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ISSN: 2152-1972 The IRM Inside... Visiting Fellow Reports 2 Current Articles 5 Santa Fe Conference 10 QuarterlyFall 2018, Vol. 28 No.3 ... and more throughout!

Commonly used experimental parameters for acquisition of anhysteretic remanent magnetization (ARM) and its anisotropy (AARM): Results and recommendations from a rock magnetic community survey Andrea R. Biedermann1,2, 0.2 Dario Bilardello2, Mike Jackson2, 3,4 2 Martin Chadima , Joshua M. Feinberg 0.1

1 Institute of Geophysics, ETH Zurich, Zurich, Switzerland 0.0 time (ms) 2 Institute for Rock Magnetism, University of (T)Field Minnesota, Minneapolis, MN, USA 3 AGICO Inc., Brno, Czech Republic -0.1 4 Institute of Geology of the Czech Academy of

Sciences, Prague, Czech Republic -0.2 0 200 400 600 Figure 1. Schematic of (partial) anysteretic remanent acquisi- 1. Motivation tion: alternating field (AF, blue line) and direct current (DC, Anhysteretic remanent magnetization (ARM) and its red line) applied over a selected time interval. anisotropy (AARM) are properties widely used in en- and AARM experiments, will also help define suitable vironmental magnetism, paleomagnetism, and magnetic parameters for automated systems. Note that different fabric studies, to gain information about magnetic re- types of studies employing ARMs, e.g. environmental manence carriers and their preferred alignment. These vs. fabric studies, may call for different sets of experi- data, in turn, can help define depositional environments mental parameters. Here, we will focus particularly on for sediments, emplacement mechanisms for intrusive how ARMs are imposed on samples. Imparting an ARM rocks, or deformation histories for metamorphic rocks. is a complex physical process, and the intensity of an In environmental magnetism, ratios of ARM to suscep- ARM will depend not only on a specimen’s inherent tibility or isothermal remanent magnetization (IRM) magnetic properties, but also on a number of experimen- (amongst other parameters) help determine the magnetic tal variables, such as the strength of the DC bias field, the mineralogy, grain size and domain state, and hence the peak alternating field (AF) used during the application of environmental processes under which rocks formed or the ARM, and the decay rate of the AF [Egli, 2006; Egli altered [Liu et al., 2012]. Being the best room tempera- and Lowrie, 2002; Sagnotti et al., 2003; Yu and Dunlop, ture equivalent for a natural thermal remanence [Potter, 2003]. These same experimental parameters can also 2004], ARM and AARM experiments are also impor- change the perceived AARM for a specimen [Bilardello tant in paleomagnetic studies, where they are widely and Jackson, 2014], which may be further affected by used to better understand remanence acquisition and its the number of directional ARMs measured, and whether anisotropy. In fabric studies, AARM as well as anisot- or not samples are demagnetized between directional ropy of partial anhysteretic remanence (ApARM) are measurements. Decay rates for AF are rarely reported in characterized to determine the preferred alignment of published studies, making it difficult to compare results the remanence-carrying grains, or a sub-population of obtained from different laboratories. remanence-carrying grains [Jackson and Tauxe, 1991]. Given these concerns, and because 15 years have For the community to continue to use these ARM passed since the inter-laboratory calibration study by and AARM applications and innovate methods, we Sagnotti et al. [2003], we conducted a survey to deter- should encourage comparable ARM and AARM proto- mine the range of experimental parameters for ARM cont’d. on cols across different laboratories. Reaching consensus and AARM measurements that are currently employed pg. 11... on the standard procedure(s) and parameters for ARM within our community. 1 in biweekly monitoring for a full year in 12 different locations (from pristine areas to heavily anthropogenic Visiting Fellow Reports environments). The amount of atmospheric deposition by mass, its chemical composition and a complete characteriza- Preparation of samples and magnetic tion of its magnetic properties are being quantified, to- analysis on filters with atmospheric gether with the relative enrichment of anthropogenic zones with respect to their closest remote counterparts. particles. The final aim of the project is to establish relationships between geochemical species and / or sources with respect to magnetic properties in order to define magnetic- Tania Mochales-Lopez geochemical proxies. Special attention is paid to events Spanish Geological Survey of interest such as episodes of Saharan dust, pollution [email protected] events or particular emission sources.

