40Ar/39Ar Ages and Geochemistry of Seamount Basalts from the Western Pacific Province

Liu, Qian; Tang, Limei; Chen, Ling; Gao, Peng

doi:10.3390/jmse10010054

Open AccessArticle

⁴⁰Ar/³⁹Ar Ages and Geochemistry of Seamount Basalts from the Western Pacific Province

¹

Key Laboratory of Submarine Geosciences, Ministry of Natural Resources, Hangzhou 310012, China

²

Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China

³

Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China

^*

Authors to whom correspondence should be addressed.

J. Mar. Sci. Eng. 2022, 10(1), 54; https://doi.org/10.3390/jmse10010054

Submission received: 26 November 2021 / Revised: 24 December 2021 / Accepted: 27 December 2021 / Published: 4 January 2022

(This article belongs to the Special Issue Origin of Oceanic Plateaus, Submarine Ridges, and Seamounts, and Their Physical and Biogeochemical Consequences)

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Abstract

:

Seamounts are features generated by hot spots and associated intraplate volcanic activity. The geochemical characteristics of igneous rocks constituting seamounts provide evidence of important details of dynamic processes in the Earth, such as mantle magma source areas, and are key to understanding how mantle plume processes control the formation and evolution of seamounts and their resulting geochemical characteristics. The Pacific Ocean contains a large number of hitherto unstudied seamounts, whose ages and geochemical characteristics remain poorly known. This study presents the geochemical characteristics of six basalt samples from five seamounts in the Western Pacific and the ⁴⁰Ar/⁹Ar ages of three samples are determined. The new analysis yielded ⁴⁰Ar/³⁹Ar ages for seamounts samples MP3D21, MP5D11, and MP5D15A of 95.43 ± 0.33, 62.4 ± 0.26, and 99.03 ± 0.4 Ma, respectively. The geochemical profiles of seamounts samples MP3D04, MP3D21, MP5D11, MP5D15A, MPID201, and MPID202 are consistent with alkaline basalts, as evidence by alkali-rich, silicon-poor compositions along with high titanium concentrations. The primitive mantle normalized rare-earth elements and trace elements spider pattern are similar to those of ocean island basalts. The Ta/Hf and Nb/Zr ratios and La/Zr-Nb/Zr discriminant diagrams indicate that the six seamounts formed from magma that originated in the deep mantle.

Keywords:

Western Pacific seamounts; basalt; ⁴⁰Ar-³⁹Ar age; geochemistry

1. Introduction

The close relationship between hot spot features and plate motion has long been a subject of intense academic research interest. Seamounts are usually described as underwater highlands more than 1000 m above the seabed. They are very common volcanic landforms on the Earth. More broadly, the term seamounts can also refer to features with relative heights of less than 1000 m [1,2,3]. Because seamounts record evidence of intraplate volcanic eruptions, they provide important information for understanding the nature of hot spot volcanic activity, and they have accordingly received high levels of research attention [4,5,6,7,8,9,10]. The magma generation is the most effective mechanism for producing geochemical differentiation. Additionally, as a product of hot spot activity and related intraplate volcanism, seamounts represent the best direct evidence of intraplate magma activity in the lower mantle. The geochemical characteristics of provide important information about dynamic processes in the Earth, such as mantle magma source areas, which are key to better understanding how mantle processes determine the formation and evolution of seamounts and their resulting geochemical characteristics [11,12,13,14,15,16,17,18,19]. Moreover, seamounts contain a large variety of materials that constitute valuable seabed resources, such as cobalt-rich crusts. Therefore, understanding seamount systems is also crucial to understanding the formation of economic seabed resources [20,21,22,23]. However, due to the wide coverage of the Pacific region, the changeable plate movement and the complex factors affecting the formation of seamounts, the geological research on most seamounts in the Pacific region is not comprehensive, especially the basic geology of seamounts in the Pacific region. Therefore, many aspects of seamounts and seamount chains, including their geochemistry, formation, and evolution, are still poorly understood.

