Keywords

1 Introduction

There have been many scientific efforts devoted for studies on active sea-floor hydrothermal fields found in Okinawa Trough (Ishibashi et al. Chap. 23). The evolution of these hydrothermal fields is one of the important issues. Dating methods have been employed for hydrothermal sulfide deposits, such as the U-Th disequilibrium method (e.g. You and Bickle 1998) applicable for the age range more than several thousand years, and 226Ra-210Pb and 228Ra-228Th method (e.g. Noguchi et al. 2011) for the range less than 150 years. The age range of several hundred years is essential to estimate the life time of hydrothermal activities. However, dating methods for the age range are lacking.

Okumura et al. (2010) made the first practical application of ESR dating technique to a sample of seafloor hydrothermal barite to obtain ages of 300 and 3,620 years, while Kasuya et al. (1991) first pointed out that barite can be used for ESR dating. Toyoda et al. (2011) determined the optimum ESR condition while Sato et al. (2011) confirmed that the signal is thermally stable enough for an age range of several thousand years.

Seafloor massive sulfide deposits are composed of sulfide minerals such as pyrite, sphalerite, chalcopyrite and galena, and also include sulfate minerals such as anhydrite and barite. The sulfate minerals precipitate by mixing of hydrothermal fluid and seawater, while the sulfide minerals precipitate from the hydrothermal fluid mainly by cooling (Hannington et al. 1995). It is generally considered that a combination and/or switch of these two modes of hydrothermal precipitation lead to growth of sulfide structures such as a chimney and mound and that of large hydrothermal deposits (e.g. Tivey 2007). Takamasa et al. (2013) determined ESR ages for barite in sulfide deposits in the South Mariana Trough hydrothermal field, and concluded that the ages are consistent with U-Th ages. In this study, the ages of various hydrothermal sulfides in the Okinawa Trough are determined by ESR dating.

2 Material and Methods

The sulfide deposit samples were obtained in the 6 research cruises; NT01-05, NT02-07, YK04-05, NT11-20, NT12-06, and NT12-10 operated by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) from Daiyon-Yonaguni Knoll field, Hatoma Knoll field, Iheya North Knoll field, Hakurei field of Izena Hole field, Yoron Hole field of the Okinawa Trough (Figs. 29.1 and 29.2, and Table 29.1). After blocks of sulfide sample were cut into pieces (Fig. 29.3), approximately 2.0 g of the pieces was crushed. The samples were soaked in 12 M hydrochloric acid for approximately 24 h. Then, 13 M nitric acid was added. Finally, after rinsing in distilled water, the sample was filtered and dried. Mineral grains other than barite were removed by handpicking. The purified mineral grains were determined by an X-ray diffraction and confirmed that the grains are pure barite.

Fig. 29.1
figure 1

A bathymetric map of the known hydrothermal fields and sampling fields in the Okinawa Trough. Red triangles indicate the sampling fields; Daiyon-Yonaguni Knoll field, Hatoma Knoll field, Iheya North Knoll field, Hakurei field of Izena Hole field, Yoron Hole field

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Fig. 29.2
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Sampling location at the five hydrothermal fields in the Okinawa Trough. (a) Daiyon-Yonaguni Knoll field, (b) Hatoma Knoll field, (c) Iheya North Knoll field, (d) Hakurei field of Izena Hole field, (e) Yoron Hole field

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Table 29.1 List of samples analyzed in the present study
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Fig. 29.3
figure 3figure 3figure 3

Overview of sulfide samples and analyzed parts in each sulfide

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The barite grains extracted from each sample were separated into 100–250 μm aliquots for gamma ray irradiation up to about 10 kGy with a dose rate of 404.4 Gy/h made at Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency (JAEA). The sample aliquots were measured at room temperature with an ESR spectrometer (JES-PX2300) with a microwave power of 1 mW, and the magnetic field modulation amplitude of 0.1 mT as indicated by Toyoda et al. (2011) to be the best measurement conditions. The equivalent natural radiation doses were obtained by extrapolating the obtained dose response curve of the SO3 − signal to the ordinate intercept.

