The chemistry and mineralogy of the clay fraction of sediments from the southern Barents Sea

The separated clay fraction (material <μ) of surface and sub-surface sediment samples from the southwestern Barents Sea is described. The partitioning of the major and minor elements within the different grain-size fractions of the sediment and between detrital and non-detrital phases demonstrates that the clay chemistry of these oxic shelf sediments is terrigenous in origin.The clays are a variable mixture of micaceous debris and illite with chloritic material, minor expandable clay and occasional kaolinite. Carbonate debris and amphibole occur locally. The semi-quantitative analysis suggests the existence of a considerable variation in the relative content of the principal mineralogical components and this is confirmed by the investigation of the major and minor elements. The terrigenous chemistry provides a more sensitive index of clay variability and allows the recognition of three distinct petrographic provinces.     

Hypoxia in the Eemian: mollusc faunas and sediment mineralogy from Cyprina Clay in the southern Baltic region

The Cyprina Clay at Mommark, southern Denmark, contains a unique record of a shallow threshold-controlled fjord environment through an entire interglacial. Today, the area suffers from periodic severe hypoxia, and the Eemian mollusc faunas (dominated by the hypoxia-tolerant Corbula gibba and Arctica islandica ), the sediments' varying content of pyrite, calcite and gypsum, as well as evidence of shell-dissolution all show that the area also suffered from oxygen stress during the Eemian. A comparison with previous faunal studies on Cyprina Clay indicates that the entire southwestern Baltic region suffered from oxygen stress of varying degree throughout the Eemian. At Mommark, the severity of hypoxia varies inversely with water depth, indicating control by sea level and water depth over the basin threshold. After an early phase of eustatic sea level rise came a prolonged period of isostatic emergence and regression. Although this isostasy pattern is different from the Holocene, the present isostatic uplift of the Baltic entranceways may have the same effect and add to the anthropogene hypoxia threat in the Baltic region.     

Clay mineralogy and geochemistry and their palaeoclimatic interpretation of the Pleistocene deposits in the Xuancheng section,southern China

A red soil profile in Xuancheng, Anhui province, southern China, in the middle to lower reaches of the Yangtze River, was investigated using X‐ray diffraction, X‐ray fluorescence spectrometer, and scanning electron microscopy. The mobile components K₂O and Na₂O and trace elements Ba and Sr of the Xuancheng section exhibit a general trend of decrease downward along the red soil profile, together with an increase downward of chemical index of alteration (CIA) values, suggesting more intense depletion in the lower portion than in the upper portion. The major components SiO₂, Al₂O₃ and Fe₂O₃, as well as SiO₂/Al₂O₃, SiO₂/Fe₂O₃ and Al₂O₃/Fe₂O₃ ratios, show notable fluctuations along the soil profile, indicating intense climatic oscillations in the area during the Pleistocene age. The clay mineral assemblage of the Xuancheng section can be generally subdivided into three groups, suggesting a general trend of three stages of climate changes. The lower portion of ～10.4–6.3 m depth has a lower illite content and higher abundance of kaolinite and illite–smectite (I/S) clays, indicating that a warm and wet climate prevailed over the episode of ca. 600–350 ka BP. A decrease in abundance of kaolinite and I/S clays and increase in illite content at a depth of ～6.3–2.2 m probably indicate a transition stage of climate change from warm/humid to cool/dry in the period ca. 350–80 ka BP. The higher illite content and lower abundance of kaolinite and I/S clays in the upper portion of ～2.2–0 m depth suggest that a relatively cool and dry climate dominated since ca. 80 ka BP. Based on changes in clay mineralogy and chemical indices of the sediments, seven warm/cold fluctuations were determined in the area since the Middle Pleistocene. Climate changes documented in the Xuancheng section are in agreement with the δ¹⁸O records of sediments from the equatorial Pacific Core V28‐238 and the loess–palaeosol sequences in the Loess Plateau of northwestern China. Correlated to the episode of S4 and S5 soil units in the Loess Plateau, the period of ca. 600–350 ka BP in the Xuancheng area was dominated by the particularly strong East Asia summer monsoon, as indicated by its most abundant kaolinite and I/S clays. Fluctuations in clay mineralogy along the Xuancheng soil profile were mainly controlled by both the East Asia summer and winter monsoons in response to the global changes in the Middle–Late Pleistocene. Copyright © 2009 John Wiley & Sons, Ltd.     

