- Email: [email protected]
Zonation of monazite in metamorphic rocks and its implications for high temperature thermochronology: a case study from the Lewisian terrain
ELSEVIER
Earth and Planetary Science Letters 171 (1999) 209–220 www.elsevier.com/locate/epsl
Zonation of monazite in metamorphic rocks and its implications for high temperature thermochronology: a case study from the Lewisian terrain X.K. Zhu Ł , R.K. O’Nions Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, UK Received 8 March 1999; revised version received 7 June 1999; accepted 11 June 1999
Abstract Monazite is a valuable chronometer for many geological processes, and a proper interpretation for the age derived from monazite requires an adequate understanding of the behaviour of this phase and its chemical and isotopic systems during geological processes. This contribution reports the results of a detailed study on chemical and isotopic zonation of monazite in Lewisian granulites, which is of general significance for monazite geochronology in high-grade metamorphic rocks. Detailed investigations on two Lewisian granulite samples using SEM, EPMA and SIMS have revealed that Lewisian monazites frequently show petrographic, chemical and age zonation. The petrographical zonation, which reflects the chemical compositional variation, can be characterised as follows on the basis of the BSE images: (1) concentric zoning, (2) patchy zoning, and (3) ‘intergrowth-like’ zoning. The chondrite-normalised REE distribution patterns are remarkably similar between different zones of single monazite grains. This rules out the possibility that any portion of a zoned monazite in these two samples is detrital in origin. Some possible mechanisms are proposed for the formation of monazite zonation. These include: (1) intergrowth of monazite crystals with different composition; (2) episodic growth or regrowth of monazite in response to the change of environment conditions during thermal event(s), which may involve recrystallisation or replacement of the original crystals. In short, the diversity of zoning patterns in monazites observed in this study cannot be explained assuming a single mechanism. Accordingly, interpretation of the chemical and age information from any zoned monazite requires an adequate understanding of its formation mechanism. Line scans of Th, U, Pb, La, Ce, Nd, Sm, Ca and Si across monazite zones have been performed on two monazite grains which experienced granulite facies metamorphism. The very sharp boundaries in these line scans indicate that the diffusive transport of these elements in monazite is low even at the temperature of granulite facies metamorphism. The effective closure of monazite grains may also depend upon other factors such as the armouring effects of the host phase, and recrystallisation of the mineral itself. Thus great care needs to be taken for geological application of the closure temperature concept. 1999 Elsevier Science B.V. All rights reserved. Keywords: monazite; crystal zoning; substitution; secondary ion mass spectroscopy; rare earths; Th=U=Pb; Lewisian
1. Introduction By virtue of its high Th–U contents, rare inheritance [1–6] and low common Pb content, monazite Ł Corresponding
has been proved to be a valuable chronometer for many geological processes [3,4,7–14]. Some recent studies [7,10–14], however, have also demonstrated that the Th–U–Pb system in this mineral phase is
author. Tel.: C44 1865 272075; Fax: C44 1865 272072; E-mail: [email protected]
0012-821X/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 1 4 6 - 6
210
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
much more complex than expected. A proper interpretation for the age deduced from monazite requires a better understanding of the behaviour of this phase and its chemical and isotopic systems during geological processes. Besides being a valuable chronometer, monazite is also an accessory phase which exerts significant influence on the Th–U and REE budgets of the host rock [15–22]. Therefore, understanding the behaviour of monazite and its Th–U–Pb and REE systems during geological processes is also of great importance in studies of petrogenesis. Zonation is a common feature of metamorphic monazite, especially where grains are large [7,12,14,23–25]. Such zonation is usually accompanied by large variations in chemical compositions. In accordance with the chemical structure, some monazites also show age zonation [7,12,13]. We report here a study of zonation in monazites from two Lewisian granulite samples based on scanning electron microscopy (SEM), electron probe micro-analysis (EPMA), and secondary ion mass spectrometry (SIMS). In this way, the patterns of the zonation are characterised, plausible mechanisms for the zonation are discussed, the types of ionic substitution which result in the chemical compositional variations are constrained, and finally the transport of elements is considered.
mode. These two samples share a similar mineral assemblage of garnet–plagioclase–perthite–quartz and secondary biotite. The general petrography of these two samples and the SEM techniques have been described elsewhere [13,26]. Although monazite grains appear to be rather homogeneous optically, they frequently show strong zonation in BSE imaging. The zoning patterns revealed by SEM are diverse and include the following broad types: (1) Concentric zonation such as shown in Fig. 1a–e, (2) Patchy zonation as shown in Fig. 1f, and (3) ‘Intergrowth-like’ zonation as shown in Fig. 1g,h. Concentric zonation as illustrated in Fig. 1a–e is the most common type. Monazite showing concentric zoning usually has a high Th rim which is bright in BSE image and relatively low Th core which is dark in BSE image (Fig. 1a,b,c), but a reverse zoning pattern where the core has a high Th content and the rim has relatively low Th content (Fig. 1e), and a zoning pattern with high Th content in the intermediate portion (Fig. 1d) have also been observed. Monazite with ‘intergrowth-like’ zonation displays an internal microstructure analogous to that of classic petrographic intergrowth and its portions with different Th contents show interlocking. Simple (Fig. 1h) and more complex (Fig. 1g) varieties are observed.
2. Monazite petrography and zonation
3. Chemical zonation and REE distribution
Two granulite samples, 86004 and 86015, from the high-grade central region of the Lewisian complex at Scourie, NW Scotland, have been studied in detail. These two samples have been subjected to extensive petrographical investigation by means of optical microscopy and SEM in both back-scattered electron (BSE) mode and secondary electron (SE)
3.1. Chemical zonation The zonation displayed by the BSE images (Fig. 1a–h) reflects the overall chemical compositional variation within a single monazite crystal as shown in Tables 1 and 2. Two zoned monazite grains, monazite #5 in sample 86004 and monazite
Fig. 1. BSE images showing the types of monazite zonation in sample 86015. (a) Simple concentric zoning with a low Th core and high Th rim, monazite #6 in section 15-1, sample 86015. (b) Complex concentric zonation with Low Th core and high Th rim, monazite #5, sample 86004. The location for the EPMA traverse is marked as A–B. (c) Complex concentric zoning with Low Th core and high Th rim, monazite #2 in section 15L, sample 86015. The location an EMPA traverse is marked as A–B. (d) Complex concentric zoning with high Th intermediate portion, low Th core and low Th rim, monazite #1 in section 15J, sample 86015. Note the cuspate boundaries between zones. The locations for SIMS analyses performed are circled and numbered. (e) Reverse concentric zoning with high Th core and relatively low Th rim, monazite #5 in section 15B, sample 86015. (f) Patchy zoning, monazite #12 in section 15-1. (g) Complex ‘intergrowth-like’ zoning, monazite #4 in section 15J. The very bright spots around monazite are remains of previous gold coating. (h) Simple ‘intergrowth-like’ zoning, monazite #5 in section 15F, sample 86015.
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
211
212
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
Table 1 Representative EMPA results of monazites in sample 86004 Label
La2 O3 Ce2 O3 Pr2 O3 Nd2 O3 Sm2 O3 Eu2 O3 Gd2 O3 Y2 O3 ThO2 UO2 PbO P2 O5 SiO2 CaO Total
M#1, core-2 M#1, core-2 M#2, core M#2, rim M#3, core M#3, rim M#4, core M#4, rim M#5, core M#5, intermediate M#5, rim M#7, core M#7, rim M#8, core M#8, intermediate M#8, rim
14.32 14.69 14.93 13.28 15.15 12.51 15.81 12.74 16.51 13.67 12.26 14.81 13.36 16.12 13.99 13.01
30.81 31.42 31.32 29.82 32.02 28.82 30.91 27.26 31.39 28.18 25.81 32.30 30.68 32.64 31.51 30.48
3.57 3.42 3.44 3.44 3.28 3.13 3.27 3.09 3.25 3.58 3.08 3.41 3.21 3.19 3.37 3.25
12.16 12.03 11.96 12.22 11.19 11.28 11.10 11.48 10.88 12.23 11.01 11.59 11.19 10.69 11.30 11.67
1.25 1.25 1.23 1.27 1.19 1.25 1.16 1.27 1.05 1.13 1.24 1.13 1.19 1.14 1.05 1.18
0.05 0.06 0.07 0.05 0.05 0.06 0.08 0.06 0.05 0.09 0.08 0.02 0.06 0.04 0.08 0.06
0.40 0.44 0.41 0.47 0.43 0.44 0.50 0.49 0.43 0.43 0.52 0.34 0.41 0.45 0.27 0.42
0.06 0.07 0.05 0.10 0.08 0.07 0.08 0.10 0.07 0.07 0.08 0.06 0.04 0.14 0.06 0.08
5.53 6.36 5.95 8.69 6.18 11.44 5.68 11.87 5.46 7.82 14.16 6.41 10.85 4.83 7.82 11.32
0.17 0.2 0.18 0.36 0.16 0.42 0.18 0.37 0.16 0.15 0.5 0.17 0.50 0.22 0.17 0.44
0.67 0.77 0.72 0.96 0.77 1.28 0.67 1.30 0.65 0.88 1.55 0.78 1.26 0.59 0.85 1.29
28.05 27.84 28.04 27.26 28.63 26.19 28.82 26.71 28.64 28.19 26.44 28.02 26.79 29.08 27.70 25.56
0.55 0.26 0.22 0.61 0.39 1.17 0.23 2.00 0.34 0.69 1.46 0.50 1.39 0.22 0.60 1.40
1.16 1.18 1.13 1.35 1.17 1.59 1.18 1.64 0.97 1.37 1.94 1.12 1.24 1.04 1.46 1.43
98.75 100.02 100.03 99.90 100.71 99.64 99.67 99.61 99.87 98.48 100.17 99.65 102.16 100.35 100.24 101.60
Note: (1) M stands for monazite, (2) Dy, Er and Er were not detected in any of the monazites analysed.
