Rare earth element geochemistry of hydrothermal deposits and Calyptogena shell from the Iheya Ridge vent field, Okinawa Trough
Rare earth element geochemistry of hydrothermal deposits and Calyptogena shell from the Iheya Ridge vent field, Okinawa Trough
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DOI：
10.2343/geochemj.35.347
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发表时间：
2001-10
影响因子：
0.8
通讯作者：
Yayoi Hongo;Y. Nozaki
中科院分区：
文献类型：
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作者：
Yayoi Hongo;Y. Nozaki
尽管热液系统的稀土元素（REE）地球化学在大洋中脊已得到深入研究，但在像冲绳海槽这样的汇聚板块区域的研究相对较少。利用“深海2000”号潜水器从冲绳海槽伊是名海岭采集的“烟囱”状矿床主要由块状碳酸盐和硅酸盐组成，含少量硫化物，根据其成分呈现黑色和白色。富含硫化物的黑色部分的稀土元素浓度比白色部分高2 - 3个数量级。两种矿床都显示出明显的正铕异常和轻稀土富集，这是无沉积物大洋中脊喷出的高温流体的特征。然而，冲绳海槽的稀土元素数据在热液喷口系统中是独特的，对于比铽重的稀土元素具有平坦的球粒陨石标准化模式。白色矿床的铕异常比黑色矿床更显著，这表明在热液溶液中，Eu²⁺比严格的三价稀土元素更具迁移性（不易被硫化物清除），并且可能通过替代方解石晶格中的Sr²⁺而进入白色矿床。深海偏顶蛤的贝壳显示出典型的海水稀土元素模式，没有任何正铕异常，表明热液溶液的影响可以忽略不计。尽管轻稀土浓度相当，但贝壳中的重稀土浓度明显高于白色矿床。这表明形成碳酸盐的热液溶液中重稀土的浓度可能低于周围海水。因此，冲绳海槽的热液系统可能不是海水中溶解稀土元素的来源，而是其汇。
Although the rare earth element (REE) geochemistry of hydrothermal systems have intensively been investigated at mid-oceanic ridges, studies are relatively few at the converging plate zones like the Okinawa Trough. The “chimney” deposits obtained by using submersible “SHINKAI 2000” from the Iheya Ridge site, Okinawa Trough are largely composed of massive carbonates and silicates with minor amounts of sulfides, and appear black and white depending upon their composition. The concentrations of REEs are 2–3 orders of magnitude higher in the sulfide-rich black portion than in the white portion. Both deposits show a pronounced positive Eu anomaly and the light REE enrichment which are characteristic of the high-temperature fluids venting from sediment-free oceanic ridges. Whilst the Okinawa Trough REE data are unique in the hydrothermal vent systems in having a flat chondrite-normalized pattern for the REEs heavier than Tb. The Eu-anomaly is greater in the white deposit than in the black deposit, suggesting that Eu2+ is more mobile (resistant to scavenging by sulfides) than stricktly trivalent REEs in the hydrothermal solution and may be incorporated into the former by substitution of Sr2+ in the calcite lattice. The Calyptogena shell shows the REE pattern typical for seawater without any positive Eu anomaly, suggesting that the influence of hydrothermal solution is negligible. Although the light REE concentrations are comparable, the heavy REEs are significantly higher in the shell than in the white deposit. This suggests that the concentrations of the heavy REEs in the carbonate-forming hydrothermal solution may be lower than those of ambient seawater. Thus, the hydrothermal system in the Okinawa Trough may not be a source but a sink for dissolved REEs in seawater.