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Single-zircon dating by stepwise Pb-evaporation constrains the Archean history of detrital zircons from the Jack Hills, Western Australia
Earth and Planetary Science Letters, 91 (1989) 286-296 Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands
286
[1]
Single-zircon dating by stepwise Pb-evaporation constrains the Archean history of detrital zircons from the Jack Hills, Western Australia B. K o b e r 1, R.T. P i d g e o n 2 a n d H.J. L i p p o l t 1 I Laboratorium ]'fir Geochronologie, Ruprecht-Karls- Universitiit, Im Neuenheimer Feld 234, D-6900 Heidelberg 1 (F.R.G.) 2 Department of Geology and Geophysics, Curtin University of Technology, Perth 6001, W.A. (Australia) Received March 22, 1988; revised version accepted October 26, 1988 Pb isotope analyses have been carried out on 42 zircon grains from a Western Australian metaconglomerate using stepwise Pb-evaporation directly in the ion source of a thermal ionization mass spectrometer. The metaconglomerate is from the Archean Jack Hills Metasedimentary Belt, and is known from ion microprobe ( " S H R I M P " ) analyses to contain a complex zircon population with ages between 4.2 G a and 3.1 Ga. The same complex pattern of ages is found by the Pb evaporation studies. Five grains yielded m i n i m u m crystallization ages from 4.17 Ga to 4.07 Ga. The main population appears significantly younger, having been generated at about 3.55-3.3 Ga. The agreement between the two analytical approaches confirms the S H R I M P results and demonstrates the value of the stepwise-evaporation technique in determining the age patterns of mixed zircon populations. In m a n y of the evaporative Pb isotope records the 207/206 ratios remained constant for all evaporation steps, which we interpret as evaporation from concordant zircon phases. However, for the majority of zircons 207/206 ratios increased with increasing evaporation temperature, and usually approached constant values during evaporation at the highest temperatures. This can be attributed to mixing of different radiogenic Pb components from either crystalline zircon phases of different ages or from domains of isotopically disturbed metamict zircon. Present results confirm > 4 G a zircon ages in the metaconglomerate from the Jack Hills and substantiate formation of crust at a very early stage in the evolution of the earth. Results also confirm a major crust-forming event 3.55-3.3 Ga ago.
1. Introduction
In the past decade techniques have been developed for the geochronological analysis of single zircon grains. The different approaches allow the dating of micro-volumes of zircon either by miniaturising conventional analytical techniques (e.g. [1,2]) or by isotopically analysing Pb and U from small areas of individual zircons vaporized in a secondary ion microprobe (e.g. [3,4]). Alternatively, single zircon crystals can be used as direct emitters of radiogenic Pb in the ion source of a thermal ion mass spectrometer by heating either of the powdered [5,6] or of the uncrushed zircons in the ion source [7,8]. The efficiency and precision of this technique has been improved by a modified ionization procedure, generating highly effective Pb ÷ emitter compounds in the ion source similar to the Si-gel technique [9]. These compounds are derived from components which are released directly from the heated zircon crystal at high temperatures. 0012-821X/89/$03.50
© 1989 Elsevier Science Publishers B.V.
Single-zircon techniques are especially important in studies of heterogeneous detrital zircon populations. Analysis of the isotopic and age distribution and the morphology of individual grains helps to identify the zircon source rocks, and to elucidate the history of the host environments. This has been spectacularly demonstrated by the Australian National University (ANU) ion microprobe ( " S H R I M P " ) analyses on detrital zircons from metasedimentary rocks from the Archean Yilgarn Block of Western Australia (Fig. 1, [10AN). These rocks were reported to contain ancient zircons with minimum ages of 4 Ga. Froude et al. [10] reported a subpopulation of 4.2-4.1 Ga old zircons (four individuals out of more than hundred analysed) in an Archean zircon population from a metaquartzite at Mt. Narryer. However, these results were not substantiated by a series of 39 conventional U / P b analyses carried out for 27 individual zircon grains and for 12 individual fragments of another 5 crystals from the same population [2].
287
spectrometer [9]. This approach uses temperature as the selection parameter for the mobilization of radiogenic Pb hosted in the domains of the zircon grains and differs from the SHRIMP technique, which vaporizes Pb from small zircon areas (spot diameter ca. 10-40 /~m). The objective of the present study was to compare the results from the different approaches and to apply additional information obtained from the stepwise evaporation technique to questions concerning the nature of the zircon grains and the ancient zircon source rocks.
PILBARA BLOCK
JACK HILLS
2. Geology of the Jack Hills Metasedimentary Belt m
ai
,
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/
/
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Fig. 1. Sketch map showing the location of the Jack Hills Metasedimentary Belt and of Mt. Narryer within the Western Gneiss Terrain of the Archean Yilgarn Block of Western Australia.
Compston and Pidgeon [11], in their SHRIMP data on 140 single grains from a metaconglomerate from the Jack Hills Belt (approximately 60 km northeast of Mt. Narryer), discovered 17 zircons (12%) with minimum ages of 4.28-3.92 Ga. Most zircons yielded ages of 3.6-3.3 Ga, many of them being nearly concordant. Compston and Pidgeon [11] concluded that the failure of the conventional study [2] on the Mt. Narryer population to observe individuals of the oldest subpopulation was due to their low abundance (< 4%) in the Mt. Narryer detritral zircon population. The higher abundance of > 4 Ga zircons in the Jack Hills metaconglomerate (ca. 12%) makes this rock more suitable for investigating the old subpopulation and for verifying their spectacular ages by alternative methods. In the present study, 42 zircons from the Jack Hills detrital population have been analysed using stepwise evaporation of radiogenic Pb from uncrushed and chemically untreated single zircons (size 100-300 ~m) in a thermal ion source mass
The Jack Hills Metasedimentary Belt is situated in the northernmost part of the Western Gneiss Terrain of the Archean Yilgarn Block of Western Australia (Fig. 1). The Western Gneiss Terrain, which forms the western margin of the Yilgarn Block, consists of repeatedly deformed and metamorphosed, generally shallow-water sediments infolded with sheets of orthogneiss and intruded by mafic and ultramafic rocks and by post-tectonic granitoids. Its rocks differ from those of the granite-greenstone provinces in the central and eastern Yilgarn Block, which consist of linear greenstone belts surrounded and intruded by diapiric granites [12]. The Jack Hills Belt is composed of psammites, pelites, cherts and banded ironstones of substantial thickness. Ultramafic rocks and amphibolites are also present. The belt is enclosed and invaded by granitic gneisses. Contacts between the belt rocks and the gneisses are invariably marked by zones of deformation. The predominance of sedimentary rocks in the Jack Hills Belt is analogous to that of the Narryer Metamorphic Belt, about 60 km to the southwest (Fig. 1), but the granulite facies metamorphic grade of the Narryer Belt contrasts with the greenschist to lower amphibolite facies metamorphic grade of the rocks of the Jack Hills Belt [13]. The analysed zircons were from the non-magnetic (on a Frantz isodynamic separator) + 135 ~m fraction recovered from a band of oligomictic quartz pebble conglomerate from within a thick psammitic sequence at the southern end of the Jack Hills Belt. It is the same zircon population which was investigated by SHRIMP analyses [11].
288
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34
37
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Fig. 2. Morphology of selected g a i n s from the analysed suite of crystals (reflected l i g h t - - p h o t o g r a p h s made before thermal treatment in the mass spectrometer). The pictures are arranged according to the age data (Table 1 and Fig. 3), starting with the youngest grains on top of the figure. The numbers given correspond to the grain n u m b e r s in Table 1. The bars give a length of 100 ~tm for each of the presented crystals. The subgrouping a - d refers to the subgrouping given in Table 1. a: 207/206 < 0.280, b: 207/206 -- 0.283, c: 0.29 < 207/206 < 0.32, d: 207/206 > 0.44.
289
3. Zircon morphology The analysed zircon population has been described in detail [11]• Therefore, only some basic observations are given here. The zircon population as a whole is heterogeneous (Fig. 2). All grains are reddish or brownish with varying intensities of color and strongly varying degrees of translucence• Many of the grains are rounded• There are, however, various relics of crystal faces, and some crystals have well-preserved crystal faces with only weakly rounded edges. Generally, the crystal surfaces appear corroded, sometimes pitted; however, well-reflecting crystal surfaces can also be observed. Various grains contain small translucent or opaque inclusions• The crystal matrices sometimes look heterogeneous in their coloration, and occasionally have schlieren.
4. Analytical procedure 42 single grains weighing 10-30 /~g each have been studied. Following the procedures described in [8,9] radiogenic Pb was evaporated over a num-
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ber of temperature steps using thermal ionization mass spectrometry with a double filament arrangement ( F I N N I G A N MAT261 instrumentation). The single crystals were encased in the evaporation filament and conditioned at ~< 1500-1600 K to strip off common Pb and some radiogenic components with low Pb activation energies weakly bound to metamict zircon or to non-zircon domains within the crystal. This method "cleans" the crystalline domains characterized by high Pb activation energies. Subsequently, during several repeated 15-minute episodes Pb was evaporated at 1700-1800 K, and deposited on the (cold) counterfilament. Each of the depositional episodes was followed by a Pb isotope analysis using the deposited salt as Pb +-ion emitter at about 1400-1500 K. The averaged 207/206 ratios for each analytical step were plotted against the evaporation temperature and against the respective 208/206 ratios (Fig. 3). The data usually were not influenced by common Pb (204/206 < 0.0001). The routine precision of the presented data is 0.5-1% (1 st. dev.). As discussed by [9] this is also a good estimation of the accuracy. Most of the analysed zircons yielded enough radiogenic Pb for 4 or more evaporative steps• Thus it was usually possible to test for internal consistency of the averaged Pb isotope ratios derived from the same crystal at different Pb evaporation temperatures• Constant 207/206 ratios can be used for age calculations corresponding to the 207/206 apparent age determinations of the conventional procedures (cf. [8, fig. 4]).
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Fig. 3. Evaporative 207/206 ratio records and 207/206208/206 relations of 42 single zircons (10-30/~g) analysed by stepwise Pb evaporation in the temperature range of 1700-1800 K. Dotted lines indicate analyses that yielded constant 207/206 ratios throughout the run. Dashed area = data array of 19 zircons whose evaporative records stabilized at 207/206 ratios of 0.280-0.286.