Table 2 Representative EPMA results of zoned monazites in sample 86015 Label
La2 O3 Ce2 O3 Pr2 O3 Nd2 O3 Sm2 O3 Eu2 O3 Gd2 O3 Y2 O3 ThO2 UO2 PbO P2 O5 SiO2 CaO Total
M#2=section15L, core M#2=section15L, inter. M#2=section15L, rim M#5=section15B, core M#5=section15B, rim M#5=section15f, dark M#5=section15f, bright M#1=section15J, dark M#1=section15J, bright M#4=section15J, bright M#2=section15-1, core M#2=section15-1,rim M#6=section15-1, core M#6=section15-1, rim M#12=section15-1, dark M#12=section15-1, bright
15.90 14.15 11.06 10.39 13.34 13.71 9.73 14.52 10.49 11.85 7.48 13.05 14.03 10.15 11.64 8.93
32.03 29.58 25.28 26.22 28.83 30.65 23.74 30.52 24.16 24.62 18.72 29.99 32.06 25.63 25.76 23.08
3.56 3.37 3.20 3.20 3.18 3.78 3.15 3.74 3.03 2.94 2.58 3.88 3.93 3.18 3.30 3.25
13.19 11.68 12.39 12.89 12.23 15.33 13.13 14.65 11.89 11.53 10.68 15.19 15.29 12.24 12.70 13.35
1.43 0.88 0.97 1.17 1.11 1.43 1.09 1.12 0.91 0.91 0.87 1.23 1.34 1.09 1.23 1.40
#2 in the section-15L of sample 86015 were chosen for further detailed investigation by means of EMPA, which were performed using a CAMECA SX-50 electron probe in the Department of Earth Sciences, Cambridge University, following the techniques described elsewhere [26]. Monazite #5 in sample 86004 is a large (500 µm across) complexly zoned grain with a broad concentric pattern (Fig. 1b). An EPMA traverse for
0.27 0.16 0.18 0.29 0.31 0.26 0.20 0.17 0.19 0.18 0.21 0.28 0.34 0.27 0.36 0.39
0.04
2.78 9.80 16.51 14.95 9.26 3.79 18.56 4.04 18.16 16.45 28.04 3.86 1.11 13.00 12.16 14.07
0.52 0.34 0.80 0.77 0.61 0.65 0.94 0.41 0.83 0.84 1.16 0.59 0.48 0.81 0.74 1.34
0.54 1.20 2.05 1.89 1.24 0.74 2.39 0.64 2.27 2.20 3.65 0.67 0.36 1.82 1.71 2.21
27.16 25.17 23.20 24.93 26.27 27.05 23.62 28.19 24.64 25.18 21.05 28.45 29.11 27.02 26.23 27.81
0.28 0.82 2.11 2.54 0.84 0.59 2.56 0.54 2.56 2.34 4.94 0.54 0.26 1.50 1.83 0.98
0.50 1.35 1.65 0.94 1.49 0.59 1.72 0.67 1.76 1.72 1.78 0.71 0.30 1.72 1.16 2.47
98.21 98.51 99.39 100.25 98.77 98.63 100.91 99.28 100.89 100.75 101.21 98.49 98.74 98.50 98.94 99.45
Th, Pb, La, Ce, Si and Ca abundances across the zones of this grain has been obtained with a 2 µm step-size. Th reaches 12 wt% at the grain rim, and 5–7% in the complex bulk core region (Fig. 2a). It shows that there are no resolvable shifts between the Th and Pb profiles, and the boundaries between these zones within the grain are sharp. Both Ca and Si also show strong zonation, and their line scans closely follow the profile of Th (Fig. 2b). The contents of
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220 20
2.0
15
1.5
10
1.0
5
0.5
0
0
50
100
150
200
250
300
0.0
1.0
2.5 Ca Si
(b) 2.0
Si(wt%)
Pb(wt%)
Pb Th
0.5
1.5
1.0
0.5
Ca(wt.%)
Th(wt%)
(a)
0.0
0.0
0
50
100
150
200
250
300
Ce La
(c) La(wt%)
3.2. REE distribution
40
25
35
20
30 25
15 20 15
10
10 5
0
50
100
150
200
250
300
5
B
A
Monazite #2 in section-15L of sample 86015 is also a concentrically zoned grain of ca. 250 µm in diameter (Fig. 1c). Results of an EPMA traverse for Th, U, Pb, La, Ce, Nd, Sm, Si and Ca across the zones of this grain are plotted in Fig. 4a–d. Th varies from ca. 2 wt% in the core to ca. 15 wt% in the rim. In the intermediate zone the Th content is about 8 wt% (Fig. 3a). The Th and Pb profiles are very similar and show sharp boundaries between the different portions of the grain. In contrast to Th, the intermediate portion of the grain shows the lowest U content of ca. 0.25 wt% (Fig. 3a), compared to ca. 0.75 wt% in the rim. Si and Ca also show strong zonation with profiles that follow the Th zoning closely (Fig. 3b). La and Ce profiles parallel each other and are inversely related to the Th zoning pattern, with the highest contents in the core and the lowest in the rim (Fig. 3c). Similarly, Nd and Sm contents are highest in the core (Fig. 3d), but show no significant differences between the intermediate portion and the rim of the grain.
-0.5
Ce(wt%)
-0.5
213
Distance (µ m)
Fig. 2. EPMA traverse of monazite #5 in sample 86004. Both the step size and beam diameter are 2 µm. See Fig. 1b for the location. (a) Profiles of Th and Pb. Note that there are no relative shifts between the two profiles and the boundaries are sharp. (b) Profiles of Ca and Si, both closely follow the profiles of Th. (c) Profiles of La and Ce, both are negatively correlated with Th profile.
La and Ce vary widely and coherently. The profiles of these elements are inversely related to the Th zoning pattern, with lowest contents in the high Th rim (Fig. 2c).
All zoned monazite grains identified in sample 86004 show concentric zonation. Fig. 4a,b are plots showing the chondrite–normalised REE distribution of monazites #4 and 5, whereas the REE composition of other monazite grains in this sample are presented in Table 1. Despite the strong compositional zonation displayed by these grains (Table 1, Fig. 2), the chondrite–normalised REE distribution patterns are remarkably similar between the zones of a single grain and between different grains. As shown in Fig. 4a,b, the chondrite–normalised values of [La]N to [Sm]N plot on smooth curves and there are strong negative Eu anomalies with the Eu=Eu* ratios varying from 0.28 to 0.22. HREE are extremely depleted and the (La=Nd)N ratios for all the monazites in this sample fall into the range of 2.1 to 2.8. The chondrite–normalised REE distribution patterns for monazites with different zoning types in sample 86015 are plotted in Fig. 5a,d. They are much similar to monazites in sample 86004 with strong negative Eu anomalies and extreme HREE depletion. As for sample 86004, the REE distribution patterns are remarkably similar between different zones and grains.
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220 (b)
1.5
15
1.0
10
Si Ca
1.5
1.5
1.0
1.0
0.5
0.5
0.0
0.0
5
0
50
A
100
150
20
-0.5
-0.5
0
0
B
Distance(µm)
100
150
Distance( µ m)
B
15
35
2.5
Ce La
(c)
50
A
Sm Nd
(d)
20
Nd(wt%)
25
15
2.0
Ce(wt%)
La(wt%)
30
10 1.5
Sm(wt%)
0.5
0.0
Ca(wt%)
Th U Pb
Th(wt%)
U,Pb(wt%)
(a)
2.0
2.0
20
2.0
Si(wt%)
214
1.0 10
15
5 0.5
10 5
5 0
A
50
100
Distance(µm)
150
B
0.0
0 0
A
50
100
Distance( µ m)
150
B
Fig. 3. EPMA traverse of monazite #2 in section-15L, 86015. The step size is 3 µm and the beam size is 2 µm. See Fig. 1c for the location. (a) Th–U–Pb line scans. (b) Si–Ca line scans. (c) La–Ce line scans. (d) Nd–Sm line scans.
4. Age zonation In situ SIMS studies [12,13] have shown that some of the above monazites also display age zonation. New SIMS data have been obtained on an additional monazite grain (monazite # 1 in section 15J of sample 86015) using the ISOLAB -120 ion probe following the techniques published elsewhere [13,27]. This monazite grain has a high Th content in its intermediate portion, similar chemical features between core and rim, and irregularly shaped boundaries between the zones (Fig. 1d). The SIMS isotope results and 207 Pb=206 Pb ages are presented in Table 3, and the analysis locations are shown in Fig. 1d. Two separate analyses performed on the high Th intermediate portion yield 207 Pb=206 Pb ages of 2447š16 and 2450š22 Ma, which are indistinguishable from ages of 2447š7 and 2451š12 Ma recorded by a monazite core in the same sample [12]. Four separate analyses
carried out on the low Th portions of the grain give ages ranging from 2408 š 25 to 2399 š 23 Ma, consistent with the ages obtained previously from other rims of zoned monazites in the same sample [12,13]. These ages are indistinguishable from each other but some 40 Ma younger than the high Th part of the grain. These younger ages are coincident with the timing of the first episode intrusion of Scourie dykes into the Lewisian [28,29].
5. Discussion 5.1. Monazite compositional variation Monazite has an ideal formula of (LREE)[PO4 ], but natural monazites show a wide range of compositions as observed in this study (Tables 1 and 2) and reported in the literature, e.g. [5,17,26,30]. For
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
Monazite/Chondrite
10
6
M4.Rim M4.Core
10
10
5
4
(a) 10
Monazite/Chondrite
Ce
Pr
Nd
Sm Eu
Gd
5
10 4 (b) 10
Th4C C Ca2C ! 2LREE3C
(1)
U4C C Ca2C ! 2LREE3C
(2)
Th4C C Si4C ! LREE3C C P5C
(3)
U4C C Si4C ! LREE3C C P5C
(4)
X
6
LowThcore Intermediate HighThrim
10
ionic size and ionic charge are the governing factors. Following the line suggested by many workers [6,23, 31,35–37], the monazite compositional variation can be explained by the following substitutions:
3
La 10
215
3
La
Ce
Pr
Nd
Sm Eu
Gd
Fig. 4. Plots showing the chondrite–normalised REE distribution patterns of monazites in sample 86004. (a) Monazite #4, which shows simple concentric zonation. (b) Monazite #5, which shows complex concentric zonation. See Fig. 1b for the zoning structure.
example, ThO2 contents vary from 1% to 28 wt% for the monazites analysed in this study, and UO2 contents are reported to vary from <0.1 ppm [8] to 16 wt% [31]. Phases isostructural with monazite are huttonite (Th[SiO4 ]) [32] and brabantite (CaTh[PO4 ]2 ) [33, 34]. Much of the compositional variation in monazite can be explained by the solid solution of these three end members [33]. More recently, Franz et al. [6] have explained the 6Y (Y group which includes Y and HREE, and 6Ce means Ce group which includes La to Eu) contents in monazite in terms of another end member — xenotime, (Y, HREE)[PO4], which is the HREE counterpart of monazite but with a tetragonal structure. The compositional variation can also be explained in terms of ionic substitution, where the effective
Y3C !
