Appearance of light carbon deep in the mantle around 3 billion years ago
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
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
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)