Illustration of the dynamo mechanism that generates the Earth's magnetic field: convection currents of fluid metal in the Earth's outer core, driven by heat flow from the inner core, organized into rolls by the Coriolis force, generate circulating electric currents, which supports the magnetic field.
The problem with trying to sustain a belief that Earth is just a few thousand years old, is that scientists keep finding evidence that it is very much older. Here, for example, is a recent paper by Oxford University and Massachusetts Institute of Technology scientists that shows not only the Earth had a magnetic field 3.5 billion years ago, but that it was not hugely dissimilar in strength to the present magnetic field.
To add insult to injury, Earth's magnetic field, and the way it deflects harmful solar radiation away from the surface if often cited by creationists as evidence for their absurd claim that Earth is fine tuned for life, so a god must have fine-tuned it. If that were true, it was fine tuned for life almost 3.5 billion years before creationists believe there was any, and before there was even a universe, let alone solar radiation and Earth.
But then what did the primitive Bronze Age hill farmers who described the universe as a small, flat planet with a dome over it centred on the Middle East in the Bible, know about solar radiation, magnetic fields or the history of the planet? Well, nothing at all, obviously!
Describe the U-Pb dating of apatite, please. U-Pb dating of apatite is a technique used in geochronology to determine the age of rocks and minerals, particularly in igneous and metamorphic contexts. Apatite is a common mineral found in a variety of rock types, including igneous, metamorphic, and sedimentary rocks. It contains uranium (U) and can incorporate lead (Pb) through radioactive decay processes.The evidence for the strength of the magnetic field 3.5 billion years ago comes in the form of magnetic particles of iron in rocks from Greenland, reliably dated using Uranium-Lead dating of apatite within the rocks. How they measured this is the subject of an open access paper in the Journal of Geophysical Research: Solid Earth and of an Oxford University news release:
Here's how the U-Pb dating of apatite typically works:
U-Pb dating of apatite is particularly useful in constraining the timing of geological events in igneous and metamorphic rocks, where other dating methods may be less applicable. It provides valuable insights into Earth's history and the evolution of geological processes over time.
- Radioactive Decay: Uranium (U) naturally occurs in several isotopic forms, including U-238 and U-235. Both isotopes decay through a series of radioactive decay steps, ultimately ending up as stable lead isotopes (Pb-206 and Pb-207, respectively). The decay of uranium to lead occurs at a known rate, making it suitable for dating purposes.
- Incorporation of Uranium and Lead in Apatite: Apatite crystals can incorporate both uranium and lead. During its formation, apatite may contain negligible amounts of lead, but over time, uranium decay will produce lead within the crystal lattice.
- Isotope Analysis: To determine the age of a sample, scientists extract apatite crystals from the rock and analyze their uranium and lead isotopic composition using techniques such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) or secondary ion mass spectrometry (SIMS). These techniques allow precise measurements of uranium and lead isotopes within the apatite crystals.
- Calculating Age: By measuring the concentrations of uranium and lead isotopes in the apatite crystals, scientists can calculate the age of the sample using radioactive decay equations. The ratio of uranium isotopes (U-238 and U-235) to their respective lead isotopes (Pb-206 and Pb-207) provides the basis for age determination.
- Interpretation: The calculated age provides information about the timing of crystallization or metamorphism of the rock containing the apatite crystals. This data contributes to understanding geological processes such as the timing of magma emplacement, metamorphic events, or the cooling history of rocks.
How can geochronologists be sure the starting level of Pb is negligible?
Ensuring that the initial level of lead (Pb) in apatite is negligible or accurately known is crucial for reliable U-Pb dating results. Geochronologists employ several methods and considerations to assess and account for the initial lead content:
By employing these methods and considerations, geochronologists can confidently assess and account for the initial level of lead in apatite crystals, ensuring the accuracy and reliability of U-Pb dating results.
