Hua-Ning Qiu

40Ar39Ar dating of the quartz samples from two mineral deposits in western Yunnan (SW China) by crushing in vacuum

Abstract The primary aim of this study is to investigate the possibility of dating fluid inclusions of quartz samples using the 40 Ar 39 Ar method by crushing in vacuo. The samples are from the Lushui Tin-tungsten Deposit and the Shangmanggang Gold Deposit in western Yunnan, SW China. Coexisting muscovite S-90MS and quartz S-90Qw in a piece of tungsten ore from the former deposit were selected, and the isochron age of S-90Qw by crushing (38.6 ± 1.9 Ma) shows excellent coincidence with that of S-90MS by stepped heating (38.6 ± 0.8 Ma). The isochron age and plateau age by crushing of the quartz SMG-8Qz from the latter deposit indicate that the gold mineralization occurred at ∼ 144 Ma. These ages are respectively agreeable with the evaluated ages from the geologic facts. In addition, since the potassium concentration (calculated from 39 Ar released by crushing) in the quartz sample SMG-8Qz was as high as 0.24%, electron-probe analysis was applied to investigate the occurrence of potassium, and the results reveal that the majority of potassium scattered in the microcrystal quartz grains.

High-precision 40Ar/39Ar age of the gas emplacement into the Songliao Basin

The problem of determining an exact isotopic age of hydrocarbon emplacement is complex because minerals suitable for dating with common isotopic methods are often lacking in the sedimentary domain. However, the igneous quartz from the Cretaceous volcanic rocks that host the gas reservoir in the Songliao Basin (northeastern China), contains abundant secondary fluid inclusions with high concentrations of K and high partial pressures of methane trapped during gas emplacement. Quartz with abundant K-rich fluid inclusions provides an excellent closed system well suited for 40 Ar/ 39 Ar dating. Three igneous quartz samples were measured by stepwise crushing to release the inclusion-based argon gas. All three samples yielded well-defined isochrons with ages in close agreement, precisely constraining the gas emplacement at 42.4 ± 0.5 Ma (2σ) below the Daqing oil field in the Songliao Basin, extending possible gas reservoirs from the upper Cretaceous to the middle Eocene.

Dating petroleum emplacement by illite 40Ar/39Ar laser stepwise heating

The timing of petroleum emplacement is crucial in understanding petroleum-forming processes and predicting new prospecting regions. This study investigates the possibility of determining the age of petroleum emplacement by an illite 40Ar/39Ar laser stepwise heating technique. A specially designed apparatus has been used to clean up the organic gases in samples contaminated with residues from oil or gas fields, resulting in credible 40Ar/39Ar data. The sandstone samples are from the Zhuhai and Zhujiang formations in the Huizhou sag in the Pearl River Mouth Basin, South China Sea. Illite 40Ar/39Ar laser stepwise heating consistently yields gradually rising age spectra at the low-temperature steps until reaching age plateaus at high-temperature steps. The first-step apparent ages of the six samples from the Zhujiang formation give a weighted mean age of 11.4 0.5 Ma (1), in agreement with an age of 12.1 1.1 Ma in the first three steps of the sample from the Zhuhai Formation. These dates are interpreted as being caused by the formation of fine authigenic illite and therefore indicative of the maximum age of petroleum emplacement in the Huizhou sag because illite is commonly one of the latest forming minerals prior to hydrocarbon accumulation. The plateau ages ranging from 58 to 276 Ma in high laser-output steps are interpreted as being caused by variable contributions from detrital K feldspar in the sandstones. The older total ages based on the illite 40Ar/39Ar stepwise heating results are equal to the K-Ar ages and are not usable in delineating petroleum emplacement because they represent mixtures with variable contributions from authigenic illite and detrital K feldspar. The 40Ar/39Ar stepwise heating technique thus can be applied to resolve important petroleum emplacement age information from illite samples that otherwise cannot be provided by traditional K-Ar methods.

Coherence of the Dabie Shan UHPM terrane investigated by Lu-Hf and 40 Ar/ 39 Ar dating of eclogites

Publisher Summary The Central China Orogenic Belt (CCOB) is the largest known ultrahigh-pressure metamorphic (UHPM) belt. It extends from Sulu in Eastern China, via the Dabie, Qinling, and North Qaidam mountains to Altyn Tagh some 3000 km to the west. Since the discovery of coesite and diamond-bearing metamorphic rocks in Dabie Shan, the belt has been studied intensively. Some of the results have led to controversy regarding the number of (U)HP metamorphic events and their absolute and relative ages. The prominence of Dabie Shan as part of the largest UHPM belt is reflected by a wealth of published work since the discovery of coesite over 20 years ago. Despite two decades of efforts to decipher its geologic secrets, interpretation of the many reported isotopic ages of the Dabie Shan terrane that aimed to address its metamorphic history is still hampered by the absence of robust thermobarometric data for the same samples. Pre-Triassic metamorphism cannot be traced by Lu–Hf Grt–Cpx geochronology in the main volume of Dabie Shan, which was subjected to Triassic (U)HP metamorphism. The Triassic event erased previous signatures of some, but not all isotopic systems, as is evident from zircon U-Pb ages.

