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Paleomagnetic studies on single crystals separated from the middle Cretaceous Iritono granite
© The Author(s) 2018
- Received: 3 July 2018
- Accepted: 25 October 2018
- Published: 13 November 2018
- Single crystals
Long superchrons of constant geomagnetic polarity are the most distinctive features at the ~ 10 Myr scale trend of the geomagnetic field and are very possibly related to the whole-mantle convection process such as the activity of mantle plumes (e.g. Larson and Olson 1991; Glatzmaier et al. 1999; Courtillot and Olson 2007; Zhang and Zhong 2011; Biggin et al. 2012). Numerical dynamo simulations indicate that non-reversing stable dynamos with strong dipole moments will occur under conditions of relatively low CMB heat flow, whereas a reversing dynamo with multipolar nature is expected under conditions with high CMB heat flow (e.g. Kutzner and Christensen 2002; Christensen and Aubert 2006; Olson and Christensen 2006). Additionally, high dipole fields could be also caused by enhanced heterogeneity of CMB heat flow (e.g. Takahashi et al. 2008; Olson et al. 2010).
Understanding the geomagnetic field intensity during superchrons is crucial for revealing the nature of the long-term change of the geodynamo and the controlling role of the mantle on it. While dynamo simulations predict stronger field during superchrons, paleointensity during the Cretaceous normal superchron (CNS) at 83–120 Ma has not reached consensus among previous paleomagnetic studies. Several studies suggest stronger field during CNS than the average for ages with frequent reversals (e.g. Tarduno et al. 2006; Tauxe 2006), while others claimed the opposite (e.g. Tanaka and Kono 2002; Shcherbakova et al. 2012). It is also challenging to get a solid conclusion from the paleomagnetic databases such as PINT (Biggin et al. 2009) and MagIC (earthref.org/MagIC) because the data deposited in them are mostly from volcanic rocks which reflect short-term geomagnetic variations and hide the long-term trends of paleointensity. In order to establish a reliable paleointensity curve for long-term variation, a brand new dataset based on appropriate samples and measurement methods is required.
To focus on the long-term variations of the past geomagnetic field, plutonic rocks could provide good candidate samples since they are likely to record the time-averaged field accurately during their long cooling history. Granites in particular have been formed at various ages and have preserved over geological time. However, paleomagnetic studies using granitic rocks are usually difficult due to weathering of the rocks and non-ideality of coarse grain multi-domain (MD) magnetite. Also, granitic rocks often contain biotite and pyrrhotite, which decompose easily upon laboratory heating and form some magnetite as a byproduct. One of the most promising approaches to overcome these difficulties is to separate single silicate crystals from them that contain magnetic mineral inclusions, and use them for paleomagnetic measurements.
Single silicate crystals with magnetic inclusions have the potential to yield reliable paleointensity data because the inclusions are more protected from chemical alternations such as oxic weathering in nature, and from thermochemical oxidation upon laboratory heating, than are the host granitic rocks. Zircon crystals have been used for paleointensity studies owing to its permanence against chemical alternation and ability to obtain direct radiometric ages on them (Tarduno et al. 2014, 2015), although Weiss et al. (2015) raised a controversy. Detailed rock-magnetic properties of zircons collected from river sand in the Tanzawa pluton, Japan, showed its adequacy for paleointensity measurements (Sato et al. 2015). Fu et al. (2017) reported that the absolute value of zircon paleointensity was consistent with the bulk rock using the Bishop Tuff of Northeastern California. Paleointensity and rock-magnetic properties have also been intensively studied on single plagioclase crystals which can contain magnetically stable fine-grained magnetite inclusions (Tarduno et al. 2006). For basalts from the 1955 Kilauea eruption, the recovered paleointensity has been compared with the whole-rock and magnetic observatory data, with good agreement (Cottrell and Tarduno 1999). Plagioclase crystals separated from lavas from the Rajmahal Traps (113–116 Ma; Tarduno et al. 2001), Strand Fiord Formation (95 Ma; Tarduno et al. 2002), Ocean Drilling Program (ODP) Site 1205 on Nintoku Seamount of the Hawaiian-Emperor volcanic chain and ODP Site 801 in the Pigafetta Basin (55.59 and 160 Ma, respectively; Tarduno and Cottrell 2005), and the Kiaman Reversed Superchron type area (~ 262–318 Ma; Cottrell et al. 2008) have been used for studies on paleointensity variation related to the reversal frequency of the dipole field. Quartz phenocrysts are also a target studied for Archean rocks (Tarduno et al. 2007, 2010, 2014). All of the paleointensity measurements mentioned above were taken using variants of the Thellier–Thellier method (Thellier and Thellier 1959; Coe 1967; Yu et al. 2004).
