Noble gases of Yamato 980459 shergottite

Isotopic ratios and concentrations of noble gases were determined for the Yamato (Y) 980459 olivine-phyric shergottite with a stepped heating extraction method. Trapped noble gas concentrations are low, and especially He and Ne are dominated by cosmogenic nuclides. Heavy noble gases, Ar, Kr and Xe, in the high temperature fractions (1000-1750°C ) show the martian atmospheric signatures: 40Ar36Ar and 129Xe132Xe ratios corrected for cosmogenic gases are >1000 and >1.4, respectively, and the data points plot along the mixing line between the Mars atmosphere and Chassigny in the system of 129Xe132Xe vs. 84Kr132Xe. Contribution of elementally fractionated Earth\u27s atmospheric noble gases is significant in the low temperature fractions (400-800°C ), which has been frequently reported for meteorites from hot deserts. Cosmic-ray exposure ages calculated based on cosmogenic 3He, 21Ne and 38Ar are 1.6, 2.5 and 2.1Ma, respectively. Considering partial loss of He from the meteorite, the exposure age of Y980459 would be around 2.1-2.5Ma. Though the terrestrial age of this meteorite has not been reported so far, the ages reported for Antarctic SNCs are 0.29Ma. Hence, the ejection age for Y980459 could be in the range of 2.1-2.8Ma, which is comparable to those of some basaltic shergottites, but different from other olivine-phyric shergottites ranging mostly 0.7-1.2Ma. Isotopic ratios of Kr show excesses in 80Kr and 82Kr, with 82Kr80Kr of 0.375, which results from epithermal neutron captures on 79Br and 81Br. The minimum pre-atmospheric size of Y980459 was calculated as 27cm in radius, based on the 21Ne exposure age and the n-capture 80Kr and Br concentrations of 3.0×1013cm3STPg and 0.205ppm, respectively. The calculated K-Ar age is 0.99Ga from the total 40Ar and reported K concentration of 157ppm. The age, however, gives an upper limit for the crystallization age of this meteorite, because of possible contribution of martian atmospheric Ar, as well as the terrestrial atmosphere

Ga3+^{3+}-substitution effects in the weak ferromagnetic oxide LaCo0.8_{0.8}Rh0.2_{0.2}O3_{3}

Magnetization and x-ray diffraction have been measured on polycrystalline samples of LaCo$_{0.8-y}$Rh$_{0.2}$Ga$_{y}$O$_{3}$ for $0 \leq y \leq 0.15$ in order to understand the spin state of Co$^{3+}$ through the Ga$^{3+}$ substitution effect. The ferromagnetic order in LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$ below 15 K is dramatically suppressed by the Ga$^{3+}$ substitution, where the ferromagnetic volume fraction is linearly decreased. The normal state susceptibility also systematically decreases with the Ga content, from which we find that one Ga$^{3+}$ ion reduces 4.6 $\mu_{\rm B}$ per formula unit. We have evaluated how the concentration of the high-spin state Co$^{3+}$ changes with temperature by using an extended Curie-Weiss law, and have found that the substituted Rh$^{3+}$ ion stabilizes the high-spin state Co$^{3+}$ ion down to low temperatures. We find that Ga$^{3+}$ preferentially replaces the high-spin state Co$^{3+}$, which makes a remarkable contrast to our previous finding that Rh$^{3+}$ preferentially replaces the low-spin state Co$^{3+}$. These results strongly suggest that the magnetically excited state of LaCoO$_{3}$ at room temperature is a mixed state of high-spin state Co$^{3+}$ and low-spin state Co$^{3+}$.Comment: 5 pages, 6 figure

Noble gases and K-Ar ages of five Rumuruti chondrites Yamato(Y)-75302, Y-791827, Y-793575, Y-82002, and Asuka-881988

