The paleomagnetic ambient temperature has been determined from samples at various distances from a Grenville diabase dike cutting late Proterozoic tonalitic gneiss near Mattawa in the Grenville Province, Canada. Using the 40 Ar/ 39 Ar technique, the intrusion age of the dike is estimated to be 570 ± 3 Ma from a 0.5‐mm‐diameter chilled margin chip. The paleo‐ambient temperature of the country rock at the time of dike intrusion, 184° ± 40°C, was calculated from Jaeger's [1964] one‐dimensional heat conduction model. The burial depth of the presently exposed rocks is estimated to be 6.5 ±1.7 km, assuming a surface temperature of 15°C and a geothermal gradient of 26°C/km. This depth at 570 Ma is discordant with the presence of Ordovician shallow‐water (<100 m) limestones about 200 km distant, suggesting that differential uplift may have occurred between the two areas,
The magnetic structure in fine grains of magnetite has been studied theoretically using a micromagnetic model which is unconstrained except for requiring Bloch‐type domain walls. Stable structures were found by second‐order numerical minimization of exchange, crystalline anisotropy, and demagnetizing energies. Single‐ (SD), two‐ (2D) and three‐domain (3D) structures were studied for particles at room temperature and in zero applied field. Two‐domain structures with relatively large domains have very small net magnetizations because of “skirts” which oppose wall moments. In particles almost filled with domain walls the net magnetization can be quite high because wall moments are uncompensated. Three‐domain structures always have a relatively large remanence due to unbalanced domains. The critical SD‐2D equilibrium size d 0 was determined to be 0.084 ± 0.012 μm. The size range over which single‐domain structures can exist is smaller than previously predicted for perfect crystals. Experimental saturation remanence data on submicron precipitated magnetite crystals can be reasonably well fit with the present results, assuming that magnetic structures assume the lowest energy state. A technique for studying the transition mechanism between stable states is developed which demonstrates that the SD reversal mechanism is coherent rotation in particles around the critical superparamagnetic size, while domain wall nucleation, propagation, and denucleation become favored at larger sizes.
An archeomagnetic study was carried out on potsherds samples from sites in Ontario with ages ranging from A.D. 90 to A.D. 1640 as determined by 14 C dating. Thellier double‐heating paleointensity experiments were performed in air on 65 specimens of 52 samples from seven sample sets. Reliable paleointensity estimates were obtained for 49 specimens. Alternating field and thermal demagnetization, temperature dependence of weak‐field susceptibility, and hysteresis measurements indicate that magnetite of pseudo‐single‐domain grain size is the carrier of natural remanent magnetization. The paleointensity results follow a half‐cycle sine curve, with a steady decrease from 54.0±5.9 μT to 37.6±5.7 μT between A.D. 90 and A.D. 885 and a monotonic increase from 52.0±6.1 μT to 59.4±1.7 μT between A.D. 1200 and A.D. 1900. The paleointensities determined yield virtual axial dipole moments (VADMs) of the Earth's magnetic field that agree well with those from other parts of North America, except between A.D. 900 and A.D. 1400, when they are systematically lower. This discrepancy is probably caused by a substantial non‐dipole field in southwestern North America from the tenth to the fifteenth century, since secular variation studies using potsherds from Arizona and lake sediments from Minnesota show different inclination variations during that period.
First-order reversal curve (FORC) and remanence-based Preisach diagrams are alternative ways of determining the Preisach distribution of a sample, which incorporates information about the coercivity spectrum and the distribution of interactions and self-demagnetizing fields. We compare results of the two methods for well-characterized synthetic and natural samples containing single-domain (SD) and pseudo-SD (PSD) magnetite, maghemite, titanomagnetite and titanomaghemite. The greater time requirements of remanence as opposed to in-field measurements limited our Preisach diagrams to a few hundred points, compared to several thousand points for the corresponding FORC diagrams. Only minimal smoothing could be applied in order to limit the regions near the axes of the diagrams in which function values must be extrapolated. In spite of these restrictions, we find excellent agreement between the essential features of the distributions determined by the two methods. The main features, the location and spreading of the distribution peak, are very consistent. However, the low-coercivity part of the Preisach distribution is sometimes poorly resolved or not imaged at all for remanence-only measurements. Features in this region can be diagnostic of PSD and multidomain (MD) grains. The essential agreement between our FORC and Preisach diagrams in the region where they overlap justifies using the much faster FORC routine instead of traditional remanence-based Preisach methods to determine the Preisach distribution of palaeomagnetic samples without strong interactions. We propose a symmetric FORC protocol that would permit separation of the irreversible and reversible parts of the Preisach distribution. The irreversible part is what is determined by remanence-only methods and what is desired for characterization of the remanence behaviour of palaeomagnetic samples. The reversible part is most significant in detecting MD behaviour and screening out samples containing large PSD and MD grains.
