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Meteorite Phosphates Show Constant 176Lu Decay Rate Since 4557 Million Years Ago

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Science  04 Nov 2005:
Vol. 310, Issue 5749, pp. 839-841
DOI: 10.1126/science.1117919

Abstract

The use of radioactive decay of 176Lu to 176Hf to study the evolution of the Earth requires a precise and accurate value for the 176Lu decay constant. Recent determinations of this decay constant by age comparison to the more precisely calibrated U-Pb isotopic system produced internally consistent but discrepant values between terrestrial minerals and meteorites. New highly radiogenic Lu-Hf data for phosphate minerals from Richardton (ordinary chondrite) and Acapulco (primitive achondrite) yield decay constant values of 1.864 × 10–11 ± 0.016 × 10–11 and 1.832 × 10–11 ± 0.029 × 10–11 year–1, respectively, identical to the value determined from terrestrial minerals.

Radioactive decay of 176Lu to 176Hf provides a potentially powerful isotopic tracer for studying early planetary differentiation. Unfortunately, the estimate for the 176Lu decay rate (λ176Lu) is controversial (14).

Numerous γ-counting studies of 176Lu decay [summarized in (1, 2, 46)] yielded a widely dispersed array of decay constant estimates. These results indicate major unresolved analytical problems in γ-counting techniques and do not constrain the λ176Lu with the required precision and accuracy for geochemical applications. Age comparison studies have produced internally consistent but discrepant λ176Lu values. Well-constrained Lu-Hf and U-Pb internal isochrons for terrestrial rocks younger than 2.7 × 109 years old yielded the λ176Lu values of 1.865 × 10–11 ± 0.015 × 10–11 year–1 (7) and 1.867 × 10–11 ± 0.008 × 10–11 year–1 (2), whereas several Lu-Hf whole-rock isochrons for chondrites and achondrites (810) yielded values in the range from 1.93 × 10–11 to 1.98 × 10–11 year–1.

Decay constants determined by age comparison are reliable only if “the initial event starting the radioisotopic clock was so short and simple as to be truly `point-like' in time, and whose subsequent perturbations were totally nonexistent” (1). It is also required that the abundance of the radiogenic isotope (expressed as 176Hf/177Hf ratio) be sufficiently high as to make any possible initial variations insignificant. The samples used in the terrestrial age comparisons (2, 7) satisfy these criteria.

Meteorites analyzed for Lu-Hf so far (810) are not so well suited for age comparison determinations of λ176Lu. Chondrites contain components of variable ages: chondrules, refractory inclusions, matrix, and metamorphic minerals. Their timing of accretion, and the nature and timing of the event that caused most Lu-Hf fractionation, are uncertain. Chondrites and basaltic eucrites have relatively small variations in 176Hf/177Hf ratio and are therefore sensitive to a real or an apparent heterogeneity in the initial 176Hf/177Hf ratio [Supporting Online Material (SOM) Text]. Cumulate eucrites, which show larger variations in Lu/Hf and 176Hf/177Hf ratios, are metamorphosed rocks with complex and prolonged geological histories possibly spanning as much as 100 to 150 million years (My) after solar system formation [e.g., (11)].

Here, I report Lu-Hf and U-Pb analyses of phosphate minerals from two meteorites, primitive achondrite Acapulco and ordinary chondrite (H5) Richardton, and discuss their implications for the 176Lu decay rate. Acapulco and Richardton are well suited for an age comparison study. These meteorites are recovered from observed falls and have low degrees of shock and weathering (1214). More importantly, both meteorites have well-documented fast cooling histories. The cooling rate of Acapulco is higher than 1000°C/My in the temperature range above 400°C. Rapid cooling continued down to 120°C, as indicated by an (U-Th)/He age of 4538 ± 32 My (1σ), indistinguishable from the 207Pb/206Pb age (15) on Acapulco phosphates [(16) and references therein]. Fast cooling makes age comparison using Acapulco insensitive to the possible difference in closure temperatures between the Lu-Hf and U-Pb systems in Caphosphates (17). The average cooling rate of Richardton was estimated to be 26° ± 13°C/My from 800° to 450°C (18) on the basis of U-Pb dates of chondrules and Ca-phosphates and experimental data for Pb diffusion in pyroxenes and apatite. This cooling rate is slower than that of Acapulco, but its influence on age comparison is still small: Even a large difference of 300°C in the closure temperature between the Lu-Hf and U-Pb systems in phosphates adds only a 12-My uncertainty to the age, corresponding to a 0.26% uncertainty in λ176Lu. Phosphates from both meteorites were previously dated with the U-Pb method, which yielded 207Pb/206Pb ages of 4557 ± 2 My for Acapulco (15) and 4550.7 ± 2.6 My for Richardton (18). The effect of uncertainty in these ages on λ176Lu determined by age comparison is insignificant. Phosphates from both meteorites have concordant (or nearly concordant) U-Pb systems, which suggests that the isotopic systems of these minerals remained closed.