Rock-magnetic properties of plagioclase separated from plutonic rocks such as granitoids (Usui et al. 2015) and gabbros (Feinberg et al. 2005; Muxworthy and Evans 2012) have also been reported. Some of them are characterized by needle-shaped tiny magnetite inclusions possibly formed by exsolution from the host plagioclase (Feinberg et al. 2005; Usui et al. 2015; Wenk et al. 2011). Several preceding studies reported paleointensity estimates using plutonic rocks in which the authors argued that the magnetic records were carried by exsolved magnetite (Selkin et al. 2008; Usui 2013). Plagioclase with exsolved magnetite is potentially an excellent recording medium of the ancient geomagnetic field, but should be treated carefully because (1) magnetic remanence anisotropy caused by needle-shaped magnetite can affect paleomagnetic results (Paterson 2013; Usui et al. 2015), (2) nonlinear thermoremanence acquisition (Selkin et al. 2007), and (3) unknown formation temperature of exsolved magnetite (Feinberg et al. 2005). Usui and Nakamura (2009) reported paleointensity using single plagioclase crystals separated from a granitic rock, although they did not claim they achieved exact, reliable estimates. Despite its potential for establishing the long-term trend of the geomagnetic field strength, paleointensity of single crystals separated from plutonic rocks have not been compared to that of the host whole rock to assess its reliability.
This study aims to assess how paleointensity measurements on single silicate crystals separated from granitic rocks are reliable compared to those on whole-rock samples. We conducted systematic rock-magnetic measurements on zircon, quartz and plagioclase grains separated from whole-rock samples collected from the Cretaceous Iritono granite, a paleomagnetically well-studied unit in northeast Japan. The results suggest that plagioclase is the most appropriate candidate mineral for paleointensity measurements among the studied minerals. We therefore conducted paleointensity experiments on plagioclase and compared the results with previously published results from the host granitic rock. Paleointensity experiments were conducted by the Tsunakawa–Shaw method (Tsunakawa and Shaw 1994; Yamamoto et al. 2003; Mochizuki et al. 2004; Yamamoto and Tsunakawa 2005; Yamamoto et al. 2015) which might be more suitable for single grain samples with exsolved magnetite than the variants of the Thellier–Thellier method. Obtained paleointensity results were consistent with the whole-rock data; thus it is suggested that plagioclase crystals separated from granitic rock have a potential to constrain the long-term variation of paleointensity.
A granite sample core 2.54 cm in diameter was crushed with a non-magnetic mortar and pestle, and sorted by 850 μm and 350 μm mesh screens. Heavy fractions of the sample smaller than 350 μm were concentrated by an aqueous panning technique. Zircons with no visible cracks or opaque particles on the surface were hand-picked under a binocular stereoscopic microscope. Quartz and plagioclase were hand-picked from samples larger than 350 μm and smaller than 850 μm. These selected crystals were leached by hydrochloric acid (HCl) to remove any tiny magnetic particles on the sample surface. HCl concentration and leaching duration was 12 N and 4 days for zircon and quartz, and 6 N and 8 h for plagioclase, respectively. Samples were then sandwiched individually between layers of magnetically clean Scotch Magic Transparent Tape in the method of Sato et al. (2015) or were mounted individually on a glass holder (see below) for rock-magnetic measurements and paleointensity experiments.
Remanence measurements using SQUID magnetometer
A superconducting quantum interference device (SQUID) magnetometer (2G Enterprises Model 755-4.2 cm) was used for remanence measurements. We followed the method of single-crystal measurements by Sato et al. (2015). A sample holder made of acrylonitrile butadiene styrene (ABS) was used for measurements. Single-crystal samples sandwiched by tape or mounted on the glass holder were fixed on the edge of the ABS holder by double-stick tape. The magnetic moments of the ABS holder and double-stick tape were measured before and after sample measurement and subtracted from the sample moment. The detection limit of the method was 2 × 10−12 Am2, so we employed 4 × 10−12 Am2 as a threshold to distinguish significant remanence intensity from noise.
