Andrew P. Jephcoat

Questioning the evidence for Earth's oldest fossils.

Structures resembling remarkably preserved bacterial and cyanobacterial microfossils from ∼3,465-million-year-old Apex cherts of the Warrawoona Group in Western Australia currently provide the oldest morphological evidence for life on Earth and have been taken to support an early beginning for oxygen-producing photosynthesis. Eleven species of filamentous prokaryote, distinguished by shape and geometry, have been put forward as meeting the criteria required of authentic Archaean microfossils, and contrast with other microfossils dismissed as either unreliable or unreproducible. These structures are nearly a billion years older than putative cyanobacterial biomarkers, genomic arguments for cyanobacteria, an oxygenic atmosphere and any comparably diverse suite of microfossils. Here we report new research on the type and re-collected material, involving mapping, optical and electron microscopy, digital image analysis, micro-Raman spectroscopy and other geochemical techniques. We reinterpret the purported microfossil-like structure as secondary artefacts formed from amorphous graphite within multiple generations of metalliferous hydrothermal vein chert and volcanic glass. Although there is no support for primary biological morphology, a Fischer–Tropsch-type synthesis of carbon compounds and carbon isotopic fractionation is inferred for one of the oldest known hydrothermal systems on Earth.

Rare-gas solids in the Earth's deep interior

Chemical inertness and surface volatility, combined with low abundance, have made the rare (noble) gases a unique trace-elemental and isotopic system for constraining the formation and evolution of the solid Earth and its atmosphere. Here I examine the implications of recent high-pressure measurements of the melting temperatures of heavy rare-gas solids—argon, krypton and xenon—with new diamond-anvil cell methods, together with their pressure–volume relationship, for the total rare-gas inventory of the Earth since its formation. The solid–liquid (melting) transition in these rare-gas solids rises significantly with pressure in the 50 GPa range,, such that melting temperatures will exceed the geotherm at pressures of the Earths transition zone and lower mantle (depths greater than 410–670 km). The densities of condensed rare-gas solids obtained from recent pressure–volume measurements at high compressions also exceed Earths mantle and core densities. These pressure-induced changes in the physical properties of rare-gas solids, combined with their expected low solubilities and diffusional growth mechanisms, suggest that dense solid or fluid inclusions of rare gases—initially at nanometre scales—would have formed in the Earths interior and may have resulted in incomplete planetary degassing. Separation of dense solid inclusions into deeper regions during early planet formation could provide a straightforward explanation for the unexpectedly low absolute abundance of xenon observed in the atmospheres of both Earth and Mars.

High-pressure Raman spectroscopic studies of FeS2 pyrite

Abstract We report micro-Raman spectroscopic studies of FeS2 pyrite in the diamond-anvil cell under hydrostatic and non-hydrostatic conditions to 55 GPa at room temperature. Four out of five Raman-active modes are resolved with helium as a pressure-transmitting medium to highest pressures. The fifth mode, Tg(2) [377 cm-1], is weak and unresolved lying ~2 cm-1 from the intense Ag mode [379 cm-1] at 1 bar. We observe an increase in the separation of the Eg [344 cm-1] and Tg(1) [350 cm-1] modes under compression. All observed frequencies increase continuously with increasing pressure showing no evidence for a structural phase transition in accord with both X-ray diffraction and shock-wave studies. The Ag and Tg(1) modes gain significantly in intensity relative to the Eg mode with increasing pressure probably resulting from Raman resonance effects. The Tg(3) mode [430 cm-1] broadens unusually compared to the other pyrite modes with pressure. The Raman data are consistent with a contraction of the S-S and Fe-S bonds under pressure. The main effect of non-hydrostatic conditions on the Raman modes is a strong pressure-induced broadening; the pressure-dependence of the frequencies and relative intensities are not affected within the error of the measurements.

The effect of pressure on partitioning of Ni and Co between silicate and iron-rich metal liquids: A diamond-anvil cell study

Abstract High-pressure and high-temperature experiments have been conducted with a laser-heated diamond-anvil cell (LHDAC) to determine the partition coefficients for Ni and Co up to 42 GPa and around 2500 K. Comparison of the present experimental data with those of multi-anvil devices shows a good agreement between the different exchange partitioning coefficients. The agreement suggests conditions in LHDAC experiments can reproduce those of multi-anvil experiments in the pressure range studied. Up to the maximum pressure reached in our work, Ni and Co become less siderophile with increasing pressure, as already observed in previous studies at lower pressures. Our data, combined with lower-pressure results, suggest a magma ocean would have extended to as much as 45 GPa (near 1200 km in depth) in order to obtain homogeneous equilibrium between core-forming metals and the silicate mantle in the early Earth.