X
Ce3C
(5)
The proposed coupled substitutions (1) to (4) can be tested through the relationships between (Th C U)–Ca, (Th C U)–Si and (Th C U)–(Si C Ca). Such plots are shown for zoned monazites in samples 86004 and 86015 in Fig. 6a,d. In the plots Pb is added to Th and U contents because it is a decay product of both Th and U, and all the elements abundances are in atom %. As illustrated in Fig. 6a,b, (Th C U)* is linearly related to (Si C Ca) with the (Th C U)* to (Si C Ca) ratio of unity. This excellent linear relationship between (Th C U)* to (Si C Ca) confirms the overall existence of the coupled substitutions (1) to (4). It is also observed that Si and Ca individually do not show a linear relationship with (Th C U)* , although they are positively correlated (Fig. 6c,d). This implies for those coupled substitutions described above the ‘deficiency’ of Si is compensated by the ‘excess’ of Ca, or vice versa. 5.2. Formation of monazite zonation Although zonation of monazite in metamorphic rocks is a well recognised phenomenon [3,6,7,12, 14,23], little is known about the mechanism of its formation. In general, the compositional zonation of a crystal may arise from a number of processes, which include overgrowth, regrowth, intergrowth, replacement and recrystallisation. As described above, zoning patterns of monazites may be broadly classified into different types. The ‘intergrowth-like’ pattern as shown in Fig. 1g,h exhibits internal microstructure similar to the intergrowth of two different minerals. The interlocking
216
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
10 6
Core Rim
10 5
10
4
Monazite/Chondrite
Monazite/Chondrite
10 6
Core Intermediate Rim
10 5
10
4
(a) 10
3
10 3 La
10
(b) Ce
Pr
Nd
Sm Eu
Gd
La
6
Ce
Pr
Nd
Sm Eu
10 6 lowThportion highThportion
Monazite/Chondrite
Monazite/Chondrite
Core Rim
10 5
10
4
10
5
10 4 (d)
(c) 10
Gd
3
10
La
Ce
Pr
Nd
Sm Eu
Gd
3
La
Ce
Pr
Nd
Sm Eu
Gd
Fig. 5. Plots showing the chondrite–normalised REE distribution patterns of monazites in sample 86015. (a) The distribution patterns of a monazite with simple concentric zonation (monazite #6 in section-1). See Fig. 1a for the zoning structure. (b) The distribution patterns of a complexly zoned monazite (monazite #2 in section-15L). See Fig. 1c for the zoning structure. Eu is artificially fixed as 0.02%, which is below the detection limit of 0.05%. (c) The distribution patterns of a monazite grain showing ‘reverse’ zonation (monazite #5 in section 15B). See Fig. 1e for the zoning structure. (d) The distribution patterns of a monazite grain with ‘intergrowth-like’ zonation (monazite #5 in section 15F). See Fig. 1h for the zoning structure.
of different portions of a monazite with different Th contents suggests that they have crystallised simultaneously. The coexistence of monazite grains with different composition in a single sample is well documented [5,6,13,17,37,38]. Thus it is considered here that intergrowth of monazites with different compositions is one of the mechanisms for monazite zonation. For the concentric zonation (Fig. 1a–e) which is the dominant zoning pattern observed in this study, explanations may include overgrowth around a detrital portion, episodic growth or replacement during metamorphic event(s). Some of the zoned monazite grains described here have previously been dated by
SIMS and show age zonation which overprints the chemical zonation [12]. From a chronological point of view, a particular concern is whether the cores which yielded the oldest ages are detrital in origin. As noted above that the REE distribution patterns of monazites are very similar between different zones of the same grain and between different grains in the same sample (Figs. 4 and 5). This would not be expected were any portion of the monazite detrital in origin, given the diversity of the REE patterns shown by natural monazite [26]. Thus from the available chemical and petrographic evidence and the previous SIMS investigations, a straightforward interpretation for these concentric zoning patterns is that they re-
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
217
Table 3 SIMS results of monazite #1 in section 15J, sample 86015 Analysis
Location
206 Pb=204 Pb
(208 Pb=206 Pb)m
(208 Pb=206 Pb)*
(207 Pb=206 Pb)m
(207 Pb=206 Pb)*
Age (Ma)
#1 #2 #3 #4 #5 #6
Low Th Core Low Th Core Low Th Core High Th intermediate High Th intermediate Low Th rim
4500 š 665 2280 š 80 54300 š 3500 12000 š 1100 11000 š 680 2100 š 120
4:573 š 0:022 4:776 š 0:037 5:777 š 0:054 8:059 š 0:059 6:879 š 0:064 5:527 š 0:033
4:580 š 0:022 4:790 š 0:037 5:778 š 0:054 8:065 š 0:059 6:886 š 0:064 5:548 š 0:033
0:1584 š 0:0017 0:1606 š 0:0018 0:1549 š 0:0021 0:1602 š 0:0015 0:1606 š 0:0021 0:1616 š 0:0022
0:1556 š 0:0017 0:1551 š 0:0019 0:1547 š 0:0021 0:1592 š 0:0015 0:1595 š 0:0021 0:1556 š 0:0022
2408 š 19 2403 š 21 2399 š 23 2447 š 16 2450 š 22 2408 š 25
All errors are quoted at 2¦ level. Common Pb corrections are based on initial Pb composition estimated using single stage Pb evolution model. Uncertainty estimated with an error propagation procedure that takes into account measurement errors and the effect of an uncertainty of 2% on the initial Pb composition. 4
3.0
(a)
(b) 2.5
(Ca+Si)
(Ca+Si)
3
2
2.0
1.5
1.0 1
0.5 0
0
1
2
3
0.0 0.0
4
0.5
1.0
(Th+U)*
1.5
2.0
2.5
3.0
(Th+U)*
2.0
1.5
(c)
(d)
1.0
(Ca+Si)
(Ca+Si)
1.5
1.0
0.5
0.5
0.0
Si Ca
Si Ca
0
1
2
3
(Th+U)*
4
0.0
0
1
2
3
(Th+U)*
Fig. 6. (a) Plot of (Th C U)* vs. (Si C Ca), monazite #2 in section 15L of sample 86015. (b) Plot of (Th C U)* vs. (Si C Ca), monazites #3, 4 and 5, 86004. (c) Plot of (Th C U)* vs. Si and Ca, monazite #2 in section 15L, 86015. (d) Plot of (Th C U)* vs. Si and Ca, monazites #3, 4 and 5, 86004. All elements are in atom%. (Th C U)* D Th C U C Pb.
flect episodic crystal growth, whereas the age structures overprinted on the chemical zonation of the monazites are the time records of those episodes.
Regrowth of monazite may result in some replacement or resorption of the original crystal, as indicated by the cuspate embayment boundaries be-
218
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
tween some monazite zones (Fig. 1a–e). Monazite #1 in section 15J of sample 86015 (Fig. 1d) provides further support for this view. As described in section 4, the high Th intermediate zone of this monazite gives the oldest age, whereas both the low Th core and rim yield ages which are some 40 Ma younger. This, together with the monazite petrography (Fig. 1d), suggests that the original monazite crystal with high Th content was subsequently replaced by low Th monazite which forms the core and the rim. However, it should be pointed out that the formation of a young monazite core does not always require replacement. As suggested by DeWolf et al. [7] recrystallisation in response to radiation damage may be another mechanism to produce young monazite cores for some monazites. In summary, monazite zonation can be formed by diverse mechanisms. A proper interpretation of the chemical and age information from a zoned monazite requires an adequate understanding of its formation mechanism. 5.3. Element transport and monazite closure Element diffusion and isotope closure in mineral phases are issues of fundamental importance in geochemical studies. The large compositional contrast within individual monazite grains provides an opportunity to examine the extent of element transport in monazite in response to thermal events. As discussed above, concentrically zoned monazites most probably result from discontinuous growth during thermal event(s). In situ SIMS chronometry [12] has shown that some zoned monazite grains in sample 86015 record a protracted series of events from ca. 2540 to 2000 Ma, whereas two episodes of granulite faces metamorphism of ½2740 and ca. 2530 Ma are recorded by the zoned monazites in sample 86004. The timing for the two episodes of granulites facies metamorphism in the central Lewisian is further supported by the ages obtained from monazite inclusions fully enclosed in garnet [13]. The preservation of the chemical and age structures in these zoned monazites implies that the element and isotope diffusion in this phase is sufficiently slow for the structures to survive a second episode of granulite faces metamorphism. This argument is further supported by the EPMA line
scans of Th, U, Pb, La, Ce, Nd, Sm, Ca and Si carried out across zones of monazite #5 in sample 86004 and monazite #2 in 86015 (Figs. 2 and 3). The very sharp boundaries in the line scans indicate that, diffusional transport of these elements on the ¾2 µm scale is hardly detectable by means of EPMA, after the formation of the high Th rim of monazite #5 in 86004 or the formation of the intermediate portion of monazite #2 in section 15L of sample 86015. It is worth recalling that these zones yield indistinguishable ages of ca. 2540 Ma [12], which were interpreted as the timing of the second episode of granulite faces metamorphism in Lewisian [12,13]. The low apparent diffusivities of some elements including Pb in monazite constrained here are broadly consistent with some previous studies [39,40]. This in principle offers the potential to deduce both chemical and age information from monazite grains formed at temperatures up to granulite facies. However it is worth emphasising that, firstly, the diffusion rates for U, Th, Pb and REE are expected to be significantly different at very high temperature as observed for zircon [41,42]. In this case, the age and chemical information derived even from the same portion of a single monazite grain may not directly correspond to each other. Secondly, the low apparent diffusivities of U, Th and Pb in monazite do not necessarily mean that the closure of U–Th–Pb system always occurs at a relatively fixed and very high temperature. As reviewed recently [43], the effective closure of a mineral phase depends also on a number of other factors such as fluid circulation in the host-rock, recrystallisation and lattice strain of the mineral itself. In the presence of these fast phenomena, volume diffusion is no longer the controlling factor for element and isotope transport in a mineral phase, and the closure temperature may be lower than expected by simple models based upon cooling rate and volume diffusion. Moreover, the armouring effect of host mineral on the closure temperature of a monazite inclusion has also been documented [7,13,25,26]. Thus even a small monazite grain (e.g. <50 µm) may survive very high temperature metamorphism if enclosed by a mineral with a higher closure temperature. This is simply because the host-mineral serves to shield monazite from reaction with other phases and to armour monazite from diffusive Pb loss. A striking example of
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
this phenomenon is observed in the Lewisian granulites, where small monazite grains fully enclosed in garnet, which is the earliest phase from a textural point of view, gave an metamorphic age of over 1000 Ma older than matrix monazite grains in a single thin section [13,26]. The application of the concept of closure temperature to monazite is further complicated by the fact that monazite is frequently zoned in high-grade metamorphic rocks as exemplified here. For a zoned monazite which preserves a polymetamorphic history, an apparent discordant pattern will be observed on a U–Pb concordia diagram if analysed by thermal ionisation mass spectrometry following techniques including conventional mechanical separation of minerals and chemical dissolution. This apparent discordant pattern however cannot be distinguished from the discordance resulting from Pb loss. In that case, the closure temperature for monazite will be under-estimated. Alternatively, if a fixed closure temperature is used, the temperature for that thermal event may be over-estimated.
