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| Titel |
Constraints on cooling of the lower ocean crust from epidote veins in the
Wadi Gideah section, Oman Ophiolite |
| VerfasserIn |
Alexander Diehl, Bastian Bieseler, Wolfgang Bach |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250153527
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| Publikation (Nr.) |
 EGU/EGU2017-18521.pdf |
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| Zusammenfassung |
| Determining the depth, extent, and timing of high-temperature hydrothermal alteration in the
ocean crust is key to understanding how the lower oceanic crust is cooled. We report data
from 18 epidote veins from the Wadi Gideah section in the Wadi Tayin block, which is a
reference section for alteration of the lower crust formed at a fast oceanic spreading center.
87Sr/86Sr ratios feature a narrow range from 0.70429 to 0.70512, while O isotope
compositions vary between - 0.7 and +4.9‰ in δ18OSMOW. These compositions indicate
uniform water-rock ratios between 1 and 2 and formation temperatures in the range
of 300 to 450˚ C. There is no systematic trend in Sr and O isotope compositions
down section. Fluid inclusion entrapment temperatures for a subset of four samples
linearly increase from 338˚ C to 465˚ C in lowermost 3 km of crust of the Wadi
Gideah section. Salinities are uniform throughout and scatter closely around seawater
values.
We developed a numerical cooling model to assign possible crustal ages to the thermal
gradients observed. For pure conductive cooling, these ages range between 4 and 20 Ma. Our
thermal model runs with a high Nusselt number (Nu) of 20 down to the base of the
crust indicate that the epidote veins may record this near-axial deep circulation in
crust of only 0.1 Ma (5-7 km off axis). When off-axis circulation is shut off in the
more distal flanks, however, massive conductive reheating of the lower crust by as
much as 200˚ C is predicted to take place. But there is no evidence for prograde
metamorphic reactions in the samples we studied (or other hydrothermally altered oceanic
gabbros). An intermediate model, in which Nu is 20 down to 2 km for the first 0.1
Ma and Nu is then 4 down to 6.5 km depth off axis to 1 Ma, is consistent with the
permeability distribution within the ocean crust and predicts a thermal gradient for
the lower crust that matches the observed one for ages between 1 and 3 Ma. The
most plausible explanation for the origin of the epidote veins is that they formed in
off-axial hydrothermal systems that reach the base of the crust within 50-150 km off
the axis. This deep circulation provides an efficient mechanism for mining heat
that escapes the crust in the young flanks of mid-ocean ridges where a sizeable
fraction of the global oceanic hydrothermal heat flux is expected to take place. |
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