The textures of chondritic meteorites demonstrate that they are not the products of planetary melting processes. This has long been interpreted as evidence that chondrite parent bodies never experienced large-scale melting. As a result, the paleomagnetism of the CV carbonaceous chondrite Allende, most of which was acquired after accretion of the parent body, has been a long-standing mystery. The possibility of a core dynamo like that known for achondrite parent bodies has been discounted because chondrite parent bodies are assumed to be undifferentiated. Resolution of this conundrum requires a determination of the age and timescale over which Allende acquired its magnetization. Here, we report that Allende's magnetization was acquired over several million years (Ma) during metasomatism on the parent planetesimal in a > ∼ 20 μT field up to approximately 9-10 Ma after solar system formation. This field was present too recently and directionally stable for too long to have been generated by the protoplanetary disk or young Sun. The field intensity is in the range expected for planetesimal core dynamos, suggesting that CV chondrites are derived from the outer, unmelted layer of a partially differentiated body with a convecting metallic core.differentiation | planetesimal | magnetic field | early solar system | paleointensity A llende is an accretionary breccia from near the surface of the CV parent planetesimal (1). Following accretion, Allende experienced minor aqueous alteration and moderate thermal metamorphism and metasomatism (2) but has remained essentially unshocked (<5 GPa) (3). Its major ferromagnetic minerals are pyrrhotite, magnetite, and awaruite, with an average pseudo single-domain crystal size (4-8). We conducted alternating-field (AF) and thermal demagnetization, rock magnetic, and paleointensity measurements on 71 mutually oriented bulk subsamples of Allende sample AMNH5056 (approximately 10-cm diameter and 8-mm thick slab surrounded by fusion crust). Of these, 51 subsamples were taken from the interior of the meteorite (>1 mm from fusion crust), whereas 20 contained some fusion crust.The differing magnetization directions of interior and fusioncrusted samples demonstrate that >95% of the natural remanent magnetization (NRM) in interior samples is preterrestrial (Figs. 1 and 2 and SI Appendix). AF demagnetization revealed that the interior samples have at least two components: a weak, low-coercivity, nonunidirectional component blocked up to 5 or 10 mT and a high-coercivity (HC) component blocked from approximately 10 to >290 mT (Fig. 1). In agreement with previous studies (4, 9, 10), the HC magnetization is unidirectionally oriented throughout the meteorite's interior (Fig. 2 and SI Appendix, Table S1). Thermal demagnetization (Figs. 1 and 2 and SI Appendix) indicates that interior samples have a low-temperature (LT) component blocked up to approximately 190°C, a dominant middle-temperature (MT) component blocked between approximately 190-300°C and oriented similarly to the HC component isolated by...

Angrites are among the oldest known pristine basaltic meteorites and record the earliest stages of planet formation and differentiation. Our paleomagnetic analysis of three angrites found that they record a past magnetic field of approximately 10 microteslas on the angrite parent body extending from 4564 to at least 4558 million years ago. Because the angrite paleomagnetic record extends beyond the expected lifetime of the early circumstellar disk, these paleofields were probably generated internally on the angrite parent body, possibly by an early dynamo in a rapidly formed metallic core.

Magnetic surveys of the martian surface have revealed significantly lower magnetic field intensities over the gigantic impact craters Hellas and Argyre than over surrounding regions. The reduced fields are commonly attributed to pressure demagnetization caused by shock waves generated during meteorite impact, in the absence of a significant ambient magnetic field. Lower than average magnetic field intensities are also observed above the Vredefort meteorite crater in South Africa, yet here we show that the rocks in this crater possess much higher magnetic intensities than equivalent lithologies found elsewhere on Earth. We find that palaeomagnetic directions of these strongly magnetized rocks are randomly oriented, with vector directions changing over centimetre length scales. Moreover, the magnetite grains contributing to the magnetic remanence crystallized during impact, which directly relates the randomization and intensification to the impact event. The strong and randomly oriented magnetization vectors effectively cancel out when summed over the whole crater. Seen from high altitudes, as for martian craters, the magnetic field appears much lower than that of neighbouring terranes, implying that magnetic anomalies of meteorite craters cannot be used as evidence for the absence of the planet's internally generated magnetic field at the time of impact.

/101.0°E).Stepwise thermal and alternating field demagnetizations isolated a stable high-temperature component (HTC) of magnetization in most specimens, which we interpret as the primary magnetization of these basaltic lava flows. The four corresponding paleopoles appear consistent with coeval paleopoles from other Asian effusive formations. However, except for the 12.7 Ma paleopole, the paleopoles are systematically far-sided from the European apparent polar wander path (APWP) with respect to site locations, corresponding to anomalously shallow inclinations in Tertiary Asian effusive formations. In the hypothesis of a dipolar magnetic field in the Tertiary, this indicates a $1000-1500 km position of the Siberia craton and Amuria block farther south than expected at 40 and 30 Ma. Tectonically, this interpretation implies decoupling and relative rotations between the western and eastern parts of Eurasia between the Cretaceous and Present. We show that if Siberia were located more to the south, the $15°-20°paleolatitude anomaly generally observed in sedimentary formations from central Asia reduces to a more reasonable average value of $7°, which could result from the superimposition of shallowing mechanisms due to sedimentary processes and northward motion of Asian blocks under the effect of ongoing penetration of India into Eurasia in the Tertiary.