Meteorite cloudy zone formation as a quantitative indicator of paleomagnetic field intensities and cooling rates on planetesimals

Metallic microstructures in slowly-cooled iron-rich meteorites reflect the thermal and magnetic histories of their parent planetesimals. Of particular interest is the cloudy zone, a nanoscale intergrowth of Ni-rich islands within a Ni-poor matrix that forms below 350{\deg}C by spinodal decomposition. The sizes of the islands have long been recognized as reflecting the low-temperature cooling rates of meteorite parent bodies. However, a model capable of providing quantitative cooling rate estimates from island sizes has been lacking. Moreover, these islands are also capable of preserving a record of the ambient magnetic field as they grew, but some of the key physical parameters required for recovering reliable paleointensity estimates from magnetic measurements of these islands have been poorly constrained. To address both of these issues, we present a numerical model of the structural and compositional evolution of the cloudy zone as a function of cooling rate and local composition. Our model produces island sizes that are consistent with present-day measured sizes. This model enables a substantial improvement in the calibration of paleointensity estimates and associated uncertainties. In particular, we can now accurately quantify the statistical uncertainty associated with the finite number of islands and the uncertainty on their size at the time of the record. We use this new understanding to revisit paleointensities from previous pioneering paleomagnetic studies of cloudy zones. We show that these could have been overestimated but nevertheless still require substantial magnetic fields to have been present on their parent bodies. Our model also allows us to estimate absolute cooling rates for meteorites that cooled slower than 10000{\deg}C My-1. We demonstrate how these cooling rate estimates can uniquely constrain the low-temperature thermal history of meteorite parent bodies.Comment: Manuscript resubmitted after revision

Magnetic meteorites and the early solar system

Today, the Earth generates a magnetic field through convection of the electrically conducting molten iron in its outer core. Core convection is governed by the thermal and chemical processes that operate deep within our planet; thus measurements of the intensity and direction of the magnetic field can provide insights into the thermochemical state of the Earth's interior. Crustal rocks can also record and preserve a memory of the field they experienced as they were forming. Paleomagnetic measurements can therefore provide records of ancient magnetic activity and, by extension, the internal conditions of our planet in the past (Tarduno et al. 2014). A combination of paleomagnetic and present-day magnetic measurements therefore allow us to study the long-term and large-scale evolution of our planet over billions of years; this method could also potentially allow us to predict how it may behave in the future

Magnetic meteorites and the early solar system

Today, the Earth generates a magnetic field through convection of the electrically conducting molten iron in its outer core. Core convection is governed by the thermal and chemical processes that operate deep within our planet; thus measurements of the intensity and direction of the magnetic field can provide insights into the thermochemical state of the Earth's interior. Crustal rocks can also record and preserve a memory of the field they experienced as they were forming. Paleomagnetic measurements can therefore provide records of ancient magnetic activity and, by extension, the internal conditions of our planet in the past (Tarduno et al. 2014). A combination of paleomagnetic and present-day magnetic measurements therefore allow us to study the long-term and large-scale evolution of our planet over billions of years; this method could also potentially allow us to predict how it may behave in the future

Constraints on asteroid magnetic field evolution and the radii of meteorite parent bodies from thermal modelling

Paleomagnetic measurements of ancient terrestrial and extraterrestrial samples indicate that numerous planetary bodies generated magnetic fields through core dynamo activity during the early solar system. The existence, timing, intensity and stability of these fields are governed by the internal transfer of heat throughout their parent bodies. Thus, paleomagnetic records preserved in natural samples can contain key information regarding the accretion and thermochemical history of the rocky bodies in our solar system. However, models capable of predicting these field properties across the entire active lifetime of a planetary core that could relate the processes occurring within these bodies to features in these records and provide such information are limited. Here, we perform asteroid thermal evolution models across suites of radii, accretion times and thermal diffusivities with the aim of predicting when fully and partially differentiated asteroids generated magnetic fields. We find that dynamo activity in both types of asteroid is delayed until ∼4.5-5.5 Myr after calcium-aluminium-rich inclusion formation due to the partitioning of 26Al into the silicate portion of the body during differentiation and large early surface heat fluxes, followed by a brief period (<12.5 Myr for bodies with radii <500 km) of thermally-driven dynamo activity as heat is convected from the core across a partially-molten magma ocean. We also expect that gradual core solidification produced compositionally-driven dynamo activity in these bodies, the timing of which could vary by tens to hundreds of millions of years depending on the S concentration of the core and the radius of the body. There was likely a pause in core cooling and dynamo activity following the cessation of convection in the magma ocean. Our predicted periods of magnetic field generation and quiescence match eras of high and low paleointensities in the asteroid magnetic field record compiled from paleomagnetic measurements of multiple meteorites, providing the possible origins of the remanent magnetisations carried by these samples. We also compare our predictions to paleomagnetic results from different meteorite groups to constrain the radii of the angrite, CV chondrite, H chondrite, IIE iron meteorite and Bjürbole (L/LL chondrite) parent bodies and identify a likely nebula origin for the remanent magnetisation carried by the CM chondrites

