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Ar/39 Ar age of Haedean zircon

HYODO, Hironobu1∗ ; SATO, Keiko2 ; KUMAGAI, Hidenori2 ; YAMAMOTO, Shinji3 ; SHIBUYA, Takazo2 ;KOMIYA, Tsuyoshi3
1

RINS, Okayama Univ. of Sci., 2 JapanAgency for Marine-Earth Science and Technology, 3 Dep. of Earth Science & Astronomy,
Univ. of Tokyo

Analyses of ages and closure temperatures using SHRIMP and LA-ICPMS U-Pb system of zircon, K-Ar system of hornblendeand micas, and fission track method in zircon and apatite in the same rock sample are the standard techniques in thermochronology. Since zircon has no potassium, it has rarely been used in K-Ar geochronology. After intensive microscopic observation andelectron microprobe analyses of Haedean zircons from the Labrador and Acasta gneiss, fine grain muscovite and other minerals
as well as fluid inclusions are recognized. The presence of such inclusions suggests a possibility of a secondary event, and it
might have caused disturbance in U-Pb system in the zircons, implying the zircons may be older. Utilizing susceptive characteristics of K-Ar system in a secondary hydrothermal event, laser step heating 40 Ar/39 Ar method was applied on the individual
zircon grains to investigate the timing of formation of such inclusions in the zircons.
During laser step heating experiment, unirradiated Haedean zircons showed little release of 40 Ar below 1000o C. However,
without exception, they released large amount of 40 Ar (an order of 10−7 ccSTP/g) above 1000o C in spite of a fact that zircon has
no potassium. This suggested that either the zircons have excess argon trapped or a potassium phase. In 40 Ar/39 Ar experiment,
the released argon isotopes were approximately 10−13 ccSTP (39 Ar) and 10−10 ccSTP (40 Ar) from a grain of 400 microgram.
The volume of 39 Ar was very small, and the relative error was large. However, one of Labrador zircon gave 4.39 + 0.34 Ga at afusion step after 1000o C, and the 39 Ar fraction was over 70% of the total release. The age could be caused by an excess argon
during initial stage of zircon formation or a secondary event. Because of localized nature of excess argon, it tends to produce
various ages in the same area rather than a uniform age. The facts that the similar age was observed in another grain, and that thefraction is above 1000o C suggest that the age may have been preserved since the formation of the zircon. The results from otherarea will be discussed together.

Keywords: Haedean zircon, 40 Ar/39 Ar age

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Two regional metamorphic belts in assemblages of subduction-related orogens in WalesEngland, U.K.
ASANUMA, Hisashi1∗ ; ITAYA, Tetsumaru2 ; FUJISAKI, Wataru1 ; SATO, Tomohiko3 ; SAWAKI, Yusuke1 ;MARUYAMA, Shigenori3
1

Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2 Research Institute of Natural Science, Okayama
University of Science, 3 Earth-Life Science Institute, Tokyo Institute of Technology
Cryogenian-Devonian orogenic belt in the British Isles consists of accretionary complex, arc-related volcano-plutonic belt with
regional metamorphic belt that have formed in relation to an oceanic-plate subduction in the Iapetus Ocean. This subductionrelated orogeny plays an important role in understanding of the geotectonic history of the pre- Ordovician orogenic belt in
Wales-England.
Wales-England consists of ca. 677-450 Ma calc-alkaline volcano-plutonic complexes and ca. 608-539 Ma accretionary complexes, suggesting that the subduction-related orogeny continued for ca. 200 My. Four metamorphic units related to this orogeny
exist in northwest Wales. However, their metamorphic ages, except for one unit, have not been constrained yet. Therefore, we
performed U-Pb dating on detrital zircon in psammitic schists and K-Ar dating on phengite in pelitic/mafic schists from the four
metamorphic units in order to constrain their sedimentary and metamorphic ages, respectively.
The K-Ar analysis was carried out at Okayama University of Science, and the U-Pb ratio was determined with LA-ICP-MS at
the Kyoto University. Three metamorphic units give K-Ar ages of 578-545 Ma, which overlaps the Ar-Ar age of 560-550 Ma reported by a previous work. During this metamorphic event, accretionary complex and calc-alkaline volcano-plutonic complexes
also had been formed. On the other hands, one metamorphic unit, called as New Harbour Group, exhibits a younger K-Ar ageof 474±9 Ma. The youngest U-Pb age of detrital zircons from the New Harbour Group is 520±31 Ma. These ages suggest that
the New Harbour Group formed in the regional metamorphic event different from that of the above three units. Calc-alkaline
igneous rocks of ca. 480-450 Ma exist in Wales-England, which coincide with the second metamorphic event. Therefore, we
conclude that at least two regional metamorphic events occurred in the geotectonic history of Wales-England.