Automatic 40 Ar/ 39 Ar Dating Techniques Using Multicollector ARGUS VI Noble Gas Mass Spectrometer with Self-Made Peripheral Apparatus

A new fully automatic 40Ar/39Ar laboratory with a Thermo Scientific© ARGUS VI mass spectrometer has been established in China University of Geosciences (Wuhan). We designed and developed a mini efficient preparation system (80 mL), a CO2 laser for heating samples, a crusher for extracting fluid inclusions within K-poor minerals and an air reservoir (31 L) and pipette (0.1 mL) system. The ARGUS VI mass spectrometer is operated by the Qtegra Noble Gas software, which can control the peripheral accessories, such as pneumatic valves, CO2 laser and crusher through a PeriCon (peripheral controller). The experimental procedures of atmospheric argon analyses, 40Ar/39Ar dating by laser stepwise heating and by progressive crushing in vacuo, can be fully automatically performed. The weighted mean of atmospheric 40Ar/36Ar ratios is 302.22±0.03 (1σ, MSWD=0.74, n=200), indicating that air reservoir and pipette system and the whole instrument system are very stable. This laboratory is a successful pioneer example in China to establish a new noble gas laboratory with self-made peripheral accessories expect for the mass spectrometer.

Structural Features and Proto-Type Basin Reconstructions of the Bay of Bengal Basin: A Remnant Ocean Basin Model

Remnant ocean basin is a key to understand the plate suturing and subsequent uplift and erosion of orogen. The Bay of Bengal Basin (BOBB) provides a typical example to analyze the remnant ocean basin structures, evolution, and relationships between depositional filling and uplifting of the Himalayan Orogen. Thirty-nine seismic profiles as well as interval velocities of well BODC3 were used to compile isopach maps of the basin. Among the seismic data, 26 seismic profiles were applied to establish 8 cross sections. The cross sections suggest the basin is asymmetric, bounded to the west by the eastern continental margin of India (ECMI) with graben-horst and to the east by the Sunda convergence margin dominated by trench-arc system. The BOBB is characterized by a prominent down flexure structures caused by huge amount of Bengal fan turbidite sediments accumulation. Our isopach maps and chronology data collected from adjacent regions reveal the initial development and fast southward growth of the Bengal fan were related to the early and major stage uplift and erosion of the Himalayan Orogen, respectively. The BOBB has experienced a critical transition from an ocean basin to a remnant ocean basin at Late Oligocene. Such basin structures and evolution features indicate the BOBB provides whole records of oblique convergence of the India and Asia plates, and the early and major stage evolution of the Himalayan Orogen.

A novel purification technique for noble gas isotope analyses of authigenic minerals

Noble and active gases are released from geological samples during gas extraction for noble gas isotope analyses. The active gases should be removed before inletting to mass spectrometers for the analyses. The normal noble gas preparation systems can clean up most geological samples. However, authigenic minerals from sedimentary rocks in oil/gas fields contain organic matter, which cannot be cleaned up by the normal preparation systems and thus influence the noble gas analyses. We introduce a novel gas purification system (PRC patent No. ZL201320117751.2), which includes several reversible purification pumps with different absorbing and degassing temperatures. It can well clean up water steam, carbon dioxide and organic gases. Mica minerals are often used for 40Ar/39Ar dating. A muscovite sample (2082MS) which could not be cleaned up by the normal preparation system with two SAES NP10® getters, becomes the test sample for a comparative experiment in this study. The experiment is assigned into 4 sections with the organic gas removal system (OGRS) “Closed/Opened” in turn. When the OGRS is closed only with two NP10 getters for purification, the 40Ar intensities increase in curves with inlet time because of impurities, the 40Ar/39Ar dating results yield age errors about ±2%–±1% (2σ). When the OGRS is opened for purification, in contrast, the 40Ar intensities decrease linearly with inlet time. This indicates that the gases have been cleaned up effectively, and the 40Ar/39Ar results yield ages with errors in ±0.4%. The OGRS is very helpful to obtain high-quality analysis data.

Metamorphic P-T path differences between the two UHP terranes of Sulu orogen, Eastern China: petrologic comparison between eclogites from Donghai and Rongcheng.

The Sulu Orogen constitutes the eastern part of the Sulu-Dabie Orogen formed by Triassic collision between the Sino-Korean and Yangtze plates. An HP Slice I and two UHP slices II and III with contrasting subduction and exhumation histories within the Sulu Orogen were postulated. This study presents the metamorphic P-T paths of eclogites from the two UHP belts constructed by petrography, mineral chemistry and Perple_X P-T pseudosection modeling in the MnC(K)NFMASHO system. Eclogites from Slice III mainly consist of omphacite, garnet and quartz, with minor rutile, ilmenite, amphibole and phengite. Eclogites from Slice II show a porphyroblastic texture with epidote porphyroblasts and garnet, omphacite, phengite, quartz and rutile in matrix. Pseudosection modeling reveals that eclogites from Slice II witness a peak metamorphism of eclogite-facies under conditions of 3.1–3.3 GPa and 660–690 ºC, and a retrograde cooling decompression process. The eclogites from Slice III record a heating decompressive P-T path with a peak-P stage of 3.2 GPa and 840 ºC and a peak-T stage of 2.4 GPa and 950 ºC, suggesting an apparent granulite-facies metamorphism overprint during exhumation. Both eclogites recorded clockwise P-T paths with peak P-T conditions suggesting a subduction beneath the Sino-Korean Plate to ~100–105 km depth. Combined with tectonic scenarios from previous studies, it is concluded that the two UHP crustal slices in the Sulu terrane have a similar geodynamic evolution, but the UHP rocks in Slice II exhumed after the eclogitic peak-pressure conditions earlier than that of Slice III. The existence of Slice II diminished the buoyancy force on Slice III, resulting in a granulite-facies overprint on Slice III. The Sulu orogenic belt is made up of different crustal slices that underwent different subduction and exhumation histories, rather than a single unit.