氩气ICP中“电荷迁移”激发机理的研究

在氩气ICP中,氩离子的密度与电子密度相当,比亚稳态氩原子的密度高1000倍左右.而且Ar~ 的有效碰撞截面和具有的能量(～16.5eV)均高于电子(～0.8eV)和Ar~m(～12.5eV)。因而以“电荷迁移”形式发生的Ar~ 对元素原子的激发应该是一个重要过程:Ar~ X→X~( *) Ar △E测量钙、镁的具有不同激发能的原子和离子谱线强度之比I_i/I_a,并与(I_i/I_a)_(LTE)相比较,发现唯有Mg(Ⅱ)279.81nm/Mg(Ⅰ)285.21nm的实验值远高于LTE值。另外,由Mg(Ⅱ)279.81nm/Mg(Ⅱ)280.27nm线对测得的激发温度高于电子温度T_e,不符合ICP中T_(exc)相似文献   

帕米尔构造结塔什库尔干碱性杂岩同位素年代学研究

利用高精度锆石SHRIMP U-Pb法测定了帕米尔构造结中东部塔什库尔干碱性杂岩的年龄,岩体不同岩石类型的年龄均为11Ma,与野外证据吻合。通过与该杂岩体钾长~(40)Ar/~(39)Ar等时线年龄(23Ma、13Ma)对比研究,~(40)Ar/~(39)Ar定年结果明显偏老,认为是由于钾长石中含有过剩氩所致,并分析了过剩氩存在的可能原因。因此塔什库尔干碱性杂岩的活动时代可以确定为11Ma,而且钾长石不宜作为该杂岩体测年的理想对象。结合岩体的成因及构造意义,认为在11Ma前,帕米尔构造结地区已具有加厚下地壳的特征。     

Dating the recent past

The dramatic environmental changes of the last 500 years are likely to continue into the future and to have an increasing impact on both the Earth and human society. Any understanding of future environmental change is thus critically dependent on our capacity to reconstruct the environmental changes of the past. Fundamental to this is an ability to place the environmental responses of the last half millennium within a reliable chronological framework. Unfortunately, this most recent part of the geological timescale presents us with some of the greatest challenges for dating. With the exception of ²³⁰Th/²³⁴U methods, whose use is restricted to the rather specific depositional environments of shallow marine and terrestrial carbonates, there is no established geochronometric tool capable of dating more than a fraction of the recent past at a resolution adequate to tackle the environmental issues of this period. This challenge has been met by refining existing procedures (including ¹⁴C, ⁴⁰Ar/³⁹Ar, event chronostratigraphy and optically stimulated luminescence) and developing new ones (such as ³²Si). These offer a means of calibrating the high-resolution environmental records of the last 500 years and answering the critical environmental questions presented by this period.     

K–Ar and 40Ar/39Ar phengite ages of Sanbagawa schist clasts from the Kuma Group, central Shikoku, southwest Japan

Conglomerates of the Kuma Group, central Shikoku, southwest Japan contain Sanbagawa schist clasts with a variety of metamorphic grades and lithologies. K–Ar and ⁴⁰Ar/³⁹Ar dating of phengite show all the pelitic schist clasts from low- to high-grade zones have similar phengite ages (82–84 Ma) that are significantly older than those from the in situ Sanbagawa sequence of central Shikoku. This is because the Kuma–Sanbagawa sequence was exhumed earlier than the in situ Asemi sequence with an exhumation process intermediate between those for the Kanto Mountains and the in situ Asemi sequences. ⁴⁰A/³⁹Ar plateau ages (103 and 117 Ma) of phengite in amphibolites indicate the timing of the early stage of the exhumation of the metamorphic pile, probably close to the peak metamorphic age.     