Project background Methods at IRM Atmospheric aerosols, once completed their cycle in the The low total particle mass collected in the filters atmosphere, return to the surface of the planet where (0.005-0.05 g) led to having to optimize sample prepara- they interact in different ways depending on the physi- tion. The main goal was to conserve the total mass of cal-chemical typology of the environment (hydrosphere, the filter to be able to estimate the concentration of the lithosphere, biosphere or anthroposphere) where they magnetic minerals and to evaluate the magnetic proper- end up depositing. Depending on the medium to which ties per unit of mass. they are transferred and based on their chemical or min- Because the samples were distributed over the 47 mm eralogical composition, the associated impacts may be (diameter) surface of the air filters, specific precautions relevant. were taken to guarantee centering of the specimens with- The DONAIRE project has carried out an integrated in the instruments’ measurement regions (Fig. 1). The study of atmospheric deposition in a wide geographic samples were folded over in four, rolled and inserted into area, using a common methodology and covering dif- 8 mm diameter straws before being compressed to a ~ 1 ferent microenvironments. The project proposed the cm high cylinder. geochemical, magnetic and mineralogical characteriza- Hysteresis loop determinations, Isothermal Remanent tion of the atmospheric deposition in the NE of Spain Magnetization acquisition and backfield demagnetization curves at ambient temperature where acquired in a Vibrating Sample Magnetometer (VSM) in maximum fields of 1-1.5 T, depending on the specimen. Because the specimens possessed weak magnetizations, averag- ing times of 20-30 seconds were used, resulting in typical experiment durations of 0.5-1 hours. Remanent magnetic measurements and AC susceptibility as a function of temperature and frequency were performed on Magnetic Properties Measuring Systems (MPMS). Proceeding sequences consisted of: i) cooling to 20 K in the presence of a saturating 2.5 T field (FC) and subsequent measuring of the remanence on warming; ii) After cooling, in absence of magnetic field (ZFC), a low temperature saturation isothermal remanent magnetization (SIRM) of 2.5 T was imparted and the remanence was measured again on warming to room temperature; iii) Subsequently, a room temperature SIRM (RTSIRM) was measured upon on cooling; iv) Lastly, frequency de- pendent (1, 10, 100 Hz) in-phase and quadrature magnetic susceptibilities were measured upon warming between 20 and 300 K.

Results The M (T) (magnetization versus temperature) curves have been used to identify magnetic mineral diagnostic transitions for matter collected in the atmospheric filters (Fig.2). In the RTSIRM curves on cooling (black curves Figure 1. Sample preparation for atmospheric filters from in Fig. 2), a transition near 120 K was determined, iden- DONAIRE project. tified as the Verwey transition and compatible with the 2 Figure 2. Examples of FC, ZFC and RT remanence on cooling analysis. Magnetic susceptibility frequency dependence. results expected for magnetite. A progressive arch decay soft, low-coercitity magnetite-type ferromagnetic miner- in the magnetization from 200 to 140 K is generally in- als. Certain specimens, however, possessed wasp-wasted terpreted as oxidized magnetite, probably reflecting oxi- loops that only saturated in stronger fields, indicating the dized magnetite and/or maghemite. The break in slope presence of high coercivity minerals. near 260 K is identified as the Morin transition, revealing Paramagnetic and diamagnetic contributions were the presence of hematite. A steady increase of the mag- identified from the high-field hysteresis slopes. Further netization from room temperature to 20 K is associated post-processing IRM and backfield data will be - per to goethite, and so is the separation of the FC and ZFC formed to determine the relative contribution of the re- remanence on warming curves, which converge towards manence carrying minerals and their grain size / coerciv- goethite’s blocking temperature (~ 300 K). Frequency ity fractions. dependence of the magnetic susceptibility has been ob- served in many of the samples, suggesting the presence Acknowledgments of superparamagnetic particles. The hysteresis loops for To IRM Institute for the opportunity to visit the lab and the samples acquired in maximum fields of 1 to 1.5 T, DONAIRE project (CGL2015-68993-R) funding. Spe- depending on the specimens, were thin, and generally cial thanks to Dario Bilardello and Mike Jackson for saturated in fields < 500 mT, indicating a dominance of their guidance.

