Robust, unbiased statistics describing long-term geomagnetic behaviour are sorely needed to elucidate the dynamics and evolution of Earth's magnetic field and the geodynamo which produces it. While palaeomagnetic data are available across much of Earth's history, their utility is hampered by highly inhomogeneous distributions in space and time and by associated uncertainties. To address this, a set of parameters based on robust statistics and describing the average strengths of the axial dipolar and non-axial dipolar components of the field, the time variability of the total field and certain ratios of these are proposed. A framework for estimating these parameters is developed whereby global data sets of palaeomagnetic directions and intensities are compared to outputs from an ensemble of numerical geodynamo simulations. A bespoke Monte-Carlo proxy-based approach allows measurement uncertainties and spatial inhomogeneity in the data to be accounted for and the framework further allows for independent validation tests to be performed. Estimates of these parameters obtained for three intervals: 0.1-1, 1-4 and 4-15 Myr ago, provide benchmarks against which field models and geodynamo simulations may be compared. Furthermore, the values obtained suggest that: (1) 0.9 Myr is an insufficient duration for fully defining the time-averaged field; (2) average axial and non-axial dipole field strengths in the interval 0.1-1 Myr were similar to 50 per cent higher than in the two preceding intervals; (3) the time-variances of the total field in the three intervals were not distinguishable. In addition to demonstrating the utility of the new framework, these findings can potentially address a longstanding question in geomagnetism: why is a polarity reversal 'overdue'?

The Ediacaran-Cambrian interval marks major environmental change and the emergence of complex life, coinciding with some of the weakest magnetic field strengths in Earth history. It has been proposed that inner core nucleation near this boundary triggered a rapid strengthening of the geodynamo during the early Cambrian, but direct constraints on magnetic field behavior across this interval remain limited. Here, we present new paleointensity results from two Cambrian igneous sequences in Brazil (~525 million years ago) and Nova Scotia (~505 million years ago). Using multiple complementary methods, both records indicate a reversing geomagnetic field that remained weak. Combined with a reanalysis of published data, our results show that weak magnetic field conditions persisted for tens of millions of years after the Cambrian radiation. These findings limit the magnitude and timing of any Cambrian geodynamo strengthening and imply that Earth's magnetospheric shielding remained strongly suppressed during a key phase of early animal evolution.

The palaeomagnetic intensity (PINT) data base documents variations in the full-vector of the ancient geomagnetic field that can be used to provide insights into the operation and evolution of the geodynamo. In this study, we report an update of PINT and the evolving behaviour of the palaeomagnetic field since 17 Ma. The update is the addition of 206 recently published site-mean data with ages between 0.06 and 2610 Ma that have been assessed using the palaeointensity quality criteria (QPI). Using this data base, we analysed, for the first time, the distribution of values of the palaeosecular variation index (PSVi) in intervals drawn from the past 17 Myr. Our results indicate that this index was enhanced prior to 5 Ma reflecting both lower average virtual dipole moments and higher angular deviations of the virtual geomagnetic pole from the geographic pole. The present Brunhes chron is highlighted as being associated with especially high measurements of dipole moment which we hypothesize may be related to its already long duration relative to most other chrons of the last 17 Myr.

Marine magnetic anomalies are pivotal to our understanding of plate tectonics, geomagnetic fields, and deep Earth dynamics. However, the question of how Ti‐rich titanomagnetites, the primary remanence carriers in oceanic crust rocks, can faithfully preserve geomagnetic field information for tens of millions of years is not well understood. Here, we combine microscopic, micromagnetic, and rock magnetic analyses, including 14‐T high‐field measurements, to show that the magnetic micro‐anisotropy of a fresh pillow lava dredged from the Juan de Fuca Ridge is dominated by internal stress throughout its chilled margin to its interior. Internal stress, which is generated primarily by the contraction of hot lava erupting into cold seawater, increased the natural remanent magnetization in this lava by a factor of ∼3. We suggest that stress‐induced magnetic domain state transformation from multidomain to single vortex (or single domain) in extrusive pillow lavas significantly enhances oceanic crustal magnetic remanence strength and stability.

