linear correlation of ba and eu contents by hydrothermal...

Research ArticleLinear Correlation of Ba and Eu Contents by HydrothermalActivities: A Case Study in the Hetang Formation, South China

Chao Chang ,1,2 Qi Fu ,3 and Xiaolin Wang 2

1Shaanxi Key Laboratory of Early Life and Environment, State Key Laboratory of Continental Dynamics, and Departmentof Geology, Northwest University, Xi’an 710069, China2State Key Laboratory for Mineral Deposits Research, Institute of Energy Sciences, School of Earth Sciences and Engineering,Nanjing University, Nanjing 210046, China3Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA

Correspondence should be addressed to Chao Chang; [email protected]

Received 28 November 2018; Accepted 11 February 2019; Published 1 April 2019

Academic Editor: Stefano Lo Russo

Copyright © 2019 Chao Chang et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A significant linear correlation between Ba and Eu contents has been observed in a set of mudrock formations in the lowerCambrian Hetang Formation in South China. Combined with petrographic features and reinvestigation of published dataobtained from the same laboratory, the analytical interference of Ba can be excluded as a main factor of this correlation.Electron microprobe analyses (EMPA) have indicated that Ba-rich minerals (hyalophane and cymrite) precipitated fromhydrothermal fluids account for the total Ba content. It is evident that significant contribution of both Ba and Eu fromhydrothermal fluids is the cause of this linear relationship, while this coexistence is ultimately controlled by their similarchemical properties and abundances in rock sequences along the fluid path. This interpretation was supported by similar linearBa-Eu correlations retrieved from barite-bearing chert in another area of the Yangtze platform during the same geologicalperiod. Therefore, linear Ba-Eu correlations may be more common than previously thought in this area, where hydrothermalactivities were active during the specific time interval.

1. Introduction

The rare earth elements (REEs) include lanthanum (La) andthe f-block elements (cerium through lutetium) in the peri-odic table, whose atomic number ranges from 57 to 71. Scan-dium and yttrium are also included in this group as they haveionic radii similar to the lighter f-block elements and arefound coexisting in the same ores (Atwood, 2012). The REEsare insoluble in most geological settings and resistant toremobilization beyond the mineralogical scale during weath-ering, diagenetic, and metamorphic processes. That makesthem important tracers for characterizing and understandinga variety of geochemical “reservoirs” ([1, 2]; Atwood, 2012).

The most stable oxidation state of all REEs is the REE(III) form, except for europium (Eu) and cerium (Ce) thatare also present in the form of Eu2+ and Ce4+, respectively[3]. The distinctive redox chemistries of Eu and Ce are ofconsiderable importance to understanding geochemical

conditions with unique insights into magmatic, aqueous,and sedimentary processes ([1, 2, 4–6]; Murray et al., 1990;Atwood, 2012). The ionic radius of Eu2+ is about 17% largerthan that of Eu3+, which leads to different substitution behav-iors of Eu2+ than other trivalent REEs (e.g., Sm and Gd) andsubsequent anomalous REE patterns (i.e., Eu anomalies).Europium occurs in the form of Eu2+ under highly reducingconditions. It exists only within certain magmatic and/orhydrothermal environments that are rarely found at thesurface of the Earth. An important geological example is thatEu can be highly concentrated in plagioclase feldspar bysubstituting into the calcium site. The anomalous Euabundance in magmatic rocks indicates relatively shallowigneous partial melting or fractional crystallization processes(Atwood, 2012). Positive Eu anomalies have also been widelyreported in modern deep-sea hydrothermal systems [7–9]and in massive deposits of submarine hydrothermal exhala-tive origin [10–12]. Consequently, it has been used as an

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effective tracer for tracking and characterizing hydrothermalactivities in sedimentary rocks and ore deposits [13–19]. Inaddition, in spite of no direct evidence showing europiumreduction under surface conditions, positive Eu anomaliesmight be present at highly reducing conditions during dia-genesis. For example, the trace and rare earth element abun-dances in phosphate nodules from the lower Cambrian stratain South China have shown positive Eu anomalies, possiblyimplying the replacement of Ca2+ by Eu2+ under reducingconditions [20].

