There have been several recently published studies of the Mg isotopic compositions of terrestrial minerals and rocks as well as various types of meteorites and their components. These studies have focused on aspects such as assessing the degree of homogeneity of Mg isotopes in the solar nebula, defining the Mg isotope composition of the bulk silicate Earth (BSE), and characterizing the mass-dependent fractionation of Mg isotopes at high temperature to constrain the effects of planetary differentiation processes. Some studies concluded that there is little variation of Mg isotopes during magmatic differentiation between phases, such as pyroxene and olivine, within the same basalt (Handler et al. 2009; Yang et al. 2009) or during granite differentiation (Liu et al. 2010). However, Mg isotopic fractionation of as much as 3.6 × 10−2‰/amu/ºC was found experimentally in silicate melt by chemical and thermal diffusion (Richter et al. 2008). Detectable Mg, as well as Fe, isotopic variations were indeed found between spinel and olivine in mantle xenoliths (Young et al. 2009) and also within zoned olivine crystals from Hawaiian basalts (Teng et al. 2011). These differences in the Mg isotope composition between cogenetic minerals were interpreted as the consequence of the temperature dependence of isotopic exchange and thermal diffusion during magmatic differentiation. It was suggested that the Mg isotopic composition of the BSE is well represented by the average composition of all the chondrite groups (Bourdon et al. 2010; Chakrabarti and Jacobsen 2010; Teng et al. 2010a). Furthermore, these studies indicate homogeneity of Mg isotopes in the protoplanetary disk.
Nevertheless, some of these studies have shown that there are variations in the compositions of the same samples analyzed in different laboratories using different analytical protocols and mass spectrometers. Figure 3 shows a comparison of the Mg isotopic compositions of several rocks and minerals (all relative to the DSM-3 terrestrial Mg standard) measured in this study with those analyzed in several different laboratories. Specifically, the compositions reported here for San Carlos (homogenized powder prepared at Harvard University or SCOL Harvard) and Kilbourne Hole olivines, the BCR-2 basalt, and the Allende CV3 whole-rock sample are in agreement (within the typical 2SD errors) with those reported for the same samples in other previous studies, with the exception of two recent investigations (Chakrabarti and Jacobsen 2010; Schiller et al. 2010). While the study by Chakrabarti and Jacobsen (2010) reports systematically lighter Mg isotope compositions for all the terrestrial and bulk chondrite samples, with BSE at −0.54 ± 0.04‰ (2SE) similar to their average chondrite composition, the study by Schiller et al. (2010) finds a significantly heavier composition only for the BCR-2 terrestrial basalt at δ26Mg = −0.03 ± 0.19‰ (2SE, n = 18). Furthermore, although not plotted in Fig. 3, our measured Mg isotope composition for the D119 shale composite also agrees with that reported previously for this same sample by Teng et al. (2007). The reason for the apparent discrepancy between the data for silica-rich terrestrial samples reported by Chakrabarti and Jacobsen (2010) and the BCR-2 data reported by Schiller et al. (2010) on the one hand, and those from other recent studies (including ours) on the other hand, is unclear at present.
Our Mg isotopic data for the Allende and Murray whole-rock samples agree with those for other carbonaceous chondrite bulk samples reported in nine other investigations (Young and Galy 2004; Baker et al. 2005; Teng et al. 2007, 2010a; Wiechert and Halliday 2007; Yang et al. 2009; Young et al. 2009; Bourdon et al. 2010; Pogge von Strandmann et al. 2011). The average composition of Allende and Murchison carbonaceous chondrites (i.e., δ26Mg = −0.31 ± 0.12‰; 2SD, n = 16) based on measurements of whole-rock samples that were analyzed in 10 different laboratories, including ours but excluding those reported by Chakrabarti and Jacobsen (2010), is shown as the black line (with the 2SD errors shown as the gray band) in Fig. 3, and is hereafter referred to as the composition of bulk carbonaceous chondrites. In the following section, we discuss the fractionation of Mg isotopes in Murchison and Murray chondrules relative to the average Mg isotopic composition of bulk carbonaceous chondrites, as this is likely to be representative of the bulk Mg isotope composition of the solar nebular reservoir from which the chondrules originated.
