9.4 The La)Ce and La)Ba systems

¹³⁸La exhibits branched decay; by $ emission to ¹³⁸Ce and by electron capture to ¹³⁸Ba. The La)Ce decay scheme is a potentially useful isotopic tracer, and the La)Ba scheme may form a useful geochronometer, but their application has been greatly hindered by the very low abundance of the parent isotope (0.089% of natural lanthanum) and its very long half-life (over 100 Byr, totalled for both routes). Nevertheless, both methods have been applied to geological problems.

In addition to the general problems mentioned above, counting determinations of the La decay constants are hampered by the low energy of emitted particles. Hence, early measurements, particularly of the $ decay branch, were scattered. To overcome this problem, the counting experiments actually measure the ( decay of isomers (excited states) of the product nuclide, rather than the isobaric decay process itself.

A further complication for counting experiments is the hygroscopic nature of La₂O₃, the material usually used in these studies. Transformations to the hydroxide or carbonate result in weight gains of 17% and 2.5% respectively, but in ten out of twelve recent counting determinations on La, no volatile data were reported (Tanaka and Masuda, 1982). However, despite a large variation in absolute values, all counting determinations since 1970 yield a ratio for $ / electron capture decay constants near 0.51 . In addition, two recent counting experiments on anhydrous La oxide yielded average values for the $ decay constant of 2.29 H 10^!12 and 2.22 H 10^!12 respectively (Sato and Hirose, 1981; Norman and Nelson, 1983). The La)Ce system was the first of the two methods to be applied geologically, but since the La)Ba case is simpler it will be discussed first.

9.4.1 La)Ba geochronology

¹³⁸Ba, the daughter product of the electron capture decay of ¹³⁸La, is also the most abundant isotope of barium, making up 88% of the natural element. In view, therefore, of the very low abundance of the parent nuclide, significant variations in the abundance of ¹³⁸Ba are only found in REE-rich and Ba-poor minerals. The first geological measurements were made by Nakai et al. (1986) on epidote, allanite and sphene from Precambrian rocks (Fig. 9.24). Nakai et al. ratioed ¹³⁸Ba against ¹³⁷Ba to yield the following decay equation:

¹³⁸Ba (¹³⁸Ba) ¹³⁸La 8_E.C.

))) = ())))) + ))) @ )))) (e^{8total t} ! 1) [9.3]

¹³⁷Ba (¹³⁷Ba)_I ¹³⁷Ba 8_total

Ba isotope analyses were normalised to a ¹³⁶Ba/¹³⁷Ba ratio of 0.6996.

Fig. 9.24. La)Ba isochron diagram for a sample of Amitsoq gneiss, western Greenland. The isochron yields an age of 2408 " 24 Myr, which dates a metamorphic event rather than the age of the rock. The initial ratio of 6.3897 is invariant in whole-rock systems. After Nakai et al. (1986).

Nakai et al. found that all analysed whole-rock samples had ¹³⁸Ba/¹³⁷Ba initial ratios within error of 6.3897, which they attributed to the very low ¹³⁸La/¹³⁷Ba ratios of all such materials. Since the Ba isotope ratio of whole-rock systems is effectively invariant over time, a La)Ba mineral age can be based simply on the analysis of one or more La-enriched minerals. Using the electron capture decay constant of 4.44 H 10^!12 determined by Sato and Hirose (1981), Nakai et al. obtained relatively good agreement between the La)Ba and Sm)Nd ages of a pegmatite from Mustikkamaki (Finland) and Amitsoq gneiss from Greenland.

9.4.2 La)Ce geochronology

The relative harmony between counting and geological determinations of the La electron capture decay branch was not matched by the La $ decay route to cerium. This branch is beset by much larger analytical problems, but has more geochemical applications. Tanaka and Masuda (1982) determined the first La)Ce isochron, ratioing ¹³⁸Ce against ¹⁴²Ce and normalising to a ¹³⁶Ce/¹⁴²Ce ratio of 0.0172 . The decay equation is:

¹³⁸Ce (¹³⁸Ce) ¹³⁸La 8_$

))) = ())))) + ))) @ ))) (e^{8total t} ! 1) [9.4]

¹⁴²Ce (¹⁴²Ce)_I ¹⁴²Ce 8_total

Because the half-lives of both decay branches are so long, they have very little effect on each other. For example, simplifying the equation to:

¹³⁸Ce (¹³⁸Ce) ¹³⁸La

)))) = ())))) + ))))) (e^{8$ t} ! 1) [9.5]

¹⁴²Ce (¹⁴²Ce)_I ¹⁴²Ce

causes only a 0.5 % over-estimate in age.

Two further technical problems are encountered in Ce isotope analysis. One is the extreme size of the ¹⁴⁰Ce peak relative to the small ¹³⁶Ce and ¹³⁸Ce peaks (e.g. ¹⁴⁰Ce/¹³⁶Ce = 464.65). Collision of the ¹⁴⁰Ce ion beam with gas molecules in the vacuum system causes down-mass peak tailing whose effect on the small peaks must be corrected.

