Chemical Geology 259 (2009) 143–151
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Chemical Geology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c h e m g e o
Pb–Pb dating of chondrules from CV chondrites by progressive dissolution
J.N. Connelly a,b,⁎, M. Bizzarro a
a
b
Geological Museum, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen, Denmark
The Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, 78712, USA
a r t i c l e
i n f o
Article history:
Received 15 April 2008
Received in revised form 25 October 2008
Accepted 1 November 2008
Editor: D. Rickard
Keywords:
Chondrules
CV chondrite
Pb-Pb
Geochronology
Progressive dissolution
a b s t r a c t
To refine absolute Pb–Pb ages of chondrules, we have experimented with progressive dissolution procedures
of chondrules in the CV chondrite Allende in an attempt to effectively remove terrestrial contamination and
generate Pb fractions of sufficient size and spread in 207Pb–206Pb space to adequately define meaningful
isochrons. Samples are extricated from the matrix, ultrasonicated, abraded and repeatedly pre-cleaned by
ultrasonicating in distilled ethanol, water and acetone. Subsequent steps of weak HCl and HBr acids
combined with ultrasonic agitation and modest heating effectively removes terrestrial contamination in
most samples we analyzed. Subsequent exposure to warm 4 M HNO3 typically isolates the least-radiogenic
component of the stepwise dissolution series. Continuing the dissolution steps with HCl of increasing
molarity with longer and higher temperature heating steps returns more radiogenic Pb fractions. The most
radiogenic Pb fraction typically corresponds to the first warm 1 M HF step with subsequent steps in stronger
HF yielding progressively less radiogenic signatures. Importantly, the Pb components extracted in the
dissolution of the final residues with 28 M HF + 14 M HNO3 typically lie slightly below the isochron defined by
previous steps, corresponding to a slightly younger 207Pb/206Pb model age than the inferred real age. The
wide range of total Pb contents (20–N 2000 pg) in different fractions returned by this method requires
analyses by TIMS using a high-efficiency Pb emitter, 202Pb/205Pb double spike and diligent monitoring of
laboratory Pb blank. As expected, U and Pb are highly and variably fractionated during the procedure such
that no U/Pb age information is recovered. A multi-chondrule fraction from the CV chondrite Allende yields
an age of 4565.32 ± 0.81 Ma, which is statistically younger than calcium–aluminum inclusions from the CV
chondrite Efremovka.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
As the earliest formed solids in the solar system, calcium–aluminum
inclusions (CAIs) and chondrules contain petrographic, geochemical and
isotopic records that constrain the evolution of the first few million years
of the solar nebula. Available petrographic, U–Pb and Al–Mg isotopic
data indicate that CAIs formed first and in a brief interval of perhaps less
than 100,000 yr at 4567.11 ± 0.15 Ma (Amelin et al., 2002; Bizzarro et al.,
2004; Thrane et al., 2006; Amelin et al., 2006). Conversely, textures in
chondrules indicate that they have a more complicated history that may
include extensive and, in some cases, repeated reworking (Scott and
Krot, 2005). As expected, this greater degree of complexity is also
expressed by the isotopic data that infer a range of ages that span the
first 3 Myr of the Solar System (MacPherson et al., 1995; Russell et al.,
1996; Kita et al., 2000; Huss et al., 2001; Kita et al., 2005). Bizzarro et al.
(2004) presented the first high-precision internal Al–Mg isochron for a
single chondrule in the CV chondrite Allende that corresponds to an age
of 0.25 Myr after CAI formation. They concluded that at least a subset of
⁎ Corresponding author. Geological Museum, University of Copenhagen, Øster
Voldgade 5-7, 1350 Copenhagen, Denmark.
E-mail address: connelly@mail.utexas.edu (J.N. Connelly).
0009-2541/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemgeo.2008.11.003
chondrules formed very early, approximately synchronous with CAI
formation.
Bizzarro et al. (2004) and Thrane et al. (2006) present compelling
evidence to support the homogeneous distribution of 26Al in at least
the CAI formation region and the accretion region of the terrestrial
planets. However, there exists an alternative proposal that 26Al and
other extinct nuclides formed heterogeneously throughout the disk by
solar irradiation such that initial 26Al contents may reflect different
production rates rather than different ages (Shu et al., 1997; Lee et al.,
1998; Shu et al., 2001; Gounelle et al., 2001). Unconstrained by any
chronological certainty, this general model allows for CAIs and
chondrules to have formed coevally and for isotopic and compositional differences to reflect different regions of a heterogeneous disk.
With CAIs from the CV chondrite Efremovka precisely dated by
Amelin et al. (2002, 2006), resolution of this debate requires a robust
absolute age for chondrules from this same meteorite group. This task
has been complicated by ubiquitous terrestrial Pb contamination in
meteorites and the model dependency of Pb–Pb ages determined from
highly-radiogenic Pb separates resulting from aggressive leaching. In
these cases, a lack of spread in 204Pb/206Pb vs 207Pb/206Pb space requires
that the initial Pb isotopic composition be modeled or approximated by
some proxy measurement. Whereas only a bonafide mineral (i.e.
144
J.N. Connelly, M. Bizzarro / Chemical Geology 259 (2009) 143–151
internal) isochron can assure fulfillment of all prerequisite conditions for
a reliable age determination, the fine-grained nature of chondrules
makes mineral separation difficult. The current study was initiated to
refine U–Pb dating methods for chondrules to produce a robust model
independent Pb–Pb age for chondrules from Allende and, thus, to test
the validity of the 26Al–26Mg chronometer. We evaluated methods to
produce internal isochrons for chondrules by stepwise dissolution that
exploits the range of solubilities of different chondrule phases. This
method provides a means to evaluate whether terrestrial contamination
is successfully removed and the projected initial Pb isotopic composition
is reasonable as well as returning sufficient spread in Pb–Pb space to
reduce the final error on calculated ages.