Using rock magnetism and thermal modeling, we evaluate the candidate minerals responsible for strong magnetic anomalies in the Terra Sirenum and Terra Cimmeria regions of Mars' southern highlands. We assume an early global dynamo field similar in strength to the present Earth's field, enduring about 500 Myr after accretion and core formation, and a basaltic crust containing no more than 4–7 weight% of magnetic minerals. Thermal evolution models with a wide variety of initial crustal thicknesses, distributions of radioactive elements, and thermal expansion coefficients all yield similar thermal histories for the crust: warming in the first ∼1000 Myr (due mainly to radioactive heating) followed by monotonic cooling for the remainder of Mars' history. Primary thermoremanent magnetization (TRM) acquired by intrusive and extrusive bodies during the first 500 Myr was in part thermally demagnetized by general crustal warming after the dynamo field disappeared, from 500 to 1000 Myr. The Curie point isotherms around 1000 Myr established the maximum depth of TRM‐bearing crust. Shock and heating due to impacts demagnetized the uppermost ∼10 km of the crust around the same time, resulting in potential magnetic layer thicknesses of 15–20 km for pyrrhotite, 40–50 km for magnetite, and 50–60 km for hematite. Other magnetic phases, such as iron and finely exsolved low‐Ti titanohematite, are possible but less likely in a basaltic crust under oxidizing conditions. The prime candidates, in order of likelihood, are single‐domain magnetite (0.2–0.4 volume% or 0.4–0.8 weight% required), single‐domain pyrrhotite (1–2 volume% or 2–4 weight%), and either multidomain (>15 μm) or 5–15 μm single‐domain hematite or a mixture of both (1.5–3 volume% or 3–6 weight%). A composite source with different combinations of these minerals at different depths is entirely possible. Viscous decay of TRM is difficult to assess without detailed knowledge of the distribution of minerals and blocking temperatures with depth but would increase the amounts of magnetic material required.
Magnetites with sizes from 1 μm to 135 μm were cooled in zero field and their magnetizations M ( T ) measured continuously. M ( T ) changed reversibly in cooling from T 0 = 300 K to 200 K, and in subsidiary warming‐cooling cycles T i → T 0 → T i for any T i . Changes in M ( T ) in cooling from 200 K to 130 K were largely irreversible due to decreasing magnetocrystalline anisotropy which promotes wall unpinning and domain nucleation. Low‐temperature demagnetization (LTD) is almost complete by 130 K in 20–135 μm magnetites but in 1–14 μm magnetites further LTD occurs on cooling to 120 K as magnetocrystalline easy axes change and domains reorganize at the Verwey transition. The observed irreversible changes are the basis of stepwise LTD as a method of paleomagnetic “cleaning.” Decrements Δ M in remanence due to cooling are most accurately measured at T 0 , requiring a set of warming‐cooling cycles T i → T 0 → T i . A less accurate method, continuous LTD, measures decrements M ( T i ) − M ( T i−1 ) from the main cooling curve below 200 K, without intermediate warming‐cooling cycles; this requires remanence measurements at T i < T 0 . Stepwise or continuous LTD curves M ( T i ) discriminate among remanence types and grain sizes. The signal of finer (PSD) grains is enhanced compared to coarser (MD) grains. Analogous to the Lowrie and Fuller [1971] test, the inverse thermoremanence (ITRM) of 1–14 μm grains is harder to stepwise LTD than saturation remanence (SIRM), while anhysteretic remanence (ARM) is harder than either; for 20–135 μm multidomain grains, ITRM is softer than SIRM.
