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Partial resetting of the UPb isotope system in monazite through hydrothermal experiments: An SEM and UPb isotope study
,NCLuDlNc
ISOTOPE GEOSCIENCE
ELSEVIER
Chemical Geology
137 (1997) 273-281
Partial resetting of the U-Pb isotope system in monazite through hydrothermal experiments: An SEM and U-Pb isotope study S tephan Teufel ‘, Wilhelm Heinrich ~~eoForschungsZentrum
Potsdam (GFZ), Telegrafenberg
Received 5 July 1996; accepted
*
A 17, D-14473 Potsdam, Germany
11 November
1996
Abstract Natural monazites of different grain sizes were treated experimentally under hydrothermal conditions and their U-Pb isotope systematics measured before and after the runs. Runs were carried out with monazite grain size fractions ranging from 40 to 125 pm in diameter, and on < 1 to 15 pm fine powdered material at 400-75O”C, 3 kbar and 28 days. U-Pb analyses of the starting material resulted in concordant U-Pb ages of 377 + 2 Ma. U and Pb concentrations were w 5500 and _ 800 ppm, respectively. All hydrothermally treated monazite grain size fractions yielded concordant U-Pb ages of u 380 Ma and no loss of Pb and U. In contrast, monazite powder revealed temperature-dependent Pb loss. Pb concentrations decreased to N 590 ppm at 750°C N 620 ppm at 65O”C, and u 710 ppm at 4OO”C, indicating Pb loss of - 26%, - 22% and N 1 l%, respectively, of the initial Pb content. No fractionation of Pb isotopes occurred; all analyses yielded ‘07Pb/ ‘06Pb ages close to 380 Ma. U concentrations remained always unchanged. All recrystallized monazites from the powdered starting ma.terial define a discordia with intercept ages of 379.3 + 2.3 and -7 + 18 Ma, the latter indicating the hydrothermally induced Pb loss at the time of the experiment. SEM observations of monazite grains from hydrothermally treated grain size fractions exhibited only minor dissolution features confined to the grain surfaces. Run products from monazite powders showed recrystallization and grain growth induced by a dissolutllon-precipitation mechanism. Increasing temperature resulted in a distinct enlargement of grain sizes at 650” and 750°C. A dissolution-precipitation process in presence of a fluid phase rather than Pb volume diffusion acts as a very efficient mechanism for the resetting of the U-Pb isotope system of monazites, even at low temperatures. This process is most important in shear zones where channelized fluid flow may occur. Keywords:
Monazite;
U-Pb
isotopes; Metamorphic
fluid; Dissolution-precipitation;
1. Introduction In geochronology, phosphate monazite
the U- and Th-bearing REEis often applied to infer the
* Corresponding author. ’Deceased, November 1995. 0009-2541/97/$17.00 Copyright PII SOOO9-2541(96100161-1
Scanning electron microscopy
timing of magmatic and metamorphic events (for a compilation see Parrish, 1990). Metamorphic monazites from metasedimentary rocks often show concordant or nearly concordant U-Pb ages (e.g., Teufel, 1988; Smith and Barreiro, 1990; Getty and Gromet, 1992) but discordant U-Pb ages are also known from a variety of metamorphic complexes (Hansen et
0 1997 Elsevier Science B.V. All rights reserved.
214
S.Teufel, W. Heinrich/
Chemicul
al., 1978; Gebauer et al., 1981; Black et al., 1984; Teufel, 1988; Getty and Gromet, 1992). Recent studies on natural samples have shown that the closure temperature for the U-Pb system in monazite is high and that monazite may record prograde growth ages and can even preserve ages that correspond to peak temperatures up to granulite-facies conditions (Mezger, 1990; Smith and Barreiro, 1990; DeWolf et al., 1993; Kingsbury et al., 1993; Zhu et al., 1994, Heinrich et al., 1996). Effective blocking temperatures were suggested to be 720-750°C (Copeland et al., 1988), 725 + 25°C (Parrish, 1990), and > 650°C and < 740°C (Mezger, 1990). The blocking ages depend both on the cooling rate of the rock and on grain size. A detailed EMP study of Pb diffusion in metamorphic monazite (Suzuki et al., 1994) revealed rimwards Pb loss in lOO-200-pm-large monazites up to some tens of micrometers. From known peak temperatures and cooling rates of these rocks, along with measured diffusion profiles, Pb diffusion coefficients in the order of lO-*l cm2 SK’ at 620°C and 10-20 cm2 s-’ at 680°C were derived (Suzuki et al., 1994). These observations suggest that, for example, in large monazite grains at a peak temperature of 700°C and a cooling rate of lO”C/Ma Pb diffusion is slow enough to allow for determining the age of the peak metamorphic episode via the U-Pb isotope systematics of monazite grains, provided that no later disturbance event occurred. An experimental attempt to estimate diffusion coefficients of Pb in monazite resulted in values in the range of lo- lo cm* s- ’ at 600°C (Shestakov, 1972). These experiments were done with sintered tablets of powdered monazite using *“Pb as a tracer. It is obvious that substantial discrepancies between field evidence and laboratory studies exist and that further experiments are needed. If the Pb diffusion coefficients were in the range of lo-*’ cm* SK’ at N 7OO”C, simple volume diffusion cannot account for the disturbed monazite ages that were observed in some metamorphic rocks. In some cases it has been shown that disturbed monazite ages plot on linear discordia arrays (cf. above) and field observations revealed that tectonic processes may have generated the discrete later event recorded at the lower intercept with the concordia. Consequently, discordant monazite ages may be used for dating shear zones (e.g., Teufel, 1988; Getty and
Grolog?
137 (19971 273-281
Gromet, 1992). Several resetting mechanisms for the U-Pb system of monazites from shear zones have been proposed, including late monazite overgrowth, late Pb loss via diffusion due to frictional heating, climb-accommodated recrystallization and crystallization due to grain boundary migration (Getty and Gromet, 1992). Selective hydrothermal leaching of Pb along subgrain boundaries in monazite grains was also proposed to be responsible for resetting (Black et al., 1984). It is clear, however, that the U-PI, discordance mechanisms in monazite and its rheological properties are poorly understood. The aim of this study is to demonstrate experimentally the hydrothermal influence on the U-Pb isotope system of monazite. For that, two different starting materials are used: (1) coarse-grained natural monazites; and (2) fine-grained monazite powder, both in presence of a fluid phase. In the first case, one would expect no significant dissolution-precipitation process, and the U-Pb systematics of these run products may reveal information on temperature-dependent volume diffusion under static conditions. In the second case, dissolution-precipitation-induced grain growth via fluid phase is most probable and we propose to show that experimentally induced recent recrystallization of monazite of early Herynian age points towards modem U-Pb ages of monazite.
