U-Pb Reverse Discordance in Zircons: the Role of Fine-Scale

Oscillatory Zoning and Sub-Micron Transport of Pb

James M. Mattinson, Cinda M. Graubard, David L. Parkinson, and William C. McClelland

Department of Geological Sciences, University of California, Santa Barbara, California

U-Pb reverse discordance in zircons has been noted for some twenty-five years, first in conventional zircon studies, more recently in ion microprobe zircon studies. In contrast to the well-understood reverse discordance effects in whole-rock samples, lunar samples, and Th-rich minerals such as monazite, reverse discordance in zircons has been poorly understood. Some studies even cast doubt on its existence as a natural phenomenon, suggesting instead that it is an analytical artifact of some kind. Step-wise dissolution experiments on zircons clearly reveal that some zircons include relatively low-U zones that contain "excess" or "parentless" radiogenic Pb, resulting in local reverse discordance. The isotopic systematics require that this excess Pb comes from redistribution of radiogenic Pb within grains (i.e., from adjacent high-uranium zones), rather than by additon of Pb from outside sources. We differentiate between two cases: "internal reverse discordance", where some parts of the zircon grain are reversely discordant, but individual grains and multi-grain populations are normally discordant or even concordant; and "external reverse discordance", where individual grains and entire populations are reversely discordant. We suggest that internal reverse discordance results entirely from internal redistribution of Pb, accompanied or followed only by minor Pb loss from the zircon grains as a whole. In contrast, external reverse discordance involves subsequent removal of the more radiation-damaged, trace-element-rich, high- U "parent" zones by corrosive hydrothermal or metamorphic fluids, or by chemical weathering. This leaves the residual zircon population enriched in the low-U, reversely-discordant zones, producing overall reverse discordance for the population. These phenomena evidently are intimately related to fine-scale oscillatory zoning of trace elements (in particular, U) in zircons. We suggest that self-organizational, disequilibrium crystallization processes are responsible for episodic crystallization of zircon. The interaction of this mode of crystallization with factors such as differences between Zr and U diffusion rates, partition coefficients that depend on the stage of crystallization, and coupled substitution of U6+ with REE's in the 3+ state, virtually require the fine-scale (microns to sub-micron) interlayering of high-U and low-U zones. Redistribution of radiogenic Pb on the scale of as little as hundredths to tenths of microns, by diffusion, alpha-recoil implantation, or both, can then produce the observed reverse discordance effects.

1. INTRODUCTION

Reversely discordant U-Pb ages are those for which the 206Pb*/238U age exceeds the 207Pb*/206Pb * age; the reverse of the much more common "normal" discordance.

Reverse discordance is common in young monazites, in which it is clearly related to the incorporation, during

Earth Processes: Reading the Isotopic Code Geophysical Monograph 95 Copyright 1996 by the American Geophysical Union

355

crystallization, of excess amounts of 230Th [Sharer, 1984; Parrish, 1990]. The 230Th is an intermediate daughter isotope in the 238U decay chain, and is incorporated preferentially, relative to U, in Th-rich minerals such as monazite. The subsequent decay of the excess 230Th results in an excess of 206Pb* in the mineral. This both slightly increases the apparent 206Pb*/238U age, and decreases the 207Pb*/206Pb * age, producing the reverse discordance. For Tertiary and even many Mesozoic monazites, the excess 206Pb, is sufficient to produce negative or "future" 207Pb*/206Pb * ages.

Reverse discordance is also well documented for

356 U-Pb REVERSE DISCORDANCE IN ZIRCONS

rock samples and a variety of lunar samples. In the case of whole-rock samples, much of the U is evidently loosely held along grain boundaries and is readily leached from the rock by chemical weathering, resulting in reverse discordance [e.g., Rosholt et al., 1973]. In the case of lunar samples, reverse discordance stems from the volatility of Pb. Highly radiogenic Pb, originally generated in situ in a host rock, is volatilized during impact events, then re- condenses in lunar soils, breccias, etc., as excess

"parentless" Pb [e.g., Silver, 1970; Nunes et al., 1973; Church and Tilton, 1974]

Reverse discordance is reported only rarely for zircons, however, and its causes are much less clear. For example, as pointed out by Martinson [1973], disequilibrium inter- mediate daughter product effects should actually produce slight normal discordance in zircons. This is due to preferential exclusion of 230Th, relative to U, from the zircon structure, the opposite of the effect documented in monazite. Preferential removal of U relative to Pb seems

unlikely since U is chemically bound in the zircon lattice, whereas Pb is an uninvited radiogenic "guest" occupying alpha-recoil damaged sites. Certainly impact/volatilization mechanisms are not applicable under "normal" terrestrial geologic conditions.

Nevertheless a variety of explanations have been proposed for zircon reverse discordance, ranging from radiogenic Pb addition, to U-loss, to analytical artifacts. We summarize these ideas in the next section. We then

present evidence for naturally occurring reverse dis- cordance in Cretaceous and Proterozoic zircon samples. Next, since we believe that reverse discordance is intim-

ately linked to fine-scale oscillatory zoning of U in zircons, we present a model for zircon crystal growth and the development of zircon oscillatory zoning. Finally, we suggest a mechanism for both "internal reverse dis- cordance" (the presence within individual zircon grains of reversely discordant zones, even though the grain as a whole might be concordant or normally discordant), and "external reverse discordance" (where the entire grain or zircon population is reversely discordant).

2. BACKGROUND

2.1. Reverse Discordance: "Conventional" Studies

Reverse discordance in zircon U-Pb ages has been noted since the early years of "conventional" U-Pb zircon geo- chronology (i.e., analyses utilizing chemical dissolution, isotope dilution, and thermal ionization mass spectrometry). Several such "conventional" studies are summarized below. Tilton et al. [1970] report reversely

discordant ages for zircon from two samples of paragneiss from the Baltimore Gneiss complex in the central Appalachian Piedmont. They suggest a possible relationship between the reverse discordance and a subsequent metamorphic event. Higgins et al. [1977] discuss the Tilton et al. [1970] data as well as additional reversely discordant data from the "Columbia Granite". They propose that uranium loss during metamorphism caused the reverse discordance in these samples.

In a study primarily focussed on U-Pb dating of apatite, Oosthuyzen and Burger [1973] report reversely discordant ages for zircons from two granitic rocks in a terrain surrounding the Barberton Mountain land in South Africa. These results stand in sharp contrast to most of their other zircon data, which range from near-concordant to strongly normally discordant. The authors fail to offer an ex- planation for the reversely discordant zircon data. In fact they make no comment on the reverse discordance at all.

Stern et al. (1981), in rather extensive study of plutonic rocks of the Sierra Nevada, report U-Pb zircon ages for 62 samples. Remarkably some 60 percent of these zircon ages are reversely discordant. The authors suggest that this widespread apparent reverse discordance is an artifact that stems from "the extreme sensitivity of the 207Pb/235U age to the corrections for common lead." Evidently they believed that their assumed isotopic composition for initial Pb in the zircons was grossly in error. Indeed the apparent reverse discordances do seem to stem from the reported 207Pb*/206Pb* ages being "too young". Stern et al. [1981] do not actually report 207Pb*/206Pb* ages, only 206Pb*/238U and 207Pb*/235U ages, but recalculation of their data shows that many of the 207Pb*/206Pb, ages are unrealistically young, even negative. It is also clear, however, that many of these results lie well outside the range that could be reasonably explained by any problems with the isotopic composition chosen for the initial Pb correction. A subsequent detailed study by Chen and Moore [1982] on zircons from Sierran plutonic rocks revealed no reversely discordant zircons. This confirms that most if not all of the Stern et al. (1981) reversely discordant results are in fact analytical artifacts, possibly related to problems measuring the small 204pb peak or the 207Pb peak.

