11.3 Argon

Initial, ‘excess’ or ‘inherited’ argon is normally regarded as a problem to be avoided in K)Ar and Ar)Ar dating (section 10.1.2). However, the isotopic composition of this argon can be a powerful geochemical tracer, especially when used alongside other rare gas data.

Atmospheric contamination is a much more serious problem in the isotopic analysis of ‘heavy’ rare gases than it is for helium and neon. Atmospheric helium has a very low abundance due to its complete escape from the atmosphere, and even atmospheric neon is believed to have been significantly depleted in the atmosphere by intense solar irradiation. Furthermore, the neon three-isotope plot allows atmospheric contamination to be monitored. On the other hand, the heavy rare gases (argon, krypton and xenon) have accumulated in the atmosphere over Earth history, and corrections for atmospheric contamination are much more difficult. Because of this problem, the first clear evidence for inherited argon was provided by the analysis of beryl, whose ring-type structure accommodates unusually large quantities of initial argon, swamping the effects of atmospheric contamination. ‘Excess’ argon was first found in beryl by Aldrich and Nier (1948) and studied in more detail by Damon and Kulp (1958). The latter workers discovered Archean beryls containing more than 99% of excess argon and with ⁴⁰Ar/³⁶Ar ratios as high as 10⁵.

11.3.1 Terrestrial primordial argon

The ⁴⁰Ar contents of beryl were observed to decrease over geological time (Fig. 11.11), leading Damon and Kulp to propose extensive early degassing of the Earth in the Archean, decreasing exponentially towards the present. The beryl data shown in Fig. 11.25 were also used by Fanale (1971) to support a more extreme model of catastrophic early degassing of the Earth. He argued that they were not consistent with models of constant degassing intensity through Earth history, such as proposed by Turekian (1964).

Fig. 11.25. Histograms of the argon content of beryl samples grouped by age to show decrease in measured ⁴⁰Ar contents with time. After Damon and Kulp (1958).

Schwartzman (1973) supported the early degassing model using Ar isotope data from the 2.7 Byr-old Stillwater complex. Because this is a basic magma it can yield more direct information about the Archean mantle than beryl-bearing pegmatites with a potentially large crustal input. A Stillwater pyroxene had an excess ⁴⁰Ar/³⁶Ar ratio of at least 17900 , corresponding to a calculated maximum ³⁶Ar/silicon ratio of 3 H 10^!11 for the 2.7 Byr-old mantle source. The ratio of atmospheric ³⁶Ar to mantle silicon at the present day is 1 H 10^!10, so the Earth was apparently outgassed to at least 70% of its present extent by the end of the Archean.

Ocean-floor basalt glasses are an important source of information about the rare gas budget of the present-day mantle because the high water pressure at the site of eruption retains initial magmatic argon in the sample (section 10.1.2). Furthermore, rapid quenching reduces contamination by atmospheric argon dissolved in seawater. In contrast, the crystalline cores of basalt pillows are largely outgassed of magmatic rare gases and contaminated with atmospheric gases during crystallisation (Fisher, 1971).

Hart et al. (1979) used the maximum ⁴⁰Ar content of 3 H 10^!6 ml/g in ocean floor basalt glasses as an estimate of the present-day ⁴⁰Ar concentration in the upper mantle source (UM). By subtracting this value from the average ⁴⁰Ar concentration in the Bulk Silicate Earth (estimated from its K abundance) and comparing this with the total ⁴⁰Ar budget of the atmosphere and crust, Hart et al. were able to calculate the mass of mantle which must have been outgassed. This calculation is shown in equation [11.1], where square brackets denote concentrations:

⁴⁰Ar_atm + ⁴⁰Ar_crust

mass of mantle outgassed = )))))))))))) [11.1]

[⁴⁰Ar_BSE] ! [⁴⁰Ar_UM]

Using this equation, a potassium abundance of 660 ppm in the Bulk Silicate Earth implies that only 25% of the total mass of the mantle need be outgassed.

