34. COMPARISON OF EXPERIMENTALLY CRYSTALLIZED AND NATURAL SPINELS FROM LEG 1351

Spinel synthesis experiments performed on samples from Ocean Drilling Program Leg 135 provide constraints for the formation of naturally occurring spinels in these rocks. We evaluated the effect of temperature, fθ2, and magma composition on the Cr# and Mg# of spinel synthesized at a pressure of 1 atm. The experimental evidence suggests that the spinels from Hole 839B must have crystallized above 1200°C to produce Mg#s of 0.70 to 0.75 (cores) and Cr#s of 0.74 to 0.76. Core-to-rim decreases in the Mg# but not in the Cr# of the natural spinels were not reproduced in equilibrium experiments and reflect disequilibrium reactions in the rocks. This zoning implies that the spinel cores grew from a parent magma that had lower A12O3 but similar Mg# to the host rock and that mixing of the parent and an evolved magma occurred shortly before eruption. We calculated a reaction constant (K,) involving Al and Cr exchange between Mg-rich spinel and liquid that is dependent on temperature and fθ2, which enables us to predict the Cr#s of the liquid in equilibrium with a given spinel composition.