Isotopic Compositions of Asteroidal Liquid Water Trapped in Fluid Inclusions of Chondrites
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Geochemical Journal, Vol. 48, pp. 549 to 560, 2014 doi:10.2343/geochemj.2.0335 Isotopic compositions of asteroidal liquid water trapped in fluid inclusions of chondrites HISAYOSHI YURIMOTO,1,2* SHOICHI ITOH,1** MICHAEL ZOLENSKY,3 MINORU KUSAKABE,4 AKIYA KAREN5 and ROBERT BODNAR6 1Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan 2Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan 3Astromaterials Research and Exploration Science, KT, NASA Johnson Space Center, Houston, TX 77058, U.S.A. 4Department of Environmental Biology and Chemistry, The University of Toyama, Gofuku 3190, Toyama 930-8555, Japan 5Toray Research Center, Inc., 3-7, Sonoyama 3-chome, Otsu, Shiga 520-8567, Japan 6Department of Geosciences, 4044 Derring Hall, Virginia Tech, Blacksburg, VA 24061 U.S.A. (Received June 2, 2014; Accepted September 17, 2014) Determination of isotopic composition of extraterrestrial liquid water provides important information regarding the origin of water on Earth and the terrestrial planets. Fluid inclusions in halite of ordinary chondrites are the only direct samples of extraterrestrial liquid water available for laboratory measurements. We determined H and O isotopic composi- tions of this water by secondary ion mass spectrometry equipped with a cryogenic apparatus for sample cooling. Isotopic compositions of the fluid inclusion fluids (brines) were highly variable among individual inclusions, –400 < δD < +1300‰; –20 < ∆17O < +30‰, indicating that these aqueous fluids were in isotopic disequilibrium before trapping in halite on asteroids. The isotopic variation of fluids shows that various degrees of water-rock interaction had been underway on the asteroids before trapping between D-rich-16O-poor aqueous fluid, D-poor-16O-rich aqueous fluid, and asteroidal rock by delivery of cometary water onto hydrous asteroids. This may be a fundamental mechanism in the evolution of modern planetary water. Keywords: fluid inclusion, hydrogen isotope, oxygen isotope, asteroid, water primitive materials in the solar system and chondrite-like INTRODUCTION materials formed the terrestrial planets, isotopic compo- The origin of the water of terrestrial planets remains sitions of chondritic liquid water provide a direct evidence one of the primary issues of planetary sciences. It is be- to reveal the origin of water of terrestrial planets. The lieved that terrestrial planets formed inside of the water fluid inclusions were found in halite (NaCl) and sylvite ice evaporation line (snow line) in the solar nebula (KCl) (hereafter collectively called “halite”) from two (Hayashi et al., 1985) where it was difficult to trap water ordinary chondrite regolith breccias (Monahans (1998), components. It is useful to use isotopic compositions as a hereafter simply “Monahans” (H5), and Zag (H3-6)) tracer in order to discuss the origin and evolution of ma- (Rubin et al., 2002; Zolensky et al., 1999, 2000). Both terials, but we have little knowledge of isotopic compo- meteorites contain millimeter to centimeter-sized aggre- sitions of extraterrestrial water components. In particu- gates of blue to purple halite containing aqueous fluid lar, there is no report of isotopic compositions of extra- inclusions in the matrix. The halite grains were dated by terrestrial liquid water. K–Ar, Rb–Sr and I–Xe systematics to be ~4.5 Ga (Bogard Aqueous fluid inclusions contained within crystals of et al., 2001; Whitby et al., 2000; Zolensky et al., 1999), halite in two ordinary chondrites are the only extraterres- and thus the trapped aqueous fluids are at least as an- trial liquid water samples available for laboratory meas- cient. These halite grains are blue to purple owing to the urements (Zolensky et al., 1999). Because chondrites are gradual accumulation of trapped electrons in cation va- cancies (Nassau, 1983), resulting, probably, from the beta decay of 40K. Since recently recrystallized halite would be colorless, fluid within the colored halite grains must *Corresponding author (e-mail: [email protected]) be pre-terrestrial in origin. Heating/freezing studies of the **Present address: Department of Earth and Planetary Sciences, Kyoto fluid inclusions in Monahans halite grains demonstrated University, Kyoto 606-8502, Japan. that they were trapped at approximately 25°C (Zolensky Copyright © 2014 by The Geochemical Society of Japan. et al., 1999), and their presence in the halite requires their 549 A B Fig. 2. Photomicrograph of synthetic fluid inclusions in hal- ite. C Fig. 3. Cryo-sample-stage installed on Cameca ims-1270. The temperature of the sample stage is cooled by liquid nitrogen, which is supplied by the frost-covered (white) tube. incorporation into the H chondrite asteroid after meta- Fig. 1. Representative photographs of fluid inclusions in hal- morphism, as heating would have released fluids from ite crystals (A) Distributions of fluid inclusions in halite of halite. Monahans (1998). Arrows point to several inclusions. (B) Fluid Large