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We have developed numerical simulations of spectroradiometric temperature measurement for refractive optical systems used in laser-heated diamond anvil cell (LHDAC) experiments. Chromatic dispersion in refractive optics is quantified by the wavelength-dependent radial displacement function. Results show that a combination of high peak temperature, steep temperature gradient, high spatial resolution, and large radial displacement can cause extreme errors in temperature, on the order of hundreds to thousands of degrees of Kelvin. Increasing detector bin size can dampen the magnitude of the error in peak temperature, but the average temperature produced in this way is still likely to be erroneous. Accurate peak temperature can be measured if the radial displacement is less than the radius of a region of constant temperature. Errors in accuracy can be reduced by locating the wavelength range that produces a minimum and symmetrical radial displacement. The precision of the fitted blackbody function is correlated positively with accuracy so that the optimal wavelength range can be located by minimizing the error in the blackbody fit. Modern spectroradiometric systems that utilize refractive optics are capable of accurate temperature measurement if proper steps are taken to identify and mitigate the effects of chromatic dispersion. [Copyright &y& Elsevier]

The spatial distribution of carbon and nitrogen isotopes in diamonds provides information on mantle residence time. Diamonds with long residence at high temperature will gradually lose their initial zoning patterns due to diffusion. Using experimentally determined carbon self-diffusion coefficients and nitrogen diffusion coefficients derived from aggregation experiments, we have modeled the diffusive relaxation of zoning profiles with a spectrum of wavelengths. Carbon isotope heterogeneity will be preserved on wavelengths greater than ∼1 μm after 1 billion years’ residence at 1400 K, and on wavelengths greater than ∼200 μm after 1 million years’ residence at 2000 K. Nitrogen isotope zoning is relaxed much more slowly, with 0.1 μm zoning preserved over the age of the Earth at 1400 K and 1 μm zoning preserved after 1 million years at 2000 K. The large difference in diffusive relaxation times between carbon and nitrogen isotopes means that initially correlated carbon and nitrogen profiles will lose their correlation after sufficient diffusion of carbon has taken place. This could be used as a thermochronometer. Carbon isotope heterogeneity in diamonds associated with lower mantle mineral assemblages has significantly smaller amplitude than nitrogen isotope heterogeneity, consistent with diffusive relaxation at high temperatures in the lower mantle. [Copyright &y& Elsevier]

The melting curve of NiSi has been determined to 70 GPa on the basis of laser-heated diamond anvil cell (LH-DAC) experiments in which changes in the gradient of temperature vs. laser power functions were used as the melting criterion. The melting curve was corroborated with in situ X-ray diffraction experiments in both the LH-DAC and multi-anvil press in which the appearance of liquid diffuse scattering in the diffraction patterns was used as the melting criterion. At all pressures, the NiSi melting curve is lower than that of FeSi, with the difference in melting temperature reaching a maximum of 900 K at 14 GPa. The location of the B31 + B20 + L triple point has been constrained to 12 ± 2 GPa and 1550 ± 100 K and the B20 + B2 + L triple point to 28.5 ± 1.5 GPa and 2165 ± 60 K. On the basis of the in situ LH-DAC experiments the Clapeyron slope of the B20 → B2 transition is estimated at -67 MPa K-1. Extrapolation of the B2-NiSi liquidus to core-mantle boundary (CMB) conditions (135 GPa) suggests the melting point of NiSi (3700 ± 400 K) will be only marginally lower than that of isostructural FeSi (4000 ± 200 K). Thus any (Fe,Ni)Si solid solution present within the D" layer is expected to remain solid, with the possible exception of the very hottest region adjacent to the CMB. [ABSTRACT FROM AUTHOR]

The phase boundary of the post-spinel transition in Mg2SiO4 was re-investigated by means of high P–T in situ X-ray diffractometry with a gold pressure marker in a Kawai-type apparatus. Rapid and continuous temperature changes were conducted to initiate dissociation of spinel, which tends to be inert after long annealing. Isothermal decompression at high temperature was conducted to form spinel from perovskite plus periclase. The phase boundary is located at ca. 22 GPa in the temperature range from 1550 to 2100 K, which is 1–1.5 GPa lower than the 660 km discontinuity. This discrepancy might be explained in terms of the pressure effect of thermocouple emf and inaccurate equation of state (EOS) for the pressure maker. The transition is found to be less sensitive to temperature than reported previously, with a Clapeyron slope ranging from −2 to −0.4 MPa/K. This small Clapeyron slope implies that the post-spinel transition would not be an effective barrier to mantle convection. [Copyright &y& Elsevier]