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| Titel |
MIRA: A new approach to measuring Delta $47 in carbonates and geothermometry of MVT type deposits |
| VerfasserIn |
P. F. Dennis, S. J. Vinen |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250023911
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| Zusammenfassung |
| Clumped isotope thermometry is based on the thermodynamics of the order-disorder reaction
for 18O and 13C:
13C16O16O16O + 12C18O16O16O = 13C18O16O16O + 12C16O16O16O
At high temperatures 18O is randomly distributed between 12C and 13C in the carbonate
anion. However, at lower temperatures there is a tendency for the 18O to cluster together with
13C. At low temperatures (T < 150oC) the degree of ordering, as measured by
Î47 of the CO2 that is produced by reaction of the carbonate with phosphoric acid
((13C18O16Osample/13C18O16Ostochastic)-1) is a potentially useful geothermometer (Eiler,
2007). However, for reliable temperature estimates to better than +/- 1oC at near earth surface
temperatures requires measurement of 1000*Î47 to better than +/-0.005. Given that the 47
isotopologue occurs at a natural abundance of just 40ppm in CO2 this is a challenging
measurement for stable isotope ratio mass spectrometry. We have developed a new instrument
MIRA to accurately measure such small isotope ratios. MIRA is configured with a 50cm
dispersion analyser, dual inlet, a high sensitivity Nier type source, 6 faraday collectors at m/z
= 44, 45, 46, 47, 48 and 49 and ultra stable and linear detection and integration
electronics. Using a dual inlet measurement precisions for 1000*Î47 are better than
+/-0.01.
To date isotopic clusters have just been used to estimate near surface and diagenetic
temperatures to 75oC. We are using MIRA and the clumped isotope thermometer to assess
formation temperatures (up to 150oC) and the timing of MVT Pb/Zn mineralisation in the
Pennines, UK orefield. Because temperature estimates are based on an internal
order-disorder reaction, they are independent of the isotopic composition of the
formation waters making it possible independently to track changes in the δ18O
composition of mineralising fluids. With sufficient resolution (+/- 2 – 5oC) we
aim to map temperature distributions in order to better constrain the hydrothermal
system.
This is the first application of clumped isotopes to ‘elevated’ temperature thermometry.
Eiler, J.M., 2007, ‘Clumped-isotope’ geochemistry – The study of naturally occurring
multiply substituted isotopologues. Earth and Planetary Science Letters, 262, 309-327 |
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