Acknowledgements We would like to thank S.J.B. Reed for his help with EPMA in the Earth Sciences Department of Cambridge University. N.S. Belshaw and A.J. Gibb are thanked for their help in running ISOLAB-120. A.S. Cohen, K.W. Burton and F. von Blanckenburg are thanked for discussions during the early stage of the study. C.F. Miller, E.B. Watson and E.J. Essene are thanked for their thoughtful reviews. A.N. Halliday is thanked for his editorial comments. The authors are particularly grateful to M.S. O’Hara for introducing us to the Lewisian and its problems. This study was supported by NERC. [AH]
References [1] W.C. Overstreet, The geologic occurrence of monazite, US Geol. Surv. Prof. Pap. 530 (1967). [2] P. Copeland, R.R. Parrish, T.M. Harrison, Identification of inherited radiogenic Pb in monazite and its implications for U–Pb systematics, Nature 333 (1988) 760–763. [3] R.R. Parrish, U–Pb dating of monazite and its application to geological problems, Can. J. Earth Sci. 27 (1990) 1431– 1450.
219
[4] H.A. Smith, B. Barreiro, Monazite U–Pb dating of staurolite grade metamorphism in pelitic schist, Contrib. Mineral. Petrol. 105 (1990) 602–615. [5] J.A. Kingsbury, C.F. Miller, J.L. Wooden, T.M. Harrison, Monazite paragenesis and U–Pb systematics in rocks of the eastern Mojave Desert, California, U.S.A.: implications for thermochronometry, Chem. Geol. 110 (1993) 147–167. [6] G. Franz, G. Andrehs, D. Rhede, Crystal chemistry of monazite and xenotime from Saxothuringian–Moldanubian metapelites, NE Bavaria, Germany, Eur. J. Mineral. 8 (1996) 1097–1118. [7] C.P. DeWolf, N.S. Belshaw, R.K. O’Nions, A metamorphic history from micron-scale 207 Pb=206 Pb chronometry of Archean monazite, Earth Planet. Sci. Lett. 120 (1993) 207–220. [8] J. Wang, M. Tatsumoto, X. Li, W.R. Premo, E.C.T. Chao, A precise 232 Th–208 Pb chronology of fine-grained monazite: Age of the Bayan Obo REE–Fe–Nb ore deposit, China, Geochim. Cosmochim. Acta 58 (1994) 3155–3169. [9] J. Evans, J. Zalasiewicz, U–Pb, Pb–Pb and Sm–Nd dating of authigenic monazite: implications for the diagenetic evolution of the Welsh basin, Earth Planet. Sci. Lett. 144 (1996) 421–433. [10] D.P.A. Hawkins, S. Bowring, U–Pb systematics of monazite and xenotime: case studies from the Paleoproterozoic of the Grand Canyon, Arizona, Contrib. Mineral. Petrol. 127 (1997) 87–103. [11] X.K. Zhu, R.K. O’Nions, N.S. Belshaw, S.J.B. Reed, S. Moorbath, Ion probe dating of monazite: examples from the Isua and the Lewisian terrains, U.S. Geol. Serv. Circ. 1107 (1994) 369. [12] X.K. Zhu, R.K. O’Nions, N.S. Belshaw, A.J. Gibb, Significance of in situ SIMS chronometry of zoned monazite from the Lewisian granulites, northwest Scotland, Chem. Geol. 135 (1997) 35–53. [13] X.K. Zhu, R.K. O’Nions, N.S. Belshaw, A.J. Gibb, Lewisian crustal history from in situ SIMS mineral chronometry and related metamorphic textures, Chem. Geol. 136 (1997) 205–218. [14] B. Bingen, O. van Breemen, U–Pb monazite ages in amphibolite- to granulite-faces orthogneiss reflect hydrous mineral breakdown reactions: Sveconorwegian Province of SW Norway, Contrib. Mineral. Petrol. 132 (1998) 336–353. [15] W.N. Sawka, J.F. Banfield, B.W. Chappell, A weatheringrelated origin of widespread monazite in S-type granites, Geochim. Cosmochim. Acta 50 (1986) 171–175. [16] S.L. Hanson, W.B. Simmons, K.L. Webber, Rare earth-element mineralogy of granitic pegmatites in the Trout Creek Pass district, Chaffee County, Colorado, Can. Mineral. 30 (1992) 673–686. [17] J.M. Montel, A model for monazite=melt equilibrium and application to the generation of granitic magmas, Chem. Geol. 110 (1993) 127–146. [18] D.A. Wark, C.F. Miller, Accessory mineral behaviour during differentiation of a granite suite: monazite, xenotime and zircon in the Sweetwater Wash pluton, southeasthern California, U.S.A, Chem. Geol. 110 (1993) 49–67.
220
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
[19] G.R. Watt, S.L. Harley, Accessory phase controls on the geochemistry of crustal melts and restites produced during water-undersaturated partial melting, Contrib. Mineral. Petrol. 114 (1993) 550–566. [20] F. Bea, M.D. Pereira, A. Stroh, Mineral=leucosome traceelement partitioning in a pereluminous migmatite (a laser ablation-ICP–MS study), Chem. Geol. 117 (1994) 291– 312. [21] F. Bea, Residence of REE, Y, Th and U in granites and crustal protoliths; Implications for the geochemistry of crustal melts, J. Petrol. 37 (1996) 521–552. [22] F. Bea, Controls on the trace element composition of crustal melts, Trans. R. Soc. Edinburgh: Earth Sci. 87 (1996) 33– 41. [23] D.V. Mohr, Zoned porphyroblasts of metamorphic monazite in the Anakeesta Formation, Great Smoky Mountain, North Carolina, Am. Mineral. 69 (1984) 98–103. [24] G.R. Watt, High-thorium monazite-(Ce) formed during disequilibrium melting of metapelites under granulite-facies conditions, Mineral. Mag. 59 (1995) 735–743. [25] X.K. Zhu, High resolution in situ SIMS study of metamorphic monazite, PhD thesis, Cambridge University, 1997. [26] X.K. Zhu, R.K. O’Nions, Monazite chemical composition: some implications for monazite geochronology, Contrib. Mineral. Petrol. (in press). [27] N.S. Belshaw, R.K. O’Nions, D.J. Martel, D.J. Burton, High-resolution SIMS analysis of common lead, Chem. Geol. 112 (1994) 57–70. [28] H.J. Chapman, 2390 Myr Rb–Sr whole-rock ages for the Scourie dykes of north-west Scotland, Nature 277 (1979) 642–643. [29] L.M. Heaman, J. Tarney, U–Pb baddeleyite ages for the Scourie dyke swarm, Scotland: evidence for two distinct intrusion events, Nature 340 (1989) 705–708. [30] F. DeMartin, T. Pilati, V. Diella, S. Donzelli, C.M. Gra-
[31]
[32] [33] [34] [35]
[36]
[37]
[38]
[39]
[40] [41] [42] [43]
maccioli, Alpine monazite: further data, Can. Mineral. 29 (1991) 61–67. C.M. Gramaccioli, T.V. Segalstad, A uranium- and thoriumrich monazite from a south-alpine pegmatite at Piona, Italy, Am. Mineral. 63 (1978) 757–761. A. Pabst, C.O. Hutton, Huttonite, A new thorium silicate, Am. Mineral. 36 (1951) 60–65. S.H.U. Bowie, J.E.T. Horne, Cheralite, a new mineral of the monazite group, Mineral. Mag. 30 (1953) 93–99. D. Rose, Brabantite, CaTh(PO4 )2 , a new mineral of the monazite group, N. Jb. Mineral. Mh. (1980) 247–257. J.N.B. Chaudhuri, H. Newesely, On the REE-bearing minerals in the beach placers of Puri, Orissa District , J. SE. Asian Earth Sci. 8 (1993) 287–291. F. Poitrasson, S. Chenery, D.J. Bland, Contrasted monazite hydrothermal alteration mechanisms and their geochemical implications, Earth Planet. Sci. Lett. 145 (1996) 79–96. G.D. Ventura, A. Mottana, G.C. Parodi, M. Raudsepp, F. Bellatreccia, E. Caprilli, P. Rossi, S. Fiori, Monazite– huttonite solid-solutions from the Vico Volcanic Complex, Latium, Italy, Mineral. Mag. 60 (1996) 751–758. R. Casillas, G. Nagy, G. Panto, J. Brandle, I. Forizs, Occurrence of Th, U, Y, Zr, and REE-bearing accessory minerals in late-Variscan granitic rocks from the Sierra de Guadarrama (Spain), Eur. J. Mineral. 7 (1995) 989–1006. K. Suzuki, M. Adachi, I. Kajizuka, Electron microprobe observations of Pb diffusion in metamorphosed detrital monazites, Earth Planet. Sci. Lett. 128 (1994) 391–405. H.A. Smith, B.J. Giletti, Lead diffusion in monazite, Geochim. Cosmochim. Acta 61 (1997) 1047–1055. J.K.W. Lee, I.S. Williams, D.J. Ellis, Pb, U and Th diffusion in natural zircon, Nature 390 (1997) 159–161. D.J. Cherniak, J.M. Hanchar, E.B. Watson, Rare-earth diffusion in zircon, Chem. Geol. 134 (1997) 289–301. I.M. Villa, Isotopic closure, Terra Nova 10 (1998) 42–47.
