Chemical and isotopic characteristics of fluids along the Cameroon Volcanic Line, Cameroon

Results of the chemical and isotopic analysis of the water and gases discharged from volcanic crater lakes and soda springs located along the Cameroon Volcanic Line were used to characterize and infer their genetic relationships. Variations in the solute compositions of the waters indicate the dominant influence of silicate hydrolysis. Na⁺ (40–95%) constitutes the major cation in the springs while Fe²⁺ + Mg²⁺ (70%) dominate in the CO₂-rich lakes. The principal anion is HCO₃ (>90%), except in the coastal springs where Cl-predominates. Lakes Nyos and Monoun have FeMgCaHCO₃⁻ type signatures; the soda springs are essentially NaHCO₃⁻ type, while all other lakes show similar ionic compositions to dilute surface waters. Dissolved gases show essentially CO₂ (>90%), with small amounts of Ar and N₂, while CH₄ constitutes the principal component in the non-gassy lakes. Active volcanic gases are generally absent, except in the Lobe spring with detectable H₂S. Stable isotope ratio evidence indicates that the bicarbonate waters are essentially of meteoric origin. CO₂ (δ¹³C = −2 to −8%₀ and He (³He/⁴He = 1 to 5.6R_a) infer a mantle contribution to the total CO₂. CH₄ has a biogenic source, while Ar and N₂ are essentially atmospheric in origin, but mixing is quite common.     

Oxygen isotope study of the Kamioka Zn-Pb skarn deposits,Central Japan

The Kamioka Zn-Pb deposits consist of clinopyroxene-rich skarns replacing limestone lenses in Hida gneissose rocks. Paleozoic metabasite, early Jurassic Funatsu granitic rocks and late Cretaceous porphyritic dikes and stocks, are the known igneous rocks in the mining area. Oxygen isotopic compositions of eight separates each of clinopyroxene and quartz from the deposits (Table 1), and some minerals and rocks from the major rock units around the deposits (Table 2), have been analyzed. The estimated oxygen isotopic ratios of the fluids responsible for the formation of the deposits, range as low as -4 to +3 (SMOW). These values are considerably lower than expected from magmatic waters, and demonstrate that none of the igneous rocks in the area could be a direct source of the skarn- and ore-forming fluids. The data instead show that the deposits are formed by a huge convective circulation of solutions of meteoric water origin, promoted by a hidden batholithic intrusion at the time of the late Cretaceous.     

Behaviors of 210Pb and 210Po around hydrothermal vents in the Okinawa Trough

Vertical profiles for the uranium-series radioisotopes 210Pb and 210Po were obtained at thetwo hydrothermal vent sites, the Iheya Ridge and the Minami-Ensei Knoll, in the Mid-Okinawa Trough in 1993 and 1994, respectively. In 1995, both radioisotopes were measured at the Minami-Ensei Knoll a-gain. At the Iheya Ridge, where the hydrothermal activity is not active as reflected by the CH_4, and 222Rn data, both the total 210Pb and 210Po activities show deficiency relative to their parents, and the mean residence time of 210Pb and 210Po is approximately equal to 20 and 2-5 a, respectively. At the Minami-Ensei Knoll, which is characterized by black smokers, the total 210Pb(0.167 × 10~(-3)-2.5 × 10~(-3)_ Bq/kg) around the plumes is deficient relative to 226Ra but the total 210Po activities (1.83 × 10 ~(-3) - 2.83 × 10~(-3) Bq/kg) are in excess relative to 210Pb. The 210Po activities are higher than those in the East China Sea and the Okinawa Trough and excess 210Po has been found. The 210Pb/226Ra and 210Po/     

闽粤晚白垩世古地磁学研究

在福建沙县至上杭、广东兴宁至河源上白垩统采集古地磁学定向样品３９０个（３９个采点）,进行了古地磁学研究,研究表明,闽粤两地的晚白垩世地磁学数据具有廊的重合性,福建：古地磁偏角１１．７°,地磁倾角４２．９°,地磁极位置北纬７９．４°,东径２１０．３°;广东;古地磁偏角１０．７°,古地磁倾角４４．０°,古地磁有位置北纬８０．１°,东径１９３．４°,该古寺磁学数据反映了中国华南地区自晚白垩世至今曾经厍过     

U-series dating of Lake Nyos maar basalts,Cameroon (West Africa): Implications for potential hazards on the Lake Nyos dam

From previously published ¹⁴C and K–Ar data, the age of formation of Lake Nyos maar in Cameroon is still in dispute. Lake Nyos exploded in 1986, releasing CO₂ that killed 1750 people and over 3000 cattle. Here we report results of the first measurements of major elements, trace elements and U-series disequilibria in ten basanites/trachy-basalts and two olivine tholeiites from Lake Nyos. It is the first time tholeiites are described in Lake Nyos. But for the tholeiites which are in ²³⁸U–²³⁰Th equilibrium, all the other samples possess ²³⁸U–²³⁰Th disequilibrium with 15 to 28% enrichment of ²³⁰Th over ²³⁸U. The (²²⁶Ra/²³⁰Th) activity ratios of these samples indicate small (2 to 4%) but significant ²²⁶Ra excesses. U–Th systematics and evidence from oxygen isotopes of the basalts and Lake Nyos granitic quartz separates show that the U-series disequilibria in these samples are source-based and not due to crustal contamination or post-eruptive alteration. Enrichment of ²³⁰Th is strong prima facie evidence that Lake Nyos is younger than 350 ka. The ²³⁰Th–²²⁶Ra age of Nyos samples calculated with the (²²⁶Ra/²³⁰Th) ratio for zero-age Mt. Cameroon samples is 3.7 ± 0.5 ka, although this is a lower limit as the actual age is estimated to be older than 5 ka, based on the measured mean ²³⁰Th/²³⁸U activity ratio. The general stability of the Lake Nyos pyroclastic dam is a cause for concern, but judging from its ²³⁰Th–²²⁶Ra formation age, we do not think that in the absence of a big rock fall or landslide into the lake, a big earthquake or volcanic eruption close to the lake, collapse of the dam from erosion alone is as imminent and alarming as has been suggested.     

