Monazite “in situ” 207Pb/ 206Pb geochronology using a small geometry high-resolution ion probe....

Monazite ‘‘in situ’’ 207Pb/206Pb geochronology using a small

geometry high-resolution ion probe. Application to

Archaean and Proterozoic rocks

Delphine Bosch a,*, Dalila Hammor b, Olivier Bruguier c, Renaud Caby a,Jean-Marc Luck d

aLaboratoire de Tectonophysique, Universite Montpellier II, CNRS-UMR 5568, cc 066, Place Eugene Bataillon,

34095 Montpellier Cedex 05, FrancebDepartement de Geologie, Universite d’Annaba, B.P. 12, El Hadjar Annaba, Algeria

cService ICP-MS, ISTEEM, Universite Montpellier II, cc 049, Place Eugene Bataillon, 34 095 Montpellier Cedex 5, FrancedLaboratoire de Geophysique, Tectonique et Sedimentologie, Universite Montpellier II, CNRS-UMR 5573,

cc 060, Place Eugene Bataillon, 34 095 Montpellier Cedex 5, France

Received 19 March 2001; accepted 2 August 2001

Abstract

This paper reports the application of secondary ion mass spectrometry (SIMS) using a small geometry Cameca IMS4f ion

probe to provide reliable in situ 207Pb/206Pb ages on monazite populations of Archaean and Proterozoic age. The reliability of

the SIMS technique has been assessed on two samples previously dated by the conventional ID-TIMS method at 2661F1

Ma for monazites extracted from a pelitic schist from the Jimperding Metamorphic Belt (Yilgarn Craton, Western Australia)

and 1083F 3 Ma for monazites from a high-grade paragneiss from the Northampton Metamorphic Complex (Pinjarra

Orogen, Western Australia). SIMS results provide 207Pb/206Pb weighted mean ages of 2659F 3 Ma (n = 28) and 1086F 6 Ma

(n = 21) in close agreement with ID-TIMS reference values for the main monazite growth event. Monazites from the

Northampton Complex document a complex history. The spatial resolution of about 30 mm and the precision achieved

successfully identify within-grain heterogeneities and indicate that monazite growth and recrystallisation occurred during

several events. This includes detection of one inherited grain dated at ca. 1360 Ma, which is identical to the age of the

youngest group of detrital zircons in the paragneiss. Younger ages at ca. 1120 Ma are tentatively interpreted as dating a

growth event during the prograde stages of metamorphism. These results demonstrate that the closure temperature for lead

diffusion in monazite can be as high as 800 �C. At last, ages down to ca. 990 Ma are coeval with late pegmatitic activity and

may reflect either lead losses or partial recrystallisation during the waning stages of metamorphism. A third unknown sample

was analysed to test the capability of the in situ method to date younger monazite populations. The sample, a pelitic

metatexite from Northwestern Hoggar (Algeria), contains rounded metamorphic monazites that provide a 207Pb/206Pb

weighted mean age of 603F 11 Ma (n = 20). This age is interpreted as recording emplacement of a gabbronoritic body during

amphibolite facies regional metamorphism and is representative of the late pulse of the Pan-African tectonometamorphic

0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0009-2541 (01 )00361 -8

* Corresponding author.

E-mail addresses: [email protected] (D. Bosch), [email protected] (O. Bruguier), [email protected]

(R. Caby), [email protected] (J.-M. Luck).

www.elsevier.com/locate/chemgeo

Chemical Geology 184 (2002) 151–165

evolution in the western part of the Tuareg shield. In situ SIMS analyses using a widely available, small geometry ion probe,

can thus be successfully used to accurately determine ages for complex Precambrian monazite populations. D 2002 Elsevier

Science B.V. All rights reserved.

Keywords: SIMS; Monazite; 207Pb/206Pb geochronology; Metamorphism

1. Introduction

Over the last decade, monazite, a lanthanide-rich

phosphate, has been widely used as a geochronometer

and this mineral is, after zircon, probably the most

used U-rich phase in geochronology. Monazite is a

common accessory mineral occurring in a wide

variety of rock types (sedimentary, metamorphic

and magmatic), therefore allowing the dating of

various events such as the emplacement of magmatic

rocks or the growth of minerals (or cooling) in

metamorphic terranes (e.g. Parrish, 1990), or the

tracing of source region for detritus that accumulated

in sedimentary basins. Monazite is thought to have a

relatively simple behaviour in comparison to zircon

and is often found in concordant position in the con-

cordia diagram, thus indicating closed system behav-

iour with respect to the U–Pb system. In contrast to

silicate minerals such as zircon, which have a ten-

dency to become metamict, monazite rarely exhibits

radiation damage of the crystal lattice in spite of very

high U and Th contents (thousands of parts per

million).

Recent studies, however, have highlighted com-

plexities in the behaviour of this mineral such as

inheritance (Copeland et al., 1988), secondary

replacement (De Wolf et al., 1993; Zhu et al., 1997;

Bingen and van Breemen, 1998) and Pb loss by

volume diffusion during a metamorphic event (Black

et al., 1984; Suzuki et al., 1994) or enhanced by

damage to the crystal lattice (Hawkins and Bowring,

1997). Implicit to this is the growing need for part

grain analyses, either by conventional method or by in

situ high-resolution ion microprobe.

Up to now, three techniques allow in situ analyses

of monazites for geochronological purposes. Sensitive

high-resolution ion microprobes (such as SHRIMP)

have been used to successfully determine U–Pb ages

(e.g. Williams et al., 1996), but this requires character-

isation of monazite standards, which should match the

Th content of the unknown samples (Zhu et al., 1998).

Moreover, these large geometry ion probes are not yet

widely available, which reduces their use as a world-

wide routine method.

Electron microprobe (EMPA) has also been shown

to be a valuable alternative to monazite dating (Mon-

tel et al., 1996; Cocherie et al., 1998). The main

advantage of this technique being the very high spatial

resolution of around 1 mm compared to the 20- to 30-

mm spots used by most ion microprobes. The preci-

sion, however, is limited to around 20 Ma, which

precludes identification of monazite-forming events

occurring in a limited time span.

There is a growing interest (e.g. Poitrasson et al.,

2000) for monazite dating by laser ablation induc-

tively coupled plasma mass spectrometry (LA-ICP-

MS) which revealed to be a very fast technique with a

spatial resolution comparable to secondary ion mass

spectrometry (SIMS). The high drilling rate (ca. 0.5–

1 mm/s) can, however, constitute a serious drawback

when analysing heterogeneous material.

