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FIELD STUDY ON CO2 FIXATION BY SERPENTINITE ROCK-BED

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FIELD STUDY ON CO2 FIXATION BY SERPENTINITE ROCK-BED Powered By Docstoc
					                                     FIELD STUDY ON CO2 FIXATION BY SERPENTINITE ROCK-BED.


                          Tatsuya Yajima 1 , Takashi Ohsumi 1 , Yukihiro Mizuochi 2 , Atushi Ninomiya 2 & Takayuki Kato 3

                                    1   Reserch Institute of Innovative Technology for the Earth (RITE), Japan
                                                         2 Sumiko Consultants Co. LTD., Japan

                                                             3 Earth Science Co. LTD., Japan




Abstract


  The sequestration of atmospheric CO2 could be an effective means of mitigating global warming. The groundwater and surface water
associated with serpentinite shows alkaline conditions and is associated with natural carbonate minerals. A technology to sequester
atmospheric CO2 could be based on the reactions forming these carbonate minerals.

  Ca-carbonate and Mg-hydrocarbonate associated with the serpentinite in the Kamuikotan metamorphic belt, Hokkaido, northern Japan,
precipitate around alkaline springs on landslide deposits, on the surface and in fractures in weathered serpentinite. The carbonates yield 14 C
ages from 22700 yBP to modern. These carbonates sometimes co-exist with serpentine mousse which is white to pale brown in color. The
relationship between δ 18 O and δ D values of the serpentine is similar to that of deweylite, which is a serpentine formed at low temperature [1].
The temperature of oxygen isotopic equilibrium between co-existing deweylite and water is approximately 16 °C corresponding to the annual
average atmospheric temperature in Hokkaido. The relationships of δ 18 O among deweylite, water and carbonate suggest low-temperature
formation of the carbonate.

   From these facts it is concluded that the alkaline water associated with serpentinite reacts with atmospheric CO2 under surface conditions and
has continued from at least 22,700 yBP to the present. In addition, the system of serpentinite and groundwater could continue to keep itself
alkaline and carbonate stable safely for a long period.

Introduction

  In order to remove anthropogenic CO2 from the atmosphere, technologies for the sequestration of CO2 in aquifers, oilfields and coal seams
have been proposed. The occurrence of carbonate minerals associated with serpentinite is a natural analogue for CO2 sequestration and
suggests one option for sequestration. It is known that Ca-carbonate and Mg-hydrocarbonate are associated with the serpentinite. The
Ca-carbonates (CaCO3 ; calcite and aragonite) precipitate by a reaction between the highly alkaline spring water and atmospheric CO2 [eg. 2]. The
highly alkaline groundwater is rich in Ca2+ and is produced by dissolution of Ca2+ and OH- during the present serpentinization [eg. 3]. On the
other hand, Mg-hydrocarbonates are distributed on the surface and in the fractures of the weathered serpentinite. Sato et al. (2000) [4]
concluded that these Mg-hydrocarbonates were formed by weathering processes related to dissolution of serpentine and brucite.

  In this study, we focus on the Ca-carbonates and Mg-hydrocarbonates associated with the weathered serpentine from the Takadomari, Niniu,
Sarugawa and Nukabira serpentinite masses in the Kamuikotan metamorphic belt, Hokkaido, northern Japan (Figure 1). We will report the
analytical results, mineral facies, 14 C ages and stable isotopic ratios for carbonates and related serpentinite and groundwater, and attempt to
estimate the possible amount of CO2 sequestrated in the serpentinite all over Japan.

Analytical methods


  The mineral facies of carbonate and accompanying minerals were identified by microscopic observations and X-ray powder diffraction
analysis. Chemical compositions of the groundwater were measured by ICP emission spectrometry and ion chromatography. The formation
temperature was estimated based on oxygen isotope relationships (δ 18 O) among co-existing serpentinite, carbonate and groundwater. 14 C
contents in the carbonates were analyzed by AMS (Accelerator Mass Spectrometry).