2. Sample selection, experimental procedure and analytical methods Monazite from a lower amphibolite-facies metapelite from NE Bavaria, Germany, was used for this study. The sample comes from the nappe complex of the vicinity of the Continental Deep Drilling location (Windischeschenbach). Rocks from this area underwent medium-pressure metamorphism N 380390 Ma ago, inferred from U-Pb age determinations of zircon suites and monazites (Teufel et al., 1985; Teufel, 1988). The timing of this metamorphic event was confirmed by 40Ar/ 39Ar age determinations of amphiboles and biotites (SchliBler et al., 1986; Kreuzer et al., 1989). The sample (Tb 104) is an andalusite-staurolite-bearing mica-schist that equilibrated at peak metamorphic conditions of N 500°C and N 3-4 kbar. The mineralogy of this sample and
S. Teufel, W. Heinrich/ Chemical Geology 137 (1997) 273-281
the chemical composition of different monazite grains were reported elsewhere (Heinrich et al., 1996). All monazite crystals from sample Tb 104 yield concordant U-Pb ages and show a highly radiogenic Pb isotopic signature (Teufel, 1988); this is a precondition for the feasibility of this study as it enabled us to detect very easily any possible Pb blank contamination that may have occurred during the experimental and analytical procedures. The separated monazite grains were mainly of anhedral roundish shape. There is no evidence of sedimentary transport such as pitted surfaces (Fig. la>. Only _ 5% of the monazite grains show crystal planes. The colour of all monazites is bright yellowish green. Small inclusions of biotite and rutile are occasionally present.
275
The sample material was split into four grain size fractions of 40-63-, 63-80-, 80-lOO- and 100-125pm grain diameter. These were tediously handpicked prior to experimental work. An amount of w 40 mg of monazite sized 40-63 Frn has been ground in an agate mortar to a fraction of variable grain size ranging from N 15 pm down to < 1 pm. This fine ground powder as well as the granulometric fractions were used as starting material for the hydrothermal experiments. About 10 mg of monazite were placed in a gold capsule along with N 20 mg of ultrapure H,O. The capsules, 25 mm in length and 3 mm in diameter with walls of 0.2 mm thickness, were sealed by welding. All experiments were carried out in stan-
Fig. 1. Scanning electron microphotographs illustrating textural characteristics observed in coarse-grained monazite starting material and in the run products: a. Starting material: Coarse-grained monazites, fraction of 40-63 km in diameter. b. Run No. 8 (75O”C, 3 kbar, 28 days): Monazite with dissolution embayments that are confined to the orginally rounded grain surfaces. There is no change in shape and size of the crystal. The surface is covered by some quench products. c. Run No. 8 (75O”C, 3 kbar, 28 days): Monazite with dissolution features is partially overgrown by newly formed idiomorphic spinel, apatite and sphene cryslals. d. Run No. 6 (750°C. 3 kbar, 28 days): Cavity in monazite is partially overgrown by sphene (sph) and spine1 (spl, and small octahedra in the upper part of the cavity).
276
S. Teufel, W. Heinrich / Chemical Geology 137 (1997) 273-281
dard cold seal hydrothermal pressure vessels at 3 kbar and temperatures between 400” and 750°C. The duration of all runs was 28 days. Maximum error in the temperature measurements was +5”C. At the end of the runs the bombs were quenched to room temperature within a few minutes. The gold capsules were reheated to 110°C and then opened. The run products were washed out with ultrapure H,O. SEM investigations of gold spattered reactants and pieces of products were made using a Hitachi S 4000 high-resolution electron microscope equipped with a field emission electron source. Acceleration voltage was between 2 and 20 kV. Secondary phases in run products such as sphene, spine1 and apatite were identified using a Kevax Delta energy-dispersive analytical system. U-W isotope analyses were performed on all monazite reactants and products using the technique of Krogh (1973). Prior to dissolution, no acid washing procedure was applied neither to the reactants nor to the products. Monazite samples were dissolved in Teflon bombs with 6 N HCl (S&tier and Allbgre, 1983). After splitting, a mixed 235U-208Pb spike was added to one part of the sample solution. First, U and Pb were separated on columns filled with 0.5 ml of anion-exchange resin AG 1X8 (lOO200 mesh). Then Pb was purified using 0.8 N HBr. U and Pb isotope ratios were measured statically on a Finnigan MAT 261 multicollector mass spectrometer. The correction factor for mass discrimination of Pb was determined by repeated analyses of NBS 981 standard resulting in a value of 1.3%0/amu. Pb isotope ratios were corrected for initial common Pb applying the model proposed by Stacey and Kramers (1975). The amount of Pb blank normally introduced to the analyses during chemical procedure has been determined as 300 pg. Isotopic composition of Pb used for blank correction was ‘08Pb/ ‘04Pb = 38.63, 207Pb/ 204Pb = 15.63 and 206Pb/ 204Pb = 18.70. Ages were calculated with the uranium decay constants as recommended by the International Commission of Geological Sciences (Steiger and Jager, 1977). Regression analysis of the data points has been carried out according to York (1969), using the computer program “GEODATE” provided by B.M. Eglington and R.E. Harmer (199 1). 206Pb/ 238U and 207Pb/ 235U ratios have 2~ uncertainties of 0.5% and an error correlation of 0.95-0.98.
3. Results Two series of experiments were performed, the first with four different grain size fractions of monazite, at constant temperatures of 750°C and the second with monazite powder at 400”, 650” and 750°C. Pressure was 3 kbar and duration was 28 days in all experiments. The experimental conditions are summarized in Table 1. Isotopic analyses and SEM studies were carried out on the run products as well as on the untreated samples. Results of the isotope analyses are given in Table 2. The experiments with different grain size fractions as starting material (run Nos. 2, 4, 6, and 8; Tables 1 and 2) revealed that hydrothermal treatment caused a complete decolorization of the monazites. SEM observations showed that hydrothermal treatment caused no growth of monazite crystals (Fig. 1). The run products still consisted of the same anhedral roundish grains as the untreated samples (Fig. la). However, the run products revealed two characteristics: (a) Formation of channels and small embayments at the grain surfaces interpreted as the result of dissolution (Fig. lb and c). These dissolution features do not penetrate deeply into the monazite crystals but are restricted to the uttermost surface of the grains. (b) In some rare cases growth of newly formed crystals of other solid phases occurring at the surface and in cavities of the primary monazites (Fig. 1~). These were identified by EDS analysis as apatite, Mg,Al-spine1 and sphene (Fig. Id). Generally, they are very small (< 1 pm), but a few of these crystals may reach up to 5 pm. The formation of these
Table 1 Experimental
conditions
of hydrothermal
runs
Run
Fraction
Monazite (mg)
H,O (mg)
Temp.
2 4 6 8 11/12 13/14 15/16
40-63 p,rn 63-80 km SO-100 km 100-125 p,m powder powder powder
8.74 9.45 8.34 9.24 11.92 9.81 10.24
18.66 20.02 16.55 18.07 19.13 20.12 19.45
750 750 750 750 400 650 750
All experiments
were performed
at 3 kbar for 28 days.