Approximately equidimensional magnetite crystals, with mean sizes of 215, 390, and 540 nm, respectively, have been produced by reducing hematite crystals. Isothermal magnetic hysteresis properties show a clear progression toward multidomain‐like behavior as the mean grain size increases. Saturation remanences M rs are only 5–10% of saturation magnetization M s , coercive forces H C are low (5.5–8 mT), and both M rs and H C have grain‐size dependences compatible with those previously established for smaller and larger hydrothermally produced magnetites. Coercivities during remanence acquisition are greater than those measured during demagnetization. The difference between acquisition and destructive fields increases in the larger grains as a result of the increasing importance of the internal demagnetizing field. The low‐temperature transition is well expressed in the M rs and H C data of the 540‐nm sample but is more subdued for smaller grains. Magnetostrictive, magnetocrystalline, and magnetostatic mechanisms in turn govern coercivity as the temperature rises. Remanence and coercivity ratios, M rs / M s and H R / H C , are almost temperature independent up to 500°C, indicating that domain wall configurations resulting from saturating fields are about the same at any temperature. A thermal fluctuation analysis of high‐temperature coercive force data suggests that regions 200–250 nm in size are thermally activated as a unit in grains of all sizes; these are likely domain walls. Apparent demagnetizing factors calculated from both low‐ and high‐temperature data are consistent with a mixture of two‐domain (2D) and three‐domain (3D) grains in all samples. However, theories of remanence in conventional 2D and 3D grains or in mixtures of 2D, 3D, and metastable single‐domain grains do not explain the data in a satisfying way.
Continuous thermal demagnetization, in which measurements of magnetization are made at high temperature T during heating, is considerably faster than the conventional palaeomagnetic method of stepwise demagnetization, in which measurements are made at room temperature T0 in a series of cooling-reheating cycles. In the case of single-domain (SD) grains, the two methods give equivalent results after the continuous measurements are converted to equivalent room-temperature values by correcting for the reversible decrease of spontaneous magnetization MS between T0 and T. To test for equivalence of the two methods in larger pseudo-SD and multidomain grains, three different samples containing magnetite of different grain sizes and origins were heated in zero magnetic field and measurements taken either continuously at T during heating or at T0 after a set of cooling steps from T. Two samples contained 100–125 μm (mean 110 μm) and 125–150 μm (mean 135 μm) sieve fractions from a crushed natural crystal of magnetite, while the third sample is a natural diabase core sample containing coarse magnetite in the dark minerals and groundmass and fine magnetite in the plagioclase. Two different vibrating-sample magnetometers were used for continuous demagnetization and a mini-furnace and SQUID magnetometer were used for stepwise measurements. Both total thermoremanent magnetization (TRM) and partial TRMs with non-overlapping blocking temperatures (TC, TI) and (TI, T0) were demagnetized. The Thellier laws of partial TRMs are approximately although not exactly obeyed. In MS(T)-corrected continuous data, there is little overlap of unblocking temperatures: pTRM (TC, TI) demagnetizes almost entirely above TI and pTRM (TI, T0) almost entirely below TI, demonstrating reciprocity and independence. The stepwise measurements decrease more rapidly in intensity with increasing T than the corrected continuous results, some of which increase slightly with heating up to T≈TI. Some additional decay of magnetization must occur during cooling from T, where the continuous measurement is made, to T0, where the stepwise result is measured. There are no high-temperature measurements of subsidiary cooling-heating cycles to confirm this deduction, but continuously recorded heating-cooling cycles below room temperature in inverse thermal demagnetization (or low-temperature demagnetization) show both reversible and irreversible features, depending on the remanence tested. The most important conclusion of this study is that MS(T)-corrected continuous demagnetization results do not exactly reproduce measured stepwise demagnetization results except for very fine grains, of SD size or close to it. Continuous thermal demagnetization cannot be used in general as a time-saving alternative to stepwise demagnetization if exact equivalence is required.
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