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The High Pressure Mineral Physics Group at the Max Planck Institute for Chemistry, Mainz, performed the first quantitative diamond cell experiments on chemical exchange processes at controlled pressure- and temperature conditions.
This study extends the pressure range of controlled chemical experiments by almost three times. The extreme smallness of the samples requires the use of unconventional analytical techniques: For the first time ionprobes were used for the chemical analysis of run products from diamond cells.
The results of this study establish unexpected strong pressure effects on chemical properties of the transition elements Ni and Co and, thus, make a major contribution to the problem of the "siderophilic element anomaly": Elements known to become dissolved preferentially in metallic iron (siderophilic elements) occur in high abundance in the Earth's upper mantle. This abundance is at odds with results from laboratory based partitioning experiments conducted at low pressures. This discrepancy is probably due to processes of chemical differentiation in the early Earth and, thus, siderophilic elements are key for understanding processes related to the formation of the Earth's core.
The "siderophilic element anomaly" has been previously explained by 1) Addition of oxidised, "primitive" meteoritic material to the mantle after core formation was almost complete (this is called the heterogeneous accretion hypothesis); 2) Change of the partitioning of siderophilic elements between iron and silicate with pressure, temperature, and O2, S2-partial pressures in such a way that the Ni- and Co-contents of the upper mantle can be reconciled with core-mantle equilibrium (this is called homogeneous accretion hypothesis).
The study of the High Pressure Mineralphysics Group at the Max Planck Institute includes the first diamond cell experiments on partitioning of elements (Ni and Co) between two different phases (here: an FeNiCo-alloy and MgSi-perovskite, the main phase of the lower mantle) at controlled pressure- and temperature conditions. The experiments were performed at a temperature of 2100 K and pressures up to 80 GPa (~ 106 times the atmospheric pressure) corresponding to a depth of ~ 1900 km in the Earth. Samples of ~ 50'50'5 µm3 volume were embedded for first time in such studies in inert, nearly hydrostatic pressure media of low thermal conductance and pressurised in a diamond cell which provide well defined pressure- and temperature conditions during the experiments. At high pressure conditions the samples were heated by a high power CO2-laser. This method of heating is possible because the diamond anvils are almost transparent for the light of CO2-lasers. After release to ambient conditions the samples were chemically analysed with an ionprobe which allows for exact chemical analysis of microscopic small samples by drilling holes of few µm diameter into the samples. These holes are drilled by bombarding the sample surface with energetic ions. During drilling the ablated sample material is analysed by a mass spectrometer.
In this study, the instruments of the Department of Cosmochemistry at the Max Planck Institute for Chemistry, Mainz, and at the Mineralogical Institute of the University of Heidelberg, were used. The study shows that the siderophilic elements Ni and Co almost loose their affinity to metallic iron at the high pressures of the Earth's lower mantle. This implies that the abundance of these elements in the Earth's mantle is not necessarily in contrast to their chemical character as commonly assumed. Such a discrepancy would not occur if a chemical exchange between silicates and metallic iron at high pressure conditions have played a role during core formation. Ongoing work shows pressure effects on the chemical behaviour of other relevant elements with respect to iron which are similar to those of Ni and Co.
Journal
Nature