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Small differences in the ratio of neodymium-142 to neodymium-144 in early formed mantle reservoirs in planetary bodies are the result of in situ decay of the extinct radionuclide samarium-146 and can be used to constrain early planetary differentiation and therefore the time scale of planetary accretion. The martian meteorite Nakhla (approximately 1.3 billion years old), the type sample of the nakhlite subgroup of the Shergottite-Nakhlite-Chassigny (SNC) meteorites, exhibits a 59 +/- 13 parts per million excess in the ratio of neodymium-142 to neodymium-144 relative to normal neodymium. This anomaly records differentiation in the martian mantle before 4539 million years ago and implies that Mars experienced no giant impacts at any time later than 27 million years after the origin of the solar system.

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— Neodymium, strontium, and chromium isotopic studies of the LEW86010 angrite established its absolute age and the formation interval between its crystallization and condensation of Allende CAIs from the solar nebula. Pyroxene and phosphate were found to contain ∼98% of its Sm and Nd inventory. A conventional 147Sm‐143Nd isochron yielded an age of 4.53 ± 0.04 Ga (2 σ) and ∍143 Nd = 0.45 ± 1.1. An 146Sm‐142Nd isochron gives initial 146Sm/144Sm = 0.0076 ± 0.0009 and ∍143 Nd = −2.5 ± 0.4. The Rb‐Sr analyses give initial 87Sr/86Sr (I87Sr) = 0.698972 ± 8 and 0.698970 ± 18 for LEW and ADOR, respectively, relative to 87Sr/86Sr = 0.71025 for NBS987. The difference, ΔI87Sr, between I87Sr for the angrites and literature values for Allende CAIs, corresponds to ∼9 Ma of growth in a solar nebula with a CI chondrite value of 87Rb/86Sr = 0.91, or ∼5 Ma in a nebula with solar photospheric 87Rb/86Sr = 1.51. Excess 53Cr from extinct 53Mn (t1/2 = 3.7 Ma) in LEW86010 corresponds to initial 53Mn/55Mn = 1.44 ± 0.07 × 10−6 and closure to Cr isotopic homogenization 18.2 ± 1.7 Ma after formation of Allende inclusions, assuming initial 53Mn/55Mn = 4.4 ± 1.0 × 10−5 for the inclusions as previously reported by the Paris group (Birck and Allegre, 1988). The 146Sm/144Sm value found for LEW86010 corresponds to solar system initial (146Sm/144Sm)o = 0.0080 ± 0.0009 for crystallization 8 Ma after Allende, the difference between Pb‐Pb ages of angrites and Allende, or 0.0086 ± 0.0009 for crystallization 18 Ma after Allende, using the Mn‐Cr formation interval. The isotopic data are discussed in the context of a model in which an undifferentiated “chondritic” parent body formed from the solar nebula ∼2 Ma after Allende CAIs and subsequently underwent differentiation accompanied by loss of volatiles. Parent bodies with Rb/Sr similar to that of CI, CM, or CO chondrites could satisfy the Cr and Sr isotopic systematics. If the angrite parent body had Rb/Sr similar to that of CV meteorites, it would have to form slightly later, ∼2.6 Ma after the CAIs, to satisfy the Sr and Cr isotopic systematics.

We have measured significant concentrations of 3c'c1, 4•Ca, 36Ar from decay of 3c'c1, and •5øSm produced from the capture of thermalized neutrons in the large Chico L6 chondrite. Activities of 3r121 and 4•Ca, corrected for a high-energy spallogenic component and a terrestrial age of ~50 ka, give average neutron-capture production rates of 208 atoms/min/g-Cl and 1525 atoms/min/kg-Ca, which correspond to thermal neutron (n) fluxes of 6.2 n/cm2/s and 4.3 n/cm2/s, respectively. If sustained for the -65 Ma single-stage, cosmic ray exposure age of Chico, these values correspond to thermal neutron fluences of ~l.3x1016 and 0.8 x1016 n/cm 2 for 3r'CI and n•Ca, respectively. Stepwise temperature extraction of Ar in Chico impact melt shows 3•Ar/3SAr ratios as large as ~9. The correlation of high 3aAr/3SAr with high C1/Ca phases in neutronirradiated Chico indicates that the excess 3•Ar above that expected from spallation is due to decay of neutron-produced 3c'c1. Excess 3•Ar in Chico requires a thermal neutron fluence of 0.9-1.7x10 • n/cm 2. Decreases in a49Sm/aS2Sm due to neutron-capture by •49Sm correlate with increases in 15øSm/152Sm for three samples of Chico, and one of the Torino H-chondrite. The 0.08% decrease in 149Sm/152Sm shown by Chico corresponds to a neutron fluence of 1.23x10 la n/cm 2. This fluence derived from Sm considers capture of epithermal neutrons and effects of chemical composition on the neutron energy distribution. Excess 3•Ar identified in the Arapahoe, Bmderheim, and Torino chondrites and the Shallowater aubrite suggest exposure to neutron fluences of --0.2-0.6x10 la n/cm 2. Depletion of 149Sm in Torino and the LEW86010 angrite suggest neutron fluences of 0.8x1016 n/cm 2 and 0.25x10 la n/cm 2, respectively. Neutron fluences of ~10 • n/cm 2 in Chico are almost as large as those previously observed for some lunar soils.Consideration of exposure ages suggests that the neutron flux in Chico may have been greater than that in many lunar soils. Neutron-capture effects, although seldom reported, may be common for large meteorites and could affect calculation of exposure ages based on cosmogenic Ar. Combining measurements of radioactive and stable species produced from neutron-capture has the potential for identifying large meteorites with complex exposure histories.Copyfight 1995 by the American Geophysical Union.Paper number 95JE00663. 0148-0227/95/95JE-00663505.00The rates at which these high-energy or spallation reactions proceed vary with depth in the meteorold, but all of them reach maxima at shielding depths less than 150 g/cm 2, some much less. The exact depth of maximum production depends on the size of the body and the specific product [e.g., Reedy, 1985; Graf et al., 1990; Bhandari et al., 1993]. Some of the neutrons produced by cosmic rays do not interact, however, until they are slowed to thermal energies. In large meteoroids (radius >200 g/cm 2) these neutron-capture products are predicted to reach maxima in their production rams at shielding depths of 100-300 g/cm 2 [e.g., Eberhardt et al.,...

