Geochemical and mineralogical investigations of the Lower Jurassic flint-clay bearing Mishhor and Ardon Formations,Makhtesh Ramon,Israel

The Lower Jurassic section at Makhtesh Ramon, in the northeastern Negev of Israel, comprises a sequence of fluviatile sediments of laterite composition. Post-depositional chemical weathering has resulted in marked facies changes whereby the clastic sequence passes laterally into high-alumina flint clays and bauxites (Goldbery, 1979). A geochemical and mineralogical investigation was initiated on 216 samples from several key sections of the facies change to obtain a more refined definition of the diagenetic overprint, to reconstruct the history of chemical weathering and finally to recognize the parent material of the unaltered laterite sediments.Results are presented herein on major and trace-element composition, normative mineralogical composition, kaolinite crystallinity and porosity and density measurements. Kaolinite crystallinity values proved to be a good indicator of intensity of leaching, showing a marked decline in ordering with increase of diagenetic reorganization. On the basis of mineralogical parameters it was possible to subdivide the section of flint clays and bauxites, generated under karstic conditions, into four units. Bivariate plots of Al₂O₃ vs. several elements within the section, plotted into four distinctive fields coinciding with the subdivision. Enrichment/depletion ratios, calculated from the element concentrations within these flint clays against element concentrations of the clastic parent material of the karstic infill, led to the reconstruction of a multi-cyclic leaching event, whose “signature” is left in each of the four units. At least 3 individual episodes of leaching, related to a rising water table and punctuated by “still-stands” could be recognized.Zr/TiO₂: Nb/Y plots of stable elements, using the diagram of Floyd and Winchester (1978) for determination of source rocks, indicated a rhyolite or alkaline rhyolite provenance for the clastic laterite detritus.     

Late Holocene hydrologic changes in northern New Zealand inferred from stable isotope values of aquatic cellulose in sediments from Lake Pupuke

Isotopic records of aquatic cellulose are becoming increasingly important for palaeohydrological reconstructions, but widespread application of this climate proxy is hampered by minerogenic contamination that affects oxygen isotope measures in cellulose. Few records of isotopes in aquatic cellulose are available from palaeoclimate archives in the Southern Hemisphere. In this study, we used a new bulk cellulose extraction method and determined the oxygen (δ¹⁸O) and carbon (δ¹³C) isotope values in cellulose from a Holocene lake sediment core segment (7.2–1.1 cal ka BP) from Lake Pupuke, Auckland, New Zealand. Isotope values from modern, potential sources of sedimentary cellulose revealed the aquatic origin of the cellulose extracted from the core, and hence enabled inference of past lake water δ¹⁸O values from the δ¹⁸O of measured cellulose in the core. A shift to a more positive water balance in the lake was identified around 2.8 cal ka BP by a decrease in inferred lake water δ¹⁸O values. At that time, greater epilimnetic primary productivity is indicated by the higher δ¹³C values of sedimentary cellulose. Greater divergence between the δ¹³C values of cellulose and bulk organic matter suggests stronger stratification of the lake, likely caused by greater freshwater input. We discuss a possible link to a solar minimum that occurred at that time.     

Properties of dust source material and volcanic ash in Iceland

The volcanic origin, primarily basaltic, of most of the surface material in Iceland influences its physical properties and appearance. Size distributions, shape analyses and melting experiments were made for surface material collected in high-erosion dust source areas and fresh volcanic ash deposits to determine whether they differ from one another and from dust from other major dust sources. The major differences found between Icelandic dust and dust from other major dust sources in the world, such as the Sahara, are in the particle shapes, lower density and darker colour. Icelandic dust particles greater than 20 μm retain volcanic morphological properties that are also found in fresh volcanic ash. Dust and fresh volcanic ash particles less than 20 μm are crystalline and blocky in nature, similar to the dust from other global source regions. The finer grained (<20 μm) Icelandic particles will have similar suspension and transport behaviours and be similarly hazardous to health and infrastructure as non-Icelandic dust. The coarser particles (>20 μm) will have different suspension and transport behaviours than other dusts due to the volcanic morphology. Icelandic surface material has between 5% and 30% glassy particles compared to fresh volcanic ash which has more than 50% glassy particles. Glassy particles were observed to melt at a lower temperature than the mineral grains; and, as a result, volcanic ash is found to be more threatening to aircraft engines than the typical dust from Iceland. Icelandic dust was observed to be blocky, or plate-like in the respirable size fraction, suggesting similar health hazards as dust from other regions.     

Experimental petrochemistry of some highly siderophile elements at high temperatures, and some implications for core formation and the mantle''s early history

The highly siderophile elements (HSE's: Ru, Rh, Pd, Re, Os, Ir, Pt and Au) and those elements with distribution coefficients between Fe-rich metal and silicate phases which exceed 10⁴. The large magnitude of these distribution coefficients makes them exceedingly difficult to measure experimentally. We describe a new experimental campaign aimed at obtaining reliable values of D_M^{mets/sil melt} for selected HSE's indirectly, by measuring the solubilities of the pure metals (or simple HSE alloys) in haplobasaltic melts as a function of oxygen fugacity.