In this paper, the capability of the more widely

available, small geometry Cameca IMS4f ion microp-

robe for rapid, in situ, isotopic analyses of selected

areas of monazite crystals has been investigated as an

alternative way to analyse complex metamorphic

populations. We present results from two well-dated

late Archaean and Proterozoic samples and from a

third unknown sample outcropping in the Hoggar

Mountains (Algeria). In addition, the results shed

some light on the behaviour of monazite, which has

implications for its use as a U–Pb geochronometer.

2. Analytical techniques

Separation of minerals was performed using stand-

ard techniques (Wifley table and heavy liquids). After

cleaning in dilute 0.5 N HNO3 and tridistilled water,

the monazite grains were subsequently mounted in

D. Bosch et al. / Chemical Geology 184 (2002) 151–165152

epoxy resin and polished to approximately half their

thickness to expose internal structures. The mounts

were then carefully washed with tridistilled water,

soap and alcohol and stored in a clean environment

before analysis. SIMS analyses were carried out on

the Cameca IMS4f ion microprobe with a spot size of

about 30 mm. To avoid sample charging by the 16O �

primary beam, the mounts were coated by ca. 100-

nm-thick gold film. Before its introduction within the

sample lock, the surface conductivity of the mount

was checked to be less than 20 V and it was then held

under vacuum overnight to ensure degassing. Before

each analysis, a 10-min rastering was conducted to

pass through the gold coat and to reach steady

sputtering conditions. The primary beam currents

ranged from 8 to 20 nA, the highest current being

used for the youngest sample. The primary beam was

accelerated onto the sample surface by a 12.65-keV

potential and stability was better than 0.6%. Positive

secondary ions were extracted using a 4.5-keV poten-

tial and the energy window was set at 50 eV to

remove low-energy ions and molecular species. The

beam passed then through a double focussing mass

spectrometer operated at a high mass resolution to

resolve molecular and isobaric interferences in the

204–208 mass range.

High-resolution mass spectrum of monazites

shows that the main interferences are mostly due to

REE-oxides. The most significant molecular interfer-

ences are related to PrPO2, GdPO and YbO2 which

occur near the 204Pb (see Fig. 1a). As the measured207Pb/206Pb ratios must be corrected from common

lead contribution by referring to the 206Pb/204Pb

measured ratios, good separation of the 204Pb peak

from these neighbouring interferences is essential.

Indeed, any unresolved interference on the small204Pb peak will be responsible for an overcorrection

of the 207Pb/206Pb ratio resulting in too young an age.

A mass resolving power of 3500 is necessary to

ensure integrity of the 204Pb as well as of the other

lead isotopes (Fig. 1a and b). Increasing the mass

resolution leads to a decrease of ions arriving to the

detector and, thus large contrast aperture (400 mm)

and field aperture (750 mm) were used during the

course of this study.

Ion beams were measured in the peak jumping

sequence with an electron multiplier operating in

pulse counting mode with a 65% yield and a 30-ns

integrated dead-time measured using Pb standards.

Under these operating conditions, the IMS4f ma-

chine achieves an overall instrumental Pb sensitivity

of 3–4 cps/ppm/nA of primary beam based on

analyses of monazites from sample W404 presented

in this study. This sensitivity is about five times low-

er than that achieved by large geometry ion probes

(e.g. Harrison et al., 1995; Williams et al., 1996) but

compares well with sensitivity of the Isolab machine

operated in the SIMS mode (e.g. De Wolf et al.,

1993).

Fig. 1. Typical mass scans of monazites obtained with the IMS4f ion

probe at a mass resolving power of 3500. (a) Molecular ions

interferences in the 204 mass range spectrum. The main inter-

ferences are from PrPO2 and to a lesser degree to GdPO and YbO2.

(b) Molecular ion interferences in the 208 mass range spectrum are

related to Sm species (SmSiO2, SmCaO, SmPSi).

D. Bosch et al. / Chemical Geology 184 (2002) 151–165 153

Data were collected in three blocks of 10 cycles

each and the total duration for one analysis was about

60 min. A background correction, monitored at

204.10 amu, as close as possible to the 204Pb peak,

was applied to the measured Pb peaks. Typical

analytical parameters for Pb analyses are listed in

Table 1. Common Pb corrections were based on the

measured 204Pb and for all the data, the assumed

common Pb composition was modelled as contempo-

raneous Pb (Stacey and Kramers, 1975). Corrected

isotopic ratios and ages were calculated after Ludwig

(1999). The quoted ages and related uncertainties

are based on weighted averages of the calculated207Pb/206Pb ages.

In the absence of U–Pb and Th–Pb analyses due

to unavailable suitable monazite standards, a review

of the possible effects that can bias the 207Pb/206Pb

ages is warranted. Indeed, although monazite gener-

ally shows a high degree of concordance, a great

number of studies have been faced to discordant

analyses, both normal and reverse. For example,

studies of the diffusion of Pb in monazite (i.e.

Suzuki et al., 1994; Smith and Giletti, 1997) indicate

that diffusive Pb loss may be experienced by the

crystals during a high-temperature metamorphic

event. Significant ancient diffusion-controlled Pb

loss would be responsible for younger ages, with

no geological meaning, whereas present-day Pb loss

will move the points towards the origin thus leaving

the 207Pb/206Pb ratio and age unaffected. In the

former case, 207Pb/206Pb ages should be considered

only as minimum values. Straddling by the ion beam

of growth zones with different ages may also result

in intermediate ages, but this can be avoided by

careful SEM imaging before analysis. Discordance

can also arise from recrystallisation which is accom-

panied by transport and migration of elements (Pidg-

eon, 1992). Although this generally results in a Pb-

free recrystallised lattice, residual radiogenic Pb can

potentially remain partially trapped in the newly

formed domain, thus leading to incomplete resetting

of the U–Pb systems and ages older than the true

age of recrystallisation. Finally, reverse discordance

has been also observed and is generally assumed to

derive from incorporation of intermediate daughter

products into the 238U/206Pb decay chain at the time

of crystal growth (Scharer, 1984). In old, Precam-

brian, monazites, unsupported thorogenic 206Pb

should be swamped by uranogenic 206Pb and thus

should not be responsible for reverse discordance.

Bingen and van Breemen (1998), however, showed

that this phenomenon could be invoked to account

for reverse discordance of monazites as old as

1 Ga. Hawkins and Bowring (1997) on the contrary

proposed that reverse discordance results from dis-

equilibrium of the U–Pb system due to postcrystal-

lisation local enrichment of Pb by diffusion. In any

case, the 207Pb/206Pb ratios do not yield reliable

ages.

