Elastic Wave Velocity of Brucite and Its Implications for Water Cycling in Subduction Zones
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摘要: 水镁石（brucite）是俯冲带水饱和橄榄岩的重要组成矿物之一，其体积分数可高达15%。研究水镁石在高压下的弹性波速，对于理解俯冲带含水橄榄岩的物质组成、速度结构以及水在深部的循环具有重要意义。以Mg(OH)2试剂为初始材料，在4 GPa、523 K的条件下热压2 h，合成了致密的多晶水镁石。在高达14 GPa的压力下，采用超声干涉法测量了水镁石的弹性波速和模量。研究发现，水镁石的弹性波速和模量随压力的增加而增大。结合地震学层析成像结果和矿物组合模型，利用Voigt-Reuss-Hill（VRH）模型，约束了日本东北俯冲带低速异常区的水含量。结果表明：俯冲板片上方地幔楔在20～40 km深度处的低速异常区水的质量分数为3.0%～10.0%，俯冲板片内部60～80 km深度处的低速异常区水的质量分数为1.0%～3.0%。Abstract: Brucite, a key constituent mineral in hydrated peridotites within subduction zones, can occupy up to the volume fraction of 15% of these water-saturated rocks. Investigating the high-pressure elastic wave velocities of brucite is thus crucial for understanding the composition, seismic velocity structure, and deep-water cycling processes of hydrated peridotites in subduction zones. In this study, dense polycrystalline brucite was synthesized from Mg(OH)2 reagent under 4 GPa and 523 K for 2 h. The elastic wave velocities and moduli of brucite were measured up to 14 GPa using ultrasonic interferometry. The results demonstrate that the elastic wave velocities and moduli of brucite increase with increasing pressure. By integrating seismic tomography with mineral assemblage modeling, we constrained the water content in the low-velocity anomaly regions of the mantle wedge using the Voigt-Reuss-Hill (VRH) model. Our estimations indicate that the water mass fraction ranges from 3.0%–10.0% in low-velocity anomaly zones of the mantle wedge above the subducting slab at depths of 20–40 km, and 1.0%–3.0% within the subducting slab at depths of 60–80 km beneath northeastern Japan.
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Key words:
- brucite /
- wave velocity /
- subduction zones /
- water content /
- high pressure
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图 1 (a) 热压实验组装，(b) 水镁石的背散射电子显微镜图和拉曼光谱，(c) 超声实验组装，(d) 7.8 GPa超声实验走时
Figure 1. (a) Assembly of hot pressing; (b) backscattered electron image and Raman spectrum of brucite; (c) assembly of ultrasonic experiment; (d) signal of travel time in the ultrasonic experiment at 7.8 GPa
图 3 水镁石的高压弹性波速(a)和模量(b)：圆圈为本研究测量数据，实线对应有限应变拟合得到的结果，虚线为Jiang等[18]的单晶布里渊测量结果
Figure 3. Velocities (a) and moduli (b) of brucite at high pressures: the circle represents the data from this study and the solid line represents the results obtained by finite strain fitting; the dashed line represents the previous single-crystal Brillouin results by Jiang, et al.[18]
图 5 日本东北俯冲带速度结构：(a)～(c)分别为日本东北俯冲带40°N处地震学层析成像的vP、vS和vP/vS剖面[1]（黑色实线为俯冲洋壳，绿色和黑色虚线为20～40 km和60～80 km深度的低波速异常区），(d)～(f) 为20～40 km异常区vP、vS和vP/vS的数据分布，(g)～(i) 为60～80 km异常区vP、vS和vP/vS的数据分布
Figure 5. Velocity structure of the subduction zone in NE Japan (a)–(c) vP, vS and vP/vS profiles obtained from seismic tomography at 40°N in the subduction zone of NE Japan[1] (Black solid line represents the subducting oceanic crust, and the green dashed line and black dashed line represents the low vP and low vP/vS anomaly areas at the depth of 20–40 km and 60–80 km.); (d)–(f) is the distribution of vP, vS and vP/vS in the anomaly area at the depth of 20–40 km; (g)–(i) is the distribution of vP, vS and vP/vS in the anomaly area at the depth of 60–80 km
图 6 日本东北俯冲带低速异常区的水含量：(a)～(c)为1.5 GPa和573 K条件下的结果，(e)～(f)为2.0 GPa和673 K条件下的结果（蓝色实线为地震学观测的速度范围，黑色实线和黑色虚线分别为利用VRH平均模型和矿物弹性性质计算的结果，蓝色虚线为地震学观测速度和矿物学计算速度共同约束得到的水含量）
Figure 6. Water content within the low-velocity anomaly zone of the subduction zone in NE Japan: (a)–(c) show the results under 1.5 GPa and 573 K; (e)–(f) present the results under 2.0 GPa and 673 K (The blue solid line represents the observed velocity by tomography. The black solid and black dashed lines represent the results calculated using the VRH average model combined with mineral elastic properties. The blue dashed line represents the water content obtained by joint constraint of seismic observation and mineralogy calculation.)
