Tertiary remagnetization of normal polarity in Mesozoic marly limestones from SE France

The results of our new paleomagnetic investigations on 21 sites in the Cévennes and Lure regions as well as previous studies demonstrate that all Mesozoic marly limestones of SE France exhibit similar paleomagnetic behavior with remagnetization disputed in age. The studied areas have the particularity to have been folded before (Late Eocene), the Alpine folding (Oligo–Miocene). Samples (201 marly limestones) dated from Lower Jurassic to Lower Cretaceous have been demagnetized by thermal treatment. They all present a well-defined component with a normal polarity which was mostly obtained between 200 and 350 °C. Numerous arguments lead from pretectonic to syntectonic widespread remagnetization related to orogenic fluid circulation affecting the whole basin. An Eocene age (between 35 and 40 Ma) is obtained for this remagnetization thanks both to the comparison of the average inclination of all regional paleomagnetic studies (+54.9°/−1.5°) with the expected paleomagnetic inclination and the syntectonic character of remagnetization.     

Magnetic susceptibility of the rock matrix related to magnetic fabric studies

Non-ferromagnetic minerals constitute what is called the rock matrix, whose susceptibility (K₁) is directly accessible using only high magnetic fields. Measurements on minerals and a wide range of rock types show that K₁ is mainly due to paramagnetism and hardly exceeds 10⁻³ SI, with an anisotropy degree (P) less than 1.35. Different methods to estimate the role of the matrix component in low-field susceptibility (K) and its anisotropy include petrological and chemical analysis, evolution of P vs K, low temperature studies and comparison of K with remanent magnetizations.     

Metamorphic control of the magnetic mineralogy of black shales in the Swiss Alps: toward the use of “magnetic isogrades”

The magnetic susceptibility of the Helvetic Jurassic black shales, sampled around the Aar and Gothard massifs in the Swiss Alps, is due to iron-bearing silicates, magnetite or pyrrhotite. It exhibits a clear correlation with the Alpine metamorphism, from zeolite to amphibolite facies: it firstly decreases due to the breakdown of original magnetite, then sharply increases when pyrite is transformed into magnetic pyrrhotite. Using numerous field measurements of the susceptibility it was possible to carry out a fast and precise mapping of the pyrrhotite-in isograde, which appears to correspond closely to the stilpnomelane-out isograde. The matrix susceptibility, obtained by means of high-field measurements with a cryogenic magnetometer, is due to iron-bearing silicates and shows only smooth variations with metamorphism. Both monoclinic and hexagonal pyrrhotites are observed in the high-grade area.     

Low paleosecular variation at the equator: a paleomagnetic pilgrimage from Galapagos to Esterel with Allan Cox and Hans Zijderveld

McFadden and Merrill (1995) suggested that the paleosecular variation (PSV) measured by the angular scatter of the virtual geomagnetic pole is minimal at the equator and should be smaller during a superchron than during the last 5 Myr. We revisited a key site of the 0–5 Ma database, the Galapagos archipelago, studied by Allan Cox in the early sixties. We obtained 79 sites with reliable mean directions on four islands (San Cristobal, Floreana, Santa Cruz and Pinzon), showing a larger proportion of transitional data than Cox (16 instead of 6%), because the sampling was concentrated on the Brunhes-Matuyama transition as delimited by Cox. This dataset allowed us to test the statistical method of Vandamme (1994) to separate PSV from transitional data. We obtained an angular scatter value of 11.2° (9.9–12.9°), instead of 16.8° for an a-priori rejection angle of 40°, compared with the 12.7° predicted from the global compilation (McFadden et al. 1991). Studies of sequences of lava flows are quite scarce in the Permian Kiaman Superchron, and the Esterel volcanics with their subequatorial paleolatitude are a good candidate to test the above prediction. We confirm the quality of the original data of Zijderveld (1975) and we improved the mean direction from one site. We also used new geological and geochronological data: Ar/Ar ages point to the period 264–278 Ma for a totally reversed volcanic sequence, in agreement with an ending of the Kiaman Superchron at 262–268 Ma. The extremely low angular scatter obtained (4 to 8°, depending on data selection) confirms the prediction, but an alternative interpretation invoking a post-volcanic Permian remagnetization is discussed.     

