A. KRISHNA SINHA, DAVID M. WAYNE,* and DAVID A. HEWITT. Department of Geological Sciences, Virginia Polytechnic Institute and State University, ...
0016-7037/92/$5.00 + .oO
Geochimica PI Cosmochimica Ada Vol. 56, pp. 3551-3560 Copyright Q 1992 Pergamon Press Ltd.Printed inU.S.A.
The hydrothermal stability of zircon: Preliminary experimental and isotopic studies A. KRISHNA SINHA, DAVID M. WAYNE,* and DAVID A. HEWITT Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 2406 1, USA (Received October 8, 199I ; acceptedin revisedform June 16, 1992)
Abstract-Experimental investigations of the stability of the U-Pb isotopic system in nonmetamict zircons show that appreciable losses of Pb and U can be induced at amphibolite-grade conditions (400°C to 6OO”C, 4 to 6 kb) in 2 M NaCl and 2% HNO3 solutions. The severity of U loss and, to a lesser extent Pb loss, varies with solution composition: in this case the 2 M NaCl solution induced more Pb and U loss than the 2% HNOs solution at the same P-T conditions. Scanning electron microscopy of the run products also revealed a range of corrosion-related surface features, which suggests that some of the observed trends in Pb and U loss must be attributed to zircon dissolution. Backscattered electron (BSE) imaging of the run products further suggests that partial homogenization of chemical zoning patterns occurred during the experiments. Microprobe analyses of treated and untreated grains show that both populations have a similar range of Hf contents. Thus, the apparent loss of sharp, well-defined zoning features is most likely due to small-scale “smearing out” of formerly sharp chemical gradients and is perhaps related to the annealing of lattice defects caused by alpha-recoil damage. Thus, experimentally induced U-Pb isotovic discordance in zircon is a complex function of zircon stability and annealing effects. INTRODUCTION
con-hosted U, Th, and Pb may undergo a considerable degree of redistribution at subsolidus conditions. In some instances, the zircon U-Th-Pb system remains undisturbed regardless of P-T conditions or kinematic regime ( PEUCATet al., 1985; TILTON et al., 1958). It is likely that variations in the degree of metamorphically induced U-Th-Pb discordance may be attributed to a number of factors: ( 1) growth of rims on preexisting zircon crystals implying either residual enrichment, or solution and transport of Zr during metamorphic or hydrothermal events (GASTIL et al., 1967; DAVIS et al., 1968; HART et al., 1968; GULSON and KROGH, 1975; SINHA and GLOVER, 1978; PEUCAT et al., 1985; GEBAUER et al., 1981; VAN BREEMAN et al., 1987; WAYNE et al., 1992); (2) radiation damage, hydroxylization, and subsequent structural annealing incurred prior to or during metamorphism (GASTIL et al., 1967; GEBAUER and GR~NENFELDER, 1976; CORFU et al., 1985; SILVER and DEUTSCH, 1963); (3) discontinuities in the bulk composition of the enclosing rocks or minerals ( GEBAUER and GR~NENFELDER, 1979; ALEINIKOFF, 1983; WAYNE et al., 1992); (4) high fluid pressure and activity during metamorphism ( GEBAUER and GR~NENFELDER, 1976; SINHA and GLOVER, 1978; SCH~~RER, 1980); (5) direct U adsorption without new crystal growth ( GRAUERT et al., 1974); (6) decrease in structural integrity of zircon crystal structure through radiation damage (HOLLAND and GOTTFRIED, 1955; GZKAN, 1976; YADAet al., 198 1) in association with volume changes may cause brittle failure (CHAKOUMAKOSet al., 1987) and enhanced leachability of U-Pb by fluids (EWING et al., 1982; KROGH and DAVIS, 1975). From these studies, it is apparent that the significance of a particular discordance-generating circumstance or mechanism may vary, and that no single factor can be correlated to consistent trends of isotopic discordance. However, the importance of fluid/rock ratios during metamorphism and the ability of hydrothermal and metamorphic fluids to dissolve and transport Zr, Hf, U, Th, Pb, REEs, and other elements is undeniable (GEBAUER and GR~NENFELDER, 1976; CUNEY, 1978;
THE CHEMICAL AND
