Tholeiitic and alkalic basalts of the oldest Pacific Ocean crust

Approximately 160 Ma old basaltic lavas obtained from ODP Site 801 in the Pigafetta Basin represent the first Jurassic oceanic crust recovered in the Pacific Ocean and the oldest in situ oceanic crust discovered anywhere. The basement consists of an upper alkali olivine basalt sequence and a lower tholeiitic sequence separated by a yellow Fe-rich hydrothermal sedimentary deposit. The aphyric and sparsely plagiodase-olivine±spinel phyric tholeiites exhibit depleted, open–system fractionated characteristics with trace element abundances and Pb–Nd isotopic compositions similar to normal mid-ocean ridge basalts (N-MORB). The aphyric alkali basalts, although showing some overlap in isotopic composition with MORB, exhibit strong similarities in terms of incompatible element abundances to ocean island basalts (OIB). They could represent either OIB-type off-axis volcanism or an alkalic event possibly associated with the waning stages of spreading axis volcanism in the Pigafetta Basin. All lavas have undergone low-grade anoxic smectite–carbonate alteration, although flows underlying the Fe-rich sediments have suffered hydrothermal alteration and fracturing.     

Biomineralization and magnetic and thermal properties of the iron concretion from the laterite bauxite deposit of Bao Lok,South Vietnam

Doklady Earth Sciences -     

Presumed magnetic biosignatures observed in magnetite derived from abiotic reductive alteration of nanogoethite

The oriented chains of nanoscale Fe-oxide particles produced by magnetotactic bacteria are a striking example of biomineralization. Several distinguishing features of magnetite particles that comprise bacterial magnetosomes have been proposed to collectively constitute a biosignature of magnetotactic bacteria (Thomas-Keprta et al., 2001). These features include high crystallinity, chemical purity, a single-domain magnetic structure, well-defined crystal morphology, and arrangement of particles in chain structures. Here, we show that magnetite derived from the inorganic breakdown of nanocrystalline goethite exhibits magnetic properties and morphologies remarkably similar to those of biogenic magnetite from magnetosomes. During heating in reducing conditions, oriented nanogoethite aggregates undergo dehydroxylation and transform into stoichiometric magnetite. We demonstrate that highly crystalline single-domain magnetite with euhedral grain morphologies produced abiogenically from goethite meets several of the biogenicity criteria commonly used for the identi?cation of magnetofossils. Furthermore, the suboxic conditions necessary for magnetofossil preservation in sediments are conducive to the reductive alteration of nanogoethite, as well as the preservation of detrital magnetite originally formed from goethite. The findings of this study have potential implications for the identification of biogenic magnetite, particularly in older sediments where diagenesis commonly disrupts the chain structure of magnetosomes. Our results indicate that isolated magnetofossils cannot be positively distinguished from inorganic magnetite on the basis of their magnetic properties and morphology, and that intact chain structures remain the only reliable distinguishing feature of fossil magnetosomes.     

Gold in the weathering crust at the Suzdal' deposit (Kazakhstan)

A comparative analysis of morphology and geochemistry was made for gold from the primary ores and weathering crust of the Suzdal' gold deposit, Eastern Kazakhstan. The deposit is localized in Carboniferous carbonaceous-terrigenous strata and is of gold-sulfide type. Study of gold from primary ores showed that it occurs mainly in two species: free and so-called invisible. Free gold is crystallomorphic segregations and irregular-shaped grains up to tens of microns in size; it occurs in intergrowths with sulfides, quartz, carbonate, and mica-chlorite aggregate. Most of gold particles have a fineness of 930–980‰, with some grains showing wide variations in composition. Invisible gold (probably chemically combined) is present in fine-acicular arsenopyrite and, less frequently, pyrite.Being transported to the weathering crust, all this gold served as a source for “neogenic” gold of diverse morphologic forms. We recognized crystalline (isometric, prismatic, acicular, and tabular) particles and drusoid gold aggregates in the form of exotic intergrowths of crystallomorphic and sinter-shelly grains. The grains tend to coarsen from bottom to top of the weathering crust. Several generations of gold of different granulometric classes are observed. We have revealed seed and layer growth and dissolution structures in crystals of early generations overgrown with fine grains. All these gold varieties are associated with hypergene minerals. Most of this gold is of high fineness (on the average, 995‰). The hypergene gold particles are chemically homogeneous high-grade, without rims.The results of studies suggest that the high-grade hypergene gold formed in the weathering crust as a result of the dissolution of invisible gold of sulfides and its local redistribution and deposition in oxidizing media. This is also evidenced from the tendency of gold to coarsen from bottom to top of the weathering crust. A distinctive feature of secondary gold is well-expressed crystals and their great diversity.     

