镁橄榄石相变长大率与水含量的关系及亚稳态橄榄石的存在深度
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摘要
通过分析镁橄榄石相变长大率方程的指前系数和亥姆霍兹活化能在不同含水条件下的实验拟合值,本文研究了橄榄石相变长大率与水含量的关系.结果表明含水量较高的镁橄榄石在相变过程中对应较低的亥姆霍兹活化能,而长大率方程中的指前系数几乎是一个与含水量无关的常数.这个常数的数值在晶界控制长大的晶体相变动力学经典理论中相应于晶界是一层或几层分子的厚度.亥姆霍兹活化能随水含量的增加而降低的结果既和利用淬火实验得到的结论相一致,也和流变学实验研究的结论相一致.应用以上结果,我们估算了含水量对亚稳态橄榄石存在深度的影响.镁橄榄石的亚稳性受到水的影响很大,具体表现是橄榄石到其高压相瓦士利石的动力学相变界面从下压了几十公里到一直下压到660 km间断面附近.地幔橄榄石与镁橄榄石的亚稳性存在差异.名义上干的地幔橄榄石实验结果显示出地幔橄榄石具有更小的亚稳性,相变完成10%的深度比镁橄榄石浅了20 km左右.以上结果仍然支持在冷的俯冲带中存在可探测的亚稳态橄榄石楔但很难达到660 km间断面深度的结论.
We investigate the relationship between water content and the parameters in the growth kinetics of forsterite phase transformation,i.e.,the pre-exponential factor and the Helmhotz free energy of activation,by analyzing the growth parameters of forsterite separately for water content varying from about 0.08 to 0.5 per cent in weight.Results show that increase of water content only decreases the Helmholtz free energy of activation for growth,while the preexponential factor of classical growth rate equation has weak dependence on the water content and is nearly a constant,of which the value corresponds to an interphase boundary with the thickness of one or a few layers of molecules.This fact is consistent with the observation of quench experiments on olivine and rheological studies.Accordingly,taking this fact as a constraint,we predict the survival depth of metastable forsterite with different water contents.The dynamic phase boundary of forsterite changes from several kilometers below the equilibrium boundary of forsterite phase transformation to almost the 660 km discontinuity.Different from forsterite, mantle olivine has smaller metastability.Under a nominal dry condition,the dynamic phase boundary of mantle olivine is 20 km shallower than that of forsterite.The current result agrees that there might exist detectable extent of metastable olivine in subduction zones although it is difficult to reach the 660-discontinuity.
We investigate the relationship between water content and the parameters in the growth kinetics of forsterite phase transformation,i.e.,the pre-exponential factor and the Helmhotz free energy of activation,by analyzing the growth parameters of forsterite separately for water content varying from about 0.08 to 0.5 per cent in weight.Results show that increase of water content only decreases the Helmholtz free energy of activation for growth,while the preexponential factor of classical growth rate equation has weak dependence on the water content and is nearly a constant,of which the value corresponds to an interphase boundary with the thickness of one or a few layers of molecules.This fact is consistent with the observation of quench experiments on olivine and rheological studies.Accordingly,taking this fact as a constraint,we predict the survival depth of metastable forsterite with different water contents.The dynamic phase boundary of forsterite changes from several kilometers below the equilibrium boundary of forsterite phase transformation to almost the 660 km discontinuity.Different from forsterite, mantle olivine has smaller metastability.Under a nominal dry condition,the dynamic phase boundary of mantle olivine is 20 km shallower than that of forsterite.The current result agrees that there might exist detectable extent of metastable olivine in subduction zones although it is difficult to reach the 660-discontinuity.