准噶尔盆地南缘上三叠统黄山街组辫状河三角洲沉积

准噶尔盆地南缘上三叠统黄山街组主要为砾岩、含砾砂岩、砂岩、粉砂岩和泥岩构成的一套碎屑岩组合,野外沉积特征研究确定为典型的辫状河三角洲沉积。通过对沉积相、亚相、微相的详细研究,认为辫状河三角洲具有类似曲流河三角洲的层序结构和微相类型,但两者间也存在很大的差别。与曲流河三角洲相比,辫状河三角洲的三角洲平原亚相不仅沉积物的粒度较粗,而且辫状河道沉积较发育,河道间洪泛平原沉积不发育。辫状河三角洲前缘亚相也可以识别出水下分流河道、河口坝、席状砂和水下分流河道间等微相,水下分流河道沉积为主体。根据野外露头的追索研究,建立了准噶尔盆地南缘上三叠统黄山街组辫状河三角洲的沉积模式。从沉积物的岩性特征看,辫状河三角洲砾岩和含砾粗砂岩体具有分布较稳定、展布较广、储集物性较好的特点。     

Clay mineralogy of the late pleistocene red sediments of the Visakhapatnam region,east coast of India

The fine-fraction mineralogy of the red sediments and of the upland soils along the Visakhapatnam—Bhimunipatnam coastal region has been studied by X-ray analysis. Dominant kaolinite with subordinate illite is the general clay-mineral assemblage in both the red sediments and the soils. The similar clay-mineral composition indicates that the upland soils are the source for the clay material in the red sediments. Montmorillonite is present in some samples and shows greater abundance in the red sediments than in the soils. The additional montmorillonite is considered to have been formed by post-depositional diagenesis in the red sediments at the expense of illite. Differences in the intensity of the weathering process controlled by the gradient in the source area have been suggested as being responsible for the variation in type and abundance of the clay minerals in the red sediments. It is inferred that humid and tropical to subtropical climatic conditions prevailed and persisted throughout the Late Pleistocene and Holocene in the Visakhapatnam region.     

Opaque mineralogy and magnetic investigation of some basaltic occurrences, Sinai, Egypt

Ore microscopic investigation of the Fe?CTi oxide minerals was carried out on samples from three Oligo-Miocene basaltic occurrences from Sinai, Egypt. These occurrences are Gebel Maghara (north Sinai), Rageibet Naama (central Sinai), and Wadi Matulla (west Sinai). The results and correlations of magnetic parameters such as NRM intensity and susceptibility, coercive force H _c, and the ratio M _r/M _s, H _c and Q value, the ratio M _r/M _s, saturation magnetization M _s, and K are discussed in light of opaque mineralogical studies. It has been found that the variations in the magnetic properties of the basaltic occurrences are strongly dependent on the crystallite size and nature and style of exsolution textures and fabrics. The latter are controlled by the cooling conditions, being most sensitive to the partial pressure of oxygen in the melt.     