Oxygen isotope signatures in bulk chondrules: Implications for the aqueous alteration and thermal metamorphism on the Allende CV3 parent body

Precise triple oxygen isotope compositions of 32 Allende bulk chondrules (ABCs) are determined using laser‐assisted fluorination mass spectrometry. Various chemically characterized chondrule types show ranges in δ¹⁸O that vary from ?4.80‰ to +1.10‰ (porphyritic olivine; PO, N = 15), ?3.10‰ to +1.50‰ (porphyritic olivine pyroxene; POP, N = 9), ?3.40‰ to +2.60‰ (barred olivine; BO, N = 4), and ?3.60‰ to +1.30‰ (porphyritic pyroxene; PP, N = 3). Oxygen isotope data of these chondrules yield a regression line referred to as the Allende bulk chondrule line (ABC line, slope = 0.86 ± 0.02). Most of our data fall closer to the primitive chondrule minerals line (PCM line, slope = 0.987 ± 0.013) and the carbonaceous chondrite anhydrous mineral line (CCAM line, slope = 0.94 ± 0.02) than the Allende anhydrous mineral line (AAML, slope = 1.00 ± 0.01) with a maximum δ¹⁸O value (+2.60‰) observed in a BO chondrule and a minimum δ¹⁸O value (?4.80‰) shown by a PO chondrule. Similarly, these chondrules depict variable ?¹⁷O values that range from ?5.65‰ to ?3.25‰ (PO), ?4.60‰ to ?2.80‰ (POP), ?4.95‰ to ?3.00‰ (BO), ?5.30‰ to ?3.20‰ (PP), and ?4.90‰ (CC). A simple model is proposed for the Allende CV3 chondrite with reference to the AAML and PCM line to illustrate the isotopic variations occurred due to the aqueous alteration processes. The estimated temperature ranging from 10 to 130 °C (mean ~60 °C) implies that the secondary mineralization in Allende happened in a warmer and relatively dry environment compared to Murchison. We further propose that thermal metamorphism could have dehydrated the Allende matrix at temperatures between >150 °C and <600 °C.     

河北平泉光头山碱性花岗岩的时代、Nd-Sr同位素特征及其对华北早中生代壳幔相互作用的意义

光头山碱性花岗岩产出在华北北部的前寒武纪基底变质岩系之中,造岩矿物组合为石英 碱性长石 纳铁闪石 霓辉石 钠铁非石±星叶石,副矿物有锆石、钛铁矿、硅钛铈铁矿等。晚期的伟晶岩囊状体由颗粒粗大的石英、碱性长石和纳铁闪石等组成,全岩Rb-Sr等时线年龄为T=200±16Ma,(~(87)Sr/~(86)Sr)_i=0.705±0.008,MSWD=11.2,代表冷却年龄,单颗粒锆石U-Pb谐和年龄为220±1Ma,代表岩体侵位时代。光头山碱性花岗岩以A/CNK<1和A/NK<1,Al_2O_3、MgO、CaO和Ba、Sr含量低,全碱含量、MnO和Rb、Ga等含量高,负Eu异常特别显著等为特征。矿物学和地球化学完全符合A型花岗岩的特征。光头山碱性花岗岩是华北地区早中生代后造山环境下岩浆活动的产物。光头山碱性花岗岩的ε_(Nd)(T=220Ma)平均值为-8.9,明显高于华北前寒武纪下地壳岩石的范围,而冀北地区前寒武纪高压麻粒岩地体虽具有大陆地幔的特征,但未经历过部分重熔,表明至少前寒武纪下地壳不可能是岩浆主要的或唯一的来源同样,现今华北下地壳由于时代较新,也不可能成为岩浆的源岩,对比时代相近的超镁铁岩和煌斑岩的Nd同位素特征,推测最可能的源区是1.8～1.9Ga形成的富集的岩石圈地幔。光头山碱性花岗岩和华北北缘早中生代侵入岩带规模很大,以富集地慢来源的岩浆为主,反映了当时的岩浆活动已经具有相当的规模和强度,如果130Ma前后中国东部大规模岩浆活动之时,是岩石圈减薄已经达到最大程度之际,那么,此前一定时间段内的幔源岩浆活动都有可能与岩石圈减薄从开始到鼎盛的过程有关,所以,华北北缘早中生代岩浆活动可能是华北中生代岩石圈减薄过程早期阶段的产物。与岩石圈减薄过程有关的早中生代岩浆活动还在中国东北地区东部和阿拉善北部形成了后造山A型花岗岩。与岩石圈减薄过程相关的早中生代侵入岩在一定范围内的带状分布,表明当时岩石圈减薄过程可能并没有涉及到整个中国东部地区只有到了侏罗纪-白垩纪,岩石圈减薄过程才在更大的区域内广泛发生。所以说,中国东部中新生代岩石圈减薄过程是在时间上从早中生代就已经开始、在空间上从华北北缘-中国东北地区东部开始向外逐渐扩展的一个深部过程。这个深部过程对应的地表表现是,先在华北北缘和中国东北东部地区形成规模很大的早中生代侵入岩带。而后,当岩石圈减薄过程扩展到整个中国东部时,岩浆活动才达到鼎盛时期,这可能就是中国东部侏罗纪-白垩纪大规模岩浆活动的深部原因所在。而以富集地幔源区为主的岩浆活动还导致了华北北缘地壳垂向生长。