ISOTOPICresponse of zircons to changes in P and T may be complicated by primary intracrystalline chemical, structural, and isotopic inhomogeneities (e.g., STEIGER and WASSERBURG, 1966; SOMMERAUER, 1974, 1976; and others), STEIGER and WASSERBURG( 1966) first suggested that the anisotropic response of zircon crystals to thermal events was due to the presence of discrete isotopic and physiochemical microdomains within each grain. SOMMERAUER ( 1974, 1976) demonstrated that zircons rich in trace elements (>2 mol% total) consist oftwo phases: a thermally stable structure with low trace element concentration and an amorphous, microheterogeneous mixture of SiO* and ZrOz which is enriched in trace components; an observation now more fully documented by the transmission electron microscopy studies of MCLAREN et al. ( 1991). Low-temperature (ca. 350°C) annealing and loss or gain of radiogenic Pb, U, Th, and other trace elements is likely to occur more readily in the latter phase (SOMMERAUER, 1976). IR and NIR spectroscopy of metamict and nonmetamict zircons ( AINES and ROSSMAN,1986) indicates that molecular water may enter radiationdamaged zircon crystals and dissociate to form (OH)- groups. The hydroxyl groups may actually stabilize the metamict state by satisfying local charge imbalances. Heating of these samples showed that (OH)- and (H)+ reconstitutes to Hz0 and escapes prior to recrystallization. Conceivably, this “water of metamictization” could carry radiogenic daughter products (e.g., Pb* ) and trace elements out of the zircon as it escapes (GOLDRICH and MUDREY, 1972). A number of geochronologic studies of metamorphic rocks (GEBAUER and GR~NENFELDER, 1976, 1979; SINHA and GLOVER, 1978; WILLIAMSet al., 1984) have shown that zir* Presentaddress:Department of Earth Sciences, The University, Leeds LS2 9JT England. 3551
3552
A. K. Sinha, D. M. Wayne, and D. A. Hewitt
SINHAand GLOVER, 1978; PAGEL, 1982; WAYNEand SINHA, 1988; ZEITLER et al., 1990; WAYNE et al., 1992). During a metamorphic episode involving fluids, the radiation-damaged portion of a zircon may be subject to a number of processes leading to isotopic discordancy. These processes include ( 1) preferential leaching of metamict zones due to the penetration of fluids along microfractures (cf. WAYNEand SINHA, 1988), (2) annealing of metamict zones, or entire crystals during metamorphism (GULSON and KROGH, 1975 ) , and ( 3 ) volume or grain boundary diffusion of Pb (or U) out of the zircon ( TILTON, 1960; WASSERBURG, 1963). The factors controlling the rate and extent of these processes must include ( 1) the physical and chemical nature of the zircons involved, (2) the intensity and duration of the P-T conditions of the metamorphic/hydrothermal episode, and (3) the composition and amount of fluid present. Interpretations of variations in metamorphically (or hydrothermally) induced isotopic discordance require thorough understanding of zircon-fluid interactions. Previous experimental studies of U-Pb isotopic discordance of zircon in hydrothermal systems (PIDGEON et al., 1966, 1974; HANSEN and FRIDERICHSEN,1989) have shown that significant degrees of U and Pb loss can occur over extremely short time periods (~24 h). The aim of this study is to further characterize UPb isotopic discordance in experimentally treated zircons as a function of solution composition, P-T conditions, and time. The data presented here represent results from a preliminary set of experiments which constitute a portion of an ongoing, comprehensive experimental study. The experiments of PIDGEON et al. ( 1966, 1974) have shown that, while Pb is easily leachable from zircon over a wide range of Tin dilute HCl and NaCl solutions, the behavior of U is less consistent. PIDGEON et al. ( 1966 ) recorded only minor U loss ( ~6% of the U concentration of the starting material) from strongly metamict Sri Lankan zircon in 2 molal NaCl at 500°C 1 kb over 3 12 h. Later experiments ( PIDGEONet al., 1974) showed that considerable U loss occurred at higher T (850°C) in dilute HCl. More recent experiments by HANSEN and FRIDERICHSEN( 1989) revealed that comparable amounts of U and Pb ( 17 and 15%, respectively) were leached from 426 Ma.-old, low U (ca. 300 ppm), nonmetamict zircons at 180°C and ambient P over a 600 h time interval. In the latter case,