A second major problem is the isobaric interference of ¹³⁸Ba onto ¹³⁸Ce. ¹³⁸Ba is six times more abundant than any other natural Ba isotope which could be used to monitor Ba interference. Therefore any interference correction for ¹³⁸Ba (even if near zero) will amplify detector noise six-fold. The solution to this problem is to analyse Ce as the oxide species CeO⁺. Because barium is divalent, the BaO⁺ species is very unfavourable. Therefore, provided that overall Ba levels are kept low by good chemistry, the Ba interference can be taken to be zero without correction. Analysing Ce as the oxide introduces other isobaric interference problems, but these are easily overcome by good chemistry (section 2.1.2).

Tanaka and Masuda (1982) attempted to date separated minerals from the Bushveld pluton, but because of the geological similarity between La and Ce, a limited range of La/Ce ratios was available and the isochron had a large analytical error. Further isochron determinations were made by Masuda et al. (1988). However, using the $ decay constant of 2.29 H 10^!12 yr^!1 obtained by Sato and Hirose (1981), the La)Ce isochrons gave old ages outside error of the corresponding Sm)Nd isochrons. If ages are taken from the Sm)Nd data then the La)Ce isochron slopes can be used to make a geological decay constant determination. Using the Bushveld result and mineral isochrons from two Finnish pegmatites (Fig. 9.25), Masuda et al. calculated an average $ decay constant of 2.77 H 10^!12 yr^!1, about 20% higher than the counting determinations.

Fig. 9.25. La)Ce isochrons for (a) the Lovbole, and (b) the Mustikkamaki pegmatites of Finland. When combined with Sm)Nd data they imply La $ decay constants of 2.70 " 0.25 and 2.93 " 0.41 H 10^!12 yr^!1 respectively. After Masuda et al. (1988).

In an attempt to provide further geological constraints on the La $ decay constant, Dickin (1987a) determined a La)Ce isochron on a suite of Lewisian whole-rock gneisses from northwest Scotland. ¹³⁸Ce was ratioed against ¹³⁶Ce, since this gives rise to more manageable isotope ratios, but an equivalent normalising factor was used for fractionation correction. It was argued that whole-rock REE systems were more advantageous for calibrating the decay constant than mineral systems, due to the greater resistance of the former to metamorphic resetting. The original sample suite contained two basic granulites and four intermediate-to-acid granulites. Using the decay constant of Sato and Hirose (1981), the La)Ce isochron age of 2.99 Byr was in good agreement with the Lewisian Sm)Nd age of 2.91 Byr (section 4.1.3).

However, further work on the Sm)Nd systematics of Lewisian gneisses has shown that the samples analysed by Dickin (1987a) comprise two suites with different geological histories. Basic gneisses preserve crustal formation ages of 2.9 Byr, whereas intermediate-to-acid whole-rock gneisses have been re-set by granulite-facies metamorphism at 2.60 Byr (section 4.1.3). Using the decay constant of Sato and Hirose, the four intermediate-to-acid granulites yield a La)Ce age of 2.65 " 0.3 Byr (2F), which is in agreement with the Sm)Nd result on this suite. No meaningful La)Ce age can be calculated on the basic granulites alone.

The most recent geological determination of the La $ decay constant (Makishima et al., 1993) has also supported the value of Sato and Hirose. Two La)Ce mineral isochrons were determined on Archean granites from western Australia, which gave more or less concordant U)Pb zircon and Rb)Sr mineral isochron ages. Sm)Nd mineral suites also yielded ages within error of the other methods, but with higher error due to large MSWD values. Using a decay constant of 2.29 H 10^!12 yr^!1, the granites gave La)Ce mineral ages of 2.76 " 0.41 and 2.69 " 0.38 Byr, which agree well with U)Pb ages of 2.665 and 2.692 Byr respectively.

9.4.3 Ce isotope geochemistry

Since La and Ce are light rare earth elements (LREE), Ce isotope data form a tracer for time-integrated LREE enrichment or depletion of geological reservoirs. Similarly, Nd isotope data are a tracer for time-integrated fractionation between the middle REE. Therefore, a combination of Ce and Nd isotope data provides a unique control of the time-integrated light-to-middle REE evolution of complex geological reservoirs in the mantle or crust. Together they may form a powerful petrogenetic tool.

Ce isotope analyses of eight ocean island basalts (OIB) from the Atlantic, Pacific and Indian oceans were presented by Dickin (1987b). When plotted against published Nd isotope analyses the data defined a linear array which fell within error of the meteoritic Bulk Earth point of Shimizu et al. (1984). The linearity of this ‘mantle array’ can be attributed to the coherent behaviour of the REE during processes of mantle evolution.

