✦ 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 provides an analogue for t he Earth before the continents formed. The R/V Atlantis is the mother ship for the manned underwater submersible Alvin. (Photo: Steve Shirey).

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).

A study published in the August 2, 2018 issue of the journal Nature (Smith, E. M., Shirey, S. B., Richardson S. H., Nestola, F., Bullock, E. S., Wang, J., & Wang, W. (2018). Blue boron-bearing diamonds from Earth’s lower mantle. Nature, 560, 84–87. http://doi.org/10.1038/s41586-018-0334-5 ) demonstrates that the world's most valuable diamonds, those that are blue and thus boron-containing, are also perhaps the world's most deeply-derived diamonds. When these diamonds do contain mineral inclusions, the inclusions are low-pressure reaction products of high pressure minerals including bridgmanite, ferropericlase, Ca-pervoskite, and stishovite among others. These minerals allow an estimate of depths of origin around or below 660 km in Earth's mantle. See the Carnegie Institution for Science press release here. See the Deep Carbon Observatory (DCO) press release here.