International Geologiical Congress - Oslo 2008

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UHP-03 Ultra-high pressure metamorphism: Minerals, microstructures and nanoscale observations

 

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

 

Shyh-Lung Hwang, National Dong Hwa University (Taiwan)
Tzen-Fu Yui, Institute of Earth Sciences, Academia Sinica (Taiwan)
Hao-Tsu Chu, Central Geological Survey (Taiwan)
Pouyan Shen, Institute of Materials Science and Engineering, National Sun Yat-sen University (Taiwan)
Yoshiyuki Iizuka, Institute of Earth Sciences, Academia Sinica (Taiwan)
Houng-Yi Yang, Department of Earth Sciences, National Cheng-Kung University (Taiwan)
Jingsui Yang, Institute of Geology, Chinese Academy of Geological Sciences (China)
Zhiqin Xu, Institute of Geology, Chinese Academy of Geological Sciences (China)
 

 

Analytical electron microscopic observations of a garnet peridotite from the Maobei area, Sulu ultra-high pressure terrane were carried out. The results showed that olivine in this garnet peridotite, with the maximum metamorphic pressure and temperature record of 50-65 kbar and 814-957oC, contains precipitates of chromian magnetite and chromian titanian hematite at dislocations and (001) faults. Specific crystallographic relationships were determined between these precipitates and olivine host, i.e., [101]Mt//[001]Ol, [110]Mt//[0 1]Ol, and [0 1]Mt//[011]Ol; and [0001]Hm//[100]Ol and [10 0]Hm//[001]Ol.

These oriented oxides are not in association with silicate/silica phases and therefore cannot be accounted for by the mechanism of olivine oxidation under oxidizing environments. It was postulated that these magnetite and hematite precipitates may most probably 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 olivine lattice, it was 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 olivine lattice. Two requirements are essential under such a scenario: ample amount of H content in olivine and an appropriate exhumation rate, probably in the range of 6-11 mm/yr, of the hosting rock. It is also noted that such dehydrogenation-oxidation processes may hamper the correct estimates of P-T conditions, and mantle oxidation state as well, based on mineral chemistries of mantle eclogite/peridotite.

The present study gives an additional example that oriented mineral inclusions may not necessarily form through exsolution processes sensu stricto, but may form through more complicated reaction mechanisms. Detailed studies on these mineral inclusions are warranted to pursue their geologic implications.

 

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