Were magnetotactic bacteria around in to the Institute for Rock Magnetism for preliminary the Precambrian? screening for magnetofossil signatures. The focus of this project were samples from ~1.9 Ga rocks in the Lake Su- perior region, specifically the Gunflint and Biwabik For- Ioan Lascu mations. The main magnetofossil trait that can be deter- Department of Mineral Sciences, National mined magnetically is the alignment of magnetofossils in chains. First-order reversal curve (FORC) diagrams Museum of Natural History, Smithsonian are sensitive to non-interacting single domain (SD) par- Institution ticles and undisrupted chains, and were the main tool in [email protected] identifying samples potentially carrying magnetofossils. In addition, low temperature magnetism was used for di- The conventional fossil record spans the recent most agnosing magnetic mineralogy, using ordering tempera- ~0.6 Ga, documenting about 15% of evolutionary his- tures or phase transitions. tory on Earth (Knoll et al., 2016). The overwhelming A number of samples were found to contain central majority of these are products of controlled biominer- ridges in FORC diagrams, indicating the presence of non- alization (CBM) by complex, multicellular life forms. interacting SD particles/chains. The most tantalizing re- Bacteria and Archaea can also biomineralize, but this sults come from a chert specimen from the Gunflint For- does not typically occur in a controlled fashion. The only mation containing magnetite (Fe3O4), iron-manganese group of bacteria that are capable of CBM are magneto- carbonates ([Fe,Mn]CO3), and goethite (FeO[OH]). The tactic bacteria, who produce iron-bearing nanocrystals magnetic signatures of Fe-Mn carbonate and magnetite in membrane-bound cellular compartments (Uebe and can be seen in the low temperature remanence curves, Schuller, 2016). Inside these compartments, magneto- where two transitions occur, respectively at ~25-35 K tactic bacteria synthesize crystals of magnetite and/or and 120 K (Fig. 1a). Goethite can be gleaned from the greigite a few tens of nanometers in size, which can be separation of the field cooled (FC) and zero-field cooled preserved in the sedimentary record as “magnetofossils” (ZFC) curves at T>120 K. The FORC diagram of this (Kopp and Kirschvink, 2008). Metagenomic analyses sample (Fig. 1b) is characterized by a narrow central and molecular dating of deep-branching magnetotactic ridge with a peak coercivity around 60 mT, superim- bacteria phyla have shown that the genes responsible for posed on a weak positive background from magnetostat- controlled magnetite biomineralization originated dur- ic interactions. Other samples also exhibit central ridges ing the Archean, more than 3 Ga ago (Lin et al., 2017). in FORC diagrams, but do not have ‘textbook’ magne- Samples from Archean and Precambrian rocks found tosome signatures. A hematite-bearing jasper specimen in the Natural History Museum collections were brought from the Biwabik Formation exhibits a sharp central 3 ridge with a coercivity tail up to 140 mT and asymmetric ization, the timing of oxygenation of the global ocean, lobes characteristic of single vortex (SV) particles (Fig. the Precambrian behavior of the magnetic field, etc. The 2a). A carbonate from the Gunflint Formation shows a magnetofossil hunt continues! central ridge with a bimodal superparamagnetic (SP) – SD distribution and no background features (Fig. 2b). References These samples are currently analyzed using electron Knoll, A. H., Bergmann, K. D., and Strauss, J. V. (2016). Life: the first two billion years. Philosophical Transactions of the Royal Society of London B, 371(1707), 20150493, doi: 10.1098/rstb.2015.0493. Kopp, R. E. and Kirschvink, J. L. (2008). The identification and biogeochemical interpretation of fossil magnetotactic bacteria. Earth-Science Reviews, 86(1-4), 42–61, doi: 10.1016/j.earscirev.2007.08.001. Lin, W., Bazylinski, D. A., Xiao, T., Wu, L.-F., and Pan, Y. (2014). Life with compass: Diversity and biogeography of magnetotactic bacteria. Environmental Microbiology, 16(9), 2646–2658, doi:10.1111/1462-2920.12313. Uebe, R. and Schüler, D. (2016). Magnetosome biogenesis in magnetotactic bacteria. Nature Reviews Microbiology, 14(10), 621–637, doi:10.1038/nrmicro.2016.99.