Accurate measurement of Eu abundances in geologicalsamples is a prerequisite to effectively obtaining importantinformation of geochemical reactions and environmental

conditions in natural processes recorded by elements. Thedetermination of Eu abundances by inductively coupledplasma-mass spectrometry (ICP-MS), however, is interferedby the coexisting element of barium (Ba). This is due to similarmass/charge ratios of Ba oxide (BaO) and hydroxide (BaOH+)species that partially overlap those of Eu isotopes (151Eu and153Eu), with 151Eu being less affected by BaO and BaOH+ than153Eu [21–23].

A routine assessment of the interference level of Ba is todetermine the yield of Ba oxide and hydroxide species usinga single-element solution of Ba (e.g., 500 ng/ml Ba in BaCl2solution) and compare the yield of potential interfering spe-cies (e.g., 135Ba16O) with the Eu isotope that has the same

100°E

Chengdu

Guiyang

Kunming Guilin

Nanning

Guangzhou

Shallow-water faciesinside the platform

Transitional facies onthe outer shelf

Slope to deep basinfacies

Uncertained faciesand tectonics

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Figure 1: Simplified paleogeographic map of the Yangtze platform during the Cambrian (after [20]).

2 Geofluids

mass number (e.g., 151Eu) in a single-element solution of Eu[21, 24]. The previous results indicated that 1000ng/g of Bacould cause an apparent Eu concentration of 0.22 ng/g in151Eu [21]. An alternative way to evaluate the extent of theinterference is analyzing the correlation between Ba and Euin the samples. Plots of Ba and Eu have been widely used inprevious studies, and linear Ba-Eu correlation has been

commonly interpreted as an indicator of apparent interfer-ence of Ba with Eu, with less attention on other possiblecauses ([5, 20, 25]; Ling et al., 2013).

To better understand the ultimate controlling factors, ifany, of Ba-Eu correlation other than analytical errors embed-ded with analytical techniques, we investigated the trace andrare earth elemental contents of a drilling profile in the lower

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Siliceousdolomite

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Carbonaceousmudrock

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Figure 2: Petrological profile of Well Wanning 2. The depth of each sample analyzed by EMPA is labelled, along with the occurrence of Ba-rich minerals.

3Geofluids

Cambrian Hetang Formation, South China. Hydrothermalactivities and Ba enrichment have been reported in previousstudies for this formation and its stratigraphic equivalents inother areas [16, 25–28]. In this study, a series of petrologicaland mineralogical analytical techniques are used to quantifythe abundances of Eu and Ba and elucidate their relationshipwith hydrothermal activities.

2. Geological Setting and Samples

During the transition between the Ediacaran period and theCambrian period, the Yangtze platform in South China grad-ually evolved from a rift to a passive continental margin basinin response to extensional tectonism [29, 30]. With sedimen-tological features, the sedimentary sequences can be classifiedinto three distinct facies: shallow water, transitional, andslope to deep basin facies (Figure 1). The shallow water faciesis represented by thick carbonate strata, while the slope todeep basin facies consists of a series of interbedded blackchert and shale. The transitional facies is composed of bothcarbonate and black shale [20, 29]. The occurrence of oredeposit beds containing Ni-Mo-PGE-Au sulfides has beenreported in this black shale-chert sequence along the NEtransitional belt at the margins of the Yangtze platform inlower Cambrian [15].

The samples in this study were taken from a drilling sec-tion (Well Wanning 2) located at Ningguo, Anhui province(Figure 1), covering lithological units of the Xijianshan andHetang formations. The sedimentary strata of the HetangFormation include mudrock, chert, and carbonates. Themajor mineralogy of mudrock varies, including carbona-ceous materials, silicates, or Ca-containing minerals. TheHetang Formation is underlain by the Xijianshan Formationwhich is mainly composed of dolostone and chert. Carbonateand quartz veins are common in this set of strata, indicatingenhanced fluid migration during diagenesis. The samplescover the whole depth of the Hetang Formation and areextended into the Xijianshan Formation (Figure 2). Toensure accurate measurements, two batches of samples withsimilar mineralogy and petrographic features at selecteddepths were collected for analyses (see below).

