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Magmatic srilankite (Ti₂ZrO₆) in gabbroic vein cutting oceanic peridotites: An unusual product of peridotite-melt interactions beneath slow-spreading ridges

Tomoaki Morishita^1,*, Jinichiro Maeda², Sumio Miyashita³, Takeshi Matsumoto^4, and Henry J. B. Dick⁵

¹ Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan
² Graduate School of Science, Hokkaido University, Hokkaido 060-0810, Japan
³ Department of Geology, Faculty of Science, Niigata University, Niigata 950-2181, Japan
⁴ Marine Science Department, Nippon Marine Enterprises, Ltd., Yokosuka 238-0004, Japan
⁵ Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, U.S.A.

Correspondence: ^* E-mail: moripta{at}kenroku.kanazawa-u.ac.jp

We report srilankite in a gabbroic vein cutting a serpentinized peridotite collected from the Atlantis II Fracture Zone, the slow-spreading Southwest Indian Ridge, using submersible SHINKAI 6500 of the Japanese Marine Science Technology Center. Srilankite occurs in small patches, <30 µm across, always coexisting with ilmenite and rutile. Zircon, apatite, and phlogopite also occur as accessory minerals in the vein. The Zr/Ti ratio of the srilankite is close to the stoichiometric value of one-half (Ti_2.00Zr_0.98Hf_0.01Fe_0.01O₆). Based on petrography, the srilankite appears to have co-crystallized with ilmenite and rutile from melts rather than through metamorphic recrystallization. Mineral assemblages and mineral compositions in the vein indicate that melts that produced the vein have high concentrations of compatible elements (MgO and Cr₂O₃) as well as incompatible elements (high-field strength elements, K₂O, and H₂O). On the other hand, TiO₂-enrichment of minerals in the peridotite host on the periphery of the gabbroic vein may have resulted from interaction with the melts. Geochemical interactions between peridotite and melt in the upper mantle may effectively concentrate incompatible elements in a modified melt, which may precipitate srilankite directly. Physical conditions under slow-spreading ridges, characterized by a highly attenuated magma supply and high rock/melt ratio, favor peridotite-melt interactions.

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