An IR Spectroscopic Study of Mg-Fe Orthopyroxenes

Serena C. Tarantino (tarantino@crystal.unipv.it)¹, M. Chiara Domeneghetti (domeneghetti@crystal.unipv.it)¹, Michael A. Carpenter (mc43@esc.cam.ac.uk)² & Vittorio Tazzoli (tazzoli@crystal.unipv.it)¹

¹ Dipartimento di Scienze della Terra, Università di Pavia, via Ferrata 1, 27100 Pavia, Italy

² Deptartment of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, United Kingdom

New insights into the solid-solution and cation-ordering behaviour of the enstatite-ferrosilite system may be obtained by comparing X-ray diffraction results with spectroscopic data. To this purpose IR and single-crystal X-ray diffraction studies were carried out on a set of natural, synthetic and heat treated orthopyroxene samples. Natural orthopyroxenes were chosen to obtain compositions representative of the binary join En-Fs and to avoid as far as possible the presence of minor elements. Orthopyroxenes of six different compositions between MgSiO₃ (X_Fs=0.00) and FeSiO₃ (X_Fs=1.00) were synthesised in a piston cylinder, at 950°C and 20 kbars, at the Bayerisches Geoinstitut, Bayreuth, Germany. Annealing experiments were carried out to obtain different degrees of order in natural crystals of the same composition.

The IR powder absorption spectra were collected under vacuum at room temperature in the 50-1400 cm^-1 frequency range. The spectra obtained change in frequency and linewidth as a function of composition. Autocorrelation analysis was used to determine variations in average linewidths of groups of peaks in the primary IR spectra.

High degrees of local heterogeneity were suggested by the _corr values of the orthopyroxene samples at intermediate compositions. Phonons at high frequencies depend on the average composition of the sample and not on local configurational changes related to the ordering of Mg and Fe in the M1 and M2 sites. Phonons at low frequencies are very sensitive to both changes in composition and degree of order and are indicative of non ideal mixing behaviour for the orthopyroxene solid solution. The spectra yielded information on the local structural states of the orthopyroxene down to unit-cell length scale, which are compared with the average structure determined by X-ray diffraction studies of the same samples.

Chemical- and Tracer-Diffusion in Granitic Melts

Astrid Tegge-Schuering (a.tegge-schuering@mineralogie.uni-hannover.de), Harald Behrens & Juergen Koepke

Institut für Mineralogie, Uni Hannover, Welfengarten 1, 30167 Hannover, Germany

Cation diffusion in the melt often controls the kinetics of magmatic processes such as crystal growth and dissolution. Since dissolved water has an enhancing effect on transport and reaction kinetics in the melt, we have examined the effect of water content on diffusion in rhyolitic and andesitic melts. Diffusion coefficients were determined by using a modified diffusion couple technique, in which chemical diffusion of major (Si, Al, Ti) and the diffusion of trace elements can be studied independently in a single experiment. Experiments with pre-equilibrated starting glasses with either 2 or 5 wt% of water have been performed in an internally heated or cold seal pressure vessel at 500 MPa in the temperature range 800°C to 1400°C. The concentration profiles of major elements (chemical diffusion) and trace elements (tracer diffusion) were determined by electron microprobe and synchrotron radiation x-ray fluorescence microanalysis, respectively. Water distribution along the diffusion profiles was analysed with infrared microscopy. No indication of uphill diffusion and concentration dependent diffusion coefficients was found in any of the chemical diffusion experiments. Therefore, we described the chemical diffusion with the concept of the effective binary diffusion coefficients (EBDC). The diffusivities as well as the activation energies are generally lower in rhyolitic than in andesitic melts with the same water content. The difference between diffusion coefficients of Eu and those of Gd and Sm is higher for rhyolitic than for andesitic melts implying a higher Eu²⁺/Eu³⁺ ratio in the former ones. Tracer diffusion coefficients of HFSE are similar to the EBDCs of Si and Al in rhyolitic melts. As a general trend we observe a strong negative correlation of diffusivity with the ionic field strength of the cation. For instance at 1100°C, 5 MPa, 5 wt% water the diffusivity decreases from log D_Rb = -10.87 over log D_Ba = -11.34 and log D_Zr = -13.15 to logD_Si = -12.69.

Determination of Water Contents of Granite Melt Inclusions by Confocal Laser Raman Microprobe Spectroscopy

Rainer Thomas (thomas@gfz-potsdam.de)

GeoForschungsZentrum Potsdam, Telegrafenberg B120, D-14473 Potsdam, Germany

A new method for determining the water content of melt inclusions using confocal laser Raman microprobe spectroscopy is described. The water content of melt inclusions can be determined in the concentration range of 0 to 20wt% with a high spatial resolution (~ 2 µm). Because the method works in reflection, only a minimum of sample preparation is necessary. The method is fast, has good accuracy and precision (± 0.25wt%), and has the potential to become a useful, high resolution spectroscopic tool for melt inclusion studies.

Melt Inclusions in Pegmatite Quartz: Complete Miscibility between Silicate Melts and Hydrous Fluids at Low Pressure

Rainer Thomas (thomas@gfz-potsdam.de)¹, James D. Webster (jdw@amnh.org)² & Wilhelm Heinrich (whsati@gfz-potsdam.de)¹

¹ GeoForschungsZentrum Potsdam, Telegrafenberg B120, D-14473 Potsdam, Germany

² Dept. Earth & Planetary Science, American Museum of Natural History, New York, NY 10024, U.S.A.

Fluorine-, boron- and phosphorus-rich pegmatites of the Variscan Ehrenfriedersdorf complex crystallized over a temperature range from about 700°C to 500°C at a pressure of about 1 kbar. Pegmatite quartz crystals continuously trapped two different types of melt inclusions during cooling and growth: a silicate-rich H₂O-poor melt and a silicate-poor H₂O-rich melt. Both melts were simultaneously trapped on the solvus boundaries of the silicate (+ fluorine + boron + phosphorus) - water system. The partially crystallized melt inclusions were rehomogenized at 1 kbar between 500°C and 712°C in steps of 50°C by conventional rapid-quench hydrothermal experiments. Glasses of completely rehomogenized inclusions were analyzed for H₂O by Raman spectroscopy, and for major and some trace elements by EMP (electron microprobe) and SIMS (ion microprobe). Both types of melt inclusions define a solvus boundary in an X_H2O-T pseudobinary system. At 500°C, the silicate-rich melt contains about 2.5 wt% H₂O, and the conjugate water-rich melt about 47 wt% H₂O. The solvus closes rapidly with increasing temperature. At 650°C, the water contents are about 10 and 32 wt%, respectively. Complete miscibility is attained at the critical point: 712°C and 21.5 wt% H₂O. Many pegmatites show high concentrations of F, B and P, this is particularly true for those pegmatites associated with highly evolved peraluminous granites. The presence of these elements dramatically reduces the critical pressure for fluid-melt systems. At shallow intrusion levels, at T >720°C, water is infinitely soluble in a F-, B-, and P-rich melt. Simple cooling induces a separation into two coexisting melts, accompanied with strong element fractionation. On the water-rich side of the solvus, very volatile-rich melts are produced that have vastly different physical properties as compared to "normal" silicate melts. The density, viscosity, diffusivity and mobility of such hyper-aqueous melts at these conditions are more comparable to an aqueous fluid.