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| Titel |
Preferential partitioning of copper into the vapor phase: fact or a natural artifact? |
| VerfasserIn |
Linda Lerchbaumer, Andreas Audétat |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056193
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| Zusammenfassung |
| Studies on natural assemblages of coexisting vapor and brine inclusions revealed that copper
(±Â Au, As, B, Mo) commonly occurs in higher concentrations in the vapor phase than in the
brine. It is believed that this is due to the formation of stable hydrosulfide-bearing complexes
in the vapor phase, a theory that is supported by experiments proving the existence of
copper-hydrosulfide complexes, experiments noting an increase in the vapor/brine partition
coefficients of Cu (DCuvap-brine) with increasing sulfur content of the fluid, and observations
showing that in natural boiling assemblages DCuvap-brine correlates positively with
DSvap-brine. On the other hand, none of the experimental studies conducted so far has
succeeded in reproducing conditions under which copper clearly fractionates into the vapor
phase (i.e., DCuvap-brine >1). An exception seemed to be the study of Nagaseki
and Hayashi (2008) who claimed to have obtained DCuvap-brine values up to 30.
However, mass balance constraints and results of experiments performed at identical
conditions in our laboratory suggest that these values are wrong and in fact are below
unity.
In view of recent experimental studies demonstrating that quartz-hosted fluid
inclusions can diffusively loose or gain copper after their formation we wanted to check
whether such a process could also lead to artificially high vapor/brine partition
coefficients. For this purpose we synthesized vapor and brine inclusions from a
Cu2S-saturated H2O-NaCl-S fluid at 800Â Ë C / 1.3Â kbar and re-equilibrated them in a
second experiment with similar fluid at 800Â Ë C / 700Â bar. After each step some of
the inclusions were analyzed by LA-ICP-MS. Although the Au capsule featured a
leak after the second experiment, the result is quite spectacular: while vapor and
brine inclusions analyzed after the first step contained only 250Â ppm Cu (n=1)
and 530 ±Â 150 ppm Cu (n=8), respectively, vapor and brine inclusions analyzed
after re-equilibration contained 5.1 ±Â 3.2 wt% Cu (n=7) and 0.4 ±Â 0.2 wt% Cu
(n=7), respectively. Results of subsequent experiments suggest that copper in these
inclusions was gained in response to a change in pH, which changed from |
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