Contrasted monazite hydrothermal alteration mechanisms and their geochemical implications

EPSL EL%VIER Earth and Planetary Science Letters 145 (1996) 79-96

Contrasted monazite hydrothermal alteration mechanisms and their geochemical implications

Franck Poitrasson a,b, * , Simon Chenery a, David J. Bland ’

a Analytical Geochemistry Group, British Geological Sun,ey, Keyworth. Nottingham NGl2 5GG. UK h Laboratoire de Gkochimie, UMR 5563 CNRS, Unicersite’ Paul Sabatier, 38, rue des 36 Pants. 31400 Toulouse, France

’ Mineralogy and Petrology Group. British Geological Survey. Keytiborth, Nottingham NG12 SGG, UK

Received 26 April 1996; revised 26 August 1996; accepted 13 September 1996

Abstract

In spite of the major importance of monazite as a repository for the rare earths and Th in the continental crust. for

U-Th-Pb geochronology, and as a possible form for high-level nuclear waste, very little work has been carried out so far on the behaviour of this mineral during fluid-rock events. This contribution describes two contrasting examples of the

hydrothermal alteration of monazite. The first case comes from a sample of the Carnmenellis granite (Cornwall, Southwest England), chloritized at 284 + 16”C, whereas the other occurs in the Skiddaw granite (Lake District, Northwest England). which underwent greisenization at 200 & 30°C.

An integrated study involving backscattered scanning electron microscopy, electron microprobe analyses, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) reveals that the chloritization event was characterized by the coupled substitution 2REE3+ =F Th4+ + Ca2+ m the altered parts of the monazite, thus leaving the P-O

framework of the crystal untouched. In contrast, greisenization led to the coupled substitution REE3+ + Psi- * Th4+ + Si4+, and therefore involved a partial destruction of the phosphate framework. The resulting rare earth element patterns are quite

different for these two examples, with a maximum depletion for Dy and Er in the altered parts of the Cammenellis monazite, whereas the Skiddaw monazite shows a light rare earth depletion but an Yb and Er enrichment during alteration. This latter enrichment, accompanied by an increase in U but roughly unchanged Pb concentrations, probably resulted from a decrease in the size of the g-coordinated site in monazite, thereby favouring the smaller rare earths.

These contrasted styles of monazite alteration show that the conditions of fluid-rock interaction will not only affect the

aqueous geochemistry of the lanthanides, actinides and lead, and the relative stability of the different minerals holding these elements. Variations in these conditions will also lead to various possible chemical exchanges between the crystalline phases and the hydrothermal fluids.

The occurrence of common lead along penetrative cracks in the Cammenellis monazite shows that only a leaching, prior to the U-Pb analyses of the whole-grain, will permit an accurate determination of the magmatic crystallization age. In

* Corresponding author. Correspondence to address b. E-mail: [email protected]

0012-821X/96/$12.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved

PII SOO12-821X(96)00193-8

80 F. Poitrasson et al/Earth and Planetary Science Letters 145 (1996) 79-96

contrast, for the Skiddaw case it may be possible to date the fluid-rock event by in situ ‘07Pb/ ‘06Pb geochronology. The observation that the altered parts of both monazite examples display Nd leaching and no significant Sm/Nd fractionation indicates that they should not affect the host whole-rock Nd isotopic signatures.

Finally, it appears that monazite-like ceramics designed for the containment of high-level nuclear wastes will retain Th

and the geochemically equivalent transuranic elements during fluid-rock events similar to those documented in this study but may release Nd, U and the corresponding radionuclides to the environment.

Keywords: monazites; alteration; hydrothermal alteration; rare earths; thorium; U/Pb; geochronology; radioactive waste; laser methods;

inductively coupled plasma methods: mass spectroscopy

1. Introduction

Monazite, (Ce,La,Nd,Th)PO,, is one of the main

mineral hosts for rare earth elements (REE) and Th

in the continental crust [1,2]. Hence, the common use of these elements and the associated radiometric

systems (i.e. Sm-Nd, and U-Th-Pb), to study either

the petrogenesis of felsic rocks, or the age and origin of the hydrothermal fluids which affected them,

makes a detailed understanding of the behaviour of this mineral when it interacts with crustal fluids

critical. Furthermore, monazite is increasingly used in U-Pb geochronology [3]. However, it has been

shown recently that the behaviour of this geochronometer, in certain cases, is not adequately

explained in terms of volume diffusion [4]. The

influence of fluid-induced recrystallization must therefore be understood for both thermochronologi-

cal applications and to date fluid-rock events. The

study of the resistance of this accessory mineral during fluid-rock interaction is also important be-

cause ceramics with the structure and composition of monazite have been proposed as a possible form for the containment of high-level nuclear wastes [5].

However, the study of the alteration of accessory

minerals in general and monazite in particular is still in its infancy, since previous work has been either

essentially preliminary in nature [6], or mostly con- cerned with the end-products of monazite alteration

[71. This contribution therefore describes a detailed

study of monazite alteration under hydrothermal in- fluences. The two extreme cases presented here show that the alteration mechanisms can be surprisingly different, and that these result in contrasting exchange of REE, Th, U and Pb between monazite and hydrothermal fluids. The geochemical, geochrono- logical and environmental implications of these find- ings will then be explored.

2. Geological background

2. I. Cammenellis granite

The first example of altered monazite is from the Cammenellis granite, Cornwall, Southwest England.

