Determination on iron species in plagioclase crystals toward the understanding on the magnetite exsolution

<p>Speciation analysis of Fe in single plagioclase crystals separated from two different gabbros was performed to understand the crystallization mechanisms of magnetite exsolution. Iron species in single crystals were measured using Fe K- and L<sub>III</sub>-edge X-ray absorption fine structure (XAFS) analysis. The K-edge pre-edge analysis showed variation in the averaged valence state of Fe in plagioclase crystals even if they had been separated from the same gabbro that was further confirmed by the L<sub>III</sub>-edge analysis. The K-edge pre-edge analysis also suggests the various degrees of contribution from tetrahedral Fe. The mixing of tetrahedral and octahedral Fe leads to an underestimation of the averaged valence state of Fe for the K-edge pre-edge analysis; thus, we adopted the L<sub>III</sub>-edge result for the valence state of Fe in plagioclase crystals. Iron K-edge extended XAFS (EXAFS) analysis of two plagioclase crystals separated from the same gabbro clearly showed different coordination environments. A weakening of EXAFS oscillation was recognized in one sample, because two Fe‒O bonds (Fe<sup>3+</sup>‒O<sub>1</sub> and Fe<sup>2+</sup>‒O2) cancelled out the oscillations of each other. The EXAFS spectrum of the other plagioclase crystal suggested a homogeneous distribution of Fe. The content of exsolved magnetite in these crystals is nearly identical, indicating that the exsolution of magnetite in plagioclase crystal had been completed before the temperature decrease that stopped the ordering of Fe ions in tetrahedral sites.</p><p>Reference: Nakada et al. (2019) G-Cubed, vol. 20 (11), 5319-5333.</p>

Analisis Tipikal Erupsi Gunung Lokon Periode Erupsi 2012-2013 Berdasarkan Karakterisasi Mikrostruktur Abu Vulkanik

Gunung Lokon yang berada di lengan utara Sulawesi adalah salah satu gunung api paling aktif di Indonesia. Perilaku erupsinya telah dipelajari melalui analisis mikrostruktur abu vulkanik. Tujuan dari karakterisasi mikrostruktur adalah untuk mengestimasi nilai dari viskositas dan permeabilitas magma. Karakterisasi mikrostruktur menggunakan XRD, FTIR, SEM/EDS/XRF dan µCT. Abu vulkanik Lokon adalah mineral polimorf yang banyak mengandung kristal plagioklase. Abu Lokon mempunyai kandungan air 0,3 -0,6 % berat dan massa dasarnya terdiri dari partikel vesikular dan non vesikular. Viskositas dari magma Lokon adalah sekitar 107Pa.s pada 10000C dan fraksi volume kristal sekitar 0,45-0,5. Hasil-hasil ini menunjukkan bahwa reologi magma Lokon adalah bersifat non Newtonian dan mekanisme fragmentasinya adalah brittle fragmentation. Berdasarkan pada permeabilitas dan porositas yang dikuantisasi dengan µCT dapat disimpulkan bahwa fragmentasi magmanya tidak dipicu oleh outgassing. Dinamika erupsi eksplosif dari Gunung Lokon pada 2012-2013 adalah erupsi vulkanian pada skala sedang.Lokon volcano where located on North arm of Sulawesi is one of the most active volcanoes in Indonesia. Behaviour of its eruptions have been learned through microstructure analysis of volcanic ash. The goal of microstructure characterization is estimate value of magma viscosity and permeability. Characterization of microstructure using XRD, FTIR, SEM/EDS/XRF and µCT. Lokon volcanic ash is a polymorph minerals which contains many plagioclase crystal. Ash has water content between 0.3 – 0.6 % wt and its groundmass contains vesicular and non vesicular particles. Viscosity of Lokon magma is about 107Pa.s at 10000C and fraction of crystal volum between 0.45-0.5. These results showed that magma rheology of Lokon is non Newtonian and the mechanism of its fragmentation is brittle fragmentation. Based on permeability and porosity that quantified by µCT, it is concluded that the brittle fragmentation is not triggered by outgassing. Dynamics of explosive eruption of Lokon volcano at 2012-2013 is moderate vulcanian eruption.