the experimental fluid was Ca-rich (ca. 160 ppm) spring water. For this study, experiments were performed at relatively high P( 4 to 6 kb) and T( 300 to 6OO”C), with two different fluids: 2 M NaCl and 2% HNOs. TECHNIQUES AND PROCEDURES Microprobe, SEM
Chemical analyses of zircons were performed on an automated Cameca SX-50 4-channel electron microprobe operated in WDS mode, at an accelerating potential of 15 kv, with a sample current of 50 na, and a beam diameter of 2 pm (point mode). Analytical standards, peak and background counting times, and the methods used in monitoring of instrumental drift and analytical reproducibility of the analyses are summarized in WAYNEet al., 1987. A scanning electron microscope (SEM) was used to obtain secondary electron images for information on surface features, and backscattered electron (BSE) images for information on variations in mean atomic number (Z-contrast) within zircon grains. Details on the application of these techniques are presented in LLOYD( 1987) and PATERSON et al. ( 1989) . All secondary electron and BSE images were collected, at 20 kv accelerating potential, on a Camscan Series 2 Scanning Electron Microscope. Hydrothermal Experimental Procedures The starting materials for the hydrothermal experiments were ( 1) zircons (labeled NB-3) from the eastern biotite granodiorite of Horton Island, Nekweaga Bay, Saskatchewan, Canada ( SINHA, 1969), and (2) zircons (LS42-B) from the post-Dickinson Group gneiss at Black Rock Quarry, Michigan (ALDRICHet al., 1965). Unlike the zircons used in the experimental study of PIDGEONet al. ( 1966, 1974), NB3 contains relatively little U and Pb (Tablet 1) and is nonmetamict. The least magnetic fraction of the NB-3 zircons were split into two naturally occurring size fractions (>75 pm and ~75 pm) and used in separate experiments involving different fluid compositions. The zircons of both size fraction are transparent, euhedral, yellow-brown in color, highly bireftingent, and contain numerous apatite inclusions (Fig. 1). The apatite inclusions (identified by spot WDS analyses using the electron microprobe) may be the cause of the low measured zo6Pb/204Pbratios of both size fractions (Table 1) Isotopic ( U-Pb ) analyses of the same separates yield a concordia upper intercept age of 1790 Ma., with ~5% discordance. The second sample, LS42-B, was chosen on the basis of its relatively high a-dosage (D* ) of approximately 1.5 X 10 ” a particles/ mg (estimated using the equation of HOLLANDand GOTTFRIED,1955 ), and high U and Pb content. Though not completely metamict (firstorder birefringence in cross-polarized light), this sample is much closer in its U-Pb contents and physical properties to the mostly
FIG.1.Transmitted light photomicrograph of NB-3 (75- 150 Frn fraction), showing numerous inclusions (mostly apatite). Scale bar in microns.
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Losses of U and Pb from zircons under hydrothermal conditions TABLE 1. P-T-t-&,, conditions, U and Pb concentrations, and Pb isotopic data for starting materials (NB-3, 75150 pm, NB-3, 45-75 grn, LS42-B) and run products of the hydrothermal experiments. Lead isotopic data were corrected for common Pb values at 1790 Ma (NB-3) and 2720 Ma (LS42-B) using the STACEYand FRAMERS( 1975) model. U-Pb and Pb-Pb ratios have been corrected for blank and mass fractionation (see WAYNE et al., 1992;WAYNE, 1990). LS42-B is significantly more metamict than NB-3, and also contains 1013 ppm Th (ALDRICHet al., 1965 ) . SAMPLE TIME
(nr)
NR-3
(75
T
P
Pb
(“C) (kb)
- 15@Im)
U
@Pm)
@Pm)
19.4
215.1
ZR-1
720
600
6
50.9
114.1
ZR-1A
720
600
6
38.9
136.6
ZR-2
200
600
4
53.6
ZR-3
720
600
4
43.1
LS42-R
+
ZR-8
686.0 200
NB-3
600
4
(45-75 pm)
-
206i’b 204Pb
357.6
-
207P b
2ggP b _
206pb
206pb
0.14741
4.108
-
207Ph’
206Ph* 207P,,*
206Pb'
238~
0.10943
0.3469
5.362
0.2213
3.353
51.0
36.5
128.3
0.3249
5.022
32.5
40.4
150.0
0.2288
3.432
45.1
30.3
0.3030
6.710 87.0
52.0
366.3
1988.0 1538.0
0.14694
0.16930
3.584
7.315
0.10988
0.16130
35.9
46.9
90.0
977.0
83.9
218.5
293.2
0.15530
3.561
0.10893
0.2819
4.429
319.3
0.15224
3.622
0.10972
0.2513
3.895
13.9
0.5
0.2271
3.407
24.4
4.3
30.2
5.7
32.1
11.7
24
300
6
72.3
217.4
ZR-5
720
300
6
63.4
209.1
ZR-6
24
600
6
58.6
206.1
265.9
0.16066
2.903
0.10959
0.2009
ZR-7
720
600
6
56.9
192.9
254.5
0.16299
2.904
0.10963
0.2076
*.