The Yamato polymict eucrite Y75011 contains several coarse-grained mesostasis-rich basaltic clasts with subophitic texture. The preservation of extensive Mg-Fe chemical zoning in pyroxenes and the presence of a dark mesostasis shows that these clasts are representative of pristine lava-like basalts extruded on the surface of the Howardite-Eucrite-Diogenite (HED) parent body. Rubidium-strontium analyses of a large clast, Y75011,84B, yielded a •recise internal isochron age of 4.60 + 0.05 b.y. for X(87Rb) = 0.0139 (b.y.) -• or 4.50 + 0.05 b.y. for X( 8 Rb) = 0.0142 (b.y.) -•. The Rb-Sr systematics of a matrix sample are nearly identical to those of the pristine clast, showing that the matrix is dominantly comminuted mesostasisrich basalt. The Rb-Sr internal isochron age of the matrix is 4.56 + 0.06 b.y. for X(S7Rb) = 0.0139 (b.y.) -• or 4.46 + 0.06 b.y. for X(S7Rb) = 0.0142 (b.y.) -•. These ages are within error limits of the Rb-Sr whole rock age of chondrites and are in agreement with 2ø6pb/2ø7pb and Sm-Nd ages of ordinary monomict eucrites for h.(87Rb) = 0.01402 (b.y.) -• based on a comparison of the Rb-Sr and U-Th-Pb ages of chondrites (Minster et al., 1982). The good precision obtained for the Rb-Sr ages of both clast and matrix is due to the presence of a mesostasis phase in which the Rb/Sr ratio is enriched 3-4 times above that of any eucrite phase previously analysed. The initial S7Sr/S6Sr ratios of clast and matrix are 0.69894 + 2 and 0.69896 + 3, respectively, and are in satisfactory agreement with the BABI value (Papanastassiou and Wasserburg, 1969). There is evidence of isotopic disturbance in both the Rb-Sr and Sm-Nd systems of the most magnetic separate analysed. A Sm-Nd internal isochron of 4.55 + 0.14 b.y. for the combined data from both clast and matrix exclusive of the most magnetic separate is in agreement with the more precise Rb-Sr age. The corresponding initial •43Nd/•44Nd = 0.50587 + 19, normalized to •48Nd/•44Nd = 0.24308. The clast data alone, exclusive of the most magnetic separate, define an isochron age of 4.52 + 0.16 b.y. and initial •43Nd/•44Nd = 0.50593 + 19. The isochron error limits quoted here for the Sm-Nd data are 20 from the York (1966) program and include the factor [S/(N-2)] • used in that program to compensate for "geological error." However, this factor is not appropriate for the Rb-Sr isochrons 143 144 and has been omitted from the quoted error limits. The initial Nd/ Nd of the pristine clast at 4.56 b.y. ago is 0.505877 + 25, in agreement within error limits with the CHUR value (Jacobsen and Wasserburg, 1984). Clast Y75011,84 represents the oldest pristine, unambigously basaltic lava as yet recovered from a solar system object. Rare earth element abundance measurements (Shimizu and Masuda , 1985) show it to be one of the most differentiated eucrites. Thus moderately evolved basaltic lavas were produced very early in the history of the HED parent body. Their preservation as clasts in the polymict eucrites suggests derivation from a different geologic setting on the parent bod...

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