This rock is a peraluminous, coarse grained, biotite-muscovite-bearing granite. A detailed de-

scription of the mineralogy and the geochemistry of

this pluton is given by Charoy [8]. The sample studied (CA94141 comes from the western side of

the Chywoon Quarry. Petrographic observations reveal two distinct chlorite populations, with different

textures, optical properties and composition. From this it can be deduced that two chloritization events

of hydrothermal origin have affected this rock. Chlo-

rite geothermometry [9] confirms this since the two chlorite types yield contrasting temperatures for these

fluid-rock events. Six electron microprobe analyses of Fe-Mg chlorites (brunsvigite) give 284 + 16°C (2 s.d.), whereas the other chlorite family yield 336 f

15°C on 10 analyses. Analyses with K,O above ca. 1 wt% were rejected because they were indicative of

chlorite-biotite intergrowths, which would have

given meaningless results [9]. The altered part of the monazite studied is enclosed within one of the lower-temperature chlorites. The fluid-rock event

responsible for the alteration of this monazite is therefore a chloritization which occurred at 284 +

16°C. For many years, two chloritization events have

indeed been recorded in the Comish granites. Hy- drothermal alteration at 284 + 16°C corresponds to the “weak hydrogen metasomatism” of Alderton et al. [lo], or the “clean chloritization” of Durrance et al. [ 1 I]. It is mostly characterized by the replacement of biotite by a pale green chlorite with few inclusions. According to the recent review of Chesley et al. [ 121, chloritization of the Comish granites at

F. Poitrasson et al/Earth and Planetary Science Letters 145 (1996) 79-96 81

200-400°C corresponds to the main episode of min-

eralization of this province (their “stage 3” mineralization). It yielded, in places, quartz-tourmaline-

chlorite-sulphide-fluorite-bearing veins with eco- nomic concentrations of Sn, Cu, Pb, Zn, Fe and As.

Although there is now a consensus on the emplacement age of the Cammenellis granite (294 + 1 Ma

by U-P\, on monazite [ 12,13]), the age of this major fluid-rock event is still a matter of debate. However, fluid-inclusion Rb-Sr and fluorite Sm-Nd isochrons

give concordant dates at 260-270 Ma 114,151 for the

Cammenellis granite.

2.2. Skiddaw granite

The second example of monazite alteration is

from the Grainsgill Cupola, a hydrothermally altered

facies of the biotite-bearing Skiddaw granite, Lake District, Northwest England. The sample used (CF7831) was taken from the underground workings

of the Carrock Fell tungsten mine, 1.5 m west from

the Harding Vein (i.e. the closest sample west of the vein taken by Ball et al. [16]; see their fig. 6). This

rock shows no evidence of tungstate mineralization, but has been strongly greisenized. The feldspars are

completely destroyed, with the accompanying devel-

opment of abundant muscovite, quartz, chalcopyrite, pyrite, apatite and r-utile. The altered parts of the

monazite studied are themselves interdigitated with chalcopyrite.

In a detailed mineralogical and geochemical study of the Carrock Fell mineralization and the associated

alteration of the host granite, Ball et al. [16] estimated the temperature of the fluid-rock event responsible for the greisenization of the Skiddaw gran-

ite and precipitation of sulphides to be 200 + 30°C.

The Skiddaw granite was intruded at 399 + 8 Ma

(K-Ar date on biotite from Shepherd et al. [17], recalculated with the new constants; see also Brown et al. [18]), and the fluid-rock event was dated at

393 + 5 Ma by a fluid-inclusion Rb-Sr isochron

[191.

3. Analytical techniques

The rock samples were prepared as polished, 300 p,rn thick sections and the monazite alteration exam-

ined using a Cambridge Instrument Stereoscan S250

scanning electron microscope @EM), mqstly in the backscattered electron mode. Chemical characteriza- tion was aided by qualitative energy dispersive X-ray

microanalysis performed with a Link Systems 860 X-ray microanalyser attached to the SEM. Electron

microprobe maps were obtained with a CAMECA

SX50, in the wavelength dispersive s ectrometry

% mode, using an accelerating potential of 3 kV and a

current of 60 nA. The monazites were analysed using a Cambridge

Instrument Microscan 5 electron microprobe with a

Link Systems ANlOO energy dispersive X-ray

spectrometer calibrated with REE oxides, pure met-

als and mineral standards [20]. X-ray spectra were collected with an accelerating potential of 15 kV, a

current of ca. 5 nA and 100 s counting time. This time was set at 60s for the chlorite analyses used for

the geothermometric estimations. The precision of

the analyses was about 1% Relative Standard Devia-

tion (RSD) for most of the major elements, except for the light rare earths and Th, where !it was esti-

mated at 2-4% RSD.

The laser ablation-inductively coupled plasma- mass spectrometry (LA-ICP-MS) analyses were

made using a VG PQ2 + ICP-MS instrument in

conjunction with a frequency-quadrupled Spectron SL803 Nd:YAG laser. A complete account of the

characteristics and the optimisation of this instru-

ment is reported elsewhere [21,22], and only the key specifications relevant to the present work will be

given here. The combination of a Q-switched UV laser, a high magnification (X36) reflebtive objec-

tive, and the optimisation of the laser parameters provides a high spatial resolution (i.e. ablation craters

of few hundred nanometres in diameter~[23]). How- ever, to strike a balance between the spatial resolu-

tion and the detection limits, the crater size was set at 4 Km. This permitted the determination of ele-

ment concentrations down to few micrograms/gram (ppm>. It is recognized that elemental fractionation during the ablation process is one of the shortcom- ings of this technique [24]. Nevertheless, the combi-

nation of a Q-Switched UV laser, and short ablation times (< 10 s) but relatively high energy pulses, reduced me chemical fractionation effects to negligible levels, even for critical ratios such ias U/Pb. A dual-flow system [25] was used for tbe calibration

