Abstract
We have investigated seven micrometeorites (MMs) from Antarctic snow collected in 2003 and 2010 by means of electron microscopy, X-ray diffraction, micro-Raman spectroscopy, transmission electron microscopy (TEM) observation, and noble-gas isotope analysis. Isotopic ratios of He and Ne indicate that the noble gases in these MMs are mostly of solar wind (SW). Based on the release patterns of SW 4He, which should reflect the degree of heating during atmospheric entry, the seven MMs were classified into three types including two least heated, three moderately heated, and two severely heated MMs. The heating degrees are well correlated to their mineralogical features determined by TEM observation. One of the least heated MMs is composed of phyllosilicates, whereas the other consists of anhydrous minerals within which solar flare tracks were observed. The two severely heated MMs show clear evidence of atmospheric heating such as partial melt of the uppermost surface layer in one and abundant patches of dendritic magnetite and Si-rich glass within an olivine grain in the other. It is noteworthy that a moderately heated MM composed of a single crystal of olivine has a 3He/4He ratio of 8.44 × 10−4, which is higher than the SW value of 4.64 × 10−4, but does not show a cosmogenic 21Ne signature such as 20Ne/21Ne/22Ne = 12.83/0.0284/1. The isotopic compositions of He and Ne in this sample cannot be explained by mixing of a galactic cosmic ray (GCR)-produced component and SW gases. The high 3He/4He ratio without cosmogenic 21Ne signature likely indicates the presence of a 3He-enriched component derived from solar energetic particles.
Highlights
Our understanding of the evolution history of our early solar system materials stands on studies of extraterrestrial materials originating from asteroids and comets
The bulk concentrations of 36Ar in most of the MMs studied are relatively higher than those reported for unmelted MMs (e.g., Osawa and Nagao 2002, Osawa et al 2003a)
When the gas abundances are normalized by the surface area remaining after the focused ion beam (FIB) cutting calculated by assuming spherical objects, the 4He concentrations are 3.9 cm3 standard temperature and pressure (STP)/cm2, 5.8 cm3 STP/cm2, and 4.8 × 10−6 cm3 STP/cm2 for D03IB068, D10IB020, and
Summary
Our understanding of the evolution history of our early solar system materials stands on studies of extraterrestrial materials originating from asteroids and comets. Asteroidal materials occurring as meteorites have been studied in detail. Okazaki et al Earth, Planets and Space (2015) 67:90 overall properties such as the combination of mineralogy, chemistry, isotopic compositions, and orbital parameters inferred from the degree of heating during atmospheric entry (e.g., Sandford and Bradley 1989; Love and Brownlee 1991, 1994; Nier and Schlutter 1992, 1993). The release pattern of noble gases is one of the most sensitive parameters in atmospheric entry heating because solar wind (SW) noble gases are implanted onto the surfaces of IDPs and MMs in interplanetary space. A comparison of noble gas release patterns and mineralogical characteristics enable discussion of the time in which the dust particles were heated, whether in the parent body, in space, or during atmospheric entry, and in addition the manner of heating revealed by peak temperature, heating duration, and oxygen fugacity
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