The Pacific Ocean contains a large number of seamounts, most of which are currently unstudied, so that little is known about their ages and geochemical characteristics. Moreover, certain technological challenges associated with seamount sampling render it difficult to achieve comprehensive sampling coverage over seamounts. Moreover, variations in hydrothermal and seawater conditions may also complicate the dating of seafloor basalts [14,24], resulting in significant difficulties in chronological analysis of seamounts. In this paper, we present the geochemical characterization of basalts from six seamounts in the Western Pacific, from which we selected three samples for ⁴⁰Ar/³⁹Ar dating. A better understanding of these seamount tracks and their evolutionary history through age and geochemical data will contribute to a better understanding of the geological processes beneath the Pacific plate.

2. Samples and Analytical Methods

The six seamount basalt samples used for dating and geochemical analysis are MP3D04, MP3D21, MP5D11, MP5D15A, MPID201, and MPID202, in which MPID201 and MPID202 are duplicate samples of MPID2 at the same sampling station (Figure 1). Backscattered electron (BSE) images and mineralogical analysis of all samples were obtained using JEOL JXA-8100 microprobes located at the Key Laboratory of Submarine Geosciences, Ministry of Natural Resources, Hangzhou. Major and trace elements in the samples were analyzed by an Axios sequence X-ray fluorescence spectrometer and an ICP-MS (Agilent 7500) at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences.

The dating samples were experimentally tested at Oregon State University and VU University in Amsterdam. They were irradiated in the cadmium shielded CLICIT facility of the TRIGA reactor and incrementally heated in the laser probe dating facility. The data processing and age calculation methods in this paper refer to the methods in [25,26].

3. Petrographic Characteristics

All rock samples used in this paper were examined as hand specimens, and the lithofacies were identified by petrographic microscope, after which the major and trace element components were analyzed and measured.

The specific sampling coordinates of the six samples used in this paper are shown in Table 1. The seamount samples MP3D04, MP3D21, MP5D11, MP5D15A, MPID201, and MPID202 collected in this paper are all basalt samples (Figure 2). Within the basalt, the minerals are uniformly distributed between massive forms and porphyritic structures. The main phenocryst phases in the samples are feldspar, pyroxene, and illite with short, columnar morphologies. The matrix is mainly composed of fine-grained crystalline feldspar and mafic minerals. Some samples show a fluidal texture with feldspar laths.

4. Results

4.1. ⁴⁰Ar/³⁹Ar Ages

For this study, three relatively fresh samples were selected for chronological analysis, and new ages of these three were determined using the Ar-Ar dating method (Figure 3). The analysis yielded ages for seamounts MP3D21, MP5D11, and MP5D15A of 95.43 ± 0.33, 62.4 ± 0.26, and 99.03 ± 0.4 Ma. Seamount MP3D21 has a shorter age plateau, which represents the minimum eruption age, while the other seamounts have suitable flat age regions and good inverse isochrons that can be interpreted as volcanic eruption ages.

4.2. Geochemistry

4.2.1. Major Elements

The major elements of the Western Pacific Ocean seamounts samples MP3D04, MP3D21, MP5D11, MP5D15A, MPID201, and MPID202 measured in this study indicate compositions rich in alkali elements, poor in silicon, and high in titanium. The range of SiO₂ contents is 39.9~49.6 wt%, the Na₂O content range is 1.5~4.5 wt%, K₂O is 0.9~2.5 wt%, CaO is 6.1~10.3 wt%, and Al₂O₃ is 12.3~15.6 wt%. TiO₂ contents are as high as 2.1%~4.3 wt%. Rock alkalinity combination index (rittman index) is usually used δ = (Na₂O + K₂O)²/(SiO₂-43) to determine the alkalinity of basalt bedrock. The results show that MP3D04, Mp3D21, MP5D11, MPID201, and MPID202 basalt are alkaline rocks (δ = 3.3~9), and this formula is not applicable to MP5D15A sample because its SiO₂ content is less than 43 wt%. The lithology of MP5D15A is shown in Figure 4. The aluminum saturation index is in the range 0.54~0.73, placing the sample in the metaluminous-rich category (A/CNK < 1, CNK > A > NK are metaluminous rocks). In the Nb/Y-SiO₂ diagram (Figure 4), all six samples (MP3D04, MP3D21, MP5D11, MP5D15A, MPID201, and MPID202) fall within the range of alkaline basalts and calc-alkaline basalts.

