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| Titel |
Alkanes and alkenes in Mediterranean volcanic-hydrothermal systems: origins and geothermometry |
| VerfasserIn |
Jens Fiebig, Walter D'Alessandro, Franco Tassi, Alan Woodland |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250035940
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| Zusammenfassung |
| It is still a matter of debate if nature provides conditions for abiogenic production
of hydrocarbons. Methane (C1) and the C2+ alkanes emanating from ultramafic
hydrothermal systems such as Lost City have been considered to be abiogenic in origin,
mainly because of the occurrence of an isotopic reversal between methane and the
C2+hydrocarbons and C1/C2+ ratios >1000 [1]. Abiogenic production of methane
has been postulated to occur under the relatively oxidizing redox conditions of
continental-hydrothermal systems, too. It was observed that temperatures received from the
H2-H2O-CO-CO2-CH4 geoindicator were coincident with temperatures derived from carbon
isotope partitioning between CO2 and CH4in gases released from the Mediterranean
volcanic-hydrothermal systems of Nisyros (Greece), Vesuvio and Ischia (both Italy) [2].
Such equilibrium pattern, if not fortuitous, can only be obtained if mantle- and
marine limestone-derived CO2 is reduced to CH4. At Nisyros, observed C1/C2+
ratios from 300-4000 are in agreement with an abiogenic origin of the methane.
Ethane and propane, however, were shown to be non-genetic with CO2 and methane.
C1/C2 and C2/C3 distribution ratios may point to the admixture of small amounts of
hydrocarbons deriving from the thermal decomposition of organic matter along with
abiogenically equilibrated methane essentially devoid of the higher hydrocarbons
[3].
Here, we provide new isotopic and hydrocarbon concentration data on several
Mediterranean volcanic-hydrothermal systems, including Nisyros, Vesuvio, Ischia, Vulcano,
Solfatara and Pantelleria. Wherever possible, we have extended our data set for the hydrogen
isotope composition of CH4 and H2, n-alkane- and alkene/alkane-distribution ratios. At
Nisyros, measured alkene/alkane- and H2/H2O concentration ratios confirm the attainment of
equilibrium between CO2 and CH4. CO2 and CH4 appear to have equilibrated in the liquid
phase at temperatures of ~360Ë C and redox conditions closely corresponding to the
metastable mineral paragenesis of fayalite-hematite-quartz. At Pantelleria and Solfatara,
apparent isotopic CH4-CO2 equilibration temperatures of ~560Ë C and ~450Ë C,
respectively, are in agreement with measured alkene/alkane- and H2/H2O ratios, too.
However, at Baia di Levante (Vulcano), these concentration ratios do not correspond to the
apparent carbon isotopic temperature. These findings imply that carbon isotopic analysis
of discharged CO2 and CH4 might be a powerful tool to determine temperatures
of volcanic aquifers. Alkene/alkane and H2/H2O concentration ratios should be
measured along with the carbon isotopic composition of CO2 and CH4 to be able
to check independently if isotopic equilibrium between CO2 and CH4 has been
attained.
[1] Proskurowski et al. (2008) Science 319, 604-607; [2] Fiebig, J. et al. (2007) Geochim.
Cosmochim. Acta 71, 3028-3039. [3] Fiebig, J. et al. (2009) Geology 37, 495-498. |
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