Keywords: British Isles, Subduction-related orogeny, Regional metamorphic belt, K-Ar phengite ages

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SGC51-03

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Ar/Ar Geochronological analysis of Paka volcano, northern Kenya Rift
SUDO, Masafumi1∗ ; STRECKER, Manfred R.1
1

Univ. Potsdam, Inst. Earth Environ. Sci.

The northern Kenya Rift constitutes one of the most prominent examples of young volcano-tectonic processes in a continental
rift system. However, the limited amount of geochronologic information on different generations of lava flows and associatederuptive centers, fissures, and extensional faults has been a major problem in assessing the degree of activity in the inner troughof the rift. Here we report on 30 new 40 Ar/39 Ar ages of lava flows associated with the evolution of the composite Paka calderashield volcano in the northern sector of the Kenya Rift.
Paka is a composite caldera-shield volcano. It rises ˜650 m above the rift floor and covers an area of ˜280 km2 with lava
flows and pyroclastics of trachyte, mugearite or basalt (Dunkley et al, 1993). Dunkley et al. (1993) suggested that the principal
volcanic eruptions on Paka occurred at 0.4 Ma, 0.2 Ma and 10 ka based on their four 40 Ar/39 Ar age determinations. The
relationship between the pristine morphology of the summit crater and associated fault systems parallel to the rift orientationsuggests that the volcano-tectonic activity at Paka is intimately associated with deformation processes in the inner rift trough, theyoungest sector of volcano-tectonic processes in the rift. Thus, knowledge of the timing of volcanic eruptions and the generationof faults is crucial for a rigorous assessment of the youngest tectono-magmatic activity along the volcano-tectonic axis of thenorthern Kenya Rift.
Thirty-two samples were collected, mostly from lava flows or pyroclastic deposits, for 40 Ar/39 Ar dating at the geochronology
laboratory at the University of Potsdam. Sampling was carried out based on the geologic map of Dunkley et al. (1993) and themorphology of lava flows on and around the flanks of Paka. Concentrated groundmass parts of the rocks after acid treatment
were sent to Oregon State TRIGA Reactor for neutron activation of samples for four hours at the CLICIT facility, then 40 Ar/39 Ar
analyses were performed at Potsdam. In most cases, high-resolution ages were successfully obtained that are broadly consistent
with Dunkley’s ages after careful evaluation of age spectra and isochrons obtained by stepwise-heating with a CO2 laser.
A total of thirty 40 Ar/39 Ar ages was obtained, except two negative ages. The age data show protracted volcanic activity during
the past 0.6 Ma years, and also three phases of pronounced volcanic activity: (I) between 0.428 and 0.372 Ma, (II) between 0.160
and 0.126 Ma, and (III) between 0.039 and 0.012 Ma (Fig. 1). Based on these three periods, all rocks were grouped in three
periods, as (1) 0.6-0.35 Ma, (2) 0.35-0.1 Ma and (3) 0.1-0 Ma. In addition, we obtained geochemical information of the samples
and their spatial distribution was investigated. We found that several basaltic lava flows of Dunkley et al (1993) were trachyte or
mugearite flows; in addition, we conclude that some stratigraphic positions should be modified. During period (1), only trachyte
and mugearite erupted on the western rift floor and the northeastern flank of Paka; instead, the younger trachytes erupted morealong a N-S trend on the flank of Paka. Many normal faults affecting flows are only found on the older lava flows in northeasternside of the volcano, which belong to period (1). The trace element ratio of Nb/Zr clearly shows different ratios among the three
age groups, which implies a gradual change of magma composition over time. Overall, our new age data are meaningful andconsistent with field observations of superposition and cross-cutting relationships and thus help to develop a structural evolution
model for the inner trough region of the northern Kenya Rift.
Reference: Dunkley P. M., M. Smith, D. J. Allen and W. G. Darling (1993): International Series, Research Report SC/93/1,
185pp, British Geological Survey