Hart et al. proposed that the upper mantle was also thoroughly degassed of ³⁶Ar, so that subsequent radiogenic ⁴⁰Ar production generated high ⁴⁰Ar/³⁶Ar ratios of up to 16 000. In contrast, the lower mantle was not significantly degassed of argon, such that radiogenic ⁴⁰Ar production was swamped by the primordial component, allowing only a modest rise in ⁴⁰Ar/³⁶Ar ratio above the atmospheric value. Hart et al. noted the similarity of these model predictions to the ⁴⁰Ar/³⁶Ar ratios observed in ocean floor glasses from ridges and mantle plumes, respectively (Fig. 11.26). This suggested to them that these data were not seriously perturbed by atmospheric contamination.

Fig. 11.26. Histograms of early ⁴⁰Ar/³⁶Ar data for MORB and OIB, showing contrasting ranges of isotope composition. After Hart et al. (1979).

The concept of a relatively less degassed or ‘un-degassed’ mantle reservoir with respect to heavy rare gasses was strongly contested by Fisher (1983; 1985). He argued that the low ⁴⁰Ar/³⁶Ar ratios measured in plume environments were a result of atmospheric contamination. This argument was initially aimed at data on xenolithic inclusions in lavas (e.g. Kaneoka and Takaoka, 1980). These appeared to contain primordial argon and helium, but were subsequently shown to be contaminated by atmospheric argon and cosmogenic helium (section 11.1.2). Submarine glasses may be more resistant to such effects, but the interpretation of these data has nevertheless provoked intense controversy.

Experience has shown that all mantle-derived rare gas samples are contaminated to some extent by atmospheric gases. Therefore, it is necessary to take stringent experimental precautions to minimise as far as possible the extent of this contamination. Samples and equipment are thoroughly baked before analysis, and frequent blanks are determined to verify the effectiveness of these procedures. It may be possible to derive a mantle signature from a contaminated sample by extracting argon from the sample in separate aliquots, which could sample isotopic heterogeneities within the sample. One approach is to perform the rare gas analysis by step heating (e.g. Staudacher and Allegre, 1982). This procedure is demonstrated for a MORB sample in Fig. 11.27. Absorbed atmospheric rare gases are released in the low-temperature heating steps, allowing an estimate to be made of the severity of contamination effects. Another approach (Hart et al., 1983) is to do separate experiments by thermal degassing and by crushing. The latter method releases gases from vesicles, which may have undergone less (but sometimes more) contamination than the rock matrix. Staudacher et al. (1986) used both step heating and crushing to carry out the most rigorous search for sample contamination.

Fig. 11.27. Plot of measured argon isotope ratio against extraction temperature for MORB glass showing probable atmospheric contamination of the low-temperature (< 1000 ^oC) fractions. After Staudacher and Allegre (1982).

In other early work on this subject, there were attempts to use elemental rare gas ratios (e.g. ⁴He/⁴⁰Ar) to distinguish between atmospheric and primordial signatures in MORB and OIB samples (e.g. Hart et al., 1983; 1985). However, it has become apparent that elemental fractionation of rare gases can occur during several phases of the evolution of the samples, including partial meting, magmatic differentiation and solidification. Hence, it appears that isotope signatures are the only reliable discriminant between atmospheric and primordial signatures, and even these can be misleading.

