Geochemical evidence for the origin of moldavites

Forty-eight moldavites and samples of rocks from the impact crater of Ries were analyzed using non-destructive neutron activation analysis. The following elements have been determined: La, Ce, Sm, Eu, Lu, Sc, Co, Cs, Hf and Th; and Rb and Cr in two moldavites. The darker moldavites, more common in Moravia, show higher contents of trace elements than those lighter in colour, from Southern Bohemia.Terrestrial igneous rocks cannot be regarded as a suitable source material, but terrestrial sandy to silty claystones show strong similarities in major and trace element abundances, and exhibit analogous inter-element variations to moldavites. The Tertiary claystones and sandstones which probably covered the Ries area before the impact, are a possible source rocks for moldavites.     

Geochemical constraints on the origin of the Moon

Hypotheses for the origin of the Moon involve variants on capture, double-planet, and fission processes. Double-planet and fission hypotheses are examined in the light of siderophile trace elements. The siderophile trace elements chosen (W, Re, Mo, P, Ga, Ge) have well understood geochemical behavior such that appropriate metal/silicate partition coefficients are available and their abundances in the lunar and terrestrial mantles 4.4–4.5 × 10⁹ years ago may be reasonably inferred. The fission hypothesis of Ringwood (1979) is not consistent with the behavior of Re, Mo, and P. The hybrid fission hypothesis of Wankeet al. (1983) overcomes many of the deficiencies of ringwood's hypothesis, but is not readily reconcilable with the behavior of Re and Ir. The double-planet hypothesis as most recently advanced by Newsom and Drake (1982, 1983) appears to be consistent with siderophile element behavior in the Moon.     

丹东花岗岩的地球化学特征及其成因

新的SHRIMP定年结果表明,原先划归于古元古代的丹东花岗杂岩现在被确定为中生代岩体。其岩浆型环锆石带的年龄代表结晶年龄,为167～157Ma;但是这些锆石的核部年龄主体为古元古代。本文侧重探讨丹东花岗岩体的岩石学和地球化学特征,论证其起源和产生的构造背景。研究结果表明,丹东花岗岩体具铕异常(Eu／Eu=0．59～1．41),轻重稀土分异明显,其微量元素特征与安第斯型构造背景下的花岗岩特征一致,其tDM和εNd(t)变化范围分别为1．93～2．30Ga和-17．95～-21．32。εNd(t)-t图解显示丹东花岗岩的源岩以古元古代中酸性壳源岩石为主。构造环境环境判别结果表明其形成于大陆边缘岩浆弧。结合中生代丹东地区所处的大地构造背景,本文认为丹东花岗岩体的起源和侵位与古太平洋板块向欧亚板块的俯冲有关。     

基于无机成因的油气勘查概念模型(BRCF)

基于超基性岩蛇纹石化成油理论,根据构造条件和沉积条件,提出了石油勘查概念模型——"BRCF"模型:B代表侵入地壳超基性岩;R代表储油层;C代表盖层;F代表深大断裂。运用该模型分析了波斯湾地区油气田和中国典型油气田油气成藏规律,并预测了中国大陆油气勘查靶区。     

Geochemical constraints on the bimodal origin of High Himalayan leucogranites

Major and trace element and Rb-Sr isotope systematics of the Manaslu leucogranite, Central Nepal, have been examined to constrain the role of mineral fractionation and fluids in peraluminous granite petrogenesis. Biotite and tourmaline are, for the most part, mutually exclusive, with a predominance of two-mica leucogranites over tourmaline leucogranites. The ⁸⁷Sr/⁸⁶Sr initial isotopic ratios (Sr_i) indicate that leucogranitic melts were derived from two different sources, the two-mica leucogranites having a metagreywacke origin (with Sr_i < 0.752 and εNd < −15) and the tourmaline leucogranites a metapelitic one (Sr_i> 0.752; εNd > − 15). Such a bimodal nature of the source zone does not directly influence the magmatic evolution, except that probably the higher initial boron content in the metapelitic rocks may increase the Na₂O/K₂O ratio. In contrast, the amount of water present during melting principally controls in part anatectic processes and element behaviour. Water-saturated conditions probably occured during melting of metagreywackeous rocks and favoured crystallization of two-mica leucogranites whereas water-absent conditions prevailed during melting of metapelitic layers and favoured biotite, plagioclase and monazite fractionation in the source zone and tourmaline crystallization in the leucogranite.     

