Solubility of alumina in orthopyroxene plus spinel as a geobarometer in complex systems. Applications to spinel-bearing alpine-type peridotites

Abstract

The addition of Fe and Cr to the simple system MgO-SiO₂-Al₂O₃ markedly affects the activities of phases involved in the equilibrium

$$\begin{gathered} {\text{Mg}}_{\text{2}} {\text{SiO}}_{\text{4}} {\text{ + MgAl}}_{\text{2}} {\text{SiO}}_{\text{6}} {\text{ = MgAl}}_{\text{2}} {\text{O}}_{\text{4}} {\text{ + Mg}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{\text{6}} \hfill \\ {\text{Olivine + Opx}}_{{\text{solid solution}}} {\text{ = Spinel + Opx}}_{{\text{solid solution}}} \hfill \\ \end{gathered}$$

. In particular, the activity of MgAl₂O₄ in many natural spinels is decreased, which gives lower calculated pressures when the formula

$$P = 1 + \left\{ {(9,116 - 5.065{\text{ }}T_0 ) - {\text{RT}}_{\text{0}} \ln \left( {\frac{{(X_{{\text{Mg}}} )_{{\text{SP}}} (X_{{\text{Al}}} )_{{\text{SP}}}^{\text{2}} (X_{{\text{Mg}}}^{{\text{M1}}} )_{{\text{OPX}}} }}{{(X_{{\text{Mg}}} )_{{\text{OL}}}^{\text{2}} (X_{{\text{Al}}}^{{\text{M1}}} )_{{\text{OPX}}} }}} \right)} \right\}/\Delta V_\Gamma$$

derived from the experimental data of MacGregor (1974) is applied to complex systems containing Fe and Cr.

Most “cold slab” alpine-type peridotites appear to have recrystallized at low to moderate pressure (≦15kb), consistent with an origin in the upper oceanic mantle and subsequent emplacement in continental crust.

Taken individually or as a group, alpine-type peridotites do not record “pyroxene geotherms”. Present methods of calculating re-equilibration pressure for spinel peridotites may have large systematic errors and should be applied cautiously.