The above discussion pertains to analyses falling

outside the main monazite population, for which a

discussion about the effects of potential discordance

on the interpretation of ages is warranted. It is clear

that only age grouping can be considered as reflecting

growth or recrystallisation events and that analyses

yielding intermediate ages should be treated with

caution as they possibly derived from complex mon-

azite grains.

3. Results and discussion

The reliability of the method has been assessed on

two Precambrian metamorphic monazite populations

previously dated by ID-TIMS (see Bosch et al., 1996;

Bruguier et al., 1999). The first sample is a late

Archaean pelitic schist from the Jimperding Metamor-

phic Belt located on the western margin of the Yilgarn

Craton (Western Australia), whereas the second sam-

ple is a paragneiss from the Proterozoic Northampton

Complex of the Pinjarra Orogen (Western Australia).

Results are reported in Table 2 and presented in

Figs. 2–4.

Table 1

Analytical parameters for Pb/Pb isotopic analyses of monazites

using the Cameca IMS4f ion probe

Mass

(amu)

Isotope Waiting time

for magnetic

settling (s)

Counting

time (s)

203.973 204Pb 3 20

204.100 background 2 20

205.974 206Pb 2 15

206.976 207Pb 2 30

207.977 208Pb 2 5


3.1. Pelitic schist W398

This sample consists mostly of muscovite (25%),

pinitised cordierite (25%), red biotite (20%), quartz

(17%), with minor sillimanite (8%), oligoclase (3%),

opaques (2%), and trace zircon and monazite. The

schist is medium grained with an average grain size

of about 0.5 mm, although biotite grains vary up

to 1.0 mm in length. The rock possesses a sinuous

subcontinuous foliation defined by biotite, muscovite

and sillimanite, which may have been crenulated.

Granoblastic areas defined by quartz and cordierite

occur between the biotite, sillimanite and muscovite

grains. Muscovite and biotite are intergrown, where-

as sillimanite is present as clusters and as inclusions

within biotite and muscovite. In places, biotite and

muscovite appear to overprint cordierite. Biotite + -

quartz and muscovite + quartz symplectites are also

present.

The sample underwent amphibolite facies regional

metamorphism with P–T conditions in the range of

650–700 �C and 2–5 kbars. It contains a population

of rounded, pale yellow monazite crystals interpreted

as metamorphic in origin. Five single crystals, pre-

viously analysed by ID-TIMS (Bosch et al., 1996), are

slightly discordant ( < 1%) with ages ranging from

2652 to 2665 Ma (Fig. 2a). The ca. 10-Ma range in

ages suggests that the schist contains a heterogeneous

monazite population and that growth or recrystallisa-

tion occurred during several events. A neighbouring

pelitic schist (W399), however, yields a homogeneous

population precisely dated by ID-TIMS at 2661F1

Ma. This ca. 2660-Ma age is interpreted as reflecting

the main monazite growth event during the prograde

stages of regional metamorphism at a temperature of

about 500 �C, similarly to cases reported for meta-

morphic monazites in pelitic schists (e.g. Smith and

Barreiro, 1990). Monazites dated at ca. 2665 Ma are

coeval with granitic intrusions (2660–2670 Ma),

whereas the younger age of 2652 Ma corresponds to

the peak of regional metamorphism in this area

(Bosch et al., 1996; Nemchin et al., 1994; Pidgeon

et al., 1996). Homogeneity of monazite ages in sample

W399 contrasts with the spread observed in sample

W398 that suggests the growth of monazite was

controlled by local conditions and that monazites

may have been armoured against Pb loss and recrys-

tallisation.

Thirty SIMS 207Pb/206Pb spot analyses were per-

formed on 15 grains. Intra-grain analyses generally

overlap each other except in two cases (see Table 2).

Analysis #4-2 gives a significantly older age (2671F6 Ma) than the two other spot analyses from the same

grain (2653F 8 and 2650F 8 Ma). The discrepancy

between these two ages is attributed to preservation of

the first monazite population (ca. 2665 Ma), which

underwent recrystallisation or resorption during the

subsequent main monazite growth event. One spot

analysis (#10-3) yields an anomalously young207Pb/206Pb corrected value of 2604F 6 Ma, whereas

the two other spots on this grain give ages of

2648F 12 and 2650F 8 Ma. This young age is not

reproduced in the present data set and is also younger

than any reported event in this part of theYilgarnCraton

and, in particular, than the apatite cooling age of

2636F 6 Ma from a nearby syenitic body (Pidgeon et

al., 1996). This suggests that ancient Pb loss during a

metamorphic event is unlikely. Moreover, the uncor-

rected 207Pb/206Pb ratio is close to the expected value

(see Table 2), but the 204Pb/206Pb ratio is among the

highest one, which suggests that this young age stems

from an overcorrection due to high count rate on the204Pb peak. This might be related to edge effects or to a

small unresolved molecular contribution possibly

related to a drift in the mass calibration, shifting the204Pb measurement towards the neighbouring PrPO2

peak. This analysiswas therefore discarded from the age

calculation. The 28 remaining analyses (Fig. 2b) have

ages ranging from 2636F 28 to 2686F 29Ma and give

a weighted mean of 0.18066F 0.00030 (2r) corre-

sponding to an ageof 2659F 3Ma (MSWD=2.2). This

mean age is well within the range of ID-TIMS values

(2652–2665Ma), and identical to the 2661-Ma age for

the main monazite growth event in this area of the

Jimperding Belt. A cumulative probability treatment

of the data (Ludwig, 1999) tends to suggest a bimodal

distribution with mean values around 2655 and 2665

Ma. One spot age excluded (#4-2), SIMS analyses,

however, did not detect unequivocally the different

monazite populations identified by ID-TIMS and the

analysed crystals would thus appear to have formed

during a single growth event. The short time span

between the three growth episodes (ca. 15 Ma) and the

relatively low precision of the SIMS analyses (from 4 to

32Maat the2r level)make theagedifferencedifficult to

resolve.


