ISSN: 2152-1972 The IRM Inside... Visiting Fellow Reports 2 Current Articles 8 ... and more throughout! QuarterlySummer 2020, Vol. 30 No. 2 Practical Magnetism III: What’s what in remanence anisotropy Dario Bilardello Institute for Rock Magnetism, University of Minnesota, Minneapolis, MN, USA [email protected] In the IRMQ28-3 article “Commonly used experimen- tal parameters for acquisition of anhysteretic remanent magnetization (ARM) and its anisotropy (AARM): Re- sults and recommendations from a rock magnetic com- munity survey” Biedermann et al. (2018) had reported results from a survey sent to our community regarding what capabilities different laboratories have to apply ARMs (peak AF fields, DC fields, AF decay rates) and which are typically used and fed into anisotropy calcula- tions. What that article left out, however, was what prop- erties are used for calculating these anisotropies, and note that I am not using the term AARM here, because the name we give these anisotropies is intrinsically de- pendent on how they are calculated.

The Rationale Figure 1. ARM acquisition curves from Sugiura (1979) for six synthetic samples Using anhysteretic remanences to compute anisotropies bearing particle concentrations of: 1, 2.64 x 10-6; 2, 1.89 x 10-5; 3, 1.46 x 10-4; 4, 7.19 x 10-4; 5, 4.25 x 10-3; 6, 2.33 x 10-2. Magnetic interactions thus increase was first proposed by McCabe et al. (1985), for what from samples 1 through 6, resulting in increased linearity of magnetization versus they termed Anisotropy of Anhysteretic Susceptibil- field, and increasingly suppressed magnetizations. ity (AAS). Susceptibility of ARM (χARM) is a property that has been used since it was first proposed by King et and was replaced by anisotropy of anhysteretic remanent al. (1982), is mostly employed in environmental mag- magnetization (AARM) (Mike Jackson, personal com- netism, and is nothing else than the ARM normalized munication). Interestingly enough, the same criticism by the DC field used to acquire it, just like magnetic has not been expressed for use of the χARM parameter for susceptibility is defined by the “in field” magnetization similar DC fields (< 1 Oe or 0.1 mT or ~80 A/m), Subir (low-field) divided by the field itself. The advantage Banerjee, personal communication) and use of the terms of such a normalization is that it inherently makes re- (and proxy) survives to this day. While the criticism sults comparable among different laboratories or when raised is legitimate, unfortunately it has created some slightly different applied fields are used, as long as the confusion surrounding the determination of the anisotro- resulting susceptibility is not field-dependent. It is thus pies and relevant terminology (AAS versus AARM, and a useful quantity to report, granted that the assumption what is actually meant by these terms). of linearity between magnetization (ARM) and DC field The problem of linearity of M versus B (in fields much less than the coercivity) has been long investi- is maintained. Using χARM for anisotropy is thus a natural segue, and also determines the units of the anisotropy gated in magnetic research and is intrinsically related principal axes. to particle interactions within a sample (e.g. Dunlop & A criticism to the term AAS, however, was that the West, 1969; Sugiura, 1979) or by thermal fluctuations condition of linearity (field dependence) was rarely met and particle volume in noninteracting samples (Egli & and thus the property should not be referred to a suscep- Lowrie, 2002). For synthetic samples, Sugiura (1979) cont’d. on tibility. Consequently, the term AAS soon faded away determined experimentally that strongly interacting pg. 13... particles (concentrations of 4.3 x 10-3 - 2.3 x 10-2) show 1 Visiting Fellow Reports

Tracing thermal alteration using rock magnetism

Huapei Wang, Junxiang Miao China University of Geosciences, Wuhan [email protected]