Keywords: Kenya Rift, Ar/Ar dating, Quaternary volcano, tectonics, whole rock chemistry

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SGC51-04

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Quantifying seawater assimilation across a single MORB pillow
BROADLEY, Michael1∗ ; BURGESS, Ray2 ; KUMAGAI, Hidenori3 ; BALLENTINE, Chris4
1

Geochemical Research Center, Graduate School of Science, The University of Tokyo, 2 SEAES, University of Manchester, M13
9PL, UK, 3 JAMSTEC, Kanagawa 237-0061, Japan, 4 Department of Earth Sciences, University of Oxford, OX1 3AN, UK
Halogens and noble gases are critical tracers of volatile interaction between geological reservoirs. The incompatibility of theheavy halogens (Cl, Br, I) coupled with their unique elemental composition and concentrations within surface reservoirs makethem suitable tracers of volatiles recycled back into the mantle [1]. However, as the halogens are concentrated within seawaterand other marine reservoirs it is possible that they are also incorporated within submarine basalts during eruption, causing theoriginal mantle volatile signature of the basalt to be overprinted [2].
This study combines halogen (Cl, Br and I), noble gas, K and H2 O concentrations within a single pillow basalt to quantify theamount of seawater assimilation during eruption, and to further elucidate the mechanisms of assimilation. In order to determine
the scale and heterogeneity of seawater contamination within the pillow, multiple sections taken at 2.5mm intervals were analysed
along a transect from the glass rind to the crystalline interior.
The two outer sections of the glass rim show enrichment in the halogens compared to the deeper glass and crystalline section of
the pillow, which have distinctly MORB-like I/Cl and Br/Cl ratios. Lower K/Cl and Br/Cl ratios measured within the outermost
sections (#1 and #2) indicate the glass has incorporated a Cl rich seawater component. The H2 O/Cl ratios within sections #1 and#2 show that the assimilant is a high salinity brine produced by hydrothermal boiling and phase separation of seawater caused by
the eruption of the basalt.
The extent of brine assimilation within the glass ranges between 8% in section #1 to >50% in section #2 indicating brine
assimilation within basaltic glasses is considerably heterogeneous. The high level of brine contamination within the glass ascalculated from the halogens is in contrast with the noble gases within the glass, which still retain their magmatic signatures.
This suggests that the halogen and noble gases within submarine basalts may be decoupled, with loss of atmospheric noble gases
from the assimilant during hydrothermal boiling.
The crystalline interior shows an increase in the Xe and I concentration relative to the glass rim. The 132 Xe/36 Ar ratio andelevated iodine concentrations when compared to the rest of the pillow indicate the pillow contains a sedimentary component
within its interior, most likely due to incorporation of fine sediments during eruption.
Combined halogen, noble gas, K and H2 O data presented here for a single pillow basalt demonstrates that contamination fromthe marine environment can be variable and extensive. This combined analytical approach demonstrates the need for carefulcharacterization of samples to ensure only samples free from contamination are chosen for analysis of mantle volatiles.
[1] Sumino et al (2010) EPSL, 294, 163 - 172 [2] Kumagai and Kaneoka (1998) Geophys. Res. Lett. 25(20), 3891 - 3894

Keywords: Halogen, Noble gas, Seawater contamination, MORB

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SGC51-05

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Halogen analysis of mantle xenoliths by extending Ar-Ar and I-Xe dating methods
KOBAYASHI, Masahiro1∗ ; SUMINO, Hirochika1 ; NAGAO, Keisuke1 ; BURGESS, Ray2 ; ISHIMARU, Satoko3 ;ARAI, Shoji4 ; YOSHIKAWA, Masako5 ; KAWAMOTO, Tatsuhiko5 ; KUMAGAI, Yoshitaka5 ; KOBAYASHI, Tetsuo6 ;NAKAMURA, Michihiko7 ; TAKAHASHI, Eiichi8
1

