Hydrocarbons help for travelling titanium in the mantle

The presence of hydrocarbons in rocks at tectonic plate boundaries could explain the movement of titanium (Ti) through the fluid above subducting plates, according to researchers at Japan’s Kanazawa University.

Shoji Arai and co-workers studied the behaviour of hydrocarbons at plate margin subduction zones – where one tectonic plate slides beneath another. In the process, they made findings that add to the debate about how titanium moves within rocks at subduction zones.

The researchers took samples of dunite rocks from the Fujiwara metamorphic belt in southwest Japan. The dunite originated from mantle which was modified by heat and water – then broken up and re-cemented. During these processes, carbon-rich matter and water from the ocean floor was incorporated into the rock.

Arai and co-workers found hydrocarbons – namely methane and propane - within olivine crystals (magnesium iron silicate) inside the dunite. They believe the hydrocarbons may be a result of progressive metamorphism of carbon-bearing sediments within the subducted slab (ocean floor).

The research also revealed thin veins of titanoclinohumite (titanium-rich magnesium silicate) running through the dunite. The movement of titanium through rocks is not well understood, partly because Ti is immobile in water-rich solutions. However, Ti can be transported by carbon-rich solutions, so organic matter in rocks could play an important role in Ti mobility.

The findings of Arai and his team suggest Ti movement upward through the mantle wedge above the slab. This indicates that Ti may move within rocks at subduction zones, if hydrocarbons and Ti-rich rocks are present together.

Publication and Affiliation
Shoji Arai1,* Satoko Ishimaru1, & Tomoyuki Mizukami1. Methane and propane micro-inclusions in olivine in titanoclinohumite-bearing dunites from the Sanbagawa high-P metamorphic belt, Japan: Hydrocarbon activity in a subduction zone and Ti mobility. Elsevier: Earth and Planetary Science Letters 353-354 (2012) 1-11 Link

1. Department of Earth Sciences, Kanazawa University, Kanazawa 920-1192, Japan

*corresponding author, e-mail address: ultrasa@staff.kanazawa-u.ac.jp

ID: 201212A006