HR: 15:10h
AN: T43E-07    [Abstracts]
TI: Searching for the Mechanism of Dehydration in Clinopyroxene
AU: * Koch-M\"{u}ller, M
EM: mkoch@gfz-potsdam.de
AF: GeoForschungsZentrum, Telegrafenberg, Potsdam, 14473 Germany
AU: Abs-Wurmbach, I
EM: irmgard.abs-wurmbach@tu-berlin.de
AF: Technische Universit\"{a}t Berlin, Ernst-Reuter Platz 1, Berlin, 10587 Germany
AU: Kahlenberg, V
EM: volker.kahlenberg@uibk.ac.at
AF: Universit\"{a}t Innsbruck, Innrain 52, Innsbruck, 6020 Austria
AU: Matsyuk, S S
EM: ssmatsyuk@hotmail.com
AF: Ukrainian Academy of Science, Pr. Palladina 34, Kiev, 252 142 Ukraine
AB: Clinopyroxene (cpx) is a major constituent of the Earth's crust and mantle. From experimental studies as well as from studies of natural systems it is well known that cpx can store hydrogen in its structure with concentrations ranging from 30 to 3000 ppm H$_{2}$O (by weight). The highest concentrations are found in clinopyroxenes that come from high pressure region (e.g. Rossman 1996). But some of our high pressure candidates show concentrations as low as 30-50 ppm H$_{2}$O (Koch-M\"{u}ller et al., 2004). Does this reflect the mantle heterogeneity in water content or did the cpx loose their water during ascent? To shed light on this geochemically and petrologically important question we investigated cpx's from a wide range of occurrences by FTIR-, M\"{o}ssbauer- and optical spectroscopy, and single crystal X-ray diffraction. Water concentrations ranged from 60 to 700 ppm H$_{2}$O (by weight). In accordance with literature data we found Fe$^{2+}$ at M1 as well as on M2, and assigned in a first step Fe$^{3+}$ to M1. We simulated hydrogen loss through heating cpx in air up to 800 $\deg$C and analyzed the stepwise heated samples by the above mentioned methods. Depending on the type of OH defect, the grade of dehydration with increasing temperature is quite different. In samples relatively poor in Fe$^{3+}$ ($< $0.1 Fe$^{3+}$ pfu), hydrogen associated with vacancies at M2 (OH bands around 3450 cm$^{-1}$) starts to leave the structure at about 550 $\deg$C and are completely gone at 700 $\deg$C. Hydrogen concentrations associated with Al$^{3+}$ at the tetrahedral site (OH bands around 3525 cm$^{-1}$, Koch-M\"{u}ller et al., 2004) remain completely unaffected by heat treatment up to 700 $\deg$C. But all hydrogen vanished at about 775 $\deg$C. However, this is different in more Fe$^{3+}$-rich samples (about 0.2 Fe$^{3+}$ pfu). Here we observe a very intense OH band at 3515 cm$^{-1}$ plus shoulder at 3450 cm$^{-1}$. This intense band exhibit a different polarisation and behaves also different in its response to pressure and heating in comparison to those of the above mentioned samples. We assume that this band is due to vibrations of an OH dipole associated with Fe$^{3+}$ at M1 and a vacancy either at M1 or M2. The OH decrease during heating is in all samples positively correlated with decrease in Fe$^{2+}$ and increase in Fe$^{3+}$. Our data indicate that dehydration occurs through iron oxidation exclusively at the M2 site following the reaction:Fe$^{2+}$$_{M2} + OH$^{-}$ = Fe$^{2+}$$_{M2} + 1/2 H$_{2}$. Whereby the amount of Fe$^{2+}$ at M1 seems to remain unchanged. This conclusion is supported by the results of Skogby and Rossman (1989) who found reversibly that in cpx heated in hydrogen H incorporation was correlated with increase of Fe$^{2+}$ in the M2 site. Thus, if dehydration of cpx is indeed correlated with Fe$^{3+}$ at M2, the amount of Fe$^{3+}$ at M2 could give us an good estimate of the lost hydrogen. References Rossman G.R. (1996) Phys. Chem. Minerals, 23, 299-304. Koch-M\"{u}ller M., Matsyuk S.S., Wirth R. (2004) Am. Mineral., 89, 921-931. Skogy H., Rossman G.R. (1989) Am. Mineral., 74, 1059 - 1069.
DE: 3620 Crystal chemistry
DE: 3670 Minor and trace element composition
DE: 3904 Defects
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 3934 Optical, infrared, and Raman spectroscopy
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting