Laser Stepwise Heating Research Articles

Fluid inclusion 40Ar/39Ar geochronology of andalusite from syn-tectonic quartz veins: New perspectives on dating deformation and metamorphism in low-pressure metamorphic belts

Syn-tectonic quartz veins are widely present in low- to medium-grade metapelitic belts, however, dating these veins is challenging due to the lack of suitable target minerals for most geochronological methods. Andalusite is a common K-poor aluminosilicate mineral in these quartz veins. In this study, we demonstrate the feasibility of dating fluid inclusions in andalusite using the 40Ar/39Ar gentle stepwise crushing technique. Three andalusite samples in syn-tectonic quartz veins from the Chinese Altai, Central Asia yielded similar gas release patterns. The first crushing steps generated significant excess 40Ar which likely originated from secondary gas inclusions. These steps gave anomalously old and quickly declining apparent ages. The following crushing steps generated less significant excess 40Ar and yielded concordant and well-defined Early Permian ages of 282–277 Ma. These steps were likely dominated by the degassing of intra-crystal liquid-rich fluid inclusion planes. Two of the crushed powder samples from the stepwise crushing experiments were selected for additional 40Ar/39Ar stepwise heating using a furnace and yielded a much younger age of 216 Ma. Laser stepwise heating of muscovite flakes in two of the investigated andalusite samples and one chloridized biotite sample from a different andalusite-bearing quartz vein yielded 40Ar/39Ar ages of 243–241 Ma and 214 Ma, respectively. Compared with gases released from the crushing experiments, gases extracted from the heating experiments showed distinct Cl–K–Ca compositional characteristics and younger ages with near atmospheric initial 40Ar/36Ar value. These observations suggest that 1) contrary to common wisdom, excess 40Ar in K-poor nesosilicates are preferably preserved in fluid inclusions rather than mineral lattices, and 2) K (Ar) contamination from K-bearing solid phases is negligible during gentle stepwise crushing. As such, the 282–277 Ma fluid inclusion 40Ar/39Ar ages could be considered as the timing of andalusite growth in the quartz veins. The younger muscovite 40Ar/39Ar ages likely record more recent cooling events. The youngest 40Ar/39Ar ages of the crushed andalusite powder and the biotite sample could reflect the timing of sericitization and chloritization of the samples, which is evidenced by petrologic observations and x-ray K mapping results. Furthermore, andalusite 40Ar/39Ar ages agree with the monazite/zircon U–Pb ages (280–273 Ma) of syn-tectonic leucogranite and pegmatite dykes and metamorphic ages (299–262 Ma) for the low-pressure metamorphic rocks of the region. These results suggest that fluid inclusion 40Ar/39Ar geochronology of andalusite can be used to constrain the timing of deformation and associated metamorphism in low-pressure metapelitic terranes, which can also provide more complete pictures of the tectono-thermal evolution in combination with traditional 40Ar/39Ar stepwise heating geochronology.

Occurrence of Excess 40Ar in Amphibole: Implications of 40Ar/39Ar Dating by Laser Stepwise Heating and in vacuo Crushing

The joint methods of 40Ar/39Ar laser stepwise heating and in vacuo crushing have been applied to date amphiboles from the North Qaidam ultra-high pressure metamorphic amphibolites. Two amphibole samples analyzed by laser heating yielded saddle-shaped age spectra with total gas ages of 574.5±2.5 and 562.5±2.5 Ma. These ages are much older than the reported zircon U-Pb ages (∼495 Ma) from Yuka eclogite, indicating the presence of excess 40Ar. In order to decipher the occurrence of excess 40Ar and constrain the age of amphibolite-facies retrogression, two duplicate amphibole samples were further employed for 40Ar/39Ar in vacuo crushing analyses. Both samples exhibit similar monotonically declining release spectra, which are characterized by rapid decline of anomalously old apparent ages in the early steps. The data of the late steps yielded concordant apparent ages with plateau ages of 460.9±1.2 and 459.6±1.8 Ma. We interpret that gases released in the early steps derive from the significant excess 40Ar containing secondary fluid inclusions (SFIs) due to their distribution characteristics along cracks leading to be easily extracted, whereas those released in the later steps represent the contribution of the small primary fluid inclusions (PFIs).

Automatic 40Ar/39Ar 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.

