All U and Pb concentrations are calculated using sample weights recorded prior to acid leaching. Because acid leaching removes external contamination but can also partially dissolve the primary minerals, the total recombined concentrations (all washes and residues for a particular fraction combined) represent the maximum, while the residues (or W4+R) represent the minimum concentrations. As the Pb isotopes for batch A81 appear to show leaching induced fractionation (discussed in section 4.2) between the 4th washes and the residues, these two samples were
numerically recombined for both the pyroxene and whole rock fractions prior to undergoing data reduction. For the remainder of the chapter these A81 samples will be referred to as W4+R. It should also be noted that problems with the analysis of A119 4R and A119 5R resulted in no useable data for these samples being collected. As a consequence, these two analyses are not shown within this study.
Total U concentrations for all A881394 recombined fractions range from 9.1 ppb to 1.1 ppb. The highest recombined U concentrations are seen in the pyroxene fractions from analysis batch A119, with U concentrations ranging from 9.1 ppb to 6.1 ppb. In contrast, the lowest recombined U concentration (1.1 ppb) is seen in the A81 tridymite fraction. The remaining A81 fractions of pure plagioclase, impure plagioclase, pyroxene and whole rock, contain recombined U concentrations of 1.6
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ppb, 4.1 ppb, 4.2 ppb and 4.3 ppb respectively. For the pyroxene fractions, most of the U is contained in the W4+R (A81) and residues (A119) with concentrations ranging from 2.4 ppb to 4.9 ppb U. In comparison, the whole rock W4+R contains 1.3 ppb U, the two plagioclase residues contain ~0.1 ppb U and the tridymite residue contains 0.4 ppb U. These concentrations of U, especially in the whole rock and pyroxene fractions, are low compared to angrites and many other achondrites. Furthermore, these recombined U concentrations are comparable to the low U concentrations seen in NWA 7325 (which can be seen in section 3.3.2) However, in contrast to NWA 7325 the U concentrations within the pyroxene and whole rock residues of A881394 are >1 ppb U.
The total Pb concentrations for all recombined mineral fractions range from 44.3 ppb in the bulk plagioclase fraction to 7.5 ppb in the tridymite fraction. The combined pyroxene and whole rock fractions contain similar concentrations of total Pb ranging from 28.1 ppb to 18.2 ppb Pb. As most of the Pb for all fractions is contained within the first washes, the Pb concentrations of the W4+R and residues are generally lower. The W4+R and residues for the pyroxene fractions contain Pb concentrations ranging from 8.7 ppb to 4.3 ppb Pb, while the whole rock W4+R contains 2.6 ppb Pb. In comparison, the trydimite and plagioclase residues show Pb concentrations of between 1.2 and 0.1 ppb. Of the 3rd washes conducted during
analysis batch A81, only the pyroxene 3rd wash contains over 1 ppb Pb (1.2 ppb),
with all other fractions 3rd washes containing 0.2 ppb Pb or less.
Because acid leaching preferentially removes non-radiogenic Pb, subsequent washing steps and residues contain progressively more radiogenic Pb. This effect is most obvious in the pyroxene and whole rock fractions as they contain significantly more radiogenic Pb relative to the plagioclase and tridymite fractions. The
206Pb/204Pb ratios for the pyroxene W4+R and residues and the whole rock W4+R
range from 1380 to 600. In contrast, the residues for the tridymite and two plagioclase fractions consist of 206Pb/204Pb ratios of between 65 and 44.
Wash 1a, wash 1b and wash 2 for analysis batch A81 all contain 206Pb/204Pb
ratios of between 37 and 19 for all mineral fractions. The washes for analysis batch A119 contain 206Pb/204Pb ratios of between 51 and 28 for wash 1 and between 161
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absence of U-rich phosphate minerals which can be present in some achondrites (e.g. Ibitira from Iizuka et al. (2014)).