3. Methods

The trace element analysis was carried out on a Finnigan Ele-ment II ICP-MS at the State Key Laboratory for MineralDeposits Research at Nanjing University. For carbonaterocks, 100mg of powdered samples was weighted in Teflonbeakers and dissolved by 1M HAc under ultrasonic bathingfor 2 hours. For mudrock and chert, 50mg of powderedsample was dissolved by a mixed HF-HNO3 solution inhigh-pressure Teflon bombs. For calcareous or dolomiticmudrock, the carbonate fractions were dissolved by 1MHAc and the solid residues were then dissolved by the samemixed acid solution in high-pressure Teflon bombs. Afterdissolution, the solutions were centrifuged and the superna-tants were transferred to clean Teflon beakers for evapora-tion. The solutions were added with 1ml of HNO3 anddried three times to remove HAc or HF. The residues were

then dissolved in 5% HNO3 solution for analysis. A solu-tion containing Rh in 10 ppb was used as an internal stan-dard to monitor signal drifting during measurements. Thedetailed analytical procedure was described in the work ofGao et al. [31], and parameters for the instrument werepresented in Table 1. Analytical uncertainties are estimatedto be less than 10%. The value of Eu/Eu∗ is calculated as2EuPAAS/ SmPAAS + GdPAAS , where EuPAAS, SmPAAS, andGdPAAS represent the abundances of Eu, Sm, and Gd,respectively, normalized by the post-Archean AustralianShale (PAAS) standard [32].

The electron microprobe analysis (EMPA) was carriedout on polished, carbon-coated thin sections to study the pet-rological and mineralogical features in the samples. It wasconducted using a JEOL JXA-8100 electron microprobe witha wavelength-dispersive system at the State Key Laboratoryfor Mineral Deposits Research at Nanjing University. Theinstrument was operated with three crystal spectrometers,with the accelerating voltage and the specimen current of15 kV and 20mA, respectively. The diameter of the beamspot used for quantitative element analysis was 1μm. Thestandards used were natural minerals supplied by the Amer-ican National Standards Institute (ANSI). The detection limitof the microprobe analysis was approximately 0.002% foreach element.

4. Results

4.1. Petrological and Mineralogical Features. The locations ofsamples analyzed by EMPA within the profile are labelled inFigure 2. Minerals containing Ba and K, such as hyalophaneand cymrite, were found in all of the mudrock and chert sam-ples with backscattered electron (BSE) observation andEMPA analyses. For each sample analyzed by EMPA, theoccurrence of Ba-rich minerals, in hydrothermal veinsand/or in matrix, is also specified (Figure 2).

Hyalophane is present as the matrix in all the samplesanalyzed, while hydrothermal veins that contain hyalophaneare observed in most samples. In matrices, the occurrence ofhyalophane includes fillings of pore spaces in chert samples(Figure 3(a)) and thin rims around clastic K-feldspar grains(Figures 3(b)–3(d)). In hydrothermal veins, hyalophane ispresent as irregular or amorphous aggregates. The close

Table 1: Parameters of the instrument.

Forward power 1200W

Reflected power 2W

Plasma gas flow rate 14.5 l/min

Auxiliary gas flow rate 0.8 l/min

Nebulizer gas flow rate 0.85 l/min

Sample uptake rate 1ml/min

Resolution 300

Points per mass 15

Integration time 0.01 s/point

Sweeps 3

4 Geofluids

association of hyalophane in veins with the whole matrix isvery common (Figure 3). In particular, the texture ofhyalophane-bearing veins penetrating into the matrix isobserved, with abundant hyalophane rims formed aroundclastic K-feldspar grains (Figures 3(b) and 3(c)). Cymritegrains show euhedral tabular forms, primarily coexistingwith hyalophane in the matrix in a number of chert samples(Figure 3(a)).

4.2. Trace and Rare Earth Elements. Samples over the wholeprofile, Batch 1, were measured by ICP-MS for abundancesof trace and rare earth elements, including carbonates,mudrock, and chert (Table 2). To limit analytical errors frominstruments, a second batch of samples, i.e., Batch 2, withsimilar mineral compositions and petrological features werealso used for analyses (Table 3).

