5:15 PM - 6:30 PM
[MIS21-P01] Origin of noble gases in surface sediments from submarine mud volcanoes off Tanegashima
Keywords:off Tanegashima, submarine mud volcano, noble gas
A submarine mud volcano is a small hill where sediments with high pore pressure rise as a mud diapir and vent on the seafloor. In Japan, mud volcanoes have been found in the Kumano area of the Nankai Trough and off Tanegashima. In this study, we analyzed noble gases collected from the sediments of four mud volcanoes out of the 15 mud volcanoes discovered in the forearc region off Tanegashima. The purpose of this study is to analyze the measurement results of noble gases in the sediments of the mud volcanoes off Tanegashima, to discuss the origin of the gases, and to compare them with noble gas data from other marine areas.
The sampling was conducted during voyages using the R/V Hakuho-Maru from August 19 to September 1, 2015 and August 9 to 18, 2019. MV1 and MV14 sediment samples were collected during the 2015 voyage, and MV2 and MV3 samples were collected during the 2019 voyage. In MV1 and MV14, after collecting the piston corer, a hole was drilled in a PVC pipe and a copper tube was set in it, filled with surface sediment, and sealed with a metal clamp. For MV2 and MV3, copper tubes were set in pre-drilled holes in the pipes of the multiple corer and sediments were collected in the same way as for piston coring. The noble gases in the surface sediments in the copper tube were measured using a noble gas mass spectrometer at ETH Zurich and the Atmosphere and Ocean Research Institute of the University of Tokyo.
As a result, the helium isotope ratio ((3He/4He)raw) ranged from 0.23 to 0.96 RA, all of which were lower than the value of 1 RA in atmospheric equilibrium seawater (ASW). 0.23 RA was the smallest value in MV1, and 0.96 RA was the largest value in MV14. In MV3, we sampled at three depths, with the deeper depths showing higher values than the shallower ones. The (3He/4He)raw values of MV2 is 0.63 RA. The results of 4He/20Ne values are as follows: MV3-1 shows the highest value of 18.4, followed by MV1, MV2, MV3-2, MV14, and MV3-3 shows the lowest value of 0.81. All the mud volcanoes are significantly higher than ASW, beyond the error range. The concentrations of heavy noble gases (Ar, Kr, and Xe) were obtained for MV1 and MV14, and it can be said that MV14 is slightly higher than MV1 for all noble gas concentrations. In both mud volcanoes, the concentrations are higher than those of ASW.
The (3He/4He)corr results corrected for the influence of ASW are about 0.2 RA for MV1, 0.6 RA for MV2, 0.3-0.6 R for MV3, and 0.9 RA for MV14. The contribution of the mantle-derived helium was about 2% for MV1, 7% for MV2, 4-7% for MV3, and 12% for MV14. In the four mud volcanoes, the influence of crustal helium was about 88-98%, suggesting that most of the collected helium was of crustal origin. Plotting the helium isotope ratio data obtained in other forearc regions shows two major ranges. The first is in the range of 0.1-0.8 RA, which is the data from the Oomine Ridge off Kumano and the mud volcanoes in the Gulf of Cadiz. The other is the data at depths of several kilometers below the seafloor with values of ~2.4 RA, suggesting the presence of mantle-derived helium. The data off Tanegashima corresponds to the lower range of the helium isotope ratio. The NGTs inferred from the heavy noble gas concentrations of MV1 and MV14 were about 150℃ and 100℃, respectively. The NGT estimated from the heavy noble gas concentration is reasonable because the temperature at which the reaction occurs is estimated to be 60-160℃. If the geothermal gradient around Tanegashima is 25-50℃/km, the temperature of 100-150℃ is estimated to be around 2-6 km below the seafloor. According to the velocity structure study, these depths are considered to be in the upper to middle crust around the Off Tanegashima, and the plate boundary is estimated to be located about 10 km below the seafloor, which is not a clear evidence for the origin of these noble gases near the plate boundary.
