Springer-Verlag 1999
Phys Chem Minerals (1999) 26:406±414
ORIGINAL PAPER
I. Daniel ´ G. Fiquet ´ P. Gillet ´ M.W. Schmidt M. Hanfland
P-V-T equation of state of lawsonite
Received: 2 June 1998 / Revised, accepted: 12 Ocotber 1998
Abstract A pressure-volume-temperature data set has been obtained for lawsonite [CaAl2Si2O7(OH)2.H2O], using synchrotron X-ray diffraction and an externally heated diamond anvil cell. Unit-cell volumes were measured to 9.4 GPa and 767 K by angle dispersive X-ray diffraction using imaging plates. Phase changes were not observed within this pressure-temperature range, and lawsonite compressed almost isotropically at constant temperature. The P-V-T data have been analyzed using a BirchMurnaghan equation of state and a linear equation of state expressed as b=±1/V0 (¶V/¶P)T. At room temperature, the derived equation of state parameters are: K0=124.1 (18) GPa K00 set to 4) and b±1=142.0(24) GPa, respectively. Our results are intermediate between previously reported measurements. The high-temperature data show that the incompressibility of lawsonite decreases with increasing temperature to 500 K and then increases above. Hence, the second order temperature derivative of the bulk modulus is taken into account in the equation of state; a fit of the volume data yields K0=123.9(18) GPa, (¶K/¶T)P=±0.111(3) GPa K±1, (¶2K/¶T2)P=0.28(6) 10±3 GPa K±2, a0=3.1(2) 10±5 K±1, assuming K00 =4. Key words Lawsonite ´ Equation of state ´ X-ray diffraction ´ High-pressure ´ High-temperature
)
I. Daniel ( ) ´ G. Fiquet ´ P. Gillet Laboratoire de Sciences de la Terre, UMR CNRS 8515, ENS Lyon and UniversitØ Claude Bernard Lyon I, Bât 402, 43 Bd du 11 Novembre, F-69622 Villeurbanne cedex, France e-mail:
[email protected] Fax: +33-4-72448593 M.W. Schmidt UMR 6224, UniversitØ Blaise Pascal, 5 rue Kessler, F-63038 Clermont Ferrand cedex, France M. Hanfland European Sychrotron Radiation Facility, BP 220, F-38043 Grenoble cedex, France
Introduction Lawsonite [CaAl2Si2O7(OH)2.H2O] is a water-rich mineral common in metabasalts and metagreywackes which have encountered high-pressure low-temperature conditions. In natural rocks, lawsonite is abundant in blueschist terrains, where it crystallizes at conditions ranging from 0.3 to 1.2 GPa and 500 to 770 K (see Evans and Brown 1986, and references therein). Lawsonite also occurs in eclogitic xenoliths estimated to originate from 2.6 GPa, 900 K (Helmstedt and Schulze 1988) from kimberlites in the Four Corner Region, USA, which have been kept at low temperature during an extremely fast exhumation process (Watson and Morton 1969). However, lawsonite is rarely found in eclogitic terrains; instead, breakdown reactions of lawsonite to zoisite documented by lawsonite-pseudomorphs are frequently reported (Newton 1986). Mostly, natural lawsonite exhibits only minor deviations from the endmember composition due to substitution of Al by less than 3% Fe and Ti (Baur 1978). Recent results from multi-anvil experiments in the CaO-Al2O3-SiO2-H2O system (CASH) have shown that lawsonite is stable up to 12.0 GPa, with a maximum temperature of 1230 K (Pawley 1994; Schmidt and Poli 1994; Schmidt 1995). The maximum stability of lawsonite in mid ocean ridge basalts amounts to 8.5 GPa, 1100 K (Schmidt and Poli 1998); lawsonite compositions being always close to the end-member CaAl2Si2O7(OH)2.H2O. Regarding the pressure and temperature conditions in subduction zones, most of thermal models for fast and cold subduction zones predict a temperature at 350 km depth lower than 1200 K (Furukawa 1993; Peacock 1990). Such low temperature would allow lawsonite to remain in the downgoing slab up to its maximum pressure stability. Hence, lawsonite which contains about 11 wt.% H2O could store a large amount water in the subducting slabs down to as much as 250 km at least and thus could contribute to water recycling in the deep mantle. Recent structural studies of lawsonite have been performed at high pressure and high temperature in order
407