青藏高原东北缘循化盆地中中新世—早上新世黏土矿物及其古气候意义

青藏高原东北缘古气候可能受控于全球变冷、青藏高原隆升及局地地形变化的影响。为解析气候演化过程及驱动因素,本文以青藏高原东北缘循化盆地西沟剖面作为研究对象,在已有古地磁年龄约束基础上,分析了中中新世—早上新世沉积物中黏土矿物的组成和微观形貌特征。结果表明,西沟剖面沉积物中黏土矿物主要由伊利石、蒙脱石、绿泥石和高岭石组成,其中伊利石含量最高,平均为59. 3％;蒙脱石次之,平均为18. 2％,绿泥石平均含量为12. 3％,高岭石平均含量为10. 2％。根据剖面中黏土矿物含量和比值的变化特征,结合循化盆地西沟剖面的沉积速率、孢粉记录、有机质碳同位素和沉积岩地球化学比值,并与深海氧同位素值（δ18O）变化曲线对比,将循化盆地14. 6~5. 0 Ma气候环境演化划分为3个阶段：14. 6~12. 7 Ma,气候干冷期,与北半球冰盖扩展引发的全球性降温事件有关;12. 7~8. 0 Ma,气候相对温暖湿润期,可能与循化盆地周围山体隆升有关,即积石山在~12. 7 Ma隆升至临界高度,成为西风带输送水汽的地形屏障,使得循化盆地内的降水增强;8. 0~5. 0 Ma,气候再次转向干冷期,该阶段气候的干旱化对应于青藏高原在8 Ma左右的快速隆升,高原进一步的隆升阻碍东亚季风西风带的暖湿气流向内陆的输送,从而引起区域干旱化。     

Clay minerals in sediments of the hydrothermally active southern trough in the Guaymas Basin (Gulf of California)

The results of the study of clay mineral alterations in Upper Pleistocene sediments of the southern trough in the Guaymas Basin (Gulf of California) due to the influence of hydrothermal solutions and heat produced by sill intrusions are discussed. Core samples from DSDP Holes 477 and 477A were taken for the analysis of clay minerals. Application of the method of modeling X-ray diffraction patterns of oriented specimens of the finely dispersed particles made it possible to establish the phase composition of clay minerals, determine their structural parameters, and obtain reliable quantitative estimates of their contents in natural mixtures. The modeling data allowed us to characterize reliably the transformation of clay minerals in sediments of the hydrothermally active southern trough in the Guaymas Basin. In Upper Pleistocene sandy–clayey sediments of the southern trough, changes in the composition of clay minerals occurred under the influence of a long-living hydrothermal system. Its lower part (interval 170.0–257.5 m) with maximum temperatures (~300°C) was marked by the formation of chlorite. Terrigenous clay minerals are not preserved here. Saponite appears at a depth of 248 m in the chlorite formation zone. Higher in the sedimentary section, the interval 146–170 m is also barren of terrigenous clay minerals. Sediments of this interval yielded two newly formed clay minerals (chlorite and illite), which were formed at lower temperatures (above 180°C and below 300°C, approximately up to ~250°C), while the relatively low-temperature upper part (110–146 m) of the hydrothermal system (from ~140°C to ~180°C) includes the mixture of terrigenous and newly formed clay minerals. Terrigenous illite is preserved here. Illitization of the mixed-layer illite–smectite was subjected to illitization. The terrigenous montmorillonite disappeared, and chlorite–smectite with 5–10% of smectite layers were formed. In the upper interval (down to approximately 110 mbsf), the composition of terrigenous clay minerals remains unchanged. They are composed of the predominant mixed-layer illite–smectite and montmorillonite, the subordinate illite, mixed-layer chlorite–smectite with 5% of smectite layers, mixed-layer kaolinite–smectite with 30% of smectite layers, and kaolinite. This composition of clay minerals changed under the influence of sill intrusions into the sedimentary cover at 58–105 m in the section of Hole 477. The most significant changes are noted in the 8-m-thick member above the sill at 50–58 m. The upper part of this interval is barren of the terrigenous mixed-layer illite–smectite, which is replaced by the newly formed trioctahedral smectite (saponite). At the same time, the terrigenous dioctahedral smectite (montmorillonite) is preserved. The composition of terrigenous clay minerals remains unchanged at the top of the unit underlying the sill base.     

