Akiko Miyazaki

Solubilities of nitrogen and noble gases in silicate melts under various oxygen fugacities: Implications for the origin and degassing history of nitrogen and noble gases in the Earth

Solubility experiments for nitrogen and noble gases (Ar and Ne) in silicate melts were conducted using two experimental configurations: one was conducted at 1 atmospheric pressure, T =1300°C and oxygen fugacity (fO2) of IW + 0.9 (i.e., 0.9 log units higher than the iron-wustite buffer) and the other at high pressures (Ptotal ∼ 2 × 108 Pa), 1500°C and fO2 ∼ IW + 6. For the former experiment, isotopically labeled-nitrogen (15N15N-enriched) was used to distinguish dissolved nitrogen from contaminating atmospheric or organic nitrogen and to examine dissolution mechanisms of nitrogen in silicate melts. The results obtained for the two series of experiments are consistent with each other, suggesting that Henrys law is satisfied for fN2 of up to ∼250 atm (2.5 × 107 Pa). The results are also consistent with our earlier results (Miyazaki et al., 1995) obtained at highly oxidizing conditions (fO2 ∼ IW + 10). All these results support physical dissolution of nitrogen as N2 molecules in silicate melts for fO2 from ∼IW + 10 down to ∼IW. The observed solubility (Henrys constant) of nitrogen (3–5 × 10−9 mol/g/atm) is comparable to that of Ar (2–4 × 10−9 mol/g/atm), and much lower than that of Ne (11–14 × 10−9 mol/g/atm) at 1300°C. A preliminary experiment was also performed for partitioning of nitrogen and noble gases between clinopyroxene (cpx) and basaltic melt using a piston cylinder-type apparatus at 1.5 GPa and at 1270 to 1350°C. The obtained cpx/melt partition coefficient of nitrogen is 0.06, slightly lower than those of noble gases (∼0.1 for Ne to Xe), suggesting that nitrogen is as incompatible as or even slightly more incompatible than noble gases. The present results imply that a large nitrogen/Ar fractionation would not be produced by magmatic processes. Therefore, the two orders of magnitude difference between the N2/36Ar ratios in the Earths atmosphere (∼104) and that in the mantle (∼106) must be explained by some other processes, such as incomplete segregation of metal blobs into the core and their later oxidation.

Widespread magmatic activities on the angrite parent body at 4562 Ma ago

Mn-Cr chronology of four quenched angrites showed that they formed nearly at the same time (4562 Ma) on the surface of the parent-body. Based on the chemical compositions, the cosmic ray exposure ages, and considerations on the thermal history of an achondrite parent-body, we suggest that the four angrites formed on top of a shallow magma ocean on the angrite parent body.

Solubilities of nitrogen and argon in basalt melt under oxidizing conditions

We measured solubilities of nitrogen and argon in basalt melt at 1300 °C under highly oxidizing conditions (P(O2)=0.16‐0.37 atm) using 15N15N‐labeled air (Ptotal=1 atm). The obtained solubilities (Henry’s constants) are K(N2)=(1.3‐2.9)×10−9 mol/g/atm and K(Ar)=(2.6‐4.5)×10−9 mol/g/atm, respectively, showing that the solubility of nitrogen is slightly lower than that of Ar under highly oxidizing conditions. The 15N15N‐labeled gas used in the solubility experiment was not isotopically in equilibrium but highly enriched in 15N15N, that is, the ratios of 14N14N: 14N15N: 15N15N in the gas were not 1: 2r: r2, where r=15Ntotal/14Ntotal. The recovered gas from the synthetic basalt glass also showed similar isotopic disequilibrium. This suggests that isotopic exchange among N2 molecules did not occur during dissolution of the gas in the melt. In other words, nitrogen dissolved in the basalt melt as molecules in the present (highly oxidizing) experimental conditions.

Heterogeneous distribution of 60Fe in the early solar nebula : Achondrite evidence

Abstract60Fe-60Ni systematics in quenched angrites and two old eucrites were investigated by secondary ion mass spectrometry. The 60Ni/62Ni isotopic compositions were normal within 2σ errors. The inferred initial 60Fe/56Fe ratios for quenched angrites was (6±9)× 10−9, and similar upper limit values were also obtained from eucrites. Using the age difference of approximately 5 Ma between the quenched angrites and Ca−Al-rich inclusions, the initial 60Fe/56Fe ratio at the start of the solar system was calculated to be approximately (6±9)× 10−8. This initial ratio is significantly smaller than previously published values obtained from chondritic materials, suggesting the heterogeneous distribution of 60Fe in the solar nebula.

Mn−Cr ages of Fe-rich olivine in two Rumuruti (R) chondrites

Mn−Cr systematics in olivine of two Rumuruti (R) chondrites was investigated. Mn/52Cr ratios up to 1800 and 1300, and δ53Cr of up to 25‰ and 7‰ were observed for NWA 753 and Sahara 99531, respectively. All data points of NWA 753 show a linear correlation between δ53Cr values and Mn/52Cr ratios on the isochron diagram. The inferred initial 53Mn/55Mn ratio for NWA 753 is (1.84 ± 0.42(2σ)) × 10−6. In the case of Sahara 99531, a positive correlation interpreted as an isochron for 53Mn/55Mn = 2.75 ± 1.55(2σ) × 10−6 was obtained for only one chondrule. Data from other chondrules in Sahara 99531 give an upper limit of 53Mn/55Mn = 0.49 × 10−6. The Mn−Cr ages of NWA 753 and a chondrule in Sahara 99531 are slightly older than that of the angrite LEW 86010 (Lugmair and Shukolyukov, 1998). Other chondrules in Sahara 99531 are at least 5 Ma younger than the LEW 86010. The Mn-Cr ages of olivine in R chondrites correspond to the time when olivine became a closed system either during slow cooling from the peak metamorphic temperature or during rapid cooling by impact excavation. In either case the olivine closure occurred earlier than the final assembly of the brecciated chondrites.