QUANTITATIVE EXTRACTION AND ANALYSIS OF CARRIERS OF MAGNETIZATION IN SEDIMENTS

We have applied an integrated procedure for quantitative magnetic mine ral extraction, based on the separation method of Petersen, von Dobene ck & Vali (1986), to a range of sediment types, to examine the efficie ncy and representative nature of the extraction process. Carriers of m agnetization have been identified by rock magnetic measurements, micro scopy and X-ray diffraction. Quantification of the extraction efficien cies is achieved by before- and after-extraction rock magnetic measure ments (susceptibility, anhysteretic and isothermal remanences). These magnetic measurements show that our modified extraction method extract s large proportions of the magnetization carriers in a range of sedime nt types (e.g. over 75 per cent for magnetite-dominated sediments). Th e extraction efficiency is dependent on the sample magnetic mineralogy and whether the magnetic grains occur as discrete grains or as inclus ions within host grains. Susceptibility extraction efficiencies are st rongly dependent on whether the susceptibility is of paramagnetic or f errimagnetic origin. The amount of material recovered in the extract s hows some inverse correlation with the density of the sediment suspens ion used during extraction. In terms of the mineralogies extracted, we identify a diverse and complex range of mineral assemblages. All size s of discrete grains of magnetite are extracted (including single-doma in and superparamagnetic grains, and chains of bacterial magnetite). O ther commonly extracted iron and titanium oxides are haematite and ilm enite. Ferrimagnetic chromites and sulphides were also obtained from s ome 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 constitu te a large proportion of the extracts, but are significantly less abun dant in other sediments, where quartz is dominant. A wide variety of p aramagnetic minerals was identified in the extracts, including pyroxen es, amphiboles, chlorites, micas, Mg-Cr-spinels, garnets, Ti-oxides, a patites, tourmaline and zircon, many of which contain ferrimagnetic in clusions, possibly less than 0.1 mu m in grain size. Dissolution of ul trafine grains of magnetite during pre-extraction carbonate dissolutio n, as suggested by Sun & Jackson (1994), does not occur in our samples .