Introduction The Thellier-series experiments [Thellier and Thellier, 1959] are widely considered the most reliable tech- nique to estimate paleointensities, which can provide crucial constraints on the behavior of the geomagnetic field. During the experiments, samples need to be heated multiple times up to the Curie temperature, in order to be thermally demagnetized and to acquire laboratory- applied thermoremanences. However, the thermally induced physical and chemical alteration of magnetic minerals during the stepwise heating treatments can se- verely bias paleointensity estimates. In this study, we use comprehensive rock magnetic measurements to de- tect the thermal alteration of sister specimens of a lava sample that have been used in a previous paleointensity study, in the hope of reaching a better understanding of Figure 1. High-temperature measurements (HT-VSM) from room how samples alter during Thellier-series paleointensity temperature to 860 K (~587 ºC) for every 20 K for specimen experiments. GA85.1w. (a) paramagnetically corrected hysteresis loops and (c) DCD curves for the 1st heating; (b) paramagnetically corrected hys- Samples and Methods teresis loops and (d) DCD curves for the 2nd heating. (e) Mr and Ms The Galapagos Archipelago consists of volcanic islands versus temperature curves; (f) Bcr and Bc versus temperature curves; on the Nazca plate just south of the Equator [Kent et al., blue curves for the 1st heating; red curves for the 2nd heating. 2010; Wang and Kent, 2013]. We focused on a sample Utilizing the above DCD curves measured at elevated GA85.1 from a Galapagos lava, which yielded a very temperatures, we performed thermal fluctuation tomo- low paleointensity (4.23±1.29 µT) [Wang and Kent, graphic (TFT) calculations [Jackson et al., 2006; Wang 2013], and was likely associated with the Santa Rosa et al., 2013] to gain more information on the thermal al- geomagnetic excursion event at 925.7±4.6 thousand teration characteristics of specimen GA85.1w (Fig. 2). years ago [Balbas et al., 2016]. Although the TFT inverse calculations from DCD mea- We carefully crushed a piece of the fresh sample into surements for plotting size-shape distribution diagrams several small chips, which were named “GA85.1w”, are only strictly valid for stable single-domain (SSD) “GA85.1u”, “GA85.1t”, etc. To track the thermal altera- and superparamagnetic particles [Jackson et al., 2006], tion during heating, we used the HT-VSM (a vibrating the inverted volume-microcoercivity diagrams and size- sample magnetometer equipped with a high-temperature shape diagrams still provide useful insight on thermal furnace in the Institute for Rock Magnetism) to measure alteration. hysteresis loops (Fig. 1a), back-field direct current de- Utilizing the hysteresis loops and DCD curves mea- magnetization (DCD) curves (Fig. 1c) at elevated tem- sured at elevated temperatures (Fig. 1), we also plotted peratures on specimen GA85.1w. After the first heating the Day diagram (Mr/Ms versus Bcr/Bc) [Day et al., round (up to 880 K), we repeated the same measure- 1976] of specimen GA85.1w for the first and second ments to gauge thermal alteration (Figs. 1b, d), which al- heating rounds (Fig. 3a). We also measured FC-ZFC lowed us to observe differences in rock magnetic proper- (field cooled, zero-field cooled) curves [Moskowitz et ties between the first and the second heating rounds. We al., 1993] and SIRM (saturation isothermal remanent also calculated the hysteresis parameters such as satu- magnetization) cooling/warming curves from 10 K to ration remanent magnetization (Mr), saturation induced 300 K for specimen GA85.1u (Fig. 3b), using a Quan- magnetization (Ms), magnetic coercivity (Bc), remanent tum Designs magnetic property measurement system magnetic coercivity (Bcr) at successive temperature (MPMS) at the Institute for Rock Magnetism. In order to steps (Figs. 1e, f) to indicate any signs of alteration. directly gauge thermal alteration, we measured FORCs 2 Figure 2. High-temperature TFT results from 300 K (27°C) to 860 K (587°C) for specimen GA85.1w. (a) volume–microcoercivity distribution and (c) effective size–shape distribution for the 1st heating; (b) volume–microcoercivity distribution and (d) effective size–shape distribution for the 2nd heating. Color scale represents linear increments (every 10%) of probability density. The misfits of the TFT calculations are ~45%. [Pike et al., 1999; Roberts et al., 2014] on specimen alteration, with a shift of the central ridge and a decrease GA85.1t before and after heating, using an AGM (al- of the peak of the microcoercivity distribution. Our com- ternating gradient magnetometer) at Rutgers University prehensive rock magnetic results of GA85.1 suggests (Figs. 3c, d). that it’s not suitable for Thellier-series experiments due to thermal alteration, which is supported by its paleoin- Results tensity results from a sister specimen GA85.1c [Wang These rock magnetic measurements at different tempera- and Kent, 2013]. tures allow us to monitor the thermal alteration of sample In conclusion, comprehensive rock magnetic mea- GA85.1 during heating treatments, which can provide surements at room temperature and elevated tempera- critical information on the reliability of its paleointensity tures can provide critical information in determining the result. From the first and second heating rounds (Fig. 1), thermal alteration of samples during heating treatments, we found that the Ms-T curves were similar, while the which can be used to determine the qualification or jus- Mr-T curves show significant differences, which indicat- tify the reliability of their paleointensity results. ed that the thermal alteration was physical (magnetic do- Supported by the IRM student visiting fellowship, data main state) but not chemical (magnetic mineralogy). The in this report were collected in the summer of 2011 by resultant Bc and Bcr curves for the first and second heat- Huapei Wang, who was a Ph.D. student at that time. Dur- ing also show clear discrepancies, which also showed ing the visit, Mike Jackson, Pete Solheid, Bruce Mos- in the Day plots (Fig. 3a). TFT plots (Fig. 2) from the kowitz, Josh Feinberg and other members of the IRM first and second heating rounds show clear changes in not only provided Huapei hands-on guidance to oper- effective volume, shape and microcoercivity, with re- ate many sophisticated instruments at the IRM, but also duced effective ferromagnetic grain size and increased shared many thoughtful and joyful discussions and the microcoercivity after the first heating. FC-ZFC curves of signature “Tea Time” with Huapei, who benefited great- specimen GA85.1u show PSD-MD magnetite behavior ly in terms of experiment design, sample measurements (Fig. 3b). FORC diagrams for specimen GA85.1t before and data analyses. and after heating (Figs. 3c, d) show relatively significant 3 ing first order reversal curves. Journal of Applied Physics. 85(9), 6660-6667. Roberts, A. P., D. Heslop, X. Zhao, and C. R. Pike, 2014. Un- derstanding fine magnetic particle systems through use of first-order reversal curve diagrams. Reviews of Geophys- ics. 52(4), 557-602. Thellier, E., and O. Thellier, 1959. Sur l’intensite´ du champ magne´tique terrestre dans le passe´ historique et ge´ologique. Annales De Géophysique. 15(3), 285-376. Wang, H., and D. V. Kent, 2013. A paleointensity technique for multidomain igneous rocks. Geochemistry Geophysics Geosystems. 14(10), 4195-4213. Wang, H., D. V. Kent, and M. J. Jackson, 2013. Evidence for abundant isolated magnetic nanoparticles at the Paleocene- Eocene boundary. Proceedings of the National Academy of Sciences of the United States of America. 110(2), 425-430.

Preservation of primary magnetic signals in regionally altered volcanic terranes or: How I learned to stop worrying and love the (maghemite) bump