Earth and Planetary Science Letters 171 (1999) 209–220 www.elsevier.com/locate/epsl
Zonation of monazite in metamorphic rocks and its implications for high temperature thermochronology: a case study from the Lewisian terrain X.K. Zhu Ł , R.K. O’Nions Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, UK Received 8 March 1999; revised version received 7 June 1999; accepted 11 June 1999
Abstract Monazite is a valuable chronometer for many geological processes, and a proper interpretation for the age derived from monazite requires an adequate understanding of the behaviour of this phase and its chemical and isotopic systems during geological processes. This contribution reports the results of a detailed study on chemical and isotopic zonation of monazite in Lewisian granulites, which is of general significance for monazite geochronology in high-grade metamorphic rocks. Detailed investigations on two Lewisian granulite samples using SEM, EPMA and SIMS have revealed that Lewisian monazites frequently show petrographic, chemical and age zonation. The petrographical zonation, which reflects the chemical compositional variation, can be characterised as follows on the basis of the BSE images: (1) concentric zoning, (2) patchy zoning, and (3) ‘intergrowth-like’ zoning. The chondrite-normalised REE distribution patterns are remarkably similar between different zones of single monazite grains. This rules out the possibility that any portion of a zoned monazite in these two samples is detrital in origin. Some possible mechanisms are proposed for the formation of monazite zonation. These include: (1) intergrowth of monazite crystals with different composition; (2) episodic growth or regrowth of monazite in response to the change of environment conditions during thermal event(s), which may involve recrystallisation or replacement of the original crystals. In short, the diversity of zoning patterns in monazites observed in this study cannot be explained assuming a single mechanism. Accordingly, interpretation of the chemical and age information from any zoned monazite requires an adequate understanding of its formation mechanism. Line scans of Th, U, Pb, La, Ce, Nd, Sm, Ca and Si across monazite zones have been performed on two monazite grains which experienced granulite facies metamorphism. The very sharp boundaries in these line scans indicate that the diffusive transport of these elements in monazite is low even at the temperature of granulite facies metamorphism. The effective closure of monazite grains may also depend upon other factors such as the armouring effects of the host phase, and recrystallisation of the mineral itself. Thus great care needs to be taken for geological application of the closure temperature concept. 1999 Elsevier Science B.V. All rights reserved. Keywords: monazite; crystal zoning; substitution; secondary ion mass spectroscopy; rare earths; Th=U=Pb; Lewisian
1. Introduction By virtue of its high Th–U contents, rare inheritance [1–6] and low common Pb content, monazite Ł Corresponding
has been proved to be a valuable chronometer for many geological processes [3,4,7–14]. Some recent studies [7,10–14], however, have also demonstrated that the Th–U–Pb system in this mineral phase is
author. Tel.: C44 1865 272075; Fax: C44 1865 272072; E-mail: [email protected]
0012-821X/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 1 4 6 - 6
210
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
much more complex than expected. A proper interpretation for the age deduced from monazite requires a better understanding of the behaviour of this phase and its chemical and isotopic systems during geological processes. Besides being a valuable chronometer, monazite is also an accessory phase which exerts significant influence on the Th–U and REE budgets of the host rock [15–22]. Therefore, understanding the behaviour of monazite and its Th–U–Pb and REE systems during geological processes is also of great importance in studies of petrogenesis. Zonation is a common feature of metamorphic monazite, especially where grains are large [7,12,14,23–25]. Such zonation is usually accompanied by large variations in chemical compositions. In accordance with the chemical structure, some monazites also show age zonation [7,12,13]. We report here a study of zonation in monazites from two Lewisian granulite samples based on scanning electron microscopy (SEM), electron probe micro-analysis (EPMA), and secondary ion mass spectrometry (SIMS). In this way, the patterns of the zonation are characterised, plausible mechanisms for the zonation are discussed, the types of ionic substitution which result in the chemical compositional variations are constrained, and finally the transport of elements is considered.
mode. These two samples share a similar mineral assemblage of garnet–plagioclase–perthite–quartz and secondary biotite. The general petrography of these two samples and the SEM techniques have been described elsewhere [13,26]. Although monazite grains appear to be rather homogeneous optically, they frequently show strong zonation in BSE imaging. The zoning patterns revealed by SEM are diverse and include the following broad types: (1) Concentric zonation such as shown in Fig. 1a–e, (2) Patchy zonation as shown in Fig. 1f, and (3) ‘Intergrowth-like’ zonation as shown in Fig. 1g,h. Concentric zonation as illustrated in Fig. 1a–e is the most common type. Monazite showing concentric zoning usually has a high Th rim which is bright in BSE image and relatively low Th core which is dark in BSE image (Fig. 1a,b,c), but a reverse zoning pattern where the core has a high Th content and the rim has relatively low Th content (Fig. 1e), and a zoning pattern with high Th content in the intermediate portion (Fig. 1d) have also been observed. Monazite with ‘intergrowth-like’ zonation displays an internal microstructure analogous to that of classic petrographic intergrowth and its portions with different Th contents show interlocking. Simple (Fig. 1h) and more complex (Fig. 1g) varieties are observed.
2. Monazite petrography and zonation
3. Chemical zonation and REE distribution
Two granulite samples, 86004 and 86015, from the high-grade central region of the Lewisian complex at Scourie, NW Scotland, have been studied in detail. These two samples have been subjected to extensive petrographical investigation by means of optical microscopy and SEM in both back-scattered electron (BSE) mode and secondary electron (SE)
3.1. Chemical zonation The zonation displayed by the BSE images (Fig. 1a–h) reflects the overall chemical compositional variation within a single monazite crystal as shown in Tables 1 and 2. Two zoned monazite grains, monazite #5 in sample 86004 and monazite
Fig. 1. BSE images showing the types of monazite zonation in sample 86015. (a) Simple concentric zoning with a low Th core and high Th rim, monazite #6 in section 15-1, sample 86015. (b) Complex concentric zonation with Low Th core and high Th rim, monazite #5, sample 86004. The location for the EPMA traverse is marked as A–B. (c) Complex concentric zoning with Low Th core and high Th rim, monazite #2 in section 15L, sample 86015. The location an EMPA traverse is marked as A–B. (d) Complex concentric zoning with high Th intermediate portion, low Th core and low Th rim, monazite #1 in section 15J, sample 86015. Note the cuspate boundaries between zones. The locations for SIMS analyses performed are circled and numbered. (e) Reverse concentric zoning with high Th core and relatively low Th rim, monazite #5 in section 15B, sample 86015. (f) Patchy zoning, monazite #12 in section 15-1. (g) Complex ‘intergrowth-like’ zoning, monazite #4 in section 15J. The very bright spots around monazite are remains of previous gold coating. (h) Simple ‘intergrowth-like’ zoning, monazite #5 in section 15F, sample 86015.
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
211
212
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
Table 1 Representative EMPA results of monazites in sample 86004 Label
La2 O3 Ce2 O3 Pr2 O3 Nd2 O3 Sm2 O3 Eu2 O3 Gd2 O3 Y2 O3 ThO2 UO2 PbO P2 O5 SiO2 CaO Total
M#1, core-2 M#1, core-2 M#2, core M#2, rim M#3, core M#3, rim M#4, core M#4, rim M#5, core M#5, intermediate M#5, rim M#7, core M#7, rim M#8, core M#8, intermediate M#8, rim
14.32 14.69 14.93 13.28 15.15 12.51 15.81 12.74 16.51 13.67 12.26 14.81 13.36 16.12 13.99 13.01
30.81 31.42 31.32 29.82 32.02 28.82 30.91 27.26 31.39 28.18 25.81 32.30 30.68 32.64 31.51 30.48
3.57 3.42 3.44 3.44 3.28 3.13 3.27 3.09 3.25 3.58 3.08 3.41 3.21 3.19 3.37 3.25
12.16 12.03 11.96 12.22 11.19 11.28 11.10 11.48 10.88 12.23 11.01 11.59 11.19 10.69 11.30 11.67
1.25 1.25 1.23 1.27 1.19 1.25 1.16 1.27 1.05 1.13 1.24 1.13 1.19 1.14 1.05 1.18
0.05 0.06 0.07 0.05 0.05 0.06 0.08 0.06 0.05 0.09 0.08 0.02 0.06 0.04 0.08 0.06
0.40 0.44 0.41 0.47 0.43 0.44 0.50 0.49 0.43 0.43 0.52 0.34 0.41 0.45 0.27 0.42
0.06 0.07 0.05 0.10 0.08 0.07 0.08 0.10 0.07 0.07 0.08 0.06 0.04 0.14 0.06 0.08
5.53 6.36 5.95 8.69 6.18 11.44 5.68 11.87 5.46 7.82 14.16 6.41 10.85 4.83 7.82 11.32
0.17 0.2 0.18 0.36 0.16 0.42 0.18 0.37 0.16 0.15 0.5 0.17 0.50 0.22 0.17 0.44
0.67 0.77 0.72 0.96 0.77 1.28 0.67 1.30 0.65 0.88 1.55 0.78 1.26 0.59 0.85 1.29
28.05 27.84 28.04 27.26 28.63 26.19 28.82 26.71 28.64 28.19 26.44 28.02 26.79 29.08 27.70 25.56
0.55 0.26 0.22 0.61 0.39 1.17 0.23 2.00 0.34 0.69 1.46 0.50 1.39 0.22 0.60 1.40
1.16 1.18 1.13 1.35 1.17 1.59 1.18 1.64 0.97 1.37 1.94 1.12 1.24 1.04 1.46 1.43
98.75 100.02 100.03 99.90 100.71 99.64 99.67 99.61 99.87 98.48 100.17 99.65 102.16 100.35 100.24 101.60
Note: (1) M stands for monazite, (2) Dy, Er and Er were not detected in any of the monazites analysed.