The distribution of Pb isotope ratios from the Jack Hills zircon population as a whole, as well as from many of the different zircon individuals is complex• At e v a p o r a t i o n t e m p e r a t u r e s of 1700-1750 K many grains yielded 207/206 ratios of 0•26-0•28, which generally increased with increasing evaporation temperatures and stabilized at the final steps (Table 1 and Fig. 3). About one third of the analysed grains yielded constant 207/206 ratios for all temperature steps free of any significant trends• The range of 0•26-0.48 for 207/206 ratios indicates that all zircon domains from which Pb was
290 TABLE 1 Lead isotope ratios of 42 zircon crystals from the Jack Hills metaconglomerate, derived by stepwise Pb evaporation from the single grains. The data are grouped according to the 207/206 ratios at which the evaporative records of the different grains stabilized a Grain No.
Translucence b
Relicts of tryst, faces c
Visible inclusions
Stepwise evaporation at temp. (K)
Number of steps
Maximum 204/206
Var. of 207/206
Var. of 208/206
(207/206) * stab. at:
Group a ((207/206) stabilized at < 0.28)." 15 medium (yes) no 4 poor no no 1 v.poor yes ? 39 poor no ?
1713-1773 1733-1783 1723-1803 1708-1748
4 3 5 5
< < < <
0.00008 0.00002 0.00006 0.00001
0.263-0.273 0.264-0.271 0.271-0.272 0.275-0.277
0.16-0.22 0.10-0.11 0.16-0.17 0.12-0.13
0.2711( ± 23) (0.271) 0.2714( ± 2) 0.2762 ( ± 9)
Group b ((207/206) stabilized at ca. 0.283): 38 poor (yes) ? 13 medium yes yes 6 poor yes no 7 medium yes yes 22 poor (yes) ? 24 poor (yes) yes 35 poor yes ? 8 good yes yes 20 poor (yes) yes 40 poor yes no 19 poor yes yes 29 medium yes no 21 poor yes yes 32 good yes yes 42 medium (yes) no 12 medium yes yes 30 good (yes) yes 17 good yes no 25 poor (yes) yes
1713-1763 1703-1788 1713-1768 1723-1753 1718-1798 1713-1793 1713-1793 1803 1713-]_798 1693-1773 1723-1778 1723-1803 1713-1803 1713-1778 1698-1743 1748-1778 1708-1788 1713-1773 1713-1773
5 4 3 2 5 5 5 1 5 6 4 4 6 4 4 3 5 4 3
< < < < < < < < < < < < < < < < < < <
0.00005 0.00016 0.00006 0.00016 0.00008 0.00021 0.00006 0.00001 0.00005 0.00004 0.00051 0.00010 0.00038 0.00057 0.00046 0.00006 0.00009 0.00008 0.00006
0.278-0.281 0.276-0.282 0.279-0.283 0.278-0.281 0.272-0.283 0.269-0.284 0.277-0.284 0.283 0.274-0.284 0.267-0.284 0.281-0.285 0.284 0.285 0.258-0.286 0.284-0.287 0.283-0.287 0.284-0.286 0.285-0.287 0.285-0.287 0.275-0.287
0.38-0.40 0.12-0.18 0.23-0.24 0.35-0.36 0.25-0.27 0.09-0.12 0.20-0.21 0.19 0.09-0.20 0.07-0.17 0.24-0.33 0.23-0.29 0.14-0.31 0.26-0.30 0.14-0.16 0.20-0.21 0.15-0.22 0.17-0.29 0.31-0.38
0.2803( _+10) 0.2804(±11) (0.281) (0.281) 0.2819(± 11) 0.2827( ± 17) 0.2827( ± 7) (0.283) 0.2832( ± 9) 0.2836( ± 7) 0.2842( _+12) 0.2843(± 12) 0.2849( ± 8) 0.2849( ± 2) 0.2850( ± 15) 0.2851( ± 9) 0.2854( ± 11) 0.2860(± 5) (0.287)
Group c ((207/206) stabilized at 0.29-0.32): 31 v.poor yes ? 9 v.poor no ? 28 poor no ? 27 poor (yes) yes 34 v.poor no ~ 37 poor yes yes 18 v.poor (yes) '~ 16 medium no yes 36 poor yes yes 33 medium no no 3 poor (yes) yes 10 medium yes yes 14 poor (yes) no 2 poor (yes) yes
1713-1743 1743-1793 1713-1798 1713-1803 1713-1783 1713-1783 1728-1773 1723-1733 1713-1763 1713-1763 1713-1758 1708-1803 1713-1763 1733-1803
3 2 6 7 7 5 3 2 4 5 3 5 5 4
< < < < < < < < < < < < < <
0.00010 0.00009 0.00028 0.00008 0.00015 0.00002 0.00025 0.00012 0.00014 0.00018 0.00001 0.00005 0.00040 0.00005
0.290-0.293 0.294-0.295 0.272-0.294 0.282-0.294 0.270-0.296 0.289-0.297 0.298-0.300 0.302-0.303 0.291-0.304 0.286-0.304 0.306-0.309 0.295-0.308 0.312-0.316 0.312-0.315
0.19-0.20 0.15-0.16 0.20-0.22 0.20-0.22 0.14-0.17 0.11-0.17 0.15-0.20 0.19-0.20 0.16-0.23 0.16-0.24 0.09-0.12 0.07-0.11 0.10-0.15 0.ll-0.12
0.2916(±17) (0.295) 0.2938( ± 12) 0.2944( ± 15) 0.2949(± 7) 0.2976( ± 14) 0.2986( ± 11) (0.303) 0.3030(± 7) 0.3034(±23) 0.3073( ± 16) 0.3076( ± 6) 0.3127(±20) 0.3140(± 13)
Group d ((207/206) stabilized at > (144): 23 poor (yes) yes 5 medium (yes) no 41 medium (yes) no 11 medium no yes 26 medium yes yes
1708-1803 1708-1778 1693-1783 1718-1768 1713-1789
15 5 8 6 11
< < < < <
0.00024 0.00021 0.00011 0.00005 0.00028
0.327-0.448 0.447-0.452 0.428-0.460 0.381-0.462 0.358-0.478
0.07-0.12 0.18-0.20 0.09-0.10 0.11-0.15 0.06-0.11
0.4469( ± 13) 0.4508(±8) 0.4597(± 4) (0.462) (0.478)
a The given maximum 204/206 ratios usually characterize the first evaporative s t e p - - t h e following steps generally yielded 204/206 ratios of 0.00001-0.00003. Consequently, a common Pb correction of the 207/206 ratios was not necessary. Most of the listed data were analysed using the multiplier. In various cases it was possible to change to the Faraday multicup for intercomparison of the results. In all these cases the results for both detector systems were in good agreement. No discrimination or fractionation corrections were carried out, which would be in the order of permil [9]. The last column gives the average of the 207/206 where ratios have stabilized over at least three evaporative steps (error given is one standard deviation). b " g o o d " = clear, " m e d i u m " = translucent to clear, " p o o r " = poorly translucent, " v . p o o r " = opaque or nearly opaque. " ( y e s ) " = relicts of crystal faces, obscured by strong rounding of the edges.
291 0,1,
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0,29 3;4
--207/206~ 0,2, 0,3, 0t,
0;31 A207/206-~ 0,kt,' 0;L~6'0:~' 31s
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' 43
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Fig. 4. Frequency histogram of 207/206 ages determined for 42 zircons by the evaporation method. The variations in the age distributions of both the Archean zircon subpopulations are far outside the routine data reproducibilityof 1%. Widths of the apparent ages classes: A(207/206)=0,01 (inset), A(207/206) = 0.0025 (expanded presentation). evaporated at high temperatures are of Archean age. In addition the wide distribution of finally stabilized 207/206 ratios indicates a complex age grouping. Most crystals belong to a heterogeneous episode of zircon generation or regeneration between 3.55 Ga and 3.3 Ga ago. Five of the 42 studied zircons (12% of the total zircon population) form a very old subpopulation with 207/206 ratios >/0.45 corresponding to ages of >i 4.07 Ga (Figs. 3 and 4, see Table 1, groups a - c and d). About 50% of the 3.55-3.3 Ga crystals have well-defined high-temperature 207/206 ratios in the range of 0.284 _+ 0.002, which corresponds to an age of 3.385 + 0.010 Ga. Hereafter these zircons are referred to as the 3.39 Ga group. About 10% of the grains yielded significantly lower ages (3.34-3.31 Ga), and 40% Of the crystals yielded higher 207/206 ratios of 0.292-0.314 when compared to the 3.39 Ga group. This range of stabilized 207/206 ratios significantly exceeds the routine 1% reproducibility of the data (cf. the width of the 3.39 Ga group age distribution, Figs. 3 and 4) and points to poly-episodic zircon generation events. This view is supported by the presence of zircons in each of the groups showing consistent 207/206 ratios over the whole range of evaporation steps. It is difficult, however, to define discrete ages of crystallization because of the lack of clear-cut and well-centered age distributions both of the 3.34-3.31 Ga zircons and the 3.55-3.43 Ga zircons (Fig. 4).
