GSA Connects 2024 Meeting in Anaheim, California

Paper No. 164-11
Presentation Time: 10:55 AM

THE GROWTH HISTORY OF A SUBLITHOSPHERIC DIAMOND REVEALED BY 3D GROWTH ZONATION, INCLUSION MINERALOGY AND GEOBAROMETRY


SMITH, Evan M., Gemological Institute of America, 50 W 47th Street, New York, NY 10036 and JONES, Daniel, Gemological Institute of America (GIA), 333 Meadowlands Pkwy, Suite 105, Secaucus, NJ 07094

Earth’s plate tectonic activity drives the exchange of materials between surface and deep mantle reservoirs through time. As the only well-preserved material exhumed from transition zone and uppermost lower mantle depths, sublithospheric diamonds provide a valuable record of this activity. However, this record is often complex and can be challenging to unravel into meaningful observations. Here we examine multiple datasets from a single sublithospheric diamond from the Karowe mine, Botswana.

The diamond is a faceted, 1.5 carat, D color, type IaB gem that passed through GIA’s grading infrastructure and was later purchased for research purposes. Its inclusions comprise olivine/ringwoodite (Mg, Fe)2SiO4, ferropericlase (Mg, Fe)O, enstatite (former bridgmanite, (Mg, Fe)SiO3, and other minor phases. The mineral assemblage is consistent with a meta-peridotitic host rock near the 660 km discontinuity at the base of the mantle transition zone. This depth is independently supported by elastic geobarometry for one of the ferropericlase inclusions.

A novel hyperspectral imaging technique has been applied to this specimen to examine its 3D spatial distribution of various photoluminescent defects. This otherwise invisible spectral and textural information provides a way to visualize the growth zonation of this diamond, as well as post-growth deformation. Taken together, the data provide a clearer picture of the formation conditions of this diamond and help place it in a broader geodynamic context.