Geochemical constraints for the origin of thermal waters from western Turkey

The combined chemical composition, B and Sr isotopes, and the basic geologic setting of geothermal systems from the Menderes Massif in western Turkey have been investigated to evaluate the origin of the dissolved constituents and mechanisms of water–rock interaction. Four types of thermal water are present: (1) a Na–Cl of marine origin; (2) a Na–HCO₃ type with high CO₂ content that is associated with metamorphic rocks of the Menderes Massif; (3) a Na–SO₄ type that is also associated with metamorphic rocks of the Menderes Massif with H₂S addition; and (4) a Ca–Mg–HCO₃–SO₄ type that results from interactions with carbonate rocks at shallow depths. The Na–Cl waters are further subdivided based on Br/Cl ratios. Water from the Cumalı Seferihisar and Bodrum Karaada systems are deep circulated seawater (Br/Cl=sea water) whereas water from Çanakkale–Tuzla (Br/Cl<sea water) are from dissolution of Messinian evaporites. Good correlations between different dissolved salts and temperature indicate that the chemical composition of the thermal waters from non-marine geothermal systems is controlled by: (1) temperature dependent water–rock interactions; (2) intensification of reactions due to high dissolved CO₂ and possibly HCl gasses; and (3) mixing with overlying cold groundwater. All of the thermal water is enriched in B. The B isotopic composition (δ¹¹B=2.3‰ to 18.7‰; n=6) can indicate either leaching of B from the rocks, or B(OH)₃ degassing flux from deep sources. The large ranges in B concentrations in different rock types as well as in thermal waters from different systems suggest the water-rock mechanism. ⁸⁷Sr/⁸⁶Sr ratios of the thermal water are used to differentiate between solutes that have interacted with metamorphic rocks (⁸⁷Sr/⁸⁶Sr ratio as high as 0.719479) and carbonate rocks (low ⁸⁷Sr/⁸⁶Sr ratio of 0.707864).     

山东德州凹陷地下热水地球化学特征及成因

典型地区地下水成因和演化机制的研究不仅对于热水资源的合理利用与开发具有重要的指导意义,而且可以为日后的地热资源勘查评价提供重要信息。笔者通过对德州凹陷的地下热水的化学成分、同位素及其水文地质特征的分析,进而对这一地区的地下热水的成岩和演化过程进行了研究。结果显示地下热水的形成受区内深大断裂和基底构造对地热形成的控制,地下热水补给是来自大气降水,为入渗变质水,水化学成分以易溶盐溶解作用为主,其气体组分主要起源于大气、地壳和地幔的混合物,反映了地下水长期径流及深循环中各种水化学作用。     

Geochemical characteristics and origin of Yangchuling porphyry W-Mo deposit

The Yangchuling porphyry W-Mo deposit is a new-type tungsten deposit discovered recently in southern China. Being of certain economic significance, this type of deposits promises some new prospects for the availability of increasing tungsten resource. The ore-bearing rock mass, intermediate to acid in composition, is a composite rock body resulting from comagmatic, multiple-stage intrusion during Middle-Late Yenshanian Period. Spatially, the rock mass is controlled by the Guangji—Anqing—Nanjing deep fault. As a typical porphyry deposit, the ore-bearing rocks (granodiorite porphyry, mozonitic granite porphyry and camouflage breccia) belong to sub-volcanic facies characteristic of near-surface environment. The rocks have suffered strong hydrothermal alteration which is closely related to mineralization. The deposit is classified as meso-epithermal deposit and our study shows that W and Mo come from the granodiorite, granodiorite porphyry and mozonitic granodiorite porphyry, which are believed to be derived through remelting from some W-rick basement rocks contaminated with minor amounts of upper mantle materials.     