1,2 Figure 3. (a) Day plot of specimen GA85.1w measured at elevated tempera- Thomas Belgrano tures with numbers indicating heating temperature in ºC. Percentages on the 1 Institute of Geological Sciences, University of dashed curve are modeled volumes of MD contribution to SSD-MD mixing curve #3 [Dunlop, 2002]; (b) Low-temperature magnetic property curves Bern, Switzerland for specimen GA85.1u; (c, d) FORC diagrams (field increment of 2 mT, 113 2 National Oceanography Centre Southampton, curves, smoothing factor of 6) for specimen GA85.1t before and after the first University of Southampton, UK heating. FORC diagrams are generated using the software package FORCinel [email protected] v3.06 [Harrison and Feinberg, 2008]. References Introduction Balbas, A., A. A. P. Koppers, D. V. Kent, K. Konrad, and P. The correlation between bulk Fe content and magnetism U. Clark, 2016. Identification of the short-lived Santa Rosa in fresh volcanic rocks is both logical and well-charac- geomagnetic excursion in lavas on Floreana Island (Gala- pagos) by 40Ar/39Ar geochronology. Geology. 44(5), 359- terized (e.g., Gee and Kent, 1998). However, pervasive 362. hydrothermal alteration, common to essentially all an- Day, R., M. D. Fuller, and V. A. Schmidt, 1976. Magnetic hys- cient submarine volcanic terranes, is generally thought teresis properties of synthetic titanomagnetites. Journal of to scatter and obscure these primary magnetic properties. Geophysical Research. 81(5), 873-880. Consequently, intra-volcanic stratigraphic interpretation Dunlop, D. J., 2002. Theory and application of the Day plot of aeromagnetic maps in these terranes is commonly un- (Mrs/Ms versus Hcr/Hc) 2. Application to data for rocks, dertaken with apprehension, or not at all. sediments, and soils. Journal of Geophysical Research: While producing a new geological map of upper Solid Earth. 107(B3), 15. oceanic crust of the Semail ophiolite (Oman–UAE), we Harrison, R. J., and J. M. Feinberg, 2008. FORCinel: An im- proved algorithm for calculating first-order reversal curve noticed that conformable aeromagnetic anomalies are distributions using locally weighted regression smoothing. often aligned with the different volcanic units (Fig. 1). Geochemistry Geophysics Geosystems. 9(5), 11. The key question which brought about my visit to the Jackson, M., B. Carter-Stiglitz, R. Egli, and P. Solheid, 2006. IRM was whether these local observations had a sound Characterizing the superparamagnetic grain distribution basis in rock magnetic properties and mineralogy, and f(V, Hk) by thermal fluctuation tomography. Journal of thus whether they could be applied to remote mapping Geophysical Research: Solid Earth. 111(B12), 33. and identifying these units under thin gravel cover. Kent, D. V., H. Wang, and P. Rochette, 2010. Equatorial pale- Using rock samples collected from the different vol- osecular variation of the geomagnetic field from 0 to 3 Ma canic units across the ophiolite, we measured magnetic lavas from the Galapagos Islands. Physics of the Earth and Planetary Interiors. 183(3-4), 404-412. susceptibility, natural remanent magnetization, high and Moskowitz, B. M., R. B. Frankel, and D. A. Bazylinski, 1993. low-T magnetic behavior and magnetic hysteresis. These Rock magnetic criteria for the detection of biogenic mag- could be compared with previously determined whole- netite. Earth and Planetary Science Letters. 120(3-4), 283- rock geochemical data. 300. Pike, C. R., A. P. Roberts, and K. L. Verosub, 1999. Charac- Relationship between bulk magnetic properties and geo- terizing interactions in fine magnetic particle systems us- chemistry 4 434 436 438 434 436 438 Mandoos N Mandoos N (a) (b) L (c) 3 deposit deposit 1 km 1 km 2732 2732 Pelagic sediments Boninitic Alley 2 (Boninites) Gt Gt Gt Gt Tholeiitic Alley uA uA (Basalt–Rhyolite series) tA Post-axial T tA 1 Lasail volcanic units (Basalts) L G L 2730 Geotimes 2730 (~MORB/FAB) 0 fA Gt SD L Gt L tA Sheeted Dyke Complex 58 km above sheeted dyke complex -1 tA U volcanic units Axial dykes & 2728 2728

Gabbros 65 RTP magnetic anomaly (µT) Thrust fault Gt Geotimes uA Undifferentiated Alley 0 0.5 1 2 Fault T Tonalite fA Felsic Alley G Gabbro tA Tholeiitic Alley Inferred contact U Ultramafic dL Depleted Lasail SD Sheeted dykes L Lasail Figure 1. Volcanostratigraphy of the Semail ophiolite and an aeromagnetic mapping example, adapted from Belgrano et al. (2019). (a) Stratigraphy of the mapped volcanics units. (b) Reduced-to-pole (RTP) aeromagnetic map, showing N–S oriented anomalies corresponding to the different volcanic units, offset by E–W faults. (c) Final geological map, confirmed by field mapping and sam- pling (circles), showing a volcanic section dipping ~40˚E in the vicinity of the Mandoos volcanogenic massive sulphide deposit (6.7 Mt ore at 1.66 wt% Cu). To test whether the map-scale relationship between the would initially have been concentrated in the glassy in- volcanic units and aeromagnetic data was reproduce- terstices of the primitive lavas (Belgrano et al., 2019). able at the sample scale, we measured the bulk magnetic Chlorite is the most dominant secondary mineral replac- properties of a series of samples differentiated on a unit- ing these interstices. As a mineral that readily incorpo- basis. Both magnetic susceptibility and natural rema- rates Mg2+, Fe2+, and a little Fe3+, chlorite, is the probable nent magnetization (NRM) intensity were found to be paramagnetic host of the Fe that previously resided in systematically lower in the units that corresponded with ferromagnetic oxides of the fresh lavas. weak magnetic anomalies in the aeromagnetic map (Bo- These conclusions seem rather straightforward in ninitic Alley and Lasail). hindsight; however, without making these magnetic As these magnetic properties vary on the basis of measurements and comparing them to the geochemical volcanic unit, they are presumably controlled by some trends, it was impossible to rule out other equally plau- primary magmatic characteristic. Despite scatter due to sible explanations, such as calc-alkaline differentiation alteration, Figure 2 shows that the magnetic suscepti- trends or conformable hydrothermal alteration. bility of the Semail volcanics is related to whole rock 6 Mg# (= molar Mg / (Mg + Fe)), which in turn varies -5 KT Magnon Boninitic Alley systematically between the different units. This relation- Transitional Alley ship is shown by the tholeiite-like trend of magnetic Felsic Alley 5 Tholeiitic Alley susceptibility with Mg#, rapidly increasing from high to Lasail

kg ) Trans. Geot.–Las.

moderate Mg#, before dropping off at evolved composi- / Felsic Geotimes 3 Geotimes tions where lavas have undergone magmatic magnetite m Einaudi et al. (2003)

-5 4 fractionation. The weakest magnetic samples however, Geotimes/V1 mostly derive from the Lasail and Boninitic Alley units, which have the highest Mg# compositions, as corrobo- 3 rated by their high Mg# relict clinopyroxenes and fresh volcanic glasses (Belgrano et al., 2019). Crucially, at Mg# >80, the relationship between sus- 2 ceptibility and Mg# is somewhat amplified, rather than obscured by alteration. Above Mg# >80, magnetic sus- Fresh Magnetic susceptibility (10 ceptibility drops to near zero, whereas below this Mg#, 1 boninite susceptibility rapidly increases. In fresh volcanic rocks, Carbonated this relationship is more linear at high Mg#, with signifi- High-Si cant magnetism persisting above Mg# 80 (Gee and Kent, 0 1998). The strong susceptibility drop above Mg# 80 in 20 30 40 50 60 70 80 90 the Semail lavas is the key reason these primitive units Mg# = molar Mg / (Mg + Fe) (%) are so distinguishable in aeromagnetic data (Fig. 1b). Evolved Primitive The majority of samples from these primitive units are Figure 2. Magnetic susceptibility vs. whole rock Mg# for the dominantly paramagnetic. This indicates that all avail- Semail ophiolite volcanic units, adapted from Belgrano et al. able Fe (5–10 wt% Fe2O3 equivalent) is sequestered in (2019), with additional data from Einaudi et al. (2003). The paramagnetic silicate phases. As magnetite is a late crys- general evolution of protolith compositions from ‘primitive’ to tallizing-phase in tholeiitic magmas, primary magnetite ‘evolved’ due to magmatic differentiation is annotated. 5 TB3-01A monly observed (Fig. 3b). This magnetite could either be (a) heating 3.0 TB3-01A 6 0 dK/dT FC remanence heating primary, following unmixing from a Ti-rich phase during ZFC remanence / ˚C)

