Low-temperature magnetic behavior of isocubanite from seafloor hydrothermal deposits in the Okinawa Trough

Low-temperature magnetic behavior of isocubanite from seafloor hydrothermal deposits in the Okinawa Trough

Abstract

The characteristic behavior of magnetic remanence correlated with mineralogical textures and composition was observed using low-temperature magnetometry, microscopy, and chemical analysis of three isocubanite samples collected from hydrothermal deposits in the Okinawa Trough and a sample transformed from natural cubanite via heating. Both zero-field remanence acquired at five kelvin and field cooling remanence acquired at three hundred to five kelvin of all samples sharply decreased with increasing temperature at approximately one hundred kelvin. In addition, low-temperature cycling of isothermal remanence at three hundred kelvin exhibited a transition at approximately one hundred kelvin; remanence increased with decreasing temperature and vice versa. The intensity of remanence at low temperature and sharpness of the transition varied across samples with different compositions and microscopic textures, that is, the presence or absence of chalcopyrite lamellae and their widths. The sample obtained from a hydrothermal chimney, in which the magnetic transition was most clearly observed, was also subjected to X-ray diffraction, Mössbauer spectroscopy, electrical resistivity, and magnetic hysteresis measurements. The obtained results were generally consistent with those reported previously for unnamed mineral CuFe three S four with an ordered cation arrangement. The low-temperature magnetic behavior of isocubanite possibly depends on the degree of cation ordering and can be regarded as an indicator of chemical composition and cooling history. Therefore, low-temperature magnetometry is useful for the detection of isocubanite and a potentially powerful technique for the prompt estimation of its composition and texture, contributing to our understanding of the formation process of hydrothermal deposits.

Introduction

Recent geological surveys have revealed seafloor massive sulfide deposits, which have attracted attention as novel mineral resources. To explore the target areas of unknown ore deposits, geoscientific model of the formation process related to hydrothermal activity must be established. As seafloor massive sulfide deposits are considered modern analogs of volcanic massive sulfide deposits, similar mineralogical studies have been conducted. To obtain constraints on the history of temperature and chemical conditions, sulfur fugacity, mineral assemblages, chemical composition of individual minerals, and microstructure of ore deposits have been investigated.

Copper is not only an important industrial target but also a key element in understanding the history of sulfide ore deposits, as it forms various minerals that reflect the environment. As some of the metal sulfides in the copper-iron-sulfur system exhibit characteristic magnetic properties, the rock magnetic approach can provide information on the mineral species, composition, and texture of the constituent minerals through rapid and non-destructive measurements. In particular, low-temperature magnetic analysis is expected to be effective for samples from hydrothermal deposits, which are vulnerable to severe alteration by heating, and useful for detecting minerals that are paramagnetic at room temperature but undergo magnetic transitions at low temperatures.

In the present study, we focus on isocubanite, which is specific to active seafloor hydrothermal fields. Isocubanite (stoichiometric formula CuFe two S three) is a high-temperature polymorph of cubanite, stable at greater than two hundred ten to two hundred seventy degrees Celsius. In the literature, isocubanite is often referred to as "intermediate solid solution" owing to the wide compositional range of the copper-iron-sulfur ternary system at high temperatures. Isocubanite has a sphalerite-type face-centered cubic lattice structure, with copper and iron atoms randomly distributed at the cation sites.

Isocubanite can be obtained by heating natural cubanite in vacuum in the laboratory. This reaction is a structural transformation from orthorhombic to cubic as well as an order-disorder transition of the cation atoms. The transition is irreversible, and the metastable isocubanite can be quenched at approximately twenty-five degrees Celsius. When annealed at temperature slightly lower than two hundred degrees Celsius, fine lamellae of chalcopyrite emerge while the matrix remains disordered. According to Putnis, in metastable isocubanite, cation ordering within the cubic structure occurs instead of transition from isocubanite to cubanite, which is very sluggish at temperature as low as approximately two hundred degrees Celsius. This reaction results in an overall reduction in free energy. Since the ordered cation occupancy of copper to iron equals one to two is not geometrically feasible in the cubic lattice system, isocubanite breaks into chalcopyrite (CuFeS two) and unnamed mineral (CuFe three S four). The decomposition of cubanite upon heating to a very fine mixture of chalcopyrite and a cubic phase, with X-ray powder diffraction pattern very similar to that of chalcopyrite, was also reported by Kaneda et al. Lamellae of isocubanite and chalcopyrite are often found in natural samples.

The magnetism of isocubanite is complex. The Mössbauer spectrum of cubic cubanite (isocubanite) is not magnetically split at two hundred ninety-five kelvin, while it shows the evidence of magnetic ordering at low temperatures but cannot be resolved into simple sextet spectra. This complexity is explained by the randomness of the occupancy of cation sites by iron two plus, iron three plus, and copper one plus ions. Synthetic isocubanite shows paramagnetic behavior.

Wintenberger et al. reported the magnetic structure and transition of unnamed mineral CuFe three S four with chalcopyrite exsolution collected from the Snake Pit hydrothermal field, Mid-Atlantic Ridge at twenty-three degrees north. They reported that their sample showed ordered cation occupancy and a tetragonal crystal structure, in which the sphalerite-type cubic lattice of isocubanite is slightly distorted. Hereafter, the abbreviations cp for chalcopyrite and np, "new phase," for CuFe three S four are used, following Wintenberger et al. They also reported that upon cooling, three magnetic transitions were observed in np at approximately two hundred eighty-five, one hundred ninety, and one hundred five Kelvin. Moreover, a magnetic transition occurs below approximately one hundred five Kelvin, where np becomes ferrimagnetic, and its magnetic moment and electric resistivity increase dramatically. Since the chemical compositions and crystal structures of CuFe three S four and isocubanite are very close to each other, isocubanite with intermediate compositions may exhibit similar magnetic properties. In particular, it may be a useful indicator for isocubanite if a characteristic magnetic transition is recognized upon low-temperature magnetometry.

In the present study, using X-ray diffraction measurements, magnetic analyses, Mössbauer spectroscopy, electrical resistivity measurements, and chemical analysis, we investigated the magnetic properties of isocubanite samples collected from submarine hydrothermal deposits in the Okinawa Trough and a sample obtained in the laboratory by heating natural cubanite.