In an attempt to resolve these components in the argon isotope signatures of MORB and OIB samples, Allegre et al. (1983) and Staudacher et al. (1986) compared argon and helium isotope ratios in glasses from ocean ridges and from the Hawaiian plume. Loihi Seamount gave the highest ³He/⁴He ratios for uncontaminated mantle-derived materials (presented in inverted form as the ⁴He/³He ratio in Fig. 11.28). Since this helium signature of Loihi glasses is regarded as primordial, Allegre et al. interpreted the low argon isotope ratio in these samples as likewise indicative of an un-degassed mantle source. In Fig. 11.28, the MORB field has a forked shape, attributed by Staudacher et al. (1986) to mixing with primordial and atmospheric helium reservoirs. Both mixing branches exhibited very strong curvature, which was attributed to the very low argon content of the degassed MORB reservoir, relative to both atmospheric and plume reservoirs. In contrast, a dunite xenolith from Loihi lay far off the MORB)plume mixing line. The argon signature of this sample is consistent with a source in oceanic lithosphere. The high ³He content of this sample cannot be cosmogenic (as for some other xenoliths), since it is submarine; therefore, it was attributed to diffusion of helium from the host magma into the xenolith before eruption.

Fig. 11.28. Plot of helium versus argon isotope ratio for submarine glasses from MORB and plume environments. Mixing lines are shown for different ³He/³⁶Ar ratios in Loihi relative to MORB sources. After Staudacher et al. (1986).

Allegre et al. (1983) observed that MORB glasses also fell on reasonable mixing lines between degassed and un-degassed (plume) sources on a plot of ⁴⁰Ar/³⁶Ar against strontium isotope ratio (Fig. 11.29). They argued that these mixing lines had a different trajectory than would be expected from surficial contamination processes, and therefore that the isotope data were indicative of processes in the mantle source. However, because rare gases may be decoupled from lithophile element systems, the trajectory of mixing lines may not be a reliable indicator of the end-member compositions.

Fig. 11.29. Plot of ⁴⁰Ar/³⁶Ar against ⁸⁷Sr/⁸⁶Sr for submarine basalt glasses, suggesting coherent mixing between MORB and OIB source mantle. Labels indicate the ³⁶Ar/⁸⁶Sr ratio in the MORB end-member relative to OIB for different mixing lines. After Allegre et al. (1986).

Patterson et al. (1990) argued that helium isotope systematics in Loihi glasses might also be decoupled from argon, due to the extreme difference in He/Ar ratio between the end-members (as demonstrated, for example, by the hyperbolic form of proposed mixing lines in Fig. 11.29). They pointed out that seawater has between two and four orders of magnitude more ³⁶Ar than Loihi glasses, but two orders of magnitude less ³He. Hence, argon isotope ratios in Loihi magmas might have been contaminated by seawater without affecting their helium signature. In MORB glasses, variable contamination of this kind generates correlations between ⁴⁰Ar/³⁶Ar ratio and 1/³⁶Ar abundances, indicative of simple mixing between atmospheric and mantle argon (Fisher, 1986). However, Loihi data do not display such a correlation. Hence, the atmospheric contamination model is hard to evaluate critically in plume environments.

Farley and Craig (1994) made a new examination of this problem, based on helium and argon measurements in olivine phenocrysts from a tholeiitic plume basalt. In this sample from the Juan Fernandez hot-spot, Farley and Craig were able to demonstrate a positive correlation between ⁴He and ⁴⁰Ar abundances released from fluid inclusions by crushing of phenocrysts (Fig. 11.30a). This correlation line is inconsistent with significant atmospheric contamination involving these isotopes, since the atmosphere has negligible ⁴He. Therefore, the correlation must be attributed to variable gas inventories sampled from a mantle source with constant ⁴He/⁴⁰Ar ratio.

In contrast, Farley and Craig observed no correlation between ⁴He and ³⁶Ar abundances (Fig. 11.30b). Therefore, they argued that ³⁶Ar abundances must have been perturbed by atmospheric contamination. However, since even gas rich samples showed this behaviour, analytical blank was an unlikely cause. Instead, Farley and Craig attributed the effect to contamination of the magma by seawater argon within the oceanic crust. Hence, they argued that the mantle source sampled by the Juan Fernandez hot-spot had a minimum ⁴⁰Ar/³⁶Ar ratio equal to the maximum observed ⁴⁰Ar/³⁶Ar ratio of 7700. However, this does not place strong constraints on the argon isotope ratio of the lower mantle (un-degassed reservoir) because the Juan Fernandez plume may have been contaminated with radiogenic argon during its ascent through the MORB source.