5 g RT IRM kg ) k cooling

kg ) -2 / 2.5 cooling, or hydrothermal. 2 /

/ RT IRM 3 warming 3 4 m m m A -8 -6

-2 Figures 3c & d are interpreted to show different 3 10

10 -4

( *

(10 ( 2.0 K 2 M mixtures of (weakly Ti-substituted) magnetite and ma- cooling -6 d K /d T 1 ghemite. In both cases an increase in susceptibility around 493˚C 1.5 0 130–140˚C, the titular ‘maghemite bump’(Kontny and 0.4 TB2-33C (b) 20 0 TB2-33C Grothaus, 2017), precedes a loss of susceptibility be-

0.3 tween 300–400˚C. This susceptibility drop is interpreted / ˚C)

15 g kg ) -20 k kg )

/ / / as the inversion of maghemite to hematite. In Figure 3c, 3 3 2 m m

m 0.2 -6 10 -8

( A the similar susceptibilities at room temperature and fol- 10 10 (

( -40 M

K lowing inversion suggest only a modest maghemite con- 5 0.1 d K /d T -60 tribution to total susceptibility. In Figure 3d, however, 574˚C 0 0 ~3/4 of susceptibility is lost during maghemite inver- (c) 24 TB3-25E 0.55 TB3-25E 130˚C 0 sion, and a small but significant fraction of susceptibility 20 0.50 / ˚C)

persists above 600˚C. The slight reversable component g kg ) 16 k kg ) /

-10 / / 3 3 0.45 2 of this high-temperature, >600˚C susceptibility is inter- m m

409˚C m -6 12 -8 ( A 10 10 (

preted as hematite, either pre-existing or the inversion ( 0.40

-20 M K 8 product of maghemite. The irreversible portion of this d K /d T 0.35 4 -30 576˚C high-temperature susceptibility therefore cannot be due 0 0.30 140˚C TB3-07C TB3-07C to hematite, and is instead interpreted as remaining Ti- (d) 10 24 bearing maghemite which unblocked at 610˚C.

/ ˚C) 0.50 20 571˚C 0 g k

kg ) The low temperature experiments shown for the same / / kg ) / 3 3 16 2 m m

-10 m samples in the right-hand column of Figure 3, support

610˚C -8 -6 0.45 12 ( A 10 10 (

(

M

K these interpretations. Only samples interpreted as con- 8 -20

d K /d T 0.40 taining magnetite exhibit a distinct Verwey transition at 4 332˚C -30 ~120 K, and this transition is broadened or suppressed 100 200 300 400 500 600 700 20 60 100 140 1 80 220 260 300 T (˚C) T (K) in the titanomagnetite or maghemite dominated samples. Figure 3. High-temperature magnetic susceptibility (K) of four representative The lack of a Besnus transition 30–35 K also militates samples with the first derivative of the heating curve (left column) and low- against the unblocking of monoclinic pyrrhotite (T Curie temperature FC and ZFC remanences and RT-IRM during cooling and warm- ≈ 320˚C) as the cause of the susceptibility drops on heat- ing of the same samples (right column). (a) Titanomagnetite in relatively fresh ing between 300–400˚C. boninite. (b) Almost stoichiometric magnetite (c) Mixed maghemite and mag- Measurements on 36 samples revealed that maghemite netite. (d) Maghemite in Geotimes. Asterisked “transition” in RTM IRMcool- is present in ~85% of the Geotimes samples, compared ing of (a) possibly caused by unintentional movement of the sample. to 30% of the Tholeiitic Alley samples, whereas primary titanomagnetite is still present in 50% of Tholeiitic Alley Magnetic mineralogy & the crucial role of maghemite samples, compared to only 15% of Geotimes. This trade- To determine the origin and thus reliability of their bulk off of fresh titanomagnetite for oxidized maghemite at magnetic properties, it was necessary to determine the similar bulk Fe contents explains how the more altered magnetic mineralogy of the different volcanic units. In Geotimes unit can have similar bulk magnetic properties particular, for geological mapping, the persistence of to the fresher Tholeiitic Alley unit. magnetism following alteration in the more evolved units is just as important as the weak primary magne- Concluding remarks tism of the primitive units. That both the Geotimes and On face value, the task of mapping subtle compositional Tholeiitic Alley units have similarly high susceptibilities differences between regionally altered volcanic units us- and NRM intensities is somewhat surprising, as the more ing aeromagnetic data seemed extremely challenging. deeply-buried basal Geotimes lavas have generally suf- Two quirks of nature, however, meant that the perva- fered more intense, higher-grade greenschist facies alter- sive alteration of these volcanic rocks was a tolerable, ation than the overlying Tholeiitic Alley lavas (Belgrano or even helpful influence. Firstly, complete chloritization et al., 2019). of magnetic oxides in primitive volcanic rocks helped to The high- and low-temperature experiments in Figure lower their magnetism and differentiate them from their 3 provide an explanation for the persistence of primary more evolved counterparts. Secondly, the prevalence magnetic signals in these altered rocks. These experi- of maghemite as an oxidized but nevertheless strongly ments firstly identified the presence of relict titanomag- magnetic phase lessened the effects of differing hydro- netite (Fig. 3a), with a Curie temperature <500˚C. As Ti thermal alteration on the two Fe-rich units. Fortunately, has a very low solubility in seawater, this titanomagne- these units are stratigraphically arranged like a bar code: tite is assumed to be primary, and is present in around strong–weak–strong–weak, so these magnetic charac- 50% of the Tholeiitic Alley samples, but <20% of Geo- teristics were generally useful for remote mapping and times samples. Almost stoichiometric magnetite, with a mapping under gravel cover. Curie temperature between 570–580˚C was also com- Far from being the unique quirks of our study area, 6 the effects of chloritization and maghemitization are ubiquitous in ancient submarine volcanic suites, so these practical observations should prove useful for volcanic Visiting Fellows mapping in other ophiolites and potentially greenstone belts. More generally, our study underscores greatly en- July - December, 2020 hanced utility of aeromagnetic surveys when combined with basic magnetic petrology of the targeted units. Maryam Abdulkarim Acknowledgements Imperial College I am tremendously grateful for receiving the generous Visiting Fellowship of the Institute for Rock Magnetism Testing the universality and scale of magnetic (IRM), University of Minnesota, and for the equally hydrocarbon migration hypothesis – generous guidance given by Dario Bilardello and Mike Lower Tertiary reservoir systems, UK North Jackson during my visit. My connection to the IRM was Sea. initiated through Andrea Biedermann, who also helped perform the measurements and to reach the conclu- sions summarized above. My other co-authors, Larryn Thomas Berndt Diamond, Yves Vogt, Sam Gilgen, and Khalid Al-Tobi Peking University built up the geological context for these conclusions over Using Time-Asymmetric First-Order Reversal many years, and helped to develop them in the resulting Curves (TAFORC) to separate biogenic from paper. I also gratefully acknowledge the support of the Public Authority for Mining (PAM), Sultanate of Oman non-biogenic signals in sediments and the Swiss National Science Foundation over the course of the University of Bern’s work in Oman. Joseph Perkins Imperial College References Belgrano, T.M., Diamond, L.W., Vogt, Y., Biedermann, A.R., Basin modelling and hydrocarbon magnetics: Gilgen, S.A., Al-Tobi, K., 2019. A revized map of volcanic Can they be used to unravel a half-century units in the Oman ophiolite: insights into the architecture North Sea mystery? A case study on the Bea- of an oceanic proto-arc volcanic sequence. Solid Earth 10, 1181–1217. https://doi.org/10.5194/se-10-1181-2019 trice Field, Inner Moray Firth Einaudi, F., Godard, M., Pezard, P., Cochemé, J.J., Coulon, C., Brewer, T., Harvey, P., 2003. Magmatic cycles and forma- tion of the upper oceanic crust at spreading centers: Geo- chemical study of a continuous extrusive section in the Sarah Slotznick Oman ophiolite. Geochemistry, Geophysics, Geosystems 4, Dartmouth College 1–25. https://doi.org/10.1029/2002GC000362 Kontny, A., Grothaus, L., 2017. Effects of shock pressure and Quantifying Goethite in Sedimentary Rocks temperature on titanomagnetite from ICDP cores and tar- get rocks of the El’gygytgyn impact structure, Russia Stud. Geophys. Geod., 61, 163-183. Gee, J., Kent, D. V, 1998. Magnetic telechemistry and mag- Sarah Widlansky* matic segmentation on the Southern East Pacific Rise. Earth and Planetary Science Letters 164, 379–385. https://doi. University of New Hampshire org/https://doi.org/10.1016/S0012-821X(98)00231-3 Magnetic mineralogy of the Sheep Pass For- mation