Pin and Lancelot [1982] report several reversely discordant fractions of zircon from metagabbro from the French Massif Central. They attribute the reverse discor- dance to analytical U losses during sample dissolution. The low-U zircons were difficult to dissolve, and "...the

microbombs were opened several times to check the dis- solution progress." The authors do not specifically state it, but presumably they infer loss of U as volatile UF6

the repeated openings of the microbombs. The slight normal discordance for subsequent single crystal zircon analyses using 205Pb spike added prior to digestion, and using more concentrated HF for the dissolution lends credence to their explanation.

Some more recent reported examples of reverse dis- cordance are difficult to dismiss as analytical artifacts, however. For example Aleinikoffet al. [1993] report slight reverse discordances for most of the zircon fractions

analyzed from two separate samples of granodiorite of the Mount Evans batholith in the Colorado Front Range. Completely independent analyses of different samples from the Mt. Evans batholith by Graubard and Mattinson [1990, and discussed later in this paper] also include some reversely discordant fractions, confirming the results reported by Aleinikoff et al. [1993]. Aleinikoff et al. [1993] propose that the observed reverse discordance is the result of addition of radiogenic Pb to the zircons. Their suggested mechanism is growth of radiogenic-Pb-rich inclusions within the zircons, presumably during a much younger hydrothermal event which brought in radiogenic Pb scavenged from the country rocks.

2.2. Reverse Discordance: "Shrimp" Studies

Since the early 1980's a large amount of U-Pb zircon geochronology has been generated by high-resolution ion microprobe, especially the remarkable "SHRIMP" inst- rument [Compston et al., 1982, 1984]. While the great majority of ion microprobe analyses appear to be con- cordant to normally discordant, a few studies have noted cases of reverse discordance. For example Williams et al. [1984] report a range of reversely discordant analyses from a single region within a 3,955 Ma zircon crystal. They conclude that the "excess" radiogenic Pb is internally derived, the result of redistribution of Pb within the zircon crystal during a later, ca. 2,500 Ma event.

Harrison et al. [1987] report SHRIMP spot analyses on nine grains of zircon from the Sunapee pluton in New Hampshire. One grain (#8) contains an intermediate zone with extremely high U concentrations (ca. 18,000- 25,000 ppm). Three spot analyses from this zone are reversely discordant. The authors attribute this to preferential loss of U from the radiation-damaged high-U zone. A second grain (#9) yields one very strongly reversely discordant SHRIMP spot analysis. This spot is much more reversely discordant, in terms of its position on the concordia diagram, than are the three grain 8 spots. Harrison et al. [1987] apparently attribute the reverse discordance in grain 9 to the same U-loss mechanism discussed in detail for the

very-high-U zone in grain 8, even though the reversely

MATTINSON ET AL. 357

discordant grain 9 spot has only ca. 500 ppm U, one- fortieth of the U in the grain 8 high-U zone.

Black et al. [1991] also report reversely discordant SHRIMP data for relatively high-U zircons, in this case from a felsic vein cutting a dolerite dike in East Antarctica. In contrast to the mechanisms suggested by Williams et al. [1984] and Harrison et al. [1987], Black et al. [1991] explain the reverse discordance as "an analytical aber- ration" related to a bias in the ion microprobe measurement of Pb and U in high-U zircons. They propose that relative Pb and U sputtering efficiencies in the zircon standard used in SHRIMP analyses are not an accurate representation of Pb and U sputtering efficiencies from high-U (damaged, metamict?) samples. Black et al. [1991] base their conclusions at least in part on the fact that some zircons that yield "almost exclusively" reversely discordant SHRIMP data, yield concordant or slightly normally discordant conventional results.

The above line of reasoning is followed up in detail in a recent study by McLaren et al. [1994]. These authors also suggest that reverse discordances observed with the ion microprobe are analytical artifacts. They propose that at least some zircons with strong radiation damage have recrystallized to a ZrO2 phase (baddeleyite) plus a silica glass phase, with the radiogenic Pb concentrated in the silica glass. They argue that the Pb is sputtered pre- ferentially from the mixture of baddeleyite and silica glass, yielding a reversely discordant result, even though the analyzed spot might be actually concordant or even normally discordant.

In summary, there is considerable dispute in the literature, based on both conventional and ion microprobe zircon analyses, as to whether or not natural reverse discordance really exists, and if so, what its mechanism might be.

3. ANALYTICAL TECHNIQUES

The present study is an outgrowth of many years of refining step-wise dissolution methods for zircon analysis [Mattinson, 1994, and references therein]. With the step- wise method, a series of partial dissolution steps is used to remove and analyze the more soluble parts of zircon grains, followed by total digestion of the remaining residue. This final digestion step is modified after Parrish [1987], utilizing Parr© bombs at 245øC. Parrish [1987] maintains that digestion for 30 hours at this temperature results in complete digestion of even "gem quality" zircons. Our experience with this technique supports Parrish's contention for the vast majority of zircon samples, although we have tended to adopt a more conservative approach,

358 U-Pb REVERSE DISCORDANCE iN ZIRCONS

using two to four days digestion time. However, even with this extended digestion time, we found that a very few samples were not quite completely digested. This was apparent after the fluorides from the HF digestion were dissolved in HC1 for ion exchange separation of Pb and U; a step where normally a completely clear solution is obtained. Obviously this was cause for concern, and we performed a separate additional four-day step to (a) complete the residue digestion, and (b) characterize the refractory material isotopically, to see whether or not failure to digest a highly refractory but minor part of zircon samples would adversely influence the U-Pb analyses and their interpretations.

The undigested residues were typically very small, compared to the original size of the zircon samples, and we considered various options as to how best they should be handled. Considering the difficulty of quantitatively handling the very small residues, we elected to pipette off typically 80 percent of the HC1 solution, leaving the remaining 20 percent plus the undigested residue behind for evaporation followed by redigestion. All of these samples used a mixed 205pb_235U tracer added prior to the original digestion, to ensure complete equilibration with the originally digested material. Thus the 80 percent aliquot of the solution characterized the zircon dissolved in the original residue digestion. Any differences in concentrations of U and/or Pb, and in the isotopic composition of Pb in the redigested 20 percent aliquot, would be entirely due to the additional contribution of U and Pb, if any, from the small refractory residue left after the first "total" digestion.

As we will detail below, the results from some samples were remarkable, indicating that the very small, refractory residues contained significant amounts of radiogenic Pb, but little or no U. We propose that the residues are zircon with extremely low U content; a result of primary igneous oscillatory zoning. The radiogenic Pb was generated in adjacent, high-U zones, then transported by diffusion or alpha-recoil implantation into the low-U zones. Con- sidering the very fine scale of oscillatory zoning in many zircons, transport need only be on the scale of hundredths to tenths of microns to produce the results observed.

4. RESULTS AND DISCUSSION

4.1. Cretaceous Sierran Pluton Sample

Five size-fractions of zircon from the Lake Harriet

granodiorite, Sierra Nevada batholith (part of an in- progress collaborative study between Mattinson and Drs. M. M. Lahren and R. Schweikert), were analyzed by step-

wise dissolution, following Mattinson [1994], as follows: 1. An initial dissolution step of 24 hours (80øC hot plate

in 50 percent HF plus a small amount of HNO3; mixed 205pb_ 235U tracer added prior to digestion) removed about 20 to 34 percent of the U and Pb from each zircon fraction. The 206pb*/238U ages for the five fractions range from about 104 to 108 Ma. The 206pb*/238U ages for the five fractions overlap with their respective 207pb*/206pb * ages (Table 1). This "concordance" is illusory, however: these initial steps, as is typical [Mattinson, 1994], contain significant amounts of common Pb, with resulting rather large errors on the 207Pb*/206Pb * ages. By comparison with the overall isotopic systematics discussed later, we infer that the 206Pb*/238U ages for the initial steps show the effects of minor Pb loss.

