Downloaded from http://sp.lyellcollection.org/ at University of California Berkeley on July 30, 2013 Rock magnetism of remagnetized carbonate rocks: another look MIKE JACKSON* & NICHOLAS L. SWANSON-HYSELL Institute for Rock Magnetism, Winchell School of Earth Sciences, University of Minnesota, Minnesota, US *Corresponding author (e-mail: [email protected]) Abstract: Authigenic formation of fine-grained magnetite is responsible for widespread chemical remagnetization of many carbonate rocks. Authigenic magnetite grains, dominantly in the super- paramagnetic and stable single-domain size range, also give rise to distinctive rock-magnetic prop- erties, now commonly used as a ‘fingerprint’ of remagnetization. We re-examine the basis of this association in terms of magnetic mineralogy and particle-size distribution in remagnetized carbon- ates having these characteristic rock-magnetic properties, including ‘wasp-waisted’ hysteresis loops, high ratios of anhysteretic remanence to saturation remanence and frequency-dependent susceptibility. New measurements on samples from the Helderberg Group allow us to quantify the proportions of superparamagnetic, stable single-domain and larger grains, and to evaluate the mineralogical composition of the remanence carriers. The dominant magnetic phase is magnetite-like, with sufficient impurity to completely suppress the Verwey transition. Particle sizes are extremely fine: approximately 75% of the total magnetite content is superparamagnetic at room temperature and almost all of the rest is stable single-domain. Although it has been pro- posed that the single-domain magnetite in these remagnetized carbonates lacks shape anisotropy (and is therefore controlled by cubic magnetocrystalline anisotropy), we have found strong exper- imental evidence that cubic anisotropy is not an important underlying factor in the rock-magnetic signature of chemical remagnetization. The usefulness of a palaeomagnetic remanence is potential geological significance (e.g. McCabe & directly related to the accuracy with which its age is Elmore 1989; Elmore & McCabe 1991). The ‘usual known (e.g. Van der Voo 1990). Primary remanence, suspects’ in remagnetization are a set of mechanisms acquiredduringorverysoonafterrockformation,pro- involving elevated temperature, stress and chemical videsdirectinformationonpalaeofieldorientationand activity, acting alone or in concert, and producing strength at that time. The possibility of partial or com- changes in the magnetic minerals themselves and/ plete remagnetization at a later time complicates or in the remanence that they carry. When the palaeomagneticinterpretation,ashaslongbeenrecog- NRM and the population of magnetic carriers are nized (e.g. Graham 1949). Every robust palaeomag- both modified by some process or event, careful netic study must therefore include some effort to rock magnetic analysis may shed light on the culprit. constrain the ages of identified components of the Our focus here is chemically remagnetized natural remanent magnetization (NRM). Relative carbonate rocks where any primary remanence is dating with respect to sedimentary processes, struc- completely obscured by ancient yet secondary tural tilting or cross-cutting relationships is the basis magnetizations. Many such units have been docu- of the classical geometric palaeomagnetic tests (fold mented in recent decades (e.g. McCabe & Elmore test and conglomerate test, Graham 1949; baked 1989). With the advent of superconducting magnet- contact test, Everitt & Clegg 1962; unconformity ometers in the 1970s and 1980s (Goree & Fuller test, Kirschvink 1978). The special circumstances 1976), carbonate rocks represented a new fron- required for application of these tests are often not tier for palaeomagnetic research. These and other available however, or positive test results may weakly magnetic sedimentary rock strata had been provide relatively loose constraints allowing magneti- unmeasurable during progressive demagnetization zation ages that significantly post-date the age of with the limited-sensitivity spinner magnetometers the rock formation. As a result, other evidence then available. Superconducting magnetometers (although generally more indirect) becomes essential enabled a rapid expansion into these untapped in evaluating the age, origin and significance of palaeomagnetic archives, but by the mid 1980s it NRM components. Such evidence commonly was already evident that many Palaeozoic carbonate includes petrographic observations, isotopic and geo- rocks had been completely remagnetized in late chemical data and rock magnetic characterization. Palaeozoic time and it became important to under- By their very existence, magnetic overprints stand the processes responsible (e.g. McCabe & provide evidence of some process or event of Elmore 1989). From:Elmore, R. D., Muxworthy, A. R., Aldana, M. M. & Mena, M. (eds) 2012. Remagnetization and Chemical Alteration of Sedimentary Rocks. Geological Society, London, Special Publications, 371, http://dx.doi.org/10.1144/SP371.3 # The Geological Society of London 2012. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ at University of California Berkeley on July 30, 2013 M. JACKSON & N. L. SWANSON-HYSELL Review of previous work minerals). The spatial/temporal association of widespread remagnetization with Appalachian– Origins of the secondary remanence and its Alleghenian–Variscan–Hercynian orogenesis (for carriers summaries see e.g. Weil & Van der Voo 2002; Tohver et al. 2008) soon led to various hypotheses Widespread remagnetization of Palaeozoic carbon- involving continental-scale flow of orogenic fluids ate strata in the Appalachian Basin was initially (Oliver 1986; Bethke & Marshak 1990; Garven recognized and documented through combined 1995) to account for not only remagnetization, but palaeomagnetic, structural and petrographic obser- also ore mineralization and hydrocarbon distri- vations. These observations included the recogni- bution in a number of different basins. Isotopic tion that the characteristic remanent magnetization and geochemical evidence in some cases ruled out (ChRM) directions sometimes clustered signifi- a significant role for exotic brines (e.g. Elmore cantly more tightly prior to correction for structural et al. 1993; Ripperdan et al. 1998), and pointed tilting, and sometimes clustered most tightly at instead to more closed-system mechanisms includ- intermediate unfolding. This led to interpretations ing clay diagenesis (Katz et al. 2000; Woods et al. that the magnetizations were acquired after and/or 2002), organic maturation (Blumstein et al. 2004) during folding (McCabe & Elmore 1989). Petro- and pressure solution (Zegers et al. 2003; Elmore graphic work led to the observation of spheroidal, et al. 2006; Oliva-Urcia et al. 2008). Kent (1985) botryoidal and framboidal magnetite grains of pure questioned whether the remagnetization necessarily end-member Fe3O4 composition that were inter- involved chemical mechanisms at all, suggesting preted to be of low-temperature diagenetic origin that viscous acquisition during the Kiaman super- (McCabe et al. 1983). Early rock-magnetic charac- chron was a viable alternative. Noting that the terization focused on acquisition and demagnetiza- range of observed unblocking temperatures could tion of isothermal remanent magnetization (IRM) be accounted for by the Viscous Remanent Magneti- and anhysteretic remanent magnetization (ARM), zation (VRM) model of Walton (1980), Kent (1985) leading in many cases to the conclusion that the demonstrated moreover that the Brunhes-aged remanence carriers were dominantly pseudo-single- viscous overprint in these rocks had unblocking domain (PSD) or multi-domain (MD) magnetite temperatures similar to those predicted by the same (e.g. McElhinny & Opdyke 1973; Kent 1979). model. However, this model was later shown to be Hysteresis measurements eventually led to the inappropriate for predicting unblocking temperatures recognition that the dominant magnetic carriers in (Dunlop et al. 1997a, b), undermining the theoretical many remagnetized carbonates are in fact very basis for a viscous origin but not the empirical obser- fine grained, mainly in the superparamagnetic (SP) vations. Similarly, Kligfield & Channell (1981) and stable single-domain (SSD) size range invoked thermoviscous mechanisms to explain wide- (Jackson 1990; McCabe & Channell 1994; Suk & spread Brunhes-age remagnetization of limestones Halgedahl 1996; Dunlop 2002b). Moreover, the from the Swiss Alps, in which the ChRM was high proportions of SP and SSD particles in the fer- carried chiefly by pyrrhotite with fine grain sizes rimagnetic inventory of these rocks result in unusual spanning the SP–SSD range. They noted that the pyr- combinations of the summary parameters conven- rhotite may have formed by alteration of early- tionally used to represent hysteresis behaviour, diagenetic pyrite, but did not address the possibility namely coercivity Bc, remanent coercivity Bcr, sat- of a chemical origin for the remanence. uration magnetization Ms, saturation remanence Subsequent work on the remagnetized Appala- Mrs and the ratios Bcr/Bc and Mrs/Ms. On the ‘Day chian Basin carbonates and remagnetized carbon- plot’ (Mrs/Ms v. Bcr/Bc; Day et al. 1977; Dunlop ates in other basins has largely supported the 2002a, b), remagnetized carbonates are largely idea that the remanence carriers are predominantly isolated in a region that is distinct from the of late diagenetic origin, and