Introduction

The U-Pb systematics of Devonian orthogneisses in the Brooks Range are complex due to the combined effects of Proterozoic inheritance and Cretaceous Pb-loss during metamorphism (Dillon et al., 1987; Aleinikoff et al., 1993). We carried out a detailed U-Pb study on zircons from one sample of the Mt. Igikpak orthogneiss. One goal was to compare the results of the step-wise dissolution method (Mattinson, 1994), and of SHRIMP-RG analyses carried out on isotopically complex zircons from the same sample.
Three conventional U-Pb analyses yielded non-linear, discordant ages between 320 and 627 Ma. In contrast, in the partial dissolution experiment four sequential steps define a line that generally parallels concordia. This pattern is interpreted to reflect the progressive removal of high-U material suffering from Pb-loss, followed by dissolution of magmatic zircon, and finally, concentration of the least soluble inherited components. The linear array defined by the first three steps reflects Pb-loss on primary magmatic zircon at approximately 120 Ma. The last step and the residue define a chord from approximately 385 Ma (the probable crystallization age) to 1375 Ma (the average age of Proterozoic inherited components). Using the SHRIMP-RG we analyzed rims and cores of 15 zircon crystals. 206Pb/238U ages range from 137 to 887 Ma with histogram maxima at 243 and at 384 Ma. The younger ages correspond to high-U, intensely metamict spots regardless of whether they are rims or cores. Some cores yield younger ages than the lower-U rims. The pattern of ages is consistent with that of the partial dissolution experiment and is amenable to the same interpretation. However, the cluster of ages at 243 Ma is difficult to explain. There is no independent geological evidence for magmatic activity at this time in the Brooks Range, yet it is unlikely that a process as complex Pb-loss would affect multiple grains of different compositions in such a way to produce a cluster of ages.

1. Geological Problem:

An extensive belt of variably deformed granitic orthogneisses extends along the Brooks Range of northern Alaska (Fig. 1). These metagranitic rocks range in composition from meta- to per- aluminous. Stratigraphic relationships constrain the orthogneisses of the northeastern Brooks Range as pre-Early Mississipian, but obtaining reliable conventional U-Pb zircon age determinations has generally proven difficult in much of the belt. Dillon et al. (1980, 1987) established that the belt is probably middle to late Devonian age, but the data set is complex. More recent work has confirmed the Devonian age of several meta-aluminous bodies along the Dalton Highway (Aleinikoff et al., 1993), but the peraluminous orthogneisses have remained intractable due to complexities associated with the combined effects of inherited components and Pb-loss.

Following up on our initial conventional analyses from a number of orthogneiss bodies in the central Brooks Range, we carried out a detailed U-Pb study involving partial dissolution and SHRIMP-RG (Sensitive High Resolution Ion Microprobe-Reverse Geometry) analytical techniques on zircons from a sample of the Mt. Igikpak (Fig. 2) orthogneiss to investigate complexities of the U-Pb systematics.

Figure 2 Mt. Igikpak, the highest peak in the central Brooks Range

The Mt. Igikpak orthogneiss is the largest metagranite in the Central Brooks Range. It is weakly per-alluminous and the dominant mineralogy is quartz-k-feldspar-plagioclase-biotite-white mica +/- garnet, clinozoisite, sphene, chlorite. The orthogneiss was metamorphosed under greenschist to lower amphibolite conditions (depending on crustal level) during Early to mid- Cretaceous collision of an island arc and the Arctic Alaska margin.

2. Methodological Problem:

Dating of isotopically complex zircons has been a challenge since the inception of the U-Pb method. The main sources of isotopic complexity are :

• Inheritance: Older zircons are incorporated in the core of newly crystallized grains.
• Pb-loss: In some geological environments (particularly fluid-rich metamorphic conditions) zircons are not closed isotopic systems. Radiogenic Pb is typically removed from portions of the zircon crystals with the highest U concentration.

These two processes contribute to produce discordant U-Pb data and ages that are difficult to interpret geologically. In samples that suffer from the effects of both inherited components and Pb-loss, the U-Pb systematics must be established before a meaningful age interpretation, if any, can be made.

Traditional strategies used to produce more easily interpretable concordant data sets determined with conventional single or multi-grain TIMS analysis include:

• Hand Picking. Because inherited components, metamict regions, and fractures that allow fluid access within individual grains are commonly visible, it is possible to visually select clean grains that are more likely to yield concordant data.
• Air Abrasion. For samples suffering from Pb-loss, mechanical abrasion removes the outer portions of zircons which commonly exhibit the highest degree of Pb-loss. Alternatively, abrasion technique can be used to demonstrate the presence of inherited components.

Two relatively new techniques that help to unravel complex U-Pb systematics in zircons are partial dissolution experiments and ion microprobe analyses (SHRIMP). In this study we compare the results of these two methods applied to zircons separated from a single sample of the Mt. Igikpak orthogneiss.

