Diapositiva 1

1
ISOTOPE COMPOSITION OF LITHIUM, BORON AND METHANE
IN HYPERALKALINE SPRINGS OF NORTHERN APENNINES
(ITALY)
Tiziano Boschetti1, Giuseppe Etiope2, Romain
Millot3, Maddalena Pennisi4, Lorenzo Toscani1
1. Earth-Sciences Department, University of
Parma, Italy (tiziano.boschetti_at_unipr.it) 2.
INGV - Istituto Nazionale di Geofisica e
Vulcanologia, Roma, Italy 3. BRGM - Metrology
Monitoring Analysis Department, Orléans,
France 4. CNR-IGGI - Institute of Geosciences and
Earth Resources, Pisa, Italy
2
W-Alps
N-Apennines

for the springs from Voltri Group Bruni et al.
2002 - App. Geochem. 17, 455-474
Figure modified from Boschetti Toscani 2008 -
Chem. Geol. 257, 76-91
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Whatre hyperalkaline waters
6.5-8.5 is the pH range in most natural waters
10 up to 12 is the pH range in the springs from
serpentinites
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and why to study them?
- the Taro-Ceno Valleys hyperalkaline springs
have an high boron content (up to 13 mg/L), quite
unusual for fresh waters (100-250 mg/L as TDS)
- deep aquifer hosting hyperalkaline waters have
low Mg content, negative (reducing) Eh and a low
PCO2 (up to 10-8 bar) due to water-rock
interaction in a closed system, therefore they
might be used to sequester anthropogenic CO2
(Bruni et al. 2002) CO2 2 OH- CO32- H2O
(travertine deposition)
- low-T serpentinization produces abiogenic CH4,
H2 and a small of other hydrocarbons (ethane)
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Chemical classification by major dissolved
constituents springs issuing from serpentinites
are characterized by 3 geochemical facies
updated from Boschetti Toscani 2008 - Chem.
Geol. 257, 76-91
- whereas springs issuing from basalts and other
formations are Ca-bicarbonate, springs from
ultramafites range from Ca-bicarbonate, passing
through Mg-bicarbonate up to hyperalkaline
Na-(Ca)-hydroxide
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Isotope composition of water molecula (Taro-Ceno
Valleys springs) all sampled waters are of
meteoric origin
updated from Boschetti Toscani 2008 - Chem.
Geol. 257, 76-91
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B and Cl concentration in the hyperalkaline
springs
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B isotope composition
B analysis by TIMS ( vs. SRM951) and B
speciation on hyperalkaline springs
Saturation indexes (SI) solution-minerals
low-T serpentinization
Some hyphotesis explaining the d11B difference in
hyperalkaline spings - in sample PR10, 10B is
scavenged as borate by precipitating minerals so,
respect to UM15, d11B increase and B content
decrease. Most simply, boric acid in bicarbonate
waters is transformed to borate (d11B similarity
between PR01 and PR10) . - high B concentration
in sample UM15 is due to the dissolution a
B-bearing phase like datolite CaBSiO4(OH) this
phase occurs in local ophiolitic breccias.
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B vs. Li isotope composition
Boschetti Toscani 2008 Chem. Geol. 257, 76-91
Boschetti et al. 2011 Aq. Geochem. 17, 71-208
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d11B vs. B/Cl looking for boron source
Boschetti Toscani 2008 Chem. Geol. 257, 76-91
Boschetti et al. 2011 Aq. Geochem. 17, 71-208
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d2H vs. d13C of dissolved methane wheres the
abiogenesis contribution?
Fields from Potter Konnerup-Madsen (2003)
In Geol.Soc. Spec. Publ. 214, 151-173 Bradley
Summons (2010) Earth Planet Sci. Let. 297,
34-41
Methane produced by (abiogenic )
serpentinization C Chimera (Turkey) LC Lost
City (Atlantis Massif, mid-Atlantic ocean) Z
Zambales (Luzon, Philippines) O Oman (Semail
Nappe)
mixing
Hydrogen and methane concentrations is depending
by various factors (T, W/R ratio, rock and fluid
composition) influencing the Fisher-Tropsch
reaction, e.g. low T reaction proceeds to the
right high T reaction to the left
Autotrophic bacterial carbonate
reduction Heterotropic bacterial
methyl-type fermentation
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Hydrocarbons in the Po plain and N-Apennine
(modified from Lindquist 1999, OFR 99-50-M)
springs from serpentinites
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Conclusions
i) Boron isotopes are fractionated due to pH
effect, while lithium due to formation of new
mineral phases, respectively ii) a (liquid)
mixing between hyperalkaline with sedimentary,
seawater-derived waters may be excluded iii) on
the contrary, the isotope composition of methane
testify the solubilization of hydrocarbons in the
aquifer at the boundary between ophiolitic units
and the below flysch and/or arenaceous
formations. This may be have overwritten the
abiotic serpentinization signature of the gas
dissolved in the hyperalkaline waters.
and future prospectives
i) d11B analysis on primary and secondary
minerals lizardite, Ca- and Mg-carbonates,
datolite CaBSiO4(OH), in the ophiolitic
breccias outcropping near to UM15 sample
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