Garnet from Cr- and Ca-saturated mantle: implications for diamond exploration

Abstract

An exotic trend of positively correlated Cr and Ca occurs in garnet from xenoliths in the Jericho kimberlite, northern Canada. The trend shows significantly less enrichment in Cr with increasing Ca than the common lherzolitic trend characteristic of kimberlite concentrate. The trend is found only in spinel–garnet peridotite xenoliths and is attributed to equilibration of garnet with spinel. The compositional variation of garnet within the trend is calculated to require buffering by equilibria involving Cr-spinel and clinopyroxene. Representation of garnet chemistry in terms of additive and exchange components shows that the chromite–clinopyroxene–garnet equilibrium (CCGE) is controlled by the exchange Mg 2Al 3↔Ca 2Cr 3. The equilibration of garnet with chromite and clinopyroxene reduces the extent of cation exchange in octahedral sites and eliminates Fe from active substitution in garnet relative to the common lherzolitic trend. A trend similar to the CCGE trend was described for spinel–garnet xenoliths from South Africa and Colorado Plateau and was produced experimentally for natural lherzolite compositions at high P–T conditions. The fact that the trend is extremely rare is explained by its low visibility and/or the lack of Ca-saturation in most spinel–garnet cratonic peridotite. The CCGE trend is more pronounced in garnet from peridotite xenoliths than in garnet from kimberlite concentrate where it is less visible due to sampling biases and dilution. The paucity of Ca-saturation results from harzburgitic composition of peridotitic mantle at depths of 120–180 km (spinel–garnet facies) under most Archean cratons. The Jericho and Colorado Plateau mantle segments are unique in having lherzolitic composition of the spinel–garnet mantle, where there is sufficient clinopyroxene to ensure the development of the CCGE trend. A CCGE trend in garnet composition reflects the presence of relatively fertile rocks at relatively shallow depths. In the Ca-saturated mantle, the CCGE trend can be used to constrain the depth interval of the mantle sampled by kimberlite. Coupled with the data on kimberlite spinel chemistry and on the local geotherm, the presence of the trend can reveal sampling of non-diamondiferous or diamondiferous mantle horizons.