Acapulco phosphates were separated (19) from a ∼0.4-g specimen provided by K. Marti (Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, USA). Fifteen phosphate fractions, including two single grains of apatite, one single grain of merrillite (SOM Text), and 12 multigrain fractions containing both of these minerals (20), were analyzed for Lu-Hf (19). Regression of all 15 Lu-Hf analyses produced an isochron (Fig. 1) with the slope of 0.08706 ± 0.0014. Six of these fractions were also analyzed for U-Pb (21) and yielded a 207Pb/206Pb-204Pb/206Pb (Pb-Pb) isochron age of 4556.5 ± 1.3 My (Fig. 2A) and a three-dimensional total Pb/U isochron age (23) of 4556.4 ± 3.2 My (Fig. 2B). Assuming that the slope of the Lu-Hf isochron corresponds to the age of 4556.5 ± 1.3 My, the decay constant of 176Lu is estimated at 1.832 × 10–11 ± 0.029 × 10–11 year–1 [with the use of the currently accepted decay constant values for U isotopes (24)]. The error of λ176Lu includes uncertainties in the Lu-Hf isochron slope, Pb-Pb isochron age, and the additional uncertainty of ±9.2 My from the errors of decay constants of U isotopes (24, 25).

Fig. 1.

Lu-Hf isochron diagram for Acapulco phosphates. Regression of all 15 Lu-Hf analyses yield an isochron with the slope of 0.08706 ± 0.0014 and the y intercept of 0.2802 ± 0.0012 [2σ, mean square of deviates weighted (MSWD) = 1.3, probability of fit = 0.21] (SOM Text). Error bars are 2σ, and are smaller than the plotting symbols for most analyses. The errors of 176Lu/177Hf and 176Hf/177Hf ratios are assumed to be uncorrelated, as usually done in Lu-Hf isochron calculations. However, the errors can be correlated for the fractions 2 and 6, where blank correction on 177Hf is significant. Assigning error correlations of 0.9 to these fractions does not noticeably change the slope of the isochron.

Fig. 2.

(A) Pb-Pb isochron for Acapulco phosphate fractions 1 to 6. 207Pb/206Pb ratios are not corrected for initial common Pb. Isochron regression yielded an age of 4556.5 ± 1.3 My, MSWD = 0.47, and probability of fit = 0.76. Error ellipses for apatite and multigrain analyses are smaller than the plotting symbols; error ellipse for the merrillite analysis is 2σ. (B) U-Pb three-dimensional linear regression for Acapulco phosphate fractions 1 to 6. The 207Pb/206Pb and 238U/206Pb ratios are not corrected for initial common Pb. The regression yielded an age of 4556.4 ± 3.2 My, MSWD = 5.0, and probability of fit = 0. Common-Pb plane intercepts at 206Pb/204Pb = 9.79 ± 0.57 and 207Pb/204Pb = 10.60 ± 0.39 give an estimate for initial Pb isotopic composition in the phosphates. Error ellipses for the U-Pb data are shown with solid lines; concordia plane projections, with dashed lines.

Richardton phosphates analyzed in this study are splits from the material previously analyzed for U-Pb (18). Ten phosphate fractions analyzed for Lu-Hf yielded an isochron with the slope of 0.08914 ± 0.0023 (Fig. 3). Exclusion of fraction 17, which contains turbid grains, gives an isochron with a slope of 0.08855 ± 0.00074. This slope, combined with the Pb-Pb isochron age of Richardton phosphates of 4550.7 ± 2.6 My (18), gives the λ176Lu estimate of 1.864 × 10–11 ± 0.016 × 10–11 year–1 (uncertainty includes all sources of errors as above).

