Magnetic Cr‐Rich Spinel in Serpentinized Ultramafic Complexes

Abstract

AbstractIn comparison to the six spinel end‐members in the spinel prism, magnetic information on the intermediate spinel compositions is relatively poorly established. To investigate magnetic properties of intermediate compositions of Cr‐rich spinel, we collected 12 samples that might contain potentially magnetic Cr‐rich spinel. On the basis of chemical composition analysis and physical magnetic behavior, magnetic Group A Fe‐rich spinel (i.e., magnetite), magnetic Group B Cr‐rich spinel, and paramagnetic Group C Cr‐Al‐rich spinel were identified. Magnetite is solely responsible for the natural remanent magnetization (NRM) in severely serpentinized samples from intraoceanic island arcs, orogenic exposures of ultramafics, and back‐arc continental lithosphere settings. Magnetic Group B Cr‐rich spinel shows maximum unblocking temperatures of 200–280°C. Such temperatures permit Cr‐rich spinel to contribute to magnetic anomalies up to about 8–12 km in terrestrial lithosphere settings, given a normal geothermal gradient of 25 K/km. The existence of magnetic Cr‐rich spinel requires certain compositional conditions including a cation ratio of [Cr]/[Fe2+ + Fe3+] from 1.33 to 1.56 as well as a low oxide ratio of (Al2O3 + MgO)/(Cr2O3 + FeO + Fe2O3) less than 10%. It is evident that compositions of spinels are related with the degree of serpentinization as Group A spinel is observed along the fractures between/among olivine grains in heavily serpentinized rocks. Spinels in Groups B and C seem to experience less severe metasomatic or hydrothermal alteration during serpentinization. Distribution of magnetic Cr‐rich spinel along the fractures of silicates (mostly olivine) may support a chemical origin of the NRM. Thus, Cr‐rich spinel is a potential NRM carrier and a source of magnetic anomalies in ultramafic complexes.