Fractionation of Mg Isotopes in Chondrules from CM2 Chondrites: Implications for Processes and Formation Environments
Previous studies have shown that isotopic fractionation of moderately volatile elements such as K, Fe, Mg, or Si in CV and LL chondrules is relatively limited (Cuzzi and Alexander 2006). We find a total variation of approximately 1‰/amu in the Mg isotopic compositions of the 23 chondrules we measured in two CM2 (i.e., aqueously altered) chondrites. Specifically, the range of Mg isotopic compositions of chondrules from Murchison (δ26Mg from −1.27‰ to +0.77‰) extends to compositions that are heavier and lighter than bulk carbonaceous chondrites (δ26Mg = −0.31 ± 0.11‰; 2SD, n = 16), while the range of δ26Mg in chondrules from Murray (from −0.95‰ to −0.15‰) is smaller and systematically lighter than bulk chondrites (Figs. 4 and 5). For comparison, the only other carbonaceous chondrite chondrules for which high-precision Mg isotope compositions have been previously reported are from the CV3 chondrite Allende (Galy et al. 2000; Young et al. 2002a; Bizzarro et al. 2004) and the CB chondrites HaH 237 and QUE 94411 (Gounelle et al. 2007). The ranges of Mg isotope compositions reported in this study for the Murchison and Murray CM2 chondrules are significantly narrower than that measured in chondrules from the Allende CV3 chondrite (δ26Mg from −3.10‰ to +1.84‰) (Galy et al. 2000; Young et al. 2002a; Bizzarro et al. 2004) (Fig. 4). However, some of these analyses of Allende chondrules were conducted in situ by laser ablation MC-ICPMS (Young et al. 2002a), and the wider range of Mg isotope compositions could be, at least in part, a sampling artifact. If only the high-precision Mg isotope data from solution analyses of Allende chondrules are considered (Galy et al. 2000; Bizzarro et al. 2004), the total range of their Mg isotope compositions is still broader than those of Murchison and Murray chondrules reported here (Fig. 4). Even if the laser ablation data are excluded from consideration, the high-precision Mg isotope database for Allende chondrules is much larger than that for the CM2 chondrules, and we cannot rule out the possibility that with additional analyses the range of CM2 chondrule compositions would approach that of Allende chondrules.
The range of compositions of 15 chondrules from CB chondrites (δ26Mg from −1.50‰ to +1.64‰; Gounelle et al. 2007) is also broader than that of Murchison and Murray chondrules. However, these analyses too were conducted by laser ablation MC-ICPMS and so may not be directly comparable to the results of the solution analyses reported here. Furthermore, these CB chondrules formed significantly later than chondrules of other carbonaceous chondrites, and their compositions have been interpreted as reflecting isotopic fractionation during their formation in an impact-related vapor plume (Krot et al. 2005a).
The range of Mg isotopic compositions in chondrules from the CM2 chondrites reported here may be due to isotopic heterogeneity in the compositions of chondrule precursors, or isotopic fractionation during evaporation and recondensation, or aqueous alteration during secondary processing on the CM2 chondrite parent body. This range is about a factor of five wider than that for bulk chondrites (approximately 0.2‰/amu; Bourdon et al. 2010; Teng et al. 2010a). The average chondrite composition is also similar to that of the BSE and other planetesimals and planets in the inner solar system, such as the Moon and Mars, and has been interpreted as evidence that Mg isotopes were homogeneously distributed in the solar nebula (Yang et al. 2009; Bourdon et al. 2010; Chakrabarti and Jacobsen 2010; Teng et al. 2010a).
The complementarity of major element compositions between chondrules and matrix has been previously established (Brearley 1996; Hezel and Palme 2010). The range of Mg isotopic compositions in chondrules and the relatively uniform compositions of bulk chondrites imply that such complementarity also exists for Mg isotopic compositions. This observed chemical and presumed isotopic complementarity of chondrules and associated matrix has implications for whether chondrules and matrix formed from the same reservoirs in the protoplanetary disk before accreting into their respective parent bodies (Hezel and Palme 2010), or if chondrules were formed closer to the Sun and were later transported and mixed with the matrix in colder accreting regions (e.g., Zanda 2004). Nevertheless, given the Mg isotopic homogeneity inferred for the solar nebula, we favor the idea that Mg isotopic variations in chondrules are likely due to fractionation processes either during their formation or during secondary processing on the chondrite parent body, and are not due to heterogeneity in their precursors. Chondrule precursors probably originated from the same reservoirs as the planetesimals and planets in the inner solar system.
On average, the Mg isotopic compositions of chondrules from Murray (δ26Mg from −0.95‰ to −0.15‰) are lighter compared to those from Murchison (δ26Mg from −1.27‰ to +0.77‰) (Fig. 4). As we only analyzed 8 Murray chondrules and 15 Murchison chondrules, this could be a sampling bias. We do not observe any clear correlation in individual CM2 chondrules between Mg isotopic composition and chondrule size, petrologic type, or the degree of alteration (Table 1). These two CM2 chondrites have similar low weathering grades, and have experienced similar degrees of aqueous alteration on their parent bodies, based on their bulk oxygen isotopic compositions (Clayton and Mayeda 1999) and petrologic properties (Rubin et al. 2007). We note, however, that if secondary processes, such as aqueous or thermal alteration, did indeed play a role in the fractionation of Mg isotopes in chondrules from CM2 and CV3 chondrites (which underwent different types of alteration), then this fractionation must have occurred in a closed system at the whole-rock scale since these chondrites have identical bulk Mg isotopic compositions (e.g., this study; Bourdon et al. 2010; Teng et al. 2010a).