Subsequently, Tanaka et al. (1987) reported Ce isotope data on eleven more ocean island, ocean ridge and island arc basalts (Fig. 9.26). Most of these were very consistent with the data of Dickin (1987b), but three samples lay outside of error of a best-fit mantle array calculated by Dickin (1988). Tanaka et al. (1987; 1988) attributed these outliers to incoherent behaviour of the La)Ce and Sm)Nd systems during the evolution of the depleted upper mantle. However, the fact that Nd, Sr and Pb isotope systems in the Ce-anomalous samples were perfectly normal led Dickin (1988) to suggest that larger analytical errors might be responsible. In contrast, Makishima and Masuda (1994) found high , Ce in MORB and rejected the low value of Tanaka et al. (1987), implying a mantle array twice as steep.

Fig. 9.26. Plot of , Nd against , Ce (part per 10⁴ deviation from Bulk Earth) for young oceanic volcanics. Error bars represent 2 SDM within-run precision. Plain crosses and those with solid centres denote data from two different labs. Solid square is a Skye plateau lava, relative to 60 Myr old Bulk Earth. Dotted error bars denote samples whose error bars do not overlap the mantle array. After Dickin (1988). NOTE error in Nd axis labels. Total range should be from zero to +10.

Because of the gradual variation of chemical properties along the lanthanide series, chondrite-normalised REE patterns for large rock reservoirs tend to define approximately linear profiles. Tanaka et al. (1987) considered the behaviour of an idealised group of rocks which underwent Ce and Nd isotope evolution starting at the Bulk Earth isotope composition. If these rocks all had linear rare earth profiles, then their Ce)Nd isotope compositions must lie on a linear array whose slope is solely a function of the relative decay constants of the parent nuclides, irrespective of age. Ideally, if such a rock suite were analysed for Ce and Nd isotope composition, the relative decay constants could be calculated without the need to know any concentration or age information.

Tanaka et al. attempted to apply this model in practice, by determining the La $ decay constant by the analysis of four unrelated continental rocks from around the world with approximately linear REE profiles. An almost perfect linear array was found, but unfortunately this must be attributed to coincidence, since the calculated initial ratios of these samples are actually more dispersed than their present-day compositions. Therefore the linearity of this particular data set is coincidental, and its slope cannot be used to determine the La $ decay constant. This concept is probably destined to remain a theoretical construct, since the principal difficulty in determining the decay constant is not determination of the La/Ce ratio, but the Ce isotope ratio itself.

A combination of Ce and Nd isotope data was used by Dickin et al. (1987) to study mixing relations during crustal contamination of continental magmas. Twelve Tertiary igneous rocks from Skye in northwest Scotland were analysed for Ce isotope composition. These were compared with theoretical mixing models based on analysed crustal end-members. The Ce versus Nd isotope diagram (Fig. 9.27) provides one way to evaluate the merits of alternative mixing models for analysed lavas. This immediately allows the exclusion of the trondhjemitic mixing line as a relevant petrogenetic model. However, the data allow a more elegant test of the end-members involved in magma mixing.

Fig. 9.27. Plot of initial Nd versus Ce isotope composition of Skye lavas (solid symbols), relative to Archean basement gneisses at 60 Myr (stars). The array of lava compositions excludes mixing of mantle-derived and trondhjemitic crustal melts. After Dickin et al. (1987).

By taking the Ce isotope compositions of competing crustal end-members, linear mixing lines can be projected back through the analysed product lavas on diagrams of Ce isotope ratio against 1/Ce concentration and La/Ce ratio to model the elemental composition of the mantle-derived precursor prior to contamination. By performing the same calculation for the Sm/Nd data, a complete model LREE profile can be determined for each precursor. These model profiles (dashed) are compared in Fig. 9.28 with a variety of REE profiles for lavas whose isotopic compositions are effectively uncontaminated (solid lines). In this way it was possible to show that the granitic sheet end-member is better able to explain the isotopic composition of the contaminated lavas than the intermediate tonalitic gneiss end-member.

Fig. 9.28. LREE profiles for isotopically uncontaminated lavas (solid lines) compared to model LREE profiles for mantle-derived precursors of contaminated lavas (dashed). The 7H model yields more consistent profiles for the two lava types. After Dickin et al. (1987).

The oceans constitute another environment where combined Ce and Nd isotope analysis can be used to study mixing processes. Ce)Nd isotope data for Atlantic and Pacific ferromanganese nodules were presented by Tanaka et al. (1986) and Amakawa et al. (1991). These isotope ratios are argued to be indicative of the composition of the ocean water from which the nodules grew. In the light of Nd and Sr isotope data (section 4.5), the Ce)Nd data are expected to reflect mixing between continental and MORB-type REE fluxes into the ocean system. However, the data were widely scattered, and did not lie on a single mixing line between reasonable MORB and continental end-members.

New Ce isotope analyses on ocean floor manganese nodules from the Atlantic Ocean (Amakawa et al., 1996) gave values which were much less scattered than previous data. Hence, the scatter of the old data may have been due to analytical error. On a plot of , Nd versus , Ce (Fig. 9.29), the new data fall close to a mixing line between MORB and continental crust, consistent with simple mixing of REE from these sources.

Fig. 9.29. Plot of , Nd versus , Ce, showing data for Pacific ( ! ) and Atlantic ( " ) manganese nodules that can be explained by mixing of REE from MORB and continental end-members. After Amakawa et al. (1996).

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