SUPPLEMENTARY INFORMATION
Pb isotope analysis
Angrite samples were gently crushed in a boron carbide mortar and whole-rock
fragments (1-3 mm) free from surficial coatings picked under a binocular
microscope (apart for one SAH99555 fragment with surficial contamination that
was deliberately analysed to constrain the contaminant composition). Pyroxene
separates were picked from a sieved 100-200 µm fraction prepared from more
intensively crushed material.
Two main sets of analytical experiments were performed – the first utilised gentle
2.0 M HCl acid washing of small samples (39-65 mg) whilst ultrasonicating for 2 x
10 min prior to digestion, whereas the second adopted considerably longer acid
washing times (5 x 10 min for whole rocks; 10 x 10 min for cpx) and larger
samples and fragments (76-175 mg). Samples were digested in HF-HNO3 acid
on a hotplate and either aliquoted and spiked (first experiments;
spike), or spiked prior to digestion (second experiments;
233U-235U-208Pb
235U-205Pb
spike). Pb
was separated by double-pass anion exchange techniques using 1M HBr to elute
matrix and 7M HCl to collect Pb. This chemistry results in a quantitative yield of
Pb and no isotopic fractionation effects due to incomplete yields1. U was
separated from the elutant of the Pb chemistry on anion exchange columns.
Most U and Pb isotopic analyses were performed by multiple-collector inductively
coupled plasma mass spectrometry (MC-ICPMS) in Copenhagen. U isotopes
were measured to <0.1% using sample-standard bracketing methods with U500
(238U/235U = 0.9954) as the bracketing standard. Pb isotopes were measured
using procedures modified after ref [1] – however, instrumental mass bias was
corrected by sample-standard bracketing techniques using SRM981 as the
bracketing standard. Over the course of an analytical session instrumental mass
bias for Pb isotopes varies < 50 ppm per atomic mass unit. In order to assure
that matrix effects do not compromise mass bias correction procedures, Pb
separated from volcanic rocks (TH29 and JB-2) was also repeatedly analysed at
similar ion beam intensities to those which we analysed angrite Pb. Pb isotope
ratios of these solutions were previously determined by precise double-spike
methods1. The standard results demonstrate that highly precise and accurate Pb
isotopic data can be obtained using this methodology, which potentially allows
dating of meteorites with a precision ≤ ±100,000 yr. Data for SRM981, Th29 and
JB-2 provide an indication of the reproducibility of
207Pb/206Pb
MC-ICPMS data,
and data for SRM983 provides an estimate of reproducibility (and accuracy) of
204Pb/206Pb
data measured on radiogenic Pb samples. Two sets of SRM983 MC-
ICPMS data were measured at different ion beam intensities (206Pb = 8 and 1 V),
and one set of data (n = 6) was also acquired by conventional thermal ionization
mass spectrometry (TIMS) in Austin. Small aliquots of SAH99555 WR4, WR5
and PX2 were re-analysed for Pb isotopes in Austin by ion counter peak jumping.
MC-ICPMS and TIMS data are corrected for spike (where appropriate),
instrumental mass bias (TIMS – 0.10±0.05% per atomic mass unit), and blank
contributions of 2 and 4 pg for the first and second set of experiments,
respectively, and the TIMS loading blank where relevant. MC-ICPMS
uncertainties are the 2 sd reproducibility (standards) or internal precision
(samples), whereas the TIMS uncertainties are the external reproducibility
estimated from the SRM983 analyses, which is considerably larger than the
internal precision of the TIMS measurements. To leave the instrumental
uncertainties clear in the Table, uncertainties on the blank corrections are not
propagated into the errors - these result in effects an order of magnitude smaller
than quoted uncertainties on Pb model ages. Model ages were calculated using
primordial Pb (ref. 2) as the initial Pb, whereas isochron calculations use no
assumed initial Pb. Model age calculations used internal errors to calculate age
uncertainties, whereas isochron calculations used 2 x internal errors as a proxy
for the external reproducibility of MC-ICPMS analyses, estimated from the
relationship between internal errors and external reproducibility on measured
standards. All age and isochron calculations were performed using the software
of ref. [3].
