 |
| Titel |
The effect of the water-to-rock ratio on REE distribution in hydrothermal fluids: An experimental study |
| VerfasserIn |
Oliver Beermann, Dieter Garbe-Schönberg, Astrid Holzheid |
| Konferenz |
EGU General Assembly 2013
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250078166
|
|
|
|
|
|
| Zusammenfassung |
| High-temperature submarine MOR hydrothermalism creates high elemental fluxes into, and
out of, oceanic lithosphere significantly affecting ocean chemistry. The Turtle Pits
hydrothermal system discovered at 5Ë S on the slow-spreading Mid-Atlantic Ridge (MAR)
in water depths of ~3000 m (~300 bar) emanates ‘ultrahot’ fluids > 400 Ë C [1] with high
concentrations of dissolved gases (e.g., H2), transition metals, and rare earth elements
(REE). The normalised REE patterns of these ‘ultrahot’ fluids are uncommon as they
exhibit depletions of LREE and no Eu-anomaly (‘special’ REE-signature in [2]),
which is in contrast to the “typical” LREE enrichment and pronounced positive
Eu-anomaly known from many MOR vent fluids observed world-wide [e.g., 3]. Although
hydrothermal fluid REE-signatures may play a key role in understanding processes during
water-rock interaction, only few experimental data have been published on REE
distribution in seawater-like fluids reacted with rocks from the ocean crust [e.g., 4, 5].
Besides temperature, the seawater-to-rock ratio (w/r ratio) strongly affects water-rock
reaction processes and, thus, has significant control on the fluid chemistry [e.g., 6,
7].
To understand how vent fluid REE-signatures are generated during water-rock interaction
processes we designed a series of experiments reacting different fluid types with mineral
assemblages from fresh, unaltered gabbro at 425 Ë C and 400 bar using cold seal
pressure vessels (CSPV). Mixtures of 125-500 μm-sized hand-picked plagioclase and
clinopyroxene grains separated from unaltered gabbro reacted in gold capsules with 3.2
wt.% NaCl(aq) fluid (similar to seawater salinity), or with natural seawater. The
w/r (mass) ratio ranged from 1 to 100 and the run durations were varied from 3
to 30Â d in the NaCl(aq) experiments, and was 3 d in the seawater experiments.
The reacted fluids were extracted after quenching and analysed by ICP-OES and
ICP-MS.
Only in the seawater experiments, the gabbro reacted considerably with the liquid
resulting in a strong REE enrichment relative to the original seawater. Increasing w/r ratios
gave rise to decreasing pH of the quench fluid and enforced the enrichment of HREE in the
fluid with relative depletion of LREE and no Eu-anomaly. The ‘special’ REE-signatures
observed in Turtle Pits vent fluids at 5Ë S MAR could be reproduced in our experiments at a
w/r ratio of 5 (pH = ~6), whereas at a w/r ratio of 1 (pHÂ =Â ~7) the fluid exhibited the
”typical” REE pattern with a positive Eu-anomaly. Fluids from experiments with w/r ratios
-¥10 (pH ~6 to ~2) showed higher HREE enrichment than observed in natural MOR vent
fluids so far.
Concluding, elevated REE concentrations in hydrothermal fluids exhibiting the ‘special’
REE pattern with relative enrichment of HREE and no Eu anomaly are indicative for leaching
processes under high w/r ratios (~5-10) that might be more common in slow-spreading
oceanic crust with focused hydrothermal fluid-flow along e.g., detachment faults
[8].
References:
[1] Koschinsky A., Garbe-Schönberg D., Sander S., Schmidt K., Gennerich H.-H., and
Strauss H. (2008) Geology 36, 615-618.
[2] Schmidt K., Garbe-Schönberg D., Bau M., and Koschinsky A. (2010) GCA 74,
4058–4077.
[3] Douville E., Bienvenu P., Charlou J. L., Donval J. P., Fouquet Y., Appriou P., and
Gamo T. (1999). GCA 63, 627-643.
[4] You C.-F., Castillo P. R., Gieskes J. M., Chan L. H., and Spivak A. J. (1996) EPSL
140, 41-52.
[5] Allen D. E. and Seyfried [Jr.] W. E.(2005) GCA 69, 675-683.
[6] Seyfried [Jr.] W. E. and Bischoff J. L. (1977) EPSL 34, 71-77.
[7] Hajash A. and Chandler G. W. (1981) Contrib Mineral Petrol 78, 240-254.
[8] McCaig A.M. and Harris M. (2012) Geology 40, 367-370. |
|
|
|
|
|
|