Low‐temperature and alternating field demagnetization of saturation remanence and thermoremanence in magnetite grains (0.037 μm to 5 mm)

This paper reports on the rock-magnetic properties of single zircon crystals, which are essential for future work establishing the reliable paleointensity method using single zircon crystals. Zircon crystals used in this study were sampled from the Nakagawa River, which crosses the Tanzawa tonalitic pluton in central Japan. Rock-magnetic measurements were conducted on 1037 grains of zircons, but many of these measurements are below the limits of the sensitivity of the magnetometers employed. Isothermal remanent magnetizations (IRMs) of 876 zircon crystal are below the practical resolution of this study; we infer that these crystals contain no or only minute quantities of ferromagnetic minerals. The other zircon crystals contain enough magnetic minerals to be measured in the DC SQUID magnetometer. Am 2 (80 zircons), combining the rock-magnetic parameter, we proposed the sample-selection criteria for future study of paleointensity experiments using single zircon crystals. In the case that the samples had high coercivity (B c ) values (>10 mT) or high M NRM /M IRM values (>~0.1), main remanence carriers are probably pyrrhotite and these samples are inappropriate for the paleointensity study. In the case that the samples had low B c values (<10 mT) and low M NRM /M IRM values (<~0.1), main remanence carriers seem to be nearly pure magnetite with pseudo-single-domain grain sizes, and these samples are expected to appropriate for the paleointensity study. Total thermoremanent magnetization (TRM) acquisition experiments were also carried out for 12 samples satisfying the above criteria. The TRM intensity was comparable with that of NRM, and a rough estimation of the paleointensity using NRM/TRM ratios shows field intensities consistent with the average geomagnetic field intensity at the Tanzawa tonalitic pluton for last 5 Myr.

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“…4b). The decrease in the IRM intensity after LTD treatment suggests the existence of nearly pure magnetite (Heider et al 1992). …”

Section: Nrm Irm Measurementsmentioning

confidence: 99%

Rock-magnetic properties of single zircon crystals sampled from the Tanzawa tonalitic pluton, central Japan

Sato

Yamamoto

et al. 2015

Earth Planet Sp

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“…MD and SD-like sources of remanence appear to coexist regardless of the origin or grain size of the magnetite. Although memory ratios (memory as a fraction of initial remanence) for thermoremanent magnetization (TRM) and saturation isothermal remanent magnetization (SIRM) are grain size dependent [Hartstra, 1983;Heider et Many factors, among them grain elongation, nonmagnetic impurities, and low-temperature oxidation [Ozdemir et al, 1993], can affect low-temperature memory, but crystal defects and resulting internal stress seem to play the major role [Heider et al, 1992]. All natural and synthetic crystals have lattice defects of one kind or another: inclusions of a nonmagnetic phase, dislocations, voids, internal grain boundaries, and so on.…”