It was previously suggested for Allende chondrules that deviations toward lighter Mg isotopic compositions from the bulk chondritic value could be associated with parent body alteration by aqueous fluids, whereas deviations toward heavier compositions may be associated with the presence of relict refractory olivines (Young et al. 2002b). While thermal metamorphism may have affected the isotopic compositions of Allende chondrules, that is not the case for the chondrules analyzed here. Among the Murchison samples, the chondrule with the lightest Mg isotopic composition (MRC2-18, δ26Mg = −1.27 ‰) is an alteration-free type IA (olivine-rich). On the other hand, we find that the chondrule with the heaviest composition (MRC1-36, δ26Mg = +0.77 ‰) is a type IB (pyroxene-rich) chondrule which is also alteration free and without any apparent relict olivines. It is possible, however, that although secondary phases are not present in the polished section, they may have been present in the split used for isotopic analysis.
We can compare the effects of aqueous alteration (i.e., CM2 chondrites) vs. thermal metamorphism (i.e., CV3 chondrites) on the Mg isotopic composition of chondrules by evaluating the compositions reported here of Murchison and Murray chondrules classified by degree of alteration (clean, light, moderate, and heavy, Table 1) with those previously reported for Allende chondrules. Because the Mg isotopic data obtained by laser ablation (Young et al. 2002a) are not representative of the bulk chondrule compositions, we compare our data only with isotopic data obtained by solution analysis on Allende chondrules (Galy et al. 2000; Bizzarro et al. 2004). For individual Murchison chondrules, there is no correlation between the Mg isotopic composition and the modal abundance of alteration products, which would otherwise reflect isotopic exchange during secondary alteration. We can instead consider the average Mg isotope compositions of groups of chondrules classified by degree of alteration. The Mg isotopic compositions vary from an average δ26Mg = −0.39 ± 0.30‰ (2SE) for the least altered chondrule group (classified as clean or with light alteration, Table 1) toward heavier compositions with an average δ26Mg = −0.11 ± 0.21‰ (2SE) for the group of chondrules classified as moderately to heavily altered. The average composition of the least-altered CM2 chondrules is slightly lighter than the bulk carbonaceous chondrites (δ26Mg = −0.31 ± 0.03‰, 2SE). The average Mg isotope compositions of both groups of CM2 chondrules are lighter than the average composition of bulk chondrules from Allende (average δ26Mg = +0.40 ± 0.24‰, 2SE) (Galy et al. 2000; Bizzarro et al. 2004), and may reflect the distinct secondary alteration histories experienced by chondrules from the CM2 and CV3 chondrules. Alternatively, this systematic difference could also reflect different isotopic compositions of chondrule precursors for the CM2 and CV3 chondrules, although we do not favor this interpretation for the reasons noted earlier in this section.
There is growing evidence from studies of terrestrial materials that the Mg isotopic compositions of silicate secondary phases precipitated from aqueous solution are enriched in heavy isotopes (Tipper et al. 2006, 2010; Teng et al. 2010b; Wimpenny et al. 2010). In contrast, it has been found that carbonates have lighter Mg isotopic compositions than their source fluids (Galy et al. 2002). Experimental studies on calcite precipitation from solutions with a range of compositions and conditions (pH and T°C) have additionally confirmed fractionation of Mg isotopes toward lighter compositions (Saulnier et al. 2012). Moreover, the lighter Mg and Fe isotopic compositions of some CV3 chondrules have been suggested to be due to alteration processes (Young et al. 2002b; Hezel et al. 2010), possibly involving the introduction of isotopically light Mg and Fe from the matrix into the chondrules. However, alteration products within chondrules are dominated by phyllosilicates and not carbonates (that occur mostly in the matrix of CM2 chondrites; e.g., De Leuw et al. 2009). Therefore, the difference in the average Mg isotopic compositions of the CM2 chondrule groups with different degrees of alteration is more likely the consequence of late isotopic fractionation toward heavier compositions in the secondary silicate phases formed within chondrules during aqueous alteration on the CM2 parent body. In the present study, even if secondary silicate alteration products were not sampled in the limited area represented by the polished sections of a given chondrule, they might still have been present in the material analyzed by MC-ICPMS.