First, we measured NRM intensity of 349, 455, and 268 grains for zircon, quartz, and plagioclase, respectively. On the basis of the NRM intensities, we then selected samples for further rock-magnetic and paleomagnetic measurements.
For the selected samples that showed significant NRM intensity (> 4 × 10−12 Am2 per grain), we conducted low-temperature remanence measurements using a magnetic property measurement system (Quantum Design model MPMS-XL5). Isothermal remanent magnetization (IRM) was first imparted at 2.5 T and 10 K after zero-field cooling from 300 K. The remanence was then measured during warming in zero-field (ZFC remanence). Subsequently, samples were cooled to 10 K in a 2.5 T field and then remanence was further measured during warming in zero-field (FC remanence).
Hysteresis loop measurements were taken for plagioclase grains and a quartz grain which contained magnetite using an alternating gradient magnetometer (LakeShore model MicroMag 2900). Samples sandwiched by tape were mounted on a transducer probe with a silica sample stage (Lake Shore model P1 probe). The blank saturation magnetization of the probe was 6 × 10−10 Am2. Maximum field during hysteresis loop measurement was 0.5 T, and the field increment was 4 mT. Diamagnetic/paramagnetic corrections were applied to the obtained hysteresis loop by subtracting the average slopes at applied field of |B| > 300 mT. Results are exhibited in the Day plot (Day et al. 1977).
Stepwise ThD of NRM was performed on selected zircon and quartz samples using a TDS-1 thermal demagnetizer (Natsuhara Giken). For plagioclase samples, stepwise ThD of laboratory-imparted thermoremanent magnetization (TRM) was performed after paleointensity experiments. TRM was given by cooling from 610 °C in a 50 μT field in air.
To investigate the NRM to IRM (NRM/IRM) distribution of plagioclase, a room-temperature IRM was imparted to 75 plagioclase samples at 2 T by an MMPM10 pulse magnetizer (Magnetic Measurements), and the IRM intensity was measured using the SQUID magnetometer.
Remanence anisotropy measurements
To assess anisotropy effect, we measured the ARM anisotropy of 19 plagioclase samples including 13 samples which were subjected to the paleointensity experiments. ARM was imparted along three orthogonal axes (ARMx, ARMy, and ARMz) to obtain the remanence anisotropy tensor. Measurements were taken after LTD treatments. TRM anisotropy tensor was also measured after paleointensity experiments for some samples and the consistency with the ARM anisotropy tensor was checked. The ARM and TRM measurements were also taken after AFD with a peak AC field of 50 mT. ARM anisotropy of whole-rock samples was checked based on measurement results obtained using a spinner magnetometer (Natsuhara Giken model SMD88).
Rock-magnetic properties of zircon
Rock-magnetic properties of quartz
Rock-magnetic properties of plagioclase
We performed magnetic hysteresis and low-temperature magnetometry measurements on four selected grains with different NRM/IRM ratios. All of the four samples exhibited similar features in both hysteresis and low-temperature magnetization. Results on the hysteresis measurements fall in the PSD region of a Day plot (Fig. 6d) and are concentrated in a narrower region of the diagram compared to the whole rock. This indicates that the magnetite in plagioclase crystals has narrower range of grain size than that in the whole rock. The Verwey transition of magnetite was clearly observed at approximately 120 K (Fig. 6e), indicating a very low titanium content of magnetite (Özdemir et al. 1993; Moskowitz et al. 1998). Larger remanence in the FC curve relative to the ZFC curve (Fig. 6e) also suggests a dominance of fine-grained magnetite (Moskowitz et al. 1993; Carter-Stiglitz et al. 2001, 2002; Kosterov 2003).
After paleointensity measurements, we took hysteresis measurements on four samples and low-temperature measurements on one sample. The results were similar to those shown in Fig. 6c, e, which implies that the double heating during paleointensity measurements did not severely affect the magnetic characteristics of plagioclase grains. A distribution of the blocking temperature was investigated on four samples after paleointensity experiments. Results show a very narrow blocking temperature distribution around 530–580 °C (Fig. 6f).