Figure 1. Gunflint chert: a) FC-ZFC remanence curves with δFC/δZFC > 2.

TN – Néel temperature; TV – Verwey transition. b) FORC diagram showing textbook magnetofossil features.

microscopy techniques to determine whether the signatures are indeed coming from magnetofossils. If suc- cessful, this would extend the fossil record of controlled Figure 2. FORC diagrams for the Biwabik jasper (a) and the biomineralization by 1 Ga, and would have a number of Gunflint limestone (b). implications for magnetotaxis, the origin of biomineral- 4 3177-3186. Hall, S. J., A. A. Berhe, and A. Thompson (2018), Order from Current Articles disorder: do soil organic matter composition and turnover co-vary with iron phase crystallinity?, Biogeochemistry, 140(1), 93-110. A list of current research articles dealing with various topics in the physics and chemistry of magnetism is a regular feature of Hasan, A., I. Hossain, M. A. Rahman, M. S. Rahman, M. N. the IRM Quarterly. Articles published in familiar geology and Zaman, and P. K. Biswas (2018), FEG-EPMA mapping and geophysics journals are included; special emphasis is given to Fe-Ti oxide mineral chemistry of Brahmaputra River sedi- current articles from physics, chemistry, and materials-science ments in Bangladesh: provenance and petrogenetic implica- journals. Most are taken from ISI Web of Knowledge, after tions, Arabian Journal of Geosciences, 11(18). which they are subjected to Procrustean culling for this news- Hernandez, A. C., J. Bastias, D. Matus, and W. C. Mahaney letter. An extensive reference list of articles (primarily about (2018), Provenance, transport and diagenesis of sediment rock magnetism, the physics and chemistry of magnetism, in polar areas: a case study in Profound Lake, King George and some paleomagnetism) is continually updated at the IRM. Island, Antarctica, Polar Research, 37(1). This list, with more than 10,000 references, is available free of Kissel, C., M. Sarnthein, C. Laj, P. X. Wang, C. Wandres, and charge. 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SANTA IS COMING BACK TO TOWN: The 2019 Santa Fe Conference on Rock Magnetism is on its way! The 11th Santa Fe Conference on Rock magnetism will be held at St. John's College in Santa Fe New Mexico from June 6 - 9th 2019.

Following the previous Santa Fe formats, the conference will feature invited talks on selected topics and provide ample space for discussion. This year's proposed topical sessions will be:

1) Highs and lows of short-term geomagnetic field behavior 2) Pitfalls of Protocols and Processing Procedures 3) Fundamental rock-magnetism 4) Magnetic imaging

On Sunday June 9th there will be an all-day MERRILL workshop on micromagnetic modeling led by Wyn Williams (University of Edinburgh) for those who wish to attend. An optional field trip will be offered on Thursday June 6th led by John Geissman (UT Dallas) and Mike Petronis (Highlands University, Las Vegas NM) to admire the local geology (and possibly archaeology)