Table 2

SIMS Pb/Pb isotopic results

Spot Percentage Pb Measured Corrected atomic ratiosa Th/U Apparent age (Ma)

206Pb 207Pb 208Pb 204Pb/206Pb 207Pb/206Pb 208Pb/206Pb F (%)

(2r)

207Pb/206Pb F (%)

(2r)

207Pb/206Pb F (2r)

W398 Schist (Jimperding Metamorphic Belt, Western Australia), main monazite growth event at 2661F1 Ma. Properties: 100–200 lm,rounded anhedral, yellow translucent

398-1-1 12.8 2.3 84.9 0.000084 0.181470 6.61 0.95 0.180441 0.70 23.8 2656.9 11.7

398-1-2 10.5 1.9 87.6 0.000340 0.182850 8.31 1.25 0.178680 1.10 29.9 2640.7 18.3

398-2-1 17.1 3.0 79.9 0.000345 0.182430 4.69 1.94 0.178193 1.72 16.8 2636.1 28.4

398-2-2 19.4 3.5 77.0 0.000123 0.182710 3.96 0.94 0.181206 0.65 14.3 2663.9 10.8

398-3-1 9.9 1.8 88.3 0.000252 0.184070 8.95 1.84 0.180986 1.19 32.2 2661.9 19.6

398-3-2 9.5 1.7 88.7 0.000248 0.183980 9.30 0.89 0.180942 0.59 33.5 2661.5 9.8

398-4-1 11.6 2.1 86.3 0.000163 0.182050 7.43 1.20 0.180055 0.50 26.7 2653.4 8.2

398-4-2 12.0 2.2 85.8 0.000107 0.183290 7.14 1.70 0.181985 0.33 25.7 2671.1 5.4

398-4-3 11.3 2.0 86.6 0.000099 0.180940 7.64 1.28 0.179724 0.48 27.5 2650.3 7.9

398-5-1 15.9 2.9 81.2 0.000158 0.183430 5.09 0.67 0.181498 0.51 18.3 2666.6 8.4

398-5-2 15.3 2.8 82.0 0.000192 0.183430 5.38 0.50 0.181085 0.31 19.3 2662.8 5.2

398-6-1 11.8 2.1 86.0 0.000332 0.185270 7.27 0.62 0.181207 1.37 26.2 2664.0 22.6

398-6-2 17.0 3.1 80.0 0.000123 0.181720 4.71 2.23 0.180216 0.57 16.9 2654.9 9.4

398-6-3 16.6 3.0 80.5 0.000302 0.184380 4.86 1.20 0.180680 0.77 17.5 2659.1 12.7

398-7-1 49.0 8.9 42.1 0.000068 0.182110 0.86 2.00 0.181278 0.25 3.1 2664.6 4.2

398-7-2 49.8 9.1 41.1 0.000081 0.182580 0.82 4.57 0.181587 0.44 3.0 2667.4 7.3

398-8-1 10.5 1.9 87.6 0.000392 0.186590 8.34 0.86 0.181802 1.25 30.0 2669.4 20.6

398-8-2 10.6 1.9 87.5 0.000393 0.183010 8.23 1.68 0.178182 1.00 29.6 2636.0 16.6

398-9 19.5 3.5 77.0 0.000290 0.183820 3.95 7.31 0.180272 0.30 14.2 2655.4 4.9

398-10-1 13.4 2.4 84.2 0.000200 0.181960 6.30 1.02 0.179512 0.72 22.6 2648.4 12.0

398-10-2 13.4 2.4 84.2 0.000070 0.180550 6.30 0.53 0.179698 0.54 22.6 2650.1 8.9

398-10-3 9.2 1.6 89.2 0.000661 0.182950 9.71 0.67 0.174798 0.40 34.8 2604.1 6.6

398-11 11.7 2.1 86.2 0.000182 0.182430 7.40 0.51 0.180204 0.85 26.6 2654.8 14.1

398-12 17.9 3.3 78.8 0.001200 0.198290 4.40 3.14 0.183682 1.75 15.8 2686.4 28.7

398-13 52.7 9.6 37.7 0.000520 0.187830 0.72 1.65 0.181478 1.26 2.6 2666.4 20.9

398-14-1 17.5 3.1 79.3 0.000260 0.182940 4.53 3.88 0.179756 0.80 16.3 2650.6 13.2

398-14-2 17.3 3.1 79.5 0.000263 0.183250 4.59 3.87 0.180027 0.84 16.5 2653.1 13.8

398-15 43.5 7.8 48.7 0.000118 0.181690 1.12 2.14 0.180243 0.55 4.0 2655.1 9.1

398-16 27.3 5.0 67.7 0.000187 0.183620 2.48 1.70 0.181333 0.82 8.9 2665.1 13.6

398-17 25.2 4.6 70.2 0.000669 0.190140 2.79 2.14 0.181974 1.95 10.0 2671.0 32.1

W404 Paragneiss (Northampton Complex, Western Australia), 1083F 3 Ma. Properties: 100–200 lm, rounded to irregular shaped,

yellow translucent

404-1-(a) 8.0 0.7 91.3 0.000312 0.091489 11.43 2.68 0.087153 2.23 38.2 1363.9 42.7

404-2-1(b) 26.7 2.1 71.3 0.000085 0.078327 2.67 2.32 0.077128 0.42 8.8 1124.5 8.3

404-2-2 9.8 0.7 89.5 0.000282 0.077925 9.13 1.02 0.073934 0.73 30.0 1039.7 14.7

404-2-3 8.4 0.6 91.0 0.000204 0.078656 10.82 0.53 0.075779 0.87 35.7 1089.2 17.3

404-3(c) 13.5 1.0 85.5 0.000288 0.077936 6.32 17.98 0.073857 0.90 20.8 1037.6 18.1

404-4 13.8 1.0 85.2 0.000391 0.078987 6.16 6.65 0.073449 1.09 20.2 1026.4 22.0

404-5 9.1 0.7 90.2 0.000476 0.079025 9.88 2.62 0.072275 0.96 32.4 993.7 19.4

404-6-1 30.8 2.3 66.9 0.000087 0.077121 2.18 2.40 0.075890 0.27 7.2 1092.2 5.4

404-6-2 31.5 2.4 66.1 0.000063 0.077012 2.10 0.82 0.076118 0.50 6.9 1098.2 10.1

404-7-1 30.1 2.3 67.6 0.000091 0.077291 2.24 0.94 0.076000 0.31 7.4 1095.1 6.3

404-7-2 31.0 2.4 66.7 0.000076 0.077114 2.15 0.51 0.076037 0.60 7.1 1096.0 11.9

404-8 25.5 1.9 72.6 0.000105 0.076202 2.85 0.84 0.074715 0.39 9.4 1060.8 7.8

404-9-1 17.3 1.3 81.4 0.000170 0.077241 4.70 2.12 0.074840 0.43 15.5 1064.2 8.7

404-9-2 12.0 0.9 87.1 0.000326 0.081815 7.26 1.61 0.077222 2.04 24.0 1126.9 40.4

404-10-1(d) 11.1 0.8 88.1 0.000320 0.080186 7.96 2.52 0.075667 2.60 26.2 1086.3 51.8


3.2. Paragneiss W404

This sample is a quartzofeldspathic gneiss yielding

a granoblastic texture, although where there is a high

proportion of phlogopite, one, and in some cases two,

foliations can be recognised. Both foliations are

defined by sparsely distributed phlogopite and ilmen-

ite, and phlogopite pressure shadows around garnets.