Mg isotope analysis
Several milligrams of powder (Earth basalts) and meteorite fragments (WR =
whole rock), and ca. 0.1-1 mg of mineral separates were digested in HF-HNO3
acid at 120°C. After complete dissolution, an aliquot was retained for
measurement of the Al/Mg ratio by MC-ICPMS4, and Mg was separated from the
other aliquot by cation exchange chemistry (AG50W x 12 resin; elution of Mg in
1M HNO3).
Mg isotopes were measured by MC-ICPMS and stable isotope data are reported
as the per mil (‰) deviation from mean 26Mg/24Mg and 25Mg/24Mg measured on
bracketing DSM-3 standards (except SAH99555 WR1a where BHVO-1 was used
as the standard). 26Mg excesses (26Mg*) are reported relative to the mean
mass-bias-corrected 26Mg/24Mg obtained on bracketing DSM-3 standards
(normalised to 25Mg/24Mg = 0.12663 using the exponential mass fractionation
law). This method of calculating 26Mg* yields indistinguishable results for these
samples as compared to calculating 26Mg* from 26Mg and 25Mg with reference
to either a kinetic or equilibrium mass fractionation line. Each digestion was
analysed 4-16 times and the reported error is 2 sd/√n. Where multiple digestions
of the same sample were undertaken the weighted average of the data are given.
Mg isotope data for angrite fragments were combined with a bulk Solar System
27Al/24Mg
= 0.10±0.02 and the terrestrial Mg isotope composition to calculate
initial abundances of
26Al
and the time of formation with respect to CAIs4,5. In
these cases, T (CAIs) are model ages for basaltic magmatism on the angrite
parent body reflecting the increase in Al/Mg ratio produced by formation of
basaltic magmas from a parent body with chondritic
27Al/24Mg
of 0.10. Mg isotope
data for the bulk angrite fragments yields model initial 26Al abundances
(26Al/27Al)0 = 2.5 to 1.6 x 10-6. The initial 26Al/27Al of CAIs (5.83 ± 0.11 x 10-5) was
recalculated from the data of ref. 4 using modified
27Al/24Mg
ratios (the values
published in ref. 4 are systematically too high by a factor of 1.11) and also the
data for bulk CAIs from ref. 5. By reference to this initial
26Al/27Al
of CAIs, angrite
magmatism took place 3.3-3.8 Myr after CAIs. Use of a supra-canonical initial
26Al/27Al
for CAIs to calculate 26Al-26Mg ages is further supported by the in situ
laser ablation CAI data of ref. 5, which yields a CAI initial 26Al/27Al ~ 6 x 10-5
when the data are treated in the same fashion as ref. 4. The feldspar age was
calculated by combining the feldspar and whole-rock analysis of SAH99555.
1.
Baker, J., Peate, D., Waight, T. & Meyzen C. Pb isotopic analysis of standards and
samples using a 207Pb-204Pb double spike and thallium to correct for mass bias with a
double-focusing MC-ICPMS. Chem. Geol. 211, 275-303 (2004).
2.
Tatsumoto, M., Knight, R. J. & Allègre, C. J. Time differences in the formation of
meteorites as determined from the ratio of lead-207 to lead-206. Science 180, 1278-1283
(1973).
3.
Ludwig, K. R. Isoplot/Ex. Version 1.00 A geochronological toolkit for Microsoft
Excel, BGC Publication 1 (2000).
4.
Bizzarro, M., Baker, J. A. & Haack, H. Mg isotope evidence for contemporaneous
formation of chondrules and refractory inclusions. Nature 431, 275-278 (2004).
5.
Young, E. D., Simon, J. I., Galy, A., Russell, S. S., Tonui, E. & Lovera, O. Supra-
canonical 26Al/27Al and the residence time of CAIs in the solar protoplanetary disk.
Science 308, 223-227 (2005).