*U.S. Student Fellows

7 section in Wachock as the key site of Vistulian loesses and palaeosols in the Holy Cross Mountains (Poland), Geologi- Current Articles cal Quarterly, 64(2), 252-262, doi:10.7306/gq.1527. Franke, C., E. Patault, C. Alary, N. E. Abriak, and F. Lagroix (2020), Magnetic Fingerprinting of Fluvial Suspended Par- A list of current research articles dealing with various topics in the physics and chemistry of magnetism is a regular feature of ticles in the Context of Soil Erosion: Example of the Canche the IRM Quarterly. Articles published in familiar geology and River Watershed (Northern France), Geochemistry Geo- geophysics journals are included; special emphasis is given to physics Geosystems, 21(5), doi:10.1029/2019gc008836. current articles from physics, chemistry, and materials-science Gao, X. B., J. Ou, S. Q. Guo, W. T. Ji, X. Q. Li, C. L. Deng, journals. Most are taken from ISI Web of Knowledge, after Q. Z. Hao, and Z. T. Guo (2020), Sedimentary history of which they are subjected to Procrustean culling for this news- the coastal plain of the south Yellow Sea since 5.1 Ma con- letter. An extensive reference list of articles (primarily about strained by high-resolution magnetostratigraphy of onshore rock magnetism, the physics and chemistry of magnetism, borehole core GZK01, Quaternary Science Reviews, 239, and some paleomagnetism) is continually updated at the IRM. doi:10.1016/j.quascirev.2020.106355. This list, with more than 10,000 references, is available free of Grilo, C. F., C. Chassagne, V. D. Quaresma, P. J. M. van Kan, charge. Your contributions both to the list and to the Current and A. C. Bastos (2020), The role of charge reversal of Articles section of the IRM Quarterly are always welcome. iron ore tailing sludge on the flocculation tendency of sedi- ments in marine environment, Applied Geochemistry, 117, Archaeomagnetism doi:10.1016/j.apgeochem.2020.104606. Guda, A. M., I. A. El-Hemaly, E. 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Sel- concentration-dependence of ARM observed by Sugiura kin, L. Meynadier, T. Schwenk, V. Spiess, and C. France- (1979) was particularly surprising, considering the very Lanord (2020), Middle to Late Pleistocene Evolution of the low concentrations (2.3 x 10-2 < C < 2.6 x 10-6) at which Bengal Fan: Integrating Core and Seismic Observations for they were observed, much below previously reported Chronostratigraphic Modeling of the IODP Expedition 354 values (e.g. Banerjee & Mellema, 1974; Dankers, 1978; 8 degrees North Transect, Geochemistry Geophysics Geo- Schmidbauer & Veitch, 1980). They concluded that, if systems, 21(4), doi:10.1029/2019gc008878. confirmed, the result would be particularly pertinent to Routledge, C. M., D. K. Kulhanek, L. Tauxe, G. Scardia, A. D. the application of the χ parameter to soils and lake Singh, S. Steinke, E. M. Griffith, and R. Saraswat (2020), ARM sediments for which those concentrations are common. A revised chronostratigraphic framework for International Ocean Discovery Program Expedition 355 sites in Laxmi Notably, Sugiura (1979) determined through Preisach Basin, eastern Arabian Sea, Geological Magazine, 157(6), models that the grains in their samples existed in bun- 961-978, doi:10.1017/s0016756819000104. dles, which does affect the distribution of the interaction Witkowski, J., D. M. Harwood, B. S. Wade, and K. Brylka field intensity. (2020), Rethinking the chronology of early Paleogene The non-linearity of ARM acquisition has been in- sediments in the western North Atlantic using diatom vestigated numerically (Dunlop & Özdemir, 1997; Egli, biostratigraphy, Marine Geology, 424, doi:10.1016/j.mar- 2006; Egli & Lowrie, 2002), and the initial slope (calcu- geo.2020.106168. lated for DC fields < 0.01 mT) has been used to deter- Xiao, C. H., Y. H. Wang, and J. Lin (2020), Constraints of mag- mine non-interacting SD behavior for a variety of natural netostratigraphic and mineralogical data on the provenance of sediments in the Parece Vela Basin of the western Pa- rocks: e.g. Dunlop and Özdemir (1997) for the Lambert- cific, Journal of Asian Earth Sciences, 196, doi:10.1016/j. ville plagioclase; Moskowitz et al. (1993) for cultured jseaes.2020.104373. magnetotactic bacteria; Till et al. (2011) for the Tiva Zhang, P., H. Ao, A. P. Roberts, Y. X. Li, Q. Sun, J. H. Zhang, Canyon Tuff; and Jackson and Swanson-Hysell (2012) P. F. Sun, and X. K. Qiang (2020a), Magnetochronology for a remagnetized limestone of the Becraft Formation of Mid-Miocene mammalian fauna in the Lanzhou Basin, (Fig. 2). Note, also, that Egli (2006) investigated the ef- northeastern Tibetan Plateau: Implications for Asian mam- fect of interactions of uniaxial SD grains to the AARM, mal migration, Geoscience Frontiers, 11(4), 1337-1344, doi:10.1016/j.gsf.2020.01.006.