(“Cl
(ppd
796.5 788.2 749.2 753.0 817.9 794.0 838.8 806.5 774.5
Pb,
1420.8 1399.0 1335.6 1334.4 1437.6 1376.9 1469.3 1399.1 1380.1
zo6Pb,hmd/g) U
‘06 Pb
-238
‘O’Pb
-23$
4.0 3.6 1.5 5.1 0.5 4.6 3.0
3.4 3.0 4.5 2.3 4.7 2.5 3.6 3.0 4.5
1.6587 1.6606 1.6599 1.6591 1.6567 1.6615 1.6574
1.6521 1.6651 1.6541 1.6689 1.7544 1.7285 1.7017 1.I275 I .6554
0.05729 0.05637 0.05663 0.05505 0.05578 0.05494 0.05498
0.05615 0.05466 0.05667 0.05480 0.18580 0.05482 0.05589 0.05484 0.05726
4680 6990 6110 16770 9830 21100 19400
7710 24300 6100 28100 111.1 27100 7750 20500 4830
5742 5407 5332 5496 5452 5610 5504
5516
815.0 711.4 710.6 632.5 614.6 594.2 586.4 810.3 708.7 107.5 631.5 612.9 593.5 585.6
1442.3 1260.3 1258.5 1123.4 1091.4 1054.7 1042.4
0.06022 0.05588 0.05659 0.04900 0.04800 0.04508 0.0454 I
0.06048 0.05976 0.0599 1 0.05989 0.06010 0.05994 0.06027 0.06035 0.05999
0.4497 0.4182 0.4232 0.3661 0.3593 0.3372 0.3395
0.4524 0.4454 0.4484 0.4482 0.4496 0.4486 0.4488 0.4504 0.4486
Analyses labelled H are from hydrothermally treated monazite. Nos. 9 and 10 are the starting materials before and after grinding. 0. I3%/amu. Uncertainties (2 (r ) are less than 0.1% unless in ratios involving ‘04Pb. Apparent ages are calculated using the constants 1977).
powder 400°C H 400°C H 650°C H 650°C H 750°C H 750°C H
Monazire powder:
10 11 12 13 14 15 16
799.3 789.1 752.5 753.7 1056.3 794.8 841.7 807.5 778.8
Pb (ppm) ‘06
Pb
“‘Pb
U
‘06Pb
-238
379.0 374.0 376.1 376.0 377.0 376.3 376.4 377.6 376.3
lo
Y
377.0 350.5 354.8 308.4 302.2 284.2 286.3
378.5 374.1 375.1 314.9 376.2 375.3 377.2 317.7 375.6
317.1 354.8 358.4 316.8 311.7 295.0 296.8
377.9
378.5 383.4 381.5 380.4
E
S
5 Z S 2
371.4 376.5 380.7
382.6 381.1
9 E. B ;; 8 s B
382.6
Measured isotopic ratios are corrected for recommended by JUGS (Steiger and JZger,
0.05417 0.05428 0.05424 0.05418 0.05430 0.05425 0.05423
0.05426 0.05406 0.05428 0.05428 0.05425 0.05428 0.05401 0.05413 0.05423
2. m. S\
5633 5613 5345 5342 5735 5507 5845
u (ppm)
Apparent ages(Ma)
382.6 382.7 381.5
*04Pb
‘06Pb
-
Atomic ratios
?? a
“‘Pb
“‘Pb
-
Concentrations
381.8 373.4
‘06Pb
“s Pb
Isotopic ratios
40-63 40-63 H 63-80 63-80 H 80-100 80-100 H loo-125 100-125 H 40-63
hg)
Weight
and products
B “Z
(km)
Sample
reactants
Grain size fracrions:
No.
Table 2 U-Pb isotope data of mon&ite
278
S. Teufel, W. Heinrich/Chemical
additional phases was due to hydrothermal reaction of former solid inclusions as biotite and rutile present in the original monazite. For U-Pb isotope analyses, splits of untreated grain size fractions have been analysed for direct comparison (Table 2). Though optical and SEM investigations did show some dissolution features, the hydrothermally treated and non-treated monazite grains display no significant difference in their U-Pb isotope systematics. The U and Pb concentrations and also the Pb isotopic compositions of both reactant and product coincide within error limits and all analyses of these grain size fractions yield concordant U-Pb ages in the range of 374-378 Ma. Aside from this striking result, there is a gradual change in common Pb content of the products. The measured 206Pb/ ‘04Pb ratios of the products increased by a factor of 3 compared to the reactants (Table 2). The untreated monazite fraction of 80-100 pm (No. 5 in
Geology 137 (1997) 273-281
Table 2) suffered considerable contamination by common Pb during the analytical procedure. Applying a corresponding higher correction of common Pb this sample also yielded a concordant U-PI, age of 376 Ma. The ground monazite starting material exhibits angular fragments with a grain size ranging from _ 15 pm to < 1 pm (Fig. 2a). Most fragments are below 1 pm. All of them show characteristic monazite {loo} and {OOl} cleavage planes. During hydrothermal treatment at 400°C all fragments lost their sharp edges and show roundish grain shapes (Fig. 2b). This is attributed to the beginning dissolution of monazite that starts at locations of high surface energy. The surfaces of larger grains often show small indentations and cavities. Monazites treated at 650” and 750°C clearly reveal advanced grain growth (Fig. 2c and d). However, in both runs relics of old crystal fragments remained. These old
Fig. 2. Scanning electron microphotographs illustrating textural characteristics observed in fine-grained monazite starting material and in the run products: a. Starting material: fine powdered monazite exhibiting irregular crystal fragments. b. Run No. 1l/12 (400°C 3 kbar, 28 days): Incipient monazite dissolution and growth of new crystals. Sharp grain edges disappeared. c. Run No. 13/14 (65O”C, 3 kbar, 28 days): Significant new grain growth of monazite. Many crystals exhibit regular crystal planes. d. Run No. 14/15 (750°C 3 kbar, 28 days): Typical monazite crystal of * 5 p,rn in size with regular crystal planes, indicating advanced grain growth.
219
S. Teufel, W. Heinrich / Chemical Geology 137 (1997) 273-281
400 Ma / Powdered =)
0.06
-
0.05
-
starting
material
8 . % $ (Y
0.3
0.4
0.6
0.5 207
Pb I 235
0.7
U
Fig. 3. 206Pb, 238~ v!i, 207pb, 235 U concordia diagram of powdered monazites from hydrothermal experiments along with starting materials. Values arc from Table 2. They define a discordia with intercept ages of 379.3 + 2.3 and - 7 f 18 Ma (MSWD = 9.9). All concordant points show U-Pb monazite data of hydrothermally treated grain size fractions.