variations of hydrogen isotopic composition inclusions in Monahans halite. (C) Fluid inclusions in Zag hal- have been observed in hydrous minerals from chondrites ite. (Robert, 2006). Cometary and interstellar water is highly D-enriched (Butner et al., 2007; Villanueva et al., 2009), 550 H. Yurimoto et al. 105 105 1H-– –50 104 104 103 103 –100 102 102 Intensity (cps) 2D– 101 101 –150 100 100 1.0070 1.0080 1.0090 2.0130 2.0140 2.0150 Mass (a.m.u) Fig. 5. Representative mass spectra of hydrogen isotopes of a fluid inclusion standard. –200 0 50 100 150 200 Time (min) EXPERIMENTAL Fig. 4. Cooling temperature curve of cryo-sample-stage as a function of time after supplying liquid nitrogen from room tem- Sample preparation perature. We have determined oxygen and hydrogen isotopic compositions of fluid inclusion fluids in halite crystals from Monahans (1998) H5 and Zag H3-6 ordinary chondrites. Both meteorites used in this study were re- representing cloud or outer solar disk chemistry. The hy- covered immediately upon falling and preserved under drogen isotopic composition of hydrous minerals in dry nitrogen, limiting terrestrial alteration. These sam- chondrites is believed to reflect inner solar system water ples have never been exposed to terrestrial liquid water. (Alexander et al., 2012; Robert, 2006). The halite crystals of 0.1 to 1 mm in size were picked Oxygen isotopes in the solar system are also highly from fresh fracture surfaces of the chondrites by a stain- variable (Sakamoto et al., 2007; Yurimoto et al., 2008), less needle. We carefully observed each halite crystal reflecting contributions of an H2O component (Kuramoto surfaces under an optical microscope and selected crys- and Yurimoto, 2005; Lyons and Young, 2005; Yurimoto tals having fluid inclusions within several tens of microm- and Kuramoto, 2004). However, it is impossible to mea- eters below the surface for isotope analysis (Fig. 1). sure isotopes of hydrogen and oxygen simultaneously in hydrous minerals, because they invariably contain struc- Synthesis of fluid inclusion standard tural oxygen not bound in water. Halite contains no struc- We prepared synthetic fluid inclusions with known tural oxygen or hydrogen, allowing the isotopic compo- isotopic compositions in halite crystals as standards in sition of oxygen and hydrogen in aqueous fluid inclusions order to calibrate measured δ-values. An ion-exchanged to be measured. Thus, fluid inclusions in halite have the pure water was prepared from meteoric water of Sapporo, potential to reveal unique information regarding the ori- Japan. Isotope ratios of the pure water were determined gin and activity of aqueous fluids in the early solar sys- by conventional gas mass spectrometry (Kusakabe and δ18 δ17 tem. This information contains, and perhaps uniquely Matsuhisa, 2008) as OSMOW = –11.17(6)‰, OSMOW identifies, the source of the aqueous fluid from which the = –5.81(3)‰ and δD = –76.7(5)‰. The pure water added halite precipitated. with supersaturated level of ground NaCl powder (JIS K Here we report direct measurement of the hydrogen 8150, Kanto Chemical Co., Inc.) was bottled up into a and oxygen isotopic compositions of the aqueous fluid in 125 ml polyethylene bottle and placed for three months halite fluid inclusion by cryo-secondary ion mass on a windowsill in a laboratory room. Millimeter sized spectrometry (Cryo-SIMS), and discuss the origin of this halite crystals were recrystallized on the bottom of the water. Preliminary results have been presented in bottle. The precipitating crystals trapped nanoliter-sized Yurimoto et al. (2010). quantities of the solution as fluid inclusions. Because the Isotopic compositions of asteroidal fluid inclusion 551 105 105 16OH– 16O– 104 104 103 103 18O– 102 102 Intensity (cps) 17 – O 16OD– 101 101 100 100 15.990 16.000 16.995 17.005 17.995 18.005 Mass (a.m.u) Fig. 6. Representative mass spectra of oxygen isotopes of a fluid inclusion standard. crystal growth occurred in closed system, the isotopic the mass spectrometer. A normal incident electron gun compositions of the trapped water must be the same as was applied to compensate positive charging of the sput- those of the pure water. We selected appropriate halite tered region due to primary beam irradiation. The pri- crystals having fluid inclusions within several of tens mary beam of 10 to 15 µm in diameter with a current of 3 micrometers below the surface to be used as standards to to 13 nA was used for excavation on the sample in order calibrate the isotopic compositions of fluid inclusions in to expose frozen fluid inclusions (ice) on the crater bot- the meteorite halite (Fig. 2). tom. The primary beam was rastered to a 50 µm-square and irradiated around the sample surface above the fro- Measurement condition of secondary ion mass zen fluid inclusions. The appearance of ice was moni- spectrometry (SIMS) tored by secondary ion images of 1H and 16O using The halite crystals to be analyzed were fastened to a stigmatic ion optics. The depths of excavation on sam- silicon wafer by epoxy, with the surface with fluid inclu- ples reached 15 to 55 µm.