Geochemical barriers: theory and practical applications

Geochemical barriers are zones in the Earth's crust with sharp physical or chemical gradients and they are often associated with concentrations of elements, sometimes to the extent of forming ore deposits. Four fundamental classes of geochemical barriers have been identified: (1) mechanical; (2) physico-chemical; (3) biochemical; (4) anthropogenic.Mechanical barriers are the simplest class and placer deposits are a good example. Physico-chemical barriers (the dominant class) are formed in zones characterized by sharp changes in the intensity of certain physico-chemical parameters of waters; tables of observed and predicted element associations have been prepared and these can be used to identify element concentrations which depend principally on the properties of waters. Biochemical barriers are specifically related to element concentrations brought about through reactions associated with life on Earth, for example, the accumulation of trace elements in effects are one variety. Although barriers consisting of combinations of several classes can occur, the main emphasis of this paper is on physico-chemical barriers with respect to ore deposits, soil geochemistry and groundwaters.     

越南西北部莱州地区新生代煌斑岩地球化学特征及其成因

越南西北部菜州地区出露的新生代煌斑岩岩脉对理解特提斯造山带东段的深部岩石圈特征和演化具有重要的地质意义.本文报道莱州地区煌斑岩的元素地球化学和Sr-Nd-Pb同位素组成特征,探讨其岩石成因.该地区煌斑岩属于钙碱性,钾质-超钾质煌斑岩特征.地球化学特征对比表明,菜州地区煌斑岩与哀牢山断裂带碱性岩具有相似的地球化学特征,但与海南和越南南部火山岩存在明显差异.分析结果表明,煌斑岩具有高的87Sr/86Sr比值、低143Nd/144Nd比值和高放射性成因Pb同位素组成特征.岩石的微量元素组成特征指示,形成煌斑岩的地幔源区可能经历过流体交代作用或沉积物组分的加入.低208Pb*/206Pb*比值暗示地幔源区富集事件是近期发生的,可能与晚古生代-早中生代印支地块向扬子地块俯冲事件有关.     

西藏那茶淌北铅锌矿床矽卡岩地球化学特征及其成因

那茶淌北铅锌矿床位于西藏冈底斯成矿带东段北部,主要由矽卡岩铅锌矿体组成,是一个典型的矽卡岩型矿床。笔者通过对矿床内的矽卡岩、大理岩(灰岩)、黑云母花岗岩的地球化学对比分析表明:在主量元素方面,显示Si、Al、Fe、Ti、Mg、Ca、Mn等元素在矽卡岩、大理岩、黑云母花岗岩之间发生明显的交换;在稀土元素方面,矽卡岩、大理岩和黑云母花岗岩均具有LREE富集,HREE亏损,并且发育Eu和Ce负异常和相似的REE配分模式,说明矽卡岩的成因与大理岩、黑云母花岗岩具有一定关系。结合三种不同岩性的地球化学特征,强烈的轻、重稀土分异,Eu、Ce负异常,推测矽卡岩由中高温还原热液交代碳酸盐而成,矽卡岩成岩过程中可能有少量海水加入。     

Geochemical constraints on the origin of the late Jurassic proto-Caribbean oceanic crust in Hispaniola