There are 7 carbonate samples in Batch 1, includingdolostone and limestone. The concentration of Ba and Eu incarbonates ranges from 76.67 to 326.92 ppm (Figure 4(a))

and 0.19 to 0.45 ppm (Figure 4(b)), respectively. Slight nega-tive Eu anomalies are observed, with Eu/Eu∗ values from0.71 to 0.99 (Figure 4(c)). There is a covariation of Sm andGd concentrations in carbonate samples, with both being inthe range of 0.50 to 3.00 ppm (Figure 4(d)).

The abundances of Ba and Eu in mudrock and chert aremuch higher than in carbonate rocks. In Batch 1, the con-centration of Ba ranges from 431.47 to 20386.00 ppm,approximately 40 times higher than that in carbonates onaverage (Figure 4(a)). The concentration of Eu is in therange of 0.27 to 6.22 ppm, which is about 9 times higherthan that in carbonates (Figure 4(b)). Values of Eu/Eu∗

range from 1.12 to 14.17, suggesting pronounced positiveEu anomalies (Figure 4(c)). In Batch 2, the concentrationof Ba and Eu ranges from 1818.70 ppm to 17316.74 ppm(Figure 4(a)) and 0.86 ppm to 3.71 ppm (Figure 4(b)),respectively. The average concentrations of Ba and Eu are30 and 7 times higher than those in carbonates, respectively,with Eu/Eu∗ values ranging from 1.08 to 11.20 (Figure 4(c)).

Vein

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Figure 3: The occurrences of Ba-rich minerals. (a) Hyalophane grains in the vein matrix and a hydrothermal vein and cymrite in the matrix,sample WN-13. Both hyalophane and cymrite in the matrix occur as fillings of interparticle pores. Note that some hyalophane aggregates inveins are connected to those in the matrix. (b) Hyalophane in veins and in the matrix, sample WN-19. Nearly all the clastic K-feldspar grainsare surrounded by hyalophane rims. Some hyalophane aggregates in veins are connected to hyalophane rims in the matrix. (c) Closeup of themarked area in (b), sample WN-19. Hyalophane rims can be easily distinguished from clastic K-feldspar cores by the difference in brightness.Hyalophane in veins and in the matrix is closely associated. (d) Hyalophane in veins and the matrix, sample WN-44. Hy: hyalophane; Cym:cymrite; Qz: quartz.

5Geofluids

The covariation of Sm and Gd can also be observed inmudrock and chert samples in both batches. The rangebecomes broader for both batches, varying from 0.78 to16.15 ppm (Figure 4(d)).

5. Discussion

5.1. Ba-Eu Correlation. In previous studies, to evaluate theinterference of Ba with Eu concentrations, correlations of

Table 2: Concentrations of Ba and selected REE elements of mudrock, chert, and carbonate samples in Batch 1.