For each of the three younger groups, grains were found with evaporative records starting at 207/206 ratios of 0.27 or even less (minimum value so far found: 0.258--grain 21). This can be interpreted by the presence of rejuvinated zircon domains and the mixing of different radiogenic Pb components during evaporation of the respective grains (see the discussion below). The > 4 Ga.zircons generally have evaporative trends which are similar to those of the younger grains. Most of the derived records start with 207/206 ratios significantly lower than the respective final steps. For instance, 207/206 ratios as low as 0.33-0.35 have been observed at evaporation temperatures of about 1700 K. The 207/206 ratios of grain 11 stabilized at ca. 0.46 in the temperature range of 1760-1780 K, but again increased significantly to about 0.475 in the last two steps (1783-1789 K). This is the largest 207/206 ratio found so far for the Jack Hills detrital population by applying the step-evaporation technique. 6. Discussion
6.1. Comparison with the results of S H R I M P analyses. The data distribution as a whole agrees well with the one derived from analyses of more than 100 grains of the same population using secondary ion mass spectrometry (SHRIMP; [11]). This holds true for the ranges of 207/206 ages (3.6-3.3 Ga, 4.28-3.92 Ga, [11]) as well as for the relative frequency of the differently aged grains (12% of the grains are very old zircons and about half of the individuals yield minimum ages of ca. 3.35 Ga [11]). However, in the present suite there is a lack of 3.1 Ga and 3.8 Ga old zircons which were reported by [11] from the Jack Hills metaconglomerate as rare components. This lack may be due to the low probability of analysing rare components given the limited number of zircons analysed by the evaporation method. The generally flat trends of the 207/206 ratios throughout the evaporative records, especially the constant 207/206 ratios of one third of the grains, support the existence of concordant zircon phases in the present population. This agrees well with the SHRIMP zircon data which mainly plot on
292 the Concordia line in the conventional Concordia diagram. 6.2. Relations between zircon morphology and Pb isotope characteristics Whereas it is not possible to divide the analysed grains into clear-cut subpopulations by mere morphological arguments, the relationship of certain morphological trends to the age groups sets boundary conditions to the age models of zirconogenesis. The existence of relics of crystal faces is a characteristic of all zircon subgroups (Fig. 2). This even holds true for the oldest zircons. The degree of crystal face corrosion and rounding of the grains cannot be clearly attributed to the different groupings. So far ovoids have not been observed for the 3.39 Ga group. This group is characterized by grains with rather well-preserved crystal faces and a mild rounding of the crystal edges. Most grains have surfaces reflecting light poorly. In contrast to the youngest group (207/206 ~< 0.275), which seems to be free of inclusions, the 3.39 Ga group zircons often contain small inclusions. Older zircon inclusions or older zircon cores are not present in these grains. Their evaporative records indicate a single (identical) crystallization event. There is no characteristic feature that distinguishes the > 4 Ga zircons from younger ones. The very old zircons also have inclusions and exhibit varying degrees of crystal rounding and surface corrosion. A relationship exists between the translucence of the grains and the 207/206 ratios (Table 1). Most of the crystals that yielded constant 207/206 ratios throughout the runs (dotted lines in Fig. 3) are characterized by good translucence. This suggests that increased opacity of grains is related to the presence of zircon domains with younger Pb components. 6.3. Phase associations in individual zircons Grain-by-grain statistics of the present study as well as of the previous S H R I M P analyses [11] show that complex U / P b isotope patterns have to be expected for each individual crystal in the zircon population of the Jack Hills metaconglomerate. Heterogeneous Pb isotope compositions in single grains have been reported for some of the very old zircons from the metaquartzite of
the Mt. Narryer [10] and from the Jack Hills metaconglomerate [11]. Compston and Pidgeon [11] concluded that variations in 207/206 ratios found by repeated S H R I M P analyses of different small areas on the same grain are due to internal redistribution of Pb and U or to loss of Pb during some early Archean event. They reported cases where analyses of zircon areas had significantly different U / P b ratios but the same 207/206 ratios, which was interpreted as recent differential Pb loss related to variations in the properties of the zircon areas. Accordingly, the crystals can be treated as an association of different zircon phases with differing response to the milieu conditions of the host reservoir. This is important in interpreting observed trends in 207/206 ratios of the evaporative records. Regular increases of 207/206 ratios (Fig. 3) can be attributed to the mixing of Pb from different zircon phases or domains with differing Pb isotope compositions during the evaporation in the ion source. The relative contribution of radiogenic Pb from the evaporated domains is systematic in that apparently "younger" domains release their Pb at slightly lower temperatures than "older" ones. Consequently the "younger" phases get exhausted somewhat earlier, and the last evaporative steps are dominated by radiogenic Pb released from "old" domains. The systematic patterns of Pb mobilization from the intergrown zircon phases characterize the differences in the activation energies necessary to remove the Pb isotopes from the respective host phase. The properties and structural character of the intergrown zircon phases can be discussed in the light of the behaviour of the U / P b system. Two explanations are suggested (cf. [8]). The first involves mixing of Pb from domains which have been open U / P b systems, with Pb from isotopically undisturbed (concordant) phases. Fission track, ion microprobe and other studies have shown that zircons from a number of rock types have a heterogeneous distribution of uranium and thorium (e.g. [2,14-16]). The structure of such zircons can be viewed as a matrix of domains with variable amounts of radiation damage [17]. Pidgeon et al. [18] showed that, other factors being equal, the susceptibility of a zircon to Pb loss is proportional to its radiation damage. During heating under metamorphic conditions radiation
293 damaged zircon can undergo annealing, possibly accompanied by Pb loss. At again lower temperatures radiation damage once again accumulates in the zircon and, as before, is enhanced in the high-uranium and -thorium domains. This is often seen as cloudy or opaque patches or zones within the zircon crystals. When the zircon is heated during mass spectrometric analysis the first Pb to be emitted is the Pb with low 207/206 ratios from the radiation damaged, high-uranium and -thorium domains. As higher filament temperatures are reached Pb from less damaged domains is emitted, culminating in emission of concordant Pb from domains which have experienced minimal radiation damage throughout their history. A second possibility is that the observed trends in the evaporative records are due to mixing of different radiogenic Pb components released from concordant crystalline domains with divergent ages (cf. [19,20]). A supporting observation is that the emission temperatures for Pb from apparently "older" and " y o u n g e r " phases are similar (1690-1800 K). The presence of different concordant phases can be explained by a variant of the radiation damage model. In this model initial zircon crystallization was followed by radiation damage in particular domains which later on was annealed during an Archean metamorphic episode, thus generating "younger" zircon phases by partial or complete resetting of the U / P b isotopic clock in these domains. Generation of apparently younger phases by metamorphic recrystallization is supported by the 208/206 ratio trends of the evaporative records (see below). This process is thought to happen also on a submicroscopic scale forming a patchwork of micro-domains (note the mentioned relation of crystal translucence to the presence of younger Pb components). It is essential for this mixing model to explain the observation that the apparently "younger" phases release their radiogenic Pb at slightly lower temperatures than the "older" phases. This can be interpreted by slightly lower Pb activation energies of the recrystallized domains possibly due to larger amounts of incorporated trace elements compared to the zircon domains which release Pb at further increased evaporation temperatures. A synthesis of the two models may best describe the observations. The zircons with increasing 207/206 ratios in the evaporative records
should be looked at as an association of concordant phases with divergent ages on the one hand, and discordant phases on the other hand. The pattern of 207/206 ratios derived by Pb evaporation at temperatures of 1700-1800 K is strongly controlled by the different Pb activation energies of the intergrown phases. 207/206 ratios of phases with the highest activation energies, which release their Pb during the final evaporation steps, are closest to the crystallization age of the original (primary) zircon lattice. 7. Geological constraints The present evaporation data can be used to discuss the nature of the unknown source rocks and models of the predepositional evolution of the detrital zircon population. The age patterns (Fig. 4), the regular trends in the evaporative records (Fig. 3), and the specific Pb isotope distributions in the 207/206-208/206 diagram (Fig. 5) con-
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gronulifes__ ~n,~e~ 0,3
O,L,
--208/206----~ Fig. 5. Plot of 207/206 ages versus 208/206 ratios. The mean ratios of the respectivelast evaporativesteps after stabilization are taken. The 208/206 ratios characterize the mean Th/U ratios in the grains. To suggest possible source rocks for the zircons, the 208/206 ratios are compared with characteristic 208/206 ratios for various Archean rock types proposed by [21]. Under this scheme most of the zircons can be attributed to tonalitic or granodioritic intrusives or greenstone belt volcanics. The 3.39 Ga subgroup may contain zircons which have been generated under high- or ultra-metamorphicconditions.
294 strain models of the early Archean history of zirconogenesis and crust formation. The availability of 208/206 ratios greatly assists the interpretation of the 207/206 ratios. The 208/206 ratio effectively measures the T h / U ratio of the zircon. Related changes in the Pb isotope ratios may indicate mixing of intergrown zircon phases with different ages and T h / U ratios. In addition the 208/206 ratios help to identify and to characterize subgroups in the zircon population. These ratios can also be used to constrain possible Archean source rocks which supplied zircons to the conglomerate ([21], see below). The 208/206207/206 correlation diagram in Fig. 3 demonstrates the following trends: Flat 207/206 ratio trends (dotted lines in Fig. 3) for the range of evaporation temperatures, are usually accompanied by only small changes in the 208/206 ratios (order of 10%). Because the 208/206 ratio is directly proportional to the T h / U ratio this indicates rather uniform T h / U ratios of the different domains of the respective crystals. They can be taken as being rather homogeneous and isotopically undisturbed. Changes in the 207/206 ratios found in the temperature range of 1700-1750 K in many cases are accompanied by only minor (10%) changes in the 208/206 ratios, comparable to the zircons with plateau-like 207/206 records. It follows that, although uranogenic Pb components of different isotope compositions occur in individual grains, these different uranogenic components seem to be hosted in domains with similar T h / U ratios in the respective crystal. This appears to rule out possible growth of zircon rims around older cores during later Archean metamorphic events because changes of growth conditions can be expected to be usually accompanied by changes of the T h / U ratios [21]. Instead, the present results favor either zircon recrystallization without severe change of the T h / U ratios, or Pb loss due to distillation under extreme metamorphism (T>~ 900-1000 K, [22]). In both the 4.1 Ga and the 3.39 Ga subpopulations crystals occur whose 208/206 ratios deviate from the general range of 0.10-0.25 (Table 1 and Fig. 5). Two of the five very old grains yield 208/206 ratios of approximately 0.05-0.06 for the first steps of the high-temperature evaporative record. This suggests that the strong changes of -
