GP21C-0787
Magnetic signatures recorded in rocks and trees located inside the Tunguska blast 100 years ago, implications for Mirror Matter, Comet, and Kimberlitic Pipe explosion hypotheses
Hundred years ago an unknown object impacted in Siberia, Tunguska region and created a seismic signature 100-1000 times stronger that the Hiroshima explosion. To this day, nothing has been found to suggest a foreign material (e.g. meteoritic) dispersion during this event. Various hypotheses were put forward, for example: Comet impact, Kimberlite Pipe explosion, and Mirror Matter interaction with the regular matter. We collected samples of black chert, conglomerate and wood from 5 different locations within 2 km from the epicenter. We used these samples for magnetic analysis and searched for any evidence of magnetic contamination that may date the Tunguska blast. All samples, wood, chert, and conglomerate showed sufficient content of magnetic material that should be capable of recording strong magnetic pulse. Our analysis shows no evidence of magnetic enhancement recorded in any of the three types (Chert, wood and conglomerate) of Tunguska samples. We will discuss this result in terms of three possible hypotheses of the created the Tunguska event.
GP21C-0788
Magnetic Anomalies Antipodal to Large Impact Basins on the Moon
The high resolution lunar-wide magnetic anomaly map derived from Lunar Prospector (LP) vector magnetometer data has revealed weak anomalies over the nearside large impact basins flooded by mare basalts. Strong anomaly features are observed over most of the Nectarian and Pre-Nectarian aged lunar highlands. In particular, regions antipodal to some of the largest basin-forming impact craters show strong magnetic anomaly concentrations. Of the 43 basins investigated here, antipodal regions of 11 basins show these anomalous features with strengths in excess of 5-25 nT at LP's mapping altitude (30 km). These distinct anomalous concentrations were previously known to occur only at the antipodes of Imbrium, Orientale, Serenitatis and Crisium basins. The mean magnetic anomaly strength within each antipodal region when plotted against increasing age of the antipodes shows two age groupings with similar magnetic behavior. The first age grouping – (Imbrium, Orientale, Serenitatis and Crisium) is of Imbrium to Late Nectarian in age. This grouping is correlative with the peak magnetic field enhancement between 3.6 and 3.9 Gyr, inferred from paleomagnetic data from the returned Apollo samples. The second age grouping ( Lorentz, Coulomb-Sarton, Tranquillitatis, Cognitum and Insularum) is of Mid to Early Pre-Nectarian age. This grouping has not been correlated to any known global magnetic field enhancement event, and needs further investigation to ascertain the origin of the anomalies. The present work supports the antipodal hypothesis as one of the possible mechanisms to explain the observed anomalies at the antipode of the impact basin. The absence of appreciable anomalies at the 32 other antipodes, however, indicates the importance of other processes, and superposition effects, that have operated on the Moon during its history.
GP21C-0789
Experimental Pressure Demagnetization of Rocks up to 1.2 GPa
Using a non-magnetic pressure cell, we carried out hydrostatic pressure demagnetization experiments up to 1.2 GPa on samples of meteorites and terrestrial rocks of different lithologies and on synthetic samples of dispersed powders of magnetite, hematite, pyrrhotite and iron. In these experiments A wide spectrum of different mineralogies (about 50 samples) was investigated to have a clearer picture of their pressure demagnetization behavior: pyrrhotite-, titanomagnetite- and FeNi-bearing meteorites; magnetite- and titanomagnetite-bearing terrestrial igneous rocks; and a variety of pyrrhotite-, hematite-, goethite- and greigite-bearing rocks. Under 1.2 GPa, these samples lost between 0 and 70% of their original saturation isothermal remanent magnetization. Although pressure demagnetization varies from one mineralogical group to the other, within each group the demagnetization at 1.2 GPa is a linear function of logBcr (coercivity of remanence).
