A POSSIBLE ARCHEAN SOURCE FOR ARKANSAS DIAMONDS?

The Cretaceous diamondiferous lamproites of southern Arkansas intrude Proterozoic lower crust of ~1.4 Ga. Analysis of garnet lherzolite xenoliths recovered during bulk testing indicate an extremely depleted ~20 km thick layer within the subcontinental lithospheric mantle (SCLM) at depths of 95-115 km. This depleted layer had olivine compositions ranging from Mg 92.5-94.0 compared to the rest of the section with less than Mg 92.0. These values suggest an older age for this depleted layer of the SCML which falls within the diamond stability field.

Re-Os model ages of sulfide grains within depleted xenoliths yield ages as old as 3.4 Ga and with major peaks at 270 Ma and 1.4 Ga. The model ages greater than 1.7 Ga are clearly older than any known crust in the area. Many of these model ages correlate with known tectonic events found in Archean cratons of Africa. It is proposed that this ancient diamondiferous SCLM has been tectonically emplaced by underplating due to its age, depletion, and resulting buoyancy within the mantle asthenosphere.

Keywords: Arkansas, xenoliths, Archean, tectonics and underplating