黔西北玄武岩风化壳稀土地球化学特征

黔西北玄武岩风化壳中的稀土矿是一个分布广泛、稀土含量较高的新类型稀土矿。本文主要通过岩相学、矿物学和地球化学的方法,系统研究了该稀土矿层的岩石组构、矿物组合及地球化学特征,探讨了该地区稀土迁移富集机理。结果显示该风化壳稀土矿层厚度大(单层厚度0.1～0.5 m,总厚度约5～15 m)。矿石中主要矿物为高岭石(60%～80%),次要矿物为蒙脱石(2%～8%)、赤铁矿(5%～20%)。稀土总量较高(144～2288×10-6),具轻稀土富集特征,并且稀土含量高的样品均为高岭石粘土岩。稀土配分模式图中显示明显负铕异常(0.22～0.85),铈异常变化较大(0.86～1.63)。综合分析认为玄武岩风化壳中的稀土元素的富集和高岭石的含量密切相关,稀土元素可能在偏还原的环境中被高岭石颗粒吸附。     

塔里木盆地北部深层风化壳储层的形成与分布

塔北地区碳酸盐岩风化壳岩溶储层发育,储层具有质量好、分布面积广、埋藏深等特点,并已在轮南、塔河和英买力等地区发现数个亿吨级规模以上的大型油气田.本文通过对塔北地区200多口重要井的钻井、录井、测井、试油等第一手资料的系统分析,结合大量的分析化验资料与岩心及镜下观察,精细研究了风化壳储层的成因特点、分布规律与评价预测等.研究认为,寒武系发育白云岩型风化壳储层,以孔洞型储层为主;奥陶系主要为灰岩型风化壳储层,储层的基质孔隙度较低,溶蚀孔洞、溶蚀扩大缝和构造缝是其主要的储集空间,储层以洞穴型储层和网状裂缝型储层为主.台地相发育的纯净碳酸盐岩、海西期构造运动导致地层的长期暴露剥蚀、断裂裂隙作用和晚期大气淡水淋溶等是控制塔北风化壳储层形成的的重要因素;并发现塔北地区风化侵蚀能够作用到的深度主要在140m以内.虽然风化壳储层基质孔隙较差,但是储层物性受埋深影响不大;在有利的古岩溶地貌和断裂发育区,即使在更大的埋藏深度,也可能发育规模较大、性能较好的储集体.     

沙尾风化壳高岭土矿床的研究

广东沙尾高岭土矿属残积型风化壳矿床,分为白云母二长花岗岩钠化高岭土矿和斑状二长花岗岩高岭土矿。矿石主要组成矿物包括高岭石、埃洛石,伊利石及石英,含少量长石。原矿石SiO_2高达67—70％,Al_2O。20—23％,精矿SiO_2 47—50％,Al_2O_3 33—36％。矿石铁、钛含量低,SiO_2/Al_2O_3在2.2—2.5之间。高岭土的成矿作用分为前期热液蚀变阶段和后期表生风化阶段,前期主要以钠长石化和白云母化为主,后期通过地表水淋洗发育成高岭土风化壳。     

Recycled crust controls contrasting source compositions of Mesozoic and Cenozoic basalts in the North China Craton

We explore Fe/Mn and Nb/Ta ratios of basalts as potential tracers for differentiating melts of recycled mafic crustal lithologies from peridotitic melts. Trace elements and Fe/Mn ratios of the Mesozoic and Cenozoic basalts from East China were analyzed by ICP-MS. Low Nb/Ta ratios (15.4 ± 0.3 (2σ, n = 45)), high Nb and Ta contents (60.1 and 4.01 ppm) and high Fe/Mn ratios (64.7 ± 1.5 (2σ, n = 45)) characterize the <110 Ma basalts. Mesozoic basalts with ages >110 Ma are characterized by superchondritic Nb/Ta ratios (20.1 ± 0.3 (2σ, n = 25)), low Nb and Ta contents (10.8 and 0.54 ppm) and slightly lower Fe/Mn ratios (60.0 ± 1.1 (2σ, n = 25)). Both the Mesozoic and Cenozoic basalts have Fe/Mn ratios higher than basaltic melt formed by partial melting of peridotite at the same MgO and CaO levels. Although both the Mesozoic and Cenozoic basalts are characterized by highly fractionated REE patterns, the >110 Ma basalts have island arc-type trace element patterns (i.e., depletion in Nb and Ta), whereas OIB-type trace element patterns (e.g., no depletion in Nb and Ta) are characteristic of the <110 Ma basalts. Based on D_Fe/Mn values for olivine, clinopyroxene, orthopyroxene and garnet, high Fe/Mn ratios and negative correlations of Fe/Mn with Yb (Y) of the <110 Ma basalts suggest clinopyroxene/garnet-rich mantle sources. The lower Fe/Mn ratios and positive correlations of Fe/Mn with Y and Yb in the >110 Ma basalts suggest orthopyroxene/garnet-rich mantle sources. Combining these data with Sr-Nd isotopes, we present a conceptual model to explain the Nb/Ta ratios and PM-normalized trace element patterns of the >110 and <110 Ma basalts. Preferential melting of recycled ancient lower continental crust during Mesozoic lithospheric thinning resulted in (1) peridotite-melt/fluid reaction that formed the orthopyroxene/garnet-rich mantle sources for the >110 Ma basalts, and (2) peridotite + rutile-bearing eclogite mixing that formed the clinopyroxene/garnet-rich mantle sources for the <110 Ma basalts. The choice of models may indeed be arbitrary and non-unique, but the goal is to seek relatively simple forward models that explain the characteristics of the lavas, and the differences between the >110 and <110 Ma basalts, in a relatively consistent geodynamic framework.     

