Ig Wood

Compositional and structural control of fission-track annealing in apatite

Geological cooling histories modelled from apatite fission-track (FT) data are dependent upon extrapolated, laboratory-based track annealing data. Annealing in apatite appears to be compositionally controlled. This study investigates how compositional variation influences apatite crystal structure (as reflected in the unit-cell parameters) and fission-track annealing, and then considers how best to monitor bulk composition in a practical way for routine fission-track analysis. New fission-track annealing data are presented for a series of 10-, 100- and 1000-h experiments on 13 apatite samples of different chemical composition. The bulk apatite composition of these samples was determined using uranium mapping, cathodoluminescence (CL), electron microprobe, inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectrometry (ICP-AES) and spectrophotometry techniques, and their cell parameters characterised by X-ray powder diffractometry (XRD). Apatite structure reflects apatite bulk composition and unit-cell dimensions are changed by the complex interactions between anion substitutions (Cl, F, OH) and cation substitution (REE, Mn, Sr). While chlorine has a dominant control on apatite structure above 0.1 anion per formula unit (∼0.35 wt.%), below this value other elements, in particular REE, exert a significant control. This study shows that the rate of fission-track annealing correlates with apatite structure, the annealing rate being slower for crystals with larger values for cell parameter a and smaller values for cell parameter c. In an earlier study, Carlson et al. [Am. Mineral. 84 (1999) 1213] found this correlation to be valid only for apatites of certain compositions. Ideally, the bulk composition and/or unit cell should be measured for each apatite grain analysed by the fission-track method to determine the appropriate track annealing parameters for use in thermal history prediction. Neither approach is a practical reality for routine analysis. The relative merits of determining chlorine content, the major influence on fission-track annealing, and using apatite solubility by measuring etch-pit sizes are discussed as practical alternatives for assessing the annealing response of individual apatite crystals.

Possible thermal and chemical stabilization of body-centred-cubic iron in the Earth's core

The nature of the stable phase of iron in the Earths solid inner core is still highly controversial. Laboratory experiments suggest the possibility of an uncharacterized phase transformation in iron at core conditions and seismological observations have indicated the possible presence of complex, inner-core layering. Theoretical studies currently suggest that the hexagonal close packed (h.c.p.) phase of iron is stable at core pressures and that the body centred cubic (b.c.c.) phase of iron becomes elastically unstable at high pressure. In other h.c.p. metals, however, a high-pressure b.c.c. form has been found to become stabilized at high temperature. We report here a quantum mechanical study of b.c.c.-iron able to model its behaviour at core temperatures as well as pressures, using ab initio molecular dynamics free-energy calculations. We find that b.c.c.-iron indeed becomes entropically stabilized at core temperatures, but in its pure state h.c.p.-iron still remains thermodynamically more favourable. The inner core, however, is not pure iron, and our calculations indicate that the b.c.c. phase will be stabilized with respect to the h.c.p. phase by sulphur or silicon impurities in the core. Consequently, a b.c.c.-structured alloy may be a strong candidate for explaining the observed seismic complexity of the inner core.

Thermal expansion and crystal structure of cementite, Fe3C, between 4 and 600 K determined by time-of-flight neutron powder diffraction

The cementite phase of Fe3C has been studied by high-resolution neutron powder diffraction at 4.2 K and at 20 K intervals between 20 and 600 K. The crystal structure remains orthorhombic (Pnma) throughout, with the fractional coordinates of all atoms varying only slightly (the magnetic structure of the ferromagnetic phase could not be determined). The ferromagnetic phase transition, with Tc ≃ 480 K, greatly affects the thermal expansion coefficient of the material. The average volumetric coefficient of thermal expansion above Tc was found to be 4.1 (1) × 10−5 K−1; below Tc it is considerably lower (< 1.8 × 10−5 K−1) and varies greatly with temperature. The behaviour of the volume over the full temperature range of the experiment may be modelled by a third-order Gruneisen approximation to the zero-pressure equation of state, combined with a magnetostrictive correction based on mean-field theory.

Ferroelastic phase transition in BiVO4 I. Birefringence measurements using the rotating-analyser method

A brief review of methods of measuring birefringence is given. Apparatus for the accurate measurement of optical birefringence at high temperatures using a modulation (rotating-analyser) method is described in detail, together with suggestions for further applications of the equipment. The technique is applied to the particular case of the ferroelastic monoclinic–tetragonal phase transition in BiVO4. The birefringence was found to show mean-field behaviour over a wide temperature range with a mean-field transition temperature of 519.9 (3) K.

Thermal expansion and atomic displacement parameters of cubic KMgF3 perovskite determined by high-resolution neutron powder diffraction

The structure of KMgF3 has been determined by high-resolution neutron powder diffraction at 4.2 K, room temperature and at 10 K intervals from 373 K to 1223 K. The material remains cubic at all temperatures. The average volumetric coefficient of thermal expansion in the range 373–1223 K was found to be 7.11 (3) × 10−5 K−1. For temperatures between 4.2 and 1223 K, a second-order Gruneisen approximation to the zero-pressure equation of state, with the internal energy calculated via a Debye model, was found to fit well, with the following parameters: θD = 536 (9) K, Vo = 62.876 (6) A3, K_{o}^{\,\prime} = 6.5 (1) and (VoKo/γ′) = 3.40 (2) × 10−18 J, where θD is the Debye temperature, Vo is the volume at T = 0, K_{o}^{\,\prime} is the first derivative with respect to pressure of the incompressibility (Ko) and γ′ is a Gruneisen parameter. The atomic displacement parameters were found to increase smoothly with T and could be fitted using Debye models with θD in the range 305–581 K. At 1223 K, the displacement of the F ions was found to be much less anisotropic than that in NaMgF3 at this temperature.