4.2.2. Trace and Rare-Earth Elements

In the primitive-mantle-normalized diagram for seamount samples MP3D04, MP3D21, MP5D11, MP5D15A, MPID201, and MPID202 (Figure 5), a slight right-dipping trend can be seen in the data (Table 2), which is similar to the trend commonly found in ocean island basalts (OIB). Among the large-ion lithophile elements (LILEs), the concentrations of Ba, K, and Sr vary moderately and are not enriched. The loss of Sr reflects either the separation crystallization of plagioclase or the mixing of crust and mantle [28]. In addition to differences among individual elements (U, Ta, Nb, and Ti), these strongly incompatible elements may be enriched or depleted by heterogeneous mantle processes, such as mantle source erosion and mantle metasomatism [29]. In the chondrite-normalized REE diagram (Figure 6), the six seamount basalt samples all show a flat right-dipping pattern, with a clear relative enrichment of LREE and relative depletion of HREE. This REE distribution pattern is consistent with typical OIB, together with the lack of obvious Ce and Eu anomalies.

5. Discussion

5.1. ⁴⁰Ar/³⁹Ar Age

For this study, the most accurate ages were determined by the ⁴⁰Ar/³⁹Ar dating method for three seamounts in the Western Pacific. The results yield an age for MP3D21 of 95.43 ± 0.33 Ma and age for MP5D15A of 99.03 ± 0.4 Ma, which belong to the formation of volcanic eruption in the late Cretaceous. Due to the short age plateau of MP3D21 seamount, the dating result of MP5D11 is 62.4 ± 0.26 Ma, which represents the minimum eruption age of the seamount and places its formation in Paleogene time.

Sample MP5D11, sample MP5D15A, and sample MP3D21 are adjacent to each other, but their age results differ by 36.9~33.3 Ma. As for the reasons for the age difference, we suggest an explanation as follows: the intraplate volcanic activity in the Western Pacific has lasted for a long time, and there are signs of volcanic activity from the Cretaceous to the modern geological period. In the long-term “broad-scale” mantle upwelling, many independent small “plumelets” are enclosed. When some small independent plumes rise to the shallow mantle, they will form short-term volcanic seamounts after decompression and melting. They will not last for a long time or form time-related hot spots trail, and the rocks of these isolated short-term volcanic seamounts also show the geochemical characteristics of E-MORB or OIB [6,34]. Moreover, at least one stem is a mantle plume with a stable position, and the plume may be affected by the extended ridge, resulting in the migration of the upper part of the upwelling plume pipeline and then the deviation of seamount age [35].

