Magnetic resonance imaging of sea-ice pore fluids: methods and thermal evolution of pore microstructure

Microstructure and thermal evolution of sea-ice brine inclusions were investigated with magnetic resonance imaging Ž . 1 MRI techniques. Ice samples were kept at temperatures between y28C and y258C during H imaging in a 4.7-T magnet Ž at 200 MHz. Measurements were completed in a 20-cm diameter cylindrical probe and actively shielded gradient coils max. y1 . 50 mT m , pixel dimensions )0.2 mm, slice thicknesses )1 mm , and for higher resolution in a mini-imaging unit with y1 Ž . a 9-cm diameter probe with gradient coils of 200 mT m pixel dimensions -0.1 mm, slice thickness -0.4 mm . Ž . Absorption of radio-frequency RF signals in the dielectrically lossy brine resulted in degraded signals and was alleviated Ž . Ž by use of a contrast agent decane . MRI data and sea-ice thin section images agree very well -5% deviation for pore . microstructural parameters . Analysis of ice grown under different current speeds indicates that pores are smaller and pore number densities larger at higher current speeds. The thermal evolution of fluid inclusions was studied on cold first-year ice samples, maintained at close to in-situ temperatures prior to experiments. Warming from y218C to y108C to y68C is Ž . associated with a distinct increase in pore size from 1.5 to 1.7 to 2.6 mm for the upper 10-percentile in the vertical and Ž . Ž elongation 4.0 to 4.2 to 6.2 for ratio of major to minor pore axes in the vertical and a decrease in number densities 0.75 to y3 . Ž . 0.62 to 0.58 mm in the vertical . Aspect ratios increased from 4:2:1 to 6:2:1 upper 10-percentile , indicating expansion and merging of pores in the vertical, possibly promoted by microscopic residual brine inclusions. q 2000 Elsevier Science B.V. All rights reserved.