Effects of weathering on single-domain magnetite in Early Pliocene marine marls

Abstract

Rock magnetic parameters are often used to recognize variations in the original magnetic mineralogy and for normalizing purposes in palaeointensity studies. Incipient weathering, however, is shown to have a profound but partly reversible influence o n the rock magnetic properties of the mark of the Early Pliocene Trubi formation in southern Sicily (Italy). The remanencc in the mark resides in single-domain (SD) magnetite grains, but the remanent coercive force ( H , , ) shows a strong variation and most values observed are anomalously high ( H ( , range 36-188 mT).The enhanced coercivities arc attributed to stress in the magnetite grains induced by surface oxidation at low temperature. Upon heating to 150°C a reduction of coercivities occurs that can be explained by a stress reduction as a result of a reduction of Fe" gradient due to a higher diffusion rate at elevated temperature. After heating to 150 "C. coercivities are quite uniform throughout the outcrop and the values are characteristic of SD magnetite ( H < , range 30-38mT). The bulk susceptibility increases by 4-24 per cent, and the isothermal remanent magnetization (IRM) decreases by 5-1 1 per cent. The increase in anhysteretic remanent magnetization (ARM) is large: 20-242 per cent. The magnitude of the changes is related to the degree of weathering.Another effect of heating the marl samples to 150°C is a substantial reduction of the coercivities of the secondary overprint in the natural remanent magnetization.After heating, separation of the secondary and primary components by alternatingfield demagnetization is more efficient. The usual difficulties of thermal demagnetization above 300 "C may thus be avoided by a combination of moderate heating to 150 "C and subsequent alternating-field dcmagnctization.

Astronomically calibrated ages for geomagnetic reversals within the Matuyama chron

“…3). This high-coercivity component is consistent with the presence of surficially oxidized titanomagnetite, as discussed above, because surface oxidation increases the internal stress and therefore the coercivity of magnetite particles (Cui et al, 1994;Van Velzen and Zijderveld, 1995). Despite the fact that this high-coercivity component is not completely removed by our AF demagnetization treatment, the identified ChRM component is linear and tends toward the origin of the plots and a straightforward magnetostratigraphy is obtained.…”

Preliminary Rock Magnetic Study of Archaeomagnetic Samples from Bulgarian Prehistoric Sites.

“…Thus, the more probable explanation of the observed low Tbs in the interval (80-150)°C is connected with unblocking of a particular grain-size fraction of magnetically soft mineral. Weathering processes acting during the burial time of archaeological remains can lead to a development of maghemite shell around the original titanomagnetite grains which carry thermoremanence (Cui et al, 1994;Van Velzen and Zijderveld, 1995). It brings about hardening of isothermal magnetic properties.…”

“…As shown by Burov et al (1986), the presence of such surface oxidised grains is reflected on the Jrs (T) curves by low unblocking temperature together with a high Tb, more characteristic for maghemite which is stable with respect to inversion than for magnetite. Also, according to Van Velzen and Zijderveld (1995), breaking down of a maghemite shell occurs most effectively at about 150°C. Since 48% of the samples studied show low unblocking temperature, we conclude that the weathering processes often influence the magnetic properties of archaeological materials and complicate the archaeomagnetic intensity investigations.…”

Paleomagnetic constraints on the timing and distribution of Cenozoic rotations in Central and Eastern Anatolia

“…To more efficiently separate secondary and primary components by AF demagnetization, specimens were heated to 150 • C to remove possible viscous or present-day field overprints caused by weathering and to reduce the coercivities of the secondary overprint in the natural remanent magnetization (NRM; Van Velzen and Zijderveld, 1995). Temperatures ranging 20-680 • C with increments of 20-50 • C were applied to thermally demagnetize the samples in a shielded ASC TD48-SC oven.…”