-
-
-
-
-
207/206 found for the first evaporative steps of these old grains may be due to (locally) enhanced contents of U and the influence of radiation on the stability of the crystalline zircon lattice. A significant number of zircons from the 3.39 Ga group yield higher 208/206 ratios (>~ 0.3) than those found in the other subgroups. The large range of 0.1-0.4 in the 208/206 ratios of the 3.39 Ga group is in agreement with the SHRIMP data [11]. The special patterns of T h / U ratios of this subpopulation may point to milieu conditions of zircon generation not experienced by the rest of the zircon population. The significance of zircon 208/206 ratios, as indicators of the parent rock, has been proposed for zircons from a number of Archean rock types [21]. On the basis of this broad scheme, the spread of 208/206 ratios from 0.10 to 0.19 for zircons from the > 4.0 Ga group suggests tonalitic to granodioritic plutonic rocks, or possibly volcanic parent rocks. The large range of 208/206 ratios of 0.1-0.4 for the 3.39 Ga zircon subgroup comply with a number of possible source rocks (Fig. 5). The approximately 3.55 Ga old zircons may indicate plutonic rocks in the source rock assemblage. In the case of the 3.50-3.43 Ga old zircons and of the zircons younger than 3.39 Ga volcanic parent rocks appear the most likely. It should be noted, however, that from the morphology of the zircon population and from the low uranium and thorium contents of the grains (mean [U] - 100 ppm, mean [Th] = 50 ppm) Compston and Pidgeon [11] proposed mafic source rocks which had experienced granulite facies metamorphism. It is significant that, although the 3.39 Ga event is obviously the most important crystallization event documented by the detrital population as a whole, the " y o u n g e r " Pb components of the grains which are older than 3.4 Ga cannot be simply attributed to the 3.39 Ga event. Two of the respective grains even yielded "first step" 207/206 ratios as low as 0.27 ( = 3.31 Ga). The lack of a clear 3.39 Ga signature in the Pb isotopic spectrum of the > 3.4 Ga zircons suggests that the zircon generation at 3.39 Ga must have taken place in an environment different from that of the > 3.4 Ga zircons. Only after 3.39 Ga could the different generations of zircons have been admixed to form the total population. This is in agreement with the conclusions of Compston and Pidgeon [11] from
295 the trends found in the SHRIMP data. They assumed the 3.35 Ga zircons identified in their data to predate the sedimentation episode, and they proposed the two 3.1 Ga zircons found in their zircon suite to set an upper age limit for the deposition of the conglomerate. As discussed above, the large variations in the 208/206 ratios of the 3.39 Ga subgroup compared to the rest of the population suggest a variety of rock types formed 3.4 Ga ago. It is not yet possible to determine whether the 3.39 Ga zircons have been generated under metamorphic or magmatic conditions. However, the frequency of zircons of the same age, and the presence of many relics of well-defined crystal faces suggests zircon generation at least partly under ultrametamorphic conditions. Taking into account the assumption of separate zircon sources (s.a.) the presence of "secondary" [21] zircons in the 3.39 Ga group can therefore most easily be modelled by generation of zircon in granitic melts which intruded the rest of the source rock assemblage 3.39 Ga ago. Alternatively, tectonically emplaced high-grade metamorphic rocks which were metamorphosed at ca. 3.39 Ga may be assumed. The different constraints on the unknown source rock assemblage can be combined to a simple speculative model of the rock suite which supplied zircons to the sediment: the oldest rocks in the 3.55-3.30 Ga rock sequence are tonalitic to granodioritic intrusives (orthogneisses). Greenstone belt volcanics have been formed during different episodes over the whole time span of the 3.55-3.30 Ga major cycle of crust formation. Part of the rock assemblage was intruded by granites at approx. 3.4 Ga. Based on the data, the presence of granulites generated at 3.4 Ga is conceivable. The youngest zircon phases or domains found in the present study yielded 207/206 ages of 3.25 Ga. This implies that post-depositional geological events like the greenschist-to-lower-amphibolite facies metamorphism younger than 3.1 Ga, the intrusion of granitoids at 2.7 Ga, shearing and pegmatite formation at 2.5-2.4 Ga [23], did not generate essential amounts of new zircon or cause the severe disturbance of the zircon U / P b systems in the Jack Hills metaconglomerate. The significant differences in the ages of the five very old zircons suggest that zircon generation more than 4 Ga ago was also a poly-episodic
event. However, it cannot be excluded that (analytical) phase mixing a n d / o r geological resetting of the U / P b systems have influenced the respective final 207/206 ratios of the ancient grains. The data distribution presented in Fig. 3 clearly indicates the presence of rejuvinated zircon domains in four of the five ancient crystals. Rejuvination of zircon domains (recrystallization or partial Pb loss due to metamictization) can be attributed to the influence of the 3.55-3.30 Ga crust-forming events during the recycling of the source rocks of the > 4 Ga zircon subpopulation. The minimum age for the zircon generation is estimated to be approximately 4.2 Ga, which is in good agreement with the measurements of [11]. Present results indicate that this first cycle of mineral formation was completed about 4.07 Ga ago. The presence of these very old zircons in a detrital population which mainly has been generated during the major cycle of crust formation at approximately 3.55-3.30 Ga demonstrates the ability of zircon to survive also early crustal processes.
8. Summary (1) Results from zircon step heating analyses identify two subpopulations within the detrital zircon suite from the Jack Hills metaconglomerate with ages of 4.17-4.07 G a and 3.35-3.3 Ga. (2) The results are in good agreement with previous S H R I M P investigations [11] demonstrating that the Pb evaporation method is a valuable technique for acquiring quantitative age information. It has an important potential for geochronological investigations of the earliest Archean crust. (3) In addition, the special features of the evaporation technique allow the study of complex mixtures of zircon phases associated in a single grain--as either intermixed phases of different lattice structures (crystalline/metamict), or as intermixed crystalline zircon domains of different physicochemical properties. (4) The investigated zircon population is composed of a complex mixture of zircons generated in different environments with differing Archean evolution. The subpopulations must have been transfered to the metaconglomerate during a sedimentation episode less than 3.3 Ga ago.
296
(5) Confirmation of the existence of ancient zircons with ages > 4 Ga substantiates models of formation of terrestrial protocrust during very early stages of the evolution of the earth. The results indicate an episode of minerogenesis from approximately 4.2 Ga to about 4.07 Ga.
9
10
Acknowledgements 11
R.T.P. gratefully acknowledges the University of Heidelberg for support as a visiting Professor in 1986. Geological studies in the Jack Hills were supported by the Australian Research Grants Scheme. F. Begemann and two anonymous reviewers made valuable comments on the original manuscript, and helped to improve the presentation. We acknowledge the assistance of D. Pingel preparing the filaments, and of K. Schacherl preparing some of the photographs.
12
13
14
15
References 16 1 J. Lancelot, A. Vitrac and C.J. All6gre, Uranium and lead isotopic dating with grain-by-grain zircon analysis: a study of complex geological history with a single rock, Earth Planet. Sci. Lett. 29, 357-366, 1976. 2 U. Sch~irer and C.J. All6gre, Determination of the age of the Australian continent by single-grain analysis of Mt Narryer metaquartzite, Nature 315, 52-55, 1985. 3 J.R. Hinthorne, C.A. Anderson, R.L. Conrad and J.F. Lovering, Single-grain 2°7pb/2°6pb and U / P b age determinations with a 10/~m spatial resolution using the ion microprobe mass analyser (IMMA), Chem. Geol. 25, 271-303, 1979. 4 W. Compston, I.S. Williams and C. Meyer, U-Pb geochronology of zircons from lunar breccia 73217 using a sensitive high mass-resolution ion microprobe, J. Geophys. Res. 89, Suppl. B, 525-534, 1984. 5 C. Kosztolanyi, Nouvelle methode d'analyse isotopique des zircons a l'6tat natural apr6s attaque directe sur le filament, C. R. Acad. Sci. Paris 261, 5849-5851, 1965. 6 A.P. Chukhonin, A mass spectrometric study of the forms taken by lead in zircon, Geochem. Int. 15, 186-189, 1978. 7 R.V. Gentry, T.J. Sworski, H.S. McKown, D.H. Smith, R.E. Eby and W.H. Christie, Differential lead retention in zircons: implications for nuclear waste containment, Science 216, 296-297, 1982. 8 B. Kober, Whole-grain evaporation for 2°7pb/2°6pb-age-
17
18
19
20
21
22
23
investigations on single zircons using a double-filament thermal ion source, Contrib. Mineral. Petrol. 93, 482-490, 1986. B. Kober, Single-grain evaporation combined with Pb + emitter bedding for 2°7pb/2°6pb-age investigations using thermal ion mass spectrometry, and implications to zirconology, Contrib. Mineral. Petrol. 96, 63-71, 1987. D.O. Froude, T.R. Ireland, P.D. Kinny, I.S. Williams and W. Compston, Ion microprobe identification of 4,100-4,200 Myr-old terrestrial zircons, Nature 304, 616-618, 1983. W. Compston and R.T. Pidgeon, Jack Hills, a further occurrence of very old detrital zircons in Western Australia, Nature 321,766-769, 1986. R.D. Gee, J.L. Baxter, S.A. Wilde and I.R. Williams, Crustal development in the Arehean Yilgarn Block, Western Australia, in: Archean Geology, J.E. Glover and D.I. Groves, eds., Spec. Publ. Geol. Soc. Aust. 7, 43-56, 1981. J.L. Baxter, S.A. Wilde, R.T. Pidgeon and I.R. Fletcher, The Jack Hills Metasedimentary Belt: an extension of the early Archean terrain of the Yilgarn Block, Western Australia (abstract), 7th Aust. Geol. Cony., 1984. P.E. Zaun and G.A. Wagner, Fission-track stability in zircons under geological conditions, Nucl. Tracks 10, 303-307, 1985. T.M. Harrison, J.N. Aleinikoff and W. Compston, Observations and controls on the occurrence of inherited zircon in Concord-type granitoids, New Hampshire, Geochim. Cosmochim. Acta 51, 2549-2558, 1987. W. Compston and A. Kr/3ner, Multiple zircon growth within early Archean tonalitic gneiss from the ancient gneiss complex, Swaziland, Earth Planet. Set. Lett. 87, 13-28, 1988. H.D. Holland and D. Gottfried, The effect of nuclear radiation on the structure of zircon, Acta. Crystallogr. 8, 291-300, 1955. R.T. Pidgeon, J.R. O'Neill and L.T. Silver, Uranium and lead isotopic stability in a metamict zircon under experimental hydrothermal conditions, Science 154, 1538-1540, 1966. R.H. Steiger and G.J. Wasserburg, Systematics in the 2°8pb-232Th, 2°7pb-235U, and 2°6pb-238U systems, J. Geophys. Res. 71, 6065-6090, 1966. R.H. Steiger and G.J. Wasserburg, Comparative U-Th-Pb systematics in 2.7× 109 yr plutons of different geological histories, Geochim. Cosmochim. Acta 33, 1213-1232, 1969. E.V. Bibikova, The most ancient rocks in the USSR territory by U-Pb data on accessory zircons, in: Archean Geochemistry. A. Kri3ner, G.N. Hanson and A.M. Goodwin, eds., pp. 235-250, Springer-Verlag, Berlin, 1984. J. Sommerauer, Die chemisch-physikalische Stabilit~it natiirlicher Zirkone und ihr U-(Th)-Pb System, Thesis ETH Ziirich, No. 5755, 151 pp., 1976 (unpublished). R.T. Pidgeon, W. Compston, S.A. Wilde, J.L. Baxter and L.B. Collins, Archean evolution of the Jack Hills Metasedimentary Belt, Yilgarn Block, Western Australia, Terra Cognita 6, 146, 1986.