Sample Depth (m) Petrology Ba Sm Eu Gd Eu/Eu∗

WN-1 820.60 Chert 2326.83 2.00 0.87 2.02 2.04

WN-2 820.50 Chert 1645.36 1.62 0.76 2.12 1.87

WN-3 820.30 Chert 11326.82 1.44 2.99 2.30 7.33

WN-4 819.10 Siliceous dolomite 88.55 1.03 0.20 1.00 0.93

WN-5 818.70 Chert 17987.58 1.97 4.31 2.14 9.81






WN-11 797.30 Carbonaceous mudrock 11742.34 1.51 2.97 1.96 7.96


WN-13 794.80 Chert 2237.81 0.20 0.68 0.24 14.17

WN-14 790.60 Siliceous mudrock 8684.80 3.38 2.44 4.05 3.05

WN-15 785.50 Chert 5499.63 3.28 1.77 4.15 2.22










WN-25 763.30 Dolomitic mudrock 13994.26 4.22 3.83 3.57 4.64




WN-29 755.80 Limestone 153.80 2.50 0.38 1.69 0.86

WN-30 754.80 Chert 2420.48 2.45 1.01 2.35 1.98

WN-31 753.70 Chert 2403.92 2.44 1.06 2.23 2.15














6 Geofluids

Ba and Eu were verified with different elemental ratios,including the relationship between Ba/Nd and Eu/Eu∗ ([5];Ling et al., 2013), Eu/Eu∗ and Ba/Sm [20], and Ba andEu/Eu∗[25]. Among them, instead of the absolute concentra-tion of Eu, the ratio of Eu/Eu∗ was the one that is frequentlyused to evaluate the Eu abundance. Due to intrinsic chemicalproperties and complex redox conditions, Sm and Gd behavedifferently than Eu in natural environments. Changes in theirabundances may not result from the same geological event(s)that impacted the Eu speciation and its distribution in differ-ent phases. Therefore, the involvement of Sm and Gd inevaluating Ba interference with the ratio of Eu/Eu∗ mayobscure the original Eu distribution and authentic Ba-Eucorrelation. The abundances of Ba and Eu are evaluateddirectly in this study.

The concentrations of Ba and Eu in mudrock and chertsamples in Batch 1 show a significant linear correlation, withthe coefficient of determination (R2) of the linear regressionline being 0.88 (Figure 5(a)). In carbonate samples withinthe same batch, however, no linear Ba-Eu correlation isobserved (Figure 5(b)). Concentrations of Ba and Eu inmudrock and chert samples in Batch 2 have also shown line-arity between them, with an R2 of 0.86 (Figure 5(c)). By com-bining data from both Batch 1 and Batch 2, a linear Ba-Eucorrelation with an overall R2 of 0.85 suggests a consistentrelation between Ba and Eu in mudrock and chert samples(Figure 5(d)), with the Ba/Eu ratios ranging from 797.32to 4806.16.

The abundance of Eu (i.e., EuT), reported as the resultcollected by ICP-MS, has two potential sources: the authenticEu content in samples (EuA) and the signal error caused byBa interference (EuF). The value of EuA is the direct result

of involved geological processes, while EuF is linearly corre-lated with the Ba concentration. If EuF is much higher thanEuA, a linear Ba-EuT correlation most likely indicates majorinterferences from Ba. This is how linear Ba-EuT correlationswere interpreted in previous studies ([5, 20, 25]; Ling et al.,2013). However, if Ba and EuA are enriched by the same geo-logical processes and the abundance of EuA is dominant inthe total Eu, a Ba-Eu linear correlation is a genuine recordof geochemical processes involving both elements. Both pos-sibilities are discussed in the following sections to unravel thecause of linear Ba-EuT correlations in this study.

5.2. Evaluation of Analytical Interferences. The interferenceof Ba in the determination of Eu was a systematic errorcaused by the overlap of peaks from Ba oxide and hydrox-ide species with Eu isotopes during ICP-MS analysis [21].Due to the difference in system errors of individual instru-ments and internal standards used, the ratio of the concen-tration of Ba over EuF could vary significantly betweendifferent instruments. For example, the Ba/EuF ratios of1000 : 0.22 and 1000 : 0.10 from ICP-MS analyses werereported by Dulski [21] and Smirnova et al. [24], respec-tively. Despite this discrepancy, for the same instrument,the Ba/EuF ratio is believed to be constant under similarworking conditions [21].

As reported above in “Introduction,” to validate theextent of Ba interference on Eu, the most effective methodis to determine the yield of Eu caused by single-elementsolutions of Ba oxide or hydroxide with varying concentra-tions [21, 24]. Such experiment has been conducted on a Fin-nigan Element XR ICP-MS in the same laboratory, withidentical pretreatment procedures and similar analytical

Table 3: Concentrations of REEs and Ba of mudrock and chert samples in Batch 2.

