Single grain noble gas analysis of Antarctic micrometeorites by stepwise heating method with a newly constructed miniature furnace

Abstract

Ten micrometeorites weighing 0.14-18.5 μg, each retrieved from surface snow near the Dome Fuji Station, Antarctica (snow-AMMs), were studied to elucidate their noble gases, mineralogy, morphology, and chemical compositions. Low densities in the range of 0.2-1.4 g/cm 3 estimated for seven samples suggested a porous inner structure. Noble gases were extracted from each particle using stepwise heating with a laboratory manufactured miniature furnace. Isotopic ratios of He and Ne indicate that the light noble gases with high 4 He concentrations ranging from 10 −2 to 10 −4 cm 3 STP/g are mostly of solar origin. The higher concentrations of 4 He observed for several samples are comparable with those of IDPs enriched in solar He, but exceed those reported for ice-AMMs. In contrast to He and Ne, heavy noble gases Ar, Kr, and Xe are primordial ones resembling Q-gas trapped in chondrites, although a small contribution of solar Ar is indicated for some samples with higher 36 Ar/ 132 Xe ratios than that for the Q-gas. Three particles released appreciable amounts of He at temperatures lower than 800• C, suggesting heating temperatures lower than 700• C at the time of atmospheric entry. Other particles released at most 10% of total He at the temperatures up to 800• C. Based on their sizes, weights, and release profiles of 4 He, initial speeds of less than 14 km/s at atmospheric entry were indicated for the particles. The slow entry speeds imply that all the snow-AMMs studied in this work were likely derived from asteroids. The present work demonstrates that the miniature furnace can be applicable to noble gas analysis of tiny grains from the Itokawa asteroidal regolith materials returned by the Hayabusa mission.

Origin of Light Noble Gases (He, Ne, and Ar) on Earth: A Review

“… 20 Ne/ 22 Ne versus (a) the heating index and (b) the 22 Ne content for some IDPs (Kehm et al, ) and micrometeorites (Bajo et al, ; Marty et al, ; Okazaki et al, ). Bajo et al (), Okazaki et al (), and Marty et al () only characterized the heating degree of their samples and we then attributed a heating index between 1 and 3 to compare with the index of Kehm et al () (see text and also supporting information Figure S‐5). IDPs are in blue and micrometeorites in orange.…”

“…IDPs and micrometeorites are also exposed to solar wind irradiation during their transport in the solar system and may also reflect the implanted solar wind composition. Several studies report the noble gas isotopic compositions of IDPs (Hudson et al, 1981;Kehm et al, 2006;Nier & Schlutter, 1990, 1992Pepin et al, 2000Pepin et al, , 2001 and of micrometeorites (Bajo et al, 2012;Marty et al, 2005;Okazaki et al, 2015;Osawa et al, 2000Osawa et al, , 2003Osawa & Nagao, 2002a, 2002b. However, the uncertainties on the neon and argon data are big and these particles have lost a fraction of their noble gases due to heating in the Earth's atmosphere (Bajo et al, 2012;F€ uri et al, 2013;Kehm et al, 2006;Marty et al, 2005;Okazaki et al, 2015;Osawa et al, 2003).…”

“…Some studies tried to define the extent of atmospheric heating for their samples (Bajo et al, 2012;Kehm et al, 2006;Okazaki et al, 2015). Kehm et al (2006) even determined a heating index from 1 to 3 based on the He and Zn contents.…”

“…Kehm et al (2006) even determined a heating index from 1 to 3 based on the He and Zn contents. To compare with the heating degree mentioned by Bajo et al (2012) estimated according to mineral composition and that of Okazaki et al (2015) determined by the solar 4 He release pattern, we attribute a value of 1 for weak or least heating, a value of 1.5 for weak to moderate heating, a value of 2 for moderate heating and a value of 3 for severe heating, as described by Bajo et al (2012) and Okazaki et al (2015). For the samples studied by Marty et al (2005), we attribute a value of 1 for the crystalline samples, a value of 1.5 for scoria/crystalline samples, a value of 2 for scoria samples, of 2.5 for scoria/cosmic spherule samples, and of 3 for cosmic spherules.…”

Mineralogy and noble gas isotopes of micrometeorites collected from Antarctic snow

“…In addition, the resin used to fix samples could cause a false increase in the background levels of a noble-gas mass spectrometer as reactive organic interference and can create an atmospheric gas trap. As exceptions, Osawa et al (2003a) and Bajo et al (2011) performed scanning electron microscopy (SEM)/energy dispersive spectrometry (EDS) and synchrotron radiation X-ray diffraction (SR-XRD) analyses to characterize the appearance, bulk chemical composition, and bulk mineralogy of the particles prior to conducting noble gas measurements. Okazaki and Nakamura (2006) conducted SEM, electron probe microanalysis (EPMA), and secondary ion mass spectrometry (SIMS) analyses on the cross sections of individual Antarctic MMs in addition to observations of their appearances and bulk chemistry measurements by SEM/EDS prior to the noble-gas analysis.…”