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Experimental constraints on magnetic stability of chondrules and the paleomagnetic significance of dusty olivine
Abstract
Dynamic crystallization experiments are conducted under a magnetic field to determine both magnetic and mineralogical properties of chondrules. The experiment reproduced synthetic dusty olivine samples that were formed by a high temperature reduction of an initially fayalitic olivine. Backscattered-electron microscopy observations confirmed that synthetic dusty olivine contains abundant fine, submicron-sized Ni-poor Fe inclusions in the cores of MgO-rich olivine grains, similar to that in natural chondrules. Alternating field demagnetization experiments of dusty olivine samples indicate mean destructive fields of up to 80 mT, suggesting the submicron-sized Fe inclusions are a carrier of stable remanence. In natural chondrules, fine Fe inclusions in the dusty olivine may have been armored against chemical alteration by surrounding host olivine crystals. Since the fine Fe inclusions were probably heated above the Curie temperature during the last chondrule forming events, the fine Fe inclusions in dusty olivine can acquire thermal remanent magnetization during the chondrule formation event. Theoretical time–temperature relation of such fine-grained Fe (kamacite) grains suggested that a paleomagnetic data observed above 490 °C in thermal demagnetization experiments of dusty olivines is reliable despite the low-grade metamorphism of unequilibrated ordinary chondrites (e.g., LL3.0). Therefore, the presence of fine Fe inclusions in dusty olivine in unequilibrated ordinary chondrites constrains that such dusty olivine in chondrules is a good candidate as an un-altered and stable magnetic recorder of the early solar magnetic field.
... Due to their unique compositional and magnetic properties, dusty olivine grains are expected to retain pre-accretional magnetization. The small grain size of dusty olivine metal implies that most are in the single domain (SD) or single vortex (SV) states, which can retain stable magnetization over the history of the solar system (Uehara & Nakamura 2006; Lappe et al. 2011;Winklhofer et al. 1997;Garrick-Bethell & Weiss 2010). ...
... The distinctive, high coercivities of dusty olivine grains allow for the isolation of their remanent magnetization during alternating field (AF) demagnetization as larger (10-100 ptm) mesostasis metal grains are expected to demagnetize at AF levels <50 mT (Gattacceca et al. 2014). Furthermore, since the post-accretional peak metamorphic temperature of Semarkona was likely only 200-260'C (Alexander et al. 1989; Cody et al. 2008), pre-accretional remanence in dusty olivine metals should be isolated upon laboratory (1 hour duration) thermal demagnetization to <450'C assuming that metamorphism lasted -5 My (Uehara & Nakamura 2006;Reisener & Goldstein 1999). ...
... The survival of this HT component up to above ~550'C shows that tetrataenite cannot be its primary carrier (Nagata 1983). The blocking temperature range of this HT component and the lack of component decay below -400'C are fully consistent with that expected for a pre-accretional TRM in the chondrule that had been thermally demagnetized in a null field on the LL parent body during 5 My of metamorphism at 200'C (Uehara & Nakamura 2006). The thermal demagnetization sequence of DOC8 therefore offers strong evidence that pre-accretional remanence is preserved in dusty olivine-bearing chondrules. ...
The first magnetic fields in the solar system were embedded in the ionized gas of the protoplanetary disk itself. Soon after, newly formed protoplanets may have harbored magnetic core dynamos. Paleomagnetic analysis of ancient samples permits direct constraints on these early solar system magnetic fields. Here I present paleomagnetic studies of several classes of meteorites. Experiments on inclusions of chondritic meteorites have led to some of the first constraints on the intensities of protoplanetary disk magnetic fields. Meanwhile, measurements of eucrites, a class of achondrites believed to originate from the asteroid Vesta, suggest that Vesta once hosted a magnetic core dynamo. New techniques developed during the course of these measurements permit ongoing and future investigations of the remanent magnetizations of new meteorites and terrestrial rocks. In support of the paleomagnetic results, I present analytical and numerical modeling of magnetic dust grain dynamics in the solar nebula and of the interior dynamics of differentiated asteroids capable of hosting magnetic dynamos.
... [3] Dusty olivine grains within chondritic meteorites have been proposed as a suitable candidate for retaining a faithful memory of pre-accretionary magnetic fields [Uehara and Nakamura, 2006]. These high-Mg olivine grains contain metallic Fe-Ni inclusions of various sizes, which were formed by a sub-solidus reduction of the fayalitic component (Fe 2 SiO 4 ) of the olivine [Boland and Duba, 1981]. ...
... Occasionally they are aligned crystallographically within the host olivine, mostly exhibit low Ni-contents and are often enclosed by an amorphous glass layer. Synthetic dusty olivines displaying similar structures have been produced and described previously [Boland and Duba, 1981;Connolly et al., 1994;Leroux et al., 2003;Uehara and Nakamura, 2006]. Black olivine bearing SD-SP Fe-Ni grains have also been found in two strongly shocked Martian meteorites [Van de Moortèle et al., 2007]. ...
... [5] The first to consider the magnetic properties of dusty olivine were Uehara and Nakamura [2006]. Their synthetic dusty olivines showed very strong remanences with Curie temperatures of 760°C, indicating a low-Ni (<7wt%) Fe-Ni alloy. ...
The magnetic properties of olivine-hosted Fe-Ni particles have been
studied to assess the potential of "dusty olivine" to retain a
pre-accretionary remanence in chondritic meteorites. Both body-centered
(bcc) and face-centered cubic (fcc) Fe-Ni phases were formed by
reduction of a terrestrial olivine precursor. The presence of Ni
complicates the magnetic properties during heating and cooling due to
the fcc-bcc martensitic transition. First-order reversal curve (FORC)
diagrams contain a central ridge with a broad coercivity distribution
extending to 600 mT, attributed to non-interacting single-domain (SD)
particles, and a "butterfly" structure, attributed to single-vortex (SV)
states. Average SV nucleation and annihilation fields of 58 ± 55
mT and 170 ± 55 mT, respectively, were extracted from the FORC
diagrams, indicating that demagnetization to >170 mT would be
required to isolate the stable SD signal. SD and SV states were imaged
directly using electron holography. The location of the SD/SV boundary
is broadly consistent with theoretical predictions. A method to measure
the volume of individual SD particles using electron holography is
presented. Combining the volume information with constraints on
coercivity, we calculate the thermal relaxation characteristics of the
particles and demonstrate that the high-coercivity component of
remanance would remain stable for 4.6 Ga, even at temperatures
approaching the Curie temperature of pure Fe. The high coercivity of the
particles, together with the chemical protection offered by the
surrounding olivine, is likely to make them resistant to shock
remagnetization, isothermal remagnetization and terrestrial weathering,
making dusty olivine a credible recorder of pre-accretionary magnetic
fields.
... The term "dusty olivine" refers to grains of olivine containing numerous sub-micrometer inclusions of metallic Fe. Dusty olivines are thought to be relict grains of olivine that were caught up in a chondrule-forming event, heated (without melting) under reducing conditions to temperatures above the Curie temperature of the Fe inclusions, and then cooled in the presence of the nebular magnetic field (Connolly et al. 1998;Leroux et al. 2003;Hewins et al. 2005;Uehara and Nakamura 2006). Recent studies of synthetic analogs of dusty olivine, created by laboratory reduction of terrestrial olivine precursors, suggest that this material has the potential to maintain a faithful record of pre-accretionary remanence (Uehara and Nakamura 2006;Lappe et al. 2011Lappe et al. , 2013. ...
... Dusty olivines are thought to be relict grains of olivine that were caught up in a chondrule-forming event, heated (without melting) under reducing conditions to temperatures above the Curie temperature of the Fe inclusions, and then cooled in the presence of the nebular magnetic field (Connolly et al. 1998;Leroux et al. 2003;Hewins et al. 2005;Uehara and Nakamura 2006). Recent studies of synthetic analogs of dusty olivine, created by laboratory reduction of terrestrial olivine precursors, suggest that this material has the potential to maintain a faithful record of pre-accretionary remanence (Uehara and Nakamura 2006;Lappe et al. 2011Lappe et al. , 2013. However, these conclusions were primarily based on transmission electron microscopy (TEM) measurements of individual Fe particles in laboratory analogs (Lappe et al. 2011). ...
Dusty olivine (olivine containing multiple sub-micrometer inclusions of metallic iron) in chondritic meteorites is considered an ideal carrier of paleomagnetic remanence, capable of maintaining a faithful record of pre-accretionary magnetization acquired during chondrule formation. Here we show how the magnetic architecture of a single dusty olivine grain from the Semarkona LL3.0 ordinary chondrite meteorite can be fully characterized in three dimensions, using a combination of focused ion beam nanotomography (FIB-nT), electron tomography, and finite-element micromagnetic modeling. We present a three-dimensional (3D) volume reconstruction of a dusty olivine grain, obtained by selective milling through a region of interest in a series of sequential 20 nm slices, which are then imaged using scanning electron microscopy. The data provide a quantitative description of the iron particle ensemble, including the distribution of particle sizes, shapes, interparticle spacings and orientations. Iron particles are predominantly oblate ellipsoids with average radii 242 ± 94 × 199 ± 80 × 123 ± 58 nm. Using analytical TEM we observe that the particles nucleate on sub-grain boundaries and are loosely arranged in a series of sheets parallel to (001) of the olivine host. This is in agreement with the orientation data collected using the FIB-nT and highlights how the underlying texture of the dusty olivine is crystallographically constrained by the olivine host. The shortest dimension of the particles is oriented normal to the sheets and their longest dimension is preferentially aligned within the sheets. Individual particle geometries are converted to a finite-element mesh and used to perform micromagnetic simulations. The majority of particles adopt a single vortex state, with "bulk" spins that rotate around a central vortex core. We observed no particles that are in a true single domain state. The results of the micromagnetic simulations challenge some preconceived ideas about the remanence-carrying properties of vortex states. There is often not a simple predictive relationship between the major, intermediate, and minor axes of the particles and the remanence vector imparted in different fields. Although the orientation of the vortex core is determined largely by the ellipsoidal geometry (i.e., parallel to the major axis for prolate ellipsoids and parallel to the minor axis for oblate ellipsoids), the core and remanence vectors can sometimes lie at very large (tens of degrees) angles to the principal axes. The subtle details of the morphology can control the overall remanence state, leading in some cases to a dominant contribution from the bulk spins to the net remanence, with profound implications for predicting the anisotropy of the sample. The particles have very high switching fields (several hundred millitesla), demonstrating their high stability and suitability for paleointensity studies.
... have a role in understanding chondrule recycling(Ruzicka et al., 2007). Dusty olivine grains within chondritic meteorites have been proposed as suitable candidates for retaining a faithful memory of pre-accretionary magnetic fields[Uehara and Nakamura, 2006; Lappe et al., 2011). Dynamic crystallization experiments confirm that dusty olivine can be produced by reduction of FeO-rich olivine in unequilibrated ordinary chondrite (UOC) material(Lofgren and Le 2002; Hewins, 2003;Uehara and Nakamura, 2006). ...
... Dusty olivine grains within chondritic meteorites have been proposed as suitable candidates for retaining a faithful memory of pre-accretionary magnetic fields[Uehara and Nakamura, 2006; Lappe et al., 2011). Dynamic crystallization experiments confirm that dusty olivine can be produced by reduction of FeO-rich olivine in unequilibrated ordinary chondrite (UOC) material(Lofgren and Le 2002; Hewins, 2003;Uehara and Nakamura, 2006). The reduction that formed the Fe-Ni inclusions is thought to be caused by the presence of organic or graphitic carbon in the chondrule precursor(Connolly et al., 1994). ...