表 1 水镁石的高压弹性性质
Table 1. Elastic properties of brucite at high pressures
p/GPa ρ/(g·cm−3) $ {v}_{\mathrm{P}} $/(km·s−1) $ {v}_{\mathrm{S}} $/(km·s−1) KS/GPa G/GPa 1.9 2.43(1) 6.35(1) 3.76(1) 52(4) 34(2) 2.5 2.45(1) 6.53(1) 3.84(1) 56(4) 36(2) 3.0 2.48(1) 6.71(1) 3.91(1) 61(4) 38(2) 4.1 2.52(1) 6.97(1) 4.02(1) 68(4) 41(2) 5.1 2.56(1) 7.20(1) 4.11(1) 75(4) 43(2) 6.1 2.59(1) 7.43(1) 4.20(1) 82(4) 46(2) 6.9 2.62(1) 7.59(1) 4.26(1) 87(4) 48(2) 7.8 2.64(1) 7.76(1) 4.33(1) 93(4) 50(2) 8.6 2.67(1) 7.92(1) 4.40(1) 98(4) 52(2) 9.4 2.69(1) 8.03(1) 4.45(1) 102(4) 53(2) 10.0 2.70(1) 8.13(1) 4.49(1) 106(4) 55(2) 10.7 2.72(1) 8.26(1) 4.53(1) 111(4) 56(2) 11.3 2.73(1) 8.38(1) 4.56(1) 116(4) 57(2) 11.8 2.75(1) 8.47(1) 4.60(1) 119(4) 58(2) 12.3 2.76(1) 8.55(1) 4.64(1) 122(4) 59(2) 12.7 2.77(1) 8.62(1) 4.68(1) 125(4) 61(2) 13.2 2.78(1) 8.68(1) 4.71(1) 127(4) 62(2) 13.6 2.79(1) 8.75(1) 4.73(1) 130(4) 62(2) 14.0 2.80(1) 8.81(1) 4.75(1) 133(4) 63(2) 
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表 2 方辉橄榄岩中主要矿物的弹性参数
Table 2. Elastic parameters of major minerals in harzburgite
Minerals $ {\rho }_{0} $/(g·cm−3) KS0/GPa $ {{K}_{\rm{{S}0}}'} $ $ ({\partial {K}}_{\rm{S}}/\partial T) $/
(GPa·K−1)G0/GPa $ {{G}_{0}'} $ $ (\partial G/\partial T) $/
(GPa·K−1)$ {\alpha }_{0} $/(10−5 K−1) Ref. Brucite 2.333 40.0 7.0 −0.011 30 2.5 −0.010 5.00 This study Antigorite 2.56 41.0 14.1 −0.010 26.48 1.3 −0.006 3.92 [6] Chlorite 2.54 79.2 3.7 −0.006 47.4 −0.3 −0.003 2.50 [29] Amphibole 2.97 98 3.5 −0.004 62 1.15 −0.011 4.94 [41] Talc 2.70 56 5.4 −0.006 29 4.7 −0.005 2.61 [3] Olivine 3.34 130.3 4.6 −0.016 77.4 1.61 −0.013 3.03 [39] Orthopyroxene 3.21 108.5 7.2 −0.026 77.9 1.7 −0.013 2.97 [31, 37] Clinopyroxene 3.3 116.5 5.0 −0.013 72.8 1.7 −0.014 2.60 [40, 42]
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