Rock magnetism of remagnetized carbonate rocks: another look

摘要: Authigenic formation of fine-grained magnetite is responsible for widespread chemical remagnetization many carbonate rocks. grains, dominantly in the super- paramagnetic and stable single-domain size range, also give rise to distinctiverock-magnetic prop- erties, now commonly used as a 'fingerprint' remagnetization. We re-examine basis this association terms magnetic mineralogy particle-size distribution remagnetized carbon- ates having these characteristic rock-magnetic properties, including 'wasp-waisted' hysteresis loops, high ratios anhysteretic remanence saturation frequency-dependent susceptibility. New measurements on samples from Helderberg Group allow us quantify proportions superparamagnetic, larger evaluate mineralogical composition carriers. The dominant phase magnetite-like, with sufficient impurity completely suppress Verwey transition. Particle sizes are extremely fine: approximately 75% total content superparamagnetic at room temperature almost all rest single-domain. Although it has been pro- posed that carbonates lacks shape anisotropy (and therefore controlled by cubic magnetocrystalline anisotropy), we have found strong exper- imental evidence not an important underlying factor signature usefulness palaeomagnetic directly related accuracy which its age known (e.g. Van der Voo 1990). Primary remanence, acquiredduringorverysoonafterrockformation,pro- videsdirectinformationonpalaeofieldorientationand strengthatthattime.Thepossibilityofpartialorcom- plete later time complicates palaeomagneticinterpretation,ashaslongbeenrecog- nized Graham 1949). Every robust palaeomag- netic study must include some effort constrain ages identified components natural remanent magnetization (NRM). Relative dating respect sedimentary processes, struc- tural tilting or cross-cutting relationships classical geometric tests (fold test conglomerate test, 1949; baked contact Everitt & Clegg 1962; unconformity Kirschvink 1978). special circumstances required application often available however, positive results may providerelativelylooseconstraintsallowingmagneti- zation significantly post-date rock formation. As result, other (although generally more indirect) becomes essential evaluating age, origin significance NRM components. Such includes petrographic observations,isotopic andgeo- data characterization. By their very existence, overprints provide process event