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Hematite and magnetite precipitates in olivine from the Sulu peridotite: A result of dehydrogenation-oxidation reaction of mantle olivine?

¹ Department of Materials Science and Engineering, National Dong Hwa University, Hualien, Taiwan, ROC
² Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC
³ Central Geological Survey, P.O. Box 968, Taipei, Taiwan, ROC
⁴ Institute of Materials Science and Engineering, National Sun Yat-sen University, Kaohsiung, Taiwan, ROC
⁵ Department of Earth Sciences, National Cheng-Kung University, Tainan, Taiwan, ROC
⁶ Key Laboratory of Continental Dynamics, Ministry of Land and Resources. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China

Correspondence: ^* E-mail: tfyui{at}earth.sinica.edu.tw

Analytical electron microscopic observations have been carried out on a garnet peridotite from the Maobei area, Sulu ultrahigh-pressure terrane. The results showed that olivine in this garnet peridotite (5.3–6.6 GPa; 853–957 °C), contains precipitates of chromian magnetite and chromian-titanian hematite at dislocations and (001) faults. Specific crystallographic relationships were determined between these precipitates and the olivine host, viz. [101]_Mt//[001]_Ol, [110]_Mt//[01]_Ol, and [01]_Mt//[011]_Ol; and [0001]_Hm//[100]_Ol and [100]_Hm//[001]_Ol. These oriented oxides are not associated with silicate/silica phases and therefore cannot be accounted for by the mechanism of olivine oxidation. It is postulated that these magnetite and hematite precipitates most likely have resulted from dehydrogenation-oxidation of nominally anhydrous mantle olivine during rock exhumation. In view of the contrasting diffusion rates of H and Fe in the olivine lattice, it is suggested that the formation process might actually take place in steps. Hydrogen diffusion with concomitant quantitative oxidation of Fe²⁺ to Fe³⁺ in olivine occurred early during initial rock exhumation and was followed by slow Fe diffusion forming magnetite/hematite at stacking faults and dislocations within the olivine lattice. Two requirements are essential under such a scenario: an ample amount of H content of the olivine, and an appropriate exhumation rate, probably in the range of 6–11 mm/year, of the host rock. It is also noted that such dehydrogenation-oxidation processes may hamper a correct estimate of the actual P-T conditions and mantle oxidation state based on mineral chemistries present in mantle eclogite/peridotite. The present study demonstrates that oriented mineral inclusions may not necessarily form through exsolution processes sensu stricto, but may form through a series of more complicated reaction mechanisms.

Key Words: Magnetite • hematite • olivine • dehydrogenation-oxidation • UHP peridotite