Sample Depth (m) Petrology Ba Sm Eu Gd Eu/Eu∗

WN2-1 794.30 Chert 17316.14 3.28 3.71 2.97 5.59

WN2-2 789.60 Siliceous mudrock 1818.70 2.07 0.85 2.41 1.76


WN2-4 752.00 Carbonaceous mudrock 5345.38 5.16 2.33 6.63 1.83





WN2-9 716.60 Calcareous mudrock 8602.37 3.29 2.13 2.91 3.24











7Geofluids

conditions to the Finnigan Element II ICP-MS used in thisstudy [33]. The results revealed a Ba/EuF ratio of 400000 : 1to 1400000 : 1 for the low-resolution (300) mode analysisand much higher Ba/EuF ratios for the mid-resolution(4000) and high-resolution (10000) mode analyses. In thiscase, the Ba/EuF ratio was supposed to be at least 100 timeshigher than the Ba/EuA ratio of analyzed samples in thisstudy, demonstrating that the extent of Ba interference onEu determination was likely minimal.

In addition, previously published data of Ba and EuTconcentrations obtained from the same instrument under

similar working conditions are also collected to evaluatethe Ba/EuF ratio. In a study of black shale in the Niuti-tang Formation (lower Cambrian) in South China, theconcentrations of Ba and EuT were reported to be in theranges of 659 to 13686 ppm and 0.33 to 3.97 ppm, respec-tively [15] (Figure 6(a)). The Ba/EuT ratio was calculatedto be varied from 391.54 to 8146.43. Both the Ba andEu concentrations in black shale, along with their ratios(Ba/EuT), are in a similar range as in samples of this study(Tables 2 and 3). However, no linear correlation betweenBa and EuT can be extracted in spite of a weak covariation

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Figure 4: The plots showing values of (a) Ba concentrations, (b) Eu concentrations, (c) Eu/Eu∗ ratios, and (d) Sm and Gd concentrations inchert and mudrock samples for Batch 1, carbonate samples for Batch 1, and chert and mudrock samples for Batch 2.

8 Geofluids

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9Geofluids

trend, suggesting that EuF is not a major component of thereported EuT values.

Another set of data comparable to this study wasreported on phosphate nodules of the Mufushan Formation,which is also in the period of lower Cambrian in South China[20]. The Ba and EuT concentrations are in a narrower rangethan other reported data, from 817 to 6144ppm and 0.05 to11.42 ppm, respectively (Figure 6(b)). The Ba/EuT ratio variessignificantly from 95.53 to 59040.00. There is no linear corre-lation between Ba and EuT (Figure 6(b)). Therefore, consid-ering the similar range of Ba concentrations and Ba/EuTratios obtained in this study, systematic analytical errorscan be excluded as a main factor attributing to the significantlinear correlation between Ba and Eu.

Furthermore, the concentrations of Sm and Eu frommostsamples in this study have also shown a positive correlation(Figure 7(a)). A similar relationship can be observed betweenEu and Gd as well (Figure 7(b)). Unlike Eu, the interferenceof Ba on Sm and Gd is believed to be negligible because of dif-ferences in mass over charge ratio [21]. Therefore, the linearBa-Eu correlation, coupled with Eu-Sm and Eu-Gd linearities,suggests the presence of an intrinsic relationship between Baand Eu which is mainly caused by geological processes. Thepositive Eu-Sm and Eu-Gd correlations in this study mayresult from the impact of the geological processes andthe original elemental feature of the sediments.

5.3. Evidence of Ba-Eu Coexistence. Europium is commonlyenriched in plagioclase as a result of substitution into the cal-cium site. Similar to Ca2+, Ba2+ can also be substituted byEu2+ due to the same electronic charge and similar cationradius [34–36]. Therefore, Eu may be enriched in mineralsor geological fluids that have high abundances of Ba. A linearBa-Eu correlation can be present as a result of their

substitutional relationship, particularly when Ba2+ is thedominant cation in minerals (or geological fluids) as shownabove (Figures 4(a), 4(c) and 4(d)).

The linear relationship between Ba and Eu suggests thatthe majority of Ba and Eu in this set of rock formations areclosely related during geological processes. The BSE observa-tion and elemental analysis by EMPA provide additional evi-dence showing the highly concentrated areas of Ba in thesamples. The Ba-rich minerals, hyalophane in particular, arepervasive in the rock sequence and present in the matrix ofall the samples analyzed by EMPA (Figure 2). Hyalophaneis believed to be formed in limited geological environments,and its origin ismostly from hydrothermal fluids (McSwiggenet al., 1994; Moro et al., 2001).