2. A second step of 24 hours (166øC oven in 50 percent HF plus a small amount of HNO3; mixed 205pb_ 235U tracer added prior to digestion) removed about 58 to 66 percent of the U and Pb from each fraction. The 206pb*/238U ages for the five fractions range from about 90 to 108 Ma. The 207pb*/206pb * ages have small errors, and range from about 114 to 117 Ma (Table 1). As discussed later, we infer that a possible very small inherited component contributed to the second step.

3. The third step was originally intended to be a total dissolution of the residue from the two partial dissolution steps (4 days, 245øC oven in 50 percent HF plus a small amount of HNO3; mixed 205pb _ 235U tracer added prior to digestion). At this stage only some 5 to 13 percent of the U in the original samples remained, and virtually all of it was removed during this step. With the exception of fraction 5, the third steps ranged from slightly normally discordant, to concordant, to slightly reversely discordant, with 206pb*/238U ages ranging from 102 to 118 Ma, and 207pb*/206pb* ages ranging from 107 + 3 to 111 + 1 (Table 1). The third step from fraction 5 is very strongly reversely discordant, with a 206Pb*/238U age of 233 Ma, and a 207pb*/206pb * age of 122 + 3 Ma. We believe that the well-constrained concordia upper intercept defined by these five steps (Figure 1) provides the best estimate of the igneous crystallization age for the granodiorite at about 110 Ma (UI = 110.1 + 0.9 Ma; LI = 22.2 + 5.9 Ma; MSWD = •.5).

4. For three of the fractions, a small residue remained

after the 3.1N HC1 dissolution for step 3. Normally at this stage the sample has been completely digested in HF, and the evaporated fluorides are easily and completely dis- solved in HC1 for ion exchange isolation of Pb and U. The presence of a residue indicated incomplete dissolution, an obvious cause for concern, as noted earlier. We decided to

attempt to analyze the small residues to determine

MATTINSON ET AL. 359

TABLE 1' Data for Lake Harriet and Mount Evans Zircons

Sample a Concentrations b Isotopic Compositions c Calculated Ages (Ma) d 238U 206Pb* 208/206 207/206 204/206 206*/238 207*/235 207*/206*

LH1 stepl 210.6 ppm 2.960 ppm 0.2320 0.08772 0.002671 103.8 104.7 124 (31) LH1 step2 672.9 9.790 0.1732 0.04996 0.0001122 107.5 107.8 114 (2) LH1 step3R 105.6 1.464 0.1721 0.05008 0.000129 102.3 102.6 109 (3) LH1 step4RR 26.4 0.5009 0.1864 0.05365 0.000364 139.7 138.3 114 (6) LH1 tot 1015.5 14.7! 0.1858 0.05800 0.000658 107.0 107.4 116 (8) LH1 tot- RR 989.1 14.58 0.1858 0.05815 0.000668 106.2 106.6 116 (8)

LH2 stepl 294.8 ppm 4.311 ppm 0.2266 0.07418 0.001768 108.0 108.1 110 (18) LH2 step2 761.8 10.457 0.1520 0.04979 0.0000994 101.4 102.0 115 (2) LH2 step3R 103.7 1.438 0.1588 0.04971 0.000106 102.4 102.6 107 (3) LH2 step4RR 25.96 0.7433 0.1646 0.05058 0.000146 209.8 202.7 121 (3) LH2 tot 1186.3 16.95 0.1726 0.05617 0.000537 105.6 105.9 113 (6) LH2 tot- RR 1160.3 16.21 0.!729 0.05643 0.000554 103.2 103.6 113 (6)

LH3 stepl 514.0 ppm 7.503 ppm 0.2750 0.07990 0.002156 107.8 108.0 111 (23) LH3 step2 873.0 12.167 0.1248 0.04923 0.0000589 103.0 103.6 117 (1) LH3 step3R 130.2 2.084 0.1301 0.04835 0.0000072 118.2 117.8 111 (1) LH3 tot 1517.2 21.75 0.1785 0.06001 0.000797 105.9 106.3 114 (9)

LH4 step 1 502.0 ppm 7.289 ppm 0.3021 0.09300 0.003038 107.2 107.7 119 (32) LH4 step2 886.0 11.608 0.1239 0.04943 0.0000726 96.9 97.6 117 (1) LH4 step3R 106.7 1.161 0.1299 0.04859 0.0000241 111.6 111.5 111 (1) LH4 tot 1494.7 20.06 0.1901 0.06544 0.001163 101.4 102.0 116 (12)

LH5 stepl 445.3 ppm 6.590 ppm 0.3229 0.10040 0.003548 109.2 109.5 114 (38) LH5 step2 883.3 10.751 0.1083 0.04900 0.0000441 90.0 91.0 116 (2) LH5 step3R 55.22 1.759 0.1325 0.04970 0.000084 233.1 223.3 122 (3) LH5 step4RR 13.84 1.001 0.1257 0.04973 0.000085 517.6 450.8 123 (2) LH5 tot !397.7 20.10 0.1849 0.06673 0.001252 106.3 106.7 116 (14) LH5 tot- RR 1383.9 19.10 0.1879 0.06760 0.001312 102.0 102.6 115 (14)

MEAl 365.8 ppm 73.48 ppm 0.1466 0.09217 0.000115 1345 1381 1437 (2) MEA2 606.2 124.2 0.1495 0.09111 0.0000477 1369 1395 1435 (2) MEA3 256.4 52.75 0.1722 0.09119 0.0000527 1374 1398 1435 (2)

MEB1 154.9 ppm 47.69 ppm 0.1864 0.11180 0.001490 1961 1724 1447 (8)

MEB2 st 1 136.8 ppm 41.36 ppm 0.1595 0.09510 0.0003214 1931 1705 1438 (2) MEB2 st 2R 33.98 7.86 0.1667 0.09524 0.000321 1527 1492 1442 (2) MEB2 st 3RR 8.89 4.038 0.1801 0.09432 0.000253 2732 2060 1442 (3) RR recalc (0.396) (2.070) 0.1928 0.09345 0.000188 12580 4405 1443 (6) MEB2 tot 179.7 53.26 0.1621 0.09506 0.000316 1899 1690 1440 (2)

MEB3 131.3 ppm 37.60 ppm 0.1424 0.09114 0.0000361 1842 1661 1439 (2)

aLH = Lake Harriet samples; ME = Mount Evans samples. Detailed sample descriptions and locations will be presented elsewhere. See text for analytical details for individual steps. "tot" designates mathematically recombined steps and residue (= original bulk zircon fraction if done in a single digestion step). b238U and 206pb* in partial digestion steps calculated using weight of the total sample to show their relative contribution to the total fraction mass balance.

cObserved isotopic compositions corrected for mass fractionation based on replicate analyses of NBS Pb isotopic reference standards, and corrected for 208pb, 207pb, 206pb, and 204pb in the 205pb/235U tracer. dAges calculated using 238U/235U = 137.88; decay constants for 238U and 235U = 1.5513 E-10 and 9.8485 E-10, respectively. Two-sigma uncertainties on 207pb*/206pb* ages following Mattinson [

360 U-Pb REVERSE DISCORDANCE IN ZIRCONS

0.0488-

0.0486-

0.0484-

0.0482-

0.048-

0.0478-

0.0476

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0.0472

140 •.

