Natural evidence of a hybrid olivine crystal-fabric (HBE type) within amphibole bearing hydrous peridotites developed below a transform fault: Marion Fracture Zone, Southwest Indian Ridge
Abstract
Two types of olivine crystal-fabrics within hydrous peridotites have been rarely identified within amphibole bearing peridotite ultramylonites dredged from Marion Fracture Zone of Southwest Indian Ridge by the PROTEA Expedition Leg 5 cruise in 1983 of the RV Melville. The ultramylonites show a hybrid (010)[001] and (001)[100] pattern (here we name HBE type) along with a typical (010)[001] (i.e. B type) olivine crystal-fabric, in contrast to an experimentally formed hybrid (100)[001] and (001)[100] pattern (here we name HCE type). EBSD analysis revealed that amphibole grains have been dynamically recrystallized into finer matrix as well as olivine grains within the ultramylonites, indicating that these peridotites should be hydrated before and/or during intense shearing. Moreover, brittle microstructures in porphyroclastic spinel grains in the very fine-grained matrix suggest that these peridotites have been sheared under higher stress conditions, presumably just below the brittle-ductile transition. Therefore, both HBE type and B type olivine crystal-fabrics could be developed in a hydrous mantle shear zone under higher stress conditions, which might be in a transitional condition between E type and B type in the water-stress diagram, in contrast to the HCE type that could be developed in a hydrous mantle shear zone under lower stress conditions. These evidences show that the uppermost mantle below the Marion transform fault has been locally but intensely hydrated during shearing associated with the transform movement.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T21D0368M
- Keywords:
-
- 1032 Mid-oceanic ridge processes;
- GEOCHEMISTRY;
- 1037 Magma genesis and partial melting;
- GEOCHEMISTRY;
- 8140 Ophiolites;
- TECTONOPHYSICS;
- 8416 Mid-oceanic ridge processes;
- VOLCANOLOGY