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51.
Mourad Essalhi Stanislas Sizaret Luc Barbanson Yan Chen France Lagroix François Demory José M. Nieto Reinaldo Sáez M. Ángeles Capitán 《Mineralium Deposita》2011,46(8):981-999
In the Rio Tinto district of the Iberian Pryrite Belt of South Spain, the weathering of massive sulfide bodies form iron caps,
i.e., true gossans and their subsequent alteration and re-sedimentation has resulted in iron terraces, i.e., displaced gossans.
To study the stucture and evolution of both types of gossans, magnetic investigations have been carried out with two foci:
(1) the characterisation and spatial distribution of magnetic fabrics in different mineralised settings, including massive
sulfides, gossans, and terraces, and (2) paleomagnetic dating. Hematite has been identified as the suceptibility carrier in
all sites and magnetic fabric investigation of four gossans reveals a vertical variation from top to bottom, with: (1) a horizontal
foliation refered to as “mature” fabric in the uppermost part of the primary gossans, (2) highly inclined or vertical foliation
interpreted as “immature” fabric between the uppermost and lowermost parts, and (3) a vertical foliation interpreted to be
inherited from Hercynian deformation in the lowermost part of the profiles. In terraces, a horizontal foliation dominates
and is interpreted to be a “sedimentary” fabric. Rock magnetic studies of gossan samples have identified goethite as the magnetic
remanence carrier for the low-temperature component, showing either a single direction close to the present Earth field (PEF)
direction or random directions. Maghemite, hematite, and occasionally magnetite are the remanence carriers for the stable
high-temperature component that is characterized by non PEF directions with both normal and reversed magnetic polarities.
No reliable conclusion can be yet be drawn on the timing of terrace magnetization due to the small number of samples. In gossans,
the polarity is reversed in the upper part and normal in the lower part. This vertical distribution with a negative reversal
test suggests remanence formation during two distinct periods. Remanence in the upper parts of the gossans is older than in
the lower parts, indicating that the alteration proceeded from top to bottom of the profiles. In the upper part, the older
age and the horizontal “mature” fabric is interpreted to be a high maturation stage of massive sulfides’ alteration. In the
lower part, the age is younger and the inherited “imature” vertical Hercynian fabric indicates a weak maturation stage. These
two distinct periods may reflect changes of paleoclimate, erosion, and/or tectonic motion. 相似文献
52.
This study investigates the isotopic composition (C, O, S and Sr) of carbonates, sulphates and sulphide cements in the rock
matrix and fracture fillings in geological formations of the Southeast basin of France, using core samples collected during
the Deep Geology of France programme (GPF Ardèche theme). The Southeast basin belongs to the Alpine Tethyan margin. It is
one of the thickest sedimentary basins in Europe, reaching upwards of 9 km in certain locations. The main fluid transfer from
the basin is related to the large Pb–Zn Mississippi Valley-type district along the southern margin of the Massif Central block.
A synthesis of the tectonic, mineralogical and petrographic investigations on the GPF boreholes shows that a major fluid circulation
event occurred across the Alpine margin of Tethys during the Early Jurassic (Hettangian–Bathonian). It produced a general
cementation of the rock porosity through precipitation of dolomite, sulphate and barite. Fracture fillings yield isotopic
signatures distinct from the matrix cements. Matrix cements have particular characteristics, i.e. δ34S and δ13C that agree with a marine origin. The δ34S values of Permo-Carboniferous to Triassic sulphides from fracture cements are interpreted as resulting from the thermo-chemical
reduction of sulphates. Fracture sulphates in the same geological formations yield δ34S values that define a relatively homogeneous end-member, whose composition is similar to sulphates in the Largentière Pb–Zn
ore deposit. The source of S is attributed to the Permo-Carboniferous succession. The borehole fracture fillings are attributed
to a major fluid circulation stage compatible with the Early Jurassic stage identified from the geological investigation of
the boreholes. The formation of the Largentière deposit is considered as resulting from the mixing of this Early Jurassic
fluid with continental hydrothermal fluids circulating in a basement horst, along its margin with the sedimentary basin. Other
Pb–Zn deposits may also be related to fluid migration along the basement/sedimentary cover interface in the eastern and western
parts of the Massif Central. This regional fluid circulation event may represent a geodynamic marker of the Jurassic extensional
phase. 相似文献
53.
