Some Recent Results
✦ R/V Atlantis of the Woods Hole Oceanographic Institution seen here in Manzanillo, Mexico at the end of the OASIS Cruise in December of 2016. Work on ocean floor rocks leads to a better understanding of the workings of Earth's mantle and serves an analogue for the Earth before the continents formed. The R/V Atlantis is the mother ship for the famous manned underwater submersible Alvin. (Photo: Steve Shirey).
Sublithospheric diamonds show fluids could contribute to deep earthquakes
Some of Earth's largest earthquakes occur at tremendous depths (500-700 km) beneath the surface, always within or near oceanic plates that have sunk back into the Earth's interior. The cause of these events has been an enduring question in geology and geophysics for more than 40 years and is one of the top 100 questions in science today. In a new paper, a team of Carnegie and University of Alberta geoscientists provide several lines of evidence that fluids contribute to the genesis of deep earthquakes. New thermal modeling shows that carbonated crust and hydrated mantle in cold slabs can transport these fluids down to where deep earthquakes occur. Evidence from diamonds provides mineralogical proof of these mobile fluids in the mantle transition zone (440 - 670 km depth).
✦ Figure S4 from the Supplemental Information for the deep focus earthquake paper showing transects 14-17 from the Sunda arc. Note that slabs that are colder (older) produce seismicity to greater depths. Four transects along the Sumatra subduction zone (Fig. S4) from west (14. SSumatra) to east (17. Flores-Sea) have been selected. The easternmost extent of sampling is defined by the easternmost extent of currently subducting oceanic plate for which a downgoing plate age can be accurately estimated. Transects from further to the west are excluded due to the apparent 3D geometry of the slab in the lowermost upper mantle and in the transition zone that could compromise our 2D slab assumptions. Original figure by Lara Wagner with caption edited here.
✦ Figure 7 from the deep focus earthquake paper showing a dehydration model for earthquake generation at 500–700 km depths from the crust to the mantle in the lithosphere. In warm slabs both the slab crust and mantle extensively dehydrate by depths of 250 km. In cool slabs the dense hydrous magnesium silicate (DHMS) phases will be stable and transport water deep into the transition zone. Once the cool slabs stall and warm up in the transition zone (gray arrows), breakdown of the DHMS phases produces fluids that initiate the peak in earthquakes at 550–650 km and provide the fluids from which aqueous sublithospheric diamonds and their inclusions precipitate. The top of the crust is warmer, fully dehydrated and intersects the carbonate-bearing basalt solidi at about the same depth as the proposed dehydration of the peridotite. Because little or no warming of the crust is needed, production of carbonatitic melt from the altered oceanic crust would occur before the cooler slab mantle has time to warm up. Original figure by Mike Walter with caption edited here.
Shirey, S. B., Wagner, L. S., Walter, M. J., Pearson, D. G., & van Keken, P. E. (2021). Slab transport of fluids to deep focus earthquake depths—thermal modeling constraints and evidence from diamonds. AGU Advances, 2, e2020AV000304. https://doi.org/10.1029/2020AV000304 Supplemental Information
Evidence for recycling from Earth's surface to the mantle transition zone
✦ Fig. 2 from Smith et al., 2021. Iron isotope measurements. Metallic inclusions in diamond samples Letseng_889 and Letseng_890 are heavier than unaltered mantle-derived peridotites and basalts but are in line with awaruite/magnetite from serpentinized peridotite. IAB, island arc basalt; OIB, ocean island basalt; MORB, mid-ocean ridge basalt.
Subducting tectonic plates carry water and other surficial components into Earth’s interior. Previous studies suggest that serpentinized peridotite is a key part of deep recycling, but this geochemical pathway has not been directly traced. Fe-Ni–rich metallic inclusions in sublithospheric diamonds from a depth of 360 to 750 km have isotopically heavy iron (∂56Fe = +0.79 to 0.90‰) and unradiogenic osmium (187Os/188Os = 0.111). These iron values lie outside the range of known mantle compositions or expected reaction products at depth. This signature represents subducted iron from magnetite and/or Fe-Ni alloys precipitated during serpentinization of oceanic peridotite, a lithology known to carry unradiogenic osmium inherited from prior convection and melt depletion. These diamond-hosted inclusions trace serpentinite subduction into the mantle transition zone. We propose that iron-rich phases from serpentinite contribute a labile heavy iron component to the heterogeneous convecting mantle eventually sampled by oceanic basalts.
Smith, E.M., Peng, N., Shirey, S.B., Richardson, SH., Wang, W., and Shahar, A. (2021) Heavy iron in large gem diamonds traces deep subduction of serpentinized ocean floor. Science Advances 7: eabe9773 https://doi.org/10.1126/sciadv.abe9773 Supplemental Information
Appearance of light carbon deep in the mantle around 3 billion years ago
✦ This box and whisker plot pairs all diamonds whose mineral inclusions have been dated by geochronology with carbon isotopic analyses on the host diamond plotted in chronological order. This is the first time that such as data set has been assembled. This figure and caption content are from Howell et al. (2020; see full reference below). The 13C data from this new study are in bold colors and data from the literature (combustion plus SIMS data) are in faint colors. Where single outliers are more than 3‰ from the remaining data, they have been marked as single data points (in Panda, Diavik and Venetia).