1.1. The U–Pb chronometer
The U–Pb isotopic system represents the only high-resolution absolute
chronometer for dating early solar system materials but its utility is
commonly compromised by complex systematics related to contamination by terrestrial Pb. Pb–Pb ages may be calculated courtesy of the duality
of the U–Pb radiogenic system where 238U decays in a series to 206Pb and
235
U decays in a series to 207Pb. Given the different decay rates of the two U
isotopes, radiogenic Pb (Pbr) will have a 207Pb/206Pb ratio that is uniquely
related to the time before present that the sample formed or when Pb
diffusion ceased in the sample. A sample comprising only Pbr would yield
an accurate and precise age that corresponds to the formation age of that
sample provided all Pbr was formed in situ and that the 238U/235U of the
sample is known. However, most, if not all, samples contain a component
of initial Pb (Pbi) such that it is not generally possible to isolate a pure Pbr
component. Therefore, the 207Pb/206Pb ratio of Pbr must be determined by
extrapolation of a line defined by analyses of fractions with different
mixtures of Pbr and Pbi plotted in 204Pb/206Pb vs 207Pb/206Pb space to the yaxis where 204Pb/206Pb, and hence Pbi, are equal to zero. Identifying the
appropriate 207Pb/206Pb ratio of Pbr with an acceptable error by this
method requires that there are sufficiently radiogenic fractions with
adequate spread to constrain the line and, therefore, the intercept. Ages
determined from a highly-radiogenic component that is isolated by
intense acid leaching exploits a short extrapolation to the y-axis to
minimize error in the final age determination, but even modest values of
204
Pb/206Pb will require an accurate estimate of the isotopic composition
of Pbi to calculate an accurate age.
Meteorites contain a third component of Pb, namely terrestrial
contaminant Pb (Pbc), which may comprise a number of different subcomponents with distinct isotopic compositions, including a ubiquitous
laboratory blank. This last sub-component should represent a well
characterized, minor constituent that may be accounted for in the
final isotopic ratios whereas extraneous contamination (field, curation,
etc) cannot be accurately subtracted. As such, determination of an
accurate and precise age from Pb isotopic analyses generally requires
that all Pbc is removed from the sample prior to dissolution such
that only binary mixtures of Pbr and Pbi are represented in fractions
used to define an isochron age. Assurances of complete removal of
Pbc is best provided by linearity of data points and the extrapolation of
lines to reasonable estimates of Pbi. It should be noted that pure
mixtures of Pbr and Pbc will also define lines that project to the correct
207
Pb/206Pb of Pbr but it is typically difficult to assess whether Pbi is
completely absent.
Recent work by Y. Amelin (e.g. Amelin et al., 2002; Krot et al., 2005;
Amelin and Krot, 2007) using the Pb isotopic system has demonstrated
that highly-radiogenic Pb can be isolated in residues of some aggressively
acid leached meteorite samples. This permits precise ages to be calculated
by either regressing multiple analyses of related materials or by
determining or assuming the isotopic composition of Pbi for purposes of
extrapolation through a single radiogenic point or cluster to a composition
reflecting a pure Pbr. Any error in assigning the initial Pbi composition will
result in an inaccurate age, despite the high precision resulting from the
short extrapolation.
This study tests whether stepwise dissolution of chondrules from
Allende (CV chondrite) will effectively first remove Pbc and then parse
the remaining Pbi and Pbr into separate fractions with sufficient spread
in Pb–Pb space to define an appropriate, well-constrained internal
isochron that would better characterize the isotopic composition of Pbr
and, therefore, the age of a sample. Linearity of points from this method
would provide an internal test of whether a single growth evolution is
likely for the sample analyzed and remove necessary assumptions about
Pbi, thereby increasing confidence in the calculated compositions of Pbr.
2. Methods
Chondrules (single or multiple) or chondrule fragments were extracted
from a single, four centimetre diameter sample of Allende meteorite that
was coarsely broken in a hydraulic press with the sample wrapped in
paper. Chondrules from the resulting fragments were then excavated from
matrix fragments and pre-cleaned using dental tools. Chondrules to be
analyzed were ultrasonicated in distilled water until matrix was removed
from the surface of the chondrule, as determined by inspection using a
binocular microscope at 25× magnification. Samples were subsequently
abraded in a microabrader lined with corundum-coated mylar until the
fragment or chondrule exterior was smooth (24–56 h). Fractions
comprising single chondrules, multiple chondrules or chondrule fragments (Table 1) were taken to the clean laboratory for a pre-cleaning
treatment that comprised repeated ultrasonicating and rinsing in
ultraclean water. This pre-cleaning procedure was repeated 3 times
beyond the point when the water was clear after 5 min of ultrasonicating.
None of these washes were processed for isotopic analyses. Some samples
were coarsely crushed using an agate mortar and pestle whereas other
samples were processed as whole chondrules or fragments (Table 1).
Samples that were crushed were pre-cleaned again by repeated
ultrasonicating and rinsing in ultraclean water. The samples were
transferred to a 7 ml Savillex® vial and were treated to a second precleaning step in water, acetone, ethanol and very dilute HCl while gently
heated (75 °C) on a hotplate and/or ultrasonicated. This was repeated for a
minimum of 5 cycles of acetone/ethanol/water. After a final rinse 3 times
in water, each fraction was subjected to a series of different acid steps as
defined in Table 1. The acid was carefully removed from the residue and
transferred into a clean 7 ml Savillex® capsule using a pre-cleaned
Eppendorf® pipette after which the residue was rinsed and ultrasonicated
twice in water – only the first water rinse was added to the preceding acid
step. To ensure dissolution of fine, suspended material that was
inadvertently transferred to the Savillex® capsule, approximately 0.1 ml
of 28 M HF was added to fractions that lacked this acid after it was isolated
from the residue. Centrifuging may be a better method of removing the
suspended material but this was not evaluated for fear of increasing the Pb
blank during handling. The fractions were dried down, re-dissolved in
concentrated HCl–HNO3 mixtures to bring them fully in solution at which
time a mixed 202Pb–205Pb–235U spike (205Pb/202Pb =0.44069; 205Pb/
206
Pb=1208; 205Pb/207Pb=1620; 205Pb/208Pb=496; 205Pb/204Pb=11410;
205
Pb/235U=119.66; 235U/238U=1405; spike prepared by F. Corfu, Oslo
University and calibrated against NBS-982) was added to each fraction to
determine Pb isotopic mass fractionation during analyses (Amelin and
Davis, 2006) and elemental abundances. This mixture was dried down and
finally re-dissolved in 1 M HBr, the appropriate acid for the first chemical
separation step. Pb was separated from the matrix in two passes through
0.055 ml anion columns following Lugmair and Galer (1992) except that
the samples were loaded in the first pass onto the columns with 1 M HBr.