radiogenic
Pb
+-
Data
Aldrich et al.
3.118 3.135
(196!i)
metamict Sri Lankan zircon used by PEGEON et al. ( 1966, 1974) than NB3. Isotopic data from a previous geochronologic study ( ALDRICHet al., 1965) yielded an upper intercept age of 2700 Ma for this sample. The 2 M (molar) NaCl and 2% HNOs solutions were prepared from Ultrex brand NaCl and Teflon-distilled (TD) Hz0 and HNOs . Platinum capsules, 3 mm in diameter, were vacuum-heated (3 to 12 h) prior to loading to remove Pb impurities by volatilization. A typical run contained 20 to 30 mg of zircon and 60 mL of solution, with the exception of ZR-IA and ZR-3 which only contained approximately 2 mg of zircon. Differences between pre- and post-welding weights of the charges did not exceed 0.1 mg. The Pt capsule was then placed in a 4.5 mm diameter Au capsule, with 25 mg of distilled H20, and sealed. The experiments were performed in standard 1 G-inch cold-seal bombs, and Hz0 was used as the pressure medium. Pressure was measured using Heise bourdon tube gauges, accurate to within approximately t50 bars. Temperature was monitored with chromelalumel thermocouples, accurate to +5”C, which were calibrated against the melting points of NaCI, KCl, and CsCI. The bombs were air-quenched to ambient temperature within approximately I h of removal from the furnace. The Pt capsules were removed, cleaned in dilute TD HN09, and opened in TD H20. Upon removal from the Pt capsules, NB-3 zircons were transparent and colorless and LS42-B zircons were white and opaque. Mass
235~
0.2872 4.440
ZR-4
from
% Pb loss % U loss
Spectrometry
The experimental products were washed in warm, Teflon-distilled (TD) 8 N HNO, for 30 min, rinsed in TD H20, and TD acetone
processed using the method OfKRCGH( 1973). Total Pb and U blanks were approximately 350 and 15 pg, respectively. The measurement of U and Pb isotopic ratios was performed on a modified, automated AVCO 35 cm radius 90” sector solid source thermal ionization mass spectrometer, using single Re filaments and standard P205-Taz05 loading techniques for U. Corrections for blanks, spike, and instrumental mass fractionation are identical to those stated in WAYNEet al. ( 1992) and WAYNE ( 1990). All decay constants used in the data reduction conform to the values recommended by ._ cal_STEIGERand JAEGER( 1977). and initial Pb compositions were culated using the model of STACEYand KRAMER~( 1975) All data was processed using the PBDAT software packages (LUDWIG,1990). RESULTS AND DISCUSSION A total of nine experiments on separate batches of zircons were run, encompassing a range of P-T conditions and solution compositions (Table 1) . Four experiments (ZR- 1, ZRIA, ZR-2, and ZR-3) using NB-3 (75-l 50 pm fraction) zircons as the starting material were run in the 2 M NaCl solution at 600°C over a range of P (4 and 6 kb) and time intervals (200 to 720 h). Two of these experiments (ZR-1 A and ZR-3 ) repeat the P-T-t conditions of earlier runs ( ZR- 1 and ZR-2, respectively), but contained considerably less ( ca. 2 mg) of the starting material. Four additional experiments (ZR-4, ZR-5, ZR-6, and ZR-7) with NB-3 zircons (45-75 pm fraction) were run in the 2% HNOs solution at constant
A. K. Sinha, D. M. Wayne, and D. A. Hewitt
3554
A 21ytNaCl solution 0 2% HN03 solution 2
I
, ‘AZR-6 3
I
4 207Pb / 235~
I
I
5
I
6
FIG. 2. Concordia diagram showing the effects of 2 M NaCI (ZR-I ,2, i A, 3) and 2% HNO, (ZR-4,5,6,7) on the U-Pb isotopic systematics of nonmetamict zircons. Hydrothermally untreated zircons (NB-3) are plotted as solid symbols. In all cases, except for ZR-1 and ZR-2, zircons preferentially lost Pb and were displaced towards higher values of discordancv. ZR-1 and ZR-2 lost U in excess of Pb and have become reversely discordant. The upper and lower intercept ages are listed as reference points only.