The gneiss is composed of quartz (25–40%), micro-

perthitic microcline (25–35%), andesine (10–20%),

phlogopite (5–15%) and garnet (5–15%), with minor

ilmenite ( < 5%) and graphite ( < 5%), and trace mus-

covite, zircon, monazite and rutile. The sample expe-

rienced granulite facies metamorphism with peak

temperatures and pressures of 850F 50 �C and 5–6

kbars.

Table 2 (continued )

Spot Percentage Pb Measured Corrected atomic ratiosa Th/U Apparent age (Ma)

206Pb 207Pb 208Pb 204Pb/206Pb 207Pb/206Pb 208Pb/206Pb F (%)

(2r)

207Pb/206Pb F (%)

(2r)

207Pb/206Pb F (2r)

W404 Paragneiss (Northampton Complex, Western Australia), 1083F 3 Ma. Properties: 100–200 lm, rounded to irregular shaped,

yellow translucent

404-10-2 8.9 0.7 90.4 0.000146 0.079072 10.13 2.23 0.077018 1.60 33.4 1121.7 31.8

404-11 10.4 0.8 88.8 0.000265 0.079178 8.52 1.49 0.075435 0.86 28.1 1080.1 17.2

404-12-1(e) 17.8 1.3 80.9 0.000167 0.077989 4.55 2.25 0.075623 0.52 15.0 1085.1 10.4

404-12-2 16.8 1.3 81.9 0.000115 0.076989 4.87 3.01 0.075359 1.29 16.0 1078.1 25.8

404-13 15.0 1.1 83.9 0.000208 0.078927 5.60 2.20 0.075989 0.68 18.5 1094.8 13.6

404-14 21.7 1.6 76.7 0.000271 0.079418 3.53 1.53 0.075594 1.02 11.6 1084.3 20.5

404-15 35.2 2.7 62.2 0.000077 0.076868 1.77 2.55 0.075785 0.44 5.8 1089.4 8.7

404-16 10.5 0.8 88.7 0.000179 0.078066 8.42 0.64 0.075536 0.90 27.7 1082.8 18.0

404-17-1(f) 28.9 2.2 68.9 0.000069 0.077211 2.38 0.69 0.076235 0.75 7.9 1101.3 14.9

404-17-2 12.9 1.0 86.2 0.000185 0.077866 6.71 4.78 0.075245 0.97 22.1 1075.1 19.3

404-17-3 14.8 1.1 84.1 0.000063 0.076959 5.70 5.71 0.076073 1.33 18.8 1097.0 26.6

404-18 20.7 1.6 77.7 0.000146 0.078235 3.75 1.75 0.076173 0.71 12.4 1099.6 14.1

404-19 14.7 1.1 84.2 0.000247 0.078574 5.74 2.19 0.075077 1.39 18.9 1070.6 27.8

404-20 28.4 2.1 69.5 0.000084 0.076761 2.45 5.92 0.075576 0.42 8.1 1083.9 8.4

C106 Gneiss (Hoggar). Properties: 80–125 lm, rounded anhedral, colourless to yellow translucent

C106-1 23.2 1.4 75.4 0.000328 0.063840 3.25 1.86 0.059115 2.36 10.41 571.4 50.9

C106-2 24.0 1.4 74.6 0.000400 0.065676 3.11 3.90 0.059914 2.55 9.97 600.5 54.7

C106-3 26.9 1.6 71.6 0.000340 0.061724 2.66 1.90 0.058070 1.38 8.51 532.4 30.1

C106-4 28.8 1.7 69.5 0.000182 0.058962 2.42 5.14 0.058953 2.86 7.74 565.4 61.7

C106-5 24.1 1.5 74.5 0.000122 0.062905 3.09 10.50 0.060930 1.14 9.94 636.8 24.4

C106-6 27.1 1.6 71.3 0.000295 0.062388 2.63 0.93 0.059052 2.16 8.43 569.0 46.7

C106-7 25.0 1.5 73.5 0.000145 0.062527 2.94 1.55 0.060442 0.89 9.45 619.4 19.1

C106-8 23.8 1.4 74.8 0.000569 0.068650 3.14 0.71 0.060352 1.28 10.09 616.2 27.5

C106-9 22.9 1.4 75.7 0.000175 0.063015 3.31 1.98 0.060497 2.42 10.64 621.4 51.8

C106-10 27.2 1.7 71.1 0.000403 0.062005 2.61 1.01 0.061513 2.64 8.40 657.2 56.1

C106-11 39.1 2.3 58.5 0.000409 0.065362 1.50 0.58 0.059463 1.16 4.80 584.1 25.0

C106-12 39.1 2.4 58.6 0.000298 0.066007 1.50 0.58 0.060312 1.14 4.82 614.8 24.5

C106-13 41.7 2.5 55.8 0.001124 0.077400 1.34 2.46 0.059977 3.52 4.29 602.8 75.3

C106-14-1 23.2 1.4 75.5 0.000159 0.064257 3.26 1.88 0.059412 1.08 10.45 582.2 23.4

C106-14-2 25.4 1.5 73.1 0.000280 0.063127 2.88 2.65 0.059086 2.85 9.22 570.3 61.4

C106-15 28.3 1.7 70.1 0.000460 0.065102 2.48 3.10 0.058455 4.49 7.92 546.9 96.6

C106-16-1 24.3 1.5 74.2 0.000572 0.067958 3.05 4.40 0.059714 1.11 9.79 593.2 24.1

C106-16-2 24.8 1.5 73.7 0.000593 0.068230 2.97 4.63 0.059687 1.11 9.54 592.3 24.0

C106-17 23.0 1.4 75.6 0.000431 0.066112 3.29 0.77 0.059895 1.33 10.56 599.8 28.7

Th/U ratios were calculated from the radiogenic 208Pb/206Pb assuming concordance between the U–Pb and Th–Pb systems.

Each analysis was labelled as follows: sample name-grain analyzed-spot number.

Letters (from a to f) into brackets refer to SEM images of Fig. 4.a Lead isotopic ratios have been corrected for background and common lead.