The bulk radium (226Ra and 228Ra) concentrations were measured by the low background pure Ge gamma ray spectrometer. Assuming that Ra is populated only in barite as confirmed by Okumura et al. (2010), the internal and external dose rates of alpha, beta and gamma rays given to the barite minerals were calculated (Toyoda et al. 2014). The alpha effectiveness of 0.043 was adopted (Toyoda et al. 2012). Corrections were made for water content, beta ray attenuation for grain sizes (a plane with 20 μm in thickness as confirmed by thin section). In the present paper, a new formula is proposed to obtain the ages which takes into account the decays of 226Ra (a half life of 1,600 years) and 228Ra (a half life of 5.75 years) with disequilibrated daughter nuclei as the following.

3 The Dose Rate Conversion Factors and the Decay Corrections for 226Ra and 228Ra

It is the most important feature of hydrothermal barite in the aspect of dose rate estimation that the only source of the radiation is internal radium (Okumura et al. 2010; Toyoda et al. 2014). The dose rate was calculated from the concentrations of radium and its daughter nuclei as the following.

The newest dose rate conversion factors, which are to be multiplied to the concentrations of U, Th, K to obtain dose rate, were reported by Guérin et al. (2011) for U and Th in radioactive equilibrium and for K together with the contributions of each nucleus in the decay chains. Toyoda et al. (2014) obtained the dose rate conversion factors from their table for 226Ra only in the case of radioactive equilibrium. By summing up the energy contributions of the nuclei in the chains listed in Guérin et al. (2011), the dose rate conversion factors for 226Ra and 228Ra were calculated so that the dose rate can be obtained in the case of radioactive disequilibrium as shown in Table 29.2.

Table 29.2 The dose rate conversion factors in mGy/y calculated, summing up the energy contributions of the nuclei in the chains listed in Guérin et al. (2011), the dose rate conversion factors for 226Ra and 228Ra were calculated so that the dose rate can be obtained in the case of radiative disequilibrium
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When the dose rate varies with time in the past, the equivalent dose, DE, obtained by the ESR measurements is expressed by the integration of a time dependent dose rate, D(t), as the following,

$$ {D}_E={\displaystyle \underset{0}{\overset{T}{\int }}D(t)dt} $$
(29.1)

As the source of the dose is Ra which decays with time, the decay has to be taken into account for the does rate, D(t). For the decay of 226Ra, Toyoda et al. (2014) obtained a formula as

$$ T=\frac{1}{\lambda}\; \ln \left(\lambda \frac{D_E}{D}+1\right) $$
(29.2)

This formula is valid for the samples with ages over 200 years where the daughter nucleus with second longest half life (210Pb, 22.3 years) in the decay series is equilibrated. However, for the younger samples, the radioactive disequilibrium and contributions from the decay series starting from 228Ra have to be considered.

When the number of nuclei of 226Ra or 228Ra is N1, and that of the daughter nuclei with second longest life (210Pb or 228Th) is N2, those are expressed as,

$$ {N}_1={N}_{10}\;{e}^{-{\lambda}_1t} $$
(29.3)
$$ {N}_2=\frac{\lambda_1}{\lambda_2-{\lambda}_1}{N}_{10}\left({e}^{-{\lambda}_1t}-{e}^{-{\lambda}_2t}\right) $$
(29.4)

where N10 is the initial number of parent nuclei, λ1 and λ2 are the decay constant of the parent and the daughter nuclei, respectively, and it is assumed that initially no daughter nuclei are present. We only know the number of nuclei at present, N1p, expressed as,

$$ {N}_{1p}={N}_{10}{e}^{-{\lambda}_1T} $$

where T is the age of the sample. Therefore, Eqs. (29.3) and (29.4) are written as,

$$ {N}_1={N}_{1p}{e}^{\lambda_1\left(T-t\right)} $$
(29.5)
$$ {N}_2=\frac{\lambda_1}{\lambda_2-{\lambda}_1}{N}_{1p}{e}^{\lambda_1T}\left({e}^{-{\lambda}_1t}-{e}^{-{\lambda}_2t}\right) $$
(29.6)

respectively.