Abstract. The palaeomagnetic field provides unique information about the deep Earth and allows past plate motions and true polar wander to be determined. While data are available across much of Earth’s history, the conversion of these into robust descriptions of palaeomagnetic field behaviour prior to 1 Ma has been hampered by their highly inhomogeneous distributions in space and time. To address this, a set of parameters termed spherical harmonic analogues are proposed that use robust statistics to capture global, time-averaged aspects of field behaviour. Global palaeomagnetic direction and intensity data may be combined with outputs from ensembles of numerical geodynamo simulations, using a bespoke Monte-Carlo proxy-based approach, to estimate values of these spherical harmonic analogues using data from long before 1 Ma. The new approach is applied to the intervals 0.1–1 Ma, 1–4 Ma, and 4–15 Ma which were chosen for the moderately uniform distributions of the ages of data within them. The resulting estimates account for measurement errors and spatial inhomogeneity in the data and perform well in independent validation tests. They therefore provide benchmarks against which statistical field models and geodynamo simulations may be compared. Obtained values suggest that the average axial and nonaxial dipole fields in the interval 0.1–1 Ma were ~50 % higher than long-term averages estimated for the preceding 14 million years. By contrast, the time-variance of the total field may have remained relatively constant over the entire interval providing a potential explanation for why the Bruhnes polarity chron is already one of the longest since 15 Ma.

Mantle circulation in the Earth acts to remove heat from its interior and is thus a critical driver of our planet’s internal and surface evolution. Numerical mantle circulation models (MCMs) driven by plate motion history allow us to model relevant physical and chemical processes and help answer questions related to mantle properties and circulation. Predictions from MCMs can be tested using a variety of observations. Here, we illustrate how the combination of many disparate observations leads to constraints on mantle circulation across space and time. We present this approach by first describing the set-up of the example test MCM, including the parameterization of melting, and the methodology used to obtain elastic Earth models. We subsequently describe different constraints, that either provide information about present-day mantle (e.g. seismic velocity structure and surface deflection) or its temporal evolution (e.g. geomagnetic reversal frequency, geochemical isotope ratios and temperature of upper mantle sampled by lavas). We illustrate the information that each observation provides by applying it to a single MCM. In future work, we shall apply these observational constraints to a large number of MCMs, which will allow us to address questions related to Earth-like mantle circulation.

Since dykes represent the main mechanism for magma movement from the Earth’s crust to the surface, understanding how they generate a path to feed an eruption is crucial for volcanic hazard assessment. To this purpose, key information can be obtained by studying fossil dykes in extinct and eroded volcanic systems where dykes show a variety of shapes, segmentation, and propagation paths due to a suite of pre-, syn- and post-emplacement physical processes (e.g. heat transfer, host rock layering, local stress variations).To discern the factors that control these complex geometries and reveal how they affect the dynamics of magma transport, we used a multi-method approach on a N-S trending fossil dyke from the Reyðarfjörður dyke swarm (eastern Iceland). We collected meso-scale geometric data from drone photogrammetry, and rock magnetic, petrographic and microstructural laboratory analyses were conducted on oriented rock cores and samples to reveal microscopic magma flow indicators (e.g., magnetic fabrics and crystal alignment). Rock cores were sampled both across the thickness and along the breadth of the dyke segments, also recording the core position relative to different cooling surfaces (i.e. from the dyke margin to its interior).The studied dyke is exposed for ˜900 m across its breadth in ˜300 m height. It comprises several segments showing different shapes (from straight to curved paths), thickness (spanning from 0.5 to 5 m) and linkage pattern (i.e. connected or not connected segments). The photogrammetry and geological field observations show the curved segments are more frequent in the shallower and thicker portions of the dyke, whereas the amount of offset, overlap and spacing between the segments is higher in the shallower portions of the dyke exposure. Anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM) were used to identify magnetic fabrics that may be related to magma flow in the rock cores. These results show that the magnetic data record complex magma flow dynamics spanning from sub-horizontal to subvertical along the dyke path, which is inferred for adjacent connected segments and from the dyke margin to its interior.We relate the geometrical variability of the dyke segments to the far-field stress (controlled by regional extension) versus the near- field stress (controlled by local magma overpressure), the latter being dominant in the shallower (and thicker) portions of the dyke. This generates a mixed mode fracturing during dyke propagation, reflecting its geometrical variability, that also controls a complex magma flow pattern within the dyke. Microstructure analysis is in progress and it is expected to complement magnetic fabric analysis and fieldwork in the interpretation of magma flow dynamics. Current results already show that a multimethod approach aimed at linking observations from the mesoscale to the microscale is required to better capture small scale and complex propagation paths and magma flow patterns providing more reliable insights on dyke propagation.