• 130 ':•ii•}iii::

"R" (Step 3) "• Fractions Only 100 '•::•;•.

iFUI = 110.1 + 0.9 Ma• 90% I LI: 22.2 +5.9 Ma! k.MSWD: 1.5 .2 .o

Lake Harriet Granodiorite, Sierra Nevadas

0 .... 1•0 .... 2•0 .... 3•0 .... dO .... 10 .... d0 .... 7'0 .... 8'0 .... 90 238U / 206Pb*

Fig. 1. Concordia diagram for Lake Harriet Step 3 (="R") zircon results. Ellipses represent two-sigma uncertainties for the 207/206 values, but are exaggerated for the 238/206 values. Regression line intercepts the concordia curve at an upper intercept of 110 Ma (inferrred to be the age of magmatic crystallization) and a lower intercept of ca. 22 Ma (see text for discussion of possible lower intercept significance). Intercept ages and uncertainties at the 95 percent confidence level are from the Isoplot program of Ludwig [ 1992].

isotopic systematics, and their impact, if any, on the overall isotopic mass balance of the zircon fractions.

Because of the very small amount, and the very fine- grained nature of the residues, we analyzed them by the technique mentioned earlier, and more fully detailed here. No new tracer was added. Instead, we carefully removed only 80 percent of the step 3 HC1 solution for the step 3 chemistry and mass spectrometry. The remaining 20 percent of the solution was left behind with the residue for evaporation, followed by an additional 4 days of digestion in HF at 245øC. The rationale is as follows: (1) if the residue were neither zircon nor any other U and Pb-bearing phase (i.e. if it were essentially an inert ingredient in terms of the U-Pb systematics), then the step 4 results would simply be duplicates of the step 3 results, albeit with only one-fourth as much U and Pb, and with a small additional

amount of blank Pb from the extra digestion step, (2) if, in contrast, the residue did contain U and radiogenic Pb and/or common Pb, then the step 4 results would reflect these additional contributions. Here we also see two

possibilities. On the one hand, the residue might simply have isotopic systematics similar or identical to those for step 3. In this case, we would simply observe slight increases in the total amounts of U and radiogenic Pb, but no significant difference in calculated ages. This further would suggest that such minor residues are generally

harmless, and could be safely ignored. On the other hand, if the step 4 residues were drastically different in their isotopic systematics from the zircon digested in step 3, then this would be clearly evident by the differences in the isotopic systematics and calculated ages between steps 3 and 4 for each fraction.

In fact the step 4 results are quite striking (Table 1; Figure 2). Comparison of the actual observed data from the Pb and U mass spectrometer runs reveals that the residues contain radiogenic Pb, but virtually no U. Even with the diluting presence of 20 percent of the step 3 zircon, which is slightly normally discordant for two of the three fractions which had residues after step 3, all of the step 4 results are reversely discordant. Based on an age of 110 Ma for the granodiorite, the step 4 206Pb*/238U ages range from 27 to 370 percent reversely discordant [here defined as 100 x (observed 206Pb./238U age- 110 Ma) / 110 Ma]. Three points are especially worth noting: (1) the 207Pb*/206Pb * ages increase with increasing degree of reverse discordance, suggestive of non-zero age redistribution of Pb as the cause of the reverse discordance, as discussed in more detail later, (2) the added Pb from the step 4 dissolutions is highly radiogenic, perhaps pure radiogenic Pb depending on the exact size of the additional blank from the step 4 digestion and chemistry, and (3) including the step 4 results in the regression with the step 3 results shown in Figure 1 does not produce any significant

0.0488

0.0486

0.0484

0.0482-

0.048-

0.0478-

0.0476-

0.0474-

140•. "TOT" and "TOT" - "RR" "•::•ii::•. Results, and typical error

LH 1 100

"RR" fractions 90 .... %• .... "RR" + "R" Fractions 8o

Reference chord is from Fig. 1

0.0472 Lake Harriet Granodiorite, Sierra Nevadas

0 .... 1•0 .... 2•0 .... 3•0 .... 4•0 .... 5'0 .... d0 .... 7•0 .... 8•0 .... 90 238U / 206Pb*

Fig. 2. Concordia diagram for Lake Harriet Step 4 (="RR") zircon results (solid ellipses); Step 3 + Step 4 (= "RR" + "R") combined zircon results (open ellipses); and total recombined results ("TOT") shown with "RR" results included (small solid triangles), and without "RR" results (small solid diamonds), with typical error for the total results shown as the shaded ellipse. See Fig. 1 caption for explanation of error ellipses. See text for further

changes in the upper or lower intercept ages (step 3's + step 4's: UI= 110.3+ 1.2Ma;LI= 18.1+_3.9Ma;MSWD=

1.7; this regression not shown in the figures). Mathematically recombining all of the partial dissolution

steps for each fraction [after Mattinson, 1994] provides a "total" or "bulk" result; the same as if the fractions had

been dissolved in a single step. All of the fractions yield very slightly normally discordant "total" results (Figure 2). This is true whether or not the step 4 results are included. The small amount of "parentless" radiogenic Pb included in the step 4 results is insufficient to significantly alter the mass balance of U and Pb in the "total" fractions, which is

dominated in each case by the large percentage of sample U and Pb contained in the step 1 and step 2 partial digestions.

Overall, the Lake Harriet granodiorite results indicate that the zircons comprise a predominantly igneous pop- ulation that crystallized at ca. 110 Ma. The slightly higher 207Pb*/206Pb * ages of the step 2 results suggest a very minor inherited component of older zircon that was removed by this step [cf. McClelland and Mattinson, 1994]. Subsequent to magmatic crystallization, small-scale internal redistribution of radiogenic Pb from high-U zones to low (or zero) U zones produced "internal reverse discordance". The non-zero lower intercept on concordia indicates that this redistribution was not a geologically recent event. The slightly normally discordant overall isotopic systematics for the "total" results indicate that the zircon population as a whole lost Pb subsequent to crystallization. Quite possibly this minor overall Pb loss and the internal redistribution of Pb were related to the

same events or processes, as considered in more detail later.

4.2. Proterozoic Mount Evans Batholith Sample

Three fractions of zircon from each of two different

samples of the Mount Evans batholith were analyzed by "conventional" one-step digestions [analyses by Graubard and Mattinson, 1990; part of ongoing dissertation research by Graubard]. Results are summarized in Table 1 and Figure 3. Prior to our work on the Mount Evans zircons, we were aware (via personal communications) that Aleinikoff and colleagues had obtained reversely discordant results for some Mount Evans zircon samples. They subsequently published these results in Aleinikoff et al. [ 1993].

Three fractions of zircon from our "sample A" were completely digested by the single-step conventional pro- cedure, and yielded slightly normally discordant results (Table 1 and Figure 3). Three fractions of zircon from our

MATTINSON ET AL. 361

i-:" Mt. Evans Samp. A 0.0928 0 Mt. Evans Samp. B1, B3

e Mr. Evans Samp. B2 Data Field From Aleinikoff 0.0918 et al., 1993

1490 .il • 1480

1470

1460

0.0908

• ß RR recalc 'RR • \

t •'•Ul = 1436.4 + 0.9 Ma• tot 1430 0.0898J I LI = 20.2 + 9.1 Ma / 1420

!•,,.MSWD = 1.4 1410 :+

Mt. Evans Granodiorite 0.0888

........ I ' ' ' I• ' ' ' I .... i .... I ........ I ........ 0 0'.5 1 '1. 2 2.5 3 315 4 4'.5 5 2381J / 206Pb*

Fig. 3. Concordia diagram for Mount Evans zircon results. Three fractions from Sample A are shown by the shaded error ellipses; two fractions from Sample B are shown by the open ellipses. The third Sample B fraction is shown in the solid ellipses: "TOT" represents the recombined results of all the digestion steps; "RR" represents the last digestion step; "RR recalc" represents the calculated position of only the final solid residue in the RR step. See Fig. 1 caption for explanation of error ellipses. The cross-hatched field is the field of zircon data from Aleinikoffet al. [1993] Figure 15C. Regression line (based on the three "A" fractions, "B3", and "B2 TOT") intercepts the concordia curve at an upper intercept of 1436 Ma and a lower intercept of ca. 20 Ma. See text for discussion. Intercept ages and uncertainties at the 95 percent confidence level are from the Isoplot program of Ludwig [ 1992].