Fig. 11.30. Plots of gas release (10^-9 cc/gram) during crushing of olivine phenocrysts from a Juan Fernandez basalt: (a) ⁴He against ⁴⁰Ar; (b) ⁴He against ³⁶Ar; showing effects of atmospheric contamination. Modified after Farley and Craig (1994).

If this seawater contamination model is correct, it casts doubt on the reliability of basaltic glasses in general as samples of primordial argon from the lower mantle. However, it does not disprove the ‘two-reservoir’ model for mantle rare gases; it simply implies that the ⁴⁰Ar/³⁶Ar ratio of the deep mantle cannot be determined directly. In this situation, neon isotope evidence may help to narrow the range of possible values by acting as a monitor for atmospheric contamination.

11.3.2 Neon–argon

Based on the ‘atmospheric contamination’ model for neon, Farley and Poreda (1993) suggested that ²⁰Ne/²²Ne ratios could be used to monitor and correct atmospheric contamination in other rare gases such as argon. However, because the end-members will have different rare gas abundance ratios (e.g. Ne/Ar), mixing will generate hyperbolic rather than linear arrays, leading to somewhat greater uncertainty in the calculation of uncontaminated end-members.

In order to constrain the curvature of mixing lines, Farley and Poreda first examined Ne)Ar isotope systematics in MORB glasses (Fig. 11.31a). Two sources of data were used. Solid symbols represent directly measured data from Hiyagon et al. (1992), whereas open symbols represent less accurate argon data calculated indirectly from Marty (1989). Mixing lines were estimated between the atmosphere point and the best estimate (at the time) of terrestrial primordial neon and argon. These were based on the composition of planetary neon (²⁰Ne/²²Ne = 12.5) and the highest ⁴⁰Ar/³⁶Ar ratio (near 30 000) measured in a volatile-rich ‘popping rock’ from the Mid Atlantic Ridge (Staudacher et al., 1989). The very high volatile contents of these magmas were thought to render them relatively immune to atmospheric contamination. Using these end-members, most MORB analyses lay between mixing lines with ²²Ne/³⁶Ar ratios (r) of 0.06 and 0.6, in atmospheric relative to mantle components.

The OIB data set (Fig. 11.31b) can be modelled in a similar way. However, based on the slightly different Ne/Ar ratios in OIB and MORB, Farley and Poreda estimated that simple two-component mixing between plume and atmospheric rare gases should fall between mixing lines with relative ²²Ne/³⁶Ar ratios of 0.07 and 0.7. Many of the Samoan samples in Fig. 11.31b have radiogenic argon signatures similar to the MORB data. However, the Samoan plume has already been shown to have a radiogenic neon isotope signature (section 11.2.3). Therefore, these data should be excluded from a search for the composition of the primordial argon reservoir. On the other hand, by fitting the mixing curves to the OIB samples with lowest argon isotope ratio, Farley and Poreda calculated a ‘revised’ ⁴⁰Ar/³⁶Ar ratio for primordial terrestrial argon, with a value around 3500. This compares with the ⁴⁰Ar/³⁶Ar ratio of 400 proposed by Allegre et al. (1983). The higher value would then imply substantial radiogenic argon growth in this ‘primordial’ reservoir.

Fig. 11.31. Plots of neon versus argon isotope compositions in submarine glasses, showing possible mixing lines due to atmospheric contamination: (a) MORB (open symbols = indirect measurements); (b) OIB (open symbols = Samoa). After Farley and Poreda (1993).