- Mineral Selection: Geochronologists often choose apatite crystals that are relatively pristine and have experienced minimal alteration or contamination. Apatite crystals from fresh igneous or metamorphic rocks are preferred over those from sedimentary environments, where lead can be introduced through secondary processes.
- Petrographic Examination: Detailed petrographic examination of the sample can help identify any potential sources of lead contamination, such as fractures or inclusions containing lead-bearing minerals. Apatite crystals showing signs of alteration or secondary mineral growth may be avoided or carefully screened.
- Chemical Analysis: Before dating, geochronologists may conduct chemical analyses to determine the concentration of lead and other trace elements in the apatite crystals. This analysis can help assess the extent of lead contamination and provide insights into the crystal's history of alteration or incorporation of lead from external sources.
- Isotopic Analysis: Isotopic analysis, particularly of lead isotopes, can help distinguish between radiogenic lead produced by uranium decay and non-radiogenic lead that may have been present initially or introduced through contamination. By measuring the isotopic ratios of lead in apatite crystals, geochronologists can assess the contribution of radiogenic lead and correct for any initial non-radiogenic lead.
- Modeling and Correction: Geochronologists often employ mathematical models to account for the presence of initial lead and its potential impact on age calculations. By incorporating data from isotopic analyses and petrographic observations, researchers can develop models that estimate the initial lead content and correct age calculations accordingly.
- Quality Control: Rigorous quality control measures, including the analysis of reference materials with known ages and lead isotopic compositions, help validate the accuracy and reliability of U-Pb dating results. Consistency between measured ages of reference materials and their known ages provides confidence in the dating technique.
A new study, led by the University of Oxford and MIT, has recovered a 3.7-billion-year-old record of Earth’s magnetic field, and found that it appears remarkably similar to the field surrounding Earth today. The findings have been published today in the Journal of Geophysical Research.
Without its magnetic field, life on Earth would not be possible since this shields us from harmful cosmic radiation and charged particles emitted by the Sun (the ‘solar wind’). But up to now, there has been no reliable date for when the modern magnetic field was first established.
Claire Nichols (left) with co-author Ben Weiss (right) collecting samples for paleomagnetic analysis using a rock-coring drill.Image credit: Claire Nichols.
In the new study, the researchers examined an ancient sequence of iron-containing rocks from Isua, Greenland. Iron particles effectively act as tiny magnets that can record both magnetic field strength and direction when the process of crystallization locks them in place. The researchers found that rocks dating from 3.7 billion years ago captured a magnetic field strength of at least 15 microteslas comparable to the modern magnetic field (30 microteslas).
These results provide the oldest estimate of the strength of Earth’s magnetic field derived from whole rock samples, which provide a more accurate and reliable assessment than previous studies which used individual crystals.
Extracting reliable records from rocks this old is extremely challenging, and it was really exciting to see primary magnetic signals begin to emerge when we analysed these samples in the lab. This is a really important step forward as we try and determine the role of the ancient magnetic field when life on Earth was first emerging.
Professor Claire Nichols, lead author
Department of Earth Sciences
University of Oxford, Oxford, UK
Whilst the magnetic field strength appears to have remained relatively constant, the solar wind is known to have been significantly stronger in the past. This suggests that the protection of Earth’s surface from the solar wind has increased over time, which may have allowed life to move onto the continents and leave the protection of the oceans.
Earth's magnetic field is generated by mixing of the molten iron in the fluid outer core, driven by buoyancy forces as the inner core solidifies, which create a dynamo. During Earth’s early formation, the solid inner core had not yet formed, leaving open questions about how the early magnetic field was sustained. These new results suggest the mechanism driving Earth’s early dynamo was similarly efficient to the solidification process that generates Earth’s magnetic field today.