Direct dating of tin–tungsten mineralization of the Piaotang tungsten deposit, South China, by 40Ar/39Ar progressive crushing

Dating ore minerals is the most direct method to determine the mineralization age of a deposit, but only a few ore minerals can be directly dated by traditional isotopic geochronometers. The aim of this study is to investigate the possibility of direct dating of the ore minerals cassiterite and wolframite from the Piaotang tungsten deposit using the 40Ar/39Ar stepwise crushing technique, comparing the age to coexisting K-rich muscovite dated by 40Ar/39Ar laser stepwise heating. The cassiterite, wolframite and muscovite samples were separated from four pieces of ore hand-specimens. The 40Ar/39Ar isochron ages of cassiterite and wolframite are concordant with ages of their coexisting muscovite, indicating that cassiterite and wolframite are suitable ore minerals for directly dating the ore-forming event by 40Ar/39Ar stepwise crushing.

40Ar/39Ar geochronology constraints on the formation age of Myanmar jadeitite

The jadeitite that occurs in Myanmar is one of the largest and most important jadeite deposits in the world. However, the formation age of the Myanmar jadeitite remains poorly constrained. Here, we conduct an integrated study using petrology and 40Ar/39Ar geochronology to analyze the Myanmar jadeitite. Petrological observations show that the studied jadeitite consists mainly of jadeite minor sodic–calcic amphibole and analcime and that the jadeitite was cut by late amphibole rock consisting of sodic amphibole and minor chlorite. 40Ar/39Ar laser stepwise heating dating of jadeite from the jadeitite yielded a plateau age of 123.9±3.4Ma, the sodic–calcic amphibole associated with jadeite yielded a plateau age of 134.8±1.4Ma, and the sodic amphibole of late amphibole rock yielded a plateau age of 92.7±1.2Ma. These results indicate that the studied jadeitite was formed during the early Cretaceous from (135Ma) high-pressure (HP) metasomatism and then experienced a late Cretaceous (93Ma) HP metasomatism. Combining our results with previous studies using petrography and geochronology, we suggest that the formation process of Myanmar jadeitite involved multistages of metasomatism.

Detrital K‐feldspar 40Ar/39Ar Ages: Source Constraints of the Lower Miocene Sandstones in the Pearl River Mouth Basin, South China Sea

Abstract:The South China Sea began to outspread in the Oligocene. A great quantity of terraneous detritus was deposited in the northern continental shelf of the sea, mostly in Pearl River Mouth Basin, which constituted the main paleo‐Pearl River Delta. The delta developed for a long geological time and formed a superimposed area. Almost all the oil and gas fields of detrital rock reservoir distribute in this delta. Thirty‐three oil sandstone core samples in the Zhujiang Formation, lower Miocene (23–16 Ma), were collected from nine wells. The illite samples with detrital K feldspar (Kfs) separated from these sandstone cores in four sub‐structural belts were analysed by the high‐precision 40Ar/39Ar laser stepwise heating technique. All 33 illite 40Ar/39Ar data consistently yielded gradually rising age spectra at the low‐temperature steps until reaching age plateaus at mid‐high temperature steps. The youngest ages corresponding to the beginning steps were interpreted as the hydrocarbon accumulation ages and the plateau ages in mid‐high temperature steps as the contributions of the detrital feldspar representing the ages of the granitic parent rocks in the provenances. The ages of the detrital feldspar from the Zhujiang Formation in the four sub‐structural belts were different: (1) the late Cretaceous ages in the Lufeng 13 fault structural belt; (2) the late Cretaceous and early Cretaceous‐Jurassic ages in the Huizhou 21 buried hill‐fault belt; (3) the Jurassic and Triassic ages in the Xijiang 24 buried hill‐fault belt; and (4) the early Cretaceous – late Jurassic ages in the Panyu 4 oil area. These detrital feldspar 40Ar/39Ar ages become younger and younger from west to east, corresponding to the age distribution of the granites in the adjacent Guangdong Province, Southern China.

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

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

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

Isotopic dating of diagenetic processes in relation to hydrocarbon charge in buried sedimentary sequences is of importance to the oil industry. Yun et al. recently published in the AAPG Bulletin (v. 94, no. 6, p. 759–771) on that topic a study entitled, “Dating Petroleum Emplacement by Illite 40Ar/39Ar Laser Stepwise Heating Technique.” This study raises questions about basic aspects of 40Ar/39Ar dating fine-grained authigenic clay minerals, describes flawed methodology, and ignores important previous work. As isotopic dating of diagenetic processes may interest readers of the AAPG Bulletin that are not necessarily specialists in either isotopic geochronology or clay mineralogy, it appeared necessary to comment on both the technical aspects and the scientific interpretations of this study. We discuss especially the fact that the authors discarded 39Ar recoil followed by an evaluation of its impact on the results and the interpretations.