Meteorite cloudy zone formation as a quantitative indicator of paleomagnetic field intensities and cooling rates on planetesimals

Metallic microstructures in slowly-cooled iron-rich meteorites reflect the thermal and magnetic histories of their parent planetesimals. Of particular interest is the cloudy zone, a nanoscale intergrowth of Ni-rich islands within a Ni-poor matrix that forms below ∼350 °C by spinodal decomposition. The sizes of the islands have long been recognized as reflecting the low-temperature cooling rates of meteorite parent bodies. However, a model capable of providing quantitative cooling rate estimates from island sizes has been lacking. Moreover, these islands are also capable of preserving a record of the ambient magnetic field as they grew, but some of the key physical parameters required for recovering reliable paleointensity estimates from magnetic measurements of these islands have been poorly constrained. To address both of these issues, we present a numerical model of the structural and compositional evolution of the cloudy zone as a function of cooling rate and local composition. Our model produces island sizes that are consistent with present-day measured sizes. This model enables a substantial improvement in the calibration of paleointensity estimates and associated uncertainties. In particular, we can now accurately quantify the statistical uncertainty associated with the finite number of islands acquiring the magnetization and the uncertainty on their size at the time of the record. We use this new understanding to revisit paleointensities from previous pioneering paleomagnetic studies of cloudy zones. We show that these could have been overestimated by up to one order of magnitude but nevertheless still require substantial magnetic fields to have been present on their parent bodies. Our model also allows us to estimate absolute cooling rates for meteorites that cooled slower than <10,000 °C My−1. We demonstrate how these cooling rate estimates can uniquely constrain the low-temperature thermal history of meteorite parent bodies. Using the main-group pallasites as an example, we show that our results are consistent with the previously-proposed unperturbed, conductive cooling at low temperature of a ∼200-km radius main-group pallasite parent body

Paleomagnetic evidence for dynamo activity driven by inward crystallisation of a metallic asteroid

The direction in which a planetary core solidifies has fundamental implications for the feasibility and nature of dynamo generation. Although Earth's core is outwardly solidifying, the cores of certain smaller planetary bodies have been proposed to inwardly solidify due to their lower central pressures. However, there have been no unambiguous observations of inwardly solidified cores or the relationship between this solidification regime and planetary magnetic activity. To address this gap, we present the results of complimentary paleomagnetic techniques applied to the matrix metal and silicate inclusions within the IVA iron meteorites. This family of meteorites has been suggested to originate from a planetary core that had its overlaying silicate mantle removed by collisions during the early solar system. This process is thought to have produced a molten ball of metal that cooled rapidly and has been proposed to have inwardly solidified. Recent thermal evolution models of such a body predict that it should have generated an intense, multipolar and time-varying dynamo field. This field could have been recorded as a remanent magnetisation in the outer, cool layers of a solid crust on the IVA parent core. We find that the different components in the IVA iron meteorites display a range of paleomagnetic fidelities, depending crucially on the cooling rate of the meteorite. In particular, silicate inclusions in the quickly cooled São João Nepomuceno meteorite are poor paleomagnetic recorders. On the other hand, the matrix metal and some silicate subsamples from the relatively slowly cooled Steinbach meteorite are far better paleomagnetic recorders and provide evidence of an intense (≳100 μT) and directionally varying (exhibiting significant changes on a timescale ≲200 kyr) magnetic field. This is the first demonstration that some iron meteorites record ancient planetary magnetic fields. Furthermore, the observed field intensity, temporal variability and dynamo lifetime are consistent with thermal evolution models of the IVA parent core. Because the acquisition of remanent magnetisation by some IVA iron meteorites require that they cooled below their Curie temperature during the period of dynamo activity, the magnetisation carried by Steinbach also provides strong evidence favouring the inward solidification of its parent core