Preliminary results for Pd, Au, Ir and Re indicate that the HSE's may dissolve in silicate melts in unusually low valence states, e.g., 2+ for Ir and 1+ for the others. These unusual valence states may be important in understanding the geochemical properties of the HSE's. Inferred values of D_M^{met/sil melt} from the solubility data at 1400°C and IW −1 are 10⁷ for Pd and Au, and 10⁹−10¹² for Ir. Metal/silicate partition coefficients are thus confirmed to be very large, and are also different for the different HSE's.

A review of the abundance of the HSE's in the Earth's upper mantle shows that they are all present at 0.8% of chondritic, i.e. they have the same relative abundance, and the ratios of their concentrations are chondritic (e.g., Re/Os). Both the low degree of depletion (compared to the high values of D_M^{met/sil melt}) and the chondritic relative abundances support the idea that the mantle's HSE's were added in a “late veneer” after the cessation of core formation. Sulfur is even more depleted in the mantle relative to CI chondrites than the HSE's: this implies a late veneer which was depleted in volatile elements, and which was added to a mantle stripped of S. Since considerable S dissolves in silicate melt, this further implies that core formation in the Earth either occurred under P−T conditions below the solicate solidus, or, if the process occurred over a range of temperatures in a cooling Earth, then the process continued down to conditions below the silicate solidus.

The chondritic relative abundances of the HSE's in the upper mantle argue for a chemically unstratified primitive mantle, unless the late veneer was mixed only into the upper mantle.     

Zur Geologie und Mineralogie der Kalk- und Gipskrusten Algeriens

Caliches: Large areas of the northern Sahara and the Algerian High Planes are covered by mostly 1–5 m thick caliches. Their age (Pliocene in the Sahara) decreases to the north and their precipitation is generally independent of groundwater. Their profile is composed (from top to base) as follows:

Upper soil, loose and mostly of eolian origin.

Upper part of caliche, with very characteristic, dense, partly layered-knobby texture, formed slowly by solutional and reprecipitational processes of ± freely outcropping caliches under addition of eolian material.

Under part of caliche, highly porous, somewhat chalky and greyish-white; precipitated mainly by capillar rise of solutions in permeable and calcareous rocks.

Substratum, preferentially calcareous sandstones, alluvial deposits and marls.

The mineralogy of the caliches (whose main components are represented in fig. 4 A-C) is rather monotonous: in addition to relicts of the substratum (partly dissolved or pushed aside by precipitation of calcite), there are only newly formed low-Mg-calcite and some quarzine (length-slow quartz). Sr-contents of calcite rise clearly from substratum to upper part of caliche. Gypsiferous Crusts (or Cementations): They are found mainly in the surroundings of Chotts (flat, ± saline lakes) and in oases of the NE-Algerian Sahara. Their formation began — mostly caused climatically — after the period of caliche formation and is still continuing in some places. Most of these gypsum-crusts are formed by evaporation of near-surface groundwaters in sandy soils. Water saturated in gypsum precipitates large crystals of gypsum (relatively low in Sr), partly filled by sand, at groundwater-surface. Fine crystalline crusts (relatively high in Sr) are formed by ascendent waters with lower gypsum content ± directly under the landsurface.     

Bücherschau

Ohne Zusammenfassung     

Characteristics and Genesis of Fine-Clastic Rock-Type Au Deposits in the Liaodong Rift

Gold deposits occurring in the Liaodong rift are located in interlayered carbonate rocks and fine-clastic rocks, belonging to the middle and upper parts of the lower Proterozoic. Compared with the Carlin-type Au deposits abroad and gold deposits in Yunnan-Guizhou-Guangxi, Qinling and western Sichuan Province, they are similar in many respects. This paper discusses the geological features of ore-bearing formations and ore-filling structures and metallogenetic characteristics. Through the discussion on the sources of ores, heat and water, it points out that the Au deposits belong to vadose reworked hydrothermal deposits. This conforms to the principle of "mineralization in the neighbouring areas", i.e., the deposits are formed in nearby ore-bearing layers, and the latest hydrothermal event was the main factor in forming the Au deposits.     

The Late Paleozoic felsic lava-dominated large igneous province in northeast Germany: volcanic facies analysis based on drill cores

In the deep subsurface of the northeast German basin products of extensive volcanism are present that formed during the waning phase of the Variscan orogeny (Permian–Carboniferous boundary). Large volumes (≥48,000 km³) of dominantly felsic magma were emplaced in the northern foreland of the Variscan mountains in a terrestrial semi-arid environment. Most of these units were inferred by previous authors to represent the products of explosive volcanic eruptions such as ignimbrites. However, a detailed analysis of the volcanic texture and lithofacies association shows that most units are best interpreted as coherent lavas emplaced either as extrusive units or as shallow intrusions, with important implications for the reconstruction of the paleo-environment. This study shows that detailed textural examination of drill cores provide important observations, which can be applied as criteria for the interpretation of the mode of volcanic eruptions. Furthermore, phenocryst logging and geochemical fingerprinting based on immobile element ratios can be employed for the classification and discrimination of individual emplacement units.