Analytical results


Carbonate and serpentine minerarls


   Two facies of Mg-hydorcarbonates are identified in the weathered serpentinite, hydrotalcite and pyroaurite (Table 1). On a landslide deposit
at the margin of the Niniu massif, white precipitates formed around the alkaline spring water (pH = 10.3, Table 2) comprise major calcite, minor
aragonite and trace amount of white serpentine and hydrotalcite (Figure 2, 3). The crystallinity of the serpentine is low on the basis of the broad
peak identified during XRD analysis (Sp in Table 1). Medium and small amounts of serpentine and pyroaurite are present as the hard, white
matrix of a river terrace deposit. This type of deposit is called ‘carbonate-cemented conglomerate’ in this study. White to milky white carbonate
precipitates fill the fractures in host serpentinite in the Nukabira massif. These carbonates are a mixture among calcite, aragonite, hydrotalcite
and pyroaurite. In the Sarugawa massif, white or light brown precipitates that fill the fractures in host serpentinite are pyroaurite and serpentine.
The low-crystallinity serpentine that is pale brown in color composes the matrix of the talus along a river in the Takadomari massif (Figure 4).

groundwater


  The groundwater from Niniu and Takadomari massifs are medium alkaline. Their chemical compositions are shown in Table 2.

Isotopic ratio and 14 C dating


  Isotopic ratios (δ 18 O, δ D and δ 13 C) of the serpentine, groundwater and carbonate are shown in Table 3. The relationship between δ 18 O and
δ D values of serpentine with low-crystallinity is similar to that of deweylite which is a serpentine formed under low-temperature conditions
(around 25 °C, [4]).

   The 14 C age of the carbonate varies from 22,700 BP to the present (Table 3). The latest carbonate is calcite and aragonite associated with an
alkaline spring on the landslide deposit in the Niniu massif.

Temperature of equilibrium condition between deweylite and water


  According to Zheng (1993) [5], the temperature of oxygen isotopic equilibrium between serpentine (deweylite) and water can be calculated
with the following formula:

  1000 ln α = 3.99X106 /T 2 - 8.12X103 /T + 2.35

  Here, T is absolute temperature (k), and α is a distribution coefficient describing partitioning of 18 O between co-existing deweylite and water.
The calculated temperature for the deweylite and water pair from the Takadomari massif is approximately 16 °C and corresponds to the annual
average surface atmospheric temperature in Hokkaido. The relationships of δ 18 O among deweylite, water and carbonate suggest
low-temperature formation of the carbonates.

Capability of CO 2 storage


  The total area of serpentinite massifs in Japan is estimated to be about 1974km2 . The preliminary estimation for the capability of CO2 storage in
the serpentinite massif all over Japan is about 700 million tons with assumptions such as injection of supercritical CO2 (density 500 kg/m 3 ),
2300m of thickness of serpentine massif, the depth for the storage between 700m and 3000m, approximately 0.02 vol. % of the available storage
space in the rock mass and 0.015 of storage ratio (possible volume for the storage / volume of interstice in the serpentine massif) by use of a
formula for the pressured permeability test.

Conclusion


   Ca-carbonate and Mg-hydrocarbonate associated with the serpentinite in the Kamuikotan metamorphic belt, Hokkaido, northern Japan,
precipitate around alkaline springs on landslide deposits, on the surface and in the fractures in weathered serpentinite. The carbonates display
14 C ages from 22,700 yBP to modern. These carbonates sometimes co-exist with serpentine mousse, which is white to pale brown in color. The

relationship between the δ 18 O and δ D values of the serpentine is similar to that of deweylite, which is a serpentine mineral formed at low
temperatures [1]. The temperature of oxygen isotopic equilibrium between co-existing deweylite and water is approximately 16 °C [5]
corresponding to the annual average surface atmospheric temperature in Hokkaido. The relationships of δ 18 O among deweylite, water and
carbonate suggest low-temperature formation of the carbonate.

  From these facts it is concluded that under near-surface and surface conditions, the alkaline water associated with the serpentinite reacts with
bicarbonate ions derived from atmospheric CO2 dissolved in groundwater. This process has continued from at least 22,700 yBP to the present.
In addition, the system of serpentinite and groundwater could continue to keep itself alkaline and carbonate stable safely for a long period.