In the rock matrix, hyalophane mainly occurs as rims ofclastic K-feldspars (Figure 3(c)). Both hyalophane and cym-rite are also present in intergranular pores (Figure 3(a)).The hydrothermal veins containing hyalophane arecommon textures, shown penetrating into the matrix(Figures 3(b) and 3(c)). Overall, the occurrence of suchBa-rich minerals suggests that they were formed by infiltra-tion of late-stage hydrothermal fluids into the rocks. Hyalo-phane of hydrothermal origin with similar occurrences(e.g., hyalophane rims around K-feldspar) was also foundin black shale in the Devonian period from the SelwynBasin, Canada [37].

In this study, both barium and europium elements areconcentrated by late-stage hydrothermal fluids passingthrough surrounding sedimentary sequences. They were thenaccumulated in hyalophane and cymrite during diagenesis.This scenario agrees well with the consistently high concen-tration of Ba in the chert and mudrock samples analyzed,with an average value of 6431.04 ppm (Tables 2 and 3). It isalso confirmed by the fact that, as hyalophane and cymrite

24000

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Figure 5: The plots showing Ba and Eu contents in samples of this study. (a) Concentrations of Ba and Eu in mudrock and chert for Batch 1.A linear correlation with an R2 of 0.88 is observed. (b) Concentrations of Ba and Eu in carbonate rocks for Batch 1. No linear correlation isobserved. (c) Concentrations of Ba and Eu in mudrock and chert for Batch 2. A linear correlation with an R2 of 0.86 is observed. (d)Concentrations of Ba and Eu in mudrock and chert for Batch 1 and Batch 2. A linear Ba-Eu correlation with an R2 of 0.85 is observed.

10 Geofluids

are not included in the trace element and rare earth elementanalyses of carbonate rocks because they are insoluble inHAC, lower Ba concentrations and nonlinear Ba-Eu correla-tion are obtained (Figures 4(a) and 5(b)).

Enrichment of Eu has been reported in modern deep-seahydrothermal systems [7–9] and in massive deposits of sub-marine hydrothermal exhalative origin [10–12]. High abun-dances of both Ba and Eu in hydrothermal fluids are presentin a variety of hydrothermal environments [7, 38–40].

Wang et al. [25] investigated the occurrences and tex-tures of chert on the marginal zone of the Yangtze plat-form in the western Hunan area, during the Ediacaran-Cambrian transition. Four lithotypes of chert are identifiedin the study, including mounded, vein, brecciated, andbedded chert. Further geochemical investigations haveshown prevalent positive Eu anomalies in chert andhomogenization temperature of fluid inclusions in the

range of 120-180°C. Consequently, the chert was suggestedto be formed by hydrothermal fluids. The variation ofoccurrences and textures was considered to be a reflectionof different formation conditions. The mounded chert wasformed at or near the vent fields, while the formation ofbrecciated chert occurred in large-scale fracture systems.The chert in veins was precipitated from silica-rich hydro-thermal fluids ascending along fractures, and the beddedchert was formed in a quiet water environment away fromvent fields. This wide range of occurrences of hydrother-mal chert was ascribed to the extensional tectonism inthe Yangtze Block during the Ediacaran-Cambrian transi-tion [25, 41].

Although the positive Eu anomaly was observed in mostchert samples, barite was only found in mounded and brecci-ated chert. Only in those two lithotypes of chert can a prom-inent linear Ba-Eu correlation be extracted (Figure 8(a)). No

14000

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10000

8000

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4000

2000

00.0 0.5 1.0 1.5 2.0

Eu (ppm)

Ba (p

pm)

2.5 3.0 3.5 4.0 4.5

(a)

Ba (p

pm)

Eu (ppm)

00

1000

2000

3000

4000

5000

6000

7000

2 4 6 8 10 12

(b)

Figure 6: The Ba and Eu data reported in previous studies using the same instrument. (a) Ba and Eu from shale in the Niutitang Formation,South China [15]. The Ba/Eu ratio was varied from 391.54 to 8146.43. (b) Ba and Eu from phosphate nodules in the Mufushan Formation[20]. The Ba/Eu ratio ranges from 95.53 to 59040.00.