grains were also affected by the dissolution-precipitation process. Many of the newly formed crystals developed crystal planes. All sharp-edged fragments disappeared, and the sample is dominated by roundish subhedral grains. U-PI, isotope analyses of the starting material and the products are given in Table 2. The comparison of values from the original fraction of 40-63-pm grain size (No. 9 in Table 2) and the powder from the same fraction ground in an agate mortar (No. 10) shows that both analyses give the same concordant U-Pb ages of 376 and 377 Ma, respectively. Both analyses show *06Pb/ ‘04Pb ratios of N 4.700, indicating that no common Pb was introduced during grinding. All three run products with powdered material gave U concentrations of N 5500 ppm (Nos. 11 to 16; Table 2) which is within the range of the starting material. In contrast, dramatic changes in the Pb concentration of the products occurred. Values of Pb concentration are 711 ppm at 4OO”C, 632 ppm at 650°C and 594 ppm at 750°C (Table 2). This implies that the sample at 400°C already lost N 11% of its original Pb content. Pb loss increased rapidly with
temperature to N 22% at 650°C and N 26% at 750°C. These changes in Pb concentration are reflected in the calculated U-Pb ages (Table 2). The intensity of Pb loss and the degree of discordancy of the U-W data points are well correlated (Table 2). Age calculations made for the different temperature steps result in younger U-Pb ages with increasing temperature (Fig. 3). Nevertheless, all analyses still show the original Pb isotopic signature as indicated Ma. Duplicate by 207Pb/ *06Pb ages of 378-383 analyses of these run products resulted in some small differences in the age determinations (Table 2). All data of hydrothermally treated monazites define a discordia with intercept ages of 379.3 f 2.3 and -7 & 18 Ma (Fig. 3). These ages agree with the primary metamorphic age of the monazites and the recent time of experimentally induced Pb loss.
4. Conclusions All our experiments are characterized fluid/solid ratio and short run durations.
by a high We found
280
s. Teufel, W. Heinrich / Chemical Geology 137 (1997) 273-281
no significant effect on the U-Pb system in the runs with different grain size fractions of monazite as reactants, even with small grain fractions of 40-63 km and high temperatures of 750°C. Thus, no Pb loss via volume diffusion occurred. Taking Shestakov’s (1972) diffusion coefficients of Pb in monazite of lo-i” cm2 SK’ at 600°C complete Pb loss should have occurred within a few days even at much lower temperatures and the U-Pb isotope system should have been reset to a zero age. Consequently, his diffusion coefficients cannot be applied to natural monazite. A possible explanation is the difference in the experimental setup. Shestakov (1972) observed the migration of radioactive *l’Pb added as a marker to a tablet of powdered and sintered monazite. Presumably, this *“Pb was not incorporated into the crystal lattice but might have migrated on the surface of monazite microcrystals within the sintered tablet and grain boundary migration was observed instead of true volume diffusion. According to our experiments, the diffusion coefficient even at 750°C must be lower than Shestakov’s values by several orders of magnitude. Our short run durations do not allow for calculation of the diffusion coefficients. However, there is no contradiction between our experiments and the values of Suzuki et al. (1994) from natural monazites where numbers in the range of lo-*i cm* s-l at 620°C 10e2’ cm* s-l at 68O”C, and extrapolated to 750°C of 10-‘910-l 8 cm* s- ’ were derived (see also Parrish and Tirrul, 1989). Also, no Pb loss via hydrothermal leaching occurred in our experiments with monazite grain fractions. Dissolution features as shown in Fig. 2c are limited to the utmost surface, and the formation of cavities (Fig. 2d) are confined to that sites where former solid inclusions reacted to new solid products via fluid phase. Both effects were obviously not large enough to disturb the U-Pb system of the monazite grains significantly. The experiments with ground monazite powders revealed strong recrystallization and growth features along with extensive temperature-dependent Pb loss. SEM images showed a dissolution-precipitation mechanism via fluid that acted to various degrees, becoming more intense with increasing temperature. Therefore, volume diffusion is also unlikely to be responsible for the Pb isotope exchange in these
runs. During the dissolution-precipitation process, Pb is not incorporated into the newly grown monazite and remains in the fluid phase. In contrast, all of the U is reincorporated into the solid and no significant amounts of U are partitioned into the fluid. This implies that the extent of Pb loss is directly correlated to the progress of the temperature-dependent dissolution-precipitation reaction. The reaction mechanism in our experiments and its effect on the U-Pb isotope system that resulted in strongly discordant U-Pb data is clearly proven. Whereas the interpretation of intercept ages of discordant U-PI, data of natural monazites often remains vague, there is no doubt about the meaning of both intercept ages of our discordia. Regional metathe starting morphism occurred at N 380 Ma point and upper-intercept age of our monazite discordia. The lower-intercept age represents the laboratory-induced metamorphism due to dissolution and precipitation at present time. This interpretation implies that the Pb distribution within relict old and newly grown grains is inhomogeneous. It is clear from SEM observations that very small monazite grains dissolved and recrystallization occurred around old, somewhat larger nuclei. If we reject volume diffusion as an effective mechanism the resulting U-Pb monazite system consists of undisturbed cores along with rejuvenated growth rims. This experimentally produced feature is probably present in many discordant monazites from metamorphic rocks (e.g., Parrish, 1990; Getty and Gromet, 1992). These results fit well into the scenario of ductile shear zones where extensive mass transfer by dissolution and reprecipitation occurs (see Getty and Gromet, 1992). This mechanism may easily produce disturbance and partial resetting of the U-W isotope system in monazites even at low temperatures of 400°C. If degree and duration of deformation in shear zones, along with enhanced fluid flow, are strong enough, the U-Pb ages of monazite will be completely reset (e.g., Teufel, 1988).
Acknowledgements Some of the analytical work was done at the Institute of Mineralogy, Geochemical Department,
S. Teufel, W. Heinrich/Chemical
Free University of Berlin. We thank H. Friedrichsen for providing analytical facilities and M. Feth for help with the analyses. U. Gemert, ZELMI, Technical University of Berlin, provided SEM facilities. G. Franz critically read an earlier version of the manuscript. The final draft profited by careful reviews of Kazuhiro Suzuki and Rolf Romer.