Extraterrestrial particulate materials on the Earth can originate in the form of
collisional debris from the asteroid belt, cometary material, or as meteoroid ablation
spherules. Signatures that link them to their parent bodies become obliterated if the
frictional heating is severe during atmospheric entry. We investigated 481 micrometeorites
isolated from ~300 kg of deep sea sediment, out of which 15 spherules appear to have
retained signatures of their provenance, based on their textures, bulk chemical compositions,
and relict grain compositions. Seven of these 15 spherules contain chromite grains whose
compositions help in distinguishing subgroups within the ordinary chondrite sources. There
are seven other spherules which comprise either entirely of dusty olivines or contain dusty
olivines as relict grains. Two of these spherules appear to be chondrules from an
unequilibrated ordinary chondrite. In addition, a porphyritic olivine pyroxene (POP)
chondrule-like spherule is also recovered. The bulk chemical composition of all the
spherules, in combination with trace elements, the chromite composition, and presence of
dusty olivines suggest an ordinary chondritic source. These micrometeorites have undergone
minimal frictional heating during their passage through the atmosphere and have retained
these features. These micrometeorites therefore also imply there is a significant contribution
from ordinary chondritic sources to the micrometeorite flux on the Earth
... Therefore, dusty olivines have a role in understanding chondrule recycling (Ruzicka et al. 2007). Dusty olivine grains within chondritic meteorites have been proposed as suitable candidates for retaining a faithful memory of preaccretionary magnetic fields (Uehara and Nakamura 2006;Lappe et al. 2012). Dynamic crystallization experiments confirm that dusty olivine can be produced by reduction in FeO-rich olivine in unequilibrated ordinary chondrite (UOC) material (Lofgren and Le 2002;Hewins 1983;Uehara and Nakamura 2006). ...
... Dusty olivine grains within chondritic meteorites have been proposed as suitable candidates for retaining a faithful memory of preaccretionary magnetic fields (Uehara and Nakamura 2006;Lappe et al. 2012). Dynamic crystallization experiments confirm that dusty olivine can be produced by reduction in FeO-rich olivine in unequilibrated ordinary chondrite (UOC) material (Lofgren and Le 2002;Hewins 1983;Uehara and Nakamura 2006). The reduction that formed the Fe-Ni inclusions is thought to be caused by the presence of organic or graphitic carbon in the chondrule precursor (Connolly et al. 1994). ...
Abstract–Extraterrestrial particulate materials on the Earth can originate in the form of
collisional debris from the asteroid belt, cometary material, or as meteoroid ablation
spherules. Signatures that link them to their parent bodies become obliterated if the
frictional heating is severe during atmospheric entry. We investigated 481 micrometeorites
isolated from ~300 kg of deep sea sediment, out of which 15 spherules appear to have
retained signatures of their provenance, based on their textures, bulk chemical compositions,
and relict grain compositions. Seven of these 15 spherules contain chromite grains whose
compositions help in distinguishing subgroups within the ordinary chondrite sources. There
are seven other spherules which comprise either entirely of dusty olivines or contain dusty
olivines as relict grains. Two of these spherules appear to be chondrules from an
unequilibrated ordinary chondrite. In addition, a porphyritic olivine pyroxene (POP)
chondrule-like spherule is also recovered. The bulk chemical composition of all the
spherules, in combination with trace elements, the chromite composition, and presence of
dusty olivines suggest an ordinary chondritic source. These micrometeorites have undergone
minimal frictional heating during their passage through the atmosphere and have retained
these features. These micrometeorites therefore also imply there is a significant contribution
from ordinary chondritic sources to the micrometeorite flux on the Earth.
... The primary remanence carriers in meteorites should, in addition, be very stable with respect to chemical and thermal alteration on the parent body, shock remagnetization and terrestrial weathering. It has been proposed that dusty olivines in chondrules fulfill these requirements [Uehara and Nakamura, 2006;Lappe et al., 2011]. Dusty olivines are high-Mg olivine grains containing submicrometer metallic Fe-Ni particles formed by subsolidus reduction of the fayalite component of the olivine during chondrule formation [Boland and Duba, 1975]. ...
... They are thought to be relict grains which survived the chondrule heating process unaltered and can be found in unequilibrated ordinary chondrites [Nagahara, 1981]. The first to consider dusty olivine as a potential carrier of preaccretionary remanence were Uehara and Nakamura [2006]. In a previous study, we investigated the mineral magnetic properties of synthetic dusty olivine and performed electron holographic imaging to determine the magnetic domain states found in Fe-Ni particles with a range of sizes and aspect ratios [Lappe et al., 2011]. ...
[1] We present a comparative study of nonheating paleointensity methods, with the aim of determining the optimum method for obtaining paleointensities from “dusty olivine” in chondritic meteorites. The REM method, whereby thermoremanent magnetization (TRM) is normalized by saturation isothermal remanent magnetization (SIRM), is shown to “over normalize” TRM in dusty olivine due to the transformation of stable single-vortex (SV) states to metastable single-domain (SD) states in a saturating field. The problem of over normalization is reduced in the REMc and REM' methods, which more effectively isolate the high-coercivity stable SD component of remanence. A calibration factor of f = 1600 (1000 < f < 2900) is derived for the REM' method. Anhysteric remanent magnetization (ARM) is shown to be a near perfect analogue of TRM in the stable SD component of dusty olivine. ARM normalization of the high-coercivity (100–150 mT) remanence with a calibration factor fARM = 0.91 (0.7 < fARM < 1.2) yields paleofield estimates within ± 30% of the actual field values for SD dominated samples. A Preisach method for simulating TRM acquisition using information extracted from first-order reversal curve (FORC) diagrams is shown to work well for SD dominated samples, but fails when there is a large proportion of SV remanence carriers. The failure occurs because (1) SV states are not properly incorporated into the Preisach distribution of remanence carriers, and (2) the acquisition of TRM by SV states is not properly modeled by the underlying SD thermal relaxation theory.
... However, it has long been recognized that geological samples should be able to provide useful paleomagnetic records for moments at least several orders of magnitude below this threshold [Kirschvink, 1981]. Examples of such samples include chondrules and inclusions in chondritic meteorites [Lappe et al., 2013;Lappe et al., 2011;Uehara and Nakamura, 2006], which may contain records of magnetic fields in the solar nebula, and detrital zircon crystals, which might provide records of the earliest history of the Earth's magnetic field [Tarduno et al., 2015;Weiss et al., 2015]. ...
Paleomagnetically useful information is expected to be recorded by samples with moments up to three orders of magnitude below the detection limit of standard superconducting rock magnetometers. Such samples are now detectable using recently developed magnetic microscopes, which map the magnetic fields above room-temperature samples with unprecedented spatial resolutions and field sensitivities. However, realizing this potential requires the development of techniques for retrieving sample moments from magnetic microscopy data. With this goal, we developed a technique for uniquely obtaining the net magnetic moment of geological samples from magnetic microscopy maps of unresolved or nearly unresolved magnetization. This technique is particularly powerful for analyzing small, weakly magnetized samples such as meteoritic chondrules and terrestrial silicate crystals like zircons. We validated this technique by applying it to field maps generated from synthetic sources and also to field maps measured using a superconducting quantum interference device (SQUID) microscope above geological samples with moments down to 10^-15 Am2. For the most magnetic rock samples, the net moments estimated from the SQUID microscope data are within error of independent moment measurements acquired using lower sensitivity standard rock magnetometers. In addition to its superior moment sensitivity, SQUID microscope net moment magnetometry also enables the identification and isolation of magnetic contamination and background sources, which is critical for improving accuracy in paleomagnetic studies of weakly magnetic rocks.
... Records of ancient dynamo activity in meteorites are preserved by ferromagnetic minerals that can, upon cooling, grain growth or chemical transformation, record a natural remanent magnetization, which can potentially be preserved over the age of the solar system (Devienne et al., 2023;Mansbach et al., 2022;Nagy et al., 2019;Shah et al., 2018). Taenite and tetrataenite are ferromagnetic minerals commonly observed in (stony-) iron meteorite groups that are promising candidates to provide reliable paleomagnetic information in extraterrestrial materials (Bryson, Church, et al., 2014;Uehara & Nakamura, 2006). Assessing their magnetic recording properties is, therefore, key to reliably reconstructing the duration and strength of planetesimal magnetism, as well as to constrain the conditions and mechanisms for dynamo generation based on records preserved in meteorites (Bryson et al., 2017;Maurel et al., 2020Maurel et al., , 2021. ...
An increasing amount of evidence suggests that the tetrataenite‐bearing cloudy zones (CZ) in iron and stony‐iron meteorites can preserve magnetic records of ancient magnetic activity of their parent bodies over solar system timescales. Tetrataenite islands in the CZ are nanometer‐sized (<200 nm) crystals that usually form through ordering from precursor taenite islands upon extremely slow cooling through 320°C. Recent micromagnetic models have shown that such precursor taenite islands form highly thermally stable single‐domain (SD) or single‐vortex states (SV). In this work we employ a 3D finite element multi‐phase micromagnetic modeling to show that tetrataenite inherits the magnetic remanence of taenite precursor when it forms over underlying SD states. When taenite forms SV states, however, tetrataenite resets the precursor magnetization and records a new remanence through chemical ordering at 320°C. We further assess the thermal stability of tetrataenite islands. We show that in cases where tetrataenite inherits the domain states of its precursor taenite, the origin of the remanence can be up to ∼10⁵ years older than previously thought in fast‐cooled meteorites, and ∼1–≳6 Myr in slowly cooled meteorites. It indicates, therefore, that different regions across slowly cooled CZ record distinct stages of planetary formation.
... These dusty olivines are not thought to have formed by crystallization within the host chondrule but represent relict grains of an even earlier time in solar system history, prior to chondrule formation (Leroux et al., 2003). The dusty olivines contain submicron-sized metallic grains of almost pure Fe kamacite (Uehara & Nakamura, 2006). The composition of metallic grains considered in this study was measured using scanning transmission electron microscopy elemental maps in previous work (Fu, Weiss, Lima, Richard, et al., 2014). ...
Recent paleomagnetic studies have constrained the strength and longevity of the magnetic field generated by the solar nebula, which has broad implications for the early evolution of the solar system. Paleomagnetic evidence was recorded by nanoscale iron inclusions in olivine crystals in the Semarkona LL 3.0 chondrite. These dusty olivines have been shown to be credible carriers of ancient magnetic remanence. The small scale of the iron inclusions presents several challenges for defining their fundamental magnetic properties. Here we present the first correlative study of the response of these magnetic structures under applied laboratory fields. Results show that the majority of particles are in a single-vortex state and exhibit stable magnetic behavior in applied fields up to 200 mT. Experimental observations using Lorentz microscopy and magnetic transmission X-ray microscopy are shown to compare well with the results of finite-element micromagnetic simulations derived from 3-D models of the particles obtained using electron tomography. This correlative approach may be used to characterize the fundamental magnetic behavior of many terrestrial and extraterrestrial paleomagnetic carriers in the single-vortex to multivortex size range, which represent the vast majority of stable magnetic carriers in rocks and meteorites.
... U naltered meteorites originating from our own protoplanetary disk acquired a thermoremanent magnetization (TRM) during formation and present an excellent opportunity to understand the extent of the early solar system magnetic field. The most likely material to have retained this field information is dusty olivine: assemblages of nanometric low-Ni kamacite grains protected from alteration by their host olivine crystal, found in the chondrules of unequilibrated primitive chondrites 1,2 . A recent estimate of the ancient magnetic field intensity (paleointensity) from dusty olivine in Semarkona 3 has provided an upper bound of 54 ± 21 μT for the magnetic field present in the chondrule-forming region (2.5 astronomical units (au)) of the protoplanetary disk during its first two to three million years 4,5 . ...
Recordings of magnetic fields, thought to be crucial to our solar system's rapid accretion, are potentially retained in unaltered nanometric low-Ni kamacite (~ metallic Fe) grains encased within dusty olivine crystals, found in the chondrules of unequilibrated chondrites. However, most of these kamacite grains are magnetically non-uniform, so their ability to retain four-billion-year-old magnetic recordings cannot be estimated by previous theories, which assume only uniform magnetization. Here, we demonstrate that non-uniformly magnetized nanometric kamacite grains are stable over solar system timescales and likely the primary carrier of remanence in dusty olivine. By performing in-situ temperature-dependent nanometric magnetic measurements using off-axis electron holography, we demonstrate the thermal stability of multi-vortex kamacite grains from the chondritic Bishunpur meteorite. Combined with numerical micromagnetic modeling, we determine the stability of the magnetization of these grains. Our study shows that dusty olivine kamacite grains are capable of retaining magnetic recordings from the accreting solar system.
