Structural, elastic and thermal properties of cementite (Fe3_3C) calculated using Modified Embedded Atom Method

Structural, elastic and thermal properties of cementite (Fe$_3$C) were studied using a Modified Embedded Atom Method (MEAM) potential for iron-carbon (Fe-C) alloys. Previously developed Fe and C single element potentials were used to develop an Fe-C alloy MEAM potential, using a statistically-based optimization scheme to reproduce structural and elastic properties of cementite, the interstitial energies of C in bcc Fe as well as heat of formation of Fe-C alloys in L$_{12}$ and B$_1$ structures. The stability of cementite was investigated by molecular dynamics simulations at high temperatures. The nine single crystal elastic constants for cementite were obtained by computing total energies for strained cells. Polycrystalline elastic moduli for cementite were calculated from the single crystal elastic constants of cementite. The formation energies of (001), (010), and (100) surfaces of cementite were also calculated. The melting temperature and the variation of specific heat and volume with respect to temperature were investigated by performing a two-phase (solid/liquid) molecular dynamics simulation of cementite. The predictions of the potential are in good agreement with first-principles calculations and experiments.Comment: 12 pages, 9 figure

Perspectives on Continental Rifting Processes From Spatiotemporal Patterns of Faulting and Magmatism in the Rio Grande Rift, USA

Analysis of spatiotemporal patterns of faulting and magmatism in the Rio Grande rift (RGR) in New Mexico and Colorado, USA, yields insights into continental rift processes, extension accommodation mechanisms, and rift evolution models. We combine new apatite (U‐Th‐Sm)/He and zircon (U‐Th)/He thermochronometric data with previously published thermochronometric data to assess the timing of fault initiation, magnitudes of fault exhumation, and growth and linkage patterns of rift faults. Thermal history modeling of these data reveals contemporaneous rift initiation at ca. 25 Ma in both the northern and southern RGR with continued fault initiation, growth, and linkage progressing from ca. 25 to ca. 15 Ma. The central RGR, however, shows no evidence of Cenozoic fault‐related exhumation as observed with thermochronometry and instead reveals extension accommodated through Late Cenozoic magmatic injection. Furthermore, faulting in the northern and southern RGR occurs along an approximately north‐south strike, whereas magmatism in the central RGR occurs along the northeast to southwest trending Jemez lineament. Differences in deformation orientation and rift accommodation along strike appear to be related to crustal and lithospheric properties, suggesting that rift structure and geometry are at least partly controlled by inherited lithospheric‐scale architecture. We propose an evolutionary model for the RGR that involves initiation of fault‐accommodated extension by oblique strain followed by block rotation of the Colorado Plateau, where extension in the RGR is accommodated by faulting (southern and northern RGR) and magmatism (central RGR). This study highlights different processes related to initiation, geometry, extension accommodation, and overall development of continental rifts.Plain Language SummaryWe identify patterns of faulting and volcanism in the Rio Grande rift (RGR) in the western United States to better understand how continental rifts evolve. Using methods for documenting rock cooling ages (thermochronology), we determined that rifting began around 25 million years ago (Ma) in both the northern and southern RGR. Rift faults continued to develop and grow for another 10 to 15 million years. The central RGR, however, shows that rift extension occurred through volcanic activity both as eruptions at the surface and as magma injection below the surface since ~15 Ma. Interestingly, RGR faulting in the north and south parts of the rift occurs on a north‐south line, while volcanism in the central RGR is along a northeast to southwest line. The differences in the location and orientation of faulting and volcanic activity may be related to the thickness of the lithosphere beneath different parts of the rift. Using these patterns of faulting and magmatism, we propose the RGR evolved through a combination of (1) oblique strain—extension diagonal to the rift and (2) block rotation—where the Colorado Plateau is the rotating block. This detailed study highlights different processes related to the accommodation of extension and the overall development of continental rifts.Key PointsInitiation of the Rio Grande rift appears to be synchronous ~25 Ma and does not support a northward propagation modelExtension is accommodated by faulting in the northern and southern Rio Grande rift and by magmatic injection in the central Rio Grande riftDifferent rift accommodation mechanisms may be controlled by preexisting weaknesses and lithospheric properties (i.e., thickness)Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/152704/1/tect21226.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/152704/2/wrcr21226-sup-00001-2019TC005635-SI.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/152704/3/tect21226_am.pd

Thermal history of the central Gotthard and Aar massifs, European Alps: Evidence for steady state, long-term exhumation

International audienceQuantifying long-term exhumation rates is a prerequisite for understanding the geodynamic evolution of orogens and their exogenic and endogenic driving forces. Here we reconstruct the exhumation history of the central Aar and Gotthard external crystalline massifs in the European Alps using apatite and zircon fission track and apatite (U-Th)/He data. Age-elevation relationships and time-temperature paths derived from thermal history modeling are interpreted to reflect nearly constant exhumation of ∼0.5 km/Ma since ∼14 Ma. A slightly accelerated rate (∼0.7 km/Ma) occurred from 16 to 14 Ma and again from 10 to 7 Ma. Faster exhumation between 16 and 14 Ma is most likely linked to indentation of the Adriatic wedge and related thrusting along the Alpine sole thrust, which, in turn, caused uplift and exhumation in the external crystalline massifs. The data suggest nearly steady, moderate exhumation rates since ∼14 Ma, regardless of major exogenic and endogenic forces such as a change to wetter climate conditions around 5 Ma or orogen-perpendicular extension initiated in Pliocene times. Recent uplift and denudation rates, interpreted to be the result of climate fluctuations and associated increase in erosional efficiency, are nearly twice this ∼0.5 km/Ma paleoexhumation rate

Is Low‐Temperature Fission‐Track Annealing in Apatite a Thermally Controlled Process?

Abstract We report a new series of experiments to explore the phenomenon of low‐temperature annealing of fission tracks in apatite that feature a number of improvements over previous work. Grain mounts were preirradiated using 252Cf to increase confined track detection and allow briefer thermal neutron irradiation. We coirradiated and etched four apatite varieties (Durango, Fish Canyon, Renfrew, and Tioga) over five time steps equally spaced from 3.66 to 15 ln(s). A length standard was coetched with all experiments to ensure that subtle differences are within detection limits. Finally, we used a standard etching protocol, allowing the data to be comodeled with extensive high‐temperature data sets and recent analyses of induced tracks that underwent ambient‐temperature annealing over year‐to‐decade time scales. Ambient‐temperature annealing occurs at two different rates, with faster annealing at early stages that decreases to a slower rate that converges with empirical fanning linear or curvilinear models. The nature of this decrease varies among the apatite species examined, but no patterns could be determined. The fitted models make geological time‐scale predictions consistent with those based on high‐temperature data only and also make predictions consistent with reasonable inferred low‐temperature histories for all four apatite varieties. The empirical fanning curvilinear equation encompasses low‐temperature annealing at month‐to‐decade time scales, but low‐temperature annealing at shorter time scales may occur by a distinct mechanism. We consider but rule out annealing by radiation from short‐lived activated isotopes. We also reconsider the notion of the initial track length, and the appropriate length for normalizing confined track length measurements