An example of the 3.7 billion year old banded iron formation found in the north-eastern part of the Isua Supracrustal Belt.Image credit: Claire Nichols
Understanding how Earth’s magnetic field strength has varied over time is also key for determining when Earth’s inner, solid core began to form. This will help us to understand how rapidly heat is escaping from Earth’s deep interior, which is key for understanding processes such as plate tectonics.
A significant challenge in reconstructing Earth’s magnetic field so far back in time is that any event which heats the rock can alter preserved signals. Rocks in the Earth’s crust often have long and complex geological histories which erase previous magnetic field information. However, the Isua Supracrustal Belt has a unique geology, sitting on top of thick continental crust which protects it from extensive tectonic activity and deformation. This allowed the researchers to build a clear body of evidence supporting the existence of the magnetic field 3.7 billion years ago.
Northern Isua has the oldest known well-preserved rocks on Earth. Not only have they not been significantly heated since 3.7 billion years ago but they have also been scraped clean by the Greenland ice sheet.
Professor Benjamin Weiss, co-author
Massachusetts Institute of Technology, MA, USA
The results may also provide new insights into the role of our magnetic field in shaping the development of Earth’s atmosphere as we know it, particularly regarding atmospheric escape of gases. A currently unexplained phenomenon is the loss of the unreactive gas xenon from our atmosphere more than 2.5 billion years ago. Xenon is relatively heavy and therefore unlikely to have simply drifted out of our atmosphere. Recently, scientists have begun to investigate the possibility that charged xenon particles were removed from the atmosphere by the magnetic field.
Whole rock drill core samples from Isua, which were measured in the MIT Paleomagentism Laboratory to extract their ancient record of Earth's magnetic field.Image credit: Claire Nichols.
In the future, researchers hope to expand our knowledge of Earth’s magnetic field prior to the rise of oxygen in Earth’s atmosphere around 2.5 billion years ago by examining other ancient rock sequences in Canada, Australia, and South Africa. A better understanding of the ancient strength and variability of Earth’s magnetic field will help us to determine whether planetary magnetic fields are critical for hosting life on a planetary surface and their role in atmospheric evolution.
The study ‘Possible Eoarchean records of the geomagnetic field preserved in the Isua Supracrustal Belt, southern west Greenland’ has been published in the Journal of Geophysical Research: Solid Earth.
Technical details and background to the research are given in the open access paper in JGR Solid Earth:
AbstractAlthough nearby cultures were navigating in boats and using 'loadstones' to find magnetic north and pinpoint the North Star, which they had even named Polaris after one of their gods, the authors of Genesis seem to have been completely unaware of such things. Indeed, how they expected people to travel in boats on a flat earth is anyone's guess. Even the story of the mythical Ark doesn't include any means of navigation or orientation (which begs the question, why do frauds like Ken Ham depict it with a keel and a prow if it had no means of propulsion or steerage?)
Recovering ancient records of Earth's magnetic field is essential for determining the role of the magnetosphere in protecting early Earth from cosmic radiation and atmospheric escape. We present paleomagnetic field tests hinting that a record of Earth's 3.7-billion-year (Ga) old magnetic field may be preserved in the northeastern Isua Supracrustal Belt as a chemical remanent magnetization acquired during amphibolite-grade metamorphism in the banded iron formation. Multiple petrological and geochronological lines of evidence indicate that the northernmost part of Isua has not experienced metamorphic temperatures exceeding 380°C since the Eoarchean, suggesting the rocks have not been significantly heated since magnetization was acquired. We use “pseudo” baked contact tests (intrusions emplaced 3.26–3.5 Ga ago) and a fold test (folding 3.6 Ga ago) to demonstrate that some samples preserve a ca. 3.7 Ga record of the magnetic field. We recover a field strength of >15 µT. This suggests that Earth's magnetic field may have been weak enough to enhance atmospheric escape during the Archean.
