Short-lived chlorine-36 in a Ca- and Al-rich inclusion from the Ningqiang carbonaceous chondrite

Abstract

Excesses of sulfur-36 in sodalite, a chlorine-rich mineral, in a calcium- and aluminum-rich inclusion from the Ningqiang carbonaceous chondrite linearly correlate with chorine/sulfur ratios, providing direct evidence for the presence of short-lived chlorine-36 (with a half-life of 0.3 million years) in the early solar system. The best inferred (³⁶Cl/³⁵Cl)_o ratios of the sodalite are ≈5 × 10^-6. Different from other short-lived radionuclides, chlorine-36 was introduced into the inclusion by solid-gas reaction during secondary alteration. The alteration reaction probably took place at least 1.5 million years after the first formation of the inclusion, based on the correlated study of the ²⁶Al-²⁶Mg systems of the relict primary minerals and the alteration assemblages, from which we inferred an initial ratio of (³⁶Cl/³⁵Cl)_o > 1.6 × 10^-4 at the time when calcium- and aluminum-rich inclusions formed. This discovery supports a supernova origin of short-lived nuclides [Cameron, A. G. W., Hoeflich, P., Myers, P. C. & Clayton, D. D. (1995) Astrophys. J. 447, L53; Wasserburg, G. J., Gallino, R. & Busso, M. (1998) Astrophys. J. 500, L189–L193], but presents a serious challenge for local irradiation models [Shu, F. H., Shang, H., Glassgold, A. E. & Lee, T. (1997) Science 277, 1475–1479; Gounelle, M., Shu, F. H., Shang, H., Glassgold, A. E., Rehm, K. E. & Lee, T. (2001) Astrophys. J. 548, 1051–1070]. Furthermore, the short-lived ³⁶Cl may serve as a unique fine-scale chronometer for volatile-rock interaction in the early solar system because of its close association with aqueous and/or anhydrous alteration processes.

Short-lived, now extinct, radionuclides have been detected in primitive meteorites (1). They have been intensively studied and are still subjects of ongoing great interest for broad scientific audiences, because short-lived radionuclides may serve as the only available fine-scale chronometers to trace processes in the early solar system (1, 2), and their relative abundance can constrain the local galactic environment of solar system formation (3–6). However, the origin of short-lived radionuclides is a long-standing issue. One scenario holds that the short-lived radionuclides originated in stellar sources like supernovae (3, 7) or asymptotic giant branch stars (5, 6) in close proximity to the forming solar system. According to such a stellar origin, shortlived radionuclides were injected homogeneously in the solar nebula, hence they may be used as chronometers. This idea is supported by the measurement of U-Pb absolute ages of Ca- and Al-rich inclusions (CAIs) and chondrules with a ≈1-million year (My) resolution (8), which yields a time interval between formation of CAIs and chondrules similar to that inferred by many ²⁶Al-²⁶Mg measurements, although a recent study reported that chondrule formation began contemporaneously with the formation of CAIs (9). On the other hand, the same short-lived radionuclides may be produced locally by intense irradiation of nebular materials by the proto-sun (10–12). The predictions of local irradiation models (4, 13) are compatible with the observed abundance of some nuclides (e.g., ¹⁰Be, ²⁶Al, ⁴¹Ca, and ⁵³Mn). According to local irradiation models, the systematically different initial (²⁶Al/²⁷Al)_o ratios between CAIs and chondrules are related to their different distances from the proto-sun, bearing no temporal significance.

Chlorine-36 has a half-life of 0.3 My and decays to either ³⁶Ar (98.1%, β^-) or ³⁶S (1.9%, ε and β⁺) (14), hence it can be detected by measuring the excess of ³⁶Ar or ³⁶S. A previous study reported the excess of ³⁶Ar in matrix of the Efremovka carbonaceous chondrite (15), which was attributed to the decay of short-lived ³⁶Cl. In this study, we provide direct evidence for the existence of live ³⁶Cl in primitive meteorites based on excess of ³⁶S and its correlation with Cl/S ratios of the samples. The suitable samples for this purpose are sodalite (Na₈Al₆Si₆O₂₄Cl₂) in CAIs, because this mineral is Cl-rich (≈7.5 wt%) and CAIs are the oldest assemblages of the solar system (16, 17).

Methods

Because only 1.9% ³⁶Cl decays to ³⁶S, the excess ³⁶S [(³⁶S/³⁴S)^*] can be calculated in the following relationship:

[1]

where (³⁶S/³⁴S)_m and (³⁶S/³⁴S)_o are the measured and initial (or normal) S isotopic ratios at the time of closure. The inferred (³⁶Cl/³⁵Cl)_o ratio can be calculated from Eq. 1.

