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: o-haruka@eps.s.u-tokyo.ac.jp

Introduction: Silica minerals have many poly-morphs including metastable phases under various temperature and pressure conditions [1]. For example, tridymite is known to have at least two different poly-morphs at room temperature: monoclinic tridymite (abundant in meteorites and so called “meteoritic” tri-dymite) and pseudo-orthorhombic tridymite (“terrestri-al” tridymite) [2]. Monoclinic tridymite transforms to orthorhombic tridymite (different from terrestrial one) above 160 oC and subsequently transforms to hexago-nal 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 en-vironment. In meteorites, Serra de Magé cumulate eu-crite is an exception, containing quartz veinlets possi-bly deposited from water [5]. In spite of the petrogene-tic importance, in meteorites they have been often re-ported 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, Yamato75011 (Y-75011), Pasamonte, and Stannern [6]. In Y-75011, 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 cris-tobalite 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 con-tain different kinds of silica polymorphs and why three kinds of silica polymorphs coexist are not clear. There-fore, in this study, we focus on the origin of silica pol-ymorphs 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 experi-ment of eucritic magma using 1 atm CO2-H2 gas mix-ing furnace. In our experiment, Millbillillie (non-

cumulate eucrite) was selected as a starting material because Millbillillie is considered to be a typical eu-crite [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 gas-mixing furnace. In order to homogenize the melt be-fore cooling, the temperature was kept at 1300 oC for 48 hours. The liquidus temperature of Millbillillie un-der fixed oxygen fugacity (logfO2=IW-1) was calculat-ed 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 fu-gacity 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 ob-served 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 plagio-clase and pyroxene (Fig. 1). Plagioclase is homogene-ous in chemical composition and it is An97.3Ab2.4. Py-roxene shows two different textural types. One is elon-gated 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 Mg-rich 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 Y-75011 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).

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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 py-roxene and plagioclase in the recovered samples also differ from that in basaltic clasts of non-cumulate eu-crites, 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 ex-perimental result. First possibility is that each silica phase in Pasamonte crystallized from magma one after another as temperature dropped. In this process, cristo-balite crystallized first, and tridymite and quartz crys-tallized later at lower temperatures due to relatively slow cooling. Cristobalite did not transform to tri-dymite at 1 oC/hr. The second possibility is that quartz was transformed from cristobalite. In this process, cris-tobalite and tridymite crystallized from magma at high temperature and subsequently cristobalite (and possi-bly 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 men-tioned in the former possibility, their textures should have been different. Regarding the aggregates of cris-tobalite and quartz in Y-75011, our previous study suggested that they occurred by either partial transfor-mation of cristobalite to quartz or co-crystallization from hydrous magma [12]. According to this study, the aggregates were likely to form by partial transfor-mation of cristobalite to quartz because cristobalite first crystallized from eucritic magma by 1 oC/hr cool-ing. However, further experiment is needed. As we reported [7], Stannern contains only quartz although other non-cumulate eucrites contain various silica pol-ymorphs. Based on this experiment, quartz in Stannern may not be crystallized from magma and probably transformed from cristobalite by thermal metamor-phism. In fact, Stannern is moderately metamorphosed and classified into grade 4 of thermal metamorphism category [13].

Conclusion: We performed dynamic crystalliza-tion 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 ini-tially cristobalite, and cristobalite was transformed to other silica phases by slow cooling process or second-ary metamorphism (including alterations).

References: [1] Sosman R. B (1965) Rutgers Uni-versity 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 Gold-schmidt Conf. Abstract, #1929. [13] Takeda H. and Graham A. L. (1991) Meteoritics, 26, 129–134.

Table 1. Quantitative chemical anal-yses of cristobalite from experimen-tal sample, Pasamonte, and Y-75011.

Experiment Pasamonte Y-75011 SiO2 98.7 98.3 98.3

Al2O3 0.44 0.5 0.56 TiO2 0.14 0.14 0.15 FeO 0.27 0.41 0.46 MnO nd nd nd MgO nd 0.03 0.02 CaO 0.23 0.09 0.23 Na2O 0.03 nd nd K2O 0.04 nd 0.07 NiO 0.07 nd nd Total 99.95 99.47 99.73

nd: not detecte

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.

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