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

Cr-rich spinels obtained from drill cores in the margins of heavily altered basaltic sills and flows contain a wealth of petrological information about the host magmas that crystallized them. 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. The compositions of these Leg 127 spinels clearly show that some units underwent syn- and post-eruption enrichment in Mg/(Mg + Fe**2+) during hydrothermal alteration and diagenesis. This study proves that Cr-spinel is a unique and powerful tool for deciphering the original magmatic characteristics of heavily altered basalts.