In secondary ion MS analysis, (³⁶S/³⁴S)^* in a sample is detected by measuring its S isotopes relative to standards. S isotopic data are reported as delta values, defined as:

[2]

and

[3]

where m indicates measured ratios and s indicates standard's normal S isotope ratios. Because of its high abundance, ³²Sisnot used here as the reference isotope. The fractionation-corrected isotopic anomaly in ³⁶S is then calculated from the linear “law”:

[4]

Sulfur isotopes (³³S, ³⁴S, and ³⁶S) and ³⁵Cl of the samples were analyzed by using the Cameca ims-6f ion microprobe at Arizona State University. A small (<10 μm) Cs⁺ primary beam of <0.1 nA at 10 keV size was used. Secondary ions were accelerated to -9 keV and collected with an electron multiplier, and dead-time correction was applied to the data. Transfer lenses were tuned for a 75-μm imaged field to increase the secondary ion transmission. Depending on the phase being analyzed, each measurement ran 30–200 cycles through the mass sequence, ³³S, ³⁴S, ³⁵Cl, and ³⁶S, with integration times set to 3, 1, 1, and 30 s, respectively. Even with the instrumental conditions set up to allow the highest possible transmission of secondary ions, a long collecting time was still required to achieve reasonable statistical precision because of the low count rates of the S isotopes in sodalite. Hydride and other molecular interferences to the S isotopes were eliminated under high mass resolution conditions with a mass resolving power (m/Δm) of ≈4,300. Relative to the sulfur isotope standard, Canyon Diablo troilite, troilite (FeS), and djerfisherite [(K,Na)₆(Fe,Ni,Cu)₂₅S₂₆Cl] in the Qingzhen enstatite chondrite should have normal sulfur isotope composition within the precision of the ion microprobe. Therefore, they were used repeatedly as standards to check instrumental mass fractionation (IMF) and verify that no ³⁶S anomaly exists in their measurements. For both standards and samples, IMF was internally corrected by using ³³S/³⁴S according to the linear law. Because there are no suitable silicate standards, the relative sensitivity factor of ³⁵Cl/³⁴S (≈0.6) used throughout this study was determined from djerfisherite in Qingzhen. This may introduce a slight uncertainty in the slopes of the lines, but does not significantly affect the results.

Because sodalite is a secondary phase replacing primary minerals of CAIs during alteration processes, a combined study of ²⁶Al-²⁶Mg systems of individual minerals of the CAI was carried out to determine the time interval between formation and alteration of the CAI. The measurements of magnesium isotopes were carried out by using the same ion microprobe with a -12.5 keV O^- primary beam of <0.4 nA for different mineral phases, including melilite (a solid solution of Ca₂MgSi₂O₇-Ca₂Al₂SiO₇), plagioclase, and sodalite. Positive secondary ions were accelerated to +9 keV and collected with an energy band-pass of ≈50 eV, an imaged field of ≈75 μm, and a mass resolving power of ≈3,500. The relative sensitivity factors of Al/Mg were determined by using terrestrial standards of melilite and plagioclase.

Results

Three CAIs from the Ningqiang carbonaceous chondrite and two EH3 enstatite chondrites (EET 87746 and ALH 77295) were studied. The Ningqiang CAI (NQJ1–1#1) has a spinel-rich core, a spinel-anorthite-Ca-pyroxene mantle, and a melilite-spinel crust (Fig. 1a). Along the boundaries between the crust and the mantle, anorthite shows a texture of replacing melilite (Fig. 1b). In turn, sodalite and much less abundant nepheline occur as alteration products in place of anorthite mainly in the mantle (Fig. 1c). The two CAIs from EET 87746 and ALH 77295 are highly altered, consisting of spinel, pyroxene, and fine-grained sodalite and nepheline.

Fig. 1.

Back-scattered electron images of the CAI (NQJ1-1#1) from the Ningqiang meteorite. (a) The whole CAI shows an annealing texture and consists of a spinel-rich core, a spinel-anorthite-Ca-pyroxene mantle, and a melilite-spinel crust. (b) Inset along the crust-mantle boundary, showing replacement of melilite (Mel) by anorthite (An) and Ca-pyroxene (thin light gray rims between dark voids and gray anorthite). The dark gray mineral is spinel (Sp). (c) Inset of the mantle, showing anorthite replaced by sodalite-nepheline assemblages (Sod-3 and Sod-4 were analyzed by the ion probe). Thin white rims in the assemblages are Ca-pyroxene (Cpx).

