1,318 research outputs found
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 Fields Recorded by Chondrules Formed in Nebular Shocks
Recent laboratory efforts (Fu et al., 2014) have constrained the remanent
magnetizations of chondrules and the magnetic field strengths at which the
chondrules were exposed to as they cooled below their Curie points. An
outstanding question is whether the inferred paleofields represent the
background magnetic field of the solar nebula or were unique to the
chondrule-forming environment. We investigate the amplification of the magnetic
field above background values for two proposed chondrule formation mechanisms,
large-scale nebular shocks and planetary bow shocks. Behind large-scale shocks,
the magnetic field parallel to the shock front is amplified by factors , regardless of the magnetic diffusivity. Therefore, chondrules melted in
these shocks probably recorded an amplified magnetic field. Behind planetary
bow shocks, the field amplification is sensitive to the magnetic diffusivity.
We compute the gas properties behind a bow shock around a 3000 km-radius
planetary embryo, with and without atmospheres, using hydrodynamics models. We
calculate the ionization state of the hot, shocked gas, including thermionic
emission from dust, and thermal ionization of gas-phase potassium atoms, and
the magnetic diffusivity due to Ohmic dissipation and ambipolar diffusion. We
find that the diffusivity is sufficiently large that magnetic fields have
already relaxed to background values in the shock downstream where chondrules
acquire magnetizations, and that these locations are sufficiently far from the
planetary embryos that chondrules should not have recorded a significant
putative dynamo field generated on these bodies. We conclude that, if melted in
planetary bow shocks, chondrules probably recorded the background nebular
field.Comment: 17 pages, 11 figures, accepted for publication in Ap
Ultra-high sensitivity moment magnetometry of geological samples using magnetic microscopy
Useful paleomagnetic 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 10ā»Ā¹āµ AmĀ². 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 samples.National Science Foundation (U.S.) (DMS-1521765)National Science Foundation (U.S.) (DMS-0934689)Peterson, Thomas F
Martian Surface Paleotemperatures from Thermochronology of Meteorites
The temporal evolution of past martian surface temperatures is poorly known. We used thermochronology and published noble gas and petrographic data to constrain the temperature histories of the nakhlites and martian meteorite ALH84001. We found that the nakhlites have not been heated to more than 350Ā°C since they formed. Our calculations also suggest that for most of the past 4 billion years, ambient near-surface temperatures on Mars are unlikely to have been much higher than the present cold (<0Ā°C) state
Further evidence for early lunar magnetism from troctolite 76535
The earliest history of the lunar dynamo is largely unknown and has important implications for the thermal state of the Moon and the physics of dynamo generation. The lunar sample with the oldest known paleomagnetic record is the 4.25 billion year old (Ga) troctolite 76535. Previous studies of unoriented subsamples of 76535 found evidence for a dynamo field with a paleointensity of several tens of microteslas. However, the lack of mutual subsample orientation prevented a demonstration that the magnetization was unidirectional, a key property of thermoremanent magnetization. Here we report further alternating field demagnetization on three mutually oriented subsamples of 76535, as well as new pressure remanent magnetization experiments to help rule out shock magnetization. We also describe new 40Ar/39Ar thermochronometry and cosmogenic neon measurements that better constrain the rock's thermal history. Although the rock is unbrecciated, unshocked, and slowly cooled, its demagnetization behavior is not ideal due to spurious remanence acquisition. Despite this limitation, all three subsamples record a high coercivity magnetization oriented in nearly the same direction, implying that they were magnetized by a unidirectional field on the Moon. We find no evidence for shock remanence, and our thermochronometry calculations show no significant reheating events since 4249āĀ±ā12 million years ago (Ma). We infer a field paleointensity of approximately 20ā40āĪ¼T, supporting the previous conclusion that a lunar dynamo existed at 4.25āGa. The timing of this field supports an early dynamo powered by thermal or thermochemical core convection and/or a mechanical dynamo but marginally excludes a dynamo delayed by thermal blanketing from radiogenic element-rich magma ocean cumulates