Five single grains previously analysed by ID-TIMS

(Bruguier et al., 1999) provided an age of 1080F 5Ma

(Fig. 3a). Discordant analyses were interpreted as

reflecting disturbances of the original monazite and,

on thegroundofoneconcordant analysis, amoreprecise

age of 1083F 3Ma was proposed for the metamorphic

growth of these minerals. This age is identical to the

zircon age of a mafic granulite (1079F 3 Ma) inter-

preted as dating granulite facies metamorphism.

Twenty-nine SIMS analyses were performed on 20

grains (Fig. 3b). The age spectrum is complex and

analyses conducted at different places of monazite

grains do not overlap completely at the 2r level. The

oldest age (1364F 42 Ma) is from grain #1 which

appears to contain a partly recrystallised central

rounded core (Fig. 4a) about 200 Ma older than the

main metamorphic population. This value falls in the

age spectrum (1150–1450 Ma) given by a group of

detrital zircons from this paragneiss (Bruguier et al.,

1999) and this, along with the rounded shape of the

core, suggests a detrital origin for the original crystal.

Preservation of this old age clearly implies that the

U–Th–Pb systems of the original monazite crystal

has not been reset and survived high-grade granulitic

conditions with peak temperatures and pressures in

the range 800–900 �C and 5–6 kbars. These con-

ditions are well above the nominal closure temper-

ature proposed by Copeland et al. (1988) and is

another example of the robustness of the U–Th–Pb

system in monazite (De Wolf et al., 1993; Bingen and

van Breemen, 1998).

SEM imaging of some monazite crystals show

evidence for recrystallisation/resorption along irregu-

Fig. 2. Isotopic results for monazites from the W398 pelitic schist

(Jimperding Metamorphic belt, Western Australia). (a) Concordia

diagram for ID-TIMS analyses. U–Pb analyses from metamorphic

monazites extracted from W399 schist are also shown. Black boxes:

W399; shaded boxes: W398. Boxes are 2r errors. (b) SIMS207Pb/206Pb diagram. Error bars are 2r.

Fig. 3. Isotopic results for monazites from the W404 paragneiss

(Northampton Metamorphic Complex, Western Australia). (a)

Concordia diagram for ID-TIMS analyses. Boxes are 2r errors.

(b) SIMS 207Pb/206Pb diagram. Error bars are 2r.


Fig. 4. SEM (BSE) imaging of selected monazite crystals from the W404 paragneiss (Northampton Metamorphic Complex, Western Australia).

Brightness is correlated with Th content such that the brighter the area, the higher the Th content and the higher the calculated Th/U ratio. The

location of the SIMS analyses are circled. (a) Image of monazite #1 showing a homogeneous high-Th rim surrounding a central rounded core.

(b) Complex monazite grain #2 showing an irregular, patchy, low-Th core resorbed by inward-directed high-Th fronts. Note the high Th content

along the fracture in the left portion of the grain. (c) Homogeneous anhedral grain #3 containing a high-Th rim in the lower part of the grain and

a central low-Th core. (d) Same as grain #2 with a bright rim in the right upper part. High-Th areas possibly reflect Th exsolution during

resorption of the core. (e) Simple homogeneous grain #12. (f) Same as grain #12 with a possible low-Th core preserved in the left part.


lar intra-grains discontinuities which look like inward-

directed reaction fronts (see Fig. 4b and d). These

processes resulted in a patchy replacement of a low-

Th core by a bright, high-Th, material and is some-

times accompanied by Th exsolution in the core (see

the bright dots in Fig. 4d), possibly occurring in the

first stages, and before completion, of this secondary

replacement. This suggests redistribution of the ele-

ments at least on a local (subgrain) scale. Preservation

of chemically distinct zones, however, implies that Th

did not homogenise completely within the grain. One

spot analysis conducted on the low-Th core of grain

#2 gave an age of 1125F 8 Ma, which although

younger than the core analysis of grain #1, is still

significantly older than the main monazite growth

event. Since losses of elements from the crystal lattice

may have accompanied the replacement processes,

this intermediate age may reflect partial lead losses

from an old core. Thus, it cannot be ruled out that

measurement was not influenced by a component of

inherited Pb from a ca. 1360-Ma-old core that has

biased the age to be too old. This age, however, is

reproduced by analyses #9-2 (1127F 40 Ma) and

#10-2 (1122F 32 Ma), the latter being associated

with an internal structure consistent with analysis

#2-1. It is unlikely that each analysed domain from

three different grains had lost the same amount of Pb

to produce identical 207Pb/206Pb ratios and ages.

Moreover, the lack of analyses yielding ages inter-

mediates between 1360 and ca. 1125 Ma, suggests

that 1125 Ma may constitute a true age grouping and

is thus unlikely to derive from crystal zones having

suffered lead losses during partial secondary replace-

ment. An alternative interpretation is to consider that

they reflect an early growth event during the prograde

stage of metamorphism. This hypothesis again implies

that radiogenic lead in monazite can be preserved at

temperatures above 800 �C and that this mineral can

thus potentially be used to calculate burial and heating

rates to provide key information on the tectonothermal

evolution of ancient orogen even for rocks subjected

to high-grade metamorphic conditions. The large

grain size of the analysed monazites (100–250 mm)

may be responsible for Pb retention under these high-

temperature conditions as suggested by experimental

modelling by Smith and Giletti (1997). The sharp age

discontinuities and complex internal structures, how-

ever, suggest that recrystallisation and the associated

element migration was a more efficient way for

resetting of the U–Pb isotope system of pre-existent

monazite than volume diffusion of Pb, although

admittedly without information on how potentially

discordant the data are, this cannot be warranted.

Most analyses (21 out of 29) yield ages ranging from

1061F 8 to 1101F15 Ma and can be combined to

give a weighted mean 207Pb/206Pb ratio of

0.07566F 0.00021 (MSWD=5.2) corresponding to

an age of 1086F 6 Ma (2r). This age is slightly older,

but similar to the 1083F 3 Ma ID-TIMS reference

value and reflects the main monazite growth event

close to the peak of granulite facies metamorphism.