The question being tackled in this study is to which extent grain rearrangement contributes to porosity reduction in very well sorted quartzose sands (ideal reservoir sands). A numerical model, RAMPAGE (an acronym of random packing generator), has been developed to address this long-standing problem. RAMPAGE represents a synthesis of various algorithms designed to simulate packing of equal-sized spheres, which have been used to represent ideal solids, liquids, and gases, as well as natural porous media. The results of RAMPAGE simulations compare favourably to theoretical and experimental data from various disciplines and allow delineation of the field of gravitationally stable random packing of equal-sized spheres in the 2-D state space of porosity (P) versus mean coordination number (N). Three end-member packing states have been identified: random loose packing (RLP: P = 45.4%, N = 5.2), random close packing (RCP: P = 36.3%, N = 7.0), and bridged random close packing (Bridged RCP: P = 39.5%, N = 5.2). Unlike previously proposed models, RAMPAGE can simulate the transition from RLP to any other point in the stability field. The RLP state is fully consistent with wet-packed porosities of synthetic sands with lognormal mass-size distributions reported in the literature. The much higher in-situ porosity values reported for modern (air-packed) sands are unlikely to be preserved at depth on geological time scales. Data on the relation between intergranular volume and burial depth indicate that the observed intergranular volume reduction in the upper ~ 800 m of the sediment column corresponds to the evolution of RLP to RCP, and is thus fully explained by non-destructive grain rearrangement. 相似文献
54.
Delphine Yeghicheyan Cécile Bossy Martine Bouhnik Le Coz Chantal Douchet Guy Granier Alexie Heimburger Francois Lacan Aurélie Lanzanova Tristan C. C. Rousseau Jean‐Luc Seidel Mickaël Tharaud Frédéric Candaudap Jérôme Chmeleff Christophe Cloquet Sophie Delpoux Marie Labatut Rémi Losno Catherine Pradoux Yann Sivry Jeroen E. Sonke 《Geostandards and Geoanalytical Research》2013,37(4):449-467
The natural river water certified reference material SLRS‐5 (NRC‐CNRC) was routinely analysed in this study for major and trace elements by ten French laboratories. Most of the measurements were made using ICP‐MS. Because no certified values are assigned by NRC‐CNRC for silicon and 35 trace element concentrations (rare earth elements, Ag, B, Bi, Cs, Ga, Ge, Li, Nb, P, Rb, Rh, Re, S, Sc, Sn, Th, Ti, Tl, W, Y and Zr), or for isotopic ratios, we provide a compilation of the concentrations and related uncertainties obtained by the participating laboratories. Strontium isotopic ratios are also given. 相似文献
55.
A recent publication on the phytolith assemblage at Fahien rockshelter, Sri Lanka (Premathilake and Hunt) is argued to represent: the exploitation of wild Musa acuminata and M. balbisiana during the Late Pleistocene; the introduction of edible diploid cultivars from the Southeast Asia–New Guinea region during the early to mid‐Holocene; the generation and cultivation of triploid banana hybrids on Sri Lanka before 6194–5994 cal bp ; and the subsequent spread of derived triploid cultivars to mainland India and westward to Africa. A careful review of the archaeobotanical research presented by Premathilake and Hunt, in the context of broader multidisciplinary evidence (agronomy, archaeobotany, genetics and linguistics) for the domestication and spread of banana cultivars, indicates that three main aspects of their argument are problematic: the lack of clarity in the characterization of banana domestication in the past; the methods used to discriminate phytoliths into banana taxa; and the promotion of Sri Lanka as a source region rather than a recipient of banana cultivars. Following reconsideration, the Fahien evidence is consistent with previous interpretations for the origins of diploid and significant triploid cultivars outside of Sri Lanka and dispersal to that island. © 2019 John Wiley & Sons, Ltd. 相似文献
56.