11Geofluids

significant Ba-Eu correlation can be recognized in the othertwo types of chert (Figure 8(b)), although the concentrationof Ba and the Ba/Eu ratio of bedded chert are in a similarrange as those of mounded and brecciated chert.

The results above provide another line of evidence sup-porting that the distinct Ba-Eu correlations in different litho-types of chert are caused by the source of elements andenvironmental conditions. In areas close to vent fields or hav-ing large-scale fracture systems, where the mounded andbrecciated chert were formed, chemical components inhydrothermal fluids can be preserved. However, beddedchert was suggested to form far away from vent fields andin a quiet water environment, where the original correlationbetween Ba and Eu contents within hydrothermal fluidscould be erased by secondary processes. Therefore, we sug-gest that the linear Ba-Eu correlation in mounded and brec-ciated chert most likely is being caused by significantcontribution from Ba-rich hydrothermal fluids.

6. Conclusions

Trace elemental analysis of the lower Cambrian Hetang For-mation in Anhui Province has shown a linear relationshipbetween Ba and Eu contents in mudrock and chert forma-tions. Combined with petrological and mineralogical obser-vation, significant contribution of Ba and Eu from Ba-richhydrothermal fluids is proposed to account for that correla-tion. Positive Eu anomaly in the rocks is suggested to be aneffective record of the influence of hydrothermal activities.

The ratio of Ba/Eu provides important information forevaluating the interference of Ba on determination of Eu con-centrations during analysis. Relatively low Ba/Eu ratios andsignificant linear Ba-Eu correlations may suggest that Baand Eu were originated from the same hydrothermal source.In addition, petrological and mineralogical investigations areessential in identifying the occurrences of Ba and Eu in rocksand facilitating understanding of Ba-Eu correlations.

12

9

6

3

00 1 2 3

Eu (ppm)

Sm (p

pm)

4 5 6 7

(a)

00 2 4 6

4

8

12

16

Gd

(ppm

)

Eu (ppm)

(b)

Figure 7: The plots showing the relationship between trace elements in this study. (a) Eu and Sm. (b) Eu and Gd. The Eu-Sm and Eu-Gdcorrelations can be observed in (a) and (b), respectively.

12 Geofluids

At the western boundary of the Yangtze platform, pro-nounced linear Ba-Eu correlations were also present inbarite-bearing chert (mounded and brecciated chert) formedby Ba-rich hydrothermal fluids. We suggest that such linearBa-Eu correlations may be common in geological environ-ments, which is ultimately constrained by the availability ofboth elements in rock sequences and the timing of hydro-thermal activities.

Data Availability

The trace and rare earth element data used to support thefindings of this study are included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This study was financially supported by the National KeyResearch and Development Program (Grant no. 2017YFC0603101), National Natural Science Foundation of China(Grant nos. 41802026, 41830425, 41890845, and 41621003),China Postdoctoral Science Foundation (Grant no. 2018M633553), and 111 Project (D17013). Research support toQF from the NSF CAREER Program under award OCE-

16000R2 = 0.91

R2 = 0.91

R2 = 0.99

Ba (p

pm)

12000

8000

4000

00.0 0.5 1.0

Eu (ppm)1.5

Mounded chertBrecciated chert

2.0

(a)

9000

6000

Ba (p

pm)

3000

0

Eu (ppm)

Bedded chertVein chert

0.0 0.4 0.8 1.2

(b)

Figure 8: Correlations of Ba and Eu contents in (a) mounded and brecciated chert and (b) vein and bedded chert in the work of Wang et al.[25]. Linear correlations with R2 of 0.99 and 0.91 are observed in mounded and brecciated chert, respectively.

13Geofluids

1652481 and the American Chemical Society PetroleumResearch Fund 54474-DNI2 is also acknowledged. Specialthanks are due to Dr. Wenlan Zhang and Qian Liu for theirhelp with analytical work.

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