GCRC, Univ. Tokyo, 2 SEAES, Univ. of Manchester, UK, 3 Dept. Earth Environ. Sci., Kumamoto Univ., 4 Dept. Earth Sci.,
Kanazawa Univ., 5 Inst. Geothermal Sci., Kyoto Univ., 6 Dept. Earth Environ. Sci., Kagoshima Univ., 7 Dept. Earth Sci., Tohoku
Univ., 8 Dept. Earth Sci., Tokyo Tech

An extension of the Ar-Ar and I-Xe dating methods enables us to determine trace amount of halogens (Cl, Br, and I), K, Ca,
Ba, and U. These elements are converted to corresponding isotopes of Ar, Kr, and Xe by neutron irradiation in a nuclear reactor.
Noble gases in the irradiated samples are analyzed by noble gas mass spectrometry and the conversion factors from parent
elements to noble gas isotopes are determined by analyses of standard samples [1]. The detection limits of this technique for
Cl, Br, and I are 10−10 to 10−11 , 10−13 to 10−14 , and 10−14 to 10−15 mol, respectively, which are several orders of magnitude
lower than those of conventional methods. Using this method, we are investigating halogen geochemistry in the mantle.
Halogens, except for F, are incompatible elements that are strongly partitioned into aqueous fluids. They are concentrated at
the Earth’s surface in distinct compositions and scarce in the mantle. These features enable us to trace water subducted into themantle using halogens. Halogens are also thought to play an important role in transporting LIL (large-ion lithophile) elements.
Enrichment of LIL elements in arc magma is explained by addition of slab-derived aqueous fluids. These elements are up to
10 times more strongly partitioned into aqueous fluids when the fluids contain halogens [2]. Because of these geochemical
characteristics, halogens are among the most important elements in slab-derived fluids. The fluid inclusions of mantle xenoliths
provide the most primary information on slab-derived fluids supplied to the mantle wedge. However, the halogen concentrations
in such materials are too low to analyze by conventional methods and thus little research has been conducted on them.
We determined the halogen concentrations in mantle xenoliths from various geological settings: Western-Pacific subduction
zones (Kamchatka, the Philippines, and N. Japan), Europe (Eifel), and North America (San Carlos and Kilbourne Hole). The
halogen compositions of fluid inclusions were obtained by crushing extraction of noble gases. Melting extraction was also
conducted to obtain those of whole samples.
The H2 O-rich fluid inclusions of the mantle xenoliths from volcanic fronts of subduction zones show high I/Cl ratios withrelatively constant Br/Cl ratios, which are similar to those of marine sedimentary pore fluids (seawater trapped in pores of deepsea sediments) [3]. Halogen compositions obtained from melting of the same samples show a small addition of Cl; however, they
are still similar to sedimentary pore fluids. Similar signatures were found in mantle peridotites from the Sanbagawa metamorphic
belt, suggesting subduction of sedimentary pore-fluid-derived halogens thorough incorporation into hydrous minerals such asserpentine in the subducting oceanic lithosphere [4]. Although the halogen concentrations are lower, some mantle xenoliths
from rear-arc regions of subduction zones also show sedimentary pore fluid-like signatures. The I/Cl and Br/Cl ratios of mantle
xenoliths from intraplate settings (Europe and North America) show common correlation, suggesting halogen fractionation from
MORB source-like composition.
The halogen signatures of mantle xenoliths from subduction zones suggest that subduction of sedimentary pore-fluid-derived
halogens [4] significantly extend into the mantle beneath volcanic front. These subducted halogens also extend to rear-arc regions,although their influence is small. The correlated I/Cl-Br/Cl ratios of mantle xenoliths from intraplate settings may have resulted
from a metasomatic event, which could generally take place in subcontinental lithospheric mantle.
[1] Turner (1965) JGR 70, 5433. [2] Kawamoto et al. (2014) EPS 66, 1. [3] Kobayashi et al. (2014) JpGU Meeting 2014,
SCG62-04. [4] Sumino et al. (2010) EPSL 294, 163.
Keywords: halogen, noble gas, mantle xenolith