82 F. Poitrasson et al. /Earth and Planetar?, Science Letters 145 (19%) 79-96

and to monitor the oxide and doubly charged ion

levels (with ‘40Ce’60 and 14’Ce2’ ). These were

usually below 0.5%. Repeated analyses of an in- house monazite standard, using the same acquisition

parameters (peak jumping on 15 masses, 30 s acquisition time), were used to estimate the precision of the LA-ICP-MS analyses. This was typically in the

range of 6-25% RSD for an element normalized to a

major mass (e.g. 14’Ce), depending on the element

and its concentration. The LA-ICP-MS concentration values were calculated using Ce data obtained

by electron microprobe as an internal standard. Con- sequently, the error bars take into account the preci-

sion on the electron microprobe determination

(estimated at 2.5% RSD for Ce) by error propaga- tion.

4. Mineralogy of the altered monazites

4.1. Carnmenellis mana:ite

Backscattered SEM images reveal that the altered parts of the monazite occur where the mineral is in contact with chlorite (Fig. 1). In contrast, those parts

bordered by quartz are unaltered and retain their original crystallographic faces. This demonstrates the

importance of textural relationships to the behaviour of monazite during a fluid-rock event. Elements

released during the chloritization of the biotite, such

as alkalis, halogens and, in particular, F (which is present at elevated levels in the biotites of the Cor-

nish granites [26]) certainly enhanced the alteration of those parts of the monazite hosted by the phyl- losilicate.

Fig. 1. Backscattered scanning electron microscope image of a monazite from the Carnmenellis granite (sample CA9414. mineral G). The

mean atomic number is higher for the brighter parts of the crystal. The alterations are characterized, morphologically, by dark areas (e.g.,

D) and the disappearance of the original crystal faces, suggesting that dissolution processes also occurred. Alteration is initiated from the

margins of the crystal enclosed in chlorite (C) and from penetrative cracks. In contrast. the mineral retained its original crystallographic

faces for those parts enclosed in quartz (Q). This monazite also shows mineral inclusions occurring as large penetrative embayments (e.g..

I, a muscovite inclusion located in the bottom, still fresh, part of the crystal), which are not related to alteration. Note that the background of

this picture was enhanced by image processing in order to highlight chlorite and quartz. A LA-ICP-MS profile is visible with the craters,

and the three representative analyses shown in Table 1 have also been located.

F. Poitrasson et al. /Earth and Planetary Science Letters 145 (1996) 79-96 83

Spatially, the alteration develops from the mar-

gins of the crystal and along penetrating fractures (Fig. 1). It is characterized on backscattered SEM images by dark and irregular areas (Figs. 1 and 2), and chemically by an enrichment in Ca and Th, and

depletion in the rare earth elements (Fig. 3).

4.2. Skidduw mona,-ite

Morphologically, alteration in the Skiddaw monazite is more advanced than for the Carnmenellis monazite. The crystal shown in Fig. 4, enclosed by

muscovite and chalcopyrite, retains no euhedral faces. The alteration is characterized by the formation of

porous zones, enriched particularly in Th. In the

most extreme cases, the monazite is totally replaced by chalcopyrite.

5. Alteration chemistry and mechanisms

5.1. Curnmenellis monazite

An electron microprobe profile across the monazite reveals that the altered zones, whether originat-

ing from a penetrative crack or the margins of the

crystal, are characterized by a pronounced decrease in Nd and increase in Th concentrations (Fig. S). In

contrast, the P content of the crystal remains unchanged (Figs. 5 and 6). This indicates that the

alteration did not destroy the P-O framework of the crystal, at least for those parts not already dissolved.

Instead, the concomitant increase in Th and Ca

concentrations, and decrease in Nd (Fig. 51 suggest

that the mechanism involved is the cationic ex-

change:

2REE3+ t Th4+ + Ca’+ (1)

in the crystallographic sites of the rare earths. This

inference is further supported by a cationic plot (Fig.

7). In addition to the increase in calcium, the alter-

ation zone associated with the fracture is also charac-

terized by an increase in iron (see representative

analysis MO67 in Table 1). Partial replacement of Fe*+ for Ca’ + in the coupled substitution ( 1) could

explain the smaller increase in Ca in this alteration

zone compared to the one developed from the mar-

Fig. 2. Detail of the Carnmenellis monazite, around the MO76 analysis site (see Pig. 1). showing the structure of the alteration. The dark

area5 (e.g.. D) have a composition approaching that of an apatite, whereas the bright nuggets a few microns in size (e.g.. N ). next to the

dark areas. are Th enriched. This picture was taken before the LA-ICP-MS craters were made.

84 F. Poirrasson et al. /Earth and Planelarv Science Lerrers 145 (1996) 79-96

F. Poitrasson et al./ Earth and Planetary Science Letters 145 (1996) 79-96 8.5

Fig. 4. Backscattered scanning electron microscope image of a monazite from the Skiddaw granite (sample CF783 1, mineral q). An example

of altered zone (i.e. porous and Th-enriched) is located (A). Chalcopyrite inclusions, probably replacing parts of the monhzite dissolved

during the extreme cases of hydrothermal alteration are also shown (0. The minerals enclosing the monazite are chalcopytite (dark grey.