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[1]
Single-zircon dating by stepwise Pb-evaporation constrains the Archean history of detrital zircons from the Jack Hills, Western Australia B. K o b e r 1, R.T. P i d g e o n 2 a n d H.J. L i p p o l t 1 I Laboratorium ]'fir Geochronologie, Ruprecht-Karls- Universitiit, Im Neuenheimer Feld 234, D-6900 Heidelberg 1 (F.R.G.) 2 Department of Geology and Geophysics, Curtin University of Technology, Perth 6001, W.A. (Australia) Received March 22, 1988; revised version accepted October 26, 1988 Pb isotope analyses have been carried out on 42 zircon grains from a Western Australian metaconglomerate using stepwise Pb-evaporation directly in the ion source of a thermal ionization mass spectrometer. The metaconglomerate is from the Archean Jack Hills Metasedimentary Belt, and is known from ion microprobe ( " S H R I M P " ) analyses to contain a complex zircon population with ages between 4.2 G a and 3.1 Ga. The same complex pattern of ages is found by the Pb evaporation studies. Five grains yielded m i n i m u m crystallization ages from 4.17 Ga to 4.07 Ga. The main population appears significantly younger, having been generated at about 3.55-3.3 Ga. The agreement between the two analytical approaches confirms the S H R I M P results and demonstrates the value of the stepwise-evaporation technique in determining the age patterns of mixed zircon populations. In m a n y of the evaporative Pb isotope records the 207/206 ratios remained constant for all evaporation steps, which we interpret as evaporation from concordant zircon phases. However, for the majority of zircons 207/206 ratios increased with increasing evaporation temperature, and usually approached constant values during evaporation at the highest temperatures. This can be attributed to mixing of different radiogenic Pb components from either crystalline zircon phases of different ages or from domains of isotopically disturbed metamict zircon. Present results confirm > 4 G a zircon ages in the metaconglomerate from the Jack Hills and substantiate formation of crust at a very early stage in the evolution of the earth. Results also confirm a major crust-forming event 3.55-3.3 Ga ago.
1. Introduction
In the past decade techniques have been developed for the geochronological analysis of single zircon grains. The different approaches allow the dating of micro-volumes of zircon either by miniaturising conventional analytical techniques (e.g. [1,2]) or by isotopically analysing Pb and U from small areas of individual zircons vaporized in a secondary ion microprobe (e.g. [3,4]). Alternatively, single zircon crystals can be used as direct emitters of radiogenic Pb in the ion source of a thermal ion mass spectrometer by heating either of the powdered [5,6] or of the uncrushed zircons in the ion source [7,8]. The efficiency and precision of this technique has been improved by a modified ionization procedure, generating highly effective Pb ÷ emitter compounds in the ion source similar to the Si-gel technique [9]. These compounds are derived from components which are released directly from the heated zircon crystal at high temperatures. 0012-821X/89/$03.50
© 1989 Elsevier Science Publishers B.V.
Single-zircon techniques are especially important in studies of heterogeneous detrital zircon populations. Analysis of the isotopic and age distribution and the morphology of individual grains helps to identify the zircon source rocks, and to elucidate the history of the host environments. This has been spectacularly demonstrated by the Australian National University (ANU) ion microprobe ( " S H R I M P " ) analyses on detrital zircons from metasedimentary rocks from the Archean Yilgarn Block of Western Australia (Fig. 1, [10AN). These rocks were reported to contain ancient zircons with minimum ages of 4 Ga. Froude et al. [10] reported a subpopulation of 4.2-4.1 Ga old zircons (four individuals out of more than hundred analysed) in an Archean zircon population from a metaquartzite at Mt. Narryer. However, these results were not substantiated by a series of 39 conventional U / P b analyses carried out for 27 individual zircon grains and for 12 individual fragments of another 5 crystals from the same population [2].
287
spectrometer [9]. This approach uses temperature as the selection parameter for the mobilization of radiogenic Pb hosted in the domains of the zircon grains and differs from the SHRIMP technique, which vaporizes Pb from small zircon areas (spot diameter ca. 10-40 /~m). The objective of the present study was to compare the results from the different approaches and to apply additional information obtained from the stepwise evaporation technique to questions concerning the nature of the zircon grains and the ancient zircon source rocks.
PILBARA BLOCK
JACK HILLS
2. Geology of the Jack Hills Metasedimentary Belt m
ai
,
Mt
H,m,,
'!;__..-< ~,,
/
/
pERTH
0
500km
Fig. 1. Sketch map showing the location of the Jack Hills Metasedimentary Belt and of Mt. Narryer within the Western Gneiss Terrain of the Archean Yilgarn Block of Western Australia.
Compston and Pidgeon [11], in their SHRIMP data on 140 single grains from a metaconglomerate from the Jack Hills Belt (approximately 60 km northeast of Mt. Narryer), discovered 17 zircons (12%) with minimum ages of 4.28-3.92 Ga. Most zircons yielded ages of 3.6-3.3 Ga, many of them being nearly concordant. Compston and Pidgeon [11] concluded that the failure of the conventional study [2] on the Mt. Narryer population to observe individuals of the oldest subpopulation was due to their low abundance (< 4%) in the Mt. Narryer detritral zircon population. The higher abundance of > 4 Ga zircons in the Jack Hills metaconglomerate (ca. 12%) makes this rock more suitable for investigating the old subpopulation and for verifying their spectacular ages by alternative methods. In the present study, 42 zircons from the Jack Hills detrital population have been analysed using stepwise evaporation of radiogenic Pb from uncrushed and chemically untreated single zircons (size 100-300 ~m) in a thermal ion source mass
The Jack Hills Metasedimentary Belt is situated in the northernmost part of the Western Gneiss Terrain of the Archean Yilgarn Block of Western Australia (Fig. 1). The Western Gneiss Terrain, which forms the western margin of the Yilgarn Block, consists of repeatedly deformed and metamorphosed, generally shallow-water sediments infolded with sheets of orthogneiss and intruded by mafic and ultramafic rocks and by post-tectonic granitoids. Its rocks differ from those of the granite-greenstone provinces in the central and eastern Yilgarn Block, which consist of linear greenstone belts surrounded and intruded by diapiric granites [12]. The Jack Hills Belt is composed of psammites, pelites, cherts and banded ironstones of substantial thickness. Ultramafic rocks and amphibolites are also present. The belt is enclosed and invaded by granitic gneisses. Contacts between the belt rocks and the gneisses are invariably marked by zones of deformation. The predominance of sedimentary rocks in the Jack Hills Belt is analogous to that of the Narryer Metamorphic Belt, about 60 km to the southwest (Fig. 1), but the granulite facies metamorphic grade of the Narryer Belt contrasts with the greenschist to lower amphibolite facies metamorphic grade of the rocks of the Jack Hills Belt [13]. The analysed zircons were from the non-magnetic (on a Frantz isodynamic separator) + 135 ~m fraction recovered from a band of oligomictic quartz pebble conglomerate from within a thick psammitic sequence at the southern end of the Jack Hills Belt. It is the same zircon population which was investigated by SHRIMP analyses [11].
288
.-
a
39
o.lmm
21
19
32
3u
b
27
34
37
C
d
23
4 I
11
2
Fig. 2. Morphology of selected g a i n s from the analysed suite of crystals (reflected l i g h t - - p h o t o g r a p h s made before thermal treatment in the mass spectrometer). The pictures are arranged according to the age data (Table 1 and Fig. 3), starting with the youngest grains on top of the figure. The numbers given correspond to the grain n u m b e r s in Table 1. The bars give a length of 100 ~tm for each of the presented crystals. The subgrouping a - d refers to the subgrouping given in Table 1. a: 207/206 < 0.280, b: 207/206 -- 0.283, c: 0.29 < 207/206 < 0.32, d: 207/206 > 0.44.
289
3. Zircon morphology The analysed zircon population has been described in detail [11]• Therefore, only some basic observations are given here. The zircon population as a whole is heterogeneous (Fig. 2). All grains are reddish or brownish with varying intensities of color and strongly varying degrees of translucence• Many of the grains are rounded• There are, however, various relics of crystal faces, and some crystals have well-preserved crystal faces with only weakly rounded edges. Generally, the crystal surfaces appear corroded, sometimes pitted; however, well-reflecting crystal surfaces can also be observed. Various grains contain small translucent or opaque inclusions• The crystal matrices sometimes look heterogeneous in their coloration, and occasionally have schlieren.
4. Analytical procedure 42 single grains weighing 10-30 /~g each have been studied. Following the procedures described in [8,9] radiogenic Pb was evaporated over a num-
I
0,45
i
;
t
I
I
" ,~
I
/.
I
;
:
:
:
:
~
~
~,2
. . . . .
,Iool
O,gO
J
3,9
ber of temperature steps using thermal ionization mass spectrometry with a double filament arrangement ( F I N N I G A N MAT261 instrumentation). The single crystals were encased in the evaporation filament and conditioned at ~< 1500-1600 K to strip off common Pb and some radiogenic components with low Pb activation energies weakly bound to metamict zircon or to non-zircon domains within the crystal. This method "cleans" the crystalline domains characterized by high Pb activation energies. Subsequently, during several repeated 15-minute episodes Pb was evaporated at 1700-1800 K, and deposited on the (cold) counterfilament. Each of the depositional episodes was followed by a Pb isotope analysis using the deposited salt as Pb +-ion emitter at about 1400-1500 K. The averaged 207/206 ratios for each analytical step were plotted against the evaporation temperature and against the respective 208/206 ratios (Fig. 3). The data usually were not influenced by common Pb (204/206 < 0.0001). The routine precision of the presented data is 0.5-1% (1 st. dev.). As discussed by [9] this is also a good estimation of the accuracy. Most of the analysed zircons yielded enough radiogenic Pb for 4 or more evaporative steps• Thus it was usually possible to test for internal consistency of the averaged Pb isotope ratios derived from the same crystal at different Pb evaporation temperatures• Constant 207/206 ratios can be used for age calculations corresponding to the 207/206 apparent age determinations of the conventional procedures (cf. [8, fig. 4]).
3,8
5. Results 0,35[ 1
O,30
3,63'7
t
tt
~,
0,25
I
0,1
0,2
:
0,3
-- 208 / 206 ~
2-----~!!-;~.__.~'-" 3,5 ........... ....................
:
1700
:
:
:
17/+0 --
:
:
:
:
:
:
3,2
1780
evuporation temperature [K ]
Fig. 3. Evaporative 207/206 ratio records and 207/206208/206 relations of 42 single zircons (10-30/~g) analysed by stepwise Pb evaporation in the temperature range of 1700-1800 K. Dotted lines indicate analyses that yielded constant 207/206 ratios throughout the run. Dashed area = data array of 19 zircons whose evaporative records stabilized at 207/206 ratios of 0.280-0.286.