... Special methods have to be used to synthesize nano-structured materials (see following sections). The study of dusty olivines in several meteorite types has shown that inclusions in the nm range may be in a single-domain state and thus carry a stable magnetization (e.g., Van de Moortèle et al. 2007;Lappe et al. 2011;Uehara and Nakamura 2006). Measuring grain sizes in the nm range can be difficult, especially when dealing with strongly magnetic powders that easily form agglomerates. ...
We present an introduction to FeNi alloys as they appear in nature and how their magnetic properties can be studied in the laboratory. Meteorites provide natural samples which can carry information about our early Solar System and the magnetic fields present at that time. Grain size, and therefore domain state, of magnetic particles is the key to understanding their ability to record magnetic information on geological time scales. Material specific properties can be easier studied and optimized for technological applications from synthetic samples. We present common synthesis methods as well as analytical procedures to analyze the composition, crystal structure, grain size, and magnetic properties of FeNi alloys. We present data compiled from the literature together with our own results from samples synthesized by mechanical alloying and melting. In particular, we demonstrate changes in hysteresis and backfield parameters as well as Curie temperatures linked to composition, pressure, and alloying. The single-domain (SD) threshold in FeNi alloys remains unknown due to methodical limits in grain size and strong magnetic interactions between individual particles.
... The metallic phases of Fe-Ni grains appear to simultaneously preserve a magnetic environment of the early solar nebula as NRMs. An experimental study suggested that abundant submicron-sized kamacite grains in a reduced "dusty" olivine acquire magnetically stable pre-accretional NRMs when the chondrules were formed (Uehara and Nakamura, 2006). Lauretta et al. (2001) suggested that silicon-bearing Fe-Ni grains in the matrix of Bishunpur (LL3.1) were formed in the reducing environment of chondrule melts. ...
... However, it has long been recognized that geological samples should be able to provide useful paleomagnetic records for moments at least several orders of magnitude below this threshold [Kirschvink, 1981]. Examples of such samples include chondrules and inclusions in chondritic meteorites [Lappe et al., 2013;Lappe et al., 2011;Uehara and Nakamura, 2006], which may contain records of magnetic fields in the solar nebula, and detrital zircon crystals, which might provide records of the earliest history of the Earth's magnetic field [Tarduno et al., 2015;Weiss et al., 2015]. ...
Paleomagnetically useful information is expected to be recorded by samples with moments up to three orders of magnitude below the detection limit of standard superconducting rock magnetometers. Such samples are now detectable using recently developed magnetic microscopes, which map the magnetic fields above room-temperature samples with unprecedented spatial resolutions and field sensitivities. However, realizing this potential requires the development of techniques for retrieving sample moments from magnetic microscopy data. With this goal, we developed a technique for uniquely obtaining the net magnetic moment of geological samples from magnetic microscopy maps of unresolved or nearly unresolved magnetization. This technique is particularly powerful for analyzing small, weakly magnetized samples such as meteoritic chondrules and terrestrial silicate crystals like zircons. We validated this technique by applying it to field maps generated from synthetic sources and also to field maps measured using a superconducting quantum interference device (SQUID) microscope above geological samples with moments down to 10^-15 Am2. For the most magnetic rock samples, the net moments estimated from the SQUID microscope data are within error of independent moment measurements acquired using lower sensitivity standard rock magnetometers. In addition to its superior moment sensitivity, SQUID microscope net moment magnetometry also enables the identification and isolation of magnetic contamination and background sources, which is critical for improving accuracy in paleomagnetic studies of weakly magnetic rocks.
... Kamacite (Fe-Ni, Ni < 5 per cent) is commonly found to be the dominant magnetic mineral in many different types of meteorites including chondritic meteorites Pechersky et al. 2012;Gattacceca et al. 2014) and lunar rocks (Garrick-Bethell & Weiss 2010). Of particular interest are the kamacite particles found inside dusty olivines in chondrules in unequilibrated chondrites, as they have the potential to carry pristine, pre-accretionary, primary remanent magnetisations (Uehara & Nakamura 2006;Lappe et al. 2011;Lappe et al. 2013), that is, kamacite is one of the most likely minerals to retain magnetic field intensity information acquired during Solar Nebular formation, because the particles are protected from chemical alteration by the encasing olivine. But for the kamacite particles to retain a meaningful palaeomagnetic signal, it is important that the particles' magnetic remanences are metastable for billions of years. ...
Kamacite particles (Fe–Ni, Ni < 5 per cent), are very common in extra-terrestrial materials, such as meteorites. It is normally assumed that for kamacite particles to be reliable recorders of magnetic fields, they need to be magnetically uniform (single domain, SD) and thermally stable. Larger particles subdivide into non-uniform multidomain (MD) magnetic structures that produce weaker magnetic signals, while small SD particles become magnetically unstable due to thermal fluctuations and exhibit superparamagnetic behaviour. In this paper we determine the first micromagnetic calculation of the stable SD range domain-state phase diagram for metallic iron; previous calculations were analytical. There is a significant increase in the critical size for the SD/MD threshold size, for example, for cube-shaped iron particles, the critical SD/MD threshold has now been estimated to be 25 nm, compared to 17 nm for previous estimates. The larger critical SD/MD threshold size for iron, agrees better with previously published nanometric observations of domain state for FeNi particles, then early analytical models.
... Occurrence of alterations induced by heating in the laboratory has limited application of thermal methods, in particular the doubleheating Thellier method, in which the natural remanent magnetization (NRM) is assumed to be dominantly a thermoremanent magnetization (TRM). The Thellier double-heating method is widely applied in terrestrial igneous rocks, where it provides accurate estimates of the thermomagnetic and microscopy techniques have been developed to investigate on the magnetic properties, mineralogy, domain state and magnetic interactions are available for igneous rocks (Davis and Evans, 1976;Day et al., 1977;Urrutia-Fucugauchi et al., 1984;Dunlop and Ozdemir, 1997), which are expanded by an array of high-resolution imaging techniques (Uehara and Nakamura, 2006;Uehara et al., 2011;Feinberg et al., 2006;Harrison et al., 2002). Analytical tests to detect heating-induced alterations and effects of anisotropy, cooling time and magnetic interactions have been developed (e.g., Dunlop and Yu, 2010;Lawrence et al., 2008;Yu, 2006;Urrutia-Fucugauchi, 1979;Coe, 1967). ...
Meteorites represent the earliest records of the evolution of the solar system, providing information on the conditions, processes and chronology for formation of first solids, planetesimals and differentiated bodies. Evidence on the nature of magnetic fields in the early solar system has been derived from chondritic meteorites. Chondrules, which are millimeter sized silicate spherules formed by rapid melting and cooling, have been shown to retain remanent magnetization records dating from the time of chondrule formation and accretion of planetesimals. Studies on different meteorite classes, including ordinary and carbonaceous chondrites, have however provided contrasting results with wide ranges for protoplanetary disk magnetic fields. Developments on instrumentation and techniques for rock magnetic and paleointensity analyses are allowing increased precision. Micromagnetic and an array of geochemical, petrographic and electronic microscopy analyses provide unprecedented resolution, characterizing rock magnetic properties at magnetic domain scales. We review studies on chondrules from the Allende meteorite that reveal relationships among hysteresis parameters and physical properties. Coercivity, remanent and saturation remanence parameters correlate with chondrule size and density; in turn related to internal chondrule structure, mineralogy and morphology. Compound, fragmented and rimmed chondrules show distinct hysteresis properties, related to mineral composition and microstructures. The remanent magnetization record and paleointensity estimates derived from the Allende and other chondrites support remanent acquisition under influence of internal magnetic fields within parent planetesimals. Results support that rapid differentiation following formation of calcium-aluminum inclusions and chondrules gave rise to differentiated planetesimals with iron cores, capable of generating and sustaining dynamo action for million year periods. The Allende chondrite may have derived from a partly differentiated planetesimal which sustained an internal magnetic field.
... Olivine is also abundant in meteorites and contains inclusions of ferromagnetic minerals , which are capable of acquiring a remanent magnetization. Therefore, there has been interest in using olivine as a recorder of the extraterrestrial magnetic field [Hoffmann et al., 2011; Lappe et al., 2011; Rochette et al., 2009; Uehara and Nakamura, 2006]. On Earth, the rheology of olivine determines flow dynamics in depths shallower than 410 km; thus, olivine properties influence mantle flow and plate tectonics. ...
Mantle flow dynamics can cause preferential alignment of olivine crystals that results in anisotropy of physical properties. To interpret anisotropy in mantle rocks, it is necessary to understand the anisotropy of olivine single crystals. We determined anisotropy of magnetic susceptibility (AMS) for natural olivine crystals. High-field AMS allows for the isolation of the anisotropy due to olivine alone. The orientations of the principal susceptibility axes are related to the olivine's crystallographic structure as soon as it contains > 3 wt.% FeO. The maximum susceptibility is parallel to the c-axis both at room temperature (RT) and at 77 K. The orientation of the minimum axis at RT depends on iron content; it is generally parallel to the a-axis in crystals with 3 – 5 wt.% FeO, and along b in samples with 6 – 10 wt.% FeO. The AMS ellipsoid is prolate and the standard deviatoric susceptibility, k', is on the order of 8*10-10 m3/kg for the samples with < 1wt.% FeO, and ranges from 3.1*10-9 m3/kg to 5.7*10-9 m3/kg for samples with 3-10 wt.% FeO. At 77 K, the minimum susceptibility is along b, independent of iron content. The shape of the AMS ellipsoid is prolate for samples with < 5 wt.% FeO, but can be prolate or oblate for higher iron content. The degree of anisotropy increases at 77 K with p77' = 7.1 ± 0.5. The results from this study will allow AMS fabrics to be used as a proxy for olivine texture in ultramafic rocks with high olivine content.
... As in equilibrated chondrites, multidomain kamacite is a very poor paleomagnetic recorder, and martensite loses any potential paleomagnetic record of its precursor taenite when the martensitic transformation occurs (Momose et al. 1984) (although for most of the range of Niabundances over martensite forms, the parent taenite will have a Curie point below room temperature). Only single-domain kamacite present in dusty olivine grains appears to be a potential source of paleomagnetic information in unequilibrated ordinary chondrites (Uehara and Nakamura 2006;Lappe et al. 2011), although accessible only through magnetic microscopy. ...
Magnetic properties are sensitive proxies to characterize FeNi metal phases in meteorites. We present a data set of magnetic hysteresis properties of 91 ordinary chondrite falls. We show that hysteresis properties are distinctive of individual meteorites while homogeneous among meteorite subsamples. Except for the most primitive chondrites, these properties can be explained by a mixture of multidomain kamacite that dominates the induced magnetism and tetrataenite (both in the cloudy zone as single-domain grains, and as larger multidomain grains in plessite and in the rim of zoned taenite) dominates the remanent magnetism, in agreement with previous microscopic magnetic observations. The bulk metal contents derived from magnetic measurements are in agreement with those estimated previously from chemical analyses. We evidence a decreasing metal content with increasing petrologic type in ordinary chondrites, compatible with oxidation of metal during thermal metamorphism. Types 5 and 6 ordinary chondrites have higher tetrataenite content than type 4 chondrites. This is compatible with lower cooling rates in the 650–450 °C interval for higher petrographic types (consistent with an onion-shell model), but is more likely the result of the oxidation of ordinary chondrites with increasing metamorphism. In equilibrated chondrites, shock-related transient heating events above approximately 500 °C result in the disordering of tetrataenite and associated drastic change in magnetic properties. As a good indicator of the amount of tetrataenite, hysteresis properties are a very sensitive proxy of the thermal history of ordinary chondrites, revealing low cooling rates during thermal metamorphism and high cooling rates (e.g., following shock reheating or excavation after thermal metamorphism). Our data strengthen the view that the poor magnetic recording properties of multidomain kamacite and the secondary origin of tetrataenite make equilibrated ordinary chondrites challenging targets for paleomagnetic study.
... Variation of REMc (NRM/IRM ratio) with progressive AF demagnetization for representative (a) Allende, (b) Bjurbole, and (c) Karoonda chondrules. by thermal alteration [Uehara and Nakamura, 2006]. Third, even if the paleointensity signal was acquired immediately after chondrule formation, the chondrules were probably rotating rather than maintaining a constant position relative to the ambient magnetic field. ...
Chondrules in chondritic meteorites record the earliest stages of
formation of the solar system, potentially providing information about
the magnitude of early magnetic fields and early physical and chemical
conditions. Using first-order reversal curves (FORCs), we map the
coercivity distributions and interactions of 32 chondrules from the
Allende, Karoonda, and Bjurbole meteorites. Distinctly different
distributions and interactions exist for the three meteorites. The
coercivity distributions are lognormal shaped, with Bjurbole
distributions being bimodal or trimodal. The highest-coercivity mode in
the Bjurbole chondrules is derived from tetrataenite, which interacts
strongly with the lower-coercivity grains in a manner unlike that seen
in terrestrial rocks. Such strong interactions have the potential to
bias paleointensity estimates. Moreover, because a significant portion
of the coercivity distributions for most of the chondrules is <10 mT,
low-coercivity magnetic overprints are common. Therefore
paleointensities based on the REM method, which rely on ratios of the
natural remanent magnetization (NRM) to the saturation isothermal
remanent magnetization (IRM) without magnetic cleaning, will probably be
biased. The paleointensity bias is found to be about an order of
magnitude for most chondrules with low-coercivity overprints.
Paleointensity estimates based on a method we call REMc, which uses
NRM/IRM ratios after magnetic cleaning, avoid this overprinting bias.
Allende chondrules, which are the most pristine and possibly record the
paleofield of the early solar system, have a mean REMc paleointensity of
10.4 μT. Karoonda and Bjurbole chondrules, which have experienced
some thermal alteration, have REMc paleointensities of 4.6 and 3.2
μT, respectively.
... Variation of REMc (NRM/IRM ratio) with progressive AF demagnetization for representative (a) Allende, (b) Bjurbole, and (c) Karoonda chondrules. by thermal alteration [Uehara and Nakamura, 2006]. Third, even if the paleointensity signal was acquired immediately after chondrule formation, the chondrules were probably rotating rather than maintaining a constant position relative to the ambient magnetic field. ...
To provide information about the magnitude of early magnetic fields and physical and chemical conditions of the solar system, we determine the magnetic properties of 32 chondrules from the Allende, Karoonda, and Bjurbole meteorites and bulk samples from M
... Since the Curie temperature of kamacite is about 750°C (Fig. 1a), single domain kamacite grains (e.g. submicron-sized kamacite in dusty olivine (Harrison et al., 2009;Uehara and Nakamura, 2006)) has the capability of recording magnetic fields during earlier stages of the solar system history (chondrule formation) and to retain this magnetization for durations in excess of several Gyr (Garrick-Bethell and . ...
Meteorites are a primary source of information about past magnetic field in the solar system. Yet, the small-scale magnetic properties of FeNi metals, which are the magnetic carriers of most meteorites, are poorly known. We study here the magnetic microstructures of FeNi metals in two equilibrated chondrites. Two types of tetrataenite-bearing microstructures are revealed: (1) Zoned taenite particles that consist of a “cloudy zone” (20–250 nm large tetrataenite precipitates embedded in Ni-poor matrix) and a 1–10 μm thick tetrataenite rim. (2) Zoneless plessite particles that consist of large tetrataenite grains (> 1 μm) embedded in a kamacite matrix. Magneto-optical imaging of saturation remanence shows that, the submicron-sized tetrataenite islands in cloudy zone carry a much stronger remanence than the μm-sized tetrataenite crystals in the tetrataenite rims and plessite. Micron-scale mapping of coercivity of remanence (Bcr) shows that the center part of the cloudy zone has finer tetrataenite grains (20 nm) and higher Bcr values (~ 1 T) than the outer part (250 nm and 400 mT, respectively). These results suggest that the micron-sized tetrataenite is in a multi domain state, whereas the submicron-sized tetrataenite in the cloudy zone are in a single domain-like state and may be regarded as a potentially good paleomagnetic recorder in meteorites. The stability of the remanent magnetization in ordinary chondrites is a function of the amount of the cloudy zones of the zoned taenite grains rather than the bulk amount of tetrataenite.
... The paleointensity recorded by the Allende chondrules is, however, well below this limit. Second, the paleofield recorded by Allende and other type 3 chondrules may have been reset by thermal alteration [Uehara and Nakamura, 2006]. Third, even if the paleointensity signal was acquired immediately after chondrule formation, the chondrules were probably rotating rather than maintaining a constant position relative to the ambient magnetic field. ...
A primary method of characterizing the magnetic properties of a sample is to measure its magnetization while gradually varying an applied ambient magnetic field between large positive values and large negative values. The change in magnetization with field can be used to map the magnetic hysteresis loop of the sample, which defines some of its bulk magnetic properties (Figure 1).
Each sample, however, is actually composed of a population of magnetic grains and domains, which may vary in their magnetic properties and which may interact with their neighbors. The micromagnetic properties of these populations, such as the distribution of micro‐coercivities and the magnetic interactions or biases, can be mapped by probing the interior of a hysteresis loop with a series of partial hysteresis curves referred to as first‐order reversal curves (FORCs) [ Mayergoyz , 1986; Pike et al. , 1999] (Figures 1 and 2). These properties can then be used in various ways ranging from characterization of the magnetic composition, grain size, and concentration of samples to understanding the mechanisms of magnetic acquisition [e.g., Pike et al. , 1999,2001; Roberts et al. , 2006; Carvallo et al. , 2006; Muxworthy et al. , 2005].
... Both samples contain abundant elongate kamacite with grain sizes of 5-20 m in length within clinopyroxene (Fig. 5a). Silicatehosted Fe-Ni phases have been documented in other meteorites (e.g., Uehara and Nakamura, 2006), and are analogous to ironoxides within silicates in terrestrial Precambrian rocks (e.g., Selkin et al., 2000;Yu and Dunlop, 2001;Renne et al., 2002). However, kamacite in the altered R3 sample (Fig. 2) is dominantly distributed along clinopyroxene boundaries (Fig. 5b). ...
Investigating mineral magnetic properties of meteorites is essential to understanding the formation and evolution of planetary bodies in the solar system. In order to decipher ancient magnetic records, demagnetization experiments were carried out for the ∼4550Ma Richardton H5 chondrite. Alternating-field demagnetization revealed a soft fraction as well as a hard fraction. Conventional thermal demagnetization in air showed severe alterations. But, a few thermal demagnetizations in vacuum were successful in isolating a stable paleomagnetic record. On the basis of microscopic analysis, we found that fine-grained clinopyroxene-hosted kamacite is abundant, responsible for the stable and permanent magnetic record of Richardton. The experimental data imply a thermal or thermochemical origin for the stable paleomagnetic record of Richardton. However, the possibility of pressure (re)magnetization cannot be evaluated because the effect of pressure on magnetization for the Fe–Ni system is unknown.
... With conventional paleomagnetic materials such as basalt, one can reasonably assume a statistically homogeneous distribution of noninteracting magnetic minerals in a typical one-inch core sample, resulting in a unique distribution of remanence. On the other hand, some chondrites (Sugiura et al., 1979;Sugiura and Strangway, 1988;Nagata and Funaki, 1983;Morden and Collinson, 1992;Gattacceca et al., 2003;Uehara and Nakamura, 2006), shocked granites (Carporzen et Stud. Geophys. ...
To identify the stable remanence carrier in rock samples, we conducted magnetic microscopic observations combined with conventional
stepwise demagnetization experiments. The instrument, which employs an amorphous wire-based magneto-impedance sensor (30 µm
diameter, 5 mm length), can document magnetic anomalies (vertical component) of the millimeter to sub-millimeter-thick rock
samples with a resolution of 500 µm. Our new technique allows identification of the sources of both stable and unstable remanence
components in meteorite and shocked granite samples. However, stray magnetic fields from the sensor magnetize the magnetic
minerals in the sample and makes serious artifacts on the magnetic images. Although the artifacts of the induced magnetization
should be solved, this new corroborative technique leads to a microscopic discrimination of stable paleomagnetic records from
terrestrial and extraterrestrial materials.
Cambridge Core - Planetary Science and Astrobiology - Chondrules - edited by Sara S. Russell
Magnetic fields are proposed to have played a critical role in some of the most enigmatic processes of planetary formation
by mediating the rapid accretion of disk material onto the central star and the formation of the first solids. However, there
have been no experimental constraints on the intensity of these fields. Here we show that dusty olivine-bearing chondrules
from the Semarkona meteorite were magnetized in a nebular field of 54 ± 21 microteslas. This intensity supports chondrule
formation by nebular shocks or planetesimal collisions rather than by electric currents, the x-wind, or other mechanisms near
the Sun. This implies that background magnetic fields in the terrestrial planet-forming region were likely 5 to 54 microteslas,
which is sufficient to account for measured rates of mass and angular momentum transport in protoplanetary disks.
We discuss the evolution of magnetic fields in chondrule-forming shocks.
These fields may impart remanent magnetization in chondrules, which can
then be used as probes of nebular fields.
Thermomagnetic and microprobe analyses are carried out and a set of magnetic characteristics are measured for 25 meteorites and 3 tektites from the collections of the Vernadsky Geological Museum of the Russian Academy of Sciences and Museum of Natural History of the North-East Interdisciplinary Science Research Institute, Far Eastern Branch of the Russian Academy of Sciences. It is found that, notwithstanding their type, all the meteorites contain the same magnetic minerals and only differ by concentrations of these minerals. Kamacite with less than 10% nickel is the main magnetic mineral in the studied samples. Pure iron, taenite, and schreibersite are less frequent; nickel, various iron spinels, Fe-Al alloys, etc., are very rare. These minerals are normally absent in the crusts of the Earth and other planets. The studied meteorites are more likely parts of the cores and lower mantles of the meteoritic parent bodies (the planets). Uniformity in the magnetic properties of the meteorites and the types of their thermomagnetic (MT) curves is violated by secondary alterations of the meteorites in the terrestrial environment. The sediments demonstrate the same monotony as the meteorites: kamacite is likely the only extraterrestrial magnetic mineral, which is abundant in sediments and associated with cosmic dust. The compositional similarity of kamacite in iron meteorites and in cosmic dust is due to their common source; the degree of fragmentation of the material of the parent body is the only difference.
Heavily shocked meteorites of shock stages S5 and S6 contain shock-induced melt veins (SMVs). SMVs might have reset the remanence of an asteroidal metamorphism at the time of giant collisions against a chondrite parent body. Here we present micropaleomagnetic and petrologic studies of SMVs in L6S5 Tenham chondrite with ∼500 μm thick black veins enclosing high-pressure minerals such as ringwoodite. Paleomagnetic data show that the high-temperature (HT, 200°C–650°C) and high-coercivity (HC, 20–100 mT) stable components of SMVs formed a cluster even from different portions of SMV, whereas the stable HT component of surrounding host rock showed a scattered orientation under stereonet projection. The host rock HC components form a girdle between the mixing of SMVs and unknown overprints, tracing the magnetic susceptibility foliation. Magnetic force microscopy and backscattered electron images confirmed kamacite and taenite assemblages in iron sulfides as remanence-carrying minerals in SMVs. Hysteresis data of SMVs revealed the presence of single-domain (SD) FeNi metals with Mrs/Ms = ∼0.1 and Hrc/Hc = ∼2, although these parameters are only applicable to magnetite. Because the metastable ringwoodite in SMVs transforms back to olivine at 188°C for 1000 Myr (metamorphism) or at 900°C for 1 h (postshock heating), the preservation of ringwoodite suggests that SMVs have not experienced either thermal condition. The magnetic time-temperature relation for SD FeNi metals suggested that 200°C unblocking temperature corresponds to the storage time of 100 years for kamacite and 4500 Myr for taenite at room temperature. The difference of HT components discards the possibility of postshock heating. Therefore, the SMV's remanence is a characteristic shock-induced thermal remanence that has newly been acquired during hypervelocity collision under a cryptic magnetic field.
During the 2867 Šteins flyby of the ROSETTA spacecraft on September 5, 2008 magnetic field measurements have been made with both the RPC orbiter magnetometer and the ROMAP lander magnetometer. These combined magnetic field measurements allow a detailed examination of any magnetic signatures caused either directly by the asteroid or indirectly by Šteins’ different modes of interaction with the solar wind. Comparing measurements with simulation results show that Šteins does not posses a significant remanent magnetization. The magnetization is estimated at less than 10−3 A m2/kg. This is significantly different from results at 9969 Braille and 951 Gaspra.