To summarize, rock-magnetic measurements of plagioclase samples indicate that the plagioclase crystals contain nearly-pure needle-like-shaped SD and PSD magnetite with width less than a few micron and various aspect ratio and are suitable for paleointensity measurements.
Paleointensity experiments of plagioclase
We conducted Tsunakawa–Shaw paleointensity experiments on 17 plagioclase grains. In consideration of the sensitivity of the instrument, samples with NRM intensities larger than 5 × 10−11 Am2 were chosen for the experiments. Taking weak remanences of single-crystal samples into account, we employed a slightly different selection criteria from the study by Yamamoto and Tsunakawa (2005) which worked on strong remanences of volcanic whole rocks. The criteria we adopted are:
A primary component found in an orthogonal plot of NRM demagnetization
f > 0.3 in a NRM–TRM1* plot
R > 0.90 in a NRM–TRM1* plot
Slope of a TRM1–TRM2* plot within 1 ± 0.1
R > 0.95 in a TRM1–TRM2* plot
Results on Paleointensity measurements of plagioclase samples
0.945 ± 0.064
1.054 ± 0.050
47.2 ± 3.2
1.142 ± 0.100
1.094 ± 0.021
57.1 ± 5.0
1.253 ± 0.083
1.048 ± 0.026
62.6 ± 4.2
0.862 ± 0.120
1.082 ± 0.021
43.1 ± 6.0
1.558 ± 0.126
1.051 ± 0.056
77.9 ± 6.3
1.457 ± 0.187
1.078 ± 0.061
72.8 ± 9.4
0.974 ± 0.096
0.968 ± 0.032
48.7 ± 4.8
1.129 ± 0.106
0.960 ± 0.031
56.4 ± 5.3
1.007 ± 0.096
1.008 ± 0.041
50.3 ± 4.8
57.4 ± 11.8
1.424 ± 0.095
0.887 ± 0.051
71.2 ± 4.7
0.714 ± 0.058
1.297 ± 0.065
35.7 ± 2.9
No primary magnetization
0.961 ± 0.035
Low linearity in NRM/TRM1* plot
0.971 ± 0.029
0.935 ± 0.086
1.232 ± 0.070
46.7 ± 4.3
1.611 ± 0.111
1.173 ± 0.065
80.6 ± 5.6
1.216 ± 0.104
0.849 ± 0.078
60.8 ± 5.2
1.310 ± 0.062
0.627 ± 0.119
65.5 ± 3.1
Two protocols were employed for handling the anisotropy bias on paleointensity ("Paleointensity experiments" section). In both protocols, it was technically difficult to impart ARM0 accurately parallel to ChRM. Therefore, the anisotropy bias on each sample would not be corrected completely. The angular differences between ChRM and ARM0 were 24° at most. The possible canceling of anisotropy bias by averaging a number of samples is discussed in "Anisotropy effect on paleointensity" section. The protocol in which ARM1, ARM2, TRM1 and TRM2 were given along the Y axis seems to be more reproducible for the present sample configuration, though the number of studied samples was not enough to determine which protocol was more suitable.
We found that ARM was larger than TRM in all plagioclase samples, in contrast to the whole-rock sample. This has been reported as a peculiar feature of exsolved magnetite by Usui et al. (2015).
Remanence anisotropy of plagioclase
Anisotropy parameters of plagioclase samples
Mean ARM (10−10 Am2)
Eigen values of remanence tensor
Anisotropy effect on paleointensity
Based on anisotropy measurements of plagioclase crystals separated from an Archean granitoid, Usui et al. (2015) demonstrated that (1) geometric mean instead of arithmetic mean should be used, and (2) tens of crystals would be needed to achieve reliable paleointensity estimates. In the present study, the geometric mean and the arithmetic mean are similar in the range of standard deviation, so fewer crystals are required. This difference can be attributed to the variation of the anisotropy effect; the anisotropy tensor they used for estimates was more anisotropic (corresponding to Pj = 6.21 and Tj = 0.34) than that used in the present study (corresponding to Pj = 3.18 and Tj = 0.04). Since the shape and fabric of exsolved magnetite vary among samples, the anisotropy effect and how to get rid of it need to be studied carefully for each rock.