Uranium was separated from the wash solution from the Pb chemistry
using U-TEVA® resin chemistry after Davis et al. (1997). Pb from different
chondrite fractions was analysed on two mass spectrometers in two
laboratories following the same protocol and using a Pb emitter made
from silicic acid after Gerstenberger and Haase (1997). All beam intensities
larger than 2 mV were analysed by faraday detectors in static mode.
Samples with a 204Pb signal intensity too small for a faraday detector but
with large enough intensities of the other Pb masses (including 202Pb
Table 1
Isotopic data for Allende chondrules
J.N. Connelly, M. Bizzarro / Chemical Geology 259 (2009) 143–151
145
146
J.N. Connelly, M. Bizzarro / Chemical Geology 259 (2009) 143–151
and 205Pb) were analysed by faraday detectors in static mode but with
204
Pb in an axial secondary electron multiplier–ion counting system
(SEM-IC; Finnigan MAT 262, Oslo) or Daly photomultiplier (VG-354,
Copenhagen). To avoid the necessity of an accurate SEM-IC vs. faraday
calibration, these samples were also analyzed dynamically at lower
filament temperatures with all masses analyzed sequentially measured
in an axial SEM-IC or Daly. Samples with all beams too small to be
effectively measured by faraday detectors were dynamically analysed
with all masses sequentially measured in an axial SEM-IC or Daly
detector. Analyses using the Daly detector in Copenhagen are less
accurate than those from Oslo as the former instrument has not been
well calibrated for small Pb samples. As such, the data from Copenhagen
from fractions C1, C2, C4 and C7 were used only to evaluate and refine
the stepwise dissolution procedures. Using information gained in these
early tests, Allende fraction C12 was subjected to our optimized step
dissolution sequence and were analysed at the University of Oslo to
determine an accurate age. The mass spectrometer protocols used for
each fraction are listed in Table 1. Final ratios, errors and error
correlations (Table 1) were calculated using an in-house program and
PbDat (v. 1.24, Ludwig, 1993). Ages were calculated using Isoplot Ex v. 3
(Ludwig, 2003), decay constants of Jaffey et al. (1971) and a natural 238U/
235
U ratio of 137.88. Chemical analyses of fractions of Allende chondrule
C12 were produced by an Elan 6100 quadrupole ICP-MS at the Geological
Survey of Denmark and Greenland.
3. Results
3.1. Allende chondrules C2, C4 and C7
To evaluate the effects of different acids and acid strengths in stepwise
dissolution of chondrules, three samples comprising abraded, pre-cleaned,
uncrushed, single or multiple chondrules were treated by slightly different
methods as described in Table 1. The results are shown graphically in
Fig. 1A–C. Chondrule fractions C2, C4 and C7 share the common attribute
that the early steps are dominated by Pbi with lesser but varying degrees of
Pbc and Pbr. The first acid steps of C4 and C7 chondrules plot on a line
between primordial Pb (PAT, Tatsumoto et al., 1973) and a subset of the
remaining fractions indicating that most of the Pbc was effectively
removed in the pre-cleaning steps (ethanol–acetone–water–weak HCl on
a hotplate and in an ultrasonicator). In general, the isotopic composition of
the Pb extracted in progressively stronger HCl becomes more radiogenic
falling on or close to a line defined by the majority of the fractions in each
sample. Consistent departures from the general pattern of pre-HF steps
becoming more radiogenic include the HBr step in C4 and C7, which
contains a more radiogenic signature than subsequent 6 M HCl steps
(Fig. 1A–C). This implies that HBr removed a radiogenic phase and, as
demonstrated in C7, may extract a phase containing Pbc that was not
affected by earlier steps of 2 M HCl and 4 M HNO3. It is also clear that HBr
does not liberate large quantities Pb of either Pbr or Pbi when applied after
2 M HCl and 4 M HNO3. 6 M HCl extracted Pb in roughly equal proportions
from Pbi and Pbr reservoirs of C4 and C7 but had apparently nearly
exhausted the Pbi component in C2 as L7 had only 6 pg of Pb and L9 and
L10 were very radiogenic.
The first application of HF, regardless of acid strength, consistently
liberates the most radiogenic Pb. Sample C2 was attacked with 28 M
HF–14 M HNO3 as the first HF step after 6 M HCl, producing a fraction
with highly radiogenic Pb consistent with the Pbi reservoir having
been nearly exhausted by this point. HF steps of chondrules C4 and C7
show the interesting and important property that the first HF step
extracts the most radiogenic Pb and that increased HF strength
becomes slightly more enriched in Pbi, indicating that this component
had not been as effectively removed in earlier steps of C4 and C7
relative to that in C2. Also important to note is that the dissolution of
the final residual fractions in C4 and C7 with 28 M HF–14 M HNO3 in a
bomb at 200 °C falls below the line defined by the majority of points
indicating that they still contained a component of Pbc that had not
been previously digested or that there was an undocumented
additional component of Pb blank in the last step. Presuming the
line defined by the majority of earlier steps corresponds to the real age
of these chondrules, it is clear that including the component
represented in the final dissolution of the residue of C4 and C7 in an
isochron would have resulted in a slightly younger age.
In summary, the approach of stepwise dissolution of samples with
increasingly stronger acids returns the desired result of producing
progressively more radiogenic fractions that define a line corresponding to an isochron. A modest pre-cleaning with ethanol, acetone,
water and very weak HCl on a hotplate and in an ultrasonic is effective
in removing most of the terrestrial contamination of these chondrules.
The most obvious difference between these three procedures lies in
the degree to which the final residues are radiogenic. Whereas this
may represent compositional variations between chondrules, it may
also reflect the aggressiveness of acid dissolution/leaching prior to the
application of HF. The major difference between C2 vs C4/C7 stepwise
dissolution is the more aggressive use of 4 M HNO3 early in the
procedure of C2, which apparently removes phases containing Pbi
more effectively than HCl.
3.2. Allende chondrule C1
After obtaining the results from the dissolution of fractions C2, C4
and C7, a large (39 mg) chondrule was broken into two pieces that
were 6.4 mg and 33.0 mg (labeled C1a and C1b, respectively).