P (6 kb), but over a range in T (300-600°C) and t (24 and 720 h). Finally, a single experiment using LS42-B zircons was run in 2 M NaCI, at 6OO”C, 4 kb for 200 h. Lead isotopic data, and U and Pb concentrations for the starting materials and the run products are listed in Table 1, and are plotted on a concordia diagram (Fig. 2). The common lead corrected 207Pb/206Pb ratios of the run products and the starting materials are similar, and show no systematic relationship to time, P-T conditions or solution composition.
’ (ZR-8:Pb
67%)
_
TIME (HOURS) FIG. 3. Percent Pb and U lost from hydro~e~ally treated zircons, plotted as a function of time. Data points from runs using 2 M NaCl are denoted by circles, and runs using HNO:, are denoted by triangles. Experiments run under similar P-T-Xauid conditions are connected by dotted, dashed, or dot-dashed lines. The connecting of ZR-2 and ZR-3 may not be valid as ZR-3 utilized approx. 2 mg of the starting material, compared to approx. 25 mg in ZR-2. As a comparison, the data of PIDGEONet al. ( 1966) are also plotted (square symbols, solid lines).
Minor variations (also noted by PIDGEON et al., 1966) are most likely due to sample heterogeneity. The run products also describe a linear trend on the concordia diagram (Fig. 2), further suggesting that no Pb isotopic f~ctionation took place as a result of hydrothermal treatment. The relative amounts of U and Pb lost by the zircons in each experiment (as percentages of the concentrations in the starting material) are plotted vs. t in Fig. 3. As was the case in similar experiments performed by PIDCEON et al. ( 1966) on metamict Sri Lanka zircons, a considerable proportion ( 14 to 30%) of the total Pb loss occurred in the first 24 h of the experiment (e.g., ZR-4 and ZR-6). This suggests that, regardless of the concentration of cu-recoil defects in the zircon lattice, a significant amount of Pb can be leached fmm zircons. Similarly, the rate of Pb loss appears to levet off after the first 24 h of the experiment, which PIDGEON et al. ( 1966) interpreted as inhibition of Pb loss as the result of annealing of a-damaged structures. The patterns of Pb loss and, to a greater extent, of U loss generated by these experiments show a dependence on solution composition. In our preliminary experiments with 2% HN03, considerable ( 14 to 30% ) Pb loss occurred, with negligible U loss over short time intervals (~24 h), and at low temperatures (300°C). Only in run ZR-7 (6OO”C, 6 kb, 720 h) does there appear to be significant U loss but still not on the scale observed in the 2 M NaCl runs. In sharp contrast, all experiments in the 2 M NaCl solution produced significant Pb (33 to 51%) and U (30 to 47%) loss. On the concordia diagram (Fig. 2) the run products of experiments ZR- 1 and ZR-2 plot above the starting material, and ZR-1 is slightly reversely discordant, as a consequence of U loss in excess of Pb loss. In the duplicate runs in 2 M NaCl (ZR- 1A, ZR-3 ) , Pb loss exceeded U loss, and the run products are normally discordant (Fig. 2). The slight decrease in U loss observed in duplicate runs may represent heterogeneity in U/Pb concentrations in our starting materials, as runs ZR- 1A and ZR-
Losses of U
and Pb from zircons under hydrothermal conditions
3 utilized only 5-10% by weight of the normal run charges of -30 mg. Although sample heterogeneity is capable of causing a certain amount of error (