References

1. Wenner, D. B. and Taylor, H. P. jr. (1973) Oxygen and hydrogen isotope studies on the serpentinization of ultramafic rocks in oceanic
    environments and continental ophiolite complexes. Am. J. Sci., 273, 207-239.
2. Barnes, I and O’Neil, J. R. (1969) The relationship between fluids in some fresh alpine-type ultramafics and possible modern

                                                                                                                                                        2
     serpentinization, Western United States. Geol. Soc. Am. Bull., 80, 1947-1960.
3. Morishita T. and Arai S. (1999) Diversity of carbonate occurrences at the Fizh massif in the northern part of the Oman ophiolite; a
     preliminary report. Sci. Rep. Kanazawa Univ., 43, 13-24.
4.   Sato T., Tada Y. and Arai S. (2000) Weathering processes of serpentinites and formation of Mg-hydrocarbonate mineral. Abstract
     (Mc-012), Japan Earth Planet. Sci. Joint Meeting.
5.   Zheng, Y. –F. (1993) Calculation of oxygen isotope fractionation in hydroxyl-bearing silicates. Earth Planet. Sci. Lett., 120, 247-263.




Figure 1 Locality map of study area




                                                                                                                                               3
                                                  Table 1 Carbonate minerals associated with the weathered serpentinite

                                                                                                             Mg-hydro-car
                 existing condition of weathere d            serpentine group           Ca-carbonate                                     other minerals
 rock mass                                                                                                     bonate
                 minerals
                                                         Chr          Liz       Sp       Cal      Ara        Htc        Pya      Br       Mao      Qz       Pl

                 precipitate from the alkaline
                                                                                C        AA        B          C
                 spring
Niniu
                 matrix of the carbonate
                                                             B         B                                                C                          C        C
                 cemented conglomerate

Nukabira         prec ipitate in the fracture                                             C        A          C         B                  AA

                 precipitate in the fracture                 A         A                                                B
Sarugawa
                 host serpentinite                           A         A                                                C            C

                 aragonite vein in the
                                                                                                  AA
Takadomari       serpentinite

                 matrix of talus                                                C

    extreme large amount A•Flarge amount B•F
AA •F                                                      small amount
                                           medium amount C•F

  Chr: chrysotile Mg 3 Si3 O5 (OH) 4                             Pyr: pyroaurite Mg 6 Fe 2 CO 3 (OH) 16 ¥ 4H 2 O

  Liz: lizardite Mg 3 Si3 O5 (OH) 4                              Br: brucite Mg(OH) 2

  Sp: low-crystallinity serpentine Mg 3 Si3 O5 (OH) 4            Mao: magnesium alumino oxide MgAl 2 O4

  Cal: calcite CaCO ‚R                                           Qz: quartz SiO 2

  Ara: aragonite CaCO3                                           Pl: plagioclase NaAlSi 3 O8 -CaAlSi 2 O8

  Htc: hydrotalcite Mg 6 Al 2 (CO 3 )(OH) 16 ¥ 4H 2 O



                                                                    Table 2 Chemical compositions of groundwater

                                                                                                                                                           (mg/l)

                                       locality         pH           Na          K         Ca           Mg         Fe         Cl -       SO4 2-   HCO3 -   CO 3 2-

                                                                                                                                                                     4
                             Niniu             10.3       2.1       <0.4    3.5           27       1.2     17          1             82     21

                             Takadomari        10.6       5.2       <0.4     1            51      0.05      8          4             88     92




                                            Table 3 Isotopic ratios and 14 C age of carbonate and serpentine samples
                                                                             δ18 O               δD        δ13 C
            sample               locality             mineral facies                                                         14   C age
                                                                           (SMOW)              (SMOW)     (PDB)

Serpenti nite (host rock)    Sarugawa           Chr, Liz>>Pya, Br                   5.7           -116

Low-crystallinity
serpentine associated with   Takadomari                                           10.4             -86
carbonates

Groundwater                  Takadomari                                           -17.8           -70.3

                             Niniu              Cal>Ara>Sp                        20.7                          -9.5          110.4 (MC)

                             Niniu              Chr, Liz>>Pya, Qt, Pl             19.1                          -7.8       1,860± 40 (BP)

Carbonate                    Nukabira           MAO>Ara>Cal                       20.9                          -7.5 22,700± 100 (BP)

                             Sarugawa           Chr, Liz>>Pya                     20.0                          -3.9       5,480± 40 (BP)

                             Takadomari         Ara                               21.1                       -12.8     17,100± 70 (BP)

SMOW: Standard Mean Ocean Water

PDM: Pee Dee Belemnite




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