Younger ages are also present in the monazite age

spectrum. These include ages in the range 1026–1040

Ma (#2-2, #3, #4) and one analysis at 994F 19 Ma

(#5). Grain #2, with three distinct ages of ca. 1125,

1040 and 1090 Ma, again illustrates the subgrain

complexity of this composite population. Ages in

the range 1020–1040 Ma are difficult to relate to

any known geological activity in the complex,

although Rb–Sr ages of 1020 and 1037 Ma have

been reported for granulites by Compston and Arriens

(1968) and Richards et al. (1985). These ages were

first interpreted as dating granulite facies metamor-

phism, but given the susceptibility of the Rb–Sr

system to fluid intervention, they can as well be

related to pegmatitic activity in the complex. An

alternative interpretation is that the in situ 207Pb/206Pb

ages of 1020–1040 Ma reflect discordance associated

with lead losses (Black et al., 1984; Suzuki et al.,

1994), or that the beam straddled zones of different

ages. This interpretation is supported by SEM images

of analysis #2-2 where the beam appears to have

struck a crack underlined by bright, high-Th, dots

and analysis of grain #3 that clearly straddled a high-

Th domain. We thus favour the interpretation that ages

in the range 1020–1040 Ma reflect discordance and

have no geological significance. However, they sug-

gest a younger disturbance event. The youngest age

from grain #5 (994F 19 Ma) is identical to the zircon

age from an undeformed pegmatitic dyke of 989F 2

Ma (Bruguier et al., 1999) and could be related to

pegmatite intrusion and fluid infiltration in the quartz-

ofeldspathic gneissic sequence. Deformation in the

complex ceased by this time (ca. 990 Ma) but ana-

tectic conditions and related fluid flows were still

active at least on a local scale. We speculate that these


conditions were sufficient to trigger partial recrystal-

lisation or Pb loss by diffusion in some monazite

grains during fluid–mineral interaction in the waning

stages of regional metamorphism and reflect the

susceptibility of monazite to fluid flows even in the

late stage of metamorphism. Such conclusion is sup-

ported by U–Pb dating of metapelites that indicates

growth of metamorphic monazites associated with

pegmatite intrusion (Lanzirotti and Hanson, 1995).

The scarcity of these young ages also indicates that

monazites from one single rock can respond differ-

ently under similar conditions, possibly because of

relatively mild conditions and/or shielding by host

minerals.

3.3. Metapelitic gneiss C106 (Egatalis/In Tassak area,

NW Hoggar)

The studied sample comes from the Egatalis/In

Tassak area of Northwestern Hoggar and was col-

lected as part of a comprehensive study to determine

the magmatic and metamorphic Precambrian evolu-

tion of this part of the Pan-African belt of the Tuareg

Shield. The western branch of the Pan-African belt in

NW Hoggar (Algeria) comprises fresh granulite facies

rocks in the Egatalis area considered as the deepest

crustal level exposed south of the Tassendjanet terrane

(Caby, 1970, 1987; Black et al., 1994). In this belt,

late low-pressure high-temperature metamorphic con-

ditions are progressively evidenced westward by the

overprint of kyanite-bearing mineral assemblages by

andalusite. The passage towards the sillimanite zone is

observed in schists and aluminous quartzites of Late

Paleoproterozoic age (Caby and Andreopoulos-

Renaud, 1983) beneath the syn-kinematic Tin Edehou

granodiorite–tonalite composite pluton that has the

geometry of a gently E-dipping, 2- to 4-km-thick

sheet.

The C106 sample is a coarse-grained pelitic meta-

texite collected at a few metres from the root of a

gabbronoritic body. It contains the very fresh mineral

assemblage quartz, garnet (alm 73-83, pyr 17-10, gro

02, spe 8-5 from core to rim), brown biotite, cordier-

ite, plagioclase (An 24%), perthitic K-feldspar, Fe

spinel (1% ZnO), ilmenite and graphite as major

phases. The leucosomes contain both antiperthitic

plagioclase and perthitic K-feldspar, and fresh cordier-

ite containing numerous inclusions of sillimanite and

green spinel, relict corundum being present in an

adjacent sample. Garnet–biotite thermometry (Ferry

and Spear, 1978) gives for this sample, temperatures

of 800–820 �C for garnet core/primary biotite inclu-

sion pairs, and of only 540–560 �C for garnet rim/

biotite (for P fixed at 4 kbars). The garnet–cordierite

pair gives consistent temperatures of 740 �C for the

same pressure. Amphibole–plagioclase geothermom-

etry (Blundy and Holland, 1990) from the adjacent

amphibolitised gabbro–norite gives rather high tem-

perature of equilibration ranging from 880 to 970 �C.The analysed monazite grains appear as metamor-

phic blasts included in cordierite. Nineteen SIMS

analyses were conducted on 17 monazite crystals

(Fig. 5). Analyses yielded a spectrum of 207Pb/206Pb

ages from 532 to 657 Ma. Of the 19 analyses, the

youngest was considered as an outlier and rejected

from calculation. The remaining 18 analyses can be

combined to provide a weighted mean 207Pb/206Pb

ratio of 0.05998F 0.00030 (MSWD = 1.8) corre-

sponding to an age of 603F 11 Ma. The low MSWD

value suggests that the sample contains a single

monazite population, although, on this basis alone, it

cannot be ruled out that heterogeneous grains with

composite age pattern are present as indicated by

scattering of the 207Pb/206Pb ages. In this age range,

the time resolution of the SIMS technique does not

allow distinguishing possible second-order events,

which are not separated by about 20 Ma. However,

these results indicate that monazite, as young as late

Fig. 5. SIMS 207Pb/206Pb diagram for monazites from the C106

metapelitic gneiss (Egatalis zone, Hoggar). Error bars are 2r.


Proterozoic in age, can be successfully investigated

with the Cameca IMS4f ion probe.

The Hoggar mountains represent the northernmost

exposure of the Trans-Saharan Pan-African belt which

formed by aggregation and suturing of continental

fragments along the eastern margin of the West

African craton. Further south in Northern Mali,

UHP metamorphism has been dated at ca. 620 Ma

(Jahn et al., in press), whereas granulite facies meta-

morphism in the Dahomeyide belt of Western Africa

and amphibolite facies metamorphism in the mobile

belt of Nigeria have been dated at 610–620 Ma

(Bruguier et al., 1994; Attoh, 1998; Affaton et al.,

2000). The 620- to 610-Ma age range thus appears to

correspond to an active period of high-grade meta-

morphism along the eastern margin of the West

African craton with subduction of continental litho-

sphere, collision and suturing of continental fragments

resulting in the formation of the Gondwana super-

continent at the end of the Proterozoic. The temporal

relationship between the 603F 11 Ma monazite age

and regional metamorphism in Northwestern Hoggar

is unclear. Although slightly younger, this age over-

laps the period of high-grade metamorphism along the

western margin of the West African Craton and may

be related to the climax of regional metamorphism,

which peaked at 800–900 �C in this part of the

Hoggar Mountains. This age is also consistent with

the migmatisation age of 609F 17 Ma of the Aleksod

eclogites further west (Barbey et al., 1989). However,

at In Tassak, the progressive passage from two-mica

schists to kinzigites and metatexites only occurs over

a distance of 150–300 m. This rather sharp thermal

paleogradient is related to the synmetamorphic

emplacement of a 300- to 500-m-thick sheet of

gabbronoritic composition (Caby, 1987). This sug-

gests that monazites record a later event that accom-

panied emplacement of the gabbronoritic body.