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decomposition of zircon and extraction of U and Pb for isotopic age determinations. Geochim. Cosmochim. Acta, 37: 485-494. Mezger, K., 1990. Geochronology in Granulites. In: D. Vielzeuf and Ph. Vidal (Editors), Granulites and Crustal Evolution. Kluwer, Dordrecht, pp. 451-470. Parrish, R.R., 1990. U-Pb dating of monazite and its application to geological problems. Can. J. Earth Sci., 27: 1431-1450. Parrish, R.R. and Tirrul, R., 1989. U-Pb age af the Baltoro granite, northwest Himalaya, and implications for monazite U-Pb systematics. Geology, 17: 1076-1079. Scharer, U. and Alltgre, C.J., 1983. The Palung granite (Himalaya); high resolution U-Pb systematics in zircon and monazite. Earth Planet. Sci. Lett., 63: 423-432. SchlBler, U., Oppermann, U., Kreuzer, H., Seidel, E., Okrusch, M., Lenz, K.L. and Raschka, H., 1986. Zur Altersstellung des ostbayerischen Kristallins Ergebnisse neuer K-ArDatierungen. Geol. Bavarica, 89: 21-47. Shestakov, G.I., 1972. Diffusion of lead in monazite, zircon, sphene and apatite. Geochem. Int., 9: 801-808. Smith, H.A. and Barreiro, B., 1990. Monazite Ub-Pb dating of staurolite grade metamorphism in pelitic schists. Contrib. Mineral. Petrol., 105: 602-615. Stacey, J.S. and Kramer% J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth. Planet. Sci. Lett., 26: 207-221. Steiger, R.H. and tiger, E., 1977. Subcommission on Geochronology: Convention on the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett., 36: 359-362. Suzuki, K., Adachi, M. and Kajizuka, I., 1994. Electron microprobe observations of Pb diffusion in metamorphosed detrital monazites. Earth Planet. Sci. Lett., 128: 391-405. Teufel, S., 1988. Vergleichende U-Pb- und Rb-Sr-Altersbestimmungen an Gesteinen des Ubergangsbereiches Saxothuringikum/Moldanubikum, NE-Bayem. Ph.D. Thesis. In: Gottinger Arbeiten zur Geologie und Palaontologie, 35: l-87. Teufel, S., Ahrendt, H. and Hansen, B.T., 1985. U-Pb und Rb-Sr-Altersbestimmungen im ijbergangsbereich Moldanubikum/Saxothuringikum, NE-Bayem. Forts&r. Mineral., 63(l): 234. York, D., 1969. Least squares fitting of a straight line with correlated errors. Earth Planet. Sci. Lett., 5: 320-324. Zhu, X.K., O’Nions, R.K. and Reed, S.J.B., 1994. REE distribution patterns of monazite: constraint on monazite origin in metamorphic rocks. 16th Gen. Meet. IMA (Int. Mineral. Assoc.), Pisa, Abstr. Vol., p.461.
ISOTOPE GEOSCIENCE
ELSEVIER
Chemical Geology
137 (1997) 273-281
Partial resetting of the U-Pb isotope system in monazite through hydrothermal experiments: An SEM and U-Pb isotope study S tephan Teufel ‘, Wilhelm Heinrich ~~eoForschungsZentrum
Potsdam (GFZ), Telegrafenberg
Received 5 July 1996; accepted
*
A 17, D-14473 Potsdam, Germany
11 November
1996
Abstract Natural monazites of different grain sizes were treated experimentally under hydrothermal conditions and their U-Pb isotope systematics measured before and after the runs. Runs were carried out with monazite grain size fractions ranging from 40 to 125 pm in diameter, and on < 1 to 15 pm fine powdered material at 400-75O”C, 3 kbar and 28 days. U-Pb analyses of the starting material resulted in concordant U-Pb ages of 377 + 2 Ma. U and Pb concentrations were w 5500 and _ 800 ppm, respectively. All hydrothermally treated monazite grain size fractions yielded concordant U-Pb ages of u 380 Ma and no loss of Pb and U. In contrast, monazite powder revealed temperature-dependent Pb loss. Pb concentrations decreased to N 590 ppm at 750°C N 620 ppm at 65O”C, and u 710 ppm at 4OO”C, indicating Pb loss of - 26%, - 22% and N 1 l%, respectively, of the initial Pb content. No fractionation of Pb isotopes occurred; all analyses yielded ‘07Pb/ ‘06Pb ages close to 380 Ma. U concentrations remained always unchanged. All recrystallized monazites from the powdered starting ma.terial define a discordia with intercept ages of 379.3 + 2.3 and -7 + 18 Ma, the latter indicating the hydrothermally induced Pb loss at the time of the experiment. SEM observations of monazite grains from hydrothermally treated grain size fractions exhibited only minor dissolution features confined to the grain surfaces. Run products from monazite powders showed recrystallization and grain growth induced by a dissolutllon-precipitation mechanism. Increasing temperature resulted in a distinct enlargement of grain sizes at 650” and 750°C. A dissolution-precipitation process in presence of a fluid phase rather than Pb volume diffusion acts as a very efficient mechanism for the resetting of the U-Pb isotope system of monazites, even at low temperatures. This process is most important in shear zones where channelized fluid flow may occur. Keywords:
Monazite;
U-Pb
isotopes; Metamorphic
fluid; Dissolution-precipitation;
1. Introduction In geochronology, phosphate monazite
the U- and Th-bearing REEis often applied to infer the
* Corresponding author. ’Deceased, November 1995. 0009-2541/97/$17.00 Copyright PII SOOO9-2541(96100161-1
Scanning electron microscopy
timing of magmatic and metamorphic events (for a compilation see Parrish, 1990). Metamorphic monazites from metasedimentary rocks often show concordant or nearly concordant U-Pb ages (e.g., Teufel, 1988; Smith and Barreiro, 1990; Getty and Gromet, 1992) but discordant U-Pb ages are also known from a variety of metamorphic complexes (Hansen et
0 1997 Elsevier Science B.V. All rights reserved.
214
S.Teufel, W. Heinrich/
Chemicul
al., 1978; Gebauer et al., 1981; Black et al., 1984; Teufel, 1988; Getty and Gromet, 1992). Recent studies on natural samples have shown that the closure temperature for the U-Pb system in monazite is high and that monazite may record prograde growth ages and can even preserve ages that correspond to peak temperatures up to granulite-facies conditions (Mezger, 1990; Smith and Barreiro, 1990; DeWolf et al., 1993; Kingsbury et al., 1993; Zhu et al., 1994, Heinrich et al., 1996). Effective blocking temperatures were suggested to be 720-750°C (Copeland et al., 1988), 725 + 25°C (Parrish, 1990), and > 650°C and < 740°C (Mezger, 1990). The blocking ages depend both on the cooling rate of the rock and on grain size. A detailed EMP study of Pb diffusion in metamorphic monazite (Suzuki et al., 1994) revealed rimwards Pb loss in lOO-200-pm-large monazites up to some tens of micrometers. From known peak temperatures and cooling rates of these rocks, along with measured diffusion profiles, Pb diffusion coefficients in the order of lO-*l cm2 SK’ at 620°C and 10-20 cm2 s-’ at 680°C were derived (Suzuki et al., 1994). These observations suggest that, for example, in large monazite grains at a peak temperature of 700°C and a cooling rate of lO”C/Ma Pb diffusion is slow enough to allow for determining the age of the peak metamorphic episode via the U-Pb isotope systematics of monazite grains, provided that no later disturbance event occurred. An experimental attempt to estimate diffusion coefficients of Pb in monazite resulted in values in the range of lo- lo cm* s- ’ at 600°C (Shestakov, 1972). These experiments were done with sintered tablets of powdered monazite using *“Pb as a tracer. It is obvious that substantial discrepancies between field evidence and laboratory studies exist and that further experiments are needed. If the Pb diffusion coefficients were in the range of lo-*’ cm* SK’ at N 7OO”C, simple volume diffusion cannot account for the disturbed monazite ages that were observed in some metamorphic rocks. In some cases it has been shown that disturbed monazite ages plot on linear discordia arrays (cf. above) and field observations revealed that tectonic processes may have generated the discrete later event recorded at the lower intercept with the concordia. Consequently, discordant monazite ages may be used for dating shear zones (e.g., Teufel, 1988; Getty and
Grolog?