The nature of the oceanic crust produced through rifting and oceanic spreading between North and South America during the Late Jurassic is a key element for the Caribbean plate tectonic model reconstruction. Located in the Cordillera Central of Hispaniola, the Loma La Monja volcano-plutonic assemblage (LMA) is composed of gabbros, dolerites, basalts, and oceanic sediments, as well as metamorphic equivalents, which represent a dismembered fragment of this proto-Caribbean oceanic crust. Petrologic and geochemical data show that the LMA have a relatively broad diversity in composition, which represent the crystallization products of a typical low-pressure tholeiitic fractionation of mid-ocean ridge basalts (MORB)-type parental magmas, ranging from N- to E-MORB. Three geochemical groups have been distinguished in the volcanic sequence: LREE-flat to slightly LREE-enriched basalts of groups II and III occur interlayered in the lower stratigraphic levels; and LREE-depleted basalts of group I in the upper levels. Mantle melt modeling suggests that group III magmas are consistent by mixing within a mantle melt column of low-degree (<1%) melts of a deep garnet lherzolite source and high-degree (>15%) melts of a shallow spinel source, and groups II and I magmas are explained with moderate to high (14–18%) and very high (>20%) fractional melting degrees of a shallower spinel mantle source, respectively. Thus, upward in the volcanic sequence of the LMA, the magmas represent progressively more extensive melting of shallower sources, in a plume-influenced spreading ridge of the proto-Caribbean oceanic crust. Nb/Y versus Zr/Y systematics combined with recent plate tectonic model reconstructions reveal that Caribbean Colombian oceanic plateau fragments in Hispaniola formed through melting of heterogeneous mantle source regions related with distinct plumes during at least from Aptian–Albian (>96 Ma) to Late Campanian.     

Geochemical constraints on the origin of the Hegenshan Ophiolite,Inner Mongolia,China

The Hegenshan ophiolite in Inner Mongolia is a remnant of oceanic lithosphere of probable Devonian age. The ophiolite consists of several blocks composed chiefly of serpentinized ultramafic rocks with lesser amounts of troctolite and gabbro, and sparse lavas and dikes. The ultramafic rocks consist chiefly of depleted harzburgite and minor dunite and are interpreted as mantle tectonites. In the Hegenshan block dunite is relatively abundant and is typically associated with podiform chromitite. Both the chromite ore and the residual chromites in this body are relatively aluminous with average Cr numbers of 44–54. A few small chromite bodies and some of the residual chromites have much higher Cr numbers (72–76). Several blocks have well-layered cumulate sequences of gabbro and troctolite. Sheeted dikes are absent but small mafic dikes are common in some of the ultramafic sections. Most of the mafic dikes have flat chondrite-normalized REE patterns and are strongly depleted in incompatible elements, similar to depleted tholeiites from immature island arcs. The basaltic lavas of the Hegenshan ophiolite have two distinctly different chemical signatures—one similar to the mafic dikes and one similar to ocean island basalts. The entire complex was probably formed within an island arc–marginal basin system that was later accreted to the southern margin of the Siberian Altaids.     

Thermal synthesis of amino acids and the origin of life

The recent review by Marshall (1994) of the production of amino acids from the interstellar components, formaldehyde and ammonia, is placed in the larger context of the origin of life. Thermal energy, being ubiquitous in the Earth, emerges as the sole necessary form of energy. To appreciate the overview of the natural evolutionary sequence it is necessary to recognize stepwiseness in evolution, a principle that has however been often ignored. Since self organization of thermal protein to cells is instantaneous, but only one step in a geochemical ladder, individual steps may be regarded as instantaneous, while the sequence requires measurable time. Two steps indicated are extrusion of a hot, dry organic magma of amino acids --> peptides into an aqueous environment in which occurs a second step of self organization. In this paper, spinoffs of the defensible theory for the origin of life have been briefly reviewed as a fundamental consequence of nonrandom thermal copolymerization of amino acids.     

江南造山带东段桃岭岩体的地球化学特征及其成因

位于江南造山带东段赣东北地区的桃岭岩体地球化学特征总体表现为高钾钙碱性系列,弱过铝质-强过铝质,高Mg#值。桃岭岩体富集轻稀土和大离子亲石元素,亏损高场强元素,具弱的Eu负异常(δEu=0.62~0.81)(样品TL-1除外δEu=0.47)。锆石LA—ICPMS U—Pb定年获得桃岭岩体的成岩年龄为早白垩世(140±1)Ma。岩体初始~(87)Sr/~(86)Sr为0.7131~0.7141,ε_(Nd)(t)值为-7.43~-6.71。结合前人研究,笔者认为早白垩世(约140 Ma)交代的地幔发生部分熔融,幔源岩浆底侵,使地壳深处(至少40 km)的变质沉积岩发生部分熔融,并且与少量的幔源岩浆发生了岩浆混合作用,形成了桃岭岩体。     