Fig. 3.

Lu-Hf isochron diagram for Richardton phosphates. Error bars are 2σ and are smaller than the plotting symbols for analyses with lower 176Lu/177Hf ratios. Regression of all 10 phosphate analyses yielded an isochron with the slope of 0.08914 ± 0.0023 and the y intercept of 0.292 ± 0.062 (MSWD = 2.5 and probability of fit = 0.011). Exclusion of fraction 17 (shown with open symbol), which contains turbid grains, gives an isochron with a slope of 0.08855 ± 0.00074 and a y intercept of 0.2792 ± 0.0019 (MSWD = 0.55 and probability of fit = 0.80). The errors of 176Lu/177Hf and 176Hf/177Hf ratios are assumed to be uncorrelated, as usually done in Lu-Hf isochron calculations. The errors can be correlated for the fractions 20, 22, and 23, where blank correction on 177Hf is significant. Assigning error correlations of 0.9 to these fractions changes the slope of the isochron to 0.08859 ± 0.00074 and MSWD = 0.62. The difference in the slopes of the isochrons calculated with uncorrelated and correlated errors is insignificant.

The content of radiogenic 176Hf is between 2.8 and 80.6% of the total amount of 176Hf in the Acapulco phosphates and between 19.6 and 97.9% in the Richardton phosphates (table S1), therefore uncertainty and possible variations of initial 176Hf/177Hf do not significantly affect the isochrons. The y intercepts of both Lu-Hf isochrons (0.2802 ± 0.0012 for Acapulco and 0.2792 ± 0.0019 for Richardton) agree within error with the solar system initial values obtained from whole-rock isochrons for eucrites [0.27966 ± 0.00002 (9)] and chondrites [0.279628 ± 0.000047 (10)], further supporting the validity of the phosphate isochrons.

A constant decay rate of 176Lu over the past 4.56 Gy has several implications. First, the initial Hf isotopic ratios of rocks and minerals of any age, including the oldest zircons, should be calculated by using the decay constant value determined from terrestrial minerals (2, 7). Second, the reliably known decay constant is a precondition for determination of the initial state of the Lu-Hf system in the Earth and the solar system.

Extreme fractionation of Lu and Hf by meteoritic phosphates (the highest measured 176Lu/177Hf ratio of 142 in Richardton fraction 22 is about 4000 times higher than the average chondritic value) places severe limits on the applicability of chondrites for determination of the solar system initial 176Lu/177Hf ratio. Equilibrated ordinary chondrites, in which a substantial portion of Lu is contained in Ca phosphates (SOM Text), are especially prone to phosphate-related Lu-Hf fractionation. Considering the degree of Lu-Hf fractionation in phosphates and association of phosphate grains with metal [(26) and references therein], it is unlikely that even the most careful crushing of equilibrated ordinary chondrites can produce powders with homogeneous Lu/Hf ratio. The most accessible group of carbonaceous chondrites, the CV chondrites, contain abundant refractory Ca-Al-rich inclusions with strong volatility-related fractionation of rare earth elements (27) and therefore possibly fractionated Lu/Hf. Primitive chondrites, which were not affected by a high-temperature metamorphism, have ratios of major refractory lithophile elements close to the solar photosphere values (28): CI, possibly CM, CR, and most primitive ordinary chondrites might be better candidates for determination of the initial solar system 176Lu/177Hf ratio.

The evidence that the spread of Lu/Hf ratios is controlled by phosphate abundance (SOM Text) is hard to reconcile with the model of enhancement of the 176Lu decay by gamma-induced photoexcitation (29). Most chondritic phosphates have ages similar to or younger than the Richardton and Acapulco phosphates analyzed here (26) and thus postdate the proposed photoexcitation. In this case, the slope of a whole rock chondritic Lu-Hf isochron is expected to be similar to the slopes of meteoritic phosphate isochrons.

Supporting Online Material

www.sciencemag.org/cgi/content/full/310/5749/839/DC1

Materials and Methods

SOM Text

Fig. S1

Tables S1 to S3

References and Notes

References and Notes

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