Figure 2. Non-linear anhysteretic remanent magnetization (ARM) acquisition for a remagnetized limestone sample of the Becraft Formation (solid line). The

initial slope gives χARM/SIRM = 2.1 x 10-3 m/A, comparable to theoretical values for non-interacting SD magnetite (Egli, 2006; Egli & Lowrie, 2002), and to values observed for cultured magnetotactic bacteria (Moskowitz et al., 1993) and for igneous materials such as the Tiva Canyon Tuff (Till et al., 2011) and the Lambertville plagioclase (Dunlop & Özdemir, 1997, fig. 11.6). Dashed line was superimposed to the initial acquisition steps to highlight the ~linear behavior of these data below ~0.05 mT (modified from Jackson and Swanson-Hysell (2012)). 13 and determined that the measured anisotropy depends on The Substance the spatial distribution of the grains, which can modu- As McCabe et al. (1985) pointed out, susceptibility of late the directional dependence of ARM intensity. For ARM can be a particularly useful parameter, and funda- strongly interacting SD particles the AARM anisotropy mentally different from the initial or low-field AC sus- parameter is heavily dependent on their concentration ceptibility. Since it is based on a remanence, it eliminates and microcoercivity, leading to complex behavior and the contribution from the paramagnetic and diamagnetic introducing further caveats to anisotropy determinations components in the sample. Additionally, the contribu- (Egli, 2006). tions of hematite and goethite are minimized (unless Considering the broad nature of rocks studied for present as nanophases) owing to their coercivities, typi- magnetic fabrics, the linearity of ARM with DC field cally much higher than the normal range of alternating is no trivial point, and given the different coercivities fields applied, and the contribution of MD grains is re- involved it should also depend on the peak AF value duced because their ability to carry remanence is mini- and the AF window over which the bias field is applied. mal compared to their susceptibility; thus enhancing the Short of conducting ARM acquisition experiments as a contribution of SD/PSD magnetite. function of DC field for each rock type for which fab- Just as for magnetic susceptibility, the measured ARM 2 3 rics are to be measured, it may be worthwhile establish- can be mass normalized (Am /kg) resulting in χARMin m / ing a qualitative cut-off field, say 0.05 mT, at or below kg when divided by the field in A/m, or volume normal-

which the M-B relations may be considered linear for ized (A/m), and resulting in χARM in a dimensionless all intents and purposes, as shown in figure 2. For ex- quantity after normalizing by the same field. Although ample, for a dominantly paramagnetic coarse grained not formally defined, deriving anisotropies may be la- gneiss sample whose remanence is carried by a range beled as “mass AAS” and “volume AAS”, respectively. of Ti-maghemite grain sizes, and a finer-grained mafic Despite the above, and owing to the criticisms con- granulite sample whose remanence is carried by MD cerning the assumption of linearity between magneti- Ti-magnetite, Bilardello and Jackson (2014) compared zation and applied field, AAS is rarely encountered in results obtained from different types of anisotropy tech- recent literature, and most commonly anisotropy of an- niques and acquired using different DC fields. For the hysteretic remanent magnetization (AARM) or anisot- two rock-types investigated they observed that DC field- ropy of anhysteretic remanence (AAR), for short, are normalized AARM results obtained using 0.05 mT DC reported instead. However, it is not always clear what fields were significantly different from results obtained these quantities actually represent, whether they are using 0.1 and 0.2 mT bias DC fields, which were instead based off of ARMs in the strict sense of the term (mass or similar to each other (Fig. 3). The effect of choice of DC volume normalized remanences), or whether they are ad- bias field on the anisotropy determination and degree of ditionally normalized by the DC field, just as originally anisotropy in particular was also briefly discussed in Bi- proposed for AAS. For example, the IRM database and lardello (2016) and Biedermann et al. (2020). For a more software allows calculating what it reports as AARM, exhaustive discussion pertaining to anisotropy the reader but in reality, is mass-normalized AAS, with principal may also refer to Cox and Doell (1967), Daly and Zins- axes in units of m3/kg. This is not intuitive for a quantity ser (1973), Stephenson et al. (1986) or Jackson (1991). termed “magnetization”. Alternatively, if the term AAS is to be dropped for good, Strictly speaking, then, AARMs should really be