Section: Introductionmentioning

confidence: 99%

Single‐domain‐like behavior in a 3‐mm natural single crystal of magnetite

Özdemir¹,

Dunlop²

1998

J. Geophys. Res.

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Abstract. We have observed single-domain (SD) like behavior in a 3-mm natural single crystal of magnetite following low-temperature demagnetization (LTD), which consists of zero-field cycling through the Verwey transition to erase remanence carried by pinned domain walls. We compared stepwise alternating field (AF) and thermal demagnetization curves of 1-mT total thermoremanent magnetization (TRM), 1-mT partial TRM (pTRM) acquired between the Curie point (Tc = 575øC) and 565øC, and saturation isothermal remanent magnetization (SIRM) measured with and without prior LTD. AF demagnetization curves of untreated TRM and SIRM decreased exponentially with increasing AF. SIRM was more resistant to demagnetization than was TRM, a multidomain (MD) result of the LowrieFuller test. After LTD the TRM and SIRM memories had AF demagnetization curves with sigmoid shapes and initial plateaus below 8 mT in which little or no demagnetization occurred. Both features are reminiscent of SD behavior. During thermal demagnetization, untreated TRM and SIRM decreased almost linearly with increasing temperature up to =500øC. Such distributed unblocking temperatures TuB are expected for pinned walls in MD grains. The remaining 60% of TRM and 40% of SIRM were lost over a narrow temperature interval concentrated between 560øC and T c. The TRM memory after LTD was very stable against thermal demagnetization. There was no decrease in remanence below 550øC and very little change until 565øC, only 10øC below T c. This very high TuB fraction of TRM seems to have SD-like character. SIRM has similar behavior. In the case of pTRM, both the untreated remanence and the memory after LTD have almost entirely high TuBs, identical to the range of pTRM blocking temperatures T B, 565øC-Tc . TuB = T B is a basic property of SD partial TRM. The pTRM memory fraction is also larger than that of total TRM or SIRM. These observations suggest that pTRM(T c, 565øC) isolates an SD-like fraction of remanence similar to that constituting TRM and SIRM memory. The high AF coercivities of the TRM and SIRM memories indicate that the source of SD-like behavior may be unusually strong domain wall pinning by crystal defects formed at monoclinic twin boundaries as a result of crystal distortion from cubic to monoclinic structure in passing through the Verwey transition.

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“…5(c)), suggesting that different behaviour of the two groups could be caused by a variation in grain size. This is of course a feature familiar from studies on magnetite (Heider et al, 1992;Muxworthy and McClelland, 2000;Muxworthy et al, 2003), but in our samples rise in SIRM memory is additionally accompanied by a progressive evolution of curves' shape, generally not observed in magnetite.…”

Section: Discussionmentioning

confidence: 63%

Low-temperature magnetic properties of andesitic rocks from Popocatepetl stratovolcano, Mexico

et al. 2009

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To contribute to the growing database of magnetic properties of rocks and minerals at cryogenic temperatures, we have measured magnetization, low-field susceptibility, and hysteresis loops as a function of temperature between 2 K (10 K for hysteresis) and 300 K for twelve representative samples from a suite of volcanic rocks of predominantly andesitic composition erupted by the Popocatepetl stratovolcano, Mexico. High temperature susceptibility measurements have yielded Curie points (T C ) mostly between 430 and 550• C, two samples additionally containing a magnetic phase with T C of 300-320• C, and one sample-with 140-170• C. Hysteresis measurements at room temperature have revealed invariably the presence of a low-coercivity mineral with coercive force ranging from 10 to 20 mT. This suggests that NRM of Popocatepetl rocks is carried by an intermediate titanomagnetite of composition between approximately TM04 and TM20. Thermal demagnetization of SIRM given at 2 K displays no evidence for the Verwey transition, further showing that samples are essentially magnetite free. At the same time, an inflection between 30-50 K reported previously for intermediate titanomagnetites (Moskowitz et al., EPSL, 157, 141-149, 1998) is seen in all studied Popocatepetl samples except one. As well, below 50 K the coercive force increases sharply with decreasing temperature reaching up to 100 mT at 10 K. On the other hand, examining the behavior of low-field susceptibility at cryogenic temperatures shows that susceptibility signal is dominated by intermediate titanomagnetites only in a part of our samples. In four out of 12 samples, however, susceptibility signal appears to be due to a hemoilmenite phase containing about 20 mole% of hematite. Caution is thus advised when interpreting low-temperature susceptibility data in terms of magnetic mineralogy.

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Low‐temperature and alternating field demagnetization of saturation remanence and thermoremanence in magnetite grains (0.037 μm to 5 mm)

Cited by 99 publications

References 60 publications

Rock-magnetic properties of single zircon crystals sampled from the Tanzawa tonalitic pluton, central Japan

Rock-magnetic properties of single zircon crystals sampled from the Tanzawa tonalitic pluton, central Japan

Single‐domain‐like behavior in a 3‐mm natural single crystal of magnetite

Low-temperature magnetic properties of andesitic rocks from Popocatepetl stratovolcano, Mexico

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