HR: 0830h
AN: GP11B-0259    [PDF]
TI: Magnetic Field Gradient Differentiation of Pedogenic Iron Oxide Minerals From Chinese Loess and Paleosols
AU: * Wagoner, L
EM: Wagoner@geology.ucdavis.edu
AF: University of California, Davis, Dept. of Geology One Shields Ave., Davis, CA 95616 United States
AU: Roth, A
EM: alroth@ucdavis.edu
AF: University of California, Davis, Dept. of Geology One Shields Ave., Davis, CA 95616 United States
AU: Singer, M J
EM: mjsinger@ucdavis.edu
AF: University of California, Davis, Dept. of Land, Air, and Water Resources One Shields Ave., Davis, CA 95616 United States
AU: Verosub, K
EM: Verosub@geology.ucdavis.edu
AF: University of California, Davis, Dept. of Geology One Shields Ave., Davis, CA 95616 United States
AB: The correlation between paleosols and enhanced magnetic susceptibility on the Chinese Loess Plateau is by now well established. However, scant effort has focussed on the interpretation of paleoclimate via the specific iron oxide mineral assemblages contributing to the enhanced magnetic susceptibility signal. This paper focuses on the separation and identification of the pedogenic ($<$ 1 micron) fraction of iron oxide/oxyhydroxide minerals from selected loess and paleosol layers of the Loess Plateau. Heretofore, it has been difficult if not impossible to isolate mixed iron oxide mineral phases due to their very similar physical and magnetic properties. Chinese loess and paleosol samples were chosen to illustrate the utility of the technique to natural soil systems. In the following method, initial size separation of mineral particles at 0.5 micron or less by gravity and centrifugation reduces the problem of overlapping magnetic susceptibilities due to mixed grain sizes. The submicron mineral fraction is then subjected to a series of high field gradient (HFG) magnetic separations utilizing a new design. Although HFG magnetic separation methods have been used before, the new design is able to differentiate submicron iron oxide mineral phases from bulk earth material. The design includes a Franz Isodynamic Separator fitted with a custom-made flow cell. A recirculating liquid is used to suspend the mineral particles between the poles of the electromagnet. By varying the strength of the field gradient, recirculation time, and flow velocity, step-wise separation of ferrimagnetic from antiferromagnetic minerals is possible. Because of the tendency for particles to aggregate during recirculation, some mixing of the oxide mineralogy has been unavoidable. Although theoretical arguments favor a narrow grain size distribution (about 50-100 nm) for stable single domain magnetite, in soil environments, and particularly for nanoscale materials, discrete particles are the exception rather than the rule. Therefore it is likely that nanoscale iron oxide minerals occur as aggregates of oxide and clay mineral particulates. To test this, HFG mineral separates were examined by SEM for morphologic identification. Although synthesized iron oxide minerals show good crystal habit, soil formed minerals do not. Energy dispersive XRF was used in conjunction with SEM for qualitative geochemical identification of oxide and aluminosilicate mineral phases. Further work is necessary to refine the HFG method to reduce aggregation and increase the separation efficiency of minerals with very similar susceptibilities. However, this is a first step toward the goal of comparing pedogenic iron oxide formation to soil paleoenvironmental conditions. With continued progress, it may be possible to correlate specific climatic properties to the resulting (stable) oxide mineral assemblages. The use of pedogenic iron oxides as paleoclimatic proxies will be broadened when the physico-chemical conditions of mineral crystallization, along with magnetic susceptibility, can be brought to bear.
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting