Geochemical characteristics and genesis of the polymetallic nodules in the Pacific survey area

doi:10.13745/j.esf.sf.2020.6.37

Abstract

Abstract:

The polymetallic nodules in China’s polymetallic nodules exploration contract area in the East Pacific Basin (EP nodules) are largely buried or semi-buried in a sedimentary environment dominated by clay and siliceous components. The manganese mineral phases of the EP nodules mainly include vernadite and todorokite, with high REY and high Cu and Ni contents and Mn/Fe ratio. These characteristics indicate the nodules are affected by interstitial water thus falling into the range of diagenesis and hydrogenesis. In comparison, the polymetallic nodules in the Northwest Pacific Basin (WP nodules) are mostly exposed to seawater, and the surrounding sediments are mainly deep-sea clay. The manganese mineral phases of the WP nodules are almost all vernadite, with high REY, high Co content and low Mn/Fe ratio. It shows that the WP nodules are of typical hydrogenic form. The NASC-normalized REY patterns of the polymetallic nodules in both regions show positive Ce and negative Y anomalies and no/weak Eu anomaly, while the REY patterns of ambient seawater show obvious negative Ce and positive Y anomalies and no obvious Eu anomaly. This inverse coupling relationship implies that the REY patterns of the polymetallic nodules are the result of selective enrichment from seawater. The WP nodules have higher Ce content and δCe than the EP nodules. Both Co and Ti correlate positively with Ce, which may collectively affect the selection of Ce and Y in seawater by polymetallic nodules. In this study, dissolved oxygen in seawater was not a factor controlling the level of positive Ce anomaly observed in the polymetallic nodules. Rather, elements such as Co and Ti and their related components can cause strong fractionation of Ce and other REY, which may be the controlling factor. The surface samples of polymetallic nodules and cobalt-rich crusts in the study area, with ε_Nd values ranging from -6.6 to -2.5, represent the most radioactive ferromanganese samples of Nd in the global sea area. Combining the ε_Nd result with REY patterns and Eu anomaly, we suggest that REY in polymetallic nodules mainly come from the surrounding continental crust with relatively high ε_Nd and can be transported to the ocean via rivers or atmospheric deposition. However, the radioactive Nd isotopes released from basalt in the Northwest Pacific Basin have only weak effect on the seawater.

Key words: polymetallic nodules, Co-rich crusts, rare earth elements, seamounts in western Pacific Ocean, Clarion-Clipperton region

CLC Number:

P736.4
P744

REN Jiangbo, DENG Yinan, LAI Peixin, HE Gaowen, WANG Fenlian, YAO Huiqiang, DENG Xiguang, LIU Yonggang. Geochemical characteristics and genesis of the polymetallic nodules in the Pacific survey area[J]. Earth Science Frontiers, 2021, 28(2): 412-425.

Figures/Tables 11

References 60

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样品	组分含量/%
样品	δ-MnO₂	钡镁锰矿	水钠锰矿	石英	斜长石	正长石	钙十字沸石
东太结核均值	52.1	27.0	0.6	3.1	4.9		12.4
WBC1404-2	49.3	27.2		2.7	2.9		17.9
WBC1419	41.2	35.6	1.1	3.5	16.4		2.3
WBC1423	61.4	24.4	0.8	2.5	3.5		7.4
PRZ1304-BC16	30.9	16.8		5.5	2.3		44.5
西太结核均值	59.0	8.3		9.4	6.5	1.1	12.0
MABC09	81.2			5.8	8.9		4.1
MABC17	79.8			5.5		12.1	2.6
MABD01	38.5	33.2		7.3		8.7	12.3
NBMC01	45.3	15.5		14.8	20.9		3.5

