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@unipr.it)2. INGV - Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy3. BRGM - Metrology Monitoring Analysis Department, Orlans, France4. CNR-IGGI - Institute of Geosciences and Earth Resources, Pisa, Italy*

*N-ApenninesW-AlpsFigure modified from :Boschetti & Toscani 2008 - Chem. Geol. 257, 76-91

* for the springs from Voltri Group: Bruni et al. 2002 - App. Geochem. 17, 455-474*

*Whatre hyperalkaline waters6.5-8.5 is the pH range in most natural waters 10 up to 12 is the pH range in the springs from serpentinites

- 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):*and why to study them?

Chemical classification by major dissolved constituents:springs issuing from serpentinites are characterized by 3 geochemical facies- 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*updated from : Boschetti & Toscani 2008 - Chem. Geol. 257, 76-91

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

B and Cl concentration in the hyperalkaline springs*

B isotope compositionB analysis by TIMS ( vs. SRM951) and B speciation on hyperalkaline springs Saturation indexes (SI) solution-mineralsSome 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.low-T serpentinization*

B vs. Li isotope compositionBoschetti et al. 2011 Aq. Geochem. 17, 71-208Boschetti & Toscani 2008Chem. Geol. 257, 76-91*

d11B vs. B/Cl: looking for boron sourceBoschetti et al. 2011 Aq. Geochem. 17, 71-208Boschetti & Toscani 2008Chem. Geol. 257, 76-91*

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, 34-41Methane produced by (abiogenic ) serpentinization:

C: Chimera (Turkey)

LC: Lost City (Atlantis Massif, mid-Atlantic ocean)

Z: Zambales (Luzon, Philippines)

O: Oman (Semail Nappe)Autotrophic = bacterial carbonate reduction

Heterotropic = bacterial methyl-type fermentationmixingHydrogen 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:*

Hydrocarbons in the Po plain and N-Apennine(modified from Lindquist 1999, OFR 99-50-M)*springs from serpentinites

Conclusionsand future prospectivesi) 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.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];*

*

*This work concerns the study of spring waters interacting with serpentinites of the Northern Apennines in Italy*Water samples come from the Taro-Ceno river valley.The ultramafic masses of this area (here represented in black) are serpentinized lherzolitic peridotites belong to an ophiolite sequence which represents the remnants of a Mesozoic ocean in the South Alpine-Apennine lithospheric plate.This figure come from an our previous study made in the 2008, when the chemical composition of the springs waters interacting with the serpentinites of this valley was studied. Also, a study of the springs interacting with the serpentinites of the Voltri Group, in the near western Alps, was published by our colleagues of the University of Genoa during the 2002. This presentation concerns a focusing on the hyperalkaline waters coming from this area, in particular from Mount Prinzera, and B area in the middle valley.

