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II. Compressibilities of Lawsonite, Zoisite, Clinozoisite, and Epidote

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II. Compressibilities of Lawsonite, Zoisite, Clinozoisite, and Epidote American Mineralogist, Volume 81, pages 341-348, 1996 Volume behavior of hydrous minerals at high pressure and temperature: II. Compressibilities of lawsonite, zoisite, clinozoisite, and epidote T.J.B. HOLLAND,l S.A.T. REDFERN,l AND A.R. PAWLEy2.. 'Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K. 'Department of Geology, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 lRJ, U.K. ABSTRACT The pressure dependence of the lattice parameters of natural zoisite [Ca2A13Si3012(OH)], clinozoisite [Ca2A13Si30'2(OH)], and epidote [Ca2A12FeSi3012(OH)] as well as synthetic lawsonite [CaAl2Si207(OH)2' H20] have been measured at ambient temperatures by en- ergy-dispersive X-ray diffraction in a diamond-anvil cell. The experimental results for each phase may be summarized concisely in terms ofthe ambient-temperature isothermal bulk modulus K298(using the Murnaghan equation of state and assuming K' = 4): Law- sonite: K298= 191 :t 5 GPa; zoisite: K298= 279 :t 9 GPa; clinozoisite: K298= 154 :t 6 GPa; epidote: K298 = 162 :t4 GPa. These new measurements, together with the new thermal expansion data in the companion paper (Pawley et al. 1996), were used to calculate some phase equilibria for lawsonite dehydration to high pressures for comparison with experimental brackets. Important discrepancies between calculated and experimentally determined reactions become evident above 3 GPa. INTRODUCTION zones, to measure their bulk moduli and extend our ca- Two factors have recently led to a renewed interest in pability of calculating mineral assemblage stabilities at the behavior of minerals at very high pressures. First, the these high pressures and temperatures. The significance ready availability of experimental apparatus capable of of three of these phases as potential hydrated silicates in achieving pressures in the range of 20 GPa has led to an subducted slabs [lawsonite (Ccmm), CaA12Si207(OH)2' increase in the number and extent of experimental deter- H20; zoisite (Pnma), Ca2A13Si30dOH); and clinozoisite minations of the stabilities of mineral phases, including (Pnma), Ca2A13Si3012(OH)]was discussed in the com- hydrous silicates to these very high pressures (e.g., Pawley panion paper (Pawley et al. 1996). 1994; Schmidt and Poli 1994). Second, the increased at- In addition to these three phases, we also investigated tention of petrologists and geophysicists on the nature the compressibility of epidote. FeH -containing epidote and behavior of the lithospheric slab as it descends into (P2/m), Ca2A12FeSi30'2(OH),is common in rocks re- the mantle has raised several important questions con- gionally metamorphosed under greenschist- and amphi- cerning the amount and distribution of fluids at deep lev- bolite-facies conditions. It also occurs in blueschists and els in subduction zones. Current thermodynamic models eclogites crystallized in subduction-zone environments for hydrous and carbonate phases are not sufficiently and in some mafic igneous rocks. In experiments unde; complete to predict the temperatures and pressures of possible subduction-zone P-T conditions, epidote of mineral decarbonation and dehydration reactions up to composition Ca2Al2.3Feo.7Si3012(OH)was found to be sta- the pressures and temperatures prevailing in the deeper ble at pressures up to 3 GPa (Pawley and Holloway 1993). parts of subduction zones. Very basic data, such as the Its identification as monoclinic epidote, and not ortho- thermal expansivity and, more importantly, compress- rhombic zoisite, was confirmed using Fourier-transform ibility of common hydrous minerals likely to be impor- infrared spectroscopy. tant components in metamorphosed rocks in the de- SAMPLES AND EXPERIMENTAL TECHNIQUE scending slab, have not been measured, and simple linear extrapolations of molar volume, which are adequate for The samples of natural zoisite, clinozoisite, and syn- modeling crustal processes, are less accurate at the very thetic lawsonite used in our measurements are the same high pressures of interest. Thus, we have selected four as those described in the companion paper (Pawley et al. common mineral phases, which are believed to be likely 1996). The sample of epidote is from a monomineralic hosts for transporting H20 into the mantle in subduction vein in high-pressure albite-amphibolites from the Gross Venediger area in the central Hohe Tauern, Eastern Alps (Holland 1979; Holland and Ray 1985). It is very close * Present address: Department of Earth Sciences, University of to the end-member composition; chemical analyses, by Manchester, Oxford Road, Manchester M13 9PL, u.K. electron microprobe, of the epidote and clinozoisite sam- 0003-004X/96/0304-0341 $05.00 341 342 HOLLAND ET AL.: COMPRESSIBILITIES OF HYDROUS MINERALS TABLE 1. Compositions of natural clinozoisite and epidote sure. Typical diffraction patterns for the four samples un- der