Abstract— A database of magnetic susceptibility measurements of stony achondrites (acapulcoite‐lodranite clan, winonaites, ureilites, angrites, aubrites, brachinites, howardite‐eucrite‐diogenite (HED) clan, and Martian meteorites, except lunar meteorites) is presented and compared to our previous work on chondrites. This database provides an exhaustive study of the amount of iron‐nickel magnetic phases (essentially metal and more rarely pyrrhotite and titanomagnetite) in these meteorites. Except for ureilites, achondrites appear much more heterogeneous than chondrites in metal content, both at the meteorite scale and at the parent body scale. We propose a model to explain the lack of or inefficient metal segregation in a low gravity context. The relationship between grain density and magnetic susceptibility is discussed. Saturation remanence appears quite weak in most metal‐bearing achondrites (HED and aubrites) compared to Martian meteorites. Ureilites are a notable exception and can carry a strong remanence, similar to most chondrites.
Abstract— The Bosumtwi impact structure (Ghana) is a young and well‐preserved structure where a vast amount of information is available to constrain any geophysical model. Previous analysis of the airborne magnetic data and results of numerical simulation of impact predicted a strongly magnetic impact‐melt body underneath the lake. Recent drilling through the structure did not penetrate such an expected impact‐melt rock magnetic source. A new 3‐D magnetic model for the structure was constructed based on a newly acquired higher‐resolution marine magnetic data set, with consideration of the observed gravity data on the lake, previous seismic models, and the magnetic properties and lithology identified in the two International Continental Scientific Drilling Program (ICDP) deep boreholes. The new model contains highly magnetic bodies located in the northeast sector of the structure, not centered onto the drilling sites. As in previous models, higher magnetization than that measured in outcropping impactites had to be assigned to the unexposed source bodies. Integration of the new model with the borehole petrophysics and published geology indicates that these bodies likely correspond to an extension to the south of the Kumasi batholith, which outcrops to the northeast of the structure. The possibility that these source bodies are related to the seismically identified central uplift or to an unmapped impact‐melt sheet predicted by previous models of the structure is not supported. Detailed magnetic scanning of the Kumasi batholith to the north, and the Bansu intrusion to the south, would provide a test for this interpretation.
Alternating field and thermal demagnetization of dolomite samples from the Silurian (Llandovery) horizontally-bedded sequence of central Estonia reveal two secondary magnetization components (A and B) both of chemical origin. A low-coercivity (demagnetized at Ł50 mT) component A (D = 60.7°, I = 7.7°, a95 = 16.6°) with high dispersion (k = 14.2), yields a palaeopole at 18.2°N and 139.5°E that points towards the Late Devonian -- Mississipian segment of the Baltica APWP (Apparent Polar Wander Path). A high-coercivity component B (D = 13.5°, I = 60.7°, k = 67.0, a95 = 4.7°) carries both normal and reversed polarities. Comparing the palaeopole (71.1°N and 173.3°E) with the European APWP reveals a Cretaceous age. These two remagnetizations are linked to mineral assemblages of magnetite and maghemite (A), and hematite (B) determined from mineralogical (X-ray, SEM and optical microscopy) and rock magnetic (acquisition and thermal demagnetization of a 3-component IRM; Lowrie-test) studies. The results suggest that the first (A) Palaeozoic remagnetization was caused by low-temperature hydrothermal circulation due to the influence of the Caledonian (more likely) or Hercynian Orogeny after the diagenetic dolomitization of carbonates. Hematite, carrying the component B, and goethite, are the latest ferromagnetic minerals that have precipitated into the existing pore space (hematite) and walls of microscopic fractures (goethite) that opened to allow access for oxygen-rich fluids during the Late Mesozoic.
Conglomerate tests are used to show that Late Precambrian Keweenawan conglomerates on the Keweenaw Peninsula, Michigan and at Mamainse Point, Ontario have suffered a partial chemical remagnetization that predated tectonic tilting. At Mamainse Point the overprint was acquired during the final normal polarity stage of Keweenawan volcanism, whereas on the Keweenaw Peninsula it occurred after cessation of volcanism and has a maximum Late Freda Sandstone age. The extent of overprinting on the Keweenaw Peninsula increases westwards along a 40 km strike length in the Copper Harbor Conglomerate, and may at least in part be related to secondary mineral zonation (including native copper) in the underlying Portage Lake volcanics. It is shown that neither of the two overprinting episodes can be responsible for the reversal asymmetry observed in Keweenawan igneous rocks.
Alternating field and thermal demagnetization of lime- and dolostones from the Lower and Middle Ordovician (Floian to Darriwilian stages) subhorizontally bedded sequences in NW and NE Estonia reveal two characteristic magnetization components (named P and S). The intermediate-coercivity (demagnetized at 30–60 mT, up to 300–350 °C) reversed polarity component P (mean of Floian Stage: Dref = 147.8 ± 10.8°, Iref = 65.8 ± 5.4°; combined mean of Dapingian and Darriwilian stages: Dref = 166.0 ± 8.4°, Iref = 56.1 ± 6.5°) is regarded as the primary remanence of early diagenetic (chemical) origin. On the Baltica's apparent polar wander path (APWP), the palaeopoles (Floian: Plat = 25.0 °N, Plon = 50.8 °E, K = 52.7, A95 = 7.2°; Dapingian and Darriwilian: Plat = 11.4 °N, Plon = 39.1 °E, K = 33.8, A95 = 6.7°) are placed on the Lower and Middle Ordovician segment. The poles indicate that Estonia was located at southerly latitudes, decreasing with time (Floian: ~48 °S; Dapingian and Darriwilian: ~37 °S), when the remanence was acquired. A high-coercivity and high-unblocking-temperature component S (mean of samples: Dref = 33.7 ± 6.3°, Iref = 51.9 ± 5.7°) that is regarded as a secondary remanence has both normal and reversed polarities. On the European APWP, its palaeopole (Plat = 52.5 °N, Plon = 157.9 °E, K = 38.9, A95 = 5.3°) gives middle to late Permian age. According to mineralogical (SEM and optical microscopy) and rock magnetic (three-component induced remnant magnetization) studies, component P is carried by magnetite (coexisting with glauconite) and component S by haematite. Magnetite is of chemical origin, formed in the course of early diagenesis and/or dolomitization. During the Permian continental period haematite, the carrier of component S, was likely precipitated from oxidizing meteoric fluids in the already existing or simultaneously formed pore space between the dolomite crystals.
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