to provide accurate basic data such as the isobaric thermal expansion and the isothermal incompressibility of lawsonite. These parameters are necessary for calculating phase relations of lawsonite and its dehydration reactions at high pressure and high temperature. A conventional heating stage for powder diffraction was employed by Pawley et al. (1996) to determine an isobaric thermal expansion coefficient a=3.16(5) 10±5 K±1. Using single-crystal X-ray diffraction techniques, Comodi and Zanazzi (1996) obtained a=3.13(9) 10±5 K±1. Holland et al. (1996) have obtained an isothermal bulk modulus of K0=191(5) GPa (K© set to 4) using energy dispersive X-ray diffraction techniques on powders in a diamond anvil cell. Using single-crystal X-ray diffraction techniques in a diamond anvil, Comodi and Zanazzi (1996) measured for lawsonite K0=96(2) GPa (K© set to 4). While the thermal expansion results of both studies are in good agreement, and are also consistent with the value previously obtained by Le ClØac'h (1990) a=3.22 10±5 K±1, the enormous difference in the compressibilities obtained from these studies demands further investigation which is the goal of this study. We have used the brightness of third-generation synchrotron radiation to perform angle-dispersive X-ray diffraction measurements on lawsonite. The cell parameters of lawsonite has been measured at simultaneous highpressure (up to 8 GPa) and high temperature (up to 760 K) with an externally heated diamond anvil cell. The isothermal bulk modulus and its temperature derivatives are derived from the experimental data.
Experimental method Sample description A powder sample was ground from a single crystal of lawsonite from the type locality, Tiburon Peninsula (California). Fragments of the same crystal were already used by Le ClØac'h and Gillet (1990) for infrared and Raman spectroscopic measurements. Electron microprobe analyses lead to a structural formula close to CaAl2 Si2O7(OH)2.H2O with a (Fe+Mg) content lower than 1 wt.%. The refined unit-cell parameters of the present sample at room pressure and temperature are in excellent agreement with those reported for samples from the same locality (Table 1) (Baur 1978; Comodi and Zanazzi 1996; Libowitsky and Armbruster 1995). High-pressure, high-temperature techniques At ambient temperature, high-pressure measurements were performed in a membrane-type diamond anvil cell (Chervin et al.
Table 1 Lawsonite, cell parameters at ambient conditions Baur (1978) Le ClØac©h (1990) Libowitsky and Armbruster (1995) Comodi and Zanazzi (1996) Pawley et al. (1996) This study
1995). This cell was equipped with type Ia diamond anvils with é 500 mm culets. The sample chambers were formed by 150 mm diameter holes drilled in a stainless steel plate preindented to about 70±100 mm thickness. Argon, nitrogen or a 16:3:1 methanol-ethanol-water mixture was used as pressure transmitting medium. Pressure was determined by the calibrated shift of the R1 fluorescence line of ruby (Mao et al. 1978) from several chips of 2±3 m in size, embedded in the pressure transmitting medium. When Ar or N2 served as pressure transmitting medium, pressure was also calculated from their respective equation of state (EoS) (Ross et al. 1986; Mills et al. 1986). For the experiments performed at high-temperature, we used a Mao-Bell type diamond anvil cell equipped with an external wire resistance heater (Mao et al. 1991). Diamonds with é 500 mm tablets were mounted in the cell. The powdered sample was mixed with NaCl in equal proportion, and subsequently loaded in a rhenium gasket. NaCl served both as a pressure transmitting medium and as a pressure calibrant. Temperature was measured by a Pt/Pt10%Rh S-type thermocouple welded on the gasket, so that it remained in contact with the diamond mounted on the cylinder. Temperature fluctuations were less than 5 K during the X-ray exposure time, which lasted for 5 min only. Pressure conditions were then inferred from the measured temperature and from the PVT equation of state of NaCl (Birch 1986). X-ray diffraction Angle-dispersive X-ray diffraction measurements were performed at the ID09 beam line of the ESRF (Grenoble, France). A monochromatic X-ray spot of wavelength l»0.45 was focused at the sample location to a beam size of 3030 m2. Diffraction rings were collected on image plates. The image plates were scanned with 100 mm resolution using a Molecular Dynamics STORM image plate reader. Sample to plate distances were 330 mm for the room temperature measurements, and 360 mm for those at high temperature, resulting in angular resolutions of 0.04 . One dimensional diffraction patterns were obtained over a 2q interval of 3 to 25 by integrating after spatial correction the two dimensional images with the program fit2d (Hammersley et al. 1996). Cell parameters for lawsonite and pressure calibrant were refined using the program package GSAS (Larson and Von Dreele 1994).