"sample B" yielded strongly reversely discordant results (Table 1 and Figure 3). Conventional digestion of one of these fractions ("B2") left a small insoluble residue. As for the Mount Harriet zircons discussed earlier, we removed 80

percent of the HC1 solution for analysis, leaving behind 20 percent for evaporation and redigestion along with the residue ("R" in Table 1). In this case, a two-day, 245øC digestion step was used, but a very small residue still remained. Once again, most (75 percent) of the HC1 solution was pipetted off for analysis, leaving 25 percent to be evaporated and redigested with the residue. A four-day 245øC redigestion finally dissolved all of the remaining material ("RR" in Table 1 and Figure 3). This analysis included only five percent of the 205Pb_ 235U tracer originally added to the total zircon fraction prior to digestion.

The RR result is very similar to the step 4 results for the Mount Harriet samples discussed earlier, in that the RR step contains a large amount of "parentless" radiogenic Pb, compared to the previous R step. The RR step also contains a small additional contribution of U beyond

362 U-Pb REVERSE DISCORDANCE IN ZIRCONS

attributable to the R step. Subtraction of the Pb and U contributions fr•m the R step to the total RR step yields, by difference, the Pb and U in the final, least soluble residue. This is plotted in Figure 3 as "RR recalc.". The result is extremely reversely discordant, plotting very near 238U/206Pb* = zero. Recombining all of the steps for the B2 fraction yields the "TOT" result which plots close to the reversely discordant results for the other sample B fractions in Figure 3.

As noted earlier, Aleinikoff et al. [1993] also report some reversely discordant zircon fractions for two Mount Evans samples. The field of data for these two samples is shown in Figure 3. The data straddle concordia, with some normally discordant points and some reversely discordant points. Aleinikoff et al. [1993, p. 791] propose that the observed reverse discordance is a result of growth of "apatite, K-feldspar, and quartz inclusions" in the zircons during a Laramide (ca. 60 Ma) ore-forming event. They suggest that radiogenic Pb was scavenged from the country rocks and incorporated in these inclusions, causing "the scatter and reverse discordance in the data". This in-

terpretation is untenable for two main reasons. First, if parentless radiogenic Pb resided in apatite, K-feldspar, and quartz inclusions, it would manifest itself in the early digestion steps, not the last step(s). Apatite, K-feldspar, and quartz are all highly soluble minerals compared to zircon; they would not survive the initial main digestion step. Second, the parentless Pb in both the Mount Evans and the Lake Harriet samples is highly radiogenic. In fact, the observed 206Pb/204Pb ratio of the Mount Evans RR analysis is higher than that for the preceding R analysis, despite the smaller size of the RR sample and the inevitable additional blank from the digestion and column chemistry. Thus the parentless Pb component is virtually pure radiogenic Pb. This is a fatal blow for any model that derives the "excess" Pb from an external source. Fluids

passing through country rocks would inevitably scavenge large amounts of common Pb as well as radiogenic Pb. This would be reflected in relatively low 206Pb/204Pb ratios for any externally derived Pb in the zircons.

The data of Aleinikoff et al. [1993] 'also show minor scatter, and trend towards higher 207Pb*/206Pb* ages, as shown in Figure 3. The authors use this to support their hypothesis of scavenging of radiogenic Pb from the older country rocks. In light of the preceding discussion; however this scatter and trend are more likely the result of actual minor inheritance of older zircon xenocrysts in their samples, with superimposed reverse discordance effects, rather than the result of incorporation of older radiogenic Pb in young inclusions. Our Mount Evans sample A and B results show no such inheritance, but we do see clear

evidence of inheritance in other Proterozoic samples in the area (Graubard and Mattinson, unpublished data).

Overall, our Mount Evans results are consistent with an

igneous crystallization age of ca. 1436 Ma, in fairly good agreement with the ca. 1442 Ma age reported by Aleinikoff et al. [1993] for a different sample of the Mount Evans batholith that appears to lack complicating inheritance and reverse discordance effects. In contrast to the total zircon

results for the Lake Harriet zircons, the results from our

sample B Mount Evans zircons are strongly "externally reversely discordant". In common with the Lake Harriet zircons, the Mount Evans zircons contain a component with a significant amount of radiogenic Pb, but with little or no U to support it. The isotopic composition of the Pb in both the Lake Harriet and the Mount Evans zircons makes

it absolutely clear that the "excess" Pb is internally derived. This is supported both by the overall isotopic systematics, compared to the more soluble parts of the zircons, as well as by the virtually purely radiogenic nature of the Pb. Any infusion of Pb from outside the individual zircon grains would contain an obvious component of common Pb. The analyses are extremely sensitive to any such component, and totally rule out this possibility.

Why should the sample B zircons be reversely discordant whereas the sample A zircons are slightly normally discordant? We have no simple answer. However, the two samples come from different parts of the batholith. Sample B was collected near a fault zone that may have served as an important conduit for fluids. We suggest that dif- ferences in fluid access, volume, and possibly compositions are important factors in producing external reverse dis- cordance, as explored more fully later. In addition, the presence or absence of minor fractures in zircon grains, possibly related to the batholith's tectonic history, are probably also important factors. As pointed out by Mattinson [1994], such fractures provide conduits for fluids to attack the internal layers of zircons. The scale of such features might be below the level of the binocular microscope we use for hand-picking the zircon grains; however. Fo• example, we recorded the 20 hand-picked grains of the B 1 sample as "clear".

In summary, in this section we have presented clear evidence for both "internal reverse discordance" and

"external reverse discordance", linked to redistribution of

radiogenic Pb within zircon grains. In the following sections, we develop a model for zircon growth and oscillatory trace element zoning, which we believe is an important key to understanding reverse discordance, then we propose mechanisms for both internal and external reverse

MATTINSON ET AL. 363

5. ZIRCON ZONING AND CRYSTAL GROWTH

5.1. Zoning

Zoning in zircons is a well-documented phenomenon. Zircon is, of course, predominantly ZrSiO4, but zoning is produced by variations in the concentrations of trace elements that substitute for Zr in the zircon lattice structure.

These include Hf, U, Th, and the rare earths, among others [e.g., see Kdippel and Sommeraur, 1974]. Two patterns of zoning are common: "normal" core to rim zoning, and oscillatory zoning. Normal core to rim zoning, especially of U and Th, is readily understood in terms of the magmatic crystallization history and fractionation trends in zircon. Silver and Deutsch [ 1963] originally demonstrated the typical core to rim increase in U that characterizes most igneous zircon populations. This evidently results from the tendency of U to build up in the residual liquid of a crystallizing melt because of its exclusion from the major minerals. The incorporation of U into zircon is insufficient to overcome this trend, in terms of the overall mass balance

of the system. Thus igneous zircons typically display high- U rims and lower-U cores, and fine-grained zircons that began crystallizing late in the magmatic crystallization history have higher bulk U concentrations than do coarser- grained zircons that began crystallizing earlier.

Also very well-documented is oscillatory zoning in zircons. Oscillatory zoning is the fine-scale alternation between zones with higher trace element concentrations and zones with lower trace element concentrations, com-

monly with sharp boundaries between them. Such zoning can be observed optically, because the variations in trace element content affect refractive index either directly, or indirectly, via radiation-induced metamictization of the lattice [e.g., Klein and Hurlbut, 1993]. The zoning can also be observed with back-scattered electron (BSE) imaging, which is sensitive to high-atomic-number species (e.g., Hf, U, and Th) and which permits resolution on a sub-gm scale [e.g., see Wayne and Sinha, 1988; Paterson et al., 1989; Hahchar and Miller, 1993], and by cathodoluminescence, which is sensitive to some of the rare earth elements [e.g., see Vavra, 1990, 1994; Hahchar and Miller, 1993]. Zoning of the more abundant trace elements can also be measured directly by electron microprobe, albeit at a considerably lower resolution than BSE imaging [e.g., Hahchar and Miller, 1993]. Finally, fine-scale zoning and other details of zircon structure are also revealed by HF etching of polished sections through zircon grains [e.g., Krogh and Davis, 1974, 1975], and by scanning electron microscopy (SEM) of partially dissolved zircons [Figures 4

-. 0.

