Mineralogical controls on garnet composition in the cratonic mantle

Garnet concentrates are a rich source of geochemical information on the mantle, but the mineralogical implications of wide ranging garnet compositions are poorly understood. We model chemical reactions between mantle minerals that may buffer the Ca-Cr lherzolitic garnet trend common in the lithospheric mantle. A harzburgitic trend of garnet compositions featuring a lower increase in Cr with Ca relative to the conventional lherzolitic trend is reported for the first time. Representation of garnet chemistry in terms of additive and exchange components in the Thompson space shows that the lherzolitic and harzburgitic trends are controlled by the cation exchanges MgFeAl <-> Ca2Cr and MgFeAl4 <-> Ca2Cr4, respectively. Various equilibrium reactions are presented to explain the trends assuming a closed or open system mantle. The compositional variability of the natural garnets from the Canastra 8 kimberlite (Brazil) is modeled by a linear system of mass balance equations. The solution returns the reaction coefficients of products (positive values) and reactants (negative values), which are then evaluated against the observed mantle mineralogy. In the isochemical mantle, the lherzolitic trend can form in the absence of clinopyroxene, but requires the presence of spinel and reflects the thickness of the spinel-garnet transition zone. This requirement contradicts observations on natural occurrences of the trend and the thermobarometry of the host peridotites. In the preferred model of a variably depleted mantle, the lherzolitic trend critically depends on the presence of clinopyroxene. The occurrence of lherzolitic garnet compositions in harzburgite can be explained by exhaustion of clinopyroxene as a result of garnet buffering. The open system behavior of the peridotitic mantle also provides a better explanation for the harzburgitic trend in garnet compositions. In an isochemical mantle, the trend can be controlled by many possible reactions, and no single mineral is essential. In the variably depleted mantle, spinel is required to make the harzburgitic trend garnet.