Noble gases and K concentrations have been determined for aliquot samples prepared from the five Antarctic R-chondrites Yamato (Y)-75302,Y-791827,Y-793575,Y-82002,and Asuka (A)-881988. K-Ar ages of about 4.2Ga were obtained for Y-75302,Y-791827,Y-793575 and A-881988,while Y-82002 showed a slightly younger age of 3.9Ga. The Y-75302,Y-791827 and Y-82002 are enriched in solar light noble gases. Four meteorites Y-75302,Y-791827,Y-82002 and A-881988 have cosmic-ray exposure ages of about 20Ma, while the age of Y-793575 is about 7Ma. Based on the noble gas compositions and K-Ar ages, Y-75302 and Y-791827 are probably paired and Y-82002 may belong to this pair. Relatively high and variable ^Xe/^Xe ratios between 2-3.7 as well as enrichments of heavier Xe isotopes were observed in all R-chondrites

Noble gas signatures of Antarctic nakhlites, Yamato (Y) 000593, Y000749, and Y000802

We have measured noble gases in three nakhlites from Antarctica, Yamato (Y) 000593, Y000749, and Y000802, by step-heating and total-melting methods.The trapped ^Ar/^Kr/^Xe ratios determined for the bulk samples are around 80/3/1, identical to those of Nakhla. The Yamato nakhlites also release noble gases showing high ^Xe/^Xe (up to 1.486) and low ^Kr/^Xe (～1.5) at 1000 and 1300 ℃, which is one of the most characteristic signatures of nakhlites. The low ^Kr/^Xe, as compared to that of the Mars atmosphere, suggests the presence of a fractionated Martian atmosphere.Cosmic-ray exposure ages based on cosmogenic ^Ne are 11.7, 11.9, and 13.0 Ma for Y000593, Y000749, and Y000802, respectively. This supports the pairing based on the mineralogical and petrographical similarities and the location of the finds. The average of the ^Ne exposure ages is 12.05±0.69 Ma. We also calculated an apparent ^Kr-Kr age as 11.8±1.0 Ma from cosmic-ray produced radioactive ^Kr and stable Kr isotopes from Y000593. The coincidence with the ^Ne exposure age indicates a short terrestrial age (Ne exposure age and terrestrial age, is 12.1±0.7 Ma. Calculated K-Ar gas retention age for the Yamato nakhlites is 1.24±0.22 Ga. The ejection time and gas retention age are close to those of non-Antarctic nakhlites and Chassigny. This suggests that the Yamato nakhlites were ejected from Mars together with other nakhlites and Chassigny. Xenon isotopes are mixtures of Chassigny Xe, fission Xe, and the Mars atmosphere. High-temperature fractions (1000-1750℃) are enriched in the Mars atmosphere and fission Xe components, compared to lower temperature fractions. There are similarities in Xe isotopes between Y000749 and Y000802 showing excesses in ^Xe and ^Xe, whereas Y000593 has only small excesses. The release pattern of ^Xe for Y000593 is also dif

Cosmic-ray exposure ages of enstatite chondrites

Eleven enstatite (E-) chondrites were analyzed for abundances and isotopic ratios of noble gases by total melting. Light noble gases, He, Ne and Ar are discussed. We obtained cosmic-ray exposure ages (T^) based on cosmogenic ^Ne that are divided into two groups, below 15 Myr and above 40 Myr. Both ranges of T^ contain various petrologic-types of E-chondrites. Isotopic ratios and concentration of Ne for Yamato (Y)-8414 and -86004 are almost the same, which suggests that they are from a common fragment. Identical exposure age T^ of 10 Myr for Y-8414 and -86004 supports their pairing. The pairing for Y-8414 and -86004 based on noble-gas signatures and T^ is consistent with their petrologic similarities. Pairing for Y-792959 and -793161 is also suggested by their similarities in mineral compositions, their close sampling location and the same petrologic-type. However, there are discrepancies in He and Ar contents between the two samples, which fails to support their pairing. Including literature data, the exposure ages show no systematic correlation with petrologic type. Additionally, both EH-and EL-chondrites are found in both two peaks of T^. The exposure age distribution has no implications about the structure of E-chondrite parent body. The parent body of E-chondrites has possibly lost the original structure after many collisions in the early solar system evolution