Shigeko Togashi

A short duration of chondrule formation in the solar nebula: evidence from 26Al in Semarkona ferromagnesian chondrules

Abstract The 26 Al– 26 Mg systems of five ferromagnesian chondrules from the least metamorphosed ordinary chondrite Semarkona (LL3.0) were studied using a secondary ion mass spectrometer. Their glass or plagioclase portions contain excesses of 26 Mg, and in two chondrules the 26 Mg excesses are well correlated with 27 Al/ 24 Mg, which demonstrates the in-situ decay of 26 Al. The initial 26 Al/ 27 Al ratios in these chondrules obtained from the slope of isochrons show a narrow range of between 6 × 10 −6 and 9 × 10 −6 , indicating their short formation duration of less than 1 My. If the solar nebula was initially homogeneous in Al isotopes, the chondrule formation ages are ∼2 My younger than those of CAIs. Our results based on the study of the least metamorphosed UOC are consistent with the previous studies on Al-rich chondrules that the chondrule formation started at least 2 My after CAIs. Alternatively, the older records before 2 My were erased by chondrule recycling process. It further suggests that the young apparent ages (3 to >5 My after CAIs) for chondrules in type 3.4 UOCs are due to the disturbance of the 26 Al– 26 Mg system, possibly during parent body metamorphism. The result is not consistent with the extended nebular time scale of >5 My and the chondrule formation by planetary processes. The Ni isotopic analysis of the FeO-rich olivines in a type II chondrule in Semarkona did not show any detectable excess 60 Ni in spite of their high Fe/Ni ratios. The upper limit of the initial 60 Fe/ 56 Fe ratio of the solar system was estimated to be 3.4 × 10 −7 , which is consistent with the previous estimate (0.2–1.9 × 10 −7 ) from eucrites. This result confirms that the 60 Ni excess previously observed from CAIs was not due to the decay of the short-lived nuclide 60 Fe, but a Ni isotopic anomaly of nucleosynthetic origin.

The behavior of gold in unaltered island arc tholeiitic rocks from Izu-Oshima, Fuji, and Osoreyama volcanic areas, Japan

Abstract High Au concentrations (4–25 ppb) in titanomagnetite concentrates from basalts and basaltic andesites of the Izu-Oshima and Fuji volcanic areas in the East Japan arc indicate that Au is strongly partitioned into titanomagnetite relative to the bulk rock. The concentration of An in volcanic rocks increases during the early stages of fractional crystallization, dominated by olivine, pyroxenes, and plagioclase, but decreases with subsequent fractional crystallization involving titanomagnetite. A delay in titanomagnetite formation during fractional crystallization subsequently results in comparatively high concentrations of Au in island arc tholeftes. Wide variations in the Au concentration are observed in rocks derived from primary magmas across a single volcanic area, and these can be positively correlated with the concentrations of incompatible elements such as Rb and Zr. These features are inherited from a S-depleted mantle source, since variable degrees of partial melting do not significantly affect the Au concentrations of the primary magmas.

Melt contribution to partitioning of trace element between plagioclase and basaltic magma of Fuji volcano, Japan

Abstract Ion microprobe analyses of trace elements in plagioclase crystals apply to partitioning between plagioclase phenocrysts and basaltic magma of Fuji volcano. The partitioning of trace elements of plagioclase is governed not only by the site preference of ions in the crystal as it is widely accepted, but also by the components of melt. The observed logarithmic count ratios of the ion microprobe analyses between trace and host elements in plagioclase, is linear with XAn (anorthite composition of plagioclase), in the range of XAn (0.55–0.85). The observed ratio is proportional with K′-value (apparent exchange partition coefficient) because of nearly constant composition of the basalt. We separate the K′-value into two terms: plagioclase component controlled term and non-plagioclase component controlled term. The second term mainly characterizes the pattern of the Onuma PC-IR-diagram in a magmatic system.