During a Partial Dissolution experiment a multi-grain zircon fraction is progressively dissolved in acid at increasing temperatures. The leachate is analyzed after each step. Dissolution at each step generally proceeds as a function of U concentration, thus the isotopic composition of increasingly more resistant portions of the zircons is determined. It has been established that this technique can document the presence of both Pb-loss and inheritance and provide useful age information (Mattinson, 1994; McClelland and Mattinson, 1996). However, because the operator has no control over which parts of the grains are being dissolved at any given step, interpretation of zircon systematics is not tied to observable variation in individual zircon grains.

In contrast during SHRIMP analysis, only a minute portion of an individual zircon is sampled by the primary beam during each analysis. Age data is directly tied to the portion of the zircon grain analyzed. The operator can utilize visual information (e.g., photomicrographs or cathodoluminescence images) to guide the sampling strategy. Once a large number of individual spot analyses is gathered, the data can be evaluated as a whole.

We contrast the results of the two methods applied to zircons from the same rock sample and discuss competing interpretations for the SHRIMP results.

3. Partial Dissolution Experiment:

Conventional analyses: The zircon population separated from a sample of the Igikpak orthogneiss consisted of white to red, cloudy to opaque grains. Three small multi-grain fractions yielded highly discordant data that form a nonlinear array, indicating complex systematics resulting from the combined effects of inherited components and Pb-loss. Previous studies have demonstrated the utility of partial dissolution methods for resolving useful age information from zircon populations with complex systematics (Mattinson, 1994; McClelland and Mattinson, 1996). In general, the variation in rate of dissolution is controlled by U concentration. Thus, the partial dissolution steps will tend to distinguish between rim material, core material, and material intermediate to core and rim (Fig. 3). The ideal scenario is to produce a concordant residue that represents the magmatic age of interest. Although this is not often the case, useful age information and interpretation of the zircon systematics can usually be derived from the partial dissolution results.

Figure 3 SEM images of zircons after several increments of stepwise dissolution (from McClelland and Mattinson, 1996).

Partial Dissolution Results: A five-step partial dissolution experiment on zircons from the Mt. Igikpak orthogneiss yielded discordant analyses with progressively increasing U/Pb and Pb/Pb ages (Fig. 4). Steps 1 through 3 define a linear array from Early Cretaceous (the expected metamorphic age) to Devonian (the expected crystallization age). The final 2 steps define a trajectory from the expected magmatic age into the Proterozoic. These results are interpreted to record (a) early removal of high-U material suffering from Pb-loss [steps 1 & 2]; (b) dissolution of magmatic zircon with minor Pb-loss and inheritance effects [steps 3 &4]; and (c) final dissolution of relatively insoluble, low-U inherited xenocrystic components [step 5]. The data are interpreted to record the presence of Proterozoic inherited components, magmatic crystallization at approximately 385 Ma, and Pb-loss associated with metamorphism at approximately 120 Ma. SHRIMP analyses from the same sample (see below) document the presence of low-U inherited cores in the zircon population and confirm the inferred 385 Ma crystallization age for the orthogneiss. Although the partial dissolution data do not provide a high-precision age determination for the Igikpak orthogneiss, the partial dissoution results clearly document that the zircon systematics are complex and allow useful age interpretation for the sample.

Figure 4 Tera-Wasserberg plot of conventional and partial dissolutino analyses of zircons from the Mt. Igikpak orthogneiss.

4. SHRIMP Analysis:

We analyzed 15 zircon crystals from one sample of the Mt. Igikpak orthogneiss. The analyses were carried out on the Stanford/USGS Ion Microprobe (SHRIMP-RG) using zircon AS57, with a known age of 1099 Ma, as a standard. The ion probe data were corrected for common Pb using the 206Pb/204Pb measured during the analysis and assuming a Pb isotopic composition according to the Cumming and Richards (1975) Pb evolution model.

Figure 5 Cathodolumicense images of zircons analysed by SHRIMP. Dark zones have high U concentrations.Click image for a larger version.

The zircons yielded 206Pb/238U ages ranging from 137 to 887 Ma with histogram maxima at 243 and 384 Ma. The younger ages correspond to high-U, intensely metamict spots regardless of whether the analyses were from rims or cores. Some cores yield younger ages than the lower-U rims (Fig. 5). The pattern of ages is consistent with that of the partial dissolution experiment and is amenable to the same interpretation. However, the cluster of ages at 243 Ma represents either (1) a distinct thermal or Pb-loss event undetected by the partial dissolution and conventional analytical techniques or (2) an analytical artifact representing a qualitative measure of the average degree of younger Pb-loss in the portions of zircon analyzed. Ongoing studies will address the differing interpretations.

Figure 6

Interpretation of SHRIMP Results: The distribution of SHRIMP age determinations is consistent with the distribution defined by the partial dissolution results: concordant and discordant analyses spread along concordia from Cretaceous to Devonian and discordant analyses defining a trend into the Proterozoic. The observed systematics defined by both the partial dissolution and SHRIMP analyses are consistent with crystallization of magmatic zircon with inclusion of inherited xenocrystic components at approximately 385 Ma and later Pb-loss induced during Cretaceous metamorphism.

