A NEW PROJECTION SCHEME AND DIFFERENTIATION INDEX FOR CR-SPINELS

The Cr# (100Cr/(Cr + Al)) and Fe2# (100Fe(2+)/(Mg + Fe2+)) of Cr-spine ls affected by low-temperature reequilibration with olivine are contro lled by isotherms with shallow slopes on the Cr# vs. Fe2# diagram. Con sequently, in slowly cooled olivine-rich plutonic rocks, the Fe2# of C r-spinel is an unreliable measure of the degree of evolution of the ma gma from which it crystallized. For example, high-Cr# spinels in a ree quilibrated dunite could have a higher Fe2# than low-Cr# spinels in a reequilibrated gabbro, despite the fact that the spinel in the gabbro originally crystallized at a lower temperature from a more evolved liq uid than did the spinel in the dunite. As a result, the distribution o f data points is difficult to interpret when minor elements in Cr-spin el (Ti, V, Fe3+, Mn) are plotted against Cr-spinel Fe2#. A solution to this problem is to project spinel compositional data onto an axis per pendicular to isotherms in Cr# versus Fe2# space. This allows better d iscrimination of spinel compositional associations and trends. The com positional changes attendant upon down-temperature reequilibration of Cr-spinels also causes problems when comparing Cr-spinels from plutoni c rocks with those from volcanic rocks or liquidus-temperature numeric al models. A solution to this problem is to recalculate high-temperatu re spinels to their low-temperature equilibrium compositions before ma king the comparison. A procedure is outlined allowing higher-temperatu re spinels to be adjusted down to the 700 degrees C isotherm. (C) 1997 Elsevier Science B.V.