it may be best to stop normalizing by the DC field, so based off of ARMs alone, not χARM, and then can be that AARMs are truly magnetizations. The linearity ca- further subdivided into mass AARM (units of Am2/kg) veat, however, must still be maintained. and volume AARM (units of A/m). These anisotropies Specific to magnetic fabrics, a “problem/non-prob- are legitimate quantities to use, and just as the IRMQ lem” is that the actual quantities used to determine an- 28:3 article specifies, as long as the DC fields used to isotropies are very rarely reported in their absolute val- acquire the ARMs are reported, then there shouldn’t be ues (and units), and readers have to make do with what any confusion as to what’s what. In general, normalizing “label” is given to the anisotropy (AARM versus AAS). by mass makes more sense in the Earth sciences given However, granted that anisotropy principal axes are the variable densities of rocks, particularly when highly rarely reported in their absolute terms, but normalized porous or vesiculated materials are investigated (as well among each other (typically so that their sum equals 1 or as measuring mass is usually easier than volume mea- 3), then the units are irrelevant. However, it does become surements). harder to determine what has been measured, especially The only caveat for using AAS should always be that when the DC fields used are not reported. For the sake of linearity between ARMs and DC fields is maintained. completeness, and so that everyone is in the know about However, if this should not be the case, then not nor- the different ways anisotropy of remanence can be cal- malizing by the DC field is, in fact, preferable, and an culated, I thought it would be didactic to write an article AARM should be used. Bilardello and Jackson (2014), that details the types of remanence anisotropies that ex- for example, reported “AARMs” that are technically ist. I apologize to all those that will find this article “a “AASs” for anhysteretic remanences acquired using festival of the obvious”, as my high-school philosophy 0.05, 0.1, and 0.2 mT bias DC fields (39.8, 79.6, and teacher used to say. 159.2 A/m, respectively) (Fig. 3). Of these, the last two were not found to be in agreement with the first, imply- 14 ing non-linear dependence of M to B, at least for fields Coarse-grained Gneiss greater than 0.05 mT, and should probably not have been normalized by the field. The same caveat applies to mea- 0.05 mT suring high-field susceptibility and its anisotropy from hysteresis loops: the high-field slope used must define a 0.05 mT linear portion of the loop and thus purely represent the paramagnetic contribution to the sample (e.g. Bilardello, 0.05 mT 2016). The same applies to high-field susceptibility de- rived from torque measurements (e.g. Martı́n-Hernández 0.05 mT & Hirt, 2001). So what does this discussion imply for the anisot- ropy of isothermal remanence magnetization (AIRM, or AIR)? IRMs are typically stronger than ARMs and the linear regime is generally surpassed. Coe (1966) evalu- ated the validity of second order tensors for AIRM and Fine-grained Mafic Granulite determined that, except maybe for hematite samples, these are inadequate to accurately describe the anisotro- py. Magnetists, including myself, have often been some- 0.05 mT what cavalier when using such remanences for anisot- 0.05 mT ropy determinations (e.g. Bilardello, 2015; Bilardello & 0.05 mT Kodama, 2009), with the justification that if anisotropies of high coercivity minerals are to be determined, then it is merely “the best one can do”. Muss es sein. In those cases, one must seek to corroborate the validity of the results by evaluating the errors of the tensor-fits and the 0.05 mT angular uncertainties of the principal axes orientations, repeatability of the measurements, and consistencies among samples (Bilardello, 2016). A further implica- tion is that susceptibilities of IRMs should never be even Figure 3. Data from Bilardello and Jackson (2014) plotted on lower hemisphere ste- contemplated and that AIRMs should only exist in two reonets and as Ramsey diagrams showing the difference in orientation and shape of flavors, mass and volume normalized, albeit just the one AARM-AAS fabrics acquired using 0.05 mT DC bias fields compared to 0.1 and 0.2 is preferable. mT for two different samples (see text for details). mT) may be considered a generally valid threshold. Recommendation - The term AARM should be used whenever the depen- The discussion above revolved solely around the prob- dence of ARM on DC field has not been determined or is lem of potential non-linearity of ARMs with field, -ef not linear, and field-normalization is not applied. In the fectively making AARMs field dependent. The reality, absence of linearity, however, the validity of second rank however, is somewhat more complex, with ARMs also tensors may be compromised (Coe, 1966). being decay-rate dependent (Biedermann et al., 2018, - Whenever there is no linearity between magnetiza- 2019; Egli & Lowrie, 2002). Combined, these effects tions and DC fields (for ARMs or IRMs), a correlation make the tensors that are reported not unique, so that between non-linearity and tensor misfit, or non-linearity no single AARM of a sample exists, but is instead inex- and changes in the tensors resulting from different DC tricably related to the DC field and AF decay rate used. fields, may be sought. The same applies to anisotropy of magnetic susceptibil- - As already stated in Biedermann et al. (2018), all ex- ity (AMS) with respect to AC field and frequency. To- perimental parameters used should always be reported gether, these considerations highlight the importance of when investigating anisotropy. the experimental parameters used, and the need to report these along the magnetic fabrics they determine. These, Acknowledgements and other “dependencies” will be the topic of a dedicated This short article has benefited from an exchange of article by Andrea Biedermann and will be discussed at ideas with Andrea Biedermann who is coincidentally length there. preparing a manuscript for peer review on similar (and more) topics. I thank Andrea for her valuable comments Following the discussion presented above it may be le- and suggestions. I also thank Bruce Moskowitz for his gitimate to propose the following guidance: careful edits and suggestions.