The distribution of Pb isotope ratios from the Jack Hills zircon population as a whole, as well as from many of the different zircon individuals is complex• At e v a p o r a t i o n t e m p e r a t u r e s of 1700-1750 K many grains yielded 207/206 ratios of 0•26-0•28, which generally increased with increasing evaporation temperatures and stabilized at the final steps (Table 1 and Fig. 3). About one third of the analysed grains yielded constant 207/206 ratios for all temperature steps free of any significant trends• The range of 0•26-0.48 for 207/206 ratios indicates that all zircon domains from which Pb was
290 TABLE 1 Lead isotope ratios of 42 zircon crystals from the Jack Hills metaconglomerate, derived by stepwise Pb evaporation from the single grains. The data are grouped according to the 207/206 ratios at which the evaporative records of the different grains stabilized a Grain No.
Translucence b
Relicts of tryst, faces c
Visible inclusions
Stepwise evaporation at temp. (K)
Number of steps
Maximum 204/206
Var. of 207/206
Var. of 208/206
(207/206) * stab. at:
Group a ((207/206) stabilized at < 0.28)." 15 medium (yes) no 4 poor no no 1 v.poor yes ? 39 poor no ?
1713-1773 1733-1783 1723-1803 1708-1748
4 3 5 5
< < < <
0.00008 0.00002 0.00006 0.00001
0.263-0.273 0.264-0.271 0.271-0.272 0.275-0.277
0.16-0.22 0.10-0.11 0.16-0.17 0.12-0.13
0.2711( ± 23) (0.271) 0.2714( ± 2) 0.2762 ( ± 9)
Group b ((207/206) stabilized at ca. 0.283): 38 poor (yes) ? 13 medium yes yes 6 poor yes no 7 medium yes yes 22 poor (yes) ? 24 poor (yes) yes 35 poor yes ? 8 good yes yes 20 poor (yes) yes 40 poor yes no 19 poor yes yes 29 medium yes no 21 poor yes yes 32 good yes yes 42 medium (yes) no 12 medium yes yes 30 good (yes) yes 17 good yes no 25 poor (yes) yes
1713-1763 1703-1788 1713-1768 1723-1753 1718-1798 1713-1793 1713-1793 1803 1713-]_798 1693-1773 1723-1778 1723-1803 1713-1803 1713-1778 1698-1743 1748-1778 1708-1788 1713-1773 1713-1773
5 4 3 2 5 5 5 1 5 6 4 4 6 4 4 3 5 4 3
< < < < < < < < < < < < < < < < < < <
0.00005 0.00016 0.00006 0.00016 0.00008 0.00021 0.00006 0.00001 0.00005 0.00004 0.00051 0.00010 0.00038 0.00057 0.00046 0.00006 0.00009 0.00008 0.00006
0.278-0.281 0.276-0.282 0.279-0.283 0.278-0.281 0.272-0.283 0.269-0.284 0.277-0.284 0.283 0.274-0.284 0.267-0.284 0.281-0.285 0.284 0.285 0.258-0.286 0.284-0.287 0.283-0.287 0.284-0.286 0.285-0.287 0.285-0.287 0.275-0.287
0.38-0.40 0.12-0.18 0.23-0.24 0.35-0.36 0.25-0.27 0.09-0.12 0.20-0.21 0.19 0.09-0.20 0.07-0.17 0.24-0.33 0.23-0.29 0.14-0.31 0.26-0.30 0.14-0.16 0.20-0.21 0.15-0.22 0.17-0.29 0.31-0.38
0.2803( _+10) 0.2804(±11) (0.281) (0.281) 0.2819(± 11) 0.2827( ± 17) 0.2827( ± 7) (0.283) 0.2832( ± 9) 0.2836( ± 7) 0.2842( _+12) 0.2843(± 12) 0.2849( ± 8) 0.2849( ± 2) 0.2850( ± 15) 0.2851( ± 9) 0.2854( ± 11) 0.2860(± 5) (0.287)
Group c ((207/206) stabilized at 0.29-0.32): 31 v.poor yes ? 9 v.poor no ? 28 poor no ? 27 poor (yes) yes 34 v.poor no ~ 37 poor yes yes 18 v.poor (yes) '~ 16 medium no yes 36 poor yes yes 33 medium no no 3 poor (yes) yes 10 medium yes yes 14 poor (yes) no 2 poor (yes) yes
1713-1743 1743-1793 1713-1798 1713-1803 1713-1783 1713-1783 1728-1773 1723-1733 1713-1763 1713-1763 1713-1758 1708-1803 1713-1763 1733-1803
3 2 6 7 7 5 3 2 4 5 3 5 5 4
< < < < < < < < < < < < < <
0.00010 0.00009 0.00028 0.00008 0.00015 0.00002 0.00025 0.00012 0.00014 0.00018 0.00001 0.00005 0.00040 0.00005
0.290-0.293 0.294-0.295 0.272-0.294 0.282-0.294 0.270-0.296 0.289-0.297 0.298-0.300 0.302-0.303 0.291-0.304 0.286-0.304 0.306-0.309 0.295-0.308 0.312-0.316 0.312-0.315
0.19-0.20 0.15-0.16 0.20-0.22 0.20-0.22 0.14-0.17 0.11-0.17 0.15-0.20 0.19-0.20 0.16-0.23 0.16-0.24 0.09-0.12 0.07-0.11 0.10-0.15 0.ll-0.12
0.2916(±17) (0.295) 0.2938( ± 12) 0.2944( ± 15) 0.2949(± 7) 0.2976( ± 14) 0.2986( ± 11) (0.303) 0.3030(± 7) 0.3034(±23) 0.3073( ± 16) 0.3076( ± 6) 0.3127(±20) 0.3140(± 13)
Group d ((207/206) stabilized at > (144): 23 poor (yes) yes 5 medium (yes) no 41 medium (yes) no 11 medium no yes 26 medium yes yes
1708-1803 1708-1778 1693-1783 1718-1768 1713-1789
15 5 8 6 11
< < < < <
0.00024 0.00021 0.00011 0.00005 0.00028
0.327-0.448 0.447-0.452 0.428-0.460 0.381-0.462 0.358-0.478
0.07-0.12 0.18-0.20 0.09-0.10 0.11-0.15 0.06-0.11
0.4469( ± 13) 0.4508(±8) 0.4597(± 4) (0.462) (0.478)
a The given maximum 204/206 ratios usually characterize the first evaporative s t e p - - t h e following steps generally yielded 204/206 ratios of 0.00001-0.00003. Consequently, a common Pb correction of the 207/206 ratios was not necessary. Most of the listed data were analysed using the multiplier. In various cases it was possible to change to the Faraday multicup for intercomparison of the results. In all these cases the results for both detector systems were in good agreement. No discrimination or fractionation corrections were carried out, which would be in the order of permil [9]. The last column gives the average of the 207/206 where ratios have stabilized over at least three evaporative steps (error given is one standard deviation). b " g o o d " = clear, " m e d i u m " = translucent to clear, " p o o r " = poorly translucent, " v . p o o r " = opaque or nearly opaque. " ( y e s ) " = relicts of crystal faces, obscured by strong rounding of the edges.
291 0,1,
0,'27 ~13
0,29 3;4
--207/206~ 0,2, 0,3, 0t,
0;31 A207/206-~ 0,kt,' 0;L~6'0:~' 31s
'-rtGol-:
'
' 43
' ~;2'
Fig. 4. Frequency histogram of 207/206 ages determined for 42 zircons by the evaporation method. The variations in the age distributions of both the Archean zircon subpopulations are far outside the routine data reproducibilityof 1%. Widths of the apparent ages classes: A(207/206)=0,01 (inset), A(207/206) = 0.0025 (expanded presentation). evaporated at high temperatures are of Archean age. In addition the wide distribution of finally stabilized 207/206 ratios indicates a complex age grouping. Most crystals belong to a heterogeneous episode of zircon generation or regeneration between 3.55 Ga and 3.3 Ga ago. Five of the 42 studied zircons (12% of the total zircon population) form a very old subpopulation with 207/206 ratios >/0.45 corresponding to ages of >i 4.07 Ga (Figs. 3 and 4, see Table 1, groups a - c and d). About 50% of the 3.55-3.3 Ga crystals have well-defined high-temperature 207/206 ratios in the range of 0.284 _+ 0.002, which corresponds to an age of 3.385 + 0.010 Ga. Hereafter these zircons are referred to as the 3.39 Ga group. About 10% of the grains yielded significantly lower ages (3.34-3.31 Ga), and 40% Of the crystals yielded higher 207/206 ratios of 0.292-0.314 when compared to the 3.39 Ga group. This range of stabilized 207/206 ratios significantly exceeds the routine 1% reproducibility of the data (cf. the width of the 3.39 Ga group age distribution, Figs. 3 and 4) and points to poly-episodic zircon generation events. This view is supported by the presence of zircons in each of the groups showing consistent 207/206 ratios over the whole range of evaporation steps. It is difficult, however, to define discrete ages of crystallization because of the lack of clear-cut and well-centered age distributions both of the 3.34-3.31 Ga zircons and the 3.55-3.43 Ga zircons (Fig. 4).
