Download `luna-24` regolith: tem study.

LAMELLAR CHROMITE-DIOPSIDE SYMPLECTITES IN OLIVINE FROM THE ‘LUNA-24’
REGOLITH: TEM STUDY. Khisina N. R.1, Nazarov M. A.1, Wirth R. 21Institute of Geochemistry and Analytical Chemistry of RAS, Kosygin st. 19, Moscow 119991, Russia; e-mail: [email protected]; 2 GeoForschungsCentrum Potsdam, Telegrafenberg, Potsdam, Germany
Lamellar chromite-diopside symplectites in olivine are rare to occur. They were described in olivine
from some terrestrial rocks [1,2] and a Martian meteorite [3]. The origin of Chr-Di symplectites is discussed up to now.No detailed investigation of Cr,Carich lamellar symplectites in lunar olivines has ever
been done.
Results. Optical observartion showed [4] that individual olivine grains from the Luna-24 regolith
contain Cr- and Ca-rich lamellae of 0.5-1 m thick
oriented parallel to the (100) of the host olivine (Fig.
1).
.Digital Formats: Line drawings AND photo
Fig. 2. Dark-field TEM image of Ca,Crlamellae. Black and white platelets are diopside and
chromite, correspondingly.
8
Fig. 1. Optical image of Ca,Cr-lamellae in
the olivine grain from the ”Luna-24” regolith.
The lamellae were investigated with EMPA, SEM
and TEM. The lamellae consist of diopside - chromite intergrowths and represent a spinel-pyroxene
symplectite of lamellar shape (Fig. 2). Alternating
platelets of diopside and chromite 40 nm and 130
nm thick, respectively, are oriented normal to the
(100) olivine/lamellae boundary, with (100)Ol //
(111)Sp //(100)Cpx; [001]Ol // [011]Sp // [010]Cpx. The
bulk mineral composition of the lamellae is close to
FeCr2O4 + 2CaMgSi2O6. The results show that (i) the
symplectitic lamellae in olivine have been formed by
a solid-state reaction; (ii) subsolidus Cr2+  Cr3+
oxidation and 2Mg = Cr + Ca cation exchange reaction were related to the symplectite formation (Fig.
3).
Discussion. Chromite and diopside are
probably the breakdown products of some preexisting phase of Ca2FeMg2Fe(Cr3+)2Si4O16 composition inferred from the bulk chemistry of the
symplectites.
Ca + Cr, at.%
6
4
2
0
14
16
18
20
22
Mg, at.%Mg, at.%
Fig. 3. The correlation of Mg versus (Ca +
Cr). EMPA data for olivine + lamellae. The point at
(Ca + Cr) =0 corresponds to the host olivine composition.
A model of a deprotonation-oxidation process associated with a {Fe, 2H-} {Fe, 2Cr3+} point defect
transformation is suggested to explain the origin of
the pre-existing phase of the symplectites. The model
seems to be a convincing explanation for the occurrence of lamellar spinel + pyroxene symplectites in
terrestrial olivines, because the latter contain commonly n101 – n102 ppm of H2O. Both {Fe, 2H-}
point defects and (100)-oriented lamellar precipitates
of hydrous olivine [MgFeH2SiO4]*n[(Mg,Fe)2SiO4]
CHROMITE-DIOPSIDE SIMPLECTITES: N. R. Khisina et al.
were found in terrestrial mantle olivine [6]. A similar
mechanism has been suggested to explain the origin
of oxide precipitates in olivine from a terrestrial garnet peridotite [5]. How can this model be applied to
lunar rocks, because the rocks are believed to be
almost free of water? Recently, some arguments suggesting an H2O presence in the lunar mantle has
come from a SIMS study of lunar volcanic glasses
[7, 8].
Conclusion. On the basis of TEM data the
formation of the symplectite due to deprotonationoxidation process is proposed. If this model of
symplectite formation is correct, then the occurrence
of chromite-diopside symplectites in lunar olivine of
deep origin should be considered as an additional
evidence for the presence of H2O in the Moon interior. We suggest that lamellar Sp-Py symplectites are
probably the fingerprints of protons escaped from the
olivine.
The investigation was supported by grant № 0905-00444-a of Russian Fondation for Fundamental
Researches and grant ОNZ-5 of theProgramm of
Earth Science Department of Russian Academy of
Sciences.
References: [2]. Moseley D. (1984) Am. Mineral., 69, 139-153. [3]. Markl G. et al. (2001) Am.
Mineral., 86, 36-46. [4]. Mikouchi T. et al. (2000)
Meteoritics & Planetary Science, 35, 937 – 942. [4]
Tarasov L.S. et al. (1980) In: Lunar soil from the
Mare of Crisium. M., “Nauka”, 359 pp. [5]. Khisina
N.R.and Wirth R. (2002). Phys. Chem. Minerals, 29,
98-111. [6]. Hwang S.-L. et al. (2008) Amer. Mineral. 93, 1051 – 1060. [7]. Saal A.E. et al. (2008) Nature, 454, 192-195. [8] Chaussidon M. (2008) Nature, 454, 170-172.