..

.

.

:.:

Fig. 4. Scanning electron micrograph of partially dissolved zircon grain showing fine-scale magmatic growth zoning. Total width of photograph is ca. 25 gm. Many individual zones are on the order of one gm or less in thickness. The zones are revealed by differences in solubility related to variations in accumulated radiation damage and trace element concentrations.

and 5; see also Todt and B•isch, 1981; and Mattinson, 1994].

Despite the abundance of observations of oscillatory zoning in zircon, there has been no comprehensive explanation of its origin. For example, Klein and Hurlbut [1993, p. ii], who show a spectacular example of an oscillatory-zoned Sri Lankan zircon on the cover of their book, say only that the oscillatory zoning "...is presumed to be the result of the crystal's having grown in a magma chamber with differing UO2 and ThO2 contents in various parts of the magma." Pigeon [ 1992, p. 463] is much more forthright: "Oscillatory zoning ... is generally believed to form during crystallisation of zircon from a magma by a process which is not completely understood."

5.2. Oscillatory Zoning in Zircons: a Model

While little serious attention has been given to oscillatory zoning in zircons, there is a substantial literature on oscillatory zoning in other minerals. In general,

364 U-Pb REVERSE DISCORDANCE IN ZIRCONS

Fig. 5. Scanning electron micrograph of a partially dissolved zircon grain. As for Fig. 4, note the very fine-scale zoning. Total width of photograph is ca. 25 gm. Note the quantitative "mining out" of sub-gm-thick zircon layers, presumably high in U and other trace elements, leaving behind the adjacent 1ow-U layers.

fall into two general categories: (1) those that appeal to either cyclical or unidirectional variations in physico- chemical conditions in the magma chamber, and their effects on the equilibrium of the growing crystal, and (2) those that appeal to "self-organization" [e.g., Nicolis and Prigogine, 1977; Ortoleva et al., 1987], in effect, the macro-scale interplay between the growing crystal and its thin local envelope of magma. Models of both types have been applied to major minerals, especially those such as plagioclase, which are solid solutions.

Models of the first type include cyclic changes in pressure owing to magma chamber convection [e.g., Carr, 1954], and periodic losses of volatiles from the magma chamber [e.g., Boone, 1959]. Anderson [1984] also appeals to a mechanism of the first type to explain oscillatory zoning in plagioclase from basaltic ash erupted from Fuego volcano, Guatemala. He proposes that decompression during the ascent of the magma controlled levels of supersaturation, and hence growth rates, while pulsations in the rates of magma flow (possibly triggered by tides) periodically "...sheared the boundary layer of melt

near the growing crystals...", thus bringing fresh magma into proximity with the crystal faces and allowing the growth of a single narrow, zoned layer with each pulse.

In sharp contrast to the above, Bottinga et al. [1966], in a classic paper, expands on a much earlier contribution by Harloff[1927], to explain oscillatory zoning in plagioclase via a mechanism of the second type: the interplay between crystal growth rates and the rates of diffusion of the species required for plagioclase crystallization, as well as those species rejected by the growing crystal. While mechanisms of the first type certainly might be adequate to explain rather small (few percent) variations in An content of plagioclase, they seem entirely inadequate to account for order-of-magnitude variations in trace element concen- trations in minerals like zircon. Accordingly, our proposed mechanism for oscillatory trace element zoning in zircon, discussed below, borrows heavily from the proponents of self-organization, such as Cahn [1960], the particularly lucid work of Bottinga et al. [1966], Chernov [1975], Ookawa [1975] and more recent proponents of geochemical self-organization such as Ortoleva et al. [1987], Wang and Merino [1992], and others. The first part of the discussion on zircon crystal growth, in particular, follows the Bottinga et al. [1966] discussion of plagioclase crystallization quite closely.

First, assume as a starting condition (1) an exact saturation level of zirconium in the melt "boundary layer" adjacent to a zircon crystal, and (2) an atomically flat or "smooth" zircon crystal face (sometimes referred to as "sharp"); i.e., one with no steps or defects to provide energetically favorable sites for crystallization. Under these conditions, the zircon will neither grow nor resorb. However, as crystallization of minerals that exclude Zr proceeds, driving up the Zr levels in the residual melt, and/or as Zr diffuses into the boundary layer from more enriched parts of the melt, Zr in the boundary layer will become supersaturated. Above some particular level, this supersaturation will provide the driving force necessary for two-dimensional nucleation on the flat ("sharp") crystal face. At that time, nucleation of a new layer of zircon on the old flat surface begins. To quote Bottinga et al. [1966]: "At this point growth commences by lateral movement of a step across the face, but since two dimensional nucleation on an incomplete surface is not much more difficult than on the originally smooth complete surface, the interface may soon extend through a large number of atomic layers. The interface becomes diffuse and now offers abundant steps, kinks, and traps. The problem of maintaining a sufficient driving force for continued growth ... is now alleviated by considering that the necessary driving force itself decreases as the interface becomes diffuse." In other words,

crystallization begins it will proceed fairly rapidly, even though the level of supersaturation in the boundary layer is dropping, because the diffuse or rough surface produced during the early stages of the crystallization cycle provides abundant favorable sites for continued crystallization. Then (again quoting): "Toward the end of the cycle the driving force decreases to the point where further growth is limited to only the most favorable sites and the rough face begins to repair itself. Eventually the face becomes sharp and further appreciable growth is precluded until diffusion again builds up the supersaturation necessary to repeat the cycle." Thus the interplay between the structure of the crystal face, the driving forces of crystallization, and the levels of critical elements in the boundary layer, provides a mechanism for periodic crystallization.

Applying the above reasoning to zircon growth, we suggest that each layer or zone is the product of (1) an initial "main growth phase" characterized by relatively rapid crystallization at high to moderate levels of Zr supersaturation and a diffuse interface, which grades into (2) a "repair phase" characterized by slower crystallization at low to very low levels of Zr supersaturation leading to an atomically flat or "sharp" interface, followed by (3) a period of repose, during which the system recovers. In the case of zircon, we can envision two different controls on

the saturation level of Zr in the melt, depending on the rate of diffusion of Zr. On one hand, if diffusion is relatively slow, the crystallization cycles will be controlled by how quickly Zr in the boundary layer can be replenished by diffusion from the adjacent, undepleted "shell" of melt. If, on the other hand, diffusion is relatively fast, such that the growing population of zircon crystals is drawing on a large proportion of the melt, then the entire volume of melt might be depleted in Zr at the end of an episode of crystallization. In this case, the next episode of zircon crystallization will not begin until Zr supersaturation in the melt has been restored by some amount of crystallization of major minerals that exclude Zr.

Next, we must consider the behavior of trace elements,

particularly U, during zircon crystallization. U4+ evidently substitutes for Zr4+ in the zircon lattice. The U ion has a

larger ionic radius than does the Zr ion [100nm for U4+ vs. 84nm for Zr4+, both in 8-fold coordination, Shannon,

1976]; thus in the sense of classical Goldschmidt geo- chemistry, U is "admitted" into the zircon lattice. In other words, U is "compatible", and partitions strongly into the zircon crystal phase. As a result, crystallization of a zircon layer or zone will result in the strong depletion of U in the adjacent melt. Thus one obvious possible control on U concentration within a zone stems from differences in the

MATTINSON ET AL. 365

relative diffusion rates of Zr and U in the magma, as discussed below.