锡石<sup>40</sup>Ar/<sup>39</sup>Ar 法直接定年探讨

The aim of this study is to investigate the possibility of dating fluid inclusions in cassiterite using the 40Ar/39Ar technique by crushing in vacuo . Paragenous muscovite and cassiterite in a piece of tungsten ore from the Piaotang tungsten deposit in southern Jiangxi Province were selected. Laser stepwise heating of the muscovite yields a flat age spectrum with a plateau age of 159.3±1.5 Ma. The 40Ar/39Ar dating result of cassiterite by crushing forms a declining age spectrum marked by the highest apparent age of 3954 Ma at the 1st stage, then shows an apparent decline step-by-step, and finally a plateau appears with a plateau age of 159.1±1.8 Ma in the last 12 stages (from 7 to 18, or about 83% of the total 39Ar). The data points of cassiterite clearly distribute into two groups corresponding to secondary and primary fluid inclusions, and their mixtures. The secondary ones contain excess 40Ar and their data points scatter on the inverse isochron diagram of 36Ar/40Ar vs. 39Ar/40Ar, while the primary ones do not contain excess 40Ar and their data points yield an excellent isochron line corresponding to an age of 158.6±2.9 Ma. The 40Ar/39Ar isochron age of the cassiterite by crushing is well concordant with the age of the paragenous muscovite by 40Ar/39Ar stepwise heating, indicating that mineralization occurred at 159 Ma. This study reveals that cassiterite is a suitable dating target for 40Ar/39Ar stepwise crushing. Moreover, the isochron age of the primary inclusions represent the mineralization age of the Piaotang tungsten deposit.

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.

Amphibolite facies retrograde metamorphism of the Zhujiachong eclogite, SE Dabieshan: 40Ar/39Ar age constraints from argon extraction using UV‐laser microprobe, in vacuo crushing and stepwise heating

AbstractThe Zhujiachong eclogite in the south‐eastern Dabieshan ultra‐high‐P terrane has been overprinted during retrograde metamorphism, with the development of garnet‐amphibolite mineral assemblages in most rocks in the outcrop. This study is focused on providing age constraints for the retrograde amphibolite facies and greenschist facies mineralogy by 40Ar/39Ar dating. By applying a novel approach of combining three different techniques for extracting argon: laser stepwise heating of single grains and small separates, a spot fusion technique by UV‐laser ablation microprobe on polished sections and an in vacuo crushing technique for liberating radiogenic argon from fluid inclusions, it is demonstrated that an internally consistent thermal history can be derived. The 40Ar/39Ar ages indicate that phengite formed before 265 Ma, probably during the ultra‐high‐P event. Ages associated with amphibolite facies retrograde metamorphism range from 242 to 217 Ma by the analyses of amphibole. Ages of c. 230 Ma were found for the symplectite matrix that formed during retrogression from eclogite pyroxene. Late stage hydrothermal activity leading to the formation of coarse‐grained paragonite and fluid inclusions in vein amphibole was dated at c. 200 Ma. These age results agree well with the mineral crystallization sequence observed from thin‐sections of the retrograded eclogite: phengite → paragonite and amphibole in matrix → amphibole in the corona.

Timing of Hydrocarbon Fluid Emplacement in Sandstone Reservoirs in Neogene in Huizhou Sag, Southern China Sea, by Authigenic Illite 40Ar- 39Ar Laser Stepwise Heating

Timing of oil or gas emplacements is a new subject in isotopic geochronology and petroleum geology. Hamilton et al. expounded the principle of the illite K-Ar age: Illite is often the last or one of the latest mineral cements to form prior to hydrocarbon accumulation. Since the displacement of formation water by hydrocarbons will cause silicate diagenesis to cease, K-Ar ages for illite will constrain the timing of this event, and also constrain the maximum age of formation of the trap structure. In this study, the possibility of authigenic illites 40Ar- 39Ar dating has been investigated. The illite samples were separated from the Tertiary sandstones in three rich oil reservoir belts within the Huizhou sag by cleaning, fracturing by cycled cooling–heating, soxhlet-extraction with solvents of benzene and methanol and separating with centrifugal machine. If oil is present in the separated samples, ionized organic fragments with m/e ratios of 36 to 40 covering the argon isotopes will be yielded by the ion source of a mass spectrometer, resulting in wrong argon isotopic analyses and wrong 40Ar- 39Ar ages. The preliminary experiments of illite by heating did show the presence of ionized organic fragments with m/e ratios of 36 to 44. In order to clean up the organic gases completely and obtain reliable analysis results, a special purification apparatus has been established by Qiu et al. and proved valid by the sequent illite analyses. All the illite samples by 40Ar- 39Ar IR-laser stepwise heating yield stair-up age spectra in lower laser steps and plateaux in higher laser steps. The youngest apparent ages corresponding to the beginning steps are reasonable to be interpreted for the hydrocarbon accumulation ages. The weighted mean ages of the illites from the Zhuhai and Zhujiang Formations are (12.1 ± 1.1) Ma and (9.9 ± 1.2) Ma, respectively. Therefore, the critical emplacement of petroleum accumulation in Zhujiang Formation in Huizhou sag took place in ca 10 Ma. Late fault activity strengthened the entrance of hydrocarbon fluids into the oil systems based on the data of the studies of the fault evolvement history, petroleum system combinations, and homogeneous temperatures of fluid inclusions.

Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: New insights from 40Ar/ 39Ar dating by stepwise crushing

The 40Ar/ 39Ar stepwise crushing technique is applied for the first time to date garnet from ultra-high-pressure metamorphic (UHPM) eclogites. Three garnet samples from the Bixiling eclogites analyzed by 40Ar/ 39Ar stepwise crushing yield regular, predictable age spectra, and a clear separation between excess 40Ar and concordant plateau and isochron ages. All three age spectra begin with high apparent ages followed by step by step decreasing ages, and finally age plateaux with apparent ages in the range from 427 ± 20 to 444 ± 10 Ma. The data points constituting the age plateaux yield excellent isochrons with radiogenic intercept ages ranging from 448 ± 34 to 459 ± 58 Ma, corresponding to initial 40Ar/ 36Ar ratios from 292.1 ± 4.5 to 294.5 ± 6.7, statistically indistinguishable from the modern air. The high initial ages are interpreted to derive from secondary fluid inclusions containing excess 40Ar, whereas the plateau ages are attributed to gas from small primary fluid inclusions without significant excess 40Ar. The plateau ages are interpreted to approximate the time of garnet growth during initial UHPM metamorphism. Phengite analyzed by laser stepwise heating yielded a complicated two-saddle age spectrum with a scattered isochron corresponding to age of 463 ± 116 Ma and initial 40Ar/ 36Ar ratio of 1843 ± 1740 indicative of the presence of extraneous 40Ar within phengite. These concordant isochron ages measured on minerals diagnostic of eclogite grade metamorphism strongly suggest that Dabie UHPM eclogites were first formed in the early Paleozoic, during the same event that caused the Qinling–Northern Qaidam Basin–Altyn Tagh eclogites.

New dates (415 Ma) for the Etive Dyke Swarm and the end of the Caledonian Orogeny in the SW Grampian Highlands of Scotland

New laser stepwise heating 40 Ar– 39 Ar dates (415 ± 1.8 and 414 ± 2 Ma) from the Etive Dyke Swarm in the SW Grampian Highlands of Scotland indicate that the dykes are 10–20 Ma older than previously assumed. The dykes constitute the final phase of the Late Caledonian Granites in the region, which themselves define the end of the Caledonian Orogeny, therefore these dates can be taken as the end of the orogeny in this area. The dates show a temporal link between the Etive magmatism and early precious metal mineralization at Tyndrum as well as emphasizing the NW to SE diachronous nature of the end of Caledonian magmatism across Scotland.

Ar–Ar ages in phlogopites from marble-hosted ruby deposits in northern Vietnam: evidence for Cenozoic ruby formation

Ruby growth in phlogopite-bearing marbles has been indirectly dated using the 40Ar/ 39Ar laser stepwise heating technique on purified syngenetic phlogopite and other micas from ruby deposits in Yen Bai, Luc Yen and Quy Chau mining districts, in northern Vietnam. The principal results indicate the following. (1) Across the Red River shear zone, the phlogopites from the Yen Bai deposits yielded Miocene cooling ages between 23.2 and 24.4 Ma identical to those previously published using the same dating method on magmatic and metamorphic rocks from the Day Nui Con Voi range. (2) Luc Yen ruby deposits in the Lo Gam zone, on the eastern flank of the Red River shear zone, yielded Oligocene cooling 40Ar/ 39Ar mica ages between 30.8 and 34.0 Ma. Regarding the age of ruby crystallisation itself, the most plausible hypothesis is that all rubies in both zones formed during the period 40 to 35 Ma. Diachronism of cooling in adjacent zones leads to the conclusion that around 35 Ma, the ductile deformation in the Lo Gam zone ended and the ruby-bearing marbles cooled rapidly while the high-temperature deformation remained in the Red River shear zone, resulting in cooling through blocking temperature, some 15 Ma later. (3) The Quy Chau ruby deposit is restricted to the Quy Chau shear zone that bounds the eastern part of the Oligocene–Miocene Bu Khang dome. Phlogopite and biotite samples reveal Miocene cooling ages between 21 and 22.5 Ma which are minimum ages for ruby formation. These ages could be linked with the end of the extension of the Bu Khang dome. Vietnamese ruby formation is linked to Cenozoic tectonics resulting from continental collision between the Asian and Eurasian plates as for other marble-hosted ruby deposits in Central and Southeast Asia.