Chondrule C1a, processed first to characterize this sample, was
subjected to a full stepwise dissolution procedure but only the final
dissolution step was analyzed as an unspiked sample. Correction for
blank Pb was based on signal intensity and experience with other
fractions of comparable size and dissolution sequences. The full
dissolution of the final residue represents the only HF step in this
sequence and produced a radiogenic signature that exceeded all
subsequent analyses of C1b (Fig. 1D).
The remaining larger piece of chondrule (C1b) was processed
differently than C1a, with the former lacking an early HNO3 step and
the dissolution of silicates was facilitated by a series of progressively
stronger HF steps. This chondrule responded differently than C2, C4
and C7 in that it lacked a systematic shift from a composition
dominated by Pbi to more radiogenic compositions and the early
steps, most notably L1-5 in 2 M HCl, are much more radiogenic than
comparable steps from the other chondrules. Conversely, none of the
steps produced a radiogenic signature as high as the final residue of
C1a. The erratic jumps in relative proportions of Pbi and Pbr in the
Notes to Table 1
Hp = hotplate.
US = ultrasonicator.
= not included in age regression.
a
- Mode of mass spectrometer.
b
- Mass spectrometer facility.
SF = static mode with all masses measured in faraday collectors.
DD = all masses measured in dynamic mode using a daly - photomultiplier ion counting system.
SF-DD6/4 = all masses measured in static mode using faraday collectors except the 206Pb/204Pb ratio which was collected in dynamic mode using a daly - photomultiplier ion
counting system.
⁎⁎ = unspiked fractions.
J.N. Connelly, M. Bizzarro / Chemical Geology 259 (2009) 143–151
147
Fig. 1. A–E. 204Pb/206Pb vs 207Pb/206Pb diagrams for Allende fractions with each fraction labeled with sequence number, acid used and total Pb (after blank correction) in parentheses.
Dark square symbols represent those fractions that define a line whereas the light square symbols represent those fractions that do not fall on a line. MT = average modern terrestrial
Pb (Stacey and Kramers, 1975), PAT = primordial Pb defined by Tatsumoto et al., 1973).
progressive steps indicates that there is a reservoir or reservoirs of Pbi
that was not effectively removed early in the stepwise dissolution
procedure and is, instead, intermittently sourced by different steps. Most
obvious is the fact that L11, which used 28 M HF and the first use of HNO3
in this sequence, contains a less radiogenic Pb than earlier steps in HCl.
The dissolution array for C1a and C1b collectively highlights the
importance of using HNO3 early in the procedure to effectively remove
a Pbi hosting phase that is not effectively removed by HCl.
3.3. Allende multi-chondrule fraction C12
Integrating the results from chondrules C1–C7, a stepwise dissolution procedure was devised for the multi-chondrule fraction C12 to
obtain an accurate and precise age by: 1) optimizing the removal of Pbc
early and with minimum loss of either Pbi or Pbr, and 2) maximizing the
dispersion in Pb–Pb space of subsequent dissolution steps by preferentially isolating Pbi early in the dissolution process with 4 M HNO3. This
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J.N. Connelly, M. Bizzarro / Chemical Geology 259 (2009) 143–151
3.4. Chemical data for chondrule C12
Fig. 2. 204Pb/206Pb vs 207Pb/206Pb diagrams for Allende chondrule fractions from C12 that
define an isochron. Fractions labeled with sequence number, acid used and total Pb (after
blank correction) in parentheses. Error ellipses represent 2-sigma errors. Errors and error
correlations were calculated using PbDat (Ludwig,1993) whereas ages, errors on ages and the
mean square of weighted deviates (MSWD) of isochron utilized Isoplot Ex v.3 (Ludwig, 2003).
fraction also tested whether the most radiogenic component is returned
in lowest molarity HF of Pbc and whether the final residue consistently
contains remnant Pbc. The results have been presented in Connelly et al.
(2008a,b) and are shown in Fig. 1E.
Recognizing that 2 M HBr has minimum effect on at least those phases
hosting Pbi, this acid was selected to start the dissolution procedure. Steps
L1 (2 M HBr) and L2 (2 M HCl) fall below the line defined by the remaining
steps (except L10) and are inferred to contain a significant component of
Pbc (approximately 33% and 27%, respectively). These early steps
successfully remove Pbc as the next 6 steps define a statistically acceptable
line that passes through PAT and is inferred to represent an isochron
corresponding to an age of 4565.32±0.81 Ma (mean square of weighted
diviates [MSWD]=0.59; Fig. 2). The first step in this progression (4 M HNO3
that was heated to 110 °C for 20 min) yielded 992 pg of Pb that was
dominated by Pbi. A subsequent step of water heated overnight contained
no measurable Pb. The next three steps in HCl (L5–L7) contain a total of
277 pg of Pb with similar isotopic compositions reflecting subequal
amounts of Pbi and Pbr. The HF steps began with cold 1 M HF that yielded
the most radiogenic component with a 204Pb/206Pb ratio of 0.0112.
Subsequent HF steps liberated a component of Pb from a Pbi source that
had not been exhausted by earlier steps. The full dissolution of the final
residue plots below the isochron (Fig. 1E), indicating that a phase with a
minor amount of Pbc must have persisted through all previous steps. A
total of 2.42 ng of Pb was extracted from this multi-chondrule fraction for
an average Pb concentration of 0.11 ppm.
To better understand the sequence in which phases were breaking
down during the stepwise dissolution procedure, the washes of each
step of C12 from the anion exchange chemistry were analysed for major
elements plus Co, Ni and P (Table 2). The results do not provide a unique
solution but rather a general guide as to which major silicate phases
broke down at different steps in the procedure. Using the fingerprint of
low Mg/Na and Mg/Al ratios of plagioclase, it is clear that the first two
steps L1 and L2 are dominated by this phase. The high Mg/Al, Mg/Na,
Mg/Ti, Mg/Ca in the subsequent steps L3 and L4 indicate that olivine
began to break down during these steps. Given the high olivine/
plagioclase modal abundances of these chondrules, it is possible that
plagioclase continued to breakdown through these steps but its
chemical signature was overwhelmed by the olivine signal. The decrease
in all these ratios in L5 but with a Mg/Na ratio higher than L1 and L2
indicates a contribution by pyroxene by this step. The remainder of the
steps contain different proportions of olivine and pyroxene except for
the last step, which appears to be dominated by pyroxene as indicated by
the high Mg/Fe and Mg/Na ratios and intermediate Mg/Al and Mg/Ca
ratios. These data do not account for sulfides or oxide phases that may
have been present. Sulphides and phosphates are expected to have low
and high 238U/204Pb (μ) values, respectively, and breakdown early in the
procedure. Oxides likely have very little effect on the U–Pb budget but
would break down late in the procedure.