Key Points
Plain Language Summary
- The north-eastern part of the Isua Supracrustal Belt experienced two metamorphic events at 3.69 Ga and 2.85 Ga and one hydrothermal event at 1.5 Ga
- Banded iron formations acquired a chemical remanent magnetization during the first thermal event that was not entirely overprinted by subsequent events
- Paleomagnetic results hint that a record of the Eoarchean geomagnetic field is preserved in the Isua Supracrustal Belt
Recovering ancient records of Earth's magnetic field is challenging because the magnetization in rocks is often reset by heating during tectonic burial over their long and complex geological histories. We show that rocks from the Isua Supracrustal Belt in West Greenland have experienced three thermal events throughout their geological history. The first event was the most significant, and heated the rocks up to 550°C 3.7-billion-years-ago. The subsequent two events did not heat the rocks in the northernmost part of the area above 380°C. We use multiple lines of evidence to test this claim, including paleomagnetic field tests, the metamorphic mineral assemblages across the area, and the temperatures at which radiometric ages of the observed mineral populations are reset. We use these lines of evidence to argue that an ancient, 3.7 billion year old record of Earth's magnetic field may be preserved in the banded iron formations in the northernmost part of the field area. The magnetization was acquired during mineral transformation associated with the first thermal event and therefore only a lower limit on the strength of the ancient magnetic field was constrained. However, we are able to conclude that the ancient magnetic field was likely comparable with the strength of Earth's magnetic field today.
1 Introduction
Recovering a reliable record of geodynamo strength throughout Earth history is key to understanding the role of magnetic fields in planetary habitability, the thermal evolution of early Earth, and the power sources required to sustain planetary dynamos for billions of years. The geodynamo is currently driven by thermochemical convection in the liquid outer core, although there remains some debate how such a field was sustained for billions of years prior to the onset of core solidification (Landeau et al., 2022; Olson, 2013). The operation of the geodynamo depends upon the heat flow across the core-mantle boundary, which is a function of both the amount of heat removed by mantle convection and the supply of heat from the core. Recent studies have inferred a gradual decrease in the strength of the dynamo from the Archean until ca. 0.5 Ga based on the existing paleointensity record, which is sparse throughout this time period. These studies propose the inner core began to solidify at the end of this decline (Bono et al., 2019; Davies et al., 2022.1; Zhou et al., 2022.2). A relatively young inner core suggests a high core thermal conductivity and a high conductive heat flow of ∼15 TW (Landeau et al., 2022). In order for thermal convection to sustain the dynamo prior to inner core nucleation would require the total heat flow out of the core to exceed 15 TW. This heat flow is approaching the upper limit of present day estimates, and would result in an increased number of mantle plumes and a higher degree of mantle melting in Earth's early history suggesting a higher heat flux into the mantle than today. However, petrological observations of both komatiites and cratonic lithosphere suggest secular cooling of Earth with a constant Urey ratio (e.g., constant ratio of radiogenic heating to surface heat flow), indicating the ratio of the heat flux into and out of the mantle has not decreased with time (Herzberg et al., 2010; Lay et al., 2008; Pearson et al., 2021). To reconcile the current paradox, trends in magnetic field strength need to be resolved with greater precision to confirm the age of the inner core. In addition, further work is needed to investigate different dynamo mechanisms and core heat flow requirements for the early geomagnetic field, and whether a decline in, or constant magnetic field strength is expected prior to core solidification (Davies et al., 2022.1; Landeau et al., 2017).
