GP21B-0502
Paleomagnetism of ~4551 Ma old Richardton chondrite
Magnetic and mineralogic variation of the Chondrites provides important information about the early evolution of the solar system and the formation of planetary objects. In the present study, we used several chips of Richardton (~4551 Ma). On the basis of rock magnetic, microscopic, and electron microprobe analyses on rock chips and mineral separates, we suggest that silicate-hosted Fe-Ni is responsible for the stable paleomagnetic record of Richardton. Small amounts of accessory opaque minerals including Cr-spinel, Fe-Ni- oxide, and Fe-sulfide are confined along the cracks between silicates, reflecting their secondary shock-origin and their little contribution to the stable remanence.
GP21B-0503
Polar ice magnetization: Comparison of results from NorthGRIP (Greenland) and Vostok (Antarctica) ice cores
Low temperature measurements of isothermal remanent magnetization (IRM) in Greenland ice spanning the last glacial and Holocene have shown that ice samples contain a measurable concentration of magnetic minerals which are part of the atmospheric aerosol. Assuming that the source materials do not change much with time, the concentration of magnetic minerals should be proportional to the measured concentration of dust in ice. We have indeed found a consistent linear relationship with the contents of dust. However, the linear relationship between low temperature ice magnetization vs. dust concentration has an offset, which when extrapolated to zero dust concentration would seemingly indicate that a significantly large magnetization corresponds to a null amount of dust in ice. Thermal relaxation experiments have shown that magnetic grains of nanometric size carry virtually all the uncorrelated magnetization. Magnetic measurements in Antarctic ice cores confirm the existence of a similar nanometric-size magnetic fraction, which also appear uncorrelated with measured aerosol concentration. The magnitude of the uncorrelated magnetization from Vostok is similar to that measured in NorthGRIP ice. Measurements of IRM at 250K suggest that the SP magnetic particles are in the size range of about 7-17 nm, which is compatible with the expected size of particles produced by ablation and subsequent condensation of meteorites in the atmosphere. The concentration of extraterrestrial material in NorthGRIP ice was estimated from the magnetic relaxation data based on a crude estimate of chondritic Ms. The resulting concentration of 0.78±0.22 ppb for Greenland is in good agreement with the outcome based on published iridium concentrations; a virtually identical concentration of 0.53±0.18 ppb has been measured in Vostok ice core.
GP21B-0504
New Low-Temperature Magnetic Data Acquired on Synthetic Lepidocrocite
Lepidocrocite (γ-FeOOH) is an iron oxyhydroxide commonly found in the environment, which is assumed to be antiferromagnetic with a small ferromagnetic-like behavior and a Néel temperature of about 50K (e.g., Hirt et al., 2002, JGR, 107, 10.1029/2001JB000242). It is currently used as starting material in bio- reduction experiments leading to the formation of Fe(II)-bearing minerals such as green rusts, magnetite, and siderite (e.g., Ona-Nguema et al., 2002, Environ. Sci. Technol., 36, 16-20). Both initial and resulting materials are being characterized using various techniques including low-temperature magnetic methods. At this meeting, results obtained on the initial synthetic lepidocrocite samples will be presented, which describe an unusual magnetic behavior. In particular, field cooled and zero field cooled induced magnetization curves (obtained using a 5mT magnetic induction) merge at a temperature around 150K (well above 50K). Below this temperature, the difference between the two curves can be qualified as a remanent magnetization, acquired during cooling of the sample in the presence of a magnetic field. As a consequence, some ferromagnetic-like behavior persists at temperatures above the admitted Néel temperature. The cooling/warming cycle of the room temperature remanent magnetization (acquired using a 2.5T magnetic induction) also indicates that some remanence can be acquired well above that temperature. Other types of measurement have been performed in order to better constrain the low-temperature magnetic behavior of these samples, in particular using a high-field VSM.
GP21B-0505
Monoclinic c-axis selection at the Verwey transition: new insights from off-axis electron holography and the delta-ratio magnetosome detection method
Given their stable single domain (SSD) configuration at room temperature and world-wide occurrence, magnetosomes produced by magnetotactic bacteria are potentially an important source for stable (post) depositional remanence. One way to detect their presence is by the so-called low-temperature delta-ratio test proposed by Moskowitz et al. [1993]. When magnetite is cooled through the Verwey transition at ~120K (Tv) it transforms from cubic to monoclinic symmetry. Associated with this crystallographic transition is the selection of a new magnetic easy axis along the c-axis in the monoclinic phase, which corresponds to one of the three original cubic [100] directions. The selection of the new c-axis is at least a function of the presence or absence of an external field during cooling. In fact, this cooling field dependence forms the basis for the delta-ratio test for detecting chains of magnetite magnetosomes. In essence, the application of the external field biases the c-axis selection and effectively produces an assemblage of partially aligned SD particles below Tv. Whereas samples containing high concentrations of magnetite magnetosomes chains are known to yield high delta ratios (>2.0), the details remain unclear. For instance, we do not know how physical deformation of the chain structure may modify the c-axis selection. Nor do we have a good understanding of how partial oxidation or magnetic relaxation affects the reorientation of magnetic easy axes on warming remanences. Here we attempt to duplicate the delta- ratio test inside a transmission electron microscope while monitoring the magnetic microstructures within magnetite magnetosome chains using off-axis electron holography. Our results will shed light on the c-axis selection process and the possible reasons for elevated delta ratios in magnetite magnetosomes.
GP21B-0506
Low Temperature Magnetic Properties of iImenite-Hematite Solid Solution Members
Magnetic properties of the ilmenite-hematite system change dramatically with increasing ilmenite content. We present new data on synthetic and natural samples with compositions ranging from nearly pure hematite to ilmenite 98%. We also characterize the rock magnetic properties in the ilmenite-rich side of the solid solution at temperatures, from 400 K to 5K. A combination of Mossbauer, x´ray, and magnetic data from synthetic and natural samples is used to Identifying which rock magnetic measurements are best for estimating various phase transitions and crossovers; and a low´temperature constrained equilibrium phase diagram is drawn.
GP21B-0507
COOLING AFTER DOUBLE HEATING: THE LTD-THELLIER PROTOCOL FOR PALEOINTENSITY DETERMINATIONS
We have combined low-temperature treatment to standard double-heating protocols for paleointensity studies of ancient rocks with significant presence of MD grains. The low-temperature demagnetization, involving at least five liquid nitrogen baths in a zero-field chamber, was used within Thellier double-heating measurements to estimate 1.2 Ga absolute paleointensity on basalts and gabbros from Nova Floresta Formation (Amazon Craton, Brazil), that contain single-domain to multi-domain (pure to nearly pure) magnetite carriers. They present a strong decrease in remanence after low-temperature demagnetization accompanied by an increase in median destructive fields. The incorporation of LTD improved paleointensity estimates and enabled paleofield recovery for otherwise sterile rock samples, giving twenty seven successful determinations for six sites. Site-mean paleofield values are within 4.90 μT and 15.55 μT (mean at 8.37 ± 4.07 μT). These results correspond to a mean VDM of 2.08 ± 0.94 × 1022 Am2, thus increasing the number of low-field determinations that dominate the (still scarce) Precambrian database.