Assuming the μ values of the three major silicates increase in the
order of plagioclase–olivine–pyroxene, the silicate breakdown modeled
after geochemical data is broadly consistent with the Pb–Pb data, except
for the last HF steps where Pbr / Pbi decreases. The Pb budget of the early
steps is likely strongly influenced by sulfides present in the chondrules
with the predominance of Pbi in the early steps indicating the effective
breakdown of sulfides with HCl and, especially, HNO3.
3.5. Evaluation of blank Pb correction
Blank subtraction is critical to the final age, error and fit of the
isochron derived from the blank corrected data. The amount of Pbc
contributed by chemistry and mass spectrometry has been determined
over the course of this study by direct measurement of blank samples
processed through chemistry – this has yielded an average blank
estimate of 1.8± 0.5 pg (n = 10) that includes the mass spectrometer
loading blank of approximately 0.4 pg. A full procedural blank is
estimated by mimicking all steps, including transfer of acids and
cleaning solutions by Eppendorf pipettes, of a full dissolution series to
yield one blank estimate for every fraction analyzed. This has yielded an
average full procedural blank of 3.0 ± 1.0 pg (n = 15).
Diligent care in sample preparation and careful monitoring of Pb
blanks through the full procedure assures that Pb blanks are consistent
and, therefore, correctable to within the assigned error on the blank. In
turn, this error is propagated through the calculation such that the blank
Table 2
Chemical analyses of washes after Pb separation for dissolution steps of Allende chondrule fraction C12
C12-L1
C12-L2
C12-L3
C12-L4
C12-L5
C12-L6
C12-L7
C12-L8
C12-L9
C12-L10
2M HBr
2M HCl
4M HNO3
H2O
2M HCl
6M HCl
6M HCl
1M HF
1M HF
28M HF
Al
Ca
Fe
Mg
Mn
K
Na
Ti
Co
Ni
P
4.788
0.299
1.965
0.258
1.538
0.147
1.957
0.600
0.164
3.764
1.694
0.136
1.118
0.632
0.952
0.181
1.492
1.105
7.563
3.412
7.702
0.803
10.998
5.884
6.614
1.573
9.288
0.605
10.352
2.394
13.044
1.104
19.080
17.216
9.206
2.320
36.858
2.118
44.960
33.680
0.038
0.005
0.056
0.042
0.024
0.007
0.073
0.012
0.330
0.062
0.204
0.011
0.037
0.008
0.013
0.002
0.011
0.001
0.007
0.004
2.570
0.159
0.552
0.080
0.200
0.020
0.176
0.052
0.178
0.224
0.077
0.011
0.044
0.014
0.038
0.014
0.044
0.056
0.587
0.131
0.018
0.002
0.044
0.019
0.005
0.002
0.027
0.001
0.041
0.006
0.348
0.043
1.174
0.504
0.100
0.043
0.488
0.013
0.887
0.123
0.071
0.011
0.022
0.001
0.026
0.002
0.005
0.001
0.004
0.002
NOTE: Concentration in ppm of solution run by an Elan 6100 quadrupole ICP-MS at the Geological Survey of Denmark and Greenland relative to 50 and 250 ppb standard solutions.
Aliquots of the washes from the Pb chemistry were dried down and redissolved in 10 ml of 1M HCl for analyses.
J.N. Connelly, M. Bizzarro / Chemical Geology 259 (2009) 143–151
149
after leaching of chondrules by Amelin and Krot (2007). This may reflect
true variation between chondrules or that Amelin and Krot (2007)
employed a leaching procedure that more effectively attacked the phases
containing Pbi prior to final dissolution of the residue. An alternative
possibility is that the more protracted dissolution procedure using a
wider range of acids and acid strengths may have compromised the
eventual integrity of the high μ phases. If correct, this may require parallel
processing of two samples, one by a protracted dissolution procedure
intended to provide a spread of Pb compositions and the other following
the procedures of Amelin and Krot (2007) intended to provide highlyradiogenic values. In some cases, mineral separates of high μ phases may
be necessary to insure highly-radiogenic phases.
4.1. Defining initial Pb (Pbi)
Fig. 3. Plot shows variation of MSWD (mean square of weighted deviates – y axis) for the
data uniformly corrected for different amounts of Pb blank (x axis). The minimum value for
the MSWD between 1.5 and 3.0 pg supports the assigned Pb blank of 3.0± 1.0 pg of Pb.
error is reflected in error on the reported age. Since the sample to blank
ratio varies from fraction to fraction, a second check of whether blank is
appropriately subtracted comes from assessment of the quality of fit of the
regression of the data array as a function of the blank assignment. An
inappropriate blank assignment would tend to randomize a data array
that would otherwise define an acceptable isochron. A plot of the linearity
of the data array (as estimated by the calculated MSWD) as a function of
different amounts of blank assigned uniformly to all fractions (Fig. 3)
shows a minimum at approximately 2.5 pg. Blank assignments less than
1.5 pg and greater than 3.5 pg results in an array that does not define an
acceptable line in Pb–Pb inverse isochron space. This is inferred to indicate
that the appropriate blank assignment lies within this range, an
assessment consistent with the independent estimate of 3.0±1.0 pg
from multiple blank analyses processed in tandem with samples.
4. Discussion
Aggressive acid cleaning of meteorite samples reported in previous
studies (e.g. Amelin et al., 2002; Krot et al., 2005) prior to full digestion of
the residue has the demonstrated advantage of removing Pbc and Pbi,
thereby producing highly-radiogenic analyses that results in a short or
no extrapolation in Pb–Pb space to an inferred pure Pbr composition.
However, even minor amounts of residual Pbi require an assumption or
definition of Pbi to calculate an accurate age.
