Melting of crustal xenoliths within ascending basalt: Example from the Kula volcanic field,western Anatolia,Turkey

During its storage or ascent, basaltic magma inevitably interacts with the surrounding rocks. In this study, schist xenoliths incorporated within ascending basalt are examined. Heating of the xenoliths combined with decompression effect of rapid magma uprise led to dehydration melting of hydrous minerals producing hercynitic spinel, melt, sillimanite and FeTi oxides. The melt is rhyolitic, strongly peraluminous (1.77 < A/CNK < 2.35) and corundum normative. It may contain up to 8 wt%FeO_t. It occurs between the foliation planes and in the intragranular environment. Dehydration melting of micas in the schist is probably related to combined effects of heating by basaltic magma and decompression due to the rapid rise. Melting of xenoliths was a progressive process at low pressure. To cite this article: H. Bayhan et al., C. R. Geoscience 338 (2006).     

Influence of cation distribution on the optical absorption spectra of Fe3+-bearing spinel s.s.–hercynite crystals: evidence for electron transitions in VIFe2+–VIFe3+ clusters

Flux-grown Fe³⁺-bearing spinel s.s.–hercynite solid-solution crystals, (Mg _1-y Fe²⁺ _y )Al₂O₄ (0 < y≤ 1), have been investigated by means of electron microprobe technique and Mössbauer and electronic spectroscopy. Obtained results show that different electronic processes cause intense optical absorption bands in the near-infrared spectral region. In addition to an electronic d–d transition in single-ion ^IVFe²⁺, observed at 5200 cm^?1, intense and broad bands at 9500 and 14 500 cm^?1 are assigned to exchange-coupled pair (ECP) and intervalence charge-transfer (IVCT) transitions in ^VI Fe ^{2+ VI} Fe³⁺clusters, respectively. The net linear extinction coefficients of these bands (α) were calibrated against Fe²⁺ and Fe³⁺ concentrations and site distributions previously defined by combined microchemical, Mössbauer, and XRD structural refinement data. The following expressions were obtained: where α is measured in cm^?1 and concentrations are expressed in mol?l^?1. The present results show that optical absorption spectroscopy may be used as a probe to obtain high spatial resolution (?～ 10 μm) information on Fe²⁺ ordering as well as on Fe³⁺ concentrations in minerals belonging to the spinel group.     

Controlled time–temperature oxidation reaction in a synthetic Mg-hercynite

The oxidation of a synthetic hercynite with composition Fe²⁺ _0.699Mg_0.301Al_1.941Fe³⁺ _0.059O₄ was investigated by X-ray single-crystal diffraction. Heating runs at 500°C up to 212 h did not produce detectable oxidation, but only a small variation in oxygen coordinate u, consistent with very limited reordering of Mg and Al in tetrahedral (T) and octahedral (M) sites, respectively. Oxidation began after the first run at 600°C, producing progressive decreases in u, cell parameter a and the mean atomic number in T site. After 1,842 h at 600°C, the transformation was close to equilibrium, with about 70% of ferrous iron transformed into ferric. This produced about 0.17 vacancies per formula unit, and caused a great increase in the displacement parameters of oxygen and M sites. Vacancies were strongly ordered in M sites, and the oxygen displacement parameter becames anisotropic, unlike stoichiometric spinels—because some of the oxygen coordination polyhedra have a vacancy instead of a cation in one vertex. The behaviour of displacement parameters in this case supports the calculated point defect distribution.