For each of the three younger groups, grains were found with evaporative records starting at 207/206 ratios of 0.27 or even less (minimum value so far found: 0.258--grain 21). This can be interpreted by the presence of rejuvinated zircon domains and the mixing of different radiogenic Pb components during evaporation of the respective grains (see the discussion below). The > 4 Ga.zircons generally have evaporative trends which are similar to those of the younger grains. Most of the derived records start with 207/206 ratios significantly lower than the respective final steps. For instance, 207/206 ratios as low as 0.33-0.35 have been observed at evaporation temperatures of about 1700 K. The 207/206 ratios of grain 11 stabilized at ca. 0.46 in the temperature range of 1760-1780 K, but again increased significantly to about 0.475 in the last two steps (1783-1789 K). This is the largest 207/206 ratio found so far for the Jack Hills detrital population by applying the step-evaporation technique. 6. Discussion
6.1. Comparison with the results of S H R I M P analyses. The data distribution as a whole agrees well with the one derived from analyses of more than 100 grains of the same population using secondary ion mass spectrometry (SHRIMP; [11]). This holds true for the ranges of 207/206 ages (3.6-3.3 Ga, 4.28-3.92 Ga, [11]) as well as for the relative frequency of the differently aged grains (12% of the grains are very old zircons and about half of the individuals yield minimum ages of ca. 3.35 Ga [11]). However, in the present suite there is a lack of 3.1 Ga and 3.8 Ga old zircons which were reported by [11] from the Jack Hills metaconglomerate as rare components. This lack may be due to the low probability of analysing rare components given the limited number of zircons analysed by the evaporation method. The generally flat trends of the 207/206 ratios throughout the evaporative records, especially the constant 207/206 ratios of one third of the grains, support the existence of concordant zircon phases in the present population. This agrees well with the SHRIMP zircon data which mainly plot on
292 the Concordia line in the conventional Concordia diagram. 6.2. Relations between zircon morphology and Pb isotope characteristics Whereas it is not possible to divide the analysed grains into clear-cut subpopulations by mere morphological arguments, the relationship of certain morphological trends to the age groups sets boundary conditions to the age models of zirconogenesis. The existence of relics of crystal faces is a characteristic of all zircon subgroups (Fig. 2). This even holds true for the oldest zircons. The degree of crystal face corrosion and rounding of the grains cannot be clearly attributed to the different groupings. So far ovoids have not been observed for the 3.39 Ga group. This group is characterized by grains with rather well-preserved crystal faces and a mild rounding of the crystal edges. Most grains have surfaces reflecting light poorly. In contrast to the youngest group (207/206 ~< 0.275), which seems to be free of inclusions, the 3.39 Ga group zircons often contain small inclusions. Older zircon inclusions or older zircon cores are not present in these grains. Their evaporative records indicate a single (identical) crystallization event. There is no characteristic feature that distinguishes the > 4 Ga zircons from younger ones. The very old zircons also have inclusions and exhibit varying degrees of crystal rounding and surface corrosion. A relationship exists between the translucence of the grains and the 207/206 ratios (Table 1). Most of the crystals that yielded constant 207/206 ratios throughout the runs (dotted lines in Fig. 3) are characterized by good translucence. This suggests that increased opacity of grains is related to the presence of zircon domains with younger Pb components. 6.3. Phase associations in individual zircons Grain-by-grain statistics of the present study as well as of the previous S H R I M P analyses [11] show that complex U / P b isotope patterns have to be expected for each individual crystal in the zircon population of the Jack Hills metaconglomerate. Heterogeneous Pb isotope compositions in single grains have been reported for some of the very old zircons from the metaquartzite of
the Mt. Narryer [10] and from the Jack Hills metaconglomerate [11]. Compston and Pidgeon [11] concluded that variations in 207/206 ratios found by repeated S H R I M P analyses of different small areas on the same grain are due to internal redistribution of Pb and U or to loss of Pb during some early Archean event. They reported cases where analyses of zircon areas had significantly different U / P b ratios but the same 207/206 ratios, which was interpreted as recent differential Pb loss related to variations in the properties of the zircon areas. Accordingly, the crystals can be treated as an association of different zircon phases with differing response to the milieu conditions of the host reservoir. This is important in interpreting observed trends in 207/206 ratios of the evaporative records. Regular increases of 207/206 ratios (Fig. 3) can be attributed to the mixing of Pb from different zircon phases or domains with differing Pb isotope compositions during the evaporation in the ion source. The relative contribution of radiogenic Pb from the evaporated domains is systematic in that apparently "younger" domains release their Pb at slightly lower temperatures than "older" ones. Consequently the "younger" phases get exhausted somewhat earlier, and the last evaporative steps are dominated by radiogenic Pb released from "old" domains. The systematic patterns of Pb mobilization from the intergrown zircon phases characterize the differences in the activation energies necessary to remove the Pb isotopes from the respective host phase. The properties and structural character of the intergrown zircon phases can be discussed in the light of the behaviour of the U / P b system. Two explanations are suggested (cf. [8]). The first involves mixing of Pb from domains which have been open U / P b systems, with Pb from isotopically undisturbed (concordant) phases. Fission track, ion microprobe and other studies have shown that zircons from a number of rock types have a heterogeneous distribution of uranium and thorium (e.g. [2,14-16]). The structure of such zircons can be viewed as a matrix of domains with variable amounts of radiation damage [17]. Pidgeon et al. [18] showed that, other factors being equal, the susceptibility of a zircon to Pb loss is proportional to its radiation damage. During heating under metamorphic conditions radiation
293 damaged zircon can undergo annealing, possibly accompanied by Pb loss. At again lower temperatures radiation damage once again accumulates in the zircon and, as before, is enhanced in the high-uranium and -thorium domains. This is often seen as cloudy or opaque patches or zones within the zircon crystals. When the zircon is heated during mass spectrometric analysis the first Pb to be emitted is the Pb with low 207/206 ratios from the radiation damaged, high-uranium and -thorium domains. As higher filament temperatures are reached Pb from less damaged domains is emitted, culminating in emission of concordant Pb from domains which have experienced minimal radiation damage throughout their history. A second possibility is that the observed trends in the evaporative records are due to mixing of different radiogenic Pb components released from concordant crystalline domains with divergent ages (cf. [19,20]). A supporting observation is that the emission temperatures for Pb from apparently "older" and " y o u n g e r " phases are similar (1690-1800 K). The presence of different concordant phases can be explained by a variant of the radiation damage model. In this model initial zircon crystallization was followed by radiation damage in particular domains which later on was annealed during an Archean metamorphic episode, thus generating "younger" zircon phases by partial or complete resetting of the U / P b isotopic clock in these domains. Generation of apparently younger phases by metamorphic recrystallization is supported by the 208/206 ratio trends of the evaporative records (see below). This process is thought to happen also on a submicroscopic scale forming a patchwork of micro-domains (note the mentioned relation of crystal translucence to the presence of younger Pb components). It is essential for this mixing model to explain the observation that the apparently "younger" phases release their radiogenic Pb at slightly lower temperatures than the "older" phases. This can be interpreted by slightly lower Pb activation energies of the recrystallized domains possibly due to larger amounts of incorporated trace elements compared to the zircon domains which release Pb at further increased evaporation temperatures. A synthesis of the two models may best describe the observations. The zircons with increasing 207/206 ratios in the evaporative records
should be looked at as an association of concordant phases with divergent ages on the one hand, and discordant phases on the other hand. The pattern of 207/206 ratios derived by Pb evaporation at temperatures of 1700-1800 K is strongly controlled by the different Pb activation energies of the intergrown phases. 207/206 ratios of phases with the highest activation energies, which release their Pb during the final evaporation steps, are closest to the crystallization age of the original (primary) zircon lattice. 7. Geological constraints The present evaporation data can be used to discuss the nature of the unknown source rocks and models of the predepositional evolution of the detrital zircon population. The age patterns (Fig. 4), the regular trends in the evaporative records (Fig. 3), and the specific Pb isotope distributions in the 207/206-208/206 diagram (Fig. 5) con-
I
4,2
i
T
i
4,0
I
3,8 3,6 3,4
J
I I I i • 9,,--'-~,
..... ~.-'"
~e
l ............
I i e"--..l
• •
,L'.._.._'.... -" ......... ".;I eo,e • e e l -
C l "-~"
3,2
I
I
•
I I I
e.e
I I
~--~vo[conics .,.fonolifes____, I grnnodiorit~s ' 0,1
ee6~
I
e-~
' i I
'3
I
I
I 0,2
i~
gronulifes__ ~n,~e~ 0,3
O,L,
--208/206----~ Fig. 5. Plot of 207/206 ages versus 208/206 ratios. The mean ratios of the respectivelast evaporativesteps after stabilization are taken. The 208/206 ratios characterize the mean Th/U ratios in the grains. To suggest possible source rocks for the zircons, the 208/206 ratios are compared with characteristic 208/206 ratios for various Archean rock types proposed by [21]. Under this scheme most of the zircons can be attributed to tonalitic or granodioritic intrusives or greenstone belt volcanics. The 3.39 Ga subgroup may contain zircons which have been generated under high- or ultra-metamorphicconditions.