Registration and travel information will be available as we finalize details, (we anticipate early Spring), on the IRM website and the usual email lists. Stay tuned! 10 cont’d. from pg. 1... parting of ARMs? 2. What AF ranges (in mT) are possible with these in- 2. Survey struments? The survey contained questions about the instruments 3. Which AF range is typically used? available in each laboratory and their typical operating 4. What is the frequency of the AF (in Hz)? parameters. The survey was circulated to the gpmag, 5. What DC fields (in mT) are possible? emrp, and latinmag e-mail lists in January and February 6. What DC fields are typically used? 2018, and asked respondents to provide information for 7. If AF decay is defined by a decay rate, then what the following questions: decay rates (in mT/per half cycle) are possible? 8. What decay rates are typically used? 1. Instruments used for AF demagnetization and im- 9. If AF decay is defined by a translation speed, then (Maximum) Possible Typically used Published AARM studies

8 45 12

4 6 15 DC field (mT) Number of experiments Number of instruments Number of instruments

0 0 0 0.1 0.2 0.38 0.5 1.5 0.01 0.05 0.1 0.2 0.5 0.01 0.05 0.1 0.3 1 M 0.17 0.27 0.4 1 0.04 0.09 0.15 0.3 1 0.03 0.06 0.15 0.5 NS 0.035 0.08 0.2

6 12

3 6 (mT) 10 max AF Number of experiments Number of instruments Number of instruments

0 0 0 100 150 170 200 300 470 50 80 100 150 180 3 30 45 60 90 140 180 90 102.5 160 180 270 400 70 90 120 170 300 15 35 50 70 95 150 200 20 40 55 80 100 160 240

12 8

6 4 Not reported Number of instruments Number of instruments 0 0 0.001 0.01 0.04 NS M Decay rate (mT/half-cycle) 0.01 0.02 0.05 NS M 0.016 0.04 0.1 0.005 0.02 0.05

4 4 Rounded to 10s Rounded to 10s

2 2

Not reported AF frequency (Hz) AF frequency Number of instruments Number of instruments

0 0 40 80 100 130 150 190 260 NS 40 80 100 130 150 190 260 NS 50 90 110 140 180 200 300 50 90 110 140 180 200 300 Figure 1: Possible, typically used, and published values for DC fields, maximum AF, decay rate and AF frequency. NS means the parameter was not specified, or is not known, M indicates that several sets of parameters were used (published studies), or that any decay rate is possible/used in a certain range. Decay rates controlled by translation speed were classified as M. 11 what translation speeds (in cm/s) are possible? 1 10. What translation speeds are typically used? AF 0-20 mT 3. Survey results AF 0-50 mT 22 laboratories participated in the survey. ARM and AF 0-100 mT AARM experiments are conducted in 21 of these, and AF 0-180 mT some have multiple instruments available. The most commonly used instruments for ARM and AARM experiments are various models of the 2G Enterprises sys-