Lateritization and ground water alteration phenomena in the Triassic Budra Formation, south-western Sinai

Field and laboratory investigations of the Triassic Budra Formation of Wadi Budra, south-west Sinai, have demonstrated the existence of surface pedogenic laterites developed within‘overbank facies’of fluviatile sediments during times of hiatus. The laterite profiles, up to 7 m thick, comprise a zone of high-iron concentrations (21.47–73.17%) which includes a concretionary unit and either overlies a zone of slightly mottled host rock or rests directly upon unaltered sandstone; a pallid zone is noticeably absent. The primary depositional horizontal fabric of the finely laminated sediments of the‘overbank facies’ has strongly influenced the final fabric of the laterite. The pronounced alternations of iron-rich pisolitic and iron-deficient laminae highlight the influence of inherent fabric (primary bedding) on pedogenesis. The micromorphology of the laterites is characterized by bimasepic plasmic fabrics, abundant elongate voids partially infilled with cryptocrystalline kaolinite and an abundance of randomly oriented skew planes. Illuviation of clay minerals to considerable depths is evident. Sesquioxidic and kaolinitic glaebules, concentrated within the concretionary units of the laterite have a flattened ellipsoidal shape and occur as both massive and concentrically structural varieties. At Gebel Mussaba Salama to the north, zoned iron mineralization occurs at various levels within the fluviatile sandstones which underlie the‘overbank facies’. This is interpreted as being a non-pedogenic, ground water alteration phenomenon induced by a rising water table within the buried alluvial channels. Iron mineralization, probably records minor fluctuations of the water table during standstill periods. Lateritization in the Triassic Budra Formation provides important evidence that the widely recorded late Triassic lateritization and bauxitization event of the neighbouring East Mediterranean countries, extended south to the continental areas bordering the Tethyan Ocean.     

Clay mineralogy of the pleistocene soil horizon in Barind Tract, Bangladesh

Pleistocene red soil horizons were exposed in different areas of the Barind Tract in north-west Bangladesh. X-ray diffractions of twenty seven samples from different depths of these soil horizons revealed that the soil horizons consisted of kaolinite, illite and chrysotile with significant amount of opal-CT. Samples from Maddhapara, Bogra, and Nachole contain kaolinite, illite, quartz and opal-CT, and the samples from Kantabari contain chrysotile instead of kaolinite. Clay mineral compositions of different soil horizons indicated two different types of clay assemblages, viz. (a) illitekaolinite and (b) illite-chrysotile. In the village of Kantabari, illite-chrysotile clay mineral assemblage indicate that soil horizons were formed under low temperatures with alkaline and reducing conditions. However, other soil horizons of illite-kaolinite clay mineral assemblage indicate that soils were possibly formed under humid, temperate and welldrained conditions. These two soil horizons were formed under different geochemical, geomorphological and climatic conditions from different parent materials. Scanning Electron Microscopy photographs showing the presence of glass shards and no opal-A were found using XRD, suggesting that the opal-A might not be a precursor to opal-CT in the red soil horizon of the study area. This opal-CT along with the general lack of fossils and presence of glass shards was indicative of a volcanogenic rather than biogenic origin for the Opal-CT in the study area, and X-ray fluorescence data reveals higher percentages of silica which is comparable to the Toba Ash of Toba Caldera, Indonesia of about 75,000 B.P.     

Geochemistry and mineralogy of a recent sabkha along the coast of Sinai, Gulf of Suez

Most efforts in the study of sea-marginal sabkhas have concentrated on the Persian Gulf, but little is known about the sediments and mineralogy of sabkhas marginal to other seas. The purpose of this paper was to present some geochemical and mineralogical observations in a recent sabkha on the coast of Sinai along the Gulf of Suez. The sabkha is composed of coarse clastic sediments with marine-derived groundwater at depth of about 1 m. The general morphology, climate and water salinity of the Gulf of Suez resemble those of the Persian Gulf, despite the fact that the content of authigenic evaporites in this sabkha is more sparse. The evaporite minerals accumulated only in the upper 30–40 cm of the sabkha, below that and down to the groundwater table, there is no accumulation of evaporites. Laterally, the salinity of the groundwater in the sabkha and the concentration of evaporites in the sediments above it increase constantly with distance from the shore. In contrast to the Persian Gulf where anhydrite is a major evaporite mineral, in Belayim gypsum is the only calcium sulphate mineral in the recent sabkha. Anhydrite is found only in an old elevated sabkha where it recrystallized from gypsum. The gypsum occurs as interstitial crystal concentrations or lithified horizons almost exclusively at the depth of 20–40 cm below the sabkha surface. Above that, in the uppermost horizons, there is in situ accumulation of interstitial halite crystals. The total concentrations of gypsum and halite are almost equal in this sabkha. The sea water recharge in El Belayim is almost exclusively by seepage through the sabkha sediments and not by flooding. The groundwater under this sabkha is only slightly more saline than the Gulf water, thus, not heavy enough for extensive downward refluxing. The major hydrodynamic process must be upward migration of the brines from the groundwater, precipitating on the way gypsum and later halite with some magnesite. Since the sediments of the sabkha are too coarse to support extensive capillary movement, the brines must, therefore, migrate upwards due to ‘evaporative pumping’.     