Presents paleointensity determinations on precisely dated Holocene submarine basaltic glass from the East Pacific Rise. Submarine basaltic glass proves to be nearly ideal for paleointensity determinations in that it produces a high success rate for Thellier experiments. The intensities vary rapidly with time excluding a westward drifting nondipole component as the source for these fluctuations. Basaltic glass is frequently recovered in both dredged and drilled material from the ocean floor. The availability of submarine basaltic glass throughout the world oceans therefore holds great potential for a better distribution of paleointensity data through time and space. -from Authors
This book explains the concepts, techniques and applications of palaeomagnetism. Following an introductory chapter on the nature and origin of the geomagnetic field, a further seven chapters cover the following topics: ferromagnetic minerals; the origin, sampling, measurement and display of natural remanent magnetism; palaeomagnetic stability; statistical analysis of palaeomagnetic data; palaeomagnetic poles; and rock magnetism. The final three chapters are devoted to the applications of palaeomagnetism to geochronology, palaeogeography and regional tectonics, respectively. Derivations referred to in the text are given in an appendix. -G.E.Hodgson
Magnetic properties and paleointensities are reported for several
low-petrologic-grade noncarbonaceous chondrites. Enstatite chondrites
are far more magnetic than others and record ancient fields of 7-16 Oe.
Abee has nearly random NRM in clasts and matrix samples, suggesting
preaccretional remanence. Indarch and Yamato-691 record high fields, but
have a single direction of magnetization, so that it cannot be
determined whether the magnetic record is of pre- or postaccretional
origin. Bjurbole, Chainpur, Mezo Madaras, and Yamato-74191 have random
(and stable) NRM components carried by plessite, indicating possible
preaccretional remanence. However, Bjurbole and Mezo Madaras are thought
to have been reheated to above 500 C after their accretion, and in that
case the random NRM in these chondrites could not be preaccretional.
Whether or not chondritic metal results from equilibrium condensation in
the nebula [1], or reflects reduction during chondrule formation [2] is
still a matter of debate. Tiny metal globules are found both in matrix
of primitive meteorites and in olivine-pyroxene bearing chondrules,
suggesting that reduction reactions could have been established either
before or during chondrule formation. The causes of reduction processes
are also controversial, either controlled by the reduced protosolar
atmosphere or due to the presence of reduced carbon in the chondrule
precursors [3]. These issues are of fundamental importance in
understanding chondrule formation processes and conditions. In addition,
because chondrules are the raw material for the Earth accretion, the
understanding of the processes controlling the formation of metal (and
its compositions) might be also essential for the understanding of the
early differentiation of the Earth. In order to shed light on metal
formation in chondrules, a series of reduction experiments were carried
out to establish 1. the timing of the reduction 2. the compositional
effects of reduction and 3. the causes of the reduction. San Carlos
olivines, Fa16 with trace amounts of Ni, Co, P, Mn, were used as
starting material, this composition being close to that of chondrules on
average. Olivines were ground to 50-100 m in order to be comparable to
chondrule precursors. Experiments were run in a 1 atmosphere vertical
furnace, in the temperature range 1550 to 1650 C and under different
oxygen fugacities (IW-1 to C-CO buffer). These reduced atmospheres were
imposed by a flux of different proportions of gases (CO, CO2, H2 and Ar)
or by using graphite capsules with a flux of pure CO gas. For a given T
and fO2, experiments were performed with time scales of 5 mn to 8 hours,
and terminated by quenching the run products in dry conditions at 500
C/sec. Each experiment produced an assemblage of olivines (Fo-rich),
silicate glass and metal globules (Fe-Ni) either as tiny blebs (< 1m)
included in olivine or as globules (1 to 50 m) located in the silicate
melt at the olivine grain boundaries. The textural features are very
similar to those observed in natural unequilibrated ordinary chondrules.
Indeed, olivines may or may not show a dusty appearance, with or without
preferential alignment of metal blebs in the same run products. Owing to
these experiments, it is also possible to specify unambiguously the
mechanism for the reduction reaction: Olivine (Fa 16) > Olivine (<
Fa16) + Si-glass + Fe metal + O2. In term of composition, olivine, Fe
metal and glass are drastically dependent on the imposed oxygen
fugacity, run duration and temperature. Within the experimental
conditions, olivines vary from Fa 16 to Fa 0.15, Fe metal from 60 wt% Ni
to 2 wt% Ni, and glass from silica-poor and iron-rich composition to
silica-rich and iron-poor composition. In general, olivine becomes more
forsteritic as oxygen fugacity decreases and run duration increases, and
for a fixed oxygen fugacity, the Ni content of metal phases shows a
drastic decrease as run duration increases. Moreover, these data show
that the rate of this reduction process is strongly sensitive to the
temperature and the nature of the reducing agent. In the light of these
textural and compositional data, this study suggests that metal in
chondrules can be produced on a time scale relevant for chondrule
formation by reduction reactions and that these processes could also
explain the main textural and compositional features of olivine and
metal observed in natural chondrules. References: [1] Grossman L. and
Olsen E. (1974) GCA, 38, 173-187. [2] Scott E. R. D and Taylor G. J.
(1983) Proc. LPSC 14th, in JGR, 88, B275-B286. [3] Connolly H. C. Jr. et
al. (1994) Nature, 371, 136-139.
The natural remanent magnetization (NRM) of 13 individual chondrules, 12
individual large grains of metal, and 9 matrix parts of ALHA 76009 L6
chondrite is examined. The NRM of the chondrules is stable for the
AF-demagnetization and the thermal demagnetization, but its direction is
widely scattered within the bulk chondrite. The NRM of large grains of
metal is unstable and its direction is widely scattered. The NRM of the
matrix is stable for the AF-demagnetization below 150 Oe. peak, but it
becomes unstable when the demagnetization field exceeds 150 Oe. peak.
The NRM direction of matrix parts is not exactly uniform but is
clustered within a hemisphere. It appears that the NRM of the chondrules
was acquired as stable thermoremanent magnetization in the presence of a
magnetic field, but a plausible mechanism for keeping stable NRM of
random orientations within the well metamorphased chondrite is not yet
clarified.
We present the results of the first numerical calculations to model magnetic decoupling in a collapsing molecular cloud core. Magnetic decoupling is the stage during which the motion of the neutrals ceases to significantly affect the magnetic field strength and the magnetic field ceases to significantly affect this motion. We have analyzed the resistivity of a weakly ionized, magnetic gas, and we have separated the contributions of ohmic dissipation and ambipolar diffusion. The chemical model used to determine the abundances of ionized particles accounts for, among other things, a distribution of grain radii. The evolution of an axisymmetric, magnetic molecular cloud core is followed from central densities of 300 to 2 × 1012 cm-3. Typically, magnetic decoupling begins at a central density of 3 × 1010 cm-3 and is complete by a density of 2 × 1012 cm-3. We find that the mechanism responsible for magnetic decoupling is ambipolar diffusion, not ohmic dissipation, and that decoupling precedes the formation of a central stellar object. When the central density is a few times 1012 cm-3, a nearly uniform magnetic field of Bdec ≈ 0.1 G extends over a region ≈20 AU in radius that contains a mass ≈0.01 M☉. This value of Bdec is consistent with measurements of remanent magnetization in meteorites. Magnetic decoupling at this stage is not accompanied by hydromagnetic shocks. We estimate that the "magnetic flux problem" of star formation is resolved by ambipolar diffusion before the magnetic field is refrozen in the matter because of thermal ionization.
We have developed a microscope to image weak magnetic fields using submillimeter pickup coils made from conventional low-temperature superconducting niobium wire coupled to the input circuit of a superconducting quantum interference device SQUID. The pickup coil and the SQUID sensor are mounted in the vacuum space of the cryostat and are thermally anchored to the liquid helium reservoir. A 25 m thick sapphire window separates the room temperature RT sample and the vacuum space. The spacing between the pickup coil and RT sample was typically less than 130 m. The spatial resolution is limited by the diameter of the pickup coil. The pickup coils are easily interchangeable, allowing us to adapt the SQUID microscope to a variety of different measurements. We have achieved a spatial resolution of 250 m with a magnetic field sensitivity of 850 fT/Hz 1/2 or a spatial resolution of 500 m with a magnetic field sensitivity of 330 fT/Hz 1/2 . We have used this instrument to measure various biomagnetic and paleomagnetic fields. © 2002 American Institute of Physics.
Measurement of natural, thermal, and saturation remanent magnetism and their stability with respect to alternating field and thermal demagnetization of concentrates of plagioclase, pyroxene, and oxide (80 percent ilmenite, 20 percent magnetite) separated from a sample of diabase from the Triassic Lambertville (N. J.) sill, reveal that the stable remanence of the whole rock is associated with the silicates and especially with the plagioclase. Measurement of Curie point and saturation moment of the feldspar indicates the presence of about 0.3 percent Fe ion in solid solution and 0.004 percent pure magnetite as a discrete phase. This magnetite cannot be identified microscopically, nor with an electron probe, and it is inferred to be present as submicroscopic, single domain particles.
Samples have been collected from eleven sites in the Modipe gabbro which crops out on the border of Bechuanaland and South Africa. After partial demagnetization in alternating magnetic fields, ten of these sites give almost vertical directions of magnetization, forming a close group which corresponds to a paleomagnetic pole at 33°S, 31°E. The gabbro is intruded by the Gaberones granite, and samples from two sites in the granite show almost horizontal magnetization. On initial measurement gabbro samples collected near the contact yield directions close to those of the granite, which suggests that the gabbro was reheated at the time of emplacement of the granite (2350 m.y. ago). After magnetic cleaning, some of these samples give directions which agree with those at the other gabbro sites. Together with detailed alternating field and thermal demagnetization studies, this agreement provides strong evidence that the magnetically cleaned directions are those of the TRM acquired when the gabbro originally cooled. The gabbro has an age of approximately 2600 m.y. and is to date the oldest rock unit that has been studied paleomagnetically; the results imply the existence of a geomagnetic field in this early stage of the earth's history. Alternating field demagnetization experiments indicate that some of the magnetic grains have very high coercivities. Thermal demagnetization and IRM studies exclude the presence of hematite and show that the magnetic mineral present is magnetite. Microscopic observations and theoretical considerations suggest that the high coercivities observed are due to the shape anisotropy of small elongated single-domain grains of magnetite.
Paleotemperature controls the maturation of coal and hydrocarbons in sedimentary basins and is also important in determining paleogeothermal gradient and hence tectonic style in exhumed metamorphic terrains. One method of estimating paleotemperature analyses the partial remagnetization of a rock due to heating in thick volcanic or sedimentary sequences, over subcrustal heat sources such as plumes, or at convergent plate margins. The overprinted natural remanent magnetization (NRM) of a rock records both the age and the paleotemperature of remagnetization, but a temperature correction from laboratory to geological time scales is required, using theoretical time–temperature relations. Time–temperature relations are well known for magnetite (Fe3O4) but are reported here for the first time for pyrrhotite (Fe7S8), another common NRM carrier. Data for each mineral separately yield independent estimates of paleotemperature if geologically reasonable estimates of heating time can be made. Paleotemperature can be estimated without geological input if data for both minerals are combined. Together with the age of remagnetization, determined from the paleomagnetic pole of the NRM overprint, these paleotemperature estimates can be used to infer the history of heating and uplift following burial. As a test case, we examine thermally acquired NRM overprints carried by pyrrhotite (Fe7S8) and magnetite (Fe3O4) in the Milton Monzonite of southeastern Australia. These overprints record a heating event about 100 Ma ago, probably thermal doming prior to rifting of the Tasman Sea, that upgraded coal rank in the Sydney Basin. Extrapolating from laboratory to geological times, using the new time–temperature contours for pyrrhotite, we estimate that the presently exposed Sydney Basin in the vicinity of the Milton Monzonite was remagnetized by heating to 165±30°C for ≈100 ka. Assuming a paleogeothermal gradient of 70°C/km appropriate for young or incipient rifts, the depth of burial at the time of remagnetization is estimated to have been 2.3±0.4 km. This figure is in excellent agreement with independent estimates based on reflectance data for the coal accessory mineral vitrinite.