137 (19971 273-281
Gromet, 1992). Several resetting mechanisms for the U-Pb system of monazites from shear zones have been proposed, including late monazite overgrowth, late Pb loss via diffusion due to frictional heating, climb-accommodated recrystallization and crystallization due to grain boundary migration (Getty and Gromet, 1992). Selective hydrothermal leaching of Pb along subgrain boundaries in monazite grains was also proposed to be responsible for resetting (Black et al., 1984). It is clear, however, that the U-PI, discordance mechanisms in monazite and its rheological properties are poorly understood. The aim of this study is to demonstrate experimentally the hydrothermal influence on the U-Pb isotope system of monazite. For that, two different starting materials are used: (1) coarse-grained natural monazites; and (2) fine-grained monazite powder, both in presence of a fluid phase. In the first case, one would expect no significant dissolution-precipitation process, and the U-Pb systematics of these run products may reveal information on temperature-dependent volume diffusion under static conditions. In the second case, dissolution-precipitation-induced grain growth via fluid phase is most probable and we propose to show that experimentally induced recent recrystallization of monazite of early Herynian age points towards modem U-Pb ages of monazite.
2. Sample selection, experimental procedure and analytical methods Monazite from a lower amphibolite-facies metapelite from NE Bavaria, Germany, was used for this study. The sample comes from the nappe complex of the vicinity of the Continental Deep Drilling location (Windischeschenbach). Rocks from this area underwent medium-pressure metamorphism N 380390 Ma ago, inferred from U-Pb age determinations of zircon suites and monazites (Teufel et al., 1985; Teufel, 1988). The timing of this metamorphic event was confirmed by 40Ar/ 39Ar age determinations of amphiboles and biotites (SchliBler et al., 1986; Kreuzer et al., 1989). The sample (Tb 104) is an andalusite-staurolite-bearing mica-schist that equilibrated at peak metamorphic conditions of N 500°C and N 3-4 kbar. The mineralogy of this sample and
S. Teufel, W. Heinrich/ Chemical Geology 137 (1997) 273-281
the chemical composition of different monazite grains were reported elsewhere (Heinrich et al., 1996). All monazite crystals from sample Tb 104 yield concordant U-Pb ages and show a highly radiogenic Pb isotopic signature (Teufel, 1988); this is a precondition for the feasibility of this study as it enabled us to detect very easily any possible Pb blank contamination that may have occurred during the experimental and analytical procedures. The separated monazite grains were mainly of anhedral roundish shape. There is no evidence of sedimentary transport such as pitted surfaces (Fig. la>. Only _ 5% of the monazite grains show crystal planes. The colour of all monazites is bright yellowish green. Small inclusions of biotite and rutile are occasionally present.
275
The sample material was split into four grain size fractions of 40-63-, 63-80-, 80-lOO- and 100-125pm grain diameter. These were tediously handpicked prior to experimental work. An amount of w 40 mg of monazite sized 40-63 Frn has been ground in an agate mortar to a fraction of variable grain size ranging from N 15 pm down to < 1 pm. This fine ground powder as well as the granulometric fractions were used as starting material for the hydrothermal experiments. About 10 mg of monazite were placed in a gold capsule along with N 20 mg of ultrapure H,O. The capsules, 25 mm in length and 3 mm in diameter with walls of 0.2 mm thickness, were sealed by welding. All experiments were carried out in stan-
Fig. 1. Scanning electron microphotographs illustrating textural characteristics observed in coarse-grained monazite starting material and in the run products: a. Starting material: Coarse-grained monazites, fraction of 40-63 km in diameter. b. Run No. 8 (75O”C, 3 kbar, 28 days): Monazite with dissolution embayments that are confined to the orginally rounded grain surfaces. There is no change in shape and size of the crystal. The surface is covered by some quench products. c. Run No. 8 (75O”C, 3 kbar, 28 days): Monazite with dissolution features is partially overgrown by newly formed idiomorphic spinel, apatite and sphene cryslals. d. Run No. 6 (750°C. 3 kbar, 28 days): Cavity in monazite is partially overgrown by sphene (sph) and spine1 (spl, and small octahedra in the upper part of the cavity).
276
S. Teufel, W. Heinrich / Chemical Geology 137 (1997) 273-281
dard cold seal hydrothermal pressure vessels at 3 kbar and temperatures between 400” and 750°C. The duration of all runs was 28 days. Maximum error in the temperature measurements was +5”C. At the end of the runs the bombs were quenched to room temperature within a few minutes. The gold capsules were reheated to 110°C and then opened. The run products were washed out with ultrapure H,O. SEM investigations of gold spattered reactants and pieces of products were made using a Hitachi S 4000 high-resolution electron microscope equipped with a field emission electron source. Acceleration voltage was between 2 and 20 kV. Secondary phases in run products such as sphene, spine1 and apatite were identified using a Kevax Delta energy-dispersive analytical system. U-W isotope analyses were performed on all monazite reactants and products using the technique of Krogh (1973). Prior to dissolution, no acid washing procedure was applied neither to the reactants nor to the products. Monazite samples were dissolved in Teflon bombs with 6 N HCl (S&tier and Allbgre, 1983). After splitting, a mixed 235U-208Pb spike was added to one part of the sample solution. First, U and Pb were separated on columns filled with 0.5 ml of anion-exchange resin AG 1X8 (lOO200 mesh). Then Pb was purified using 0.8 N HBr. U and Pb isotope ratios were measured statically on a Finnigan MAT 261 multicollector mass spectrometer. The correction factor for mass discrimination of Pb was determined by repeated analyses of NBS 981 standard resulting in a value of 1.3%0/amu. Pb isotope ratios were corrected for initial common Pb applying the model proposed by Stacey and Kramers (1975). The amount of Pb blank normally introduced to the analyses during chemical procedure has been determined as 300 pg. Isotopic composition of Pb used for blank correction was ‘08Pb/ ‘04Pb = 38.63, 207Pb/ 204Pb = 15.63 and 206Pb/ 204Pb = 18.70. Ages were calculated with the uranium decay constants as recommended by the International Commission of Geological Sciences (Steiger and Jager, 1977). Regression analysis of the data points has been carried out according to York (1969), using the computer program “GEODATE” provided by B.M. Eglington and R.E. Harmer (199 1). 206Pb/ 238U and 207Pb/ 235U ratios have 2~ uncertainties of 0.5% and an error correlation of 0.95-0.98.