tems (configured for either in-line, or offline treatment), ARM the ASC Scientific/Precision Instruments D-Tech (for AF/ARM treatment), and the Molspin and Magnon systems. Figure 1 offers a snapshot of the ranges of values NxG1141 that are currently possible within our community, and typically used for DC fields, AF windows, decay rates, 0 and AF frequencies. These values are also compared to experimental parameters reported in AARM literature from 72 studies published between 1985 and 2017. Maximum possible DC fields vary between 0.1 mT and 1.5 mT, although the most commonly used DC fields 1 are 0.05 mT (i.e. close to the Earth’s field) and 0.1 mT. BG2.05a Note that even in these low bias fields, ARM is not always a linear function of field (Figure 2). The maximum possible AF fields range from 90 to 470 mT. It is interesting to note that – although AF fields up to 470 mT are possible for some instruments in some laboratories, the maximum field used in published AARM studies is 240 mT. Most laboratories report using a maximum AF ARM field of 100 mT. This common range of 0-100 mT is not dictated by the instrumental capability or scientific ratio- nale. Many of the available instruments are capable of applying larger AF fields of 200 mT or 300 mT. How- ever, for many years the Schoenstedt AF demagnetizers, 0 which had a peak field of 100 mT, were very common, 0 0.1 0.2 and their limitation led to a de facto experimental stan- DC field [mT] dard. It is worth noting that it is often desirable to apply ARMs using a smaller peak AF field than the maximum Summary of known magnetic susceptibility observations for different minerals Figure 2: DC dependence of ARM acquired over different AF AF field, to ensure complete demagnetization of the lab- windows by two samples from igneous intrusions. Sample NxG1141 oratory remanence, if needed. is an oxide gabbro from the Duluth Complex, MN, USA, and sample Decay rates can be defined in several different ways BG2.05a a gabbronorite from the Bushveld Complex, South Africa. depending on the instrument used. Please note that the a fixed rate of 0.001 or 0.005 mT/half-cycle, or vary the survey question about decay rates was only answered by decay rate depending on the AF window). The Molspin 14 out of 21 laboratories employing ARM/AARM sys- offers a smaller range of decay rates, 0.002-0.016 mT/ tems. There are three general classes of instruments that half-cycle, and typically used are 0.004 mT/half-cycle, are used to impart ARMs. The first class of instrument is similar in magnitude to fields used on the DTech when common on 2G systems with in-line AF/ARM systems they are not varied with AF. The Sapphire Instruments and produces a field that simply varies between positive SI-4 has a faster decay, with rates of 5-40 mT/cycle, i.e. and negative directions for a peak value. The electronic 2.5-20 mT/half-cycle, and the rates are adjusted based on components of this kind of instrument do not control the the AF window. The third group of instruments reports rate of field decay. Instead, decay rates are determined the decay rate in units of mT/s with values of 5, 10 and 20 by the translation speed over which the samples are mT/s (Magnon), or 1, 1.5, 3 and 9 mT/s (Agico LDA5 & moved through the alternating field. Translation speeds PAM1), with decay rates of 3, 5 and 10mT/s being used can be as high as ~20 cm/s, although more commonly most often. Because the AF frequency is known, these used speeds are around 8 - 15 cm/s, chosen depending mT/s decay rates can be converted into mT/half-cycle on the strength of the samples’ magnetization. (Strongly decay rates by DecayRate [mT/half-cycle] = DecayRate magnetized samples are more likely to cause flux jumps [mT/s] / (2* AF Frequency [Hz]). With this conversion, in SQUIDs at higher translation speeds.) A second class the 5, 10 and 20 mT/s of the Magnon, operating at 105 of instruments reports the decay rates in mT/half-cycle. Hz, translate to 0.024, 0.048 and 0.095 mT/half-cycle, Laboratories using the DTech instrument have access to and the 1, 1,5, 3 and 9 mT of the LDA5 & PAM1 operat- decay rates of 0.0001-0.1 mT/half-cycle, and either use 12 1 ing at 43 Hz translate to 0.012, 0.017, 0.035, and 0.10 (2) how long the sample is exposed to the maximum AF AF 0-20 mT mT/half-cycle. The AF frequencies at which the instru- (and whether or not the DC field is on during that time), ments operate vary by over almost an order of magni- (3) whether the DC field is turned on at the maximum AF AF 0-50 mT tude, from 43 and 300 Hz, and are not always known. or a slightly lower field, and (4) the shape of AF decay, in AF 0-100 mT addition to the decay rate. The complexity of the physics AF 0-180 mT 4. Discussion of ARM acquisition is such that neither analytical mod- The survey shows that our community uses a number of els (limited to SD populations, e.g. Egli [2006]), nor mi- different instruments for ARM and AARM experiments. cromagnetic models [e.g. Conbhui et al., 2018] predict The most frequently used DC field, 0.05 mT, is smaller ARM response to these factors, and experimental work