- The term AAS should be used whenever the linearity of References ARMs to DC fields is maintained within the AF window Banerjee, S. K., & Mellema, J. P. (1974). A new method for the used, and the results are normalized by that field. To this determination of paleointensity from the ARM properties extent, a DC field of 0.05 mT (see figure 2) over a 100 of rocks. Earth and Planetary Science Letters, 23, 177–184. mT AF window (or any partial ARM window within 100 Biedermann, A. R., Bilardello, D., Jackson, M., Chadima, M., 15 & Feinberg, J. M. (2018). Commonly used experimental 136(1), 1–28. https://doi.org/10.1007/BF00878885 parameters for acquisition of anhysteretic remanent mag- Jackson, Mike, & Swanson-Hysell, N. L. (2012). Rock mag- netization (ARM) and its anisotropy (AARM): Results and netism of remagnetized carbonate rocks: another look. recommendations from a rock magnetic community survey. Geological Society, London, Special Publications, 371(1), The IRM Quarterly, 28(3), 1–14. 229–251. https://doi.org/10.1144/SP371.3 Biedermann, A. R., Bilardello, D., Jackson, M., Tauxe, L., & King, J., Banerjee, S. K., Marvin, J., & Özdemir, Ö. (1982). Feinberg, J. M. (2019). Grain-size-dependent remanence A comparison of different magnetic methods for determin- anisotropy and its implications for paleodirections and pa- ing the relative grain size of magnetite in natural materials: leointensities – Proposing a new approach to anisotropy Some results from lake sediments. Earth and Planetary Sci- corrections. Earth and Planetary Science Letters, 512, 111– ence Letters, 59(2), 404–419. https://doi.org/10.1016/0012- 123. https://doi.org/10.1016/j.epsl.2019.01.051 821X(82)90142-X Biedermann, A. R., Jackson, M., Bilardello, D., & Feinberg, Martı́n-Hernández, F., & Hirt, A. M. (2001). Separation of fer- J. M. (2020). Anisotropy of Full and Partial Anhysteretic rimagnetic and paramagnetic anisotropies using a high-field Remanence Across Different Rock Types: 2—Coercivity torsion magnetometer. Tectonophysics, 337(3–4), 209–221. Dependence of Remanence Anisotropy. Tectonics, 39(2), https://doi.org/10.1016/S0040-1951(01)00116-0 1–19. https://doi.org/10.1029/2018TC005285 McCabe, C., Jackson, M., & Elwood, B. B. (1985). Magnetic Bilardello, D. (2015). Isolating the anisotropy of the character- anisotropy in the Trenton limestone: results of a new tech- istic remanence-carrying hematite grains: a first multispeci- nique, Anisotropy of Anhysteretic Susceptibility. Geophys- men approach. Geophysical Journal International, 202(2), ical Research Letters, 12(6), 333–336. 695–712. https://doi.org/10.1093/gji/ggv171 Moskowitz, B. M., Frankel, R. B., & Bazylinski, D. A. (1993). Bilardello, D. (2016). Magnetic Anisotropy: Theory, Instru- Rock magnetic criteria for the detection of biogenic mag- mentation, and Techniques. Reference Module in Earth netite. Earth and Planetary Science Letters, 120(3–4), 283– Systems and Environmental Sciences. Elsevier Inc. https:// 300. https://doi.org/10.1016/0012-821X(93)90245-5 doi.org/10.1016/B978-0-12-409548-9.09516-6 Schmidbauer, E., & Veitch, R. J. (1980). Anhysteretic rema- Bilardello, D., & Jackson, M. J. (2014). A comparative study of nent magnetization of small multidomain Fe3O4 particles magnetic anisotropy measurement techniques in relation to dispersed in various concentrations in a non- magnetic rock-magnetic properties. Tectonophysics, 629(C). https:// matrix. Journal of Geophysics - Zeitschrift Fur Geophysik, doi.org/10.1016/j.tecto.2014.01.026 48(3), 148–152. Bilardello, D., & Kodama, K. P. (2009). Measuring remanence Stephenson, A., Sadikun, S., & Potter, D. K. (1986). A theo- anisotropy of hematite in red beds: Anisotropy of high-field retical and experimental comparison of the anisotropies of isothermal remanence magnetization (hf-AIR). Geophysi- magnetic-susceptibility and remanence in rocks and miner- cal Journal International, 178(3). https://doi.org/10.1111/ als. Geophysical J. Royal Astronomical Soc., 84, 185–200. j.1365-246X.2009.04231.x https://doi.org/10.1111/j.1365-246X.1986.tb04351.x Coe, R. S. (1966). Analysis of magnetic shape anisotropy Sugiura, N. (1979). ARM, TRM and magnetic interactions: using second-rank tensors. Journal of Geophysical Re- Concentration dependence. Earth and Planetary Science search, 71(10), 2637–2644. https://doi.org/10.1029/ Letters, 42(3), 451–455. https://doi.org/10.1016/0012- JZ071i010p02637 821X(79)90054-2 Cox, A., & Doell, R. R. (1967). Measurement of High-Coer- Till, J. L., Jackson, M. J., Rosenbaum, J. G., & Solheid, P. (2011). civity Magnetic Anisotropy. In D. W. COLLINSON, K. M. Magnetic properties in an ash flow tuff with continuous CREER, & S. K. B. T.-D. in S. E. G. RUNCORN (Eds.), grain size variation: A natural reference for magnetic parti- Methods in Palaeomagnetism (Vol. 3, pp. 477–482). New cle granulometry. Geochemistry, Geophysics, Geosystems, York: Elsevier. https://doi.org/10.1016/B978-1-4832-2894- 12(7), n/a-n/a. https://doi.org/10.1029/2011GC003648 5.50082-3 Daly, L., & Zinsser, H. (1973). Étude comparative des an- isotropies de susceptibilité et d’aimantation rémanente isotherme: Conséquences pour l’analyse structurale et le paléomagnétisme. Annales Geophysicae, 29, 189–200. Dankers, P. H. (1978). Magnetic properties of dispersed natural iron-oxides of known grain-size. University of Utrecht. Dunlop, D. J., & Özdemir, Ö. (1997). Rock Magnetism: Fun- RAC News damentals and Frontiers (p. 573). Cambridge: Cambridge University Press. After 4 years as chair of the IRM's Re- Dunlop, D. J., & West, G. F. (1969). An experimental evalu- ation of single domain theories. Reviews of Geophysics, view and Advisory Committee (RAC) 7(4), 709. https://doi.org/10.1029/RG007i004p00709 Mark Dekkers (Utrecht University) re- Egli, R. (2006). Theoretical considerations on the anhysteretic tires from his position. We thank Mark remanent magnetization of interacting particles with uniaxi- al anisotropy. Journal of Geophysical Research: Solid Earth, for his contribution to the IRM! 111(B12), n/a-n/a. https://doi.org/10.1029/2006JB004577 Egli, R., & Lowrie, W. (2002). Anhysteretic remanent mag- Please join us in welcoming Beatriz netization of fine magnetic particles. Journal of Geophysi- cal Research: Solid Earth, 107(B10), EPM 2-1-EPM 2-21. Ortega Guerrero (Universidad Nacio- https://doi.org/10.1029/2001JB000671 nal Autónoma de México) as new RAC Jackson, Michael. (1991). Anisotropy of magnetic remanence: A brief review of mineralogical sources, physical origins, member! and geological applications, and comparison with suscepti- bility anisotropy. Pure and Applied Geophysics PAGEOPH, 16 University of Minnesota John T. Tate Hall, Room 150 116 Church Street SE Nonprofit Org. Minneapolis, MN 55455-0149 phone: (612) 624-5274 U.S Postage e-mail: [email protected] PAID www.irm.umn.edu Twin Cities, MN Permit No. 90155

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. Short proposals would like to contribute (e.g., titles plus are accepted semi-annually in spring and abstracts of papers in press), or if you have fall for work to be done in a 10-day period any suggestions to improve the newsletter, during the following half year. Shorter, less please notify the editor: formal visits are arranged on an individual basis through the Facilities Manager. Dario Bilardello The IRM staff consists of Subir Baner- Institute for Rock Magnetism jee, Professor/Founding Director; Bruce University of Minnesota Moskowitz, Professor/Director; Joshua 150 John T Tate Hall Feinberg, Assistant Professor/Associate 116 Church Street SE Director; Maxwell Brown, Peat Sølheid Minneapolis, MN 55455-0128 and Dario Bilardello, Staff Scientists. phone: (612) 624-5049 Funding for the IRM is provided by the e-mail: [email protected] National Science Foundation, the W. M. www.irm.umn.edu

Keck Foundation, and the University of Minnesota. The U of M is committed to the policy that all people shall have equal access to its programs, facilities, and employment without regard to race, religion, color, sex, national origin, handicap, age, veteran status, or sexual orientation.

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