The neon–argon diagram forms the basis of ongoing efforts to pin-point the argon isotope compositions of pristine mantle sources. However, new argon data for the ‘popping rock’ from the Mid Atlantic ridge (Burnard et al., 1997) showed that even this very volatile-rich sample had suffered severe atmospheric contamination. Burnard et al. released the gases from vesicles in a section of rock 0.3 mm thick, using short pulses from a Nd:YAG laser. They found that ⁴⁰Ar abundances in individual vesicles were well correlated with ⁴He and CO₂, indicating a mantle-derived source. In contrast, ³⁶Ar abundances were generally poorly correlated with these other quantities, leading Burnard et al. to suggest that perhaps none of the measured ³⁶Ar was mantle-derived. This seems to be a rather extreme interpretation, because a cluster of ⁴⁰Ar/³⁶Ar ratios was observed at a value around 40 000. Nevertheless, this places a new lower limit on the ⁴⁰Ar/³⁶Ar ratio of the MORB source.

This new ⁴⁰Ar/³⁶Ar ratio for the MORB source was supported by additional studies of the argon–neon systematics of the popping rock, using gases extracted by stepwise crushing (Moreira et al., 1998). This work yielded a good correlation line consistent with previous MORB data on the neon three-isotope plot, which was attributed to mantle–atmosphere mixing. However, an excellent hyperbolic mixing line was also observed on the argon–neon isotope diagram (Fig. 11.32) which allows the most accurate determination yet of the argon isotope ratio of the MORB source. Based on the intersection of the mixing line with the solar composition (rather than the planetary neon composition used by Farley and Poreda), a ⁴⁰Ar/³⁶Ar ratio of 44 000 was determined.

Fig. 11.32. Argon–neon isotope plot showing a determination of the argon isotope ratio of the MORB source. Open points were omitted from the calculation of the mixing line because they had high argon blanks. After Moreira et al. (1998).

Since the work of Farley and Poreda (1993), several new studies have also been made on argon–neon isotope systematics in OIB glasses. Most of the data fall fairly close to the mixing line observed by Farley and Poreda. However, if the mixing line is extrapolated to a solar neon isotope ratio (²⁰Ne/²²Ne = 13.8) rather than the neon-B planetary value (²⁰Ne/²²Ne = 12.5), it necessarily implies that the ⁴⁰Ar/³⁶Ar of the plume source will be more radiogenic.

Valbracht et al. (1997) made such a study on glassy pillow rims and olivine phenocrysts, collected from water depths of 3 to 5 km on Loihi seamount. The best results were obtained by step heating, and reached ²⁰Ne/²²Ne ratios as high as 13. These authors plotted argon isotope data against ²¹Ne/²²Ne, but the result is equivalent to using ²⁰Ne/²²Ne. Two mixing lines were drawn, with the minimum and maximum curvature that could fit the data. Using a solar ²¹Ne/²²Ne ratio of 0.41 (calculated from the three-isotope plot), the inferred ⁴⁰Ar/³⁶Ar composition of the plume source is between 2500 and 6000.

In a somewhat different approach, Marty et al. (1998) were able to successfully sample plume-type rare gases from 380 Myr old carbonatites in the Kola region of northern Russia. Neon and argon were extracted from a variety of rock types by crushing, and defined an array on the neon three-isotope plot with a slope slightly lower than Loihi. On a neon–argon isotope plot the data define an array with ²⁰Ne/²²Ne ratios as high as 12. Extrapolation to the solar neon composition led to estimated ⁴⁰Ar/³⁶Ar ratios between 5000 and 6000, depending on the curvature of the mixing line drawn through the data. These results clearly show the important role of deep mantle plumes in carbonatite magmatism, as well as supporting the argon isotope composition of the plume source obtained from ocean islands.