Four sodalite-rich alteration assemblages in the Ningqiang CAI with very high ³⁵Cl/³⁴S ratios (up to 57,000) show clear ³⁶S excesses that correlate with ³⁵Cl/³⁴S ratios (Table 1 and Fig. 2), suggesting in situ decay of ³⁶Cl and excluding possible anomaly of sulfur isotopes or spallogenic ³⁶S. The inferred (³⁶Cl/³⁵Cl)_o ratios are determined from the slopes of the correlation lines in Fig. 2, calibrated for 1.9% decay of ³⁶Cl to ³⁶S. The inferred (³⁶Cl/³⁵Cl)_o ratios (±2 σ) are: (4.6 ± 0.6) × 10^-6, (5.1 ± 1.0) × 10^-6, (7.7 ± 2.5) × 10^-6, and (1.1 ± 0.2) × 10^-5, respectively. Sodalite-3 yields the highest inferred (³⁶Cl/³⁵Cl)_o value, although the data show large scatter, probably indicating later disturbance in this grain. Therefore, the two assemblages with the highest Cl/S ratios (Fig. 2 b and d) yield the best determination of the inferred (³⁶Cl/³⁵Cl)_o ratios of ≈5 × 10^-6 in the sodalite. The alteration assemblages in the two CAIs from the enstatite chondrites show no resolvable ³⁶S excess because of their low ³⁵Cl/³⁴S ratios (<100). Secondary ion images revealed the presence of very fine-grained hot spots of ³⁴S, suggesting the presence of tiny sulfides, which would mask any evidence of ³⁶Cl.

Table 1. Sulfur isotopic compositions and Cl/S ratios.

Analysis	35Cl/34S, ±2 σ	36S/34S × 103, ±2 σ
Sod-1a	2,905 ± 217	3.81 ± 0.38
Sod-1b	4,287 ± 210	4.09 ± 0.58
Sod-1c	7,031 ± 385	4.33 ± 0.60
Sod-1d	6,289 ± 435	4.48 ± 0.46
av. Tr-1	<0.01	3.434 ± 0.003
av. Dj-1	0.75 ± 0.01	3.420 ± 0.002
Sod-2a	26,872 ± 2493	5.64 ± 1.15
Sod-2b	10,031 ± 1045	4.84 ± 1.00
Sod-2c	22,942 ± 1475	5.59 ± 0.98
Sod-2d	24,977 ± 1496	5.72 ± 0.94
Sod-2e	38,533 ± 3777	7.37 ± 1.98
Sod-2f	32,931 ± 2936	7.12 ± 1.54
av. Tr-2	<0.01	3.436 ± 0.013
Sod-3a	10,141 ± 637	5.70 ± 0.94
Sod-3b	13,151 ± 800	6.17 ± 0.78
Sod-3c	7,924 ± 447	5.63 ± 0.85
Sod-3d	7,816 ± 332	6.11 ± 0.73
Sod-3e	9,147 ± 426	4.52 ± 0.73
Sod-3f	5,894 ± 372	3.93 ± 0.75
av. Tr-3	<0.01	3.436 ± 0.002
A_sod-1a	18 ± 4	3.57 ± 0.09
Sod-3a	19,958 ± 1,198	6.33 ± 1.19
Sod-3b	36,306 ± 3,490	6.92 ± 1.29
Sod-3c	49,896 ± 2,506	7.36 ± 0.94
Sod-3d	57,486 ± 4,797	7.51 ± 1.59
Sod-3e	52,159 ± 4,215	8.34 ± 1.39
Sod-3f	44,581 ± 3,808	8.39 ± 1.67
av. Tr-3	<0.01	3.431 ± 0.003

Except for one analysis (A_sod-1a) of sodalite (Sod) in ALH 77295, others are in the Ningqiang CAI. Analyses of troilite (Tr) and djerfisherite (Dj) in the Qingzhen enstatite chondrite are given as averages (av.).

Fig. 2.

Sulfur isotopic compositions of four sodalite-rich assemblages of the CAI (NQJ1-1#1) from the Ningqiang meteorite. All four analyses show clear excess of ³⁶S that linearly correlates with the Cl/S ratio, indicating in situ decay of ³⁶Cl in the assemblages. Error bars are 2 σ. The inferred (³⁶Cl/³⁵Cl)_o ratios of the assemblages at the time of alteration are determined from the slopes of the best-fit correlation lines and are calibrated in proportion to the decay of ³⁶Cl to ³⁶S.