In the southern part of Central Hoggar (Laouni

Terrane), troctolites, olivine-bearing gabbros and nor-

ites (Cottin et al., 1998) have been emplaced at the

end of regional metamorphism within syn-kinematic

Pan-African granitoids dated at 630–600 Ma by

Bertrand et al. (1986). In this area, field relationships

have been used to bracket emplacement age of the

mafic–ultramafic intrusions between 600 and 520

Ma, the latter corresponding to the age of the N–S

elongated late orogenic Taourirt granites (Paquette et

al., 1998). In the Tassendjanet terrane of Western

Hoggar, the Tin-Zebane dyke swarm, which includes

dykes and stocks of gabbros, has been dated at

592F 8 Ma (Hadj-Kaddour et al., 1998). All these

mafic–ultramafic rocks were emplaced during asthe-

nospheric upwelling related to a rapid lithospheric

thinning which affected most of the Hoggar (Cottin et

al., 1998; Hadj-Kaddour et al., 1998). The 603F 11

Ma monazite age is consistent with the 592F 8 Ma

Rb–Sr age of the Tin-Zebane dyke swarm, and the

closeness of the C106 metapelite sample with a

gabbro–norite massif suggests that monazite may date

some point in the retrograde path of the regional

metamorphism following metamorphic peak that

was, in the present case, closely related to crystallisa-

tion of the gabbronoritic magma at ca. � 10–12 km

depth. Taking into account the 620-Ma U–Pb zircon

age of the syn-kinematic diorite–granodiorite associ-

ation typical of the same belt farther south in Mali

(Caby and Andreopoulos-Renaud, 1989), this age is

representative of the late pulse of Pan-African tecto-

nometamorphic evolution in the western part of the

Tuareg shield.

4. Conclusions

The Cameca IMS4f ion microprobe has been

successfully used to determine in situ 207Pb/206Pb

ages on monazite crystals from Archaean to Late

Proterozoic ages. Molecular interferences can be sep-

arated with a mass resolving power of ca. 3500 and an

energy filtering of 50 eV. Large field and contrast

apertures have been used to increase the number of

ions arriving at the detector resulting in an overall

instrumental sensitivity of 3–4 cps/ppm of Pb/nA of

primary beam. Measurements can be routinely

achieved with a spatial resolution of ca. 30 mm. As

a single measurement requires only 60 min of data

acquisition, a comprehensive study of a monazite

population can be performed in a few days even for

heterogeneous monazite populations such as those

often present in metamorphic rocks.

Two sets of Proterozoic and Archaean monazites,

analysed earlier by ID-TIMS technique, yielded

nearly identical SIMS and TIMS ages for the main

monazite growth events. SIMS analyses provided207Pb/206Pb weighted mean ages with uncertainties


ranging from 3 Ma (2r) for Archaean monazites to 6

Ma (2r) for Grenvillian population and to 11 Ma (2r)for Pan-African monazite grains. In addition, SIMS

analyses make it possible to analyse separately dis-

tinct age domains present within one single grain.

Although second-order events that are not separated

by ca. 20 Ma cannot be resolved successfully, the age

spectrum given by SIMS analyses shows a greater

complexity and the technique makes it possible to

unravel complex age patterns presented by composite

monazite populations. The technique is thus capable

of dating discrete events that are not represented by

new growth or complete recrystallisation of pre-exist-

ing grains. In situ analyses of 207Pb/206Pb ratios in

such monazite populations can therefore provide a

wealth of information on the timing of metamorphic

events and have implications for U–Pb systematic in

monazite. In the examples presented above, monazite

crystals show complex internal structures, comparable

to those commonly observed for zircons (e.g. Hanchar

and Miller, 1993). As for zircon, new growth and

recrystallisation appear to be a very efficient phenom-

enon in monazite and substantiate the usefulness of in

situ analyses. In metamorphic environments, monazite

growth can occur at several periods during the whole

metamorphic history, from the prograde stages to the

retrograde part of the metamorphic path. In amphib-

olite facies metapelites, monazite grows during the

prograde stage and, due to the robustness of the U–

Th–Pb systems, generally preserves information on

this part of the P–T– t path. In such environments and

providing textural relationship can be established,

monazite can be used to estimate burial and heating

rates. In granulite facies rocks, recrystallisation of

crystals inherited from source regions or grown during

the prograde stages of metamorphism is almost com-

plete, but preservation of radiogenic lead in monazite

domains substantiates robustness of the U–Pb system

in this mineral which was not completely reset by

peak temperature of 800–900 �C. In situ analysis of

such domains constitutes a window to look back into

parts of the metamorphic history, which are generally

inaccessible due to blotting out of primary assemb-

lages. Monazite populations can even show a greater

complexity due to possible new growth, recrystallisa-

tion or partial Pb loss during the retrograde part of

metamorphism, which is often dominated by an

important magmatic activity. In the case of the North-

ampton Complex, pegmatite intrusion during the

waning stage of metamorphism was possibly respon-

sible for discrete disturbances or even partial recrys-

tallisation in pre-existing minerals. In the close

vicinity of magmatic bodies, as documented by the

Hoggar sample C106, resetting of the U–Pb isotope

system can reach completion, possibly due to fluid

flows and high-temperature gradients.

The polycyclic growth and complexity of monazite

has opposite consequences, as it can constitute a

serious drawback to the use of this mineral in U–Pb

geochronology or, on the contrary, provide a wealth of

information that can open up new perspectives in our

understanding of metamorphic processes, providing

tools can be developed to unravel such within-grain

complexity.

Acknowledgements

We thank J. Kieffer and E. Lebeau from the

‘‘Service Commun National du SIMS de l’Universite

de Montpellier II’’ for their help when running the

samples. Helpful and constructive reviews by Antonio

Lanzirotti, Alexander Nemchin, and Roberta Rudnick

are greatly appreciated. RR

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Monazite “in situ” 207Pb/ 206Pb geochronology using a small geometry high-resolution ion probe....

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