Description of a very dense meteorite collection area in western Atacama: Insight into the long‐term composition of the meteorite flux to Earth

We describe the geological, morphological, and climatic settings of two new meteorite collections from Atacama (Chile). The “El Médano collection” was recovered by systematic on‐foot search in El Médano and Caleta el Cobre dense collection areas and is composed of 213 meteorites before pairing, 142 after pairing. The “private collection” has been recovered by car by three private hunters and consists of 213 meteorites. Similar to other hot desert finds, and contrary to the falls and Antarctica finds, both collections show an overabundance of H chondrites. A recovery density can be calculated only for the El Médano collection and gives 251 and 168 meteorites larger than 10 g km^?2, before and after pairing, respectively. It is by far the densest collection area described in hot deserts. The Atacama Desert is known to have been hyperarid for a long period of time and, based on cosmic‐ray exposure ages on the order of 1–10 Ma, to have been stable over a period of time of several million years. Such a high meteorite concentration might be explained invoking either a yet unclear concentration mechanism (possibly related to downslope creeping) or a previously underestimated meteorite flux in previous studies or an average terrestrial age over 2 Myr. This last hypothesis is supported by the high weathering grade of meteorites and by the common terrestrial fragmentation (with fragments scattered over a few meters) of recovered meteorites.     

Track of fluid paleocirculation in dolomite host rock at regional scale by the Anisotropy of Magnetic Susceptibility (AMS): An example from Aptian carbonates of La Florida,Northern Spain

The present study aims to apply the AMS method (Anisotropy of Magnetic Susceptibility) at a regional scale to track the fluid circulation direction that has produced an iron metasomatism within pre-existing dolomite host rock. The Urgonian formations hosting the Zn–Pb mineralizations in La Florida (Cantabria, northern Spain) have been taken as target for this purpose. Sampling was carried out, in addition to ferroan dolomite host rock enclosing the Zn–Pb mineralizations, in dolomite host rock and limestone to make the comparison possible between magnetic signals from mineralized rocks, where fluid circulation occurred, and their surrounding formations. AMS study was coupled with petrofabric analysis carried out by texture goniometry, Scanning Electron Microscopy (SEM) observations and also Shape Preferred Orientation (SPO) statistics. SEM observations of ferroan dolomite host rock illustrate both bright and dark grey ribbons corresponding respectively to Fe enriched and pure dolomites. SPO statistics applied on four images from ferroan dolomite host rock give a well-defined orientation of ribbons related to the intermediate axis of magnetic susceptibility K₂. For AMS data, two magnetic fabrics are observed. The first one is observed in ferroan dolomite host rock and characterized by a prolate ellipsoid of magnetic susceptibility with a vertical magnetic lineation. The magnetic susceptibility carrier is Fe-rich dolomite. These features are probably acquired during metasomatic fluid circulations. In Fe-rich dolomite host rock, ?c? axes are vertical. As a rule, (0001) planes (i.e. planes perpendicular to ?c? axes) are isotropic with respect to crystallographic properties. So, the magnetic anisotropy measured in this plane should reflect crystallographic modification due to fluid circulation. This is confirmed by the texture observed using the SEM. Consequently, AMS results show a dominant NE–SW elongation interpreted as the global circulation direction and a NW–SE secondary elongation that we have considered as sinuosities of the fluid trajectory. The second type of magnetic fabric is essentially observed in the limestone and characterized by an oblate form of the ellipsoid of magnetic susceptibility, a horizontal magnetic foliation and mixed magnetic susceptibility carriers. It is interpreted as a sedimentary fabric.     

Planning the sea: The French experience. Contribution to marine spatial planning perspectives

As marine spatial planning is developing, prompted by the European Union through common principles prescribed in a road map, it is important to disseminate and analyse what has been put forward as the French experience in this field. Hence, the experience of the schémas de mise en valeur de la mer immediately proves to be original in its framework and deeply rooted in French specificities. However, a further analysis of this procedure implemented over several decades highlights the perspectives that would be worth integrating into French and European considerations. This paper thus provides information referring to the SMVM experience, presents an analysis of its results, and attempts to draw lessons from it with regard to recent developments in French marine policy.     