C) and muscovite (black, M). The two representative analyses shown in Table I are located. The profile shown in Fig. 8 starts with the

analysis made above MO 140 and finishes with the analysis located at the extreme right of the monazite.

gins (Table 1). However, although this replacement would maintain the electrostatic equilibrium of the

crystal lattice, the ionic radius of iron is much smaller

than that of the LREE3+, Th4+ or Ca2+ [27], and its electronic structure prevents it from fitting into a

9-coordinated polyhedron such as the monazite-REE

site [28]. It is therefore concluded that the iron present in the vicinity of the fracture occurs as

impurities in the crystal lattice. Thus, for both alteration zones, the main mechanism involved is the

coupled substitution (11, leading towards the braban-

tite end-member (CaTh (PO,),; [29]) of the monazite

group. Nevertheless, the Ca content of analysis MO76

is too high to be accounted for purely by a solid solution between monazite and brabantite; small

amounts of apatite may also have replaced monazite

in this alteration zone, as previously suggested on the basis of SEM investigations (Fig. 2, and see also

[6]). This leads toward a brockite-like composition

(Table 1; typical formula: (Ca,Th,REEXP,Si)O, . H,O, and see representative analyses in [2], for comparison).

The decrease in Nd in the altered parts of the monazite compared with the increase in Th (Fig. 5)

Fig. 3. Th (top) and Nd (bottom) X-ray electron microprobe maps for the Cammenellis monazite. Note that the central part of the monazite.

which is the most altered, is characterized by localised enrichments in Th (bright areas) and depletions in Nd (dark areas). The Nd leaching

associated with a penetrative crack, and detected with an electron microprobe profile (Fig. 5) is easily observable on the1 Nd map (black arrow).

86 F. Poitrasson et al. /Earth and Plunetary Science Letters 145 (19961 79-96

probably results from the higher solubility of lanthanides in the hydrothermal fluids. While the REE

are removed in solution from the altered zones, the

Th released from parts of the crystal undergoing dissolution is fixed in the crystallographic sites va- cated by the rare earths according to the coupled

Unaltered

Crack

? Altered Unaltered Altered Unaltered

-_

20 ~, , ) / ( / I-, t---+-,-+~+--+?_r_i

4 -;

t 4.5

tCa

1I -m-P/10

4.0

B 3s

z 3.0 E 1 ‘g 2.5 f

z

s 2

2.0 i

s 1.5

1 .o

0.5 b

0.0 A--~+-~ .-,+-LLL , L 1_1+ UI -1 +._LLLL+L r A I ,

40 60 60 100 120 140 160 180 200 220 240 260 280 300

Distance (microns)

Fig. 5. EIectron microprobe profile across the Carnmeneilis monazite for Nd, Th, Ca and P. The representative analyses shown in Table 1 are indicated (see Fig. I for location). The gaps in the profiles correspond to mineral inclusions not directly related to the alteration process.

These analyses have therefore been removed for clarity. The altered parts of the crystal are characterized by an enrichment in Th and Ca and

a concomitant depletion in Nd. In contrast, P does not appear to be affected by the alteration, suggesting that the phosphate framework of

the monazite was not destroyed. One of the alteration zones is related to a crack penetrating the crystal, located on this profile, visible on the

SEM picture next to analysis MO67 (Fig. 1). and also well defined on the Nd X-ray electron microprobe map (Fig. 3). See text for details.

F. Poitrasson et al. /Earth and Planetap Science Letters 145 f 1996) 79-96

L_~ +- ~. -I-- pi-- t-- I I

Ca P Y Si Al Th U 206Pb *07Pb 208Pb

Fig. 6. Representative analyses for the Carnmenellis monazite (top) and the Skiddaw monazite (bottom) showing the contrakting effects of

alteration for these two examples. Ca, P, Si, Al, Th were obtained by electron microprobe, and Y. U, ro6Pb, ‘“‘Pb, ‘“*Pb were obtained by

LA-ICP-MS. Data from Table 1. See text for discussion.

1 1.5

&E+Y(pi$

3 3.5

05

i-

04 “” ! “I’ 1 ‘j “/ “’

Fig. 7. Cationic plots (per formula units for 16 oxygens) for the electron microprobe analyses made within and close to the /rItered zones of

(a) the Cammenellis monazite and (b) the Skiddaw monazite. The analyses representing mixtures between the altered~ monazites and

replacing material were not included. The straight lines representing the cationic substitutions (a) 2REE’+ + Th’+ it Ca’+ and (b) REE3+ + Ps+ % Th’+ + Si”+ are reported for reference.

88 F. Poitrasson et al./Earth and Planetary Science Letters 145 (1996) 79-96

substitution (Eq. (I)). There is thus no need to invoke an external source of Th relative to the monazite in order to explain its observed increase in concentration in the altered mineral (Figs. 5 and 6).

5.2. Skiddaw monazite

Electron microprobe analyses also reveal a depletion in Nd and enrichment in Th in the altered zones of the monazite crystal (Fig. 8). However, in contrast to the Cammenellis example, the altered parts of the Skiddaw monazite show a sharp decrease in P concentrations, whereas their Si contents can be up to about 6 times higher than the fresh areas (Table 1; Figs. 6 and 8). This indicates that the coupled substi-

tution:

12EE3+ + P5’ + Th4’ + Si4+

Table 1

Representative electron microprobe and LA-ICP-MS analyses of monazite

occurred, leading towards the huttonite end-member (ThSiO,; [30]) of the monazite group. This is also evidenced by a cationic plot (Fig. 71, although the number of analyses available is limited here. The increase in Ca concentrations in the altered parts of the crystal (Fig. 8) suggests that coupled substitution (Eq. (1)) may also have occurred to a lesser extent. In this example, the cationic exchanges involved both the REE- and P-sites of the monazite, indicating a partial destruction of the P-O framework during alteration.