北大别黄土岭麻粒岩中黑云母过剩Ar的成因解释和Ar同位素热年代学研究启示

应用阶段加热脱气技术测定了北大别黄土岭麻粒岩的黑云母和共存花岗岩岩体中斜长角闪岩包体角闪石的Ar-Ar年龄。角闪石给出的坪年龄为(124.9±4.6） Ma,与区域上早白垩世120~130 Ma的变质-岩浆事件年龄相一致。黑云母给出的坪年龄为(176.9±0.8) Ma,年龄大于Ar封闭温度较高的角闪石的年龄,这表明黄土岭麻粒岩黑云母中含有过剩Ar。较低温度和富流体环境生长的晚世代黑云母是过剩Ar的主要载体。因此,在应用Ar-Ar热年代计重塑多成因同类含钾矿物的岩石地质体的冷却速率时,应用激光探针对代表峰期世代矿物进行原位测定应是正确的选择。     

Triact spicules in proterozoic rocks of the Northern Territory of Australia

Triact fragments, probably of sponge spicules, occur in a siliceous rock found in the Proterozoic succession along the coast of the Gulf of Carpentaria in the Northern Territory of Australia.     

Hydrothermal processes associated with meteorite impact structures: evidence from three Australian examples and implications for economic resources

Meteorite impacts cause conversion of kinetic energy into thermal energy. Part of this thermal energy is used to form a melt sheet, part is dissipated to heat the target rocks and these together with the hot rocks that elastically rebound from the depth of several kilometres (central uplift) activate hydrothermal circulation. Impact-generated hydrothermal systems have been documented from several impact structures world-wide. Three Australian examples—Shoemaker, Woodleigh and Yarrabubba—provide evidence of hydrothermal fluid flow both within and around the structures. Field observations, and petrographic and geochemical data suggest a common evolutionary trend of post-impact hydrothermal activity from early high-temperature alkali metasomatism to a later lower temperature H⁺ metasomatism, resulting in the overprinting by hydrous mineral assemblages. Hydrothermal systems activated by meteorite-impact events are important because they may also form economic mineral deposits, as is documented for several impact structures in the world. A working model of hydrothermal circulation in terrestrial impact structures posits two main stages: (i) initial high-temperature fluids percolate downward causing widespread alkali metasomatism of the shattered target rocks below the melt sheet, resulting in their modification to rocks of syenitic affinity; and (ii) inflow of meteoric water and progressive cooling of the melt sheet leads to a lower temperature stage, in which hydrothermal fluid flow tends to move upward, resulting in mineral assemblages and alteration patterns that resemble those of epithermal systems. In addition, these fluids can discharge at the surface as hot springs.     

Timing of brittle faulting and thermal events,Sydney region: association with the early stages of extension of East Gondwana

Structural studies in the Sydney region have revealed the presence of vertical to near-vertical, north-northeast-striking faults that are manifest as joint swarms and highly brecciated zones in which gouge of varying thickness is developed. Strike-slip movement accompanied by minor dip-slip, normal movement occurred on these faults. Timing of movement on these faults by K–Ar dating of illite and illite–smectite in fractions extracted from fault gouges, was attempted. These dates were compared with dates obtained from the host-rocks. K–Ar ages determined from the 2–10 μm to <0.1 μm fractions produced from the gouge and host-rocks, range from 159.5 ± 3.2 to 106.6 ± 2.1 Ma (n = 26). In <0.5 μm fractions extracted from the gouges that are less contaminated by detrital phases, K–Ar ages vary from 138 ± 4.4 to 106.5 ± 2.1 Ma (mean 121 Ma; n = 6) which are similar to ages obtained from host-rocks in the Sydney region. The similarity in age between the host rocks and gouge suggests that the K–Ar system has been reset. The resetting is attributed to a thermal event at ca 120 Ma related to the underplating of felsic intrusions associated with early stages of breakup of East Gondwana. Subsequent to this event, dykes of Early Eocene age (K–Ar whole-rock: 51.0 ± 1.1 Ma) exploited north-northeast-striking faults and subsequently developed brecciated margins. These observations and the fact that gouge formed before the thermal event suggests that movement took place on north-northeast-striking faults prior to 120 Ma and after 51 Ma.     

