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Register a new userThe effect of diffusive re-equilibration time on trace element partitioning between alkali feldspar and trachytic melts
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We present new experimental data on major and trace element partition coefficients between alkali feldspar and trachytic melt. Experiments were conducted at 500 MPa, 870 890 {deg}C to investigate through short disequilibrium and long near equilibrium experiments the influence of diffusive re-equilibration on trace element partitioning during crystallization. Our data show that Ba and Sr behave compatibly, and their partition coefficients are influenced by re-equilibration time, orthoclase (Or) content, growth rate and cation order-disorder. High field strength elements (HFSE) and rare earth elements (except Eu) are strongly incompatible, but alkali feldspar efficiently fractionates light (LREE) from heavy rare earth elements (HREE). Our crystallization experiments reveal a strong influence of disequilibrium crystal growth on the partitioning of Ba and Sr. In particular, short-duration experiments show that rapid alkali feldspar crystal growth after nucleation, promotes disordered growth and less selectivity in the partitioning of compatible trace elements that easily enter the crystal lattice (e.g., Ba and Sr)....
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The interdiffusion of six major elements (Si, Ti, Fe, Mg, Ca, K) between natural shoshonite and a high-K calc-alkaline rhyolite (Vulcano island, Aeolian archipelago, Italy) has been experimentally measured by the diffusion couple technique at 1200{deg}C, pressures from 50 to 500 MPa and water contents from 0.3 (nominally dry) to 2 wt%. The experiments were carried out in an internally heated pressure vessel, and major element profiles were later acquired by electron probe microanalysis. The concentration-distance profiles are evaluated using a concentration-dependent diffusivity approach. Effective binary diffusion coefficients for four intermediate silica contents are obtained by the Sauer-Freise modified Boltzmann-Matano method. At the experimental temperature and pressures, the diffusivity of all studied elements notably increases with dissolved H2O content. Particularly, diffusion is up to 1.4 orders of magnitude faster in a melt containing 2 wt.% H2O than in nominally dry melts. This effect is slightly enhanced in the more mafic compositions. Uphill diffusion was observed for Al, while all other elements can be described by the concept of effective binary interdiffusion. Ti is the slowest diffusing element through all experimental conditions and compositions, followed by Si. Fe, Mg, Ca and K diffuse at similar rates but always more rapidly than Si and Ti. This trend suggests a strong coupling between melt components. Since effects of composition (including water content) are dominant, a pressure effect on diffusion cannot be clearly resolved in the experimental pressure range.
Pyroxene is the principal host of Fe$^{3+}$ in basalt source regions, hosting 79 and 81% of the Fe$^{3+}$ in spinel and garnet lherzolite, respectively, with opx and cpx hosting 48% and 31%, respectively, of the total Fe$^{3+}$ in spinel peridotite. To better understand partitioning of Fe$^{3+}$ between pyroxene and melt we conducted experiments at 100 KPa with f$_{O2}$ controlled by CO-CO$_2$ gas mixes between $Delta$QFM -1.19 to +2.06 in a system containing andesitic melt saturated with opx or cpx only. To produce large (100-150 $mu$m), homogeneous pyroxenes, we employed a dynamic cooling technique with a 5-10$deg$C/h cooling rate, and initial and final dwell temperatures 5-10$deg$C and 20-30$^circ$C super and sub-liquidus, respectively. Resulting pyroxene crystals have absolute variation in Al$_2$O$_3$ and TiO$_2$ <0.05 wt.% and <0.02 wt.%, respectively. Fe$^{3+}$/Fe$^T$ in pyroxenes and quenched glass were measured by XANES. We used a newly developed XANES calibration for cpx and opx by only selecting spectra with X-ray vibrating on the optic axial plane at $50 pm 5^circ$ to the crystallographic c axis. Values of DFe$^{3+}$ cpx/melt increase from 0.03 to 0.53 as fO2 increases from $Delta$QFM -0.44 to 2.06, while DFe$^{3+}$ opx/melt remains unchanged at 0.26 between $Delta$QFM -1.19 to +1.37. In comparison to natural peridotitic pyroxenes, Fe$^{3+}$/FeT in pyroxenes crystallized in this study are lower at similar f$_{O2}$, presumably owing to lower Al$^{3+}$ contents. This study shows that the existing thermodynamic models implemented in pMELTS and Perple_X over-predict the stability of Fe$^{3+}$ in pyroxenes, causing an anomalous reduced character to spinel peridotites at calculated conditions of MORB genesis.