The timing of the formation of the Earth's inner core remains a major mystery in the history of Earth's deep interior, with estimates ranging from >1 billion years to <500 million years. Inner core nucleation is expected to be characterized in the paleomagnetic record by a drop in magnetic field strength prior to inner core nucleation, followed by a sharp increase in strength after nucleation. Paleointensity data from the 1 Ga–500 Ma age range are, however, exceptionally sparse (with fewer than a dozen studies), which largely prevents any definitive conclusions. This study presents new paleointensity results from whole rocks aged 510 ± 5 Ma from the Florida Mountains in southwestern New Mexico. Exceptionally low paleointensity estimates (<2 μT) were measured from three sites in granite and syenite outcrops of the southwestern portion of the Florida Mountains. Detailed rock magnetic and imaging investigations, including Quantum Diamond Microscopy, suggested that the two were unreliable because their remanence was unlikely a pure thermoremanent magnetization. The third site is more trustworthy and gave an estimate of 1.2 ± 0.2 μT ( N = 5) corresponding to a virtual dipole moment of , which is of a similar magnitude to the lower bound of estimates from the Ediacaran (635–541 Ma). Such a low estimate at 510 Ma appears inconsistent with recent claims that the field strengthened rapidly following inner core nucleation in the late Ediacaran. Nevertheless, the risk of this single estimate being unrepresentative of the long‐term field should be recognized alongside the urgent need for more paleointensity data spanning the interval 540–440 Ma.

The Kiaman Reversed Superchron (similar to 260-318 Ma) is the longest known period of single geomagnetic polarity in Earth history (similar to 55 million years). It is associated with anomalously low dispersion of virtual geomagnetic poles and some high estimates of Earth's dipole moment. However, many of these strong paleointensity data are of poor or unknown quality. Here we report full-vector paleomagnetic measurements from a series of mid-Kiaman (similar to 282-302 Ma) lamprophyre dykes from Orkney, Scotland. A total of 258 paleointensity experiments were performed alongside rock magnetic experiments and scanning electron microscopy. Eleven dykes produced virtual dipole moment estimates indicating that the field was weak (between 0.1 and 2.9 x 1022 Am2) at 302 Ma and only moderately stronger (between 2.7 and 7.1 x 1022 Am2) at 282 Ma. These new data challenge the paradigm of a uniquely strong field in the Kiaman superchron and are especially intriguing when considered alongside recent studies of geomagnetic field behavior during the later Cretaceous Normal Superchron (similar to 84-121 Ma). Average dipole moment may be marginally elevated and paleosecular variation moderately suppressed during the superchrons but, in other respects, the field can appear similar to that encountered during other times. The deep-Earth conditions allowing for the generation of a geomagnetic field that is capable of weak, unstable behavior and transitory polarity inversions, while nevertheless maintaining a dominant single polarity for tens of millions of years, is not yet clear. The challenge of explaining superchrons and their geodynamic origin motivates further study integrating paleomagnetic observations with predictions from geodynamo simulations.

Since dykes represent the main mechanism for magma movement from the Earth’s crust to the surface, understanding how they generate a path to feed an eruption is crucial for volcanic hazard assessment. To this purpose, key information can be obtained by studying fossil dykes in extinct and eroded volcanic systems where dykes show a variety of shapes, segmentation, and propagation paths due to a suite of pre-, syn- and post-emplacement physical processes (e.g. heat transfer, host rock layering, local stress variations). To discern the factors that control these complex geometries and reveal how they affect the dynamics of magma transport, we used a multi-method approach on a N-S trending fossil dyke from the Reyðarfjörður dyke swarm (eastern Iceland). We collected meso-scale geometric data from drone photogrammetry, and rock magnetic, petrographic and microstructural laboratory analyses were conducted on oriented rock cores and samples to reveal microscopic magma flow indicators (e.g., magnetic fabrics and crystal alignment). Rock cores were sampled both across the thickness and along the breadth of the dyke segments, also recording the core position relative to different cooling surfaces (i.e. from the dyke margin to its interior). The studied dyke is exposed for ˜900 m across its breadth in ˜300 m height. It comprises several segments showing different shapes (from straight to curved paths), thickness (spanning from 0.5 to 5 m) and linkage pattern (i.e. connected or not connected segments). The photogrammetry and geological field observations show the curved segments are more frequent in the shallower and thicker portions of the dyke, whereas the amount of offset, overlap and spacing between the segments is higher in the shallower portions of the dyke exposure. Anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM) were used to identify magnetic fabrics that may be related to magma flow in the rock cores. These results show that the magnetic data record complex magma flow dynamics spanning from sub-horizontal to subvertical along the dyke path, which is inferred for adjacent connected segments and from the dyke margin to its interior. We relate the geometrical variability of the dyke segments to the far-field stress (controlled by regional extension) versus the near- field stress (controlled by local magma overpressure), the latter being dominant in the shallower (and thicker) portions of the dyke. This generates a mixed mode fracturing during dyke propagation, reflecting its geometrical variability, that also controls a complex magma flow pattern within the dyke. Microstructure analysis is in progress and it is expected to complement magnetic fabric analysis and fieldwork in the interpretation of magma flow dynamics. Current results already show that a multimethod approach aimed at linking observations from the mesoscale to the microscale is required to better capture small scale and complex propagation paths and magma flow patterns providing more reliable insights on dyke propagation.