Size Ranges of Magnetic Domain States in Tetrataenite

Paleomagnetic studies of meteorites provide unique constraints on the evolution of magnetic fields in the early solar system. These studies rely on the identification of magnetic minerals that can retain stable magnetizations over ≳4.5 billion years (Ga). The ferromagnetic mineral tetrataenite (γ''-Fe0.5Ni0.5) is found in iron, stony-iron and chondrite meteorite groups. Nanoscale intergrowths of tetrataenite have been shown to carry records of paleomagnetic fields, although the effect of magnetostatic interactions on their magnetic remanence acquisition remains to be fully understood. Tetrataenite can also occur as isolated, non-interacting, nanoscale grains in many meteorite groups, although the paleomagnetic potential of these grains is particularly poorly understood. Here, we aim to improve our understanding of tetrataenite magnetization to refine our knowledge of existing paleomagnetic analyses and broaden the spectrum of meteorite groups that can be used for future paleomagnetic studies. We present the results of analytical calculations and micromagnetic modeling of isolated tetrataenite grains with various geometries. We find that tetrataenite forms a stable single domain state at grain lengths between 6 and ∼160 nm dependent on its elongation. It also possesses a magnetization resistant to viscous remagnetization over the lifetime of the solar system at 293 K. At larger grain sizes, tetrataenite's lowest energy state is a lamellar two-domain state, stable at Ga-scale timescales. Unlike many other magnetic minerals, tetrataenite does not form a single-vortex domain state due to its large uniaxial anisotropy. Our results show that single domain and two-domain tetrataenite grains carry an extremely stable magnetization and therefore are promising for paleomagnetic studies

The top‐down solidification of iron asteroids driving dynamo evolution

The cores of some small planetesimals, such as asteroid (16) Psyche, are thought to have been exposed through collisions during the early solar system that removed their mantles. These small bodies likely solidified from the top down representing a fundamentally different solidification regime to that of Earth's core. Here we derive simplified models of the downwards solidification of the metallic crust, and consider thermal convection and the potential for viscous delamination of the weak, warm base of the crust to provide a buoyancy flux sufficient to drive a dynamo. Thermal buoyancy is very short lived ( ∼1000 years), and therefore cannot be the source of measured paleomagnetic remanence. In contrast, viscous delamination is found to provide a long‐lasting buoyancy flux sufficient to generate an intense, multipolar magnetic field, while not greatly affecting the crustal solidification time. Our results suggest that a Psyche‐sized (150 km radius) body solidified in roughly 6.7 ‐ 20 Myr, and that delamination produced a strong magnetic field over much of this time. Finally, including light, insoluble impurities, such as sulfur, results in a partially solid mushy zone at the base of the crust. This further weakens the base of the crust and results in smaller scale delamination events. Despite a significant change in the dynamics of delamination, the time to total solidification and the predicted properties of the magnetic field are broadly comparable to the sulfur‐free case, though we argue this may result in observable compositional stratification of the body

Pallasite paleomagnetism: Quiescence of a core dynamo

Recent paleomagnetic studies of two Main Group pallasites, the Imilac and Esquel, have found evidence for a strong, late-stage magnetic field on the parent body. It has been hypothesized that this magnetic field was generated by a core dynamo, driven by compositional convection during core solidification. Cooling models suggest that the onset of core solidification occurred ∼200 Ma after planetary accretion. Prior to core solidification, a core dynamo may have been generated by thermal convection; however a thermal dynamo is predicted to be short-lived, with a duration of ∼10 Ma to ∼40 Ma after planetary accretion. These models predict, therefore, a period of quiescence between the thermally driven dynamo and the compositionally driven dynamo, when no core dynamo should be active. To test this hypothesis, we have measured the magnetic remanence recorded by the Marjalahti and Brenham pallasites, which based on cooling-rate data locked in any magnetic field signals present ∼95 Ma to ∼135 Ma after planetary accretion, before core solidification began. The cloudy zone, a region of nanoscale tetrataenite islands within a Fe-rich matrix was imaged using X-ray photoemission electron microscopy. The recovered distribution of magnetisation within the cloudy zone suggests that the Marjalahti and Brenham experienced a very weak magnetic field, which may have been induced by a crustal remanence, consistent with the predicted lack of an active core dynamo at this time. We show that the transition from a quiescent period to an active, compositionally driven dynamo has a distinctive paleomagnetic signature, which may be a crucial tool for constraining the time of core solidification on differentiated bodies, including Earth

Meteorite evidence for partial differentiation and protracted accretion of planetesimals.

Modern meteorite classification schemes assume that no single planetary body could be source of both unmelted (chondritic) and melted (achondritic) meteorites. This dichotomy is a natural outcome of formation models assuming that planetesimal accretion occurred nearly instantaneously. However, it has recently been proposed that the accretion of many planetesimals lasted over ≳1 million years (Ma). This could have resulted in partially differentiated internal structures, with individual bodies containing iron cores, achondritic silicate mantles, and chondritic crusts. This proposal can be tested by searching for a meteorite group containing evidence for these three layers. We combine synchrotron paleomagnetic analyses with thermal, impact, and collisional evolution models to show that the parent body of the enigmatic IIE iron meteorites was such a partially differentiated planetesimal. This implies that some chondrites and achondrites simultaneously coexisted on the same planetesimal, indicating that accretion was protracted and that apparently undifferentiated asteroids may contain melted interiors