Table 2 Representative EPMA results of zoned monazites in sample 86015 Label
La2 O3 Ce2 O3 Pr2 O3 Nd2 O3 Sm2 O3 Eu2 O3 Gd2 O3 Y2 O3 ThO2 UO2 PbO P2 O5 SiO2 CaO Total
M#2=section15L, core M#2=section15L, inter. M#2=section15L, rim M#5=section15B, core M#5=section15B, rim M#5=section15f, dark M#5=section15f, bright M#1=section15J, dark M#1=section15J, bright M#4=section15J, bright M#2=section15-1, core M#2=section15-1,rim M#6=section15-1, core M#6=section15-1, rim M#12=section15-1, dark M#12=section15-1, bright
15.90 14.15 11.06 10.39 13.34 13.71 9.73 14.52 10.49 11.85 7.48 13.05 14.03 10.15 11.64 8.93
32.03 29.58 25.28 26.22 28.83 30.65 23.74 30.52 24.16 24.62 18.72 29.99 32.06 25.63 25.76 23.08
3.56 3.37 3.20 3.20 3.18 3.78 3.15 3.74 3.03 2.94 2.58 3.88 3.93 3.18 3.30 3.25
13.19 11.68 12.39 12.89 12.23 15.33 13.13 14.65 11.89 11.53 10.68 15.19 15.29 12.24 12.70 13.35
1.43 0.88 0.97 1.17 1.11 1.43 1.09 1.12 0.91 0.91 0.87 1.23 1.34 1.09 1.23 1.40
#2 in the section-15L of sample 86015 were chosen for further detailed investigation by means of EMPA, which were performed using a CAMECA SX-50 electron probe in the Department of Earth Sciences, Cambridge University, following the techniques described elsewhere [26]. Monazite #5 in sample 86004 is a large (500 µm across) complexly zoned grain with a broad concentric pattern (Fig. 1b). An EPMA traverse for
0.27 0.16 0.18 0.29 0.31 0.26 0.20 0.17 0.19 0.18 0.21 0.28 0.34 0.27 0.36 0.39
0.04
2.78 9.80 16.51 14.95 9.26 3.79 18.56 4.04 18.16 16.45 28.04 3.86 1.11 13.00 12.16 14.07
0.52 0.34 0.80 0.77 0.61 0.65 0.94 0.41 0.83 0.84 1.16 0.59 0.48 0.81 0.74 1.34
0.54 1.20 2.05 1.89 1.24 0.74 2.39 0.64 2.27 2.20 3.65 0.67 0.36 1.82 1.71 2.21
27.16 25.17 23.20 24.93 26.27 27.05 23.62 28.19 24.64 25.18 21.05 28.45 29.11 27.02 26.23 27.81
0.28 0.82 2.11 2.54 0.84 0.59 2.56 0.54 2.56 2.34 4.94 0.54 0.26 1.50 1.83 0.98
0.50 1.35 1.65 0.94 1.49 0.59 1.72 0.67 1.76 1.72 1.78 0.71 0.30 1.72 1.16 2.47
98.21 98.51 99.39 100.25 98.77 98.63 100.91 99.28 100.89 100.75 101.21 98.49 98.74 98.50 98.94 99.45
Th, Pb, La, Ce, Si and Ca abundances across the zones of this grain has been obtained with a 2 µm step-size. Th reaches 12 wt% at the grain rim, and 5–7% in the complex bulk core region (Fig. 2a). It shows that there are no resolvable shifts between the Th and Pb profiles, and the boundaries between these zones within the grain are sharp. Both Ca and Si also show strong zonation, and their line scans closely follow the profile of Th (Fig. 2b). The contents of
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220 20
2.0
15
1.5
10
1.0
5
0.5
0
0
50
100
150
200
250
300
0.0
1.0
2.5 Ca Si
(b) 2.0
Si(wt%)
Pb(wt%)
Pb Th
0.5
1.5
1.0
0.5
Ca(wt.%)
Th(wt%)
(a)
0.0
0.0
0
50
100
150
200
250
300
Ce La
(c) La(wt%)
3.2. REE distribution
40
25
35
20
30 25
15 20 15
10
10 5
0
50
100
150
200
250
300
5
B
A
Monazite #2 in section-15L of sample 86015 is also a concentrically zoned grain of ca. 250 µm in diameter (Fig. 1c). Results of an EPMA traverse for Th, U, Pb, La, Ce, Nd, Sm, Si and Ca across the zones of this grain are plotted in Fig. 4a–d. Th varies from ca. 2 wt% in the core to ca. 15 wt% in the rim. In the intermediate zone the Th content is about 8 wt% (Fig. 3a). The Th and Pb profiles are very similar and show sharp boundaries between the different portions of the grain. In contrast to Th, the intermediate portion of the grain shows the lowest U content of ca. 0.25 wt% (Fig. 3a), compared to ca. 0.75 wt% in the rim. Si and Ca also show strong zonation with profiles that follow the Th zoning closely (Fig. 3b). La and Ce profiles parallel each other and are inversely related to the Th zoning pattern, with the highest contents in the core and the lowest in the rim (Fig. 3c). Similarly, Nd and Sm contents are highest in the core (Fig. 3d), but show no significant differences between the intermediate portion and the rim of the grain.
-0.5
Ce(wt%)
-0.5
213
Distance (µ m)
Fig. 2. EPMA traverse of monazite #5 in sample 86004. Both the step size and beam diameter are 2 µm. See Fig. 1b for the location. (a) Profiles of Th and Pb. Note that there are no relative shifts between the two profiles and the boundaries are sharp. (b) Profiles of Ca and Si, both closely follow the profiles of Th. (c) Profiles of La and Ce, both are negatively correlated with Th profile.
La and Ce vary widely and coherently. The profiles of these elements are inversely related to the Th zoning pattern, with lowest contents in the high Th rim (Fig. 2c).
All zoned monazite grains identified in sample 86004 show concentric zonation. Fig. 4a,b are plots showing the chondrite–normalised REE distribution of monazites #4 and 5, whereas the REE composition of other monazite grains in this sample are presented in Table 1. Despite the strong compositional zonation displayed by these grains (Table 1, Fig. 2), the chondrite–normalised REE distribution patterns are remarkably similar between the zones of a single grain and between different grains. As shown in Fig. 4a,b, the chondrite–normalised values of [La]N to [Sm]N plot on smooth curves and there are strong negative Eu anomalies with the Eu=Eu* ratios varying from 0.28 to 0.22. HREE are extremely depleted and the (La=Nd)N ratios for all the monazites in this sample fall into the range of 2.1 to 2.8. The chondrite–normalised REE distribution patterns for monazites with different zoning types in sample 86015 are plotted in Fig. 5a,d. They are much similar to monazites in sample 86004 with strong negative Eu anomalies and extreme HREE depletion. As for sample 86004, the REE distribution patterns are remarkably similar between different zones and grains.
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220 (b)
1.5
15
1.0
10
Si Ca
1.5
1.5
1.0
1.0
0.5
0.5
0.0
0.0
5
0
50
A
100
150
20
-0.5
-0.5
0
0
B
Distance(µm)
100
150
Distance( µ m)
B
15
35
2.5
Ce La
(c)
50
A
Sm Nd
(d)
20
Nd(wt%)
25
15
2.0
Ce(wt%)
La(wt%)
30
10 1.5
Sm(wt%)
0.5
0.0
Ca(wt%)
Th U Pb
Th(wt%)
U,Pb(wt%)
(a)
2.0
2.0
20
2.0
Si(wt%)
214
1.0 10
15
5 0.5
10 5
5 0
A
50
100
Distance(µm)
150
B
0.0
0 0
A
50
100
Distance( µ m)
150
B
Fig. 3. EPMA traverse of monazite #2 in section-15L, 86015. The step size is 3 µm and the beam size is 2 µm. See Fig. 1c for the location. (a) Th–U–Pb line scans. (b) Si–Ca line scans. (c) La–Ce line scans. (d) Nd–Sm line scans.
4. Age zonation In situ SIMS studies [12,13] have shown that some of the above monazites also display age zonation. New SIMS data have been obtained on an additional monazite grain (monazite # 1 in section 15J of sample 86015) using the ISOLAB -120 ion probe following the techniques published elsewhere [13,27]. This monazite grain has a high Th content in its intermediate portion, similar chemical features between core and rim, and irregularly shaped boundaries between the zones (Fig. 1d). The SIMS isotope results and 207 Pb=206 Pb ages are presented in Table 3, and the analysis locations are shown in Fig. 1d. Two separate analyses performed on the high Th intermediate portion yield 207 Pb=206 Pb ages of 2447š16 and 2450š22 Ma, which are indistinguishable from ages of 2447š7 and 2451š12 Ma recorded by a monazite core in the same sample [12]. Four separate analyses
carried out on the low Th portions of the grain give ages ranging from 2408 š 25 to 2399 š 23 Ma, consistent with the ages obtained previously from other rims of zoned monazites in the same sample [12,13]. These ages are indistinguishable from each other but some 40 Ma younger than the high Th part of the grain. These younger ages are coincident with the timing of the first episode intrusion of Scourie dykes into the Lewisian [28,29].
5. Discussion 5.1. Monazite compositional variation Monazite has an ideal formula of (LREE)[PO4 ], but natural monazites show a wide range of compositions as observed in this study (Tables 1 and 2) and reported in the literature, e.g. [5,17,26,30]. For
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
Monazite/Chondrite
10
6
M4.Rim M4.Core
10
10
5
4
(a) 10
Monazite/Chondrite
Ce
Pr
Nd
Sm Eu
Gd
5
10 4 (b) 10
Th4C C Ca2C ! 2LREE3C
(1)
U4C C Ca2C ! 2LREE3C
(2)
Th4C C Si4C ! LREE3C C P5C
(3)
U4C C Si4C ! LREE3C C P5C
(4)
X
6
LowThcore Intermediate HighThrim
10
ionic size and ionic charge are the governing factors. Following the line suggested by many workers [6,23, 31,35–37], the monazite compositional variation can be explained by the following substitutions:
3
La 10
215
3
La
Ce
Pr
Nd
Sm Eu
Gd
Fig. 4. Plots showing the chondrite–normalised REE distribution patterns of monazites in sample 86004. (a) Monazite #4, which shows simple concentric zonation. (b) Monazite #5, which shows complex concentric zonation. See Fig. 1b for the zoning structure.
example, ThO2 contents vary from 1% to 28 wt% for the monazites analysed in this study, and UO2 contents are reported to vary from <0.1 ppm [8] to 16 wt% [31]. Phases isostructural with monazite are huttonite (Th[SiO4 ]) [32] and brabantite (CaTh[PO4 ]2 ) [33, 34]. Much of the compositional variation in monazite can be explained by the solid solution of these three end members [33]. More recently, Franz et al. [6] have explained the 6Y (Y group which includes Y and HREE, and 6Ce means Ce group which includes La to Eu) contents in monazite in terms of another end member — xenotime, (Y, HREE)[PO4], which is the HREE counterpart of monazite but with a tetragonal structure. The compositional variation can also be explained in terms of ionic substitution, where the effective
Y3C !
