Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation

The Earth’s inner core grows by the freezing of liquid iron at its surface. The point in history at which this process initiated marks a step-change in the thermal evolution of the planet. Recent computational and experimental studies1,2,3,4,5 have presented radically differing estimates of the thermal conductivity of the Earth’s core, resulting in estimates of the timing of inner-core nucleation ranging from less than half a billion to nearly two billion years ago. Recent inner-core nucleation (high thermal conductivity) requires high outer-core temperatures in the early Earth that complicate models of thermal evolution. The nucleation of the core leads to a different convective regime6 and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner-core nucleation to be estimated directly. Previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements, by the lack of an effective means of assessing their reliability, and by shorter-timescale geomagnetic variations. Here we examine results from an expanded Precambrian database of palaeomagnetic intensity measurements7 selected using a new set of reliability criteria8. Our analysis provides intensity-based support for the dominant dipolarity of the time-averaged Precambrian field, a crucial requirement for palaeomagnetic reconstructions of continents. We also present firm evidence for the existence of very long-term variations in geomagnetic strength. The most prominent and robust transition in the record is an increase in both average field strength and variability that is observed to occur between a billion and 1.5 billion years ago. This observation is most readily explained by the nucleation of the inner core occurring during this interval9; the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the Earth

Palaeolatitudes of glacial deposits and palaeogeography of Neoproterozoic ice ages

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Chemostratigraphy of the Neoproterozoic Mirassol d'Oeste cap dolostones (Mato Grosso, Brazil): An alternative model for Marinoan cap dolostone formation

International audienceWe have conducted a detailed study of the Neoproterozoic Mirassol d'Oeste cap dolostones that overlay the glacial diamictites of the Puga Formation (˜ 635 Ma, Amazon craton, Brazil) in order to understand the formation of these post-glacial dolostones. Petrographic features indicate that the dolostones are primary to early diagenetic in origin and precipitated in a moderately shallow-water platform corresponding to a carbonate ramp during transgressive conditions. Major and trace element contents, as well as C and O isotopic signatures, are consistent with an anoxic sediment influenced by sulphate-reducing bacteria. Such an environment is known to provide favourable conditions for the precipitation of dolomite as observed nowadays in modern hypersaline lagoons. Isotopic compositions of tube-like structures suggest local upward fluid seepage from the underlying cap dolostone. Our data concur with geochemical data from other Neoproterozoic cap dolostones to support a microbially-mediated model in specific environmental conditions for the formation of these unusual deposits worldwide

Absolute paleointensity at 1.27 Ga from the Mackenzie dyke swarm (Canada)

International audiencePaleointensity studies have been conducted on 6 mafic dykes from the 1270 Ma Mackenzie swarm in the Slave Province (Canada). The mean direction of the characteristic magnetization coincides with results of an earlier study in which the primary origin of the magnetization was established on the basis of a contact test. High unblocking temperatures, magnetic mineralogy, and grain-size experiments suggest that the magnetization is dominated by pseudo-single domain or single domain grains of magnetite. Paleointensity experiments were conducted with a specially designed oven, using a revised version of the Thellier-Coe method. Thirteen successful determinations of paleointensity were obtained for 4 dykes. The paleofield estimates vary between 4.3 and 22.1 μT, yielding virtual dipole moments (VDMs) between 1.3 ± 0.2 and 4.5 ± 0.9 × 1022 Am2. These new results increase the number of low field determinations during the Precambrian, which largely dominate the database with an averaged field of 3.1 ± 2.5 × 1022 Am2. They also emphasize the importance of additional studies to understand the differences with the strong paleointensities obtained using new techniques

Spatialisation of urban airborne particulate matter pollution using magnetic properties: case study of Paris (France)

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Incipient charnockitisation due to carbonic fluid transfer related to late Pan-African transcurrent tectonics in Madagascar; implications for mobility of Fe, Ti, REE and other elements

Incipient charnockitisation of an A-type granite protolith has been recognized in a spectacular outcrop 35 km north of Antananarivo. The charnockites form structurally-controlled dark greenish patches suggesting channelized fluid transfer, during and after partial reworking of the granite coeval with the Antananarivo virgation zone (ca 560 Ma) and/or the Angavo shear zone of late-Panafrican age (ca 550 Ma). They are characterized by a significant increase of the bulk magnetic susceptibility. The granitic protolith contains quartz, perthitic alkali feldspar, high-Ti hastingsitand accessory minerals (apatite, allanite, magnetite and zircon). The charnockitic granite contains quartz, perthitic alkali feldspar and ghost (altered) orthopyroxene crystals, as well as secondary low-Ti hastingsite surrounding orthopyroxene. The large increase of magnetic susceptibility magnitudes is related to the formation of pockets of secondary magnetite, spatially associated with quartz and other accessories, such as fluorine, calcite, bastnaesite, sphalerite, Ti-oxide and (Ca, REE)fluor-carbonates, in fluid percolation zones or in reaction rims around ghost orthopyroxene. Fluid inclusions entrapped in quartz grains witness the presence of CO2-rich hydrocarbonic fluids of low salinity, that are more abundant in charnockites than in granites. It is suggested that the rocks underwent a rather long history of fluid percolation, leading to prograde and then retrograde transformations. The corresponding metasomatic changes point to the mobility of Ti, Fe, Ca, Zn, F and REE. These changes are consistent with the CO2-rich nature of the percolating fluids

Magnetic properties of PM in relation to combustion sources from west Africa.

International audienceThe particulate matter (PM) presents in air has well known health hazards, finer fractions being particularly harmful. The magnetic methods present advantages in PM investigation, being quick and cost effective, and also sensible enough to investigate finer fractions of the iron oxides. West African cities are being subject to increasing air pollution problems, which are expected to get worse in the coming decades. In order to address the evolution of air quality in two major West African cities (Cotonou and Benin) an extensive monitoring campaign was conducted by Djossou et al. (2018) for about two years. Sites with different combustion sources were chosen in order to address the impacts in air quality. We present here the use of environmental magnetism methods in the air filters previously collected. The magnetic protocol includes the acquisition of anhysteretic remanent magnetization (ARM), saturation isothermal magnetization (SIRM) and the calculation of the S ratio in order to assess concentration, mineralogy and grain size of the iron oxides captured by the filters. Volume (air pumped in the filter) normalized ARM and SIRM data present a temporal variation that seems to be related to the season, increasing in the dry seasons, and decreasing in the wet seasons, especially on the traffic and waste burning sites. The magnetic properties are mainly dominated by high coercive magnetite-like grains

Low paleointensities recorded in 1 to 2.4 Ga Proterozoic dykes, Superior Province, Canada

Earth and Planetary Science Letters, v. 213, n. 1-2, p. 79-95, 2003. http://dx.doi.org/10.1016/S0012-821X(03)00243-7International audienc

Environmental magnetism as a tracer of urban metallic pollution in Seine River sediments (Paris, France)

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