The ranges of delta18O and delta17O in components of the Murchison (C2) chondrite exceed those in all other meteorites analyzed. Previous authors have proposed that C2 chondrites are the products of aqueous alteration of anhydrous silicates. A model is presented to determine whether the isotopic variations can be understood in terms of such alteration processes. The minimum number (two) of initial isotopic reservoirs is assumed. Two major stages of reservoir interaction are required: one at high temperature to produce the 16O-mixing line observed for the anhydrous minerals, and another at low temperature to produce the matrix minerals. The isotopic compositions severely constrain the conditions of the low-temperature process, requiring temperatures 44%. Extension of the model to the data on C1 chondrites requires aqueous alteration in a warmer, wetter environment.
Evidence from overprinting of magnetizations of Late Permian and Mesozoic rocks and from the rank of Permian coals and Mesozoic phytoclasts (coal particles) suggests that surface rocks in the Sydney Basin, eastern Australia, have been raised to temperatures of the order of 200°C or higher. As vitrinite reflectance, an index of coal rank or coalification, is postulated to vary predictably with temperature and time, estimates of the paleotemperatures in the Sydney Basin based on observed vitrinite reflectance measurements can be made in conjunction with reasonable assumptions about the tectonic and thermal histories of the basin. These estimates give maximum paleotemperatures of present day surface rocks in the range 60-249°C, depending on factors such as location in the basin, the thickness of the sediment eroded, and the maximum paleogeothermal gradient. Higher coal rank and, consequently, larger eroded thicknesses and paleogeothermal gradients occur along the eastern edge of the basin and may be related to seafloor spreading in the Tasman Sea on the basin's eastern margin. A theory of thermal activation of magnetization entailing the dependence of magnetic viscosity on the size distribution of the magnetic grains is used to obtain an independent estimate of the maximum paleotemperatures in the Sydney Basin. This estimate places the maximum paleotemperature in the range 250-300°C along the coastal region. Both coalification and thermal activation of magnetization models provide strong evidence of elevated paleotemperatures, which in places exceed 200°C, and the loss of sediment thicknesses in excess of 1 km due to erosion.
We present a quantitative relationship between blocking temperature and time that, in principle, provides a calibration of thermal remagnetization in nature. For a given metamorphic temperature-time regime, one can decide whether a given laboratory blocking temperature (or for paleointensity work, a range of blocking temperatures) is consistent with primary natural remanence (NRM) or with a metamorphic overprint. Independent of the domain structure or the chemical composition of the magnetic minerals, two general types of behaviour are predicted. If the primary NRM possesses laboratory (or primary cooling) blocking temperatures within 100°C or so of the Curie temperature, thermal remagnetization at lower temperatures, even over times as long as 106 years, is improbable. If the blocking temperatures are lower, viscous remagnetization is pronounced at temperatures well below those indicated by laboratory thermal demagnetization. An approximate scale of the ``survival potential'' of primary NRM in rocks of different metamorphic grades indicates that primary paleointensities are unlikely to be recovered from rocks metamorphosed above high-greenschist facies if the predominant magnetic mineral is nearly pure magnetite, or above middle-amphibolite facies if nearly pure hematite is predominant. Evidence from laboratory experiments and paleomagnetic field studies in metamorphic regions suggests, however, that these survival estimates are unduly optimistic. Chemical remagnetization through the destruction of primary magnetic minerals, and not thermal remagnetization, probably sets an effective upper temperature for the survival of primary NRM.
The origins of multiple magnetizations of the Archean Stillwater Complex have been investigated through scanning electron microscopy and scanning transmission electron microscopy observations of mineralogical relations, using representative samples from nine sites in mafic Banded series rocks. On the basis of directional grouping and demagnetization behavior, three magnetizations (here labeled A, B, and C) have been recognized. The natural remanent magnetization (NRM) is typically dominated by only one of these magnetizations and multicomponent behavior in individual specimens is rare. The A remanence resides in magnetic grains of high median destructive fields and high, discrete laboratory unblocking temperatures and is inferred to be a primary thermoremanent magnetization, of circa 2.71 Ga age. The B and C magnetizations, of lower median destructive fields and more distributed unblocking temperatures, are inferred to be secondary and related to alteration, including serpentinization, involving limited, moderate to low-temperature fluid interaction, perhaps in response to thermotectonic events (e.g., mafic dike emplacement). Samples with NRM dominated by the A magnetization contain titanium-free magnetite needles (width
Texture of chondrules in the type 3 ordinary chondrites are described. Conditions for the formation of most of them can be roughly estimated by comparison with the experimental results. The chondrules studied are classified by their textures into the following four groups: (1) those formed by complete melting of the precursor materials; (2) those formed by incomplete melting of the precursor materials; (3) 'lithic fragments' or 'lithic chondrules' formed through low temperature heating of the precursor materials; and (4) complex chondrules formed by accretion of two chondrules or by squeezing out the liquid. Probable heating histories for the first three groups are indicated.
The Michikamau anorthosite possesses very stable natural remanent magnetization, some of which resists alternating fields up to 1800 Oe. The rock contains two types of opaque grains, fine opaque needles of order 10 × 0.5 μ in the plagioclase felspar, and large equidimensional magnetite particles. Ore microscope studies suggest, but do not establish, that the needles are composed of magnetite. Saturation isothermal remanence and thermal demagnetization studies indicate magnetite as the carrier of remanent magnetization. In order to distinguish the effects of the large grains from those of the needles, mineral separation was used to show that an artificial specimen of essentially pure plagioclase had very similar isothermal remanent magnetization properties to the whole rock. Both indicated magnetite as the magnetic mineral. Thermoremanent properties of the separated mineral fractions indicated magnetite as the dominant magnetic constituent but showed some evidence of laboratory-produced hematite. Theoretical models of grains elongated along [111] and [110] axes are used to show that magnetite needles can exist in stable single-domain configuration in the size and shape ranges of the needles observed in the Michikamau anorthosite. There is thus considerable experimental and theoretical evidence for the conclusion that the stable remanent magnetization of the Michikamau anorthosite is carried by fine single–domain needles of magnetite in the plagioclase felspar.
The authors discuss the characteristics of agglomeratic chondrules and discuss their implications for understanding chondrule precursors and chondrule evolution. Additionally, they review some of the other aggregational components in chondrites in order to emphasize their ubiquity in many chondrites. Finally, the authors evaluate the evidence for chondrule formation by complete vs. incomplete melting and try to assess which of these processes was dominant.
The important effects of oxidation conditions upon the stability and
physical properties of iron-bearing olivine have not been fully
considered in recent geophysical studies of this mineral. Calculated on
the basis of existing thermodynamic data, a T-ƒo2 phase
diagram for olivine at 1 atm total pressure shows the conditions under
which olivine of any given composition is thermodynamically stable. Due
regard for these conditions must be given in the design and
interpretation of geophysical experiments involving olivine.
The memoir is devoted to a brief theoretical study of the most typical magnetic properties of rocks. In particular §§ 3–16 are on ferrimagnetism, §§ 17–35 on single domain particles and §§ 36–57 on large multi-domain particles.
Theoretical studies are made of the following aspects of the subject and compared with the experimental results: remanent magnetization (§ 38), initial susceptibility (§ 39), variation with applied field of thermoremanent magnetization (abbreviated to T.R.M.) (§§ 40, 41, 57), the ratio Qk of T.R.M. acquired in a given field to the induced magnetization in the same field (§ 42), the additivity of partial T.R.M.'s in the case both of small grains (§ 28) and large grains (§ 57).
Considerable space is devoted to the magnetic ‘viscosity’ due to thermal agitation in small grains (§§ 24–27) and in larger ones (§§ 49–56). Expressions are given for magnetic ‘viscosity’ in the range of Rayleigh's relations (§ 51) particularly with a demagnetizing field present (§ 54). The theoretical and experimental results on the irreversible decrease in isothermal remanent magnetization are briefly quoted both for small (§ 30) and large (§ 55) grains.
Different reversing mechanisms are reviewed which could cause a negative T.R.M., that is one directed in the opposite sense to the field applied during cooling. Some are related to negative Weiss-Heisenberg exchange forces: reversal by diffusion involving ionic exchange between the two sub-lattices in a ferrimagnetic (§ 7), reversal by anomalous thermal variation is spontaneous magnetization (§§ 11, 12), reversal by diffusion with complete change of composition (§ 16). The others are effects of the demagnetizing field: reversal in mixtures of two constituents with different Curie Points (§§ 31–34), reversal by segregation, allotropy and chemical alteration (§ 35). The actual examples so far known are recalled.
The opaque magnetic constituents are re-examined for Allende chondrite specimens before their magnetic properties are measured. The main interest is concerned with natural remanent magnetization (NRM) and the most probable paleointensity of the Allende chondrite. The composition and structure of opaque minerals of both chondrules and matrix are examined in order to confirm the ferromagnetic (or ferrimagnetic) constituents in the Allende specimen. Results of Konigisberger-Thellier experiments of a number of bulk and matrix specimens and of a chondrule having stable NRM (assuming that NRM is partial thermoremanent magnetization acquired during cooling from about 320 degrees C in the presence of a magnetic field) lead to a conclusion that the paleointensity of the Allende chondrite is given by a magnetic field of 1-3 oersteds.
Experimental simulations of chondrules have been used in attempts to define both chondrule heating and cooling conditions and hence throw light on the heating mechanism and formation environment.
Submicroscopic, needle-shaped titanomagnetite inclusions exsolved in silicate minerals commonly occur in mafic intrusive rocks and are protected from alteration by their silicate hosts, making them excellent candidates for paleomagnetic studies. A suite of samples containing clinopyroxene- and plagioclase-hosted magnetite inclusions from five geologically diverse sites was examined using magnetic force microscopy to image the inclusions' magnetic domain state. Alternating field demagnetization experiments indicate that some inclusions are more stable recorders than others. The two factors controlling the remanence behavior of the inclusions are internal microstructures and inclusion dimensions. Magnetite-ulvöspinel unmixing within an inclusion subdivides the original titanomagnetite solid solution into a boxwork structure composed of 103 105 magnetite prisms separated by thin ulvöspinel lamellae. The conversion of multidomain-sized needles into assemblages of interacting single domains increases the coercivity (and hence relaxation time) of the inclusions, and results in a thermochemical magnetic remanence. In samples without this exsolution microstructure, the inclusions' diameters determine coercivity and their magnetization is thermoremanent. Both styles of high-coercivity inclusions successfully record paleomagnetic directions in Mesozoic rocks, and their ubiquity within silicate minerals (clinopyroxene and plagioclase) of mafic intrusive rocks indicates their value as chemically and magnetically stable tools for elucidating the ancient magnetic field, marine magnetic anomalies, and crustal kinematics.
The focus of this paper is to present a brief summary of experiments that were designed to study some aspect of chondrule precursors and the constraints that can be derived from them. Furthermore, this paper also presents the reader with knowledge of what one cannot say about chondrule precursors based on experiments.
Crystallographically oriented and highly elongate magnetite inclusions in clinopyroxene are the dominant source of highly stable remanent magnetization in gabbros of the Early Cretaceous Messum Complex, Namibia. Rock magnetic properties determined for individual pyroxene crystals indicate a high proportion of single-domain magnetite, consistent with the observed sizes and shape anisotropy of the magnetite inclusions. As in previous studies of similar inclusions, these are inferred to have formed by exsolution. Two arrays of inclusions are regularly present in the Messum clinopyroxenes, inclined at about 74°, consistent with formation at about 800°C deduced from optimization of phase boundary orientations. Virtual geomagnetic poles from these rocks are consistent with reference data, confirming that the magnetization is of thermoremanent origin. Bipolar magnetizations are recorded at one site as well in individual clinopyroxene crystals, suggesting that remanence acquisition upon initial cooling of the gabbro spanned a geomagnetic polarity reversal.