Whatre hyperalkaline waters?The term hyperalkaline is related to the pH of the waters.Most of natural waters have a pH from 6.5 to 8.5, while spring waters from serpentinites range from pH 10 to 12.This pH range is very rare in nature, but its typical of all the fluids interacting with serpentinites.(CLICK) In fact, in addition to the springs from Italian Northern Apennine, we have several examples of these waters in the world: in particular in the Mediterranean basin, but also in the Western United States, Lost-City in the mid-ocean-ridge, Oman sultanate, and so on.**Why to study hyperalkaline waters?In the specific case of the Taro-Ceno springs, the boron content of these waters is up to 13 mg/L, which is unusually high for freshwaters.Other characteristics of these waters are the low Mg concentrations, negative redox potential and low carbonate or, in other terms, very low CO2 partial pressure values up to 10-8 bar.The C depletion was attributed to carbonate precipitation, although the possible reduction of CO2 to CH4 might also be important. Moreover, also hydrogen is present as dissolved gas, probably cause to the reduction of water during the late stage of water-rock interaction following this reaction.*These ternary plots represent the chemical classification of the Taro-Ceno springs using dissolved cations and anions, that is calcium, magnesium and sodium+potassium and sulfate, chlorine and total alkalinity.While most of waters have a Ca-bicarbonate composition like most of meteoric waters (for example surface waters (open square), springs issuing from basalts (grey square) and other formations (open circles), the springs from serpentinites start as Ca-bicarbonate (green squares), interact with magnesium bearing minerals of ultramafites becoming Mg-bicarbonate (yellow triangles); after than, magnesium is precipitated as silicates like clays, neoformed serpentine and brucite and waters become Na-hydroxide hyperalkaline (represented by red diamonds). Black diamonds are hyperalkaline springs from Voltri Group and they have a Calcium-hydroxide composition.*The analysis of isotope composition of the water molecula shows that all the waters sampled from the TaroCeno Valleys are meteoric in origin, since they are consistent with the meteoric water lines of central and northern Italy.Surface waters sampled from main stem and tributaries of Taro and Ceno rivers (open squares) are quite similar to the isotope composition of the springs from basalts and sedimentary formations (represented by grey squares and open circles, respectively).Springs from serpentinites (colored symbols) show a wider range, whit alkaline waters (red diamonds) are shifted to more negative values probably cause to longer and deeper circuit.In this table are reported the boron and chlorine content of some hyperalkaline springs from different countries: Mediterranean area, Oman sultanate, and western united states.The Taro-Ceno springs show the highest Boron/Chloride molar ratio.So, now the principal question is: where the boron comes from and whats the reason of this higher ratio in the Taro-Ceno Springs?(CLICK) Before to show the boron isotope composition, its interesting to take a look to this diagram representing the Cl and B concentrations in serpentinites rock-forming minerals.Hyperalkaline springs fall amongst the mineral samples, so its most probably that chlorine and boron come from the rocks during water-rock interaction.*In order to confirm this hypothesis, we have analysed the B isotope composition.These are the results of hyperalkaline springs obtained by two laboratories, in Italy and France, by chemical separation and TIMS analysis.The match between laboratories is encouraging, but the difference between two samples of same chemical composition is very strange.Cause to the high pH of the solutions, the borate is the dominant specie in the two samples.So its probably that the evolution of boron isotope composition from bicarbonate to hyperalkaline UM15 sample may be due to a pH fractionation, as is evident by the curves representing the typical trigonal(3)-tetragonal(4) (boric acid borate) fractionation.The higher boron isotope composition and the lower boron concentration in PR10 sample may be due to a boron-10 (borate) adsorption/scavenging by precipitating minerals like clays, calcite, and low temperature neoformed serpentine. In fact, this sample is oversatured in these minerals.*Li isotope can be used as another tool to investigate the effects caused by secondary minerals.Ca-bicarbonate waters are trending towards the isotope composition of local serpentinites, while Mg-bicarbonate and hyperalkaline waters are more scattered.The increase of Li isotope values showed by hyperalkaline water samples is typical of fractionation caused by Li-adsorption on secondary minerals; however, should be noted that PR10 is within the field of sedimentary brines.A look to the Li/Cl molar ratio reveals that its trending lower during the evolution of water-serpentinite interaction, and also that the sedimentary formation brines have are similar ratios.Therefore, a mixing between serpentinite and sedimentary waters may be probable, at least for the PR10 sample.However, this clash with the hypothesis stated in the previous slide.(CLICK) In fact, the same similarity is not evident if we look to B/Cl ratios.*If we join the B/Cl ratio with boron isotope composition, we obtain another efficient tool to discriminate the different sources of boron.But in this diagram the B/Cl ratios of the springs from serpentinite fall out of the plot so(1 CLICK) if we update the diagram adding the samples of this study plus the field of sedimentary brines, we observe that the springs have a proper and distinct field, where the highest boron isotope composition is imputable to pH fractionation. (2 CLICK) Besides pH fractionation or water-rock interaction (datolite?), the quite high B/Cl ratio of UM15 sample maybe caused by boron desorption from clays.*We have analyzed also the hydrogen and carbon isotope composition of methane dissolved in hyperalkaline springs.This hydrogen-carbon isotope plot of methane is useful to check the hypothetical abiogenic origin of the methane, typical of serpentinized area, and to check the possibility of mixing with hydrocarbon-bearing fluids from sedimentary formation presents below the ophiolite units in Northern Apennine and in the near so called sedimentary Po plain basin.In the plot, the PR10 hyperalkaline sample falls amongst the cluster of sedimentary methane samples, which is formed by a mixing between methane of thermogenic (yellow) and biogenic (pink) origin.Abiogenic methane has a quite different isotope composition, here represented by this gray field enclosing data from most famous area of serpentinization like Chimera, Lost City, Oman.The contribution of methane of sedimentary origin to hyperalkaline springs is not an extraordinary event if we consider the geology of the area because* the Po Basin and the N-Apennine form an hydrocarbon system, in this figure delimited by a red line, in which the hyperalkaline springs fall.(CLICK) In this area, the Northern Apennines is a fold-and-thrust belt, bounded to the NE by the Po Plain that represents the filling of the present-day foredeep.The majority of biogenic/diagenetic hydrocarbons gas is from shallow Pliocene and Pleistocene units, while minor thermogenic hydrocarbon contributions are from deeper Mesozoic and Miocene units.**