Introduction
Fifty-seven samples of manganese nodules from the GH78-1 area were analysed in bulk rock for major and some minor metal elements to provide the data for the evaluation of the chemical features of the nodules. Sample preparation was carried on following those for previous cruises (e.g. MORITANI et al., 1979; Fujinuki et al., 1977). While the analyses for Mn, Fe, Co, Ni, Cu, Zn and Pb were done by atomic absorption spectrometry (Terashima, 1978). Outline of the precedure is given below. Air-dried sample of 0.1 g was decomposed with a mixture of 5 ml of hydro- chloric acid and 5 ml of nitric acid in a covered 50 ml beaker on a boiling bath for about 30 minutes. After cooling, the solution was filtered into a 100 ml calibrated flask and diluted to the mark with water. The atomic absorbance of each element in the sample solution was measured by atomic absorption spectrometer using an air-acetylene flame. The concentra- tion of each element was determined from a calibration graph, which was prepared using a series of synthetic standard solutions that has a composition similar to the sample solution. The total water was analyzed gravimetrically as reported in the previous work (FUJINUKI et al., 1977).
Results and discussion
Table XIV-1 shows the contents of each element including total water and Mn/Fe ratio, and Table XIV-2 does average and standard deviation of each element except water, with regard to three morphological types of manganese nodules classified into smooth, intermediate and rough ones. In Table XIV-1, intermediate type of surface structure is noted by the mark "s r" attached to the capital letter or letters indicating the shape of manganese nodules. Relations between nodule chemistry and morphological type Among those elements analyzed, Fe, Co and Pb are apt to have higher con¬ centrations in the nodules of smooth surface structure type, while Mn, Ni, Cu and Zn are in rough type, as shown numerically by the ratios of averages of each element between rough and smooth types. This tendency is also shown in Fig. XIV-1(1)-(4) for Co, Zn, Pb and Cu. Although Mn is not a typical element of which concentration shows close rclation to the surface structure type of manganese nodules, Mn/Fe ratio indi- cates closer relation as shown in Fig. XIV-1(6) and Fig. XIV-2. Mn/Fe ratio is 1.50 in average, 0.94 at the minimum and 4.02 at the maximum (Table

— 227 —
(Continued able XIV-1

— 228 —

XIV-3). Almost all samples of the smooth type show rather low Mn/Fe ratio less than 2, while all those of rough type shows high value more than 2 (Fig. XIV-2). Another chemical aspect distinguished with regard to surface structure types of the nodule is Ni plus Cu grade. Although that difference is somewhat unclear because of the bulk rock analysis, but still there is recognized a characteristic relation between the grade and occurrence of nodule, i.e. as high grade-low abundance or low grade-high abundance (Fig. XIV-3). Fig. XIV-3 suggests that the morphology (shape) of the nodule is not so important factor related to Ni plus Cu grade, excepting that the nodules having IS or IDP shapes show invariably rather low grade of Ni plus Cu less than 1.0%. Relations among some elements Table XIV-3 shows average, minimum and maximum values of some ratios
— 229 —
between elements, in addition to Ni plus Cu grade. While, the relation of Ni and Cu contents to Mn/Fe ratio, and relation between Fe and Co are shown in Figs. XIV-4 and XIV-5 respectively. Those figures represent intensely posi- tive correlations between Ni or Cu content and Mn/Fe ratio, and Fe and Co, in the same manner as those in our previous works (Fujinuki et al., 1977 and Moritani et al., 1979). The positive correlation between Fe and Co had tended to lead us to an assumption of Co substitution for Fe (e.g. ARRHENIUS et al., 1964). According to Usui (1979), however, Co substitution for Mn in 8-MnO 2 phase, which was demonstrated synthetically by Burns (1976), is more reason- able to interprete that phenomenon. Areal distribution of nodule chemistry Fig. XIV-6 shows distribution of Mn/Fe ratio in manganese nodules in this area (represented as the average at each station) together with the topography and surface sediment facies (Nakao and Suzuki, 1981), including the data for three stations in the GH77-1 area. Mn/Fe ratio is selected here as the representative factor which well indicates the chemical characteristics of manga- nese nodules regardless of the kind of analytical data base.
- 231 -
Though the data are not enough to discuss the relation between sedimentary facies or topography and Mn/Fe ratio of the nodules, it is indicated that relatively high Mn/Fe ratio (> 2) appears dominantly in the southern part of this area, to the south of the parallel of latitude, 9°N, and it may be possible to point out the following suggestions. Topographic highs, like a ridge, seem to be not favorable place, for (a) the distribution of high Mn/Fe nodules. Parallel distribution of high Mn/Fe nodules to the topography is ex- (b) pected in deeper sites. Siliceous surface sediment including siliceous ooze seems to have some (c) genetic relation to high Mn/Fe nodules. 15 N!(%)
Cu(%) Arrhenius, G., Mero, G. and Korkisch, J. (1964) Origin of oceanic man- ganese minerals. Science, vol. 144, p. 170-171. Burns, R. G. (1976) The uptake of cobalt into ferromanganese nodules, soils, and manganese (IV) oxides. Geochim. Cosmochim. Acta, vol. 40, p. 95- 102. Fujinuki, T., Mochizuki, T. and Moritani, T. (1977) Chemical composi- tion of manganese nodules. In A. Mizuno and T. Moritani (eds.), Geol. Surv. Japan Cruise Rept., no. 8, p. 162-171. Moritani, T., Mochizuki, T., Terashima, S. and Maruyama, S. (1979) Metal contents of manganese nodules from the GH77-1 area. In T. Mori- TANI (ed.), Geol. Surv. Japan Cruise Rept., no. 12, p. 206-217. Nakao, S. and Suzuki, T. (1981) Bottom sediments in the GH78-1 area. In T. MORITANI and S. NAKAO (eds.), Geol. Surv. Japan Cruise Rept., no. 17, p. 75-102. Atomic absorption analysis of Mn, Fe, Cu, Ni, Co, Terashima, S. (1978) Pb, Zn, Si, Al, Ca, Mg, Na, K, Ti, and Sr in manganese nodules. Bull. Geol. Surv. Japan, vol. 29, p. 401-411. Usui, A. (1979) Minerals, metal contents, and mechanism of formation of manganese nodules from the Central Pacific Basin (GH76-1 and GH77-1 areas). In J. L. Bishoff and D. Z. Piper (eds.), Marine Geology and Oceanography of the Pacific Manganese Nodule Province, Plenum Publ. Corp., p. 651-679.