Origin

Characteristics

analysis

Cammenellis granite

(sample CA9414, mineral G)

Unaltered Altered

MO57 MO67

Altered

MO76

Skiddaw granite

(sample CF783 1, mineral P)

Unaltered Altered

MO141 MO140

La,% Ce203 Pr20, Nd,O - .3

Sm&

Gd,O,

DY,O,

Er#,

Yb,O,

Y20, ThO,

uo, ‘06Pb0

“‘PbO

‘08 PbO

pzos

SiO

Al,:

FeO ’

CaO

MnO

K2O

cue

S Total

12.1 7.58 5.02 15.1 6.99

25.2 16.7 10.9 30.2 14.2

2.63 1.67 1.26 3.01 1.59

10.6 6.58 4.35 10.8 5.04

2.18 1.42 0.748 2.35 1.08

1.96 1.49 < 0.9 1.73 1.22

0.575 0.352 0.130 0.364 0.347

0.114 0.0669 0.0265 0.0375 0.126

0.0338 0.0212 0.0169 0.0070 0.108

1.58 0.981 0.397 0.646 1.24

9.38 21.2 16.9 7.16 28.9

1.30 0.981 0.366 0.124 0.612

0.0744 0.304 0.0826 0.0064 0.0078

0.0158 0.195 0.0630 0.0009 0.0014

0.155 0.558 0.190 0.1135 0.0604

29.6 27.3 29.5 25.8 15.0

1.58 0.7 1 3.36 1.24 6.59

0.5 I 0.30 1.28 < 0.2 < 0.2

< 0.4 3.95 1.75 < 0.4 1.12

1.85 4.63 20.9 0.53 1.85

< 0.5 < 0.5 < 0.5 na na

na na 0.55 < 0.1 < 0.1

na na na < 0.5 1.16

na na na < 0.1 < 0.1

101.44 96.99 97.79 99.22 87.24

0.125 0.120 0.117 0.119 0.122

0.2 1 0.64 0.76 0.14 0.18

Ca. P, La. Ce, I’r, Nd, Gd, Si, Al, Th. Fe, Mn, K, Cu. S by electron microprobe and Sm. Dy, Er, Yb. Y, U and Pb by LA-ICP-MS, normalized to Ce obtained by electron microprobe. Data in wt%. na = not analysed. See text for analytical details and precisions. lJ7Sm/ ‘44Nd and 207 Pb/ 206Pb by LA-ICP-MS; precisions of ca. 5% and 25%. respectively.

F. Poitrasson et al. / Earih and Planetam Science titters 145 (19%) 79-96 89

6. Behaviour of REE, U and Pb during monazite alteration

The altered zones of the Carnmenellis monazite

are characterized by a depletion of all the lanthanides but this is particularly marked for Dy and Er (Fig. 9).

Interestingly, the analysis of a strongly transformed

Altered Unaltered

-- ~~

30 -. ,--. / , / ,. _I

MO1 40

zone (M076) shows that Yb is one of the least

depleted rare earths, together with Ce. In contrast, the altered zones of the Skiddaw monazilte show a LREE depletion (Fig. 9) as well as an enrichment of

the heaviest rare earths (Er and Yb) with’respect to

the fresh parts of the crystal. According to Bowie and Home [31] and Hughes

Altered Unaltered Mus. mixed

-- -

, i_, --7-, .-- ,.,: -., ,

i+Nd

I-cTh

: tCa

I_ i tP/loi

r (

30 40 50 60 70 80 90 100 110 120 130 140 150 160

Distance (microns)

Fig. 8. Electron microprobe profile across the Skiddaw monazite for Nd. Th. Ca. P and Si. The representative analyses showd in Table I are

indicated (see Fig. 4 for location). The altered parts of the crystal are characterized by an enrichment in Th. Ca, Si and a depl$tion in Nd and

P. The phosphate framework of the monazite has therefore been affected during the alteration process in this example. dub to a Si for P

substitution. (Mus. mixed = analyses ‘contaminated’ by muscovite intergrowths).

90 F. Poitrasson et al./ Earth and Planetary Science Letters 145 (1996) 79-96

et al. [32], the substitution of the smaller ‘9’Th4+ and t9]Caz+ for t9]LREE3+ is accompanied by a size reduction of the 9-coordinated site. In the case of the

Skiddaw monazite, this effect should be further en-

hanced by the pronounced substitution of the larger [41Si 4f for t4]P5+, as shown by the high concentra-

tions of Si in the altered parts of the mineral (Table

1, Figs. 6 and 8). Consequently, small REE (i.e. the heavy rare earths) become relatively more stable

than the larger ones in the 9-coordinated sites during the alteration of monazite. This explains why HREE

are not depleted, and actually become enriched, when

the substitution (2) occurs (Fig. 9). The enrichment

-I--

observed for Yb and Er in the monazite alteration

zones may result from the replacement of the LREE by the HREE released from the parts of the crystal

undergoing dissolution. This process is analogous to that already proposed above for Th. It may thus be

inferred that the HREE were less soluble than the LREE in the fluids involved in the greisenization of the Skiddaw granite. Interestingly, this increase of

stability in the monazite lattice toward the HREE

was also observed from La to Gd during the substitu-

tion (1) for medium-grade metamorphic monazites [33]. In the altered zones of the Carnmenellis mon-

azite, the reduction in size of the REE site is less

’ -t-

Carnmenellis I ?

