September 14, 2017

Gaining Insight into the Atlin Ophiolite

Posted by larryohanlon

By Rebecca Fowler

This is the second and final dispatch from Rebecca Fowler, a science writer documenting the work of Texas A&M University scientists conducting fieldwork at the Atlin ophiolite in British Columbia.

Masako Tominaga, a geophysicist at Texas A&M, with a magnetometer. She’s in the midst of walking a transect across an outcrop at the summit of Monarch Mountain. Photo: Rebecca Fowler

Last week, I joined a group of scientists from Texas A&M University for fieldwork in Atlin, British Columbia, collecting data that will help scientists better understand fundamental earth system processes. Geophysicist Masako Tominaga is the project PI; she aims to create a field-based method that will enable scientists to directly observe and track the naturally occurring process of mantle rock carbonation, where one type of rock, peridotite, is transformed into another. This process has only been done in the lab with field samples, not in nature.

Atlin was chosen for our field site because it’s home to an ophiolite, a rare place on earth where the crust and mantle are exposed at the surface. Our group also explored the ophiolite at Atlin in 2016. That year, we primarily looked for exposed outcrops using a geological map of the area created about 15 years earlier. We quickly discovered that many outcrops or lithological contacts — where one type of rock meets another — indicated on the map were undiscoverable. And the outcrops we were able to locate weren’t ideal for making geophysical measurements because they were covered in tangled undergrowth or located in dense forested areas.

Noah Vento and the gravimeter. Variations in the processed gravity data will enable this team of scientists to see how certain geochemical processes transform periodotite, a rock found in this outcrop, into other types of rock. Photo: Patrick Fulton

But one perfect exposure of mantle rock was found at the summit of Atlin’s Monarch Mountain. This assemblage ranges in color from gray-green to rusty red and contains a sequence with each of the rock types this science team is interested in: peridotite, serpentinite, soapstone, and listvenite. The summit is scattered with shrubby vegetation but is free of the trees found at lower elevations in Atlin, making it suitable for both geophysical measurements and rock samples.

So, for several days last week, our team of four woke at 7:00 a.m. and made the 1.5 hour climb up Monarch Mountain with all of the equipment and instruments and snacks needed for a day of data collecting.

Once we reached the summit, Masako Tominaga, the project’s lead scientist, identified transects across the exposed mantle outcrop. Gravity and magnetics data would be gathered at regular intervals along these lines and use to create an image of how carbonation and serpentinization processes alter the rocks.

Patrick Fulton and Masako Tominaga discuss the outcrop at the summit of Monarch Mountain. Photo: Rebecca Fowler

The strategy for these tasks: divide and conquer. Masako Tominaga roamed the outcrop, chipping away at it with her rock hammer so she’d have a suite of rock samples from the marked transects and a few other places. With her small, portable magnetometer she then slowly walked the lines, measuring the abundance of magnetic minerals in the outcrop at one-meter intervals. I followed alongside her, recording these measurements in a notebook.

Meanwhile, Noah Vento was responsible for mapping the mantle formation. And together with Patrick Fulton, Noah recorded variations in gravity along each transect Masako was measuring. In addition to Patrick and Noah, our team had another new member this year: the gravimeter. Project scientist James Kinsey of Woods Hole Oceanographic Institution loaned us this instrument (thanks, James!) to collect data that will help determine how carbonation transforms periodiotite in other serpentinite, soapstone, and listvenite.

Masako and her colleagues have now done field studies and documented the natural carbon sequestration processes at three ophiolite formations: Atlin, Norway and northern California. Though there are differences among these ophiolite formations, she sees enough similarities to believe that it’s possible to develop the sort of in situ, carbonation monitoring technique she proposed. The next steps in this project — data processing and analysis — will reveal if this true. If it is, scientists will have a new tool for understanding a mantle rock processes and other vital earth science processes related to the role of carbon on our planet.

Thanks to NASA and the National Science Foundation for supporting this project. If you’d like to see more photos from our expedition, check out the AGU Instagram account (search for #Atlin17).

Part 1 of this two-part series is here.