The Effect of Metal Composition on Fe-Ni Partition Behavior between Olivine and FeNi-Metal, FeNi-Carbide, FeNi-Sulfide at Elevated Pressure

Metal-olivine Fe-Ni exchange distribution coefficients were determined at 1500 C over the pressure range of 1 to 9 GPa for solid and liquid alloy compositions. The metal alloy composition was varied with respect to the Fe/Ni ratio and the amount of dissolved carbon and sulfur. The Fe/Ni ratio of the metal phase exercises an important control on the abundance of Ni in the olivine. The Ni abundance in the olivine decreases as the Fe/Ni ratio of the coexisting metal increases. The presence of carbon (up to approx. 3.5 wt.%) and sulfur (up to approx. 7.5 wt.%) in solution in the liquid Fe-Ni-metal phase has a minor effect on the partitioning of Fe and Ni between metal and olivine phases. No pressure dependence of the Fe-Ni-metal-olivine exchange behavior in carbon- and sulfur-free and carbon- and sulfur-containing systems was found within the investigated pressure range. To match the Ni abundance in terrestrial mantle olivine, assuming an equilibrium metal-olivine distribution, a sub-chondritic Fe/Ni-metal ratio that is a factor of 17 to 27 lower than the Fe/Ni ratios in estimated Earth core compositions would be required, implying higher Fe concentrations in the core forming metal phase. A simple metal-olivine equilibrium distribution does not seem to be feasible to explain the Ni abundances in the Earth's mantle. An equilibrium between metal and olivine does not exercise a control on the problem of Ni overabundance in the Earth's mantle. The experimental results do not contradict the presence of a magma ocean at the time of terrestrial core formation, if olivine was present in only minor amounts at the time of metal segregation

Magmatic evolution of the Jbel Boho alkaline complex in the Bou Azzer inlier (Anti-Atlas/Morocco) and its relation to REE mineralization

Highlights • The Jbel Boho complex is shown to have an alkaline, intraplate geochemical signature. • At least three magma generations are responsible for forming the extrusive-intrusive complex. • The highly evolved and LREE-rich rhyolitic dykes are associated with synchysite-(Ce) mineralization. Abstract The Jbel Boho complex (Anti-Atlas/Morocco) is an alkaline magmatic complex that was formed during the Precambrian-Cambrian transition, contemporaneous with the lower early Cambrian dolomite sequence. The complex consists of a volcanic sequence comprising basanites, trachyandesites, trachytes and rhyolites that is intruded by a syenitic pluton. Both the volcanic suite and the pluton are cut by later microsyenitic and rhyolitic dykes. Although all Jbel Boho magmas were probably ultimately derived from the same, intraplate or plume-like source, new geochemical evidence supports the concept of a minimum three principal magma generations having formed the complex. Whereas all volcanic rocks (first generation) are LREE enriched and appear to be formed by fractional crystallization of a mantle-derived magma, resulting in strong negative Eu anomalies in the more evolved rocks associated with low Zr/Hf and Nb/Ta values, the younger syenitic pluton displays almost no negative Eu anomaly and very high Zr/Hf and Nb/Ta. The syenite is considered to be formed by a second generation of melt and likely formed through partial melting of underplated mafic rocks. The syenitic pluton consists of two types of syenitic rocks; olivine syenite and quartz syenite. The presence of quartz and a strong positive Pb anomaly in the quartz syenite contrasts strongly with the negative Pb anomaly in the olivine syenite and suggests the latter results from crustal contamination of the former. The late dyke swarm (third generation of melt) comprises microsyenitic and subalkaline rhyolitic compositions. The strong decrease of the alkali elements, Zr/Hf and Nb/Ta and the high SiO2 contents in the rhyolitic dykes might be the result of mineral fractionation and addition of mineralizing fluids, allowing inter-element fractionation of even highly incompatible HFSE due to the presence of fluorine. The occurrence of fluorite in some volcanic rocks and the Ca-REE-F carbonate mineral synchysite in the dykes with very high LREE contents (Ce ∼720 ppm found in one rhyolitic dyke) suggest the fluorine-rich nature of this system and the role played by addition of mineralizing fluids. The REE mineralization expressed as synchysite-(Ce) is detected in a subalkaline rhyolitic dyke (with ΣLREE = 1750 ppm) associated with quartz, chlorite and occasionally with Fe-oxides. The synchysite mineralization is probably the result of REE transport by acidic hydrothermal fluids as chloride complex and their neutralization during fluid-rock interaction. The major tectonic change from compressive to extensional regime in the late Neoproterozoic induced the emplacement of voluminous volcaniclastic series of the Ediacran Ouarzazate Group. The alkaline, within-plate nature of the Jbel Boho igneous complex implies that this extensional setting continued during the early Cambrian