iii 22

I ; --›t I I , ----_+__- / I

J

0 10 --

w , + MO1 41 (unaltered) Skiddaw B / t MO1 40 (altered) g

i_~_

“E

P 1

, i-_*___ ----__--_-I__--_--_--__--_

Cm ; _+ ~_ 1 I ~~ 1 1~ 1 ~~ , ~~ - 1 ~_~ , _i

La Ce Nd Sm DY Er Yb

Fig. 9. Representative rare earth element analyses obtained by LA-ICP-MS for the Carnmenellis monazite (top) and the Skiddaw monazite

(bottom). Note the contrasted effect of the alteration for these two examples. See text for discussion.

F. Poitrusson et al. /Earth and Planetan Science Letters 145 (1996i 79-M 91

because the Si3+ + P5+ substitution did not occur.

This probably explains why the altered parts of this monazite show no HREE enrichment, with only Yb displaying a smaller depletion compared to the other

rare earths (Fig. 9). Further, the higher solubility of the HREE compared to the LREE in F-rich fluids (e.g. [34] and references therein) would also tend to reduce any HREE enrichment in the altered parts of

this mineral. The net result was thus a preferential leaching of Dy and Er over the other rare earths (Fig. 9). since the hydrothermal fluids that altered the

Cammenellis monazite were likely to contain elevated concentrations of fluorine (see above).

These two examples also show a contrasting behaviour with respect to their U and Pb contents.

Uranium was depleted and Pb enriched during the alteration of the Carnmenellis monazite, whilst the U content of the Skiddaw monazite is higher in the altered parts, and Pb shows small variations (Fig. 6

and Table 1). U [91 ” has an ionic radius of 1.05 i

which is similar to those of the heaviest rare earths (e.g. 1.052 A for “‘Tm3+ and 1.042 A for 19’Yb’+

1271). Therefore, the crystallographic mechanism in- voked for the HREE should also be valid for ura-

nium. This explains the parallel behaviour of U and the HREE in both examples. Recent experimental work showing that the incorporation of U in the monazite lattice is accompanied by a decrease in the

9-coordinated site of this mineral [35] provides further evidence. In addition. the precipitation of sulphide minerals during the alteration of the Skiddaw granite indicates reducing conditions. This probably

prevented uranium from occurring in the very soluble U6’ oxidized state. and can be used to explain the lower mobility of this element during the alter-

ation of the Skiddaw monazite compared to the

Cammenellis case. Unlike uranium, “‘Pb’+ is larger than the largest LREE (La) normally accommodated

by the monazite lattice (i.e. 1.35 vs. 1.216 i [27]). Hence. the important reduction of the 9-coordinated site associated with substitution (2) during the alter-

ation of the Skiddaw monazite probably explains why chemical exchange with the hydrothermal fluids was not accompanied by an increase in the concentration of lead. as in the Cammenellis case. This effect may have been further enhanced by the precipitation of chalcopyrite during the alteration of the Skiddaw monazite (Fig. 4). because semi-quantita-

tive LA-ICP-MS analyses suggest that this sulphide

is a better acceptor for lead than monazit&.

7. Discussion

7. I. Cmsequences ,for the mobility of RJ!?E cud Th during ,fluid-rock events

It is well established that monazite is :one of the

main repositories for REE and Th in the ~continental

crust (e.g. [ 1.21). Hence, its alteration mechanisms

are of critical importance for understandhng the behaviour of these elements during fluid-rpck events,

either for the study of the hydrothermal fluids themselves. or to assess whether geochemicali data based

on lanthanides and thorium can still tie used for petrogenetic studies.

The effect of fluid-rock interactions qn REE and

Th is often interpreted in terms of the nature of the hydrothermal fluids at the scale of the iwhole rock

(i.e. decimetre scale; e.g. 1361). Howeker. in the

Cammenellis monazite only that part ofi the crystal that is enclosed in chlorite is altered (Fig. 1). This suggests that. in this case, REEs releaied into the fluid during monazite alteration are mbbilized by

very localized high concentrations of F. Qlue to chloritization of biotite (see above and [26]), Extrapola-

tion of the F contents of the hydrotharmal fluids from the micrometre scale to the decimetre scale should therefore be treated with caution.

Although the contrasting exchange biehaviour of

the REE between the altered monazites alnd the fluid phase observed in the two examples studied (Fig. 9) are likely to be due ultimately to differences in

temperature and fluid compositions. it ia worth not- ing that they can be explained in terms of crystallographic substitutions. Thus, if one wants to under-

stand and accurately predict the behaviour of the REE during tluid-rock events. account should be taken of the various possible exchange mechanisms between fluids and the main mineralogi@al repositories of these elements in the rocks, as well as the important controls exerted by pH. tempeq’ature, redox conditions, the ligands present in the flpid, and the relative stability of different REE-bedring phases

[7.34.36].

92 F. Poitrasson et al. /Earth and Planetary Science Letters 145 (I 996) 79-96

These results also demonstrate the importance of monazite alteration mechanisms on the behaviour of Th during hydrothermal alteration. For instance, in addition to its low solubility in hydrothermal fluids, the limited mobility of Th during fluid-rock events [37] can also be explained by the fact that the altered monazite will act as a good acceptor for the Th released by other parts of the crystal undergoing dissolution. Hence, even in rocks where both the lanthanides and Th are hosted mainly by monazite, the observed concurrent hydrothermal mobility of REE and immobility of Th at the whole-rock scale [38] are not inconsistent, and can be explained by this mechanism.