Regional-Scale Excess Ar wave in a Barrovian type metamorphic belt, eastern Tibetan Plateau

Laser step heating ⁴⁰Ar/³⁹Ar analysis of biotite and muscovite single crystals from a Barrovian type metamorphic belt in the eastern Tibetan plateau yielded consistent cooling ages of ca. 40 Ma in the sillimanite zone with peak metamorphic temperatures higher than 600°C and discordant ages from 46 to 197 Ma in the zones with lower peak temperatures. Chemical Th‐U‐Total Pb Isochron Method (CHIME) monazite (65 Ma) and sensitive high mass‐resolution ion microprobe (SHRIMP) apatite (67 Ma) dating give the age of peak metamorphism in the sillimanite zone. Moderate amounts of excess Ar shown by biotite grains with ages of 46 to 94 Ma at metamorphic grades up to the high‐grade part of the kyanite zone probably represent incomplete degassing during metamorphism. In contrast, the high‐grade part of the kyanite zone yields biotite ages of 130 to 197 Ma. The spatial distribution of these older ages in the kyanite zone along the sillimanite zone boundary suggests they reflect trapped excess argon that migrated from higher‐grade regions. The most likely source is muscovite that decomposed to form sillimanite. The zone with extreme amounts of excess argon preserves trapped remnants of an ‘excess argon wave’. We suggest this corresponds to the area where biotite cooled below its closure temperature in the presence of an elevated Ar wave. Extreme excess Ar is not recognized in muscovite suggesting that the entrapment of the argon wave by biotite took place when the rocks had cooled down to temperatures lower than the closure temperature of muscovite. The breakdown of phengite during ultrahigh‐pressure (UHP) metamorphism may be a key factor in accounting for the very old apparent ages seen in many UHP metamorphic regions. This is the first documentation of a regional Ar‐wave spatially associated with regional metamorphism. This study also implies that resetting of the Ar isotopic systems in micas can require temperatures up to 600°C; much higher than generally thought.     

40Ar/39Ar and K–Ar age constraints on the timing of regional deformation,south coast of New South Wales,Lachlan Fold Belt: Problems and implications

Four slate samples from subduction complex rocks exposed on the south coast of New South Wales, south of Batemans Bay, were analysed by K–Ar and ⁴⁰Ar/³⁹Ar step‐heating methods. One sample contains relatively abundant detrital muscovite flakes that are locally oblique to the regional cleavage in the rock, whereas the remaining samples appear to contain sparse detrital muscovite. Separates of detrital muscovite yielded plateau ages of 505 ± 3 Ma and 513 ± 3 Ma indicating that inheritance has not been eliminated by metamorphism and recrystallisation. Step‐heating analyses of whole‐rock chips from all four slate samples produced discordant apparent age spectra with ‘saddle shapes’ following young apparent ages at the lowest temperature increments. Elevated apparent ages associated with the highest temperature steps are attributed to the presence of variable quantities of detrital muscovite (<1–5%). Two whole‐rock slate samples yielded similar ⁴⁰Ar/³⁹Ar integrated ages of ca 455 Ma, which are some 15–30 million years older than K–Ar ages for the same samples. These discrepancies suggest that the slates have also been affected by recoil loss/redistribution of ³⁹Ar, leading to anomalously old ⁴⁰Ar/³⁹Ar ages. Two other samples, from slaty tectonic mélange and intensely cleaved slate, yielded average ⁴⁰Ar/³⁹Ar integrated ages of ca 424 Ma, which are closer to associated mean K–Ar ages of 423 ± 4 Ma and 409 ± 16 Ma, respectively. Taking into account the potential influences of recoil loss/redistribution of ³⁹Ar and inheritance, the results from the latter samples suggest a maximum age of ca 440 Ma for deformation/metamorphism. The current results indicate that recoil and inheritance problems may also have affected whole‐rock ⁴⁰Ar/³⁹Ar data reported from other regions of the Lachlan Fold Belt. Therefore, until these effects are adequately quantified, models for the evolution of the Lachlan Fold Belt, that are based on such whole‐rock ⁴⁰Ar/³⁹Ar data, should be treated with caution.