A subduction wedge origin for Paleoarchean peridotitic diamonds and harzburgites from the Panda kimberlite, Slave craton: evidence from Re-Os isotope systematics

An extensive study of peridotitic sulfide inclusion bearing diamonds and their prospective harzburgitic host rocks from the 53 Ma Panda kimberlite pipe, Ekati Mine, NWT Canada, has been undertaken with the Re-Os system to establish their age and petrogenesis. Diamonds with peridotitic sulfide inclusions have poorly aggregated nitrogen (< 30% N as B centers) at N contents of 200-800 ppm which differs from that of chromite and silicate bearing diamonds and indicates residence in the cooler portion of the Slave craton lithospheric mantle. For most of the sulfide inclusions, relatively low Re contents (average 0.457 ppm) and high Os contents (average 339 ppm) lead to extremely low Os-187/Os-188, typically << 0.05. An age of 3.52 +/- 0.17 Ga (MSWD = 0.46) and a precise initial Os-187/Os-188 of 0.1093 +/- 0.0001 are given by a single regression of 11 inclusions from five diamonds that individually provide coincident internal isochrons. This initial Os isotopic composition is 6% enriched in Os-187 over 3.5 Ga chondritic or primitive mantle. Sulfide inclusions with less radiogenic initial Os isotopic compositions reflect isotopic heterogeneity in diamond forming fluids. The harzburgites have even lower initial Os-187/Os-188 than the sulfide inclusions, some approaching the isotopic composition of 3.5 Ga chondritic mantle. In several cases isotopically distinct sulfides occur in different growth zones of the same diamond. This supports a model where C-O-H-S fluids carrying a radiogenic Os signature were introduced into depleted harzburgite and produced diamonds containing sulfides conforming to the 3.5 Ga isochron. Reaction of this fluid with harzburgite led to diamonds with less radiogenic inclusions while elevating the Os isotope ratios of some harzburgites. Subduction is a viable way of introducing such fluids. This implies a role for subduction in creating early continental nuclei at 3.5 Ga and generating peridotitic diamonds.