X
Ce3C
(5)
The proposed coupled substitutions (1) to (4) can be tested through the relationships between (Th C U)–Ca, (Th C U)–Si and (Th C U)–(Si C Ca). Such plots are shown for zoned monazites in samples 86004 and 86015 in Fig. 6a,d. In the plots Pb is added to Th and U contents because it is a decay product of both Th and U, and all the elements abundances are in atom %. As illustrated in Fig. 6a,b, (Th C U)* is linearly related to (Si C Ca) with the (Th C U)* to (Si C Ca) ratio of unity. This excellent linear relationship between (Th C U)* to (Si C Ca) confirms the overall existence of the coupled substitutions (1) to (4). It is also observed that Si and Ca individually do not show a linear relationship with (Th C U)* , although they are positively correlated (Fig. 6c,d). This implies for those coupled substitutions described above the ‘deficiency’ of Si is compensated by the ‘excess’ of Ca, or vice versa. 5.2. Formation of monazite zonation Although zonation of monazite in metamorphic rocks is a well recognised phenomenon [3,6,7,12, 14,23], little is known about the mechanism of its formation. In general, the compositional zonation of a crystal may arise from a number of processes, which include overgrowth, regrowth, intergrowth, replacement and recrystallisation. As described above, zoning patterns of monazites may be broadly classified into different types. The ‘intergrowth-like’ pattern as shown in Fig. 1g,h exhibits internal microstructure similar to the intergrowth of two different minerals. The interlocking
216
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
10 6
Core Rim
10 5
10
4
Monazite/Chondrite
Monazite/Chondrite
10 6
Core Intermediate Rim
10 5
10
4
(a) 10
3
10 3 La
10
(b) Ce
Pr
Nd
Sm Eu
Gd
La
6
Ce
Pr
Nd
Sm Eu
10 6 lowThportion highThportion
Monazite/Chondrite
Monazite/Chondrite
Core Rim
10 5
10
4
10
5
10 4 (d)
(c) 10
Gd
3
10
La
Ce
Pr
Nd
Sm Eu
Gd
3
La
Ce
Pr
Nd
Sm Eu
Gd
Fig. 5. Plots showing the chondrite–normalised REE distribution patterns of monazites in sample 86015. (a) The distribution patterns of a monazite with simple concentric zonation (monazite #6 in section-1). See Fig. 1a for the zoning structure. (b) The distribution patterns of a complexly zoned monazite (monazite #2 in section-15L). See Fig. 1c for the zoning structure. Eu is artificially fixed as 0.02%, which is below the detection limit of 0.05%. (c) The distribution patterns of a monazite grain showing ‘reverse’ zonation (monazite #5 in section 15B). See Fig. 1e for the zoning structure. (d) The distribution patterns of a monazite grain with ‘intergrowth-like’ zonation (monazite #5 in section 15F). See Fig. 1h for the zoning structure.
of different portions of a monazite with different Th contents suggests that they have crystallised simultaneously. The coexistence of monazite grains with different composition in a single sample is well documented [5,6,13,17,37,38]. Thus it is considered here that intergrowth of monazites with different compositions is one of the mechanisms for monazite zonation. For the concentric zonation (Fig. 1a–e) which is the dominant zoning pattern observed in this study, explanations may include overgrowth around a detrital portion, episodic growth or replacement during metamorphic event(s). Some of the zoned monazite grains described here have previously been dated by
SIMS and show age zonation which overprints the chemical zonation [12]. From a chronological point of view, a particular concern is whether the cores which yielded the oldest ages are detrital in origin. As noted above that the REE distribution patterns of monazites are very similar between different zones of the same grain and between different grains in the same sample (Figs. 4 and 5). This would not be expected were any portion of the monazite detrital in origin, given the diversity of the REE patterns shown by natural monazite [26]. Thus from the available chemical and petrographic evidence and the previous SIMS investigations, a straightforward interpretation for these concentric zoning patterns is that they re-
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
217
Table 3 SIMS results of monazite #1 in section 15J, sample 86015 Analysis
Location
206 Pb=204 Pb
(208 Pb=206 Pb)m
(208 Pb=206 Pb)*
(207 Pb=206 Pb)m
(207 Pb=206 Pb)*
Age (Ma)
#1 #2 #3 #4 #5 #6
Low Th Core Low Th Core Low Th Core High Th intermediate High Th intermediate Low Th rim
4500 š 665 2280 š 80 54300 š 3500 12000 š 1100 11000 š 680 2100 š 120
4:573 š 0:022 4:776 š 0:037 5:777 š 0:054 8:059 š 0:059 6:879 š 0:064 5:527 š 0:033
4:580 š 0:022 4:790 š 0:037 5:778 š 0:054 8:065 š 0:059 6:886 š 0:064 5:548 š 0:033
0:1584 š 0:0017 0:1606 š 0:0018 0:1549 š 0:0021 0:1602 š 0:0015 0:1606 š 0:0021 0:1616 š 0:0022
0:1556 š 0:0017 0:1551 š 0:0019 0:1547 š 0:0021 0:1592 š 0:0015 0:1595 š 0:0021 0:1556 š 0:0022
2408 š 19 2403 š 21 2399 š 23 2447 š 16 2450 š 22 2408 š 25
All errors are quoted at 2¦ level. Common Pb corrections are based on initial Pb composition estimated using single stage Pb evolution model. Uncertainty estimated with an error propagation procedure that takes into account measurement errors and the effect of an uncertainty of 2% on the initial Pb composition. 4
3.0
(a)
(b) 2.5
(Ca+Si)
(Ca+Si)
3
2
2.0
1.5
1.0 1
0.5 0
0
1
2
3
0.0 0.0
4
0.5
1.0
(Th+U)*
1.5
2.0
2.5
3.0
(Th+U)*
2.0
1.5
(c)
(d)
1.0
(Ca+Si)
(Ca+Si)
1.5
1.0
0.5
0.5
0.0
Si Ca
Si Ca
0
1
2
3
(Th+U)*
4
0.0
0
1
2
3
(Th+U)*
Fig. 6. (a) Plot of (Th C U)* vs. (Si C Ca), monazite #2 in section 15L of sample 86015. (b) Plot of (Th C U)* vs. (Si C Ca), monazites #3, 4 and 5, 86004. (c) Plot of (Th C U)* vs. Si and Ca, monazite #2 in section 15L, 86015. (d) Plot of (Th C U)* vs. Si and Ca, monazites #3, 4 and 5, 86004. All elements are in atom%. (Th C U)* D Th C U C Pb.
flect episodic crystal growth, whereas the age structures overprinted on the chemical zonation of the monazites are the time records of those episodes.
Regrowth of monazite may result in some replacement or resorption of the original crystal, as indicated by the cuspate embayment boundaries be-
218
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
tween some monazite zones (Fig. 1a–e). Monazite #1 in section 15J of sample 86015 (Fig. 1d) provides further support for this view. As described in section 4, the high Th intermediate zone of this monazite gives the oldest age, whereas both the low Th core and rim yield ages which are some 40 Ma younger. This, together with the monazite petrography (Fig. 1d), suggests that the original monazite crystal with high Th content was subsequently replaced by low Th monazite which forms the core and the rim. However, it should be pointed out that the formation of a young monazite core does not always require replacement. As suggested by DeWolf et al. [7] recrystallisation in response to radiation damage may be another mechanism to produce young monazite cores for some monazites. In summary, monazite zonation can be formed by diverse mechanisms. A proper interpretation of the chemical and age information from a zoned monazite requires an adequate understanding of its formation mechanism. 5.3. Element transport and monazite closure Element diffusion and isotope closure in mineral phases are issues of fundamental importance in geochemical studies. The large compositional contrast within individual monazite grains provides an opportunity to examine the extent of element transport in monazite in response to thermal events. As discussed above, concentrically zoned monazites most probably result from discontinuous growth during thermal event(s). In situ SIMS chronometry [12] has shown that some zoned monazite grains in sample 86015 record a protracted series of events from ca. 2540 to 2000 Ma, whereas two episodes of granulite faces metamorphism of ½2740 and ca. 2530 Ma are recorded by the zoned monazites in sample 86004. The timing for the two episodes of granulites facies metamorphism in the central Lewisian is further supported by the ages obtained from monazite inclusions fully enclosed in garnet [13]. The preservation of the chemical and age structures in these zoned monazites implies that the element and isotope diffusion in this phase is sufficiently slow for the structures to survive a second episode of granulite faces metamorphism. This argument is further supported by the EPMA line
scans of Th, U, Pb, La, Ce, Nd, Sm, Ca and Si carried out across zones of monazite #5 in sample 86004 and monazite #2 in 86015 (Figs. 2 and 3). The very sharp boundaries in the line scans indicate that, diffusional transport of these elements on the ¾2 µm scale is hardly detectable by means of EPMA, after the formation of the high Th rim of monazite #5 in 86004 or the formation of the intermediate portion of monazite #2 in section 15L of sample 86015. It is worth recalling that these zones yield indistinguishable ages of ca. 2540 Ma [12], which were interpreted as the timing of the second episode of granulite faces metamorphism in Lewisian [12,13]. The low apparent diffusivities of some elements including Pb in monazite constrained here are broadly consistent with some previous studies [39,40]. This in principle offers the potential to deduce both chemical and age information from monazite grains formed at temperatures up to granulite facies. However it is worth emphasising that, firstly, the diffusion rates for U, Th, Pb and REE are expected to be significantly different at very high temperature as observed for zircon [41,42]. In this case, the age and chemical information derived even from the same portion of a single monazite grain may not directly correspond to each other. Secondly, the low apparent diffusivities of U, Th and Pb in monazite do not necessarily mean that the closure of U–Th–Pb system always occurs at a relatively fixed and very high temperature. As reviewed recently [43], the effective closure of a mineral phase depends also on a number of other factors such as fluid circulation in the host-rock, recrystallisation and lattice strain of the mineral itself. In the presence of these fast phenomena, volume diffusion is no longer the controlling factor for element and isotope transport in a mineral phase, and the closure temperature may be lower than expected by simple models based upon cooling rate and volume diffusion. Moreover, the armouring effect of host mineral on the closure temperature of a monazite inclusion has also been documented [7,13,25,26]. Thus even a small monazite grain (e.g. <50 µm) may survive very high temperature metamorphism if enclosed by a mineral with a higher closure temperature. This is simply because the host-mineral serves to shield monazite from reaction with other phases and to armour monazite from diffusive Pb loss. A striking example of
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
this phenomenon is observed in the Lewisian granulites, where small monazite grains fully enclosed in garnet, which is the earliest phase from a textural point of view, gave an metamorphic age of over 1000 Ma older than matrix monazite grains in a single thin section [13,26]. The application of the concept of closure temperature to monazite is further complicated by the fact that monazite is frequently zoned in high-grade metamorphic rocks as exemplified here. For a zoned monazite which preserves a polymetamorphic history, an apparent discordant pattern will be observed on a U–Pb concordia diagram if analysed by thermal ionisation mass spectrometry following techniques including conventional mechanical separation of minerals and chemical dissolution. This apparent discordant pattern however cannot be distinguished from the discordance resulting from Pb loss. In that case, the closure temperature for monazite will be under-estimated. Alternatively, if a fixed closure temperature is used, the temperature for that thermal event may be over-estimated.