ARM than the maximum DC field that can be applied in most is required to document the effects and their dependence laboratories, 0.1 mT. Similarly, 100 mT is often used as on other key factors such as particle size and interac- the upper limit of the AF window, although available in- tions. Additionally, viscosity may affect the measured NxG1141 strumentation would allow higher fields. Again, this sug- ARMs. gests that the instrument capabilities are not the limiting 0 factors in these experiments. 5. Summary and recommendation Decay rates are defined in two profoundly different A wide range of parameters are currently being used in ways, (1) via the speed with which a sample is moved (A)ARM experiments, and a universal understanding of through a constant-amplitude AF field so that the field how each of these parameters affects the resulting mag- strength experienced by the sample depends on its posi- netization and its anisotropy has yet to be established. 1 tion, or (2) by applying an AF that slowly decreases in When comparing results from different studies, it is im- BG2.05a intensity over many cycles. In particular, the latter de- portant to be aware that experimental variables can and cay rates vary over several orders of magnitude. Some will affect the reported results. For example, Sagnotti laboratories report using a decay rate of (maximum AF)/ et al. [2003] reported a range of ~75% in ARM inten- (1000 half-cycles). Note that decay rates given in mT/s sities acquired on identical PSD samples in different are not strictly comparable, because the AF frequency labs, using nominally identical AC and DC fields (AF has an additional influence. of 100mT and DC bias field of 0.05 mT. The most im- It is interesting to compare these self-reported values portant recommendation for our community is to report with experimental parameters described in the AARM all experimental parameters in our future studies. While ARM literature [Biedermann et al., in review]. For example, many researchers report the DC bias field and peak AF most published studies report having used DC fields of magnitude, we also encourage researchers to report the 0.1 mT, which corresponds to the maximum DC field frequency of the AF and its decay rate (in mT/half-cycle available in many laboratories. Conversely, the most or the translation speed). 0 commonly used DC field by far, according to our survey, The most suitable set of experimental parameters will 0 0.1 0.2 is 0.05 mT. We are not sure what the reasons for this depend on the specific goals of any given study. Never- DC field [mT] discrepancy are, however, some possibilities include the theless, we present some considerations below that may following: (1) The survey respondents may tend to work help in determining the optimal parameters. in different laboratories than the authors of previously Larger DC fields lead to stronger ARMs, and therefore published AARM studies. This may reflect different also a better signal-to-noise ratio. On the other hand, practices, for example, in labs that do a lot of environ- ARM intensity is not always linear with DC field, and mental magnetic work versus those that focus more on to avoid any problems due to non-linearity, smaller DC anisotropy. (2) The standards may have evolved broadly fields may be favourable (Fig 2). How the ARM varies across the whole community, i.e. DC fields on 0.1 mT with DC field, and whether or not this variation is linear, were used earlier, but nowadays fields of 0.05 mT are can be determined experimentally. It has been shown that more frequently used. AARM can vary with DC field [Bilardello and Jackson, Survey responses agreed well with published studies 2014]. Therefore, when AARMs are used to anisotropy- for the most typical AF window during ARM experi- correct paleomagnetic data, it is advisable to use a DC ments: 0-100 mT. However, published studies report a field close to the geomagnetic field. larger number of AF windows than our survey. This is The influence of the AF window on AARMs and most likely related to the different sizes of the datasets. ApARMs is related to different sub-populations of Decay rates and AF frequencies are generally not report- grains carrying distinct anisotropies. This can be used in ed in published studies, and can therefore not be com- fabric studies, e.g. to characterize different deformation pared to our survey results. stages. Anisotropy