Isotopic fractionation associated with diffusion in crystals is the most reliable means of understanding the origin of mineral zoning in igneous and metamorphic rocks. We have experimentally determined the relative diffusivities of iron isotopes in olivine as a function of crystallographic orientation, composition, and temperature. For two isotopes i and j of an element, the isotope effect for diffusion is parameterized as D-i/D-j = (m(j)/m(i))(beta), where beta is a dimensionless parameter, and D and m stand for diffusivity and mass, respectively. A series of single crystal diffusion couple experiments were conducted at an oxygen fugacity of QFM - 1.5 at temperatures of 1200, 1300, and 1400 degrees C. For the Fo(83.4)-Fo(88.8) composition pair, beta(Fe) is isotropic and a value of 0.16 +/- 0.09 can be used to describe diffusion along all major crystallographic axes in olivine. Based on our experiments and previously reported coupled Mg-Fe isotopic data, we also estimate beta(Mg) = 0.09 +/- 0.05 for this range of olivine composition. For the Fo(88.8)-Fo(100) composition pair, beta(Fe) becomes anisotropic with beta(Fe [100]) = 0.11 +/- 0.03, beta(Fe [010]) = 0.14 +/- 0.03 (both within error of the value measured for the Fo(83.4)-Fo(88.8) pair), and beta(Fe [001]) = 0.03 +/- 0.03. For Fo# between 83.4 and 100, beta(Fe [100]) and beta(Fe [010]) are thus independent of composition. The reason why beta(Fe) ([001]) transitions from similar to 0.16 to similar to 0.03 close to the Mg-endmember is unclear. Over the temperature range studied, a dependence of beta(Fe) on temperature was not resolved. General analytical expressions are introduced to calculate isotopic fractionation as a function of distance, time, beta, and the concentration contrast between the diffusing media for spherical, cylindrical, and planar geometries. (C) 2018 Elsevier Ltd. All rights reserved.

We have conducted high-pressure, high-temperature isotope exchange experiments between molten silicate and molten Fe-Si-C-alloys to constrain the effect of Si on equilibrium Fe isotope fractionation during planetary core formation. The values of Delta Fe-57(Metal-Silicate) at 1850 degrees C and 1 GPa determined by high-resolution MC-ICP-MS in this study range from -0.013 +/- 0.054 parts per thousand (2SE) to 0.072 +/- 0.085 parts per thousand with 1.34-8.14 atom % Si in the alloy, respectively. These results, although not definitive on their own, are consistent with previous experimental results from our group and a model in which elements that substitute for Fe atoms in the alloy structure (i.e., Ni, S, and Si) induce a fractionation of Fe isotopes between molten silicate and molten Fe-alloys during planetary differentiation. Using in situ synchrotron X-ray diffraction data for molten Fe-rich alloys from the literature, we propose a model to explain this fractionation behavior in which impurity elements in Fe-alloys cause the nearest neighbor atomic distances to shorten, thereby stiffening metallic bonds and increasing the preference of the alloy for heavy Fe isotopes relative to the silicate melt. This fractionation results in the bulk silicate mantles of the smaller terrestrial planets and asteroids becoming isotopically light relative to chondrites, with an enrichment of heavy Fe isotopes in their cores, consistent with magmatic iron meteorite compositions. Our model predicts a bulk silicate mantle delta Fe-57 ranging from -0.01 parts per thousand to -0.12 parts per thousand for the Moon, -0.06 parts per thousand to -0.33 parts per thousand for Mars, and -0.08 parts per thousand to B -0.33 parts per thousand for Vesta. Independent estimates of the delta Fe-57 of primitive mantle source regions that account for Fe isotope fractionation during partial melting agree well with these ranges for all three planetary bodies and suggest that Mars and Vesta have cores with impurity (i.e., Ni, S, Si) abundances near the low end of published ranges. Therefore, we favor a model in which core formation results in isotopically light bulk silicate mantles for the Moon, Mars, and Vesta. The processes of magma ocean crystallization, mantle partial melting, and fractional crystallization of mantle-derived melts are all likely to result in heavy Fe isotope enrichment in the melt phase, which can explain why basaltic samples from these planetary bodies have variable delta Fe-57 values consistently heavier than our bulk mantle estimates. Additionally, we find no clear evidence that Fe isotopes were fractionated to a detectable level by volatile depletion processes during or after planetary accretion, although it cannot be ruled out. (C) 2019 Elsevier B.V. All rights reserved.