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Across-arc variation in noble gas and halogen compositions of volcanic rocks from the
Izu-Ogasawara subduction zone
SUMINO, Hirochika1∗ ; BURGESS, Ray2 ; JEPSON, Lisa2 ; CAVRIT, Deborah2 ; SHIMIZU, Aya4 ;MACHIDA, Shiki3 ; BALLENTINE, Chris5
1
4

GCRC, Grad. Sch. Sci., Univ. Tokyo, 2 SEAES, Univ. Manchester, 3 Sch. Creative Science and Engineering, Waseda Univ.,
Tokyo Metropolitan Industrial Technology Research Institute, 5 Dept. Earth Sciences, Univ. Oxford

Recently, subducted halogens and noble gases with seawater and sedimentary pore-fluid signatures were discovered in exhumed mantle wedge peridotites and eclogites from the Sanbagawa-metamorphic belt, southwest Japan [1,2]. These findings
along with seawater-derived heavy noble gases (Ar, Kr, and Xe) in the convecting mantle [3] provide observations that allow us
to investigate the processes that control the return of volatile and highly incompatible elements into the mantle. Serpentinizedlithosphere of subducting oceanic plates can transport noble gases and halogens acquired from pore water in the overlying sediment [1,2,4]. To verify whether and how such subduction fluids modify the composition of the mantle beneath subduction zones,
we determined the noble gas and halogen compositions of olivines in arc lavas of the northern Izu-Ogasawara subduction zone
and IODP sediments and basalts recovered from the northwestern margin of the Pacific plate. Trace amounts of halogens (Cl, Br,
and I) in the olivine samples were measured by a combination of neutron irradiation and noble gas mass spectrometry [5].
The 3 He/4 He ratios of samples from the volcanic front (Izu-Oshima, Miyakejima, Mikurajima, Hachijojima, and Aogashima)and rear-arc (Niijima, Higashi-Izu monogenetic volcanoes, Nanzaki, and en-echelon seamount chains) regions are in the range ofthe mid-ocean ridge basalt (MORB) value, without systematic differences among the regions. This indicates a considerably low
contribution to the mantle wedge beneath the arc of radiogenic 4 He in the subduction fluids observed in the Sanbagawa samples
exhumed from a depth ranging from 30 to 100 km [1,2].
In contrast, the 40 Ar/36 Ar ratios for each volcano (299-620) are significantly lower than the MORB source (up to 32,000[3]), indicating significant involvement of atmospheric Ar (40 Ar/36 Ar = 296) in the magmas. Systematically higher 40 Ar/36 Ar
ratios in the rear arc than in the volcanic front, and a comparison with those of subducting materials, suggest that subductionof seawater-derived Ar significantly affects the noble gas composition of the magma-generation region. A simple mass balancecalculation of subducted and mantle-derived Ar isotopes reveals that higher subduction flux than that of seawater-derived Ar
in the pore fluids in the subducting sediment/crust is required. The serpentinized lithosphere in the subducting slab is the best
possible carrier of seawater-derived Ar with high subduction flux.
Although the halogen compositions of most of the olivines are close to that of MORB-source mantle, some samples fromthe en-echelon seamount chains show a significant contribution from pore-fluid-derived halogens. Combined with the noble
gas results, halogen-poor fluid associated with atmospheric noble gases may be dominantly released from the subducting slab
beneath the Izu-Ogasawara arc, while halogen-rich fluid significantly contributes to the magma generation region beneath somevolcanoes in the rear arc.
It remains unclear whether noble gases and halogens in the subducting materials are completely released beneath the rear arc.
However, the presence of seawater-like noble gases in the convecting mantle [3] implies that a small portion of seawater-dissolved
atmospheric noble gases, carried in the serpentine, might be transported to greater depths in the deeper mantle.
[1] Sumino et al. (2010) Earth Planet. Sci. Lett. 294, 163-172. [2] Sumino et al. (2011) Mineral. Mag. 75, 1963. [3] Holland& Ballentine (2006) Nature 441, 186-191. [4] Kendrick et al. (2011) Nature Geosci. 4, 807-812. [5] Turner (1965) J. Geophys.
Res. 70, 5433-5445.
Keywords: Noble gas, Halogen, Subduction zone, Izu-Ogasawara arc, Pore fluid

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SGC51-07

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Noble gas isotopes; Solar evolution parameterOZIMA, Minoru1∗
1

University of Tokyo
Noble gas isotopic parameters are essential reference parameters for planetary evolution.

