Tsanyao Frank Yang

Exhalation of radon and its carrier gases in SW Taiwan

Abstract Gas compositions of mud volcanoes reveal multiple sources for gas exhalation in SW Taiwan. For comparison, two sites, Yan-chao (YC) and Chung-lun (CL), were chosen for measurements of soil Rn concentrations using a portable radon detector. The 222 Rn concentrations at the YC site were ca. 5200 Bq/m 3 . However, the average 222 Rn concentrations at the CL site exhibited higher value of ca. 16,800 Bq/m 3 . With the reference of the gas flux and compositions from the nearby mud pool, the soil 222 Rn concentrations are largely controlled by the flux of carrier gases exhaled from deep reservoirs.

Newly discovered eastern dispersal of the youngest Toba Tuff

Volcanic glasses with minor mafic mineral fragments, such as biotite and hornblende, found in deep-sea sediments of the South China Sea Basin (SCSB) have been clearly identified as eruptive products of the Youngest Toba Tuff (YTT), northern Sumatra, Indonesia. The tephra layer occurs between marine oxygen isotopic event 5.1 (79.3 ka) and event 4.22 (64.1 ka), with an interpolated age of 74.0 ka, which is in good consistence with previous radiometric dating (73‐75 ka) and ice-core dating (71 ^ 5 ka) of the YTT. The tephra consists predominantly of bubble-wall shards with minor elongated vesicles of pumice fragments. Geochemical characteristics of the tephra, such as high total alkali content, high 87 Sr/ 86 Sr ratio and uniformity of their compositions, all suggest that the recovered tephra is of the Youngest Toba Tuff. This finding supports an extended dispersal of coarse (.63 mm) glass shards over 1500 km northeast of the Toba caldera, a direction opposite to what previously conceived. While providing a better documentation of the distribution extent of the Toba ash, this report points to the need to reestimating the eruptive volume of the YTT and re-evaluating its environmental impact. q 2000 Elsevier Science B.V. All rights reserved.

Geochemical variation of soil–gas composition for fault trace and earthquake precursory studies along the Hsincheng fault in NW Taiwan

The present study is proposed to investigate geochemical variations of soil-gas composition in the vicinity of the geologic fault zone of Hsincheng in the Hsinchu area of Taiwan. Soil-gas surveys have been conducted across the Hsincheng fault, to look for the degassing pattern of this fault system. During the surveys, soil-gas samples were collected along traverses crossing the observed structures. The collected soil-gas samples were analysed for He, Rn, CO(2), CH(4), Ar, O(2) and N(2). The data analysis clearly reveals anomalous values along the fault. Before selecting a monitoring site, the occurrence of deeper gas emanation was investigated by the soil-gas surveys and followed by continuous monitoring of some selected sites with respect to tectonic activity to check the sensitivity of the sites. A site was selected for long term monitoring on the basis of coexistence of high concentration of helium, radon and carrier gases and sensitivity towards the tectonic activity in the region. A continuous monitoring station was established at Hsinchu National Industrial Science Park (HNISP) in October 2005. Preliminary results of the monitoring station have shown possible precursory signals for some earthquake events.

Radon, helium and uranium survey in some thermal springs located in NW Himalayas, India: mobilization by tectonic features or by geochemical barriers?

Radon, helium and uranium measurements have been carried out in hot water springs in the Parbati and Beas valleys of Himachal Himalaya. Most of these hot springs are known as famous pilgrimage centers. The activity of dissolved radon in the liquid phase is found to vary widely, by an order of magnitude, between 10 and 750 Bq L(-1), whereas, the dissolved helium content in these thermal springs varies between 10 and 100 ppm. The uranium contents are low and vary from <0.01 to 5 microg L(-1). The measured values of radon, helium and uranium are possibly controlled by structural geology, namely the presence of pervious fault systems, and by the lithology of the leached host rocks. Redox-potential geochemical barriers cause the mobilization of uranile ions in solution (UO2+); the most plausible hypothesis is when the conditions are oxidising, confirming the importance of physico-chemical conditions up to the supergenic environment, to control the fluid geochemistry of the U-He-222Rn system. Some evidence is available from both geothermometric considerations and geochemical data which will be reported elsewhere, whereas the present study is focused on U decay series-noble gas geochemistry. The first analysis of collected 3He/4He data is consistent with a crustal signature at the studied thermal springs.

