Some authigenous phases in sediments are formed immediately after deposition at bottom water-sediment interface recording the conditions of sediment formation. The main goals of this work are to study (1) rare earth and minor elements composition in authigenous phases (micronodules (MN), fish bone apatite and phillipsite) of pelagic sediments and (2) the influence of the formation conditions of sediments on authigenous minerals composition.
The sediments of station 35 (29 o 36' S, 149 o 58' W, depth 3880 m) were recovered by piston corer and presented by coccolith-foraminiferal oozes (0-10 cm) with nodules. Under the hydrogenous crust (10-12 cm) the pelagic non-carbonate zeolitic clays (12-190 cm) with three horizons (64-68, 158-159 and 165-166 cm) of the small (10-15 mm) nodules were found. MN, fish bone debris and phillipsite were obtained from sediments (hor. 37-40, 105-110, 165-175 and 189-190 cm) by using wash residues (size fraction >0.05 mm). After drying at 60-70o C they were hand-picked using binocular. The REE contents were determined by inductively coupled plasma source mass-spectrometry (Dubinin, 1993).
Chemical composition of sediments supports the results of lithological investigations. The amount of phillipsite and MN in size fraction >0.05 mm are decreased with the depth from 9.2% to 1.2% of total sediment mass. This is accompanied by the decrease of Al content and the simultaneous increase of Fe and Mn content and is resulted in hydrothermal input to sediments identified by Al/(Al+Mn+Fe) criterion (Bostrom, 1973). The most interest feature of sediments is the great accumulation of REE (La up to 411 ppm). Their composition with negative Ce anomaly (Ce/Ce*=0.47-0.36) also supports the hydrothermal material. REE composition in the fish bone apatite has ocean water like pattern. The sum of REE contents reaches 0.93% (hor. 37-40 cm) and is decreased down to two times. The decrease of apatite amount down in sediment size fraction >0.05 mm together with their REE content argues for the increase of sedimentation rate. The REE composition of phillipsites from hor. 37-40 cm has shale-like pattern. Negative Ce anomaly was detected in phillipsite from other three horizons reflecting the admixture of hydrothermal Fe oxyhydroxides. With increasing depth and hydrothermal component amount in sediments there are Fe (from 10 to 20%), P (from 0.46 to 1.16%) and trivalent REE content increase and Ce and Co decrease in MN composition. The most specific feature of MN is the dependence of REE composition from MN size fraction in sediments with hydrothermal input. There are the gradual increase of Ce anomaly and decrease of Ln/Fe ratio from fine to coarse MN fraction. The differentiation between REE composition of MN from one horizon depends on hydrothermal/hydrogenous oxyhydroxide ratio in sediments and sedimentation rate. In sediments without hydrothermal input there are no variations in REE compositions between MN fractions.
Bostrom, K., Stockholm Contrib. Geol. 27, 149-243 (1973).
Dubinin, A.V., Geochemistry N11, 1605-1619 (1993).