Factors controlling chemistry of magmatic spinel: An empirical study of associated olivine, Cr-spinel and melt inclusions from primitive rocks

Compositions of similar to 2500 spinel-olivine pairs and 400 melt inclusion -spinel pairs have been analysed from 36 igneous suites from oceanic, arc a nd intraplate tectonic settings. Our data confirm that Cr-spinel mg-number is largely controlled by melt composition, but also influenced by octahedra l site substitutions, and rate of cooling. Lavas quenched in submarine envi ronments tend to have higher mg-number at a given cr-number than slowly coo led subaerial lavas and peridotites. Unlike mg-number, Cr-spinel Al2O3 and TiO2 contents show good correlations with melt composition, with only limit ed post-entrapment modifications. Out data suggest that increased activity of Al2O3 decreases the partitioning of TiO2 into spinels. The Al2O3 content of Cr-spinel is a useful guide to the degree of partial melting of mantle peridotites; however, this same relationship is obscured in volcanic rocks. Al2O3 contents of volcanic Cr-spinels are mostly determined by melt compos ition rather than mantle source composition. The data also suggest that mos t spinels from residual mantle peridotites can be readily differentiated fr om those hosted in volcanic rocks. Mantle peridotite spinel tend to have lo wer TiO2 and higher Fe2+/Fe3+ ratios than spinel from volcanic rocks. The s pinel compositions in our database can be subdivided on the basis of tecton ic setting and mode of occurrence using an Al2O3 vs TiO2 diagram. A total o f seven fields can be distinguished with varying degrees of overlap. This d iagram can then be used to determine the tectonic setting of spinel from al tered mafic igneous rocks such as serpentinites or meta-basalts, or detrita l spinel in sandstones.