3. Results Two series of experiments were performed, the first with four different grain size fractions of monazite, at constant temperatures of 750°C and the second with monazite powder at 400”, 650” and 750°C. Pressure was 3 kbar and duration was 28 days in all experiments. The experimental conditions are summarized in Table 1. Isotopic analyses and SEM studies were carried out on the run products as well as on the untreated samples. Results of the isotope analyses are given in Table 2. The experiments with different grain size fractions as starting material (run Nos. 2, 4, 6, and 8; Tables 1 and 2) revealed that hydrothermal treatment caused a complete decolorization of the monazites. SEM observations showed that hydrothermal treatment caused no growth of monazite crystals (Fig. 1). The run products still consisted of the same anhedral roundish grains as the untreated samples (Fig. la). However, the run products revealed two characteristics: (a) Formation of channels and small embayments at the grain surfaces interpreted as the result of dissolution (Fig. lb and c). These dissolution features do not penetrate deeply into the monazite crystals but are restricted to the uttermost surface of the grains. (b) In some rare cases growth of newly formed crystals of other solid phases occurring at the surface and in cavities of the primary monazites (Fig. 1~). These were identified by EDS analysis as apatite, Mg,Al-spine1 and sphene (Fig. Id). Generally, they are very small (< 1 pm), but a few of these crystals may reach up to 5 pm. The formation of these
Table 1 Experimental
conditions
of hydrothermal
runs
Run
Fraction
Monazite (mg)
H,O (mg)
Temp.
2 4 6 8 11/12 13/14 15/16
40-63 p,rn 63-80 km SO-100 km 100-125 p,m powder powder powder
8.74 9.45 8.34 9.24 11.92 9.81 10.24
18.66 20.02 16.55 18.07 19.13 20.12 19.45
750 750 750 750 400 650 750
All experiments
were performed
at 3 kbar for 28 days.
(“Cl
(ppd
796.5 788.2 749.2 753.0 817.9 794.0 838.8 806.5 774.5
Pb,
1420.8 1399.0 1335.6 1334.4 1437.6 1376.9 1469.3 1399.1 1380.1
zo6Pb,hmd/g) U
‘06 Pb
-238
‘O’Pb
-23$
4.0 3.6 1.5 5.1 0.5 4.6 3.0
3.4 3.0 4.5 2.3 4.7 2.5 3.6 3.0 4.5
1.6587 1.6606 1.6599 1.6591 1.6567 1.6615 1.6574
1.6521 1.6651 1.6541 1.6689 1.7544 1.7285 1.7017 1.I275 I .6554
0.05729 0.05637 0.05663 0.05505 0.05578 0.05494 0.05498
0.05615 0.05466 0.05667 0.05480 0.18580 0.05482 0.05589 0.05484 0.05726
4680 6990 6110 16770 9830 21100 19400
7710 24300 6100 28100 111.1 27100 7750 20500 4830
5742 5407 5332 5496 5452 5610 5504
5516
815.0 711.4 710.6 632.5 614.6 594.2 586.4 810.3 708.7 107.5 631.5 612.9 593.5 585.6
1442.3 1260.3 1258.5 1123.4 1091.4 1054.7 1042.4
0.06022 0.05588 0.05659 0.04900 0.04800 0.04508 0.0454 I
0.06048 0.05976 0.0599 1 0.05989 0.06010 0.05994 0.06027 0.06035 0.05999
0.4497 0.4182 0.4232 0.3661 0.3593 0.3372 0.3395
0.4524 0.4454 0.4484 0.4482 0.4496 0.4486 0.4488 0.4504 0.4486
Analyses labelled H are from hydrothermally treated monazite. Nos. 9 and 10 are the starting materials before and after grinding. 0. I3%/amu. Uncertainties (2 (r ) are less than 0.1% unless in ratios involving ‘04Pb. Apparent ages are calculated using the constants 1977).
powder 400°C H 400°C H 650°C H 650°C H 750°C H 750°C H
Monazire powder:
10 11 12 13 14 15 16
799.3 789.1 752.5 753.7 1056.3 794.8 841.7 807.5 778.8
Pb (ppm) ‘06
Pb
“‘Pb
U
‘06Pb
-238
379.0 374.0 376.1 376.0 377.0 376.3 376.4 377.6 376.3
lo
Y
377.0 350.5 354.8 308.4 302.2 284.2 286.3
378.5 374.1 375.1 314.9 376.2 375.3 377.2 317.7 375.6
317.1 354.8 358.4 316.8 311.7 295.0 296.8
377.9
378.5 383.4 381.5 380.4
E
S
5 Z S 2
371.4 376.5 380.7
382.6 381.1
9 E. B ;; 8 s B
382.6
Measured isotopic ratios are corrected for recommended by JUGS (Steiger and JZger,
0.05417 0.05428 0.05424 0.05418 0.05430 0.05425 0.05423
0.05426 0.05406 0.05428 0.05428 0.05425 0.05428 0.05401 0.05413 0.05423
2. m. S\
5633 5613 5345 5342 5735 5507 5845
u (ppm)
Apparent ages(Ma)
382.6 382.7 381.5
*04Pb
‘06Pb
-
Atomic ratios
?? a
“‘Pb
“‘Pb
-
Concentrations
381.8 373.4
‘06Pb
“s Pb
Isotopic ratios
40-63 40-63 H 63-80 63-80 H 80-100 80-100 H loo-125 100-125 H 40-63
hg)
Weight
and products
B “Z
(km)
Sample
reactants
Grain size fracrions:
No.
Table 2 U-Pb isotope data of mon&ite
278
S. Teufel, W. Heinrich/Chemical
additional phases was due to hydrothermal reaction of former solid inclusions as biotite and rutile present in the original monazite. For U-Pb isotope analyses, splits of untreated grain size fractions have been analysed for direct comparison (Table 2). Though optical and SEM investigations did show some dissolution features, the hydrothermally treated and non-treated monazite grains display no significant difference in their U-Pb isotope systematics. The U and Pb concentrations and also the Pb isotopic compositions of both reactant and product coincide within error limits and all analyses of these grain size fractions yield concordant U-Pb ages in the range of 374-378 Ma. Aside from this striking result, there is a gradual change in common Pb content of the products. The measured 206Pb/ ‘04Pb ratios of the products increased by a factor of 3 compared to the reactants (Table 2). The untreated monazite fraction of 80-100 pm (No. 5 in
Geology 137 (1997) 273-281
Table 2) suffered considerable contamination by common Pb during the analytical procedure. Applying a corresponding higher correction of common Pb this sample also yielded a concordant U-PI, age of 376 Ma. The ground monazite starting material exhibits angular fragments with a grain size ranging from _ 15 pm to < 1 pm (Fig. 2a). Most fragments are below 1 pm. All of them show characteristic monazite {loo} and {OOl} cleavage planes. During hydrothermal treatment at 400°C all fragments lost their sharp edges and show roundish grain shapes (Fig. 2b). This is attributed to the beginning dissolution of monazite that starts at locations of high surface energy. The surfaces of larger grains often show small indentations and cavities. Monazites treated at 650” and 750°C clearly reveal advanced grain growth (Fig. 2c and d). However, in both runs relics of old crystal fragments remained. These old
Fig. 2. Scanning electron microphotographs illustrating textural characteristics observed in fine-grained monazite starting material and in the run products: a. Starting material: fine powdered monazite exhibiting irregular crystal fragments. b. Run No. 1l/12 (400°C 3 kbar, 28 days): Incipient monazite dissolution and growth of new crystals. Sharp grain edges disappeared. c. Run No. 13/14 (65O”C, 3 kbar, 28 days): Significant new grain growth of monazite. Many crystals exhibit regular crystal planes. d. Run No. 14/15 (750°C 3 kbar, 28 days): Typical monazite crystal of * 5 p,rn in size with regular crystal planes, indicating advanced grain growth.