The preservation of a temperate climate and liquid water on early Earth depends critically upon the strength of the magnetosphere (Sterenborg et al., 2011; Tarduno et al., 2014). Recent atmospheric escape models have suggested that both weak (<10 μT) and strong (>1 mT) magnetic fields could substantially enhance atmospheric escape under present-day solar wind conditions via the polar wind or cusp escape, respectively (Gronoff et al., 2020; Gunell et al., 2018; Lundin et al., 2007). During the Archean, the Sun was rotating faster, generating a stronger stellar dynamo and therefore the solar wind was more intense than today (Vidotto, 2021.1). An increased solar wind strength causes greater interaction with the upper atmosphere and greater escape of ions assuming a constant level of protection from Earth's magnetosphere. Previous magnetohydrodynamic simulations have suggested that if Earth's magnetic field was half its present day strength 3.5 Ga ago, the area of the polar cap (the area containing open dipolar magnetic field lines, allowing atmospheric escape via the polar wind) could increase by up to 50% (Sterenborg et al., 2011). In order to quantify the extent to which atmospheric escape in the Archean was driven by the solar wind, we therefore need robust observations of Earth's magnetic field strength during this time period.
The evolution of Earth's atmosphere played a pivotal role in developing life as we know it; initially the atmosphere was depleted in oxygen (i.e., reduced), creating conditions favorable for the origins of life (Catling & Zahnle, 2020.1; Sasselov et al., 2020.2). However, complex life was able to develop following the Great Oxidation Event (GOE) ca. 2.5 Ga, and this may at least in part have been driven by hydrogen loss (Catling, 2013.1; Zahnle et al., 2013.2). There is evidence for loss of ionized xenon and hydrogen throughout the Archean (Avice et al., 2018.1; Zahnle et al., 2013.2, 2019.1), and the loss of ionized species is inherently linked to the magnetosphere (Gronoff et al., 2020). In order to assess the extent to which hydrogen and xenon loss were mediated by Earth's magnetic field, we must recover accurate records of its strength prior to 2.5 Ga ago. The maximum amount of hydrogen and xenon escape via the polar wind can be determined from the lower limit on magnetic field strength. Therefore, recovering these limits for the early magnetic field will allow us to determine the relative importance of this escape mechanism for the evolution of Earth's atmosphere prior to the GOE. Atmospheric escape models can rely upon paleomagnetic observations to assess the size of the polar cap under increasing solar wind strengths, and therefore the role of Earth's magnetic field in mitigating or enhancing past atmospheric escape.
Extending the paleomagnetic record back through time becomes increasingly challenging, as ancient rocks have inevitably experienced multiple metamorphic and metasomatic events during their lengthy geological histories. Modern paleointensity studies often focus on fresh lava flows, where the lava acquires a thermal remanent magnetization (TRM) during cooling and crystallization (Valet, 2003). The age of magnetization therefore corresponds to the crystallization age of the lava and a paleointensity can be reliably recovered using thermal demagnetization (Dunlop & Ozdemir, 1993). However, when considering rocks that have undergone metamorphism or metasomatism, rocks will acquire a thermochemical remanent magnetization (TCRM) during mineralogical transformations associated with these events. A TCRM introduces uncertainty in both the timing of magnetization, since the magnetization post-dates the formation age of the rock and, if unrecognized may lead to underestimates of the strength of the recovered magnetization, given that a TCRM has a lower remanence susceptibility than a TRM (Stokking & Tauxe, 1987, 1990).
The previous oldest whole-rock paleomagnetic studies were conducted on rocks from the 3.5 Ga Barberton Greenstone Belt in South Africa and the Duffer Formation, Australia (Biggin et al., 2011.1; Herrero-Bervera et al., 2016; Tarduno et al., 2010.1). A paleointensity of 6.4 μT was recovered from the Duffer Formation (Herrero-Bervera et al., 2016), although it remains unresolved whether this value represents a genuinely weak geomagnetic field acquired as a TRM, or a lower estimate on the true paleointensity acquired as a TCRM. Paleomagnetic studies on 4.2–3.2 Ga old single zircon crystals have argued for evidence of an active geodynamo during the Archean and Hadean with a similar field strength to today (Tarduno et al., 2015, 2020.3, 2023). However, other studies have demonstrated that the magnetic carriers in these zircons are secondary in origin and the magnetization is likely an overprint that post-dates the formation of the zircons by billions of years (F. Tang et al., 2019.2; Weiss et al., 2015.1, 2018.2; Borlina et al., 2020.4; Taylor et al., 2023.1). An additional limitation of these single-crystal paleomagnetic studies is that no directional information is preserved (the zircons are detrital). In contrast, for whole-rock samples, the age of magnetization can be assessed using paleomagnetic field tests. The aim of this study is to extend the ancient whole-rock paleomagnetic record beyond 3.5 Ga.