Results and discussion All diffraction data presented in this study were obtained within the stability field of lawsonite, as defined by Schmidt (1995) (Fig. 1). Data were acquired at variable pressures, for temperatures close to the isotherms 298 K, 486 K, 662 K and 759 K (Fig. 1). Typical diffraction patterns recorded near 5 GPa at the four different temperatures are displayed in Fig. 2. Phase changes have not been detected for the pressure-temperature range reported in this study. We looked in detail for the strong 107, 105 and 324 reflections which are characteristics of the Pcmn
a ()
b ()
c ()
V (3)
8.795(3) 8.791(3) 8.790(1) 8.797(5) 8.790(1) 8.794(1)
5.847(1) 5.849(2) 5.847(1) 5.852(2) 5.840(1) 5.849(1)
13.142(6) 13.131(4) 13.128(1) 13.126(6) 13.133(2) 13.134(1)
675.8(7) 675.3(5) 674.7(3) 675.7(9) 674.1(3) 675.6(3)
408
7.8 GPa and 767 K. For each refinement, of the order of 100 reflections were used for the unit-cell determination. This allows for an accurate calculation of the relative axes compressibilities. Lattice parameters and unit-cell volumes for the various P-T conditions achieved in this study are listed in Table 2.
Results at room temperature
Fig. 1 Temperature-pressure paths followed during the experiment. All measurements were performed within the stability field of lawsonite defined by Schmidt (1995), in order to avoid destabilization of lawsonite. The dashed line represents the upper pressure limit of stability of lawsonite defined by Pawley (1994). Data points for which diffraction pattern is presented in Fig. 2, are circled
structure of lawsonite, described at low temperature (Libowitsky and Armbruster 1995). None of these reflections, which are forbidden in the Ccmm symmetry (h+k=2n), could be found. Hence, full profile Le Bail refinements were applied using the space group Ccmm up to the maximum pressure and temperature conditions of Fig. 2 Selected X-ray diffraction patterns of lawsonite at 5 GPa, for the different temperatures investigated. The 2q range of the figure is limited to 3±16 for sake of clarity, while peaks up to 24 have been used in the determination of unit-cell parameters. Lawsonite diffraction peaks are indicated with their indexing in the Ccmm space group; asterisks refer to the indexing of peaks from the pressure transmitting medium NaCl
The deformation of lawsonite is linear up to 8 GPa (Fig. 3), with an almost isotropic compression of the cell edges. Such a behavior had already been noticed by Comodi and Zanazzi (1996) to pressures of 3.8 GPa. At 298 K, under fully hydrostatic conditions, i.e., in the experiments carried out with alcohol mixture as a pressure transmitting medium, linear regressions yield mean axial compressibility coefficients of ba=2.56(4) 10±5, bb=2.43(4) 10±5, bc=2.17(3) 10±5 GPa±1 defined as bl=±1/l0 ¶l/¶P. The bulk modulus b±1, calculated as the reciprocal of the cell-volume linear compressibility, is 142.0(24) GPa (Table 3). The same data can be fitted with a least-squares method to an eulerian finite strain BirchMurnaghan equation of state given by: 2 39 2 38 7 5 < 2 = 3 3 3 ÿ V V0 5 V0 ÿ15 1 34 4 ÿ K00 4 P 32K0 4 0 ÿ : ; V V V