The thermal history of the lhasa block, South Tibetan Plateau based on FTD and ArAr dating

Abstract Twelve basement samples were collected from South Tibet Plateau for FTD and ArAr analysis to demonstrate their uplifting history since Cenozoic era. The preliminary results from different minerals with different closure temperatures, including apatite and zircon for fission-track dating, and K-feldspar, biotite for ArAr dating, show that at least four stages of thermal history can be recognized in the studied area.

Volcanism above a subducted extinct spreading center: a reconnaissance study of the North Luzon Segment of the Taiwan-Luzon Volcanic Arc (Philippines)

Abstract The aseismic ridge system of the South China Sea, extinct since about 17 Ma, subducts below the central part of the Taiwan-Luzon volcanic arc, North Luzon Segment (NLS). The buoyancy of the relatively young and warm crust of the ridge system may be responsible for the high elevation of the NLS. Volcanism in this part of the arc is characterized by the eruption of small volumes of intermediate to felsic magma (57–65% SiO 2 ). The eruption products from the volcanic front in the central part of the Cordillera form a low-K, calc-alkaline suite, characterized by high Sr and low HREE and Y contents. Some workers consider these characteristics to be the geochemical fingerprints of magmas generated by slab-melting. However, model calculations require the amount of residual garnet in the source to be small (⩽ 10%), in contrast to experimental work that indicates high amounts of residual garnet (⩾ 30%). Alternatively, the melts may have been generated by melting of amphibolitic lower crust. The magmas erupted in the eastern and southern parts of the segment are calc-alkaline suites with moderate to high K contents and Sr and Y contents typical for arc suites. The small volume and evolved composition of the erupted magmas may be related to the uplift of the NLS with the crust acting as a density filter, which prevents more mafic magmas from reaching the surface.

Radon-thoron monitoring in Tatun volcanic areas of northern Taiwan using LR-115 alpha track detector technique: Pre-calibration and installation

In the present study, experiments have been carried out to calibrate LR-115 alpha detector films in bare and cup-mode exposure for the measurement of radon and thoron concentrations in soil gas. Results showed non-uniformity in track formation on the films in bare-mode exposure. However, in cup-mode exposure the non-uniformity was reduced to a greater extent. The calibration factors obtained for radon in bare- and cup-mode exposures are 0.049 and 0.034 tr. cm−2 per Bq m−3 d, respectively. An attempt has been made to calibrate the radon-thoron discriminative cup with LR-115 films for simultaneous measurements of radon and thoron. This paper also presents the preliminary results of radonthoron monitoring in Tatun volcanic areas of northern Taiwan for the first time using radon-thoron discriminators with LR-115 films. The results show that the safe temperature to install the LR-115 films in volcanic areas is ≤ 65°C and thoron concentration in the study area is low.

Production, consumption, and migration of methane in accretionary prism of southwestern Taiwan