Mercury, the Solar System's innermost planet, has an unusually massive core prompting speculation that the planet lost silicate after it formed. Using the unusually high sulfur and low iron composition of its surface and space geodetic constraints on its core composition, we show Mercury's chemistry to be compatible with formation in a larger planet at minimum 1.4-2.5 times Mercury's present mass and possibly 2-4 times its mass by similarity with other rocky Solar System bodies. To do this, we apply an experimentally determined metal-silicate partitioning model for sulfur to Mercury's silicate. The model is validated by applying it to Vesta, which, when evaluated at the conditions of Vestan self-differentiation, yields sulfur contents in its silicate in the range of HED meteorites. Mercury could have lost a substantial fraction of its rocky material through impacts or by being itself a remnant impactor. Independent of any stripping, because a significant amount of silicon resides in Mercury's core, silicate meteoritic debris from Mercury would likely be characterized by Si-30 isotopic enrichment >+ 0.10 parts per thousand relative to parent sources that could aid identification of a new meteorite class.

Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is one of the most popular techniques for determining trace element concentrations in sulfides. Due to the lack of matrix-matched standards, standardization of sulfide analyses are usually based on silicate glass calibrant materials. Matrix effects during ns-LA-ICP-MS analyses of Fe-rich sulfides were quantified for many trace elements by comparison of elemental concentrations obtained by LA-ICP-MS and electron microprobe (EPMA) for many synthetic sulfides. The data was used to obtain the fractionation indices (F-i, the ratio between the EPMA- and LA-ICP-MS-determined concentrations of element i) for many elements while considering Fe, Cu and Ni as internal standards. The results show that significant (>15% RD) matrix effects arise during ns-LA-ICP-MS analyses of Ti, Zn, Ge, Se, Mo, Cd, In, Sb, Te, Pb, Bi in sulfides when using Fe as the internal standard. The use of Ni as an internal standard yields on average higher F-i values for most elements, resulting in more pronounced matrix effects for refractory elements and less so for volatile elements, relative to Fe. The use of Cu as an internal standard yields overall more significant matrix effects for volatile elements (i.e., lower F-i values). The F-i values for most elements remain constant with increasing concentrations, and matrix correction factors for these elements can therefore be applied across the ppm to wt% range. In agreement with previous observations for Fe-rich metals and silicate glasses, the magnitudes of the matrix effects for the various elements are strongly correlated with elemental volatility. This correlation was used to obtain a predictive model for describing F-i for Fe-rich sulfides. The results were used to assess the effects of matrix effects on calculated sulfide liquid-silicate melt partition coefficients derived from experiments. Matrix effects arising through the use of non-matrix-matched standards will result in significant discrepancies between measured and true partition coefficients, the extent mainly depending of the volatility of the element considered. Corrections on ns-LA-ICP-MS derived element concentrations therefore need to be performed to obtain true abundances in the absence of matrix-matched standards.

Subducting tectonic plates carry water and other surficial components into Earth's interior. Previous studies suggest that serpentinized peridotite is a key part of deep recycling, but this geochemical pathway has not been directly traced. Here, we report Fe-Ni-rich metallic inclusions in sublithospheric diamonds from a depth of 360 to 750 km with isotopically heavy iron (delta Fe-56 = 0.79 to 0.90 parts per thousand) and unradiogenic osmium (Os-187/Os-188 = 0.111). These iron values lie outside the range of known mantle compositions or expected reaction products at depth. This signature represents subducted iron from magnetite and/or Fe-Ni alloys precipitated during serpentinization of oceanic peridotite, a lithology known to carry unradiogenic osmium inherited from prior convection and melt depletion. These diamond-hosted inclusions trace serpentinite subduction into the mantle transition zone. We propose that iron-rich phases from serpentinite contribute a labile heavy iron component to the heterogeneous convecting mantle eventually sampled by oceanic basalts.

Interpreting isotopic signatures documented in natural rocks requires knowledge of equilibrium isotopic fractionation factors. Here, we determine equilibrium Fe isotope fractionation factors between several common rock-forming minerals using a comparative approach involving three independent methods: (i) isotopic analyses of natural minerals from a metapelite from Mt. Moosilauke, New Hampshire, for which equilibration temperature and pressure are well constrained to be near the aluminosilicate triple point (T similar or equal to 500 degrees C, P similar or equal to 4 kbar), (ii) Nuclear Resonant Inelastic X-ray Scattering (NRIXS) measurements of Fe force constants of minerals, and (iii) Density Functional Theory (DFT) ab initio calculations of Fe force constants of minerals.

Experiments show that the iron isotopic composition of iron meteorites can be explained by core crystallization, and suggest the presence of sulfur-rich core material that remains unsampled by meteorite collections.

As a transition metal that is moderately volatile at high temperatures, copper shows limited isotopic fractionation in terrestrial mantle-derived rocks but significant enrichment in its heavier isotope (up to 12.5 parts per thousand for Cu-65/Cu-63) in objects that experienced volatile loss during formation, such as tektites, trinitite glasses, and lunar rocks. Previous efforts to model the Cu isotope fractionation trend from measurements of delta Cu-65 in tektites found that the trend cannot be explained by the theoretical isotope fractionation factor (alpha) for free evaporation of Cu, making it necessary to experimentally study Cu isotope fractionation under conditions similar to tektite formation.