Modes of occurrence and geochemistry at DSDP Holes 69-505A and 69-505B

The compositions of chrome spinels of Costa Rica Rift basalts from Deep Sea Drilling Project Site 505 vary depending on their occurrences as (1) inclusions in olivine crystals, (2) inclusions in Plagioclase crystals, and (3) isolated crystals in variolitic or glassy samples. The variations are a consequence of (1) changes of melt compositions as crystallization proceeds, and (2) contrasting behavior of olivine and Plagioclase in competition with spinels for Al and Mg. Some spinels have skeletal rims compositionally less magnesian than mineral cores; however, the cores do not appear to be xenocrysts, unlike some texturally similar spinels in Mid-Atlantic Ridge basalts.