ALBERT

Description: The partitioning of REE,Y and Sc (R3+) between olivine and melt has been investigated experimentally during basalt-carbonate interaction. Three synthetic basalts (meltMg#72, meltMg#75 and meltMg#78) were doped with 0, 10 and 20 wt% CaCO3 and then equilibrated for 72 h at 1 atm, 1,150, 1,200 and 1,250 °C, and the QFM oxygen buffer. The thermal decomposition of CaCO3 produced CaO contents in the melt up to ~22 wt%. Regular relationships are found between the ionic radius and the partition coefficient (DR3+), showing typical near-parabolic patterns. DR3+ is weakly dependent on temperature, but decreases with increasing CaCO3 in the starting material (e.g., DSc decreases from 0.20 to 0.13). From the point of view of the lattice strain theory, DR3+ is described in terms of the radius of the crystal site (r0), the Young Modulus (E) due to the elastic response of that site to lattice strain caused by cation insertion, and the strain-free partition coefficient (D03+). The value of r0 decreases as Ca cations are accommodated into the more distorted M2 site of olivine via progressive CaFe substitutions. This mechanism is accompanied by a higher proportion of Mg cations entering into the smaller M1 site, making the optimum ionic radius smaller and favoring the crystallization of more forsteritic olivines from decarbonated melts. The enrichment of Ca in the crystal lattice is also proportional to the number of Si and Ca cations available in the melt. This causes E to be anticorrelated either with Ca in olivine or the activity of CaO in the melt. R3+ cations behave as network modifiers and, during basalt-carbonate interaction, the increasing abundance of non-bridging oxygens enhances the solubility of REE, Y and Sc in the melt. As a consequence, D03+ is negatively correlated with the degree of melt depolymerization. Additionally, the strain of the crystal lattice dominates the DR3+ parabolic patterns and D03+ is strongly controlled by forsterite and aluminium concentrations in olivine. The accommodation of REE, Y and Sc in the crystal lattice requires maintenance of local charge-balance by the generation of vacancies, in accord with a paired substitution of R3+ and a vacancy for Mg in octahedral sites.

Description: Published

Description: 327-340

Description: 3V. Proprietà chimico-fisiche dei magmi e dei prodotti vulcanici

Description: JCR Journal

Description: The purpose of this review study is to reappraise in a more comprehensive form the thermodynamic principles behind the partitioning of trace elements between clinopyroxene and melt. The original corollary is that the partitioning energetics controlling the crystal-melt exchange are described by two distinct but complementary contributions: ΔGpartitioning = ΔGstrain + ΔGelectrostatic. ΔGstrain is the excess of strain energy quantifying the elastic response of the crystallographic site to insertion of trace cations with radius different from that of the major cation at the site. ΔGelectrostatic is the excess of electrostatic energy requiring that an electrostatic energy penalty is paid when a trace cation entering the lattice site without strain has charge different from that of the resident cation. Lattice strain and electrostatic parameters for different isovalent groups of cations hosting the same lattice site from literature have been discussed in comparison with new partitioning data measured between Tschermak-rich clinopyroxenes and a primitive phonotephritic melt assimilating variable amounts of carbonate material. Through such a comparatively approach, we illustrate that the type and number of trace cation substitutions are controlled by both charge-balanced and -imbalanced configurations taking place in the structural sites of Tschermak-rich clinopyroxenes. A virtue of this complementary relationship is that the control of melt composition on the partitioning of highly charged cations is almost entirely embodied in the crystal chemistry and structure, as long as these crystallochemical aspects are the direct expression of both ΔGstrain and ΔGelectrostatic. A size mismatch caused by cation substitution is accommodated by elastic strain in the surrounding lattice of clinopyroxene, whereas the charge mismatch is enabled via increasing amounts of charge-balancing Tschermak components, as well as the electrostatic work done on transferring the trace cations from melt to crystallographic sites, and vice versa. The influence of the melt chemistry on highly-charged (3+ and 4+) cation partitioning is greatly subordinate to the lattice strain and electrostatic energies of substitutions, in agreement with the thermodynamic premise that both these energetic quantities represent simple-activity composition models for the crystal phase. The various charge-balanced and -imbalanced configurations change principally with aluminium in tetrahedral coordination and the clinopyroxene volume change produced by heterovalent cation substitutions. In contrast, for low-charged (1+ and 2+) cations, the role of melt chemistry cannot be properly deconvoluted from the structural changes of the crystal lattice. The incorporation of these cations into the clinopyroxene lattice depends on the number of structural sites critically important to accommodating network-modifying cations in the melt structure, implying that the partitioning energetics of monovalent and divalent cations are strictly controlled by both crystal and melt properties. We conclude that the competition between charge-balanced and charge-imbalanced substitutions may selectively change the ability of trace elements to be compatible or incompatible in the clinopyroxene structure, with important ramifications for the modeling of natural igneous processes in crustal magma reservoirs which differentiate under closed- and open-system conditions.

Description: Published

Description: 103351

Description: 3V. Proprietà chimico-fisiche dei magmi e dei prodotti vulcanici

Description: JCR Journal