Magnetic Cr‐Rich Spinel in Serpentinized Ultramafic Complexes

Abstract

In 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]/[Fe 2+ + Fe 3+ ] from 1.33 to 1.56 as well as a low oxide ratio of (Al 2 O 3 + MgO)/ (Cr 2 O 3 + FeO + Fe 2 O 3) 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.

Focal Mechanisms of Intraslab Earthquakes: Insights From Pseudotachylytes in Mantle Units

“…Previous studies reported that chrome‐rich spinel is ferrimagnetic (Kądziałko‐Hofmokl et al., 2008; Rais et al., 2003), however, this phase generally owes its magnetization to magnetite inclusions or exsolution features, while common chrome‐rich spinels are paramagnetic above 70 K at 1 atm (Gattacceca et al., 2011). Furthermore, chrome‐rich spinel is ferrimagnetic only if (Al 2 O 3 + MgO)/(Cr 2 O 3 + FeO + Fe 2 O 3 ) < 0.1 (Yu & Tikoff, 2020), a condition that is not met in the Cima di Gratera chrome spinels (Andersen & Austrheim, 2006). Hence chrome‐rich spinel is paramagnetic in these pseudotachylytes.…”

Weathering-driven porosity generation in altered oceanic peridotites

“…Natural samples of oceanic peridotites are typically heavily altered and the primary textures and mineralogic associations are not preserved, except for spinel (Yu and Tikoff, 2020). Alteration results from the chemical and mineralogic changes that occur during the interaction between ultramafic rocks and seawater and/or hydrothermal fluids.…”

Holocene Paleosecular Variations Recorded by Relict Magnetic Minerals in the Anoxic Black Sea Sediments

“…Hounslow (1996) reported that ferrimagnetic Fe-Cr and Fe-Mn spinels are the dominant remanence carrying phases in the Lunde Formation, North Sea. And the Fe-Cr spinel is also a potential carrier of a natural remanent magnetization (NRM) and a source of magnetic anomalies in ultramafic complexes (Yu & Tikoff, 2020). From sediments which are dominated by ferrous ilmenites, (titano-)magnetite, Cr-Fe spinels, and impure magnetite, Scheidt et al (2017) also reconstructed a reliable magnetic polarity stratigraphy.…”