Chondrites as samples of differentiated planetesimals
Chondritic meteorites are unmelted and variably metamorphosed aggregates of the earliest solids of the solar system. The variety of metamorphic textures in chondrites motivated the āonion shellā model in which chondrites originated at varying depths within a parent body heated primarily by the short-lived radioisotope 26Al, with the highest metamorphic grade originating nearest the center. Allende and a few other chondrites possess a unidirectional magnetization that can be best explained by a core dynamo on their parent body, indicating internal melting and differentiation. Here we show that a parent body that accreted to >~200 km in radius by ~ 1.5 Ma after the formation of calciumāaluminum-rich inclusions (CAIs) would have a differentiated interior, and ongoing accretion would add a solid undifferentiated crust overlying a differentiated interior, consistent with formational and evolutionary constraints inferred for the CV parent body. This body could have produced a magnetic field lasting more than 10 Ma. This hypothesis represents a new model for the origin of some chondrites, presenting them as the unprocessed crusts of internally differentiated early planetesimals. Such bodies may exist in the asteroid belt today; the shapes and masses of the two largest asteroids, 1 Ceres and 2 Pallas, can be consistent with differentiated interiors, conceivably with small iron cores with hydrated silicate or iceāsilicate mantles, covered with undifferentiated crusts.National Science Foundation (U.S.) (NSF Astronomy CAREER grant)Mitsui & Co. (U.S.A.), Inc. ( Mitsui Career Development Professorship)United States. National Aeronautics and Space Administration (NASA Origins grant)Massachusetts Institute of Technology (Victor P. Starr Career Development Professorship)United States. National Aeronautics and Space Administration (NASA/Dawn co-investigator grant
Magnetic tests for magnetosome chains in Martian meteorite ALH84001
Transmission electron microscopy studies have been used to argue that magnetite crystals in carbonate from Martian meteorite ALH84001 have a composition and morphology indistinguishable from that of magnetotactic bacteria. It has even been claimed from scanning electron microscopy imaging that some ALH84001 magnetite crystals are aligned in chains. Alignment of magnetosomes in chains is perhaps the most distinctive of the six crystallographic properties thought to be collectively unique to magnetofossils. Here we use three rock magnetic techniques, low-temperature cycling, the Moskowitz test, and ferromagnetic resonance, to sense the bulk composition and crystallography of millions of ALH84001 magnetite crystals. The magnetic data demonstrate that although the magnetite is unusually pure and fine-grained in a manner similar to terrestrial magnetofossils, most or all of the crystals are not arranged in chains
History of the Solar Nebula from Meteorite Paleomagnetism
We review recent advances in our understanding of magnetism in the solar
nebular and protoplanetary disks (PPDs). We discuss the implications of theory,
meteorite measurements, and astronomical observations for planetary formation
and nebular evolution. Paleomagnetic measurements indicate the presence of
fields of 0.540.21 G at 1 to 3 astronomical units (AU) from the Sun
and 0.06 G at 3 to 7 AU until >1.22 and >2.51 million years (Ma) after
solar system formation, respectively. These intensities are consistent with
those predicted to enable typical astronomically-observed protostellar
accretion rates of 10 M yr, suggesting that
magnetism played a central role in mass and angular momentum transport in PPDs.
Paleomagnetic studies also indicate fields <0.006 G and <0.003 G in the inner
and outer solar system by 3.94 and 4.89 Ma, respectively, consistent with the
nebular gas having dispersed by this time. This is similar to the observed
lifetimes of extrasolar protoplanetary disks.Comment: Revised version published in Science Advance
Magnetic Microscopy Promises a Leap in Sensitivity and Resolution
Twenty years ago, Kirschvink argued that
many paleomagnetic studies were limited by
the sensitivity of the magnetometer systems
then in use [Kirschvink, 1981]. He showed that
sedimentary rocks could preserve detrital
remanent magnetizations at levels of 10^(-14) to
10^(-15) Am^2, about 100-1000 times below the
noise level of today's best superconducting
(SQUID) rock magnetometers. If a more sensitive
magnetometer could be built, it would
dramatically expand the range and variety of
rock types amenable to paleomagnetic analysis.
Just such an instrument is now on the horizon:
the low-temperature superconductivity (LTS)
SQUID Microscope
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