In addition to remanence anisotropy, nonlinear TRM acquisition is a major issue of exsolved magnetite paleomagnetism. In the case of the studied sample, the NRM/IRM ratio was lower than the TRM/IRM ratio of previously studied plagioclase crystals (Usui et al. 2015) and rocks containing exsolved magnetite (Selkin et al. 2007). This implies that the nonlinear TRM acquisition may be insignificant for the obtained paleointensity range (~ 60 μT). Also, there is a possibility that NRM of exsolved magnetite is a thermochemical remanent magnetization (TCRM) rather than a TRM since the formation temperature of exsolved magnetite in plagioclase is not clear (Feinberg et al. 2005). In that case, obtained paleointensity could give the lower limit of the field strength at the age, as TCRM acquisition is less efficient than TRM acquisition (Stacey and Banerjee 1974; Usui and Nakamura 2009).
Comparison of magnetic carriers of plagioclase and whole-rock samples
Wakabayashi et al. (2006) and Tsunakawa et al. (2009) predicted that most of the stable remanence of the Iritono granite was carried by magnetite inclusions in plagioclase. However, detailed rock-magnetic experiments on plagioclase grains compared to the previously reported whole-rock studies revealed that the distribution of blocking temperatures and grain sizes are different between plagioclase crystals and the whole rock. The pTRM distributions do not show any concentration in a particular temperature interval below 550 °C for plagioclase samples, while about 10% of the whole-rock TRM is carried by a low blocking temperature (300–500 °C) component. Because the Iritono granite contains magnetite and pyrrhotite ("Rock-magnetic properties of zircon, Rock-magnetic properties of quartz, Rock-magnetic properties of plagioclase" sections; Wakabayashi et al. 2006; Tsunakawa et al. 2009), the low blocking temperature component above 350 °C found in the whole-rock TRM can be attributed to coarse-grained PSD and MD magnetites. Hysteresis loop measurements also indicate that magnetite in plagioclase has a narrow range of grain size compared to the whole rock (Fig. 6d). In addition, the results of the whole-rock experiments exhibit a bimodal distribution according to different paleointensity methods which suggest the influence of non-ideal magnetic minerals, and alternation of such minerals which could not be detected or suppressed completely. On the other hand, plagioclase samples mostly contain nearly-pure, fine-grained magnetite as the magnetic carriers.
Nevertheless, the magnetic carrier of plagioclase crystals and the whole rock was different in terms of distribution of grain size and blocking temperatures, and the estimated paleointensity was consistent among them. Therefore, we conclude that a reliable paleointensity was obtained successfully. Because of the more ‘ideal’ magnetic carrier, paleointensity experiments on single plagioclase with exsolved magnetite inclusions can potentially give more reliable and informative paleointensity results than the conventional whole-rock experiments.
Effect of cooling rate on paleointensity
The extremely slow cooling of granitic rocks compared to laboratory timescales may require a correction to the paleointensity estimate due to the time dependence of the acquisition of TRM, which varies by size and aspect ratio of the magnetic grains (Halgedahl et al. 1980; Selkin et al. 2000; Yu 2011). Results on magnetic hysteresis measurements of plagioclase grains are plotted in the PSD region on the Day plot (Fig. 6d), which could be interpreted as a mixture of a range of grain size and aspect ratio. The stable remanence that is involved in paleointensity measurements is carried by SD to PSD magnetite. Based on SD theory (Halgedahl et al. 1980; Selkin et al. 2000) and the estimated cooling time of the Iritono granite body, Tsunakawa et al. (2009) argued that the ratio of TRM in nature to TRM in laboratory would be 1.5 for the SD components. On the other hand, PSD grains have insignificant cooling rate dependence on TRM acquisition (Yu 2011). The cooling rate corrected paleointensity value of 38.2 ± 7.9 μT assuming SD magnetite gives the lower limit of the paleointensity, since the PSD magnetite should give higher paleointensity value. Therefore, corresponding cooling rate corrected VDM value of 8.9 ± 1.8 × 1022 Am2 using the paleointensity value of plagioclase crystals and inclination of the H component of the whole rock in Wakabayashi et al. (2006) can impose a constraint on the lower limit of paleointensity at the age of 115 Ma.
