14:15 〜 14:30
[PPS08-03] シリカに富む多孔質南極微隕石：未知のタイプの太陽系外縁天体起源ダストか？
キーワード：南極微隕石、惑星間塵、シリカ、太陽フレアトラック
Mid-infrared (MIR) spectra of some T Tauri stars (TTS) suggest the presence of
silica minerals [e.g., 1, 2]. Spectral fitting indicates that tridymite and cristobalite
are the dominant forms in the TTS [2]. Silica minerals in chondritic meteorites are
rare in ordinary and carbonaceous chondrites [e.g., 3] and minor (1-4 vol.%) in
enstatite chondrites [e.g., 4]. Some Antarctic micrometeorites (AMMs) with
chondrule-like textures contain pyroxene and amorphous silica [5]. Some
Stardust samples recovered from the comet 81P/Wild 2 contain silica: one
contains tridymite and cristobalite [6] and a grain named Ada comprising mainly
tridymite and fayalite [7]. It is not clear whether this assemblage was formed
during high- or low-temperature processes. An interplanetary dust particle (IDP)
W7010*A2 contains tridymite [8]. However, its mineralogical context is also
unclear. An exceptional case is a silica mineral that was formed in the proto-solar
disk: quartz in an amoeboid olivine aggregate (AOA) in Y-793261 CR chondrite
[9]. They inferred that cristobalite had condensed after the isolation of olivine from
the ambient gas and that cristobalite was transformed into quartz at a lower
temperature. Here we report the two porous AMMs with unique mineralogy
containing abundant silica.
Two AMMs D10IB324 and D12IB086 were found in the surface snow near the
Dome Fuji Station in 2010 and 2012, respectively. Because we collected snow
that fell on the earth within ~1 year, these AMMs fell on the earth in different years.
Both of them are porous and composed of abundant filamentous SiO2-rich
material with a variable amount of MgO, relatively abundant silica mineral grains,
abundant ~200–~400-nm across spheroidal objects composed of amorphous
SiO2 embedding many ~10–~100-nm Fe sulfide grains, Mg-Fe carbonate
(decomposed to aggregates of ~10-nm sized oxide grains), isolated pyrrhotite,
and minor twinned low-Ca clinopyroxene, and they lack olivine. Because
filamentous material with chemical compositions similar to low-Ca pyroxene was
selectively altered to phyllosilicate with ~1.0-nm lattice fringes, they experienced
aqueous alteration, which is consistent with the presence of Mg-Fe carbonate.
Different from the vast majority of AMMs and IDPs, they contain abundant SiO2-
rich phases: one is amorphous SiO2-rich material, which often has almost pure
SiO2 compositions and another is crystalline silica mineral. The silica mineral has
a characteristic square cross-section in ultrathin sections, suggestive of
pseudomorphs of cubic high-cristobalite, and is now quartz based on selected
area electron diffraction patterns. Their morphologies suggest that they were
crystallized in free space without any melt and that they might have been formed
by vapor growth. Their major phases suggest that their bulk chemical
compositions deplete Al and Ca. Mg may be also depleted to some degree
because low-Ca pyroxene is rare in these AMMs. In the case of D10IB324, we
found roedderite (Na,K)2(Mg,Fe)5Si12O30. Its presence may be related to the
depletion of these elements. These AMMs were exposed to solar activity for long
periods because low-Ca pyroxene in D12086 and roedderite in D10IB324 contain
high track densities >5 x 1010 cm-2. According to [10], IDPs and AMMs having
such track densities might originate from Edgeworth-Kuiper belt objects. If true,
there are Edgeworth-Kuiper belt objects that are composed mainly of materials
that experienced fractionation of refractory elements.
References [1] Honda et al. (2003) ApJS 154, 18. [2] Sargent et al. (2009) APJ
690, 1193–1207. [3] Brearley and Jones (1996) Rev. Mineral. 36, Ch. 2. [4]
Kimura and Lin (1999) AMR 12, 1–18. [5] Imae et al. (2013) GCA 100, 116-157.
[6] Mikouchi et al. (2007) 38th LPSC 1946 (abstract). [7] Matrajt et al. (2008)
MaPS 43, 315–334. [8] Rietmeijer & McKey (1986) Meteoritics 743 (abstract). [9]
Komatsu et al. (2018) PNAS 115, 7497–7502. [10] Keller and Flynn (2022) Nat.