The dose rate, D(t), given by the parent and daughter nuclei is written as,

$$ D(t)={Q}_1{\lambda}_1{N}_1(t)+{Q}_2{\lambda}_2{N}_2(t) $$
(29.7)

where Q1 and Q2 are the dose rate conversion factors for the nuclei from the parent to the one just before the daughter with the second longest life time in the decay series, and for those from that daughter and after, respectively. Please note that a conversion factor is given per unit activity, λN. From Eqs. (29.5)–(29.7) is written as,

$$ D(t)={\lambda}_1{N}_{1p}{e}^{\lambda_1T}\left\{{e}^{-{\lambda}_1t}\left({Q}_1+{Q}_2\frac{\lambda_2}{\lambda_2-{\lambda}_1}\right)-{Q}_2{e}^{-{\lambda}_2t}\right\} $$
(29.8)

The DE is given by integrating the dose rate as the following,

$$ \begin{array}{l}{D}_E={\displaystyle \underset{0}{\overset{T}{\int }}D(t) dt}\\ {}={\lambda}_1{N}_{1p}{e}^{\lambda_1T}\left\{\frac{1}{\lambda_1}\left({Q}_1+{Q}_2\frac{\lambda_2}{\lambda_2-{\lambda}_1}\right)\left(1-{e}^{-{\lambda}_1T}\right)-\frac{Q_2}{\lambda_2}\left(1-{e}^{-{\lambda}_2T}\right)\right\}\end{array} $$
(29.9)

For the present case, two decay series have to be taken into account, one starting from 226Ra (half life: 1,600 years) where the daughter nucleus with the second longest life time is 210Pb (22.3 years), and the other from 228Ra (5.75 years) where the daughter is 228Th (1.91 years). In the present work, the dose rates from both series are summed and T is obtained by Eq. (29.9) where the present radium activities (λ1N1) were used.

4 Results and Discussions

A typical ESR spectrum is shown in Fig. 29.4. The principal g factors are calculated from this powder spectrum to be 1.9995, 2.0023, and 2.0031, being consistent with the g factors for SO3 − radical obtained by Krystec (1980), which are 1.9995, 2.0023, and 2.0032. The peak-to-peak height of the spectrum (Fig. 29.4) was used as the signal intensity. An example of dose response of the signal intensity is shown in Fig. 29.5. Extrapolating the dose response to the zero ordinate, being fitted by a saturating exponential curve, the equivalent doses are obtained as shown in Table 29.3.

Fig. 29.4
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A typical ESR spectrum in hydrothermal barite (natural sample of HPD#1331G01d)

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Fig. 29.5
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A typical dose response of the SO3 − signal on gamma ray dose (HPD#1331G01d)

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Table 29.3 Dating results of sulfide samples
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The 226Ra and 228Ra concentrations were measured by a low background pure germanium gamma ray spectrometer for bulk samples as shown in Table 29.3. The 226Ra concentrations were obtained from the peak counts for 214Bi (609, 1,120, 1,765 keV) and for 214Pb (295 and 352 keV) in comparison with a standard uranite sample with radioactive equilibrium with known uranium concentration. The 228Ra concentrations were from the peak counts for 228Ac (338, 911, 969 keV) (Yonezawa et al. 2002). The ages of the samples were determined using the Eq. (29.6) as shown in Table 29.3.

The ages ranged from 4.1 to 16,000 years while the ages of pieces from a sample varies more than estimated analytical errors (Table 29.3). As discussed by Toyoda et al. (2014), microscopic observation revealed that sulfide minerals with various size and various occurrence fill the spaces among barite crystals, suggesting repeated stages of mineralization, which starts with sulfate precipitation and followed by sulfide precipitation. Thus the samples would consist of minerals in various ages. It would be more probable that the determined ages are somewhat “averaged” within the portion of the sample, implying that the variation in ages corresponds to the ratio of the younger to older crystals. Therefore, when the ages vary within a sample portion, it would mean that repeated hydrothermal activities have formed the sample at the time of around those ages. If this is the case, the oldest age would correspond to the youngest limit of the oldest activity.

The determined ESR ages are young up to 56 years for those samples in which 228Ra is detected. It is consistent that younger samples contain 228Ra with a short half life of 5.75 years. It is also reasonable that the all ESR ages for sulfide samples of active vent site are younger than those for sulfide samples of inactive vent site. 226Ra-210Pb and 228Ra-228Th ages obtained for part of the present samples are discussed in comparison with the present ESR ages by Uchida et al. (Chap. 47).

4.1 Daiyon-Yonaguni Knoll Field

The age of active chimney (818-S-01) from an active vent site ranges from 580 to 990 years, while the samples from active vent sites (2K1267L1, 2) show younger ages, 200 and 260 years.