Analyses of the unaltered melilite/spinel-rich crust of the Ningqiang CAI show clear ²⁶Mg excesses that linearly correlate with the ²⁷Al/²⁴Mg values of the minerals (Fig. 3a), indicating in situ decay of ²⁶Al. The inferred (²⁶Al/²⁷Al)_o ratio (±2 σ) is (5.1 ± 1.4) × 10^-5, consistent with previous observations in Ningqiang CAIs (18) and identical to the canonical (²⁶Al/²⁷Al)_o value (5 × 10^-5) (19). However, neither anorthite nor the sodalite-rich assemblages in the CAI mantle show resolvable ²⁶Mg excess, providing a maximum inferred (²⁶Al/²⁷Al)_o ratio of ≈0.7 × 10^-5 (Fig. 3b). Assuming the (²⁶Al/²⁷Al)_o ratios of melilite and sodalite reflect a temporal difference, then sodalite formed ≥2 My after the formation of melilite.

Fig. 3.

Mg isotopic compositions and Al/Mg ratios of the same CAI. (a) Melilite in the unaltered crust of the inclusion shows clear ²⁶Mg excess that correlates with the Al/Mg ratios. Spinel and Ca-pyroxene occur in the crust. The inferred (²⁶Al/²⁷Al)_o ratio is determined from the slope of the best-fit line of the analyses. Error bars are 2 σ.(b) Anorthite and the sodalite-rich alteration assemblage show no significant ²⁶Mg excess. The dashed line refers to a (²⁶Al/²⁷Al)_o ratio of 0.7 × 10^-5, the upper limit; the dot horizontal line refers to the normal ²⁶Mg/²⁴Mg ratio of 0.139. Error bars are 2 σ. The analysis data are given in Table 2, which is published as supporting information on the PNAS web site.

Discussion

Because of the high Cl concentration of sodalite, ³⁶Cl may be cosmogenically produced by reaction of ³⁵Cl(n,γ)³⁶Cl during cosmic-ray exposure of the meteorite. However, the cosmogenic ³⁶Cl can be ignored according to the noble gas isotopic data. If there had been cosmogenic ³⁶Cl, we would have an excess of ³⁶Ar from its decay. This excess would increase the ³⁶Ar/³⁸Ar ratio that is used to calculate cosmogenic ³⁸Ar concentration. An increase in ³⁶Ar/³⁸Ar ratio causes the cosmogenic ³⁸Ar to be too low if we use a trapped ³⁶Ar/³⁸Ar ratio of 5.32. That means the cosmic-ray exposure (CRE) age calculated from cosmogenic ³⁸Ar will be lower than that from ³He and ²¹Ne. However, this is not the case: the CRE ages of the Ningqiang meteorite based on ³⁶Ar, ³He, and ²¹Ne are 43.5 ± 5.3, 43.4 ± 5.4, and 39.6 ± 4.0 My, respectively (20). Similarly, the estimated cosmogenic ³⁶Ar component decayed from the cosmogenic ³⁶Cl in the Efremovka matrix is also very low, <3% of the excess ³⁶Ar from decay of short-lived ³⁶Cl (15).

The inferred (³⁶Cl/³⁵Cl)_o ratios of (5–11) × 10^-6 for the Ningqiang CAI alteration assemblages are four to eight times higher than a previous estimate (1.4 ± 0.2) × 10^-6, which was derived from measurements of ³⁶Ar excess in the matrix of Efremovka (15). There are several possible explanations for the difference. First, the vast majority (94–96%) of ³⁶Ar in meteorites is a mixture of a trapped component, a spallation component, and the decay of cosmogenic ³⁶Cl. Estimation of ³⁶Ar excess from the decay of short-lived ³⁶Cl hence highly depends on accurate determination and subtraction of these components. A recent study of isotopic compositions of noble gases in meteorites showed a large variation of ³⁸Ar/³⁶Ar ratios caused by experimental artifact, requiring reassessment for the entire reported ³⁶Ar excess (21). Thus, it is likely that the previous report of ³⁶Cl decay products was inaccurate.