5.2. Magma Source

Both the primitive-mantle-normalized distribution of trace elements and the normalized REE distribution of MP3D04, MP3D21, MP5D11, MP5D15A, MPID201, and MPID202 seamounts in the Western Pacific (Figure 5 and Figure 6) exhibit similar patterns to those of OIB and are distinct from normal mid-ocean ridge basalts (N-MORB) and enriched ocean ridge basalts (E-MORB). OIB are generally considered to be the product of mantle plumes (hot spots) [19,36]. A comparison of trace element partitioning between the samples analyzed in this study with OIBs shows that the samples have been influenced by minor mixing with the crust, and the Nb/Yb versus Th/Yb diagram (Figure 7) also shows that the samples are not mixed with a large amount of circulating crustal components. In the Zr-Nb diagram (Figure 8), it can be seen that all six samples fall in the transition-enriched mantle region. At the same time, the Zr/Nb ratios of these seamounts range from 6.58 to 8.21, LA/Nb ratios range from 0.74 to 1.06, and Th/LA ratios range from 0.06 to 0.11, which are close to the EMI end elements of the mantle [37,38]. Indicating that these rocks were formed by enrichment and/or partial melting of transitional mantle sources. MP3D04, MP3D21, MP5D11, MP5D15A, MPID201, and MPID202 have no Nb and Ta deficits, and Nb and Ta are more enriched than Zr and Hf, which is characteristic of mantle plume basalts [39]. Basalts with Ta/Hf ratios greater than 0.3 and Nb/Zr ratios greater than 0.1 are considered to represent mantle plumes, indicating that the magma originated in the deep mantle [40,41]. The La/Zr–Nb/Zr structural discrimination diagram (Figure 9) shows that the sample points all lie in the mantle plume basalt region, indicating that the magma source that formed all six seamounts originated from a mantle plume. The variation of garnet and spinel content and the depth of magmatic origin are further determined by a La/Yb–Sm/Yb diagram (Figure 10); these samples in the Western Pacific are close to the melting curve of garnet peridotite, indicating that the magma may come from the partial melting of garnet phase mantle [42,43].

6. Conclusions

Through an integrated analysis of the petrology, ⁴⁰Ar/³⁹A chronology, and elemental composition of seamounts samples MP3D04, MP3D21, MP5D11, MP5D15A, MPID201, and MPID202 in the Western Pacific, this study obtained the following findings:

(1): The ⁴⁰Ar/³⁹A ages of seamounts samples MP3D21, MP5D11, and MP5D15A were found to be 62.4 ± 0.26, 95.43 ± 0.33, and 99.03 ± 0.4 Ma, respectively, placing the time of eruptions in a period ranging from the Late Cretaceous to the Paleocene. The age difference of 36.9~33.3 Ma between the samples may be due to the formation of independent seamounts in different periods by small independent plumes on the mantle plume under the Pacific plate.
(2): The basalt samples from seamounts MP3D04, MP3D21, MP5D11, MP5D15A, MPID201, and MPID202 in the Western Pacific are low-silicon and high-alkali rocks that are weakly enriched in rare-earth elements and weakly depleted in LIL elements such as U, K, and Sr, with no Ce or Eu anomalies. These characteristics are typical of OIB, and the composition was likely influenced by a small degree of crustal mixing during the magmatic evolution of the plume volcanic system. The petrological characteristics and various geochemical diagrams of the samples indicate that seamounts MP3D04, MP3D21, MP5D11, MP5D15A, MPID201, and MPID202 formed from magma that originated in the deep mantle and resulted from the presence of a mantle plume in the Western Pacific.

Author Contributions

Conceptualization, Q.L. and L.T.; Data curation, Q.L., L.T. and P.G.; Funding acquisition, Q.L., L.T. and P.G.; Investigation, Q.L. and L.T.; Project administration, Q.L. and L.T.; Resources, Q.L. and L.T.; Supervision, L.C.; Writing—original draft, Q.L. and L.T.; Writing—review & editing, Q.L., L.T. and L.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China under contract No. 41976072; Second Institute of Oceanography, Ministry of Natural Resources under contract Nos QNYC1901 and JG2002.

Data Availability Statement

All the datas have been shown in Table 2.

Acknowledgments

We would like to acknowledge the China Ocean Sample Repository for providing the samples.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Bathymetric map of the study area in the Western Pacific (a) and sampling stations; (b) the numbers marked in the figure are the age of seamount basalt.

Figure 2. Tapical photographs of basalt hand specimen (a) and orthogonal–polarized light photomicrograph (b) of sample from Western Pacific seamounts.