294 strain models of the early Archean history of zirconogenesis and crust formation. The availability of 208/206 ratios greatly assists the interpretation of the 207/206 ratios. The 208/206 ratio effectively measures the T h / U ratio of the zircon. Related changes in the Pb isotope ratios may indicate mixing of intergrown zircon phases with different ages and T h / U ratios. In addition the 208/206 ratios help to identify and to characterize subgroups in the zircon population. These ratios can also be used to constrain possible Archean source rocks which supplied zircons to the conglomerate ([21], see below). The 208/206207/206 correlation diagram in Fig. 3 demonstrates the following trends: Flat 207/206 ratio trends (dotted lines in Fig. 3) for the range of evaporation temperatures, are usually accompanied by only small changes in the 208/206 ratios (order of 10%). Because the 208/206 ratio is directly proportional to the T h / U ratio this indicates rather uniform T h / U ratios of the different domains of the respective crystals. They can be taken as being rather homogeneous and isotopically undisturbed. Changes in the 207/206 ratios found in the temperature range of 1700-1750 K in many cases are accompanied by only minor (10%) changes in the 208/206 ratios, comparable to the zircons with plateau-like 207/206 records. It follows that, although uranogenic Pb components of different isotope compositions occur in individual grains, these different uranogenic components seem to be hosted in domains with similar T h / U ratios in the respective crystal. This appears to rule out possible growth of zircon rims around older cores during later Archean metamorphic events because changes of growth conditions can be expected to be usually accompanied by changes of the T h / U ratios [21]. Instead, the present results favor either zircon recrystallization without severe change of the T h / U ratios, or Pb loss due to distillation under extreme metamorphism (T>~ 900-1000 K, [22]). In both the 4.1 Ga and the 3.39 Ga subpopulations crystals occur whose 208/206 ratios deviate from the general range of 0.10-0.25 (Table 1 and Fig. 5). Two of the five very old grains yield 208/206 ratios of approximately 0.05-0.06 for the first steps of the high-temperature evaporative record. This suggests that the strong changes of -
-
-
-
-
-
207/206 found for the first evaporative steps of these old grains may be due to (locally) enhanced contents of U and the influence of radiation on the stability of the crystalline zircon lattice. A significant number of zircons from the 3.39 Ga group yield higher 208/206 ratios (>~ 0.3) than those found in the other subgroups. The large range of 0.1-0.4 in the 208/206 ratios of the 3.39 Ga group is in agreement with the SHRIMP data [11]. The special patterns of T h / U ratios of this subpopulation may point to milieu conditions of zircon generation not experienced by the rest of the zircon population. The significance of zircon 208/206 ratios, as indicators of the parent rock, has been proposed for zircons from a number of Archean rock types [21]. On the basis of this broad scheme, the spread of 208/206 ratios from 0.10 to 0.19 for zircons from the > 4.0 Ga group suggests tonalitic to granodioritic plutonic rocks, or possibly volcanic parent rocks. The large range of 208/206 ratios of 0.1-0.4 for the 3.39 Ga zircon subgroup comply with a number of possible source rocks (Fig. 5). The approximately 3.55 Ga old zircons may indicate plutonic rocks in the source rock assemblage. In the case of the 3.50-3.43 Ga old zircons and of the zircons younger than 3.39 Ga volcanic parent rocks appear the most likely. It should be noted, however, that from the morphology of the zircon population and from the low uranium and thorium contents of the grains (mean [U] - 100 ppm, mean [Th] = 50 ppm) Compston and Pidgeon [11] proposed mafic source rocks which had experienced granulite facies metamorphism. It is significant that, although the 3.39 Ga event is obviously the most important crystallization event documented by the detrital population as a whole, the " y o u n g e r " Pb components of the grains which are older than 3.4 Ga cannot be simply attributed to the 3.39 Ga event. Two of the respective grains even yielded "first step" 207/206 ratios as low as 0.27 ( = 3.31 Ga). The lack of a clear 3.39 Ga signature in the Pb isotopic spectrum of the > 3.4 Ga zircons suggests that the zircon generation at 3.39 Ga must have taken place in an environment different from that of the > 3.4 Ga zircons. Only after 3.39 Ga could the different generations of zircons have been admixed to form the total population. This is in agreement with the conclusions of Compston and Pidgeon [11] from
295 the trends found in the SHRIMP data. They assumed the 3.35 Ga zircons identified in their data to predate the sedimentation episode, and they proposed the two 3.1 Ga zircons found in their zircon suite to set an upper age limit for the deposition of the conglomerate. As discussed above, the large variations in the 208/206 ratios of the 3.39 Ga subgroup compared to the rest of the population suggest a variety of rock types formed 3.4 Ga ago. It is not yet possible to determine whether the 3.39 Ga zircons have been generated under metamorphic or magmatic conditions. However, the frequency of zircons of the same age, and the presence of many relics of well-defined crystal faces suggests zircon generation at least partly under ultrametamorphic conditions. Taking into account the assumption of separate zircon sources (s.a.) the presence of "secondary" [21] zircons in the 3.39 Ga group can therefore most easily be modelled by generation of zircon in granitic melts which intruded the rest of the source rock assemblage 3.39 Ga ago. Alternatively, tectonically emplaced high-grade metamorphic rocks which were metamorphosed at ca. 3.39 Ga may be assumed. The different constraints on the unknown source rock assemblage can be combined to a simple speculative model of the rock suite which supplied zircons to the sediment: the oldest rocks in the 3.55-3.30 Ga rock sequence are tonalitic to granodioritic intrusives (orthogneisses). Greenstone belt volcanics have been formed during different episodes over the whole time span of the 3.55-3.30 Ga major cycle of crust formation. Part of the rock assemblage was intruded by granites at approx. 3.4 Ga. Based on the data, the presence of granulites generated at 3.4 Ga is conceivable. The youngest zircon phases or domains found in the present study yielded 207/206 ages of 3.25 Ga. This implies that post-depositional geological events like the greenschist-to-lower-amphibolite facies metamorphism younger than 3.1 Ga, the intrusion of granitoids at 2.7 Ga, shearing and pegmatite formation at 2.5-2.4 Ga [23], did not generate essential amounts of new zircon or cause the severe disturbance of the zircon U / P b systems in the Jack Hills metaconglomerate. The significant differences in the ages of the five very old zircons suggest that zircon generation more than 4 Ga ago was also a poly-episodic
event. However, it cannot be excluded that (analytical) phase mixing a n d / o r geological resetting of the U / P b systems have influenced the respective final 207/206 ratios of the ancient grains. The data distribution presented in Fig. 3 clearly indicates the presence of rejuvinated zircon domains in four of the five ancient crystals. Rejuvination of zircon domains (recrystallization or partial Pb loss due to metamictization) can be attributed to the influence of the 3.55-3.30 Ga crust-forming events during the recycling of the source rocks of the > 4 Ga zircon subpopulation. The minimum age for the zircon generation is estimated to be approximately 4.2 Ga, which is in good agreement with the measurements of [11]. Present results indicate that this first cycle of mineral formation was completed about 4.07 Ga ago. The presence of these very old zircons in a detrital population which mainly has been generated during the major cycle of crust formation at approximately 3.55-3.30 Ga demonstrates the ability of zircon to survive also early crustal processes.
8. Summary (1) Results from zircon step heating analyses identify two subpopulations within the detrital zircon suite from the Jack Hills metaconglomerate with ages of 4.17-4.07 G a and 3.35-3.3 Ga. (2) The results are in good agreement with previous S H R I M P investigations [11] demonstrating that the Pb evaporation method is a valuable technique for acquiring quantitative age information. It has an important potential for geochronological investigations of the earliest Archean crust. (3) In addition, the special features of the evaporation technique allow the study of complex mixtures of zircon phases associated in a single grain--as either intermixed phases of different lattice structures (crystalline/metamict), or as intermixed crystalline zircon domains of different physicochemical properties. (4) The investigated zircon population is composed of a complex mixture of zircons generated in different environments with differing Archean evolution. The subpopulations must have been transfered to the metaconglomerate during a sedimentation episode less than 3.3 Ga ago.
296
(5) Confirmation of the existence of ancient zircons with ages > 4 Ga substantiates models of formation of terrestrial protocrust during very early stages of the evolution of the earth. The results indicate an episode of minerogenesis from approximately 4.2 Ga to about 4.07 Ga.
9
10
Acknowledgements 11
R.T.P. gratefully acknowledges the University of Heidelberg for support as a visiting Professor in 1986. Geological studies in the Jack Hills were supported by the Australian Research Grants Scheme. F. Begemann and two anonymous reviewers made valuable comments on the original manuscript, and helped to improve the presentation. We acknowledge the assistance of D. Pingel preparing the filaments, and of K. Schacherl preparing some of the photographs.
12
13
14
15
References 16 1 J. Lancelot, A. Vitrac and C.J. All6gre, Uranium and lead isotopic dating with grain-by-grain zircon analysis: a study of complex geological history with a single rock, Earth Planet. Sci. Lett. 29, 357-366, 1976. 2 U. Sch~irer and C.J. All6gre, Determination of the age of the Australian continent by single-grain analysis of Mt Narryer metaquartzite, Nature 315, 52-55, 1985. 3 J.R. Hinthorne, C.A. Anderson, R.L. Conrad and J.F. Lovering, Single-grain 2°7pb/2°6pb and U / P b age determinations with a 10/~m spatial resolution using the ion microprobe mass analyser (IMMA), Chem. Geol. 25, 271-303, 1979. 4 W. Compston, I.S. Williams and C. Meyer, U-Pb geochronology of zircons from lunar breccia 73217 using a sensitive high mass-resolution ion microprobe, J. Geophys. Res. 89, Suppl. B, 525-534, 1984. 5 C. Kosztolanyi, Nouvelle methode d'analyse isotopique des zircons a l'6tat natural apr6s attaque directe sur le filament, C. R. Acad. Sci. Paris 261, 5849-5851, 1965. 6 A.P. Chukhonin, A mass spectrometric study of the forms taken by lead in zircon, Geochem. Int. 15, 186-189, 1978. 7 R.V. Gentry, T.J. Sworski, H.S. McKown, D.H. Smith, R.E. Eby and W.H. Christie, Differential lead retention in zircons: implications for nuclear waste containment, Science 216, 296-297, 1982. 8 B. Kober, Whole-grain evaporation for 2°7pb/2°6pb-age-
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23
investigations on single zircons using a double-filament thermal ion source, Contrib. Mineral. Petrol. 93, 482-490, 1986. B. Kober, Single-grain evaporation combined with Pb + emitter bedding for 2°7pb/2°6pb-age investigations using thermal ion mass spectrometry, and implications to zirconology, Contrib. Mineral. Petrol. 96, 63-71, 1987. D.O. Froude, T.R. Ireland, P.D. Kinny, I.S. Williams and W. Compston, Ion microprobe identification of 4,100-4,200 Myr-old terrestrial zircons, Nature 304, 616-618, 1983. W. Compston and R.T. Pidgeon, Jack Hills, a further occurrence of very old detrital zircons in Western Australia, Nature 321,766-769, 1986. R.D. Gee, J.L. Baxter, S.A. Wilde and I.R. Williams, Crustal development in the Arehean Yilgarn Block, Western Australia, in: Archean Geology, J.E. Glover and D.I. Groves, eds., Spec. Publ. Geol. Soc. Aust. 7, 43-56, 1981. J.L. Baxter, S.A. Wilde, R.T. Pidgeon and I.R. Fletcher, The Jack Hills Metasedimentary Belt: an extension of the early Archean terrain of the Yilgarn Block, Western Australia (abstract), 7th Aust. Geol. Cony., 1984. P.E. Zaun and G.A. Wagner, Fission-track stability in zircons under geological conditions, Nucl. Tracks 10, 303-307, 1985. T.M. Harrison, J.N. Aleinikoff and W. Compston, Observations and controls on the occurrence of inherited zircon in Concord-type granitoids, New Hampshire, Geochim. Cosmochim. Acta 51, 2549-2558, 1987. W. Compston and A. Kr/3ner, Multiple zircon growth within early Archean tonalitic gneiss from the ancient gneiss complex, Swaziland, Earth Planet. Set. Lett. 87, 13-28, 1988. H.D. Holland and D. Gottfried, The effect of nuclear radiation on the structure of zircon, Acta. Crystallogr. 8, 291-300, 1955. R.T. Pidgeon, J.R. O'Neill and L.T. Silver, Uranium and lead isotopic stability in a metamict zircon under experimental hydrothermal conditions, Science 154, 1538-1540, 1966. R.H. Steiger and G.J. Wasserburg, Systematics in the 2°8pb-232Th, 2°7pb-235U, and 2°6pb-238U systems, J. Geophys. Res. 71, 6065-6090, 1966. R.H. Steiger and G.J. Wasserburg, Comparative U-Th-Pb systematics in 2.7× 109 yr plutons of different geological histories, Geochim. Cosmochim. Acta 33, 1213-1232, 1969. E.V. Bibikova, The most ancient rocks in the USSR territory by U-Pb data on accessory zircons, in: Archean Geochemistry. A. Kri3ner, G.N. Hanson and A.M. Goodwin, eds., pp. 235-250, Springer-Verlag, Berlin, 1984. J. Sommerauer, Die chemisch-physikalische Stabilit~it natiirlicher Zirkone und ihr U-(Th)-Pb System, Thesis ETH Ziirich, No. 5755, 151 pp., 1976 (unpublished). R.T. Pidgeon, W. Compston, S.A. Wilde, J.L. Baxter and L.B. Collins, Archean evolution of the Jack Hills Metasedimentary Belt, Yilgarn Block, Western Australia, Terra Cognita 6, 146, 1986.