Remarkably little is known about absolute diffusion rates in silicate magmas, but the larger, more massive U4+ ion presumably will diffuse more slowly than Zr4+. In fact, it is likely that both of these ions will be complexed (with SiO4) in the magma because of their high field strengths, making their diffusion much more sluggish, but still in the same relative relationship to each other. Under oxidizing conditions, U would most likely be in the +6 state in the magma, however, and much more highly complexed than Zr, which only occurs in the +4 state. In this case, we would expect very much slower diffusion for U, relative to Zr. Growth of a layer of zircon would thus be accompanied by a strong zonation in U, owing to the rapid depletion of U in the boundary layer during a zircon growth cycle. The period of repose between growth cycles would allow the diffusional replenishment of U and other trace elements.

A possible reinforcing mechanism for zoning of U, when it is present in the magma in the +6 state, partially follows the line of reasoning of Wang and Merino [1992] who studied oscillatory zoning of trace elements in calcite. Rapid crystallization during the "main growth phase" would result in not only the depletion of U, described above but also the build-up of rejected (incompatible) cations in the magma at and near the interface. This build- up of positive charge would tend to repel highly charged cations such as U6+ in the boundary layer, inhibiting their diffusion towards the interface. Such inhibition would

reinforce the zoning patterns related purely to relative diffusion rates. The period of repose would allow the diffusional dispersal of the rejected cations, along with the replenishment of the compatible species such as U, setting the stage for crystallization of the next zone.

A second possible major mechanism for strong zoning of U involves a dependence of the partition coefficient for U (and other trace elements) on the crystallization stage. We suggest that during the "main growth phase" at high supersaturation levels, high growth rates, and with a high density of favorable sites on the diffuse interface, the zircon structure is relatively "indiscriminate"; i.e., it readily admits a range of trace elements. In effect, the partition coefficients for the trace elements are relatively high during this phase. In contrast, during the later stages of the "repair phase" at very low supersaturation levels, very low growth rates, and with few favorable sites for attachment, the structure is much more selective. In effect, the partition coefficients for the trace elements are relatively low during this phase. In short, we propose that the normal concept of a single partition coefficient does not apply to

366 U-Pb REVERSE DISCORDANCE IN ZIRCONS

organized, disequilibrium, episodic growth. Instead, dif- ferent, perhaps greatly different partition coefficients apply, depending on the exact phase of crystal growth. Note that this second mechanism would tend to reinforce the zoning tendencies of the first mechanism.

Finally, a third major mechanism possibly operates in parallel with the second. Electron microprobe studies of zircons have for many years [e.g., KOppel and Sommerauer, 1974; Wayne et al., 1992; Hanchar and Miller, 1993] shown a clear positive correlation between U concentrations and the concentrations of other trace

elements, including the rare earth elements (REE's), particularly the heavy REE's, and P. The REE's are predominantly +3 valence state ions. Their incorporation into the zircon lattice requires a coupled substitution with ions of a +5 or +6 valence state. These coupled substitutions undoubtedly involve P5+. We suggest that, under oxidizing conditions, U6+ could also provide the charge balance necessary for incorporation of the +3 REE's. This possibility is particularly intriguing. For example, U6+ has an ionic radius of 86nm for eight-fold coordination [Shannon, 1976], virtually identical to the Zr4+ ionic radius of 84nm. The combination of similar

ionic radius and higher charge (stronger bonding) for U would make this an extremely favorable coupled substitution in the zircon lattice. In effect, these conditions

would result in a very high partition coefficient for U6+; far higher than for U4+ (IR = 100nm) under the same conditions. Once again, this mechanism would tend to reinforce the U zoning tendencies of all the previously discussed mechanisms. In addition, such a mechanism

would strongly decouple U from Th, which only occurs in the +4 state. The "...U and Th...uncoupled variations ranging up to a factor of five or more." noted in zircons by Silver [1992] lend support to our ideas about the possible importance of U valence state in zircon zoning.

In summary, strong oscillatory zoning of U and other trace elements in zircon can be understood in terms of self-

organizational crystallization behavior. The interplay among the stage of crystal growth, the nature of the crystal- liquid interface, the degree of supersaturation, and rates of diffusion dictates that crystal growth be episodic. This being the case, we suggest that a variety of mechanisms, acting individually or in combination, then produce very strong zoning of U and other trace elements within each episodically grown layer. These mechanisms involve relative diffusion rates, growth-rate-dependent distribution coefficients, and coupled substitutions that allow incor- poration of U in higher valence states under oxidizing conditions. All appear to act in such a way as to juxtapose

the highest U part of one zone with the lowest U part of the adjacent zone.

6. REVERSE DISCORDANCE MECHANISM

6.1. Internal Reverse Discordance

Once radiogenic Pb has been formed by the decay of U in the high-U zones of oscillatory zoned zircons, transport of some of this Pb from the high-U zones to adjacent low- U zones will produce "internal reverse discordance". Such internal redistribution of radiogenic Pb was previously suggested by Williams et al. [1984] to explain reversely discordant SHRIMP analyses in one region within an Archean zircon grain. Silver [1992] also discusses "Local internal diffusion and redistribution of radiogenic lead within individual zircon crystals during episodes of dis- turbance .... ". Although Silver does not specifically relate this to the problem of reverse discordance, his ideas are clearly along the same lines as ours.

Given the very fine scale of zoning in some zircons, the transport necessary to produce internal reverse discordance need only be on a very short scale. For example, Hanchar and Miller [1993, Figure 7] show a back-scattered electron zircon image with sharply bounded -- 0.25gm-thick zones of high average atomic number. These zones presumably are rich in heavy trace elements such as U, and are juxtaposed with zones of low average atomic number, presumably low in heavy trace elements. Transport of radiogenic Pb on a scale of only hundredths to tenths of gm would effectively produce alternating zones of normal discordance (high-U zones with net Pb losses) and reverse discordance (low-U zones with net Pb gains).

Diffusion coefficients for Pb in zircon are not

particularly well known. For example, Cherniak et al. [1991, Figure 10] show that diffusion coefficients for Pb determined by various workers range over several orders of magnitude at any given temperature. Rates of diffusion for Pb in zircon are low, requiring laboratory experiments to be carried out at very high temperatures. In some cases, zircon is not stable under the experimental conditions, leading to erroneous results [e.g., see the discussion of this problem by Chapman and Roddick, 1994]. Even if zircon were stable under the experimental conditions, alpha recoil damage and fission tracks are rapidly annealed under high temperature experimental conditions. Thus measured diffusion parameters would not account for the presence, under geologic conditions, of easy diffusion paths or "short circuits" such as fission tracks or micro fractures [e.g., Lee, 1994]. Nevertheless, even using relatively low diffusion coefficients [e.g., D = 5 x 10-21 cm2/sec at ca. 700øC

MATTINSON ET AL. 367

Harrison et al., 1987] yields penetration distances for Pb of almost one gm in 10,000 years using the Bose-Einstein relation for a "thin film" source [e.g., Richardson and McSween, 1989, p. 135]. Diffusional transport of Pb of this order or even much less is quite adequate to produce internal reverse discordance in zircons with fine oscillatory zoning, as noted earlier.

If diffusion of Pb within a zircon is directly related to a single short-lived thermal event (i.e., an "episodic distur- bance" in the classic sense), then the slope of the discordia trajectory and its upper and lower intercepts with concordia will have geological significance. As with normally dis- cordant zircon U-Pb results, multi-stage histories of Pb diffusion, with or without the added complexity of an older inherited component of zircon, will result in U-Pb sys- tematics that are more difficult to interpret.

