Takahashi, and 5 co-authors (2002) reported that abundances of rare earth elements plus Y (REE) were determined for granitic rocks, Tertiary sedimentary rocks, and related groundwater in the Tono area, central Japan. Tetrad effects, concave curves in REE patterns, were found for these rocks and groundwater samples. Conjugate M- and W-types of tetrad effects were simultaneously observed in these specimens which constitute the water–rock system; the granitic rocks show the M-type tetrad effects while the groundwater and the sedimentary rocks exhibit the W-type tetrad effects.
Non-chondritic Y/Ho ratios were found especially in the groundwater; Y/Ho ratios showed larger values than the chondritic value for the samples showing W-type tetrad effects, and vice versa. These results suggest that:
(1) preference of the groundwater for a W-type tetrad effect and larger Y/Ho ratio than chondrite produces an M-type tetrad effects and lower Y/Ho ratios in the granitic rocks during weathering processes;
(2) the W-type tetrad effect and larger Y/Ho ratio for the sedimentary rocks is brought about by the fixation of groundwater REE onto the sedimentary rocks. The variation of the tetrad effect and Y/Ho ratio was confirmed by laboratory experiments on the leaching of REE from the granitic rocks. The W-type tetrad effect and large Y/Ho ratio in the U-mineralization zone in the sedimentary rocks were also found by SIMS analysis, which is consistent with the W-type tetrad effect and larger Y/Ho ratio in the whole rock abundances of the sedimentary rocks.
Disequilibrium of the U series (234U/238U), analyzed by SIMS in the U-mineralization zone, suggests that the U-bearing minerals were formed through precipitation from groundwater as it became reducing. These results suggest that the REE in the sedimentary rocks were transported by the groundwater from the granitic rocks in the Tono area. Based on this study, it is implied that the degree of the tetrad effects and the variation of Y/Ho ratio can be valuable tools to investigate the migration of REE during water–rock interactions in natural systems.
The systematics of the tetrad effects and Y/Ho ratios were observed in the Tono area and their implications summarizes the averaged values of Y/Ho ratios and the magnitudes of tetrad effects for the weathered granitic rocks obtained from core samples (KNA-6 and BH-1), the groundwater with and without particulate matter, the sedimentary rocks, and its U-mineralized zone.
It should be noted that conjugate W- and M-type tetrad effects were observed in the Tono area. The variation of Y/Ho ratios was consistent with that of the magnitude of the tetrad effect. The latter, expressed by (Dy)CN/(Dy*)CN, was plotted against the Y/Ho ratio for the granitic rocks, the groundwater with and without particulate matter, the sedimentary rocks, and the U-mineralized zone. These results allow us to speculate about the migration of REE in the Tono area.
The groundwater leaches REE from the granitic rocks, where W-type tetrad effects and higher Y/Ho ratios than chondrite are produced in the groundwater. The weathered granitic rocks, the residue of the water–rock interaction, exhibit M-type tetrad effects and slightly lower Y/Ho ratios. The REE in the U-mineralized zone preserve W-type tetrad effects and higher Y/Ho ratios, transmitted from groundwater. This influences the REE pattern of the bulk sedimentary rocks, which consequently have W-type tetrad effects and slightly higher Y/Ho ratios. In these processes, the preference of groundwater for W-type tetrad effects and larger Y/Ho ratios is primarily responsible for producing REE variations in the granitic rocks and the sedimentary rocks in the Tono area, from which we can assess the REE behavior.
Irrespective of these systematic variation of Dy/Dy*, further study is needed for the points below. Firstly, we need to be careful to discern the extent of tetrad effect in REE patterns if it is slightly observed. Secondly, we need to quantify the contribution of the effect through granite crystallization at its late stage to the degree of M-type tetrad effect found in granite samples.
The original working URL: https://www.sciencedirect.com/science/article/pii/S0009254101003886
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