Reconstructions of long-term time-averaged geomagnetic field structures are important to understand geomagnetic field evolution and to identify the longevity and scale of non-dipolar field morphology. This study presents MTAM1, a non-zonal time-averaged field model for the Miocene era (5.3-23 million years ago), or indeed any time period prior to 5 million years ago. The time averaged field model for the Miocene is an inverse model based on a directional data compilation comprising 38 different localities, each with a minimum of 10 sites, called PSVM (Engbers et al., 2022a). The data were separated into normal (PSVMN) and normalised reversed (PSVMR) datasets, yielding two corresponding models MTAM1N and MTAM1R. Allowing for the opposite sign, no substantial differences were found between these two models, suggesting symmetry between the morphology of the normal and reversed fields and no evidence for non-reversing features in the geomagnetic field. Under this assumption of symmetry, the normal and reversed data can be modelled together, enhancing the data distribution and thus the robustness of the complete time-averaged field model for the Miocene. The broad structure of the models resembles previous time-averaged field models for the past 5 Myr but there are some clear differences, particularly under the South Atlantic, where all Miocene models include a reversed flux patch (RFP). To investigate whether this difference is well defined, or could result from differences in modelling methodology or data distribution, the data of the last 5 Myr were inverted with our normal model for the Miocene as a prior constraint. We find no evidence that the Miocene model is inconsistent with the field structures required by data from the past 5 Myr, suggesting an overall stability in the averaged geomagnetic field morphology for the past 23 Myr. This is consistent with long-term mantle control on geomagnetic field morphology leading to consistent deviations from the geocentric axial dipole on a multi-million-year timescale on the Core-Mantle boundary (CMB).

Plain Language Summary Nearly synchronous global changes in geomagnetic polarity give both a detailed irregular pacing to geological time and provide a glimpse into heat transfer processes across the core—mantle boundary which drives the Earth's geodynamo. Although the Late Carboniferous is characterized by some well‐studied reversals, details of the tempo of polarity changes in the Early Carboniferous are unknown. This work addresses this by providing a detailed record of polarity changes over a ∼2 million year interval at around 334.5–332.5 million years ago‐from the Trowbarrow Quarry section in NW England. We demonstrate that these limestones likely preserve magnetization from close to their time of formation and record at least 31 polarity reversals. These observations support the idea that the Earth's dynamo was in a hyperactive reversing state similar to those sustained for tens of Myr in the Late Jurassic, parts of the Cambrian and the Late Ediacaran. It further corroborates a ∼200 Myr cyclicity in paleomagnetic field behavior since the Precambrian, potentially linked to variable core heat flow forced by mantle convection.

Paleosecular variation analysis is a primary tool for characterizing ancient geomagnetic behavior and its evolution through time. This study presents a new high-quality directional dataset, paleosecular variation of the Paleogene (PSVP), with and without correction for serial correlation (SC), compiled from 1,667 sites from 45 different localities from the Paleogene and late Cretaceous (84 – 23 Ma). The dataset is used to study the variability, structure, and latitude dependence of the geomagnetic field during that period by varying selection criteria and PSV models. Modeled values for the equatorial virtual geomagnetic pole (VGP) dispersion have over-lapping uncertainty intervals within their uncertainty bounds between 8.3° and 18.6° 30 for the past 250 Ma. We investigate the suitability of two descriptive models of PSV, Model G-style quadratic fits and covariant GGP models, and find that both styles of model fail to satisfactorily reproduce the latitude dependent morphology of PSV, but suggest that estimates of the equatorial VGP dispersion may still robustly characterize aspects of Earth’s long-term field morphology. During this time where the PSV behavior has not changed substantially, the reversal frequency has varied widely. The lack of a clear relationship between PSV behavior and reversal frequency is not trivially explained in the context of published findings regarding numerical geodynamo simulations. This manuscript is the version accepted for publication in G-Cubed on 16th May, 2024.

