Quantitative extraction and analysis of carriers of magnetization in sediments

We have applied an integrated procedure for quantitative magnetic mineral extraction, based on the separation method of Petersen, von Dobeneck & Vali (1986), to a range of sediment types, to examine the efficiency and representative nature of the extraction process. Carriers of magnetization have been identified by rock magnetic measurements, microscopy and X-ray diffraction. Quantification of the extraction efficiencies is achieved by before- and after-extraction rock magnetic measurements (susceptibility, anhysteretic and isothermal remanences). These magnetic measurements show that our modified extraction method extracts large proportions of the magnetization carriers in a range of sediment types (e.g. over 75 per cent for magnetite-dominated sediments). The extraction efficiency is dependent on the sample magnetic mineralogy and whether the magnetic grains occur as discrete grains or as inclusions within host grains. Susceptibility extraction efficiencies are strongly dependent on whether the susceptibility is of paramagnetic or ferrimagnetic origin. The amount of material recovered in the extract shows some inverse correlation with the density of the sediment suspension used during extraction. In terms of the mineralogies extracted, we identify a diverse and complex range of mineral assemblages. All sizes of discrete grains of magnetite are extracted (including single-domain and superparamagnetic grains, and chains of bacterial magnetite). Other commonly extracted iron and titanium oxides are haematite and ilmenite. Ferrimagnetic chromites and sulphides were also obtained from some samples. Considerable amounts of quartz and feldspar are extracted, due to the presence of magnetic inclusions within these diamagnetic host grains. In the deep-sea sediments we examined, feldspars constitute a large proportion of the extracts, but are significantly less abundant in other sediments, where quartz is dominant. A wide variety of paramagnetic minerals was identified in the extracts, including pyroxenes, amphiboles, chlorites, micas, Mg–Cr-spinels, garnets, Ti-oxides, apatites, tourmaline and zircon, many of which contain ferrimagnetic inclusions, possibly less than 0.1 μm in grain size. Dissolution of ultrafine grains of magnetite during pre-extraction carbonate dissolution, as suggested by Sun & Jackson (1994), does not occur in our samples.