Expanding Family of Litharge-Derived Sulfate Minerals and Synthetic Compounds: Preparation and Crystal Structures of [Bi2CuO3]SO4 and [Ln2O2]SO4 (Ln = Dy and Ho)

During the last decades, layered structures have attracted particular and increasing interest due to the multitude of outstanding properties exhibited by their representatives. Particularly common among their archetypes, with a significant number of mineral and synthetic species structural derivatives, is that of litharge. In the current paper, we report the structural studies of two later rare-earth oxysulfates, [Ln(2)O(2)]SO4 (Ln = Dy, Ho), which belong indeed to the grandreefite family, and a novel compound [Bi2CuO3]SO4, which belongs to a new structure type and demonstrates the second example of Cu2+ incorporation into litharge-type slabs. Crystals of [Bi2CuO3]SO4 were obtained under high-pressure/high-temperature (HP/HT) conditions, whereas polycrystalline samples of [Ln(2)O(2)]SO4 (Ln = Dy, Ho) compounds were prepared via an exchange solid-state reaction. The crystal structure of [Bi2CuO3]SO4 is based on alternation of continuous [Bi2CuO3](2+) layers of edge-sharing OBi2Cu2 and OBi3Cu tetrahedra and sheets of sulfate groups. Cu2+ cations are in cis position in O5Bi(2)Cu(2) and O6Bi(2)Cu(2) oxocentered tetrahedra in litharge slab. The crystal structure of [Ln(2)O(2)]SO4 (Ln = Dy, Ho) is completely analogous to those of grandreefite and oxysulfates of La, Sm, Eu, and Bi

A Calorimetric and Thermodynamic Investigation of the Synthetic Analogue of Mandarinoite, Fe₂(SeO₃)₃·5H₂O

Thermophysical and thermochemical calorimetric investigations were carried out on the synthetic analogue of mandarinoite. The low-temperature heat capacity of Fe 2 ( SeO 3 ) 3 · 5 H 2 O ( cr ) was measured using adiabatic calorimetry between 5.3 and 324.8 K, and the third-law entropy was determined. Using these C p , m o ( T ) data, the third law entropy at T = 298.15 K, S m o , is calculated as 520.1 ± 1.1 J∙K−1∙mol−1. Smoothed C p , m o ( T ) values between T → 0 K and 320 K are presented, along with values for S m o and the functions [ H m o ( T ) − H m o ( 0 ) ] and [ Φ m o ( T ) − Φ m o ( 0 ) ] . The enthalpy of formation of Fe 2 ( SeO 3 ) 3 · 5 H 2 O ( cr ) was determined by solution calorimetry with HF solution as the solvent, giving Δ f H m o ( 298   K ,   Fe 2 ( SeO 3 ) 3 · 5 H 2 O ,   cr ) = −3124.6 ± 5.3 kJ/mol. The standard Gibbs energy of formation for Fe 2 ( SeO 3 ) 3 · 5 H 2 O ( cr ) at T = 298 K can be calculated on the basis on Δ f H m o ( 298   K ) and Δ f S m o ( 298   K ) : Δ f G m o ( 298   K ,   Fe 2 ( SeO 3 ) 3 · 5 H 2 O ,   cr ) = −2600.8 ± 5.4 kJ/mol. The value of ΔfGm for Fe2(SeO3)3·5H2O(cr) was used to calculate the Eh⁻pH diagram of the Fe⁻Se⁻H2O system. This diagram has been constructed for the average contents of these elements in acidic waters of the oxidation zones of sulfide deposits. The behaviors of selenium and iron in the surface environment have been quantitatively explained by variations of the redox potential and the acidity-basicity of the mineral-forming medium. These parameters precisely determine the migration ability of selenium compounds and its precipitation in the form of solid phases