We can also consider the possibility of "alpha recoil implantation" of radiogenic Pb. Both the 238U_ 206Pb and the 235U_ 207Pb decay chains include several energetic alpha decays. With each alpha decay, the heavy nucleus recoils, driving into the zircon lattice to create a short track. This alpha recoil track is similar to a fission track but much shorter, only on the order of 0.02gm [Fleischer et al., 1975, p. 209]. Since the 238U- 206pb and the 235U_ 207Pb decay chains include eight alpha decays and seven alpha decays, respectively, many 206Pb and 207Pb atoms will have been transported several hundredths of a gm to over a tenth of a gm from the sites of their original parent U atoms. Again, as discussed above, such distances are highly significant in zircons with zoning on a gm to sub-gm scale. The difference in the number of alpha decay events for the two decay chains is almost perfectly offset by the fact that the average energy per alpha decay is greater for the seven alpha decays in the 235U _ 207Pb decay chain than for the eight alpha decays in the 238U_ 206Pb decay chain. The total energy released by alpha decay mode for each complete decay chain is almost identical: ca. 41.9 MeV/atom for 238U - 206pb; ca. 41.4 MeV/atom for 235U- 207Pb [calculated using data from Heath, 1968]. Thus we expect that any physical fractionation of the Pb isotopes as a result of this process would be minor. It cannot be totally ruled out, however.

As a first approximation, alpha recoil processes would instantaneously sample (via implantation into a low-U layer) some small proportion of all the Pb produced by decay. If the sampling efficiency remained the same over the life of the system, the isotopic composition of the implanted Pb would be more or less identical to the bulk radiogenic Pb in the system. On a concordia diagram, this would result in an apparent zero-age discordia trajectory. Increased sampling efficiency with time (e.g., from prog-

ressively greater track lengths in an increasingly alpha- recoil damaged lattice structure with time) might actually result in "future" lower intercept ages. Alpha-recoil move- ment of radiogenic Pb to grain boundaries, micro cracks, fission tracks, or other diffusion "short circuit" sites from

which it is later extracted could possibly be a factor in apparent zero or even future age lower intercepts for normally discordant zircon samples (either single crystal or multi-grain fractions).

Combinations of alpha-recoil mechanisms and "normal" diffusional redistribution of Pb during geological thermal events would likely result in lower intercept ages inter- mediate between the age of the thermal event and the zero or future apparent age from alpha recoil processes.

6.2. External Reverse Discordance

Generating "external reverse discordance" requires one further step. We propose that once internal reverse discor- dance has been established, as discussed above, external

reverse discordance can be produced by preferential removal of some fraction of the original high-U zones. As shown by Mattinson [1984, 1994] zircon zones that are highest in U, and presumably other trace elements, are preferentially and selectively dissolved during laboratory partial dissolution experiments. Figure 5 in particular shows the remarkably selective removal of sub-gm high-U layers while adjacent low-U layers appear virtually untouched. Undoubtedly these same zones are preferen- tially removed by corrosive hydrothermal or metamorphic fluids as well. For example, Wayne et al. [1992, Figure 4B] show severe pitting and etching of zircon, evidently the result of attack by metamorphic fluids with high HF fugacity. Preferential removal of the high U zones within a zircon after they have "donated" some of their radiogenic Pb to adjacent low-U zones will leave the remaining zircon enriched in low-U zones containing an excess of parentless radiogenic Pb. Hence, individual zircon grains, possibly the entire zircon population, will be externally reversely discordant. This is evidently the case for the Mount Evans zircons. We suggest that the hydrothermal fluids appealed to by Aleinikoff et al. [1993] might in fact be responsible for the reverse discordance, but not by bringing in excess radiogenic Pb from outside sources as suggested by those authors. Instead, the hydrothermal fluids attacked and dissolved out the relatively high-U layers in the zircons, leaving behind the relatively low-U layers with their excess of parentless radiogenic Pb; Pb that was derived from the formerly adjacent high-U layers, now "missing in action". The degree of external reverse discordance then is chiefly a balance among several factors, including the scale

368 U-Pb REVERSE DISCORDANCE IN ZIRCONS

degree of oscillatory U zoning, scale of radiogenic Pb redistribution between high-U and low-U zones, and subsequent degree of removal of the high-U zones.

7. CONCLUSIONS

Reverse discordance in the U-Pb system in zircons is a real, naturally occurring phenomenon. At least in some cases it is related to fine-scale oscillatory zoning in zircons. The zoning itself is a product of self-organizational crys- tallization processes which dictate that crystallization of zircon is episodic, with a "main growth phase" under con- ditions of high to moderate supersaturation, followed by a "repair phase" under conditions of low to very low super- saturation, followed by a period of repose. Strong zoning of U and other trace elements is generated during each cycle of growth by variations in relative diffusion rates, growth-stage-dependent distribution coefficients, and possible coupled substitutions of U in higher oxidation states (6+) along with 3+ REE's.

The strong, fine-scale zoning of U in zircons juxtaposes very high U layers with very low U layers. After ingrowth of radiogenic Pb, diffusion over distances of only hund- redths to tenths of gm is sufficient to produce "internal reverse discordance"; i.e., the presence within zircon grains of reversely discordant low-U layers containing "excess" or "parentless" radiogenic Pb that was derived from adjacent high-U layers. Alpha recoil processes are also energetic enough to displace Pb by hundredths of [tm to a tenth of a gm or so. Thus, in zircons with strong, fine-scale zoning, internal reverse discordance might be the rule rather than the exception. Detection of such small-scale Pb redist- ribution is another matter, however. Both conventional

techniques, even applied to single zircon crystals or fragments of single zircon crystals, and ion microprobe techniques, which analyze U and Pb from 20 to 30 gm diameter spots, result in average analyses for tens or even hundreds of individual growth zones. In most cases, the mixture of reversely discordant low-U zones and normally discordant high-U zones will result in a net concordant to normally discordant analysis, and the presence of internal reverse discordance will go undetected. Partial dissolution techniques appear to be selective enough to separate the high- and low-U zones in some cases, even on a sub- micron scale; however [e.g., Figure 5; and Mattinson, 1994, Figures 1A and lB], and reveal the internal reverse discordance.

"External reverse discordance" occurs when hydro- thermal or metamorphic fluids, or possibly chemical weathering, or even strong laboratory acid washing treatments, attack zircon grains and selectively remove

some of the high-U material. This is not a simple U-loss, but the actual removal of high-U zircon, along with its U and remaining radiogenic Pb. Thus the residual zircon is left enriched in low-U, highly insoluble zones that contain excess radiogenic Pb, rendering the zircon population as a whole reversely discordant.

It is important to realize that the demonstration of naturally occurring reverse discordance does not rule out the possibility of reverse discordance resulting from analytical artifacts. This is especially true for the ion microprobe, where the measured U-Pb ratio is critically dependent on identical efficiencies of sputtering for standards and samples. Future ion microprobe work will undoubtedly help clarify this problem.

Finally, we believe that we are still in the early stages of understanding the nature and causes of fine-scale zoning in zircons, and its significance for understanding and interpreting zircon U-Pb systematics. In particular, we are continuing our work aimed at characterizing the geo- chemistry of the high-U and low-U zircon zones, the relationship of U-Pb systematics to this geochemistry, and the possible role of inclusions of other minerals within zircon grains, either as sources or sinks for radiogenic Pb. Some of our ideas on zircon growth and trace element zonation are speculative at this stage. We hope that these ideas will stimulate other workers to investigate these problems.

Acknowledgments. It is a distinct pleasure and privilege to honor two of the giants of U-Pb isotope geochemistry, George Tilton and Mitsunobu Tatsumoto. I (JMM) have had the very good fortune to have enjoyed the mentorship and friendship of George Tilton for some thirty years. I can confirm everyone's perception that in addition to being one of the brightest in our field, George Tilton is also one of kindest and most generous.

Some of the ideas in this paper were presented at the Eighth ICOG meeting [Mattinson et al., 1994]. We thank Randy Parrish and an anonymous reviewer for thorough reviews and helpful comments.

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