Here, we begin the effort to extend the whole-rock paleomagnetic record to 3.7 Ga ago by recovering natural remanent magnetizations (NRMs) from banded iron formations (BIFs) in the Isua Supracrustal Belt (ISB), southwest Greenland. We compile existing geochronological and petrological observations to determine a thermal history for the ISB and identify three metamorphic and hydrothermal episodes that may have remagnetized these rocks. We present results from paleomagnetic field tests that allow us to verify whether magnetization pre- or post-dates the emplacement of igneous intrusions and folding. We also discuss how the thermal history of the area can be constrained from Pb-Pb isochrons for magnetite, and U-Pb ages for apatite in the BIF.
We argue that the magnetite in Isua should carry a TCRM formed during amphibolite-grade metamorphism ca. 3.7 Ga (Frei et al., 1999; Dymek, 1988; Nutman et al., 2022.3). The BIF has a whole-rock Rb-Sr age of 3.7 ± 0.14 Ga (Moorbath et al., 1973) and a Pb-Pb apatite age of 3.9 ± 0.2 Ga (Nishizawa et al., 2005). However, it is now commonly accepted that magnetite in BIF is not a primary phase formed directly via precipitation from the water column. Instead, the majority of magnetite in BIFs is now considered to be the product of metamorphism and diagenesis of precursor ferro-ferric oxides and hydroxides (Rasmussen & Muhling, 2018.3; Konhauser et al., 2017.1; Nutman, 2017.2). The magnetite may have grown via direct crystallization, acquiring a CRM by grain growth through a blocking volume (Kobayashi, 1959; Stokking & Tauxe, 1987, 1990). Alternatively, magnetite may have replaced existing phases in the BIF, acquiring a phase-transformation CRM. How this type of CRM becomes magnetized is poorly understood, and in this case is further hampered by the ongoing debate regarding the primary mineralogy in BIFs, which could include green rust, ferrihydrite and greenalite (Halevy et al., 2017.3; J. E. Johnson et al., 2018.4; Nutman, 2017.2; Tosca et al., 2016.1).
Our results tentatively suggest that the BIF in the northernmost part of the northeastern ISB has escaped metamorphic events exceeding 400°C since 3.7 Ga, and therefore the high temperature component of magnetization preserves a record of the Eoarchean geomagnetic field. We highlight the importance of combining detailed field observations with petrological and geochronological analyses and paleomagnetic field tests; without this context the recovered paleodirections are ambiguous and cannot be reliably interpreted. The approach presented here can now be applied to other Archean terranes in order to build up a robust picture of Earth's earliest geomagnetic field record.
1.1 U-Pb Ages of Magnetite and Apatite as Thermochronometers
The Isua BIF contains abundant magnetite and apatite, for which U-Pb thermochronometry can be used to estimate metamorphic temperatures. U-Pb dating of apatite is a well-established method, and the Isua BIF has previously been dated using this approach (Nishizawa et al., 2005). Magnetite can also contain low but measurable concentrations of U (0.2–0.4 ppm) and Pb (0.2–0.7 ppm) with radiogenic Pb representing ∼2% of total Pb (Gelcich et al., 2005.1). The low amount of U and radiogenic Pb makes it challenging to directly recover a U-Pb isochron from magnetite. However, stepwise leaching allowed both uranogenic and thorogenic arrays to be successfully recovered and a Pb-Pb isochron to be calculated (Frei et al., 1999).