Figure 3. ⁴⁰Ar/³⁹Ar age analysis results for basalts from seamounts in the Western Pacific. (a) sample MP5D11; (b) sample MP5D15A; (c) sample MP3D21.

Figure 4. Nb/Y-SiO₂ diagram of seamount basalts from the Western Pacific (after Winchester, J.A. et al., 1997 [27]). I: subalkaline basalt, II: alkaline basalt, III: basanite/nepheline, IV: andesite, V: rhyolite-dacite/dacite, VI: rhyolite, VII: Sodium diorite alkali flow rock/alkali flow rock, VIII: trachyte, IX: phonolite.

Figure 5. Primitive-mantle-normalized diagram of trace elements (ppm) in seamount basalt samples from the Western Pacific. (Primitive mantle datas are after Taylor, S.R. et al., 1985 [30]).

Figure 6. Internationalized REE (ppm) distribution patterns of seamount basalt samples from the Western Pacific. (Cl chondrite datas are after Taylor, S.R. et al., 1985 [31], OIB datas are after Taylor, S.R. et al., 2002 [32], and E-MORB and N-MORB datas are after Klein, E.M., 2003 [33]).

Figure 7. Nb/Yb–Th/Yb discrimination diagram of seamount basalt in the Western Pacific (after Pearce, J.A., 2007 [44]).

Figure 8. Zr–Nb diagram of seamounts in the Western Pacific (after Barbero, E. et al., 2021 [45]).

Figure 9. La/Zr–Nb/Zr structural discrimination diagram of seamount basalts from the Western Pacific (after Wu, L.N. et al., 2003 [46]) I: N-morb region of divergent plate boundaries; II: convergence plate boundaries; III: ocean island and seamount basalt region, including T-morb and E-morb oceanic plate regions; IV: continental plate interiors, of which IV₁ is the intracontinental rift and marginal rift tholeiitic basalt area, IV₂ is the intracontinental rift alkaline basalt area, and IV₃ is the continental tension zone (or initial rift) basalt region; V: mantle plume basalt region.

Figure 10. La/Yb–Sm/Yb diagram of seamount basalt in the Western Pacific (after Xu, Y. G. et al., 2005 [42]).

Table 1. Sampling locations of seamounts in the Western Pacific Ocean.

Samples	LON	LAT
MP3D21	−165.8769	14.1453
MP5D15A	−168.0404	10.6113
MP5D11	−168.2445	10.5086
MP3D04	−165.4146	13.6551
MPID201-020	−160.0526	19.5635

Table 2. Basalt major element (%) and trace element (ppm) concentrations of seamounts in the Western Pacific (×10⁻⁶) as well as rare-earth (×10⁻⁶) element abundances.

