Lunar and Planetary Science XLVIII (2017)
1854.pdf
CRYSTALLIZATION EXPERIMENT OF SILICA MINERALS IN EUCRITES. H. Ono1, A. Takenouchi1, T.
Mikouchi1 and A. Yamaguchi2, 1Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan, 2National Institute of Polar Research (NIPR), 10-3 Midori-cho, Tachikawa,
Tokyo 190-8518, Japan. E-mail: [email protected]
Introduction: Silica minerals have many polymorphs including metastable phases under various
temperature and pressure conditions [1]. For example,
tridymite is known to have at least two different polymorphs at room temperature: monoclinic tridymite
(abundant in meteorites and so called “meteoritic” tridymite) and pseudo-orthorhombic tridymite (“terrestrial” tridymite) [2]. Monoclinic tridymite transforms to
orthorhombic tridymite (different from terrestrial one)
above 160 oC and subsequently transforms to hexagonal tridymite above about 400 oC [3]. In our previous
study, we found orthorhombic tridymite lamellae in
monoclinic tridymite in Moama (cumulate eucrite) and
orthorhombic tridymite in Pasamonte (non-cumulate
eucrite), suggesting rapid cooling below 400 oC [4].
While silica minerals are often crystallized from
late-stage magma, they also form by metamorphism
and/or hydrothermal alteration. For example, in the
Earth’s crust, quartz is ubiquitous in hydrothermal environment. In meteorites, Serra de Magé cumulate eucrite is an exception, containing quartz veinlets possibly deposited from water [5]. In spite of the petrogenetic importance, in meteorites they have been often reported simply as “silica” even in eucrites that contain
more abundant silica minerals compared to other
achondrites. Therefore, it is important to identify silica
phases and to reveal their origins.
In our previous studies, we studied silica minerals
in basaltic clasts of 3 non-cumulate eucrites, Yamato
75011 (Y-75011), Pasamonte, and Stannern [6]. In Y75011, we found aggregates of cristobalite and quartz.
These aggregates showed local hackle fracture patterns,
suggesting transformation from cristobalite to quartz
by rapid cooling [7]. Alternatively, aggregates of cristobalite and quartz formed from hydrous magma [8].
Pasamonte contains coexisting subhedral cristobalite,
tridymite and quartz, while Stannern contains only
anhedral quartz. The reasons why these eucrites contain different kinds of silica polymorphs and why three
kinds of silica polymorphs coexist are not clear. Therefore, in this study, we focus on the origin of silica polymorphs in basaltic clasts in brecciated eucrites and
performed a crystallization experiment to verify the
occurrence of silica in eucrites by crystallization of
eucritic basaltic magma by rapid cooling.
Method: We performed a crystallization experiment of eucritic magma using 1 atm CO2-H2 gas mixing furnace. In our experiment, Millbillillie (non-
cumulate eucrite) was selected as a starting material
because Millbillillie is considered to be a typical eucrite [9]. A Millbillillie chip was crushed to 1~10 µm
sized powder by an agate mortar and compressed into
two pellets (each weighing 125 mg). These pellets
were put on Pt wire holders and suspended in a gasmixing furnace. In order to homogenize the melt before cooling, the temperature was kept at 1300 oC for
48 hours. The liquidus temperature of Millbillillie under fixed oxygen fugacity (logfO2=IW-1) was calculated from its bulk composition [10] by MELTS [11] as
1273 oC. The melt was cooled down to 850 oC at
1oC/hr and subsequently quenched to air. Oxygen fugacity was fixed at IW-1.0 during the experiment using
gas mixture of CO2-H2. Experimental temperatures
were measured with a Pt-Rh thermocouple.
The recovered samples were sliced and polished
thin sections (PTSs) were made. The PTSs were observed by an optical microscope and silica phases were
identified by EBSD patterns using a FE-SEM (JEOL
JSM-7100F) and Raman spectra using a micro Raman
spectrometer (JASCO NRS-1000), both at NIPR.
Quantitative chemical analyses and elemental mapping
were performed using the JEOL JXA-8530F electron
microprobe at the University of Tokyo.
Results: Our SEM observation revealed that the
recovered sample is mainly composed of lathy plagioclase and pyroxene (Fig. 1). Plagioclase is homogeneous in chemical composition and it is An97.3Ab2.4. Pyroxene shows two different textural types. One is elongated and has relatively homogeneous compositions
compared to the other. The other is radial shaped and
chemically zoned (Fe- and Ca-rich at the rims and Mgrich at the cores). Their compositions are shown in Fig.
2. Silica mineral is present as euhedral crystals. EBSD
and Raman analyses reveal that all silica minerals are
cristobalite and no other silica polymorphs are found.