To systematically quantify the production, consumption and migration of methane along a continental margin, 210 sediment cores were collected from offshore southwestern Taiwan and analyzed for their gas and aqueous geochemistry. These data, combined with published results, were used to calculate the diffusive methane fluxes across different geochemical transitions and to develop scenarios of mass balance to constrain deep microbial and thermogenic methane production rates within the accretionary prism. The results showed that methane diffusive fluxes ranged from 2.71 × 10−3 to 2.78 × 10−1 and from ‒1.88 × 10−1 to 3.97 mmol m−2 d−1 at the sulfate-methane-transition-zone (SMTZ) and sediment-seawater interfaces, respectively. High methane fluxes tend to be associated with structural features, suggesting a strong structural control on the methane transport. A significant portion of ascending methane (>50%) is consumed by anaerobic oxidation of methane at the SMTZ at most sites, indicating effective biological filtration. Gas compositions and isotopes revealed a transition from the predominance of microbial methane in the passive margin to thermogenic methane at the upper slope of the active margin and onshore mud volcanoes. Methane production and consumption at shallow depths were nearly offset with a small fraction of residual methane discharged into seawater. The flux imbalance arose primarily due to the larger production of methane through deep microbial and thermogenic processes at a magnitude of 1,512 – 43,096 Tg Myr−1 methane and could be likely accounted for by the sequestration of methane into hydrate forms, and clay absorption.

Active mud volcanoes in the gas hydrate potential area of the upper Kaoping Slope, off southwest Taiwan

Thirteen mud volcanoes are identified from the multibeam bathymetric data in the gas hydrate potential area of the upper Kaoping Slope, off southwest Taiwan. The heights of the mud volcanoes range from 65 m to 345 m and the size of their bases range from 680 m to 4,100 m in diameter. The slopes of the mud volcanoes are very steep (from 5.3° to 13.6°), suggesting that the mud volcanoes are fed by high-viscosity flows. ROV observations reveal active eruptions of the mud volcanoes MV1, MV5 and MV12, and two modes of eruption (explosive and effusive eruptions) were identified. The MV1 and MV5 are characterized by explosive eruptions and the eruptive cycles are about 3~5 and 5~8 minutes, respectively. The MV12 shows an effusive eruption, characterized by the continuous outpouring of mud together with gas plume. The gas plumes were found on tops of mud volcanoes MV1, MV4, MV5, MV10 and MV12, as indicated by the high-frequency sonar (EK500, EK60 sonars and multibeam echo sounder) images. The mud flows on the flank of six mud volcanoes (MV1, MV3, MV5, MV6, MV9 and MV10), illustrated by the high backscatter intensity bands from the sidescan sonar images. As evidenced by the results of investigation, the mud volcanoes in the upper Kaoping Slope off SW Taiwan are very active.

PAST AND PRESENT RECORDS OF GAS HYDRATE GEOCHEMICAL SIGNATURES IN A TERRIGENOUS MATERIALS DOMINATED ACTIVE MARGIN, SOUTHWEST OF TAIWAN

Temporal variations in gas hydrate related geochemical signatures under different deposition conditions are the primary purposes of this study. Accreted wedge located offshore Southwestern Taiwan receives high terrigenous river materials, 100 MT/yr, at present time. It is not clear how seep environment varied during the past glacial. A 25 meters long piston core was taken offshore Southwestern Taiwan on r/v Marion DuFresne. Short piston cores and box cores were also taken on r/v OR-1. Samples were analyzed for pore water dissolved sulfide, sulfate, methane, chloride, del O18, calcium, magnesium, alkalinity, pH, and sediment AVS, pyrite, inorganic carbon, del O18, C13. Changes in deposition environment play a major role in the study area. Three stages of geochemical processes are identified in the 25 meters long core, interchange between reduce and oxic depositional environments, with reducing condition in the top 10 m, oxic in between 10-20 meter and reducing below the 20 meter. High concentrations of dissolved sulfide, rapid sulfate depletion, increase of methane, decrease of calcium were found in pore water in the top 10 m of sediments together with high concentrations of pyrite, relatively higher proportion of coarsegrained sediment. Concentrations of pyrite were very low in sediments between 15 to 20 meters but increased rapidly from 20 to 25 meters with a maximum concentration at 400 umol/g. Chloride concentrations also increased to a maximum concentration of 630 mM at 20 m. The rapid increase of chloride indicated gas hydrate formation at this depth. Authigenic carbonate