Background

The Earth is divided into four distinct layers: the atmosphere, crust, mantle and core. The upper three – atmosphere, crust, and mantle, are connected to each other over geologic time through plate tectonics. The mantle constitutes 84% of the Earth’s volume and extends from the base of the crust (at 10-60 km) all the way to 2,900 km depth, where the isolated, iron-rich core is found. Because water cycles between the atmosphere, crust, and mantle, understanding the composition of the mantle, potentially containing the largest chemical “reservoir” of H₂O, is therefore critical to understanding our dynamic Earth. Unfortunately, we have no direct access to these depths. The deepest cave on Earth extends only 2 km, barely scratching the surface of the crust and even the deepest hole ever drilled (~12 km) punctures only one-quarter of the thickness of the continental crust (average crustal thickness ~35 km). Therefore, we rely on two primary ways to study the Earth’s interior composition: samples from volcanic eruptions and laboratory experiments. My research involves both lines of study.

Samples from Earth’s deep interior

My research focuses on studying mineral inclusions encapsulated in diamonds. Diamonds act as vessels bringing up mantle minerals, trapped as inclusions, to the surface. Due to diamond’s unique properties, such as its chemical inertness and its hardness, it protects these inclusions from alteration, whereas mantle minerals brought up in xenoliths are heavily altered upon their journey to the surface. These mineral inclusions in diamond serve as the only natural samples from the deep mantle. The non-destructive methods I have developed to characterize mineral inclusions encapsulated in diamonds strive to preserve high-pressure phases, oxidation states and volatile contents. This is information that would be loss using conventional cracking/extraction methods. By studying these inclusions using non-destructive methods we gain a direct snapshot into the earth’s mantle where diamond formation takes place; and therefore potentially also where fluids are abundant.

What are super-deep diamonds?

While most diamonds originate from the upper 200 km of the mantle, some so called super-deep diamonds originate from depths >300 km, providing a glimpse into Earth’s deep mantle. There are five known super-deep localities: Jagersfontein, South Africa, Juína, Brazil, Kankan, Guinea, Lac de Gras, Canada and Monastary, South Africa (Brenker et al., 2007). The super-deep diamond samples I work with are from Juína, Brazil. Within the past five years alone, diamond inclusion research has made significant geochemical discoveries such as the first terrestrial ringwoodite, a high-pressure form of olivine (Mg₂SiO₄) (Pearson et al., 2014), and more recently the discovery of CaSiO₃-perovskite (Nestola et al., 2018). Such discoveries demonstrate the continued value in diamond inclusion research.

 

What can super-deep diamonds tell us about Earth’s mantle?

Elucidating Earth’s deep water cycle

In 2014, inside a super-deep diamond from Juína, Brazil, Pearson et al. (2014) discovered the first natural example of the mineral ringwoodite, a high-pressure form of olivine (Mg₂SiO₄). Ringwoodite is only stable from 520-660 km depth, and most surprisingly the crystal was found to contain ~1.5% H₂O water (in the form of hydroxyl, OH), nearly the maximum amount possible based on laboratory experiments (Smyth et al., 2003). Following the discovery of hydrous ringwoodite (Pearson et al. 2014), in 2016 a diamond inclusion of ferropericlase-(Mg,Fe)O, thought to have come from 1,000 km in depth, displayed signatures of water-bearing fluids, representing the deepest evidence for water to date (Palot et al., 2016). These recent studies provide sufficient evidence that signatures of water can be found in super-deep diamonds. What remains to be determined, is the extent and heterogeneity of water in the mantle. This leads to big picture questions including the origin of Earth’s water. Is water being recycled from the oceans, or is there a primordial trap of water in melt layers above and below the transition zone (410-660 km) of the mantle? My research aims to shed light on the hydration state of the mantle.

 

Recreating conditions that inclusions are under using Diamond Anvil Cells

I aim to better understand how some of these high-pressure mineral phases, only stable in narrow ranges of the mantle, survive the ascent to the surface. To do this I use diamond anvil cells to recreate the conditions that these minerals experience. By utilizing resistive heating methods high-temperature and high-pressures can be created simultaneously in diamond anvil cells. To reach temperatures over 600 degrees Celsius we use laser heating methods at the synchrotron light source at the Advanced Photon Source at Argonne National Lab.

 

 

Diamond inclusions is a cutting-edge research field gaining media attention:

Aug 1st, 2018 ” Rare blue diamonds reveal secrets from hundreds of miles below the Earth’s surface”

 

 

 

July 11th, 2018 “The Hunt for Earth’s Hidden Oceans”

 

 

 

 

 

 

March 7th, 2018 “This Tiny Diamond Contains a Mineral That’s Never Been Seen Before”

 

 

November 28th, 2016  “Earth’s Deepest Water May Be 1000 Kilometers Below the Surface”

 

 

 

 

 

References

Brenker, F.E., Vollmer, C., Vincze, L., Vekemans, B., Szymanski, A., Janssens, K., Szaloki, I., Nasdala, L., Joswig, W., and Kaminsky, F. (2007) Carbonates from the lower part of transition zone or even the lower mantle. Earth and Planetary Science Letters, 260, 1–9.

Nestola, F., Korolev, N., Kopylova, M., Rotiroti, N., Pearson, D.G., Pamato, M.G., Alvaro, M., Peruzzo, L., Gurney, J.J., Moore, A.E., and Davidson, J. (2018) CaSiO₃ perovskite in diamond indicates the recycling of oceanic crust into the lower mantle. Nature, 555, 237–241.

Palot, M., Jacobsen, S.D., Townsend, J.P., Nestola, F., Marquardt, K., Harris, J.W., Stachel, T., McCammon, C.A., and Pearson, D.G. (2016) Evidence for H2O-bearing fluids in the lower mantle from diamond inclusion. Lithos, 265, 1–7.

Pearson, D.G., Brenker, F.E., Nestola, F., McNeill, J., Nasdala, L., Hutchison, M.T., Matveev, S., Mather, K., Silversmit, G., Schmitz, S., and others (2014) Hydrous mantle transition zone indicated by ringwoodite included within diamond. Nature, 507, 221–224.

Shirey, S.B., Cartigny, P., Frost, D.J., Keshav, S., Nestola, F., Nimis, P., Pearson, D.G., Sobolev, N. V., and Walter, M.J. (2013) Diamonds and the Geology of Mantle Carbon. Reviews in Mineralogy and Geochemistry, 75, 355–421.

Smyth, J.R., Holl, C.M., Frost, D.J., Jacobsen, S.D., Langenhorst, F., and Mccammon, C. (2003) Structural systematics of hydrous ringwoodite and water in Earth’s interior. American Mineralogist, 88, 1402–1407.