GP21C-0790
Shock recovery experiments in the range of 10 to 45 GPa - comparison of results of synthetic magnetite and terrestrial diabase
Shock-induced changes in magnetic properties of rocks, minerals and meteorites play an important role in modelling the magnetic anomalies of impact structures (e.g. Vredefort), in interpretation of the magnetic anomalies of planetary bodies (e.g. Mars) and in understanding the paleomagnetic data of meteorites. To shed further light on these problems we report results of experimentally shocked samples of synthetic fine grained magnetite. We used cylindrical surface-polished discs (d 10 mm, h 4 mm) of the well characterized magnetite with SD-PSD grain size range. The magnetite powder was mixed with Al2O3 and sintered into disktype pellets. A series of shock recovery experiments from 10 to 45 GPa (nominal pressure) using a conventional high-explosive set-up with a steel (ARMCO) sample container, surrounded by a momentum trap of the identical material. As the samples were shocked inside the highly magnetic containers, the prevailing magnetic field was roughly five times higher than the ambient field. After the shock, the containers cooled down slowly to ambient temperatures. The estimated post-shock temperatures of the samples range from nearly ambient temperature (10 GPa) up to about 1400 K (45 GPa). Evaluating the "real" pressures reached in these experiments requires a model to account for the high porosity of the pellets. The porosity also affects significantly the post-shock temperature. Independent of the fact that pressure, shock- and post- shock tmperatures are not sufficiently constrained yet, the experiments form a well-characterized series of shocks with systematically increasing pressure. Surprisingly enough, the sample discs were not friable and could be removed by retaining shape largely unchanged. The shock induced changes in sample properties show, with the exception of the 45 GPa sample, with increasing shock pressure: 1. Reduction of bulk density and significant increase in porosity 2. Minor increase in magnetic susceptibility (10 GPa, 15 GPa and 45 GPa) compared to pre-shock value and a decreasing trend in susceptibility as a function of shock (except 45 Gpa) 3. Decrease of the Median Destructive Field values of SIRM and ARM but simultaneously a progressive "hardening" of SIRM in shocked samples. 4. Decrease in SIRM but increase in ARM. 5. Progressive shock demagnetization of the pre-shock remanence (SIRM). 6. Decrease in the value of the coercive force (Hc) and a trend towards MD-field but no shock-dependent trend in the ratios of Mrs/Ms and Hcr/Hc (Day-plot parameters). While the first three results confirm our previous shock experiments on natural Laanila diabase (Pesonen et al., 1997), decrease of SIRM is opposite to what has been previously seen. We are now investigating what part of this decrease is due to shock. The results of the 45 GPa sample are not in line with those of the 10 GPa, 15 GPa, 25 GPa and 35 GPa samples. We are now investigating the reasons for these discrepancies. One possibility is that it is contaminated by the container material.
GP21C-0791
Natural Remanent Magnetization in LL6 St. Severin Chondrites
The natural remanent magnetization (NRM) of LL6 St. Severin chondrite is extremely stable against the alternating-field (AF) demagnetization. Approximately half of the NRM remained undemagnetized even at the highest available peak AF of 100 mT. A suite of rock magnetic tests, low-temperature saturation isothermal remanence cycling, microscopic observations, and electron microprobe analyses were carried out on rock chips and mineral separates of St. Severin. We found that St. Severin contained three different opaque phases: Kamacite, Tetrataenite, Troilite. Kamacite dominantly existed as inclusions within Ca-rich pyroxene. On the other hand, Tetrataenite and Troilite were distributed along the cracked boundaries of silicate minerals. Because Troilite is non-magnetic, abundance of Tetrataenite may suggest that a slowly cooled St. Severin recorded chemical remanent magnetization from the first place. Then, it is likely that a later shock- induced high pressure reset the earlier remanent magnetization, resulting in a remagnetization of St. Severin.
GP21C-0792
Evidence for internally generated magnetic fields on the CV chondrite parent planetesimal
It has long been known that some carbonaceous chondrites carry a stable natural remanent magnetization. The origin of this magnetization has been a longstanding mystery because it was in many cases acquired following accretion of the parent bodies. The spinning, orbiting parent body would have been unlikely to record the spatially and temporally variable magnetic fields associated with the early sun and the protoplanetary disk. On the other hand, the possibility of an internally generated dynamo field has been discounted due to the traditional assumption that chondrites are samples of undifferentiated bodies. We have conducted a paleomagnetic study on mutually oriented samples of Allende CV3 carbonaceous chondrite. We found that the fusion crust is magnetized in a different direction from the interior, which constitutes a baked contact test demonstrating that the magnetization in the interior is preterrestrial. Our thermal demagnetization analyses demonstrate that most of the natural remanent magnetization is carried by pyrrhotite with a small contribution from magnetite and other phases. This confirms results from numerous previous investigators that the magnetization in Allende must have been acquired after accretion. A variety of petrologic and geochronometric suggest that thermal metamorphism to peak temperatures of 550- 600°C continued for millions of years afterward, consistent with our thermal demagnetization data. Other new geochemical datasets indicate that metallic cores had already formed in planetesimals by this time. Therefore, the magnetization Allende and other CV samples is naturally explained by an internal core dynamo on a partially differentiated parent body.
GP21C-0793
Micromagnetic and Microstructural Analyses in Chondrules of the Allende Chondritic Meteorite
Results of micromagnetic and microstructural studies of individual chondrules from the Allende carbonaceous meteorite are presented. Allende is a CV3 carbonaceous chondrite consisting mainly of chondrules, matrix with CAIs, olivine inclusions, opaque minerals, and lithic and mineral fragments. The macroscopic texture of Allende reflects abundance of chondrules of sub-millimeter size within the aphanitic black matrix. Allende is part of the oxidized chondrites with an anhydrous mineralogy, with hydrous phases restricted to chondrules and CAIs. Studies indicate secondary alteration processes affecting Allende mineralogy and composition. Chondrules separated for this study have diameters varying from 0.08 to 0.45 cm. Micromagnetic analyses in the chondrules identify magnetite as a main magnetic phase. Magnetic hysteresis data in terms of plots of parameter ratios suggest pseudo-single domain and multidomain states. Relationships of hysteresis parameters with chondrule size, mass, density and shape are obtained. Morphology, internal structure and elemental composition are investigated by scanning electron microscope and WDS spectrometer analyses using a JEOL microprobe. Relationships are correlated to the internal microstructures and elemental compositional variation within the chondrules. Results show distinct mineralogical assemblages with spatial compositional variation, which are correlated to chondrule size and shape.