Significance of Shaw-type paleointensity methods on single crystals
This is the first report applying the Tsunakawa–Shaw paleointensity method to single grain samples. Considering that several results were rejected because of severe alternation during laboratory heating, a Shaw-type method, in which number of heating in laboratory are minimized, seems to be more appropriate than a Thellier-type method. Furthermore, the ThD curves of plagioclase crystals (Fig. 6f) show a very narrow distribution of blocking temperature below the Curie temperature of magnetite (530–580 °C), while the AFD curve (Top-right diagram in Fig. 8) show broad distribution of coercivity (50–150 mT). This emphasizes an advantage to estimate a paleointensity not in a blocking temperature space (by a Thellier-type method) but in a coercivity space (by a Shaw-type method), especially for a magnetically weak sample such as single crystals. Thus the Tsunakawa–Shaw method could be more suitable than the Thellier–Thellier methods in the case of plagioclase sample containing exsolved magnetite, while these methods should be compared in the future paleointensity study using appropriate samples.
Paleointensity during middle CNS
Considering the possible TCRM origin of NRM and the contribution of PSD grains on the cooling rate correction, the VDM value of 8.9 ± 1.8 × 1022 Am2 gives the lower limit of the time-averaged field strength during the middle age of CNS. Average field strength of the periods of frequent reversals have been estimated as the VDM value of the past 5 million years from the Society Islands volcanic rocks (3.6× 1022 Am2; Yamamoto and Tsunakawa 2005), and the virtual axial dipole moment (VADM) value of 0–160 Ma excluding CNS period from submarine basalt glass samples (4.8× 1022 Am2; Tauxe 2006). The present result suggests that the time-averaged field strength during middle CNS was several times as large as that of non-superchron periods, supporting the prediction by dynamo models and simulations (e.g. Larson and Olson 1991; Glatzmaier et al. 1999; Kutzner and Christensen 2002; Christensen and Aubert 2006; Olson and Christensen 2006; Courtillot and Olson 2007; Takahashi et al. 2008; Olson et al. 2010). By applying the present paleointensity method to various granitic rocks from different ages, we may be able to improve our understanding of the long-term behavior of the geomagnetic field concerning the mantle convection process without the complications of unideal magnetic minerals that often compromise such work.
We have evaluated the utility of using single silicate crystals separated from granitic rocks in the exploration the long-term evolution of the intensity of the geomagnetic field. We studied the rock-magnetic properties of zircon, quartz and plagioclase separated from the Iritono granite whose paleointensity was already well constrained by past studies using whole-rock samples. In our samples we found that plagioclase was the most suitable mineral phase to study, which was more reliably and stably magnetic than other minerals like zircon or quartz. We conducted paleointensity experiments on 17 plagioclase grains using the Tsunakawa–Shaw method. Nine samples were successful and gave mean paleointensity values of 57.4 ± 11.8 μT. This value is consistent with the previously reported whole-rock paleointensity, suggesting that an assembly of single plagioclase crystals separated from a granitic rock has the ability to yield the accurate paleointensity data. Considering the unknown forming temperature of exsolved magnetite and cooling rate effect on TRM acquisition, time-averaged VDM is estimated to be higher than 8.9 ± 1.8× 1022 Am2 at the age of 115 Ma, suggesting high dipole strength during the middle age of CNS.
YY and HT collected the samples. CK conducted the magnetic measurements. All contributed to discussion and writing the manuscript. All authors read and approved the final manuscript.
We thank Shinji Yamamoto for petrological discussions. The microscopic photograph of plagioclase sample (Fig. 7) was taken by Yujiro Tamura. We thank lead guest editor John Tarduno and two anonymous reviewers for their constructive comments. Rock- and paleomagnetic measurements were taken under the cooperative research program of Center for Advanced Marine Core Research (CMCR), Kochi University (Accept Nos. 16A009, 16B009, 17A028 and 17B028). This work was supported by the Japan Society for the Promotion of Science (JSPS) Research Fellowship for Young Scientists (DC1) No. 15J11812.
The authors declare that they have no competing interests.
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The data and materials used in this study are available on request to the corresponding author, Chie Kato (firstname.lastname@example.org).
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This work was supported by the Japan Society for the Promotion of Science (JSPS) Research Fellowship for Young Scientists (DC1) No. 15J11812.
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