Altered spinel as a petrotectonic indicator in abyssal peridotite from the easternmost part of Southwest Indian Ridge

The easternmost part of Southwest Indian Ridge (SWIR) has special crustal structure, magmatic and tectonic processes. Abyssal peridotite from the easternmost part of Southwest Indian Ridge (63.5ºE/28ºS) is serpentinized spinel lherzolite. The accessory spinel has zoned texture, which was studied by petrography, electron probe micro-analysis (EPMA), and backscattered electron (BSE) imaging to reconstruct the petrotectonic and hydrothermal metamorphic history of the host abyssal peridotite. The fresh core is magmatic Al-spinel with low Cr^#. The average extent of melting of the abyssal peridotite is about 5.9%. The composition of fresh magmatic spinel core indicates the studied area to be an anomalously thin crust with a melt-poor system. Hydrothermal reaction modifies the chemical composition of magmatic spinel. Ferritchromit is the first product forming the inner rim during pre-serpentinization. The abyssal ferritchromit crystalized as micro- to nano-sized particle with no triple grain boundary, indicating they crystalized in a rapid cooling process during hydrothermal alteration. Chemical compositions of ferritchromit indicate a hydrothermal metamorphism in amphibolite facies. Magnetite in the outer rim was formed by replacement of ferritchromit during syn- or post-serpentinization. Authigenic chlorites crystallized in two events: (1) after formation of ferritchromit crystallized as vein in fracture-zone near the core of spinel and (2) after formation of magnetite crystallized at outermost rim. They are different in compositions, indicating their formation temperature was about 289 ºC and declined to 214 ºC. These results show that the abyssal peridotite had undergone amphibolite to lower-greenschist facies hydrothermal events during pre- to syn-serpentinization or post-serpentinization.