In the mud volcanoes off Tanegashima, the results of 3He/4He in the sediments suggest that about 90% of the helium is of crustal origin, although there are differences in MV1, MV3 and MV14. From the heavy noble gas concentration, the origin temperature of noble gas in MV1 and MV14 is estimated, and the helium corrected from sediments is considered to have come from about 2-6 km below the seafloor. Therefore, the helium isotope ratios and heavy noble gas concentrations in the sediments do not provide clear evidence that the noble gases originated near the plate boundary in either case.
The sampling was conducted during voyages using the R/V Hakuho-Maru from August 19 to September 1, 2015 and August 9 to 18, 2019. MV1 and MV14 sediment samples were collected during the 2015 voyage, and MV2 and MV3 samples were collected during the 2019 voyage. In MV1 and MV14, after collecting the piston corer, a hole was drilled in a PVC pipe and a copper tube was set in it, filled with surface sediment, and sealed with a metal clamp. For MV2 and MV3, copper tubes were set in pre-drilled holes in the pipes of the multiple corer and sediments were collected in the same way as for piston coring. The noble gases in the surface sediments in the copper tube were measured using a noble gas mass spectrometer at ETH Zurich and the Atmosphere and Ocean Research Institute of the University of Tokyo.
As a result, the helium isotope ratio ((3He/4He)raw) ranged from 0.23 to 0.96 RA, all of which were lower than the value of 1 RA in atmospheric equilibrium seawater (ASW). 0.23 RA was the smallest value in MV1, and 0.96 RA was the largest value in MV14. In MV3, we sampled at three depths, with the deeper depths showing higher values than the shallower ones. The (3He/4He)raw values of MV2 is 0.63 RA. The results of 4He/20Ne values are as follows: MV3-1 shows the highest value of 18.4, followed by MV1, MV2, MV3-2, MV14, and MV3-3 shows the lowest value of 0.81. All the mud volcanoes are significantly higher than ASW, beyond the error range. The concentrations of heavy noble gases (Ar, Kr, and Xe) were obtained for MV1 and MV14, and it can be said that MV14 is slightly higher than MV1 for all noble gas concentrations. In both mud volcanoes, the concentrations are higher than those of ASW.
The (3He/4He)corr results corrected for the influence of ASW are about 0.2 RA for MV1, 0.6 RA for MV2, 0.3-0.6 R for MV3, and 0.9 RA for MV14. The contribution of the mantle-derived helium was about 2% for MV1, 7% for MV2, 4-7% for MV3, and 12% for MV14. In the four mud volcanoes, the influence of crustal helium was about 88-98%, suggesting that most of the collected helium was of crustal origin. Plotting the helium isotope ratio data obtained in other forearc regions shows two major ranges. The first is in the range of 0.1-0.8 RA, which is the data from the Oomine Ridge off Kumano and the mud volcanoes in the Gulf of Cadiz. The other is the data at depths of several kilometers below the seafloor with values of ~2.4 RA, suggesting the presence of mantle-derived helium. The data off Tanegashima corresponds to the lower range of the helium isotope ratio. The NGTs inferred from the heavy noble gas concentrations of MV1 and MV14 were about 150℃ and 100℃, respectively. The NGT estimated from the heavy noble gas concentration is reasonable because the temperature at which the reaction occurs is estimated to be 60-160℃. If the geothermal gradient around Tanegashima is 25-50℃/km, the temperature of 100-150℃ is estimated to be around 2-6 km below the seafloor. According to the velocity structure study, these depths are considered to be in the upper to middle crust around the Off Tanegashima, and the plate boundary is estimated to be located about 10 km below the seafloor, which is not a clear evidence for the origin of these noble gases near the plate boundary.
In the mud volcanoes off Tanegashima, the results of 3He/4He in the sediments suggest that about 90% of the helium is of crustal origin, although there are differences in MV1, MV3 and MV14. From the heavy noble gas concentration, the origin temperature of noble gas in MV1 and MV14 is estimated, and the helium corrected from sediments is considered to have come from about 2-6 km below the seafloor. Therefore, the helium isotope ratios and heavy noble gas concentrations in the sediments do not provide clear evidence that the noble gases originated near the plate boundary in either case.