite core in 86015. These results are supportd by in situ SIMS chronometry of monazite micro-inclusions in garnet (Zhu et al., in press), and are broadly consistent with the results of the high-precision single zircon dating (Corfu et al., 1994). Thus the ages of N 2740-27 10 and N 2540 Ma provide further estimates for the timing of the the first and second episodes of granulite thermal events in the central Lewisian. The ages N 2410 Ma are considered to date another geological event temporally coincident with the first episode of Scourie dyke intrusions.

6. Conclusions (1) Zonation is common in metamorphic monazite. Monazite is able to retain chemical and isotopic zoning up to granulite facies. (2) The behaviour of the Th-Pb system in Lewisian monazite shows no difference to that of the U-Pb system during thermal events up to granulite facies metamorphism. (3) The closure temperature for the U-Th-Pb system in the monazite studied is not compositionally dependent. (4) The complex zoning patterns preserved in Lewisian monazites reflect the complex thermal his-

X.K. Zhu et al./ Chemical Geology 135 (19971 35-53

52

tory of the central Lewisian. A metamorphic history of more than 300 Ma from N 2740 to _ 2400 Ma is recorded by the zoned monazite grains.

Acknowledgements

where e*3’l -

1

R, = ___ e*l’l -

1

If a fraction (f,) of Pb exchange occurred at time the “*Pb/ ‘06Pb ratio of the mixture at the present day is:

t’ between the two zones, then for zone-l

We would like to thank AS Cohen and K.W Burton for the fruitful discussions and their suggestions during the study. D.J. Mattel is thanked for the help on computer. Stephen Reed is thanked for his help on EMPA. Comments on the manuscript by F. Corfu, Pete Kinny and John Tarney are gratefully acknowledged. This study was supported by NERC and the Royal Society.

232Th, = -R; 238 U,

(A-4)

where

Appendix A. Equations for model calculations (1) The effect of Pb loss on measured 207Pb/206 Pb ratios

232Th2 a=

238 u2

Assume monazite formed at time t, then: *” Pb

q

eA,2- 1 -=--

238u (-1‘06Pb true= -

eA,r -

1

1

eQ - 1

137.88 e*l’- 1

Then:

(A-l) If a fraction (f, ) of Pb was lost at time t’, then: ‘07Pb i-1“‘Pb

=measured

1

eA2’_Qe&t

- eV)

- 1

go*Pb/ *“Pb

=

137.88 ehlr -f,(e*l’ - e*l”) - 1 (A-2)

(2) The effect of Pb transport on “‘Pb / 206Pb ratios

u,

(A-5) For zone-2, the analogous expression is:

goaPb/

‘06Pb =

(A-6)

238

unmixed

!

x 100

relative to U and Th

Transport of Pb between two zones (zone-l and zone-2) of a monazite formed at times t, and t,, respectively. Th,, Th, and U,, U, are Th and U concentrations in zone-l, zone-2 at the present day, respectively. If no Pb exchange has occurred between the two zones, then for zone-l the *‘sPb/ *06Pb ratio at the present day is: 232Th, =-R

-1

l

X.K. Zhu et al. / Chemical Geology 135 (1997) 35-53

where

eh3t2 --fa(eh3’2 - e*S”) + bfa(eh3’l - ehlt’) - 1 R; = eh,c* _-fa(eAlt2 _ ehIr’) + !!jT(eAf’l - e*l”) - 1

References Belshaw, N.S., O’Nions, R.K., Martel, D.J. and Burton, D.J., 1994. High-resolution SIMS analysis of common lead. Chem. Geol., 112: 57-70. Chapman, H.J. and Moorbath, S., 1977. Lead isotope measurements from the oldest recognised Lewisian gneisses of northwest Scotland. Nature (London), 268: 41-42. Cohen, AS.., O’Nions, R.K. and O’Hara, M.J., 1991. Chronology and mechanism of depletion in Lewisian granulites. Co&b. Mineral. Petrol., 106: 142-153. Compston, W., Williams, IS. and Meyer, C., 1984. U-Pb geochronology of zircons from lunar breccia 73217 using a sensitive high mass-resolution ion probe. Proc. 14th Lunar Planet Sci. Conf., J. Geophys. Res., 89: B525-534. Copeland, P., Parrish, RR. and Harrison, T.M., 1988. Identification of inherited radilagenic Pb in monazite and its implications for U-Pb systematics. Nature (London), 333: 760-763. Corfu, F., Heaman, L.M. and Rogers, G., 1994. Polymetamorphic evolution of the Lewiisian complex, NW Scotland, as recorded by U-Pb isotopic compositions of zircon, titanite and rutile. Contrib. Mineral. Petrol., 117: 215-228. DeWolf, C.P., Belshaw, N.S. and O’Nions, R.K., 1993. A metamorphic history from micron-scale “‘Pb/ *06Pb chronometry of Archean monazite. Earth Planet. Sci. Lett., 120: 207-220.

53

Friend, C.R.L. and Kinny, P.D., 1995. New evidence for protolith ages of Lewisian granulite, northwest Scotland. Geology, 23: 1027-1030. Harrison, T.M., McKeegan, K.D. and LeFort, P., 1995. Detection of inherited monazite in the Manaslu leucogranite by “‘Pb/ 232Th ion microprobe dating: crystallisation age and tectonic implication. Earth Planet. Sci. Lett., 133: 271-282. Hinton, R.W., 1990. Ion probe trace element analysis of silicates: measurement of multi-element glass. Chem. Geol., 83: 1 l-25. Kinsbuty, J.A., Miller, CF., Wooden, J.L. and Harrison, T.M., 1993. Monazite paragenesis and U-Pb systematics in rocks of the eastern Mojave Desert, California, U.S.A.: implications for thermochronometry. Chem. Geol., 110: 147-167. Mohr, D.V., 1984. Zoned porphyroblasts of metamorphic monazite in the Anakeesta Formation, Great Amoky Mountain, North Carolina. Am. Mineral., 69: 98- 103. Overstreet, W.C., 1967. The geologic occurrence of monazite. U.S. Geol. Surv., Prof. Pap. No. 530. Parrish, R.R., 1990. U-PI, dating of monazite and its application to geological problems. Can. J. Earth Sci., 27: 1431-1450. Parrish, R.R., 1994. U-Pb problematics of very high-U accessory minerals: examples from the Himalaya and Cordillera and implications for U-Pb geochronology. U.S. Geol. Surv. Circ. (ICOG 8 Abstr.), 1107: 243. Pidgeon, R.T. and Bowes, D.R., 1972. Zircon U-PI, ages of granulites from the central region of the Lewisian, northwestem Scotland. Geol. Mag., 109: 47-258. Shimizu, N. and Hart, S.R., 1982. Isotope fractionation in secondary ion mass spectrometry. J. Appl. Phys., 53: 1303-1311. Smith, H.A. and Barreiro, B., 1990. Monazite U-Pb dating of staurolite grade metamorphism in pelitic schist. Contrib. Mineral. Petrol., 105: 602-615. Zhu, X.K., O’Nions, R.K. and Belshaw, N.S., Reed, S.J.B. and Bleser, T., 1993. History of the Lewisian complex from SIMS microchronometry of monazite (abstract). Eos (Trans. Am. Geophys. Union), 74: 678. Zhu, X.K., O’Nions, R.K. and Belshaw, N.S. and Gibb, A.J., Lewisian cmstal history from in situ SIMS mineral chronometry and related metamorphic textures. Chem. Geol., in press.