样品	组分含量/%
样品	δ-MnO₂	钡镁锰矿	水钠锰矿	石英	斜长石	正长石	钙十字沸石
东太结核均值	52.1	27.0	0.6	3.1	4.9		12.4
WBC1404-2	49.3	27.2		2.7	2.9		17.9
WBC1419	41.2	35.6	1.1	3.5	16.4		2.3
WBC1423	61.4	24.4	0.8	2.5	3.5		7.4
PRZ1304-BC16	30.9	16.8		5.5	2.3		44.5
西太结核均值	59.0	8.3		9.4	6.5	1.1	12.0
MABC09	81.2			5.8	8.9		4.1
MABC17	79.8			5.5		12.1	2.6
MABD01	38.5	33.2		7.3		8.7	12.3
NBMC01	45.3	15.5		14.8	20.9		3.5

样品	参数值类型	w_B/%										w_B/10^-6
		Al₂O₃	SiO₂	P₂O₅	CaO	TiO₂	Mn	Fe	Co	Ni	Cu	Ce	∑REY	δCe	Y/Ho
东太结核	最小值	3.30	12.68	0.30	2.08	0.49	16.78	4.41	0.16	0.32	0.20	217	587	0.96	14.1
	平均值	5.41	15.48	0.56	2.62	1.27	25.00	10.30	0.29	1.07	0.79	588	1 164	1.55	17.3
	最大值	6.96	19.47	1.00	3.46	2.42	32.57	19.09	0.49	1.51	1.61	1 368	2 050	3.17	21.5
西太结核	最小值	3.99	15.23	0.56	2.03	1.37	15.02	11.80	0.31	0.25	0.18	626	1 016	2.28	16.6
	平均值	6.23	18.48	0.71	2.60	2.13	18.01	17.01	0.45	0.42	0.26	1 140	1 751	2.88	18.6
	最大值	9.41	29.02	1.50	3.52	2.67	23.2	21.05	0.55	0.79	0.59	1 556	2 393	3.44	20.9
西太结壳	最小值	0.73	1.94	0.96	3.10	1.28	17.22	9.41	0.23	0.22	0.05	829	1 583	1.31	18.4
	平均值	1.92	7.87	5.85	11.06	1.62	23.11	12.89	0.47	0.46	0.12	1 297	2 392	2.37	29.4
	最大值	4.81	15.27	14.34	23.92	2.2	27.72	16.95	0.72	0.64	0.30	2 001	3 141	3.21	52.6

样品	参数值类型	w_B/%										w_B/10^-6
		Al₂O₃	SiO₂	P₂O₅	CaO	TiO₂	Mn	Fe	Co	Ni	Cu	Ce	∑REY	δCe	Y/Ho
东太结核	最小值	3.30	12.68	0.30	2.08	0.49	16.78	4.41	0.16	0.32	0.20	217	587	0.96	14.1
	平均值	5.41	15.48	0.56	2.62	1.27	25.00	10.30	0.29	1.07	0.79	588	1 164	1.55	17.3
	最大值	6.96	19.47	1.00	3.46	2.42	32.57	19.09	0.49	1.51	1.61	1 368	2 050	3.17	21.5
西太结核	最小值	3.99	15.23	0.56	2.03	1.37	15.02	11.80	0.31	0.25	0.18	626	1 016	2.28	16.6
	平均值	6.23	18.48	0.71	2.60	2.13	18.01	17.01	0.45	0.42	0.26	1 140	1 751	2.88	18.6
	最大值	9.41	29.02	1.50	3.52	2.67	23.2	21.05	0.55	0.79	0.59	1 556	2 393	3.44	20.9
西太结壳	最小值	0.73	1.94	0.96	3.10	1.28	17.22	9.41	0.23	0.22	0.05	829	1 583	1.31	18.4
	平均值	1.92	7.87	5.85	11.06	1.62	23.11	12.89	0.47	0.46	0.12	1 297	2 392	2.37	29.4
	最大值	4.81	15.27	14.34	23.92	2.2	27.72	16.95	0.72	0.64	0.30	2 001	3 141	3.21	52.6