The carbon isotopic composition of all radiometrically dated diamonds was just published in a new paper in GCA (reference below) by Dan Howell and colleagues from the Diamonds and the Mantle Geodynamics of Carbon (DMGC) consortium. In this new dataset, lithospheric diamonds of both peridotitic and eclogitic paragenesis are compared. If we consider eclogitic diamonds to be crystallized from slab-derived fluids, then this new dataset shows clearly the first appearance of light, organic carbon in the mantle transported deep enough by slabs to generate diamond-forming fluids. From the literature, we know that kerogens with isotopically light carbon are available on Earth's surface by 3.5 Ga (Des Marias, RiMG 2001). So the circa 400-500 myr delay before such organic carbon appears in slab fluids may have broader mantle convective significance or simply be due to the lack of subducted slabs before 3 Ga.
Howell, D., Stachel T., Stern R. A., Pearson D. G., Nestola F., Hardman M. F., Harris J. W., Jaques A. L., Shirey S.B., Cartigny P., Smit K. V., Aulbach S., Brenker F. E., Jacob D. E., Thomassot E., Walter M. J., and Navon O. (2020). Deep carbon through time: Earth’s diamond record and its implications for carbon cycling and fluid speciation in the mantle. Geochimica et Cosmochimica Acta 275: 99-122. https://www.sciencedirect.com/science/article/pii/S0016703720301216
Subduction to the MTZ: the new paradigm for superdeep diamonds
✦ Left, histogram of δ13C values of transition zone diamonds from Jagersfontein and Monastery (South Africa), the Juina area in Brazil (containing either majorite or Ca-rich inclusions) and Kankan (Guinea). Histogram of δ15N values of transition zone diamonds from Jagersfontein, Monastery, Brazil and Kankan. Right, schematic history of diamond formation in the transition zone, illustrating the deep recycling of surficial carbon and nitrogen in the mantle. (Figure by Mederic Palot and Graham Pearson with caption as modified from the book).
A new summary chapter covering some of the most exciting work of the Diamonds and the Mantle Geodynamics of Carbon (DMGC) group of the DCO has just appeared in the new book, Deep Carbon: Past to Present. Among the highlights is a summary by Palot and Pearson of the light carbon and heavy nitrogen isotope data on superdeep diamonds which contribute to the overwhelming evidence that superdeep diamonds result from deep subduction into the mantle transition zone. It was not that long ago that subduction itself was questioned for diamond formation. The idea of such deep subduction from the diamond evidence was first advocated by Ben Harte (U Edinburgh) and Thomas Stachel (U Alberta).
Shirey, S., Smit, K., Pearson, D., Walter, M., Aulbach, S., Brenker, F. E., Bureau, H., Burnham, A. D., Cartigny, P., Chacko, T., Frost, D. J. , Hauri, E. H., Jacob, D. E., Jacobsen, S. D., Kohn, S. C., Luth, R. W., Mikhail, S., Navon, O., Nestola, F., Nimis, P., Smith, E. M., Stachel, T., Stagno, V., Steele, A., Thomassot, E., Thomson, A. R., Weiss, Y. (2019). Diamonds and the Mantle Geodynamics of Carbon: Deep Mantle Carbon Evolution from the Diamond Record. In B. Orcutt, I. Daniel, & R. Dasgupta (Eds.), Deep Carbon: Past to Present (pp. 89-128). Cambridge: Cambridge University Press. doi:10.1017/9781108677950.005. (This is an open-access publication for free download) https://www.cambridge.org/core/books/deep-carbon/diamonds-and-the-mantle-geodynamics-of-carbon/E46212484DDAA32B1DA14B796EB3D9BC
How continental mantle keels form
✦ Sulfur isotopes emerge as a tool to understand the geodynamics of cratonic mantle construction. MIF sulfur has both a distinctive Hadean-Archean time stamp and an atmospheric link that emerges as a very sensitive way to track Archean surficial sulfur into the deep mantle below continents. (A) Sulfides in Paleoarchean diamonds from the Slave craton do not contain MIF sulfur, supporting models for craton construction that did not involve incorporation of recycled surficial material. (B) Younger diamonds from the West African, Kaapvaal, and Zimbabwe cratons contain MIF sulfur, which suggests construction of the cratonic mantle through subduction-style horizontal processes. (Figure 3 from Smit, K.V., Shirey, S.B., Hauri, E.H., and Stern, R.A. (2019) Sulfur isotopes in diamonds reveal differences in continent construction. Science 364, 383-385. http://doi.org/10.1126/science.aaw9548)
A study published in the April 26, 2019 issue of the journal Science (ref. below) demonstrates that the sulfur isotopic composition of sulfides in diamonds is sensitive to whether the cratonic keel to the continents was constructed by lateral accretion or deep mantle upwelling. In this study we argue that because we see sulfur isotopic compositions in diamonds that can only come from the Archean atmosphere, the only feasible way such sulfur could have been collected is by incorporation into the surface sedimentary layer of an the oceanic plate that was then accreted by some lateral process such as subduction. The construction of a mantle keel that serves to stabilize the overlying crust from destruction and gives it elevation to emerge above sea-level is considered one key to habitability on Earth.
(Smit, K.V., Shirey, S.B., Hauri, E.H., and Stern, R.A. (2019) Sulfur isotopes in diamonds reveal differences in continent construction. Science 364, 383-385. http://doi.org/10.1126/science.aaw9548)
Blue diamonds – the deepest diamonds ever
✦ A blue, boron-bearing diamond, with dark inclusions of a mineral called ferropericlase that was one of 26 inclusion-containing blue diamoids examined as part of the study. This gem weighs 0.03 carats. (Photo: Evan M. Smith/© 2018 GIA).