Keywords: noble gas isotopes, planetary evolution, meteorite

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SGC51-08

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New analytical methods and a system for noble-gas measurements of extraterrestrial materials
OKAZAKI, Ryuji1∗
1

Department of Earth and Planetary Sciences, Kyushu University

Noble gas isotopes in extraterrestrial materials, meteorites, cosmic dust (micrometeorites), and spacecraft-return samples, are
one of the most useful tools of cosmochemistry. Radiogenic (e.g., 40 Ar, 129 Xe, and 136 Xe) and cosmogenic (21 Ne, 81 Kr, 83 Kr,
and 128 Xe) isotopes are used for dating when gases have begun to be accumulated in the relevant minerals or phases. Primordialor trapped (neither radiogenic nor cosmogenic) isotopes are informative to infer the origin of the materials. Conditions of physical processes, such as evaporation and condensation, will be discussed based on elemental/isotopic fractionation effects.
The mass spectrometry system at Kyushu Univ. consists of a conventional mass spectrometer (MS) and a resonance ionizationMS, called RING (Resonance Ionization Noble Gas mass spectrometer). The conventional MS is mainly used to measure lightnoble gases (He, Ne, and Ar) in micro- or submicro-gram samples enriched in solar-wind noble gases, such as mineral separates from regolith breccia meteorites, Antarctic micrometeorites [1], lunar soils, and the Itokawa dust particles returned by the
Hayabusa spacecraft [2]. Extraction of noble gases is performed using infrared radiation lumps for 50-200 ℃ and a resistantfurnace (called Pot-pie furnace) for 200-2000 ℃. In addition, the MS system is equipped with a Nd:YAG laser that produces
energy of 200 mJ/pulse (with a 10 nsec width) at 1064 nm and a spot with ∼50 µm in diameter. In the ablation of samples, plasma
lights are emitted due to the high energy density of the laser pulse, and have wavelengths depending on the chemical compositionof the fused area. The plasma emission spectra are measured using a LIBS (Laser-induced Breakdown Spectroscopy, e.g., [3])system prepared for the laser extraction system. Combination of the conventional MS and LIBS enables to determine K-Ar
radiometric ages for the specific areas (e.g., shock-melt veins or feldspathic phases) of polished sections. Experimental analyses
using terrestrial rocks containing K-rich phases (e.g., K-feldspar) and equilibrated ordinary chondrite samples are now on going.
The RING mass spectrometer is designed to measure extremely small amounts of Kr and Xe (>100 atoms), as modeled after
RELAX (Refrigerator Enhanced Laser Analyzer for Xenon) developed by the Manchester group (e.g., [4] and [5]). In our system,a dye laser equipped generates 3.5 mJ/pulse at ∼216 and ∼256 nm of wavelengths that ionize resonantly Kr and Xe, respectively.
At this moment, we have got mass spectrum of Xe isotopes but not those of Kr yet.
In addition to the improvements and developments of instruments for noble-gas mass spectrometry, sample preparation methods have also been developed for combinational (multidiscipline) analyses for individual samples with micro- and submicro-grammasses. For example, we have performed TEM and noble gas analyses for individual micrometeorites [1] and the Itokawa dust
particles [2].
References: [1] Okazaki et al. (2015) submitted to Earth Planet. Space. [2] Noguchi et al. (2015) this issue. [3] Loree andRadziemsky (1981) Plasma Chem. Plasma Process. 1, 271-279. [4] Gilmour et al. (1994) Rev. Sci. Instrum. 65, 617-625. [5]Strashnov et al. (2011) J. Anal. At. Spectrom. 26, 1763-1772.
Keywords: noble gas, extraterrestrial material, resonance ionization mass spectrometry, laser-induced breakdown spectroscopy,
K-Ar dating