Astronom. 6, 731-735.
silica minerals [e.g., 1, 2]. Spectral fitting indicates that tridymite and cristobalite
are the dominant forms in the TTS [2]. Silica minerals in chondritic meteorites are
rare in ordinary and carbonaceous chondrites [e.g., 3] and minor (1-4 vol.%) in
enstatite chondrites [e.g., 4]. Some Antarctic micrometeorites (AMMs) with
chondrule-like textures contain pyroxene and amorphous silica [5]. Some
Stardust samples recovered from the comet 81P/Wild 2 contain silica: one
contains tridymite and cristobalite [6] and a grain named Ada comprising mainly
tridymite and fayalite [7]. It is not clear whether this assemblage was formed
during high- or low-temperature processes. An interplanetary dust particle (IDP)
W7010*A2 contains tridymite [8]. However, its mineralogical context is also
unclear. An exceptional case is a silica mineral that was formed in the proto-solar
disk: quartz in an amoeboid olivine aggregate (AOA) in Y-793261 CR chondrite
[9]. They inferred that cristobalite had condensed after the isolation of olivine from
the ambient gas and that cristobalite was transformed into quartz at a lower
temperature. Here we report the two porous AMMs with unique mineralogy
containing abundant silica.
Two AMMs D10IB324 and D12IB086 were found in the surface snow near the
Dome Fuji Station in 2010 and 2012, respectively. Because we collected snow
that fell on the earth within ~1 year, these AMMs fell on the earth in different years.
Both of them are porous and composed of abundant filamentous SiO2-rich
material with a variable amount of MgO, relatively abundant silica mineral grains,
abundant ~200–~400-nm across spheroidal objects composed of amorphous
SiO2 embedding many ~10–~100-nm Fe sulfide grains, Mg-Fe carbonate
(decomposed to aggregates of ~10-nm sized oxide grains), isolated pyrrhotite,
and minor twinned low-Ca clinopyroxene, and they lack olivine. Because
filamentous material with chemical compositions similar to low-Ca pyroxene was
selectively altered to phyllosilicate with ~1.0-nm lattice fringes, they experienced
aqueous alteration, which is consistent with the presence of Mg-Fe carbonate.
Different from the vast majority of AMMs and IDPs, they contain abundant SiO2-
rich phases: one is amorphous SiO2-rich material, which often has almost pure
SiO2 compositions and another is crystalline silica mineral. The silica mineral has
a characteristic square cross-section in ultrathin sections, suggestive of
pseudomorphs of cubic high-cristobalite, and is now quartz based on selected
area electron diffraction patterns. Their morphologies suggest that they were
crystallized in free space without any melt and that they might have been formed
by vapor growth. Their major phases suggest that their bulk chemical
compositions deplete Al and Ca. Mg may be also depleted to some degree
because low-Ca pyroxene is rare in these AMMs. In the case of D10IB324, we
found roedderite (Na,K)2(Mg,Fe)5Si12O30. Its presence may be related to the
depletion of these elements. These AMMs were exposed to solar activity for long
periods because low-Ca pyroxene in D12086 and roedderite in D10IB324 contain
high track densities >5 x 1010 cm-2. According to [10], IDPs and AMMs having
such track densities might originate from Edgeworth-Kuiper belt objects. If true,
there are Edgeworth-Kuiper belt objects that are composed mainly of materials
that experienced fractionation of refractory elements.
References [1] Honda et al. (2003) ApJS 154, 18. [2] Sargent et al. (2009) APJ
690, 1193–1207. [3] Brearley and Jones (1996) Rev. Mineral. 36, Ch. 2. [4]
Kimura and Lin (1999) AMR 12, 1–18. [5] Imae et al. (2013) GCA 100, 116-157.
[6] Mikouchi et al. (2007) 38th LPSC 1946 (abstract). [7] Matrajt et al. (2008)
MaPS 43, 315–334. [8] Rietmeijer & McKey (1986) Meteoritics 743 (abstract). [9]
Komatsu et al. (2018) PNAS 115, 7497–7502. [10] Keller and Flynn (2022) Nat.
Astronom. 6, 731-735.