The thermal history of four spinel lherzolites (Lhz-13. Lhz-28, Lhz-29 and Lhz-53) from tuff breccia of the Ichinomegata crater, northeast Japan, has been studied in detail. Lhz-13 and Lhz-53 showed nearly perfect chemical homogeneity of the constituent minerals, and increase of Ca near the rim of olivine is the only disequilibrium evidence observed. In addition to the Ca zoning in olivine, Lhz-28 and Lhz-29 revealed compositional zoning in the Mg/Mg + Fe ratio and Ca content in ortho- and clinopyroxenes. Lhz-13 and Lhz-53 equilibrated at about 800°C in the upper mantle, based on Fe/Mg partitioning between olivine/spinel and olivine/clinopyroxene, and on the mutual solubility of Ca between olivine and pyroxenes. Lhz-28 and Lhz-29 also equilibrated originally at about 800°C, but were preheated at about 1000°C prior to their entrapment by the ascending host magma. The Fe/Mg partitioning between olivine /spinel and olivine/clinopyroxene reequilibrated during the preheating event: however, the Ca solubility did not reequilibrate. Olivine alone has rehomogenized with a high-Ca content but pyroxenes were compositionally zoned with Ca. The preheating event, indicated by the high-Ca content in the core of olivine, is recognized from about a half of the Ichinomegata Iherzolites (50 xenoliths were studied). The duration of heating during the transport of the xenolith by the magma (estimated from the width of the Ca zoning in the rim of olivine) ranges between several hours to a year depending on the rock specimen. From the requirement to reset olivine core compositions, the duration of the preheating event was estimated as greater than 1000 yr.
New time-temperature relations for remagnetization of the mineral kamacite show that much of the magnetization observed in ancient lunar rocks is stable over billions of years and almost certainly originated on the Moon.
(1) The basic features of the geomagnetic reversal chronology of the last 160 million years are well established. The relationship between this history and other features of the field, however, has been elusive. The determination of past field strength (paleointensity) is especially challenging. Commonly accepted results have come from analyses of bulk samples of lava. Historic lavas have been shown to faithfully record the past field strength when analyzed using the Thellier double-heating method. Data from older lavas, however, tend to show effects of in situ and laboratory-induced alteration. Here we review an alternative approach. Single plagioclase crystals can contain minute magnetic inclusions, 50-350 nm in size, that are potential high-fidelity field recorders. Thellier experiments using plagioclase feldspars from an historic lava on Hawaii provide a benchmark for the method. Rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples. This resistance is likely related to the lack of clays. In addition, magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well-defined thermoremanent magnetization into an irresolute chemical remanent magnetization. If there is a relationship between geomagnetic reversal frequency and paleointensity, it should be best expressed during superchrons, intervals with few (or no) reversals. Thellier data sets based on single plagioclase crystals from lavas erupted during the Cretaceous Normal Polarity Superchron (� 83-120 million years ago) suggest a strong (>12 � 1022 Am2), stable field, consistent with an inverse relationship between reversal frequency and paleointensity. Superchrons may represent times when the pattern of core-mantle boundary heat flux allows the geodynamo to operate at peak efficiency, as suggested in some numerical models. Thellier data from single plagioclase crystals formed during times of moderate (10 reversals/million years) reversal occurrence suggest a weaker and more variable field. These paleointensity data, together with a consideration of paleomagnetic directions, suggest that geomagnetic reversals, field morphology, secular variation, and intensity are related. The linkages over tens of millions of years imply a lower mantle control on the geodynamo. On even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the Proterozoic and Archean geomagnetic field during the onset of growth of the solid inner core. Data from plagioclase crystals separated from mafic dikes, together with directional data from whole rocks, indicate a dipole-dominated field similar to that of the modern, 2.5-2.7 billion years ago. Older Archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow. Most rocks of this age have been affected by low- grade metamorphism; investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo. Absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization, anisotropy, and slow cooling. Fortunately, prior work in rock magnetism, together with advances in analytical equipment and techniques, provides a solid foundation from which these frontier issues can be approached.
The intensity of natural remanent magnetization (NRM) is nearly
proportional to the intensity of saturation remanent magnetization in
achondrites and carbonaceous chondrites. The NRM in these meteorites is
stable against alternating field demagnetization. This NRM is thought to
reflect magnetic fields that were present in the early solar system. The
correlation between the NRM intensity and the intensity of saturation
remanent magnetization in ordinary chondrites is weak. Since the
coercive force of kamacite in ordinary chondrites is very small, it is
possible that the extraterrestrial remanence in many ordinary chondrites
is dominated by a soft, spurious remanence. Theoretically,
interplanetary magnetic fields of appropriate strength could have
existed for 10 Myr in the early solar system, causing meteorites to be
magnetized during the accretion and cooling stages.
Abstract— We present a detailed petrographic and electron microprobe study of metal grains and related opaque minerals in the chondrule interiors and rims of the Bishunpur (LL3.1) ordinary chondrite. There are distinct differences between metal grains that are completely encased in chondrule interiors and those that have some portion of their surface exposed outside of the chondrule boundary, even though the two types of metal grains can be separated by only a few microns. Metal grains in chondrule interiors exhibit minor alteration in the form of oxidized P-, Cr-, and Si-bearing minerals. Metal grains at chondrule boundaries and in chondrule rims are extensively altered into troilite and fayalite.
The results of this study suggest that many metal grains in Bishunpur reacted with a type-I chondrule melt and incorporated significant amounts of P, Cr, and Si. As the system cooled, some metal oxidation occurred in the chondrule interior, producing metal-associated phosphate, chromite, and silica. Metal that migrated to chondrule boundaries experienced extensive corrosion as a result of exposure to the external atmosphere present during chondrule formation. It appears that chondrule-derived metal and its corrosion products were incorporated into the fine-grained rims that surround many type-I chondrules, contributing to their Fe-rich compositions. We propose that these fine-grained rims formed by a combination of corrosion of metal expelled from the chondrule interior and accretion of fine-grained mineral fragments and microchondrules.
Chondrule precursors encompass solids that may have condensed from
nebular gas, as well as large grains of unknown origin, but they were
not composed of material formed by crystal-fractionation. While
planetary models of chondrule formation on the basis of impact heating
or asteroidal volcanism fail to satisfy the mineralogical, chemical,
isotopic, and physical constraints on the energy source of the formation
process, neither can nebular models forming chondrule liquids by direct
condensation from the gas phase be reconciled with existing data. A
dusty solar nebula before or after planetesimal formation is suggested
as a superior alternative setting for chondrule formation.
Iron–nickel metallic particles have been reported in meteorites1 and lunar2–5 and terrestrial6,7 rocks. The origin of these metallic particles is not unique as they may be formed by (1) condensation from a primordial solar nebula8; (2) crystallization from a melt; and (3) subsolidus reduction reactions under low oxygen or sulphur fugacity. We report here an electron microscopy study of the solid-state microstructural development in olivine single crystals (Fo92) in which half of the iron has been reduced to the metallic state by a gas–solid interaction in the temperature range 950–1,500 °C. The reaction, Fo92Fo96+metallic Fe(Ni in solid solution)+pyroxene, begins with a homogeneous transformation involving fine-scale metallic precipitates resembling Guinier–Preston zones9. The microstructure develops by the growth of the first-formed precipitates during an Ostwald ripening process9 in which the precipitates located in the dislocation sub-boundaries develop in preference to precipitates in the subgrains. On the other hand, pyroxene is first observed to nucleate heterogeneously at pre-existing dislocations and its coarsening rate is more than an order-of-magnitude faster than that of the metallic phase. Besides the textural similarity of the observed microstructures with that reported for some of the lunar materials2, these results have important implications for the physical models of accretion of terrestrial planets, planetesimals and meteorites10, especially with respect to the distribution of siderophile elements. The rate of reaction observed here places constraints on models for the formation of the Earth's core by segregation of a metallic phase with or without reduction.
Although chondrules have been extensively studied chemically1,2, petrologically3–5 and experimentally6–8, their origin is still not certain. I discuss here whether chondrules were condensed directly from the primitive solar nebula9–11 or were formed from precursory materials by various mechanisms, such as high velocity impact of small bodies12,13, impact on the surface of a parental body14,15 or dust fusion16–18. Investigations of an Antarctic meteorite suggest that chondrules could have formed through melting of pre-existing materials.
CHONDRULES are millimetre-sized spheroidal bodies composed mainly of olivine and orthopyroxene, which comprise the dominant fraction of most chondritic meteorites. They are the products of partial melting of aggregates of fine-grained silicates with minor contributions from metals, sulphides and oxides. Although the formation conditions of chondrules are not well understood, these are thought to involve a transient melting event in the solar nebula13. The ubiquity of reduced carbon in interstellar clouds and primitive meteorites implies that it was also present in the early solar nebula, and may thus have been a potential constituent of chondrule pre-cursor material. We describe here experiments in which carbon and magnesian silicate precursor material of primitive chondrule composition are 'flash-heated' together and then crystallized. The resulting material shows many mineralogical features character-istic of natural chondrules, which are not produced in the absence of carbon412. Our results suggest not only that carbon was present in the solar nebula, but also that it played a key role in chondrule formation by creating within the melt a reducing environment that was decoupled from the nebula gas.
Abstract— An analytical transmission electron microscopy (ATEM) study was undertaken in order to better understand the formation conditions of dusty olivines (i.e., olivines containing abundant tiny inclusions of Fe-Ni metal) in primitive meteorites. Dusty olivines from type I chondrules in the Bishunpur chondrite (LL3.1) and from synthetic samples obtained by reduction of San Carlos olivines were examined. In both natural and experimental samples, micron size metal blebs observed in the dusty region often show preferential alignments along crystallographic directions of the olivine grains, have low Ni contents (typically <2 wt%), and are frequently surrounded by a silica-rich glass layer. These features suggest that dusty olivines are formed by a sub-solidus reduction of initially fayalitic olivines according to the following reaction:
Some volatilization of SiOgas may account for the apparent excess of metal relative to silica-rich glass observed in both experimental and natural samples. Comparison with experimentally produced dusty olivines suggests that time scales of the order of minutes usually inferred for chondrule formation are also adequate for the formation of dusty olivines. These observations are in agreement with the hypothesis that at least part of the metal phase in chondrites originated from reduction during chondrule formation.
Abstract— We address the origin of “dusty,” metal-bearing relict olivine grains in chondrules. It has been suggested previously that these grains may be either primitive condensates or derived from a previous generation of chondrules. In this paper, we infer the original composition of dusty olivine grains, before they were reduced, and examine the possibility that they were derived from a previous generation of chondrules. Original compositions of dusty grains, including their estimated initial FeO contents and their minor element contents, match closely with compositions of olivines from chondrules in unequilibrated chondrites. In addition, the cores of some dusty grains are unaltered, and the compositions of these cores are also consistent with a chondrule origin. Therefore, we conclude that a derivation from a previous generation of chondrules is a plausible origin for these relicts. Although alternative origins, such as condensates or interstellar grains, cannot be ruled out on the basis of the available data, chondrules are an obvious source, and we suggest that this is the most likely interpretation. If this is the case, it is additional evidence for the importance of recycling of chondrule material in the chondrule-forming region.
Abstract— In order to investigate the distribution of 26A1 in chondrites, we measured aluminum-magnesium systematics in four calcium-aluminum-rich inclusions (CAIs) and eleven aluminum-rich chondrules from unequilibrated ordinary chondrites (UOCs). All four CAIs were found to contain radiogenic 26Mg (26Mg*) from the decay of 26A1. The inferred initial 26Al/27Al ratios for these objects ((26Al/27Al)0 ≅ 5 × 10−5) are indistinguishable from the (26Al/27Al)0 ratios found in most CAIs from carbonaceous chondrites. These observations, together with the similarities in mineralogy and oxygen isotopic compositions of the two sets of CAIs, imply that CAIs in UOCs and carbonaceous chondrites formed by similar processes from similar (or the same) isotopic reservoirs, or perhaps in a single location in the solar system. We also found 26Mg* in two of eleven aluminum-rich chondrules. The (26Al/27Al)0 ratio inferred for both of these chondrules is ∼1 × 10−5, clearly distinct from most CAIs but consistent with the values found in chondrules from type 3.0–3.1 UOCs and for aluminum-rich chondrules from lightly metamorphosed carbonaceous chondrites (∼0.5 × 10−5 to ∼2 × 10−5). The consistency of the (26Al/27Al)0 ratios for CAIs and chondrules in primitive chondrites, independent of meteorite class, implies broad-scale nebular homogeneity with respect to 26Al and indicates that the differences in initial ratios can be interpreted in terms of formation time. A timeline based on 26Al indicates that chondrules began to form 1 to 2 Ma after most CAIs formed, that accretion of meteorite parent bodies was essentially complete by 4 Ma after CAIs, and that metamorphism was essentially over in type 4 chondrite parent bodies by 5 to 6 Ma after CAIs formed. Type 6 chondrites apparently did not cool until more than 7 Ma after CAIs formed. This timeline is consistent with 26Al as a principal heat source for melting and metamorphism.