Stepwise dissolution procedures defined here appear to effectively
remove Pbc in the first steps while preserving Pbi and Pbr in the residue
that can be partitioned in subsequent steps allowing for the definition of
true internal isochrons in Pb–Pb space. More work is involved in
processing multiple steps from a single sample but linearity of different
fractions by this method tests whether: 1) Pbc has been successfully
removed early in the procedure, 2) an appropriate Pbi is inferred from the
isochron, and 3) any initial Pb is present with a highly-radiogenic
signature resulting from an initial in growth due to a high U/Pb (so-called
“aborted high µ Pb”). Failure to remove Pbc early in the procedure will
result in Pb in subsequent steps being derived from a three-component
system, which will cause scatter of points in Pb–Pb space. Furthermore,
nothing is theoretically lost in this approach as samples are processed
such that the final residue should represent a radiogenic fraction
comparable to what would have been obtained by analyzing a single final
residue after aggressive leaching/cleaning procedures. However, we note
that, except for fractions C2-L9 and C2-R, the most radiogenic fractions
analysed in this study are less radiogenic than the analyses of residues
Pb–Pb dating requires either direct measurement or robust
estimate of the 207Pb/206Pb of Pbr. As such, it is a common goal of
most Pb–Pb chronological studies to select and process samples to
derive a highly-radiogenic Pb fraction that minimizes the extrapolation to the intercept, which corresponds to the best estimate of the
pure Pbr component. However, even a sample with a highlyradiogenic composition corresponding to a 204Pb/206Pb ratio of 0.001
will return an offset of ca. 1 Myr for a model age depending on
whether the initial composition is inferred to be that of averaged
modern terrestrial Pb (MT, Stacey and Kramers, 1975) or that of
primordial Pb (PAT, Tatsumoto et al., 1973). MT is never preferentially
assigned instead of PAT as an estimate of Pbi in a meteorite when
calculating model ages but some data arrays defined by multiple
analyses project through MT rather than PAT (Baker et al., 2005;
Amelin and Krot, 2007). This infers that Pbi in that sample was either
more primitive than PAT (i.e. pre-PAT; see Tera and Carlson, 1999) or
that MT was not fully removed from the fractions processed. The
prospect of Pb isotopic heterogeneity in the early Solar System allows
for the possibility of a pre-PAT Pb composition, but there is no data to
directly support such a scenario. Attempts to measure the Pb isotopic
composition of iron meteorites as proxies for the Pb isotopic
composition in the early Solar System have all concluded that the
values of Canyon Diablo measured by Tatsumoto et al. (1973) are
representative (Oversby, 1973; Göpel et al., 1985; Connelly et al.,
2008a,b). A more primitive Pb isotopic composition than that of PAT
might be confirmed by Pb isotopic analyses of CAIs, chondrules and/or
differentiated meteorites that are both more and less radiogenic than
MT (to ensure that the line is not a mixture of highly radiogenic Pb and
MT) and that projects back to a “pre-PAT” isotopic composition. That
no such analyses are known infers that a pre-PAT reservoir did not
exist and that data arrays defined by radiogenic fractions that
extrapolate through MT are likely caused by Pbr mixed with variable
amounts of Pbc rather than Pbi.
Except in rare cases in which a sample contains no Pbi, the inclusion
of MT in analyses used to constrain Pbi will return an age older than the
true age. A linear array of points in 204Pb/206Pb vs 207Pb/206Pb space that
projects through or near PAT rather than MT provides assurance that MT
has been successfully removed and that a permissive Pbi component is
inferred by the isochron. The step dissolution method defined here
provides sufficient spread in 204Pb/206Pb vs 207Pb/206Pb space to evaluate
whether MT has been successfully removed in pre-cleaning and early
steps and that the inferred isotopic composition of Pbi is reasonable with
respect to our estimate of PAT.
The issue of defining and evaluating Pbi is seemingly overcome in
cases where aggressive leaching returns residual fractions that, within
error, have a 204Pb/206Pb ratio of zero or near zero. However, since this
scenario does not allow for evaluation of whether an internal isochron
exists in Pb–Pb space, it allows for the possibility that a highlyradiogenic Pbi component from the precursor or source reservoir is
present. Undetected pre-crystallization radiogenic Pbi incorporated in
any meteorite will make the final model age older than the true age of
150
J.N. Connelly, M. Bizzarro / Chemical Geology 259 (2009) 143–151
crystallization. In such a scenario, stepwise leaching is anticipated to
generate a spread of points along the y-axis of a Pb–Pb plot.
4.2. Matrix Pb (Pbm)
The matrix in Allende has Pb concentrations of ca. 1 ppm,
approximately 10 times that of chondrules, and has varying Pb isotopic
compositions that fall on or near the line defined by the preferred
fractions of chondrule C12 (Tatsumoto et al., 1976 and Chen and Tilton,
1976). Given this concentration contrast, it is vital that all matrix is
removed prior to analyses to avoid the non-radiogenic component being
dominated by Pb from the matrix, thus voiding the basic assumption for
an accurate isochron age calculation that the non-radiogenic Pb
component must be initial Pb. Since matrix Pb will be approximately
collinear with analyses of materials formed in the first few million years
of the solar system, alternative means must be used to ensure matrix Pb
was not included in the analyses. First, we are confident that the
ultrasonicating and abrasion treatments prior to the stepwise dissolution was effective in removing matrix material from the chondrule.
Inspection at 25× magnification using a binocular microscope confirmed
that no dark matrix was visible against the contrast of the lighter
chondrule at this resolution after these treatments. Secondly, chondrule
C12 contained 1.6 ng of non-radiogenic Pb, representing approximately
67% of the total amount of Pb. With concentrations of Pb in the matrix at
1 ppm (Tatsumoto et al., 1976 and Chen and Tilton, 1976), approximately
1.6 mg of matrix (or 8% of our sample) would have been inadvertently
included in the analyses if all non-radiogenic Pb was derived from
matrix. The lack of any observable matrix at 25× magnification precludes
this possibility. As such, we conclude that the non-radiogenic Pb in C12
represents initial Pb that was incorporated into the chondrule when it
formed such that this basic assumption for calculating an accurate
isochron is fulfilled.