引文
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[3] Schelming H, Monz R, Rubie D C. The influence of olivine metastability on the dynamics of subduction. Earth Planet.Sci. Lett. , 1999, 165(1) : 55-66
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[5] Burnley P C. The effect of nonhydrostatic stress on the olivine-spinel transformation in magnesium germinate [Ph.D. thesis]. Davis: University of California, 1990． 187
[6] Rubie D C, Tsuchida Y, Yagi T, et al. An in situ X ray diffraction study of the kinetics of the Ni_2SiO_4 olivine-spinel transformation. J. Geophys. Res., 1990, 95 (BIO): 15829-15844
[7] Brearley A J, Rubie D C, Ito E. Mechanisms of the transformations between the α, β and γ polymorphs of Mg_2SiO_4 at 15 GPa. Phys. Chem. Miner. , 1992, 8: 343-358
[8] Fujino K, Irifune T. TEM studies on the olivine to modified spinel transformation in Mg_2SiO_44． In: Syono Y, Manghnani M H, eds. High Pressure Research; Application to Earth and Planetary Sciences. Terra; AGU, 1975
[9] Brearley A J, Rubie D C. Transformation mechanisms of San Carlos olivine to ( Mg, Fe)_2SiO_4 β-phase under subduction zone conditions. Phys. Earth Planet. Int. , 1994, 86(1-3) ,45-67
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[11] Kubo T, Ohtani E, Funakoshi K I. Nucleation and growth kinetics of the α-β transformation in Mg_2 SiO_4 determined by in situ synchrotron powder X-ray diffraction. Am. Miner. ,2004, 89: 285-293
[12] Hosoya T, Kubo T, Ohatani E, et al. Water controls the fields of metastable olivine in cold subducting slabs. Geophys. Res. Lett. , 2005, 32: L17305, doi: 10． 1029/2005GL023398
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[14] Jing Z C, Ning J Y, Wang S G, et al. Phase and thermal structures of the subduction zones. Geophys. Res. Lett. ,2002, 29(22) : 2045
[15] O'Connell R J. On the scale of mantle convection. Tectonophysics, 1977, 38(1-2) : 119-136
[16] Christian J W. The Theory of Transformations in Metals and Alloys Part I: Equilibrium and General Kinetic Theory. 2nd ed. Oxford; Pergamon Press, 1975． 449
[17] Porter D, Easterling K. Phase Transformations in Metal and Alloys. New York: Chapman and Hall, 1975
[18] Kubo T, Ohtani E, Kato T, et al. Effects of water on the or ft transformation kinetics in San Carlos olivine. Science,1998, 281(5373) : 85-87
[19] Wang S G, Ning J Y. Kinetics of olivine phase transformation and the role of water. Eos Trans. AGU, Fall Meet. Suppl. , Abstract, DI53A-1105． 2007, 88(52)
[20] Diedrich T, Sharp T G, Leinenweber K, et al. The effect of small amounts of H_2O on olivine to ringwoodite transformation growth rates and implications for subduction of metastable olivine. Chem. Geol. , 2007, 262(1-2) : 87-99
[21] Mosenfelder J L, Marton F C, Ross C R II, et al. Experimental constraints on the depth of olivine metastability in subducting lithosphere. Phys. Earth Planet. Inter. , 2001, 127(1-4) : 165-180
[22] Kubo T. Transformation kinetics in the earth' s interior. Rev. High Press. Sci. Technol. , 1999, 9: 26-33
[23] Ando J I, Tomioka N, Matsubara K, et al. Mechanism of the olivine-ringwoodite transformation in the presence of aqueous fluid. Physics and Chemistry of Minerals , 2006, 33(6) : 377-382
[24] Jung H, Karato S I. Water-induced fabric transitions in olivine. Science, 2001, 293(5534) : 1460-1463
[25] Karato S I, Jung H. Effects of pressure on high-temperature dislocation creep in olivine. Philos. Mag. , 2003, 83(3) : 401-414
[26] Nishihara Y, Shinmei T, Karato S I. Grain-growth kinetics in wadsleyite: Effects of chemical environment. Phys. Earth Planet. Inter. , 2006, 154(1) . 30-43
[27] Morishima H, Kato T, Ohtani E, et al. The phase boundary between α-and β-Mg_2SiO_4 determined by in situ X-ray observation. Science, 1994, 265(5176) : 1202-1203
[28] Liu M, Kerschhofer L, Mosenfelder J L, et al. The effect of strain energy on growth rates during the olivine-spinel transformation and implications for olivine metastability in subducting slabs. J. Geophys. Res., 1998, 103 ( B10) :23897-23909
[29] Katsura T, Yamada H, Nishikawa O, et al. Olivine-wadsleyite transition in the system ( Mg, Fe)_2SiO_4． J. Geophys. Res., 2004, 109 ( B2 ): B02209, doi:10． 1029/003JB002438
[30] Wood B J. The effect of H_2O on the 410-kilometer seismic discontinuity. Science, 1995, 268(5207) : 74-76
[2] Kirby S H, Stein S, Okal E A, et al. Metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere. Rev. Geophys. , 1996, 34(2) : 261-306
[3] Schelming H, Monz R, Rubie D C. The influence of olivine metastability on the dynamics of subduction. Earth Planet.Sci. Lett. , 1999, 165(1) : 55-66