The Campi Flegrei caldera (southern Italy) is one of the most hazardous volcanic systems on Earth, having produced >60 eruptions in the past 15 ka. The caldera remains active and its potential for future eruptions is high, posing a danger to the dense population living nearby. Despite this, our understanding of pre-eruptive processes and the architecture of the sub-volcanic system are poorly constrained. Here, we combine established petrological techniques, geothermobarometric evaluation, and high resolution trace element crystal mapping, to present a multifaceted, coherent reconstruction of the complex pre eruptive dynamics and eruption timescales of Astroni volcano located in the eastern sector of Campi Flegrei caldera. The Astroni volcano is an important case study for investigating plumbing system processes and dynamics at Campi Flegrei caldera because it produced the most recent (ca. 4 ka ago) Plinian eruption within the caldera (Astroni 6); current long-term forecasting studies postulate that a similar sized event in this location is a probable future scenario. Geothermobarometric results indicate interaction between an evolved, shallow magma chamber, and a less evolved, deeper pocket of magma, in agreement with previous studies focused on the Astroni 6 eruption products. In addition, a range of textural and trace element zoning patterns point to a complex evolution of both magmas prior to their subsequent interaction. High resolution trace element crystal maps reveal discrete zonations in compatible elements. These zonations, combined with knowledge of Kfeldspar growth rates, highlight a recharge event in the shallow plumbing system a few hours to days before the Astroni 6 eruption.
We are currently measuring the dissolution kinetics of albite feldspar at 100 deg C in the presence of high levels of dissolved CO_2 (pCO_2 = 9 MPa) as a function of the saturation state of the feldspar (Gibbs free energy of reaction, Delta G). The experiments are conducted using a flow through reactor, thereby allowing the dissolution reactions to occur at a fixed pH and at constant, but variable saturation states. Preliminary results indicate that at far-from-equilibrium conditions, the dissolution kinetics of albite are defined by a rate plateau, with R approx 5.0 x 10^{-10} mol m^{-2} s^{-1} at -70 < Delta G < -40 kJ mol^{-1}. At Delta G > -40 kJ mol^{-1}, the rates decrease sharply, revealing a strong inverse relation between the dissolution rate and free energy. Based on the experiments carried out to date, the dissolution rate-free energy data correspond to a highly non-linear and sigmoidal relation, in accord with recent studies.
Seismic attributes calculated by conventional methods are susceptible to noise. Conventional filtering reduces the noise in the cost of losing the spectral bandwidth. The challenge of having a high-resolution and robust signal processing tool motivated us to propose a sparse time-frequency decomposition while is stabilized for random noise. The procedure initiates by using Sparsity-based adaptive S-transform to regularize abrupt variations in frequency content of the nonstationary signals. Then, considering the fact that a higher amplitude of a frequency component results in a higher signal to noise ratio, an adaptive filter is applied to the time-frequency spectrum which is sparcified previously. The proposed zero adaptive filter enhances the high amplitude frequency components while suppresses the lower ones. The performance of the proposed method is compared to the sparse S-transform and the robust window Hilbert transform in estimation of instantaneous attributes by applying on synthetic and real data sets. Seismic attributes estimated by the proposed method is superior to the conventional ones in terms of its robustness and high resolution image. The proposed approach has a vast application in interpretation and identification of geological structures.
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