NOBLE GAS DISTRIBUTION IN OLIVINES IN UDACHNAYA KIMBERLITE. H by ggl16746

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									NOBLE GAS DISTRIBUTION IN OLIVINES IN UDACHNAYA KIMBERLITE. H. Sumino1, K. Matsufuji1,
K. Nagao1, H. Kagi1, I. Kaneoka2, V.S. Kamenetsky3, M.B. Kamenetsky3 and A. V. Sobolev4, 1Laboratory for
Earthquake Chemistry, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan, (su-
mino@eqchem.s.u-tokyo.ac.jp), 2Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan,
3
  ARC Centre of Excellence in Ore Deposits and School of Earth Sciences, University of Tasmania, Hobart, Tasma-
nia 7001, Australia, 4Max Planck Institute for Chemistry, Geochemistry Division, Mainz 55020, Germany.


    Introduction: In order to identify the parental             First, major element composition of the polished
magma or source mantle, noble gases in mafic miner-         olivine slabs were analyzed by SEM-EDX in order to
als such as olivine and pyroxene have been widely           distinguish their phenocrystic/xenocryst-like origin in
used to investigate the isotopic signature of volcanic or   terms of Fo zoning [6], and then distributions of car-
mantle-derived rocks. Generally magmatic noble gases        bonate and water were investigated using micro FT-IR
are considered to concentrate in fluid/melt inclusions      spectrometer with an aperture of 60×60 μm2. Compos-
in the minerals, whereas in situ radiogenic/cosmogenic
noble gas isotopes may relatively homogeneously dis-
tribute in the solid phase of mineral lattices and/or          (A)                         0.4 ± 0.1
melt/mineral inclusions [1]. Recently, a combination of
low-blank and high-sensitivity noble gas mass spec-                                                1.0 ± 0.3
trometry and laser gas extraction has greatly improved
spatial resolution of noble gas isotope analysis. How-
ever, the low concentration of the noble gases in ter-
restrial samples has hindered the practical application
of laser-microprobe analysis to such samples except in
the case of 40Ar-39Ar and (U-Th)/He dating methods.
For this reason, there have been few attempts to inves-
tigate the distribution of mantle-derived noble gases in
minerals in terrestrial volcanic or mantle-derived rocks
using laser-microprobe analysis [2, 3], and the inclu-                                                 0.9 ± 0.4
sion-hosted component is often extracted by crushing        1.2 ± 0.5
several hundred to thousands of grains together in vac-                                                        500
                                                               (B)
uum [1]. Here we present an attempt to reveal the no-
ble gas trapping sites in single olivine crystals sepa-
rated from the Udachnaya kimberlite from Siberia with
non-destructive analytical methods such as SEM-EDX,
micro FT-IR and Raman spectroscopy.
    Samples and experiments:              Fresh olivine
phenocrysts in kimberlite collected from the deep
levels (~500 m) of the Udachnaya-East kimberlite pipe
in Yakutia, Russia, have been revealed to preserve
magmatic noble gases possibly in low density CO2
fluid inclusions associated with carbonate-chloride
inclusions [4]. Since a preliminary noble gas laser-
microprobe analysis on a 400-μm thick doubly-
                                                                                                                0
polished section of bulk kimberlite revealed that re-
lease of abundant radiogenic 4He which had accumu-                                            200 μm
lated in matrix after the kimberlite emplacement (ca.
                                                            Figure 1. (A) An optical microscope image of a thin
350 Ma [5]) considerably increased blank level even at
                                                            olivine slab analyzed in this study. Boxes indicate
room temperature, olivines separated from the kimber-
                                                            laser-ablated areas for noble gas extraction. Numbers
lite were doubly polished into thin slabs (approxi-
                                                            are 4He/3He (in unit of 106 with 1σ errors) observed
mately 200-300 μm thick) and resin used to mount
                                                            for each laser pit. (B) A color map of CO32- content
them on glass slides during the polishing was com-          (ppm) of the same slab obtained by micro FT-IR
pletely removed.                                            spectrometer.
ite inclusions of carbonate, chloride, sulfate and fluid     mantle-derived rocks, especially when it is combined
[6, 7] were investigated in detail using micro Raman         with other non-destructive microscopic analytical
spectroscopy with several microns resolution. After          methods.
these non-destructive analyses, the samples were                 References: [1] Kurz M.D. (1986) Nature 320,
loaded into a vacuum chamber equipped with a sap-            435-439. [2] Burnard P.G. et al. (1994) Earth Planet.
phire glass window and connected to a noble gas puri-        Sci. Lett., 128, 243-258. [3] Sumino H. et al. (2008) J.
fication and separation line. Noble gases were ex-           Volcanol. Geotherm. Res., 175, 189-207. [4] Sumino H.
tracted by drilling and carving out a sample area ap-        et al. (2006) Geophys. Res. Lett., 33, L16318. [5] Maas
proximately several hundreds of microns in size by an        R. et al. (2005) Geology, 33, 549-552. [6] Kamenetsky
ultraviolet laser beam (213 nm wavelength). Ablated          V.S. et al. (2007) J. Petrol, 49, 823-839. [7] Golovin
volumes of each laser pit were about 10-5 cm3, corre-        A.V. et al. (2003) Dokl. Earth Sci., 388, 369-372. [8]
sponding to several tens of micrograms of olivine.           Thomas J.B. et al. (2008) Chem. Geol., 253, 1-22.
Average procedural blank values of 4He and 40Ar were
5 × 10-11 cm3 STP and 3× 10-11 cm3 STP, respectively.
    Results: Amounts of 4He and 40Ar extracted from
each laser pit vary by more than an order of magnitude
(0.6-16 × 10-10 cm3 STP for 4He and 0.5-9 × 10-10 cm3
STP for 40Ar) and are heterogeneous even in a single
crystal. From their homogeneous Fo content [6] the
xenocryst-like olivines systematically show low noble
gas concentrations. 4He/3He ranging from 1.1 × 105 to
2.6 × 106 are also heterogeneous in a single crystal.
Based on He-Ar isotope systematics, three components
can be recognized. The first one bears similar 4He/3He
and 4He/40Ar*, where 40Ar* is non-atmospheric 40Ar,
to those obtained by bulk in-vacuo crushing of
phenocrystic olivines [4], suggesting its residence in
the fluid phase of inclusions. Another component has
higher 4He/40Ar* and 4He/3He than the fluid inclusion-
hosted component, implying a significant part of 4He
in the second component is in situ radiogenic origin. A
rough correlation between CO32- content determined
by micro FT-IR analysis and 4He/3He in a single crys-
tal (Fig. 1) suggests the radiogenic component is en-
riched in the carbonate phase in inclusions. This is
consistent with a relatively high concentration of U in
carbonates. The last component shows atmospheric
40
  Ar/36Ar and was predominantly observed when ca. 5
μm depth of the whole area of the sample surface was
planed away by laser beam scanning or a laser pit
penetrated to the bottom surface of the sample thin
slab. These features suggest that this atmospheric
component is absorbed on the sample surface or at-
mospheric noble gases are incorporated into the up-
permost several hundred nm of surface layers of the
olivine crystal [8].
    Although the spatial resolution of this work, which
is mainly limited by noble gas detection limits, is too
low to analyze noble gases in each single inclusion
(from 2 to 80 μm in size [7]), the preliminary results
show the potential of noble gas laser-microprobe
analysis in distinguishing several noble gas compo-
nents trapped in distinct sites of minerals in volcanic or

								
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