As seen in Fig. 4.2, on a plot of 207Pb/206Pb vs. 204Pb/206Pb, all analyses lie
(within uncertainty) between the most radiogenic data, modern crustal Pb (Stacey and Kramers 1975) and primordial Pb (Tatsumoto et al., 1973). This suggests there is no multi-stage Pb isotopic evolution and that all Pb components in Asuka 881394 are a mixture of these three components. The wash steps for the pyroxene, tridymite and pure plagioclase fractions plot almost directly along a line connecting radiogenic Pb and primordial Pb, suggesting undetectable quantities of “terrestrial” Pb. In contrast, the washes for the impure plagioclase and whole rock fractions show significant scatter, indicating a significant “terrestrial” Pb component.
Figure 4.2. Plot of 207Pb/206Pb vs. 204Pb/206Pb for all A881394 washes and residues. The isochron
regression and age is constructed using all pyroxene analyses. The isotopic composition of primordial Pb for Canyon Diablo troilite (CDT) (Tatsumoto et al., 1973) and modern terrestrial Pb (MT) (Stacey and Kramers, 1975) are also shown with tie lines connecting to the most radiogenic point. For clarity, the errors for each single point and the isochron regression are not shown. A81: Wash 1a = red, wash 1b = brown, wash 2 = blue, wash 3 = light green, wash 4 = dark green, W4+R and residue = black. A119: Wash 1 = red, wash 2 = blue, residue = black
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4.3.2
Pb-Pb dates
All Pb-Pb ages for Asuka 881394 were calculated using the measured 238U/235U
ratio of 137.768 ± 0.038 (2 SE) (Wimpenny et al., 2013). A plot of 207Pb/206Pb vs. 204Pb/206Pb for the plagioclase, pyroxene and whole rock fractions can be seen in Fig
4.2. Tie lines connecting the most radiogenic point to both the primordial Pb composition measured by Tatsumoto et al. (1973) and the modern terrestrial Pb composition measured by Stacey and Kramers (1975) are also present. An isochron regression using the residues, W4+R and all leachates from the pyroxene fractions passes within error of primordial Pb and yields and apparent age of 4565.1 ± 2.0 Ma, MSWD = 129 (Fig 4.2.).
Producing an accurate and precise Pb-Pb isochron age for A881394 requires the careful selection of suitable mineral fractions. As the plagioclase and tridymite fractions are very low in total and radiogenic Pb, and produce a scattered array, they were considered unsuitable for a Pb-Pb isochron. In contrast, the pyroxene residues and the pyroxene and whole rock W4+R analysis are high in radiogenic Pb and produce a very precise array. As a result, the pyroxene and whole rock residue and W4+R analyses were combined with a selection of points from the previous analysis by Wadhwa et al. (2009) in order to produce the final Pb-Pb isochron. An isochron using these selected points can be seen in Fig. 4.3. and yields an age of 4564.95 ± 0.35 Ma, MSWD = 1.4. A more detailed discussion surrounding the final Pb isochron point selection can be seen in section 4.4.2.
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Figure 4.3. Plot of 207Pb/206Pb vs. 204Pb/206Pb for A881394 showing the 16 selected residues from
Wadhwa et al. (2009) (light grey), the A81 pyroxene and whole rock W4+R analysis (black) and the A119 pyroxene residues (red). The age uncertainty in brackets includes the uncertainty of the
238U/235U ratio.
The model 207Pb*/206Pb* dates of all leachates and residues are presented in
Table 4.1 (238U/235U ratio of 137.768). Additionally, the weighted average model 207Pb*/206Pb* date for all analyses included in the final Pb-Pb isochron can be seen in
Fig. 4.4. All 207Pb*/206Pb* model ages were calculated assuming that all non-
radiogenic Pb after blank subtraction is initial primordial Pb with the isotopic composition measured by Tatsumoto et al. (1973). As seen in Fig. 4.4 the weighted average model 207Pb*/206Pb* date for A881394 (4564.98 ± 0.17 Ma) completely agrees
with the Pb-Pb isochron age of the same analyses (4564.95 ± 0.35 Ma). This is a strong indication that the analyses used in the final Pb-Pb isochron contain no detectible quantities modern terrestrial Pb.
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Figure 4.4. Plot of the 207Pb*/206Pb* model ages for A881394 showing the 16 selected residues from
Wadhwa et al. (2009) (light grey), the A81 pyroxene and whole rock W4+R analysis (black) and the A119 pyroxene residues (red).