Additional argon–neon isotope data were obtained by Trieloff et al. (2000) from sub-glacially erupted basalt glasses from Iceland and volatile-rich dunite xenoliths from Loihi. These sample suites defined arrays slightly steeper than previous Loihi data on the neon three-isotope plot, but on the neon–argon plot (Fig. 11.33) the results were similar to previous work. The upper ends of these data arrays (²⁰Ne/²²Ne = 12.5) fell within error of the neon-B composition, leading Trieloff et al. to suggest that this was the composition of ‘solar neon’ in the Earth. However, subsequent data (section 11.2.2) have exceeded this value, showing that the Earth contains true solar neon with ²⁰Ne/²²Ne ratios as high as those of the solar wind. Using this value to determine the argon isotope composition of the plume source leads to a ⁴⁰Ar/³⁶Ar value around 7000 for Iceland. On this occasion the Loihi data pointed to a higher ⁴⁰Ar/³⁶Ar value around 15 000.

Fig. 11.33. Neon–argon isotope plot for Icelandic subglacial basalts ( ! ) and Loihi dunites ( " ), showing mixing lines between atmospheric and plume-source (solar) rare gases. Modified after Trieloff et al. (2000).

Unfortunately these data leave the argon isotope composition of the plume source in a certain amount of doubt. It is demonstrably higher than that of air and lower than the MORB source, but a precise value remains elusive. The steady state model of Porcelli and Wasserburg (1995b) predicts that the ⁴⁰Ar/³⁶Ar ratio of the (lower mantle?) primordial rare gas source should be about one third of the upper mantle value. Given a ⁴⁰Ar/³⁶Ar value of ca. 45 000 for the latter reservoir, a ratio between 5000 and 15 000 may be reasonable for the lower mantle.

11.3.3 Argon-38

In principle, ³⁸Ar data could be combined with ³⁶Ar and ⁴⁰Ar to make a three-isotope system analogous to neon. Unfortunately, ³⁸Ar is about five times less abundant than ³⁶Ar, which is itself a very rare isotope. Furthermore, the total variation between atmospheric and solar compositions is only 5%. Therefore, until recently, no ³⁸Ar/³⁶Ar ratios had been observed that were distinct from the air composition. Two research groups have recently claimed to find such deviations (Valbracht et al., 1997; Pepin, 1998). However, in a third study of comparable methodology (Kunz, 1999), no such deviations were found, so these positive signals remain disputable.

Valbracht et al. (1997) observed a weak correlation between ³⁸Ar/³⁶Ar ratios and ²⁰Ne/²²Ne ratios in Loihi glasses, which they interpreted as a mixing line between atmospheric and solar rare gases. Niederman et al. (1997) also presented ³⁸Ar/³⁶Ar data for MORB glasses from the East Pacific Rise (EPR), but did not discuss the data in detail, believing them to be within error of the atmospheric composition. However, Pepin (1998) pointed out that the EPR argon data define a relatively strong correlation with neon isotope data, between the atmospheric and solar compositions (Fig. 11.34a). Pepin also plotted the Loihi and EPR data on the argon three-isotope plot (Fig. 11.34b), on which mixing lines are linear. On this plot, both the MORB and OIB data display a fan between the predicted mixing lines from atmospheric argon to upper- and lower-mantle sources.

Fig. 11.34. Plots showing claimed ³⁸Ar/³⁶Ar variations in mantle-derived samples. (a) MORB samples on the argon–neon plot; (b) MORB and OIB samples on the argon 3-isotope plot. Error bars are 1F. After Pepin (1998).

Kunz (1999) pointed out a problem with the EPR data on the argon 3-isotope plot; which is that these data do not lie on the mixing line to MORB sources, but below that line, trending towards OIB. This could be attributed to contamination of the MORB source with plume material, but some of the EPR data lie surprisingly close to the atmosphere–plume source mixing line to be explained in this way. Therefore, Kunz argued that these analyses are simply MORB samples with heavy atmospheric contamination, and that the low ³⁸Ar/³⁶Ar ratios (which have large error bars) are products of analytical noise. Furthermore, in a test for ³⁸Ar/³⁶Ar variations in the popping rock sample from the Mid Atlantic Ridge, Kunz found no data outside error of the atmospheric composition, even though ²⁰Ne/²²Ne values were as high as 12.5. Therefore, the suggested ³⁸Ar/³⁶Ar variations remain unproven.

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