 |
| Titel |
The rheological behaviour of fracture-filling cherts: example of Barite Valley dikes, Barberton Greenstone Belt, South Africa |
| VerfasserIn |
M. Ledevin, N. Arndt, A. Davaille, R. Ledevin, A. Simionovici |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1869-9510
|
| Digitales Dokument |
URL |
| Erschienen |
In: Solid Earth ; 6, no. 1 ; Nr. 6, no. 1 (2015-02-20), S.253-269 |
| Datensatznummer |
250115401
|
| Publikation (Nr.) |
 copernicus.org/se-6-253-2015.pdf |
|
|
|
|
|
| Zusammenfassung |
In the Barberton Greenstone Belt, South Africa, a 100–250 m thick complex
of carbonaceous chert dikes marks the transition from the Mendon Formation to the Mapepe
Formation (3260 Ma). The sub-vertical- to vertical position of the
fractures, the abundance of highly shattered zones with poorly rotated
angular fragments and common jigsaw fit, radial structures, and multiple
injection features point to repetitive hydraulic fracturing that released
overpressured fluids trapped within the shallow crust. The chemical and
isotopic compositions of the chert favour a model whereby seawater-derived
fluids circulated at low temperature (< 100–150 °C) within
the shallow crust.
From the microscopic structure of the chert, the injected material was a
slurry of abundant clay-sized, rounded particles of silica, carbonaceous
matter and minor clay minerals, all suspended in a siliceous colloidal
solution. The dike geometry and characteristics of the slurry concur on that
the chert was viscoelastic, and most probably thixotropic at the time of
injection: the penetration of black chert into extremely fine fractures is
evidence for low viscosity at the time of injection and the suspension of
large country rock fragments in the chert matrix provides evidence of high
viscosity soon thereafter. We explain the rheology by the particulate and
colloidal structure of the slurry, and by the characteristic of silica
suspensions to form cohesive 3-D networks through gelation.
Our results provide valuable information about the compositions,
physical characteristics and rheological properties of the fluids that
circulated through Archean volcano-sedimentary sequences, which is an
additional step to understand conditions on the floor of Archean oceans, the
habitat of early life. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|