Summary of Durham et al for COMPRES website REV2

New measurements of activation volume in olivine under anhydrous conditionsW.. It can be expected that the properties of olivine control much of Earth’s character because it is the major

New measurements of activation volume in olivine under anhydrous conditions

W B Durham1, S Mei2, D L Kohlstedt2, L Wang3, N A Dixon1

1Massachusetts Institute of Technology, Cambridge, MA 02127

2University of Minnesota, Minneapolis, MN 55455

3Stony Brook University, Stony Brook, NY 11794

Critical parameters in geodynamics are many, but few are as elusive as the activation volume for creep of olivine It can be expected that the properties of olivine control much

of Earth’s character because it is the major phase of the upper mantle, which transcends a pressure range from approximately 0.3 to 15 GPa to a depth of 400 km In particular, the rheological properties of olivine are key to the thermal structure and dynamic behavior of the upper mantle Measurement of the pressure dependence of the viscosity of olivine has been a goal of experimentalists almost since the earliest test of olivine-bearing rock under pressure nearly a half-century ago (Griggs et al., 1960)

Activation volume, V*, which measures the response of high-temperature, steady-state deformation of a rock to hydrostatic pressure, P, is given by

&



where & is creep rate, T is temperature, and R is the gas constant The greatest barrier to a well-resolved measurement of V* has been a limit on achievable pressures in the

laboratory, which, until the recent advent of a new generation of high-pressure

deformation apparatus, was about 2 GPa using the solid-medium Griggs apparatus We report here first results from the deformation-DIA, which raises this limit by over a factor

of two

The particular advance reported in this paper has come about through two recent changes to the sample cell assembly, both aimed at correcting earlier attempts to correct shortcomings in earlier D-DIA experiments: (1) a choice of cell materials that maintains anhydrous conditions around the sample, and (2) elimination of the thermocouple from the assembly The anhydrous assembly (Figure 1) is a sphere of mullite embedded in a cube of unfired pyrophyllite, where the diameter of the sphere matches the edge length of the cube Elimination of the thermocouple from the cell assembly results in a

demonstrably more symmetric and mechanically stable environment in and around the deformation column throughout an experiment To determine temperature we use

carefully calibrated relationships between furnace power and temperature For measuring the calibration curves we use identical equipment and reproduce all run conditions except sample material: pressure, temperature, D-DIA ram displacement rate, and run time The effect of time and deformation is in fact substantial

Deformation experiments designed to measure V* in Equation (1) were carried out in

a D-DIA apparatus (Wang et al., 2003) installed at beam line X17B2, National

Synchrotron Light Source, Brookhaven National Lab We carried out 3 experiments using the hybrid assembly, at a narrow range of temperatures near 1473 K, strain rates of 2

10-6 to 2 10-5 s-1, and pressures of 2.7 to 4.9 GPa Strain rate and pressure were

stepped individually in two of the experiments, giving 5 sets of (&, , P, T) The data allow us to constrain V* in Equation (1) Invoking values for stress and temperature

sensitivity from the literature (Hirth and Kohlstedt, 2003), we calculate from our results

V* = 9.5  10-6 m3/mol with an The fit to the data is illustrated in Figure 7 along with the uncertainty range of ±7  10-6 m3/mol defined by the error bars

Comparison to other measurements in the D-DIA There have been other recent

attempts to measure activation volume of olivine using synchrotron x rays and the D-DIA

or similar instruments, including our own work using predecessors to the anhydrous, thermocouple-free cell used here for the first time (Mei et al., manuscript in preparation)

The experiments of Li et al (2004) determined a very low value for V* of < 5  10-6

m3/mol using stress relaxation experiments in a hydrostatic DIA Because those

measurements were made at high stresses and low plastic strains, the results may not be directly comparable to the measurements reported here In experiments under stresses and

to plastic strains similar to those in our experiments, and using the same D-DIA, Li et al