4.3. Age of Allende chondrules
We report an age of 4565.32±0.81 Ma for a multi-chondrule fraction
from the CV chondrite Allende. Amelin and Krot (2007) report a preferred
Pb–Pb age for multigrain fractions of Allende chondrules of 4566.6±1.0 Ma
based on the average age of seven wash – residue sets (2 washes, 1
residue). From the same data set, the regression of the eight residues yields
an age of 4567.3±2.4 Ma (MSWD=0.76) whereas the average of eight
residue model ages (using PAT as Pbi) is 4565.9±0.8 Ma (MSWD=0.85).
Their preferred age of 4566.6±1.0 Ma and the age reported here for a
multigrain fraction of chondrules overlap within error but the age cited by
Amelin and Krot (2007) is skewed to a slightly older value and overlaps
with the accepted age of CAI formation (Amelin et al., 2002). Reasons for
this offset may include: 1) a real difference in ages of the chondrules in the
two studies, 2) a systematic bias in mass spectrometry results of one or
both of the analytical data sets, 3) an inappropriate correction for
instrumental mass fractionation in one or both of the data sets, 4)
inappropriate laboratory blank correction and/or 5) an inappropriate
assumption that the chondrule washes accurately represent the initial Pb
in the chondrules analysed by Amelin and Krot (2007). Given that both
studies analysed multiple chondrule fractions of the same meteorite, it
seems unlikely that there is a systematic age difference in the average of
these two data sets. We presume that both studies report accurate raw
ratios within their respective stated external reproducibilities. A significantly inappropriate blank correction would cause a bias in the data but
both studies report low and reproducible blanks such that this seems
unlikely to generate a systematic bias. The definition of a statistically valid
line in this study for chondrule fraction C12 also suggests that an
appropriate amount of blank Pb has been subtracted.
Consequently, the offset in ages between the two studies most likely
reflects either an inappropriate correction for instrumental mass
fractionation or an inappropriate assumption that the washes accurately
reflect initial Pb in the chondrules. Given that a double spike was not yet
available to Amelin and Krot (2007), the accuracy of the mass fractionation
correction applied to their samples cannot be assessed. Use of the washes
to constrain the isochron requires that the washes reflect a simple binary
mix of initial and radiogenic Pb, a condition that necessitates that all
terrestrial contamination was removed in earlier cleaning/leaching steps.
Assuming that the isotopic composition of terrestrial contaminant would
lie beneath a true isochron, a residual component of terrestrial
contamination would result in an older estimated 207Pb/206Pb age. The
HNO3 washes appear to form a field rather than a line with an upper
bounding surface in 204Pb/206Pb vs 207Pb/206Pb space corresponding to the
line defined by the accepted C12 fractions of this study, which
approximately intersects primordial Pb. As such, it appears that some of
the washes of Amelin and Krot (2007) contain a minor component of
terrestrial contamination that skews their wash-residue ages to ages that
are slightly older than the age determined here.
Recognizing that the data of Amelin and Krot (2007) is more radiogenic
than any points defined in this study but lacks sufficient spread to provide
an evaluation of Pbi, Connelly et al. (2008a) combined the two data sets to
define an isochron corresponding to an age of 4565.45±0.45 Ma
(MSWD=1.02). The isochron projects through PAT, indicating that Pbc
was successfully removed and these chondrules evolved as a closed
system since 4565.45±0.45 Ma. This age is 1.66±0.48 Myr younger than
CAI formation in CV chondrites (Amelin et al., 2002), requiring that these
chondrules formed or were reprocessed after CAIs. As discussed in
Connelly et al. (2008a), this age offset is consistent with that predicted by
the short-lived 26Al–26Mg chronometric system and, thus, validates the
underlying assumptions of homogeneous distribution of 26Al in the early
Solar System (see Kita et al., 2005 for review).
5. Conclusions
The method of stepwise dissolution of chondrules outlined in this
paper provides a means to remove terrestrial contamination and create
sufficient spread in Pb–Pb space to define an internal isochron and the
radiogenic 207Pb/206Pb composition. Acceptable linearity of a sufficient
number of data points provides assurance that terrestrial Pb contamination was successfully removed and that Pb evolved in a single-stage and as
a closed system. This method is made viable by the recent capacity to
accurately and precisely analyze small amounts of Pb by TIMS (sub 50 pg)
using a 202Pb–205Pb double spike and high-efficiency emitter while
maintaining a consistently low blank.
This method was successfully applied to a multi-chondrule fraction
from CV chondrite Allende. Experiments with single and multiple
chondrule fractions from Allende served to define the appropriate acid
sequence that was ultimately used to date chondrule fraction C12 at
4565.32 ± 0.81 Ma. Combining this data with the more radiogenic
analyses of Amelin and Krot (2007), Connelly et al. (2008a) report an age
of 4565.45 ± 0.45 Ma for Allende chondrules that supports 26Al/26Mg
relative age offsets between chondrules and CAIs in CV chondrites (Kita
et al., 2005). Lastly, this method appears to be effective for Pb–Pb
geochronology on other matrixes including a differentiated meteorite
from the angrite group (SAH99555; Connelly et al., 2008b).
Acknowledgements
Financial support for this project was provided by the Danish
National Science Foundation, the Carlsberg Foundation and the Jackson
School of Geosciences at The University of Texas at Austin. Fernando
Corfu is gratefully acknowledged for generously providing 202Pb–205Pb
double spike and access to his mass spectrometry facilities at the
University of Oslo.
References
Amelin, Y., Davis, W.J., 2006. Isotopic analysis of lead in sub-nanogram quantitites by
TIMS using a 202Pb–205Pb spike. Journal of Analytical Atomic Spectrometry 21,
1053–1061.
J.N. Connelly, M. Bizzarro / Chemical Geology 259 (2009) 143–151
Amelin, Y., Krot, A.N., 2007. Pb isotopic age of the Allende chondrules. Meteoritics and
Planetary Science 42, 1321–1335.
Amelin, Y., Krot, A.N., Hutcheon, I.D., Ulyanov, A.A., 2002. Lead isotopic ages of
chondrules and calcium–aluminum-rich inclusions. Science 297, 1678–1683.
Amelin, Y., Wadhwa, M., Lugmair, G., 2006. Pb-isotopic dating of meteorites using
202
Pb–205Pb double spike: comparison with other high-resolution chronometers.
Lunar Planetary Science Conference, XXXVII, #1970.