Abstract— We present a model for the thermal processing of particles in shock waves typical of the solar nebula. This shock model improves on existing models in that the dissociation and recombination of H2 and the evaporation of particles are accounted for in their effects on the mass, momentum and energy fluxes. Also, besides thermal exchange with the gas and gas-drag heating, particles can be heated by absorbing the thermal radiation emitted by other particles. The flow of radiation is calculated using the equations of radiative transfer in a slab geometry. We compute the thermal histories of particles as they encounter and pass through the shock.
We apply this shock model to the melting and cooling of chondrules in the solar nebula. We constrain the combinations of shock speed and gas density needed for chondrules to reach melting temperatures, and show that these are consistent with shock waves generated by gravitational instabilities in the protoplanetary disk. After their melting, cooling rates of chondrules in the range 10–1000 K h−1 are naturally reproduced by the shock model. Chondrules are kept warm by the reservoir of hot shocked gas, which cools only as fast as the dust grains and chondrules themselves can radiate away the gas's energy. We predict a positive correlation between the concentration of chondrules in a region and the cooling rates of chondrules in that region. This correlation is supported by the unusually high frequency of (rapidly cooled) barred chondrules among compound chondrules, which must have collided preferentially in regions of high chondrule density. We discuss these and other compelling consistencies between the meteoritic record and the shock wave model of chondrule formation.
The petrologic and geochemical properties of chondrules as well as results of experimental studies provide strong constraints on chondrule-formation models. Nebular formation is indicated by the non-mass-fractionated oxygen isotopic compositions of bulk chondrules. Chondrule formation from a melt is required by the prototypical spheroidal shapes and the presence of euhedral phenocrysts and glassy mesostases. Incomplete melting is indicated by the abundance of porphyritic chondrules (which experiments demonstrate require relict nuclei) and coarse relict grains. The length of time that chondrules were hot is constrained by their retention of relict grains and moderately volatile elements. Rapid cooling of chondrules after formation is supported by the presence of zoned phenocrysts, isotopic anomalies and dynamic crystallization experiments. It is clear from the presence of relict grains, enveloping compound chondrules and igneous rims that many chondrules were heated again after cooling. The heating mechanism responsible for chondrule formation seems to have operated at varying intensities over large regions of the inner solar nebula for at least the time it took ambient nebular temperatures to cool from above ∼900 to below ∼600 K. The chondrule-formation mechanism provided a repeatable source of energy capable of highly localized melting, characteristic of flash heating. The occurrence of ferroan microchondrules with low melting temperatures within some chondrule rims indicates that chondrule formation did not occur exclusively in high-temperature regions near the Sun as required in bipolar outflow models. Mechanisms for forming chondrules that are consistent with the constraints include various flash-heating models: nebular lightning, magnetic reconnection flares, gas dynamic shock waves and radiative heating.
It appears that the highly unequilibrated Bishunpur ordinary chondrite preserves phase relations acquired during solar nebular processes to a relatively high degree; metamorphic temperatures may not have exceeded 300–350°C. The major categories of metal are: 3 kinds of metal in the metal matrix, three kinds in chondrule interiors and 2 kinds in chondrule rims. The fine-grained matrix metal is highly variable in composition: the kamacite Co content (7.8 ± 2.0 mg/g) is within the L-group range (6.7–8.2 mg/g) but extends well above and below; its Ni content (38 ± 5 mg/g) is considerably lower than in more equilibrated chondrites and taenite is Ni-rich ( > 450 mg/g) and unzoned. These compositions imply equilibration at very low temperatures of about 300–350°C. It seems unlikely that volume diffusion could account for the observed relatively unzoned phases; a better model involves mass transport by grain boundary diffusion and grain growth at the indicated temperatures. We find no evidence that the matrix was ever at higher temperatures. Large (50–650 μm) polycrystalline metal aggregates consisting of individually zoned crystals are also found in the matrix; they probably represent clusters formed in the solar nebula. A few large (50–250 μm) round monocrystalline grains are also present in the matrix.
The magnetic properties of eleven ordinary chondrites (eight LL-chondrites and three L-chondrites) have been analysed. The samples were repeatedly fragmented and oriented to a common reference direction, natural remanent magnetisation (NRM), susceptibility (χ), and anisotropy of susceptibility measured at each stage. The response of some fragments to alternating field (a.f.) demagnetisation, thermal demagnetisation, isothermal remanent magnetisation (IRM), and magnetic hysteresis has been measured. It was found that the orientation of the stable NRM was random, down to a scale of ∼ 1 mm3. The dominant magnetic carrier in types 4–6 was the ordered iron-nickel mineral γ″-tetrataenite. The type 3's measured contained no tetrataenite and the NRM was carried by fine-grained taenite. A magnetic fabric, analogous to a physical fabric, was found in all samples. In all but two, the fabric was foliated. Tuxtuac and Wold Cottage showed lineation. Thermal demagnetisations of NRM showed a trend towards higher blocking temperatures for type 6 chondrites over types 3–5. Because the fabric was continuous in the majority of samples, and the NRM randomly orientated, it was concluded that the magnetic carriers were magnetised before emplacement in the meteorite, and that the meteorites are not finescale breccias. The preservation of the random NRM leads to the preference of hot accretion as the mechanism for producing chondritic textures, as opposed to metamorphic reheating, as this would tend to erase the random magnetisation.
Constraints placed on chondrule formation have largely been generated from experiments which use a long duration, below liquidus isothermal melting (minutes to hours) rather than a short duration, above liquidus flash melting event (seconds to minutes). In this paper we examine how a short duration, superliquidus heat pulse can produce chondrule textures. By incompletely melting material with a type of flash melting we show that the maximum temperature limit of chondrule formation was approximately 2100°C, almost 400°C higher than previously constrained. Previous experiments also have not studied the effect of variations in precursor grain size on the formation of chondrule textures. For this reason we simultaneously investigate the effect of variations in the grain size of a starting composition on the formation of chondrule textures. We show how MgO-rich (Type IA) chondrules and other fine-grained chondrules could only have been formed from the incomplete melting of a rather uniformly grain sized precursor of less than 63μm. Because fine-grained, MgO-rich chondrules have the some of the highest chondrule liquidus temperatures, we proposed that these types of textures define a minimum melting temperature for chondrule formation.
The ordinary chondrites, Semarkona and Bishunpur, appear to have experienced in situ hydrous alteration. Here we report the results of the first detailed TEM examination of them, which supports this conclusion. In Semarkona, all but the most magnesian mafic minerals have been destroyed, and primary sulphides, most likely troilite, have been partially altered to phyllosilicates and Ni-rich pyrrhotite. In Bishunpur, alteration is confined to the production of smectite from an amorphous feldspathic material in the chondrule rims and interchondrule matrix. During hydrous alteration of both meteorites, the temperature probably did not exceed 260°C. The equilibrium composition of the gas present during the alteration of Semarkona was not solar but was dominated by H2O. The formation of smectite does not require the presence of liquid water. However, evidence for the redistribution of elements, such as Ca, Zn, and Si, over tens of microns or more implies that there was at least some transient grain-boundary fluid present. The alteration in Semarkona and Bishunpur is far from complete. Thermodynamic modelling suggests that more altered ordinary chondrites would be mineralogically similar to the CM and CI carbonaceous chondrites.Superficially, the mineralogy, mineral chemistry, and bulk chemistry of Semarkona matrix-rims resemble those of some chondritic interplanetary dust particles (CIDPs). The ordinary chondrite parent bodies may, therefore, be a source of some CIDPs.
Theoretical blocking curves for hematite and magnetite are given over temperature ranges from room temperatures to Curie temperature and over time scales from 1 second to 109 years. In principle these curves predict how remanent magnetization in rocks is gradually lost during the heating accompanying burial and metamorphism, and how it is replaced by a new remanence during uplift and cooling. A series of experiments are described which give the decay of remanent magnetization and the build up of viscous magnetization during long-term heating at temperatures up to 400°C. Tests showed that in the selected samples minimal chemical changes occured. The treatment is considered to be analogous to the burial of rocks to 5–10 km for several millions of years. It is shown that burial magnetizations acquired in this way should be removed by standard magnetic cleaning procedures. A second series of experiments at somewhat higher temperatures (400–530°C) were carried out in order to observe the unblocking of magnetite and hematite in long-term heating experiments, and to compare the with the theoretically derived curves. In the second experiments there is a discrepancy with theory, and a compromise is proposed which indicates that very little, if any remanence can survive high greenschist facies metamorphism, and that the age of the remanence in high-grade metamorphic rocks is between the Rb—Sr whole-rock isochron and K—Ar mica ages. It is also argued that burial magnetizations are being generated in the present geomagnetic field in rocks at depths of the order of 10 km and are a potent source of magnetic anomalies over the continental crust.
Taenite with 48–57% Ni is magnetically soft, but when transformed via atomic ordering below 320°C it becomes the new magnetic mineral tetrataenite with considerable magnetic hardness and other distinctive magnetic properties. Tetrataenite is ubiquitous in chondrite meteorites, occurring as discrete grains, as a clear taenite rim, and in the etched cloudy zone. Measured remanent coercivities range from ∼ 30 mT in Appley Bridge (LL4) to ∼ 600 mT in some Bjurbole (L4) chondrules. This range suggests a considerable variation in tetrataenite properties that might be related to degree of order, size and properties of the ordered regions, possible shock induced effects, and other causes, including the initial grain size, particularly in the cloudy zone.
Although Mars does not presently appear to have a global dynamo magnetic field, strong crustal fields have recently been detected by the Mars Global Surveyor above surfaces formed ∼3 or more Ga. We present magnetic and textural studies of Martian meteorite ALH84001 demonstrating that 4 Ga carbonates containing magnetite and pyrrhotite carry a stable natural remanent magnetization. Because 40Ar/39Ar thermochronology demonstrates that most ALH84001 carbonates have probably been well below the Curie point of magnetite since near the time of their formation [Weiss et al., Earth Planet. Sci. Lett. (2002) this issue], their magnetization originated at 3.9–4.1 Ga on Mars. This magnetization is at least 500 million years (Myr) older than that known in any other planetary rock, and its strong intensity suggests that Mars had generated a geodynamo and global magnetic field within 450–650 Myr of its formation. The intensity of this field was roughly within an order of magnitude of that at the surface of the present-day Earth, sufficient for magnetotaxis by the bacteria whose magnetofossils have been reported in ALH84001 and possibly for the production of the strong crustal anomalies. Chromite in ALH84001 may retain even older records of Martian magnetic fields, possibly extending back to near the time of planetary formation.
The abundance of metallic iron is highly variable in different kinds of chondrites. The precise mechanism by which metal fractionation occurred and its place in time relative to chondrule formation are unknown. As metallic iron is abundant in most Type I (FeO-poor) chondrules, determining under what conditions metal could form in chondrules is of great interest. Assuming chondrules were formed from low temperature nebular condensate, we heated an anhydrous CI-like material at 1580°C in conditions similar to those of the canonical nebula (PH2 = 1.3 × 10⁻⁵ atm). We reproduced many of the characteristics of Type IA and IIA chondrules but none of them contained any iron metal. In these experiments FeO was abundant in charges that were heated for as long as 6 h. At a lower temperature, 1350°C, dendritic/cellular metal crystallized from Fe-FeS melts during the evaporation of S. However, the silicate portion consisted of many relict grains and vesicles, not typical of chondrules.
In early solar system history there are several electromagnetic processes (electric discharges, pressure shock waves, electric discharges and currents) capable of magnetizing the primitive solid particles condensating from the solar nebula. The record of these magnetic events is the main objective of rock magnetic laboratory studies of meteorites found on the Earth. However, terrestrial environment can affect the magneto-mineralogy, can cause changes in magnetic parameters and can overprint the primary magnetic record.