4.2 Hatoma Knoll Field

The ESR ages of inactive chimney (HPD#1331G01) were 2,400–3,700 years, which are statistically indistinguishable. 210Pb in this sample was in radioactive equilibrium with 226Ra. One piece from sulfide mound sample, HPD#1331G03, shows an age of 990 years, which was statistically younger than other two ages of 3,000 and 5,700 years. The age of 2K1353R2 was determined to be 970 years. The age of HPD#1331G07 taken from an active chimney was determined to be of 7.1 years. U-Th ages were determined for sulfide minerals extracted from three pieces of HPD#1331G01, but they vary much more than ESR ages.

4.3 Iheya North Knoll Field

A sample from an inactive chimney (HPD#1358R3) shows ages from 3,000 to 4,300 years, while the other sample taken from a sulfide mound (HPD#1358R2) shows younger age 560–1,000 years, which are also statistically indistinguishable.

4.4 Izena Hole Field

Two samples from an inactive chimney (HPD#1329G01, G03) show ages over 10,000 years. On the other hand, a sample from an active chimney (HPD#1313G05) shows statistically consistent ages from 12 to 16 years, in which 228Ra was also detected. 228Th was equilibrated with 228Ra.

4.5 Yoron Hole Field

The samples analyzed in the present study were all from active chimneys. Older ages over 70 years were determined for HPD#1333G05 and G06 in which no 228Ra was detected while younger ages up to 56 years were determined for HPD#1333G03, G07, G08 and for HPD#1333G11 in which 228Ra was detected. The two consistent ages of 400 years were determined from two pieces of the sample, HPD#1333G05, however, the age of 200 years from one piece is significantly younger than these ages. Another age of 330 years between these ages was also determined. It seems that there are two distinct ages of 70–80 and 120–150 years in HPD#1333G06, which are statistically distinct. U-Th ages for this sample were less than 20 or 80 years, younger than ESR ages. 226Ra-210Pb ages were determined for these two samples using the same barite samples for ESR measurements. The ages were 71–77 years for HPD#1333G05 and 23–27 years for HPD#1333G06, significantly younger than ESR ages, but the relative order is consistent. The ESR ages of 200–400 years for HPD#1333G05 are older than 70–150 years for HPD#1333G06, while 226Ra-210Pb ages for HPD#1333G05 are 71–77 years, older than 23–27 years for HPD#1333G06 (Uchida et al. Chap. 47).

The sample HPD#1333G03 shows consistent ESR ages of 4.1–5.2 years, while 228Ra-228Th ages were 4.5–5.2 years, being coincided with ESR ages. Three ESR ages of HPD#1333G07 were in agreement between 50 and 56 years while one piece shows significantly younger age of 28 years. 228Th was in radioactive equilibrium with 228Ra in these samples, which is consistent with the results that ESR ages are older than HPD#1333G03, in which 228Ra and 228Th are in disequilibrium. Two ESR ages, 35 and 39 years, of HPD#1333G08 are statistically consistent but one age of 27 years is younger, for which 228Ra-228Th was determined to be 4.2 years. 228Th was in radioactive equilibrium with 228Ra in the former two pieces, being consistent with ESR results.

4.6 ESR Ages of Hydrothermal Field in the Okinawa Trough

Figure 29.6 shows the determined ESR ages plotted as a geographical position of longitude of the hydrothermal fields. The order of ages of the 5 hydrothermal fields would be arranged, from young to old as follows; Yoron Hole field < Daiyon-Yonaguni Knoll field < Hatoma Knoll field ≒ Iheya North Knoll field < Izena Hole field. The samples from Hakurei site showed the high variation of ESR ages, indicating possible variation in the age of the hydrothermal fields, therefore, a conclusion based on analysis of a few samples from active sites may not be appropriate. More systematic dating works are necessary in each specific hydrothermal field in order to reconstruct the history of the hydrothermal activities.

Fig. 29.6
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The obtained ESR ages plotted as a function of longitude of the hydrothermal fields

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4.7 ESR Dating of Hydrothermal Barite

Our attempt is the first study to determine such a large number of ESR ages using hydrothermal barite of many sulfide breccia samples. The results indicate that ESR dating of barite would be practically useful to investigate the history of sea-floor hydrothermal systems. The technique determines the ages more promptly than using isotopes, and is especially useful in the range from 100 to 10,000 years where 226Ra-210Pb method is not applicable. The present study showed that the method reveals relative order of the ages among the samples, however, additional efforts should be needed to establish the method for absolute age determination through further comparative dating studies. Moreover, it is also necessary to investigate the relationship and between the variation in ages and the occurrence of the minerals.