Abstract
Srilankite (Ti2ZrO6), a mineral rich in the high field strength elements (HFSE), was originally described in pebbles found in the washing concentrate of a gemstone mine in Rakwana in the province of Sabaragamuva, Sri Lanka (Willgallis et al., 1983). It was also reported as an inclusion in garnets from the Yagodka lamprophyre pipe in Tobuk-Khatystyr field (Kostrovitskiy et al., 1993) and from Colorado Plateau ultramafic diatremes in the Navajo Volcanic Field (Wang et al., 1999). Recently, Bingen et al. (2001) reported srilankite from crustal mafic granulite and interpreted that it was a reaction product between baddeleyite and ilmenite controlled by very local conditions where no free silica was available. Thus occurrence of srilankite is an indicator of the high HFSE/SiO2 conditions in its formation. Here we first report srilankite from the ocean floor.
The studied sample was directly collected from an outcrop at 2605m depth exposed on the eastern rift valley wall of the Atlantis II Fracture Zone, the Southwest Indian Ridge (SWIR), during Dive #643 of the ABCDE cruise using submersible SHINKAI 6500 of the Japan Marine Science Technology Center (JAMSTEC). The SWIR is an ultraslow spreading ridge with a 14 mm/year full spreading rate (Hosford et al., 2003). The Atlantis II Fracture Zone is a 199-km offset of the SWIR with a 6-km deep transform-valley. Srilankite occurs as small patches, < 30 micron meter, always coexisting with ilmenite and rutile. Zircon, apatite and phlogopite also occur as accessory minerals in the vein. A Zr/Ti ratio of the srilankite is close to a stoichiometric value of one-half. Based on petrography, the srilankite appears to be directory crystallized with ilmenite and rutile from melts rather than metamorphic recrystallization. Mineral assemblages and mineral compositions in the vein indicate that melts which produced the vein have high concentrations of compatible elements (MgO and Cr2O3) as well as incompatible elements (high-filed strength elements, K2O and H2O). On the other hand, TiO2-enrichment of minerals in the peridoitite host on the periphery of the gabbroic vein may result from the interaction with the melts. Geochemical interactions between peridotite and melt in the upper mantle may lead to effectively concentrate the incompatible elements in a modified melt, which in the extremely case is able to precipitate srilankite directly. Physical conditions under slow-spreading ridges, characterized by a highly attenuated magma supply and high rock/melt ratio, are favorable for the peridotite-melt interactions.
