ionization mass spectrometry and high-resolution
inductively coupled plasma source mass spectrometry, respectively (16). We measured the
238U/235U ratios of three CAIs from the reduced
CV chondrite Efremovka, three chondrules from
the oxidized CV chondrite Allende, and whole-rock chondrites and differentiated meteorites in
an attempt to understand the extent and origin
of 238U/235U variations in the early solar system
(Fig. 1 and Table 1). The Efremovka CAIs show
a range of 238U/235U ratios (Table 1), confirming the presence of U isotope variability in refractory inclusions. In contrast, our analyses of
meteorites derived from chondritic and differentiated asteroids as well as three individual chondrules from Allende show identical 238U/235U
ratios within analytical uncertainty (Fig. 1), defining a weighted mean of 137.786 T 0.013.
These observations indicate a uniform 238U/235U
ratio in the inner solar system outside the CAI-forming region. This is consistent with an earlier
study (17) but at odds with the solar system
initial 247Cm/235U value of ~1.1 × 10−4 inferred
from 238U/235U variability in Allende CAIs (15).
Moreover, our analyses of Efremovka CAIs show
significant departure from the apparent correlation between the 144Nd/238U [an assumed proxy
for Cm/U (18)] and 235U/238U ratios of Allende
CAIs (15), similarly to another recent study (19).
Thus, we infer that the 238U/235U variability in
CAIs largely reflects mass-dependent fractionation associated with the CAI-forming process
and not 247Cm decay (supplementary materials
The subset of chondrite meteorites we ana-
lyzed includes the Ivuna carbonaceous chondrite,
a member of the rare clan of primitive mete-
orites referred to as CI chondrites. Composed
of matrix material with the highest abundances
of presolar grains, CI chondrites are generally
considered to represent the least chemically frac-
tionated and least thermally processed mete-
orites: They have solar abundances of most
elements (20) and, by extension, have the solar
247Cm/235U ratio. Therefore, we interpret the
238U/235U value of 137.786 T 0.013 obtained for
bulk inner solar system materials as represent-
ing the best estimate of the bulk 238U/235U ratio
of the solar system and hence that of the Sun.
Table 1. Summary of Pb-Pb ages, 238U/235U ratios used in age calculations, and 54Cr compositions
of individual CAIs and chondrules. The Pb concentrations are based on the total amount of Pb
analyzed. m = 238U/204Pb. The e54Cr values represent 104 deviation of the 54Cr/52Cr value of a sample
relative to the terrestrial chromium reference standard and were acquired following Trinquier et al.
(41). Uncertainties reflect the external reproducibility of 9 ppm. The e54Cr value for the 31E CAI is from
Larsen et al. (13).
Weight (mg) m
Pb (ppb) Age (My) 238U/235U e54Cr
178.8 4567.35 T0.28 137.627 T0.022
119.4 4567.23 T0.29 137.770 T0.022 6.8 T1.2
322.3 4567.38 T0.31 137.832 T0.022
24.1 4567.32 T0.42 137.786 T0.013 –0.58 T0.09
78.3 4566.67 T0.43 137.786 T0.013 –0.60 T0.09
40.8 4566.24 T0.63 137.786 T0.013 –0.36 T0.09
27.6 4566.02 T0.26 137.786 T0.013 –0.87 T0.09
77.7 4564.71 T0.30 137.786 T0.013 –0.24 T0.09
Fig. 1. 238U/235U ratios of individual chondrules,
bulk chondrites, and achondrites. These samples
define a mean of 137.786 T 0.013 [mean square
of weighted deviations (MSWD) = 1.2], which we
interpret as the present-day solar 238U/235U ratio.
The vertical gray band reflects the 2 SD uncer-
tainty of the 238U/235U solar value. Uncertainties
of sample measurements reflect external repro-
ducibility or the internal precision of individual
analyses, whichever is larger.