[4] Burnley P C, Green H W. Stress dependence of the mechanism of the olivine-spinel transformation. Nature,1989, 338(6218) : 753-756
[5] Burnley P C. The effect of nonhydrostatic stress on the olivine-spinel transformation in magnesium germinate [Ph.D. thesis]. Davis: University of California, 1990． 187
[6] Rubie D C, Tsuchida Y, Yagi T, et al. An in situ X ray diffraction study of the kinetics of the Ni_2SiO_4 olivine-spinel transformation. J. Geophys. Res., 1990, 95 (BIO): 15829-15844
[7] Brearley A J, Rubie D C, Ito E. Mechanisms of the transformations between the α, β and γ polymorphs of Mg_2SiO_4 at 15 GPa. Phys. Chem. Miner. , 1992, 8: 343-358
[8] Fujino K, Irifune T. TEM studies on the olivine to modified spinel transformation in Mg_2SiO_44． In: Syono Y, Manghnani M H, eds. High Pressure Research; Application to Earth and Planetary Sciences. Terra; AGU, 1975
[9] Brearley A J, Rubie D C. Transformation mechanisms of San Carlos olivine to ( Mg, Fe)_2SiO_4 β-phase under subduction zone conditions. Phys. Earth Planet. Int. , 1994, 86(1-3) ,45-67
[10] Rubie D C, Ross C R II. Kinetics of the olivine-spinel transformation in subducting lithosphere: experimental constraints and implications for deep slab processes. Phys.Earth Planet. Inter. , 1994, 86(1-3) : 223-241
[11] Kubo T, Ohtani E, Funakoshi K I. Nucleation and growth kinetics of the α-β transformation in Mg_2 SiO_4 determined by in situ synchrotron powder X-ray diffraction. Am. Miner. ,2004, 89: 285-293
[12] Hosoya T, Kubo T, Ohatani E, et al. Water controls the fields of metastable olivine in cold subducting slabs. Geophys. Res. Lett. , 2005, 32: L17305, doi: 10． 1029/2005GL023398
[13] Turnbull D. Phase Transformations in Metal and Alloys. In:Seitz F, Turnbull D, eds. Phase Change, in Solid State Physics, vol. 3． New York: Elsevier, 1956
[14] Jing Z C, Ning J Y, Wang S G, et al. Phase and thermal structures of the subduction zones. Geophys. Res. Lett. ,2002, 29(22) : 2045
[15] O'Connell R J. On the scale of mantle convection. Tectonophysics, 1977, 38(1-2) : 119-136
[16] Christian J W. The Theory of Transformations in Metals and Alloys Part I: Equilibrium and General Kinetic Theory. 2nd ed. Oxford; Pergamon Press, 1975． 449
[17] Porter D, Easterling K. Phase Transformations in Metal and Alloys. New York: Chapman and Hall, 1975
[18] Kubo T, Ohtani E, Kato T, et al. Effects of water on the or ft transformation kinetics in San Carlos olivine. Science,1998, 281(5373) : 85-87
[19] Wang S G, Ning J Y. Kinetics of olivine phase transformation and the role of water. Eos Trans. AGU, Fall Meet. Suppl. , Abstract, DI53A-1105． 2007, 88(52)
[20] Diedrich T, Sharp T G, Leinenweber K, et al. The effect of small amounts of H_2O on olivine to ringwoodite transformation growth rates and implications for subduction of metastable olivine. Chem. Geol. , 2007, 262(1-2) : 87-99
[21] Mosenfelder J L, Marton F C, Ross C R II, et al. Experimental constraints on the depth of olivine metastability in subducting lithosphere. Phys. Earth Planet. Inter. , 2001, 127(1-4) : 165-180
[22] Kubo T. Transformation kinetics in the earth' s interior. Rev. High Press. Sci. Technol. , 1999, 9: 26-33
[23] Ando J I, Tomioka N, Matsubara K, et al. Mechanism of the olivine-ringwoodite transformation in the presence of aqueous fluid. Physics and Chemistry of Minerals , 2006, 33(6) : 377-382
[24] Jung H, Karato S I. Water-induced fabric transitions in olivine. Science, 2001, 293(5534) : 1460-1463
[25] Karato S I, Jung H. Effects of pressure on high-temperature dislocation creep in olivine. Philos. Mag. , 2003, 83(3) : 401-414
[26] Nishihara Y, Shinmei T, Karato S I. Grain-growth kinetics in wadsleyite: Effects of chemical environment. Phys. Earth Planet. Inter. , 2006, 154(1) . 30-43
[27] Morishima H, Kato T, Ohtani E, et al. The phase boundary between α-and β-Mg_2SiO_4 determined by in situ X-ray observation. Science, 1994, 265(5176) : 1202-1203
[28] Liu M, Kerschhofer L, Mosenfelder J L, et al. The effect of strain energy on growth rates during the olivine-spinel transformation and implications for olivine metastability in subducting slabs. J. Geophys. Res., 1998, 103 ( B10) :23897-23909
[29] Katsura T, Yamada H, Nishikawa O, et al. Olivine-wadsleyite transition in the system ( Mg, Fe)_2SiO_4． J. Geophys. Res., 2004, 109 ( B2 ): B02209, doi:10． 1029/003JB002438
[30] Wood B J. The effect of H_2O on the 410-kilometer seismic discontinuity. Science, 1995, 268(5207) : 74-76
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