TY - JOUR
T1 - High fluences of neutrons determined from Sm and Gd isotopic compositions in aubrites
AU - Hidaka, Hiroshi
AU - Ebihara, Mitsuru
AU - Yoneda, Shigekazu
N1 - Funding Information:
Of the meteorite samples for this study, Bishopville (USNM2669) and Cumberland Falls (USNM604) were provided by the National Museum of Natural History, Smithsonian Institution, U.S.A., and ALH78113 was from the National Institute of Polar Research, Japan. Lunar samples were offered by NASA, Johnson Space Center, under the contract for the sample request No. 2340. We also thank Prof. Masatake Honda for donating the Norton County sample. Critical comments and reviews by J. Masarik, J. DeLaeter and an anonymous reviewer were greatly useful to improve the manuscript. H.H. acknowledges financial support from a Grant-in-Aid for Scientific Research of Ministry of Education, Science, Sports and Culture, Japan (No. 10640476). [ AH ]
PY - 1999/11/15
Y1 - 1999/11/15
N2 - Large isotopic deviations of 149Sm-150Sm and 157Gd-158Gd derived from neutron capture effects were observed in five enstatite achondrites (aubrites); Norton County, ALH78113, Cumberland Falls, Bishopville and Mayo Belwa. The isotopic variations of these aubrites (ε149Sm = -32 to -7, ε150Sm = +11 to +57, ε157Gd = -42 to -14 and ε158Gd = +9 to +26) correspond to irradiation with neutron fluence of (1.17 - 3.99) x 1016 n cm-2, which is within the range of estimated neutron fluence values of some lunar samples. These results indicate that aubrites may have been located near the surface of their parent body for several hundreds of Ma. Furthermore, the neutron energy spectrum obtained from the combination of Sm and Gd isotope systematics (ε(Sm)/ε(Gd)) suggests that the energy of thermalized neutrons in five aubrites was in the range of 200-400 K, and was lower than those in lunar samples A-12 and A-15. In addition, we analyzed one anomalous aubrite (Mt. Egerton), and two enstatite chondrites (Happy Canyon and Abee) to investigate the relationship of their origin with aubrites. However, these three meteorites showed no evidence for intensive neutron irradiation observed in the aubrites.
AB - Large isotopic deviations of 149Sm-150Sm and 157Gd-158Gd derived from neutron capture effects were observed in five enstatite achondrites (aubrites); Norton County, ALH78113, Cumberland Falls, Bishopville and Mayo Belwa. The isotopic variations of these aubrites (ε149Sm = -32 to -7, ε150Sm = +11 to +57, ε157Gd = -42 to -14 and ε158Gd = +9 to +26) correspond to irradiation with neutron fluence of (1.17 - 3.99) x 1016 n cm-2, which is within the range of estimated neutron fluence values of some lunar samples. These results indicate that aubrites may have been located near the surface of their parent body for several hundreds of Ma. Furthermore, the neutron energy spectrum obtained from the combination of Sm and Gd isotope systematics (ε(Sm)/ε(Gd)) suggests that the energy of thermalized neutrons in five aubrites was in the range of 200-400 K, and was lower than those in lunar samples A-12 and A-15. In addition, we analyzed one anomalous aubrite (Mt. Egerton), and two enstatite chondrites (Happy Canyon and Abee) to investigate the relationship of their origin with aubrites. However, these three meteorites showed no evidence for intensive neutron irradiation observed in the aubrites.
KW - Chladnite
KW - Gadolinium
KW - Isotope ratios
KW - Neutrons
KW - Samarium
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U2 - 10.1016/S0012-821X(99)00221-6
DO - 10.1016/S0012-821X(99)00221-6
M3 - Article
AN - SCOPUS:0033570679
SN - 0012-821X
VL - 173
SP - 41
EP - 51
JO - Earth and Planetary Sciences Letters
JF - Earth and Planetary Sciences Letters
IS - 1-2
ER -