Second, the difference may be related to partial loss of gaseous ³⁶Ar during mild thermal metamorphism in the asteroidal body. The partial loss of ³⁶Ar in Ningqiang would be consistent with its significantly lower gas retention ages (⁴⁰K-⁴⁰Ar age of 4,260 ± 70 My and U/Th-⁴He age of 4,170 ± 160 My, in comparison with the chondrite formation age of ≈4,560 My) (20). Although Efremovka may have a different parent body from that of Ningqiang, it probably lost some ³⁶Ar as well, because both meteorites are carbonaceous chondrites and experienced comparable mild thermal metamorphism (with the same petrographic type of 3) in their parent bodies. In contrast, little sulfur was lost during the same metamorphism event. Third, the different ³⁶Cl/³⁵Cl ratios of the Efremovka matrix and the Ningqiang sodalite may be caused by their distinct cosmic-ray exposure histories. However, as discussed above, the cosmogenic ³⁶Cl is negligible. Finally, it is possible that the difference may represent a time interval of 0.6–1.2 My (i.e., two to four times of the half-life of ³⁶Cl) between the alteration of the CAI and the formation of Efremovka matrix, with assumption of homogeneous distribution of ³⁶Cl in the nebula.

The inferred (³⁶Cl/³⁵Cl)_o ratios of the sodalite cannot represent the initial value at the time when CAIs formed, because this Cl-rich mineral is an alteration product of the CAI (22). To deduce the initial (³⁶Cl/³⁵Cl)_o value when the CAI first formed, the time interval between the formation and alteration of the Ningqiang CAI is required. Such a time interval ≥2 My was determined based on the canonical (²⁶Al/²⁷Al)_o ratio and the maximum inferred (²⁶Al/²⁷Al)_o ratio of the anorthite and sodalite-rich assemblages. However, the lack of ²⁶Mg excesses in the anorthite and the sodalite-rich assemblages may be partially caused by metamorphism that occurred much later on the parent body of Ningqiang. The ²⁶Al-²⁶Mg systematics of plagioclase in a plagioclase- and fassaite‡‡-rich (type C) inclusion and plagioclase- and olivinerich inclusions from the same meteorite also show little ²⁶Mg excess (23). In accordance with self-diffusion rates of Mg in anorthite, it takes <2 My to homogenize Mg isotopes of the anorthite layers (<10 μm, in width) in the CAI at 500°C (23). Therefore, the (²⁶Al/²⁷Al)_o ratios in anorthite or sodalite might reflect redistribution of Mg isotopes during metamorphism on the parent body. However, the ³⁶Cl-³⁶S system may not be significantly disturbed by the same event, because there are no S-bearing minerals surrounding the sodalite-rich assemblages. Assuming NQJ1-1#1 was an unmelted CAI that endured the same heating event that produced type C inclusions (22), a time interval of 1.5 My between the formation of the unaltered crust of NQJ1-1#1 and the heating event can be estimated according to the canonical value and the maximum (²⁶Al/²⁷Al)_o ratio of type C inclusions (≈1 × 10^-5) (19, §§). The same time difference (1.5 My) can be referred to as the minimum interval between the formation and alteration of the Ningqiang CAI. Using this time difference and the best inferred (³⁶Cl/³⁵Cl)_o value (≈ 5 × 10^-6) of sodalite, the initial (³⁶Cl/³⁵Cl)_o ratio at the time when CAIs formed is then deduced to be ≥1.6 × 10^-4.

The initial (³⁶Cl/³⁵Cl)_o value can be used to constrain the source of short-lived radionuclides and the setting of solar system formation. Wasserburg et al. (3) assumed that the solar system formation was triggered by explosion of a nearby supernova, and ³⁶Cl and other short-lived radionuclides were injected into the protosolar nebula. The predicted initial (³⁶Cl/³⁵Cl)_o ratios of the solar nebula range from 3 × 10^-6 to 2 × 10^-4 (3), with consideration of a dilution factor for the supernova material to force a canonical (²⁶Al/²⁷Al)_o ratio and by using the yields of massive stars reported in ref. 24. The predictions are consistent with the initial (³⁶Cl/³⁵Cl)_o ratio of ≥1.6 × 10^-4 estimated in this study. Recent calculations report somewhat higher yields of ³⁶Cl/³⁵Cl relating to ²⁶Al/²⁷Al for massive stars (25). The new data may enhance the initial (³⁶Cl/³⁵Cl)_o values, but are still consistent with our observations. On the other hand, ³⁶Cl might be synthesized in a nearby low-mass asymptotic giant branch star instead of a supernova. However, a low-mass asymptotic giant branch star source predicted much lower initial (³⁶Cl/³⁵Cl)_o ratios (≤ 7.85 × 10^-7 to 2.4 × 10^-6) (5, 6).