1 where K0, K00 and V0 are the isothermal bulk modulus, its pressure derivative and the unit-cell volume at zero pressure and 298 K. The limited pressure range over which this study was performed leads us to consider K00 as a con-
409 Table 2 Unit cell parameters of lawsonite and NaCl used in the determination of the EoS Lawsonite a ()
b ()
c ()
V (3)
8.79413(1)
5.84901(8)
13.13419(17)
675.583(10)
16:3:1 8.7916(5) 8.7621(3) 8.7283(4) 8.6821(2) 8.6400(3) 8.6187(3) 8.6454(2) 8.7018(2)
5.8488(3) 5.8274(2) 5.8057(3) 5.7803(2) 5.7523(2) 5.7373(2) 5.7549(2) 5.7922(2)
13.1330(6) 13.0941(4) 13.0493(5) 12.9952(3) 12.9407(4) 12.9112(4) 12.9468(3) 13.0195(3)
675.302(37) 668.590(22) 661.260(28) 652.165(14) 643.154(21) 638.437(21) 644.150(14) 656.212(14)
Ruby scale 0.038(1) 1.36(3) 2.73(6) 4.62(10) 6.72(14) 7.77(16) 6.65(14) 3.88(8)
In Ar or in N2 298 298 298 298 298
8.7471(7) 8.6753(7) 8.7314(3) 8.6929(5) 8.6347(7)
5.8170(5) 5.7744(4) 5.8111(2) 5.7876(3) 5.7526(6)
13.0744(8) 12.9798(9) 13.0611(3) 13.0119(5) 12.9401(13)
665.250(54) 650.216(48) 662.706(17) 654.643(29) 642.759(78)
2.14(5) 5.60(12) 2.93(6) 4.57(9) 7.68(15)
In NaCl 496 488 487 484 483 482 483 482 660 662 661 661 662 662 662 666 667 753 738 753 764 766 759 771 767
8.8187(1) 8.7909(1) 8.7398(1) 8.7147(1) 8.6968(1) 8.6717(2) 8.6432(2) 8.6078(2) 8.6163(1) 8.6623(1) 8.6716(1) 8.6791(1) 8.7095(1) 8.7127(1) 8.7299(1) 8.7369(1) 8.7898(1) 8.8285(3) 8.8029(5) 8.7829(6) 8.7711(9) 8.7502(9) 8.7384(10) 8.7104(11) 8.6878(12)
5.8602(1) 5.8385(1) 5.8052(1) 5.7890(1) 5.7742(1) 5.7465(1) 5.7295(2) 5.7159(2) 5.7200(1) 5.7479(1) 5.7549(1) 5.7606(1) 5.7814(1) 5.7832(1) 5.7949(1) 5.8017(1) 5.8395(1) 5.8674(2) 5.8452(4) 5.8279(5) 5.8204(6) 5.8086(6) 5.8008(7) 5.7731(8) 5.7556(10)
13.1606(1) 13.1232(1) 13.0578(1) 13.0248(1) 13.0083(2) 12.9538(2) 12.9335(3) 12.9024(3) 12.9277(1) 12.9762(1) 12.9868(1) 12.9975(1) 13.0299(1) 13.0343(1) 13.0575(1) 13.0655(1) 13.1275(1) 13.1690(3) 13.1341(5) 13.1035(8) 13.0894(10) 13.0625(10) 13.0519(12) 13.0068(14) 12.9664(14)
680.135(18) 673.556(28) 662.511(43) 657.091(65) 653.233(89) 645.51(11) 640.49(15) 634.81(18) 637.151(62) 646.089(35) 648.090(26) 649.840(30) 656.096(26) 656.763(27) 660.558(24) 662.275(25) 673.812(21) 682.165(18) 675.809(41) 670.714(53) 668.237(75) 663.925(67) 661.588(83) 654.065(98) 648.36(11)
T (K) 298 In meth-eth-water 298 298 298 298 298 298 298 298
stant equal to 4 in the subsequent data analysis. We calculated an isothermal bulk modulus K0=124.1(18) GPa associated with a reference unit-cell volume V0=675.6(2) 3 (Table 3). The present values b±1=142.0(24) GPa and K0=124.1 (18) GPa are intermediate between the previous results obtained under similar hydrostatic conditions by Comodi and Zanazzi (1996) and by Holland et al. 1996 (Fig. 3). Part of the discrepancy between the three studies might be interpreted in terms of differences in starting materials, or in compression conditions but the large discrepancy between our results and those of Holland et al. (1996) remains unresolved. If the present experiment is to be compared to the study performed by Comodi and Zanazzi (1996) who determined for lawsonite b±1=110.0(40) GPa, and K0=96.0(20) GPa, two differences separate these studies.
