Magnetism of Cr-rich Spinel

As accessory minerals in igneous rocks, spinels are useful as petrographic or provenance indicators. As the first phase to crystallize in mafic-ultramafic magma sources, Cr-rich spinel (CRS) is important to unravel the isotopic evolution of planetary lithosphere. In order to investigate magnetic properties of CRS, twelve Cr-rich spinel bearing samples were studied. We found that CRS can contribute to natural remanent magnetization (NRM). One important compositional condition for the presence of magnetic CRS can be quantitatively proposed as follows: CRS is magnetic when less than 20% of Al+Mg as well as more than 30% of Fe are available. Microscopic observation provides another clue that the magnetic CRS occurs within the serpentine seams developed along the massive olivine grains in ophiolites. Observed Curie point of CRS ranges from ~200 to ~350C, permitting its magnetic preservation at depths up to 10-30 km in an Earth-like thermal gradient. In the future, CRS should not be neglected as a potential source of magnetic anomaly in planetary lithosphere.