Towards the identification of siderite, rhodochrosite, and vivianite in sediments by their low-temperature magnetic properties

Abstract

Siderite (FeCO 3), rhodochrosite (MnCO 3), and vivianite ([Fe 3(PO 4) 2] · 8 H 2O) are well-known authigenic minerals in a number of sedimentary settings. Here, we explore the potential of low-temperature mineral magnetic techniques for their identification at low concentration in bulk samples thus expanding mineral magnetic proxies for environmental purposes. The basic rock magnetic properties of these minerals, which are paramagnetic at ambient temperature, were determined with a ‘Magnetic Properties Measurement System’. Well-crystalline chemically analyzed material of natural origin was used to gather these data. The diagnostic value of the observed specific magnetic properties was tested on two mid-Eocene sediment samples from the Norwegian Sea (ODP Leg 104, Site 643) known to contain these minerals. The observed Néel temperatures of siderite (37 K) and rhodochrosite (34 K) conform with literature data. Both carbonates show a fairly strong spin-canted remanence (∼0.4 Am 2/kg) from cooling in a 5 T magnetic field. Different ratios of field-cooled and zero-field-cooled remanences, however, allow a discrimination between the two minerals. A characteristic of rhodochrosite is its extremely high magnetic susceptibility just below the Néel temperature. An almost vertical slope in very low fields of the hysteresis loop also testifies to this high susceptibility. It is assigned to a weak anisotropic ferromagnetism confined to the basal plane in which the spontaneous magnetization can almost freely rotate. A prominent magnetic property of siderite is its metamagnetism, resulting in a progressively upward bending of the hysteresis curve in magnetic fields above 5 T. Vivianite also shows an onset of metamagnetic transition below 5 K in 5 T fields and a ‘two-stage’ increase in susceptibility between 2 and 12 K attributed to successive short- and long-range magnetic ordering. The magnetic properties of the two authigenic marine sediments could be largely explained by combining characteristics of rhodochrosite and manganosiderite. Shifting of the Néel points to lower temperatures and less well pronounced magnetic phenomena are attributed to element substitution and non-stoichiometry which occur commonly in sedimentary environments.