Sample	MP3D04	MP3D21	MP5D11	MP5D15A	MPID201	MPID202
Al₂O₃	14.3	15.6	13.4	12.3	13.8	13.8
CaO	9.6	6.1	9.1	10.3	7.9	7.9
Fe₂O₃	14.6	14.7	15.5	13.6	14.3	14.2
K₂O	0.8	2.5	0.9	1.5	1.8	1.8
MgO	5.1	1.4	3.9	9.3	2.9	2.9
MnO	0.2	0.2	0.2	0.6	0.1	0.1
Na₂O	2.3	4.5	3.2	1.5	3.2	3.2
P₂O₅	0.4	1.1	0.7	0.4	2.0	2.0
SiO₂	44.9	49.6	47.5	39.9	47.3	47.3
TiO₂	3.2	2.1	4.3	3.1	3.0	3.0
L.O.I	4.0	1.5	0.8	6.7	3.2	3.1
Total	99.4	99.4	99.4	99.4	99.4	99.4
Sc	37.6	14.3	32.8	32.9	23.9	24.5
Ti	19,202.3	13,012.5	27,014.0	17,140.3	18,301.1	18,356.2
V	336.6	122.6	343.8	173.1	107.1	108.9
Cr	90.6	19.3	12.7	76.8	4.3	5.5
Mn	1958.9	1768.3	1730.2	2307.1	997.6	1047.1
Co	68.9	28.1	51.9	62.7	47.2	25.9
Ni	116.3	40.9	27.6	148.1	19.2	20.8
Cu	93.4	402.5	110.1	151.6	82.5	87.9
Zn	147.5	430.7	243.1	194.1	263.7	269.8
Ga	23.5	31.2	29.2	21.6	31.1	31.5
Ge	1.6	2.0	2.1	1.6	1.9	2.0
Rb	12.0	41.1	14.4	53.5	64.9	67.1
Sr	274.2	463.1	378.1	377.9	408.3	411.1
Y	34.1	60.4	55.0	40.9	81.7	82.7
Zr	203.8	498.2	372.7	189.7	458.2	469.6
Nb	31.0	66.7	48.2	24.3	55.9	57.2
Cs	0.6	0.5	0.8	1.9	3.9	3.9
Ba	349.8	316.5	138.5	394.0	117.3	123.4
La	23.8	51.9	35.7	25.4	59.4	59.6
Ce	53.1	115.4	82.8	51.1	125.9	129.3
Pr	6.9	16.2	11.5	6.5	16.8	16.7
Nd	29.9	68.7	50.7	28.2	72.9	73.6
Sm	7.0	15.7	12.2	6.7	17.1	16.6
Eu	2.3	4.9	3.8	2.3	5.4	5.4
Gd	7.3	15.1	12.4	7.3	17.2	17.2
Tb	1.1	2.3	1.9	1.1	2.5	2.6
Dy	6.7	12.8	10.5	6.4	14.8	15.0
Ho	1.3	2.4	2.1	1.3	3.0	2.9
Er	3.3	6.0	5.2	3.4	7.6	7.7
Tm	0.5	0.8	0.7	0.5	1.0	1.0
Yb	2.9	5.0	4.4	2.9	6.5	6.4
Lu	0.4	0.7	0.6	0.4	0.9	1.0
Hf	4.9	11.3	8.5	4.4	9.7	9.9
Ta	2.1	4.2	3.0	1.7	3.8	3.8
Pb	4.7	22.8	5.0	2.4	2.8	3.0
Th	2.5	5.5	3.1	1.6	3.9	3.9
U	0.4	0.6	0.7	0.9	2.0	2.0

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Liu, Q.; Tang, L.; Chen, L.; Gao, P. ⁴⁰Ar/³⁹Ar Ages and Geochemistry of Seamount Basalts from the Western Pacific Province. J. Mar. Sci. Eng. 2022, 10, 54. https://doi.org/10.3390/jmse10010054

AMA Style

Liu Q, Tang L, Chen L, Gao P. ⁴⁰Ar/³⁹Ar Ages and Geochemistry of Seamount Basalts from the Western Pacific Province. Journal of Marine Science and Engineering. 2022; 10(1):54. https://doi.org/10.3390/jmse10010054

Chicago/Turabian Style

Liu, Qian, Limei Tang, Ling Chen, and Peng Gao. 2022. "⁴⁰Ar/³⁹Ar Ages and Geochemistry of Seamount Basalts from the Western Pacific Province" Journal of Marine Science and Engineering 10, no. 1: 54. https://doi.org/10.3390/jmse10010054

APA Style

Liu, Q., Tang, L., Chen, L., & Gao, P. (2022). ⁴⁰Ar/³⁹Ar Ages and Geochemistry of Seamount Basalts from the Western Pacific Province. Journal of Marine Science and Engineering, 10(1), 54. https://doi.org/10.3390/jmse10010054

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⁴⁰Ar/³⁹Ar Ages and Geochemistry of Seamount Basalts from the Western Pacific Province

Abstract

1. Introduction

2. Samples and Analytical Methods

3. Petrographic Characteristics