The mineralogy and geochemistry of the sediments of northwestern Lake Victoria

In the nearshore area of northwestern Lake Victoria a thin strip of quartzarenite sand occurs which grades lakeward into silty clay consisting of quartz with subordinate amounts of K-feldspar, plagioclase, kaolinite, illite, vermiculite and organic matter. Varimax matrix determinations of the element concentrations in the lake-bottom sediments extractable by aqua regia indicate that: (1) there is a strong association of Cr, Cu, Zn and Ni; (2) there is a strong association of Fe, Mn and Co; (3) Cu and Ni show moderate to slight associations with organic matter; (4) Ca is relatively independent of the other elements. The general decrease in the pH values of the surface and bottom waters outward from the lake shore, with consistently higher values for the surface water relative to the bottom waters, results from decreasing levels of photosynthetic activity.     

Early formation of K-feldspar in shallow-marine sediments at near-surface temperatures (southern Israel): evidence from K-Ar dating

Authigenic K‐feldspar was investigated in two Albian to Turonian sections in Israel using K‐Ar and Ar‐Ar dating, X‐ray diffraction, scanning electron microscopy and chemical analysis. Both sections comprise a similar succession of shallow‐marine limestones, dolomites and marls, with some sandstone and shale beds of continental origin. The HCl‐insoluble residue fraction of the studied samples consists of clays, quartz, feldspars, goethite and trace amounts of heavy minerals. Most of the insoluble residues have a relatively high K‐feldspar content that has an adularia habit and is concentrated in the 4–7 µm size fraction. The authigenic origin of the K‐feldspar in the fine silt fraction is indicated by its high content relative to quartz, the uniform and idiomorphic shape of the crystals and its limited size range. Of the fine silt (4–7 or 4–10 µm) separates, 40% have ages that are similar to stratigraphic ages within the analytical and biostratigraphic errors. Ar‐Ar dating of a fine silt fraction with 94% K‐feldspar (4–10 µm, sample GYP 7) yields a plateau age identical to the total gas age and similar to the stratigraphic age. These results indicate that the K‐Ar age is not a mixture between detrital and late diagenetic K‐feldspar ages, but is rather an age of formation within a few million years after deposition. It is suggested that the early formation of the K‐feldspar was associated with dolomitization and was induced by residual brines as part of a reflux process.     

Clay mineralogy, organic carbon burial, and redox evolution in Proterozoic oceans

Clay minerals formed through chemical weathering have long been implicated in the burial of organic matter (OM), but because diagenesis and metamorphism commonly obscure the signature of weathering-derived clays in Precambrian rocks, clay mineralogy and its role in OM burial through much of geologic time remains incompletely understood. Here we have analyzed the mineralogy, geochemistry and total organic carbon (TOC) of organic rich shales deposited in late Archean to early Cambrian sedimentary basins. Across all samples we have quantified the contribution of 1M and 1M_d illite polytypes, clay minerals formed by diagenetic transformation of smectite and/or kaolinite-rich weathering products. This mineralogical signal, together with corrected paleo-weathering indices, indicates that late Archean and Mesoproterozoic samples were moderately to intensely weathered. However, in late Neoproterozoic basins, 2M₁ illite/mica dominates clay mineralogy and paleo-weathering indices sharply decrease, consistent with an influx of chemically immature and relatively unweathered sediment. A late Neoproterozoic switch to micaceous clays is inconsistent with hypotheses for oxygen history that require an increased flux of weathering-derived clays (i.e., smectite or kaolinite) across the Precambrian-Cambrian boundary. Compared to previous studies, our XRD data display the same variation in Schultz Ratio across the late Neoproterozoic, but we show the cause to be micaceous clay and not pedogenic clay; paleo-weathering signals cannot be recovered from bulk mineralogy without this distinction. We find little evidence to support a link between these mineralogical variations and organic carbon in our samples and conclude that modal clay mineralogy cannot by itself explain an Ediacaran increase in atmospheric oxygen driven by enhanced OM burial.     