Furthermore, the relative enrichment of the HREE in the altered parts of the Skiddaw monazite shows that a stronger hydrothermal leaching of the other rare earths at the whole-rock scale cannot only be explained by the presence of HREE-rich accessory phases which are thought to be resistant to crustal fluids (e.g. zircons [39,40]). The REE fractionation during fluid-rock events by LREE holders like monazite can also affect the whole-rock HREE budget. Accordingly, Ward et al. [2] have demonstrated that monazite holds a non-negligible fraction of the HREE in granites (see their fig. 8).

For both examples, alteration of monazite should theoretically result in a whole-rock REE depletion

and fractionation (Fig. 9). However, Jefferies [6] has shown that those parts of the Cammenellis granite chloritized during the ‘weak hydrogen metasomatism’ were not significantly depleted in REE compared to the fresh granite. Similarly, Ball et al. [16], concluded that greisenization did not significantly affect the Skiddaw granite REE contents. This appar- ent discrepancy can be explained in different ways. In the Cammenellis example, the fact that only that part of the monazite enclosed in chlorite was altered suggests that a quantitative estimate of the propor- tion of monazite in contact with chlorite has to be taken into account when extrapolating the alteration effects at the whole-rock scale. Furthermore, the observation that REE were remobilized during fluid-rock interactions has no bearing at all on the distance of transport of these lanthanides by the fluids. These lanthanides may either reprecipitate next to their mineral source, or be transported over distances exceeding several hundred metres, depending on the fluid-rock interaction conditions [40]. For Carnmenellis, the majority of the rare earths released by the monazite may have reprecipitated within a few centimetres as fluorite (see [61X thereby barely affecting the whole-rock REE patterns. In the Skid- daw case, the comparison of averages of REE analyses [16] is probably not accurate enough because new whole-rock REE data reveal that monazite alter-

0.8 i

o.oi- ,-- -,---. c- r ~ +- ~+--__, _--,

La Ce Pr Nd Sm ELI Gd w

Fig. 10. Whole-rock rare earth element analyses for two representative altered and unaltered samples of the Skiddaw granite. Note the large

depletion of LREE and the relative enrichement of the heaviest rare earths in the greisenized granite, as observed for the altered parts of the

Skiddaw monazite with respect to the fresh parts (Fig. 9). This monazite comes from the altered granitic sample CF783 1, thus demonstrating

that the whole-rock REE pattern is closely related to the style of REE exchange between monazite and hydrothermal fluids. The relative

depletion in Eu in the altered granite is probably due to feldspar alteration during the greisenization event. Data from Table 2.

F. Poitrasson et al. /Earth and Planetary Science Letters 145 (1996) 79-96 93

Table 2

Representative whole-rock REE analyses of the Skiddaw granite

Characteristics Unaltered Altered

analysis SK9606 CF7831

La (kg/g) 44.1 17.9

Ce 83.0 39.1

Pr 9.41 4.05

Nd 29.2 13.9

Sm 5.28 2.78

Eu 1.10 0.38

Gd 4.10 2.66

DY 3.05 3.00

Er 1.64 1.78

Yb 1.71 1.95

LU 0.22 0.25

(La/Sm), 5.15 3.97

b/Y’& 17.3 6.14 Eu/Eu * 0.70 0.42

Eu/Eu * = 2(Eu/Eu,,)/((Sm/Sm,,)+(Gd/Gd,,)), where cn

indicates chondritic values (from [41]). See [42] for analytical

details.

ation has dramatic effects at the whole-rock scale (Table 2 and Fig. IO’).

7.2. Implications for geochronology and isotope geochemistry

Although the selection of monazite crystals for

U-Pb geochronology is in general careful, some weakly altered grains may not be easily recognised

optically. Hence, only a detailed examination of the isotopic results may suggest that some of them have

been affected by a fluid-rock event. Interestingly, such a case occurred with some of the monazite grains used for the U-Pb dating of the Cammenellis

granite [12]. The evaluation of the alteration effects

on the U-Th-Pb systematics of this mineral is there-

fore important. Although no precise isotopic determinations have

been made during the present study, the very high amounts of lead observed in the altered part of the

Cammenellis monazite, as well as its very high “‘Pb/ ‘06Pb ratios (Table l), show that common lead has been incorporated in the crystal. This means that a monazite altered in this way will not be suitable for an accurate dating of the host-rock crystallization. Even more critical is that this alteration develops not only from the rims of the crystal but also from penetrative fractures (Figs. 1, 3 and 5).

Consequently, if monazite grains with small fractures

are inadvertently selected, an abrasion procedure [43] will only partially solve the problem of incorporated

common lead. This is because, in contrast to zircon, monazite rarely becomes metamict during alteration

151. Hence, even if the crystal partially breaks up in

the abrader, the altered zones do not necessarily yield softer, and therefore more abradable parts com-

pared to the fresh zones of the crystal (see also 131). It is also noteworthy that even the chemical Th-Pb

dates obtained by electron microprobe [44,45] are

also likely to be wrong, owing to the very large

amounts of lead incorporated during alteration (Ta- ble 1 and Fig. 6). In contrast, the Skiddaw monazite

does not appear to have gained large amounts of

common lead in its altered areas. This, together with the incorporation of large amounts of Th and U, may

permit the dating of the fluid-rock event using these altered areas. Further precise in situ [4] or step-leach-

ing [46] U-Th-Pb isotopic work is required to assess this.

