Geochemistry of Cr-spinels from ODP Leg 127/128 basalts

These host igneous units, cored during Leg 127 of the Ocean Drilling Program in the Japan Sea, underwent extensive hydrothermal alteration that makes petrogenetic interpretations difficult. Most spinels in these samples exhibit minor or no zoning and are similar in composition to others reported from mid-ocean ridge basalt and backarc basin basalt tholeiites.

Most show a range in Cr/(Cr + Al) of 0.26 to 0.62, a range in Mg/(Mg + Fe**2+) of 0.53 to 0.68, low TiO2 (<0.7%), and low Fe**3+/(Cr + Al + Fe**3+) (<0.1), which is consistent with crystallization from a tholeiitic liquid at a magmatic oxygen fugacity 2 log units or more below the nickel-nickel oxide oxygen buffer. By recasting the composition of these spinels in terms of an ideal reciprocal solution model and by deriving a Delta G(ex) for spinel-liquid Mg-Fe**2+ exchange, direct determination of magmatic Mg/(Mg + Fe**2+) is possible.