Variations in the magnetic properties of meteoritic cloudy zone

Iron and stony‐iron meteorites form the Widmanstätten pattern during slow cooling. This pattern is composed of several microstructures whose length‐scale, composition and magnetic properties are dependent upon cooling rate. Here we focus on the cloudy zone: a region containing nanoscale tetrataenite islands with exceptional paleomagnetic recording properties. We present a systematic review of how cloudy zone properties vary with cooling rate and proximity to the adjacent tetrataenite rim. X‐ray photoemission electron microscopy is used to compare compositional and magnetization maps of the cloudy zone in the mesosiderites (slow cooling rates), the IAB iron meteorites and the pallasites (intermediate cooling rates), and the IVA iron meteorites (fast cooling rates). The proportions of magnetic phases within the cloudy zone are also characterized using Mössbauer spectroscopy. We present the first observations of the magnetic state of the cloudy zone in the mesosiderites, showing that, for such slow cooling rates, tetrataenite islands grow larger than the multidomain threshold, creating large‐scale regions of uniform magnetization across the cloudy zone that render it unsuitable for paleomagnetic analysis. For the most rapidly cooled IVA meteorites, the time available for Fe‐Ni ordering is insufficient to allow tetrataenite formation, again leading to behavior that is unsuitable for paleomagnetic analysis. The most reliable paleomagnetic remanence is recorded by meteorites with intermediate cooling rates ( urn:x-wiley:ggge:media:ggge22125:ggge22125-math-0001 2–500 °C Myr urn:x-wiley:ggge:media:ggge22125:ggge22125-math-0002) which produces islands that are “just right” in both size and degree of Fe‐Ni order

A New Depositional Framework for Massive Iron Formations After the Great Oxidation Event

Abstract The oldest recognized proxies for low atmospheric oxygen are massive iron‐rich deposits. Following the rise of oxygen ∼2.4 billion years ago (Ga), massive iron formations (IFs) largely disappear from the geologic record, only to reappear in a pulse ∼1.88 Ga, which has been attributed to sea‐level transgressions, changing ocean chemistry triggered by intense volcanism, or lowered atmospheric oxygen levels. The North American Gogebic Range is one of the rare records of this pulse and even more uniquely has exposures of both volcanics and IF, providing an ideal field locality to investigate triggers for this pulse of IF. To determine the environmental context and key factors driving IF deposition after the initial rise in oxygen, we made detailed observations of the stratigraphy and facies relationships and present updated mapping relationships of the Gogebic Range Ironwood Iron Formation and the Emperor Volcanics. This work expands existing mine datasets and logs to constrain variations in stratigraphy. Our results are the first to quantitatively constrain thickness variations along the entire Gogebic Range and tie them to syn‐sedimentary faulting along listric normal faults and half grabens. Furthermore, our dataset suggests that initiation of intense syn‐basinal volcanism linked to a large igneous province does not coincide with initial iron deposition, thus cannot be invoked as a causal trigger. Finally, the possibility of iron deposition in a shallow‐water environment suggests that the post‐GOE IF pulse may not reflect global marine transgressions, but instead a chemocline shallowing due to changing global atmospheric oxygen

Paleocene latitude of the Kohistan–Ladakh arc indicates multistage India–Eurasia collision

© 2020 National Academy of Sciences. All rights reserved. We report paleomagnetic data showing that an intraoceanic Trans- Tethyan subduction zone existed south of the Eurasian continent and north of the Indian subcontinent until at least Paleocene time. This system was active between 66 and 62 Ma at a paleolatitude of 8.1 ± 5.6 °N, placing it 600-2,300 km south of the contemporaneous Eurasian margin. The first ophiolite obductions onto the northern Indian margin also occurred at this time, demonstrating that collision was a multistage process involving at least two subduction systems. Collisional events began with collision of India and the Trans-Tethyan subduction zone in Late Cretaceous to Early Paleocene time, followed by the collision of India (plus Trans-Tethyan ophiolites) with Eurasia in mid-Eocene time. These data constrain the total postcollisional convergence across the India-Eurasia convergent zone to 1,350-2,150 km and limit the north-south extent of northwestern Greater India to <900 km. These results have broad implications for how collisional processes may affect plate reconfigurations, global climate, and biodiversity

A time‐resolved paleomagnetic record of Main Group pallasites: Evidence for a large‐cored, thin‐mantled parent body

Several paleomagnetic studies have been conducted on five Main Group pallasites: Brenham, Marjalahti, Springwater, Imilac, and Esquel. These pallasites have distinct cooling histories, meaning that their paleomagnetic records may have been acquired at different times during the thermal evolution of their parent body. Here, we compile new and existing data to present the most complete time-resolved paleomagnetic record for a planetesimal, which includes a period of quiescence prior to core solidification as well as dynamo activity generated by compositional convection during core solidification. We present new paleomagnetic data for the Springwater pallasite, which constrains the timing of core solidification. Our results suggest that in order to generate the observed strong paleointensities (∼65–95 μT), the pallasites must have been relatively close to the dynamo source. Our thermal and dynamo models predict that the Main Group pallasites originate from a planetesimal with a large core (>200 km) and a thin mantle (<70 km).This work was supported by the European Research Council under the European Union's Seventh Framework Programme (FP/2007–2013)/ERC grant agreement numbers 320750 and 312284. We acknowledge the Helmholtz-Zentrum Berlin for the use of the synchrotron radiation beam time at beamline UE49 of BESSY II. We thank the Natural History Museum, London, the American Museum of Natural History, and the Harvard Museum of Natural History for loan of samples. CION thanks the Simons Foundation (award #556352), The Geological Society, the Mineralogical Society of Great Britain, the Mineral Physics Group of Great Britain, Jesus College Cambridge, and the Royal Astronomical Society for funding. BPW thanks the NASA Solar System Workings program and T.F. Peterson, Jr. for support. JH-A thanks the Spanish MCINN Project (DWARFS MAT2017-83468-R). Work at the University of Rochester was supported by NSF grant EAR1656348 and NASA grant 80NSSC19K0510.Peer reviewe