corrections on samples with multiple In addition to the parameters discussed above, other sub-fabrics are challenging, and several ApARM tensors details of the experimental setup may cause changes in will be needed, with the AF windows defined e.g. by de- the measured data, and further work will be needed until termining the magnetization directions during an ARM we thoroughly understand every factor that influences an demagnetization. ARM measurement. Examples of such possible factors Decay rates should ideally be chosen based on the are (1) how the AF is increased to its maximum value maximum AF field and the instrumental limitations of prior to the AF decay (rate of increase, shape of increase, the equipment in a given laboratory. Weaker AF peak whether or not the DC field is on during the increase), fields call for slower decay rates, whereas higher fields 13 require faster decay rates. (Too slow decay rates in high Egli, R. (2006), Theoretical considerations on the anhysteretic fields would lead to overheating coils, whereas too fast remanent magnetization of interacting particles with uni- decay rates in weak fields would be unable to demagne- axial anisotropy, Journal of Geophysical Research, 111, tize the sample completely.) Some instruments automati- B12S18, doi: 10.1029/2006JB004577. Egli, R., and W. Lowrie (2002), Anhysteretic remanent magne- cally change the decay rates as a function of the peak AF tization of fine magnetic particles, Journal of Geophysical designated by a user, e.g. when the decay rate is defined Research, 107(B10), 2209, doi: 10.1029/2001JB000671. via a fixed translation speed. Other instruments require Jackson, M., and L. Tauxe (1991), Anisotropy of magnetic that decay rates be adjusted manually. susceptibility and remanence: Developments in the char- Ultimately, we hope that this study will help our commu- acterization of tectonic, sedimentary, and igneous fabric, nity achieve more comparable ARM and AARM results Reviews of Geophysics, 29, 371-376. through a higher awareness of the variety of experimen- Liu, Q., A. P. Roberts, J. C. Larrasoana, S. K. Banerjee, Y. tal parameters associated with ARMs. Guyodo, L. Tauxe, and F. Oldfield (2012), Environmental magnetism: Principles and applications, Reviews of Geo- physics, 50(4), doi: 10.1029/2012RG000393. Acknowledgments Potter, D. K. (2004), A comparison of anisotropy of magnetic We are grateful to the community for answering our sur- remanence methods - a user's guide for application to pa- vey. ARB was supported by the Swiss National Science leomagnetism and magnetic fabric studies, in Magnetic Foundation, projects 167608 and 167609. Fabrics: Methods and Applications, edited by F. Martín- Hernández, C. M. Lüneburg, C. Aubourg and M. Jackson, pp. 21-35, The Geological Society, London, UK. References Sagnotti, L., P. Rochette, M. Jackson, F. Vadeboin, J. Dinarès- Biedermann, A. R., M. Jackson, D. Bilardello, and J. M. Fein- Turell, A. Winkler, and M.-N. S. Team (2003), Inter-lab- berg (in review), Anisotropy of full and partial anhysteretic oratory calibration of low-field magnetic and anhysteretic remanence across different rock types: 2. Coercivity-depen- susceptibility measurements, Physics of the Earth and Plan- dence of remanence anisotropy. etary Interiors, 138, 25-38. Bilardello, D., and M. J. Jackson (2014), A comparative study Yu, Y., and D. J. Dunlop (2003), Decay-rate dependence of of magnetic anisotropy measurement techniques in relation anhysteretic remanence: Fundamental origin and paleo- to rock-magnetic properties, Tectonophysics, 629, 39-54, magnetic applications, Journal of Geophysical Research, doi: 10.1016/j.tecto.2014.01.026. 108(B12), doi: 10.1029/2003JB002589. Conbhui, P. O., W. Williams, K. Fabian, P. Ridley, L. Nagy, and A. R. Muxworthy (2018), MERRILL: Micromagnetic Earth Related Robust Interpreted Language Laboratory, Geochemistry Geophysics Geosystems, 19(4), 1080-1106, doi: 10.1002/2017GC007279. QuarterlyThe IRM The Institute for Rock Magnetism is dedi- The IRM Quarterly is published four cated to providing state-of-the-art facilities times a year by the staff of the IRM. If you and technical expertise free of charge to any or someone you know would like to be on interested researcher who applies and is ac- our mailing list, if you have something you cepted as a Visiting Fellow. 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