Geochemical characterization and origin of granitoids from the South Bohemian Batholith in Lower Austria

Major and 31 minor elements have been determined in 39 large samples of Variscan granitoids from 6 plutons or intrusions from the South Bohemian Batholith (Rastenberg, Weinsberg, Mauthausen, Schrems, Eisgarn and Gebharts). The granitoids are mainly granites but also diorites, tonalites, trondhjemites, granodiorites. Average concentrations of Ba, Th, U, La, Ce, Pb, Nd, Sr and K in the Weinsberg, Mauthausen and Schrems granites exceed those in average felsic I- and S-type granites by factors ranging between 2.1 and 1.3. The granites melts formed at waterundersaturated conditions and intruded at 10 to 15 km depth during late-tectonic and post-tectonic phases of the Variscan orogeny (about 330 to 300 Ma ago). Hydrothermal or low temperature alteration is excluded for the majority of samples from a study of oxygen isotopes. The thickness of the plutons is estimated at about 6 km from heat balance constraints. By analogy with experimental partial melting, three different sources of the granitoids can be identified and chemically characterized: (1) The trondhjemites, tonalites and diorites in the early Rastenberg pluton are products of 15 to 40% melting respectively of a mafic (partly amphibolitic) lower crust. Redwitzites from the West Bohemian Massif which are comparable in age partly resemble the Rastenberg rocks. The mafic sources of the Rastenberg granitoids and redwitzites are crustally contaminated as reflected in their Sr-Nd isotopes. (2) The very large syn-tectonic Weinsberg pluton was formed from about 30% partial melting of a tonalitic lower crust at 800 to 850°C. Its low proportion of ca. 10% restite has a ferrodioritic composition. The post-tectonic fine-grained Mauthausen and Schrems granites which tend to a granodioritic mode, are very low in restite and are also products of melting of a tonalitic source. (3) The youngest (leuco-)granite, the Eisgarn pluton (high in Si, P, Li, Rb, Cs, U,⁸⁷Sr/⁸⁶Sr and low in Ca, Sr, Ba) reflects a pelitic source. The change from mafic to tonalitic to pelitic source composition for the granitoid sequence may indicate that the depth of melt formation decreased with time. The concentration of heavy rare earth elements decreased from Weinsberg to Eisgarn granites which indicates an increasing proportion of garnet in the source. The orogenic heat conformable with a heat flow of about 100 mWm^-2 was provided by mafic intrusions. An alternative would be a drastic increase of the crustal thickness which cannot be recognized by barometry of the associated metamorphic rocks. Exposed metamorphic country rocks occur in higher amphibolite facies indicating about 5 kbar pressure. Mafic intrusions contain gabbros (Kleinzwettl) or have formed (quartz-)diorites (Gebharts), the latter being contaminated by granitic melts from partial melting of the wall rocks (MASH process). Largescale contamination by crustal materials can be observed in ¹⁸O and in Sr-Nd isotopes. The major mafic activity was probably caused by depression of solidus temperatures in the mantle wedge above a subduction zone where water was available from dehydration of subducted ocean crust. This water initiated partial melting of ultramafic rocks and metasomatism in the uppermost mantle above the level of melting. The water also mobilized highly incompatible elements (Ba, Th, U, La, Ce, Pb, Nd, Sr and K) from the uppermost mantle and transported them into the lower crust. Indicators of a nearby subduction or collision zone of Late Variscan age in addition to the specific association of granitoidal rocks are abundant upper mantle tectonites. An alternate or additional source of metasomatic fluids may have been dehydration of lower crustal rocks during Variscan high-grade metamorphism.Dedicated to Prof. Dr J. Zemann on the occasion of his 70th birthday