Strewn field,mineralogy, and petrology of Al Haggounia 001: A unique enstatite chondrite

In this work, we investigate macroscopic characteristics, magnetic susceptibility, mineralogy, and mineral composition of Al Haggounia 001. The samples were collected during eight field missions in the period between 2015 and 2019. In the strewn field of about 65 km in length, the specimens are found either on the surface or shallowly buried in loose sediments, which rules out the previous suggestions that this meteorite is a fossil meteorite. Macroscopically, the samples exhibit three major lithologies with various colors, porosities, and distributions of oxidized veins. The data obtained using transmitted and reflected light microscopy, scanning electron microscopy, and electron microprobe analysis confirm the macroscopic observations and show a heterogenous distribution of silicates and metal sulfides. Al Haggounia 001 is composed of enstatite, plagioclase, kamacite, taenite, schreibersite, daubreelite, troilite, graphite, sinoite, and silica polymorphs. We identified a new type of chondrules that are flattened and composed of rods of albite and enstatite, as well as elongated nodules of metal and sulfides, in addition to compression fractures in the form of subparallel veinlets. These features presumably reflect the deformation caused by shock. The magnetic susceptibility of Al Haggounia 001 (4.39 ± 0.20) is much lower than that of usual EH (5.48 ± 0.16) and EL (5.46 ± 0.04) chondrites but is in the range of E finds (5.05 ± 0.43). The thermomagnetic and hysteresis measurements are controlled by type, size, distribution of metal-sulfide nodules, arrangement of oxyhydroxide veins, and weathering. Al Haggounia 001 is an anomalous meteorite with a polymict nature. It records multiple events revealing its unique origin which expends the groups of enstatite chondrites and provides insights into the complex formation and evolution history of their parent body.     

Impact of density fluctuations on flux measurements of trace gases: Implications for the relaxed eddy accumulation technique

A simple and fast approach to determine when density fluctuations are non-negligible in the calculation of the flux of trace gases (F _c ) is proposed. The correction (F _c – F _c (raw)), when expressed as the percentage of the flux, is dependent on the ratio of background concentration of the trace gas over its flux (% MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeikaiabeg% 8aYnaaBaaaleaacaWGJbaabeaakiaab+cacaWGgbWaaSbaaSqaaiaa% dogaaeqaaOGaaeykaaaa!3CBC!\[{\rm{(}}\rho _c {\rm{/}}F_c {\rm{)}}\], on the partitioning of available energy between sensible (F _T ) and latent (F _v ) heat fluxes, and on the flux measuring system. An increase from 100 to 200 W m^-2 in available energy and from 0 to 20% in F _T /(F _T + F _v ) led to a threefold reduction in the required value of % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaacq% aHbpGCdaWgaaWcbaGaam4yaaqabaaaaOGaai4laiaadAeadaWgaaWc% baGaam4yaaqabaaaaa!3B6D!\[\overline {\rho _c } /F_c \] to have a density correction of 10%. A trace gas with a % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaqWaceaaca% WGgbWaaSbaaSqaaiaadogaaeqaaaGccaGLhWUaayjcSdGaai4lamaa% naaabaGaeqyWdi3aaSbaaSqaaiaadogaaeqaaaaaaaa!3E91!\[\left| {F_c } \right|/\overline {\rho _c } \] value above 0.014 m s^-1 has a density correction on flux of less than 10%, for even the worst case scenario. Values of % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOramaaBa% aaleaacaWGJbaabeaakiaac+cadaqdaaqaaiabeg8aYnaaBaaaleaa% caWGJbaabeaaaaaaaa!3B6D!\[F_c /\overline {\rho _c } \] for several trace gases computed from typical situations show that the fluxes of N₂O, NO, CO₂, CH₄ and O₃ need to be corrected, while those of pesticides and volatile organic compounds, for example, do not. The corrections required with the newly developed relaxed eddy accumulation technique are discussed and equation development is shown for two sampling systems.Land Resource Research Centre Contribution No 91-61.