Acknowledgements We would like to thank S.J.B. Reed for his help with EPMA in the Earth Sciences Department of Cambridge University. N.S. Belshaw and A.J. Gibb are thanked for their help in running ISOLAB-120. A.S. Cohen, K.W. Burton and F. von Blanckenburg are thanked for discussions during the early stage of the study. C.F. Miller, E.B. Watson and E.J. Essene are thanked for their thoughtful reviews. A.N. Halliday is thanked for his editorial comments. The authors are particularly grateful to M.S. O’Hara for introducing us to the Lewisian and its problems. This study was supported by NERC. [AH]
References [1] W.C. Overstreet, The geologic occurrence of monazite, US Geol. Surv. Prof. Pap. 530 (1967). [2] P. Copeland, R.R. Parrish, T.M. Harrison, Identification of inherited radiogenic Pb in monazite and its implications for U–Pb systematics, Nature 333 (1988) 760–763. [3] R.R. Parrish, U–Pb dating of monazite and its application to geological problems, Can. J. Earth Sci. 27 (1990) 1431– 1450.
219
[4] H.A. Smith, B. Barreiro, Monazite U–Pb dating of staurolite grade metamorphism in pelitic schist, Contrib. Mineral. Petrol. 105 (1990) 602–615. [5] J.A. Kingsbury, C.F. Miller, J.L. Wooden, T.M. Harrison, Monazite paragenesis and U–Pb systematics in rocks of the eastern Mojave Desert, California, U.S.A.: implications for thermochronometry, Chem. Geol. 110 (1993) 147–167. [6] G. Franz, G. Andrehs, D. Rhede, Crystal chemistry of monazite and xenotime from Saxothuringian–Moldanubian metapelites, NE Bavaria, Germany, Eur. J. Mineral. 8 (1996) 1097–1118. [7] C.P. DeWolf, N.S. Belshaw, R.K. O’Nions, A metamorphic history from micron-scale 207 Pb=206 Pb chronometry of Archean monazite, Earth Planet. Sci. Lett. 120 (1993) 207–220. [8] J. Wang, M. Tatsumoto, X. Li, W.R. Premo, E.C.T. Chao, A precise 232 Th–208 Pb chronology of fine-grained monazite: Age of the Bayan Obo REE–Fe–Nb ore deposit, China, Geochim. Cosmochim. Acta 58 (1994) 3155–3169. [9] J. Evans, J. Zalasiewicz, U–Pb, Pb–Pb and Sm–Nd dating of authigenic monazite: implications for the diagenetic evolution of the Welsh basin, Earth Planet. Sci. Lett. 144 (1996) 421–433. [10] D.P.A. Hawkins, S. Bowring, U–Pb systematics of monazite and xenotime: case studies from the Paleoproterozoic of the Grand Canyon, Arizona, Contrib. Mineral. Petrol. 127 (1997) 87–103. [11] X.K. Zhu, R.K. O’Nions, N.S. Belshaw, S.J.B. Reed, S. Moorbath, Ion probe dating of monazite: examples from the Isua and the Lewisian terrains, U.S. Geol. Serv. Circ. 1107 (1994) 369. [12] X.K. Zhu, R.K. O’Nions, N.S. Belshaw, A.J. Gibb, Significance of in situ SIMS chronometry of zoned monazite from the Lewisian granulites, northwest Scotland, Chem. Geol. 135 (1997) 35–53. [13] X.K. Zhu, R.K. O’Nions, N.S. Belshaw, A.J. Gibb, Lewisian crustal history from in situ SIMS mineral chronometry and related metamorphic textures, Chem. Geol. 136 (1997) 205–218. [14] B. Bingen, O. van Breemen, U–Pb monazite ages in amphibolite- to granulite-faces orthogneiss reflect hydrous mineral breakdown reactions: Sveconorwegian Province of SW Norway, Contrib. Mineral. Petrol. 132 (1998) 336–353. [15] W.N. Sawka, J.F. Banfield, B.W. Chappell, A weatheringrelated origin of widespread monazite in S-type granites, Geochim. Cosmochim. Acta 50 (1986) 171–175. [16] S.L. Hanson, W.B. Simmons, K.L. Webber, Rare earth-element mineralogy of granitic pegmatites in the Trout Creek Pass district, Chaffee County, Colorado, Can. Mineral. 30 (1992) 673–686. [17] J.M. Montel, A model for monazite=melt equilibrium and application to the generation of granitic magmas, Chem. Geol. 110 (1993) 127–146. [18] D.A. Wark, C.F. Miller, Accessory mineral behaviour during differentiation of a granite suite: monazite, xenotime and zircon in the Sweetwater Wash pluton, southeasthern California, U.S.A, Chem. Geol. 110 (1993) 49–67.
220
X.K. Zhu, R.K. O’Nions / Earth and Planetary Science Letters 171 (1999) 209–220
[19] G.R. Watt, S.L. Harley, Accessory phase controls on the geochemistry of crustal melts and restites produced during water-undersaturated partial melting, Contrib. Mineral. Petrol. 114 (1993) 550–566. [20] F. Bea, M.D. Pereira, A. Stroh, Mineral=leucosome traceelement partitioning in a pereluminous migmatite (a laser ablation-ICP–MS study), Chem. Geol. 117 (1994) 291– 312. [21] F. Bea, Residence of REE, Y, Th and U in granites and crustal protoliths; Implications for the geochemistry of crustal melts, J. Petrol. 37 (1996) 521–552. [22] F. Bea, Controls on the trace element composition of crustal melts, Trans. R. Soc. Edinburgh: Earth Sci. 87 (1996) 33– 41. [23] D.V. Mohr, Zoned porphyroblasts of metamorphic monazite in the Anakeesta Formation, Great Smoky Mountain, North Carolina, Am. Mineral. 69 (1984) 98–103. [24] G.R. Watt, High-thorium monazite-(Ce) formed during disequilibrium melting of metapelites under granulite-facies conditions, Mineral. Mag. 59 (1995) 735–743. [25] X.K. Zhu, High resolution in situ SIMS study of metamorphic monazite, PhD thesis, Cambridge University, 1997. [26] X.K. Zhu, R.K. O’Nions, Monazite chemical composition: some implications for monazite geochronology, Contrib. Mineral. Petrol. (in press). [27] N.S. Belshaw, R.K. O’Nions, D.J. Martel, D.J. Burton, High-resolution SIMS analysis of common lead, Chem. Geol. 112 (1994) 57–70. [28] H.J. Chapman, 2390 Myr Rb–Sr whole-rock ages for the Scourie dykes of north-west Scotland, Nature 277 (1979) 642–643. [29] L.M. Heaman, J. Tarney, U–Pb baddeleyite ages for the Scourie dyke swarm, Scotland: evidence for two distinct intrusion events, Nature 340 (1989) 705–708. [30] F. DeMartin, T. Pilati, V. Diella, S. Donzelli, C.M. Gra-
[31]
[32] [33] [34] [35]
[36]
[37]
[38]
[39]
[40] [41] [42] [43]
maccioli, Alpine monazite: further data, Can. Mineral. 29 (1991) 61–67. C.M. Gramaccioli, T.V. Segalstad, A uranium- and thoriumrich monazite from a south-alpine pegmatite at Piona, Italy, Am. Mineral. 63 (1978) 757–761. A. Pabst, C.O. Hutton, Huttonite, A new thorium silicate, Am. Mineral. 36 (1951) 60–65. S.H.U. Bowie, J.E.T. Horne, Cheralite, a new mineral of the monazite group, Mineral. Mag. 30 (1953) 93–99. D. Rose, Brabantite, CaTh(PO4 )2 , a new mineral of the monazite group, N. Jb. Mineral. Mh. (1980) 247–257. J.N.B. Chaudhuri, H. Newesely, On the REE-bearing minerals in the beach placers of Puri, Orissa District , J. SE. Asian Earth Sci. 8 (1993) 287–291. F. Poitrasson, S. Chenery, D.J. Bland, Contrasted monazite hydrothermal alteration mechanisms and their geochemical implications, Earth Planet. Sci. Lett. 145 (1996) 79–96. G.D. Ventura, A. Mottana, G.C. Parodi, M. Raudsepp, F. Bellatreccia, E. Caprilli, P. Rossi, S. Fiori, Monazite– huttonite solid-solutions from the Vico Volcanic Complex, Latium, Italy, Mineral. Mag. 60 (1996) 751–758. R. Casillas, G. Nagy, G. Panto, J. Brandle, I. Forizs, Occurrence of Th, U, Y, Zr, and REE-bearing accessory minerals in late-Variscan granitic rocks from the Sierra de Guadarrama (Spain), Eur. J. Mineral. 7 (1995) 989–1006. K. Suzuki, M. Adachi, I. Kajizuka, Electron microprobe observations of Pb diffusion in metamorphosed detrital monazites, Earth Planet. Sci. Lett. 128 (1994) 391–405. H.A. Smith, B.J. Giletti, Lead diffusion in monazite, Geochim. Cosmochim. Acta 61 (1997) 1047–1055. J.K.W. Lee, I.S. Williams, D.J. Ellis, Pb, U and Th diffusion in natural zircon, Nature 390 (1997) 159–161. D.J. Cherniak, J.M. Hanchar, E.B. Watson, Rare-earth diffusion in zircon, Chem. Geol. 134 (1997) 289–301. I.M. Villa, Isotopic closure, Terra Nova 10 (1998) 42–47.