The apatite observed in the Isua BIF is considered to be associated with early hydrothermal events ca. 3.63 Ga (Frei et al., 1999). Three previous studies have investigated the potential of Pb-Pb dating for magnetite in BIFs (Erel et al., 1997; Frei et al., 1999; Frei & Polat, 2007.1). These studies were carried out on the Isua BIF and the Brockman Iron Formation from the Hamersley basin, western Australia. Studies on the Isua BIF recovered Pb-Pb isochron ages of 3.691 ± 0.049 Ga and 3.691 ± 0.022 Ga (Frei et al., 1999; Frei & Polat, 2007.1). The Pb-Pb age of the magnetite has not been perturbed or reset since this early metamorphic event and no additional magnetite nucleation or growth since this time. However, previous studies were unable to interpret these ages in terms of the subsequent thermal history of the area, since the Pb diffusion rate in magnetite was unconstrained.
Based on Pb diffusion measurements for apatite and magnetite (Cherniak et al., 1991; E. B. Watson et al., 2023.2), assuming a maximum cooling rate of 100°C Ma−1 following peak metamorphic conditions (Figures S1 and S2 in Supporting Information S1), and a maximum grain size of 27 and 100 μm for magnetite and apatite in the Isua BIF, respectively (Figures S3 and S4 in Supporting Information S1), we estimated the peak closure temperature for each system. Pb-Pb isochrons for magnetite will be reset by heating to >380°C, and U-Pb ages in apatite will be reset by heating to >530°C. Since both of these temperatures are below the Curie temperature for magnetite (580°C), these serve as useful reference points for assessing which thermal events may have remagnetized the Isua BIF.
1.2 Geologic Setting
The northeastern part of the ISB is subdivided into three main terranes separated by faults (Figure 1). The 3.7 Ga northern terrane, the focus of this study, is sandwiched between a northwest terrane and the 3.8 Ga southwest terrane (Nutman & Friend, 2009). The southern part of the northern terrane is dominated by metamorphosed boninites, interspersed with dolomites, conglomerates, and basalts. Further north in the field area, magnetite-bearing cherts begin to dominate, and the northernmost extent of the area is almost exclusively made up of BIF. These 3.7 Ga sediments and volcanics were intruded by dykes that are assumed to be part of the Ameralik dyke swarm (based on their composition, distribution and thickness; see Discussion for further details) emplaced 3.26–3.5 Ga ago (ages constrained by U-Pb zircon dating) across much of the Nuuk district of southwest Greenland (Nutman et al., 2004; Nutman & Friend, 2009). The final major intrusive event in the area was the emplacement of a large (>100 m wide) noritic dyke, which trends north-south across the northeast part of the ISB cross-cutting all the major lithologies (Nutman & Friend, 2009). Zircons from the dyke have a U-Pb age of 2.214 ± 0.010 Ga (Nutman et al., 1995).
The Isua BIF, the main focus of this study, has a simple mineralogy comprised of alternating bands of quartz and magnetite with minor amphibole at the boundary between the two phases. The northernmost part of the northeast ISB (north of 65°11′N) is exceptionally well preserved, with localized regions of low-strain where pillow structures and original sedimentary features are still observable (Nutman & Friend, 2009). We outline several lines of evidence below that suggest this part of the belt (Figure S5 in Supporting Information S1) only experienced one significant (>380°C) metamorphic event ca. 3.69 Ga.