The effect of pressure upon hydrogen bonding in chlorite: A Raman spectroscopic study of clinochlore to 26.5 GPa

Abstract The effect of pressure upon hydrogen bonding in synthetic end-member clinochlore, (Mg5Al)(Si3Al)O10(OH)8, has been studied in situ by high-pressure micro-Raman spectroscopy in a moissanite-anvil cell to 26.5 GPa at 300 K. The ambient spectrum consists of three OH-stretching bands between 3400 and 3650 cm-1, attributed to the hydrogen-bonded interlayer OH, and a narrow band at 3679 cm-1 that is assigned to the non-hydrogen-bonded OH groups of the talc-like 2:1 layer. The pressure dependence of the OH modes is linear up to 6 GPa. Near 9 GPa a major discontinuity occurs in the pressure dependence of the interlayer OH-stretching modes. It involves frequency increases >100 cm-1 that indicate major changes in hydrogen bonding. The OH mode of the 2:1 layer does not show discontinuous behavior at 9 GPa. A further discontinuity occurs at ~16 GPa. This discontinuity affects both interlayer and 2:1 OH, and is likely to be associated with a change in the overall compression mechanism of clinochlore. The spectroscopic behavior is a completely reversible function of pressure. Predictions based upon recent high-pressure diffraction studies of hydrogen bonding and compression of clinochlore suggest that the 9 GPa transition is associated with attainment of an O2--O2--contact distance of 2.7 Å.

Aluminium control of argon solubility in silicate melts under pressure

Understanding of the crystal chemistry of the Earths deep mantle has evolved rapidly recently with the gradual acceptance of the importance of the effect of minor elements such as aluminium on the properties of major phases such as perovskite. In the early Earth, during its formation and segregation into rocky mantle and iron-rich core, it is likely that silicate liquids played a large part in the transport of volatiles to or from the deep interior. The importance of aluminium on solubility mechanisms at high pressure has so far received little attention, even though aluminium has long been recognized as exerting strong control on liquid structures at ambient conditions. Here we present constraints on the solubility of argon in aluminosilicate melt compositions up to 25 GPa and 3,000 K, using a laser-heated diamond-anvil cell. The argon contents reach a maximum that persists to pressures as high as 17 GPa (up to 500 km deep in an early magma ocean), well above that expected on the basis of Al-free melt experiments. A distinct drop in argon solubility observed over a narrow pressure range correlates well with the expected void loss in the melt structure predicted by recent molecular dynamics simulations. These results provide a process for noble gas sequestration in the mantle at various depths in a cooling magma ocean. The concept of shallow partial melting as a unique process for extracting noble gases from the early Earth, thereby defining the initial atmospheric abundance, may therefore be oversimplified.

Raman observations of the OH stretching region in hydrous wadsleyite (b-Mg2SiO4) to 50 GPa

Abstract Raman spectra of hydrous β-Mg2SiO4 (1.65 wt% H2O) have been measured in a diamond-anvil cell with helium as a pressure-transmitting medium at room temperature to 50 GPa. We observe three OH-stretching modes, a doublet with components at 3329 and 3373 cm−1, which decrease linearly with pressure, and a single mode at 3586 cm−1, which remains nearly constant up to 24 GPa before decreasing at higher pressures. Assessment of the mode frequencies and their pressure dependence, together with previous results from X-ray and IR data, are consistent with protonation of the O1 site in agreement with previous studies. Strict assignment of Raman activity awaits detailed structural models. The nature of the protonation in wadsleyite may require more specific experimental probes for full solution of the hydrogen-site problem.

High pressure, high resolution synchrotron x-ray powder diffraction with a position-sensitive detector

Abstract We report results of high-pressure experiments with a new diamond-anvil cell in a monochromatic, high-resolution x-ray scattering geometry with alinear position-sensitive detector. The experiments make possible the study of factors controlling line widths of diffraction profiles at pressures in the 100 GPa range, and demonstrate the potential for the use of line profile analysis and Rietveld refinement techniques with high-pressure powder diffraction data. Combined data for various materials indicate that relative contributions to linewidths due to particle size, intrinsic material strength, pressure and state of stress in the sample can be resolved. With light rare-gas solids as pressure-transmitting media, measured FWHMs of the order 0.03∘ 2 θ corresponding to resolution Δd/d of 2.5 × 10−3 for 2θ∼10-15∘ are reported. Formation of a high pressure phase appears to involve growth of submicron domains, judging from substantially broadened diffraction peaks under quasihydrostatic conditions. Detaile...

Synthesis of a novel zeolite through a pressure-induced reconstructive phase transition process.

The first pressure-induced solid-phase synthesis of a zeolite has been found through compression of a common zeolite, ITQ-29 (see scheme, Si yellow, O red). The new microporous structure, ITQ-50, has a unique structure and improved performance for propene/propane separation with respect the parent material ITQ-29.

High-resolution synchrotron X-ray powder diffraction and Rietveld structure refinement of two (Mg (sub 0.95) , Fe (sub 0.05) )SiO 3 perovskite samples synthesized under different oxygen fugacity conditions

Abstract This paper presents high-resolution synchrotron X-ray powder diffraction data at 290 K on two Fe-bearing. polycrystalline silicate perovskite samples with approximate compositions (Mg0.95Fe0.05)SiO3 synthesized at 25 GPa and 1920 K in a multi-anvil press at different oxygen fugacity conditions. Mössbauer smdies have indicated that Fe3/∑Fe for the samples are 0.09 ± 0.01 and near 0.16 ± 0.03. Rietveld structural refinements confirm that Fe2+ and Fe3+ dominantly substitute for Mg2+ in the 8-fold to 12-fold coordinated A site for both compositions. There appears to be no significant differences in the bond distances for these amounts of Fe3+ and no conclusive structural evidence to support indications from Mossbauer experiments that Fe3+ may occupy both A and B sites. To explore the effect of valence state further, this study also reports the first diffraction patterns of (Mg-Fe)SiO3 perovskite collected at a wavelength near the Fe absorption edge.