塔巴庙地区奥陶系风化壳识别与结构分析

水平洞穴砂砾岩是识别多期次岩溶旋回水平岩溶带的标志,是岩溶旋回连接沉积旋回及周期性海平面变化的纽带,故塔巴庙地区奥陶系水平洞穴岩溶带产出的层位、厚度,以及井深,则是水平岩溶发育状况的体现。这里从地层学、岩石学、矿物学、钻井录井、测井、地球化学等标志识别风化壳,根据三个水平岩溶带特征确定三个水平岩溶带的分布,进而对其进行分析对比。确定出三个水平岩溶带差异性及成因,使保存好的风化壳作为油气水储集的有利场所。     

Urban and rural limestone weathering; the contribution of dust to black crust formation

A novel approach of studying dust, black crust and host limestones by using various techniques is presented in this paper. Samples were collected from limestone monuments located in rural, urban and industrial areas in Germany and Hungary. The paper focuses on differences in the mineralogical composition, major and trace element distribution of materials and their total polyaromatic hydrocarbons (PAH) concentration having different exposure to air pollutants. High concentration of gypsum was found in laminar and dendritic black crusts and even in the dust of both urban and rural areas, despite the low concentration of SO₂ in rural atmospheric environments. The black crusts show a typical microfabric with distinct layers of various gypsum–calcite mineral associations, silt-sized mineral fragments and black particle content. In dust, newly formed gypsum crystals were found along with mineral fragments, and siliceous and, less frequently, carbonaceous fly ash particles. High concentration of lead was found both in the samples of urban and rural areas of Germany and Hungary. Lead mostly accumulates in dust and also in the black crust. Highest lead concentrations were found not on the surface of the black crust, but close to the crust/limestone boundary according to LA-ICPMS analyses. It indicates that despite the ban of leaded petrol, lead is still present in the dust and the in the soiled zones of the built environment. Similarly to lead, no significant difference in the total PAH content of dust and crust samples were found in the two countries. Dusts are equally enriched in PAH in both countries, whereas black crusts contain fewer amounts. Most of the aqueous extracts of the samples were saturated with respect to sulphate, which also indicate the presence of gypsum, whilst in some German samples unsaturated conditions were detected. In summary, by using a combination of these analytical techniques, it was possible to distinguish rural and urban samples and to outline the role of dust in bringing pollutants to the stone surface. It is suggested that the analysed dust and stone samples indicate not only present, but also the past, pollution fluxes and as a consequence stones similar to sediments or surface waters can be used as environment indicators.     

Ground air of the bauxite-bearing lateritic weathering crust (Republic of Guinea)

The ground air of the bauxite deposit in West Africa was tested for its chemical composition. The obtained data provide the first insight into the air effect on the mineralogical-geochemical zoning of the lateritic weathering profile. The air was tested in areas with different geomorphologic features, which allowed us to study the effect of relief and landscape on the ground-air formation.     

Nature of magnetic grains in basalts and implications for palaeomagnetism

Journal of Earth System Science - Investigations involving temperature dependence of low-field susceptibility and of low- and high-field hysteresis have been carried out on more than one thousand...     

鄂尔多斯盆地延安地区奥陶系风化壳古地貌恢复

鄂尔多斯盆地东南部延安地区奥陶系马家沟组顶部发育风化壳岩溶储层,因古地貌代表了地层沉积和成岩作用的背景,因而在储层的形成和演化过程中具有重要意义。在区域构造背景和地层展布的基础上,综合利用残余厚度法和印模法,并结合铝土岩厚度分布和古沟槽识别结果,准确恢复出延安地区奥陶系风化壳古地貌分布格局。结果显示,延安地区西南部表现为古岩溶高地,中部大部分地区表现为古岩溶斜坡,东部属岩溶斜坡与岩溶盆地之间的过渡带。古地貌格局和古沟槽的分布影响了储层的形成、发育和天然气的富集。延安地区岩溶古高地、靠近侵蚀古沟槽的斜坡位置含气性较好,可能与大气水淋滤等因素导致的储层发育程度、物性好坏、不整合面之上盖层的封盖性等因素有关。