NaCl a ()
5.6863(1) 5.5942(1) 5.4759(1) 5.4259(1) 5.3974(1) 5.3556(1) 5.3058(1) 5.2687(1) 5.2600(1) 5.3184(1) 5.3408(1) 5.3556(1) 5.3975(1) 5.4092(1) 5.4368(1) 5.4669(1) 5.5908(1) 5.7376(1) 5.5912(1) 5.5296(1) 5.4884(1) 5.4456(1) 5.4144(1) 5.3600(1) 5.3214(1)
Pressure scale (GPa)
EoS NaCl 0.002(1) 1.17(6) 3.20(16) 4.26(22) 4.93(25) 6.01(30) 7.46(37) 8.65(43) 9.45(47) 7.59(38) 6.93(35) 6.52(33) 5.44(27) 5.16(26) 4.52(23) 3.89(19) 1.73(9) 0.23(1) 1.93(9) 2.95(15) 3.74(19) 4.63(23) 5.31(26) 6.72(33) 7.80(39)
We used powder as a starting material instead of a single crystal, and we measured pressure by the ruby fluorescence method whereas Comodi and Zanazzi (1996) employed the Sm2+:BaFCl as pressure calibrant. Discrepancies between high-pressure single crystal and powder data have already been observed on many types of minerals, and can be ascribed to differences in the pressure calibration methods, or to the presence of deviatoric stresses (Fiquet and Reynard 1998; Reynard et al. 1996). In the present study, we took particular care to load a very small amount of powder in the diamond anvil cell to avoid contacts between grains upon compression. It is therefore difficult to assign the inconsistency of the results to deviatoric stresses. However, we think that differences in the pressure measurements are responsible for the higher bulk modulus obtained in the present study (Comodi personal communication).
410
Fig. 3 Compression of the lawsonite unit-cell parameters at 298 K. Solid and open symbols are data recorded under hydrostatic conditions in methanol-ethanol-water, and under quasi-hydrostatic compression in Ar or N2, respectively. Curves are fits showing linear compression between 0 and 8 GPa, along with results reported by Comodi and Zanazzi (1996) and Holland et al. (1996)
From powder diffraction data also collected in a fully hydrostatic pressure transmitting medium, Holland et al. (1996) derived a much higher isothermal bulk modulus K0=191(5) GPa. However, the latter obtained also bulk moduli much higher than that of Comodi and Zanazzi (1997) on natural samples of zo and clinozo, suggesting a technical origin for the large discrepancy between the results. In the present study, lawsonite powder was also compressed under quasi-hydrostatic conditions, in a soft solid
Table 3 Axes compressibilities and bulk modulus of lawsonite obtained at various temperatures. A linear regression of the data yields ba, bb, bc, b, b±1, while an adjustment to a second order Birch-Murnaghan EoS provides values for KT,0 and the associated VT,0
pressure-transmitting medium such as Ar or N2. In that case, pressures have been estimated simultaneously by the ruby fluorescence method and by the equation of state of the pressure transmitting medium, and no significant difference between the two pressure scales could be found within the pressure range studied. However, in such conditions where deviatoric stresses are assumed to be moderate, (s1±s3)