Alternatively, local irradiation models predict an upper limit of (³⁶Cl/³⁵Cl)_o = 1.3 × 10^-4 (4). Although the predicted value is consistent with the observation from this study, the volatile nature of chlorine and the observation of ³⁶Cl in sodalite probably are not compatible with local irradiation models. According to the model by Shu et al. (10, 11), intense irradiation of the proto-sun evaporated Mg-Fe-silicate dust balls to form CAIs and simultaneously produced short-lived radionuclides (e.g., ¹⁰Be, ²⁶Al, and ⁴¹Ca) through the bombardment of CAI materials by energetic particles. After their formation, short-lived radionuclides bearing CAIs were ejected to distant locations where chondrites accreted. Sodalite, a volatile-rich (Na and Cl) alteration product, could not form together with the CAI primary minerals during the intense irradiation near the proto-sun. Instead, it almost certainly formed in the chondrite-accreting locations in the nebula and/or the asteroidal bodies. Therefore, in the local irradiation model, ³⁶Cl itself would have to be produced in a gaseous phase close to the proto-sun by intense irradiation, and then a mechanism to transport the gaseous ³⁶Cl, coupled with solids, from the CAI-forming region to distant chondrite-accreting regions must be invoked. Finally, the transported gaseous ³⁶Cl would have to be incorporated into alteration assemblages through solid-gas reaction. However, the transported gaseous ³⁶Cl must have been much diluted before it was incorporated into the alteration assemblages. Furthermore, according to the local irradiation model (10), gases and small particles would be thrown into interstellar space with finite escape speeds, decoupled from the large CAIs that fell back to the disk. Therefore, the observation of ³⁶Cl in this study presents a serious challenge to local irradiation models.

CAIs and chondrules in carbonaceous chondrites are commonly altered to produce sodalite and other secondary minerals (17). However, where the alteration took place, in the nebula or the asteroidal bodies, is a highly controversial issue. One approach is to determine ages of sodalite and other alteration products. The I-Xe (half-life of ¹²⁹I, 15.7 My) data of Allende sodalite-rich samples suggest an apparent closure time of 5 My after Murchison magnetite (26), marginally longer that the predicted lifetime of the solar nebula. However, the long closure age may be caused by partial loss of gaseous ¹²⁹Xe during thermal metamorphism in the parent body. Other data on Al-Mg systems of sodalite suggest the time intervals of several My after formation of CAI primary minerals (19, §§, ¶¶), but they also may be explained by diffusion of normal Mg isotopes from common surrounding Mg-rich phases as discussed above. ³⁶Cl has a shorter half-life than those of ²⁶Al and ¹²⁹I, and discovery of the short-lived ³⁶Cl in sodalite favors for a short interval (e.g., a few half-lives of ³⁶Cl), otherwise it would have decayed and little can be detected. This short time interval favors a nebular origin of sodalite in the Ningqiang CAI, consistent with petrography and mineral chemistry (22).

Conclusions

Sodalite-rich alteration assemblages in a Ningqiang CAI show clear excess of ³⁶S that linearly correlates with Cl/S ratios of the assemblages, providing direct evidence for the presence of short-lived ³⁶Cl in the early solar system. The best inferred (³⁶Cl/³⁵Cl)_o ratio of sodalite is 5 × 10^-6. Combined with the ²⁶Al-²⁶Mg systems of the individual minerals and petrographic settings of the CAI, the initial (³⁶Cl/³⁵Cl)_o ratio of the solar nebula is determined to be ≥1.6 × 10^-4.

The presence of ³⁶Cl in the sodalite-rich alteration assemblages of the Ningqiang CAI and the initial (³⁶Cl/³⁵Cl)_o ratio are consistent with a supernova source of short-lived radionuclides, whereas a low-mass asymptotic giant branch star source predicts a much lower (³⁶Cl/³⁵Cl)_o ratio. Although ³⁶Cl can be produced by local irradiation of the proto-sun, its volatile nature and occurrence in alteration assemblages of CAIs present a serious challenge for local irradiation models.

Unlike other short-lived radionuclides (e.g., ²⁶Al and ⁴¹Ca), chlorine is closely associated with secondary alteration in CAIs, chondrules, and matrix. ³⁶Cl may serve as a unique fine-scale chronometer, especially for volatile-rock reactions in the nebula and/or on asteroidal bodies. The discovery of ³⁶Cl in sodalite favors a short time interval after formation of CAI primary minerals, consistent with a nebular origin of the alteration in the Ningqiang CAI.