The Paleoproterozoic era is the longest in Earth's history, with significant changes hypothesised to have occurred in the deep Earth's physical and chemical conditions at this time. It has been suggested that the paleomagnetic field became weaker at this time (-2.4 Ga) and remained weak for the next billion years. Paleomagnetism is intrinsically linked to, and is able to inform on, ancient deep Earth processes; a weak dipole strength sustained over this time period may have implications for both core and mantle evolution. We test this hypothesis here in a two-fold approach: (1) A paleointensity study on the widespread ca. 1.6 Ga diabase/dolerite Melville Bugt dyke swarm. The swarm extends along the west coast of Greenland for more than 1000 km and intruded over -13 million years, capturing polarity reversals of Earth's magnetic field. (2) A detailed statistical analysis on the long-term trend in average dipole moment from an improved paleointensity dataset (PINT.org) that has recently undergone a major update. Five of the Greenland dykes produce paleointensity results ranging from 1.4 mu T to 5.1 mu T (virtual dipole moment range 0.3-1.2 x 1022 Am2) during the mid-point of this extended period of 'dipole low'. Our statistical study robustly confirms that this one-billion-year period was indeed associated with an anomalously weak dipole moment (2.7 x 1022 Am2) relative to 500-million-year intervals before and after, which were almost twice as strong. Sampling of more geographically diverse rocks from this time is needed to yield a clear picture of the long-term time evolution of the dipole moment.

The late Asbian appears to mark the initial, well-documented, onset of far-field glacio-eustatic changes in equatorial Mississippian strata. This work unravels the nature of cyclicity in upper Asbian shallow marine carbonates, using a combination of petrographic study, rock magnetic proxies and astrochronological testing on samples from the Trowbarrow section, NW England. Rock magnetic data express the content of two types of siliciclastic sources; a marine-delivered magnetite-dominated source, and an eolian-delivered, hematite-dominated source. The eolian-sourced material generally peaked during regressive and low-stand parts of the carbonate rhythms. Astrochronologic testing methods based around the average spectral misfit and TimeOpt methodology show the magnetite abundance proxies are principally carrying the astronomically forced signal. Two likely sedimentation rate models are derived from the five better magnetic proxies using evolutive methods. In addition, a set of three likely major hiatus levels are inferred in the sedimentation rate models, based on testing possible major hiatus scenarios with TimeOpt methods, using eccentricity modulation. From these, using the three best proxies, an average astrochronologic duration for the Trowbarrow section suggests a late Asbian duration of 1976 +/- 86 kyr (1 sigma), and a basal late Asbian age of 334.48 +/- 0.35 Ma (2 sigma). Coupled atmosphere-ocean models for the late Paleozoic, suggest that lows in short eccentricity correspond to glacials, when inferred delivery of siliciclastic sediment to the carbonate ramp is generally at a maximum. The glacial and lower sea-level intervals also coincide with maximum delivery of eolian siliciclastics, likely linked to increased aridity and less vegetation cover on adjacent and distal parts of Laurentia. Around 332-333 million years ago changes in sea-level driven by changes in polar ice volume were an important control on the sedimentation patterns through time in carbonates from low paleolatitudes. Understanding the pacing of these changes has implications for timescales and paleoceanographic processes. These cyclical changes in carbonate lithology are related to changes in magnetic properties within the late Asbian (late Visean) section at Trowbarrow in NW England. The magnetic changes principally express differences in the small content of silica-based clastics, but also reflect changing Fe-oxide mineralogy between times of hematite-rich and magnetite-rich clastic input. The changes are responding to differences in eolian delivery and marine dispersal of the clastics. Eolian delivery of clastics was at its peak near sea-level lowstands, when nearby terrestrial systems were the most arid and supplying the most dust. Statistical assessments show that the magnetite abundance signal principally expresses changes in astronomical eccentricity. Primarily using the expected eccentricity pacing, two age models are constructed, which includes three levels of detectable hiatus in the section. Using these two age models and the three best magnetic proxies, the average late Asbian duration is 1976 +/- 86 kyr, with the base of the late Asbian at 334.48 +/- 0.35 Ma. Rock magnetic properties show the included siliciclastics are from hematite-rich eolian dust and marine-dispersed sources The magnetite mineral abundance proxies principally carry the primary eccentricity-driven astronomical signal An astrochronology for the late Asbian indicates its duration is 1976 +/- 86 kyr