Baker, J., Bizzarro, M., Wittig, N., Connelly, J., Haack, H., 2005. Early planetesimal melting
from an age of 4.5662 Gyr for differentiated meteorites. Nature 436, 1127–1131.
Bizzarro, M., Baker, J.A., Haack, H., 2004. Mg isotopic evidence for contemporaneous
formation of chondrules and refractory inclusions. Nature 431, 275–278.
Chen, J.H., Tilton, G.R., 1976. Isotopic lead investigations on the Allende carbonaceous
chondrite. Geochimica et Cosmochimica Acta 40, 635–643.
Connelly, J.N., Amelin, Y., Krot, A.N., Bizzarro, M., 2008a. Chronology of the solar
system's oldest solids. Astrophysical Journal 675, L121–L124.
Connelly, J.N., Bizzarro, M., Thrane, K., Baker, J.A., 2008b. The Pb–Pb age of Angrite
SAH99555 revisited. Geochimica et Cosmochimica Acta 72, 4813–4824.
Davis, W.J., McNicoll, V., Bellerive, D.R., Santowski, K., Scott, D.J., 1997. Modified chemical
procedures for the extraction and purification of uranium from titanite, allanite and
rutile in the Geochronological Laboratory. Radiogenic Age and Isotopic Studies: Report
10. Geological Survey of Canada, Current Research 1997-F, pp. 33–35.
Gerstenberger, H., Haase, G., 1997. A highly effective emitter substance for mass
spectrometric Pb isotope ratio determinations. Chemical Geology 136, 309–312.
Gounelle, M., Shu, F.H., Shang, H., Glassgold, A.E., Rehm, K.E., Lee, T., 2001. Extinct
radionuclides and protostar cosmic rays: Self-shielding and light elements. The
Astrophysical Journal 548, 1051–1070.
Göpel, C., Manhès, G., Allègre, C.J., 1985. U–Pb systematics in iron meteorites: uniformity
of primordial lead. Geochemica et Cosmochemica Acta 49, 1681–1695.
Huss, G.R., MacPherson, G.J., Russell, S.S., Srinivasan, G., Wasserburg, G.J., 2001. 26Al in
CAIs and chondrules from unequilibrated ordinary chondrites. Meteoritics and
Planetary Science 36, 975–997.
Jaffey, A.H., Flynn, K.F., Glendenin, L.E., Bentley, W.C., Essling, A.M., 1971. Precision
measurements of halflives and specific activities of 235U and 238U. Physical Review C
4, 1889–1906.
Kita, N.T., Nagahara, H., Togashi, S., Morishita, Y., 2000. A short duration of chondrule
formation in the solar nebula: evidence from 26Al in Semarkona ferromagnesian
chondrules. Geochimica et Cosmochimica Acta 64, 3913–3922.
Kita, N.T., Huss, G.R., Tachibana, S., Amelin, Y., Nyquist, L.E., Hutcheon, I.D., 2005.
Constraints on the origin of chondrules and CAIs from short-lived and long-lived
radionuclides. Chondrites and the Protoplanetary Disk, Astrophysical Society of the
Pacific Conference Series, vol. 341, pp. 558–587.
151
Krot, A.N., Amelin, Y., Cassen, P., Meiborn, A., 2005. Young chondrules in CB chondrites
from a giant impact in the early Solar System. Nature 436, 989–992.
Lee, T., Shu, F.H., Shang, H., Glassgold, A.E., Rehm, K.E., 1998. Protostellar cosmic rays and
extinct radionuclides in meteorites. The Astrophysical Journal 506, 898–912.
Ludwig, K.R., 1993. PBDAT: a computer program for processing Pb–U–Th isotope data.
United States Geological Survey Open File Report 88-524.
Ludwig, K.R., 2003. Isoplot/Ex version 3.00, a geochronological Toolkit for Microsoft
Excel. Berkeley Geochronology Center Special Publ. 4, May 30, 2003.
Lugmair, G.W., Galer, S.J.G., 1992. Age and isotopic relationships among the angrites
Lewis Cliff 86010 and Angra dos Reis. Geochimica et Cosmochimica Acta 56,
1673–1694.
MacPherson, G.J., Davis, A.M., Zinner, E.K., 1995. The distribution of aluminum-26 in the
early Solar System – A reappraisal. Meteoritics 30, 365–386.
Oversby, V.M., 1973. Redetermination of lead isotopic composition in Canyon Diablo
troilite. Geochimica et Cosmochimica Acta 37, 2693–2696.
Russell, S.S., Srinivasan, G., Huss, G.R., Wasserburg, G.J., MacPherson, G.J., 1996. Evidence
for widespread 26Al in the solar nebula and time constraints for nebula time scale.
Science 273, 757–762.
Scott, E.R.D., Krot, A.N., 2005. Chondritic meteorites and the high-temperature nebular
origins of their components. Chondrites and the Protoplanetary Disk, Astrophysical
Society of the Pacific Conference Series, vol. 341, pp. 15–53.
Shu, F.H., Shang, H., Glassgold, A.E., Lee, T., 1997. X-rays and fluctuating X-winds from
protostars. Science 277, 1475–1479.
Shu, F.H., Shang, H., Gounelle, M., Glassgold, A.E., Lee, T., 2001. The origin of chondrules
and refractory inclusions in chondritic meteorites. The Astrophysical Journal 548,
1029–1050.
Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a
two-stage model. Earth and Planetary Science Letters 26, 207–221.
Tera, F., Carlson, R.W., 1999. Assessment of the Pb–Pb and U–Pb chronometry of the
early solar system. Geochimica et Cosmochimica Acta 63, 1877–1889.
Tatsumoto, M., Knight, R.J., Allegre, C.J., 1973. Time differences in the formation of
meteorites as determined from the ratio of Lead-207 to Lead-206. Science 180,
1279–1283.
Tatsumoto, M., Unruh, M., Desboroug, G.A., 1976. U-Th-Pb and RbSr systematics of
Allende and U-Tb-Pb systematics of Orgueil. Geochimica et Cosmochimica Acta 40,
617–634.
Thrane, K., Bizzarro, M., Baker, J.A., 2006. Extremely brief formation interval for
refractory inclusions and uniform distribution of 26Al in the early solar system.
Astrophysical Journal 646, L159.