Mineralogy and magnetism of Fe-Cr spinel series minerals from podiform chromitites and dunites from Tąpadła (Sudetic ophiolite, SW Poland) and their relationship to palaeomagnetic results of the dunites

Abstract

S U M M A R YThis paper presents the mineralogy and magnetic properties of two varieties of chromitite, sampled in the same exposure of a serpentinite massif regarded to be a fragment of the Sudetic ophiolite. The varieties differ mineralogically and magnetically. One of them, labelled TaA and altered to a low degree according to scanning electron microscope (SEM) and microprobe results, comprises an unaltered Al-Cr spinel core, some secondary chromite and less abundant Cr-magnetite. It has a high magnetic susceptibility and natural remanence (NRM), and its dominating magnetic phase is ferrichromite with T b /T c of about 530 • C occurring in grains in the single domain (SD) + pseudo-single domains (PSDs) and superparamagnetic (SP) states. The second variety, labelled TaB, is highly altered and comprises, apart from Al spinel, two kinds of secondary chromites that do not differ much from the primary Al spinel and no Cr-magnetites. This chromitite variety does not reveal one well defined T b /T c , but a series ranging between 200 and 450 • C, their grain sizes correspond to the SD + SP domain states. The same pattern was observed in chromitite TaA after annealing in air at a temperature of 700 • C. Dunites associated with the chromitites contain, apart from magnetite, chromite grains. The majority of specimens reveal magnetic characteristics similar to the TaA-like variety, but in some specimens TaB-like characteristics were also observed. A standard palaeomagnetic study performed earlier on the dunites showed that their NRM has three components-Lower Devonian carried by magnetite, Permian and Tertiary-Recent. The results presented here lead us to conclude that the latter two are carried by ferrichromites similar to those found in the TaA and TaB chromitites.

Magnetic Cr‐Rich Spinel in Serpentinized Ultramafic Complexes

“…Cr‐rich spinel can entirely constitute the NRM (Figure 4) or it can dominate the NRM while coexisting with magnetite (Figure 5). In particular, Cr‐rich spinel often records T c of 200–350°C both in terrestrial (e.g., Hounslow, 1996; Kądziałko‐Hofmokl et al, 2008; Kumar & Bhalla, 1984a; Murthy & Krishnamacharyulu, 1994; O'Driscoll & Petronis, 2009; Xu et al, 1997; Yu et al, 2001) and in extraterrestrial (e.g., Yu & Gee, 2005) materials. In this study, Group B spinel shows maximum unblocking temperatures of 200–280°C.…”

Magnetic Properties of Ferritchromite and Cr‐Magnetite and Monitoring of Cr‐Spinels Alteration in Ultramafic and Mafic Rocks

“…This limit (n = 0.6) has therefore been applied to define the superior compositional boundary of ferritchromite in terms of Cr content ( Figure 1a). Kądziałko-Hofmokl et al (2008) defined the Cr-magnetite according to its magnetic properties, that is, a ferritchromite with a magnetic behavior close to that of magnetite in terms of saturation magnetization and T c . Yet, Cr-magnetite is still not defined in terms of its chemical composition, and a better constraint on this transition and the associated n value is needed.…”

The inverse microconglomerate test: Further evidence for the preservation of Hadean magnetizations in metasediments of the Jack Hills, Western Australia

“…Previous studies of synthetics indicate that Cr‐Fe spinels can have a range of Curie temperatures [ Robbins et al , ]. Altered phases in nature commonly have Curie temperatures that range from 120 to 450°C [ Kądziałko‐Hofmokl et al , ]. Therefore, these particles are ideal for the inverse microconglomerate test because they should have been reset by the peak metamorphic temperatures experienced by the JH sediments.…”