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Relationships between morphological features and isotopic signatures of noble gases offour Itokawa particles
NOGUCHI, Takaaki1∗ ; OKAZAKI, Ryuji1 ; MITSUNARI, Takuya2 ; TOBIMATSU, Yu1 ; UESUGI, Masayuki3 ;YADA, Toru3 ; KAROUJI, Yuzuru3 ; HIDAKA, Hiroshi4 ; KIMURA, Makoto2
1

Kyushu University, 2 Ibaraki University, 3 JAXA, 4 Hiroshima University

Introduction: Fine-grained particles of the asteroid Itokawa preserve the effects related to the processes on and near thesurface of an asteroid. Various types of surface modification were found on the very surface (<100 nm) [1-6]. The modification
textures suggest that irradiation by solar wind (SW) plays an important role for their formation. Number density of solar flare
tracks in them ranges from a few × 109 to a few x 1010 tracks/cm2 [2, 5], which are considerably lower than expected. These
data suggest the mechanisms and periods of the surface modification of Itokawa grains are still unresolved. In addition to the
SW irradiation, there seems to have been another mechanism that modified the morphology and surface features of the grains.
Matsumoto et al. [3] found some Itokawa grains were aggregates composed of subgrains with stepped surfaces, which suggests
evaporation and/or condensation. Noble gas mass spectrometry is another tool to infer the individual history of each Itokawa
grain. Nagao et al. [7] argued that a grain experienced multiple processes: exposure to SW, burial in the regolith layer, andremoval of its external surface. We intend to investigate the relationships among morphology, surface modifications, and noble
gas isotopic signatures to discuss the history of each Itokawa particle from various perspectives.
Samples and methods:. We fixed four Itokawa particles on tantalum plates by acetone-soluble glue. We observed the surfaceof the particles and prepared thin foils by FIB-SEM at Ibaraki University. After preparation of FIB foils, all the Ta plates withsamples rinsed by acetone in a N2 filled glove box. They were transferred to Kyushu University and noble gas mass spectrometry
was performed for all the noble gas elements using a modified VG-5400. Noble gas extraction from each sample was conducted
stepwise at 50, 100, 200, 300, 600 and 1400 ℃.
Results and discussion:. SEM observation of the four Itokawa grains investigated shows various morphology and surface
features. Edges observed on the surfaces of three grains vary from sharp to dull. Additionally, one grain is composed of two
subgrains with stepped surfaces. Morphology and surface features are related to the He release patterns. The three particles have
similar 4 He release patterns; they release most 4 He below 200 ℃ fractions and a small amount of 4 He above 600 ℃. Although
the gas-release profiles are similar to each other, there is a difference in the bulk concentrations of 4 He; one of the three grain
having dull edges contain more abundant 4 He than the others having sharp edges. Because bulk 3 He/4 He ratios in the three grainsare 2.5 to 6.5 x 10−4 , which is comparable with that in the solar wind of 4.57 x 10−4 [8], it is likely that the implanted He in
the grains increased as the surfaces were degraded by solar wind sputtering. On the other hand, the grain composed of subgrains
with stepped surfaces shows a different 4 He release pattern; 4 He was released only below 200 ℃. This fact suggests that deeply
implanted He, which is probably related to He released above 600 ℃, was completely reset by severe heating. The steps on
the surfaces of this grain may have been formed by evaporation and/or evaporation and subsequent recondensation by a heating
event, which released He from the interior of the grain. These results strongly suggest that micrometer-size surface features are
related to the amounts and the release patterns of the implanted He.
References: [1] Noguchi et al. (2011) Science 333, 1121-1125. [2] Noguchi et al. (2014) Meteorit. Planet. Sci. 49, 188-214.
[3] Matsumoto et al. (2013) 44th Lunar Planet. Sci. Conf. abstr. #1441. [4] Thompson et al. (2014) Earth Planet. Space 66,
89-99. [5] Keller and Berger (2014) Earth Planet. Space 66, 71-78. [6] Noguchi et al. (2014) Earth Planet. Space 66, 124-134.
[7] Nagao et al. (2011) Science 333, 1128-1131. [8] Heber et al. (2009) Geochim. Cosmochim. Acta 73, 7414-7432.

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