An extension of Joint Phase Diverse Speckle image restoration is presented. Multiple realizations of multiple objects having
known wavefront relations with each other can now be restored jointly. As the alignment of the imaging setup does not change,
near-perfect alignment can be achieved between different objects, thus greatly reducing false signals in the determination
of derived quantities, such as magnetograms, Dopplergrams, etc. The method was implemented in C++ as an image restoration
server, to which worker clients can connect and disconnect randomly, so that a large number of CPUs can be used to speed up
the restorations. We present a number of examples of applications of the restoration method to observations obtained with
the Swedish 1-m Solar Telescope on La Palma. 相似文献
57.
Henrik Svensen Gray Bebout Long Li Luc Chevallier 《Geochimica et cosmochimica acta》2008,72(20):4929-4947
We have investigated the N geochemistry of minerals and rocks from contact metamorphic aureoles and hydrothermal vent complexes (HVC) in the Karoo Basin in South Africa. The HVC formed during phreatic eruptions associated with rapid devolatilization and pressure build-up in contact aureoles around early Jurassic sill intrusions. By combining outcrop data from a HVC and core data from contact aureoles, we investigate the relationship between light element release during metamorphism and vertical fluid migration. Sandstone and breccia from the HVC contain early-diagenetic ammonium -bearing feldspar (buddingtonite) and illite. Ammonium occupies up to 95% of the A site in feldspar, corresponding to concentrations up to 5.2 wt% N. Bulk-rock N isotope data for rocks from inside and outside the hydrothermal vent complex fall into two distinct groups. Background samples have δ15Nair between +1.5‰ and +4.9‰, whereas minerals from the vent complex have δ15N in the range +7.5 to +10.6‰. The N geochemistry of contact metamorphic shale from the lower stratigraphic units of the Karoo Basin shows that the vitrinite reflectance and δ15N values are positively correlated. Shale with reflectivity values >4%Ro are enriched in 15N, with δ15N values between +6‰ and +14‰, implying the release of isotopically light N into metamorphic fluids (probably as N2). We suggest that the relatively high δ15N values of the early-diagenetic buddingtonite in the HVC reflect exchange of buddingtonite with N-bearing fluids ascending from greater depth after their release during contact metamorphism and dehydration. We present a qualitative model whereby hydrothermal vent complexes represent fluid flow structures after their formation, focusing N-bearing metamorphic fluids sourced in deeper levels of the basin. The release of organic N from sediments at depth in volcanic basins could play a role in the geochemical cycle of N, becoming particularly important during periods of intense volcanic activity. 相似文献
58.
Ftima Gonzlez Snchez Luc R. Van Loon Thomas Gimmi Andreas Jakob Martin A. Glaus Larryn W. Diamond 《Applied Geochemistry》2008,23(12):3840-3851
The effect of temperature and ionic strength on the diffusion of HTO parallel to the direction of compaction through 5 highly compacted clay minerals (bulk dry density, ρb,d = 1.90 ± 0.05 Mg/m3), namely montmorillonite (Na- and Ca-form), illite (Na- and Ca-form), and kaolinite, was studied. The diffusion experiments were carried out at temperatures between 0 °C and 60 °C and at ionic strengths of 0.01 M and 1 M NaCl for the Na-form clays and kaolinite, and of 0.005 M and 0.5 M CaCl2 for the Ca-form. The ionic strength had an insignificant influence on the values of the effective diffusion coefficient (variation by less than 10%) for the clays under study at this degree of compaction. The effective diffusion coefficients followed the order Na-montmorillonite < Ca-montmorillonite < Ca-illite < Na-illite kaolinite. It is thought that the differences between Na- and Ca-montmorillonite originate from the larger size particles, and thus the lower tortuosity of the latter; whereas the differences between Na- and Ca-illite are related to the different degree of solvation of the Na and Ca cations. The activation energies were successfully calculated using the Arrhenius law. Swelling clays (Na- and Ca-montmorillonite) had slightly larger activation energy values (20 kJ/mol) compared to bulk water (17 kJ/mol); Ca-illite (16 kJ/mol), Na-illite (13 kJ/mol) and kaolinite (14.4 kJ/mol) lower values than that of bulk water. The low activation energies of the last three clays may be related to weaker H-bonds between water and the clay surfaces compared to those in bulk water. 相似文献
59.
60.