The systematic but not unambiguous nature of the relationship between degree of alteration (as represented by the phyllosilicate abundance) and the Mg isotope composition may be due to the fact that the effects of secondary alteration are superimposed upon Mg isotope compositions resulting from the primary processes involved in chondrule formation. More specifically, it is possible that the Mg isotopic compositions of CM2 chondrules record some degree of fractionation by volatilization and recondensation processes followed by the effects of secondary aqueous alteration. Chondrule precursor materials were partially melted during flash heating events, during which peak heating temperatures of up to approximately 2000 K were reached (Hewins et al. 1996). Evaporation from partial melts at high temperature is recognized as the process that resulted in fractionation of Mg and Si isotopes in CAIs toward heavier compositions (Grossman et al. 2000; Richter et al. 2009). Therefore, evaporative processes could explain the heavier compositions (relative to bulk chondrites) found in some of the chondrules analyzed here. Conversely, partial recondensation may produce some of the chondrule compositions with lighter Mg isotopic compositions than bulk chondrites. It has been proposed that impact-related vapor plumes may be involved in the formation of all the chondrules (e.g., Fedkin et al. 2012; Grossman et al. 2012), and the volatilization and recondensation processes leading to isotopic fractionation of Mg isotopes may have occurred in this context. As it has been suggested previously for chondrules from other chondrite types, to account for the limited degree of Mg isotope fractionation in CM2 chondrules would require high gas pressures (Galy et al. 2000; Young et al. 2002a; Hezel et al. 2010), and/or enhanced dust/gas ratios (Cuzzi and Alexander 2006; Alexander et al. 2008) in their formation environment.
26Al-26Mg Systematics and Time scales of Chondrule Formation
Constraining the high-resolution chronology of chondrules is important for assessing the lifetime of the nebular disk and the potential problem of chondrule storage in the solar nebula over an extended period. At present, besides the Pb-Pb absolute chronometer, the only other chronometers that offer the time resolution for addressing such issues are those based on the extinct radionuclides, such as the 26Al-26Mg system.
Most chondrules studied here have subchondritic to near-chondritic 27Al/24Mg ratios (i.e., approximately 0.1; Thrane et al. 2006) and, therefore, it was not possible to resolve significant variations of δ26Mg* values from the terrestrial standard outside our external long-term reproducibility (as represented by our 2SD uncertainty of ± 0.05‰ based on repeat measurements of the San Carlos olivine; Table 2). Results are illustrated in Fig. 6. Only two chondrules from Murchison, MRC1-45 (δ26Mg* = 0.06 ± 0.03‰, with a slightly sub-chondritic 27Al/24Mg ~ 0.08) and MRC2-45 (δ26Mg* = −0.06 ± 0.02‰, with a slightly super-chondritic 27Al/24Mg ~ 0.16), have variations in δ26Mg* that are outside of the reported 2SD errors for these samples. Nevertheless, these variations only slightly exceed our 2SD external reproducibility (±0.05‰). The conservative interpretation would be that there is no statistical significance to these small apparent variations in δ26Mg* for these two chondrules and, therefore, no inferences regarding their time of formation may be obtained. Alternatively, if these small variations in these two samples are taken at face value, they may hint toward the presence of a small degree of isotopic heterogeneity in the initial abundance of 26Al or Mg isotope compositions in these chondrules as suggested recently by Larsen et al. (2011).
Figure 6. Al-Mg systematics of CM2 chondrules from Murchison and Murray (black squares). For reference, the solid and dashed lines labeled as “T = 0 Ma after CAIs” represent a canonical isotopic evolution and associated error envelope, respectively, for CAIs (26Al/27Al0 = 5.23 × 10−5 and δ26Mg*0 = −0.040 ± 0.029 ‰; Jacobsen et al. 2008). An Al-Mg isochron for 1 Ma after CAI formation labeled “T = 1 Ma after CAIs”) is also shown.
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The absence of a resolvable 26Mg* excess in the chondrule (MRC2-39) with the highest 27Al/24Mg ratio of 0.316 may be interpreted either as indicative of formation >1 Ma after CAIs or of resetting of the Al-Mg isotope systematics by secondary alteration processes on the CM2 parent body. This particular chondrule does not show petrographic evidence for secondary alteration (Fig. 1).
Recent in situ, high-precision Al-Mg analyses of chondrules from primitive ordinary and carbonaceous chondrites indicate that chondrule precursors formed as early as 0.9 ± 0.2 Ma after CAI formation, while chondrule crystallization from molten droplets lasted from 1.2 to 4.0 Ma, with formation peaks between 1.5 and 3.0 Ma, after CAI formation (Villeneuve et al. 2009). Such late episodes of formation could not be resolved in the low Al/Mg chondrules analyzed here (Fig. 6). Nevertheless, we may deduce from the lack of clearly resolved variations in 26Mg* in the low-Al chondrules analyzed in this study that these chondrules were originally formed or were equilibrated >1 Ma after CAI formation.