Figure 7

An unexpected result from the SHRIMP analyses is the second cluster (Cluster 2 in Fig. 7) defined by the SHRIMP 206Pb/238U ages at about 243 Ma. There are two possible interpretations of this cluster with regard to the significance of the age. Either the peak records an Early Triassic thermal event that has not been previously resolved by conventional analyses or the peak represents an artifact of the analytical procedure.

HYPOTHESIS 1: Standard interpretation of SHRIMP analyses favors the first interpretation. Although no Early Triassic thermal event has been previously recognized in Northern Alaska, there is no geological evidence that supports or definitively refutes this interpretation. The available stratigraphic evidence indicates that during this time the area was a simple passive margin with little tectonic activity (Moore et al. 1995). In contrast, on the Chukotka Peninsula of Northeastern Russia, which is considered part of the same tectonic province as Northern Alaska, there is a large complex of basaltic dikes thought to be related to incipient continental rifting (Natalin et al. 1999).

HYPOTHESIS 2:The second interpretation is suggested in order to reconcile the apparent lack of evidence for an Early Triassic thermal event in the Brooks Range and is based on several observations. Firstly, data that statistically define the second cluster actually show a significant spread in ages and include analyses that are clearly discordant. The 206Pb/238U ages of discordant analyses should have no geological significance. Secondly, there are examples where the event is recorded by the core and not the rim (e.g., Grain 4; Fig. 3.1). This observation requires that the Early Triassic event is a Pb-loss rather than zircon growth event. Thirdly, the current data set represents a biased sampling procedure. The specific aim of the analyses was to obtain age determinations that document Pb-loss, the presence of inherited material, and a Devonian magmatic age as defined by the partial dissolution results. Analyses from domains of higher-U concentration typically yielded poorer quality analyses due to elevated 204Pb concentrations. Thus these regions were generally avoided. For given grains, there is a correlation between U concentration and degree of observed Pb-loss. Therefore, the data set is biased toward analyses from zones with low to intermediate degrees of Pb-loss and the Early Triassic age cluster effectively represents a qualitative measure of the average degree of younger Pb-loss in the portions of zircon analyzed.

Additional analyses are required to resolve uncertainty regarding the significance of the Early Triassic age recorded by the second age cluster. Detailed transects of grains from the Mt. Igikpak orthogniess and additional analyses from other units within the region will likely distinguish between the disparate interpretations.

4. Conclusion:

The partial dissolution technique relies on variability in dissolution rate, primarily as a funtion of U concentration, to generate a pattern of ages that resolves the effects of Pb-loss and inheritance on primary magmatic zircons. Because the portion of zircon undergoing dissolution can not be controlled, the experiment is entirely objective.

SHRIMP analyses generate age information from small areas within individual zircon grains. The operator has control over what part of a grain is analysed and can use visual information to guide the sampling strategy during the analysis. Thus, in contrast to the partial dissolution technique, data collection is subjective.

At the general level both methods yielded similar results:

• Results of the partial dissolution experiment indicate that zircons record
  inherited components and have suffered from Pb-loss. Ages range
  from Proterozoic to Early Cretaceous.
• Analyses from both data sets define a swath subparallel to concordia.
• Both methods provide a Devonian crystallization age for the Mt. Igikpak
  orthogneiss.

However in detail there are important differences. The partial dissolution experiment data point to a crystallization age for the orthogneiss of about 385 Ma. This corresponds to the lower intercept of a chord drawn between the insoluble residue and the last dissolution step and is supported by the ages of the leach step which dissolved the largest volume of zircon.

In contrast, the SHRIMP data are not evenly distributed along concordia, but define several clusters of ages that are not apparent in the partial dissolution experiment. One cluster (at 384 Ma, 8 ages) agrees with the crystallization age independently interpreted from the partial dissolution experiment.

A second cluster at 243 Ma (Early Triassic, 9 ages) does not correspond to any previously recognized thermal event in Northern Alaska. Standard interpretation of the data suggests that the cluster represents the effects of Pb-loss during a geologically significant event.

An alternative hypothesis is that the Early Triassic cluster of ages is an analytical artifact produced by sampling procedure. This explanation requires that nine zircons spots with different U concentrations underwent the correct amount of Pb-loss to yield the same age. Given the complexities of diffusive Pb-loss, this hypothesis seems unlikely. However, the obvious bias in data collection (e.g., neglecting obvious high-U zones), inclusion of discordant data in to the statistical analysis, and the question of whether an older Pb-loss event is expected to be preserved when overprinted by a younger event support the alternative hypothesis.

We will carry out further studies to determine if this age pattern can be substaniated by detailed traverses across zircons from the Mt. Igikpak orthogneiss and other units within the region. The most significant conclusion from this study is that despite the technique, interpretation of geochronologic data must take into account the geological context of the sample analyzed, the systematics of the zircons analyzed must be determined, and potential sources of error outside of analytical uncertainty must be considered.

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