219
S. Teufel, W. Heinrich / Chemical Geology 137 (1997) 273-281
400 Ma / Powdered =)
0.06
-
0.05
-
starting
material
8 . % $ (Y
0.3
0.4
0.6
0.5 207
Pb I 235
0.7
U
Fig. 3. 206Pb, 238~ v!i, 207pb, 235 U concordia diagram of powdered monazites from hydrothermal experiments along with starting materials. Values arc from Table 2. They define a discordia with intercept ages of 379.3 + 2.3 and - 7 f 18 Ma (MSWD = 9.9). All concordant points show U-Pb monazite data of hydrothermally treated grain size fractions.
grains were also affected by the dissolution-precipitation process. Many of the newly formed crystals developed crystal planes. All sharp-edged fragments disappeared, and the sample is dominated by roundish subhedral grains. U-PI, isotope analyses of the starting material and the products are given in Table 2. The comparison of values from the original fraction of 40-63-pm grain size (No. 9 in Table 2) and the powder from the same fraction ground in an agate mortar (No. 10) shows that both analyses give the same concordant U-Pb ages of 376 and 377 Ma, respectively. Both analyses show *06Pb/ ‘04Pb ratios of N 4.700, indicating that no common Pb was introduced during grinding. All three run products with powdered material gave U concentrations of N 5500 ppm (Nos. 11 to 16; Table 2) which is within the range of the starting material. In contrast, dramatic changes in the Pb concentration of the products occurred. Values of Pb concentration are 711 ppm at 4OO”C, 632 ppm at 650°C and 594 ppm at 750°C (Table 2). This implies that the sample at 400°C already lost N 11% of its original Pb content. Pb loss increased rapidly with
temperature to N 22% at 650°C and N 26% at 750°C. These changes in Pb concentration are reflected in the calculated U-Pb ages (Table 2). The intensity of Pb loss and the degree of discordancy of the U-W data points are well correlated (Table 2). Age calculations made for the different temperature steps result in younger U-Pb ages with increasing temperature (Fig. 3). Nevertheless, all analyses still show the original Pb isotopic signature as indicated Ma. Duplicate by 207Pb/ *06Pb ages of 378-383 analyses of these run products resulted in some small differences in the age determinations (Table 2). All data of hydrothermally treated monazites define a discordia with intercept ages of 379.3 f 2.3 and -7 & 18 Ma (Fig. 3). These ages agree with the primary metamorphic age of the monazites and the recent time of experimentally induced Pb loss.
4. Conclusions All our experiments are characterized fluid/solid ratio and short run durations.
by a high We found
280
s. Teufel, W. Heinrich / Chemical Geology 137 (1997) 273-281
no significant effect on the U-Pb system in the runs with different grain size fractions of monazite as reactants, even with small grain fractions of 40-63 km and high temperatures of 750°C. Thus, no Pb loss via volume diffusion occurred. Taking Shestakov’s (1972) diffusion coefficients of Pb in monazite of lo-i” cm2 SK’ at 600°C complete Pb loss should have occurred within a few days even at much lower temperatures and the U-Pb isotope system should have been reset to a zero age. Consequently, his diffusion coefficients cannot be applied to natural monazite. A possible explanation is the difference in the experimental setup. Shestakov (1972) observed the migration of radioactive *l’Pb added as a marker to a tablet of powdered and sintered monazite. Presumably, this *“Pb was not incorporated into the crystal lattice but might have migrated on the surface of monazite microcrystals within the sintered tablet and grain boundary migration was observed instead of true volume diffusion. According to our experiments, the diffusion coefficient even at 750°C must be lower than Shestakov’s values by several orders of magnitude. Our short run durations do not allow for calculation of the diffusion coefficients. However, there is no contradiction between our experiments and the values of Suzuki et al. (1994) from natural monazites where numbers in the range of lo-*i cm* s-l at 620°C 10e2’ cm* s-l at 68O”C, and extrapolated to 750°C of 10-‘910-l 8 cm* s- ’ were derived (see also Parrish and Tirrul, 1989). Also, no Pb loss via hydrothermal leaching occurred in our experiments with monazite grain fractions. Dissolution features as shown in Fig. 2c are limited to the utmost surface, and the formation of cavities (Fig. 2d) are confined to that sites where former solid inclusions reacted to new solid products via fluid phase. Both effects were obviously not large enough to disturb the U-Pb system of the monazite grains significantly. The experiments with ground monazite powders revealed strong recrystallization and growth features along with extensive temperature-dependent Pb loss. SEM images showed a dissolution-precipitation mechanism via fluid that acted to various degrees, becoming more intense with increasing temperature. Therefore, volume diffusion is also unlikely to be responsible for the Pb isotope exchange in these
runs. During the dissolution-precipitation process, Pb is not incorporated into the newly grown monazite and remains in the fluid phase. In contrast, all of the U is reincorporated into the solid and no significant amounts of U are partitioned into the fluid. This implies that the extent of Pb loss is directly correlated to the progress of the temperature-dependent dissolution-precipitation reaction. The reaction mechanism in our experiments and its effect on the U-Pb isotope system that resulted in strongly discordant U-Pb data is clearly proven. Whereas the interpretation of intercept ages of discordant U-PI, data of natural monazites often remains vague, there is no doubt about the meaning of both intercept ages of our discordia. Regional metathe starting morphism occurred at N 380 Ma point and upper-intercept age of our monazite discordia. The lower-intercept age represents the laboratory-induced metamorphism due to dissolution and precipitation at present time. This interpretation implies that the Pb distribution within relict old and newly grown grains is inhomogeneous. It is clear from SEM observations that very small monazite grains dissolved and recrystallization occurred around old, somewhat larger nuclei. If we reject volume diffusion as an effective mechanism the resulting U-Pb monazite system consists of undisturbed cores along with rejuvenated growth rims. This experimentally produced feature is probably present in many discordant monazites from metamorphic rocks (e.g., Parrish, 1990; Getty and Gromet, 1992). These results fit well into the scenario of ductile shear zones where extensive mass transfer by dissolution and reprecipitation occurs (see Getty and Gromet, 1992). This mechanism may easily produce disturbance and partial resetting of the U-W isotope system in monazites even at low temperatures of 400°C. If degree and duration of deformation in shear zones, along with enhanced fluid flow, are strong enough, the U-Pb ages of monazite will be completely reset (e.g., Teufel, 1988).
Acknowledgements Some of the analytical work was done at the Institute of Mineralogy, Geochemical Department,
S. Teufel, W. Heinrich/Chemical
Free University of Berlin. We thank H. Friedrichsen for providing analytical facilities and M. Feth for help with the analyses. U. Gemert, ZELMI, Technical University of Berlin, provided SEM facilities. G. Franz critically read an earlier version of the manuscript. The final draft profited by careful reviews of Kazuhiro Suzuki and Rolf Romer.
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