(2006) again measured a value of V* near 0 We cannot reconcile our data with theirs,

although we can point out a difference in water content Their experiments were

conducted using a pressure medium of amorphous boron powder in epoxy, in contrast with the mullite-based hybrid cell used in our runs Their samples had a significantly higher water content (~3500 ppm) than ours (≤40 ppm) On the other hand, the results of Borch and Green (1989) suggest that water enhances the pressure sensitivity of creep, at

least to P = 2 GPa

Raterron et al (2008) have recently found that two important slip systems in olivine

have contrasting values for V* For a-slip, that is, (010)[100], V * = 12±4  10-6 m3/mol;

while for c-slip, that is (010)[001], V* = 3±4  10-6 m3/mol These authors observe that

a-slip dominates below P ≈ 8 GPa at high temperature In our experiments, a-a-slip was probably quite active, and thus our value of V* = 9.5 ± 7  10-6 m3/mol should be

compared to that of V * = 12±4  10-6 m3/mol from the Raterron et al (2008) study Thus, the possibility that creep in the polycrystal is controlled by a-slip cannot be eliminated

Acknowledgements

of Materials Sciences and Division of Chemical Sciences, under Contract No DE-AC02-98CH10886 This rheology research was partially supported by COMPRES, the Consortium for Materials Properties Research in Earth Sciences under NSF Cooperative Agreements EAR 01-35554 and 06-49658 using cell assemblies fabricated in the Multi-Anvil Cell Development project at the Arizona State University

References

Borch, R.S and Green, H.W., II, 1989 Deformation of peridotite at high pressure in a new molten cell: Comparison of traditional and homologous temperature treatments Phys Earth Planet Interiors, 55: 269-276

Griggs, D.T., Turner, F.J and Heard, H.C., 1960 Deformation of rocks at 500˚ to 800˚C In: D.T Griggs and J.W Handin (Editors), Rock Deformation, Geol Soc Am

Memoir 79 Geol Soc Am., New York, pp 39-104

Hirth, G and Kohlstedt, D.L., 2003 Rheology of the Upper Mantle and the Mantle Wedge: A View From the Experimentalists In: J Eiler (Editor), Inside the Subduction Factory American Geophysical Union, Washington, D.C., pp 83-105

Li, L., Weidner, D., Raterron, P., Chen, J.H and Vaughan, M., 2004 Stress measurements

of deforming olivine at high pressure Physics of the Earth and Planetary Interiors, 143-44: 357-367

Li, L et al., 2006 Deformation of olivine at mantle pressure using the D-DIA European Journal of Mineralogy, 18(1): 7-19

Raterron, P., Amiguet, E., Chen, J., Li, L and Cordier, P., 2008 Experimental

deformation of olivine single crystals at mantle pressures and temperatures Physics

of the Earth and Planetary Interiors, in press

Wang, Y., Durham, W.B., Getting, I.C and Weidner, D.J., 2003 The deformation-DIA: A new apparatus for high-temperature triaxial deformation to pressures up to 15 GPa Rev Sci Instrum., 74: 3002-3011

Publication

Durham, W B., S Mei, D L., Kohlstedt, L Wang and N A Dixon, New measurements

of activation volumes in olivine under anhydrous conditions, Phys Earth Planet Interiors, in press, 2008

FOR MORE INFORMATION

Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of

Technology 54-720, 77 Massachusetts Ave., Cambridge, MA 02127

Email: wbdurham@mit.edu

Figure 1 The new hybrid cell A 6-mm diameter mullite sphere is cradled and lightly cemented between two pieces of unfired pyrophyllite to form a solid cube of 6-mm edge length Coring of the hole for furnace and deformation column then follows

Figure 2 The 5 measurements from the hybrid cell reduced to a common T and &

as indicated using n = 3.5 and E* = 500000 J/mol (Hirth and Kohlstedt, 2003) in Equation (2) The value of V* = 9.5  10-6 m3/mol (solid line) is defined by the two points from the one run conducted at two different pressures (circled) Error bars for those two points allow values from 2.5 to 17  10-6 m3/mol (dashed lines) For purposes of plotting, uncertainty in temperature has been converted to an

additional uncertainty in stress using the same values of n and E* (see text)