Petrology and mineralogy of ‘laterites’ in southern and eastern Australia and southern Africa

Field observations of ‘laterites’ in southern and eastern Australia and in southern Africa reveal a variety of ferruginous horizons and crusts referred to herein as ferricretes. Their geomorphic and stratigraphic relationships with bedrock, sediments and soils indicate formation throughout long intervals of geological time in landscapes which are also characterised by zones of bleached and iron-mottled materials. There does not appear to be a genetic relationship between the ferricretes and the weathered zones in the sense of the so-called ‘laterite profile’. Many of the ferricretes form part of existing soil profiles.

Petrographic studies of a variety of ferricretes have identified three broad categories: (a) ferruginised bedrock; (b) Fe-impregnated and -indurated sediments, including sands, clays and organic sediments; and (c) ferricretes of complex sedimentary and pedogenic origin. Type-(a) and -(b) ferricretes characteristically have simple fabrics, often with single-generation, secondary Fe-oxides. Type-(c) ferricretes have complex fabrics, with many generations of hematite, goethite and in some variants, gibbsite, in the matrix and in ferruginous clasts and pisoliths. Maghemite is a common constituent of the pisoliths. The characteristics of ironstone gravelly duplex soils, which are common in the contemporary landscapes, provide the framework for a model involving multiple stages in the development of these ferricretes.

The origins of the various secondary oxide minerals in ferricretes are assessed on the basis of knowledge about the formation of these minerals in pedogenic environments. Examples are given of the intricate patterns of distribution of the minerals in thin section from which definitive data may be obtained on environmental conditions for integration with field-based geomorphic studies.     

Lithofacies and cyclicity of Mohilla evaporite basins on the rifted margin of the Levant in the Late Triassic,Makhtesh Ramon,southern Israel

Marine‐connected basins with evaporites occur beneath most extensional continental margins that originated at low‐latitudes and often are of major economic significance. Cyclicity in the evaporite lithofacies reflects the degree of restriction of the basin, overprinted by sea‐level changes, and caused by structural movements in the barrier region, whether by fault‐block rotation, footwall uplift or hanging wall subsidence, in both extensional and compressional basins. The Upper Triassic evaporites of the Ramon section in southern Israel model cyclic sedimentation in such environments. The Mohilla Formation is a carbonate–evaporate–siliciclastic succession of Carnian age that fills a chain of basins extending along the Levant margin from southern Israel to Jordan and Syria. The basins developed in half‐grabens adjacent to normal faults that formed during a period of regional extension. Evaporites of this formation are well‐exposed in outcrops at Makhtesh Ramon, the southernmost of these basins. The M2 Member of the Mohilla Formation is composed of 42 sub‐metre cycles of alternating dolostone, gypsum and calcareous shales. Field and microfacies analysis showed these cycles to conform mostly to restricted shallow and marginal marine environments, spatially limited by the uplifted shoulders of the half‐graben systems. A total of 10 facies types belonging to six depositional environments have been identified. From stacking patterns and analysis of bed to bed change, cycles can be categorized into three groupings: (i) low frequency exposure to exposure cycles that developed under eustatic or climate control; (ii) high frequency deepening/shallowing‐upward cycles, characterized by gradual transitions due to short‐term sea‐level or runoff‐event oscillations possibly referable to orbital forcing; and (iii) high frequency shallowing‐upward cycles, characterized by abrupt transitions, attributable to sporadic tectonic events affecting accommodation space or barrier effectiveness. The way facies and cycling of the sedimentary environments was deciphered in the Mohilla evaporite basin can be used to unravel the genesis of many other evaporite basins with barriers of tectonic origin.     