Figure 1
A simplified geological map [after Nutman and Friend (2009)] depicts the northeastern part of the ISB. The two smaller maps show the entire extent of the ISB and its location in Greenland. Previous tectonothermal constraints on the metamorphic history of the ISB are shown by the colored symbols, where pink, blue and yellow colors represent evidence for Eoarchean metamorphism, Neoarchean metamorphism and Proterozoic hydrothermal activity, respectively. Petrological constraints from metabasites (squares) and metapelites (circles) and the inferred metamorphic boundaries (gray lines; Arai et al. (2014.1); Komiya et al. (2002)); garnet-biotite thermometry (5-point stars; Rollinson (2002.1, 2003.1)); Sm-Nd pillow basalt ages (upwards pointing triangle; Polat et al. (2003.2)); Pb-Pb apatite ages (pentagon; Nishizawa et al. (2005)); Sm-Nd plagioclase amphibole ages (7-point star; Gruau et al. (1996)); and Pb-Pb magnetite BIF ages (downwards pointing triangle; Frei et al. (1999); Frei and Polat (2007.1)). The sites where paleomagnetic data were collected as part of this study are labeled 8A/A, B, C, D, 3AA, 4A, 5A and 6A.
The northern terrane has experienced two metamorphic events during the Eoarchean and Neoarchean, and a hydrothermal event during the Proterozoic, evidence for each event is summarized in Table 1. The temperature and timing of the events are summarized in Figure 2. The Eoarchean metamorphic event was upper-greenschist to amphibolite grade, resulting in the formation of a single generation of garnets (Rollinson, 2003.1) and the growth of grunerite and magnetite in the BIF at 3.69 Ga (Dymek, 1988; Frei et al., 1999). Garnet-biotite thermometry indicates a peak temperature of 470–550°C (Rollinson, 2002.1). This metamorphic event was likely the result of the collision between the 3.7 Ga northern terrane and the 3.8 Ga southern terrane at 3.69–3.66 Ga based on zircon U-Pb ages (Nutman & Friend, 2009).
A Neoarchean metamorphic event followed the juxtaposition of the Isukasia and Kapisilik terranes occurred ca. 2.98–2.95 Ga (Nutman et al., 2015.2; Frei et al., 1999; Gruau et al., 1996; Polat et al., 2003.2). The metamorphic grade increases from north to south toward the mylonitized region between the two terranes, which lies >20 km south of the ISB. The southernmost part of the ISB experienced temperatures of 500–600°C (Gruau et al., 1996). However, peak metamorphic temperatures in the northernmost region remained below 380°C, since neither the BIF apatite or magnetite Pb-Pb ages were reset during this time period (Nishizawa et al. (2005); Frei et al. (1999)). The Ameralik dykes were metamorphosed in this event, with their doleritic assemblage being transformed to actinolite, chlorite, epidote and magnetite-bearing assemblage, indicating lower greenschist grade metamorphism (360–400°C; Komiya et al. (2004.1); Arai et al. (2014.1)).
A subsequent thermal perturbation at 1.5–1.6 Ga is not observed in most of the metamorphic assemblages across the area, with the 2.2 Ga norite dyke retaining its primary igneous mineralogy (Nutman et al., 2022.3). The only evidence for this later event is in a partial resetting of the Pb-Pb apatite age in the BIF, and the complete resetting of the Rb-Sr age in the pillow basalts (Nishizawa et al., 2005; Polat et al., 2003.2). Since neither system has undergone complete homogenization and resetting, this event is interpreted to have been a low temperature (∼320°C) overprint and not sufficient to produce new mineral growths or reaction rims on the existing metamorphic mineral assemblages.
But of course, the authors weren't describing a boat; they were describing a seal box (an ark is a box hence the 'Ark of the Covenant', which wasn't a boat either).
But that's bye the bye: what's significant here is not that Earth has a magnetic field of which the Bible's authors were unaware, but that the scientists have found evidence of it in rocks reliably dated to 3.5 billion years before those ignorant authors believed the Universe was created.
The other little embarrassment for creationists here is that what they like to claim is evidence that their god created a planet 'fine-tuned for life' in that the magnetic field deflects harmful solar radiation, has been shown to be present 3.5 billion years before there was life to be fine-tuned for, according to creationist superstition.
Overall, then, just another comprehensive refutation of creationism, for creationist frauds to lie about and their dupes to ignore.
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