There are two main mechanisms which will alter the Nd isotopic signatures of granites during a

fluid-rock event [40]: (a> if the Sm/Nd ratio is

changed several million years after the magmatic crystallization of the pluton or; (b) if the hydrothermal fluid introduced Nd with a significantly different

lA3Nd/ ld4Nd ratio compared to the original granite isotopic signature. For both the Skiddaw and the

Cammenellis examples, process (a) probably had little effect. In the Skiddaw granite case, geochrono- logical determinations suggest that the fluid-rock event occurred soon after magmatic crystallization

(see above). The Sm/Nd ratios of the altered parts of the Cammenellis monazite are not significantly

different from the fresh parts of the crystal (Table 1).

Hence, monazite alteration would have only a small effect, if any, on the Sm/Nd ratio of the Cammenel- lis granite on a whole-rock scale. The observation that the altered parts of both monazites are character-

ized by a depletion of Nd (Figs. 5 and 8) means that the altered monazites themselves will not change the whole-rock eNdi, because they probably did not incorporate significant amounts of exotic Nd with a contrasting isotopic signature (i.e. process (b) inef- fective). It is thus concluded that, for both examples, monazite alteration probably did not affect the Sm- Nd radiometric system at the whole-rock scale.

94 F. Poitrasson et al. /Earth and Planetury Science Letters 145 (1996) 79-96

7.3. Implications for monazite-like ceramics as con- tainments of nuclear wastes

The temperatures of the hydrothermal fluids which

have reacted with the samples studied are similar to those that are likely to be encountered around certain

high-level nuclear waste disposal sites [47,48]. The

present study can therefore be used to infer the

behaviour of ceramics with the structure and composition of monazite [5] during such an event. An important observation is that, in both cases, Th was

retained, even from those parts of the crystal which

had already been dissolved, as the altered zones reincorporated the Th just released (Figs. 5 and 8, and see above). Consequently, it is likely that the Th

present in high-level nuclear wastes, together with the transuranic elements which are thought to have a

geochemical behaviour equivalent to Th4+ (e.g. Pa5+,

Np”+ and Pu4+ [47]), will be retained if the ceramic

has been exposed to similar fluids. In contrast, a ceramic with the structure and composition of mon-

azite does not seem to be a suitable form for the rare earth elements, and the nuclides geochemically equivalent to Nd3+ (e.g. Pu3+, Cm”+ and Am3+

[47]), if the disposal site undergoes the type of fluid-rock events documented in the present work. A similar conclusion can be drawn for U and the

related transuranic elements, since this element was leached from one of the two examples studied.

8. Conclusions

This work shows that, depending on the conditions of the fluid-rock interaction, the mechanisms

of monazite alteration are different. This, in turn, leads to contrasted exchange of REEs, U and Pb between this mineral and the co-existing hydrothermal fluids. The implication is that studies that aim to provide an improved understanding and predictabil- ity to the behaviour of these elements during fluid- rock interactions should take into account these mechanisms. It was also demonstrated for one example that the release of REEs into the fluids was controlled by very localized chemical conditions, implying that extrapolation of this process from the micrometre to the decimetre scale should be treated with caution. For the chloritized example studied,

monazite alteration was accompanied by the incorporation of common lead within penetrative cracks.

Preparation of the sample using an abrasion procedure may therefore not be adequate for obtaining a reliable magmatic crystallization age by U-Pb

geochronology. In this case, initial leaching to re- move all the altered parts of the crystal before

isotopic analysis would certainly provide more reli-

able results. In contrast, the data obtained from monazite altered during a greisenization event suggests that it may be possible to date the fluid-rock

event using in situ *“Pb/ *06Pb techniques. For the two examples studied, monazite alteration in itself should not significantly affect the Nd isotopic signa-

tures of the host whole rocks. Finally, it appears that a monazite-like ceramic would be an excellent repository for high-level nuclear wastes containing Th and

the geochemically equivalent transuranic elements, but not for Nd or U and their equivalents under the

fluid-rock interaction conditions pertaining to this study. It can therefore be suggested that a nuclear waste package involving such a ceramic should not be deposited under geological conditions where a combination of fluid composition and the heat pro-

duced by the nuclear wastes may lead to fluid-rock interaction conditions similar to those described in

this paper.

Acknowledgements

A. Cavy carried out whole-rock sample preparation and F. Cantagrel REE analyses at the University

of Clermont-Ferrand, URA 10 CNRS. T.J. Shepherd kindly provided the sample CF783 1. This work ben- efited from analytical discussions with J.J. Robinson and M. Styles. J.M. Cook and T.J. Shepherd provided advice and a careful internal review of this article. K. Mezger, J.P. Respaut and an anonymous

referee are thanked for their journal reviews. P.D. Wetton is acknowledged for the maintenance and help with the SEM. J. Pearce and M. Styles are thanked for the access to the SEM and the electron microprobes. This study was carried out while the first author was funded by a Human Capital and Mobility contract number ERBCHBICT941027 from the European Commission at BGS, and a Training and Mobility of Researchers contract number ERBFMBICT960687 from the EC at the University

F. Poitrusson et al. / Earth and Planetap Science Letters I45 (I YY6) 7Y-Y6 95

of Clermont-Ferrand. URA 10 CNRS. This work is

published with the permission of the Director of the British Geological Survey, Natural Environment Re- search Council. [CL]

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Documents

Contrasted monazite hydrothermal alteration mechanisms and their geochemical implications