A. Nitrogen isotope (δ 15 Ntot) and %N data from Mirror Lake N-20 and Little Bear N-09 wells B. Carbon and nitrogen isotope (δ 13 Corg and δ 15 Ntot) and %CN data from Loon Creek O-06 well C. Carbon isotope data (δ 13 Corg), %C data, and decarbonation report from Prohibition Creek composite section D. X-ray diffraction data from Prohibition Creek composite section E. Elemental data from Prohibition Creek composite section F. HAWK pyrolysis-combustion data from Prohibition Creek composite section G. HAWK dynamic pyrolysis reports and pyrograms from Prohibition Creek composite section H. Bulk magnetic susceptibility data from Prohibition Creek composite section I. Prohibition Creek composite section and cross

SUMMARY Precambrian palaeointensity measurements provide fundamental constraints on the evolution of the deep Earth. Core evolution models predict trends in dipole moment on billion-year timescales that can be tested by palaeomagnetic records. Here, we report new palaeointensity results from the recently identified ∼2.62 Ga Yandinilling dyke swarm of the Yilgarn Craton, Western Australia, and consider them alongside published measurements spanning 500 Myr across the late Archaean to earliest Proterozoic. Rock magnetic and scanning electron microscopy analysis confirm that the magnetic mineralogy is fine-grained magnetite, appearing mostly as exsolved lamellae with ilmenite. Six sites produced acceptable palaeointensity estimates from thermal and microwave IZZI protocol Thellier experiments and from double-heating technique Shaw experiments. These site mean values of 9–26 µT translate to virtual dipole moments of 11–44 ZAm2 that are considerably lower than today's dipole moment of ∼80 ZAm2 and the value predicted for this time period by some thermal evolution models. Their average (median = 41 ZAm2) is, however, similar to the long-term average during both of the intervals 2300–2800 Ma (median = 44 ZAm2; N = 103) and 10–500 Ma (median 41 ZAm2; N = 997). While there is little evidence for a substantial net change in average dipole moment between the late Archaean and Phanerozoic, there is preliminary evidence that its variance has increased between the two intervals. This lower variance more than two billion years ago supports the idea that the geodynamo, even while not producing a stronger magnetic field, was more stable on average at the Archaean–Proterozoic transition than it is today.

Dykes form key pathways for the transport and emplacement of magma within the crust. We have identified syn- and post-emplacement processes recorded across a similar to 2 m thick basaltic dyke on the Isle of Skye, Scotland. We measured the rock magnetic properties, anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic magnetisation (AARM) across two dyke-thickness profiles spaced 13 m apart along the dyke strike (sites G5 and G6). At 20-25 cm intervals, our samples are very closely spaced compared to standard sampling protocols. Our results show that the dyke's magnetic fabrics originate from two mineral groups: titanomagnetite, which is abundant in the central dyke region at site G5, and iron sulphides (pyrite and pyrrhotite) that dominates the margin regions at both sites. The titanomagnetite occurs in unaltered dyke rock; its magnetic fabrics are primary, having formed during magma solidification, and record lateral magma flow. The pyrrhotite occurs in jointed and hydrothermally altered dyke rock; its petrological and magnetic fabrics are secondary, having originated from a sulphide-rich fluid which infiltrated cooling joints oriented perpendicular to the dyke margins and locally modified the primary magnetic fabrics. At site G6, pyrrhotite also occurs in the dyke centre, suggesting that locally the post-emplacement sulphide-rich fluid permeated into this region; this site is located close to a branch in the dyke and has increased joint frequency, which could explain the enhanced alteration. The presence of syn- (primary) and post-emplacement (secondary) fabrics was only identified due to our high frequency sampling regime and use of both AMS and AARM techniques. We highlight that future magnetic anisotropy studies of dykes may benefit from high sample frequency combined with sampling along-strike and across-thickness. Using both AMS and AARM techniques to detect more variations in magnetic fabrics can reveal more complete syn- and post-emplacement dyke histories. (C) 2022 Elsevier B.V. All rights reserved.