Clay mineralogy, grain size distribution and their correlations with trace metals in the salt marsh sediments of the Skallingen barrier spit, Danish Wadden Sea

To understand the behavior of trace metals in the salt marsh at Skallingen, Danish Wadden Sea, we investigated a profile from surface to 25?cm depth of the salt marsh sediment, focusing primarily on clay mineralogy and grain size distribution of the sediments and their relationship with trace metals. The clay assembly of the sediment consists of illite, kaolinite and much less chlorite and smectite. The major clay minerals of illite, kaolinite as well as chlorite correlate very poorly with all the trace metals investigated, due probably to the weak competing strength of these clays compared with the other adsorbents and to low availability of the mobile trace metals in the system. Correlation between trace metals and clay minerals may therefore be used as an indicator in environmental assessment. Fine grain fractions of the sediment increased markedly after salt marsh invasion in about 1931 but decreased after 1964. This increase was contributed mainly by the grains finer than 20???m while the fraction of 40?C63???m actually decreased soon after the invasion. The relation between most of the trace metals and grain size distributions is close in general but far from linear. The drastic turning point is observed at about 30???m, around where the correlation coefficient r drops from 0.8 to 0.1. Adsorption is the controlling mechanism for the behavior of trace metals in the salt marsh. Fe/Mn (hydr)oxides and organic matter play the key role.     

南秦岭晚三叠世胭脂坝岩体岩石磁学及磁组构可靠性约束

胭脂坝岩体是秦岭造山带内具典型代表性意义的晚三叠世花岗岩,已有的年代学和地球化学研究对该岩体的侵位机制有不同认识。对岩体的岩石磁学进行分析,结合其岩相学特征,探讨了用磁组构约束岩体内部组构这一方法的可靠性。研究表明,胭脂坝岩体样品的平均磁化率Km值和校正磁化率各向异性度PJ值总体较低,与顺磁性花岗岩类一致。磁滞回线分析表明,绝大部分样品的磁化率来自顺磁性矿物的贡献,少部分高磁化率样品含有铁磁性矿物的贡献,但其浓度极低。高磁化率值样品的等温剩磁曲线获得和反向退磁曲线及χ-T曲线分析表明,铁磁性矿物为磁铁矿,且为多畴颗粒。结合岩相学特征,认为胭脂坝岩体低场磁组构反映了顺磁性的云母类矿物的组构特征,磁组构特征真实地记录了样品的内部组构特征。     

Clay mineralogy of the ocean sediments from the Wilkes Land margin,east Antarctica: implications on the paleoclimate,provenance and sediment dispersal pattern

Core U1359 collected from the continental rise off Wilkes Land, east Antarctica, is analyzed for the clay mineralogy and carbon content. The temporal variation of the clay mineralogical data shows a dominance of illite with chlorite, smectite and kaolinite in decreasing concentration. Clay mineral illite is negatively correlated with smectite which shows enrichment during 6.2–6.8, 5.5–5.8, 4.5 and 2.5 Ma. The mineralogical analyses on the silt size fraction (2–53 μm) of some selected samples were also carried out. The combined result of both the size fractions shows the presence of chlorite and illite in both size fractions, smectite and kaolinite only in clay size fraction (<2 μm) and similarity in the crystallinity and chemistry of illite in both fractions. Similar nature of illite in both fractions suggests negligible role of sorting probably due to the deposition from the waxing ice sheet. During times of ice growth, nearby cratonic east Antarctica shield provided biotite-rich sediments to the depositional site. On the other hand, the presence of smectite, only in the clay size fraction, suggests the effective role of sorting probably due to the deposition from distal source in ice retreat condition. During times of ice retreat, smectite-rich sediment derived from Ross Orogen is transported to the core site through surface or bottom water currents. Poor crystallinity of illite due to degradation further corroborates the ice retreat condition. The ice sheet proximal sediments of U1359 show that in the eastern part of Wilkes Land, the ‘warming’ was initiated during late Miocene.