Strong Premelting Effect in the Elastic Properties of hcp-Fe Under Inner-Core Conditions

On the Brink of Melting The considerable pressures and temperatures of Earths iron-rich inner core make it a challenge to compare measurements made in experimental systems with observed seismic data. Computational simulations of core materials may reconcile any apparent differences. Martorell et al. (p. 466, published online 10 October) used ab initio simulations to predict the elastic properties of iron at core pressures. As temperatures approached the melting point of pure iron, the material was predicted to weaken to the point that seismic waves would be slowed considerably. An inner core with a small percentage of light elements like oxygen and silicon near its melting temperature would correspond well with measured seismic velocities. Elastic weakening of iron just before melting explains variations in the seismic structure of Earth’s inner core. The observed shear-wave velocity VS in Earth’s core is much lower than expected from mineralogical models derived from both calculations and experiments. A number of explanations have been proposed, but none sufficiently explain the seismological observations. Using ab initio molecular dynamics simulations, we obtained the elastic properties of hexagonal close-packed iron (hcp-Fe) at 360 gigapascals up to its melting temperature Tm. We found that Fe shows a strong nonlinear shear weakening just before melting (when T/Tm > 0.96), with a corresponding reduction in VS. Because temperatures range from T/Tm = 1 at the inner-outer core boundary to T/Tm ≈ 0.99 at the center, this strong nonlinear effect on VS should occur in the inner core, providing a compelling explanation for the low VS observed.

The equation of state of CsCl‐structured FeSi to 40 GPa: Implications for silicon in the Earth's core

21.74 ± 0.02 Au, with a K0 of 184 ± 5 GPa and K 0 = 4.2 ± 0.3. The measured density and elastic properties of CsClFeSi are consistent with silicon being a major element in the Earth’s core. INDEX TERMS: 1212 Geodesy and Gravity: Earth’s interior—composition and state (8105); 3919 Mineral Physics: Equations of state; 8124 Tectonophysics: Earth’s interior—composition and state (old 8105). Citation: Dobson,

The high-pressure phase diagram of ammonia dihydrate

We have investigated the P–T phase diagram of ammonia dihydrate (ADH), ND3·2D2O, using powder neutron diffraction methods over the range 0–9 GPa, 170–300 K. In addition to the ambient pressure phase, ADH I, we have identified three high-pressure phases, ADH II, III, and IV, each of which has been reproduced in at least three separate experiments. Another, apparently body-centred-cubic, phase of ADH has been observed on a single occasion above 6 GPa at 170 K. The existence of a dehydration boundary has been confirmed where, upon compression or warming, ADH IV decomposes to a high-pressure ice phase (ice VII or VIII) and a high-pressure phase of ammonia monohydrate (AMH V or VI).

Ab initio simulation of ammonia monohydrate (NH3⋅H2O) and ammonium hydroxide (NH4OH)

Ab initio pseudopotential plane-wave calculations on the ambient pressure monohydrate of ammonia were carried out. Significant differences in the pressure dependence of covalent O-H and N-H bond lengths from ice VIII and solid ammonia were observed. Simulated structure spontaneously transforms to an ionic solid (NH4OH) at ∼5 GPa. It was found that the enthalpy difference between AMH I, and ammonium hydroxide at zero Kelvin is ∼15 KJ mol-1.

Mid-Cretaceous oceanic anoxic events in the Pacific Ocean revealed by carbon-isotope stratigraphy of the Calera Limestone, California, USA

The paleoceanographic history of the Pacific Ocean during the mid-Cretaceous is poorly constrained due to the loss of much of the contemporaneous Pacific seafloor to subduction and difficulties in recovering chert-rich sediments by ocean drilling. Pelagic sediments that were originally deposited in the Pacific Ocean but that have been subsequently accreted during subduction potentially provide an alternative paleoceanographic archive. This study presents micropaleontological and carbon-isotope data from the Calera Limestone, part of the classic Franciscan Complex exposed in Permanente Quarry, central California, USA. In the three stratigraphic sections studied, pelagic limestones with low organic-carbon contents are the dominant lithology. However, two stratigraphic intervals are recognized that contain organic carbon, and these date to the early Aptian and late Albian–early Cenomanian. These time intervals correspond to two mid-Cretaceous oceanic anoxic events (OAEs): early Aptian OAE1a (equivalent to the “Selli level”) and late Albian OAE1d (equivalent to the “Breistroffer event”). It is well established that both of these events were associated with significant carbon-isotope excursions, which are also shown to exist in the Calera Limestone. The record of OAE1a from the Calera Limestone complements ocean drilling records by providing further evidence for variability in the sedimentary and stratigraphic record of this event. The carbon-isotope data from the late Albian–early Cenomanian provide the first detailed chemobiostratigraphic record of this period for the Pacific Ocean, confirming that environmental change occurred at this time in the Pacific, possibly related to OAE1d.