Abstract— High‐purity separates of presolar diamond were prepared from 14 primitive chondrites from 7 compositional groups. Their noble gases were measured using stepped pyrolysis. Three distinct noble gas components are present in diamonds, HL, P3, and P6, each of which is found to consist of five noble gases. P3, released between 200 °C and 900 °C, has a “planetary” elemental abundance pattern and roughly “normal” isotopic ratios. HL, consisting of isotopically anomalous Xe‐HL and Kr‐H, Ar with high 38Ar/36Ar, and most of the gas making up Ne‐A2 and He‐A, is released between 1100 °C and 1600 °C. HL has “planetary” elemental ratios, except that it has much more He and Ne than other known “planetary” components. HL gases are carried in the bulk diamonds, not in some trace phase. P6 has a slightly higher median release temperature than HL and is not cleanly separated from HL by stepped pyrolysis. Our data suggest that P6 has roughly “normal” isotopic compositions and “planetary” elemental ratios. Both P3 and P6 seem to be isotopically distinct from P1, the dominant “planetary” noble‐gas component in primitive chondrites. Release characteristics suggest that HL and P6 are sited in different carriers within the diamond fractions, while P3 may be sited near the surfaces of the diamonds. We find no evidence of separability of Xe‐H and Xe‐L or other isotopic variations in the HL component. However, because ∼1010 diamonds are required to measure a Xe composition, a lack of isotopic variability does not constrain diamonds to come from a single source. In fact, the high abundance of diamonds in primitive chondrites and the presence of at least three distinct noble‐gas components strongly suggest that diamonds originated in many sources. Relative abundances of noble‐gas components in diamonds correlate with degree of thermal processing (see companion paper), indicating that all meteorites sampled essentially the same mixture of diamonds. That mixture was probably inherited from the Sun's parent molecular cloud.

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Abstract-In order to investigate the distribution of 26A1 in chondrites, we measured aluminummagnesium systematics in four calcium-aluminum-rich inclusions (CAIs) and eleven aluminum-rich chondrules from unequilibrated ordinary chondrites (UOCs). All four CAIs were found to contain radiogenic 26Mg (26Mg*) from the decay of 26A1. The inferred initial 26Al/27Al ratios for these objects ((26Al/27Al)o = 5 x 10-5) are indistinguishable from the (26APAl),, ratios found in most CAIs from carbonaceous chondrites. These observations, together with the similarities in mineralogy and oxygen isotopic compositions of the two sets of CAIs, imply that CAIs in UOCs and carbonaceous chondrites formed by similar processes from similar (or the same) isotopic reservoirs, or perhaps in a single location in the solar system. We also found 26Mg* in two of eleven aluminum-rich chondrules. The (26Al/27Al)o ratio inferred for both of these chondrules is -1 x 10-5, clearly distinct from most CAIs but consistent with the values found in chondrules from type 3.0-3.1 UOCs and for aluminumrich chondrules from lightly metamorphosed carbonaceous chondrites (-0.5 x 10-5 to -2 x 10-5). The consistency of the (26A1/27Al)o ratios for CAIs and chondrules in primitive chondrites, independent of meteorite class, implies broad-scale nebular homogeneity with respect to 26A1 and indicates that the differences in initial ratios can be interpreted in terms of formation time. A timeline based on 26A1 indicates that chondrules began to form 1 to 2 Ma after most CAIs formed, that accretion of meteorite parent bodies was essentially complete by 4 Ma after CAIs, and that metamorphism was essentially over in type 4 chondrite parent bodies by 5 to 6 Ma after CAIs formed. Type 6 chondrites apparently did not cool until more than 7 Ma after CAIs formed. This timeline is consistent with 26A1 as a principal heat source for melting and metamorphism.

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60 Fe, which decays to radiogenic 60 Ni ( 60 Ni*), is a now extinct radionuclide. 60 Fe is produced only in stars and thus provides a constraint on the stellar contribution to solar system radionuclides. Its short half-life [t p 1/2 yr (1.49 Myr)] makes it a potential chronometer for the early solar system. We found clear evidence 6 1.49 # 10 for 60 Ni* in troilite (FeS) grains from the Bishunpur and Krymka chondrites, two of the least metamorphosed (LL3.1) ordinary chondrites. The weighted means of inferred initial 60 Fe/ 56 Fe ratios [( 60 Fe/ 56 Fe) 0 ] for the troilites are and for Bishunpur and Krymka, respectively. We compare ourϪ7 Ϫ7(1.08 ‫ע‬ 0.23) # 10 (1.73 ‫ע‬ 0.53) # 10 data with upper limits established previously on ( 60 Fe/ 56 Fe) 0 for a chondrule in an unequilibrated ordinary chondrite, Semarkona, and for troilites in a relatively metamorphosed chondrite, Ste. Marguerite, taking into account their 26 Al-26 Mg ages. The 60 Fe and 26 Al chronometers can be combined to produce a consistent chronology for CaAl-rich inclusions, which are thought to be the earliest solar system solids, chondrules, troilites, and Ste. Marguerite. The initial 60 Fe/ 56 Fe for the solar system is inferred from this chronology to have been to Ϫ7 2.8 # 10 . This is at or below the low end of predictions for a supernova source.Ϫ7 4 # 10