Cristobalite is present at grain boundaries of pyroxene
and plagioclase, and its size is ~100 µm (Fig. 1). They
have many irregular cracks texturally similar to the
subhedral cristobalite in Pasamonte. Our previous
study showed that cristobalite in Pasamonte and Y75011 has many opaque inclusions less than 1 µm,
however there are no such inclusions in the recovered
sample in this study. Comparing chemical composition
of each cristobalite, experimental sample is a little
poorer in FeO (Table 1).
Lunar and Planetary Science XLVIII (2017)
Discussion: This experiment revealed that only
cristobalite crystallizes from eucritic magma at the
cooling rate of 1oC/hr. This result suggests that simple
cooling at 1oC/hr does not explain the co-existence of 3
silica polymorphs in Pasamonte. The textures of pyroxene and plagioclase in the recovered samples also
differ from that in basaltic clasts of non-cumulate eucrites, suggesting that 1 oC/hr cooling from 1300 to
850 oC is not able to form the texture of eucritic basalt.
Therefore, for example, slower cooling rates and/or
secondary processes are needed to explain co-existence
of the subhedral 3 silica polymorphs.
At least two possibilities are suggested by our experimental result. First possibility is that each silica
phase in Pasamonte crystallized from magma one after
another as temperature dropped. In this process, cristobalite crystallized first, and tridymite and quartz crystallized later at lower temperatures due to relatively
slow cooling. Cristobalite did not transform to tridymite at 1 oC/hr. The second possibility is that quartz
was transformed from cristobalite. In this process, cristobalite and tridymite crystallized from magma at high
temperature and subsequently cristobalite (and possibly tridymite) transformed to quartz during the cooling.
We favor the second process because the texture
and size of three silica phases in Pasamonte are similar.
If they successively crystallized from magma as mentioned in the former possibility, their textures should
have been different. Regarding the aggregates of cristobalite and quartz in Y-75011, our previous study
suggested that they occurred by either partial transformation of cristobalite to quartz or co-crystallization
from hydrous magma [12]. According to this study, the
aggregates were likely to form by partial transformation of cristobalite to quartz because cristobalite
first crystallized from eucritic magma by 1 oC/hr cooling. However, further experiment is needed. As we
reported [7], Stannern contains only quartz although
other non-cumulate eucrites contain various silica polymorphs. Based on this experiment, quartz in Stannern
may not be crystallized from magma and probably
transformed from cristobalite by thermal metamorphism. In fact, Stannern is moderately metamorphosed
and classified into grade 4 of thermal metamorphism
category [13].
Conclusion: We performed dynamic crystallization experiment of eucrite magma and revealed that
euhedral cristobalite is the only silica phase in a fast
cooling run of 1oC/hr. This result suggests that silica
polymorphs found in non-cumulate eucrites were initially cristobalite, and cristobalite was transformed to
other silica phases by slow cooling process or secondary metamorphism (including alterations).
1854.pdf
References: [1] Sosman R. B (1965) Rutgers University Press, 388 pp. [2] Graetsch H. and Flörke O. W.
(1991) Z. Kristallogr., 195, 31–48. [3] Nukui A et al.
(1978) American Mineral., 63, 1252–1259. [4] Ono H.
et al. (2016) 7th NiPR Symp. Polar Sci. [5] Treiman A.
H. et al. (2004) EPSL, 219, 189–199. [6] Seddio M.S.
(2015) American Mineral., 100, 1533–1543. [7] Ono H.
et al. (2016) 79th MetSoc. Abstract, #6336. [8] Leroux
H. and Cordier P. (2006) Meteoritics & Planet. Sci., 41,
913–923. [9] Yamaguchi A. et al. (1994) Meteoritics,
29, 237–245. [10] Miura Y. N. et al. (1998) GCA, 62,
2369–2387. [11] Ghiorso M. and Sack R. (1995) CMP,
119, 197–212. [12] Ono H. et al. (2016) 26th Goldschmidt Conf. Abstract, #1929. [13] Takeda H. and
Graham A. L. (1991) Meteoritics, 26, 129–134.
Table 1. Quantitative chemical analyses of cristobalite from experimental sample, Pasamonte, and Y-75011.
SiO2
Al2O3
TiO2
FeO
MnO
MgO
CaO
Na2O
K2O
NiO
Total
nd: not detecte
Experiment
98.7
0.44
0.14
0.27
nd
nd
0.23
0.03
0.04
0.07
99.95
Pasamonte
98.3
0.5
0.14
0.41
nd
0.03
0.09
nd
nd
nd
99.47
Y-75011
98.3
0.56
0.15
0.46
nd
0.02
0.23
nd
0.07
nd
99.73
Fig. 1. BSE image of experimental run product. Px:
pyroxene. Plag: plagioclase. Xtb: Cristobalite
Fig. 2. Chemical composition of pyroxene in the run
product cooled at 1 oC/hr from 1300 to 850 oC.