Testing an inverse Thellier method of paleointensity determination

2007

J. Geophys. Res.

Self Cite

Insight into the size and morphology of assemblages of magnetite particles can be gained by comparing temperature variations of remanence or susceptibility after zero‐field cooling (ZFC) and after field cooling (FC) through the Verwey transition around Tv = 120 K. At 10 K a sample is demagnetized following ZFC, but in the FC initial state before warming the sample has a transition cooling remanence (TrCRM) acquired in crossing Tv. The matching transition warming remanence (TrWRM) acquired as a result of heating a demagnetized sample from low temperature across Tv is often called inverse thermoremanent magnetization (ITRM). In TrCRM experiments, initially demagnetized samples were cooled in a 2 mT field from 300 K to 10 K. Magnetization M was measured at 1 K to 5 K intervals, the highest‐resolution data being taken between 140 K and 90 K. The field was zeroed at 10 K, and the TrCRM was monitored during zero‐field warming back to 300 K. The properties of TrCRMs are generally similar to those of TrWRMs produced by heating a ZFC sample in a 2 mT field from 10 K. In 10 of 12 samples (grain sizes from 0.6 to 135 μm), M of monoclinic magnetite produced by field cooling through Tv exactly equals M of cubic magnetite produced by field warming through Tv, even though the ultimate TrCRM and TrWRM values when H → 0 are entirely different. Mirror‐image symmetry was observed between in‐field warming curves tracking the acquisition of TrWRM and zero‐field warming curves of TrCRM between 10 and 300 K. The symmetry, with increases in the field‐on M curves mirroring decreases in the field‐off Mr curves, was almost perfect from 10 to 110 K. Approximate symmetry was also observed between in‐field cooling curves tracking TrCRM production and zero‐field cooling curves of TrWRM between 300 K and Tv. Detailed study of the properties and mechanism(s) of transition remanences will help clarify why the ZFC/FC method is diagnostic in some instances and not in others.

Section: Discussionmentioning

confidence: 72%

Transition warming and cooling remanences in magnetite

2007

J. Geophys. Res.

Self Cite

“…Low‐temperature cycling curves for ITRM, measured as the first (zero‐field) of two cycles to T i in an inverse Thellier paleointensity experiment [ Dunlop and Yu , 2003], have similar shapes to the ARM curves. However, SIRM curves for all samples have the shape illustrated in Figure 3, regardless of the varied shapes of the ARM and ITRM cycling curves.…”

Section: Low‐temperature Cycling Resultsmentioning

confidence: 99%

Stepwise and continuous low‐temperature demagnetization

Geophysical Research Letters

2003

Self Cite

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 T0 = 300 K to 200 K, and in subsidiary warming‐cooling cycles Ti → T0 → Ti for any Ti. 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 ΔM in remanence due to cooling are most accurately measured at T0, requiring a set of warming‐cooling cycles Ti → T0 → Ti. A less accurate method, continuous LTD, measures decrements M(Ti) − M(Ti−1) from the main cooling curve below 200 K, without intermediate warming‐cooling cycles; this requires remanence measurements at Ti < T0. Stepwise or continuous LTD curves M(Ti) 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.

“…As a result, ITRM can be stepwise thermally demagnetized either by heating above T 0 or cooling below T 0 . Conventional thermal demagnetization is described in this paper, while demagnetization by cooling has been studied in some detail in developing stepwise LTD [ Dunlop , 2003] and the inverse Thellier method of determining paleointensity [ Dunlop and Yu , 2003].…”

Section: Discussionmentioning

confidence: 99%

“…In general, pITRM( T 1 , T 2 ) did not completely disappear after zero‐field cooling from T 2 to T 1 . Nonreciprocity, which is of vital importance in paleointensity determination, is documented in more detail by Dunlop and Yu [2003]. The vectorial independence of neighboring pITRMs was not tested because of the difficulty of rotating the sample relative to the field H during pITRM production.…”

Section: Partial Itrmsmentioning

confidence: 99%

Inverse thermoremanent magnetization

2006

J. Geophys. Res.

Self Cite

[1] Inverse thermoremanent magnetization (ITRM) is reversed to the thermoremanent magnetization (TRM) process: ITRM results from warming from low temperature T in a magnetic field, while TRM results from field cooling from high T. The development of ITRM was studied in magnetites of grain sizes from submicron to 135 mm, in pyrrhotites and in hematite crystals. All three minerals acquired ITRM after warming through their magnetic transitions (35 K for pyrrhotite, 120 and 130 K for magnetite, 250 K for hematite). However, when an impacting meteorite's cold interior warms to ambient T in the geomagnetic field, magnetite is the most likely candidate for acquiring ITRM. The magnetite ITRM blocking temperature distribution was determined from 12 neighboring partial ITRMs in nested field-on warming plus field-off cooling cycles (300-20 K). The largest partial ITRMs are produced in T intervals around magnetite's Verwey transition (T V = 110-120 K) and isotropic point (T K = 130 K). Both transitions involve large changes in crystalline anisotropy and renucleation of magnetic domains. ITRM is blocked when initially broad domain walls narrow and are pinned by dislocations. ITRM has contrasting properties to TRM, which is mainly due to blocked single-domain moments. ITRM is strongest for 3-to 20-mm grains, whereas TRM peaks for submicron magnetites. Only 10-20% of ITRM survives low-temperature demagnetization (LTD) at 77 K or AF demagnetization to 10-15 mT, compared to 30-90% for TRM. ITRM decreases quasi-linearly with T in thermal demagnetization. The median unblocking temperature T UB is %300°C and 20-25% survives at 550°C. The low-T UB part of ITRM could mimic extraterrestrial NRM of low T UB , cited as evidence of negligible heating of meteorites in their transfer to Earth. The high-T UB ITRM would contaminate paleointensity determinations up to the highest T steps. The best cure for ITRM contamination is AF or LTD pretreatment.