pressure loading are shown in Figure 1. Clinozoisite Epidote wt% Cations wt% Cations RESULTS Si02 39.58 2.989 38.01 3.013 Values of cell parameters as a function of pressure are Ti02 0.0 0.0 0.04 0.002 listed in Table 2 and plotted in Figures 2-5 as relative AI203 33.65 2.996 21.41 2.001 Cr203 0.0 0.0 0.09 0.006 compressions. Uncertainties are shown only for relative Fe203 0.34 0.019 16.12 0.961 volume compression for the sake of clarity in the figures. FeO 0.0 0.0 0.26 0.017 MnO 0.0 0.0 0.06 0.004 Values of the isothermal bulk modulus, K, and its pres- MgO 0.0 0.0 0.0 0.000 sure derivative, K', are commonly derived by fitting the CaO 24.70 2.000 23.48 1.996 pressure-volume data to the Birch-Murnaghan equation Na20 0.0 0.0 0.0 0.0 K20 0.0 0.0 0.0 0.0 7 5 2 3 Va 3 Va 3 3 Va 3 p K 1 - (4 = 2 [(V) - (V)]{ "4 - K') [(V-I) ]} pIes used in this study are given in Table 1. All the small or to the simpler Murnaghan equation amount of Fe in clinozoisite was assumed to be FeH in the recalculation of the microprobe data; for the epidote sample, however, only a small proportion of Fe was as- sumed to be Fe2+, and this quantity was calculated on which may be conveniently rearranged into volume-ex- the basis of eight cations per formula unit with 12.5 a plicit form, atoms. Clean and clear individual crystals of zoisite, cli- nozoisite, and epidote were hand-picked from the natural I V K' - K' samples, and they and a batch of the synthetic lawsonite P . Va= [ 1 + K ] were crushed and ground to a fine powder for X-ray dif- fraction experiments. Individual powder samples were We fitted our measured volume data with both the mixed with powdered NaCl, which acted as an internal Birch-Murnaghan and Murnaghan equations and found standard and pressure calibrant for the high-pressure that, over the pressure ranges of this study, both equa- measurements. tions yield identical results within error. Rather than fit High-pressure powder diffraction was performed, in a the data to the Birch-Murnaghan equation in the form manner identical to that used in the study of Redfern et above by minimizing residuals in pressure, we adapted a al. (1993), on the wiggler station 9.7 of the SERC syn- nonlinear least-squares routine to refine initial guesses for chrotron radiation source at Daresbury, U.K. The pow- Va, K298, and K' by minimizing the residuals in volume dered sample was loaded into a preindented, heat-treated (the measured parameter). The values for K298and K' are Inconel gasket (200 mm hole) in a lever-arm diamond- often different when residuals in V rather than P are min- anvil cell. A 4:1 nondried methanol-ethanol mixture was imized, and the former is much the more useful because used as the pressure-transmitting medium. Measure- we wish to predict the volume from the pressure and not ments were made in energy-dispersive mode at room vice versa. Regression for three parameters, Va, K298,and temperature between ambient pressure and 16GPa. White K', is not warranted by the quality of the data especially radiation, collimated by alSO J.Lmpinhole, was diffracted because the parameters are very highly correlated (e.g., at a fixed 28 angle (accurately determined for each exper- increasing K' reduces K298and Va). The scatter in our iment from the diffraction pattern ofNBS silicon powder) measured volumes is also too large to enable very precise and further collimated using 500 mm long molybdenum values of K' to be determined, and attempts to obtain receiving flats shimmed 100 J.Lmapart. Spectra were col- precise values for epidote and zoisite led to physically lected between 5 and 120 keY using a lithium-drifted unrealistic (negative) values, the uncertainties of which germanium detector with a resolution varying from 149 were larger than the absolute values. Thus, we elected to eVat 5.9 keY to 468 eV at 122 keY. set the pressure derivative K' to 4, a commonly accepted Pressure was calibrated with the use of the measured assumption that is based on the results of many mea- cell volume of the internal NaCl standard (determined surements on silicate and oxide phases of geological in- from least-squares refinement of a minimum of three terest and equivalent to reducing the Birch-Murnaghan Bragg peaks) and the equation of state for NaCl given by equation to a simpler form by eliminating the final term Decker (1971). The estimated uncertainty in the pressure (Anderson 1989; Liu and Bassett 1986). Nonlinear re- readings is :t2%. Sample and NaCl diffraction peak po- gression of the volume data can be found in Table 3. sitions were measured by Gaussian peak fitting of dif- fracted intensity. Values for unit-cell parameters were re- DISCUSSION fined by a nonlinear least-squares method using the The two mineral end-members of the monoclinic epi- program UnitCell (Holland and Redfern in preparation), dote solid-solution series, clinozoisite and epidote, show and from 7 to 15 peaks depending on sample and pres- virtually identical compression behavior, with c being HOLLAND ET AL.: COMPRESSIBILITIES OF HYDROUS MINERALS 343 TABLE2.
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